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The Bosch ProcessBrian Vanderelzen
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
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Bosch Process Overview– U.S. Patent #5,501,893– Assigned to Robert Bosch Gmbh– 8/5/94– A mechanism for anisotropically etching silicon in
a plasma environment– The mechanism employs alternating a semi-
isotropic etch step with a polymerizing step– Initial chemistry involved SF6 & Ar for the etch,
CHF3 & Ar for the polymerization– The Bosch process offers significant advantages
over prior art including repeatability, etch rate, selectivity, and aspect ratio
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
3Bosch Process Overview (Cont.)
• Processed licensed initially by STSystems, Inc. who continued to advance the process in conjunction with Bosch
• Process currently licensed by a wide variety of Semiconductor tool manufacturers
• A primary enabler of MEMS technology• Enables very deep etching in silicon with high
selectivity– Depths > 1mm– Rates > 10 microns per minute– Aspect ratios > 50:1– Selectivities
• >50:1 Photoresist mask• >200:1 SiO2 mask
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
4Anatomy of a directional etch
• Reactive Ion Etching is a bit of a misnomer. Etching is primarily done by neutral reactive species. This chemical component is more or less isotropic.
• In order to achieve anisotropy, there must be some form of resistance to the chemical component
• A delicate balance must then be achieved such that the chemical etch can only proceed where the directional physical ion bombardment overcomes this etch resistance
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
5Mechanisms of Anisotropy
• Generally etch resistance is referred to as passivation – an etch resistant layer deposited during the etch– Polymer forming gas – fluorocarbons– O2 to form etch resistant oxide– Self passivating etchants such as HBr
• Reacted species exhibit low volatility• Require physical bombardment to be released from the
surface
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
6Limitations of Single Step Process
• Passivation inhibits vertical etching at the same time that it prevents lateral etch, resulting in slow etch rates
• Passivating chemistry often reacts with the etch mask reducing selectivity– Fluorocarbons etch oxide– O2 etches photoresist
• The physics of deposition is significantly different than that of etch– As aspect ratio increases, the demands of each change
independently and frequently in opposite directions– Low pressures and high bias voltage improves directionality,
allowing ions to reach the bottom of narrow trenches. These same parameters reduce efficiency and conformality of deposition.
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
7Do the Two Step
• The Bosch Process overcomes these limitations by segregating the etch and passivation into independently controllable steps
• The typical process alternates a highly chemical SF6 based etch step with a teflon-like polymer forming passivation step
• The passivation step conformally coats all surfaces• The etch may then only proceed where the energetic
ions break through this passivation• Typical step times range from 5 to 20 seconds• The etch step typically exhibits poor anisotropy,
however, by keeping the steps short, one builds an anisotropic etch from stacked isotropic ‘blocks’
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
8The Etch Step
• Directional ions and non-directional reactive species etch silicon for several seconds
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
9Passivation
• ICP power breaks down C4F8, with little to no bias power
• A fluorocarbon polymer precipitates out on all surfaces
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
10Repeat Etch
• Directional energetic ions break through passivationon horizontal surfaces
• Reactive neutral species do not etch until silicon is exposed
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
11Repeat Etch 2
• Repeat isotropic etches stack to form an anisotropic etch
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
12Scalloping
• The stepped process typically results in a scalloped sidewall
• This SEM shows typical undercut and scalloping for a moderately high rate process
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
13Parameter Ramping
• The physics of the process changes with aspect ratio• It is desirable to ramp parameters as the process
proceeds • Pressure is routinely decreased with time to allow
gases to get in and out of narrow features and to increase ion directionality
• Cycle times, gas flows, and power may also be adjusted as the etch progresses
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
14Issues – Undercut and Scalloping
• Undercut and scalloping are a result of the isotropic nature of the etch step
• May be reduced by shortening cycle times• Adding C4F8 or O2 to the etch will increase step anisotropy and
reduce both scalloping and undercut at the expense of selectivity
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
15Undercut & Scalloping
• Image on left shows scalloping < 20nm• Image on the right ‘probably’ better• Results achieved by adding high O2 flow to etch step• Dramatically reduces PR selectivity – will require hard mask
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
16Issues – ARDE (RIE Lag)
• Etching is highly aspect ratio dependent• High aspect ratio features tend to etch much slower than small
aspect ratio features• It is very difficult to optimize an etch for varying feature sizes
– Etches that are optimized for small features tend to widen large features
– Etches optimized for large features tend to cause etch stopping or grass in small features
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
17Issues - Notching
Slide courtesy of CMI - Center of MicroNanoTechnology, Ecoles Polytechniques fédérales de Lausanne
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
18Notching Solution – lf pulsing
Slide courtesy of CMI - Center of MicroNanoTechnology, Ecoles Polytechniques fédérales de Lausanne
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
19Issues – Loading and Microloading
• The etch is significantly chemically limited
• The rate at which gas reaches the surface determines etch rate
• Thus patterns with large open areas may etch slower than denser patterns– Using the same recipe, a wafer with
80% open area may etch as slow as ¼ the rate of a wafer with 10% open area
– Etch recipes must be optimized for specific pattern density
• Local density or microloading also an issue– Ideally solved in the design phase
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MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu
20The Modern Bosch Process
• The Bosch process has been developed and improved over the years primarily in University and R & D type environments
• Tool manufacturers are now working on creating production ready tools and processes
• Etch rate has seen dramatic increases recently to over 40 microns per minute
• Tools for 200mm and 300mm wafers are now being produced• Many of typical Bosch process issues have now been resolved
– Etch rates > 40 microns per minute– Uniformity < 3%– Aspect ratios near 100:1– Sidewall roughness < 15nm– SOI notching greatly improved or eliminated– ARDE issues greatly reduced
© Surface Technology Systems plc, January 06 STS Confidential
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Recent Process Results
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Pegasus: An Enabling Technology
Power Devices
RF MEMSDe-couplingcapacitors
Ink Jet heads
Micro Fluidics‘Lab on a chip’
Silicon Inertial sensors
Optical MEMSswitching
AdvancedPackaging
MEMS Pressuresensors
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Via SOI
50µm vias, 200mm wafers
Etch Characteristic Target AchievedDepth (µm) 250
>10
<3
<200
89
<500
250
Etch Rate (µm/min) 10.06
Uniformity (±%) <1
Sidewall Roughness (nm) <186
Profile angle (o) 90
Notching (nm/edge) ~760
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Very High Aspect Ratio
Aspect Ratio: 73:1!
