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

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

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

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.

Silicon oxide etch data

	BHF	5% HF	40 % HF	SIO
General description	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
Chemical solution	BHF	5% HF	40% HF	BHF with wetting agent
Process temperature	Room temperature	Room temperature	Room temperature	Room temperature
Possible masking materials	Photoresist Silicon nitride PolySi Blue film	In plastic beaker or PP-bath in the fume hood in cleanroom 2: Photoresist Silicon nitride PolySi Blue film In RCA bench: No masking material	In plastic beaker with magnetic stirrer in the fume hood in cleanroom 2: Photoresist Silicon nitride (Only on quartz wafers) PolySi(Only on quartz wafers) Blue film Chromium and gold	Photoresist Silicon nitride PolySi Blue film
Etch rate	Wet thermal oxide:~80nm/min PECVD1 (standard):~147nm/min TEOS:~265nm/min	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
Life time of the photoresist	~½ hour	~½ hour	~½ hour	~½ hour
Batch size	Plastic beaker: 1-5 wafer of 4" at a time PP-bath: 1-25 wafers of 4" at a time BHF-bath in cleanroom3: 1-25 wafers of 4" at a time	Plastic beaker: 1-5 wafer of 4" at a time PP-bath: 1-25 wafers of 4" at a time RCA bench: 1-25 wafers of 4" at a time	1-7 wafer of 4" at a time	1-25 wafer of 4" at a time
Size of substrate	4" wafers	2"-6" wafers	4" wafers	4" wafers
Allowed materials	In plastic beaker or PP-bath in the fume hood in cleanroom 2: All materials BHF-bath in cleanroom3: Silicon Silicon nitrides Silicon oxides Photoresist	In plastic beaker or PP-bath in the fume hood in cleanroom 2: All materials In RCA bench: Only new wafers from the box or during RCA cleaning	In plastic beaker in the fume hood in cleanroom 2: All materials	Silicon Silicon nitrides Silicon oxides Photoresist

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

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.