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

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Wet Silicon Oxide Etch (BHF, HF and BOE 7:1 Etchant VLSI with Surfactant (wetting agent))

Silicon oxide can be etched using HF. At DTU Nanolab it is mainly used in a buffered version BHF (premixed). You can find safety datasheets on Kemibrug here 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.

BOE 7:1 Etchant VLSI with Surfactant (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 BOE 7:1 Etchant VLSI with Surfactant. 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 using a plastic in Fume hood 01 or 02 (Acids/bases) in D-3. Samples with III-V materials (e.g. InP and GaAs) must be etched in Fume hood 07: III-V Avids/bases in D-3.

The user manual for Oxide etch 1: BHF, APV's and contact information can be found in LabManager by clicking here

The user manual for Oxide etch 2: BHF (clean), APV's and contact information can be found in LabManager by clicking here

The user manual for various HF baths in RCA and Fume hoods, APV's and contact information can be found in LabManager by clicking here

Process information:
See more etch rates in HF solutions.

Etchant		BHF	5% HF	40% HF	BOE 7:1 Etchant VLSI with Surfactant
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
Purpose	Link to safety APV and SDS (You must be logged in to Kemibrug to view SDSs)	see APV here see SDS here	see APV here see SDS here	see APV here see SDS here	see APV here see SDS here
Performance	Isotropic	Isotropic	Isotropic	Isotropic	Isotropic
	Etch rates See more	Wet thermal oxide:75-80nm/min	Wet thermal oxide:~25nm/min	Boronfloat and quartz: ~3-4 μm/min	Thermal oxide 80 nm/min
	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	BOE 7:1 Etchant VLSI with Surfactant (BHF with a wetting agent)
Process parameter range	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.

Different places to do wet silicon oxide etch
BHF clean wetbench 04 in D-3. Wet silicon oxide etch bath positioned to the right in the wetbench. This bath can also be used for BHF with wetting agent.
BHF in wetbench 01 Si etch in D-3 (Oxide etch 1: BHF.) The BHF bath is positioned in the middle of the bench. This is primarily used to remove oxide before and after a Si etch.
HF baths in RCA bench in B-1.
The 5% HF bath in 'Fume hood(RCA)' in cleanroom B-1 is only for use during special RCA cleaning processes inside the fume hood.

Comparing different BHF baths

	BHF (KOH) in D-3(Oxide etch 1: BHF)	BHF clean in D-3 (Oxide etch 2: BHF)	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	1-7 wafers at a time
Size of substrate	2"-6" 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	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	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

BOE 7:1 Etchant VLSI with Surfactant bath

	SiO etch bath
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
Remark	Amorphous silicon can be used as a mask fro SiO2 etching. For very long and deep etches of SiO2/Quartz (several hours/micrometers) we have indications that show that it is not good to use the SiO etch. The amorphous silicon has small holes (etch pits) which the SiO etch penetrates and the SiO2 below is etched.