Bosh Process

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The Bosch ProcessBrian Vanderelzen

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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 semiisotropic 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Bosch 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Anatomy 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Mechanisms 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Limitations 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.

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Do 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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The Etch Step Directional ions and non-directional reactive species etch silicon for several seconds

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Passivation ICP power breaks down C4F8, with little to no bias power A fluorocarbon polymer precipitates out on all surfaces

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Repeat Etch Directional energetic ions break through passivation on horizontal surfaces Reactive neutral species do not etch until silicon is exposed

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Repeat Etch 2 Repeat isotropic etches stack to form an anisotropic etch

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Scalloping The stepped process typically results in a scalloped sidewall This SEM shows typical undercut and scalloping for a moderately high rate process

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Parameter 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Issues 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Undercut & 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Issues 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Issues - Notching

Slide courtesy of CMI - Center of MicroNanoTechnology, Ecoles Polytechniques fdrales de Lausanne MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Notching Solution lf pulsing

Slide courtesy of CMI - Center of MicroNanoTechnology, Ecoles Polytechniques fdrales de Lausanne MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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Issues 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 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

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The 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

MICHIGAN NANOFABRICATION FACILITY, THE UNIVERSITY OF MICHIGAN, ANN ARBORmnf.umich.edu

Surface Technology Systems plc, January 06

STS Confidential

Compound Semiconductors

Photonics

Advanced Packaging

Data Storage

Recent Process Results

MEMS

Surface Technology Systems plc, January 06

STS Confidential

Data Storage

Pegasus: An Enabling TechnologyOptical MEMS switching Silicon Inertial sensors Ink Jet heads

Photonics

Advanced Packaging

RF MEMS De-coupling capacitors Advanced Packaging

Compound Semiconductors

MEMS

Micro Fluidics Lab on a chip Surface Technology Systems plc, January 06

MEMS Pressure sensorsSTS Confidential

Power Devices

Data Storage

Via SOI

Photonics

Advanced Packaging

50m vias, 200mm wafers

Compound Semiconductors

Etch Characteristic Depth (m) Etch Rate (m/min) Uniformity (%) Sidewall Roughness (nm) Profile angle (o)

Target 250 >10 300:1 to SiO2 mask Etch Rate 3 m/min Wafer is 200 m thick, underlayer is SiO2

MEMS

Compound Semiconductors

Photonics

Advanced Packaging

Surface Technology Systems plc, January 06

STS Confidential

Smooth Sidewalls

Surface Technology Systems plc, January 06

STS Confidential

Data Storage

Ramping Gas Flow

Advanced Packaging

Without Parameter Ramping Undercut/edge =320nm Scalloping = 230nm

Photonics

Compound Semiconductors

With Parameter Ramping Undercut/edge =100nm Scalloping = 40nm

MEMS

Surface Technology Systems plc, January 06

STS Confidential

High