In semiconductor technology, silicon oxide layers are mainly used as dielectrics or, more recently, for MEMS applications (micro electro mechanical systems). The simplest way to produce silicon oxide layers on silicon is the oxidation of silicon by oxygen. This process takes place in tube furnaces, nowadays mostly in vertical furnaces. If silicon oxide is to be formed on a substrate other than silicon, deposition of both elements from the gas phase is required. Processes are distinguished between so‑called LPCVD methods (low pressure chemical vapor deposition), which are carried out in vertical furnaces usually at higher temperatures, and processes that run at low temperatures supported by plasma in PECVD systems (plasma enhanced chemical vapor deposition).
Atmospheric thermal oxidation of silicon in the diffusion furnace
The oxidation of Si proceeds in three steps: transport of oxygen to the surface, diffusion of oxygen through the already grown oxide, and finally the reaction of oxygen with silicon at the silicon-silicon dioxide interface. As oxide thickness increases, the growth rate slows down because diffusion through the oxide becomes rate‑limiting. Very thin oxides can also be grown at reduced pressure or in RTP systems (rapid thermal anneal). During oxidation, silicon is consumed and the interface moves into the substrate. Oxidation of silicon can be performed dry or wet.
Si + O2 → SiO2
Dry oxidation takes place at temperatures between about 850 and 1200°C and proceeds relatively slowly but with very good uniformity. By adding small amounts of HCl or other chlorine‑containing gases such as trans‑DCE (dichloroethylene) or TCA (trichloroethane), the incorporation of contaminating metal atoms can be prevented and the number of crystal defects reduced. However, a small amount of chlorine is incorporated into the oxide layer. In wet oxidation the oxide deposition is greatly accelerated and growth rates increase substantially, allowing the formation of thick oxide layers. The moisture is usually introduced via a torch burner, i.e., hydrogen and oxygen are reacted immediately before entering the furnace so that the produced water is of very high purity. To operate this process safely, the burner flame must be monitored continuously and potential hydrogen leaks detected promptly. That makes this technology more expensive.
LPCVD deposition of silicon oxide in the tube furnace
Thermal oxide deposition processes mostly take place at reduced pressure (LPCVD). There are several common methods. In the LTO method (low temperature oxidation) diluted silane is reacted directly with oxygen at around 430°C (pyrolytic decomposition of silane):
SiH4 + O2 → SiO2 + 2 H2
Unfortunately this reaction is diffusion‑controlled, i.e. the concentration of the gas close to the surface determines the deposition rate. Because reactants are depleted at the deposition itself, it is difficult to maintain uniform conditions across the entire reactor. For this process, JTEKT (formerly Koyo) therefore uses injection cages that ensure fresh gas flows into the furnace chamber from all sides simultaneously. Only in this way can uniformly thick layers be achieved across a complete batch of processed wafers.
At higher temperatures (900°C) SiO2 can be formed in the so‑called HTO method (high temperature oxidation) or from a combination of dichlorosilane SiH2Cl2 and nitrous oxide N2O:
SiH2Cl2 + 2 N2O → SiO2 + decomposition products
TEOS method. A widely used precursor for depositing silicon dioxide is TEOS (tetraethoxysilane), which decomposes thermally relatively easily:
Si(OC2H5)4 → SiO2 + decomposition products
PECVD deposition of silicon oxide
Often the high temperatures required for the above formation of silicon oxide layers are undesirable. Activation of reactants in the plasma enables deposition at much lower temperatures. PECVD systems are used. In plasma‑assisted oxide deposition, silane SiH4 and nitrous oxide N2O are used:
3 SiH4 + 6 N2O → 3 SiO2 + 4 NH3 + 4 N2
Plasma deposition of silicon oxide from TEOS is also possible:
Si(OC2H5)4 → SiO2 + decomposition products
Furthermore, plasma deposition of silicon oxide using a triode configuration, as also used for plasma nitride deposition, allows control of film stress (stress control). Stress especially accumulates in thicker films, which can lead to wafer bow and can be problematic for MEMS applications. Stress is influenced by hydrogen incorporation, deposition temperature and ion bombardment during deposition.
To better control film stress a triode configuration of the plasma reactor is used, also known as dual‑frequency PECVD. The top electrode is driven with an RF voltage of 13.56 MHz while the sample holder is biased at 360 kHz. The reaction chamber itself is grounded. This allows a high plasma density to be set via the high‑frequency generator, while the low‑frequency generator accelerates ions toward the substrate. Frequencies below 1 MHz allow ions to follow the oscillation of the plasma - at 13.56 MHz only the electrons can do so.
