Fused Silica bonding protocol - by fjwuxn

VIEWS: 20 PAGES: 11

									Wafer prep

  1. Select a wafer from the canister and a sacrificial glass plate. Make sure the plate is
     clean (wipe down with IPA). Handle the wafer using gloves. The features of the
     wafer are protected in a layer of photoresist.

  2. Cover hotplate with aluminum foil. Set to about 90 degrees. Place glass plate onto
     the hotplate.

  3. Place aqua bond 55 puck in the center of the glass plate. Let sit until a small
     amount of the bonder melts and pools below the puck. Then remove the puck. Let
     the pool of the melted bonder settle a bit. Pool should be about 3 inches in
     diameter and free of bubbles.

  4. Hold the wafer, features side down above the plate. Orient the wafer such that the
     flat edge of the wafer is pointed towards one of the corners of the glass plate. This
     ensures that the wafer features are angled about 45 degrees from the Sherline
     orientation.

  5. Carefully drop the wafer onto the melted bonder in the center of the glass plate -
     features side down. Try to avoid trapping bubbles between the plate and the
     wafer. Allow the wafer to settle on the top of the melted bonder. The bonder
     should start to ooze towards the outside of the wafer. Make sure the wafer doesn't
     float away from the center of the glass plate.

  6. Place the large Teflon block on the back of the wafer. The added weight alone
     should help push more of the bonder outward between the wafer and the glas. Be
     careful to not allow the wafer to move laterally on the glass since at high
     temperatures, the protective layer of photoresist may not be as effective. If
     necessary you can carefully push downward on the block to push more of the
     bonder and any bubbles outward.

  7. Once the wafer looks properly bonded with a thin (transparent or very close to
     transparent) virtually bubble free layer of bonder between the wafer and the glass
     plate, remove the teflon block and turn off the hotplate. Monitor the wafer as it
     cools to make sure it doesn't move laterally on the plate.

  8. Allow the bonded wafer to reach room temperature and sit for at least an hour so
     the bonder can continue to harden before drilling.




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Pre-modrilla Setup                                                           V3.1

10/22/09
   1. Open CNC Sherline software (inches version) on the desktop of the CNC
       computer. Make sure it is in manual movement mode.

   2. Turn on the mill by changing the switch on the side of the computer to the on
      position. The stepper motors on the mill may hum a bit. This is ordinary.

              NB: Do not turn the mill on before opening the CNC Software (Step 1
              above).

   3. Mount wafer-glass plate assembly onto the mill stage and clamp down using the
      clips in 4 corners.

              NB: Make sure the “street” pattern on the wafer is at about a 45º
              angle in reference to the XY direction of the mill.

   4. Insert the collet and drill bit (do not use a chuck) into the mill without over
      tightening.

   5. Move the stage using the CNC software to a position with the drill bit above an
      unused part of the wafer.

              NB: Take note of the X and Y positions and do not change this
              position until step 15.

   6. Place a drop of water onto the wafer directly under the bit.

   7. Turn on the drill to the highest speed.

   8. Lower the drill slowly (-Z button in the CNC software) while closely watching
      the space between the bit and the wafer. Once the bit is inside the water drop,
      lower the advance speed to very close to the smallest possible value) Drill a small
      test hole in the surface of the wafer (it doesn’t have to go very deep).

   9. Raise the drill above the wafer and turn off the drill.

   10. Raise the drill roughly to mark labeled focus on the Z travel.



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           NB: Mill will stop automatically and make buzzing noise if the
           extreme end is reached. This is not damaging, but it the Z readings
           will continue to change even though the mill isn’t moving.

11. Remove drill bit and collet.

12. Screw adapter onto the mill, insert and tighten centerscope.

13. Adjust Z such that test hole at the surface of the wafer is in focus on the
    centerscope.

14. Adjust the centerscope dial to best center the test hole in the field of view. Note
    the position of the test hole in relation to the dotted line in the field of view. You
    may have to turn the centerscope in the mill 90 degrees to best find the center.

