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STANDARD OPERATING PROCEDURE: NPGS E-BEAM LITHOGRAPHY

Purpose of this Instrument: Electron beam lithography of complex patterns at nanometer scale precision on resist coated surfaces.

Location: Engineering Science Building (ESB) Room G-73 (cleanroom)

Primary Staff Contact: Dr. Weiqiang Ding 304-685-1938 Office: ESB G75D weding@mail.wvu.edu

The Shared Research Facilities are operated for the benefit of all researchers. If you encounter any problems with this piece of equipment, please contact the staff member listed above immediately. There is never a penalty for asking questions. If the equipment is not behaving exactly the way it should, contact a staff member. NOTE: The purpose of this manual is for e-beam lithography process with the JEOL 7600F SEM. Users need to take photolithography training and SEM training before being trained for e-beam lithography. This SOP covers the basic e-beam lithography operations. Please refer to the photolithography SOP and SEM SOP for details of some of the specific processes involved (e.g., spinner operation, SEM focus and astigmatism correction, etc).

PATTERN DESIGN

There are two copies of the software program (DesignCAD Express/NPGS) installed within the Shared Research Facilities available for pattern design. One copy is installed on the NPGS desktop in the SEM room (the computer in the middle of SEM workbench). Another one is on the computer in ESB G75D.

NOTE: Users should use the software program in G75D for pattern design whenever possible.

1. Select Project > Create New Project from the NPGS program menu to create a new project.

2. Click on “DesignCAD Express” in the NPGS program commands list (left column, Figure 1) or select Commands > DesignCAD Express in the NPGS program menu to start the DesignCAD Express program.

3. Refer to the NPGS program manual for DesignCAD operation. The NPGS manual (pdf version) is located on the desktop of the NPGS computer in SEM room and on the computer in ESB G75D. Contact shared research facility staff for help if you encounter problems.

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Figure 1. NPGS program interface.

4. Only use Line types 0, 1, 3, 4, 5 in DesignCAD for your pattern design. Refer to page 65 in NPGS manual for detail description of each line type.

Line type 0 Solid Line: single pass and wide lines

Line type 1 Dashed Line: filled polygon (serpentine fill) (recommended)

Line type 5 Dotted Line: filled polygon (one side fill)

Line type 3: wide lines (perpendicular)

Line type 4: filled circle (out-from-center)

5. Select different colors for lines/patterns that you want to use different exposure dosages. You will define exposure dosage for each color used in the pattern later in the Run File Editor.

6. Select NPGS > Save command in the DesignCAD Express menu to save the pattern file. Do NOT use the standard File > Save command to save the patterns.

7. In NPGS program window, select “DesignCAD Files” in the Display File Types drop down box (at the upper right corner of the program window, Figure 2a).

8. Select the DesignCAD file to be processed. Click right mouse button and select Run File Editor in the pop-up menu (Figure 2a).

9. In the left column of the Run File Editor window (Figure 2b), set the proper XY Move to Pattern Center values (initial stage offset) in micrometer unit. At the beginning of E-

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beam lithography, the stage will move to a new region for patterning according to the pre-set initial stage offset value.

(a)

(b)

Figure 2. (a) Pop-up menu with right mouse click; (b) Run File Editor interface.

10. Click on the Entity Type section in the left column of the Run File Editor window. The detailed entity data will show up in the right column of the window (Figure 2b).

11. Set proper processing parameters in the entity data:

Origin Offset: allows the entire pattern to be shifted from the origin (center of the screen).

Magnification: automatically set based on your pattern size.

Center-to-Center Distance (nm): distance between adjacent exposure points (see Figure 3).

Line Spacing: distance between adjacent passes of the beam (see Figure 3).

Configuration Parameter: set 1

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Measured Beam Current: 40 pA. The beam current will be measured with a Faraday cup and be adjusted to 40 pA during the SEM setup process.

NOTE: User should refer to pages 55-56 in the NPGS manual for more information about these parameters.

Figure 3. Center-to-center and line spacing settings (from NPGS user’s manual).

Figure 4. Set doses for different dwell colors used in pattern design.

12. If you have used multiple colors in the layer, input the desired Line Dose for each dwell color used in the pattern design (Figure 4). If you want to define a series of doses, click on the “Set Doses” button at the bottom of the Run File Editor window to open up the “Define a Series of Doses” dialog (Figure 4).

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NOTE: The recommended line dose for 950k PMMA at 30 kV is about 1.3 nC/cm (default line dose). The recommended initial dose range for testing purpose is from 0.6 to 3 nC/cm (from too low to too high).

13. Click on “Save” at the bottom of the Run File Editor to save the changes and then click on “Exit”.

14. In the NPGS program window, select “Run Files” in the Display File Types drop down box (Figure 1).

15. Click on the run file to be processed from the list.

16. Click on “Estimate Total Time” in the NPGS program commands list (left column, Figure 1) to check the time needed to perform e-beam lithography of the pattern. If the required total time is longer than the time available for your session, you need to modify your design or increase the center-to-center and line spacing.

