477 lab manual

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Table of Contents

1. Introduction. .............................................................................................................................................................. 12. Equipment and Time Requirements. ......................................................................................................................... 13. Safety Considerations. ............................................................................................................................................... 2

3.1 Lab Safety. ........................................................................................................................................................... 23.2 Clothing ............................................................................................................................................................... 33.3 General Handling of Laboratory Chemicals. ....................................................................................................... 33.4 Handling of Solvents. .......................................................................................................................................... 43.5 Handling of Acids and Bases. .............................................................................................................................. 43.6 Emergency Eye Wash Information. ..................................................................................................................... 53.7 Furnace Safety. .................................................................................................................................................... 5

4. Laboratory Reports and Records. .............................................................................................................................. 65. Lab Week 1: Wafer Documentation. ......................................................................................................................... 6

5.1 Overview ............................................................................................................................................................. 65.2 Wafer Handling. .................................................................................................................................................. 75.3 The 4-point probe ................................................................................................................................................ 75.4 Measurements ...................................................................................................................................................... 85.5 Analysis ............................................................................................................................................................... 9

6. Lab Week 2: Field Oxidation [100 min]............................................................................................................... 116.1 Overview ........................................................................................................................................................... 116.2 Process Steps ..................................................................................................................................................... 116.3 Analysis ............................................................................................................................................................. 12

7. Lab Week 3: Lithography with mask 1 [100 min]................................................................................................ 137.1 Overview ........................................................................................................................................................... 137.2 Measurement ..................................................................................................................................................... 137.3 Process Steps ..................................................................................................................................................... 137.4 Analysis ............................................................................................................................................................. 15

8. Lab Week 4:Boron diffusion (pre-deposition): [115 min]...................................................................................... 178.1 Overview ........................................................................................................................................................... 178.2 Process Steps ..................................................................................................................................................... 178.3 Analysis ............................................................................................................................................................. 18

9. Lab Week 5: Diffusion (drive-in): [85 min] ............................................................................................................ 199.1 Overview ........................................................................................................................................................... 199.2 Measurement ..................................................................................................................................................... 199.3 Process Steps ..................................................................................................................................................... 199.4 Analysis ............................................................................................................................................................. 20

10. Lab Week 6: Lithography (mask 2): [65 min] ....................................................................................................... 2110.1 Overview ......................................................................................................................................................... 2110.2 Process Steps ................................................................................................................................................... 21

11. Lab Week 7: Gate oxidation: [110 min] ................................................................................................................ 2311.1 Overview ......................................................................................................................................................... 2311.2 Process Steps ................................................................................................................................................... 2311.3 Analysis ........................................................................................................................................................... 24

12. Lab Week 8: Lithography using mask 3 for contact cuts: [115 min], metallization. ............................................. 25

12.1 Overview ......................................................................................................................................................... 2512.2 Measurement ................................................................................................................................................... 2512.3 Process Steps ................................................................................................................................................... 2612.4 Analysis ........................................................................................................................................................... 28

13. Lab Week 9: Lithography using mask 4 for patterning metal [120 min]. .............................................................. 2913.1 Overview ......................................................................................................................................................... 2913.2 Process Steps ................................................................................................................................................... 2913.3 Analysis ........................................................................................................................................................... 32

14. Device Testing ....................................................................................................................................................... 3314.1 Curve Tracer Introduction ............................................................................................................................... 33

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Prepared byDon Reinhard and Dean Aslam; F08

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14.2 Using the curve-tracer to measure the current-voltage characteristics of a resistor. ....................................... 3414.3 Using the Curve-Tracer to measure the current-voltage characteristics of a diode. ........................................ 3514.4 Using the Curve-Tracer to measure the current-voltage characteristics of a MOSFET. .................................. 3814.5 Capacitance vs. voltage measurements for MOS capacitors ........................................................................... 39

16. Solution Mixing Guidelines ................................................................................................................................... 40References: .................................................................................................................................................................. 42

Notes: ........................................................................................................................................................................... 42

List of Figures

Figure 1 - The Four-Point Probe Method ...................................................................................................................... 7Figure 2 - Resistivity vs. Doping [See reference 4]. Note that the vertical scale is logarithmc. .................................. 10Figure 3 - Photoresist Data from Shipley Corp. .......................................................................................................... 15Figure 4 - Wafer Pattern for Mask 1 ............................................................................................................................ 16Figure 5 - Detail of Mask 2 Result on Wafer............................................................................................................... 21Figure 6 - Detail of Features on the Glass of Mask 3 .................................................................................................. 26Figure 7 - Detail of the Features on the Glass Mask 4 ................................................................................................. 30Figure 8 - A 'Chip' on the Processed Wafer ................................................................................................................. 30Figure 9 - Identification of Devices ............................................................................................................................. 32

Figure 10 - Front View of the Curve Tracer ................................................................................................................ 33Figure 11 - Microprobe Contacts to a Resistor Fabricated in the MSU Student Lab .................................................. 34Figure 12 - Curve-Tracer Display for a Resistor ......................................................................................................... 35Figure 13 - Microprobe Contacts to a Diode when a Surface n+ Contact is Available ............................................... 36Figure 14 - Forward I-V Characteristics for a Student-Fabricated Diode (top) and Reverse Characteristics (bottom).

............................................................................................................................................................................. 37Figure 15 - Family of Curves for a Student-Fabricated P-Channel MOSFET ............................................................. 38Figure 16 - Capacitance vs. Voltage for a Student-Fabricated MOS Structure ........................................................... 39

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1. Introduction.

In this Laboratory Manual, experiments are described which allow the student to learn

several of the concepts basic to microeelctronic fabrication in a learning-by- doing environment.

Topics addressed include doping principles, diffusion, oxidation, metallization,

photolithographic pattern definition, etching, the MOS interface, threshold voltageconsiderations, MOSFETI-Vcharacteristics, MOS capacitor diagnostics, diffused resistors, and

diffused junction diode I-Vcharacteristics. In addition, the student is provided with a unique

opportunity to compare theoretical predictions with actual device performance since he or shehas carried out the fabrication and knows exactly what went into the making of the chip.

During the testing and analysis portion of the experiment, properties of the MOS system,

diffused layers, p-n junctions, and field-effect transistors are studied. Therefore, in spite of thesimplicity of the fabrication steps, hands on experience with several key issues is obtained. The

experiments have formed the laboratory portion of a combination lecture and laboratory course

offered at Michigan State University for the past several years, primarily to seniors in electricalengineering.

Lego-based demos will be used during the lectures to demonstrate the device fabricationand operation concepts. Some demos of complete microsystems will also be used.

2. Equipment and Time Requirements.

One academic semester provides sufficient time to finish the oxidation experiment andthe diffused devices experiment with the laboratory section meeting once a week for three hours

a week. Experience shows that the schedule below is a suitable minimum progress schedule.

Lab Week 1 Wafer characterization.

Lab Week 2 Wafer cleaning and oxidation.Lab Week 3 Mask 1: photoresist exposure and development for boron diffusions.

Lab Week4 Oxide etch and boron pre-deposition.

Lab Week 5 Boron drive-in.Lab Week 6 Mask 2: photoresist exposure and development for gate oxidation; oxide

etch.

Lab Week 7 Gate oxidation.

Lab Week 8 Mask 3: photoresist exposure and development for contact cuts; oxideEtch. (Aluminum evaporation done by TA.)

Lab Week 9 Mask 4: photoresist exposure and development for metallization; etch

metal.Lab Weeks 10 - 11 Testing and analysis.

The laboratory is equipped with four furnaces for diffusions and oxidations, a photoresistspinner, three photoresist ovens, a mask aligner, an aluminum evaporator, a deionized water

supply, a four point resistivity probe, and wafer probe stations. An incident light microscope

provides magnification levels from 65X to 400X. Also instrumentation for measurement of

capacitance vs. voltage and current vs. voltage are used with the wafer probe stations. As isoutlined in the following sections, each of these equipment items plays an important role in the

experiments.

