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.
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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