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Electronics Manufacturing – M528

Semiconductor devices and fabrication




Key discoveries:

Key discoveries for semiconductor development




Properties of materials

Solid state physics
  • Configuration of electrons determine the chemical and physical properties.

Naturally occurring atoms have negative electrons that orbit a positively charged nucleus
  • In uncharged atoms, the number of electrons exactly balance the positive charge of the nucleus.
  • In positive ions, the number of electrons are less than the positive charge of the nucleus.
  • In negative ions, the number of electrons are greater than the positive charge of the nucleus.

Table of elements
  • The atomic number indicates the number of protons in the nucleus.
  • The number of electrons is normally equal to the number of protons.
 Nucleus of an atom




Electrical conduction

Electron shells
  • Electrons start to occupy shells with the lowest energy level (innermost shell).

The movement of electrons form currents
  • From areas of surplus electrons (negative charge),
  • To holes, which are areas lacking electrons (positive charge).

Conduction describes the property to transport current
  • Electricity is a stream of flowing electrons.

Conductor materials
  • Low resistivity.
  • e.g. metals

Insulator materials
  • High resistivity.
  • e.g. glass, plastics

Semiconductor materials
  • Intermediate resistivity.
  • e.g. silicon, germanium
 



Source: Semiconductors. (n.d.). Retrieved 12 April 2008 from
http://nobelprize.org/educational_games/physics/semiconductors/4.html




Silicon

Silicon is naturally common
  • Second most adundant element on the planet.
  • Nearly a quarter of the planet crust by weight.
  • Element symbol is Si.
  • Atomic number is 14.
  • Group number is IV.

Properties of silicon
  • Semi-metallic.
  • Semiconductor.
  • Stable.
  • Can be heated to a high degree without losing material properties.

Pure semiconductor
  • Doping is the process of adding tiny amounts of atoms from another material to a pure semiconductor to increase its conductivity.
  • Conductivity of silicon is easy to control via doping.
 Silicon structure




Silicon isolation

Silicon needs to be isolated from naturally occuring silicon oxide materials

Addition processing is required to convert polycrystalline silicon to monocrystalline silicon known as an ingot

The Czochralski process is a common technique for single crystal growth

20% of wafer fabrication costs is for cleaning!

Silicon ingot




Two critical innovations

Monolithic production
  • All devices fabricated on same semiconductor material.
  • Jack Kilby

Metallisation step
  • Apply monolithic production technique to interconnects.
  • Robert Noyce

Planar processing
  • Combination of both ideas.
  • Processing steps depend on the geometric form of silicon wafers being a plane
  • Each fabrication process is performed on the entire plane of the substrate or layer

Fabrication processes could be categorised as:
  • Altering property of substrate or layer
  • Depositing a new layer
  • Removing an existing layer
 Planar processing
Source: (top) Rumelt, R. (2003). Semiconductor technology.
Los Angeles: University of California.
(bottom) International Roadmap for Semiconductors.
ITRS Press Conference, Dec 2004, 31.




Transistors + interconnects = Integrated circuits

Task specific IC
  • ASIC (application specific integrated circuit)
  • DSP (digital signal processor)

General purpose IC
  • FPGA (field programmable gate array)
  • microprocessors

IC integration scale
  • 1970s: Large scale integration
    • 10^3–10^4 devices/chip
  • 1980s: Very large scale integration
    • 10^4–10^6 devices/chip
  • 1990s: Ultra large scale integration
    • 10^6–10^8 devices/chip
  • 2000s: Giga scale integration
    • 10^9–10^10 devices/chip
 Xilinx Virtex FPGA
Source: Rabaey, J., Chandrakasan, A., Nikolic, B. (2003). Digital integrated circuits (2nd ed.)
New York: Prentice Hall.




Packaging levels:

Packaging levels




Fabrication processes

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Semiconductor cross section
Source: International Roadmap for Semiconductors.
ITRS press conference, Dec 2004, 25.