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All the above equipment, except for the diffusion furnaces and the evaporator, are housed

in a room under positive pressure with class 100 filtered air. However, only a few basic precau-tions are taken to maintain adequate clean room conditions for a student laboratory. Students

wear lab coats while working in the area and if shoes are dirty, disposable shoe covers are

available. Full cleanroom apparrel is not used, so class 100 conditions do not exist in the working

area. However, the simple Fabrication process used in these experiments, along with fairly largedimensions (the smallest critical dimension is approximately 10 m), combine to provide a fairly

robust process. The diffusion furnaces open into a laminar flow hood with class 100 air. Thishood also has a sink and most of the wet chemistry is performed in this hood.

For sake of safety, diffusions are carried out with commercial solid planar sources in

which the active dopant component is dispersed in an inert, refractory matrix. Toxic gases arenot used in these exercises.

Students usually work in groups of two or three and there are three groups in each

section. Each group has its own wafer and is responsible for carrying the wafer through all thefabrication steps. However, at some points in the fabrication sequence a batch process involves

all the wafers in a section. For example each diffusion furnace is dedicated to a given purpose.

One is used only for oxidations, one is used only for boron pre-depositions, and one is used onlyfor boron drive- ins. The fourth is for aluminum annealing and also for optional n-type

diffusions. Thus, although each group is individually responsible for the handling, cleaning,

photolithography, mask alignment, etching, and testing of their wafer, it is not practical for each

group to separately do a boron drive-in, for example. Therefore, the sections activities areplanned such that all the wafers go into the drive-in at the same time. Usually all furnace

operations are done in a batch process, as well as the metallization.

3. Safety Considerations.

3.1 Lab Safety.

These laboratory exercises contain hazards not normally encountered in an electrical

engineering laboratory. Principally, the hazards involve the use of chemicals including strong

acids, solvents, and bases. With proper precautions, these chemicals can be used quite safely. But

without due respect for the chemicals, serious problems could result. It is important that thestudent reads and is aware of the characteristics of each chemical used in the laboratory. Also, in

order to be prepared for the unlikely event of an accident, he or she should be aware of a variety

of emergency information such as that listed below.

Know the emergency phone number for the institution, and the location of a phone

which is always available when the laboratory is in session. Know the location of the Material Safety Data Sheets (MSDSs) for chemicals usedin the laboratory.

An emergency shower, an eye wash fountain, a fire extinguisher, respirator mask, anda first aid kit should be available near the work area. Know their location.

In any accident, get assistance. If acid or base splashes into the eye, flush the eyeprofusely at the eye wash station for 15 minutes. The first few seconds are critical.Your instructor or lab partner should make sure that your eyes are open during the

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rinse and should hold them open if necessary. Section 3.6 contains more specific

information about eyewash procedures.

In case of a chemical spill, alert everyone in the specific spill area and notify the TAor ECE technicians.

If acid or base spills, or splashes, onto the skin, rinse immediately with cold water for

15 minutes.Notify the TA or ECE technicians. Wear proper clothing. If an acid or base spills on clothing, remove the clothing.

3.2 Clothing

Shoes should be worn rather than sandals. Long pants should be worn instead of shorts ordresses.

Minimum personal protective equipment includes a lab coat, disposable gloves, andsafety glasses. Do not wear contact lenses.

Addiionally use a faceshield when working with wet chemicals. Face shields offerprotection for the face but not the eyes.

Additinally use rubber gloves when working with wet chemicals. Remove gloves before leaving the laboratory and always wash hands after removing the

gloves.

3.3 General Handling of Laboratory Chemicals.

Several basic guidelines for the general use of chemicals such as used in these exercises arelisted below.

Acids, bases, and strong solvents should always be used in a hood. Photoresist relatedchemicals should be used in well ventilated areas.

All chemicals should be disposed of according to the MSU ORCBS guidelines.

Chemicals such as solvents, acids, and bases require special pick-up. Do not pourthem down the drain.

Generally, it should not be necessary for a student to mix any of the chemicals. Thiswill be done by staff before the laboratory starts. Likewise, the disposal of chemicals

is normally the responsibility of the staff. In special cases where the student is given

and accepts the responsibility of preparing and mixing chemical solutions, see thedetailed instructions on solution mixing guidelines.

When in doubt of the proper procedure, never guess. Always ask your instructor.

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3.4 Handling of Solvents.

Solvents used in these exercises primarily may include methanol (methyl alcohol),

acetone, and 2-propanol (isopropyl alcohol). Some particular comments pertaining to solvents

are listed below.

Rubber gloves are important when using solvents, not necessarily because they arehazardous to the skin, but because they can be absorbed through the skin.

Solvents are volatile and the fumes they give off are highly flammable. If a solvent in abeaker should happen to ignite, a cover may be placed over the top to extinguish the

flames.

Use solvents in a well ventilated area, preferably under a hood. Avoid breathing thevapors as much as possible.

Never mix an acid and a solvent. Never heat an acid and solvent on the same hot plate.

Some characteristics of the solvents used in these exercises are listed below. For

additional information, see reference 1, Dangerous Properties of Industrial Materials by Sax andthe MSDS sheets.

Acetone(CH3COCH3) is a mild skin irritant and can cause headaches due to prolonged

inhalation, but its primary hazard is that it is very flammable when exposed to

heat or flames. Also it can react violently with certain acids and bases.

Isopropyl Alcohol (CH3CHOHCH3), or 2-propanol, is in most medicine cabinets as themain ingredient of rubbing alcohol. In spite of its familiarity, it should be noted

that it can cause eye irritation and eye damage. It also is flammable and can react

violently with acids.Methyl Alcohol (CH3OH), or methanol, is flammable and reacts violently with several

acids. On very severe exposures, methanol shows toxicity for the nervous system,especially for the optic nerves.

3.5 Handling of Acids and Bases.

Strong acids and bases may represent the most serious hazard in the laboratory. As a gen-eral rule, always wear rubber gloves (without holes), lab coats or aprons, and a face shield along

with eye protection when working with any acid or base. Again, always work under the hood.

Some particular high-lights of the acids and bases that may be used in these exercises are listedbelow. Again, for more information, see Dangerous Properties of Industrial Materials, by Sax

and the MSDS sheets.

Sulfuric Acid(H2SO4) is extremely corrosive and toxic to body tissues. It will rapidly

attack the skin and cause severe burns and blistering.

Nitric Acid(HNO3) can also destroy tissue and cause burns, but is not quite asaggressive as sulfuric acid. Nitric acid leaves a characteristic yellow stain on the

skin.

Hydrofluoric Acid(HF) produces severe skin burns which are slow in healing. It is

extremely corrosive to tissues and is all the more dangerous in that burns may not

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be noticed until some time after exposure. Therefore, HF should be handled with

great caution. Rinse your hands carefully after using HF even if you did userubber gloves. Avoid the fumes.

Hydrochloric Acid(HCl) is strongly corrosive and causes burns on contact with the

skin.

Phosphoric Acid(H3PO4) should not be heated to the boiling point since the POXfumes are toxic. It will cause burns if in contact with the skin.

Ammonium Hydroxide (NH4OH) is a powerful base capable of causing severe burns. It

should always be used with adequate ventilation.Acetic Acid(CH3COOH) is highly corrosive and causes severe burns on contact with

the skin. Inhalation of concentrated vapors may cause serious damage to the

lining of the nose, throat, and lungs. Neither odor nor degree of irritation areadequate to indicate vapor concentration. Use only with adequate ventilation.

Generally the above acids and bases will be encountered in solutions containing morethan one chemical. Some solutions contain hydrogen peroxide, described below.

