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Bosch DRIE Silicon Processing and STS Results

Jim McVittie<mcvittie@stanford.edu>

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