4 THIN FILM VACUUM COATING UNIT

Thin films are low dimensional materials. They generally lie in the thickness range of few nanometers to several micrometers. Production of ultra high vacuum (of the order of 10-6 torr) is an essential requirement for the production of high quality thin films. High quality thin films are the pre-requisite for the manufacturing of many electronic and optical devices. There are several pumps available in the market to create the vacuum of desired amount. Generally these pumps are categorized in two categories: Fore Vacuum pumps and High Vacuum pumps. Example of fore vacuum pump is Rotary Pump and that of high vacuum pumps are Turbo Molecular Pump and Diffusion Pump. There are various techniques which use these pumps to produce ultra high vacuum in order to deposit quality thin films. Some of them are explained below:

1.    Pulsed Laser Deposition – Pulsed Laser Deposition (PLD) is a typical technique for the preparation of thin films of complex stoichiometry, well-controlled interfaces or multilayered structures. In PLD, laser shots of the intense laser with specific wavelength and high energy are focused on the target of the material to be used for the deposition. The energy of the laser should be high enough so that it can ablate the target material. The deposition of the films can be performed under high vacuum conditions. The required vacuum is achieved with the aid of two pumps. First one is a fore vacuum pump e.g. rotary pump, required to create the base vacuum up to the order of 10-2 Torr followed by a high vacuum pump e.g. turbo molecular pump which can create vacuum up to the order of 10-6 Torr. However, responsive background gases, such as oxygen (for the preparation of oxide films) or nitrogen (for nitride films) could also be used to maintain the required stoichiometry. An inert background gas such as argon can also be utilized for the deposition of thin films using PLD. When high intensity laser is made to irradiate the target, it ablates the material which it ablates the material which leads to the development of a plume which spreads in a flow perpendicular to the surface and is collected on an appropriate substrate. The full PLD process can be portrayed by five stages, which are as follows:

(1) Light absorption in the solid

(2) One-dimensional plume expansion of the ablated material during laser irradiation

(3) Free three-dimensional expansion into vacuum or a dilute background gas

(4) Slowing down and stopping of the plume in a background gas and

(5) Collection of ablated atoms on a substrate and subsequent film growth

The deposition of thin films can be controlled by varying a few parameters such as pressure, substrate temperature, background gas, target-substrate distance etc. Figure below shows the schematic of the PLD system. A high power Nd:YAG Laser (Make: Continuum) has been used, operating at a wavelength, λ = 266 nm and energy = 100mJ. The deposition chamber is fitted with a turbo molecular pump (Make: Pfeiffer) and creates a high vacuum of the order of 10-6 torr in the chamber. Also, the chamber is equipped with a rotating target carousel which can accommodate six 1 inch diameter targets. This setup enables multilayer deposition of different materials on the same substrate without breaking the vacuum. The substrate holder is available with PLD controlled heater assembly for substrate heating up. All six targets can be controlled externally via a microcontroller and can be programmed for the desired thickness of thin film by setting the number of laser shots and laser frequency.

The substrate holder is held parallel to the target and the laser beam is made incident at an angle of 45⁰ to the target surface through focusing optics. Target to substrate distance can be varied as per requirement by adjusting the substrate holder arrangement.

2.     Thermal Evaporation – Thermal evaporation is in literal sense a physical deposition technique. As per the name the material to be deposited is brought in the vapor phase by the virtue of thermal energy to the atoms of the material in solid form and these atoms are transferred to a cooler substrate from the heated source. Evaporation is achieved by joule’s heating by applying low voltage and high current. The synthesis process need to be carried out in a properly designed vacuum system, i.e. under high vacuum environment (10-6 torr) created by rotary and diffusion pump; so as to avoid uncontrolled oxidation of source materials and final product as well as that of the components of the synthesis system. Mean free path (mfp) of the particles also increases in vacuum system, which is often desired, as it should be greater than the dimensions chamber and the source-to-substrate distance. Under such conditions, the transport of the material from the source to the substrate occurs by molecular beams. Sometimes reactive gases are used in certain cases of depositions, in that cases, it is useful first to evacuate the system to very low pressure so that the materials to be synthesized do not get contaminated by undesirable atoms and then pressurize the system to desired value by introducing the high purity gases in the synthesis chamber. Illustrative layout of the thermal evaporation unit has been presented in figure below.