Hydrogen Peroxide (H2O2) is also found in many household medicine cabinets in

dilute solution as a topical antiseptic. However, it is used in concentrated form inthese exercises. Solutions of H2O2 of 35 weight% and over, such as are used in

this laboratory, will easily cause blistering of the skin. In solution with acids,

hydrogen peroxide can be highly volatile. Also, it is important to keep containersof this material covered since the water from an H2O2 solution can evaporate,

concentrating the material, and increasing the fire hazard of the remainder.

3.6 Emergency Eye Wash Information.

In severe contamination cases, the eye may involuntarily clamp shut, and the individual

may be in a state of shock. Therefore the person may need assistance from the instructor andfellow students in getting to and using the eye wash station. See also the instructions on the eye

wash station. When the eyes are in the streams of solution, the thumb and forefinger of each

hand should be used to open the eyelids. Hold the eyes open until thoroughly flushed - 15

minutes is recommended. Simultaneously, emergency medical personnel should be summoned.Also notify the EC Shop, the Laboratory Supervisor, or the ECE Office.

The emergency eye wash unit is intended for the emergency first aid of victims of fluid chemicalor small soft particle material eye contamination until medical treatment is obtained. In all cases

the victim should seek medical attention immediately after flushing. When transporting the

victim to professional medical care, the eyes should continue to be irrigated with a buffered,physiologically correct solution.

3.7 Furnace Safety.

Furnace temperatures may be as high as 1100 C in the flat zone region inside the tube.

Observe the following procedures when working with a furnace:

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Although the sides of the furnace are generally cool to the touch, the quartz ends may bequite hot. When removing an end cap, use a thick cloth, or thick glove, for protection.When a push rod is removed from the furnace, the end will be too hot to touch for about

five minutes.

Glass components on the furnace set-ups are actually fused silica, which means they areexpensive. Handle them carefully. Do not apply a torque; the push rods are especially

fragile. The quartz may be slippery so be certain of your grip.

There is a continuous flow of gases through the furnaces. Gas cylinders may be underpressures of over 2000 psi, so must be properly secured and handled. Generally the

student should not adjust flowmeter or regulator controls unless supervised by the

instructor.

Scavenge the output of the furnace tubes to an external exhaust.

3.7 Electrical Safety.

Use caution regarding electrical hazards as with any electrical engineering laboratory.Some particular notes are as follows.

Some pieces of test equipment, such as curve tracers, are capable of producing overhundred volts and fatal shocks. Keep the voltage at zero until the leads are safely

connected to the device or circuit being tested.

Equipment such as furnaces and mask aligners may have high voltage circuitry behindprotective panels which is not intended to be worked with during the laboratory.

4. Laboratory Reports and Records.

Each group is individually responsible for a laboratory notebook. The laboratory note-book should contain a daily record of laboratory work, observations, measurements taken, andresponses to analysis questions. All data, records of experiments, and observations during the lab

should go directly into the notebook in ink, with the instructor initialing the pages at the end of

the period. See handout #1 for grading details.

5. Lab Week 1: Wafer Documentation.

5.1 Overview

Safety First:

1) Wear the minimum personal protective equipment, i.e. a lab coat, disposable gloves, andsafety glasses.

Goal: Determine the carrier concentration of the p-type wafer that you will be using this

semeseter.

Flow: -> make 4-point probe measurements -> determine the sheet resistance -> find the

corresponding donor concentration.

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5.2 Wafer Handling.

The starting point for this experiment is a uniformly doped, polished, n-type silicon wafer

whose background doping is reasonable for basic MOS and junction devices; an approximate

range is 2 x 1014

to 5 x 1015

cm-3. It will be necessary to know the actual carrier concentration inlater analysis of results. In the steps that follow, you will be handling the wafer with tweezers

and carrying it from one work area to another. Clean the tweezers at the beginning of the lab byrinsing in acetone, methanol, and deionized water in that order. Dry the tweezers with N 2 gas.

When carrying the wafer, always hold it over a piece of filter paper, or a KimwipeTM

, in case the

wafer slips from the tweezers.

During these experiments, never touch the wafer with bare hands and never touchanything that will come in contact with the wafer, such as the quartz furnace components (push

rods, boats, etc.) with your bare hands. When working with dry wafers or quartz furnace

components, use clean gloves. When working with wet processes, use rubber gloves. Smallamounts of impurities, particularly sodium ions from the skin or from impure water, will wreck

an MOS process. Always make sure beakers are clean. One improperly cleaned beaker, orinadvertent contact with tap water can send MOSFET threshold voltages well out of usefulranges of values.

5.3 The 4-point probe

A common method for the measurement of sheet resistance is the four point probe method

shown in Figure 1 for the case of both a circular substrate (wafer) and a rectangular substrate.

Here four equally spaced point probes are brought into contact with the wafer surface and aknown dc current is passed through the two outer probes. The inner two probes are connected to

a high impedance voltmeter. This 4 point arrangement offers the advantage that contact

resistance effects are largely eliminated since the voltage measurement probes draw negligiblecurrent. The relationship between the measured current and voltage and the material resistivity is

a calculable function of the probe spacing and the substrate geometry.

Figure 1 - The Four-Point Probe Method

If the substrate dimensions are much larger than the probe spacing, s, then the relationship

between V, I, and the sheet resisance RS is

RS = (/ln 2)(V/I) = 4.53(V/I).

For smaller dimension samples, a different constant of proportionality must be used as listed inTable 1. Additional information is in reference 2 (Smits). Alternatively, the sheet resistance may

be determined by incorporating a 4 probe test pattern into the wafer layout as a test site.

TABLE 1 Constant of proportionality between RS and V/I for a rectangular sample of

dimensions a and d, and for a circular sample of diameterd(see Figure 1).

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d/s ------------RECTANGLES--------------------- CIRCLES--

a/d = 1 a/d = 2 a/d = 3 a/d = 4

1.0 0.9988 0.99941.25 1.2467 1.2248

1.5 1.4788 1.4893 1.48931.75 1.7196 1.7238 1.72382.0 1.9454 1.9475 1.9475

2.5 2.3532 2.3541 2.35413.0 2.4575 2.7000 2.7005 2.7005 2.26624.0 3.1137 3.2246 3.2248 3.2248 2.9289

5.0 3.5098 3.5749 3.5750 3.5750 3.36257.5 4.0095 4.0361 4.0362 4.0362 3.927310.0 4.2209 4.2357 4.2357 4.2357 4.1716

15.0 4.3882 4.3947 4.3947 4.3947 4.364620.0 4.4516 4.4553 4.4553 4.4553 4.436440.0 4.5120 4.5129 4.5129 4.5129 4.5076

4.5324 4.5324 4.5324 4.5324 4.5324

5.4 Measurements

When carrying out the four point probe measurement, note that the probe tips are rather

fragile. Once the probes are in contact, the wafer should not be moved. Place the probe over themiddle of the wafer. Lower the probes until contact is made and record the voltage and current

readings. The readings may drift with time, but the ratio of V/I should be relatively constant. Do

this 5 times, shifting the wafer slightly every time you lift the probe for another measurement.Record the experimental values, and any observations of interest in your lab book.

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5.5 Analysis

Calculate the average of your (V/I) values, the maximum value, the minimum value andthe standard deviation.

Record the probe spacing, s.

Record the wafer diameter, d. Determine the 4-point probe correction factor.

Calcuate the wafers sheet resistance and standard deviation.

Record the wafer thickness.

Calculate the silicon resistivity based on average values and max/min values.

Uses the graph of resistivity vs impuirity concentration in Figure 2 to determine the netdonor conctration. Estimate the uncertainly associated with this value based on max/minconsiderations.

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Figure 2 - Resistivity vs. Doping [See reference 4]. Note that the vertical scale is logarithmc.

