CHAPTER 2

APPARATUS AND EXPERIMENTAL TECHNIQUES USED IN THE PRESENT STUDY

2.1 lntroductio~~

Bismuth, antimony, bismuth oxide and antimony oxide thin films have wide

range of applications in electronic circuits and in optical systems. There are a

number of deposition techniques used for making these films. Since the electrical

and optical properties very much depend on the crystal structure and the impurities

present along with the stoichiometry of oxygen, different techniques may yield

different film properties. Also the substrate material on to which the films are

evaporated is found to influence the film properties. In this chapter, the apparatus

and experimental techniques used in the present study are dealt with.

2.2 Methods of preparation of films

The basic steps involved in the thin film deposition are: creation of material,

transpot-t of material and deposition of material. It is ~ossible to classify these

techniques in a variety of ways, such as, physical vapour deposition (PVD),

chemical vapour deposition (CVD) and electrochemical deposition (ECD). Physical

methods cover deposition techniques which depend on the evaporation or ejection

of material from a source, whereas chemical methods depend on a specific chemical

reaction [I].

The physical vapour deposition is classified into thermal evaporation,

sputtering, electron beam evaporation, molecular beam epitaxy, reactive

evaporation, flash evaporation and ion plating. The objective of these deposition

processes is to transfer atoms from a source to substrate where film formation and

growtlr proceed atornistically. In evaporation, atoms are removed from the source

by thermal means, whereas in sputtering they are dislodged from target surface

through impact of gaseous ions. The molecular beam epitaxy produces epitaxial

films by condensation of atoms from Knudsen source under ultra high vacuum. If

the evaporated material is transported through a reactive gas, the deposition

technique is called reactive evaporation. Flash evaporation technique is used when

we have to deposit a multicomponent material, that cannot be heated to the

evaporation point together. Ion plating refers to a process in which the substrate and

film are exposed to a flux of high energy ions during deposition.

Chemical vapour deposition is the process of chemically reacting a volatile

compound of a material to be deposited, with other gases or condensation of a

compound from the gas phase onto substrate where reaction occurs to produce a

solid deposit. The various chemical reactions are thermal decomposition, hydrogen

reduction, nitridation, disproportionation, chemical transport reactions and

con~bination of one or more of these reactions. Each of the above methods has its

own advantages and disadvantages and we will restrict our discussion to those

methods which are used in the present study. We have employed the resistive

heating technique for the preparation of thin film in the present investigation and

are discussed in detail below.

2.3 Resistive heating evaporation

A large number of materials can be evaporated in vacuum using refractory

materials such as W, Mo, Ta and Nb. Resistively heated evaporation sources are

available in a wide variety of forms utilizing refractory metals singly or in a

combination with ceramic compound crucibles. As the temperature of the charge

rises, its vapour pressure rises and a significant evaporation rate develops.

Schematic diagram of the resistive heating set up is given in figure 2.1. The vapour

thus formed is condensed onto substrates held at a suitable distance and desirable

temperature

This method has the following advantages.

1. Impurity concentration in the film will be minimum.

: 2.1 scllcmatic cli:lgsalll ol n rcsistivc heating Set

S , : SOIIICC S2: Substrate

p : TO Pumping systclrl c : Evapos;llion chamber

2. The material boils at low temperature under vacuum.

3. The growth rate can be controlled effectively.

4. There is a reduction in the amount of effective oxides formed on the boiling

surface.

5. Mean free path of the vapour atoms is considerably large at low pressure and

hence a sharp pattern of the film is obtained.

6. The selection of substrate is wide.

On heating a material in vacuum it evaporates at a rate G given by

Langmuir Dushman equation [2],

G = P (MI2 I1 R T ) " ~ ... 2.1

where, P is the vapour pressure of the material at temperature T, M is the molecular

weight and R is the gas constant per mole.

The film deposition is not uniform because the amount of material reaching

the substrate depends o n the angle (0 ) between the source and the substrate. For a

point source the deposition rate is proportional to cos 0 /r2 [I], where 0 is the angle

between source and substrate and r is the source to substrate distance. The purity

and morphology of the film can be influenced by residual gas pressure, evaporation

rate, temperature and nature of the substrate. For the formation of the film with

reproducible properties these parameters must be constant. The rate of evaporation

and hence condensation can have wide limits, depending upon the type of source

and the material used. Another advantage of this method is that single evaporation

can give films of different thicknesses if the substrates are kept a t different distances

from the evaporation source. Film thickness can also be controlled by quartz crystal

thickness monitor.