15. Zero the Z value on the mill.

16. Move the mill stage to the first Modrilla landmark position so that the mark is in
    the same position in the field of view as the test hole in step 14.

17. Zero the X and Y values on the mill.

18. Repeat step 16 for the second landmark position. Record X and Y values for
    Modrilla.




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Modrilla Setup

  1. Open Modrilla software. Although the software is written in Java and should be
     platform independent, it doesn’t seem to display correctly in platforms other than
     windows (fields are often inaccessible).

  2. Make the following 2 files:

         i. Hole Positions only (no drilling instructions) ie “pos.ngc”

               Setup:
                    Alignment Mode:          Drill bit alignment
                    Input:                   DXF
                    Input file units:        Check DXF file (mm for T-mixer
                                                             µm for the serpentine)
                     Output:                 G-code
               Mill setup:
                     Mill units:           inches
               DXF marks:
                     First alignment mark: Check DXF file
                                           (30, 50 for T-mixer works well)
               Mill marks:
                     First alignment:      0, 0
                     Second alignment mark: Use coordinates from step 18 of Pre-
               modrilla setup.
               Drill nick:
                     Z:                    0
               Options:
                     Mill Z speed units: 0.1
                     Glass Thickness:      1
                     Overdrill             0
                     Peck distance:        100
                     Non-drilling speed: 10
                     Repeats per peck:     1
               input file:                 full mask.dxf (for T-mixer)
               NB: Make sure to check the output .ngc file to be sure it was
               computed correctly. Also rename the file before making the next
               one.

               Open the file in an editing program and remove all the lines that move
               the drill up 0.200” (G0 Z: +0.200)

               Rename the file as pos.ngc.




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       ii. Drilling instructions for every hole position ie. holes.ngc (changes from
           previous in bold)

             Setup:
                  Alignment Mode:          Drill bit alignment
                  Input:                   DXF
                  Input file units:        Check DXF file (mm for T-mixer
                                                           µm for the serpentine)
                   Output:                 G-code
             Mill setup:
                   Mill units:          inches
             DXF marks:
             First alignment mark:      Check DXF file
             (30, 50 for T-mixer works well)
             Mill marks:
                   First alignment:     0, 0
                   Second alignment mark: Use coordinates from step 18 of Pre-
             modrilla setup.
             Drill nick:
                   Z:                   0
             Options:
                   Mill Z speed units: 0.015
                   Glass Thickness:     1000
             (500 µm wafer + ~10 µm photoresist and bonder + 360 µm (.014”)
             from paper and extra space)
                   Overdrill            0
                   Peck distance:       25
                   Non-drilling speed: 10
                   Repeats per peck:    2
             input file:                full mask.dxf (for T-mixer)
             NB: Make sure to check the output .ngc file to be sure it was
             computed correctly. Also rename the file before making the next
             one.

3. From the file output in step 2.ii and renamed to all.ngc edit and create the
   following 2 files.

       i. Drilling instructions for the first 2 or 4 hole positions only. ie first2.ncg

       ii. Drilling instructions for all the rest of the hole positions. ie rest.ncg

               i. It may also be a good idea to split this file into smaller files. Eg.
                  3rds




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  4. Save all files to a USB Flash drive. Make sure all have filenames no more 8.3
     characters long.




Drilling steps

  1. Save all the g-code files to the mill computer (ie the g-code folder on the
     desktop).

  2. Set the CNC mill software to auto movement mode

  3. Load and verify the positions (pos.ngc) file.

  4. Step through each drilling position step, checking the hole position through the
     scope to make sure they’re correctly lined up and no hole is skipped. NB. Make
     sure the Z value has been zeroed correctly from step 15 of the pre-modrilla setup.)

  5. Load and verify the first holes file (first2.ngc). Run the program (the mill should
     go through the entire drilling process far above the surface of the wafer). Leave
     the stage at this XY coordinate.