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6. Lab Week 2: Field Oxidation [100 min]

6.1 Overview

Safety First:

1) See section 3 for details on safety. You will work with hazardous chemicals which can beharmful if you are not careful.2) Know the location of a phone, eye wash fountain, fire extinguisher, respirator, and a first aid

kit near your work area in the lab.3) Remember to use face shields and rubber gloves with chemicals.

Goal: Grow field oxide which will act as a masking layer for p-type diffusion.

Process Flow: RCA clean, oxidize

6.2 Process Steps

1) Place your n-Si wafer in a carrier basket. Using the basket, immerse the wafer in a beakercontaining acetone. Use mild agitation for 20 seconds.

Acetone and methanol are solvents that are volatile, flammable, hazardous to skin and can

be absorbed through skin. All chemicals will be prepared and disposed off by the TA. All

beakers must be labeled.

2) Remove the wafer and basket from the acetone beaker and immerse them in a beakercontaining methanol. Use mild agitation for 20 seconds. This removes the acetone residue.

3) Remove the wafer and basket from the methanol beaker and immerse them in a beakercontaining DI water. Use mild agitation for 20 seconds.

4) Take the wafer out of the basket and place it in a wafer holder.5) RCA clean (see reference 3). The RCA clean is a 2-step cleaning procedure (degrease and

demetal etch solutions) which is usually performed before any high temperature (800 1100C) process step: a. Rinse the wafer in running DI water for 3 min, dry the wafer with N2 and

transfer it to a basket labeled degrease etch. b. Immerse wafer in freshly prepared degrease etch for 10 min at 50 C (10%). Degrease etch, which removes organic impurities, consists of75 ml

H2O:15 ml H2O2 (30% solution):15 ml NH3OH.

Degrease or demetal etch solution must stay inside the hood. If it spills on clothes,

remove clothes. Never mix an acid and a solvent or heat them on the same hot plate.

See section 3 for details.

c. Remove the wafer and basket from degrease etch beaker and immersethem in beaker containing DI water for 30 seconds. d. Remove the wafer and basket from the DI water beaker and rinse them inrunning DI water for 3 minutes.

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7. Lab Week 3: Lithography with mask 1 [170 min]

7.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember proper lat attire.

Goal: Measure oxide thickness, etch a pattern (holes) in the field oxide.

Process Flow: -> measure oxide thickness -> spin PR -> expose and develop to generate

pattern -> etch oxide -> remove PR

7.2 Measurement

1) Measure the oxide thickness using the ellipsometer:a) Turn on the ellipsometer power (turn the key to clockwise).

b) Place the wafer and set polarizer drum to 85 and analyzer to 45 degree.c) Adjust the gain control until meter reads approximately 140 to 200.d) Rotate the analyzer drum (staying in the red region) to yield the lowest reading. If meter

reading is over 100 use gain control to go below 100. Rotate polarizer drum, (staying inthe red region) to get an even lower reading. Repeat the process, going back and forth

between the analyzer rotation and polarizer rotation, until a minum is reached. Read the

analyzer drum reading (this is called the A1 value). Read the polarizer drum reading (thisis called the P1 value).

e) A program to utilize these values is available on the computer next to the ellipsometer.The program is started via a desktop icon. Utilize the F5 key and enter A1 and P1 values.

The program will then provide approximately analyzer drum and polarizer drum values

for the second minimum, referred to as A2 and P2 values respectively. Set the drumreadings to these values and iterate back and forth between the analyzer drum and the

polarizer drum to get a minimum again. These are the actually A2 and P2 values.f) Enter the A2 and P2 values in the computer. The software will provide you with a oxide

thickness. You can also request the program to provide all possible oxide thicknesses

using a list option at the bottom of the screen.g) If there is ambiguity ad to the actual oxide thickness, one may repeat with a different

angle and determine the most likely thickness.

7.3 Process Steps

1) Put the wafer in carrier and immerse the wafer in acetone for 20 s.2) Immerse the wafer in methanol for 20 s.3) Immerse the wafer in DI for 20 s.4) Rinse the wafer in DI for 3 min.5) Blow dry the wafer carefully with N2.6) Bake the wafer at 65 C for 15 min (this step will make sure that photoresist adhesion to wafer

is good).

7) Spin positive photoresist (PR)10

on the wafer: (Exposed areas of positive photoresist will be

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etched away by the developer solution.)

a) Using a dropper, cover the entire wafer with positive PR.b) Spin the wafer for 30 s at 3000 rpm.

8) Bake the wafer for 20 min at 65 C.9) Now you will use the mask aligner and mask 1 to generate a pattern in PR:

a) Turn on the vacuum pump providing vacuum for the mask aligner. Direct the vacuum toaligner.b) Turn on the low and high pressure N2 gas.c) Turn on the two toggle valves on the manometer box.d) Turn on the power switch on the UV lamp controller. Wait for ready display and press

start button.

e) Turn on the mask aligner, video monitor, CCD camera, and microscope.f) Place the mask on the holder (Pattern face up on the table so that the mask pattern will be

adjacent to the wafer when the holder is placed in the stage).

g) Turn on the vacuum switch (toggle switch is down for vacuum on).h) Place the mask holder in the stage and turn on maskholder button.

i) In the auto mode push start button, this will move the lamp house to left.j) Place the wafer on the wafer stage and press the foot switch.k) Contact light will light up / if not press separate button.l) Press exposure button. A typical exposure time is 7 seconds for Shipley 1830 resist for a

lamp power of approximately 275 watts.

m)Remove the wafer and put it in a wafer holder. [This step will be completed by TA]: Turnoff the mask holder button and remove mask from stage. Turn off the mask aligner,

monitor, UV lamp controller. Wait for 15 to 30 min. And turn down two manometer

toggle valves. Turn off the vacuum pump and close the N2 gas tanks.10)The exposed areas of the positive PR can now be dissolved in developer to generate a pattern

in PR:a) Immerse the wafer and basket in the developer agitating continuously for 45 - 50

seconds.

b) Immerse the wafer in a DI beaker immediately. Leave it in the beaker for about 15seconds, then remove the wafer and rinse it in running DI for about 1 min.

c) Blow dry with N2. If you inspect the wafer with the microscope, use the yellow filter.d) Postbake for 15 min at 135 C.

11)Etch window: Using PR as a mask, you will now use the following sequence to etchwindows in the field oxide (p-diffusion will take place in n-Si through these windows in the

next experiment):

a) Prepare 3 beakers; Buffered Oxide Etch, DI-1 and DI-2. Buffered etch is a mixture of 1HF (50 %):7 NH4F (40%).

b) Immerse the wafer and basket into Buffered Etch (Etch rate is approximately 100 nm permin. Estimate the etch time using the oxide thickness).

c) Immerse the wafer into DI-2 for SiO2 etching test: Observe the back surface of the wafer.If it is wet (i.e. covered with a continuous film of water) thenthe SiO2 is not completely

etched. If a few isolated drops are found on the backside, then the SiO2 is completely

etched away.d) If SiO2 is still remains, immerse the wafer into Buffered Etch again.e) When SiO2 is completely etched away, rinse the wafer in DI for 3 min.

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f) Blow dry with N2.12)Inspect the wafer under microscope: etched away areas should appear white and spot-free.13)Remove PR:Now that windows have been etched in SiO2, PR has served its purpose and

needs to be removed:

a) Immerse the wafer in the PR remover for 3 min.

b) Rinse with Acetone for 20 sec.c) Rinse with Methanol for 20 sec.d) Rinse with DI for 3 min.e) Blow dry with N2.

7.4 Analysis

Compare the measured oxide thickness to the calculated value from last weeks lab.

Based on the manufactures information for the photoresist and the spin speed and timeused in the lab, estimate the photoresist thickness.

Figure 3 - Photoresist Data from Shipley Corp.

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The mask 1 wafer pattern is shown in Figure 4. Draw a cross-sectional view for theportion indicated and numerically label the horizontal and vertical dimensions of interest.