2.4 Production of vacuum

Vacuum is necessary for the preparation of thin films. Various degrees of

vacuum are classified according to the pressure range as follows [3].

1. Low vacuum: 760 - 25 torr

2. Medium vacuum: 25 - 1 0 ~ ~ torr

3. High vacuum: - l o 6 torr

4. Very high vacuum: 1 0 ~ ~ - 1 0 ~ ~ torr

5. Ultra high vacuum: below 1 0 ~ ~ torr

Two different principles are employed for the production of vacuum. One is

the physical removal of gases from the vessel and exhausting the gas to outside. The

other is the condensation of gas molecules on some part of the inner surface.

Cryogenic, Cryosorption, Sublimation and Getter ion pumps work on this principle.

We have used the oil-sealed rotary pump and diffusion pump for the production of

high vacuum and are discussed below.

2.5 Oil Sealed Rotary Pump

Figure 2.2 shows the cross section of a sliding vane rotary pump. An

eccentrically placed slotted rotor turns in a cylindrical stator driven by a directly

coupled electric motor. In the slots there are two sliding vanes which are in constant

contact with the walls of the stator. Two spring loaded vanes (blades) sliding in

I'ig 2.2 CI-oss-section of oil-sealctl rotary pump

1. Valve 4. Gas ballast valve 2. Oil 5. Air filter 3. Non -return valve

diametrically opposite slots in the rotor press against the inner surface of the stator.

Friction is minimized by a thin oil film which lubricates all parts of the pump and

also seals the minute gap. The exhaust is normally closed by pressure valve leading

to an oil reservoir. During operation air enters to the vacuum connection and passes

into the volume created by eccentric mounting of the rotor in the stator. Two vanes

mounted in the rotor, sweep this volunle and the trapped air is compressed to a

pressure just above one atmosphere, which causes the discharge valve to expel it

through the oil seal to the atmosphere. This type of pump can attain a vacuum of

4.5 x 10 torr only owing to back leakage of air across the stator and rotor seating.

This limitation is over-come by providing a second stage pump in series with the first

stage. To reduce condensation of vapour during this compression cycle, gas

ballasting is used. A controlled a~nount of suitable non-condensable gas is admitted

during the compression cycle. The most important characteristics of a rotary pump

are the speed at which it will remove the gas from the system and the lowest

pressure to which it will exhaust the vacuum system.

2.6 Diffusion Pump

The idea of evacuating a vessel by molecular momentum transfer was first

described by Gaede 141. A schematic diagram of the diffusion pump is shown in

figure 2.3. The heater vaporizes the work fluid and hot vapour rises in the chimney.

The direction of flow of vapour is reversed at the jet cap so that it issues out through

an annular nozzle with supersonic speed. This is due to the pressure difference

IGg. 2.3 Schenlatic diagram of cross-scclion of a diffusion pump.

I . To vacuuln systcin 6 . Hcnter 2. Barrel pump casing 7. Boiler 3. Water cooling 8. Oil 4. To l'orc-pump 9. Nozzles 5. Fore-arm 10. Jet assembler

between the inner and outer side of the chimney. The high speed jet of fluid

molecules imparts n~omenturn to the random moving incoming gas molecules. Thus

the gas molecules move towards the outlet where it is removed by a backing pump

(rotary pump). The vapour jet condenses on the cooled pump walls and returns to

the boiler. The gas molecules diffuse to the vapour molecules, hence the name

diffusion pump.

The working fluid used in the diffusion pump should have high molecular

weight, desired low vapour pressure and necessary thermal stability. Commonly

used fluids are hydrocarbons, silicon fluids, polyphenyl ether and perflouro

polyether. We have used the silicon oil 704 DC as the working fluid. This oil is

superior to other fluids because of its low vapour pressure and high resistance to

oxidation at high working temperature.

