Note: Always check CAR and cracker motors for PID vibrations.

What is the Junk growth, and why do it?

The junk growth is the first growth performed when bringing up a system. The philosophy is after opening up the system for maintenance (i.e. sources are cool and the chamber is at atmosphere), there is a a lot of "junk" that has landed and built up on, among other things, the source material, cells, and cell shutters. The junk growth is a chance to heat all of these up past their typical operating temperature to blow off all junk and to test whether the cells are operating within an acceptable range.

The growth itself has the wafer (full 3" Si (100)) facing the sources. The shutters on the sources are left open while the cells heat up. Once the cells reach the junk growth temperatures, a series of shutter cycling occur. Cycling heats up and cleans the backside of the shutters using the cell's heat. A group V needs to always be on the shutter during growth; otherwise, the sample becomes drippy with molten group 3 material.

Substrate rotation is not used because the orientation of the wafer needs to be preserved. Problems with the shutters and sources can be seen at the different regions of the wafer -- this can be a potential troubleshooting tool is something goes abnormal happens during the growth.

In addition to monitoring AMBER and the rest of the electronics during the growth, also watch the actual shutters and listen to them when actuating during the cycling portion of the recipe. This is a good time to figure out if there are any shutter problems.

Day before junk growth

1. Load 2 full 3" Si (100) wafers and 4 1/4" SI GaAs wafers onto the trolley. The Si wafers will be used for the pre- and post-melting of aluminum. The 1/4 GaAs pieces with be used for the Std. quantum well and superlattice (SL) growths both pre- and post-melting of aluminum

   1. Work with an older student if you are unfamiliar with the loading process

   2. The 3" Si wafers need another 3" Si baking wafer

   3. The 1/4 SI GaAs pieces need sapphire backing wafers

   4. After loading samples back into the LC, perform a LC bake (standard recipe on growth computer in C:\AMBER\recipes)

2. There is no STO (source turn on) for the junk growth

3. Check your recipe with the older students. Sometimes junk growths temperatures and the number of shutter cycles change, e.g. replacing a shutter likely will require more shutter cycling to check its performance.

4. The junk growth takes most of the day to complete. You need to stay in the lab (or have another student monitoring the machine) during the shuttering section. You will not need to unload the trolley at the end of the day (if there are no problems).

Growth Day

Recipes

Below are examples of previous junk growth recipes. Check with older students for the most recent and correct recipe to run as the junk growth parameters change over time, based on the status of the machine...

1. Junk growth without aluminum

   1. http://lase.mer.utexas.edu/share/Stephen/amber_log_files/junk_and_qualification_growths/junk%20growth%20no%20Al%20Recipe%20--%20best%20recipe.txt

2. Junk growth with aluminum

   1. Not actually a junk growth, do an outgassing recipe

   2. If there's a power failure but Al stays melted, you need to do another junk growth with Al (rather than outgassing)

      1. http://lase.mer.utexas.edu/share/Stephen/amber_log_files/recipes/B160912A%20-%20Bravo%20junk%20growth%20with%20Al%20after%20unplanned%20power%20outage%20Recipe.txt

Bake Si Wafer on heater station

1. Move Trolley into buffer chamber (after LC lamp bake)

2. Move Si wafer onto heater station (HTS)

3. Move trolley to Echo buffer or to the far side of the Bravo BC (i.e. closest to the LC gate valve). The Si wafer is unclean and will outgas crud back onto the trolley if the trolley is not moved.

4. End and Save RGA scans on GC and BC, if applicable. Ask the older students before stopping the RGA scans, because they may be monitoring them.

   1. Save scans (use the YYYY-MM-DD convention followed by a brief description of the scan)

   2. Turn off electron multiplier then filaments

   3. Disconnect the COM in the software

   4. Turn off (i.e. flip switch) on the actual RGA head/box

   5. If needed, unplug the serial cable to the RGA head/box then unplug the power cord

   6. Follow the steps above in reverse for turning the RGA back on

5. Leave chamber ion gauges (IGs) on

6. In the AMBER config menu, adjust the max temp set point of DD Be from 400 --> 850

   1. We de not intend to use the DD Be cell for anything, so we intentionally keep the max temp low

   2. To properly clean out the cells, we change the max temp before starting the junk growth, because you cannot adjust AMBER config settings while a recipe is running

   3. Once the recipe is finished at the end of the growth day, we need to change the AMBER DD Be config max temp back from 850 --> 400

