Thermco Furnaces – Standard Operating Procedures Change Record Originated: E. J. Stewart 6/25/03 Modified: R. L. Peterson, rev 2, 4/15/05 Modified: C. L. Silvestre, rev 3, 12/10/05 Modified: C. L. Silvestre, rev 4, 08/03/12
Table of contents Overview Standard Operating Procedure Appendix I – Temperature setpoints Appendix II – Cleaning details Appendix III – Dry oxidation procedure Appendix IV – Wet oxidation procedure
Overview The Thermco furnaces are a stack of 3 horizontal furnaces used for anneal and oxidation of 4” Si wafers, as well as smaller Si samples. Non-silicon samples can be used in furnace 2. Each furnace is designated for a specific kind of anneal/oxidation, and can flow different gases as described below: Furnace
Use
1 (TOP)
Clean anneal of silicon
2 (MIDDLE) 3 (BOTTOM)
Anneal and oxidation of any sample Oxidation of CLEAN SILICON ONLY
Thermco tube instructions, rev. 4
Gases
Restrictions
N2, Al/Ti metal only forming gas Silicon substrates only: No organics, no photoresist, no glass, no steel, no heavy metals, no III-Vs, etc. N2, No restrictions. Dry O2 N2, Clean Si ONLY. Dry O2, No exceptions. Wet O2
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Required clean Solvent clean
Solvent clean if possible RCA or Piranha + HF
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One Line diagrams
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Standard Operating Procedures IMPORTANT: MAKE SURE TO CHECK NO ONE IS USING THE FURNACE BEFORE STARTING AN ANNEAL! The log sheets list start and finish times for the previous users, so you can know if and when they finished. I. Furnace heat-up and bubbler preparation (1) Increase furnace setpoint to desired temperature. For all furnaces, this is the middle of the 3 setpoint knobs, located on the side of the furnaces near the bottom. See Appendix I for how to get the desired temperature. (2) Clean the table surface using an alpha-wipe sprayed with methanol. Note: only Thermco and Tystar furnace glassware is allowed on the table surface. Put sample boxes, lab notebooks, wipes, gloves, etc. on the lower surface. (3) (Furnace 3 wet oxidation only) Prepare bubbler. See Appendix IV for details. II. Sample clean (while furnace heating up) Furnace 1 and 2: Solvent clean (Acetone, Isopropanol/Methanol) Furnace 3: H2O2/H2SO4+HF OR RCA. See Appendix III for details. III. Loading (1) Wait until furnace at setpoint temperature. (2) Turn up the N2 or O2 to 120mm. Wait 5 minutes before removing end cap. (3) Prior to working at the furnace load station, put on fresh disposable nitrile gloves. Use thick fabric gloves when handling hot end caps and elephants. Remove end cap and place opening down on the stainless steel table top. Place elephant on end of tube. PLACE END CAP AND ELEPHANTS ON TUBE VERY LOOSELY! Several end caps have become completely stuck on the end of the tubes, even without being put on very tightly. Also, USE THE CORRECT ELEPHANTS AND PUSH RODS DESIGNATED FOR EACH FURNACE. This avoids cross contamination from one furnace to another. (4) Pull boat out onto elephant with long push rod. Insert push rod so that the last scratch mark (closest to the end where you grip the rod) lines up with the hole the rod goes through on the elephant. With the rod in this position, you should be able to lower the hook end of rod into the hole on the boat, and pull the boat out. Hold the push rod ONLY by the end away from the hook. Be careful, the push rod is tip heavy and will exert a high torque on your hands when held properly. There are multiple scratch marks on all of the push rods. Line up to the scratch mark closest to the end where you grip the rod. Only touch the rod behind the closest scratch mark. (5) Remove elephant, place on metal table, and replace end cap. When the end cap is on the end of the tube, rotate it so that its outlet vent points to the right, towards the square exhaust inlet hole on the inside of the furnace cabinet.
