SCIENCE AND TECHNOLOGY OF THIN FILMS
Prof. V K Vaidyan
Department of Physics, University of Kerala, Trivandrum
–695 581
email: vaidyan@vsnl.in
http://www.oocities.org/vaidyan_kurian/
Thin films are two dimensional micrometrial and thin
signifies the order of the wavelength of light. The science and technology of thin solid
films have made tremendous advance during the last three decades due to the demand for
reliable optical, magnetic, electronic and superconducting thin film devices. The
scientific and technological impact of thin films in the modern world is so great that
without which the whole communication network around the globe stands still, the digital
computer world comes to halt.
Three basic steps involved in thin film deposition are:
creation of material, transport of material and deposition of material. A possible
classification of the different deposition techniques is:
Physical methods cover deposition techniques that depend
on the evaporation or ejection of material from a source, while chemical methods depend on
a specific chemical reaction.
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 growth
follow atomistic ally. In evaporation, atoms are removed from the source by thermal means,
where as in sputtering they are dislodged from target surface through impact of gaseous
ions. The molecular beam epitaxy produces epitaxial films by condensation of atoms. If the
evaporated material is transported through a reactive gas, the 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, chemical transport reactions and combination of one or
more of these reactions. Each of these methods has its own advantages and disadvantages.
Film thickness is the most important parameter that
controls the properties of a film. Of the different methods for thickness measurement, the
most commonly used techniques are Optical methods and Quartz thickness monitor.
Tolansky’s 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. The interferometer consists of two slightly inclined optical
flats one of them supporting the film that forms a step on the substrate When the system
is illuminated with a parallel beam of monochromatic light (wavelength l), multiple beam interference fringes with a
distance ‘d’ appears. The fringes are shifted by a distance ‘Dd’ in the region of sharp edge of
the film. The film thickness ‘t’ is then calculated by the formula t
= (Dd/d)(l/2).
Quartz crystal thickness monitor: Quartz
crystal thickness monitor is used for controlling the rate of evaporation and thickness of
the film during evaporation. An oscillating quartz crystal of frequency ‘f’ MHz
is kept near the substrate inside the vacuum coating unit. The film deposition takes place
on substrate and the exposed area of the crystal. The mass added to the exposed side of
the crystal shifts its resonance frequency. This change in frequency is measured
continuously, from which growth rate and film thickness can be monitored.
Stages
of film growth
The sequence of the nucleation and growth steps to form a continuous film, which emerges from nucleation theory and electron-microscopic observations, are as follows:
1. Formation of adsorbed monomers
2. Formation of sub critical embryos of various sizes
3. Formation of critically sized nuclei (nucleation step)
4. Growth of these nuclei to supercritical dimensions with the resulting depletion of monomers in the capture zone around them
5. Concurrent with step 4, there will be nucleation of critical clusters in areas not depleted of monomers
6. Clusters touch and coalesce to form a new island occupying an area smaller than the sum of the original two, thus exposing fresh substrate surface
7. Monomer adsorbs on these freshly exposed areas, and secondary nucleation occurs
8. Large islands grow together, leaving channels or holes of exposed substrate
9. The channels or holes fill via secondary nucleation to give a continuous film
CHARACTERIZATION OF FILMS
Every atom in a crystal scatters an x-ray
beam incident upon it in all directions. As the atoms in a crystal are arranged in a
regular repetitive manner, the condition for diffraction is given by the Bragg equation, nl
= 2d sinq derived by the English physicists Sir W H
Bragg and his son Sir W L Bragg in 1913 to explain why the cleavage faces of crystals
appear to reflect x-ray beams at certain angles of incidence q . The variable d is the
distance between atomic layers in a crystal, and the variable l is the wavelength of the incident x-ray
beam; n is an integer. For the first order diffraction pattern, n = 1.
This observation is an example of x-ray
wave interference, commonly known as x-ray diffraction (XRD), and was direct evidence for
the periodic atomic structure of crystals. Although Bragg law was used to explain the
interference pattern of x-rays scattered by crystals, diffraction has been developed to
study the structure of all states of matter with any beam, e.g., ions, electrons,
neutrons, and protons, with a wavelength similar to the distance between the atomic or
molecular structures of interest.
Thus, the position of the diffraction
beams from a crystal depends on the size and shape of the unit cell and the wavelength of
the incident x-ray beam, the intensities of the diffracted beams also depend on the type
of the atoms in the crystal and the location of the atoms in the unit cell. Hence, no two
substances have the same diffraction patterns, satisfying both direction and intensity of
all diffracted beams. The diffraction pattern is thus the fingerprint of a
crystalline compound and the components of a mixture can be identified individually
comparing them with the standard diffraction patterns.
A
substance that is present in the pure state or as one constituent of a mixture always
produces its characteristic diffraction pattern. The intensities of the diffraction lines
due to one phase of a specimen depend on the proportion of that phase in the mixture. Qualitative
analysis of a substance is hence based on the identification of its diffraction
pattern, while its quantitative analysis is on the intensities of diffraction lines
of that substance. The advantage of x-ray diffraction method is that it identifies the
substance as such and not as its constituent elements. So, we can make use of this method
to identify the different allotropic modifications of a substance. Compared to ordinary
chemical analysis, XRD is much faster and non-destructive. It requires only a very small
quantity of the sample.
