Chapter 7.14

Ionmill Procedures

(ionmill)

1.0         Title

Ionmill Procedures

2.0         Purpose

3.0         Scope

The following settings work well for the ionmill. You may need to modify them for your particular task (e.g., to achieve uniformity on large samples, see note under Step 9.5.3). Nevertheless finding a set of suitable conditions and staying consistent will enable you to predict etch rates and required mill times for a variety of materials. For (0001) sapphire (c-axis), the following settings will give a depth of ≈ 0.65 µm in 45 min. Shipley 1818 photoresist (soft baked for 20 min at 90°C and hard baked for 20 min at 120°C) will mill at 2.0 µm/h, so samples coated with the normal procedure (30 s at 5500 rpm which yields ≈ 1.8 µm thick photoresist) should not be ion milled any longer than 45 min since overetching may begin to round corners (of course over-etching may be desired in special circumstances). Glass will etch at ≈ 1.5 µm/h using the following settings.

4.0         Applicable Documents

5.0         Definitions and Process Terminology

6.0         Safety

7.0         Statistical/Process Data

8.0         Available Process, Gases, Process Notes

9.0         Equipment Operation

9.1         General Preparation

9.1.1          Check cryo pump temperature (it must be < 18 K).

9.1.2          Check if the vacuum pressure is low. It should read 0 mtorr on the digital gauge. Turn on the ion gauge filament via the switch on the ion gauge controller. Base pressure for the ionmill should be in the low 10-6 torr range on the ion gauge.

9.1.3          Check the argon pressue: There is a small, round gauge mounted on the Ar feedline, near the hoist. Ar pressure should be 3-5 psi.

9.1.4          Ion milling transfers considerable heat to the substrate being etched. When polymer masks are used, some method of  heat transfer to the rotating water-cooled substrate table is required.

Cool Grease™ is a thermally conductive paste suitable for ion milling. A jar of Cool Grease™ is available at the ionmill. If  it needs to be replaced, lab members may check out additional Cool Grease™ from the office. Information about Cool Grease™ can be found at: http://www.aitechnology.com/

Loading Sample into Ionmill

9.1.5          Close high vacuum valve and wait for pop (clank) to insure it is closed. It may take ≈ 30 s for the pop to sound, and its loudness has changed over time, but it is very important to wait for this.

9.1.6          Vent chamber and wait for digital gauge to stabilize at atmospheric pressure (≈ 700 - 760 torr) to insure complete ventilation.

9.1.7          Raise hoist and turn vent valve off. Make sure the gasket (giant O-ring) stays in place, if not put in place. It is usually a good idea to always push it down around the entire chamber wall even if it does not come undone completely.

9.1.8          Apply Al paste to stage so that most of plate (or larger wafer) will have contact with the paste.

9.1.9          Push on plate (or large wafer) so that paste is smashed around. Blow off surface with N2 gun.

9.1.10      Lower the hoist. Make sure the stage apparatus does not hit the chamber walls during lowering.

9.2         Pumping Down Chamber to Sufficient Vacuum

9.2.1          Open roughing valve and pump down to 100 and 200 mtorr on pressure gauge.

9.2.2          When pressure gets between 100 and 200 mtorr close roughing valve, make sure pressure is stable (i.e., there are no major leaks) then open high vacuum valve. (In the old manual it states to rough the chamber to 200 - 400 mtorr; however, I routinely rough vacuum furnaces to less than 100 mtorr before starting a turbo pump or diffusion pump, and so I have selected 100 - 200 mtorr as a compromise.)

9.2.3          Turn ion gauge on and allow system to pump down to < 4 × 10-6 torr.

Depending on your application, you may need to let the system pump down to a better vacuum; using the procedure in the next section and Steps 9.7.6 and 9.7.7 to flush the chamber with argon can help attain a better vacuum. However, recently I’m not sure the ionmill will pump much better than about 2 × 10-6  - torr, even after leaving it overnight.

9.3.4     While waiting for the system to pump down, turn on table rotation and coolant. Verify that vacuum is not lost due to rotation, or a water leak. Also, verify water flow. Previously, the rotation and water could be turned on at any time; however, after some modifications to the equipment, it appears as if they will not come on until the high vacuum valve is opened.

9.3         Preparing Chamber with Proper Argon Pressure

9.3.1          Make sure all (not the limiters though, these should be all the way up) knobs for ion beam controls are completely turned down, and that all devices are turned off. Some people are very sloppy about shutting the machine down.

9.3.2          Turn main power on (breaker).

9.3.3          Make sure argon pressure is set to 2 × 10-5 torr.

9.3.4          Turn argon power and filament on. (Make sure the pressure button for the Ar-ion gauge is depressed; i.e., not the deviation, calibration, or some other setting, unless of course you prefer to monitor these.)

