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  • Date updated:August 8, 2000

CP4 Etch and Surface Barrier Application Standard Operating Procedure

  • authored by Douglas McGregor


CP4A is used to etch silicon rapidly. A Schottky barrier is formed by coating the silicon surface with iodine.

Material Requirements

Equipment needed for process

  • A plastic water tank
  • A plastic beaker
  • A DI water cascade rinse
  • A plastic wide mouth bottle
  • 3-inch plastic (or TFE) wafer holders

Chemicals needed for process

Safety CP4 Etch and Surface Barrier Application

Engineering controls

Protective equipment needed

Acid gloves, apron, and face shield to mix the CP4A etch. General gloves for the iodine handling.


  • The CP4 etch is:
    3:5:3 hydrofluoric acid: nitric acid: glacial acetic acid, generally at 300 ml: 500 ml: 300
    ml to make a beaker of 1100 ml solution.

  • '''The solution is mixed relatively slowly in a plastic beaker. It is disposed of after use.
  • The amount produced is enough to completely submerge a 3-inch diameter wafer of silicon
    within a 2000-ml plastic beaker.

CP4 Etch

  1. The plastic beaker is placed into an 8-inch x 8-inch plastic tank that has been partially filled with DI water.
  2. Plastic freezer packs are used in the water to chill the water bath. Silicon wafers are etched for about 5–8 minutes in the
    CP4A etch.
  3. Afterwards, they are removed and rinsed in a DI water cascade.
  4. The CP4A etch is discarded by flushing it down the drain.

Before Schottky contacts are evaporated onto the Si wafers, the surface must be treated with iodine vapor. Iodine in the solid form (flakes) is kept is a small chemical bottle.

Iodine Flakes

  1. A few grams of iodine are placed in a plastic tray about 1 inch in diameter.
  2. This container is then placed in a wide mouth 6-inch diameter plastic bottle with a lid.
  3. A silicon wafer is secured to the lid and the lid is closed.
  4. The iodine sublimed and coats the wafer. It does
    not escape into the room from the air-tight container.
  5. The process is performed overnight within a fume hood.
  6. The silicon wafer is then removed and the residual iodine is poured into a separate air-tight glass container for storage.


The acids are stored in the acid storage cabinet. The iodine is stored with other general chemicals in properly assigned cabinets.
© 2011 Lurie Nanofabrication Facility, University of Michigan