Figure 4 - Wafer Pattern

for Mask 1

Show cross-

section

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8. Lab Week 4:Boron diffusion (pre-deposition): [170 min]

8.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember the furnace rod end will be hot. Dont touch it for 5 minutes after removal fromthe furnace.

3) Remember proper lab attire.

Goal: Carry out a p-type diffusion (pre-deposition) into n-type Si.

Process Flow: RCA clean -> diffusion -> etch borosilicate glass

8.2 Process Steps

1) RCA clean: You will now do RCA clean before the wafer goes in the furnace for pre-deposition diffusion:a) Rinse the wafer in running DI water for 3 min, dry the wafer with N2 and transfer it to a

basket labeled degrease etch.b) Immerse wafer in freshly prepared degrease etch for 10 min at 50 C (10 %). Degrease

etch, which removes organic impurities, consists of75 ml H2O:15 ml H2O2 (30%

solution):15 ml NH3OH.

Degrease or demetal etch solution must stay inside the hood. If it spills on

clothes, remove clothes. Never mix an acid and a solvent or heat them on the

same hot plate. See data sheet # 1 for details.

c) Remove the wafer and basket from degrease etch beaker and immerse them in beakercontaining DI water for 30 seconds.

d) Remove the wafer and basket from DI water beaker and rinse them in running DI waterfor 3 min.

e) Immerse the wafer and basket in freshly prepared demetal etch for 10 min at 50 C (10%). Demetal etch, which removes metalic impurities, consists of75 ml H2O:15 ml H2O2(30% solution):15 ml HCl.

See section 3 for safety details.

f) Remove the wafer and basket from demetal etch beaker and immerse them in beaker

containing DI water for 30 seconds.g) Remove the wafer and basket from DI water beaker and rinse them in running DI water

for 3 min.

2) Take the wafer out of the basket and place it in a wafer holder.3) Dry the wafer with N2. The wafer should be spotless with no stains or blemishes.4) Pre-deposition diffusion:

a) Place the wafer in quartz boat.

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b) Check that the furnace temperature is 925 C and flow rate is 400 sccm of N2.c) Slide the boat into the center of furnace slowly at 5cm per 10 sec. (3 min. total).d) Do pre-deposition for 60 min.e) Pull the boat out to end of the tube and let it cool for 3 minutes.f) Pull the boat out of the furnace and let it cool another 3 minutes.

5) Switch the furnace gas flow and temperature back to idle condition (750 C, 30 sccm).6) Etch borosilicate glass:a) Place the wafer in basket and immerse the wafer in 9:1 H20:HF solution for 40 s.b) Rinse for 3 min in running DI.c) Immerse the wafer in Borosilicate Etch solution [50ml H2SO4: 50ml HNO3] for 7

minutes.

d) Rinse for 3 min in running DI.e) Immerse the wafer in 50:1 H20:HF solution for 30 s.f) Rinse for 3 min in running DI and blow dry with N2.

7) Store the wafers in a clean area for the next experiment.

8.3 Analysis

Assuming a constant-source diffusion with N0 determined by the solid solubility, useequation 4.4 of Jaeger (course text) and the original n-type concentration of your wafer to

determine the depth of the p-n junction resulting from the pre-deposition diffusion.

Use the Irvin curves of Jaegers chapter 4 to estimate the sheet resistance associated withthe pre-deposition diffusion.

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9. Lab Week 5: Diffusion (drive-in): [85 min]

9.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember the furnace rod end will be hot. Dont touch it for 5 minutes after removal from

the furnace.

3) Remember to use a face shield and rubber gloves.

Goal: Test area evaluation, drive-in diffusion.

Process Flow: test area evaluation -> RCA clean -> drive-in diffusion

9.2 Measurement

1) Check at least two test regions with the four point probe. Take 5 measurements for each,lifting the wafer slightly each time but remembering not to more the wafer when the tips are

down. Record the V and I values

9.3 Process Steps

1) Inspect the wafer und microscope. If you see some spots on the wafer go through the acetoneand methanol cleaning procedure before starting the RCA clean.

2) RCA clean:a) Rinse the wafer in running DI water for 3 min, dry the wafer with N2 and transfer it to a

basket labeled degrease etch.b) Immerse wafer in freshly prepared degrease etch for 10 min at 50 C (10 %). Degrease

etch, which removes organic impurities, consists of75 ml H2O:15 ml H2O2 (30%solution):15 ml NH3OH.


remove clothes. Never mix an acid and a solvent or heat them on the same hot plate.

See data sheet # 1 for details.



e) Immerse the wafer and basket in freshly prepared demetal etch for 10 min at 50 C (10%). Demetal etch, which removes metalic impurities, consists of75 ml H2O:15 ml H2O2(30% solution):15 ml HCl.

See section 3 for safety details.

f) Remove the wafer and basket from demetal etch beaker and immerse them in beakercontaining DI water for 30 seconds.

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g) Remove the wafer and basket from DI water beaker and rinse them in running DI waterfor 3 min. Blow dry with N2.

3) Drive-in diffusion:a) Set the Oxygen flow rate at 400 sccm and furnace temp at 1100 C (run it for 1 hr).b) Place the wafer in the quartz boat of the p-type drive in furnace

c) Slide the boat in slowly (5cm per 10sec) total 3 minutes.d) Do the following oxidation procedure; 5 min dry, 10 min wet and 10 min dry.e) Switch the flow to N2 and set it to 400 sccm. Anneal the wafers at 1100 C for 15 min.f) Pull the boat out of the center of the furnace taking 3 min and leave the boat near the end

of the furnace for 3 min to cool down. Set the furnace to idle condition (750 C, 30 sccm).

Remove the wafer from the boat and place it in a quartz holder.

4) Store the wafer in dark clean place for the next experiment.

9.4 Analysis

Calculate the average of your (V/I) values, the maximum value, the minimum value andthe standard deviation for the test areas.

Determine the 4-point probe correction factor based on the probe spacing and the testarea dimensions..

Calcuate test areas average sheet resistance and standard deviation.

Use the methodology of equation 4.6 of Jaeger (course text) and the original n-typeconcentration of your wafer to determine the depth of the p-n junction resulting from the

pre-deposition diffusion.

Use the Irvin curves of Jaegers chapter 4 to estimate the sheet resistance associated withthe pre-deposition diffusion. Compare to the measured result

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10. Lab Week 6: Lithography (mask 2): [65 min]

10.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember proper lab attire.3) Use great caution with HF.

Goal: generate pattern using for gate oxides using mask 2.

Process Flow: align and expose using mask 2 -> develop to generate pattern, -> etch window ->

remove PR

Figure 5 - Detail of Mask 2 Result on Wafer

10.2 Process Steps

1) Spin PR on the wafer:a) Using a dropper, cover the entire wafer with positive PR.b) Spin the wafer for 30 s at 3000 rpm.c) Bake wafer for 20 min.

2) Now you will use the mask aligner and mask 2 to align and then generate a pattern in PR:a) Turn on the vacuum pump providing vacuum for the mask aligner. Direct the vacuum to

aligner.b) Turn on the low and high pressure N2 gas.a) Turn on the two toggle valves on the manometer box.b) Turn on the power switch on the UV lamp controller. Wait for the ready display and

press start button.c) Turn on the mask aligner, video monitor, CCD camera, and microscope.d) Place the mask on the holder (Pattern face up on the table so that it will be adjacent to the

wafer)

Gate oxide region

for a MOSFET

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e) Turn on the vacuum switch (toggle switch is down for vacuum on).f) Place the mask holder in the stage and turn on maskholder button.g) In the auto mode push start button, that will move the lamp house to left.h) Place the wafer on the wafer stage and press the foot switch.i) Contact light will light up / if not press separate button.

j) Check for the alignment; the alignment is done between the structure on the wafer andthe structure on the mask using the alignment marks which are provided on all masks.k) Press exposure button.l) Remove the wafer and put it in a holder. [This step will be completed by TA]: Turn off

the mask holder button and remove mask from stage. Turn off the vacuum mask switch

up and remove the mask. Turn off the mask aligner, monitor, UV lamp controller. Wait

for 15 to 30 min. And turn down two manometer toggle valves. Turn off the vacuumpump and close the N2 gas tanks.

