Basics of SemiconductorMEMS Fabrication

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Basics of Semiconductor/MEMS Fabrication

• Demetrios Papageorgiou
• Electrical and Computer Engineering Department
• Northeastern University

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How it all began

• William Shockley, John Bardeen, and Walter
  Brattain, all from Bell Labs, are credited with
  the creation of the transistor (1947).
• It didnt look pretty

Images from Bell Labs
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and where we are today.
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Ingot formation
Images from www.egg.or.jp/MSIL/english/index-e.htm
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Wafer slicing and polishing
Images from www.egg.or.jp/MSIL/english/index-e.htm
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Now the fun begins

• The processes available are all planar.
• Each layer requires some combination of
  patterning, deposition/growth, and removal.
• The combination and order determine the end
  result.

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Patterning (Lithography)

• The designer creates the circuit by identifying
  where material will remain and where it will be
  removed.
• This information is then separated out by
  processing step for pattern transfer to the wafer
• Masks
• Contact (typically used with features greater
  than 1mm)
• Projection (typically used with features less
  than 1mm)
• Direct-Write
• Uses an electron beam so they are usually part of
  a scanning electron microscope (SEM) system.
• Typically used with features less than 100nm

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

alignment marks
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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Contact Alignment

• The masks are used on an aligner which aligns the
  mask to the previous patterns transferred.
• The mask is placed on the aligner, moved into the
  proper position, and brought into contact with
  the wafer.
• An ultraviolet light then exposes the unprotected
  photosensitive material on the wafer surface.

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Stepper (Projection)

• A stepper is similar to contact aligner in that
  an ultraviolet light source is used to expose
  photoresist on the wafer surface.
• There is no physical contact made between the
  mask and wafer. The pattern is projected onto the
  wafer surface.
• Typically, there is a reduction in the
  magnification of the image to improve resolution.
  The image is then step-and-repeated across the
  entire wafer until it has all been exposed.

http//www.eecs.umich.edu/ssel
http//snf.stanford.edu/Equipment/ultratech/ultrat
ech.html
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Film Growth

• Oxidation
• Silicon dioxide it the most common film grown in
  cleanrooms.
• The silicon wafer is placed in a furnace
  operating between 900-1200 C and oxygen gas or
  steam are passed through the furnace.
• Epitaxial layers
• MBE (Molecular Beam Epitaxy) uses vapor-phase
  material which falls into place in the
  substrates crystal lattice.
• Laser Re-Crystallization uses a laser to scan
  polycrystalline silicon on the wafer surface,
  melting it and allowing it to re-crystallize
  forming a single-crystal material matched to the
  substrate.

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Thin-Film Deposition

• CVD (Chemical Vapor Deposition)
• PECVD (Plasma Enhanced CVD)
• LPCVD (Low-Pressure CVD)
• APCVD (Atmospheric Pressure CVD)
• Sputtering
• Evaporation

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Sputtering
www.ee.ttu.edu/mems/

• DC bias accelerates ion of an inert gas to the
  target.
• Impact releases target atoms.
• Relatively uniform coverage.

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Evaporation

• The material (metal) is heated in a vacuum.
• Evaporation of material sends atoms out in all
  directions.
• Non-uniform coverage.

www.ee.ttu.edu/mems/
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Doping

• Dopants are added to semiconductors to alter its
  electrical properties (and sometimes etch
  behavior and film stress).
• Two/three methods used
• Diffusion the source is placed in close
  proximity to the wafer and the temperature is
  raised.
• Diffusion a gas which contains the dopant is run
  through the furnace at an elevated temperature.
• Ion Implantation dopant atoms are projected at
  the wafer and become lodged in the material.

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Etching

• Wet
• Dry
• RIE (Reactive Ion Etching)
• Ion Milling
• CMP (Chemical Mechanical Polishing)
• Lift-off

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Wet Etching

• Typically the most selective way to etch
  material.
• Sometimes the only efficient way to etch a
  material.
• Usually isotropic.

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RIE (Reactive Ion Etching)
ions
Anode
feed gases
Exhaust
Ex
E-Field
absorption
desorption
Reaction with Surface
Subsequent surface reactions
Cathode
RF Power
Material removal
13.56 MHz
Straight Sidewalls
www.ee.ttu.edu/mems/Sample20Notes.ppt
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Ion Milling

• Semiconductor industrys version of
  sand-blasting.
• Very unselective. (bad)
• Can etch pretty much anything. (good)

http//www.eofoundry.com/Foundry/ion_mill.htm
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CMP (Chemical Mechanical Polishing)

• The name says it all polishing through chemical
  and mechanical etching.
• Wafers mounted on a chuck and inverted.
• The exposed surface is then placed in contact
  with a slurry-coated disk.
• As the disk turns, the wafer surface is polished.

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Lift-off

• Involved deposition of metal over patterned
  photoresist followed by the dissolution of the
  photoresist.
• Used primarily for hard to etch metals.

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Lift-off

• Involved deposition of metal over patterned
  photoresist followed by the dissolution of the
  photoresist.
• Used primarily for hard to etch metals.

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Making Integrated Circuits
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Making Integrated Circuits
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Making Integrated Circuits
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Making Integrated Circuits
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Making Integrated Circuits
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MEMS
IC Fabrication
MEMS Fabrication
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MEMS (Microelectromechanical Systems)
Pictures stolen from everywhere (Univ. of
Michigan, Analog Devices, Akoustica, Univ. of
Wisconsin, MEMSIC).
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MEMS Processes

• MEMS has branched off significantly from the IC
  industry
• MEMS-specific equipment and processes have been
  created
• Polyimide
• Plastics
• KOH
• EDP
• TMAH
• SU-8
• Foundaries have been altered and created for
  dedicated MEMS work.

• LIGA
• Wafer bonding
• Supercritical CO2 Release
• DRIE
• XeF2 etcher
• etc.

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Wet Anisotropic Etch

• Potassium Hydroxide (KOH)
• Selective to lt111gt crystal plane
• Very smooth sidewalls
• Ethylenediamine Pyrocatechol (EDP)
• Selective to lt111gt crystal plane
• Rough surfaces
• REALLY nasty
• Tetramethyl Ammonium Hydroxide (TMAH)
• Selective to lt111gt crystal plane
• Less selective than EDP
• A lot nicer to work with.
• Etch rates are on the order of 7mm/min

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Wet Anisotropic Etch
V-Groove
Anisotropic etchant
54.7
HF etch
SiO2
Si (100)
www.ee.ttu.edu/mems/Sample20Notes.ppt
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Dry Anisotropic Etch

• DRIE
• Creates plasma and uses a separate power supply
  to direct ions to the wafer.
• Capable of 5001 selectivity
• 501 aspect ratio
• Etch rates of 22mm/min

http//www.sensorsmag.com/
www.adixen.com
Stella Pang, Univ. of Michigan, Ann Arbor
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Dry Isotropic Etchants

• XeF2
• Purely chemical etch
• Extremely selective
• Etch rates of 10mm/min

www.xactix.com
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LIGA (LIthgraphie Galvanoformung Abformung)

• Formation of patterns over a seed layer using
  x-rays to expose a thick layer of photoresist (up
  to 1mm thick).
• Electroplating fills up openings in the
  photoresist.
• The photoresist then removed leaving behind the
  structure which is typically used as is or then
  used as a mold.

http//www.fzk.de
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Using a lot of these technologies
Wise, Univ. of Michigan