INTRODUCTION:
Aluminum is used to make contacts between your devices and the outside world. Aluminumn is deposited by thermal evaporation, a physical vapor deposition technique.
SAFETY:
There are no chemical hazards associated with the vacuum system. The main concerns are maintaining the integrity of the vacuum. Plastic gloves should be worn at all times when handling anything ont he interior of the vacuum chamber. It is very important to follow the pumpdown sequence below EXACTLY in order not to damage the system (and save you many hours of cleaning up the chamber). In addition, heat and cool the system at the rate specified so as not to burn out the heating wire or shatter the crucible(if you are using one with a tungsten basket).
PROCEDURE:
I. Load sample and pump down
1. Check vacuum gauge - system should be pumped down to somewhere < 5x10-6 torr. If it is higher contact your instructor.
2. Turn off the ion gauge - needle will swing all the way to the right.You have to hold the down the OFF button for 3-4 seconds.
3. Toggle HI VAC switch to the closed position. You will hear a 'click'.
4. Wait for 10 seconds. The valves on the Varian unit are operated pneumatically and do not respond instantly. By waiting we ensure that they have time to fully close or open.
5. Toggle AIR RELEASE switch to the open position. It will take a few minutes for the chamber to come up to pressure. Wait until you can no longer hear air entering the chamber.
6. Raise the bell jar using the hoist toggle switch. When working inside the jar make sure you wear gloves at all times.
7. Open shutter (MAN OPEN) and load aluminum shot into the tungsten basket. Check electrical connections to ensure that they are solid.
8. Mount your samples and a glass slide (for thickness monitoring) in the sample holder.
9. Lower bell jar, ensure that it is well fit. Close shutter.
10. Toggle AIR RELEASE switch to the closed position.
11. Toggle FORE (foreline) switch to the closed position.Wait for 10 seconds.
12. Toggle ROUGH (roughing) switch to the open position.
13. Wait for the chamber pressure to fall < 10 mTorr (TC-1 gauge). Wait until it essentially reaches zero. This will take 3 - 5 minutes.
14. Toggle ROUGH switch to the closed position. Wait for 10 seconds.
15. Toggle FORE switch to the open position. Foreline pressure (TC-2) should fall to zero.
16. Toggle HI VAC switch to the open position. The foreline pressure will quickly jump up to a few hundred mTorr and then it should start to decrease. Monitor foreline pressure gauge (TC-2) to ensure that the pressure does not exceed 500 mTorr. If the value exceeds 500 mTorr and does not drop close the high vacuum valve and contact instructor.
17. Turn on the ion gauge by holding the button down until the green EM light goes on. The pressure should read somewhere in the 10-5 and be dropping. Wait until pressure drops below 5x10-6 befiore going on to deposition. (takes ~ 30 minutes.)
II. Deposition
1. Ensure that the pressure is < 5x10-6 torr. Open shutter.
2. Check to see that the SCR power knob is at 0% and turn on the power supply - Red LED lights up.
3. Slowly increase the power to 60% in increments of 10% per 30 seconds. (If using a ceramic crucible increase 10% each minute). After reaching 60% increase in increments of 5 % until you see the tungsten basket starting to glow a dull red (typically around 70%).
4. You will be able to observe the onset of deposition by watching the transparency of the glass slide.
5. There are two methods to deposit the aluminum film.
A. Flash Deposition: The goal of this technique is to rapidly evaporatorate the entire amount of aluminum. In this technique the final amount is determined by how much was initially loaded into the tungsten basket. (Note 1 aluminum slug will deposit ~1200 Å.) From the position that you observe a dull glow, suddenly increase the SCR 10-15%, and wait till you can no longer observe Al in the basket (typically < 30 s).
B. Timed Deposition: In this technique the evaporation rate is slow and the thickness is determined by time. Increase the power by ~1-5 % above the glow power. Leave the SCR at this position for a specificied length of time. The exact power and time need t be determined empirically for a desired thickness.
6. When deposition is complete (either method A or B) decrease the power supply 15% to quench the evaporation. Following this decrease 10 % per 30s (basket) or per minute (crucible).
7. When the power is back to 0% turn off the SRC power supply. Make sure the red LED is extinguished.
III. Remove sample and pump down
1. Follow the instructions in section I, except remove sample instead of loading sample.Do NOT touch the tungsten wire - it is still hot. Leave machine under high vacuum..
2. Measure the thickness of the aluminum on the glass slide using the profilometer.
3. Measure the sheet resistance.