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SEMICONDUCTOR MANUFACTURING - CLEAN ROOMS, WAFER
         CLEANING AND GETTERING- Chapter 4



           Year of Production              1998     2000     2002    2004     2007     2010     2013     2016     2018

     Technology Node (half pitch)         250 nm 180 nm     130 nm   90 nm    65 nm    45 nm    32 nm    22 nm    18 nm

       MPU Printed Gate Length                     100 nm   70 nm    53 nm    35 nm    25 nm    18 nm    13 nm    10 nm

      DRAM Bits/Chip (Sampling)           256M     512M      1G       4G      16G      32G      64G      128G     128G

      MPU Transistors/Chip (x106)                                     550     1100     2200     4400     8800     14,000

           Critical Defect Size           125 nm   90 nm    90 nm    90 nm    90 nm    90 nm    65 nm    45 nm    45 nm

     Starting Wafer Particles (cm-2 )                                <0.35    <0.18    <0.09    <0.09    <0.05    <0.05

   Starting Wafer Total Bulk Fe (cm-3 )   3x1010   1x1010   1x1010   1x1010   1x1010   1x1010   1x1010   1x1010   1x1010

     Metal Atoms on Wafer Surface         5x109    1x1010   1x1010   1x1010   1x1010   1x1010   1x1010   1x1010   1x1010

          After Cleaning (cm-2 )

    Particles on Wafer Surface After                                  75       80       86       195      106      168

           Cleaning (#/wafer)
Three Tiered Approach

     Modern IC Factories Employ a Three Tiered Approach to
                 Controlling Unwanted Impurities
• 1. Clean Factories, Clean Room with Clean Environment:
  Air is Filtered, Machines Designed to Produce Minimum Particles,
  and Ultra Pure Chemicals and Gases Are used in Wafer
  Processing
• 2. Wafer Cleaning is Done Often and Thoroughly to
  Remove Particles and Contaminant Films on the Wafer Before
  They Get in to the Devices
• 3. Gettering: A Method by which unwanted impurities that
  get into wafer are pushed to non critical parts of wafer, typically
  wafer backside or wafer bulk, far away from active devices on the
  top wafer surface
• Contaminants may consist of
  particles, organic films (photoresist),
  heavy metals or alkali ions.
Level 1 Contamination Reduction: Clean Factories

                                                             • Particles may originate from
                                                               People, Machines, Chemicals, and
                                                               Process Gases
                                                             • Particles may be Airborne or
                                                               Suspended in Liquids and Gases
                                                             • Air Quality is Measured by the
                                                               “Class” of the Facility
                                                             • Class X Means in Cubic Foot of Air
                                                               There are Less Than X Total
                                                               Particles Greater than 0.5 um.
                                                             • Typical Office Bldg is Class 100,000
(Photo courtesy of Stanford Nanofabrication Facility.)


                                                         Factory environment is cleaned by:
                                                         • Hepa Filters and Recirculation for Air,
                                                         • “Bunny Suits” for Workers.
                                                         • Filtration of Chemicals and Gases.
                                                         • Manufacturing Protocols.
• In Clean Rooms Most Concern are 10 nm and 10 um Particles
• People Typically Emit Several Hundred Particles per Minute from
  Each cm2 Surface Area (5-10 million Particles per Minute per Person
  Typical). [Robots to Minimize Particle Generation]
• Bunny Suits to Cover Bodies and Clothing to Block Particle
  Emission
• Face Masks to Prevent Exhale of Particles
• Air Showers at Entrance
• Specially designed Machines to Minimize Particle generation
• Material Used Inside Clean Rooms are also Special
• Constant Air Filtration to Remove Particles as They are Generated
• Filtration of Chemicals to Minimize Particles coming from there
Level 2 Contamination Reduction: Wafer Cleaning




                             • RCA clean is “standard process”
                               used to remove organics, heavy
                               metals and alkali ions.

                             • Ultrasonic agitation is used to
                               dislodge particles.
Level 3 Contamination Reduction: Gettering
• Gettering is used to remove metal ions and alkali ions from device active regions.




   • For the alkali ions, gettering generally uses dielectric layers on the topside
     (PSG or barrier Si3N4 layers).
   • For metal ions, gettering generally uses traps on the wafer backside or in the
     wafer bulk.
   • Backside = extrinsic gettering. Bulk = intrinsic gettering.
PSG Layer
Devices in near
 surface region
Denuded Zone                          10 - 20 µm
 or Epi Layer




   Intrinsic
   Gettering
    Region                            500+ µ m




   Backside
   Gettering
    Region




• Heavy metal gettering relies on:
   • Metals diffusing very rapidly in silicon.
   • Metals segregating to “trap” sites.
SiO2 precipitates (white dots) in bulk
           of wafer.


• “Trap” sites can be created by SiO2 precipitates
  (intrinsic gettering), or by backside damage
  (extrinsic gettering).

• In intrinsic gettering, CZ silicon is used and SiO 2
   precipitates are formed in the wafer bulk through
   temperature cycling at the start of the process.
Modeling Particle Contamination and Yield


• ≈ 75% of yield loss in modern VLSI fabs is due to particle
contamination.

• Yield models depend on information
  about the distribution of particles.

• Particles on the order of 0.1 - 0.3 µm are
  the most troublesome:
     • larger particles precipitate easily
     • smaller ones coagulate into larger
       particles
Summary of Key Ideas
• A three-tiered approach is used to minimize contamination in wafer
  processing.

• Particle control, wafer cleaning and gettering are some of the "nuts and bolts"
  of chip manufacturing.

• The economic success (i.e. chip yields) of companies manufacturing chips
  today depends on careful attention to these issues.

