Raith Freehand Leads Patterning for Random Graphene Flakes

Roger Robbins                                                                                                                                                               3/19/2015


This is an instructional paper describing the process of generating Leads patterns and aligning them to randomly located Graphene flakes using the Raith e-beam lithography tool’s pattern design capability of overlaying patterns on SEM images of the target Graphene flake.


It has become common practice to deposit a solution of liquid-suspended Graphene flakes onto a specially prepared substrate and allow the Graphene to settle and adhere to the substrate as the fluid evaporates.  The substrate will have already-prepared arrays of metal pads to serve as electrical contact points for external circuitry to test properties of the Graphene.  The problem is to connect the randomly located Graphene to the nearest metal pads via electron beam lithography.  This means that the substrate will have to be scanned by the SEM to locate the selected Graphene flake and then, the operator has to design, in real time, a set of lead patterns that will connect the Graphene to the pads with sufficient alignment accuracy to effect a satisfactory connection.

Figure 1.  SEM image of Gold contact pads with Graphene flakes showing typical flake dimensions.

Preparation Details

The normal procedural steps follow a plan as follows:

  • Prepare the substrate by using optical lithography to define a rectangular matrix of numbered metal contact pads such as 300 micron squares stepped on 600 micron grid spacing, thus giving a nice size pad to make signal contacts and an area in between the pads for Graphene flakes to settle in an electrically isolated space.
  • Deposit the Graphene flakes and dry the substrate.
  • Examine the substrate under an optical microscope and determine if there is sufficient Flake attachment to examine in an SEM for flake integrity and selection.
  • Examine the flake with an SEM at ~low voltage and locate flakes suitable for electrical testing. Rank the flakes according to quality and link their location to the contact pad number.
  • Load the substrate into the Raith e-beam lithography tool and align the origin and coordinate angle to the contact pad array. Use the corners of the pads and align by nestling the edges up to the cross-hairs on the SEM screen.

Figure 2.  Method of manual alignment to contact pads for coordinate location.  Accuracy error should be less than a half micron.

  • From previously collected data select the top ranking Graphene flake locations by pad number and decide on a Global Coordinate location that encompasses the region of the flakes. Then align to the 3 pad corners corresponding to the P1, P2 and P3 alignment points.  Watch out for image drift by re-checking the 3 points several times and performing final adjustments until they are stably located.  Record the pad numbers linked to the P1, P2, and P3 alignment locations.
  • Drive back to P1, burn some spots, and do the final write-field alignment for a 100 micron field size.
  • Drive to the first flake location and set the stage under the cross hair so that the flake is shown in a scan field that includes 4 corners of the adjacent pads around the flake. (See the right side image in Figure 5.)
  • Choose a “Write Field” size big enough for an image to capture the flake and 4 corners of the adjacent contact pads. (500 microns at 200X mag in the case of 300 micron pads separated by 300 microns of space)

Figure 3.  For defining Write Field size during pattern creation, select the 500 x 200 um size to incorporate the area between the corners of adjacent contact pads.

  • Open a new Position List (P.L.).
  • Open the Scan manager window and select “Images”

Figure 4.  Location of Scan Manager “Images” menu.

  • Under the “images” menu select the appropriate write field size (500 microns) and drag this selection into the Position List.
  • Execute the position list by clicking the blue square in the P.L. command line. This will cause the tool to make a high resolution SEM image of the field which will then vanish when it is complete.

  • Click on the GDSII icon and select the “New Geometry” icon. This will open a new Edit window for designing a pattern.
  • Double click on the P.L. image item line and another window will pop up with the image of the flake and pad corners.

Figure 5.  Raith window showing both GDSII Editor and P.L image simultaneously after image transfer to GDSII window.  “Show Video” command comes from main top line command menus under “Options”.

  • Select “Options” from the top icon line of the main window and choose the “Show Video” from the drop-down menu. This will copy the flake and pad image to the GDSII edit window.
    • Note: the image will land at the Global coordinates where it lives relative to the Global coordinate origin, so you may have to go find it via the “show all” command in the GDSII editor toolbox, or just reduce the magnification of the edit field.
  • Once you have found the image, magnify it to fill the editor window, showing the flake and the 4 contact pads.
  • Create the connection leads geometries: (Figure 5 – Left side image with blue pattern lines )
    • Select the “line” icon in the editor toolbox and draw a line from an anchor location on the 45 degree diagonal precisely 50 microns into the lower left corner pad and drag the other end of the line to an appropriate location next to the Graphene flake and stop.  Select “Properties” of the line. And define the pad end as -50 microns in both u and v coordinates from the crosshair edge corner coordinates of the pad (not the rounded corner edge).  Also set the width of the line to something like 2 or 3 microns.  The pad end of this line will determine the accuracy of the alignment – be careful with this location.  You also should be able to calculate the pad corner location from the number of pads and spaces from the P1 alignment origin.
      • Note: If the line inside pad corner is widened, then the Working Area location is pushed farther into the pad area by L, which is related to the angle of the line by L = ½ W*tan(θ), where W is the line width.  If this is significant for the flake alignment, then you must adjust the position of the LL end of the line by moving the end-point towards the upper right @ 45 deg by a distance of L, (which will be W/2 if the angle is truly 45 deg).

  • Repeat the line creation procedure for the other connection lines, making sure that the anchor points of the other lines do not extend to coordinates points less than the lower left corner anchor.
  • Finish the connections to the Flake with smaller width lines from the ends of the major lead lines.
  • Save this GDSII pattern with a name linked to the Flake ID.
  • Proceed to repeat the pattern creation with all the other flakes you desire to work with.

Patterning Details

The next step is to coat the substrate with positive e-beam resist, align and pattern the leads, develop and apply metal for a lift-off procedure.  The detail procedure for alignment and exposure follow:

  • Coat the substrate with PMMA 950 A4 e-beam resist spun at 3000 rpm with an acceleration of 3000 rpm/sec. Soft bake at 180 C for 2 minutes.
  • Load sample into the Raith e-beam lithography tool, first noting which way the contact pad numbers are facing so you can find the previous alignment points.
  • Set the high voltage to the appropriate value for exposure.
  • Select a 100 micron write field size – since we designed the leads patterns to overlap the corner pads by 50 microns, we need the 100 micron write field size to match for proper alignment. (The Raith software lands the center of the lower left corner write field of the GDSII patterns at the P.L. placement coordinates.)
  • Drive to the lower left corner of the chip and focus on the surface.
  • Drive to the exact P1 pad location used in designing the Leads patterns, and set the origin location and then drive to the P2 location and set the chip coordinate system angle.
  • Return to the P1 point and align the chip to the cross hair edges at a magnification of something like 10K X. Keep this magnification for each of the 3-point alignment targets.
  • After completing the 3 point alignment adjustments, repeat the measurements to avoid inadvertent image drift misalignment issues.
  • Create some burn spots at the P1 location and perform the write field alignment procedure at this time.
  • Open a new Position List window.
  • Drag the GDSII patterns into the Position List and set the U,V coordinates to the pad corner coordinates appropriate for each of the patterns.
  • Measure the Faraday cup and drive back to P1.
  • Click on the calculator icon in the top row of commands and set the exposure values, then calculate the dwell time. This will transfer to all of the P.L. entries.
  • Make sure your electron beam is still focused and expose the patterns.
  • Develop the pattern and examine under the optical microscope to assess the accuracy of the alignment.

Figure 6.  Example of freehand designed connecting leads to “pseudo-Graphene” flake of photoresist using the above alignment methodology.

  • Proceed to the metal deposition and lift-off process.