In sputter deposition, a target made of a material to be deposited is physically bombard by a flux of inert – gas ions ions (orgon) in a vaccume chamber at a pressure of 0.1 – 10 Pa.
Atoms or molecules from the target are ejected and deposited onto the wafer.
There are several general classes of sputter tools differing by the ion excitation mechanist
In direct – current (DC) glow discharge, suitable only for electrically conducting materials, the inert gas ions are accelerated in a dc electric fields between the target and the water.
In planar RF, the target and the wafer form two parallel plates with RF excitation applied to the target.
In ion-beam deposition ions are generated in a remote plasma, then accelerated at the target.
RF planar sputtering and ion-beam methods work for the deposition of both conducting and insulating materials, such as SiO2.
In planar and cylindrical magnetron sputtering, a externally applied magnet field increases the ion density near the target, thus raising the deposition rates.
Nearly any inorganic material can be sputtered.
Sputtering is a favored method in the MEMS for the deposition at low temperature (<1500 C) of thin metal films such as Al, titanium, Al/Si & Ti/W alloys, amorphous silicon , insulators including glass and piezo-electric ceramics(eg. PZT and ZnO).
In a variation known as reactive sputtering, a reactive gas such as nitrogen or oxygen is added during the sputtering of a metal to form compounds such as titanium nitride or titanium dioxide.
The directional randomness of the sputtering process, provided that the target size is larger than the wafer, results in good step coverage. The uniformity of the thin film over a geometrical step- through some thinning occurs near corners.
The deposited film has a very fine granular structure and is usually under stress, the stress levels vary with the sputter power and chamber pressure during deposition, with tensile stress occurring at higher power and lower pressure.
The transition between the compressive and tensile regions is sharp over a few tenths of a Pa, making the crossover an ideal point for zero stress deposition difficult to control. Heating the substrate during deposition is sometimes used to reduce film stress.
Many metals, particularly inert ones such a gold, silver and platinum, do not stick(adhere) well to silicon, SiO2 or silicon nitride, immediately after deposition or during handling.
A thin (5 to 20 mm) adhesion layer, which bond to both the underlying material and the metal over it, enables the inert metal to stick.
The common adhesion layers are Cr, Ti alloy. The inert metal must be deposited on the adhesion layer without breaking the vacuum an oxygen in the air would immediately oxidizes the adhesion layer, making it useless.
Evaporation involves the heating of a source material to a high temperature, generating a vapor that condenses on a substrate to form a film.
Nearly any element (Al, Si, Ti, Au) including many high- melting-point metals and compounds (eg. Cr, Mo, Ta, Pd, Pt, Ni/Cr, Al2O3) can be evaporated.
Deposited films consisting of more than one element may not have the same composition as the source material because the evaporation rates may not corresponds to the stoichiometry of the source.
Process of Evaporation:
Evaporation is performed in a vacuum chamber with the background pressure typically below 10-4 Pa to avoid contaminating the film.
Target heating can be done “resistively” by passing an electrical current through a tungsten filament, strip or boat holding the desired material or scanning a high- voltage (eg. 10 kV) electron beam over the source material.
Resistive evaporation is simple but can result in spreading impurities or other contaminants present in the filament.
E-beam evaporation by contrast, can provide better- quality films and slightly higher deposition rates (5-100 nm/min), but the deposition system is more complex, requiring water cooling of the target and shielding from x-rays generated when the energetic electrons strike the target.
Radiation that penetrates the surface of the silicon substrate during the deposition process can damage the crystal and degrade the characteristic of electronic circuits.
Evaporation is a directional deposition process in the specific angle deposition of material particles. On the substrate, causing poor step coverage and learning corners and side walls exposed.
This is undesirable if thin film continuity is required, Rotating the substrate to face the source at different angles during deposition reduces the effect.
Sometimes shadowing can be used deliberately to selectively deposit material an one side of a step.
Thin films deposition by evaporation exhibits tensile stress, increasing with higher material melting point.
Evaporated niobium and platinum films, can have tensile stress in excess of 1 G Pa, sufficient to cause curling of the wafer or even peeling, like sputtering, an adhesion layer must be used with many metals.