To prevent back diffusion of gas from dense to the rare zone, the vapour jet

should retain as much of its density as possible. To reconcile this requirement with

wide throat area for maximum gas intake, the cross-section of the lower zone is

narrowed through aerodynamically shaped tapering stacks. The outer walls are

water cooled to recover the work fluid back and to produce a denser boundary layer

by removing vapour molecules which travel laterally without contributing to the jet

action. To enhance the directionality and speed of the vapour the pumps employ

multi-stage stacks, with three jets working in series.

24 2.7 Vacuum Coating Plant

The vacuuIn evaporation apparatus consists of pumping system, coating

chamber and electrical services. Detailed reviews of various types of vacuum

systems and their ultimate pressures are given by Dushman [5], Holland (61, Roth

[7] and Casewell [8]. A brief description of vacuum coating system used in this

investigation is given below.

The system is 'HIND HIVAC' Vacuum coating unit (model No. 12 A 4) which

consists of 0.1 M diffusion pump in conjunction with backing rotary pump. The

ultimate pressure achieved in a 0.3 M diameter stainless steel bell jar is of the order

of 6 x 10 torr. It has set-ups for electron beam evaporation, flash evaporation and

reactive evaporation. A L.T. transformer of 20 V, 50 A is used for filament heating.

Substrates are cleaned by ionic bombardment in this system. The thickness of the

deposited film can be monitored by a quartz crystal thickness monitor. The

measurement of pressure in the system is done by means of Pirani and Penning

vacuum gauges provided within the system. Figure 2.4 is the schematic diagram of

vacuum coating unit used for film preparation and figure 2.5 is the photograph of

the plant.

2.8 Substrate cleaning

For deposition of films, highly polished and thoroughly cleaned substrates

are required. A variety of cleaning processes are available 17, 9, 101. First, the

substrates are cleaned using liquid detergent. Then it is kept in dilute nitric acid for

some time. After this the substrates are cleaned using distilled water. The substrates

Fig. 2.4 Schematic diagram of a vacuum coating unit.

Bell jar, Substrate,

Crystal of thickness monitor 10. Sourse shutter 11.

Evaporation sourse 12. (EBG or resistive heater)

Current feed through penning gauge (Ionisation gauge) Roughing valve

Baffle valve (High vacuum isolation valve) Diffusion pump Highvac (Backing valve) pirani gauge (Thermal conductivity gauge) ore-line trap, Isolation valve Rotary pump.

Figure 2.5 : Photograph of the thin film unit a n d the other instruments used in the laboratory

are then agitated ultrasonically in acetone. They are then rinsed in isopropyl alcohol

and dried in hot air. Inside the bell jar the substrates are subjected to ionic

bombardment for five minutes as final cleaning before deposition.

2.9. Substrate heater and temperature measurement

Substrate heating is provided usingNichromestrips wound over a thick mica

sheet sandwiched between two mica sheets (figure 2.6). The dimensions of the

heater used is 10 x 6 cm and the substrates are held against the heater surface with

stainless steel clamps. The maximum temperature attainable is 500 "C. With this

heater an area of 5 x 4 cm of the substrate can be uniformly heated to within + 5

"C. The temperature of the substrate is measured by a chrome1 - alumel

thertnocouple. The current to the substrate heater is controlled by a variable voltage

transformer.

2.10 Preparation of Films

The films are evaporated onto clean substrates. Due to the degassing of the

material on initial heating, the pressure in the chamber increases slightly and the

initial deposited film is impure. A shutter is introduced in between the source and

the substrate to avoid the film deposition on the substrates during degassing period.

The shaft of the shutter is passed through a vacuum tight seal, which is filled on the

base plate of the coating unit. lnorder to avoid the overheating of the substrate

while evaporation, it should be kept at a height of 0.16-0.18 m above the heating

I : ig , 2.0 Schclllalic diagram of thc substrate heater used in the laboratory.

I . Mica sheet 2. Nichrome heater strip

source. In the present work, Bi, Bi203 , Sb, and Sb2O3 are used as source materials.

During the evaporation the vacuum is maintained at < 10.~ torr. Films are

deposited at a rate of 13-15 nm. per minute. Ohmic contact has been given using

high purity silver thick films at either end of the substrate, before deposition of the

film.