Issues:- Bowing at top of trench
Process development still in progress.
0.8µm trench, 150mm wafer (note: ¼ piece)
Etch Characteristic Specification Achieved Depth (µm) 100 ~81Rate (um/min) >3 1.49Uniformity (±%) 5 2.2Selectivity Si:SiO2 >50:1 >54:1Initial mask undercut (nm) <50 <60Sidewall roughness (nm) <50 ~40Profile (º) 89-90 89.9
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Wafer Level Packaging
180µm trench, 200mm wafer
Etch Characteristic Target Achieved
Depth (µm) 75
>10
55-60
75
Etch Rate (µm/min) ~15
Profile angle (o)
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Results of recent work to increase etch rate
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Pegasus High Rate Switched Process
Close to practical application
100µm via
69µm deep
etched at 35µm/min
on standard Pegasus
(200mm wafer, open area 10%)
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Pegasus High Rate Switched Process
Champion data
35µm trench
92µm deep
etched at 46µm/min
on standard Pegasus
(200mm wafer, open area 1%)
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Pegasus High Rate Switched Process
Champion data
80µm trench
100µm deep
etched at 50µm/min
on standard Pegasus
(200mm wafer, open area 1%)
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Pole Position in DRIE
© Surface Technology Systems plc, January 06 STS Confidential
STS ASE-SR Process Capabilities
Prepared forUniversity of Michigan NNIN Meeting
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Contents
Examples of process capability of standard rate system at University of Michigan
Deep etching at high etch ratesHigh aspect ratio etchingSmooth sidewallsMinimising RIE lag
STS Contact details
© Surface Technology Systems plc, January 06 STS Confidential
Deep Etching
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Through Wafer / Deep Etch
Main Requirements:High Etch-rateHigh Selectivity to MaskProfile Control of >10:1 Aspect RatioSidewall Roughness Control
Silicon micromachined fuel atomiser showing smooth
sidewalls and base of silicon. A magnification of the exit hole
is shown top right.
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Deep Etching
Cross-section of a 350µm deep bore through silicon.Etch rate 2.8µm/min, anisotropy >0.99.
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Through Wafer Etching
Anisotropy >0.99Selectivity >300:1 to SiO2 maskEtch Rate » 3 µm/minWafer is 200 µm thick, underlayer is SiO2
© Surface Technology Systems plc, January 06 STS Confidential
Smooth Sidewalls
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Ramping Gas Flow
Without Parameter Ramping
Undercut/edge =320nm
Scalloping = 230nm
With Parameter Ramping
Undercut/edge =100nm
Scalloping = 40nm
© Surface Technology Systems plc, January 06 STS Confidential
High Aspect Ratio Etching
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High Aspect Ratio Via Etch
30 µm via,324 µm depth:
Typical results:90.2°>50:1 Si:PR
Etch rate, depth limited by sidewall break-down
100mm wafer diameter, 5% exposed area,7 µm photoresist + 3 µm TEOS mask
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High Aspect Ratio Via Etch - Breakdown
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High aspect ratio viasHigh Aspect Ratio Etch
Typical application: Trench capacitors, trench isolation, MOS decoupling capacitors in RF circuits
Typical requirements:2-20 µm holes/trenches
Up to 100 µm depthVertical/positive slopeSmooth sidewallsRounded base
Results89.4° profile Selectivity >20:1 Si:Ox;>10:1 Si:PR<80nm scallops; <270nm/edge CD loss
Typical example:1.5 µm diameter holes>33 µm depth150 mm wafer diameter~15 % exposed Si area
Courtesy of F. Roozeboom, Philips
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RIE Lag
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Reducing RIE Lag
Control of RIE lag is difficult however there are some general process trends which help.
Increase the deposition characteristicThe deposition at the base of large features is greater than that in smaller features.
Reduce etch rate in larger features
STS Confidential
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Examples of Minimal RIE Lag
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If You Need More Information Contact…
Steve HallMid West Regional Sales Manager
Cell: 608 234 2934Email: [email protected]