Mathies:RIE

RIE silicon etch in ptherm (for thin Si films)

General

Si etching is usually done with SF6 plasma in the ptherm, a RIE system. The technics-c etching system can also be used.

Notes:

• The chiller unit integrates to the RIE and no longer needs to turn on separately.
• Do not try to lift the lid while the chamber is under vacuum/below atmospheric pressure. This can break the lift arm.
• To raise or lower the chamber lid, press & hold both buttons on the left control panel, then flip the switch to UP or DOWN as appropriate. Hold the buttons (and switch, if necessary) until the lid is all the way up or down.

Wafer preparation

• If the wafers have not already been marked with a # for identification, do this before taking the wafers to ptherm for etching. The standard numbering system is *date* - *wafer #*, plus your initials. Carefully carve the ID # into the photoresist on the front of the wafer with the edge of a razor blade, then blow off the cracked PR debris with the nitrogen gun.
• Before putting wafers into the ptherm, put blue Disco tape on the backs of the wafers. The blue tape prevents the Si on the backs of the wafers from being etched off.

o Lay out a sheet of blue tape, then lay the wafers on top of the tape. Trim around the edges with a razor blade, then use the back of the blade to squeeze the bubbles out from under the tape (moving from center to edge). Blue-taping trick: instead of laying the wafer down flat on the tape, hang one corner of the tape over the edge of the glass plate, then gently slide the wafer over the edge, pressing the tape between the edge of the glass and the back of the wafer. o Thin wafers (0.55mm and thinner) can easily break during this step, so be particularly careful when handling the wafers and smoothing the bubbles out of the tape.

Before Silicon etch

Note: The booster pump comes on automatically when the chamber reaches 1 torr. The gas lines are labeled, with O2 being on Channel 1 on the flow controller and SF6 on Channel 3. The standby state of the ptherm system should be:

• Chamber under vacuum
• Vacuum breaker switched OFF
• On the power control box, AC LINE and RF POWER should be OFF, POWER ADJUST dial set at zero.

Chamber cleaning and Silicon Etching

It's a good idea to do a chamber clean before etching, especially before stripping the Si layers off of the wafers prior to drilling and bonding. Otherwise, a thin-film residue will sometimes remain on the wafer surface. Recipe:

   * Set O2 flow to 20%, SF6 flow to 60%
* 5-10 min @ 300W


RIE etch of a silicon hard mask layer:

   * O2 ashing (100% flow): 1 min @ 100-150W
* SF6 silicon etch (100% flow): 1.5 min @ 200W


These times can be adjusted if necessary - for example, if S1805 is used as a masking layer, the both steps should be shortened (to ~15 sec for the O2 ashing and 30-45 sec for the SF6 etch). Information can be written on a bare Si surface (backside of the wafer only) with Sharpie, and if the etch times are shortened, (to ~20 sec for the O2 and ~1 min 15 sec for the SF6) the Si under the ink will remain.

General procedure notes

1. To vent the chamber, enable the tool, turn off the vacuum (green button) and turn on the vent valve (red button). Wait for the pressure to come up to atmospheric (~760 torr) before opening the chamber lid. 2. Open the chamber by holding down both yellow HOIST buttons (top left control panel) and holding the hoist switch to UP until the hydraulic arm has moved the chamber lid to its highest position. 3. Arrange your wafer(s) on the chamber floor. The ptherm can hold four 4" wafers or one 6" wafer per run. 4. Close the vacuum chamber. (Hold down both yellow HOIST buttons and flip the hoist switch to DOWN.) 5. Press VENT again to close the vent valve, then VACUUM to open the vacuum valve and begin pumpdown. Wait for the pressure to decrease below ~50 mtorr. 6. Switch the valve for the desired gas ON on the gas control panel, then use the touchscreen to turn on the gas flow.

• From the main screen, press SELECT, then select the desired CHANNEL (1 for O2, 3 for SF6), then press SET, then ENTER. Make sure the set flow value is 100%, then press START.

7. On the power control box, switch AC LINE and RF POWER to ON, then turn up the POWER ADJUST dial to the desired wattage. When finished, turn the RF POWER switch to OFF first, then turn down the POWER ADJUST dial.

• Make sure to check the Reverse Power reading. The Reverse Power should be no more than 10-15% of the Forward Power. (Note that with the SF6 plasma, the Reverse Power tends to be very high for the first several seconds, but then decreases quickly.)

8. Turn off the gas flow at the mass flow controller (press OFF, then ENTER), then switch the gas valve OFF at the gas control panel. 9. When you are finished with the etch, purge the chamber and then vent it (as described at the beginning), then lift the chamber lid and remove your wafer(s).

• There is no longer a separate N2 purge line. To clear residual SF6 out of the chamber, turn the VACUUM valve off, then quickly open and close the VENT valve, then press VACUUM again to pump down the chamber again. Alternately, select N2 on the MFC (gas #1, toggle switch up), set the flow to 100%, and allow the gas to flow for a minute or so.

10. Close and pump down the chamber, then leave the system in the appropriate standby state and disable the tool.

Alternate Si etch: technics-c

If the ptherm RIE equipment is down, you can also use technics-c (in the VLSI room) to do a plasma etch of the Si layer.

Printed operating instructions and recipe information for technics-c are located on the lab bench beside the equipment.

Clean the chamber prior to use with an O2 scourge.

Recipe 1: Place your wafers in the chamber, run an O2 descum for 1 min, then use the Nitride etch recipe to etch the a-Si layer. The etch rate is somewhat variable, but the 2000A a-Si layer will be etched away in 3 - 3.5 min.

Recipe 2: Run an O2 descum for 30-45sec @ 150W. Then etch a-Si for 1-1.5 min @ 180W.

General procedure notes

1. Before venting the chamber, make sure the solenoid is closed. Enable the tool, turn on the vent. Wait for a few second to let the chamber vent. The big crack on the lid signifies atmospheric pressure.

2. Grab the handle to open the chamber. The lid will stay standing with a 90degree position. (Be careful! The lid is heavy.)

3. Make sure the plate is not hot. It may liquidize the photoresist. Put your wafers on the slots. It fits four 4" wafers.

4. Close the chamber slowly. Turn on the solenoid. Vent should be open before turning on the solenoid.

5. Wait for 5 seconds to pump the chamber. Then close the vent. Let the chamber vacuum reach ~30mTorr (0.030Torr).

6. Turn the gas INLET to O2 and turn on channel (O2) on the PD module. The green light will go on. Then open the GAS #1 switch on the PE module. The pressure will rise as gas flows in, and then stabilize.

7. Once flow into the chamber is stable. Strike the plasma by switching the power toggle on and turning the dial clockwise until the desired power is reached. You can see the plasma through the window on the front of the chamber. Begin timing your run.

8. Once the run is complete, turn off the power first. Turn off the gas on the PD module first, and let the vacuum pump pump out the gas line for a few seconds. Then close off the GAS #1 switch and allow the chamber to pump down to ~30 mtorr.

9. You may switch to SF6 (channel 2) now. No need to purge. The process is the same as O2 plasma. But both SF6 and He should be turned on on the PD module. And open the GAS #1 switch on the PE module. All gases come in from the same line.

10. To vent the system. Turn off the vacuum pump by switching the SOLN toggle to the closed position. Then vent the chamber by switching the VENT toggle on.

11. The chamber pressure will now come up to atmosphere and you may remove your sample.

12. Close and pump down the chamber, then leave the system in the standby state following the steps 4 and 5. Reset the system for standard O2 use. You may disable technics-c now.