Aluminum Etch SOP

Parameters

Etch Rate

>1 nm/s

Etch Temp

40C

Stir RPM

350

Purpose

Before reading further, please see the Safety section for more information.

Thermal evaporation deposits aluminum over the entire chip. We want to etch away specific areas, and leave aluminum only to act as contacts for the source, drain, and gate. These aluminum contacts will allow us to probe or package the chip and apply a voltage to the doped semiconductor underneath.

If etching aluminum is unsuccessful, then portions of the chip can be shorted and the device won’t function as we intended.

Aluminum etch has proven difficult to keep consistent. There are many problems with peeling and bad adhesion of both photoresist on top and aluminum to silicon below. Proper cleaning of the chip before evaporation, evaporation parameters, photoresist developing parameters, and etching parameters are necessary to have a successful aluminum etch run.

Tools

  1. Fume hood

  2. Hot plate with spinner

  3. Magnetic stirrer

  4. Glass beaker

  5. Appropriate container for acid (polypropylene)

  6. Nitrile Gloves (under)

  7. Neoprene Gloves (over)

  8. Nitrile Splash Apron

  9. Spill kit and all other required safety equipment, disposal equipment, and building utilities

Materials

  1. Aluminum etchant (16:1:1:2 phosphoric, nitric, acetic, DI water)

  2. DI water

Procedure

Preparation

  1. Place your chip on the cleanroom wipe taped to the inside of the acid hood. Do not leave the tweezers inside the fume hood.

  2. Put on nitrile gloves

  3. Put on a splash gown.

  4. Put on a face shield.

  5. Put on the neoprene gloves. Avoid touching the outside of the gloves.

  6. Pre-heat the hot plate.

  7. Place the evaporating dish on the hot plate slightly off center so that the magnetic stirrer is on one side.

Etching

  1. After the acid has had a few minutes to come to temperature, use the tweezers labeled “acid” to pick up the chip and put it in the dish on the opposite side from the stirrer.

  2. Start the timer.

  3. Turn on the stirrer. Make sure the chip sits face up on the bottom of the dish and doesn’t get sucked up by the stirrer.

  4. Prepare to take the chip out 30 seconds before the timer is up. It can be tricky to grab the chip, so start early and drop it into the DI water when the timer expires.

  5. Swish the chip around a couple of times and then take it out and let it dry on the wipe.

  6. Turn off the hot plate.

  7. Take off the PPE.

  8. Use your outside tweezers to pick up the chip and dry it off with the nitrogen gun.

Inspection

Incomplete aluminum etch looks black and rough.

Peeling is very obvious as well.

Safety

While the acids involved in this step are not as aggressive as hydrofluoric acid, there are still many overlapping safety protocols to be followed. This document is NOT qualified to teach you all the required safety protocols and correct operating procedures - please refer to an SOP and more disposal and safety resources.

This document is an operating procedure for our specific lab, and is a reference for students to know how to consistently complete a successful etch.

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