|Silicon oxide. LPCVD and PECVD deposition processes. Stress regulation.|
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In semiconductor technology, silicon oxide layers are mainly used as dielectrics or latterly also for MEMS (micro electro mechanical systems) applications. The most simple way to produce silicon oxide layers on silicon is the oxidation of silicon by oxygen. This process is performed in tube furnaces, today mainly vertical furnaces. If silicon oxide is to be formed on a different substrate than silicon, a deposition of both elements from the gas phase is necessary. One differentiates between so called LPCVD processes (low pressure chemical vapor deposition), which are mostly performed in vertical furnaces at higher temperatures and processes which are run plasma-enhanced at lower temperatures by PECVD systems (plasma enhanced chemical vapor deposition)
|furnaces for wet deposition||LPCVD furnaces||Triode PECVD reactor|
The oxidation of Si takes place in three steps: transport of the oxygen to the surface,
diffusion of the oxygen through the already grown oxide and finally the reaction of the oxygen with the silicon at
the interface between silicon and silicon oxide. With growing oxide thickness, the growing rate slows down because
the time of the diffusion through the oxide depends on its thickness, and therefore becomes relevant for the
oxidation rate. Very thin oxides can also grow at reduced pressure or in RTP systems
(rapid thermal anneal). By the oxidation of the silicon, the silicon is consumed and
the interface moves into the substrate. The oxidation of the silicon can be carried out dryly or wetly.
Si + O2 → SiO2
Dry oxidation takes place at temperatures from 850 - 1200 °C and is carried out rather slowly but with good evenness. By adding small amounts of HCL or other chloric gases, such as TCE (trichloroethylene), integration of contaminated metal atoms can be prevented and the number of crystal defects can be reduced - however; a small amount of chlorine is integrated in the oxide layer.
By wet oxidation, deposition is highly accelerated and growth rate is significantly increased. Thus, thick oxide layers can be produced. Moisture is usually inserted by an oxyhydrogen burner called "torch", that is hydrogen and oxygen reacts immediately before insertion into the furnace, creating the desired water in high purity. In order to perform the process safely, the flame of the burner has to be monitored constantly and it has to be assured that leaks of hydrogen are discovered early enough. Unfortunately, this raises the costs of this technology.
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
Often, the necessary high temperatures for the formation of silicon oxide layers
described above is not desired. The activation of plasma makes significantly lower temperatures for the
deposition possible. PECVD systems are used. For oxide deposition, silane
SiH4 and laughing gas N2O are used:
3 SiH4 + 6 N2O → 3 SiO2 + 4 NH3 + 4 N2
Additionally a plasma deposition of silicon oxide from TEOS is possible:
Si(OC2H5)4 → SiO2 + decomposition products
Furthermore; plasma deposition of silicon oxide at use of the triode configuration allows, like the deposition of plasma nitride, the adjustment of the layer tension (stress control). Stress is usually created by deposition of thicker layers, which may lead to a deflection of the whole wafer and is particulary disturbing at MEMS processes. Stress is affected by the integration of hydrogen, the deposition temperature and the bombardment with particles.
For better adjustment of the layer tension, a triode configuration of the plasma reactor is used, also known as doublefrequency PECVD. A RF of 13,56 MHz is put on the upper electrode, while 360 kHz are put on the chuck. The reaction chamber itself is earthed. Thus, a high plasma density can be reached by a highfrequency generator, while an acceleration of the ions towards the substrate can be attained by a lowfrequency generator. Frequencies below 1 MHz enables ions to follow direction changes of the plasma - at 13,56 MHz only electrons are able to do so. The triode configuration is available for SNTEK machines
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