Diffusion and Deposition Processes.
Diffusion processes.
This process changes the electrical properties of the semiconductor. Sometimes a reactive element is added, for example oxygen, which causes the Si wafer to oxidize. Si is then consumed and converted into silicon dioxide, SiO2, which can be used in many applications.
Oxidation processes (oxide thicknesses from 16 Å up to 20 µm):
– Dry oxidation
– Pyrogenic oxidation with water-cooled external torch
– Liquid flow DI water injection system for thick oxides (up to 20 micron)
– Wet oxidation (steam oxidation using water bubbler system)
All oxidations can be provided with DCE or HCl
Annealing process Ar or N2
1200 °C clean Ar anneal + low O2
Alloy/Sinter process (forming gas to 100% 1-12)
Liquid source deposition process (POCl3, BBr3)
Custom-designed processes also available
(Metal Alloy) anneal
An anneal is a heat treatment in general and is often used to reduce stress or redistribute dopants. A metal alloy anneal is used to reduce the electrical resistance between Si and Al contacts.
Dry oxidation
The dry oxidation process is used to grow layers on a wafer by a chemical reaction between oxidants and silicon atoms. The layer acts as a gate dielectric, a protective coating, a mask during diffusion or as an insulating layer. The dry oxidation rate is limited and therefore mainly applied for thin oxide films up to 1000 Å.
Pyrogenic oxidation
Pyrogenic oxidation, also known as wet oxidation, is used to create a thick insulating SiO2 layer. Because the high-purity steam generated from H2 and O2 is highly acidic its oxidation rate is much faster. The SiO2 layer is used as a mask during dopant diffusion, as a junction passivation and as an insulating field oxide.
DI water injection oxidation
DI water injection oxidation, also called wet oxidation, is used to create extremely thick insulating SiO2 films and as a cheap alternative to the pyrogenic oxidation.
This process can be used to create a mask for dopant diffusion, a junction passivation layer, an insulating field oxide and optical insulator films. Typical SiO2 thickness for the last application is 8-20 microns and may require 40 days or more of continuous processing. Using DI-water instead of H2 is much more economical and safer.
Phosphorus doping using POCl3
POCl3 is used to create a n-type conducting layer by first converting POCl3 into a deposited P205 layer.
The P205 film supplies the P-dopant which is subsequently diffused into the base material. This process is used in the production of solar cells and to generate resistors.
Cleaning and enhanced oxidation using DCE
Dichloroethylene and trans 1,2-dichloroethyIene are liquid source materials used for the in-situ generation of ultra-high-purity HCI and used in (tube) cleaning and improved oxidations.
The generated HCI is capable of removing mobile ions and transition metals from the furnace environment and wafer surface. This allows for the growth of oxides with lower defect densities, lower mobile ion concentrations and a significant improvement of minority carrier lifetime.
LPCVD Processes.
The products of that reaction are deposited on top of the wafer and will therefore not consume atoms from the base material.
Poly silicon
– Flat
– Ramped
– Phosphorus-doped
– Boron-doped
SiPOS
Nitride
– Low stress nitride
– Oxi-nitride
TEOS (BPSG)
LTO (BPSG)
HTO
Tantalum oxide
Ramped polycrystalline silicon
Polycrystalline Si (poly in short) is used as self-aligned gate electrodes and as masking material. Resistors and electrodes can also be fabricated with the appropriate amount of (in-situ or ex-situ) doping.
The term ‘ramped’ refers to the tilted temperature profile that is generally used to counter-effect the depletion caused by the consumption of SiH4, DCS or Si2H6.
Flat polycrystalline silicon
A flat poly process is used in situations with a demand for precise grain dimensions. The term flat refers to the flat temperature profile. The depletion effect is eliminated with either injectors or high gas flows.
The use of poly as “bulk material” in Thin Film Transistor (TFT) applications typically requires uniform grains to improve the electron mobility.
Silicon nitride
Silicon nitride is used as an insulating or masking layer in electrical and mechanical applications because of its excellent chemical stability and step coverage, and as an anti-reflecting coating in optical applications.
For thick films (>1 µm), the Low Stress Nitride process is available, while doping nitride films with oxygen results in oxynitride films that have special applications in the optical industry.
TEOS
TEOS is used to deposit an insulating SiO2 layer in electrical applications that require excellent step coverage. Its medium process temperature of around 700°C minimizes the redistribution of (implanted) dopant profiles.
The BPSG-doped TEOS process is used for the deposition of medium temperature (650-700°C) boron- and phosphorus-doped SiO2 films that are used as cladding and passivatio