Acid Etching Documentation

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Documentation

The following with discuss all parts of the acid etching process. This includes the equipment and chemicals needed, along with the procedure and safety requirements for performing this experiment.

Equipment and Chemicals

Personal Protective Equipment

  • Lab Coat
  • Lab Apron
  • Heavy Rubber Gloves (Preferrably Elbow Length)
  • Face Shield (8-inch)
  • Chemical Splash Goggles
  • Respirator
    • If performing work in fume hood, respirator not required
  • Standard lab attire (ie. pants, close toed shoes, etc.)

Procedure Equipment

  • Forceps
  • Pipet
  • Evaporating dish (150 ml)
  • Hot Plate
  • Ceramic Dish (as large as you want, usually around 35-150 ml)

Chemicals

  • Acetone (Any Grade will do)
  • Red Fuming Nitric Acid (RFNA)
    • Note that Nitric Acid used should be higher than 90% grade

Users should take note, RFNA is federally regulated and requires certain training to utilize. Extreme caution should be taken when using RFNA as improper handling of chemical can be deadly. Users should refer to the Fuming Nitric Acid MSDS before handling.

Procedure

This procedure is fairly straight forward once you obtain the equipment and chemicals. All work should be done under a fume hood unless noted otherwise. This procedure contains two main areas: CNC Pocket Milling, and Acid Etching.

CNC Pocket Milling

In order to contain where the acid dissolves the packaging of the chip, a small "pocket" must first be milled into the top of the chip. To do this, you must first come up with a 3D model of the chip in a CAD software that is compatable with the CNC machine you are going to use (we used SolidWorks for this). Once you have modeled the chip in the CAD program, draw up a 10x10x0.25 mm pocket that you will cut out of the top of the chip (The depth of 0.25 mm can vary, the absolute deepest you should go to avoid damage to the die is 0.25 mm). At a width of 10mm, you will be able to see the entire die plus all of the bond wires. In order to protect the bond wires better, perhaps consider using a 5x5x0.25 mm pocket centered on the chip. This way, all that should be visible after the milling is the die. An example of what the 3D model should look like can be found here: Fpga pocket mill.PNG

Once this 3D model is complete, take the chip to a CNC machine (the mechanical engineering department has CNC labs and techs that can do this work for you) to have the pocket milled out from the chip.

Acid Etching

The fume hood that you will be performing the work in should be setup as follows:

  • Place FPGA(s) into ceramic dish, place dish on hot plate, and begin to warm to approximately 100 degrees C
  • Fill 150 mL evaporating dish about 2/3 full with acetone. We will use this to rinse the chip off so the more acetone, the better.

Once the hot plate is warmed up, you may begin adding RFNA to the pocket on top of the chip using a pipet. This should be done in very small amounts (ie. One drop at a time). After 30 seconds of the acid being on the chip, remove the chip from the ceramic dish using forceps, and rinse off in dish containing acetone. Now, if you read the RFNA MSDS, you may be thinking "Woah! Doesn't RFNA react violently with Acetone??". Although you are 100% correct, the amount of RFNA remaining on the chip is so minimal that you need not worry about any explosions or violent reactions. Once chip is rinsed of all dissolved epoxy, place it back into the ceramic dish on the hot plate, and wait until most of the left over acetone has evaporated from it before adding another drop of RFNA.

Repeat this process until you can clearly see the die with little to no epoxy remaining on top. If you leave the acid on the die for too long at one time, it can begin to eat away at the die resulting in an unusable chip. In our experience, the total amount of time required, in 30s increments, for the acid to eat away all of the packaging without damaging the die is 3 minutes.

Once you have completed the experiment, be sure to clean up all equipment and store RFNA in safe location specified by your lab supervisor.

Safety

RFNA is extremely toxic and any unnecessary contact should be limited. At room temperature, RFNA gives off highly toxic vapors which can be deadly if inhaled. When not in use, container should be tightly sealed. Below are listed specific health effects and first aid measures.

Potential Health Effects

  • Inhalation
    • May be fatal if inhaled. Material is extremely destructive to the tissue of the mucous membranes and upper respiratory tract. Causes respiratory tract irritation.
  • Skin Contact
    • May be fatal if absorbed through skin. Causes skin burns. Causes skin irritation.
  • Eye Contact
    • Causes eye burns. Causes severe eye burns. Causes eye irritation.
  • Ingestion
    • May be harmful if swallowed.

First Aid Measures

  • In case of Inhalation
    • If breathed in, move person into fresh air. If not breathing, give artificial respiration. Consult a physician.
  • In case of Skin Contact
    • Take off contaminated clothing and shoes immediately. Wash off with soap and plenty of water. Take victim immediately to hospital. Consult a physician.
  • In case of Eye Contact
    • Rinse thoroughly with plenty of water for at least 15 minutes and consult a physician.Continue rinsing eyes during transport to hospital.
  • In case of Ingestion
    • Do NOT induce vomiting. Never give anything by mouth to an unconscious person. Rinse mouth with water. Consult a physician.