Specific Process Knowledge/Etch/Wet Silicon Oxide Etch (BHF)

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Wet Silicon Oxide Etch (BHF, HF and SIO Etch (wetting agent))

Wet Silicon Oxide Etch (BHF): positioned in cleanroom 3

SIO etch (BHF with wetting agent): positioned in cleanroom 4

BHF between the KOH baths: positioned in cleanroom 3. This is primarily used to remove oxide before and after a KOH etch

PP-bath: positioned in fume hood in cleanromm 2

Silicon oxide can be etched using HF. At Danchip it is mainly used in a buffered version BHF (premixed). The BHF has a more stable etch rate and is more gentle to photoresist due to an almost pH neutral solution which makes photoresist a good masking material for the oxide etch.

SIO etch (BHF with wetting agent) can also be used for etching silicon oxide. This is mainly for etching small holes. If you have wetting problems in BHF try the SIO etch. It is also a good idea to wet the sample in water for a few minutes before etching.

Wafers with metals or other materials, not allowed in the dedicated oxide etch baths, can be etched in the PP-bath positioned in the fume hood in cleanroom 2 or in a plastic beaker.

Etchant		BHF	5% HF	40% HF	SiO etch
Purpose	Etching of silicon oxide	Etching of silicon oxide with a stable etch rate	Mainly for removing native oxide	Mainly for etching deep into borofloat or quartz wafers	Etching of silicon oxide - especially for etching small holes
	Link to safety APV and KBA	see APV here see KBA here	see APV here see KBA here	see APV here see KBA here	see APV here see KBA here
Performance	Isotropic	Isotropic	Isotropic	Isotropic	Isotropic
	Etch rates	Wet thermal oxide:~80nm/min PECVD1 (standard):~147nm/min TEOS:~265nm/min Stoichiometric Si3N4:~0.75nm/min (Morten Bo Mikkelsen, March 2013)	Wet thermal oxide:~25nm/min PECVD1 (standard):~87nm/min TEOS:~153nm/min	Boronfloat and quartz: ~3-4 μm/min	A little higher etch rates than BHF
	Lifetime of photoresist	~½ hour	~½ hour	~½ hour	~½ hour
Process parameter range	Chemical solution	BHF 12%HF with Ammoniumflouride	5% HF	40% HF	SiO etch (BHF with a wetting agent)
	Temperature	Room temperature	Room temperature	Room temperature	Room temperature
Substrates	Masking material	Silicon nitride Poly silicon photoresist Blue film	Silicon nitride Poly silicon photoresist Blue film	Photoresist Silicon nitride (Only on quartz wafers) Poly silicon (Only on quartz wafers) Blue film Chromium and gold	Silicon nitride Poly silicon photoresist Blue film

Life time of the photoresist and blue film in BHF

Photoresist delaminates from the substrate in BHF. For standard photoresist this process starts after about or less than ½ hour but the time can vary with resist pattern. It starts by under etching the photoresist at the edges. The life time can be prolonged a little to ½ hour - 40 minutes if the photoresist is baked at 120^oC.

Blue film also has a limited life time in BHF. Blue film is mainly used for back side protecting the wafer while etching on the front side. The life time of Blue film is also about ½ hour.

Comparing different BHF baths

	BHF in Cleanroom3 (KOH bench 1+2)	BHF clean in Cleanroom3	BHF in Cleanroom4 (KOH bench 6”)	BHF in RCA Bench	BHF in PP-bath	BHF Plastic beaker
Batch size!	1-25 wafers at a time	1-25 wafer at a time	1-25 wafers at a time	1-25 wafers at a time	1-25 wafers at a time	1-7 wafers at a time
Size of substrate	2"-4" wafers	2"-4" wafers	2"-6" wafers	2"-6" wafers	2"- 4" wafers or any that fits in a dedicated holder	2"- 4" wafers or any that fits in a dedicated holder
Allowed materials	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride	All materials	All materials
Restrictions	Wafers have to be cleaned in 7UP or RCA before further processing.	No wafers with metal are allowed in this bath	Wafers have to be cleaned in 7UP or RCA before further processing.	Only for wafers with phosphor glass or boron glass that comes directly from one of the furnaces in stack A. All other substrates and material are strictly forbidden to go into the tank.	None	None

Comparing different 5% HF baths

	5% HF in RCA Bench	5% HF PP-bath	5% HF Plastic beaker
Batch size!	1-25 wafers at a time	1-25 wafer at a time	1-7 wafers at a time
Size of substrate	2"-6" wafers	2"- 4" wafers or any that fits in a dedicated holder	2"- 4" wafers or any that fits in a dedicated holder
Allowed materials	Only for wafers that are being RCA cleaned	All materials	All materials

Comparing different 40% HF baths

	40% HF PP-bath	40% HF Plastic beaker
Batch size!	1-25 wafers at a time	1 wafer at a time
Size of substrate	Any that fits to a dedicated holder	Any that fits to a dedicated holder
Allowed materials	All materials	All materials

SiO etching bath

	SiO tank
Batch size!	1-25 wafers at a time
Size of substrate	4" wafers
Allowed materials	Silicon Poly silicon Silicon nitrides Silicon oxynitrides Silicon oxides Photoresist