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In VLSI processing it is required to deposit thin layers of various materials for different purposes. These can be insulators for isolation of devices, contacts, and metal lines and can be metals for making contacts and epitaxial films for stress engineering and parasitic resistance reduction etc. We had seen in one of the earlier lectures that the device is constructed within a micro meter of the top
- f the Si wafer. This is followed by several levels of interconnects on top. All these layers can be
classified as thin films and are deposited by one of the technique we would discuss in this module.
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This is a brief outline of the module. We would look at various aspects of physical vapor deposition and chemical vapor deposition. Atomic layer deposition, which has gained recent popularity, would also be discussed.
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SLIDE 3 Vapor is a common feature of both the techniques. In PVD, you would typically use solid source of the material which is then converted into a vapor by various techniques like heating, sputtering and laser ablation. The material condenses on the surface of the substrate on which the film has to be deposited. Chemical reactions on the wafer are rare in the case of PVD. However there can be reactive PVD processes in which various materials can react in the vapor phase to form the product which then would condense on the substrate surface. Typically PVD processes are high vacuum processes with pressure in the range of 10-6 Torr or lower. In the case of CVD a chemical reaction between the reactants on the surface of the substrate leads to the end product which is deposited. The eventual sources transported to the reaction chamber must be in vapor phase. CVD can be carried
- ut at atmospheric pressure, high pressure (for deposition of artificial diamond for
example) or low pressure. The low pressure would be in the range of 0.1 Torr.
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SLIDE 4 Thin film processes are also widely applied in other fields. They find applications in a large variety of fields. For example watches, ornaments etc. A typical VLSI process upto metal 1 would contain about 19 thin film deposition
- steps. Out of these about 4 are sputter deposition processes and the rest are CVD
- r ALD processes.
It must be noted that all the subsequent layers above metal 1 are also deposited by thin film processes.
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Step coverage is an important consideration for various applications. In VLSI device fabrication, in general films have to be deposited over non planar surfaces. Hence line of sight deposition techniques like MBE and evaporation are not of any interest.
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If the energy on the surface is much larger than that in the vapor phase, the gas molecules would not be adsorbed on the surface. Rather they would be bounced off the surface. The energy and momentum must be conserved on the impact of the adsorbate on the surface. Essentially sinks of energy and momentum are required. Rearrangements: Let us say a dimer is physisorbed on the surface. For example, it is adsorbed on the surface and attached to the surface by weak van der Waals forces (forces between molecules, surface etc due to permanent or induced dipoles). The dimer can gain energy from the surface due to thermal energy on the surface, impact of other atoms on the surface etc, and it can move to the curve of chemisorption, if the meeting point of the two curves indicated by the circle is below the energy in the vapor phase. Further the dimer may move to strongly chemisorbed curve by absorbing energy. However this may require a breakage of the dimer into a monomer and breakage of surface bonds.
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Nucleation is pretty much like peoples behavior in a party.
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