SOP for Thinning

Chemical/Mechanical Thinning

**SAFETY**: Bromine is extremely corrosive.
Bromine reacts explosively with acetone.

General preparation.
- Set Corning hotplate temperature to ~50^oC greater than adhesive flow temperature.
  - Flow Temperatures:
  - Crystalbond 509 (clear/amber/acetone soluable) : 121^oC.
  - Crystalbond 555 (white/hot water soluable) : 54^oC.
  - Hotplate Control Dial Setting (flow temp + ~50^oC)
  - Crystalbond 509 : ~1.75
  - Crystalbond 555 : ~1
- Sample and quartz disk should be clean: TCA - Acetone - Methanol - IPA.
- Measure and record sample thickness with digital dial micrometer apparatus.
- Clean the polisher's white teflon disk and firmly affix a tan felt polishing pad to it. Place the teflon disk/pad into the polisher's black plastic slurry cup, and mount the cup to the polisher under the reciprocating arm.
- In the bromine processing fume hood, using the bromine processing glassware already in the hood:
  - To a 50 ml beaker add:
  - 40 ml methanol.
  - 2 ml bromine.
  - Cover beaker with a small petri dish cover, inverted.
- For later dismounting from Crystalbond 555, in a 3 inch diameter petri dish place a 3 inch disk of filter paper. Fill the petri dish with DI water.
- For later dismounting from Crystalbond 509, in a 3 inch diameter petri dish place a 3 inch disk of filter paper. Fill the petri dish with acetone.
- Fill a methanol wash bottle and place it in the bromine processing fume hood.
- Place a timer in the bromine processing hood.
Affix sample to quartz disk. With hotplate at setpoint:
- Place quartz disk on hotplate.
- Melt adhesive on quartz disk. Cover entire suface, and wait until the adhesive is brought to hotplate temperature.
- Place sample on disk and into adhesive. Lightly press sample into adhesive. Wait until sample is brought to hotplate setpoint.
- Remove disk/sample from hotplate and set on a wiper. Do not turn off hotplate. Again press sample into adhesive carefully to eliminate air bubbles from the adhesive under the sample. Also attempt to make the adhesive under the sample thin and uniform. Thin adhesive bonds the sample to the disk better than thick adhesive. All of this must be done before the adhesive begins to cool and thicken.
- Remove excess adhesive. With the sample/adhesive/disk at room temperature, and using a wiper under your work to control adhesive debris, use a single edge razor blade to scrape away all of the adhesive on the quartz disk surrounding the affixed sample. Also remove any adhesive from the reverse side and edges of the disk.
Mount the disk/sample to the polisher's cylindrical white teflon disk carrier.
Measure and record the adhesive thickness. Zero the micrometer on the quartz disk, measure the sample + adhesive thickness, subtract the sample thickness recorded previously.

Target Thickness 100 um +/-20 um

Example for unmounted sample:
Sample Thickness = +600 um
Target Thickness = -100 um
Reduction        =  500 um
Thinning Rate    =  20 um/min. approximate
Thinning Time    =  25 min. approximate

When measuring the sample thickness during a thinning
process, the sample is mounted to the quartz disk with
adhesive. Therefore, the thickness of the adhesive must
be subtracted from the measurement:
	
Example for mounted sample:
Measured thickness      =    110 um
adhesive thickness      =   - 10 um
Actual sample thickness =    100 um

The thinning rate is widely variable due to changing current conditions such as the concentration of the bromine/methanol solution, the geometry of the sample edges, the speed of the polisher, and the roughness of the polishing pad. Therefore, the thinning is executed in repeated steps of (1) thin (2) measure (3) recalibrate etch rate.

Thin:
- Mount the carrier/disk/sample upside-down to the polisher under the reciprocating arm so that the end of the arm inserts into the dimple in the bottom of the carrier.
- Place the previously prepared petri dish/filter paper/DI water on the hotplate.
- Pour approximately 20 ml of the bromine/methanol solution into the polisher's slurry cup. Always re-cover the solution beaker with the petri cover.
- Set the polisher speed to 2 and press the start button.
- Start the timer.
- Slowly ramp up the speed to 4.

Monitor the initial thinning run for a sufficient amount of time to ensure the carrier doesn't tip-over, nor the sample detach from the quartz disk. Either of these may occur early in the thinning because the very sharp edges of the sample produce considerable friction at the polishing pad. If the polisher can be stopped quickly in the event, the sample might escape damage. The events become less of a concern after the process begins to round the sample edges.

During thinning, with a plastic pipet, add 1 or 2 ml of the bromine/methanol solution to the slurry cup every 5 minutes or so to help maintain its concentration.

It is normal for the polishing pad to become all or partially detached from the teflon disk at some point during thinning. This is not a problem. Do not replace the pad as it will continue to perform acceptably. However, as it lifts the edges will curl up which will form a cup, preventing the free flow of solution throughout the slurry cup. Add sufficient solution as necessary to ensure the solution doesn't get trapped in the pad cupping, leading to stale and weak solution being trapped at the sample.

Upon thinning for one-half of the previously calculated total thinning time:
- Stop the polisher by setting its speed to zero (off).
- Stop the timer.
- Remove the carrier from the cup using a blue plastic forcep gripped in the carrier dimple and around the carrier edge.
- Rinse the carrier/sample with methanol over a beaker dedicated to collecting the rinse.
- N₂ blow dry.
- Measure and calibrate:
  - Do not remove the sample from the carrier.
  - Zero the micrometer on the quartz disk.
  - Measure the sample + adhesive.
  - Calculate etch rate:
    [original sample/adhesive - current sample/adhesive] / thinning time
  - Calculate new approximate total etch time:
    [current sample/adhesive - target sample/adhesive] / new etch rate
Repeat the thinning sequence of (1) thin (2) measure (3) calibrate until the target sample thickness is acheived. Be careful not to over-thin. Counter-intuitively, as the sample becomes very thin it has sometimes been observed that the thinning rate accelerates even as the thinning solution is probably weaker. Its best to approach the target thickness very carefully towards the end of the process by performing frequent measurements and calibrations.

Upon reducing the sample to the target thickness:

Dismount and clean the sample:
- Remove the quartz disk from its carrier.
- Crystalbond 555: Place the disk/sample into the hot water contained in the petri dish/filter paper on the hot plate.
- Crystalbond 509: Place the disk/sample into the petri dish/filter paper of acetone.
- While waiting for the disk/sample wax to detach, clean the bromine hood processing area. Rinse all bromine contaminated equipment with methanol. Pour all the liquid bromine/methanol into the recovery bottle in the hood which is labelled for WASTE bromine/methanol solution. Leave all pipets and pads in the hood to evaporate the solution before disposing in a trash container. Turn off the micrometer, and clean it if necessary.
- Upon the sample detaching, carefully remove the sample from the solvent, edgewise because surface tension on a broad plane of the sample may be sufficient to break it.
- Place the sample on a wiper.
- Turn off the hotplate. Clean and return the petri dish.
- Clean the sample: TCA - Acetone - Methanol - IPA.