Polishing and Faceting Techniques
author: Anthony Poole 2023
Steps:
1. Initial Lap using loose grit (optional depending on material):
• Place approximately 1/8 teaspoon of grit onto the bare glass in a white bowl.
• Add approximately 3 drops of water onto the grit and mix the two.
• Using light pressure, low speed (3-4), and short >me dura>on (3-5), begin the lap.
• Add water as needed to prevent the solution from drying out.
• Wash the sample off thoroughly to avoid cross-contamination, which can cause poor
results in later stages.
• Observe the sample under a microscope for potential issues.
• Common issues and their solutions:
o Rough/torn up edges: caused by some combination of too much grit, speed, and
duration. Softer materials such as GaAs will be more delicate and using grit may
be overkill and cause chunks of sample to falls off.
o Sample Breaking: caused by far too much pressure. The grit will do its job with
minimal pressure; there is no need to use too much.
o Scratching of Surface: caused by too much grit, not enough water, or too high of
speed. This is a common issue that is created by the solution "washing up" along
the surface of the sample as it rotates. As it rotates, it dries and becomes
abrasive to the surface of the sample. Too much grit will create a surplus of
abrasive solution, and too high of speed will cause the solution to wash up the
surface.
2. Metallurgical Paper Polish (or initial lap if skipping the grit):
• Using the black dish with the double-sided tape, cut out a circular sec>on of
metallurgical sandpaper.
• Ensure that the paper is tightly attached to the double-sided tape and is not lifting at any
points. If it is lifting replace the tape.
• If an initial lap has already been performed, place the chuck back on the hot plate and
move up the sample to expose more material.
• Using medium dura>on (6-7), speed (6-7), and medium to light pressure, perform the
polish/lap.
• Observe the sample under a microscope for potential issues. If using this step to polish,
there should already be a decent polish. If there are large amounts of pitting or
scratching, then clean the bowl and sample and re-do this step.
• Common issues and their solutions:
o Pitting/Scratching/General Poor results: caused by cross-contamination with grit
paper or previous samples in the dishes. To ensure proper results, cross-contamination must be minimized as much as possible. Wash dishes,
change/clean gloves, separate chuck and sample, and clean both are all possible
steps to take to reduce contamination.
o Surface scratching: like in the initial lap, it is possible to get surface scratching of
the sample near the point of contact. This is most likely caused by the paper
lifting off the tape and scratching the surface or grit/removed material being
brushed up onto the sample.
o Rounded Facets: it is possible that the sample has acquired a rounded parabolic
shape from the shape of the glass dish that it is lapped/polished on. Because of
the repeated use of the dishes, they have a small but consequential parabolic
shape. It is possible that the shape is being passed down to the sample as a
result of too much material being exposed and/or too light of pressure. If you
skip the ini>al lap and use the metallurgical paper to lap, and the sample comes
out as rounded go back and re-do the lap using the grit it is likely that your
material is too hard for metallurgical paper to lap.
3. Polish with Micro Alumina
• Wash down chuck, sample and bowl as well as possible along with gloves/hands reduce
cross contamination.
• Place Polishing felt on the glass of the dedicated micro alumina bowl (blue one) and
place ~1/4 teaspoon of micro alumina on top using a clean object to create an even coat
along the surface.
• Allow the micro polish to full absorb into the pad before star>ng.
• Use the minimum pressure along with maximum >me and speed. Monitor the process
to ensure that the polish felt is not being ripped up by the sample.
• Common Issues and their solutions.
o Poor Polish: Most likely cross contamination, use more caution and clean tool
and hands be]er. It may even help to remove the sample from the chuck and
wash down the whole thing before performing the micro polish.
o Tearing felt/Fibers on sample: A combination likely of too much pressure too high
speed and too much micro polish causing the felt to get too wet and the sharp
edges of the sample tearing it up.
4. Cleaning
• After final polish was off chuck return to hot plate and fill two separate petri dishes with
acetone and IPA (isopropyl alcohol)
• Place the sample first in the acetone allowing to sit for ~45 seconds flip and allow to sit
again.
• Remove sample and using a clean chem wipe hold the sample down with tweezers and
use a scrubbing s>ck (make sure to wet the s>ck prior with acetone) to scrub the surface
and facet carefully. Flip and repeat on other side.
• Place sample in IPA doesn’t have to be long, flip. Repeat the same scrub procedure.
• Dry by air drying or using air hose.
• Common Issues and their solutions.
o Scratching on surface: the bottoms of the petri dished contain residue that can
scratch the surface of the samples. Try to make sure the samples are not moving
around inside of dishes.
o Residue: Scrub for longer and allow to sit in acetone for longer.
Important Notes
• The process varies depending on the material. Using the grit on some materials will
likely cause a very rough polish that may be unsalvable depending on the size of the
sample. If you do not know how hard the material maybe I would advise first star>ng
with the metallurgical paper and if you get an uneven rounded lap, then go back and use
the grit.
• Very li]le pressure is needed for every and all steps. It is possible to use the minimum
pressure for every step and s>ll ge`ng a good polish. When in doubt err on the side of
cau>on and use less pressure.
• When placing the sample in the chuck ensure that it is placed correct. Using the
broadside of the tweezers to push the sample flush with the chuck is a good trick to
ensure it won’t dislodge or break during lap/polish.
• Cleanliness is very important and cross contamina>on can ruin a sample quickly washing
down tools, hands and samples is necessary. Old chem wipes can be used to wipe down
the inside of the bowls and dishes to great effect.
Materials:
• White, black and blue bowls + glass dishes
• dish with double-sided tape
• Chuck and sample
• Metallurgical sandpaper
• Micro alumina
• Polishing felt
• Acetone
• IPA (isopropyl alcohol)
• Scrubbing sticks
• Chem wipe
• Tweezers
Opinions from Alex:
- Tips for polishing GaSb substrates
- I found that three rounds at speed 6 time 6 on the typical lapping paper was enough to flatten the facet to 45 degrees
- Use a different sheet of polishing paper each time
- Start with (referencing Anthony's terminology for applied pressure) "very low", "low", and "between low and medium" for the three rounds of lapping
- Between each round of lapping, use a water-wet cleanroom swab to brush the dust from the edge of the facet. This dust/contamination can lead to a rough polish in the paper stage as well as the final polish
- Using max speed with no applied pressure for time six, I was able to get a decent final polish with the Al goop with the TexMetC pads
- The Al polish is best removed with water, I soaked the substrate and chuck for a few minutes which allowed easier removal of the residue with a water-wet cleanroom swab
- I found that three rounds at speed 6 time 6 on the typical lapping paper was enough to flatten the facet to 45 degrees