THIN FILM TECHNOLOGY

Introduction

Thin film science now covers a wide span of disciplines such as solid-state physics, surface science, crystallography, crystal growth, optics, electronics, and materials science etc.,These investigations have led to numerous inventions in the form of active devices and passive components, piezo-electric devices, micro miniaturization of power supply, rectification and amplification, sensor elements, storage of solar energy and its conversion in other forms, magnetic memories, super conducting films, interference films, reflecting and anti reflecting coatings and many others. The present development trend is towards newer types of devices, monolithic and hybrid circuits, Field Effect Transistors (FET), Metal Oxide Semiconductor Transistors (MOSFET), sensors for different applications, switching devices, cryogenic applications, high-density memory systems for computers etc.,

Advantages

Further because of compactness, better performance and reliability coupled with low cost of production and low package weight, thin film devices and components are performed over their counter parts.

COMPARISON TO THICK FILM

A casual comparison of film behavior and physical chemical characteristics revealed the sensitivity of film properties on the preparative conditions, film structures, and presence of defects and impurities and even on film thickness especially for ultra thin films. In fact various physical constants reveal to bulk material properties may not often be the same for corresponding films prepared from the bulk. However with increasing film thickness these tend to assume corresponding bulk values. A transition from the bulk to the thin film states may even cause a drastic change in its properties as illustrated by the behaviors of alkali metals and also noble metals. For example the highly conducting sodium, potassium, rubidium, gold and platinum having positive temperature coefficient of resistance in the bulk form show negative temperature coefficient of resistance when in thin film states thus behaving as semi conducting films. Bulk bismuth and antimony, which are metallic in nature, behave as semiconductors in the thin film states. Buckel and Hilsch observed that thin bismuth films unlike bulk showed super conducting behavior properties at low temperature. Highly disordered or amorphous films have electrical or magnetic properties, which may differ by several orders from that of the bulk single crystals. All films whether prepared by vacuum deposition or by other techniques are invariably associated with some growth defects or imperfections such as lattice defects stacking defects twinning, disorders in atomic arrangements, dislocation, grain boundaries, foreign atom inclusion et.., There is often a considerable deviation from that of the bulk behavior and sometimes new phenomenon are observed in thin film states.

Definition of thin film

What ever be the film thickness limit an ideal film can mathematically be defined as a homogeneous solid material contained between tow parallel planes and extended infinitely in two dimensions say x and y but restricted along the third direction z, which is perpendicular to the x-y plane. The dimension along z direction is known as the film thickness (d or t). Its magnitude vary from a limit d®0 to any arbitrary value say 10 mm but always remaining much less than those as long the two direction x and y.

THICKNESS OF THIN FILM

Thin films can be divided mainly into three types based on their thickness. The first one is the ultra thin film, which has a thickness of 50-100 Angstrom units. The second one is the thin or very thin film, which has a thickness of 100-1000 Angstrom units and the third one, is comparatively thicker one which has a thickness greater than 1000 Angstrom .

DEFECTS IN thin films

A real film however, deviates considerably from the ideal core since its two surfaces are never exactly parallel even when formed in the best experimental deposition condition and also the material contained between the two surfaces are rarely homogeneous, neither uniformly distributed nor of the same species. It is also a common experience that a film may also contain many imperfections, impurities, dislocations, grain boundaries and various other defects and may also be discontinuous. Further the top surface of a film often develops some topographical features and characteristic of the growth conditions and these features can no doubt be minimized but cannot altogether be avoided. The bottom surface of a film in contact with a substrate generally takes up the topographical features of the latter. Some of the factors, which determine the physical, chemical, optical, and other properties of the film, are the following: The rate of deposition, substrate temperature, environmental condition, residual gas pressure in the system, purity of the material to be deposited, inclusion of foreign material in the deposit, inhomogeneity of the film, structural and compositional variations of the film in localized or wider area etc., some of which have been actually observed. It is well known that the resistance of a freshly deposited metal film and in fact almost any other film when kept in vacuum changes irreversibly to a lower value with time. But on log ageing at room temperature or even a short annealing at a little higher temperature in vacuum its resistance attains a constant value, which is reversible with temperature. Vand reported that the resistance of a metal film when plotted against its temperature of deposition showed a maximum and minimum. But on heat treatment of the film tend to a more stable, no doubt, due to annealing of its defects stains etc. The film resistance then becomes more or less reversible with temperature

