microsystems designece434/winter2008/434_1.pdf · 2008. 4. 3. · microsystem technology examples:...

(C) Andrei Sazonov 2005, 2006 1

Microsystems Design

Aim of the course:

-to introduce you to broad variety of microsystems;

- to give you the basic skills in microsystems design.


Microsystem:

i) a small-scale system;

ii) integrates several components;

iii) transforms one form of signal into another.

What is a microsystem?


i) a small-scale system:

Microsystems are based on the technologies (and often on the materials) initially developed primarily for IC fabrication.

Microsystem technology examples: - thin film deposition;- etching (wet and plasma);- photolithography;- printing.

Example: “Swiss watch” (mechanical wrist watch):

- a small-scale system;- integrates several components (gears, stones, spring,…); - transforms the potential energy of rolled spring into the hand motion…

BUT:- uses traditional metal processing technologies.

Therefore, it is NOT a microsystem.

Electronic wrist watch IS a microsystem (fabricated using IC technology, integrates display, sound alarm, sensors (heart rate sensor, etc.)


ii) integrates several components:

Usually, microsystem includes:

- transducers (sensors/actuators);

- power supply;

- control and processing circuitry;

- input/output channels.

• In microsensor system, input signal from environment is transformed into output signal.

Example: digital camera – transforms the light intensity into electric charge.

• In microactuator system, the signal generated by the system is transformed into output signal.

Example: in electronic wrist watch, the signal generated by the chip, is transformed into the image on LCD display, and into the alarm sound.


TRANSDUCER SYSTEM COMPONENTS

Transducers

Power Supply and Management

Control and Processing Circuitry

Input/Output Channels/Protocol

USER

ENVIRONMENT (optional)


iii) converts one form of signal into another:

The key elements of a microsystem are the transducers that by definition convert one form of energy into another.

Transducers are divided onto sensors and actuators.

Sensors measure outside parameter and convert it into electrical signal that is related to that outside parameter.

Actuators convert an electric signal into another form of signal.

Example:

Piezoelectric effect can be used for both sensors and actuators. In electronic wrist watch, the quartz resonator is an actuator that converts electric field oscillations into sound waves. Based on the same effect, pressure sensor converts mechanical strain into the voltage. Worth to note that the strain/polarization relationship is linear, i.e., the input and output signals are proportional.


Microsystems history:

1970s

1980s

late 1980s

1990s

Progress in IC technology (planar process)

Large-scale IC manufacturing (batch process) –drastic cost reduction

IC integration with other forms of signal

Development of affordable microsystems for automotive and industrial applications

Further expansion of microsystems to new application areas (biomedical, computer, consumer)

1980s

2000sFurther scale reduction of microsystems to nanosystems


Microsystems:

mechanical

chemical biological fluidic

magnetic

force, pressure, velocity,

acceleration, position.

Example: piezoelectric

quartz resonator

radiant / optical

IR/VIS/UV/X-ray,γ-ray, α-particle.

Examples: night vision

imaging, liquid crystal displays

thermal

temperature, heat, heat flow.

Examples: flow sensors, thermoelectric

coolers

magnetic field intensity, flux density,

magnetization. Examples:

magnetic read/write heads

concentration of chemicals, composition, reaction rate.

Example: CHEMFETs,

thin film batteries

neural probes. Example:

biochip

flow/viscosity/density.

Example: DNA analysis


Microsystems: When Hot and When Not?

Microsystems are not almighty! Following factors affect their competitiveness:

- Scale (some physical effects are not scalable);

- Cost (not all microsystems can be batch processes and/or tested);

- Limitations (i.e., shape – not all shapes can be obtained using micromachining);

- Performance (complexity).


Application markets for microsystems:

automotive

biomedical consumer

computer

sensor systems: airbags, ABS, light intensity,

heat, …

industrial

position sensors, pressure, velocity, …

integration with environment:

displays, wireless, …

analytical systems: biochips,

DNA/protein assays, …

electronic watch, home security systems, heart

rate monitors, …


Microsystems Design: What Is It (1)?

Product definition

Design concept(initial configuration)

Design constraints Fabrication process selection

Signal mapping and conversion Circuitry

Materials selection

Packaging

Initial design considerations


Microsystems Design: What Is It (2)?

Design concept(initial configuration)

Sensor/actuator physical design Circuit design

Fabrication process design

Integration Mask design

Design verification

Product

Design analysis


Microsystems Design

Sensor/actuator physical designCircuit design

Fabrication process design IntegrationMask design

Micromachining

Sensors:

mechanical optical thermal chemical magnetic


Micromachining.Micromachining = a set of microsystem fabrication technologies.