17. Click on “Simulate Writing” in the NPGS program commands list (left column, Figure 1) to observe how the patterns will be written.

SUBSTRATE PREPARATION (SPIN COATING)

1. Silicon wafer is the recommended substrate for e-beam lithography. Cleave a wafer to

obtain a piece of 1 cm x 1 cm and blow off debris with dry nitrogen.

2. Sign in on the FOM for your Spinner Hood session. Write down your name and other required information on the log sheet.

3. Clean the substrate through bath sonication. A bath sonicator is located in the organic solvent hood in the wet processing room (ESB G75C1). Typical cleaning procedure for Silicon substrate is as follow:

(1) Sonicate in Acetone for 5 min;

(2) Sonicate in IPA for 5 min;

(3) Sonicate in water for 5 min.

After cleaning, pour the chemical waste into the proper waste collection jar stored under the organic solvent hood.

4. Bake the substrate at 150 oC for 30 min on a hotplate located in the acid hood in the wet processing room.

5. Spin coat photo resist with the spinner in the spinner hood in the photolithography room (ESB G75C2). Refer to the specific photo resist data sheet for the spinning speed setting.

For instance, for 495 PMMA A4 resist, spin at 2500 rpm will result in a film of 200 nm thick. Note: Please refer to the SOP of the spinner for operation instruction.

6. Clean up the inside of the spinner with wipers and acetone supplied (located on the spinner hood).

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7. Bake the photo resist film according to instruction in the photo resist data sheet. For

example, the 495 PMMA should be baked at 180 oC for 11.5 min on a hotplate.

8. Make a 1 mm scratch perpendicular to an edge on the photo resist film surface with a sharp object such as the tweezers. It will serve as a reference mark for focusing the e-beam and locating the pattern areas.

9. Sign out on the FOM for your Spinner Hood session. Sign out on the log sheet. Report any problem in the comment section and fill out the Service Request Form accordingly.

SPECIMEN LOADING & SEM SETUP

1. Sign in on the FOM for your SEM session. Write down your name and other required information on the log sheet.

2. Mount the NPGS specimen holder (with Faraday cup, Figure 5) to the SEM specimen holder. Align the left edge of the notch on the NPGS holder to a scratch line on the SEM specimen holder.

3. Fix the substrate to the NPGS specimen holder with the copper clips. Look from the side to ensure that the substrate is flat against the holder.

4. Load the specimen holder in the SEM in a way that the Faraday cup (Figure 5) is in the back when you slide the specimen holder into the transfer chuck. The Faraday cup will be in the front when the holder is inserted into the vacuum chamber. NOTE: Refer to the SEM SOP for proper sample loading procedure.

Figure 5. NPSG holder with specimen loaded.

5. Select the NPGS holder in SEM specimen holder selection dialog window and specify the specimen offset (distance from the top of the specimen holder to the sample surface). Please refer to the SEM SOP for the specimen offset description.

6. Initialize the SEM stage by selecting Maintenance > Maintenance > Stage Setting in the SEM program menu and then clicking on Stage Initialize.

7. Click on the “Observation” tab on the right side of the SEM image window.

8. Select “HBAJ_Writing_NPGS_holder” within the User section of the Recipe window at

Faraday cup

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the lower right corner of the SEM program window (Figure 6). Click on the “Set Conditions” button.

9. Turn on the electron beam. Set the SEM operation condition (recommended) as follow: Accelerating Voltage 30 kV; Probe Current setting 7; Working Distance 8.5 mm.

10. Start the NPGS program on the NPGS computer desktop (the monitor in the middle) if it is not already on.

11. Click on the "Lens Clear" button in the NPGS program commands list (left column, Figure 1) to remove lens hysteresis, if any.

12. Click on the “SEM Mode” in the NPGS program commands list (left column, Figure 1).

13. Find the Faraday cup in the SEM (Figure 6).

14. Move the Faraday cup to the center of the screen. Zoom into the cup (Figure 6).

Figure 6. Absorbed current reading and adjustment.

15. Click on the “Cursor” and then “Spot” buttons in SEM program menu to focus the beam in the cup (Figure 6).

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16. Click on the tab under Probe Current in the “Probe Current” section under the SEM image window. Check the absorbed current reading (Figure 6).

17. Fine adjust the beam current with the slide bar to obtain an absorbed current of 40 pA.

18. Select the Run file to be processed in the NPGS program window.

19. Select the “Exposure Scale” option at the lower left corner of the NPGS program window

(Figure 1). Input the measured beam current (40 pA). It will be used to scale all the current information in the selected Run file.

E-BEAM OPTIMIZATION

1. Locate the scratch made on the sample surface in the SEM. Find some small particles near the end of the scratch for e-beam alignment purpose.

2. Adjust the focus, beam alignment (wobble) and beam astigmatism to get a sharp image up to 200,000× magnification. Refer to the standard operation procedure of the SEM (located on the SEM workbench) for the beam alignment and astigmatism adjustment instructions.

3. Reduce to 100,000× magnification and record an image for future reference.

4. Insert the beam blanker by pressing the “BLANK” button on the DEBEN beam blanker controller box (located on the top of the workbench).

