MINIBRUTE ANNEAL TUBE STANDARD OPERATION PROCEDURE

Transcription

1 Arizona State University NanoFab MINIBRUTE ANNEAL TUBE STANDARD OPERATION PROCEDURE Rev A

2 Title: MINIBRUTE ANNEAL TUBE STANDARD OPERATION PROCEDURE Table of Contents Issue: Rev A Contents Table of Contents Purpose / Scope Reference Documents Equipment / Supplies / Material Safety Set Up Procedures Operation Procedures Process Data Revision History...6 Page 1

3 1. Purpose / Scope 1.1. The Thermco Minibrute tube furnace is used to anneal 4 inch wafers and smaller samples for times greater than five minutes in a controlled ambient of either argon, nitrogen, oxygen, or forming gas (5% H2 / N2) at temperatures up to 700C. This SOP provides the procedures to setup and anneal a film or multiple films on any substrate. 2. Reference Documents none 3. Equipment / Supplies / Material 3.1. Tweezers 3.2. Pushrod and boat 3.3. Insulated gloves 4. Safety 4.1. Follow all safety procedures outlined in the CSSER Handbook 4.2. This system operates at high temperatures (up to 700C) and contact with samples, the boat, the pushrod, and the endcap will result in severe burns if care is not taken. Be extremely careful not to touch any of these parts until sufficient time has been given for them to cool. Seek medical attention for severe burns. 5. Set Up Procedures 5.1. Four gases are available for use in the system: argon, nitrogen, oxygen, or forming gas (5% H2 / N2). There are shutoff valves located on the floor behind the system for the desired gases and these should normally be found in the open positions. Each gas has a toggle valve shutoff and flowmeter (rotometer) on the front panel for user control. Nitrogen will typically be used for purging the tube during heating and cooling steps and sometimes in combination with a process gas during the timed anneal Check the system logbook and temperature readout to confirm that the system is not in use and that the temperature is low enough to safely handle the hardware Temporarily open the gas toggle valves on the front of the system for the gases that will be used and confirm that the purge and process gas flows are set correctly (N2 setting = 50; Ar setting = 70; H2N2 = 3; O2 = variable). Adjust the rotometer needle valves if needed and then shut off the gas toggle valves. 6. Operation Procedures 6.1. Furnace heat-up Turn on both the control and element switches on the front panel. Adjust the exhaust center load setpoints to heat to the desired temperature which will be indicated by the digital readout on top of the system. Control setpoints can be taken from the table below or from user s notes in the logbook and do not directly indicate the temperature. It will take approximately 15 to 20 minutes for the furnace to reach temperature and stabilize. Page 2

4 A calibration T/C measurement can be taken by the staff to provide a more accurate indication of the real sample temperature. This is done by inserting a thermocouple into the core of the tube and measuring the temperature over a +/- 10 cm distance from the system centerpoint. Request help from the NanoFab staff if this needs to be done. Temperature (C) Setpoints Exhaust Center Load Calibration (C) 6.2. Loading Select one of the four quartz boats that is most appropriate for your samples. Load the boat and samples in the cool zone at the load end while the furnace is heating to the setpoint temperature. Remove the end cap, load the samples, and replace the end cap, gently hand tightening the bolts holding it in place Switch on the nitrogen (or argon) purge gas to a flow of 7 LPM (N2 setting = 50 or Ar setting = 70) and wait for the furnace to reach setpoint and stabilize. Nitrogen is preferred as a purge gas since it is house supplied whereas argon is supplied from a bottle and will deplete that bottle in 19 hours Use the quartz pushrod to push the samples into the center zone of the furnace. Remove the small flange on the endcap to insert the rod, push to the mark, and remove the pushrod. Caution!! The end may become quite hot so be careful both where you touch it and where you place it upon removal. Replace the flange and continue purging with nitrogen (or argon) for at least 10 minutes to allow the samples to heat to temperature Processing Switch on (using the toggle valves) the oxygen, or forming gas as desired for your process and turn off the nitrogen (or argon) purge. The recommended flow for forming gas is 3 LPM, which is a setting of 3 on the rotometer. Oxygen typically will be mixed with argon or nitrogen for a total flow of 3 LPM. The oxygen flowmeter has a maximum flow of 2 LPM and reads directly Time the anneal process as needed. Typical times are 20 to 30 minutes. An upper limit is defined by depletion of the gas cylinder, which takes about 40 hours for a full tank. Please discuss very long anneals with the NanoFab staff Switch (using the toggle valves) on the nitrogen (or argon) purge gas to a flow of 7 LPM and switch off the oxygen or forming gas, if they have been used Switch off the furnace heater elements and control circuit. Use the quartz pushrod to pull the samples into the cool zone at the load end of the furnace. Remove the small flange on the endcap to insert the rod, pull the boat from the hot zone, remove the pushrod, and Page 3

5 replace the flange. Caution!! The pushrod end may be quite hot so be careful both where you touch it and where you place it upon removal. Continue purging with nitrogen (or argon) for at least 10 minutes (or more) to allow the samples and endcap to cool to a safe handling temperature before you proceed to remove them Switch off all gases to the system and remove the endcap to retrieve your samples System shutdown Replace the endcap Confirm that all gas toggle valves are closed and that control and element power switches are turned off. 7. Process Data 7.1. Minibrute Brooks flow meter tubes There are four Brooks rotometers located on the front panel to control gas flows to the system. These are rated as shown in the table below when the gas pressure is regulated to 15 psi. The recommended flow rates for purging and for processing are 7 LPM and 3 LPM, respectively. The oxygen flowmeter is sized to allow controllable mixtures with nitrogen or argon. Gas Max. Flow (SLPM) Notes: Argon LPM = setting of 70 Nitrogen 43.8 Min flow 50/150 setting Oxygen 2 Direct read LPM Forming gas 5 Direct read LPM 7.2. Calculate flow rate / purge time for tube to reach clean conditions: Calculate the tube volume (V) in liters: a. Tube length L = 140 cm b. Tube inner diameter D = 14 cm c. V = (πd 2 x L)/4000 = 21.5 L Choose the exchange rate time (defined as the time to achieve one gas exchange within the process tube) that will reduce the atmospheric contamination level to an insignificant number. Air is about 20% oxygen, which is reduced by dilution by half for each gas change of the chamber. After N changes of gas the oxygen level is expected to be below (20%) x (0.5)N. Six gas exchanges will drop the atmospheric level to ½ 6 = 1/64 and it takes approximately 15 minutes for the sample to reach full temperature. So, the exchange rate time should be about 3 minutes The main gas flow rate is then calculated by dividing the volume of the process tube by the exchange rate time. Since the volume of the process tube is 21.5 liters and an exchange rate time of 3 minutes was chosen, then the main carrier gas flow rate will be 7 SLPM. Page 4

6 A size 1A argon cylinder from Matheson contains 7930 liters of gas the cylinder will last 19 hours at this purge rate Maintenance flow to use during processing: Flows chosen by various labs vary widely. A reasonable choice is 3 SLPM which (for Argon) would deplete the cylinder in 44 hours. Page 5

7 Title: MINIBRUTE ANNEAL TUBE STANDARD OPERATION PROCEDURE 8. Revision History Effective Date Originator DESCRIPTION OF REVISION Issue 4/6/15 C. Tracy Initial Release A Issue: Rev A B C D E F G H Page 6