Micromachining:

Bulk micromachining

Surfacemicromachining LIGA

Etching the bulk of large single-crystal

silicon substrate

Building up and patterning thin film

layers, i.e., the entire device is thin-film

fabricated

Lithography, electro-plating, molding. Deep photoresist,

x-ray lithography, then molding or e-plating.

Applications: strain gauges,

cantilevers,accelerometers, fluidic sensors

Applications: displays, CHEMFETs,

capacitive sensors

Applications: Sensors involving thick structures

or deep trenches (gears, turbines)


Bulk micromachining.This technology uses silicon wafers (can also be SiC, GaAs, or quartz wafers), which are etched through the mask to form the desired three-dimensional geometry of device (we either “dig” the cavities in the wafer, or remove everything around our structure).

Key elements: - photolithography;- etching.

Advantages:- straightforward, fabrication process is well known and reliable;- low fabrication cost in bulk manufacturing.

Drawbacks:- high material loss (etched away);- suitable only for simple geometry (beams);- limited to low aspect ratio (wafer thickness).


Bulk micromachining.Examples of structures fabricated by bulk micromachining:

1. A microgear – silicon wafer is etched away around the structure.

2. A resistive heater – a cavity was etched underneath the heater to avoid heating of the rest of wafer, where the circuit is located. Si substrate SiO2

Poly-Si heater

Conductive wires

Contact pads


Surface micromachining.

This technology builds the device structure by adding materials layer by layer on top of the substrate. The substrate itself is not affected.

Key elements: - thin film deposition;- photolithography;- etching.

Advantages:- not constrained by the thickness of the wafer;- wide choice of thin film materials;- more complex geometries can be achieved (microvalves, actuators).

Drawbacks:- complex fabrication involving many masks;- higher fabrication cost;- engineering problem (interfacial stresses, stiction);- involves etching of sacrificial layers.


Surface micromachining.Examples of devices fabricated by surface micromachining:

1. Microgears – poly-Si thin film was deposited on top of SiO2 sacrificial layer, then patterned using photolithography and dry etching, then sacrificial layer was etched away.

2. Capacitive accelerometer - poly-Si thin film was deposited on top of SiO2 sacrificial layer, then comb-shaped patterned using photolithography and wet etching, then sacrificial layer was etched away.


LIGA.This technology involves deep x-ray lithography (synchrotron radiation) to pattern thick layer of photoresist (PMMA). After developing, deep trenches are either covered by electroplated metal (seed metal should be at the bottom of trench), or filled up by molding.

Key elements: - electroplating;- x-ray lithography;- molding.

Advantages:- high aspect ratio structures are possible;- wide variety of geometries and configurations available;- allows fabrication of bulk metallic structures.

Drawbacks:- requires a special synchrotron radiation facility;- the most expensive process;- requires a facility for microinjection molding and for mass production.


LIGA.Examples of devices fabricated by LIGA:

1. Microgears – formed using electroplating metal inside PMMA form.

2. Heat exchanger (Joule-Thomson microrefrigerator); made by electroplating metal inside PMMA form.


Materials for Microsystems:

1.Substrates:• Silicon (Ge, GaAs, InP, …);• Oxidized/nitridized silicon;• Metals;• Glass/quartz;• Ceramics;• Plastics and polymers;• Biological tissues (skin?).

2.Additive materials:• Conductors (metals, metal compounds, conductive oxides,…);• Semiconductors (c-Si, poly-Si, μc-Si, a-Si,…);• Insulators (Si- compounds, ceramics,…);• Organics (polymers, enzimes, DNA, RNA,…).


Material σy, 109N/m2

E, 1011N/m2

ρ, g/cm3

c, Jg-1K-1

k, Wcm-1K-1

α, 10-6K-1

TM, ºC Applications

Si 7 1.9 2.3 0.7 1.57 2.33 1400SiC 21 7 3.2 0.67 3.5 3.3 2300SiO2 8.4 0.73 2.27 1 0.014 0.5 1700Si3N4 14 3.85 3.1 0.69 0.19 0.8 1930GaAs 2.7 0.75 5.3 0.35 0.5 6.86 1238Quartz 0.6 0.85 2.66 1 0.09 7.1 1710Al 0.17 0.7 2.7 0.942 2.36 25 660Steel 2.1 2 7.9 0.47 0.329 17.3 1500Cu 0.07 0.11 8.9 0.386 3.93 16.56 1080

σy – yield strength;E – Young’s modulus;ρ – mass density;c – specific heat capacity;

k – thermal conductivity;α – thermal expansion coefficient;TM – melting point.