Heating of the material can be carried out through some suitable filament, crucible, boat etc. (collectively called as “evaporation source” or “crucible”) which is in intimate contact with the evaporant and which is heated electrically. Evaporation sources are generally made of refractory materials such as tungsten (W), molybdenum (Mo), tantalum (Ta), and niobium (Nb) (with or without a ceramic coating); because they have a low vapour pressure at the evaporation temperature and low reactivity with the evaporant. The thickness of the deposited thin film is monitored with the help of quartz crystal oscillator placed in the vicinity of the substrate. The final film produced by evaporation technique is in general “non-stoichiometric” because it is difficult to achieve “conformal coverage” during evaporation. It is very likely that the compound used for evaporation dissociates before evaporation. Therefore, evaporation technique for thin film deposition has been superseded in many instances by sputtering and CVD methods. A Hind Hivac thermal evaporation system is shown in figure below. It consists of a 4 inch oil based diffusion pump backed by a rotary pump to attain high vacuum of ~ 4 x 10-6 torr. The vacuum system is equipped with quartz crystal based thickness monitor to control the in-situ thickness of the deposited thin film.

3.   Sputtering – Sputtering is a high-energy fabrication method to produce thin films, especially for obtaining stoichiometry thin films (i.e. without changing the composition of the original material) from target material. Target material may be some metal, alloy, ceramic or any composite material. Sputtering is also effective in producing non-porous compact films. It is an efficient technique to deposit multilayer films for mirrors or magnetic films for spintronics (devices using spin of electron and hole along with the charge on them) applications. Atoms from a solid target source are ejected or sputtered via the process of momentum exchange inside the plasma by the action of high-energy ions, usually originating from some inert gas ambient like argon. The ejected particles are then deposited (condensed) on the surface of a substrate to produce a thin film. Plasma (taken from the Greek word “plassein” meaning “to mold” or “to spread”) is an ionized gas that is considered to be a distinct phase of matter. Plasma is an ionized state of matter containing ions, electrons and neutral species. Plasma is electrically conductive and is strongly influenced by electric and magnetic fields. Plasma is used to derive ionization creating a large number of ions and free electrons. Plasma is generated by applying DC or AC voltages and a bias voltage is applied to the target to promote acceleration of ions.

Inert gas ions like Ar+ are made incident on the target at a very high energy. Depending on the energy of ions and the ratio of ion mass to that of target atoms, the ion-target interaction can be a complex phenomenon (i.e. the kinetic energy of the impinging particle largely dictates what event would take place). The interaction of ions with the target material is dependent on the energy of the colliding ions. The incident ions may get bounced back when their energy is very low (< 5 eV), whereas for energy greater than 10 keV, the collision is nearly head-on and they get embedded in the target material (basis of ion implantation). If the kinetic energy lies in the range of the above two extremes, a number of interaction mechanisms are possible, i.e. it can create collision cascades in target atoms, displace some of the atoms in the target creating vacancies, interstitials and other defects, desorb some adsorbates, create photons while losing energy to target atoms and sputter out some target atoms/molecules, clusters, ions and secondary electrons. Figure shows a schematic picture of various possibilities. For deposition of materials, one is interested in the sputtering yield, which is defined as the number of ejected species per incident ion and increases with the energy and mass of the bombarding ions. Sputter yield for different elements with same incident ion having same energy varies in general. Therefore, for a target consisting of two or more than two different elements, the one having more sputter yield is incorporated in more amount than the others. However, high sputter yield elements get depleted fast and other elements make higher contribution. Thus, the stoichiometry is achieved in the deposited film.

Sputter deposition can be carried out using Direct Current (DC), Alternating Current (AC) (or Radio Frequency (RF) sputtering) or magnetron sputtering. In all the above cases glow discharge or plasma of some inert gas atoms are used to create large number of ions and free electrons. The deposition is carried out in a high vacuum (HV, (10-3 – 10-7 mbar or torr) since 1 mbar = 3/4 torr approximately) or ultra-high vacuum (UHV, (10-7 – 10-12 mbar or torr)) system equipped with electrodes, one of which is a sputter target (held at high negative voltage (cathode)) and the other is a substrate (held at positive, ground or floating potential with the walls of chamber (anode)), etc. Although the system during deposition is at high pressure, which ensures that the adequate purity is obtained.

In general, the sputter yield is greatest for the following set of conditions:

(i)   High atomic weight process gas.

(ii)   Low atomic weight cathode material.

(iii)   Low concentration of reactive gas species in the vessel.

“Argon” is the most commonly employed process gas for sputter deposition processes, as it has a high sputter yield for most metals, is chemically inert and non-toxic, and is relatively inexpensive compared with the other noble gases (Krypton (Kr) and Xenon (Xe)). On the other hand, oxygen gas is known to react chemically with metals, forming metal oxides easily. Thus, in the present work, argon and oxygen gases are used in deposition of metal and metal oxide thin films respectively.