Design complexity is expensive

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Design complexity is expensive
Source: Rabaey, J., Chandrakasan, A., Nikolic, B. (2003). Digital integrated circuits (2nd ed.)
New York: Prentice Hall.




Fabrication is iterative

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 These steps repeat until all devices are fabricated on the wafer.




Lithography

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 
  • Lithography operates similar to a sophisticated (and expensive) reduction camera.
  • The condenser lens delivers light to the mask with specified energy and directionality.
  • The objective lens picks up some of the diffraction from the mask and projects an image on the wafer.
  • Early days of lithography used 456 nm wavelength light.
  • Lithography today is using 193 nm wavelength light.

Lithography
Source: H. Geng (Ed., 2005), Semiconductor manufacturing handbook.
Blacklick: McGraw-Hill.




Lithography in action

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 
Lithography in action
Source: Rabaey, J., Chandrakasan, A., Nikolic, B. (2003). Digital integrated circuits (2nd ed.)
New York: Prentice Hall.




Etching techniques

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Chemical etching are isotropic processes which have the advantage of being low cost.
Wet chemical etching


Dry etching tend to be anisotropic processes. They are required for advanced IC fabrication.
Dry plasma etching

Source: H. Geng (Ed., 2005), Semiconductor manufacturing handbook.
Blacklick: McGraw-Hill.




Testing

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 IC fabrication use in-process test structures which are typically an integral part of production.




Packaging techniques

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Packaging reconciles a wide range of requirements:
  • Electrical: Minimal leaks.
  • Mechanical: Reliable and robust.
  • Thermal: Effective heat removal.
  • Economical: Cheap for production and distribution.

Wire bonding

Source: Rabaey, J., Chandrakasan, A., Nikolic, B. (2003). Digital integrated circuits (2nd ed.)
New York: Prentice Hall.




Soldering techniques

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Pin-in-hole (PIH)
  • Since 1950s
  • Hold larger components
  • Wave soldering finish

Soldering techniques

Surface mounted (SMT)
  • Since 1960s
  • Less hole drilling
  • Higher board density
  • Double sided boards
  • Leadless components
  • Reflow soldering finish




Flip chip assembly

Front end processes
  • Design
  • Photomask development
  • Feature development
    • Oxidation layering
    • Lithography
    • Etching
      • Wet chemical
      • Dry plasma
    • Doping

Back end processes
  • Metallisation
    • Lithography
  • Testing
  • Packaging
 Flip chip assembly technique
  • Attachment of bare chip directly to a substrate in a face down configuration
  • Electrical connection between chip and substrate achieved by conducting “bumps”
  • Provides design and economic advantages
    • Higher feature density
    • Less interconnect distances
    • Less fabrication steps

Flip chip assembly




Future of fabrication

Today
  • Integrated circuits
    • Thin-film fabrication (single dimension nanoscale layers)
    • Electron beam lithography fabrication
    • Solid state memory devices (flash/USB)
  • Microelectromechanical systems (MEMS)
    • Surface micromachining
    • Flip chip assembly
    • Nanoscale sensors

Tomorrow
  • Carbon nanotubes
    • Transistors
    • Logic structures
  • Molecular electronics
    • Transistors
    • Switches
    • Interconnects
    • Memory
  • Plastic-based electronics

 
Microelectromechanical systems (MEMS)

Source: Micromachines image gallery. (n.d.). Retrieved 24 Apr 2005
from http://mems.sandia.gov/scripts/images.asp.




Future of fabrication

Today
  • Integrated circuits
    • Thin-film fabrication (single dimension nanoscale layers)
    • Electron beam lithography fabrication
    • Solid state memory devices (flash/USB)
  • Microelectromechanical systems (MEMS)
    • Surface micromachining
    • Flip chip assembly
    • Nanoscale sensors

Tomorrow
  • Carbon nanotubes
    • Transistors
    • Logic structures
  • Molecular electronics
    • Transistors
    • Switches
    • Interconnects
    • Memory
  • Plastic-based electronics

 Stretchable Silicon

Stretchable silicon

Source: Greene, K. (2006, March/April). Stretchable silicon. Technology Review, 70.


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