Instrumentation for faBrication, Cfiaracterization,

and Irradiation of'Devices Chapter- 2

2.1 Introduction

This thesis is based on the study of effects of radiation on semiconductor devices and

involves various instruments used for fabrication, characterization and irradiation of

devices. MOS devices were fabricated in the Class-1000 clean room facility at CEERI,

Pilani. Utmost care was taken during fabrication so that the silicon wafers are not

contaminated at any stage. Some of the main instruments used for fabrication of MOS

capacitors were Thermoco Oxidation Furnace for oxidation of samples and Varian e-

beam evaporation unit for Gate metallization. Ellipsometer and Talystep were used for

oxide thickness measurement. Four probe wafer testing unit was used to measure the

sheet resistance of the wafer. Chip level testing of MOS devices were performed using

Prover wafer level testing system. The Current-Voltage (I-V) characteristics of the

devices were measured using Keithley 236/228 source measure unit. Capacitance-

Voltage (C-V) measurements were performed using HP 4284A LCR meter. The I-V and

C-V measuring units were interfaced with computer using LabVIEW platform. The

schematic and working of the instruments used for gamma, electron and heavy ion

irradiation of the devices are also discussed in detail.

2.2 Instrumentation Used During Fabrication of MOS Capacitors

2.2.1 Thermal Oxidation Setup

Thermal oxidation is an important process in VLSI technology which is generally carried

out in oxidation furnace that provides the sufficient heat needed to elevate the oxidizing

ambient temperature. The furnace which was used for thermal growth of Si02 on n-Si

typically consists of;

• a fool proof cabinet

• a heating assembly

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a fused quartz horizontal process tubes where the wafers undergo oxidation

a digital temperature controller and measurement system

a system to monitor the flow of gas into and out of the process tubes and a loading station used for loading (or unloading) wafers into (or from) the process tubes as shown in Figure 2.1.

DryNj/OjGas

Line

Quartz Boat —i Wafer

End Cap

Ci C> u 0 U U U LI O 0 C 0 "at~ • • : *

0 0 0 0

0 0 0 0 U Lt LI LI 0 C 0 0 vr i:-i 0 0 0 0

>.t M.I.

c o 00

0 0 0 , Timp R«s4 Pv

Knob Gas Inlet

"etiperature Control er

Figure 2.1: Schematic diagram and photograph of horizontal oxidation furnace

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The heating assembly consists of several heating coils that control the temperature

around the furnace quartz tube. There are three different zones in the quartz tube i.e. left,

right and center. The temperature of both end zones (lefl and right) was fixed at 400

"C±50 "C throughout the process. For the ramp up and ramp down of furnace

temperature, there are three digital control systems for three zones. The furnace consists

of two different gas pipe lines, one is for N2 gas and other is for dry/wet O2 gas. To

control the gas flow, there are MATHESON'S gas flow controllers.

2.2.2 Determination of Oxide Thickness

EUipsometer

Spectroscopic ellipsometry is a non-contact, non-destructive optical technique that

enables the determination of material refractive indices and layer thicknesses by

measuring the change in polarization of a probing light beam upon reflection from a

sample.

Principle of ellipsometry

•

•

•

•

When linearly polarized light reflects from a surface, elliptically polarized light is

generated under certain conditions.

The amount of induced ellipticity depends on the surface properties (refractive

index, bulk or layered sample).

Ellipsometry technique measures the phase and amplitude relationships between

two orthogonal polarizations (p and s waves) upon reflection.

When p and s waves are reflected, they experience a phase shift and an amplitude

reduction.

The experimental data are expressed as tanv|/ (relative amplitude ratio) and A

(relative phase shift), related to the Fresnel-reflection-coefficients Rp and Rs for p-

and s- polarized light, which are complex functions of the angle of incidence

The ellipsometric parameter Delta is defined as dl - d2 where dl and d2 denotes

the induced phase shift difference between p and s waves, respectively while the

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ellipsometric parameter tan(Psi) is defined as the ratio of the complex ampHtude of

the total reflection coefficient of the p and s waves (|RP| / |RS|).

Ellipsometry is said to be self-referencing because measurements do not require any

reference sample and are largely insensitive to variations in the beam intensity and

ambient environment, making this technique highly accurate and reproducible.

Ellipsometry exploits phase information and the polarization state of light, and can

achieve angstrom resolution. This technique can be applicable to thin films with

thickness less than a nanometer to several micrometers. The Schematic of the

ellipsometer is given in Figure 2.2.