Material Applications

Si Integrated circuits (signal processing), piezoresistors, VIS/IR sensors, pressure sensors, etc.

SiC Passivation films (high T protection)

SiO2 Thermal and electric insulation, masking layer for Sietching, sacrificial layer in surface micromachining, optical waveguides.

Si3N4 Passivation films (moisture barriers), masking, insulation layers, optical waveguides.

GaAs Integrated circuits (signal processing), piezoelectric sensors/actuators.

Quartz Thermal and electric insulation, piezoelectric sensors, optical waveguides, fluidic devices.

Al Conductive layers.

Steel Microgears

Cu Heat conductors


Microsystem Fabrication Processes:

1.Deposition.

2.Lithography.

3.Etching.

4.Electroplating.

5.Printing.


Definitions:

• Aspect ratio – the ratio of the depth to the width of etched pit or of a grown structure.

• Isotropic /anisotropic processes differ by how normal to the wafer surface the sidewalls or etched pits are. Isotropic process spreads with equal rate in all directions. Anisotropic process has preferred direction.

Aspect ratio: low; high. substrate

isotropic

anisotropic


1. Deposition: adds a uniform layer on top of the substrate. Added layer may be of different nature or of the same nature as the substrate.

substrate

added layer

Deposition:- Chemical Vapor Deposition (CVD):

i) Epitaxy;ii) Thermal CVD;iii) Plasma Enhanced CVD.

- Physical Vapor Deposition (PVD):i) Evaporation;ii) Sputtering.


2. Lithography: is used to pattern the top layer of our structure and thus get desired shape.

Involves the deposition of photosensitive polymeric material – photoresist(PR) – followed by PR exposure to the radiation (UV or x-rays) through the mask. Radiation triggers a rearrangement in polymer chains, making possible to remove exposed areas by wet etching (develop).

added layerPRmask

UV light

Lithography:- photolithography (UV light used);- x-ray lithography;- electron beam lithography


3. Etching: actual process of patterning the top layer of our structure. Consists of the top layer removal through the mask (the mask is a material with extremely low etching rate in that particular etchant).

substrate

maskpatterned layer

Etching:- wet (liquid chemical based);- dry (plasma based).


4. Electroplating: a process of metal deposition from the electrolyte by applying a negative potential to the object of deposition relative to an inert counterelectrode.

- +

electrolyte

anode

cathode

mask


5. Printing: a process of “ink” transfer in desired patterns to an underlying substrate.

Printing:- screen printing;- transfer printing.

Screen printing Transfer printing

substrate substrate

“ink”

mask


Deposition.1. Chemical Vapor Deposition (CVD).

Used for deposition of non-metallic thin films:- silicon dioxide (SiO2);- silicon nitride (SiNx);- poly-Si.

exhaustRF induction coils

substrates

wafer holder quartz tube

Ar HCl SiCl4,SiH4

H2 PH3 or B2H6


CVD principle: gaseous precursors are introduced into a heated furnace (for example, inductively heated quartz tube). Chemical reactions occur on the substrate surface resulting in the film deposition.

The properties of CVD materials are strongly process dependent. The main factor here is the substrate temperature.

At ~1200 ºC, we get epitaxy – crystalline silicon growth on top of c-Si wafer.

At ~500-850 ºC, we get thermal CVD, where precursors are thermally activated; poly-Si, thermal SiO2 are deposited by this technique.

Below ~450 ºC, precursors need to be plasma activated. By plasma enhanced CVD (PECVD) we deposit amorphous silicon, silicon nitride and oxide.

Deposition.


Deposition.Plasma Enhanced Chemical Vapor Deposition (PECVD).

Used for deposition of non-metallic thin films (SiO2, SiNx, nc-Si, a-Si).

Voltage oscillations at high frequency (13.56 MHz) force free electrons to accelerate in electric field and collide with source gas molecules cracking them onto reactive radicals.

substrate holder

rf electrode

~ plasmaplasmaplasma

reactive gases (SiH4, NH3, H2, …)

exhaust

Vacuum chamber


SiO2 deposition:

•Thermally grown (Si + O2 = SiO2) at 850-1150 ºC;

•PECVD (SiH4 + O2 = SiH4 + N2O = SiOx + 2H2 + N2) at ~450 ºC.

SiNx deposition:

CVD (SiH4+4NH3 = Si3N4+12H2) at 700-800 ºC;

PECVD (SiH4+NH3 = SiNH+3H2) at 250-350 ºC.

Poly-Si deposition:

CVD (SiH4 = Si+2H2) at 600-650 ºC;

PECVD (SiH4= Si+2H2) at ~450 ºC.

