New Lapping Process

Laser Lapping Procedure

Written from NTS notes NTS, 9/29/2015

Note: Must have a laser sample top side processed and ready to lap.

Note: Everything from the process goes in the As trash.

1. Step 1 - SiC grit paper, 600 grit. Time = 5 minutes. Speed = 20. Force = 0 N. Softstop = On.

   1. Replace the grit paper after every 5 minutes. Check the sample thickness with the drop micrometer. Stop after sample reaches 150 microns thick.

2. Step 2 - TexMet C Lapping Pad. Time = 30 minutes. Speed = 20. Force = 5 N. Softstop = On.

   1. Place about 1 teaspoon worth of 1 um alumina solution on center of pad. (about 1 dime diameter).

   2. Lightly spread solution around with edge of glass slide.

   3. After 30 minutes, clean and check the finish of the sample. Keep doing 30 minute steps until the wafer is mirror-like with minor visible defects.

   4. Use the same TexMet C lapping pad for the entire step. Add a pea-sized drop of 1 um alumina if the pad surface is looking dried out

3. Step 3 - Microcloth Lapping Pad. Time = 30 minutes. Speed = 20. Force = 0 N. Softstop = On.

   1. Place about 1 teaspoon worth of 0.3 um alumina solution on center of pad. (about 1 dime diameter).

   2. Lightly spread solution around with edge of glass slide.

   3. After 30 minutes, clean and check the finish of the sample. Keep doing 30 minute steps until the wafer is mirror-like.

   4. Use the same Microcloth lapping pad for the entire step. Add a pea-sized drop of 0.3 um alumina if the pad surface is looking dried out

Prep

Cleave Glass Slide

1. Cut in half or thirds for samples

2. Make one straight, non-stop scribe, then break over scribe

Prep Crystal bond

1. Turn on hotplate to 5

2. Cleave small crystal bond (CB) piece: quarter of a pea / one grain of rice equivalent for a glass slide

   1. CB is hard when cold, can fracture and “’splode”

   2. CB is soft when hot

Prep lapping chuck

1. Put CB on lapping chuck, mostly centered

2. Put lapping chuck on hot plate to let CB melt

   1. Wait for it to melt

3. Put glass slide on CB, centered on chuck

4. Roll swab on slide a few times to make sure it’s super flat (roll out)

5. Remove chuck from hotplate with tongs and roll out slide a couple times

   1. Roll out periodically as it cools

6. Check centering as you go and always roll out right after a shift

Affix sample

1. Get CB for sample, want about 3-4 grains of sand equiv for a thumbnail sized sample

2. Put CB in middle of slide

   1. Might start to melt from residual heat

   2. If it’s not centered, move/scrape it over a little

3. Put second small piece off to side of slide as a melt test piece

   1. Far enough from center that it won’t touch sample when it melts

4. Put chuck on hotplate and wait for melt test to melt

5. Remove with tongs and roll out sample to flatten CB and sample

   1. Can also put glass slide on sample then roll out (idea being it might be more uniform pressure)

6. Remove melt test bead with razor blade

Drop Micrometer Use

1. Battery powered, turn ON and OFF after use

   1. Press ON once to turn ON

   2. Press again to Zero

   3. Press and hold to turn OFF

2. 16. Zero on glass slide

3. 17a. Check thickness uniformity of sample

4. 17b. Measure middle and four corners or sides

5. 17c. Want about within 5um across sample

6. 17d. Want measured number to be reasonable for the substrate (plus epi (small) and CB)

7. 18a. Will be used for checking thickness

8. 18b. Turn off til next use

Lapping Tool Prep

1. Choose bowl and grit paper for lapping tool

   1. Lapping tool only goes down to 600 grit which is the largest we can use

   2. But there are polishing cloths and particulate suspensions we can use

   3. We can use 600 grit paper down to 150 um thick

   4. Beyond that we use a cloth and particulate suspension to go down to ~110 um, then polish

Replace grit pad if needed

1. Grit pads are adhesive backed, peel off backing

   1. Minimize how much surface you touch

2. Apply grit pad to glass platen

   1. Grit pad is a bit smaller than glass platen so try to center

   2. Make it very flat

   3. Minimize how much surface you touch

3. Place glass platen back in bowl

   1. Glass platen is motched to indicate where it goes in bowl

   2. Using both thumbs, apply pressure on opposite edges of the platen to ensure it is secured and not lopsided.

4. Place bowl back on lapper

   1. Bowl has square bottom piece and mark on rim to indicate reload direction

5. Drip a ~silver dollar sized droplet of DI water into bowl near the center

Tool Parameters

1. Time: 30 sec to 99 min in 10 sec increments, or can do Run mode (time manually)

   1. Speed: 10 to 50 RPM in 5 RPM increments

   2. Force: 0 to 50 Newtons in 5 N increments (we use 0 N)

   3. Soft stop: Steps down force twice for final portions of run, see manual for details, we don’t use

   4. Polishing cloths are characterized by hardness and nap (also depends on particulates)

      1. The harder, the more uniform

      2. The longer the nap (fiber length), the better the polish (kind of)

      3. Particulates embed in cloths with time

Lapping

Note: When lapping do not cross contaminate, esp. from larger grit to smaller grit. Smaller to larger is not a deal breaker.

