Protocols - SOPs

Electron-Beam Lithography (@ EM facility, NEU)

Notes on making CAD files and run files:

Before writing a pattern, one needs a pattern to write. For the Hitachi SEM, these patterns are created with the NPGS software, which also controls SEM writing. To make a pattern, open the software, and click the “DesignCAD” button on the left. It will open up NPGS’s cad software. Within this software, one can make dots, lines, polygons, arrays, etc . . . Make sure when saving a pattern that the save command under the “NPGS” tab is used, and not the one under File. Otherwise, the pattern won’t be recognized for writing.

After making a pattern in CAD, press the “Edit Run File” button on the left hand side of the NPGS window. This will bring up the run file editing window. The run file tells the SEM what to do during writing: move the stage, expose, other external commands, etc . . . Each of these commands will be one entity on the left side of the run file editing window. The number of entities can be changed at the top of the left side column of the “Edit Run File” window. A pattern writing entity is called a “Pattern” step. Clicking on the step in the left hand column brings up the parameters for that step in the right hand column. For a pattern, the parameters are the beam writing density (which I’ve historically not change from the defaults for our relatively large patterns), the magnification, the measured current from the ebeam, and the different doses of the layers or colors in the selected CAD file. One may specify which CAD file a pattern step calls in the left hand side of the Edit Run File window.

Sample Preparation:

1. Before spinning resist on an EBL sample, clean it with piranha, or if the sample is not dirty, solvents followed by drying and 5-10 minutes of ozone

2. After cleaning, place sample on the spinner. Because EBL samples are often small compared to entire wafers, it is important to make sure the sample makes contact with the vacuum, and stays on the chuck. To do this, place the sample over the vacuum hole, and press “START” on the spinner to initiate spinning. Make sure the sample does not move laterally on the chuck. If it does, stop spinning by pressing “RESET”, and reposition the sample until the vacuum holds it on the chuck while spinning.

3. While the vacuum is on, but sample is not spinning, use the dropper bottle of 950 PMMA to drop enough PMMA resist on the sample to cover it. Avoid using an excess of resist. Make sure to avoid contact with the PMMA as it is immersed in a highly toxic solvent.

4. Press “START” to initiate spinning, but only let the sample spin for a few seconds to spread the resist. Stop spinning to make sure resist is spread over the whole sample. Then press “START”, and let the sample spin for the whole 50 seconds at 4500 RPM. I have never known the spinner to be set for any other parameters.

5. Remove the sample from the spinner. The resist will often make it difficult to remove. My preferred method is to slide a razor blade under the sample to jar it loose, then remove it with tweezers.

6. Bake the sample at 180 °C for 10 minutes. Note that it takes a long time (>10 minutes) for the hot plate to reach 180 °C.

7. Remove sample from hot plate with tweezers and mount it on a SEM sample holder stub with carbon tape.

8. Make a scratch in the resist using a scribe or razor. Make sure the scratch goes from the edge of the sample to the region where the resist is uniform in color (and thickness). This scratch will be for focusing the SEM prior to writing. The end of the scratch should be near (millimeter scale) the area where the pattern will be written. If the sample is a set of chips with membranes, make a separate scratch for each membrane.

Exposure in the SEM:

1. Before exposing the sample, align the SEM with the Au standard. Try to get a goodlooking image of gold nanoparticles at 150k magnification or more. The working distance of the microscope should be 8 mm when aligned. If it is far off from 8 mm when in focus, move the stage until the image is in focus within 0.2 mm of 8 mm working distance. After getting a good image, do not touch the stig or aperture alignment again. Make sure the beam is aligned at the settings (aperture, voltage, etc . . .) that will be used for writing. Traditionally, aperture 3 and condenser lens 5 have been used for writing with ebeam voltage 10 kV and filament current 10 uA.

2. Measure the ebeam current at the writing settings by putting the Faraday cup in the SEM, and grounding the SEM stage through the current meter. Note that the high voltage will be disabled while switching the grounding cable.

3. Mount the sample on the SEM sample mount. Note that, while holding the sample holder with the part that connects to the SEM insertion rod pointing down and looking at the sample, you are viewing it with the same orientation that you will see in the same. That is, from this point of view, the upper left hand of the sample as you view it will also be the upper left hand corner in the SEM view.