The identification of a substance can be done by comparing its diffraction pattern
with known patterns and to identify the one, which matches with the XRD pattern of the
sample. This procedure necessitates the collection of known patterns and was initiated by
Hanawalt, Rinn and Frevel at the Dow Chemical Company. From 1941 to 1969 several technical
societies including the American Society for Testing Materials (ASTM) had continued the
work of acquiring and disseminating diffraction data. Since 1969 the Joint Committee on
Powder Diffraction Standards (JCPDS) has taken up this activity.
By TEM
The transmission electron microscope (TEM)
operates on the same basic principles as the light microscope but uses electrons instead
of light. What we can see with a light microscope is limited by the wavelength of light.
TEMs use electrons as "light source" and their much lower wavelength makes it
possible to get a resolution a thousand times better than with a light microscope. We can
see objects to the order of a few Å (~10-10 m). For example, you can study small
details in the cell or different materials down to near atomic levels. The possibility for
high magnifications has made the TEM a valuable tool in materials research.
A "light source" at the top of the microscope emits the electrons that
travel through vacuum in the column of the microscope. Instead of glass lenses focusing
the light in the light microscope, the TEM uses electromagnetic lenses to focus the
electrons into a very thin beam. The electron beam then travels through the specimen we
want to study. Depending on the density of the material present, some of the electrons are
scattered and disappear from the beam. At the bottom of the microscope the unscattered
electrons hit a fluorescent screen, which give rise to a "shadow image" of the
specimen with its different parts displayed in varied darkness according to their density.
The image can be studied directly by the operator or photographed with a camera.
Ever since the award of Nobel Prize in 1986 for the
invention of Scanning Tunneling Microscope (STM) a new frontier research has been opened.
The capability to feel a surface, down to a resolution of 10-11 m makes STM a
versatile and sensitive surface analytical instrument with a high spatial sensitivity both
in the plane and in the vertical direction.
An extremely fine conducting probe is held close to the sample. Electrons tunnel between the surface and the stylus, producing an electrical signal. The stylus is extremely sharp, the tip being formed by one single atom. It slowly scans across the surface at a distance of only an atom's diameter. The stylus is raised and lowered in order to keep the signal constant and maintain the distance. This enables it to follow even the smallest details of the surface it is scanning. Recording the vertical movement of the stylus makes it possible to study the structure of the surface atom by atom. A profile of the surface is created, and from that a computer-generated contour map of the surface is produced.
APPLICATIONS
Thin film resistors
The use of thin films for the fabrication of resistors goes back at least 80 years. When used for fabrication of discrete resistors, thin films offer improved performance and reliability as compared with resistors of composition type – and lower cost for comparable performance when compared with precision wire-wound resistors. In the area of integrated circuitry, thin film resistors have come into their own.
Most film-resistor requirement can be met with films having sheet resistance in the range 10 to 1,000 W/?. Besides a suitable sheet resistance, films must possess a low temperature coefficient of resistance. They must also be sufficiently stable. Finally, it should be possible to produce them at a reasonable cost.
Thin film capacitors
The increasing quantity and complexity of electronic systems have stimulated the development of thin film microelectronics. This approach to integrated circuits involves the deposition of many resistors, capacitors, and their connections on stable, insulating substrates. Thin film dielectrics that are extensively used in circuit applications are mainly inorganic substrates, viz., oxides and halides of metals and semiconductors. The two most common dielectric materials that have been thoroughly evaluated in thin film capacitors are evaporated SiO and anodised Ta2O5.
Capacitor is the basic component for storing electric charge. Thin film capacitors usually have a parallel plate configuration, where two conducting layers separated by a suitable dielectric are supported by an insulating substrate 100 to 1,000 times thicker. The value of the capacitance (C) is a function of the over lapping area (A), the thickness of the dielectric d, and the dielectric constant e of the dielectric layer, C = e0 e (A/d), where e0 is the permitivity of free space.
Active devices
The deposition of thin film resistors upon the same substrate with passive components permits the fabrication of complex integrated circuits entirely by evaporation. Thin film circuits having thousands of active and passive components - including diodes, FETs, memories, superconductive tunnelling devices - have been produced with excellent yields.
Integrated Circuits
An integrated circuit (IC) can simply be described as a tiny block of semiconductor, usually silicon, which contains precisely doped regions and on the surface of which have been placed precisely formed, geometrically patterned thin films of metals and insulators.
The IC consists of a silicon single crystal substrate upon which has been grown an epitaxial layer of additional silicon. In the epitaxial layer, doped regions have been formed by diffusion. These doped regions are the PN junction regions and constitute the active electrical part of the circuit. The individual active regions are diodes and transistors, which form the IC, when they are interconnected. Isolation diffusion regions electrically separate the active regions from each other. As a by-product of the diffusion process, a thin film of SiO2 is grown on the surface of silicon. This film of SiO2 performs the vital basic function of protecting the PN junction from the outside world. The SiO2 also serves as the insulating layer upon which is placed a thin film of metal aluminium. The aluminium when properly geometrically patterned makes electrical contact to the underlying silicon through openings or windows in the oxide and interconnects the active areas in silicon. Metal films may also be used from time to time as resistors or capacitors.
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