3) The exposed areas of the positive PR can now be dissolved in developer to generate a patternin PR:a) Immerse the wafer in the developer agitating continuously for 45 -60 seconds.

b) DI rinse in a beaker 1 min.c) Blow dry with N2.d) Postbake for 15 min at 135 C.

4) Etch window: Using PR as a mask, you will now use the following sequence to etchwindows in the field oxide so that p-diffusion can take place in n-Si through these windows:

a) Prepare 3 beakers; Buffered Oxide Etch, DI-1 and DI-2. Buffered etch is a mixture of7HF (40 %):1NH4F (50%).

b) Immerse the wafer and basket into Buffered Etch (Etch rate is approximately 100 nm permin.).

c) Immerse the wafer into DI-2 for SiO2 etching test: Observe the back surface of the wafer.If it is wet (i.e. covered with a continuous film of water) then SiO2 is not completelyetched. If a few isolated drops are found on the backside, then SiO2 is completely etched

away.

d) If SiO2 is still remains, immerse the wafer into Buffered Etch again.e) When SiO2 is completely etched away, rinse the wafer in DI for 3 min.f) Blow dry with N2.g) Inspect the wafer under microscope: etched away areas should appear white and spot-

free.5) Remove PR: Now that windows have been etched in SiO2, PR has served its purpose and

needs to be removed:

a) Immerse the wafer in the PR remover for 3 min.b) Rinse with acetone for 20 sec.c) Rinse with methanol for 20 sec.d) Rinse with DI for 3 min.e) Blow dry with N2.

6) Store wafers for the next experiment.

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11. Lab Week 7: Gate oxidation: [110 min]

11.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember the furnace rod end will be hot. Dont touch it for 5 minutes after removal fromthe furnace.

Goal: grow gate oxide in selected areas.

Process Flow: RCA clean -> gate oxidation and N2 anneal

11.2 Process Steps

1) RCA clean: You will now do RCA clean before the wafer goes in the furnace for the gate

oxidation:a) Rinse the wafer in running DI water for 3 min, dry the wafer with N2 and transfer it to abasket labeled degrease etch.

b) Immerse wafer in freshly prepared degrease etch for 10 min at 50 C (10 %). Degreaseetch, which removes organic impurities, consists of75 ml H2O:15 ml H2O2 (30%

solution):15 ml NH3OH.


remove clothes. Never mix an acid and a solvent or heat them on the same hot

plate. See section 3 for details.



e) Immerse the wafer and basket in freshly prepared demetal etch for 10 min at 50 C(10%). Demetal etch, which removes metalic impurities, consists of75 ml H2O:15 ml

H2O2 (30% solution):15 ml HCl.

See data section 3 for safety details.

f) Remove the wafer and basket from demetal etch beaker and immerse them in beakercontaining DI water for 30 seconds.

g) Remove the wafer and basket from DI water beaker and rinse them in running DI waterfor 3 min.

2) Take the wafer out of the basket and place it in a wafer holder.3) Dry the wafer with N2. The wafer should be spotless with no stains or blemishes.4) Dry oxidation:

a) Place the wafer in the center of quartz boat of dry oxidation furnace.b) Set the furnace temperature at 1100 C and Oxygen flow rate at 400 sccm. Oxygen flow

should have been on for at least an hour prior to this point.

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c) Slide the boat into the furnace slowly (5 cm per 10second) taking a total of 3 minutes topush the boat into the center of furnace.

d) Carry out dry oxidation for 30 min.e) Anneal in N2 for 30 min.f) Pull the boat out of the hot zone taking 3 min.

5) Set the furnace to idle conditions (750 C, 30 sccm).

11.3 Analysis

Use the methodology of Equation 3.9 of Jaeger (course text) to calculate the oxidethickness in the gate oxide regions

Use methodology of Equation 3.9 of Jaeger to calculate the oxide thickness in the fieldoxide regions

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12. Lab Week 8: Lithography using mask 3 for contact cuts: [115 min], metallization.

12.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember proper lab attire.3) Use great caution when using HF.

Goal: generate pattern using mask 3, etch contact holes, then coat waver with aluminum.

Process Flow: measure oxide thickness -> pin PR -> generate pattern with mask 3 -> etch holesin oxide -> coat with Al

12.2 Measurement

2) Measure the oxide thickness using the ellipsometer:a) Turn on the ellipsometer power (turn the key to clockwise).b) Place the wafer and set polarizer drum to 85 and analyzer to 45 degree.c) Adjust the gain control until meter reads approximately 140 to 200.d) Rotate the analyzer drum (staying in the red region) to yield the lowest reading. If meter

reading is over 100 use gain control to go below 100. Rotate polarizer drum, (staying inthe red region) to get an even lower reading. Repeat the process, going back and forth

between the analyzer rotation and polarizer rotation, until a minum is reached. Read the

analyzer drum reading (this is called the A1 value). Read the polarizer drum reading (thisis called the P1 value).


The program will then provide approximately analyzer drum and polarizer drum valuesfor the second minimum, referred to as A2 and P2 values respectively. Set the drum

readings to these values and iterate back and forth between the analyzer drum and thepolarizer drum to get a minimum again. These are the actually A2 and P2 values.

f) Enter the A2 and P2 values in the computer. The software will provide you with a oxidethickness. You can also request the program to provide all possible oxide thicknessesusing a list option at the bottom of the screen.

3) If there is ambiguity ad to the actual oxide thickness, one may repeat with a different angleand determine the most likely thickness. Measure the oxide thickness using the ellipsometer:a) Turn on the ellipsometer power (turn the key to clockwise).b) Place the wafer and set polarizer drum to 85 and analyzer to 45 degree.c) Adjust the gain control until meter reads approximately 140 to 200.d) Rotate the analyzer drum (staying in the red region) to yield the lowest reading. If meter

reading is over 100 use gain control to go below 100. Rotate polarizer drum, (staying in

the red region) to get an even lower reading. Repeat the process, going back and forth

between the analyzer rotation and polarizer rotation, until a minum is reached. Read theanalyzer drum reading (this is called the A1 value). Read the polarizer drum reading (this

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is called the P1 value).


The program will then provide approximately analyzer drum and polarizer drum values

for the second minimum, referred to as A2 and P2 values respectively. Set the drum

readings to these values and iterate back and forth between the analyzer drum and thepolarizer drum to get a minimum again. These are the actually A2 and P2 values.

f) Enter the A2 and P2 values in the computer. The software will provide you with a oxidethickness. You can also request the program to provide all possible oxide thicknessesusing a list option at the bottom of the screen.

a) If there is ambiguity ad to the actual oxide thickness, one may repeat with a differentangle and determine the most likely thickness.

12.3 Process Steps

1) Pre-PR cleaning:

a) Immerse the wafer in acetone for 20 sec.b) Immerse the wafer in methanol for 20 sec.c) Rinse with DI for 3 min.d) Blow dry with N2.e) Bake the wafer at 65 C for 15 min.

2) Spin positive PR on the wafer:a) Using a dropper, cover the entire wafer with positive PR.b) Spin the wafer for 30 s at 3000 rpm.

3) Pre-bake the wafer for 20 min at 65 C.