1. Attach chuck to lapping tool arm/pin

2. Start lapping, e.g. 5 min, 15 RPM, 0 force

   1. Can see wear pattern in pad in bowl

   2. Harold: occasionally give a quick squirt of DI water whenever I observe the slurry to be thick, dark grey and opaque. Don’t flood the sample with water or else the lapping rate will slow down. ?

   3. Harold: If the grit paper loosens from the glass platen, which is typically due to too much water, then stop the lapping and replace the grit paper immediately.

3. About five minutes is the time after which you should switch pads [5, 10, more?]

Check sample

1. Remove bowl so tip doesn’t get in gunk

2. Remove sample from bowl

3. Squirt sample with DI to remove gunk, water goes in special drain

4. Blow dry with N2

5. Check thickness on micrometer

   1. E.g. 5 min on 600 grit did ~9 um, but this is highly variable (see Harold’s work)

6. Repeat process until target thickness is achieved.

Rinse or Switch Pads

1. Squirt bowl and pad with DI, til most of the gunk is removed, water goes in special drain

2. Remove glass platen and pad from bowl

   1. Remove pad from glass platen and throw away pad

3. Harold: Pad can be reused for about ~10 minutes of lapping if you don’t mind slower lapping rates (and any other unknown issues)?

4. Squirt rinse out bowl

   1. Squirt rinse glass platen

5. Dry platen and bowl

6. See Steps 20-24 above

Old Lapping Process

Laser processing procedures:

Safety procedures

• make sure to use gloves when touching anything in hood, including bottles

  • check carboy under sink for fullness, swap out when full

  • All trash is As trash

Prepping chuck

• Zero out drop micrometer with center of bare chuck

  • If there is leftover crystal bond wax on the chuck, load it onto lapping and use scrap 600 grit paper to even out chuck

  • Put chuck onto hot plate, turn up to 5, put a small (1/4 pea) sized piece of xtal bond wax on the center of chuck, wait until it melts

  • use tweezers to load wafer piece (8 x 12) onto the chuck epi-down, smoosh and move the wafer piece around to evenly distribute the xtal bond wax over the bottom of the wafer

  • turn off hotplate and take wooden dowel roll across surface of wafer to ensure flatness. careful to not get xtal bond wax on the wafer surface. make sure wafer piece is centered on the chuck

  • transfer to aluminum "heat-sink" to cool down before checking on drop micrometer for flatness.

Lapping tool procedure

• make sure lapping tool is cleaned (avoid contaminating samples)

  • place glass backing plate on multiple cloth kimwipes (to soak up excess water)

  • load first SiC grit paper (see grit progression section) and wet with DI water

  • load sample into lapping tool by inserting the rod and tightening.

  • loosen side set-screw

  • set the lapping tool down so sample is touching grit paper.

  • turn the micrometer counter-clockwise until it extends above the lapping tool housing.

  • gently tighten the micrometer clockwise until you get resistance. this is the "zero" point of the sample.

  • lift the lapping tool off the grit paper and then turn the micrometer counter-clockwise by 1/3 of the 25 micron tick mark (~10 microns).

  • turn the locking nut at the top of the lapping tool clockwise until it is snug against the micrometer.

  • keep the side set-screw loose.

  • using figure 8s and applying NO pressure to lapping tool, slowly rotate the lapping tool around grit paper.

  • continue lapping in figure 8s.

  • every 100 microns, remove sample and check thickness on drop micrometer.

  • for the last 100 microns of milling, check sample on drop micrometer every ~25 microns.

  • when switching to a finer grit of paper, clean tool, chuck, glass plate, change out kimwipes, change gloves (anything that could have the larger grit on it).

• GaSb SiC paper standard

  • start with 600 grit paper for most of the lapping process.

  • for rough polish, allow for ~10-20 microns of thickness to remove with 1200 grit

  • for mirror polish, allow ~25-50 microns of thickness to remove with 1000 grit, ~10-20 microns to remove with 1200 grit, and then use alumina polish

• Laser thicknesses

  • optically pumped laser: 130 micron thickness

  • index-guided ridge laser: 110 micron thickness (the thinner the better, as it is easier to cleave facets. Do not go below 100 microns. The wafers are too fragile.)