M9

• Semiconductor electrical performance and yields

  Result of the ability to introduce only the desired impurities into the wafer and to precisely reproduce mask dimensions on the wafer's surface
• Semiconductors are very sensitive to unwanted contamination, even from amounts as small as several parts per million
• Cleanliness is the key, so a lot of money and technology is invested in maintaining a "clean" production environment
• Key Areas of Contamination Control

  • Device Processing
  • Device Performance
  • Device Reliability
• Device processing in a contaminated environment

  Can cause wafers to fail in in-process quality checks
• Dirty air or chemicals

  Can contaminate wafers in process
• Microscopic size dirt

  Can cause holes in photoresist layers
• Failures in device processing

  Result in low fabrication yields and higher production costs due to reworking of the wafers
• Contamination

  Can cause electrical-related failures in chips such as soft or leaky junctions and premature breakdown
• Low levels of contaminants

  Can be introduced during wafer fabrication and not be detected during normal circuit testing, but may migrate to the active devices and reach a level high enough to cause circuit failure
• Contamination Sources

  • Air
  • Production Facility
  • People
  • Water
  • Chemicals
  • Gases
  • Static Charge
• Contamination is defined as anything that results in a failure mode
• An element or compound desired in one step may be a contaminant at another
• Airborne contaminants

  Plain everyday air (even non-smoggy air) is dirty and contains a large amount of particulates, water vapor, as well as photochemical smog
• The dimensions used in photomasking are smaller than the diameter of the most common particulates in the air
• Air Class Number

  Air cleanliness levels are defined in Federal Standard (FS) 209a, with two factors: maximum size particulate allowed and maximum quantity allowed
• LSI semiconductor stations must filter the air to a 0.5 micron level
• Air Class Numbers

  • VLSI area: Class 10 @ 0.3 micron
  • VLF hood: Class 100 @ 0.5 micron
  • Typical Fab area: Class 10,000
  • House room: Class 100,000
  • Outdoors: >Class 100,000
• Vertical laminar flow (VLF) hoods

  Airflow in the room is controlled by these
• High-efficiency particulate air (HEPA) filter

  Air entering the fabrication areas are filtered using this, which has a filtering efficiency of 99.99%
• Clean room construction

  • Wall and ceiling materials are non-shedding and smooth surfaced
  • Piping holes are sealed and caulked
  • Vacuum and mechanical pumps should be located outside
  • Flat surfaces must be minimized to prevent collection of dust
• Fabrication area isolation techniques

  • Air pressure balancing
  • Double door entrances
  • Air showers
  • Tacky floor mats
• Temperature

  Must be controlled to ±2 °C for photomasking operations
• Humidity

  High humidity can cause adhesion problems, while low humidity can cause static charge buildup. Typical humidity is specified at 15 - 50%
• People

  The largest source of contaminants, giving off a variety of vapors and particulates
• Personnel entering fabrication areas

  Don various garments (e.g., bunny suit, lab smock) and pass through air showers to knock off particulates
• Water

  Contains a large amount of contaminants such as dissolved minerals, particulates, and bacteria
• Reverse osmosis and deionization

  Techniques used to remove ions from water
• Demineralization (deionization)

  Changes water from a conductive medium to a resistive one
• Filtration, diatomaceous earth filtration, and membranes with 0.1 micron pores

  Methods used to remove solid particles from water
• Bacteria and fungi

  Removed from water by filtration, after being killed by ultraviolet light sterilizers
• Chemicals used to clean wafers

  Contain other elements that function as contaminants, so the chemical purity must be very high
• Gas purity

  Measured in four categories: purity, water vapor content, particulates, and metallic ions
• High purity is required of the reactant gases used in oxidation, diffusion, plasma etching, CVD, reactive ion etch and others
• Gas purity specification

  Typically 99.99% to 99.9999%, depending on the gas and application
• Dry gases

  Essential in semiconductor processing due to the easy oxidizing nature of SiO2, as trace amounts of H2O can cause growth of SiO2 which can block diffusions
• Gases must be particulate free

  To the 0.2 micron level, as any particulate will be implanted into the wafer along with the dopant in ion implantation
• Static charge

  Can reach up to 50,000 volts and attract unacceptable levels of particulates from the air and work surfaces
• Static charge control approaches

  • Prevent static buildup
  • Static discharge methods
• Antistatic wafer carriers, garments, and topical antistatic solutions

  Used to prevent static buildup