Figure 6 - Detail of Features on the Glass of Mask 3

4) Now you will use the

mask aligner and mask 3 toalign and then generate apattern in PR:

a) Turn on the vacuumpump providing vacuum forthe mask aligner. Direct the

vacuum to aligner.b) Turn on the low and

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high pressure N2 gas.

c) Turn on the two toggle valves on the manometer box.d) Turn on the power switch on the UV lamp controller. Wait for ready display and press

start button.

e) Turn on the mask aligner, video monitor, CCD camera, and microscope.

f) Place the mask on the holder (Pattern face up).g) Turn on the vacuum switch (toggle switch is down for vacuum on).h) Place the mask holder in the stage and turn on maskholder button.i) In the auto mode push start button, that will move the lamp house to left.j) Place the wafer on the wafer stage and press the foot switch.k) Contact light will light up / if not press separate button.l) Check for the alignment; the alignment is done between the structure on the wafer and

the structure on the mask using the alignment marks which are provided on all masks.

m)Press exposure button.n) Remove the wafer and put it in a holder. [This step will be completed by TA]: Turn off

the mask holder button and remove mask from stage. Turn off the vacuum mask switch

up and remove the mask. Turn off the mask aligner, monitor, UV lamp controller. Waitfor 15 to 30 min. And turn down two manometer toggle valves. Turn off the vacuum

pump and close the N2 gas tanks.5) The exposed areas of the positive PR can now be dissolved in developer to generate a pattern

in PR:

a) Immerse the wafer in the developer agitating continuously for 45 -60 seconds.b) DI rinse in a beaker 1 min.c) Blow dry with N2.d) Postbake for 15 min at 135 C.

6) Etch window: Using PR as a mask, you will now use the following sequence to etchwindows in the oxide so that metal contact can be made:a) Prepare 3 beakers; Buffered Oxide Etch, DI-1 and DI-2. Buffered etch is a mixture of

7HF (40 %):1NH4F (50%).

b) Immerse the wafer and basket into Buffered Etch (Etch rate is 100 nm per min. Find theetch time using the oxide thickness ~300 nm).

c) Immerse the wafer into DI-2 for SiO2 etching test: Observe the back surface of the wafer.If it is wet (i.e. covered with a continuous film of water) then SiO2 is not completely

etched. If a few isolated drops are found on the backside, then SiO2 is completely etchedaway.

d) If SiO2 is still remains, immerse the wafer into Buffered Etch again.e) When SiO2 is completely etched away, rinse the wafer in DI for 3 min.f) Blow dry with N2.g) Inspect the wafer under microscope: etched away areas should appear white and spot-

free.

7) Remove PR: Now that windows have been etched in SiO2, PR has served its purpose andneeds to be removed:a) Immerse the wafer in the PR remover for 3 min.b) Rinse with Acetone for 20 sec.c) Rinse with Methanol for 20 sec.d) Rinse with DI for 3 min.

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e) Blow dry with N2.8) Evaporate Al (done by TA).

12.4 Analysis

Compare the gate oxide thickness measurement result from this lab with the calculated

value from last week. Discuss possible reasons for differences. Calculate the expected MOS voltage given your initial wafer doping and your gate oxide

thickness. Use the methodology of Equation 9.2 from Jaeger (course text).

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13. Lab Week 9: Lithography using mask 4 for patterning metal [120 min].

13.1 Overview

Safety First:

1) See section 3 for details on safety.2) Remember appropriate lab attire.

Goal:pattern Al and anneal.

Process Flow: spin PR -> generate pattern with mask 4 -> etch Al -> low-T anneal

13.2 Process Steps

1) Pre-PR cleaning:a) Immerse the wafer in Acetone for 20 sec.

b) Immerse the wafer in Methanol for 20 sec.c) Rinse with DI for 3 min.d) Blow dry with N2.

2) Bake the wafer at 65 C for 15 min.3) Spin positive PR on the wafer:

a) Using a dropper, cover the entire wafer with positive PR.b) Spin the wafer for 30 s at 3000 rpm.

4) Pre-bake the wafer for 20 min at 65 C.5) Now you will use the mask aligner and mask 4 to align and then generate a pattern in PR:

a) Turn on the vacuum pump providing vacuum for the mask aligner. Direct the vacuum toaligner.

b) Turn on the low and high pressure N2 gas.c) Turn on the two toggle valves on the manometer box.d) Turn on the power switch on the UV lamp controller. Wait for ready display and press

start button.e) Turn on the mask aligner, video monitor, CCD camera, and microscope.f) Place the mask on the holder (Pattern face up).g) Turn on the vacuum switch (toggle switch is down for vacuum on).h) Place the mask holder in the stage and turn on maskholder button.i) In the auto mode push start button, that will move the lamp house to left.j) Place the wafer on the wafer stage and press the foot switch.k) Contact light will light up / if not press separate button.l) Check for the alignment; the alignment is done between the structure on the wafer and

the structure on the mask using the alignment marks which are provided on all masks.

m)Press exposure button.n) Remove the wafer and put it in a holder. [This step will be completed by TA]: Turn off

the mask holder button and remove mask from stage. Turn off the mask aligner, monitor,

UV lamp controller. Wait for 15 to 30 min. And turn down two manometer toggle valves.

Turn off the vacuum pump and close the N2 gas tanks.

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Figure 7 - Detail of the Features on the Glass Mask 4

6) 5. The exposed areas ofPR can now be dissolved in developer to generate a pattern in PR:

a) Immerse the wafer in the developer agitating continuously for 45 -60 seconds.b) DI rinse in a beaker 1 min.c) Blow dry with N2.

7) Postbake for 15 min at 135 C.8) Al etching:

a) Immerse the wafer and basket in Aluminum etch at 40 to 50C. Aluminum Etch Solution

(to be prepared by TA): 80ml H3PO4, 5ml HNO3, 5 ml acetic acid, 10 ml H2O.b) Check for complete etching (see when Al just disappears).c) Rinse gently in running DI for 3 min.d) Blow dry with N2.

9) 7. Remove PRa) Immerse the wafer in the PR remover for 3 min.b) Rinse with Acetone for 20 sec.c) Rinse with Methanol for 20 sec.d) Rinse with DI for 3 min.e) Blow dry with N2.

10)8. Al anneal:

a) Anneal for 45 min at 450 C in forming gas. Forming gas consists of 5 % hydrogen and 95% nitrogen.b) Pull out wafer slowly (in approximately 2 min).c) Place wafer in a holder.

11)Store the wafer for measurements.

Figure 8 - A 'Chip' on the Processed Wafer

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Prep

ared byDon R inhard and De n Aslam; F08

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13.3 Analysis

Figure 9 - Identification of Devices

Referring to Figure 9, draw the MOSFET cross-section where indicated by the red arrows

showing both vertical and horizontal dimensions of features of interest.

Diode

MOSFETs

Ca acitors

Resistors

MOSFET Blowup

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14. Device Testing

14.1 Curve Tracer Introduction

Figure 10 - Front View of the Curve Tracer

The curve tracer is an electronic instrument that plots current vs. voltage. For example, if aresistor is connected to the terminals, a linear plot is expected. In the picture above, this is in fact

the test condition. The vertical axis is current (1 mA/division) and the horizontal axis is voltage(1 V/division). One can control a number of things such as the volts/division and

amperes/division on the display. Also, one can specify whether the plot should include only

positive voltages and currents, only negative voltages and currents, or both positive and negative

voltages and currents. Moreover, the curve-tracer can also plot the current-voltage characteristicsof 3 terminal devices and display a family of curves. A full treatment of the instrument is

somewhat complicated. However, the curve tracer is programmable and your lab instructor has

set up several programmed modes for various test situations. Several examples are treated in thishandout.

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14.2 Using the curve-tracer to measure the current-voltage characteristics of a resistor.

When measuring a resistor, connect one end of the resistor to the C terminal on the curve tracer

and the other end of the resistor to the E terminal. The B terminal is left open. The C

terminal voltage is swept from negative to positive voltages and the E terminal is grounded.. In

Program One the current scale is 1 mA per division and the voltage scale is 1 V/division. InProgram Two, the current scale is 100 A per division and the voltage scale is 1 V/division. So

a 1 K resistor would have a slope of unity when using Program One and a 10 K resistorwould have a slope of unity when using Program Two.

