LPCVD. Dépôt chimique en phase vapeur à pression sous-atmosphérique
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For the manufacturing of integrated circuits on silicon substrate, it is necessary to prepare various layers:
1) Conductive layers consisting of semiconductor material or metals
2) Insolating, dielectric layers, mainly silicon oxide and nitride
Oxide can be formed by oxidation of substrate material if the layer should be grown directly on silicon substrate. On other substrates or for nitride layer formation, it is necessary to deposite the material from the vapor, using an LPCVD process (low pressure chemical vapor depostion).
Thermal oxide deposition is almost always carried out at low pressure (LPCVD).
There are several established methods: In the LTO process
(low temperature oxide) depleted silane reacts with oxygen at approximately 430 °C
(pyrolysis of silane):
SiH4 + O2 → SiO2 + 2 H2
Unfortunately, this reaction is diffusion controlled, that is the concentration of the gas determines the deposition rate. During the deposition process the concentration of the reactants decreases; therefore, it is difficult to create the same conditions for the deposition inside the whole reactor. In this process, Koyo therefore uses cages for the injection of the gases, which assure that fresh gas flows into the furnace chamber from all sides at the same time. Only that way, evenly thick layers are deposited on all processed wafers of the batch.
At higher temperatures (900 °C), SiO2 can be created in the so called HTO process (high temperature oxide), but also by a combination of dichlorosilane SiH2Cl2 and laughing gas N2O:
SiH2Cl2 + 2 N2O → SiO2 + decomposition products
TEOS process. An often used compound for formation of silicon oxide layers is TEOS (Tetraethylorthosilicate), which can be decomposed very easily:
Si(OC2H5)4 → SiO2 + decomposition products
LPCVD nitride can easily be deposited in a reproducible, very pure and uniform way. This leads to layers with good electric features, very good coverage of edges, high thermal stability and low etch rates. However; high temperatures are necessary for deposition and reaction rate is slower.
Deposition is performed in several steps: gas transport to the surface - adsorption - reaction of surface reactants (without involvement of substrate atoms) - desorption of byproducts. Since the reaction at the surface at given temperatures determines the deposition rate(this is called reaction controlled), depletion of reaction material in the gas phase by consumption, and therefore the induction of fresh gas, only plays a subordinate role. Hence it is possible to process many wafers, which are arranged side by side in the gas flow, without great problems. The formation of silicon usually results from dichlorosilane (DCS) and ammonia at 700-850°C.
3 SiH2Cl2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2
Poly-silicon can be formed by decomposition of silane at temperatures around 600°-700°C.
SiH4 → Si + 2 H2
In-situ-doping is possible by adding doping gases like phoshine PH3 or borane BH3. In this case it has to be made sure, that the doping gas concentration can be kept stable over the length of the furnace tube. This can be reached by using gas injectors.
Also metals can be deposited by LPCVD-processes:
WF6 + 3 H2 → W + 6 HF
LPCVD systems are available from the company Koyo Thermo Systems. Several vertical furnaces and horizontal furnaces can be used.