150mm Wafer Transfer

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150mm Wafer Transfer

• Keith Warner, Andy Loomis
• April 7, 2000

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Transfer Process Goals
Adhesive-based wafer-to-wafer bonding 2 5 µm
bondline thickness, thermal limits on
post-transfer processing, /- 2 µm alignment
accuracy with existing tools Complete transfer
of NU processes, also useful for other device
types Oxide fusion bonding (Adopt existing
process from CCD work) lt 1 µm bond layer,
post-transfer processing 400C (Tungsten CVD
plugs, hot Al), /- 1 µm alignment with tool to
be purchased Determine technology limits on this
program Sub-micron via placement with oxide
bonding Build/specify tools using stepper
technology techniques
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Wafer Bonding - Voids

• Voids are formed after the SOI handle wafer is
  removed
• Particulate - Cleaning, clean area, inspection,
  filtration
• Pinholes and oxide quality - Starting material
• Adhesion Cleaning
• Bubbles Vacuum outgassing of epoxy

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Wafer Bonding - Voids
(After wafer thinning decorative etch)
12/16/99
3/2/00
Void free wafer transfer achieved
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Wafer Bonding - Alignment

• Alignment method depends on type of tool
• Early work used Research Devices flip-chip bonder
  
• Limitations of optical system restricted accuracy
  to /- 5 µm

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Wafer Bonding - Alignment

• Karl Süss mask aligner with infrared optics
• Better than /- 2 µm accuracy, repeatable

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Wafer Bonding - Alignment
(Same wafer without voids)
Accurate alignment achieved
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Wafer Bonding Bondline Uniformity
(2-5 µm required across wafer)

• Wafer flatness Double side polished ultra-flat
  wafers
• Adhesive curing cycle Experiments started
  (Research Devices tool is not designed for
  wafer-to-wafer bonding available pressure is
  inadequate)
• Spacers Latex microspheres
• Particulate Cleaning, inspection, filtration

Still working on optimization
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Wafer Bonding Apparatus and Operation
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Status

• Void-free transfer successful
• Alignment accuracy adequate for devices currently
  available (North2)
• Method identified to meet bondline requirements
  experiments are underway
• Full-time effort has started