Trench refill is the coating of high aspect ration features, mainly deep trenches, in oxide. This can be used for high density electrically isolated capacitors, thick thermal isolation, and optical gratings. Combining this with in suti doped LPCVD polysilicon allows for electrically isolated, sealed, through-wafer vias.
The ability to refill a trench and minimize keyholes (gaps) starts with the etch. If the etch has a negative profile, or a ledge it will be impossible to completely refill.
At the LNF we recommend you start with LNF recipe 4 on either the STS Pegasus 4 or STS Pegasus 6. The etch rate and quality is dependent on your feature size, shape, and density. You may need to work with the tools engineers to optimize your etch. You can see the difference between the etch and it's impact in on the refill in the following pictures.
Recipe 1 after TEOS Refill Recipe 4 after TEOS Refill
Oxide can either be thermal grown or deposited. At the LNF we have both LP and PE CVD deposition techniques. The pro's and con's of each technique is listed below.
This is a deposited silicon oxide formed from the reaction of Tetraethyl Orthosilicate (Si(OC2H5)4) at 690°C in S4 T4 - TEOS 100mm only.
Si(OC2H5)4 --> SiO2 + 2(C2H5)2O
TEOS is a highly conformal oxide and can completely refill a trench with oxide. As deposited oxide has the following properties:
- >75% Refill with 20:1 aspect ration DRIE trenches
- Refractive Index: 1.43
- 50MPa Compressive Stress
- BHF etch rate: 5000Å/min
The image on the right is TEOS after densification. As you can see the stress increase and can cause deformation of your structure. Material properties after densification are:
- Refractive Index 1.46
- 230MPA Compressive Stress
- BHF etch rate: 1300Å/sec
HTO, High Temperature Oxide
HTO is formed by the reaction of N2O and DCS (Dichlorosilane, H2SiCl2) at 900°C.
N2O + H2SiCl2 --> Si02 + 2N2 + 2 HCL
HTO can be deposited on 4 and 6 inch wafers in the LNF. Four inch wafers can be processed in wafer S2 T3 - Nitride-HTO-Oxynitride 100mm only, and 6 inch wafers in S2 T2 - Nitride-HTO 150mm only
LTO, Low Temperature Oxide
LTO is formed by the reaction of Silane (SiH4) and Oxygen (O2).
SiH4 + O2 → SiO2 + 2 H2
LTO can be deposited on 4 and 6 inch wafers in the LNF. Four inch wafers can be processed in wafer S1 T2 - LTO 100mm only (No Metals), and 6 inch wafers in S1 T4 - LTO 150mm only (No Metals). If your wafers have metal or have had metal on them you can use S6 T4 - LTO (some metals allowed) or D4, LTO (some metals allowed).
We have three PECVD (Plasma Enhanced Chemical Vapor Deposition) tools in our cleanroom. All three can deposit Silicon Oxide.
- P5000 PECVD - 6 inch substrates
- GSI PECVD - 4 inch substrates, 200° and 380°C recipies
This will oxidize the sidewall of your trench. If you size things correctly you can end up with a completely oxidized area. For more information read: Fabrication of thick silicon dioxide layers using DRIE, oxidation, and trench Refill. Thick thermal oxidation can be done in S3 T2 - Wet Oxidation 100mm and 150mm.
Zhang, Chunbo, and K. Najafi. "Fabrication of Thick Silicon Dioxide Layers Using DRIE, Oxidation and Trench Refill." Proc. of The Fifteenth IEEE Conference on Micro Electro Mechanical Systems, 2002, Las Vagas, NV, USA. 160-163. Web.