Asics are normally based on a monocrystalline group IV silicon semiconductor structure. There are three materials which are widely used:
- Si (Silicium): Cheap, widely used, versatile
- SiC : High speed electronics
Thin Film
Epitaxy growth
Thin film deposition
Thin films are extensively used to apply dopants and sealants to wafers. Thin films can be deposited with a thickness of between a few nanometres to about 100μm. The film can subsequently be locally etched using Lithography and Etching. To guarantee adhesion, the waver must be clean and have an optimal roughness on the surface. The deposition process normally causes intrinsic stress, which can lead to corrosion and cracking. Therefore, proper deposition is critical. Thin films are very sensitive to thermo-mechanical stress (different materials ⇒ different expansion coefficients) Deposition are classified into two reaction types, chemical and physical.
- Chemical
- Chemical Vapour Deposition (CVD)
- Electrodeposition
- Epitaxy
- Thermal oxidation
- Physical
- Physical Vapour Deposition (PVD)
- Casting
Thermal oxidation and masking
Allows to create stable insulation and conformal passivation layers.
Lithography
Etching
Is the process where the material is removed from the wafer (from the silicon substrate itself or from any film or layer). There are two types of etching:
- Wet etching
- material is dissolved when immersed in a chemical solution
- Dry etching
- material is sputtered or dissolved using reactive ions or a vapour phase.
Lift-off processing
Lift-off creates patterns on the wafer surface through an additive process. Lift-off is used in patterning metal films for interconnections. Lift-off consists of forming an inverse image of the pattern desired on the wafer using a stencil layer, which covers certain areas on the wafer and exposes the remainder. The layer to be patterned is then deposited over the stencilled wafer. In the exposed areas of the stencil, the layer material is deposited directly on the wafer substrate, while in the covered areas, the material is deposited on the stencil film. After the layer material has been deposited, the wafer is immersed in a liquid that dissolves the stencil layer. Once the stencil is dissolved by the liquid, the layer material lifts off (hence the term lift-off), leaving behind the layer material that was deposited directly onto the wafer substrate itself, which forms the final pattern on the wafer.(Book Williams)
The stencil layer normally gets created by photolithography. In this process, there are two types of resist (positive and negative) used. The developer is used to remove either the exposed (positive resist) or the unexposed (negative resist) material. Currently, positive resist has supplanted negative because it can achieve far better resolution. As a developer, an NAOH solution is normally used, which causes the exposed resist to etch away. After that the remaining resist is cured by baking at ∼125°C in order to harden it.
Resistor Fabrication
A resistive component can be manufactured in the following ways:
- Diffused resistors
- ion-implanted resistors
- thin-film resistors
- polysilicon resistors
Planarization
Is the process of improving the flatness or planarity of the surface.