• Level 1 control - clean factories through air filtration and highly purified
  chemicals and gases.

• Level 2 control - wafer cleaning using basic chemistry to remove unwanted
  elements from wafer surfaces.

• Level 3 control - gettering to collect metal atoms in regions of the wafer far
  away from active devices.

• Bottom line is chip yield. Since "bad" die are manufactured alongside "good"
  die, increasing yield leads to better profitability in manufacturing chips.

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Ch04

  • 1. SEMICONDUCTOR MANUFACTURING - CLEAN ROOMS, WAFER CLEANING AND GETTERING- Chapter 4 Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018 Technology Node (half pitch) 250 nm 180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm DRAM Bits/Chip (Sampling) 256M 512M 1G 4G 16G 32G 64G 128G 128G MPU Transistors/Chip (x106) 550 1100 2200 4400 8800 14,000 Critical Defect Size 125 nm 90 nm 90 nm 90 nm 90 nm 90 nm 65 nm 45 nm 45 nm Starting Wafer Particles (cm-2 ) <0.35 <0.18 <0.09 <0.09 <0.05 <0.05 Starting Wafer Total Bulk Fe (cm-3 ) 3x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 Metal Atoms on Wafer Surface 5x109 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 1x1010 After Cleaning (cm-2 ) Particles on Wafer Surface After 75 80 86 195 106 168 Cleaning (#/wafer)
  • 2. Three Tiered Approach Modern IC Factories Employ a Three Tiered Approach to Controlling Unwanted Impurities • 1. Clean Factories, Clean Room with Clean Environment: Air is Filtered, Machines Designed to Produce Minimum Particles, and Ultra Pure Chemicals and Gases Are used in Wafer Processing • 2. Wafer Cleaning is Done Often and Thoroughly to Remove Particles and Contaminant Films on the Wafer Before They Get in to the Devices • 3. Gettering: A Method by which unwanted impurities that get into wafer are pushed to non critical parts of wafer, typically wafer backside or wafer bulk, far away from active devices on the top wafer surface
  • 3. • Contaminants may consist of particles, organic films (photoresist), heavy metals or alkali ions.
  • 4. Level 1 Contamination Reduction: Clean Factories • Particles may originate from People, Machines, Chemicals, and Process Gases • Particles may be Airborne or Suspended in Liquids and Gases • Air Quality is Measured by the “Class” of the Facility • Class X Means in Cubic Foot of Air There are Less Than X Total Particles Greater than 0.5 um. • Typical Office Bldg is Class 100,000 (Photo courtesy of Stanford Nanofabrication Facility.) Factory environment is cleaned by: • Hepa Filters and Recirculation for Air, • “Bunny Suits” for Workers. • Filtration of Chemicals and Gases. • Manufacturing Protocols.
  • 5. • In Clean Rooms Most Concern are 10 nm and 10 um Particles • People Typically Emit Several Hundred Particles per Minute from Each cm2 Surface Area (5-10 million Particles per Minute per Person Typical). [Robots to Minimize Particle Generation] • Bunny Suits to Cover Bodies and Clothing to Block Particle Emission • Face Masks to Prevent Exhale of Particles • Air Showers at Entrance • Specially designed Machines to Minimize Particle generation • Material Used Inside Clean Rooms are also Special • Constant Air Filtration to Remove Particles as They are Generated • Filtration of Chemicals to Minimize Particles coming from there
  • 6. Level 2 Contamination Reduction: Wafer Cleaning • RCA clean is “standard process” used to remove organics, heavy metals and alkali ions. • Ultrasonic agitation is used to dislodge particles.
  • 7. Level 3 Contamination Reduction: Gettering • Gettering is used to remove metal ions and alkali ions from device active regions. • For the alkali ions, gettering generally uses dielectric layers on the topside (PSG or barrier Si3N4 layers). • For metal ions, gettering generally uses traps on the wafer backside or in the wafer bulk. • Backside = extrinsic gettering. Bulk = intrinsic gettering.
  • 8. PSG Layer Devices in near surface region Denuded Zone 10 - 20 µm or Epi Layer Intrinsic Gettering Region 500+ µ m Backside Gettering Region • Heavy metal gettering relies on: • Metals diffusing very rapidly in silicon. • Metals segregating to “trap” sites.
  • 9. SiO2 precipitates (white dots) in bulk of wafer. • “Trap” sites can be created by SiO2 precipitates (intrinsic gettering), or by backside damage (extrinsic gettering). • In intrinsic gettering, CZ silicon is used and SiO 2 precipitates are formed in the wafer bulk through temperature cycling at the start of the process.
  • 10. Modeling Particle Contamination and Yield • ≈ 75% of yield loss in modern VLSI fabs is due to particle contamination. • Yield models depend on information about the distribution of particles. • Particles on the order of 0.1 - 0.3 µm are the most troublesome: • larger particles precipitate easily • smaller ones coagulate into larger particles
  • 11. Summary of Key Ideas • A three-tiered approach is used to minimize contamination in wafer processing. • Particle control, wafer cleaning and gettering are some of the "nuts and bolts" of chip manufacturing. • The economic success (i.e. chip yields) of companies manufacturing chips today depends on careful attention to these issues. • Level 1 control - clean factories through air filtration and highly purified chemicals and gases. • Level 2 control - wafer cleaning using basic chemistry to remove unwanted elements from wafer surfaces. • Level 3 control - gettering to collect metal atoms in regions of the wafer far away from active devices. • Bottom line is chip yield. Since "bad" die are manufactured alongside "good" die, increasing yield leads to better profitability in manufacturing chips.