Deposition.


2. Physical Vapor Deposition (PVD) – sputtering and evaporation.

Used for deposition of metallic and non-metallic thin films.

shutter

substrate

Si targetpump pump

Ar+ ions

-

Sputtering Evaporation

shutter

substrate

Si “boat”

pump pumpheater


Materials deposited by PVD:

Evaporation – metals (Au, Ag, Pt, Cu, etc.).

Sputtering – metals (Al, Cr, Mo, W, etc.), silicon, oxides (ITO – indium tin oxide, ZnO), piezoelectric materials (PZT – lead-zirconate-titanate).

PVD issues:

1. Poor step coverage.

2. Shadowing.

3. Film composition/stoichiometry.substrate

structure

metal


Photolithography

PR

mask

UV light The photoresist can be positive or negative. In case of positive PR, exposed areas become soluble; in case of negative PR, exposed areas become insoluble.

Photoresist is sensitive to light with wavelenghts between 300-500 nm. The light source is mercury lamp. To get higher resolution, the wavelength has to be reduced:-150-300 nm – deep UV;- 0.4-5 nm – x-rays.

A photomask contains a pattern of opaque and transparent regions. To achieve best resolution, the mask must be in contact or in close proximity with the photoresist.


Photolithography: Design RulesIn device fabrication process, multiple photolithographic steps are used. Each mask must be aligned with previous patterns. There always are small misalignments at each step that accumulate in complex process. Design rules determine minimum offsets between mask features and serve to provide process reliability.

Example of misalignment:

Mask top view Structure cross section after patterning

Mask 1

Mask 2

Mask 1

Mask 2 – acceptable misalignment

Mask 2 –unacceptable misalignment


Etching1. Wet etching.The structure is simply immersed into liquid chemical which attacks unprotected

(unmasked) regions.

Si etching:

a) Isotropic: - HF/HNO3/CH3COOH – “HNA etchant”. Overall reaction:18 HF + 4HNO3 + 3Si = 2H2SiF6 + 4NO + 8H2O. Drawbacks: - it attacks SiO2;

- etching rate depends on doping (lighter doping – lower rate).

b) Anisotropic: - Alcali hydroxides (KOH, NaOH; NH4OH). Overall reaction:Si + 2OH- + H2O = SiO2(OH)22- + 2H2. Drawbacks: - alcali ions are detrimental to electronic devices;

- hydrogen slows down etching.Anisotropy: in KOH, (100) plane can be etched up to 400 times faster than (111).


.The physical origin of Si anisotropic etching is still not clear.

Different planes in silicon unit cell:

(100) (111)(110)

- Tetramethyl ammonium hydroxide (TMAH; (CH3)4NOH). Anisotropy: (100) plane can be etched up to 35 times faster than (111). Perfect etch stops for TMAH are PECVD SiNx and SiOx.

- Ethylene diamine pyrochatechol (EDP).

Top view Side view(111)

(100)

54.74º


SiOx etching:

HF solutions (buffered with NH3F) – BHF. Overall reaction: SiO2 + 6HF = H2SiF6 + 2H2O.

SiNx etching:

BHF or H3PO4.

Organic films etching:

Oxidizers (e.g., H2SO4/H2O2 – “piranha”).

Metal etching:

Depends on the metal, but usually strong acids or bases (e.g., NaOH, HCl).


2. Plasma etching (“dry etching”).

Reactive Ion Etching (RIE) – the most important dry etching technique.

Same reactor configuration as PECVD except that the substrate holder is RF driven (in PECVD, substrate holder is grounded).

In plasma, gas molecules (either chlorine containing, e.g., CHCl3, CCl4, or fluorine containing, e.g., CF4, SF6, NF3) are ionized resulting in highly reactive species.

~

plasma

e- + SF6 → SF5- + F+ + e-;e- + SF5- → SF42- + F+ + e-;

etc.Then chemically active radicals and ions attack the

unprotected areas on the wafer surface:SiFx + F+ → SiFx+1; x = 0…3.

Si atoms are removed from the wafer as SiF4. This etching is isotropic. To get anisotropy, the sidewalls

need to be protected. This can be achieved by the polymer buildup on the sidewalls while etching. The polymer can be generated as a by-product of etching if we have some carbon source in the gas mixture (e.g., using CF4 or CHF3).


Dry etching mixtures:

For Si: SF6 (isotropic), C4H8 or CHF3 (polymer formation);

For SiO2: CF 4+ O2, CHF3, C2H6 (anisotropic);

For SiNx: SF6 + O2 (isotropic), CF4 + O2 (anisotropic);

Organic films (e.g., photoresist): O2;

Aluminum, GaAs: chlorine containing gases (CCl4, BCl3, …).