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(6) Slide boat from elephant to tray using short push rod. (7) Load samples 4" wafers: Place vertically in boat. Small samples: Place samples on top of a 4” wafer laying horizontally on boat. We leave a clean 4” wafer in each furnace for those who are using small samples. Please leave these in the furnaces. If for whatever reason you need to remove it, when finished clean a new one (according to the specifications of that furnace) and replace it. (8) Slide boat back into elephant with short push rod. (9) Remove end cap, place elephant on end of tube. (10) Slide boat slowly and gently into furnace using long push rod. As when removing boat, insert push rod so that the scratch mark closest to end where you grip the rod lines up with the hole the rod goes through on the end of the elephant. This places the boat in the center of the temperature-controlled region of the furnace. If at any time you feel the boat hit something, STOP. The boat legs must be aligned to go around (on both sides of) the thermocouples, which lay inside a quartz sheath at the bottom of the tubes. If the boat hits the sheath, pull the boat a few inches towards you, then gently realign the boat by pushing it back in with the push rod at a slight angle to the right or left. If the boat hits the sheath hard, it can break and the thermocouples can be damaged. To repair a broken sheath, the entire furnace tube must be uninstalled and repaired or replaced. So, please be careful not to run into the sheath. If any glassware does break, contact cleanroom staff or a Thermco superuser. Do not dispose of items with minor damage (chips), as it may be repairable. (11) Remove elephant; replace end cap. III. Annealing/oxidation (1) Switch on desired gases, set to desired flow rates. Standard processing flow rate for all furnaces and gases (except for furnace 3 wet oxidation) is 120 mm. For furnace 3 wet oxidation, use 70 mm. When switching gases, first turn the gas flow to zero (on the rotameter) for the current gas, then turn the valve, then slowly turn up the flow for the new gas. This prevents a sudden pressure surge the next time the valve is switched. When flowing oxygen, a 120 mm flow will deplete the oxygen tank at a rate of 180 psi/hour. If you are doing a long oxidation (several hours), make sure to check that there is enough gas. When flowing forming gas, the cylinder tank valve and first manual valve (between the tank and the regulator input) must be opened. When the forming gas anneal is done, close both valves. If you are not familiar with these valves, ask Joe Palmer or another user. Contact Joe Palmer if a tank is empty, almost empty, or if you need more gas for your run. Nitrogen comes from the house supply and thus there need be no concern about running out of nitrogen gas.
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(2) Wait required time for anneal/oxidation. During the anneal/oxidation, check the furnace at least every 30 minutes. IV. Unloading and shut down (1) Unloading: Same procedure as loading, except sample removal (boats are left in furnaces). Leave 4" support wafer on boat if using small samples. (2) Shut valves on process gas rotameters except for House N2. Leave N2 flowing at 20 mm in all furnaces. (3) Shut manual cylinder valves on the Forming Gas Cylinder. (There is a slow leak somewhere in the plumbing, so make sure the cylinder is shut when not in used.) (3) Reduce temperature setpoint to stand-by value designated for each furnace: Furnace 1: 4350C (setpoint = 000) Furnace 2: 2000C (setpoint = 000) Furnace 3: 6000C (setpoint = 180) (4) Replace elephant and tray into cabinet once cool. Do not put hot elephants directly back into cabinets when finished. Wait ~ 30 min for them to cool. Hot elephants may melt the plastic in the cabinets or burn someone who, not knowing they are hot, picks one up without insulting gloves on. (5) Make sure to complete the logbook. Remember to record when you finish so that the next person knows the furnace is free.
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Appendix I: Temperature setpoints Each furnace setpoint knob has a positive offset (furnace 1 and 3 ~ +435, furnace 2 ~ +200), meaning that, for example, the setpoint on furnace 3 must be 4350C LESS than the desired temperature. So, for a furnace 3 temperature of 9000C, the knob setpoint should be ~465. But these are not quite exact, and can depend on the temperature; see the table below of previously used setpoints and temperatures. When the furnace has heated up and the temperature stabilized, you may have to make minor adjustments to the setpoint to get the exact temperature you want. Note 1: These temperatures are those indicated by the digital readout box, located on top of furnace 1 near the back, for the center zone. This display reads the 3 (front, center, rear) thermocouples lying within the quartz tubes, which we believe are more accurate than the ones read by the needle gauges by the setpoint knobs. Note 2: The front and center thermocouples in tube 2 have been damaged by user breakage of the thermocouple protective quartz sheath. So, if your temperature for tube 2 is critical, it may be best to push the boat in a little farther than normal (a little beyond the center zone, closer to the rear zone, where the thermocouple does work) to make sure the readout and sample temperatures are the same.
Furnace 1 (Bottom) Furnace 2 (Middle) Furnace 3 (Top) Temp 415 450 500 550 650 700 800 850 1000 1150
Setpoint 00 20-25 70-80 120-130 255 275-280 380-385 435 804-808 950
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Temp 250 350 400 600 700 850 900 950 1050 1100
Setpoint 40 130 188-200 360-370 490 640 710 754-757 649 685
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Temp 550 600 650 750 800 850 900 1000
Setpoint 123 180-185 223 330 375-385 431-438 486-487 585-595
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Appendix II: Gas Flow details
Tube #3 (Top) N2 and Forming Gas (10% Hydrogen in Nitrogen) Standard Flow Rates: 20 mm (1.5 lpm) N2 Processing: 120 mm (10 lpm) N2 and Forming Gas
Tube #2 (Middle) N2 and O2 Standard Flow Rates: 20 mm (1.5 lpm) N2 Processing: 120 mm (10 lpm) N2 and O2
Brooks Instrument R-2-15C Meter Reading N2 O2 (mm) (liter/min) (liter/min) 20 1.41 3.01 40 2.87 5.80 100 7.70 15.14 150 11.42 21.98 O2 Flow of 120 mm depletes cylinder at 180 psi/hour.
