Microfabrication

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Microfabrication
Nathaniel J. C. Libatique, Ph.D. nlibatique_at_gmail.
com
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Process Steps

• Start with polished wafers of chosen r and
  crystal orientation
• Films epitaxial, thermal oxides, polysilicon,
  dielectrics, metals
• Doping via diffusion or ion implantation
• Lithography shadow masked or projection
• Etching Wet and Dry
• Sequential Mask Transfer
• Stepper Iteration

Sze, Semiconductor Devices, John Wiley and Sons
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Wafer ? Die ? Device
Sze, Semiconductor Devices, John Wiley and Sons
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Ingredients

• Clean Rooms
• Exposure Techniques
• Masks
• Photoresist
• Pattern Transfer
• Etching

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Clean Room Technology

• Pinholes
• Constriction of I
• Short ckt

• Epitaxy Dislocations
• Gate Oxide Low Vb

Rule of Thumb particles greater than 1/10 of
Lmin is disruptive. Lmin 5 mm requires lt 0.5
micron dust particles
Sze, Semiconductor Devices, John Wiley and Sons
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Clean Room Technology

• Dust count should be four orders of magnitude
  lower than ordinary room air.
• Class 100 100 particles (half micron or
  greater) per cubic foot 3500 particles per
  cubic meter
• If we expose a 125 mm wafer for 1 minute to a
  laminar flow air stream at 30 m/min, how many
  dust particles will land on the wafer in a class
  10 clean room?

Sze, Semiconductor Devices, John Wiley and Sons
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Particle Emission
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Clean Room Classes
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Design

• Keep critical areas very small
• Separate working areas
• Slight overpressure in white areas
• Laminar flow boxes in poor air quality areas

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Comb Structure
White area for wafer and chip processing
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Ball Room Structure
Ceiling
Floor
HEPA filter high efficiency particulate air
filter, Ceiling to floor laminar flows,
Perforations in floor
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Exposure
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Goals

• Resolution
• Registration
• Throughput

? Yield and cost, complexity-function, power
dissipation, speed
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Shadow Printing

• lm (lg)1/2
• the gap g includes the resist layer
• l 0.4 um, g 50 um, 4 um
• l 0.25 um, g 15 um, 2 um
• Dust dimensions gt g can damage the mask!

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Projection Printing

• Avoids mask damage
• To increase resolution ? image a small portion at
  a time
• Large masks followed by 101 demag or
• 11 masks
• Tradeoff defect free masks vs. simpler optics

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Annular Field Scan
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Small-Field Raster Scan
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Reduction Step and Repeat
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11 Step and Repeat
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Resolution and DOF
Sze, Semiconductor Devices, John Wiley and Sons
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f/ f/D
D
f
f/5
f/32
http//en.wikipedia.org/wiki/F-number
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D
f
q
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CAD used to generate mask artwork Secondary chip
sites for process evaluation as well as for
alignment-registration
Mask defect density is a concern in mask
fabrication
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Yield vs Defect Density
Semicons Dirty Secret Y e-DA for one mask
level For multiple mask levels Y e-NDA
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Photolithography
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Response Curve

• Vertical axis Remaining after exposure and
  development
• Horizontal Axis Exposure

100
Solubility increases with exposure for a positive
resist
Completely soluble. Measure of sensitivity for
ve resist
ET
E1
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Finite Solubility
Negative resist cross linked polymers
insoluble Positive resist exposed areas become
soluble
ET threshold energy, E1 drawn from tangent at
ET (ve)
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Post-Etch
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gamma solubility with incremental energy
increase, contrast ratio, sharpness
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• Negative resists lower exposure times due to
  higher sensitivity ? high throughput
• Positive resists does not swell significantly
  unlike negative resists ? high resolution

CRM Grovenor, Microelectronic Materials
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Sites

• http//jas.eng.buffalo.edu/education/fab/NMOS/nmos
  .html
• http//www.ecse.rpi.edu/schubert/Course-ECSE-6290
  
• http//www.nikon.com/about/technology/core/optical
  _u/evanescent_e/index.htm