EVAPORATION SOURCES OR HEATING SOURCES

There are several types of practicing heating sources made from refractory metals such as tungsten, molybdenum, tantalum etc., These sources may be made from single or multi stranded filament to various shapes or from straight strips with or without any dimple at the center or in the form of a boat and some times having the shape of a cylindrical etc.,

METHODS OF THIN FILM PREPARATION

THERMAL EVAPORATATION

This method is used to deposit metals, alloys and also many other compounds. This involves the evaporation of the material in vacuum by thermal energy and allowing the vapour stream of the change to condense on the substrate so as to form a continuous and adherent deposit of desired thickness. Refractory metals like tungsten, molybdenum or tantalum are generally used in the form of a wire or strip having different shapes. The choice of a particular refractory metal as a heating source depends on the material to be evaporated so that the evaporant material does not react with the refractory metal at the high temperature of evaporation. How ever the formation of alloy in the source cannot be avoided. Hence coating of refractory oxides such as Al₂O₃, BeO or other suitable materials is often given over the filament or strips so as to present the direct contact between the molten charge and the refractory metal. In any case when a direct heating of the charge is made the filament or strip is pre-cleaned by passing a heavy current through it so as to make it whit hot or incandescent for a very short period so that all the surface impurities of the filament or the strip are removed by evaporation and this is called flash cleaning. Usually a shutter is placed in between the heating source and the substrate so that no vapour stream of the charge can reach substrate.The quality and characteristic of the deposit will depend on the rate of deposition, substrate temperature, ambient pressure and the uniformity of the film and the geometry of the evaporant source and its distance from substrate.

FLASH EVAPORATION

This method is generally adopted when a material has a tendency to decompose or dissociate during evaporation as in the case of some alloys, group of III- V compounds such as InSb, AlSb, etc., titanates, molybdates and even some oxide metals etc., Basically the process is similar to the thermal evaporation techniques with the difference that only a small amount of the charge in powder form is fed at a time to a white hot boat of tungsten, modybdenum or tantalum so that an instantaneous evaporation of the total charge takes place without leaving any residue. Because of the high temperature of the boat as well as the limited amount of the charge fed at a time to the boat there will be no time for constituents to build up by differential vapour pressure. Hence composition of the gaseous phase will be more or less the same as that of the charge and it is expected that on condensation the deposits will retain the composition of the evaporant.

This method of deposition is called flash evaporation since there is no accumulation of the charge on the heated boat before the arrival of the next charge the question of the enrichment of the residue by one constituent as in the normal process does not arise at all.The charge in powder form is fed from a reservoir or a hopper to a heated boat through a chute and the feeding rate can be made continuous or intermittent by using a vibrator or suitable cam arrangement. All other requirements for deposition are more or less similar to those of the normal evaporation process. For some oxides or titanates often a part of the molten charge is maintained over the heated boat, which is then constantly fed, by small amount of fresh charge depending on the rate of evaporation. By this method a constant composition thin film can be produced

ELECTRON BEAM METHOD

This is one of the best methods for the deposition of metals, alloys, refractory metal at a high rate and is now routinely used for the production of metal film resistors and others. In this technique an electron beam accelerated with a voltage of say 2-10V is focused on the surface of a charge which is normally kept inside a graphite crucible placed on water cooled copper block. Thus high-energy electron beam emitting from the cathode impinges on the charge, which is at the same potential as the anode is converted into intense heat energy, which melts the charge. By suitable focusing the electron beam and controlling its intensity it is possible to keep the top surface of the charge in a molten condition from which evaporation can take place. The temperature attained by the charge at the surface can be as high as 3000 C or more and hence refractory metals such as. tantalum, molybdenum, tungsten etc., can be melted and evaporated by this technique.