Figure 11 shows a view of an MSU student fabricated wafer in which a resistor is contacted bymicro-probes. These probes are connected by test leads to the curve tracer. (Mask lithography

patterns may vary in details from semesteor to semester.

Figure 11 - Microprobe Contacts to a Resistor Fabricated in the MSU Student Lab

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With the curve-tracer set to the Program One conditions, as described earlier, the following

current-voltage characteristic is obtained on the curve-tracer display.

Figure 12 - Curve-Tracer Display for a Resistor

Although Figure 12 does not show the divisions on the screen, the slope can be seen to be near

unity, indicating an approximately 1 K resistor. The curve-tracer has additional cursor featuresthat allow one to measure the slope.

14.3 Using the Curve-Tracer to measure the current-voltage characteristics of a diode.

Although the diode is also a two terminal device, unlike the resistor its current-voltage

characteristics are not symmetric. It is often desirable to use one current and voltage scalecombination to examine the forward characteristics and a different combination for the reverse

characteristics.Program three uses 10 A per division and 100 mV per division and sweeps theC terminal voltage from zero to positive values. It is used to measure the forward

characteristics. Program fouruses 1 A per division and 1 V per division and sweeps the Cterminal voltage from zero to negative values. It is used to measure the reverse characteristics.

Recall that the cathode of the diode on the chip is the n-type substrate. An ohmic contact to thesubstrate would require a phosphorus n

+diffusion. If that was not done, then an approximate

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ohmic contact can be the metallized back surface of the wafer. The C terminal on the curve-

tracer is connected to the diode anode and the E terminal on the curve-tracer is connected to thediode cathode. Figure 13 shows the case when both anode and contact pads are on the top surface

of the wafer.

Figure 13 - Microprobe Contacts to a Diode when a Surface n+ Contact is Available

Figure14 shows the curve-tracer display for the forward diode characteristics and the reversecharacteristics. If the turn-on voltage for the diode is taken to be the forward voltage bias at

which the diode current is 100 A, then the results indicate a turn-on voltage between 0.6 and

0.7 volts. The reverse characteristics for this diode shows negligible leakage current (at leastnegligible compared to 1 A) at 10 volt reverse bias.

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Figure 14 - Forward I-V Characteristics for a Student-Fabricated Diode (top) and Reverse

Characteristics (bottom).

(The Origin (0 V, 0A0 is in the Lower Left Corner for the Former and the Upper Right Cornerfor the Latter)

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14.4 Using the Curve-Tracer to measure the current-voltage characteristics of a MOSFET.

The MOSFET is a three terminal device, so its current-voltage characteristics are often illustratedas a family of curves. Typically, the drain current is plotted vs. drain-to-source voltage for

various values, or steps, of gate-to source voltage. In Program six, the drain-to-source scaling is

500 mV/division and the drain-current scaling on the vertical axis is 100 A/div. The drain ofthe MOSFET is connected to the C terminal of the curve-tracer, the gate of the MOSFET is

connected to the B terminal of the curve-tracer, and the source is connected to the E terminal

of the curve tracer which is grounded. The C terminal is swept from zero to negative voltages.The gate-to-source voltage is stepped in 10 steps, at 2 V per step, from zero volts to 20 volts.

The results are shown below. Knowing that the most negative gate-to-source voltage is 20 volts,and that there are 10 steps, one can deduce that the threshold voltage between 4 and 6 volts for

this MOSFET. In the figure below, the origin (0 V, 0 A) is in the upper right hand corner of the

display.

Figure 15 - Family of Curves for a Student-Fabricated P-Channel MOSFET

QuickTime and aPhoto - JPEG decompressor

are needed to see this picture.

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14.5 Capacitance vs. voltage measurements for MOS capacitors

The capacitance vs. voltage of the MOS capacitors yields the flat-band voltage, the threshold

voltage, and an electrical measurement of the gate oxide thickness. One may identify three

distinct regions as shown in Figure 16: the inversion region; the depletion region; and the

accumulation region. See also the discussion in the ECE 474 text by Streetman. The ECE 477 labhas a dedicated capacitance bridge and probe station, computer interfaced to automatically

measure capacitance as the voltage is swept.

Figure 16 - Capacitance vs. Voltage for a Student-Fabricated MOS Structure

In the accumulation region, the capacitance is given by C=OXA

tOX

where tOX is the oxide

thickness. A is the capacitance area, and OX is the oxide permitivitty.

accumulation

depletion

inversion

threshold voltage

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16. Solution Mixing Guidelines

Generally all solutions will be prepared prior to the laboratory by technical staff from the

Electrical Engineering Department. However, in special cases, a student who is authorized to

mix solutions should observe the following guidelines:

1. Use a face shield and goggles. Always wear rubber gloves and a lab coat. Mix

chemicals under a hood with a fume exhaust. Never peer into the top of a

beaker, always view it from the side.

2. Pour chemicals slowly. Make sure you have the right chemicals. Always pour

acid into water, never water into acid. Use Teflon or an appropriate plastic

beaker for HF solutions.

3. Never mix acids and solvents, or heat them on the same hot plate.

4. Always label beakers. Do not let used chemicals accumulate in beakers. When

you are finished, dispose of the solution unless someone else will be using it

that lab period. (An exception to this rule is photoresist strip solution.)

5. Empty beakers should be rinsed thoroughly.6. Always start with clean beakers which are stored top down in a clean area. If

beakers must be cleaned, an adequate procedure for purposes of these exer-

cises is as follows.

a. Scrub in hot water and AlconoxTM

with a brush.b. Rinse in hot water.

c. Rinse in distilled water.

d. Rinse in deionized water.e. Bake out glass beakers in clean area until dry. Dry plastic and Teflon beakers at

room temperature.(For higher purity procedures, chromic acid is used to clean glassware.)

Some commonly used solutions in amounts appropriate for single wafer, beaker process-

ing are as follows. In all cases below where water is used, deionized water is assumed.

Degrease Etch (5H2O:1H2O2:1NH4H4):75 ml H2O

15 ml H2O2 (30% solution)15 ml NH4OH

Demetal Etch (8H2O:2H2O2:1HCL):80 ml H2O

20 ml H2O2 (30% solution)

10 ml HCl

Aluminum Etch (2H2O:16H3PO4:1HNO3:1 Acetic Acid)

80 ml H3PO4

5 ml HNO35 ml Acetic acid

10 ml H2O

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Buffer Etch (mixture of HF and ammonium fluoride7 parts 40% NH4F to 1 part 50% HF

Borosilicate Etch (1H2SO4:1HNO3)

50 ml H2SO450 ml HNO3

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References:

1. N. I. Sax, Dangerous Properties of Laboratory Materials, 5th Edition, Van Nostrand Reinhold

Company, New York, 1979.

2. F. M. Smits, Measurement of Sheet Resistivities with the Four Point Probe, Bell System

Technical Journal, 711-718, May, 1958.3. W. Kern and D. A. Puotinen, Cleaning Solutions Based on Hydrogen Peroxide for use in Sili-

con Semiconductor Technology, RCA Review, 187-206, June, 1970.

4. W. E. Beadle, J.C.C. Tsai, and R. D. Plummer, Quick Reference Manual for Silicon IntegratedCircuit Technology, John Wiley and Sons, New York, 1985.

Notes:

1. Alconox is distributed by VWR Scientific Company, P.O.Box 3200, San Francisco,

California, 94119.2. Kimwipe is a trademark of Kimberly Clark Corporation, Roswell, Georgia, 30076.

3. Signatone probe stations are available from Signatone, 3687 Enochs Street, Santa Clara, Cali-fornia 95051.

477 lab manual

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