Tube #1 (Bottom) N2 and O2 Standard Flow Rates: 20 mm (1.5 lpm) N2 Processing: 120 mm (10 lpm) N2 and Dry O2, 70 mm (5 lpm) Wet O2 O2 Flow of 120 mm depletes cylinder at 180 psi/hour.
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Appendix III: Cleaning details If you're not familiar with these cleans and the chemicals used, have someone show you first. H2O2/H2SO4 + HF clean: 1. [H2O2(30%) : H2SO4] = [1 : 1] soak for 15 minutes 2. DI water rinse 3. [HF(49%) : DI water] = [1 : 50] dip for 15 seconds 4. N2 blow dry RCA clean: 1. [NH4OH : H2O2 : DI water] = [1 : 1 : 5] @ 700C 2. DI water rinse 3. [HCl : H2O2 : DI water] = [1 : 1 : 6] @ 700C 4. DI water rinse 5. [HF(49%) : DI water] = [1 : 50] dip for 15 seconds 6. N2 blow dry
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Appendix VI – Dry oxidation procedure (1) (2) (3) (4) (5) (6) (7)
Wait for furnace to reach desired temperature. 5 minutes before loading, switch on dry O2, set flow rate to 120 mm. Load wafers with dry O2 flowing. Wait required anneal time. Switch off dry O2, switch on N2, set flow rate to 120 mm. Anneal samples in N2 for 25 minutes. Unload wafers, follow standard shut down procedure.
Measured oxide thickness (by ellipsometer) vs. dry oxidation time for furnace 3: Temperature Dry Oxidation Time Thickness (ºC) (min) (Angstroms) 800 20 45 850 120 120 900 10 70 1100 270 2900
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Appendix V – Wet oxidation procedure (1) Prepare bubbler. Check bubbler heating mantle temperature controller set point by pushing down the black button and observing the set point temperature. Heating mantle temperature controller will display current temperature when the black button is not held down. (If the set point temperature needs to be changed, push down and turn the black button.) Place 9 inch square TexWipes at the 2 o’clock and 9 o’clock positions. Remove the thermocouple from the bubbler and place on the metal shelf. Disconnect O2 outlet and place on the 2 o’clock TexWipe. Tip the bubbler to ease the removal of the outlet. Disconnect O2 inlet and place on the 9 o’clock TexWipe. Tip the bubbler to ease the removal of the inlet. Remove bubbler from heating mantle. Take to the Silicon Wet Bench. Run the Wet Bench DI water for 5 minutes. Pour out any residual water. Rinse 3 times by filling with DI water and pouring the DI water out. Fill bubbler about 2/3 full with DI water. Also fill the thermocouple well about 2/3 full with water. Dry the outside of the bubbler with TexWipe. Return bubbler to heating mantle. Reconnect O2 inlet, gas outlet, and thermocouple. (2) Wait for furnace to reach desired temperature. Clean samples using Piranha or steps 1 and 2 of RCA clean. Wait to do the HF dip until immediately before you load the sample onto the boat. (3) 10-15 minutes before loading, switch on the heating mantle for the bubbler. Set temperature to 95ºC, by pushing in and turning the knob. The heater will take 10-15 minutes to heat up. It is a good idea to be ready to load your samples and start your oxidation when the water gets to temperature and begins to evaporate. (4) 5 minutes before loading, switch on N2, set flow rate to 120 mm, and unload boat. (5) HF dip samples and load with N2 flowing. (6) Switch off N2, switch gas from “N2” to “O2”. Flow O2 at 70 mm. (7) Switch oxidation from “dry” to “wet.” Set O2 flow rate to 70 mm. (8) Wait required time. (9) Switch from “wet” to “dry.” Switch gas from “O2” to “N2.” Set flow to 120 mm. (10) Anneal in N2 for 5 minutes. (11) Unload wafers, follow standard shut down procedure. Measured oxide thickness (by ellipsometer) vs. wet oxidation time for furnace 3: Temperature Wet Oxidation Time Thickness (ºC) (min) (Angstroms) 1000 30 1800 1000 35 2200 1000 45 2340 1000 65 2975
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