There are three types of electron beam guns. In the first type the work accelerated gun where electron gun where electron beam coming out of a loop type of a filament is accelerated directly towards the charge or through an appropriate shield thus concentrating the beam. In the second method there is a self-accelerated gun where electrons are emitted from hairpin type of filament and focused through a Wehnelt cylinder on the material. In the third type there is a bent beam electron gun where a beam of electron is bent by an appropriate magnetic field and then focused on the charge. It is also possible to have an oscillating electron beam to facilitate the melting of the charge. In all cases necessary precautions are taken so that evaporation is discontinuous before the charge is exhausted since the last residue may contain some of the impurities coming out of the container vessel such as graphite or other material. It may be mentioned here that because of the high temperature of working the electron beam gun assembly is water-cooled.

CATHODIC SPUTTERING

The ejection of atoms from the cathode surface by impinging of energetic positive ions of noble gases such as Helium, Argon, Neon, Krypton, Xenon at a reduced pressure under high DC voltage gives rise to sputtering phenomenon. It is now possible to make various resistive, semi conducting, super conducting and magnetic films by this method in a better way.

There are two types of sputtering. If the process does not involve any chemical reaction between bombarding gas ions and cathode then it is known as physical sputtering.If on the other hand some reaction s are involved then it is termed as reactive sputtering. Both these types of sputtering are carried out in a comparatively poor vacuum condition and are known as high pressure sputtering.The mechanism of the process involves a momentum transfer between impinging energetic ions or neutrals and the cathode surface atoms as a result of which a physical removal of atoms takes place. Various workers especially by Wehner have amply verified this momentum transfer theory of Stark Conclusions drawn are that the sputtering yield increase with the energy and the mass of the bombarding ions and also with the decrease of angle of incidence to the target. A minimum energy is required to start the sputtering process. Sputtered atoms have much higher energies than those of the thermally evaporated ones and the former are, however ejected along the crystallographic directions of the cathode metal lattices.

HETROJUNCTION THIN FILMS TECHNOLOGY

Based on the hetrojunction thin film technology, integrated circuits are fabricated. Integrated circuit technology is a branch of microelectronics. With this technology we can produce miniature electronic equipment in a large extent. Integrated circuits on a small single chip of silicon of size 5 to 50 mils (1 mil=25 m m) contain a large number of both active and passive elements with interconnections. In this small chip a single transistor can be accommodated in an area 1 mil by 1 mil which means that a chip may contain 50 components with inter connections. The process of fabrication of integrated circuits is similar to the process of fabrication of individual transistors or diodes.The basic structure of integrated circuits is as shown in figure, which consists of four distinct layers.The bottom layer is a p-type silicon of thickness 6 mils known as substrate upon which the whole integrated circuit is built.

The second layer is a thin (1 mil) N type silicon formed over the substrate. Active and passive components are built within this N type layer. These components are transistors, diodes, capacitors and resistors. These are made within the N type layer by diffusing a p-type and N-type impurities suitable in certain precisely defined regions. The selective diffusion of impurities over the second N-type layer over the precisely defined regions is possible with the help of third SiO₂ layer, which is formed over the second layer. This layer prevents the diffusion of impurities over the second N-type layer. Therefore from the selected regions SiO₂ is etched out by a process called photo lithography and impurities can be allowed to diffuse to these regions. Therefore the other regions are protected form impurity diffusions. The fourth layer is a metallic layer, which is added to make interconnections between various components that are fabricated.

ADVANTAGES OF THIN FILMS

Small in size

Low cost due to processing of large quantities

Low power requirements

Lesser weight due to miniature circuit

Higher reliability, because all components are fabricated simultaneously and no soldering joints

The maintenance of thin film based IC system is easier than the discrete system

Interconnection errors are non-existant

Temperature difference between parts of a circuit is small.