Specific Process Knowledge/Etch/Etching of Bulk Glass

Etching of Glass can be done either wet or dry. For wet etching please see below on this page. Dry etching can be done either with AOE using Flourine chemistry (only fused silica) or with IBE by sputtering with Ar ions and/or using Flourine chemistry.

At Danchip, we have two types of bulk glass substrates: Borosilicate glass (Borofloat 33 (like pyrex)) and fused silica glass which in cleanliness is similar to quartz. Both types are etched wet in a special set-up placed in a fumehood using a strong HF-solution (isotropic etch). The set-up consists of a 5L plasic beaker placed on a stirring plate (magnetic stirring) and a special horizontal wafer holder. Normally a 40% pre-mixed HF solution is used.

Masking materials and pre-treatment of the glass surface prior to the deposition of the masking material is a special concern in particular for deep etching (> 10µm).

Due to the high cleanliness fused silica is allowed access to basically all machines meaning that e.g. LPCVD silicon can be deposited as masking material. This is an excellent mask even for quite deep etches.

Regarding borosilicate glass masking is more tricky. The following sequence has been used with some success (using sputtered silicon from the Alcatel):

• Piranha clean
• Bake-out at 250 ^oC (>2.5 hours)
• Plasma ashing
• Sputter-deposit in Alcatel: Power: 550W, Ar-pressure: 10$^{-2}$ mbar (base pressure: 10$^{-6}$ mbar)
• Patterning of the silicon using either wet (poly-etch) or dry etching

Process Advice

• Deep etch in Glass

Overview: Wet HF-etch of bulk glass

	Fused silica	Borofloat glass
General description	40% pre-mixed HF	40% pre-mixed HF
Possible masking materials	LPCVD silicon (good adhesion and step coverage)	Sputtered silicon (Alcatel) (bad adhesion and step coverage) We are testing other masks at the moment (ask BGE)
Etch rate	~700 nm/min (patterned silica, slow stirring) ~800 nm/min (non-patterned silica, slow stirring)	~3.9 µm/min
Uniformity	~ 2% (slow stirring, horizontal wafer)
Batch size	1 wafers at a time	1 wafer at a time
Size of substrate	4" wafers	4" wafers
Allowed materials	No restrictions	No restrictions

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Comparing methods for etchng bulk glass at Danchip

Etching of Glass can be done either wet or dry. Wet etching is done with HF. Dry etching can be done either with AOE using Flourine chemistry (only fused silica) or with IBE by sputtering with Ar ions and/or using Flourine chemistry.

At Danchip, we have two types of bulk glass substrates: Borosilicate glass (Borofloat 33 (like pyrex)) and fused silica glass which in cleanliness is similar to quartz. Both types are etched wet in a special set-up placed in a fumehood using a strong HF-solution (isotropic etch). The set-up consists of a 5L plasic beaker placed on a stirring plate (magnetic stirring) and a special horizontal wafer holder. Normally a 40% pre-mixed HF solution is used.

Masking materials and pre-treatment of the glass surface prior to the deposition of the masking material is a special concern in particular for deep etching (> 10µm).

Due to the high cleanliness fused silica is allowed access to basically all machines meaning that e.g. LPCVD silicon can be deposited as masking material. This is an excellent mask even for quite deep etches.

Regarding borosilicate glass masking is more tricky. Sputtered Si, Cr or Cr/Au can be used. It is a challange to avoid delamination, pinholes and cracks in the masking material.

• HF etch of bulk glass (borofloat vs. fused silica)
• Deep etch in Glass

Compare the methods for bulk glass etching

	Wet Silicon Oxide etch (BHF/HF)	RIE (Reactive Ion Etch)	AOE (Advanced Oxide Etch)	IBE/IBSD Ionfab 300
Generel description	Isotropic etch	Anisotropic etch: vertical sidewalls	Anisotropic etch: vertical sidewalls	Primarily for pure physical etch by sputtering with Ar-ions
Possible masking materials	Photoresist PolySilicon Silicon nitride (LPCVD) Blue film Cr/Au for deeper etches (plastic beaker)	Photoresist DUV resist E-beam resist (Poly)Silicon Silicon Nitride Metals if they cover less than 5% of the wafer area (ONLY RIE2!)	Photoresist DUV resist E-beam resist (Poly)Silicon Silicon Nitride Aluminium Chromium (Please try to avoid this)	Any material that is accepted in the machine
Etch rate range	~75 nm/min (Thermal oxide) in BHF ~90 nm/min (Thermal oxide) in SIO Etch ~25 nm/min (Thermal oxide) in 5%HF ~3-4µm/min in 40%HF	Process dependent Tested range: ~20nm/min - ~120nm/min	Process dependent Tested range: ~230nm/min - ~550nm/min	Process dependent Tested once ~22nm/min
Substrate size	#1-25 100mm wafers in our 100mm bath What can be fitted in a plastic beaker	As many small samples as can be fitted on the 100mm carrier. #1 100mm wafer (or smaller with carrier) #1 150mm wafer (only RIE2 when set up for 150mm)	As many small samples as can be fitted on a 100mm wafer #1 50 mm wafer fitted on a 100mm wafer #1 100 mm wafer #1 150 mm wafers (only when the system is set up to 150mm)	As many samples as can be securely fitted on a up to 200mm wafer #1 50 mm wafer with special carrier #1 100 mm wafer with special carrier #1 150 mm wafers with special carrier #1 200 mm wafer
Allowed materials	In the dedicated bath: Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist Blue film In a plastic beaker: No limits cross contamination wise	Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist DUV resist E-beam resist Other metals if they cover less than 5% of the wafer area (ONLY RIE2!) Quartz/fused silica	Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Photoresist DUV resist E-beam resist Aluminium Chromium (try to avoid it) Quartz/fused silica	Silicon Silicon oxides Silicon (oxy)nitrides Metals from the +list Metals from the -list Alloys from the above list Stainless steel Glass III-V materials Resists Polymers Capton tape

Wet HF-etch of bulk glass

	Fused silica	Borofloat glass
General description	40% pre-mixed HF	40% pre-mixed HF
Possible masking materials	LPCVD silicon (good adhesion and step coverage)	Sputtered silicon (Alcatel) (bad adhesion and step coverage) We are testing other masks at the moment (ask BGE)
Etch rate	~700 nm/min (patterned silica, slow stirring) ~800 nm/min (non-patterned silica, slow stirring)	~3.9 µm/min
Uniformity	~ 2% (slow stirring, horizontal wafer)
Batch size	1 wafers at a time	1 wafer at a time
Size of substrate	4" wafers	4" wafers
Allowed materials	No restrictions	No restrictions