Specific Process Knowledge/Etch/AOE (Advanced Oxide Etch)

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Etching using the dry etch technique AOE (Advanced oxide etch)

AOE: positioned in cleanroom B-1, Photo: DTU Nanolab internal

Name: M/PLEX ICP - AOE (Advanced Oxide Etcher)
Vendor: STS (now SPTS)
The AOE can be used for dry etching silicon oxide, silicon (oxy) nitride and quartz. Look in the manuals for the AOE to see how to operate the machine (you can find the manuals in LabManager on the AOE page).

The user manual, quality control procedure and results, user APV, technical information and contact information can be found in LabManager:

AOE in LabManager - requires login

Process information

• Etch of Silicon Oxide
• Etch of Si
• Remove/etch resist/barc
• Etch of Fused Silica
• Etch of Quartz - special very thick samples
• Etch of Silicon Nitride

Limitations using the AOE

Wafer bow

There is a limit to how much the wafer can bow and still be clamped on the chuck. The limit can maybe vary a little over time and may also depend on the material on the backside of the substrate. On a 100mm Si wafer with SiO2 on the backside (<10µm) we expect the limit to be around 50µm bow (when the back side surface is convex).

A bow will be created when etching the top oxide layer on a wafer with oxide on both sides. For a larger etch load the bow will be more severe for a specific etch depth when for a smaller etch load. I have been able to etch much deeper in SiO2 with a P-Si mask than with a photo resist mask on a wafer with 50% load. When using photoresist the wafer stopped clamping during the etch after just a few µm. With P-Si I could etch 15µm without problems. I expect this to be due to a combination of P-Si on the back side clamping much better and P-Si on the back side helping to reduce the bow.

Transparent wafers

Transparent wafers are a challenge for two reasons. 1. In the load lock the LASER detection system that is used to detect the wafer during mapping cannot detect a completely transparent wafer. 2. A transparent wafer is either quartz or fused silicon. These materials are very difficult to clamp electrostatically and will therefore not be able to pass the He leak up test successfully.

1. The first issue may be overcome by using a non-transparent masking material or adding a non-transparent material on the back side of the wafer
2. The second issue may be overcome by reducing the He back side pressure or reducing the He back side cooling completely. Another way to solve it is to either bond the transparent wafer to a silicon wafer before transferring it into chamber or deposit a more conducting/semiconducting layer on the backside of the wafer (could be silicon, maybe Chromium, please ask). 1-2µm P-Si may be enough, maybe even less.

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

Purpose	Dry etch of	Silicon oxide Silicon (oxy)nitride Quartz Silicon mask etching
Performance	Etch rates	~0.05-0.6 µm/min
	Anisotropy	Typical profiles: 86-90 degrees
Process parameter range	Process pressure	~0.2-95 mTorr
	Process power	Coil power: up to 3000W Platen power: up to 600W
	Platen temperature	-10 to 60 degrees Celcius
	Gas flows	C4F8: 0-40 sccm O2: 0-100 sccm CF4: 0-100 sccm H2: 0-30 sccm He: 0-500 sccm N2: 0-1000 sccm SF6: 0-300 sccm
Substrates	Batch size	1 6" wafer per run (only when the system is setup to 6") 1 4" wafer per run 1 2" wafer per run (needs carrier) Or several smaller pieces (needs carrier)
	Substrate material allowed	Silicon with layers of silicon oxide or silicon (oxy)nitride Quartz wafers
	Possible masking material	Photoresist/e-beam resist Silicon/PolySi Aluminium Chromium