Wednesday, March 16, 2011

Wet delayering process

My preferred method for delayering chips is wet etching in dilute hydrofluoric acid (HF), commonly available in grocery stores as Whink brand rust remover:

~2% technical grade HF from Price Chopper
As the MSDS makes quite clear, this stuff is not something you want to splash on your hands (or skin in general). While it's extremely dilute compared to the 45% concentrated solution used in some laboratories, I've been through the HF safety talk at my school's cleanroom enough times that I'd rather not take chances.

I have a pair of Norfoil (Silver Shield) gloves around that I use for this kind of work. They're rather stiff so a common practice to improve dexterity is to double-glove with an XL nitrile glove over the Norfoil. When combined with a lab coat, splash goggles, and a face shield there's little chance of anything getting through. (EDIT: A fume hood is significantly safer; at the time of this post I didn't have one so I just tried to work with the smallest volume possible. I do now and wouldn't repeat the experiment outside it.)

Nitrile outer glove (blue) over Norfoil glove (silver), tucked into sleeve of Tyvek lab coat (white)

My standard lab PPE

Before getting dressed I placed a 10 ml beaker of distilled water on my hot plate and preheated it to a warm but not boiling temperature (exact temp isn't critical).

The next step is to pour a bit under 1 ml of the HF solution into a plastic test tube. HF will eat glass so using glass labware with it isn't a good idea!

HF solution in the test tube
Drop the die into the tube, cap it, and place it in the water bath. Etch rate depends on temperature, strength of the acid (Whink's strength isn't precisely controlled and I often will re-use the acid several times) and a few other factors so it's difficult to accurately predict. I usually will etch for 30 seconds at a time on modern planarized processes and 60-90 seconds on a large non-planarized chip.
Sample etching in the water bath

When the time is up, remove the tube from the heat and suction the HF with a pipette. The acid can usually be re-used for many etches, though it does get weaker over time. Drop the die into a beaker of acetone to remove any acid residue.

Remove the sample from the acetone using solvent-resistant plastic tweezers. (Many common plastics, such as polycarbonate, will dissolve into the acetone and contaminate your sample. Metal tweezers have a nasty habit of chipping edges of dies.)

Rapidly blow-dry the sample, holding it down with tweezers so it doesn't go flying. I used a can of R-134 duster spray. (If you let the solvent evaporate slowly large crystals can form from dissolved materials.)

Drying the sample
Image the die under a microscope to see if you've etched far enough. (The die I used in this demonstration was actually a bit over-etched as I paid more attention to camera angles than etch timing!)

The other main delayering method I plan to explore is CMP with colloidal silica. At the moment MTI is sold out, but when a new shipment arrives expect a post on CMP!

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