Specific Process Knowledge/Thin film deposition/Deposition of Silicon Nitride

Deposition of Silicon Nitride can be done with either LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition).

Deposition of Silicon Nitride using LPCVD

LPCVD silicon nitride can be deposited in a LPCVD nitride furnace. Danchip has two LPCVD nitride furnaces: A new furnace (installed in 2008) for deposition of stoichiometric nitride on 4" or on 6" wafers and an older furnace (installed in 1995) for deposition of stoichiometric nitride and low stress nitride on 4" wafers.

The LPCVD nitride deposition is a batch process, meaning that nitride can be deposited on a batch of up to 17 wafers (in the old nitride furnace) or 25 wafers (in the new nitride furnace) at a time. The deposition takes place at temperatures of 780-845 degrees Celsius and at a pressure of 120-200 mTorr. The LPCVD silicon nitride has a good step coverage, and the film thickness is very uniform over the wafers. On the furnaces there are standard processes for deposition of stoichiometric nitride (Si₃N₄) (at the moment only on the new nitride funace) and for deposition of low stress nitride (SRN) (only on the old nitride funace).

• Deposition of Silicon Nitride using LPCVD

Deposition of Silicon Nitride using PECVD

PECVD nitride and oxynitride can be deposited in one of the PECVD systems at Danchip. You can run 1-3 wafers on several smaller chips at a time depending on which one of the PECVD's you use. The deposition takes place at 300 degrees Celsius. This can be of importance for some applications, but it gives a less dense film compared to LPCVD nitride, and the stoichiometry is on the following form: Si_xN_yO_zH_v. The step coverage and the thickness uniformity of the film are not as good as for the LPCVD nitride. In one of our PECVD systems (PECVD3) we allow small amounts of metal on the wafers entering the system; this is not allowed in the LPCVD furnace and in the clean PECVD (PECVD1). We also have a PECVD for deposition on III-V materials (PECVD2).

• Deposition of Silicon Nitride using PECVD - or oxynitride

Comparison of LPCVD and PECVD for silicon nitride deposition

	LPCVD	PECVD
Stoichiometry	Si3N4 SRN (old nitride furnace, only 4" wafers) Si3N4: Stoichiometric nitride SRN: Silicon rich nitride (low stress nitride)	SixNyHz SixOyNzHv Silicon nitride can be doped with boron, phosphorus or germanium
Film thickness	Si3N4: ~50 Å - ~3000 Å SRN: ~50 Å - ~10000 Å Thick nitride layers have to be deposited over more runs	~40 nm - 10 µm
Process temperature	780 oC - 835 oC	300 oC
Step coverage	Good	Less good
Film quality	Deposition on both sides of the substrate Dense film Few defects	Deposition on one side of the substrate Less dense film Incorporation of hydrogen in the film
Batch size	Old nitride furnace: 1-35 4" wafers per run (dependent on the size of the quarz boat) New nitride furnace: 1-25 4" or 6" wafers per run	1-3 4" wafers or one 6" wafer or many smaller chips per run
Substrate materials allowed	Silicon wafers (new wafers from a new box or RCA cleaned wafers) with layers of silicon oxide or silicon (oxy)nitride (RCA cleaned) from furnaces in stack A or B in cleanroom 2 Quartz wafers (RCA cleaned)	Silicon wafers with layers of silicon oxide or silicon (oxy)nitride Quartz wafers Small amounts of metal < 5% of the wafer coverage (ONLY in PECVD3!)
Etch rate in 80 oC KOH	Expected <1 Å/min	Dependent on recipe: ~1-10 Å/min
Etch rate in BHF	Very low	Very high compared to the etch rate of LPCVD nitride