Electron_Beam_Lithography_Protocol by stariya

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									                                                                                                    C. Rochford
                                                                                                      May 2009

                               Electron Beam Lithography: Protocol
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Note: These are basic directions. Check the NPGS manual or ask an experienced user to show you how to use
the advanced features in order to make your EBL more automated and streamlined. If you intend to do liftoff,
ask an experienced user about using a double layer resist or see Single_Nanowire_Sample_Prep.doc

Cleanroom Sample Preparation

    1)   Turn on the hotplate and set the temperature to 120 °C.
    2)   Take a substrate approximately 1 cm2 in size (cut using the diamond saw or scratching with a diamond
         scribe) and ultrasonicate the sample for 5 minutes in 20-30 mL of acetone in a small beaker.
    3)   Remove the substrate from the acetone and promptly place it into a small beaker filled with ~20-30
         mL of isopropanol (2-propanol). Ultrasonicate the substrate for 5 minutes.
    4)   Remove the substrate from the beaker and promptly rinse it with isopropanol.
    5)  Dry the substrate using clean N2 gas. Be careful that your substrate does not blow away.
    6)  Turn on the vacuum pump sitting on the floor to the left of the fume hood. This allows suction
        through the small hole in the spinner stage.
    7) Open the top of the spinner and place your sample over the small hole. Make sure that your sample is
        level and does not move easily.
    8) Remove the PMMA bottle from the cabinet underneath the fume hood and use a glass capillary tube
        to quickly deposit ~10-15 L of PMMA on the top of your sample. Spread the PMMA around, but try
        not to touch the tube to the substrate surface.
    9) Close the spinner lid and flip both switches to start rotation. Spin at 4000 rpm for 60 seconds.
    10) Turn off the vacuum and remove your sample from the spinner. It is helpful to make a scratch from
        the edge of your chip to a position near where you would like to do your writing. This will help you
        navigate to the appropriate position and optimize your focus. Place your chip on the hotplate surface
        and cover it with a clean beaker or Pyrex Petri dish. Allow the substrate to soft-bake and remove the
        volatile solvent for 5 minutes.
    11) Remove your substrate from the hotplate (it should be light green/blue in color with no spots if on
        (100) silicon), place your sample in a covered container for transport to the SEM, and clean up the
        fume hood work surface.

Designing a Pattern and Run File

    1)   Make sure that the NPGS (right) PC is on. Double click on the NPGS icon.
    2)   Go to your directory, which should have been created during your EBL training.
    3)   Within the NPGS interface, click on the „DesignCAD‟ button on the left hand side. Use DesignCAD
         to create your pattern as a collection of lines or polygons. Ensure that you use the functions under the
         NPGS tab to create your features. Arrange the layers so that your finest features are written first.
    4)   It is critical to save your pattern under the „NPGS‟ tab NOT under the standard „File‟ tab.
    5)   Close DesignCAD and find your file within the appropriate NPGS directory. Right click on the
         pattern that you just created and create a run file for this pattern.
    6)   Within the Run File Editor, you can create as many copies of different patterns as you would like. The
         most important features are the „Magnification‟, „Beam Current‟, and „Dosage‟ values on the right
         hand side of a given pattern.
    7)   The optimal magnification for best signal to noise ratio is 1400x, but you may write at whatever
         magnification you want. NPGS will not let you enter a magnification which is too high. Also, select a



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                                                                                                     C. Rochford
                                                                                                       May 2009

        value for the beam current at which you would like to write. This is somewhat arbitrary and
        determines the speed of your writing (any values between 10-150 pA are reasonable).
    8) The beam dosage is the amount of time you will spend at each pixel in your pattern. This value is
        highly dependent upon the material of your substrate. Also, this value varies depending on if you are
        writing a line (line dose) or a polygon (area dose). For writing on Si coated with 2% PMMA, it is
        recommended that you use 2-6 nC/cm for lines and 200-250 mC/cm2 for polygons. You should have
        optimized these parameters during training.
    9) Make sure you save the run file when you are done. Also, under the main NPGS interface, simulate
        writing in order to perform an error-check of your run file, as well as to get a writing time estimate.
    10) Consult JC Nabity‟s NPGS manual if you have any questions.

