Specific Process Knowledge/Etch/ASE (Advanced Silicon Etch)

The ASE

The ICP-DRIE tool at Danchip: STS ASE - positioned in cleanroom2

The ICP-DRIE (Inductively Coupled Plasma - Deep Reactive Ion Etcher) tool at Danchip is manufactured by STS and is called the ASE (Advanced Silicon Etcher). Originally the main purpose of the ASE was etching of silicon using the Bosch process. However, after the acquisition of the DRIE-Pegasus the ASE now only serves as backup silicon dry etcher. It has therefore been decided to diversify the etching possibilities on the ASE by adding a CO₂ gas line in order to open up for polymer etching.

The Bosch process: Etching of silicon

The Bosch process uses alternation between an etch cycle and a passivation cycle. Introducing a passivation step in an etch process is very beneficial for the control of the angle of the sidewalls in the etch process because it allows us to cover them with a protective layer that suppresses the isotropic etching. Combined with the high plasma density in the ICP chamber, the excellent sidewall control enables us to etch high aspect ratio structures in silicon with very high etch rates.

In the case of the silicon etching on the ASE, an etch phase with SF₆ and O₂ alternates with a passivation phase with C₄F₈.

Process information

• Etch of Silicon using ASE
• Etch of polymers using ASE

An overview of the performance of the ASE and some process related parameters

Purpose	Dry etch of	Silicon Polymers such as polyimide, PDMS, PMMA, BCB and resists
Performance	Etch rates	Silicon: ~4-6 µm/min (depending on features size and etch load)
	Anisotropy	Good
Process parameter range	Process pressure	~0.1-95 mTorr
	Gas flows	SF_6: 0-600 sccm O_2: 0-100 sccm C_4F_8: 0-300 sccm Ar: 0-100 sccm
Substrates	Batch size	1 6" wafer per run 1 4" wafer per run 1 2" wafer per run Or several smaller pieces on a carrier wafer
	Substrate material allowed	Silicon wafers with layers of silicon oxide or silicon (oxy)nitride Quartz wafers
	Possible masking material	Photoresist/e-beam resist PolySilicon Silicon oxide or silicon (oxy)nitride Aluminium