2.1 1 Sample Annealing

The samples have been annealed in a specially designed furnace. It consists

of Kanthal (A1 grade temperature range 1150 - 1350 "C). To avoid heat loss it is

surrounded by thick package of fire brick silica whose working temperature is 1100

"C and melting point is 1710 "C. The width of the heating element is about 20 cm.

The filament is also covered with sillmate (&03 - Si02) tube, maximum working

temperature is 1500 "C and melting point is 1710°C. It helps to provide uniform

heating region at the centre of the tube. In addition, it avoids any thermal shock

during the annealing process. The temperature of the heater is controlled and

recorded by a digital temperature controller cum recorder.

Figure 2.7 shows the block diagram of the temperature controller-cum

recorder. The thernio couple used is chrome1 alumel type. The output of the

thermocouple is calibrated to 0.04 mVPC and fed directly to the comparator circuit

as shown in the block diagram. The comparator consists of the IC LN 324 and its

associated circuitry. By adjusting the hysteresis loop of the comparator, using a

hysteresis voltage regulator one can control, set and reset voltage for the relay

Fig. 2.7 Block diagram of the temperature controller cum recorder

L a d Heater -

- Relay switch

C0l l -

L o - - .

Voltcge regulator

. . ,

N c-- Power --

~ U P P I Y +

-12 \I

- 12

- + 5 5

- 7 - - - -1- - - - - - - - - - - - ------' I -r 1 LN 324

+i

-+; -; \,

1

W n t d e c o d e r

I 7 . .-

Cr. Al I

ADC

- TC - - I

i* L- . 1

Comparator I I

I I

I Ic 7107 I I I I 7 segm-mr I

I d r ~ v e r

switch. The voltage corresponding to the setting temperature is referred by the

comparator. The heater coil is connected through the relay switch and the power to

the heater and thereby the temperature is controlled by the comparator circuitry.

The analog signal from the thermocouple is converted to a digital one with

the help of an PJD converter, using 3 digital single chip PJD converter IC 7107

(intersil) having high accuracy. The AID converter provides a built-in seven segment

display unit. The temperature can be displayed digitally. When the temperature

reaches the pre-set temperature, the heater cuts off automatically, by action of relay

switch. After a few seconds the heater is again switched on and the process is

repeated, thus maintaining a constant temperature at the centre of the furnace.

2.12 Measurement of Thickness of Film

Thickness of the film is the most important parameter which controls the

properties of the film. There are different methods for measuring the thickness of the

film. Of these, we use the optical method and the quartz crystal thickness monitor

method.

2.12(1) Optical Method (Multiple Beam Interferometry)

The phenomenon of interference occurs in a transparent film.

Interference is also observed in reflection and transmission. The condition for

maxima in the reflection will be the condition for minima in the case of transmission

and vice versa. In the case of multiple beam interference by reflection, the

interference pattern forms, is just the opposite of that seen in transmission. In other

words, where there is a sharp bright fringe on a dark background in transmission,

observation of reflected light gives sharp dark fringes on a bright background.

For practical purposes fringe systems are identified according to the method of

formation, and two cases are distinguished in multiple beam interferometry. Fizeau

fringes are generated by monochromatic light and represent contours of equal

thickness in an area of varying thickness 't' between two glass plates. This is

accomplished by contacting the two glass plates such that they form slight wedge at

an angle a so that the thickness between two plates vary. The angle a is generally

made very small so that consecutive fringes are spaced as far apart as possible. For

the normal incidence of monochromatic light, this spacing between fringes

corresponds to a thickness of 112, where h is the wavelength of monochromatic

radiation used.

The second multiple beam interferometry technique is referred to as fringes of

equal chromatic order (FECO). In this white light is used at angle of incidence of

zero degree and the reflected or transmitted white light is dispersed by a

spectrograph. Here the fringes are formed for certain values of tJh [Ill. The FECO

fringes can be obtained with the two silvered surfaces parallel to each other, where

the plate is adjusted to get Fizeau fringes.

Tolansky's Fizeau Fringe Method

When two reflecting surfaces are placed close to each other, interference

fringes are produced, the measurement of which makes it possible to determine the

film thickness and surface topology with high accuracy. This interference fringes

method has been developed by Tolansky [12, 131 and is now accepted as the

absolute standard method.

