Bosch DRIE Silicon Processing and STS
Results
Jim McVittie
<[email protected]>
Stanford Nanofabrication Facility
Stanford University
2008 NNIN Etch Workshop
1
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:
Flow Rate
SF 6
C4 F 8
Etch
Passivate
Etch
Time
Fig from Arturo A. Ayón PEUG talk May 2001
3
Wafer
Flow Rate
Bosch Deep RIE High Aspect Ratio Silicon Etching
SF6
C4F8
Etch
Depo.
Time
•
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
4
Best Case Bosch Etch results
90 deg Walls
Scallops
A. A. Ayón et al, 1999
From Ayón’s PEUG talk
5
Parameters Shallow trench process STS-HRM
Etching Cycle:
Time: 3.5s
Gas: 450sccm SF6 + 45 O2
Press: ~40 mT
Coil Pw: 2500W
Bias Pw: 40W
Chuck Temp: 10C
Cycles: 65 (6 min)
Passivating Cycle:
200sccm C4F8
~15 mT (APC fixed at 15%)
2000W
0W
Same
Results:
4.7 um/min for 2 um wide trench
90.2deg
250 nm undercut
140 nm scollops
76:1 PR Sel
From Ayón’s PEUG talk
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SEM Results from Shallow Trench Process
on STS-2
1 % Exposed area
Trenches
30um
25
20
15
12
10
7
6
4
3
2 1
27.3
1.3
2.3
Trench
12um
Holes
12
10
7
Trench
7
1.3 um
170nm
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DOE Test For STS1
High Rate Process
3 to 200 um Trenches
20 to 200 um Vias
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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
12
Lag -- 2
13
Undercut
14
Micrograss
15
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Process Sensitive for STS HRM
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Advances in Bosch Process
Lots 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
Photoresist
Polysilicon
SiO2
Si
• Separates the effects of the ion flux and neutral fluxes
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Polymer Deposition (Wide Opening Overhang)
•
C4F8 flow rate = 85 sccm, P = 15 mTorr, Coil Power = 600W for 15 min.
Ions
Ions
Polymer
Photoresist
Polysilicon
5 µm
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)
No definitive conclusion
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Polymer Deposition (Narrow Opening Overhang)
Ions
Ions
Polymer
2 µm
Photoresist
Polysilicon
α=6
o
Bias Power = 0 W
α=3.5
o
Bias Power = 8W
• 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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Polymer Deposition in Previously Etched Trenches
Ions
Dep part of Bosch etch in STS
C4F8 Flow = 85 sccm
P = 15 mTorr
Coil Power = 600W
Bias Power = 8W
Time = 15 min.
(No switching,
Deposition only)
15 µm
•
The starting point of significant deposition on the sidewalls depends on the
trench width
• Energy ions reflected from opposite wall is driving sidewall dep
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Ion Reflection and Polymer Deposition
Ions
Polyimide
Tape
7.5 µm
15 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
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