/VLight source

Polarizer \ / \ / >

Compensator (optional)

<D i

Sample

Detector/\

^ V / V Analyzer

Compensator (optional)

Figure 2.2: Schematic diagram and photograph of spectroscopic ellipsometer

Talystep

Talystep is a surface profiler which measures the step height of oxide on the wafers. It

consists of a stylus which scans the surface and the variation on the surface is recorded

and converted into electrical signal. The profiler has sharply, pointed, conical diamond

with a rounded tip stylus, resting lightly on the surface, is traversed slowly across it, and

the up and down movement of the stylus relative to a suitable datum are magnified and

recorded on a base and a graph representing the cross-section will be obtained. Figure

2.5 shows the schematic diagram of surface profiler.

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Oxide Thickness

]

Figure 2.3: Schematic diagram and photograph of Talystep surface profiler

2.2.2 Metallization

The metallization on silicon wafers have been carried out using Varian's 112B e-beam

evaporation unit (Figure 2.4). In this process, the source material is heated in a vacuum

chamber which has initially been pumped down to less than 10'̂ torr. Evaporated atoms

from the source condense on the surface of the wafer. The system has three vacuum

pumps: Sorption pump. Vac-ion piunp and Titanium sublimation pump. Sorption pumps

were used for rough vacuum of the order of 10'' torr while Vac-ion pumps were used for

high vacuum of the order of 10"̂ torr. For ultra high vacuum (UHV) or to achieve fast

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vacuum Titanium sublimation pump was used. The used system has 3 kW electron gun

capabilities. Using e-beam heaters, a high energy electron beam is focused onto source

material in a crucible using magnetic fields. E-beam heater can achieve higher

temperatures so that a wider range of materials can be evaporated. A schematic of E-

beam evaporation system is given in Figure 2.4.

Vacuum

PurriD

^Substrate

^&

~l •Thermionic

Filament

Figure 2.4: Schematic diagram and photograph of Varian E-beam metallization

system

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2.3 Instrumentation for I-V, C-V and DLTS Measurements

2.3.1 LabVIEW Based I-V and C-V Characterization Setup

The computer aided test & measurement setup was developed on LabVIEW platform.

Figure 2.5 shows the complete block diagram of the hardware interface and the

experimental setup. I-V characteristic of two terminal devices like MOS capacitors were

measured using single SMU, but for the measurements of three terminal devices like

Bipolar Junction Transistor and MOSFETs, two SMU's are programmed to work in pair.

In our setup, Agilent 4284A LCR meter was programmed to measure C-V characteristic

at different frequencies with a different voltage level. All instruments were connected to

computer in parallel configuration via GPIB bus. The instruments were controlled by the

LabVIEW based custom written program, which sends the control words (command) to

the instrument via USB/GPIB interface and receives the data from the instruments.

Program is written to select required functions of the instrument and acquire data from it.

The LabVIEW environment is based on the concept of virtual instruments (Vis), which

can be defined as layers of software and hardware, added to a personal computer, in such

a way that computer acts as a custom-designed instrument.

I

/ s

)

o c

Computer with installed

LabVIEW Software

- < — •

• * — •

•^—•

Keithley 236 SMU

Keithley

228 SMU

Agilent 4284A

LCR Meter

D

U

T

18

• • • ^ M ^ ^ ^ ' -̂s.. 11

IH^^^^^ />

L

i'

'''^•••••H^H^I^^^HBI^^H

• -

^̂ 1

Hi^HI^Hi

i • 1

-̂ ^̂ ^̂ ^̂ ^̂ H

Figure 2.5: LabVIEW based Computer aided I-V and C-V measurement setup

2.3.2 DLTS Measurement Setup

The Deep Level Transient Spectrometer, IMS-2000 (shown in Figure 2.6) consists of the following units,

• A Cryostat - capable of maintaining and controlling the device chamber

temperature in the range 80 K to 500 K; Pt-100 temperature sensor is used to

measure the temperature with an accuracy of 0.01 K. The entire operation of the

cryostat is computer controlled.

• Capacitance meter.

• High speed pulse generator - The pulse generator is capable of generating pulses of

widths ranging from 100 ns to 10 s. The pulse height could be programmed from -

12 V to+12 V.

• Boxcar averager - The Boxcar averager is capable of generating seven rate

windows at 1.3, 3.3, 8.3, 20.8, 51.7, 127.9 and 312.7 s''.

• Interface electronics.