The interferometer consists of two slightly inclined optical flats one of them

supporting the film which forms a step on the substrate. The film is deposited on to

a glass substrate. A sharp edge within the film is produced by masking with a razor

blade during deposition. Over this film a highly reflecting silver layer is coated. This

forms the step. The optical flat brought in contact with the flat containing step is

semi-silvered which is called match flat. The flats are brought into contact in such a

way that the coated surfaces are facing each other. The whole system is illuminated

with a parallel bsam of monochromatic light of wavelength h (589.3 nm) from a

sotliu~n vapour lamp. By slightly tilting the match flat, the multiple beam interference

fringes appear with a distance X. The fringes are shifted by a distance AX in the

region of sharp edge of the film. It corresponds to a shift of AX in x for thickness

step of LIZ. The thickness of the film is given by

t = AXIX . hi2 .... 2.2

The experimental arrangement and pattern obtained are shown in figure 2.8

2.12(2) Quartz Crystal Thickness Monitor

Quartz crystal thickness monitor is used for controlling the rate of

evaporation and thickness of the film during evaporation. Because of its sensitivity

and simplicity it has become a standard method in various film fabrication processes

(14-171. This type of monitor utilizes the thickness shear mode of a piezo electric

PARTIAL REF LECTOR 1

IGg. 2.8 Arrangeincnt for Fizeau Ssinges.

a) Fringe pattern c) Sa111ple with step and inatch-flat b) arrangelnent

quartz crystal. The mass added to the exposed side of the crystal shifts the

resonance frequency irrespective of the thickness, density, elastic constants or

stiffness of the added material. The AT cut crystal, which has low temperature

coefficient for the resonance frequency is generally used for the monitor. The

frequency of fundamental resonance for AT cut crystal is given by

f = 112d [ClpQ] 'I2 = N/d .... 2.3

where d is the thickness of the crystal, p~ is its density, C is its shear elastic constant

and IU is a constant which is equivalent to ( ~ 1 4 ~ ~ ) " ~ . If a deposit of mass m is

added to the exposed surface area A produces a change in frequency Af, then

Af = -f Q mipQAd .... 2.4

where Q is a constant and the negative sign is the indication of decrease in

frequency [17]. Combining equations 2.3 and 2.4 we get

Af = -f Q m/N p~ A = -Cf d A

here CI = f Q / N p~ is a constant of the crystal. Assuming uniform film thickness

and constant density pl of the film

m/A = pi t

Af = -Cf pi t .... 2.5

or t = - AflCi p, .... 2.6

A block diagram of a quartz crystal thickness monitor is shown in figure 2.9.

An oscillating quartz crystal of frequency 6 MHz is kept near the substrate inside the

vacuum coating unit. The deposition of the vapour can take place both on the

substrate and on the exposed area of the crystal surface. A second crystal of

4 M H Z c r y s t a l

: inside vacuum

/ chamber

10 kHz - Frequencey t o - v o l t a s k c o n v e r t e r

6 . 5 MHz o s c i l l a t o r

& mixer 5 0 0 kHz

R a t e meter

L o c a l o s c i l l a t o r

4 7 5 k H z - 4 0 0 kHz

Fig. 2.9 Block diagram of quartz crystal thickness monitor.

frequency 6.5 MHz is mounted in the control unit of the thickness monitor. The

difference in the crystal frequency is amplified and fed into the electronic circuit

where it is mixed with the frequency of a variable oscillator to produce a final

diflerence in frequency of 0.1 to 100 KHz.

noted from a frequency counter. The mass

causes a reduction in the natural resonant fr

converted to a DC signal which activates both

meter. Thus both the thickness and rate of deposition of the film are measured. At

the end of the deposition the frequency shift meter is brought to zero by adjusting

the variable oscillator frequency.