In order to promote the polymer buildup on the trench sidewalls the substrate temperature has to be kept low. Water-cooled or even liquid He-cooled substrate holders are being used for this purpose (using so-called Bosch process, c-Si etch rates about 2 um/min can be achieved with vertical sidewalls).


Lift-off patterning

A metal is deposited on top of thin film (e.g., photoresist, amorphous silicon) isotropically patterned with undercuts. Due to poor step coverage, metal layer is discontinuous. Then the underlaying layer (called “sacrificial”) is dissolved through the discontinuities, and unwanted metal is lifted off.

Example 1: sharp tips fabrication for the field-emission TV screens.Example 2: cantilever beam based capacitive sensor fabrication.

This technique utilizes the poor step coverage of sidewalls in PVD processes.

1,2 – sacrificial layer;3 – metal;4 – substrate.

124

3


BondingSometimes we need to firmly join 2 or more wafers to create a wafer stack (for

example, SOI technology) or to package.

1) Direct bonding – wafer surfaces are H saturated, then brought together and annealed at 1000 ºC. H-H bonds provide initial adhesion, and after annealing, H diffuses into wafers, and chemical bonds form instead on the interface. Used to bond together Si wafers, Si and SiO2, etc.

2) Anodic (field-assisted) bonding – to bond together glass and silicon. Si wafer is positively biased (300-700 V) and heated (500 ºC). Positive ions (e.g., Na+) are repelled from the glass interface, leaving uncompensated negative charge; Coulomb attraction between glass and Si assisted by heating results in chemical bonding.

3) Bonding with intermediate layer – adhesive layers are used to promote bonding. Used mainly in packaging.

Wafer 1

Wafer 2adhesive


Electroplating

This process is used mainly for metal deposition: Cu, Ni, Ag, Au, Pt, Pd, etc.

Example: Cu plating from (CuSO4 + H2O) solution:

Cu2+ + 2e- → Cusolid.

Film quality is mainly controlled by the current; if the current density is too high, then the side reactions (usually H2O electrolysis) result in porous film with poor properties.Templates (masking layers) are usually sacrificial; the deposited film uniformity depends on the potential difference between different openings.


Process Design (process integration)

Input: i) Design constraints;ii) Materials.

Output: i) Process sequence (process flow);ii) Mask set.

Criteria: i) Device performance;ii) Manufacturability;iii) Cost.

Issues: i) Device geometry;ii) System partitioning and packaging;iii) Thermal and contamination constraints;iv) Material properties control;v) Process reliability (mechanical stability, accuracy, alignment easiness, location on the wafer, die separation).


Sample Process Flow 1: Junction DiodeStep DescriptionSubstrate material: (100) Si, p-type, 1x1015 cm-3 boron.

1. Clean Standard RCA clean with HF dip.2. Oxide Grow 0.1 μm SiO2.

3. Protect Front Spin cast photoresist on front, pre-bake.

4. Backside implant Implant boron to achieve 1019 cm-3 at surface after all anneals

5. Strip Strip photoresist. 6. Photolithography Spin cast photoresist, pre-bake, expose Mask 1 (n-region),

develop, post-bake.7. Implant Implant phosphorus to achieve 1019 cm-3 at surface after

drive-in8. Strip Strip photoresist.

9. Clean Standard RCA clean without HF dip.

10. Drive-in Drive in diffusion to achieve 1 μm junction depth.11. Photolithography Spin cast photoresist, pre-bake, expose Mask 2 (contacts),

develop, post-bake.


Sample Process Flow 1: Junction Diode

Step Description12. Contact opening Wet etching, buffered HF (BHF).

13. Strip Strip photoresist.

14. Clean Standard RCA clean without HF dip.

15. Top metal Al evaporation, 1 μm.

16. Photolithography Spin cast photoresist, pre-bake, expose Mask 3 (top metal pattering), develop, post-bake.

17. Top metal patterning PAN etch (phosproric-acetic-nitris acids), 35-38 ºC.

18. Strip Strip photoresist.

19. Backside metal Al evaporation on backside, 1 μm.

20. Sinter Anneal contacts at 425 ºC, 30 min.


SiO2 deposition and B backside implantation

SiSiO2

p+ Si


Mask 1: P implantation

PR

n+ Si


Drive-in

n+ Si


Mask 2: via opening for top metal contact


Mask 3: top metal patterningaluminum

Bonding pad

Microsystems Design

microsystems designece434/winter2008/434_1.pdf · 2008. 4. 3. · microsystem technology examples:...

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