9.3.5          Make sure valve closed light is on.

9.3.6          Switch from valve to control mode by pressing the control/close button.

9.3.7          Wait for argon pressure to get near 2 × 10-5  - torr, then turn pressure setting up to 8 × 10-5 torr. It will usually take about a minute for the automatic valve to engage and for the pressure to reach the initial set point of 2 × 10-5  - torr.

9.3.8          Wait for pressure to equilibrate at 2 × 10-5 torr; meanwhile, make sure the shutter is closed.

9.4         Setting Ion Beam Controls

9.4.1          Turn the isolation transformer on (lights will come on if it is indeed on). If it does not turn on, try flipping the lever slowly.

9.4.2          Turn cathode knob up to a current of 19 A, which should correspond to a power of 150 W and 7.5 V.

9.4.3          Turn magnet on and up to 0.4 A.

9.4.4          Turn the arc up to 45 V; this should correlate with about 1 A. You can now adjust the cathode slightly to get 1 A reading on the arc.

9.4.5          Make sure all three (cathode current, magnet current, & arc current) are stable.

9.4.6          Turn suppressor on, and up to 250 V.

9.4.7          Turn accelerator main on, then press HV on button. (You may need to turn dial up and down in order to get the HV to turn on properly.)

9.4.8          Adjust the acceleration potential to 1000 V. This should correspond to about 1 0 A/cm2 on the current density display. (At this time, if the suppressor voltage oscillates, it is due to the degassing of species inside the chamber. By waiting a short while these oscillations should stop, but they are not a major concern.)

9.4.9          Adjust shutter until maximum is read on current density display.

9.4.10      Turn neutralizer on, adjust until there is zero readout for current density and target current. This will usually be in the vicinity of 10-15 V.

9.4.11      After these adjustments, the acceleration potential should be about 900-1000 V. The arc current will be ≈ 1.1 A.

9.5         Milling Samples by Opening Shutter for Desired Time

During milling, make sure all gauges are stable. In particular watch the target current. Small little spikes in the current are okay; however if a current > 0 develops, compensate with neutralizer. Also, keep an eye on the suppressor, sometimes it extends past 250 V (i.e., off the gauge range), so you will need to adjust it. Fluctuations in the controls tend to occur in the early stages of milling. You should keep an eye on the controls for the first 20-30 min of milling, but if you need to mill for an extended time (say a few hours) check it every 20-30 min after the initial time. Close shutter after desired time period.

9.6         System Shutdown

9.6.1          Turn down the acceleration all the way, push HV off button, and then turn off main.

9.6.2          Turn down the arc all the way.

9.6.3          Turn cathode down all the way.

9.6.4          Turn the magnet, suppressor, and neutralizer off in any order. Always turn one knob down, then turn that control off before proceeding to the next control.

9.6.5          Turn off the isolation transformer.

9.6.6          Turn the argon pressure setting to 2 × 10-5 torr.

9.6.7          Switch to valve closed mode (press control/closed).

9.6.8          Turn off the filament and then the power to the argon controller.

9.6.9          Let system cool for 5-10 min, meanwhile, turn off table rotation.

9.6.10      Turn off master power (breaker), coolant, and filament.

9.7         Removing Sample and Leaving Chamber under High Vacuum Conditions

9.7.1          Close high vacuum valve (again wait for pop to insure it is closed).

9.7.2          Vent chamber.

9.7.3          Raise hoist and turn off vent, make sure gasket (giant O-ring) is in place after hoist has risen.

9.7.4          Remove sample/plate.

9.7.5          Clean stage with razor blade and wipe with acetone or isopropanol. Also, clean off the copper plate if you used it.

9.7.6          Lower hoist.

9.7.7          Open roughing valve.

9.7.8          When pressure is between 100 and 200 mtorr, close roughing valve, then open high vacuum valve.

10.0      Troubleshooting Guidelines

10.1   Problem:    Neutralizer not working.

Solution:    Replace broken neutralizer wire. The .008" Tungsten wire is located in the sea locker drawer lableled ionmill in the back of the old lab. Cut wire to length adequate to wrap around posts. First, unloosen nuts and remove old wire. Wrap with new wire around post twice and stretch to next post, and wrap again. Secure nuts to wire. Cut off excess wire.

11.0      Figures and Schematics

12.0      Appendices

Rev. 00 – 9/02, X. Meng

Rev. 01 – 10/05, R. Hamilton – Changes on 9.1 General Preparation.

Rev. 02 – 2/06, R. Hamilton – Sections 9.1.4 and 9.1.5 deleted and replaced with new information on Cool Grease™.

Rev. 03 – 2/07, J. Donnelly – Added one problem in Section 10.0 – Troubleshooting.