2.13 Measurement of Electrical Conductivity

The resistance R of a film of length I, breadth b and thickness t is given by

where p is the resistivity of the film. The col~ductivity IS of the film is

If I=b, Equation 2.7 becomes

R = plt = Rsh .... 2.8

so that the resistance Rsh of the square film is independent of the size of the square

i.e; it depends only on the resistivity and thickness. The quantity Rsh is called sheet

resistance of the film and is expressed in ohm per square unit. If the thickness of the

film is known, the resistivity is obtained from

p = t Rsh

:.Conductivity CT = 1/11 = 1

- - - -. . .

t Rsh

The direct method of measuring Rsh is to prepare rectangular samples of film

and measure its resistance as shown in figure 2.10 (a). Now find out the number of

squares between the end contacts. If R = V/I is the resistance between the two

voltage terminals, sheet resistance is given by,

Rsh = R -ivbi--

.2.10

A lour terminal method is necessary to avoid contact resistance between the film

and the end terminals as shown in figure 2.10 (b). The configuration shown in figure

2.10 (a) and 2.10 (b) have been used for the conductivity measurements in this

thesis.

2.14 Conductivity cell

A schematic diagram of the cross section of the conductivity cell fabricated

in the laboratory is shown in figure 2.11. The cell consists of a thick walled

cylindrical chamber with a bottom flange and four side tubes made of stainless steel.

Tl~ree side tubes are closed air-tight with glass windows and are used in

spectroscopic studies. The remaining side tube is connected to a rotary vacuum

pump and the chamber can be evacuated to a pressure of m bar. The inner

tube is made of stainless steel pipe which has been welded to a large copper finger.

Tlle liquid nitrogen cavity and heater coil help the sample to attain the required

SAMPLE .-.. FlLM

FlLM <.- 1.5

7

THICK Ag ..-.. FlLM

T 1.5

I v I

i , 2.10 Scllemrlic diagram of electrical co~lductivity measurements. ,411 dimensions are in mm.

;I) 'l-Wo-lel-lllill:lJ lxl~~llod h ) Four-ter~lli~~al 111cLhod.

IGg.2.1 1 Schc~natic diagram ol the cross-sectio~l of the conductivity cell.

I . Cylindrical chamber 9. Side tube 2. Inner tube 10. Bottom flange 3. Liquid nitrogen cavity I I . To ~ot;lry pump 4. Coppcr finger 12. Neoprine "Omring 5. Mica insulator 13. BNC 6. Heater coil 14. Ther~nocouple 7. Sample holder 15. Connecting leads to BNC 8. Glass window 16. Substrate with film

temperature very quickly. The outer enclosure is made leak proof by using a

neoprene '0' ring which rests inside the groove as the flanges. A sample holder

fixed at the copper finger can hold the film on a substrate in the form of strip with

the help of screws. Mica sheets are placed in between the sample holder and

substrate. The outer surface of the copper finger is covered with mica sheets and the

heater coil is wound over it. The electrical leads are taken out through teflon

insulation. A dc power supply is used to heat the heater coil. The electrical leakage

current through the mount is by-passed to earth by grounding the inner tube. The

leads of the electrodes are taken out using BNC connector. A copper- constantan

thermocouple in contact with the sample senses the temperature. Temperature of

the sample in the cell can be varied from liquid nitrogen temperature to 400°C.

2.15 Electrometer

The electrical conductivity measurements are carried out using the Keithley

programmable e!ectrometer (Model No:617). It has special characteristics such as

high input resistance and high sensitivity which give the instrument much better

capability than those of the ordinary digital multimeters (DMM). The input resistance

of an ordinary DMM is of the order of 10 MCZ while that of the Keithley electrometer

is greater than 200 TI1 ( 2 X 10'"l ) . The electrometer can detect current as low as

0.1 fA (10~16 A), while a typical DMM might be limited to current measurement in

the range of pA.

Due to tile high input impedance, the electrometer can be used to make

measurements as high as 200 G n with a constant current mode. By using constant

voltage mode the range of ~neasurement of resistance using the Keithley

electrometer can be extended up to 1016Cl. Also for a given resistance range, the Vll

mode will be faster. In V/I mode, the built in voltage source of the instrument can be

used to apply a current 1, through the unknown resistance R. The insulation

resistance is then a~~tomat ica l l~ calculated by the instrument as follows, R = V/l,

where I is the current through the resistance as measured by the instrument and V is

the programmed voltage. The programmable voltage from the internal voltage

source can be varied from -102.35 V to + 102.4 V in 50 mV steps.

