Chemical SOP - Silicon Etching Procedures and Guidelines

This document outlines basic guidelines for the safe etching of silicon at CHESS.  The intent is to supplement a specific etch procedure so qualified personnel will accomplish their task in a manner safe to themselves, and those who later uses the same facilities.

Prepare as follows:  Typical silicon etches contain hydrofluoric (HF) and other strong acids.  Nalgene and/or Teflon holders and tools must be used.  Important: Review and understand the contents of standard operating procedures and MSDS's that apply to the chmicals being used.

Before starting to mix etch ingredients, put the sign on the outside of the CHESS Chemical Room door stating, "Caution:  Etching in progress.  Strong acids in use."  Also notify the CHESS Operator, the CHESS Chemical Safety Officer, or another senior or technical staff member that you will be working with strong acids.  If you need to leave the chemical room while the etching is in progress, this person should also be informed and another "Caution:  Etching in progress" sign should be place on the hood sash.  Both signs will be stored in the draw with the glove selection.

Plan what needs to be done, from start to finish!  Silicon can be very smooth and slippery after etching, so make sure you have the right tools (basket or tongs) to pick up and handle the finished parts.

All work is done in the chemistry room fume hood.  Check that it is operational and clear of equipment that you are not using.

In the hood sink, use the water hose to ensure a continuous cold water flow into an oversized wash tray.

A timer should be running and easily read from where you perform the etch.

Personal Protective Equipment:  Wear a Neoprene apron, long pants, shoes that protect the feet, and a full face shield!  Neoprene gloves should be put on last after they have been thoroughly inspected to ensure that they are free of any defects. (Although not required, double gloving with Nitrile gloves underneath is highly recommended.)

The correct quantity of etch is based on surface area of the parts and the amount of silicon to remove.  Volume proportions for the typical etchant used at CHESS are given below.  Make only enough for the job at hand.  Mix etch ingredients in the container used for etching.

The standard room temperature etch used at CHESS contains 1 part 49% HF mixed into 19 parts concentrated Nitric acid.  (For other approved mixtures of etching solutions, check with Ken Finkelstein or another qualified CHESS staff member.)

Perform the etch with the container positioned next to the hood sink.

When etching is complete place silicon and tools in the overflowing wash tray, and rinse your gloves.

Remove the silicon after it is thoroughly washed.  From here, handle silicon outside the hood only.

Clean Up:  Neutralize used etchant:  After etch is complete, use the wash tray, 3/4 full with water, to neutralize used etchant.  From experience, to neutralize 1/2 liter of etch, thoroughly mix 1 cup sodium bicarbonate in the water.  (All mineral acids should be neutralized to a pH of grater than 5.5, before pouring down the sink.  Test with pH paper if necessary.)

Remove water tube from wash tray but leave water flowing into sink.

Slowly add small amounts of used etchant to water.  After reaction (bubbling) subsides, stir water and wait until reaction slows.  Repeat this procedure until all etchant has been neutralized.  Some bicarbonate should remain in the wash tray after this process is completed.  If not, slowly and carefully add more sodium bicarbonate to the wash tray and stir.  Sodium bicarbonate may also be added to bottom of the sink.  When any additional reaction has subsided, dump wash tray.  Keep water flowing throughout.

Finally:  To ensure that all small spills in the hood are neutralized, wipe the fume hood surface (floor) with a damp paper towel or sponge covered with bicarbonate of soda.  Thoroughly rinse and wipe up the fume hood floor.  Rinse all tools and containers with water.  Wash out the sink and make sure no bicarbonate of soda remains in the bottom.  Wipe down protective gear.  Return all equipment to drying racks and/or its assigned location.

Prepared by:  David J. Jones
Date:  9 July, 2001

Last Update: 2006-09-21