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==LPCVD TEOS oxide deposition==

[[Image:teos1.jpg|300x300px|thumb|Figure 1: TEOS molecule structure]]
[[Image:TEOS.jpg|300x300px|thumb|Figure 2: LPCVD TEOS oxide deposited on Si trenches]]

DTU Nanolab has one furnace for deposition of LPCVD TEOS oxide. The furnace is a Tempress horizontal furnace, and it was installed in the cleanroom in 1995.

In the furnace LPCVD TEOS oxide can be deposited on up to 15 4" wafers. It is not possible to deposit 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(C<sub>2</sub>H<sub>5</sub>O)<sub>4</sub>. The molecular structure can be seen figure 1. It is a liquid that is stored in a bubbler. When a deposition is started, the bubbler is heated to 75 <sup>o</sup>C, and TEOS is then vaporized and introduced into the furnace. The TEOS flow is controlled by an MFC (mass flow controller). 

In the furnace, TEOS is thermally decomposed on the sample surface, so that a layer of silicon dioxide (SiO<sub>2</sub>) is deposited there:

Si(C<sub>2</sub>H<sub>5</sub>O)<sub>4</sub> ''(l)'' → SiO<sub>2</sub> ''(s)'' + 4 CH<sub>2</sub>H<sub>4</sub> ''(g)'' + 2 H<sub>2</sub>O ''(g)''

Carbon can be incorporated in the TEOS oxide layer, but this can be reduced by an annealing - see below. 

The difference between TEOS and silane gas (used for deposition of silicon nitride) 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, and for that to happen a relative high temperature of 720 <sup>o</sup>C is needed.    

TEOS molecules have a very high surface mobility enabling the TEOS oxide to fill holes that have a large aspect ratio and leaving the surface quite smooth, see figure 2, hence it also covers corners and side walls very well. However very small nanometer trenched and/or very deep trenches will be challenging to fill.

TEOS oxide has an amorphous crystal structure. If TEOS oxide is annealed at a high temperature, ~1000 <sup>o</sup>C, carbon is removed, and the crystal structure becomes more dense, and thus the thickness decreases. The quality of an annealed TEOS oxide film is actually almost similar to the quality of a thermal oxide film.

There is one standard process for deposition of LPCVD TEOS oxide on the furnace, and this recipe is called "TEOS". The Thin Film group will have to be involved, if depositions have to done with other process parameters. The exact TEOS flow is unfortunately not know, but it is expected to be much higher than the setpoint in the recipe, because the MFC is not calibrated for TEOS (it has not been possible to buy a correctly calibrated MFC).

On the furnace there are also two standby recipes, which are used for wafer loading and unloading. One standby recipe is called "STANDBY". The other standby recipe is called "STB-SLOW", and this is being used, if a thicker TEOS oxide layer (> 1 µm) is deposited, because the furnace then has to open slower to avoid stress and thus cracks in the deposited film.

==Process parameters standard deposition recipes and the standby recipes on the TEOS furnace:==

{| border="1" cellspacing="0" cellpadding="4" 
!Recipe name
!Wafer size and number of wafers
!Temperature [<sup>o</sup>C]
!Pressure [mTorr]
!TEOS gas flow [sccm]
!O<sub>2</sub> gas flow [sccm]
!Comments
|- 
| "STANDBY" and "STB-SLOW" 
(standby recipes)
| 4" wafers
1-15 wafers
| 560 <sup>o</sup>C
(wafer loading temperature)
| Atmospheric pressure
(wafer loading pressure)
| 0 sccm
| 0 sccm
| For wafer loading and unloading
|-
| "TEOS" 
(deposition recipe)
| 4" wafers
1-15 wafers
| 715, 712, 720 <sup>o</sup>C
(temperature zone 1, 2, 3)
| 175 mTorr
| 50 sccm. <i>The exact flow is not know - the setpoint is much lower than 50 sccm, but the MFC is not calibrated for TEOS</i>
| 0 sccm
| 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 oxide 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.