I would like to etch a perfectly vertical square silicon mesa (tower) using RIE, but so far the most vertical mesa I have been able to etch has an angle of 85 degrees between the sidewall of the Si mesa and the plane of the wafer, in other words the mesa’s sidewall is sloping out as you look from the top of the mesa to the bottom, as if mask erosion or maybe a sidewall inhibitor is being deposited. Any suggestions for making the silicon side wall perfectly vertical and achieving a 90 degree angle would be appreciated .
The mesa’s top view dimensions are 40 by 40μm, the mesa’s are spaced 1000μm apart, the depth of the silicon etch is 0.3μm, only the silicon under the 40 by 40μm mask should not be etched, the rest should be etched 0.3μm deep.
The RIE system (Dual chamber Plasma Therm 790) has an eight-inch cathode, a maximum power of 500W and a minimum chamber etching process pressure of 2mTorr. Chamber 1 has the following gases: Cl2= 20sccm, HBr=50sccm, O2=20sccm, CF4=100sccm and He=100sccm. Chamber2 has: O2=20sccm, CHF3=100sccm and H2=20sccm.
I use a two-layer mask. First I deposit a 0.3μm LTO (low temperature silicon dioxide) mask, then I spin on a layer of photo resist to pattern the LTO with the following RIE etch:
Chamber2 pressure= 40mTorr
Power = 200W
CHF3 gas flow= 40 sccm
Oxygen gas flow = 3 sccm
Then remove wafer and strip the photo resist with a piranha clean. Now only the SiO2 mesa mask remains.
The following two silicon etches have produced an angle of 85 degrees between the sidewall of the Si mesa and the plane of the wafer; I would like a 90-degree angle.
Etch A
Chamber1 pressure= 5mTorr
power = 500W
HBr gas flow= 27 sccm
He gas flow = 3 sccm
Etch B
Chamber1 pressure= 2mTorr
power = 400W
HBr gas flow= 14 sccm
He gas flow = 6 sccm
Cl2 gas flow = 2 sccm
Any RIE etch recipe suggestions to achieve a more vertical silicon etch are appreciated.
_ Joseph P. Donnelly
_| ~-. off : (512)-471-8658
\, *_} fax : (512)-471-5625
\( e-mail : JoeSmiles@mail.utexas.edu
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