4.   Electron-beam evaporation: E-beam evaporation is an evaporation technique in which an accelerated beam of electrons from cathode is made to hit the target at anode leading to vaporization of target material. These vapors get condensed on the surface of substrate to form a thin film of target material. Tungsten filament is used as a source of electrons. This filament is heated by passing a high current through it which leads to thermionic emission of electrons. These electrons are focused to form a beam by applying magnetic field. The beam then strikes the source material which is placed in molybdenum/Graphite crucible. The beam is swept across the surface of the source material to uniformly heat all of the material. The material is then evaporated and condenses over the substrate surface. E-beam evaporation is similar to thermal evaporation but has some advantages over thermal evaporation. For instance, in e-beam a large amount of energy is supplied to the target materials, thus, materials having high melting temperature can also be evaporated. Also, e-beam evaporation process involves heating of only the source material, so very low degree of contamination from the crucible occurs, as compared to that in thermal evaporation.

5.  Molecular Beam Epitaxy: It is a technique to deposit a mono crystalline film. Epitaxy means the growth of crystalline layers on crystalline substrate. It involves the epitaxial growth via the interaction of one or more molecular or atomic beams that occur on a surface of a heated crystalline substrate. It is performed in ultra high vacuum (10-11 Torr). It is a very slow growth process, so very precise control over major compositional variations and impurity incorporation can be achieved. There are two types of epitaxy: Homoepitaxy (Substrate and material are of same composition), used to fabricate layers with different doping levels and Heteroepitaxy (Substrate and material are of different kinds like Ga-As) used to fabricate integrated crystalline layers of different materials.

 

In MBE process, the term ‘beam’ means the evaporated atoms do not interact with each other or the chamber walls or with other vacuum chamber gases until they reach the wafer. Epitaxial growth takes place due to interaction of molecular or atomic beams on the surface of a heated crystalline substrate. The vacuum system consists of a stainless steel chamber. The pumping system usually consists of ion pump, rotary pump, turbo molecular pump and cryogenic pump for the pumping of specific gas species. Ultra high vacuum is used to obtain sufficiently clear epi-layer. MBE generally has application in the manufacturing of hetero junction Bipolar Transistor used in satellite communication, electronic and optoelectronic devices like LEDs, in construction of quantum wells, dots and wires, and low temperature super conductors.

6.    PECVD (Plasma Enhanced Chemical Vapor Deposition): As the name suggest, it is a chemical deposition technique. In this process, thin films of various materials can be deposited on substrates at lower temperature than that of standard chemical vapor deposition. In PECVD processes, deposition is achieved by introducing reactant gases between parallel electrodes—a grounded electrode and an RF-energized electrode. The capacitive coupling between the electrodes excites the reactant gases into plasma, which induces a chemical reaction and results in the reaction product being deposited on the substrate. The substrate, which is placed on the grounded electrode, is typically heated to 250°C to 350°C, depending on the specific film requirements. In comparison, CVD requires 600°C to 800°C. The lower deposition temperatures are critical in many applications where CVD temperatures could damage the devices being fabricated.

Typically, PECVD is used to deposit Silicon Nitride (SixNy), Silicon dioxide (SiO2), Silicon oxy-nitride (SiOxNy), Silicon carbide (SiC) and amorphous silicon (a-Si). Silane (SiH4), the silicon source gas, is combined with an oxygen source gas to form silicon dioxide or a nitrogen gas source to produce silicon nitride.

Silicon dioxide and silicon nitride are dielectric (insulating) materials commonly used in the fabrication of electronic devices to isolate multiple conductive layers, capacitors, and for surface passivation. These films are also used for encapsulation to protect devices from corrosion by atmospheric elements such as moisture and oxygen.

Summary:

In this module, we have learnt about different techniques which incorporate high vacuum in order to deposit thin films for numerous applications. Each of these techniques has its own advantages and disadvantages. For e.g. Thermal evaporation is simple and cheap but there is considerable amount of waste of source material during the process. In e-beam evaporation, multilayer deposition is possible but it is very difficult to control the deposition rate. PLD has the advantage that it maintains the stoichiometry of the target material thereby making it the best suited technique to deposit complex materials on the substrate in the same chemical composition as they are in target material. The only major disadvantage it has is that the uniform deposition occurs only in very small area (∼ 1 cm2). So based on their pros and cons, different techniques are used to fabricate thin films of various materials.

REFERENCES

1. “The Material Science of thin films” by Milton Ohring.
2. “Coatings on Glass” (volume 6) by H. K. Pulker.
3. “Langmuir Blodgett films” (volume 3) by C. W. Pitt, G. G. Roberts.
4. “Handbook of thin film Technology” by Frey, Hartmut, Khan and Hamid R.
5. “Thin film Technology and Application” by K. L. Chopra & L. K. Malhotra.
6. “Deposition Technology for films and coatings” by Rointan F. Bunshah.
7. “High vacuum techniques” J. Yarwood (Chapman & Hall) 1967.
8. “Vacuum technology” A. Roth (North-Holland Publishing Company, Amsterdam) 1982