4. Insert the sample into the SEM, turn on the ebeam blanker and set the beam to “Off”.

5. Press “Home” in SEM software to bring sample under ebeam.

6. Turn on high voltage, and move the sample several centimeters to one side so that, when the beam is turned on, it doesn’t expose the areas which are to be written.

7. Unblank the beam and navigate to the edge of the sample in Low Mag mode. Use the dumbbell tool on the SEM software to make sure the top edge of the sample is aligned with the xaxis of the stage. This will make it much easier to navigate with the beam blanked.

8. Find the scratch at the edge of the sample. Switch to high mag mode and zoom into ~1k magnification. Navigate using the mouse to the end of the scratch. Look for some piece of dirt from the scratch to focus on. Small particles are good to use because they tend not to be too far off the resist surface, so focusing on them will not put the beam’s focus far above the plane which is to be written. Focus on the piece of dirt. Get a good image at 70k mag or more. If, when in focus, the working distance is much different from what it was while aligning on the Au standard, move the stage in the appropriate direction and refocus. Specifically, if the working distance is less than 8 mm, move the stage z so that the sample is farther away from the gun and refocus. Do this until the beam is focused at the same working distance where it was aligned.

9. It is now time to write the pattern. First, a general note on navigating on the sample and exposing resist. The Hitachi SEM has 16 different condenser lens settings, numbered 1 to 16. The higher the number, the less the ebeam current coming out of the gun. Condenser lens 5 is used for writing. However, for navigating on the resist, use condenser 16. This results in the least possible current, and hence least chance of inadvertently exposing resist. Note that, in general, condenser 16 will be misaligned, and there is a slight offset between 5 and 16. It still works fine for navigating at relatively low mag though.

10. Blank the beam and go to the spot for writing. If the sample is a chip with a membrane, the following method is useful. Go to a corner and set the condenser to 16. Blank the beam. Because the chips are 5 mm on a side, and the membrane is in the middle, the membrane should be roughly 2.5 mm from the corner in the x and y directions. Move 2.5 mm in x and y on the stage to get to the membrane. Briefly unblank the beam. The membrane should stand out as a black square. Move the stage until it is centered on the membrane. The crosshairs, which are turned on by checking the “Area Marker” box under the righthand Utility tab, are useful here. Move up to 1000X mag and make sure the membrane is still roughly centered. Blank the beam, and switch back to condenser setting 5.

11. Edit the run file to be used in NPGS to include the current measured with the Faraday Cup.

12. Bring the mag to the necessary magnification for the pattern being written. Flick the beamblanker to the setting “Ext”. The blue LED should light up. This allows the NPGS software to control the blanker. Flick the switch on top of the blanker from “SEM” to “NPGS”. This allows NPGS to control the SEM.

13. Select the run file to be written from the NPGS menu on the NPGS computer. Click the button “Process Run File”. When prompted with a dialog box, click “Yes”. The pattern being written should display quickly on the NPGS computer screen. The computer will make a beeping sound when the pattern is finished.

14. Write additional patterns if necessary. When doing membranes, before writing on each one, focus on the adjacent scratch. Otherwise, the patterns will become increasingly out of focus as you move around the chip. Keep in mind that when navigating, the beam should be on condenser lens 16, and when writing, it should be on 5.

15. When finished writing, turn off high voltage, turn off the blanker, and make sure the switch on top of the blanker is set to “SEM”. Remove sample from SEM.

Development and SF6 Etching:

1. Develop samples in a 1:3 mixture of MIBK:Isopropanol for 1 minute. Perform this development as soon as possible after exposure.

2. Rinse sample thoroughly with Isopropanol after development and dry with nitrogen.

3. If the samples are membranes to be etched with SF6, place them in the RIE after development and etch. Note: do not do an oxygen plasma before SF6 etching, because it will eat some of the PMMA and cause features to be larger than intended.

4. Place samples in acetone after SF6 etch. Leave them for at least 30 minutes. Remove with acetone, rinse with acetone, followed by isopropanol, and dry with nitrogen.

5. Clean sample with piranha to remove residual PMMA.