Unit 3 Crystal Growth and Wafer Preparation

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Unit 3 Crystal Growth and Wafer Preparation
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Material Preparation

• Material preparation is the beginning of the
  process in making an IC chip .
• The goal for this part of the process is to grow
  the ingot that will be sliced up into wafers.
• The wafer is a round solid silicon disc that will
  have all of the processing performed on it.

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Step 1 Obtaining the Sand

• The sand used to grow the wafers has to be a very
  clean and good form of silicon.
• For this reason not just any sand scraped off the
  beach will do.
• Most of the sand used for these processes is
  shipped from the beaches of Australia.

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Step 2 Preparing the Molten Silicon Bath

• The sand is taken and put into a pot where it is
  heated to about 1600 degrees C where it melts.
• The molten sand will become the source that will
  ultimately produce the raw poly-crystalline
  silicon.

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Raw Polysilicon

• Raw polycrystalline silicon produced by mixing
  refined trichlorosilane with hydrogen gas in a
  reaction furnace.
• The poly-crystalline silicon is allowed to grow
  on the surface of electrically heated tantalum
  hollow metal wicks

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Polysilicon Ingots

• The polycrystalline silicon tubes refined by
  dissolving in hydrofluoric acid producing
  polysilicon ingots.
• Polycrystalline silicon has randomly oriented
  crystallites, electrical characteristics not
  ready for device fabrication.
• Must be transformed into single crystal silicon
  using crystal pulling

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Step 3 Making the Ingot

• A pure silicon seed crystal is now placed into
  the molten sand bath.
• This crystal will be pulled out slowly as it is
  rotated.
• The result is a pure silicon tube that is called
  an ingot

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Creating the Single Crystalline Ingot

• Crushed high-purity polycrystalline silicon is
  doped with elements like arsenic, boron,
  phosphorous or antimony and melted at 1400 in a
  quartz crucible surrounded by an inert gas
  atmosphere of high-purity argon.
• The melt is cooled to a precise temperature, then
  a "seed" of single crystal silicon is placed into
  the melt and slowly rotated as it is "pulled"
  out.

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Creating the Single Crystalline Ingot (cont.)

• The surface tension between the seed and the
  molten silicon causes a small amount of the
  liquid to rise with the seed and cool into a
  single crystalline ingot with the same
  orientation as the seed.
• The ingot diameter is determined by a combination
  of temperature and extraction speed

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Examples of completed ingots
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Ingot Sizes

• Most ingots produced today are 150mm (6") and
  200mm (8") diameter,
• For the most current technology 300mm (12") and
  400mm (16") diameter ingots are being developed.

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Ingot Characterization

• Single Crystal Silicon ingots are characterized
  by the orientation of their silicon crystals.
  Before the ingot is cut into wafers, one or two
  "flats" are ground into the diameter of the ingot
  to mark this orientation.

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Different flats for orientation

• Each of the wafers is given either a notch or a
  flat edge
• This will be used in orienting the wafer into the
  exact position for later procedures.

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lt100gt Lattice Orientation

• This lattice orientation is used for MOS
  (metaloxide semiconductor), Bi-CMOS, GaAs types
  of chips.

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Lattice Orientation

• The lattice orientation refers to the organized
  pattern of the silicon crystals in the wafer and
  their orientation to the surface.
• The orientation is obtained based on the
  orientation of the crystal that is placed into
  the molten silicon bath.
• The different orientations have different
  benefits and are used in different types of
  chips.

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lt111gt Lattice Orientation

• This orientation is used for Bipolar types of
  chips

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Step 4 Preparing the Wafers

• The ingot is ground into the correct diameter for
  the wafers.
• Then it is sliced into very thin wafers.
• This is usually done with a diamond saw.

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Some wafers in storage trays
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Wafer Lapping

• The sliced wafers are mechanically lapped using a
  counter-rotating lapping machine and aluminum
  oxide slurry. This flattens the wafer surfaces,
  makes them parallel and reduces mechanical
  defects like saw markings

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Wafer Lapping Machine
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Wafer Etching

• After lapping, wafers are etched in a solution of
  nitric acid/ acetic acid or sodium hydroxide to
  remove microscopic cracks or surface damage
  created by the lapping process.
• The acid or caustic solution is removed by a
  series of high-purity RO/DI water baths

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Wafer polishing

• Next, the wafers are polished in a series of
  combination chemical and mechanical polish
  processes called CMP
• The wafers are held in a hard ceramic chuck using
  either wax bond or vacuum and buffed with a
  slurry of silica powder, RO/DI water and sodium
  hydroxide

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Wafer Cleaning

• Most wafer manufacturers use a 3-step process
  which starts with an SC1 solution (ammonia,
  hydrogen peroxide and RO/DI water) to remove
  organic impurities and particles from the wafer
  surface.
• Next, natural oxides and metal impurities are
  removed with hydrofluoric acid.
• Finally, the SC2 solution, (hydrochloric acid and
  hydrogen peroxide), causes super clean new
  natural oxides to grow on the surface.

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Growth of Epitaxial Silicon

• The purpose of EPI growth is to create a layer
  with different, usually lower, concentration of
  electrically active dopant on the substrate. For
  example, an n-type layer on a p-type wafer.
• This layer is of a much better quality then the
  slightly damaged or unclean layer of silicon in
  the wafer
• It is called the Epitaxial layer - where the
  actual processing will be done.

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An Epitaxial reactor.
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Epitaxial Reactors