apparatus and experimental techniques used in the...
Post on 29-Mar-2020
12 Views
Preview:
TRANSCRIPT
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$', Academic Press, (1978).
3. L.I.Maissel and R.Glang, "Hand Book of Thin film Technolog,", Mc.Graw Hill,
New York (1970).
4 . W.Gaede, Ann Physil{., 46 (1915) 357
5. S.Dushman, "Scientific Foundatio~~ o f Vacuum Techniqud', John Wiley &Sons,
Inc., New York (1962).
6. L.Holland, "Vacuum Deposition of Thin Fimi', Chapman and Hall, London
(1956).
7. A.Roth, "Vacuum Technolo&', Amsterdam, North Holland (1976).
8. IH.C.Caswel1 "Pllysics of Thin filmit, Academic Press, New York 1 (1963)
9. J.R.Rairden and C.A.Neugebauer, Proc. IEEE, 52 (1964) 936.
11. W.A.Pliskin and S.J.Zanin, "Hand Book of Thin Film Technolo&', Mc.Graw
tjill. New York (1970).
12. S.Tolansky, "M~rltiple Beam ir~ferferometry o f Surfaces and Films", Oxford Univ.
Press. London (1948).
13. S.Tolansky, "AII l~ifroducfion to l~~terfero~netr$' Longmans, Green & Co.Ltd,
London (1955).
14. C.A. Evans Jr., J.Vac.Sci.Technol. 12 (1975) 144.
15. Chin-Shun Lu, J. Vac.Sci. Techno1 12 (1975) 578.
16. A.Piegari and E.Masetti, Thin Solid Films, 124 (1985)249.
17. A.W.Warner and C.D.Stockbridge, JAppl. Phys., 34 (1963) 437.
top related