  6. Move the drill up to the focus line on the Z travel.

  7. Remove the centerscope, install the collet and bit.

  8. Find Z0

         i. Move the drill to a position directly above the wafer away from any
            features.

         ii. Slowly, in very very small increments lower the drill to just above the
             surface.

         iii. Place a fresh piece of copy paper (1 sheet of 20lbs paper is .004”) between
              the bit and the wafer surface.

         iv. Set the software from continuous to incremental movement and move the
             drill down (-Z) in .0001 - .001 increments until the paper doesn’t move
             freely under the bit. Be very very careful not to crash the bit. DO NOT
             SET THE INCREMENT GREATER THAN .001 INCHES!



                                           6
          v. Back off (move in the +Z direction .010” from the surface

                  a. Note Z value.

                  b. Zero Z

  9.   Raise the drill back above the wafer (increase Z value)

  10. Place a drop of water at the positions of the holes in the first holes program on the
      surface of the wafer.

  11. Turn on the drill to the fastest setting.

  12. Reload, verify, and run the first hole file.

  13. Turn off the drill.

  14. Raise drill to a higher position (Z+ direction). Remove water from wafer surface.

  15. Using the mirror tool and a headband mounted magnifier, verify the hole
      positions. Assuming everything is aligned…

  16. Cover the surface of the wafer with water. The mill stage provides a well so that
      the water won’t drain into the mill.

  17. Load and verify the next (more extensive) drilling .ncg file

  18. Set the drill speed to the highest value

  19. Run the program.

Post Drilling

  1. Rinse the glass plate and wafer thoroughly under DI water.

  2. Attach the syringe needle assembly to the DI water tap and tighten the hose
     clamps. Rinse each hole with high pressure DI water.

  3. Soak the plate and wafer in soapy (one capful of Micro-90 or alconox) DI water.
     Bring all into the cleanroom. Leave for at least an hour.



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4. Fill a large glass container with about 2-3 inches of Kwik Stip photoresist
   remover. Cover and heat the solution to 60 C.

5. Place a few pieces of scrap teflon in the container and immerse the plate and
   bonded wafer in the solution on top of the teflon pieces such that the glass plate is
   held above the bottom of the container and the wafer is free to fall to the bottom
   of the container once it is released.

6. Cover the container and monitor occasionally. It may take a few hours for the
   wafer to be released and all of the photoresist and bonder to be dissolved.

7. Preheat the vacuum oven. Set to 120 C.

8. After the wafer has been released, first remove the glass plate from the container,
   and rinse it in DI water. Then remove the wafer and rinse in DI water and soak in
   warm micro-90 solution.

       a. Note, after allowing the Kwik Stip to reach room temp, even though it is
          discolored, it is still usable a number of times. It should therefore be kept
          in a container and used again. Only after a few uses should it properly be
          disposed in the same manner as other solvent waste.

9. Attach and tighten the syringe needle assembly and carefully rinse each hole
   under high-pressure spray. Start with the back-side of the wafer and then repeat
   with the wafer facing features up. Be especially careful not to direct the spray
   onto the features.

10. Dry the wafer using the dry nitrogen gun.

11. Inspect the wafer (holes and features) with the microscope. Take note of any grit
    around the holes.

12. Measure feature depths using the Dektak. Take note of at least a good sample
    of chip depths. For especially deep chips (30+ µm) make sure that the Dektak
    is set to measure valleys only and not hills and valleys.

13. Assemble a disposable tray out of tinfoil and place the wafer in the tray and the
    tray inside the vacuum oven.

14. Pump down the vacuum oven and let the wafer dehydrate for at least one hour
    (longer is better).




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  15. Remove wafer and let cool in tray on a table for a couple minutes. Don’t let it
      reach room temp however.

  16. With the dry nitrogen gun, blow under as high a pressure as possible through each
      hole from both the front and back thoroughly trying to remove as much grit as
      possible.

