Diffusion and Deposition 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.
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, 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.
The products of that reaction are deposited on top of the wafer and will therefore not consume atoms from the base material.
– Low stress nitride
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 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 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 passivation layers. The deposited films exhibit excellent step coverage and reflow properties.
Low Temperature Oxide (LTO) is mainly used as a passivation layer over devices, which already have metal contacts.
The low temperature reduces the electrical and mechanical properties (step coverage) of the deposited SiO2 film, which makes it typically less suitable for electrical applications.
Tantalumoxide, Ta2O5, can be used to replace SiO2 and Si3N4 as gate dielectric in electrical applications such as DRAMS because of its excellent electrical properties at very thin layers.
Hard, scratch-resistant masking films can be generated by crystallizing the Ta2O5 layer with an O2 anneal. Tantalumoxide can also be used in combination with silicon dioxide layers to form high index-contrast multilayer structures for UV laser applications.
Semi-insulating Polycrystalline Silicon (SIPOS) is used in power electrical devices, where high voltages and high currents are present. The concentration of oxygen atoms in the polycrystalline silicon layer is decisive for the insulating effect of the layer. The atomic oxygen concentration in the polysilicon layer should be between 15 and 35 at%. Above 35% the properties of SiO2 are becoming too strong.
To measure the atomic oxygen % accurately special tools will be required. Alternatively a simple procedure called Differential Mass Method or Differential Thickness Method can be used to determine the average oxygen-doping quite accurately, but additional furnace tubes (thermal oxidation, LPCVD nitride) and etch tools will be necessary.
High Temperature Oxide (HTO) is an LPCVD process operating at or around 900°C that produces deposited SiO2 films with excellent electrical and step coverage properties, resembling those of thermally grown oxides. Due to the high deposition temperature HTO is less suitable for wafers that contain sensitive diffusion profiles, but at the same time it could be used as a possible alternative for thermally grown oxides.
Polyimide curing (PI curing)
Polyimides are lightweight high performance plastics that are flexible and resistant to heat and chemicals. Polyimides are used as mechanical stress buffers, electronic passivation layers, adhesive films in chip bonding, interlayer dielectrics and flexible cables in laptops and mobile phones.
Polyimides are typically spin coated in liquid form, baked and thermally cured at temperatures between 200 and 400°C. The bake-out removes remaining solvents and the curing anneal creates the cross-linking bonds in the polymers to give the film its final properties.
Low oxygen values are necessary during the bake-out and curing process to get bright and uniformly coloured films that have a good adhesion.
Atomic Layer Deposition techniques yield excellent step coverage and build a film from layers of 1 atom thick at a time. ALD, unlike LPCVD or PECVD processes, does not mix precursors but alternates the precursors in pulses. The nature of the self-limiting surface reaction of the precursors makes a simple process set-up.
Aluminiumoxide is one of the many materials to be produced with the ALD technique. In vacuum conditions, TriMethylAluminium (TMA) reacts with H2O to form Al2O3. One pulse of TMA adsorbs and saturates the surface with a monolayer of TMA molecules. The excess TMA is removed by purging the process-chamber. One pulse of H2O reacts with the adsorbed TMA and produces 1 atomic layer of Al2O3. The excess H2O is removed by purging the process-chamber. A typical TMA+ H2O pulse gives 1Å of Al2O3 , repeating the pulses gives the desired film thickness.
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