Specific Process Knowledge/Etch/Etching of Bulk Glass

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Comparing methods for etching bulk glass at DTU Nanolab

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 fluorine chemistry (only fused silica) or with IBE by sputtering with Ar ions and/or using fluorine chemistry.

At DTU Nanolab, 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 concentrated HF-solution (isotropic etch). The set-up consists of a 5L plastic 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 challenge to avoid delamination, pinholes and cracks in the masking material.

Compare the methods for bulk glass etching

	Wet Silicon Oxide etch (BHF/HF)	ASE	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	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 ~80 nm/min (Thermal oxide) in BOE 7:1 Etchant VLSI with Surfactant ~25 nm/min (Thermal oxide) in 5%HF ~3-4µm/min in 40%HF	Process dependent Expected range: ~ less than 20 nm/min - ~200 nm/min	Process dependent Expected range: ~ less than 230 nm/min - ~550 nm/min	Process dependent Expected ~10-30 nm/min
Substrate size	#1-25 100 mm 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 (bad/no cooling!) #1 100mm wafer (or smaller with carrier) #1 150mm wafer (only 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 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