  17. Inspect holes under microscope and look for grit. Compare with earlier
      inspection.

  18. It may be necessary to repeat these steps again, starting with step 8. If the wafer
      passes inspection...

Pre Bonding

  1. Don protective clothing and equipment (vinyl smock, gloves, goggles, facemask).
     Prepare the following three solutions in the largest Pyrex beakers, each placed on
     its own hotplate.

         a. 1500 ml sulfuric acid. Heat to 70 C (This will be turned into Piranha
            solution in a later step)

         b. 1500 ml H20, 300 ml HCl. Heat to 65 C (This will be RCA 2)

         c. 1500 ml H20, 300 ml NH4OH. Heat to 75 C (RCA 1)

  2. Place the wafers in the wafer boat (holder) features side facing forward (away
     from the handle). Place the 170 µm coverslips in the boat including one extra also
     good side forward (the back sides of each coverslip should be scored with the
     letters F0. If the letters read reversed, then the wafer is good side up).

  3. Add 500 ml of H2O2 to the heated sulfuric acid. Allow bubbling to subside a little.
     Temperature can shoot up very high so make sure to use a thermometer capable of
     measuring temperatures greater than 200 C.Carefully lower the boat into the
     piranha solution. Soak for at least a ½ hour (1-2 hours preferred). Monitor
     temperature occasionally.

  4. Remove the boat from the Piranha solution and rinse in DI water for 5 minutes.
     Make sure to rinse the handle as well.




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 5. Add 300 ml H2O2 to the RCA 2 solution. Turn off heat (RCA 2 is self heating).
    Remove the boat from the DI bath and carefully lower the boat into the solution.
    Let the wafers soak for 10 minutes.

 6. Remove boat and rinse in DI for 5 minutes.

 7. Add 300 ml H2O2 to the RCA 1 solution. Let the solution start bubbling
    vigorously and then remove the boat from the DI bath and carefully lower the
    boat into the solution. Let the wafers soak for 22 minutes.

 8. Remove boat and rinse in DI for 5 minutes. Let sit in DI bath following rinse.

 9. Carefully move the solutions out of the way in the hood and thoroughly clean the
    workspace.

Bonding

 1. Change gloves. Don facemask.

 2. Bring supplies to workspace (~20-30 cleanroom wipes, extra gloves, glass plate,
    bonding only pair of tweezers (labeled BO)). Don a second pair of gloves.

 3. Place a wafer features side up on top of a stack of 3-4+ cleanroom wipes and dry
    the wafer with the N2 gun. Make sure to get the back as well but don’t let the
    features side come in contact with anything. The wafer must be completely dry so
    it is best to time this process and go for no less than 5 entire minutes.

 4. Place a single cleanroom wipe on top of the glass plate and then place the dried
    wafer features side up on top of the wipe.

 5. Change the outer pair of gloves.

 6. Place a coverslip good side up on top of a stack of 3-4+ fresh cleanroom wipes
    and dry the wafer with the N2 gun. Make sure to get the back as well but don’t let
    the good side come in contact with anything. The wafer must be completely dry
    so it is best to time this process and go for no less than 5 entire minutes.

 7. Go over both the coverslip and the wafer one last time with the N2 gun incase any
    dust settled on either surface.




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  8. Carefully pick up the coverslip clasping it by its edge (do not touch the surface)
     and hold it good side down a cm above the wafer. Orient the coverslip so that the
     flat edge is lined up with the flat of the wafer.

  9. Carefully drop the coverslip onto the wafer and allow the coverslip to settle onto
     the wafer. At this point any downward force could start bonding so be very
     careful using the tweezers to nudge the coverslip if necessary into alignment.

  10. Once the coverslip is properly aligned with the wafer, carefully push down with
      one finger in the center of the wafer. One should see a bonding front begin to
      radiate outward if looking down directly at the wafer. If necessary, help the
      bonding front by applying additional pressure in other positions around the wafer.

Annealing

  1. Bake for 2 hours at 1100 C (Program is: ramp 20 800C hold, ramp to 1100C hold,
     return to room temp.)




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