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

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Wet Silicon Oxide Etch (BHF, HF and SIO Etch (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.

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 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 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 SDS (You must be logged in to Kemibrug to view SDSs)
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 110 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
  • 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 120oC.

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 (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


SiO etch 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.