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Bosch DRIE Silicon Processing and STS Results
Jim McVittie<[email protected]>
Stanford Nanofabrication FacilityStanford University
2008 NNIN Etch Workshop
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Outline• Bosch Process Overview• STS HRM Shallow Trench Process• DOE performed on STS1• Process Sensitivities for HRM• Experiments on the role of Ions on the deposition cycle• Summary
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Bosch Deep RIE High Aspect Ratio Silicon Etching
• High Aspect Ratio of Silicon Etching is a critical MEMS technology.
• Separated the etching and Sidewall passivation into two steps
• Time Multiplexed passivating and etching processes:
Time
Flow Rate
SF6
C4F8
Etch Passivate Etch
Fig from Arturo A. Ayón PEUG talk May 2001
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Bosch Deep RIE High Aspect Ratio Silicon Etching
• Inductively Coupled High Density Plasma (ICP)
• The etching process switches back and forth between etch (using SF6) and deposition (using C4F8) cycles
• The deposition phase protects the sidewalls and makes the etching process anisotropic
Wafer Flow
Ra t
e
Time
SF6
C4F8
Etch Depo.
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Best Case Bosch Etch results
A. A. Ayón et al, 1999
From Ayón’s PEUG talk
90 deg Walls Scallops
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Etching Cycle: Passivating Cycle:Time: 3.5sGas: 450sccm SF6 + 45 O2 200sccm C4F8Press: ~40 mT ~15 mT (APC fixed at 15%)Coil Pw: 2500W 2000WBias Pw: 40W 0 WChuck Temp: 10C SameCycles: 65 (6 min)
Results: 4.7 um/min for 2 um wide trench90.2deg250 nm undercut140 nm scollops76:1 PR Sel
From Ayón’s PEUG talk
Parameters Shallow trench process STS-HRM
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SEM Results from Shallow Trench Processon STS-2
25 20 1015 7 6 4 3 2 11230um
1 % Exposed area
1.3 um
Trenches
Trench Holes Trench12um 12 10 7 7
2.3
27.3
1.3
170nm
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DOE Test For STS1
3 to 200 um Trenches
20 to 200 um Vias
High Rate Process
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Effect Of Pump Speed on Bosch Si Etch Process
200 um Trench Results
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Effect of Deposition on Bosch Si Etch Process
200 um Trench Results
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Effect of Etch Cycle Time
200 um Trench Results
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Lag or ARDE -- 1
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Lag -- 2
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Undercut
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Micrograss
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Process Sensitive for STS HRM
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Advances in Bosch ProcessLots of variation on basic process for specific needs
•Smooth •High Rate•High AR•Vias•SOI – Addition of low freq RF bias to reduce side notch
at bottom oxide interface•Through wafer•Pillars• High Exposed Area
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Overhang Test Structure
Si
SiO2
Polysilicon
Photoresist
• Separates the effects of the ion flux and neutral fluxes
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Polymer Deposition (Wide Opening Overhang)
5 µm
• C4F8 flow rate = 85 sccm, P = 15 mTorr, Coil Power = 600W for 15 min.
Bias Power = 0 W Bias Power = 8W
• Less spread for deposition with higher Bias power
• Deposition thickness is almost the same (10% more for high bias power)
Ions Ions
PolysiliconPhotoresist
Polymer
• No definitive conclusion
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Polymer Deposition (Narrow Opening Overhang)
Bias Power = 0 W Bias Power = 8W
α=6 o α=3.5 o
Ions Ions
Polysilicon
Photoresist
Polymer2 µm
• Ion enhanced deposition is dominant dep mechanism• Dep on ion shaded surfaces << on exposed surfaces
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Trench Before Deposition
15 µm
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• The starting point of significant deposition on the sidewalls depends on thetrench width
• Energy ions reflected from opposite wall is driving sidewall dep
15 µm
C4F8 Flow = 85 sccmP = 15 mTorrCoil Power = 600WBias Power = 8WTime = 15 min.(No switching,Deposition only)
Ions
Polymer Deposition in Previously Etched TrenchesDep part of Bosch etch in STS
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Ion Reflection and Polymer Deposition
IonsPolyimideTape
15 µm
7.5 µm
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Summary
• Reviewed Bosch process sensitivities for STS tools at Stanford• Polymer Deposition Experiments
•Polymer deposition is an ion-driven process
•Ion reflection plays an important role in the polymer deposition on the sidewalls of trenches