Mathies:Drilling a Glass Microfluidic Wafer

Overview

Silicon at the backs of the wafers should be removed by RIE. The etched wafers must then be bonded to a backing wafer with pine resin (“optical polishing pitch”).

There are three common methods for drilling. These are, in order of complexity:

1. The manual drill press (good for wafers that have few holes and don't require precise drilling) 2. Use the CNC milling machine (good for 4" wafers with many holes and/or tight tolerances) 3. The machine shop (necessary for 6" wafers, not used for 4" wafers)

All three of these methods require that the etched wafer be resin-bonded to a blank float glass wafer, as described in Resin Bonding.

Resin Bonding

1. Wear gloves. The resin is, unsurprisingly, very sticky.

2. Turn on the hotplate to ~225°C, let heat for 15-20 min.

3. Put a piece of Al foil on the hotplate; put the patterned wafer on the foil face-up (Si/pattern side up).

4. Pour pine resin powder onto the wafer, let melt.

5. Get a clean backing wafer & place it on top of the patterned wafer.

6. The pine resin will form bubbles in between the wafers. Let the wafers sit on the hotplate until most of the bubbles are fairly large.

7. Move the Al foil and wafers carefully off the hotplate, then use the block of wood to press the wafers together (start at center), squeezing out the bubbles. It is especially important that no bubbles remain near where holes will be drilled.

• Put a piece of wood down on the counter by the hotplates, then move the Al foil and wafers onto the wood instead of directly onto the counter. The wafers will cool more slowly, allowing you more time to press bubbles out of the resin.
• If there are still bubbles in the pine resin after it has cooled and become very viscous, move the Al foil and wafers back to the hotplate and repeat the heat/press process.

8. Make sure the two wafers are aligned as closely as possible by pressing in on the edges with the Al foil, then allow the wafers/resin to finish cooling.

9. Peel off the Al foil, scrape off any excess resin, then clean the wafer with IPA and a technicloth. Make sure that the back side of the backing wafer is especially clean, as it needs to fit and seal to the vacuum chuck on the mill.

General notes on drilling

• When using the regular drillbits, clip the bit in half before putting it into the drill chuck. This will reduce flex in the bit while drilling.
• When using the CNC mill, remember to make a note of the number of holes drilled on the sheet beside the mill. If your drilling brings the total number of holes since last mill lubrication to ~500, it's your turn to re-lubricate the mill.
• Remember to put the blue lubrication liquid on your wafer before drilling.
• The bits will wear down with drilling, so there's a limited number of holes per bit that can be drilled.
• When drilling with the manual high-speed drillpress, note that the drillbits will wear faster and thus not last as long – approximately 15 holes/bit for the 1.5mm bits, and 5-6 holes/bit for the 2mm bits. The hole size will also decrease as you drill because of wear on the bit.

CNC Mill Documentation

The drilling procedure for the CNC mill is:

• Bond your wafer to a backing wafer with pine resin (“optical polishing pitch”).
• Make a DXF file with the pattern of holes to be drilled.
• Match the pattern in the DXF file to the actual alignment of the wafer on the chuck, then drill using the FlashCut program.
• Clean up.

Detailed notes on preparing the DXF file, the dxf2fgc conversion program, and drilling on the CNC mill are included in a separate PDF file.

For FlashCut2.0, http://www.ocf.berkeley.edu/%7Ewgrover/cnc.pdf

For FlashCut3.0, http://www.cchem.berkeley.edu/ramgrp/protocol/CNCOperation.pdf

Cleaning up

After the holes have been drilled in each wafer, the patterned wafers must be separated from the backing wafers and cleaned.

1. Place the resin-bonded wafers on Al foil on the hotplate (again, at ~225°C). Allow to heat for a few minutes, until the resin melts again.

2. Using a razor blade, carefully pry the two wafers apart. (Be careful – this step requires handling hot wafers.)

3. Set the wafers on Al foil on the counter until cool.

4. Clean wafers with acetone, then IPA, then DI water. (The acetone and IPA cleaning should be done under a fume hood; use a dish to catch the excess solvents.) Make sure that the acetone does not evaporate and leave residue on the chip, and also rinse well with the DI water. Blow dry with the N2 gun.

After this inital cleanup, the wafers require a final cleaning process before bonding.

Drilling Supplies

Extra pine resin is located on the shelves above the hotplates, in Lewis 306. Blank 100mm soda-lime glass wafers are in the drawer under the drybox labeled "New Glass Wafers".

Amplex drill bits can be ordered from: http://www.amplexabrasives.com/home/default.asp

Crystalite/Triple-Ripple drill bits are ordered from Abrasive Technology, at http://www.abrasive-tech.com

Contact

• Eric Chu 16:45, 15 July 2009 (PDT):