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Figure 2.6: Photograph of DLTS (IMS 2000) measurement setup

2.4 Instrumentation for Radiation Experiments

2.4.1 Gamma Ray Irradiation Facility

^Co- gamma irradiation was done using Blood Irradiator-2000 facility available at ISRO

Satellite Centre (ISAC), Bangalore. The schematic of the gamma irradiator has been

given in Figure 2.7. The Blood Irradiator-2000 is a compact, portable, self-shielded type

Cobalt-60 gamma-ray irradiator. Atomic Energy Regulatory Board (ABRB), India,

approves the safe design and use of self-contained dry source storage gamma irradiator

(category-!). The unit is designed to house Cobalt-60 source of 675 Ci and provides an

irradiation volume of about 2000 c.c. approximately. The doubly encapsulated sealed

radioactive source is used in cylindrical form, which is completely contained in a dry

container called flask unit. The sealed sources are shielded at all times, making it human

accessible. The irradiation chamber is located in vertical shaft (Drawer). The shaft moves

up and down with the help of a drive system, which enables exact positioning of the

irradiation chamber in the center of the radiation field. Access holes of 8 mm diameter

are provided in the vertical shaft for introduction of connecting wires for electrical

parameter measurements and thermocouple sensors etc. for temperature measurement

inside irradiation zone.

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Specifications of the Gamma-Irradiator (Blood Irradiator-2000):

60/-Energy of Co gamma - photon: 1.17 MeV & 1.30 MeV Cobalt-60 source

capacity: 675 Ci.

Dose rate at maximum capacity at the time of installation: 9.23 Gy/min.

Irradiation volume: 2000 cc approximately.

1. Sample chamber 2. Control panel 3. Biological shield lor the source 4. Source cage 5. SuppoHnig table 6. Central shaft iiicoixioraTing access tube 7. Tension ananaement for wire rojie S. Drive system 9. Hand crank

U_ M Figure 2.7: Schematic diagram of Cobalt-60 gamma irradiator

2.4.2 Electron Irradiation Facility

Electron irradiation was performed using the variable energy Microtron facility at the Microtron Centre, Mangalore University, Mangalore. The facility provides electrons and bremsstrahlung radiation of 8 MeV energy.

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The important features of the machine are:

Beam energy : 8 MeV

Pulse current : 50 mA (max)

No. of electron orbits : 14

Beam size

Pulse duration

Pulse repetition rate

Average beam power

Magnetic field strength

Magnetron power

Operating frequency

3 mm X 5 mm

2.5 ^s

250 Hz (max)

375 W (max)

1927 G

2MW

2998 MHz

In Microtron, electrons move in circular orbits, all orbits having common tangent at the

axis of accelerating RF cavity. The synchronization of electron motion with accelerating

field is achieved by the period of each succeeding orbit to be larger than the former by an

integral multiple of the RF period. Figure 2.8 shows the basic principle and photograph

of Microtron.

Accelerating cavily

W«veguide

Magnetic chaonei for particle extraction

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Figure 2.8: Schematic diagram and photograph of Microtron

2.4.3 Heavy Ion Irradiation Facility

Ion irradiation was performed using a 15 UD Pelletron facility available at Inter

University Accelerator Centre (lUAC) previously called Nuclear Science Centre (NSC),

New Delhi. It is capable of accelerating almost any ion from hydrogen to uranium to

energies ranging from few MeV to hundreds of MeV. In this machine, negative ions are

produced and are pre-accelerated to energy ~ 400 keV and injected into strong electric

field inside an accelerator tank filled with SF6 insulating gas. The center of the tank is a

terminal shell which is maintained at high voltage (-15 MV). The negative ion on

traversing through the accelerating tubes from the top of the tank to the positive terminal

gets accelerated. On reaching the terminal they pass through a stripper, which removes

some electrons from the negative ions, thus transforming the negative ions into positive

ions. These positive ions are then repelled away from the positively charged terminal and

are accelerated to ground potential to the bottom of the tank. In this manner, same

terminal potential is used twice to accelerate the ions. On exiting from the tank, the ions

are bent into horizontal plane by analyzing magnet, which also select a particular beam

of ion. The switching magnet diverts the high-energy ion beams into various beam lines

into the different experimental areas of the beam hall. The entire machine is computer

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controlled and operated from the control room. Figure 2.9 shows the schematic and

photograph of Pelletron.

Interchangeable Ion Sources - # ^ :

Ion accelerating tube

High Voltage Terminal

Sulphur Hexa Fluoride Gas

Pellet Chains

Injector Deck

Injector Magnet

elon

Accelerator Tank

Charge Stripper

Ecjuipotential Rings

+ ve Ion

Analyser Magnet

^ To Sv/itching Magnet

Figure 2.9: Schematic diagram and photograph of 15UD Pelletron

All the instruments were well calibrated for greater accuracy. Necessary care and

precaution were taken while handling the instruments.

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