Motivation
It may be desirable to thin the substrate off a wafer, either for electrical/thermal reasons, or to ease the wafer cleaving step. To do so, we use a lapping machine to slowly grind away at the substrate until the sample reaches the desired thickness.
Instructions and Notes
Note: Our machine has been designated for the InP material system. Do not grind any other compounds.
Note: InP is toxic. Make sure that the grit is wet to avoid InP dust, and ensure that the wet slurry drains into the designated InP waste container.
Lapping and Polishing Machine
See the links below for the manual and demo video. They should be sufficient for a good understanding of this machine.
Polishing Fixture
While there are demo videos showing the operation of this fixture (see links below), how to operate, and how it operates, are not necessarily obvious from these videos, especially with regard to the three knurled nuts on the fixture.
- Fix the empty sample holder within the fixture. If necessary, use the the topmost knurled nut to lock it into position.
- Turn the lowest knurled nut counter-clockwise (so that it moves up) so that the sample holder drops fully. If micrometers are affixed to the fixture, the dial on them should stop moving, otherwise you may note that a visible gap forms between the lowest knurled nut and the fixture. Zero the micrometer to this point, or else make note of its reading.
- Start turning the lowest knurled nut clockwise, so that it moves the sample holder up. There should not be a gap visible between the lowest knurled nut and the fixture. Keep turning until you observe that the micrometer reading has changed by the target thickness. This raises the bottom limit of the sample holder’s motion, preventing the sample from being thinned below that thickness.
- Turn the middle knurled nut counter-clockwise, so that it locks the bottom knurled nut in position. Note that the micrometer reading may change, so it may be necessary to adjust both of the nuts to return the reading (and the sample holder) to the desired height (note that the mounting wax has some thickness).
- At this point, the fixture should be calibrated to a target thickness. Now you can remove the sample holder, but avoid shifting the bottom two knurled nuts.
- Heat the sample holder on a hotplate, and then apply the wax to its surface (note that the temperature is wax dependent). Ensure that enough wax is applied so that the entire sample will have wax between it and the chuck. Failure to do so may result in chipping during grinding, and excessive wax coverage should not cause major issues.
- Mount the sample holder back into the fixture, ensuring that it is firmly fixed (using the top knurled nut if necessary).
Mounting Samples using Carrier
As InP is rather delicate, especially after thinning, it is helpful to have the samples mounted on a (thicker, Si) carrier wafer as a way of handling them during and after thinning (particularly for the following metal deposition step). This process assumes that you have two mounting waxes of different mounting/removal temperatures:
- Set the hotplate to the higher of the two wax mounting temperatures, and place the carrier wafer on the hotplate.
- Apply the higher temperature wax to the carrier to the full area where you wish the samples to be placed (failure to fully cover the carrier-sample overlap with wax can lead to sample damage during lapping). The wax should melt nicely.
- Place the samples (substrate side up) on the carrier so that they are fully on the wax. Ensure that they are flat.
- Remove carrier with samples from the hotplate.
- Set the hotplate to the lower of the two wax mounting temperatures, and place the sample holder on the hotplate.
- Once the sample holder is sufficiently heated, you should be able to apply the lower temperature wax to its surface, and should do so.
- Place the carrier on the sample holder. Ensure that it is flat.
At this point, you can mount the sample holder in the fixture. Now, setting the fixture is trickier with a carrier, as setting the target thickness would have to take into consideration the thickness of the carrier and extra mounting wax. It may be simpler to “zero” the fixture to the height of all things mounted, and then calibrate to allow it to lower the sample a set distance (i.e. the amount the dials move between when the fixture is flat on the ground and when the fixture is raised, allowing the sample to be lowered).
Removing the samples
The samples can generally be removed with a combination of heat and acetone. It is important to develop a good technique to do so as removal is the step when the samples are most likely to break. Using a carrier wafer (as described above) can be helpful the carrier is easier to remove from the sample holder using heat (place the sample holder on the hot plate, and once sufficiently heated it should be possible to slowly slide the carrier off the sample holder without disturbing the samples), and then removing the samples from the carriers may be easier to do using chemical means.