The resistance R of the film kept inside the conductivity cell under running

vacuum of about 1 0 ~ ~ m bar can be measured out by connecting it to the

electronieter as shown in figure 2.12.

2.16 UV - Visible Spectrophotometer

To study the optical transmittance of the films in the ultra violet-visible

range, Shimadzu 160 A spectrometer has been employed. It is a double beam

system employing a static beam splitting half mirror, which sends the light beam

from the rnonochror~~ator equally through the sample and the reference substrate.

The light beam emitted from the light source (Deuterium lamp D2 or Halogen lamp

W,) is reflected by the mirror MI and is directed into the monochromator. The

de~~ter ium lamp produces wavelength from 200 nm. The halogen lamp produces

model 4801 cable

Fig 2. I2 Measuren~ent of resistance using Keithley propmmmable electrometer (Model No 61 7) in V/I mode

wavelength upto 1100 nm. The light source switching wavelength can be set to any

value within the range of 295 to 364 nm. Its initial value is 350.5 nm. These lamps

call be automatically interchanged according to the wavelength range needed. All

tlie optical elements except the light source are isolated from the external

atmosphere by the window plate W so as to be dust free. The slit width of the

monochromator is fixed at 2 nm. G is a 900 linelnm aberration corrected concave

holographic grating. The light beam from the monochromator is passed through the

stray light cut off filter F, reflected by the mirror M2 and split by the half mirror M3

into the sample and reference beams. Each beam passes through the respective cell

to the detector. Two voltages are produced by the detector which are proportional

to light intensities of the reference and sample beams respectively. These two

voltages are amplified and fed to tlie electrical system.

Figures 2.13 and 2.14 give the simplified optical diagram and block

diagram of the electrical system used in the Schimadzu 160 A spectrophotometer

respectively. The output absorbance or transmittance can be seen in the video

display and printed out using a chart recorder.

2.17 X-ray Diffractometer

Shimadzu 610-XD has been used to obtain X-ray diffraction spectra of samples

in the present work. The detector here is a proportional counter connected to a

pc~lse height analyser. Radiation from the filtered Cu Ka is used as the source. The

accelerating potential applied to X-ray tube is 2 5 KV and the tube current is 10 mA.

1 ; i e 2.13 ol)tica\ diagt-;llr of the slrctraplloto~neter (Schimadzu UV 160 A) Dl: Deuterium lamp G: Grating

W : Window plate Sam: Sample cell

W ,: kialogen lamp SI: Entrance slit

MI-M5: Mirrors S2: Exit slit M3: Ilalf-nlirror Ref.: Reference cell

P.D.: Photo diode i F: Filter

\

~ c l c c l o r Loy ornplificr

Sornple beam

Arnplllier Dclecior Log ompi f ie r

Video R A M

-

1:ig. 2.14 Block diagra~n of the electrical system of the spectrophotometer ( Shimadzu UV 160 A).

A chart recorder running synchronous with the goniometer is used for recording the

spectra. The Bragg-Brentano geometry is employed for X-ray analysis. This is such

that when the X-ray beam falls at angle B to the substrate, the detector is brought to

an angle 28. The specimen and detector are rotated at angular velocities wand 2w

respectively to get the various diffraction planes. When thin films are used the

effective thickness of the X-ray beam sees is (diSin8) where, d is the film thickness

and U is the angle of incidence. Consequently the scattered intensities will be angle

dependent which has to be taken into account while comparing the intensities with

ASTM data.

References

1. K.L.Chopra andL.K.Malhotra, " Thin film Technology and Applications" Mc

(;raw Hill, New York (1985).

2. T.J.Coutts, "Active and Passive Thin Film Device$', Academic Press, (1978).

3. L.I.Maissel and R.Glang, "Hand Book of Thin film Technolog,", Mc.Graw Hill,

New York (1970).

4 . W.Gaede, Ann Physil{., 46 (1915) 357

5. S.Dushman, "Scientific Foundatio~~ o f Vacuum Techniqud', John Wiley &Sons,

Inc., New York (1962).

6. L.Holland, "Vacuum Deposition of Thin Fimi', Chapman and Hall, London

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