5. Re-focus and correct the beam astigmatism to obtain a sharp image. Do not change the Z-axis position.

6. Move to a nearby unexposed area. Zoom to over 200K magnification. Click on the

“Cursor” and then “Spot” buttons on the SEM program menu (Figure 6). Wait for

10 seconds to allow the e-beam to make a spot on the sample surface.

7. Focus on the spot. Adjust the beam focus and beam astigmatism to get a nice focused image. The spot should be symmetric if beam astigmatism is eliminated. NOTE: Refer to the SEM SOP for astigmatism adjustment process.

8. Move to another unexposed area nearby. Make another spot with the “Cursor” & “Spot” functions for a shorter exposure time (a few seconds). Adjust the focus and beam astigmatism to fine adjust the beam to get a focused image. Check the spot condition.

9. Repeat step 8 as many times as necessary until the spot left on the film is nice and symmetric.

10. Make a spot on the film with “Cursor” & “Spot” functions for one second exposure.

Measure the spot size with the Ruler function on the SEM program window. The spot should be less than 20 nm in diameter if the beam condition is optimized. Otherwise, repeat the beam adjustment process in Step 8.

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E-BEAM LITHOGRAPHY

1. Click on the “NPGS mode” in the NPGS program commands list (left column).

2. Select the run file to be processed from the list.

3. If necessary, use the Run File Editor to set proper initial offset (XY Move to Pattern Center values) to position the patterns to an unexposed area. Exit Run File Editor.

4. Select the run file to be processed. Click on “Process Run File” in the NPGS program commands list (left column) to perform e-beam lithography.

5. Once finish, turn off the e-beam in the SEM program.

6. Click on the “SEM mode” in the NPGS program commands list (left column). NOTE: It is important that you set operation mode to “SEM mode” after finishing the E-beam lithography. The next SEM user cannot perform SEM observation if the system is left under the “NPGS mode”.

7. Take out the beam blanker by pressing the “OUT” button on DEBEN beam blanker controller.

8. Unload the specimen from the SEM vacuum chamber. NOTE: Refer to the SEM SOP for proper sample removal procedure.

9. Sign out on the FOM for the SEM session. Sign out on the log sheet. Report any problem in the comment section and fill out the Service Request Form accordingly.

PATTERN DEVELOPMENT

1. Check the photo resist data sheet for the proper developer and development conditions. Below are recommended developing conditions for the 495K PMMA photo resist for your references.

Developer Time Conditions

MIBK:IPA 1:3 70 s for more sensitive features

MIBK:IPA 1:1 70 s For larger features (stronger developer)

IPA 20 s

The PMMA developers are stored under the lithography processing hood in the wet processing room.

2. Dip the sample into the proper developer solution and swirl constantly during the entire processing time (e.g., 70 seconds).

3. Dip the sample into pure IPA solution or DI water and swirl for 20 seconds to stop the reaction.

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4. Quickly wash the substrate with water and blow dry with the Nitrogen gas from the gas gun on the process workbench. If you have sensitive features, decrease the flow of Nitrogen gas with the adjustment knob before blow dry the substrate.

5. Pour the used developer and IPA into proper waste storage containers located under the lithography processing hood.

6. Clean up the lithography processing hood area.

7. Observe the substrate under the optical microscope located in the wet processing room. First locate the scratch mark at lower magnification. Use the scratch as a reference to locate the pattern (near the end of the scratch) at high magnification.

8. Turn off the optical microscope. Clean up the optical microscope bench before leaving the room.

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EMERGENCY PROCEDURES

If a user ever has a problem, even if slightly unsure about, ASK someone who knows and can help. There are no penalties for asking for help but there may be for not reporting damage to the equipment that may delay or prevent others from working. If, at any time, the user needs to contact someone for help, call or locate the following staff of the Shared Research Facility (SRF):

Dr. Weiqiang Ding (SRF Staff) Office: ESB G75D Phone: (304) 685-1938 cell

Dr. Kolin Brown (SRF Staff) Office: ESB G75D Phone: (304) 366-6551 cell

If no one is available and the SEM is not acting as expected, the user should do the following:

Shut OFF the electron beam (OFF button under “Observation” on top of the PC_SEM program window)

If the PC_SEM program freezes, open the front door of the SEM enclosure chamber and press the GUN button in front of the specimen exchange chamber to shut off the electron beam.

Exit the PC_SEM program

Shut down the computer Then, if possible, the user should stay with the SEM while trying to contact the above individuals. If it becomes necessary to leave the instrument then the user should leave a large, legible note on both the SEM and at least one of the above individuals’ offices, stating:

The problem (describe what happened and steps taken)

When it occurred (date and time)

User name and phone number

If a dangerous situation is evident (smoke, fire, sparks, etc.), ONLY if it is safe to do so, the user should press the RED emergency off button at the front of the SEM control console to turn OFF power to the entire SEM system and notify the proper emergency personnel. In any case, the user should leave the facility and contact emergency personnel as soon as possible from a safe place.