7. Compile and run the junk growth recipe

   1. Recipe has heater station (HTS) bake, waitop command, cell temperature and ramp rates

   2. HTS bake is 450 C for 1 hour

   3. Make sure all viewport lights on the BC are turned off when starting a HTS bake

Prep for Growth

1. Wait for HTS to drop below 250 C before transferring wafer of the HTS and back onto the trolley

2. Turn off the Beam Flux IG

3. Transfer wafer to GC (CAR) after HTS bake

   1. CAR must be in 'local' and the 'transfer' position. The CAR rotation must be in 'local' and the rotation turned off

   2. Set CAR temp Eurotherm to 'manual' with the output power at ~5-7% for transfer

   3. Open GC to BC after HTS is under 200 C to prevent outgassing into the GC from the BC

      1. Open GC gate valve (GV) first, then GC shutter (fully open)

      2. Reverse order to close after loading

      3. Always close the shutter!

   4. Remove RHEED block first and put it onto the trolley.

   5. Transfer wafer in

   6. Set CAR temp Eurotherm to 'auto', which will bring the substrate temp back to 200 C

4. Rotate CAR to growth position

   1. Flip switch to CAR position, should be at 'transfer' or ~352 degrees

   2. Put in 'manual' (from 'local') and rotate it (by hand) down to under 5

      1. Do not tighten the set screw on the CAR rotation knob! Leave it loose

   3. Put into 'local' and hit 'growth' to send it to the growth position or 0 degrees

5. There is no away bake for Junk Growth

6. The CAR temperature is 200 C (idle) during the entire junk growth

   1. You don't want to heat the wafer and redeposit the junk all over the rest of the system

7. Leave the HVP on at the idle value of 105 Watts

8. There is NO rotation for junk growths (so we can see if there are any issues with cells and shutters that can be debugged with nonuniformities on the wafer)

9. Start the RGA scan if needed. Leave the shutter closed

10. Make sure all GC viewports shutters are closed

Junk Growth

1. Continue the recipe by finishing the 'Starting junk growth' waitop command

2. The recipe will heat up the cells with shutters open

   1. Check sublimator power/temp mode, and double check cell set points and ramp rates

   2. Adjust the trend charts so you can see all the cell temps at once

   3. NOTE: just because the cell temperature has flatlined, it may be at the wrong temp!!!

      1. Check that the cells are reaching the desired temp

      2. There may be max temp or power setting that needs to be adjusted

3. After the cells heat up, there is a waitop command to re-home the cracker valves

   1. Reopen the Cracker valves to 250 before continuing the waitop

   2. Note: Don’t close Group V cell shutters when the Group V cell needle valve is open.

   3. Keeping a Group V overpressure is necessary to prevent an accumulation of molten Group III on the wafer.

   4. The recipe will keep either As or Sb open at all times

4. Recipe will cycle cell shutters, closes effusion cells, and run source turn off

   1. Listen to and watch the shutters (in AMBER, on the switch panel, and the actual shutter rods) to identify any malfunctioning shutters

5. After the source turn off starts, there is a waitop command to re-home the cracker valves

   1. 'Remote' --> 'Local'

   2. Hold down 'Local' and 'Home' simultaneously

   3. You will be prompted with 'Do you want to do manual calibration?' --> 'No'

   4. 'Test full travel' --> 'No'

   5. Set back to 'Remote'

6. After re-homing the cracker valves and hitting continue on the recipe, the As, Sb shutters will close

7. Finish procedure by removing Junk growth wafer from GC

8. Change the max temp set point of DD Be back to its lower value, i.e. change from 850 --> 400 in the AMBER config menu

   1. Prepare system for the standard QW and SL growths. Assuming you also loaded the 1/4" GaAs pieces for the QW and SL growths, you do not need to unload/reload any wafers at the end of the current growth day

Growth Parameters

• From Wiki page System_Bravo_Junk_Growth_Temps

• NOTE: the temps below change over time!!!!

  • We adjust a temp up if more of a particular source is needed. See the last junk growth recipe and discuss what temp is best with the older students.

  • As described in the "What is junk growth and why do it?" section, we heat the cells past their warmest operating temperatures during growth. Determine what the hottest temp is during growth for each cell by talking to all MBE growers in the group, then increase the junk growth temp by ~50-100 C.

  • Adjusting the max temp might require you to adjust the max temp setpoints on the system (Sorensens, Eurotherms, AMBER). Ask an older student how to check these if you decide to increase the max temp from the last junk growth.