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Prepping Si Wafers

Prepping Si Wafers

Last edited by ESW 05/08/2018

Introduction

The Si(111) wafers currently in use for Bi growths are stored in the left-hand desiccator in the batcave (same area as the Si backing wafers). Please do not confuse them with the Si backing wafers, which are (100)-oriented. They are currently not tracked by the database.

Si(111) wafer specs:

  • Supplier: Nova Wafers

  • 3" wafers, 380 +/- 25 um

  • Intrinsic

  • Single side polished

Note: Sometimes it is advantageous to grow on a more conductive Si (111) substrate (for example, for STM measurements). Be careful if you later use this wafer for electrical measurements (especially for Bi thin films), and generally while ordering wafers. A 30 nm Bi film has a resistivity of ~1E-4 Ohm-cm, putting it's sheet resistance at ~3E+3 Ohm. Make sure to use a Si (111) substrate with a resistance at least 2 orders of magnitude higher than that of the film you're trying to measure.

Note #2: The HF dip procedure is necessary because neither of our current systems have a substrate heater sufficient for deoxing Si (which requires a temperature of ~1200 C). Our CARs can go up to a thermocouple temperature of 990 C, which is equivalent to a pyro temperature of ~765 C. A CAR with an upgraded substrate heater that may be able to reach this temperature will allegedly arrive by the end of 2016, hopefully making the procedure obsolete. (as of 2018, the procedure is not yet obsolete :) ).

HF Dip

  1. This is easier with 2 people - otherwise you need to abandon cleanup of the acid bench while you get the wafers under vacuum. Depending on the previous grower, removing the faceplates and backing wafers can take more time than the etch. The time the etched wafers are exposed to atmosphere should be minimized as much as possible in order to prevent re-oxidation and general contamination of the passivated Si (111).

P1

  1. Use the acid bench for undoped Si (closest to the SRD spinners). We typically use the equipment that comes with the bench, with tweezers reserved for Si. Never pour HF into a glass beaker. Use the plastic tank at the acid bench for HF.

  2. Load 3" wafer cassette/boat holder (metal) into SRD spinner. Use long metal tool on the wafer holder shelf to remove screws. Do not drop screws into SRD spinner.

  3. Put on acid bench PPE

  4. Clean the acid bench if necessary

  5. Load wafers into acid bench wafer cassette

  6. Mix 15:1 HF dip

    1. Add 1800 mL of DI water to HF tank

    2. Add 120 mL of 49% HF to the tank

  7. Dip wafer cassette with Si(111) wafers into HF for ~ 15 seconds.

  8. Spray water (with spray gun, gently) onto wafers to check that they are hydrophobic - indicative of oxide removal. Repeat as needed.

  9. Fill quick dump rinser (QDR) with DI water

  10. Remove wafers and place into QDR

  11. Run rinse process for 5 times

  12. Remove rinsed wafers and hand off to P2.

  13. Clean acid bench and put tools away.

P2

  1. Sign into the SRD spinner

  2. Unload the MBE following the typical procedure

  3. Prepare each block that will be used with a Si(100) backing wafer and no faceplates

  4. Pick up rinsed wafers from P1

  5. Load rinsed wafers into SRD spinner. The 3" wafer cassette holder should already be in place. The green dot on the wafer cassette faces you when the wafer is loaded correctly.

  6. Dry for 2 minutes, with nitrogen dry selected. Currently recipe 6. Wait for nitrogen dry to finish after spinning completes to avoid water droplets

  7. Move dried wafers to wafer carrier to return to clean room (?)

  8. Load Si wafers ASAP

  9. Finish loading procedure as usual

  10. Use an LC bake of 450 C instead of the typical 550 C. Otherwise the hydrogen passivation may be desorbed in the load chamber.

For more info about growth, see Bi and Bi1-xSbx Project

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