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LEFT: schematic of the inductively coupled plasma (ICP) argon ion source, user sample (red) is placed to the right of the accelerator grid. “Neut.” Refers to the neutralizer. RIGHT: schematic of the Hiden Ion Milling Probe using Secondary Ion Mass Spectrometry (SIMS) for etch End Point Detection (EPD).
Tool Highlights
- Direct physical milling of thin and thick films using argon atoms.
- Ion beam source : inductively-coupled argon plasma, accelerated up to 900 V in a 14cm broad beam with currents up to 620 mA (Kaufman & Robinson, Inc.).
- Ar+ ions neutralized using a matched electron beam current before reaching the sample.
- water-cooled rotating sample carrier can be oriented 0-90 degrees from normal beam incidence.
- Secondary ion mass spectrometer (SIMS) mounted on the system can be used for manual or automatic endpoint detection (EPD) of milled material by discerning elements based on charge/mass ratio of positive ions (Hiden Analytical).
- Sample load-lock system saves time with processing.
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The shared carrier and mask wafers are dirty since they collect all the sputtered material from users' samples.
If you care about contamination of your sample due to re-sputtering of material, you need to provide your own carrier and masks.
- To order your own "rotary substrate carrier" from AJA International (www.ajaint.com) (email), use the part # SH-H2O-R-ASSY .
- About Si wafers for masking the sample holder
About etching gold (Au): please discuss with Neil or the BNC staff in charge before etching Au.
Sample chuck cooling during etch: Note that thick films require high etch rates (and hence high amounts of heating), so this may mean one needs to load the sample directly on the chuck by venting the chamber (instead of using load lock). This will permit users to apply more torque and get better cooling of the chuck. However, it requires special training.
For EPD, note that argon atomic mass is the same as MgO, so the EPD is not able to sense (ionized) MgO in the large Ar ion background.
Standard Operating Procedure
Process Library
Recipes should be submitted at bottom of this page in comments section or emailed to Neil Dilley ndilley@purdue.edu ; they need to include:
- mask type and how to prepare it (e.g., photoresist type, spin speed, bake temp/time, exposure, development)
- milling
- program #
- duty cycle (on/off times)
- angle of substrate
From Qi Wang
Mask: Photoresist AZ9260/2000rpm/bake at 110C for 3min
Material to etch: SmNiO3
Depth: 50nm
Process used in PhaseIIJ software (program #, angle, and milling duty cycle): Program 3_-30deg_2min
A 5min ultrasonic cleaning in acetone could remove most of the photo resist. The stubborn residue near pattern edges was able to be removed by soaking in remover-PG at 80C for 1h and ultrasonic cleaning for 2min.
From Sen Dai, deep etching ~0.5um of LiNbO3 (special SOP, ask staff)
Mask: SPR220 or any PR that fits your needs.
Our requirement is that the PR is at least ~2x thicker than the material you want to etch so that it will not be etched through.
If using SPR220:
Spin at 3000rpm, then bake 115degree 90sec. This will produce 4 um film. Expose your pattern at 350mJ/cm2. Develop using MF24A 60sec.
Milling: program #3 (VB=600 V, (mill 3'30" / cool 12' ) repeated 8x.
From Chengzi Huang
Mask: SPR220-4.5 as the photoresist, spin speed 5000rpm, bake 115C for 90s.
Milling: 50nm/40nm/150nm Pd/BYZ/NNO with program 3, angle -30, 3 cycles of (2 min etch +12min cooling)
When etching Si, PR residuals can be easily removed by 5s ultrasonic cleaning in acetone. When etching the above Pd/BYZ/NNO stack, post cleaning requires 1min ultrasonic cleaning + 5min branson asher O2 plasma clean.
From Steve Novakov (John Heron group, U. Michigan)
Mask: SPR 220-3.0 on the spinner with the following recipe: Step 1. 500 rpm, 500 rpm/s, 5s Step 2: 4500 rpm, 1250 rpm/s, 35s. Bake on hotplate at 90 C for 2 minutes. This may not be 100% mandatory for projection exposure tools and large devices, but is mandatory for contact mask exposure and small devices. NOTE: The final spin speed can be varied/tuned. I prefer as thin of a resist as possible to shorten development/exposure time. However, in the case of TMD’s, having a modest edge/corner bead serves as an additional protective element, reducing pressure on the film from the mask aligner, etc.
Milling: Al2O3 | Bi2Se3 (~ 10 nm)| MnSe (~ 5 nm)| Se (~ 10 nm) : etch to Al2O3 substrate. Program #2, -30 degrees. Duty cycle milling on 30sec / off 30 sec, for a total milling time of 4 minutes until Bi and Se signals level off in EPD. Sample was likely milled after 2 minutes but the Bi/Se trends at EPD were tailing off slowly.
More complete description of their litho/liftoff process is here.
Ozan Erturk (Bhave group)
PMN-PT Etching
Mask: SPR220-7 at 4000rpm, bake 110C for 30 sec hold ~1cm above the hotplate then 90s in contact.
Milling: Si/PMN-PT/ PR with program 3, angle -14, 2 cycles of ( 210 sec etch +7min cooling) + angle -70, 2 cycles of ( 210 sec etch +7min cooling)
This package that contains 2 cycles of -14 degree followed by -70 degree etch can be repeated as many times required to achieve the desired vertical depth.
Etched material: 1.8um (Etch rate ~52nm/min)
Etched PR: ~1.2um (Selectivity is 1:1.5 PR : PMN-PT)
Note: selectivity significantly decreases for longer cycle times. For deeper etch depths, it is recommended to perform batches of 10 cycles with ~2hrs of cooling in between.
Si Etching
Mask: SPR220-7 at 4000rpm, bake 110C for 30 sec hold ~1cm above the hotplate then 90s in contact.
Milling: Si/PR with program 3, angle -14, 10 cycles of ( 150 sec etch +7min cooling)
Etched material: 1.1um (Etch rate ~45nm/min)
Etched PR: ~1.1um (Selectivity is 1:1 PR : Si)