Specific Process Knowledge/Thin film deposition/Deposition of Silicon Oxide/Deposition of Silicon Oxide using LPCVD TEOS
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LPCVD TEOS Furnace (B3)
DTU Nanolab has one furnace for deposition of LPCVD TEOS oxide. The furnace was installed in the cleanroom in 1995.
In the furnace LPCVD oxide can be deposited on 4" wafers. It is not possible to deposit LPCVD TEOS oxide on 6" wafers in he cleanroom.
TEOS is is Tetra-Ethyl-Ortho-Silicate, sometimes also referred to as Tetra-Ethoxy-Silane, and it has the chemical formula Si(C2H5O)4. It is a liquid that is stored in a bubbler. When a deposition is started, the bubbler is heated to 75 C, and TEOS is then vaporized and introduced into the furnace.
In the furnace, TEOS is thermally decomposed on the sample surface, so that a layer of silicon oxide (SiO2) is deposited on the wafer surface:
Si(C2H5O)4 (l) → SiO2 (s) + 4 CH2H4 (g) + 2 H2O (g)
Carbon can be incorporated in the film, but this can be reduced by an annealing.
The difference between TEOS and silane (used for deposition of silicon nitrid) is essentially that in TEOS the silicon atom is already oxidized. Therefore the conversion of TEOS to silicon dioxide is a rearrangement rather than an oxidation. As can be seen from figure 1 what is basically required to deposit silicon dioxide is a removal of two oxygen atoms for that a relative high temperature of 725 oC.
There one standard process for deposition on the LPCVD TEOS furnace called TEOS. Anyway there are two standby recipe, which used for load and unload the wafers : one called "Standby" for standard process and other one called "Stb-slw" which is for unloading thicker oxides. And on this recipe the furnace will be opened significantly slower than normal standby recipe. Thick TEOS layers have a tendency to form cracks when they are moved to fast out from the furnace.
TEOS can be used as an alternative to thermally grown or PECVD oxide, it has a dielectric constant of 3.56 (For thermal oxide it is 3.46). Furthermore TEOS has a very high surface mobility enabling it to fill holes that has a large aspect ratio and leaving the surface quite smooth see figure 2, hence it also covers corners and side walls very well.
Process parameters for the two standard deposition recipes on the TEOS furnace:
| Recipe name | Wafer size and number of wafers | Temperature [oC] | Pressure [mTorr] | TEOS gas flow [sccm] | O2 gas flow [sccm] | Comments |
|---|---|---|---|---|---|---|
| "STANDBY" and "STB-SLOW"
(standby recipe) |
4" wafers
1-17 wafers |
560
(wafer loading temperature) |
Atmospheric pressure | 0 | 0 | For wafer loading and unloading |
| "TEOS"
(deposition recipe) |
4" wafers
1-17 wafers |
715, 712, 720
(temperature zone 1, 2, 3) |
175 | 50 | 0 | Process recipe |
Deposition rate and refractive index:
The deposition rate is normally a little below 10 nm/min. But especially for short depositions, the deposition rate is higher and depends on the deposition time. The mean reason for this is that there is some TEOS (about 20-30 nm) will be deposited during the pre- and post-deposition steps, and this does not count as deposition time. The pre-deposition step is when the TEOS is introduced into the furnace tube before the deposition, and the pre-deposition step is when the TEOS gas line and mass flow controller are emptied through the furnace tube after the deposition.
Before you use the furnace, it is recommended to look at process log to find the latest deposition rates.
The refractive index is around 1.44.
TEOS has compressive stress (around 100-300 MPa).
Using LPCVD TEOS as a masking material for KOH etching
It is possible to use LPCVD TEOS pxide as a masking layer for KOH etching. However, it is not as suitable as LPCVD silicon nitride for deep KOH etching, as the the etch rate for TEOS oxide is higher than the etch rate for silicon nitride.