Getting the Beam Ready for Writing

    1)    Make sure that your PMMA-coated substrate is affixed to an aluminum stub. You may use silver
          paste, carbon tape, or the metal clips on the raised stub. If using silver paste allow your sample to dry
          in air or clean N gas for at least 20 minutes before placing in the SEM chamber, this will prevent
          excessive pumping times and gas bubbles which can disrupt vacuum quality during viewing.
    2)    It is extremely helpful to place your sample on the special EBL stub. This large stub has a small
          wedge-shaped piece of silicon coated with a thin layer of gold nanoparticles. These nanoparticles
          allow for better focusing and astigmatism correction. The raised stub also has a Faraday cup, which
          allows you to accurately determine your beam current and improve reproducibility of your
          lithography.
    3)    Because you don‟t want to expose your sample to the beam until everything is ready, draw a detailed
          map of the different components on your stub. This allows for easy navigation between substrates
          (especially if you‟re using a standard sample for focusing).
    4)    Follow steps 2-5 on the JEOL SEM directions. Select a spotsize that provides roughly the desired
          current listed in your run file.
    5)    It is fundamentally important that you don‟t want to expose your sample until you are ready to do so.
          Align the stage so that when you turn on the electron beam it does not expose your sample.
    6)    When closing the SEM chamber door, double-check that the stage/stub-holder will not be hitting any
          components in the chamber and you will not be aimed at your sample immediately when the beam is
          turned on.
    7)    Connect the BNC from the Keithley picoammeter to the SEM chamber door. The supplied BNC cap
          should be unconnected. This will allow you to monitor the beam current.
    8)    If you‟ve just turned on the picoammeter, be sure that the „ZC‟ setting is not activated (this shorts the
          connection to view the noise in the signal which will hover at around 0.5-1.0 pA regardless of the spot
          size). Having the „CZ‟ setting on is okay, because this is just the zero-level subtraction of the noise.
          Also, it is often useful to convert the readout to scientific notation by pressing „Menu‟ and then
          scrolling down using the arrows and changing from „ENG‟ to „SCI‟.
    9)    Follow steps 7 and 9 on the JEOL SEM directions.
    10)   It is suggested that you use the Au nanoparticle standard to get an extremely good focus and
          stigmation at 50kx to 100kx. You should use an EHT of 30 kV except in rare cases.
    11)   After you optimize your beam, double check the beam current in the Faraday cup. If the spotsize
          needs to be adjusted, go back and re-optimize your beam before proceeding to your chip.
    12)   Use your map to navigate to the edge of your PMMA covered substrate. Refocus on this new location.
          If you made a scratch, follow it to the end under appropriate magnification in order to limit unwanted
          exposure. Focus on a small speck on the PMMA at the end of the scratch.
    13)   It is critical to make sure that you have the magnification set to the exact value given in the run file.
          Otherwise your sample will not be the correct size and will not be exposed properly. Check the
          manual to see how NPGS can do this for you.



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                                                                                                C. Rochford
                                                                                                  May 2009

    14) In order to check your beam, go to a blank region of PMMA near your scratch (still zoomed in to limit
        exposure), and press the “3s Dot” button on the NPGS front panel. This will let the beam sit in one
        spot for 3 seconds. When it is done, zoom in and check for a donut shaped contamination spot. You
        can then focus on this spot, which represents the surface of the PMMA. If the spot is not round, then
        you have some astigmatism. If you can‟t see it spot, then your focusing is probably bad.

The Writing Procedure

    1) Set the ScanService blanker to “External”
    2) Under the SEM „Setup‟ tab click on the „Setup Ext Scan‟ utility. Change the external Scan Control to
       CH1. This will now allow the NPGS program to control the SEM beam and stage.
    3) On main NPGS interface select the „NPGS mode‟ button on the left-hand side.
    4) Check the picoammeter to verify that the beam is blanked. Use the stage control to move to your
       desired location (This can also be done as a “Move only” element in your run file.)
    5) Process your run file.
    6) When you are done using the SEM, perform the SEM shutdown procedures given in steps 11- 14 on
       the JEOL SEM directions. Also be sure to turn off the External Scan Control and set the ScanService
       blanker back to “On”

Developing your sample (Requires two clean beakers)

    1) Fill a clean beaker with ~30 mL of the 3:1 IPA/MIBK developer.
    2) Use your tweezers to dip your sample completely in the developer for 70 seconds. Be sure to
       vigorously move the sample around to ensure uniform development.
    3) Remove your sample from the developer and promptly rinse with ~40 mL of isopropanol. This will
       stop the developing process.
    4) Dry your sample immediately with clean N2 gas.
    5) Your sample is now ready for metal deposition or etching.




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