Overviewsnsf.stanford.edu/equipment/fab/snp/SOP_Nabity.docx · Web viewThis manual describes how to perform basic e-beam lithography with the Nabity Pattern Generation System (NPGS)

Standard Operating ProcedureNabity Pattern Generation SystemContentsOverview.....................................................................................................................................................2

Operations...................................................................................................................................................2

Preliminary steps.....................................................................................................................................2

Prepare design files.................................................................................................................................3

Modifications to pattern files..................................................................................................................3

Modifications to alignment files..............................................................................................................5

Prepare sample........................................................................................................................................6

Create run file..........................................................................................................................................7

Prepare microscope settings for writing................................................................................................10

Execute the run file................................................................................................................................10

Tidy up...................................................................................................................................................11

Preparing a dose array...........................................................................................................................11

Trouble Shooting.......................................................................................................................................13

Contact:.....................................................................................................................................................13

About this document.................................................................................................................................14

Nabity Pattern Generation System v1.0 10/22/2012

OverviewThis manual describes how to perform basic e-beam lithography with the Nabity Pattern Generation System (NPGS) that is installed on the Nova NanoSEM in SNC. NPGS contains a number of advanced features, such as pattern fracturing and automatic alignment, which are fully functional but are not described here. For instructions on using these advanced features, please refer to the NPGS documentation published by Joe Nabity.

This manual assumes knowledge of the basic principles of e-beam lithography; users who are new to e-beam lithography should contact SNC staff or another labmember for assistance.

NPGS is an aftermarket product designed to give e-beam lithography capability to an SEM. NPGS takes as input a pattern file and a few user-supplied parameters such as beam current, dose, and dot spacing. It then calculates a sequence of beam movements and beam blanks that will write the pattern on the sample.

The main components of our lithography system are:

NPGS computer. This computer has DAC outputs to control e-beam deflection, and can send other commands to the SEM (e.g., set magnification) via an Ethernet connection to the SEM computer. It also has an analog input for reading image data from the SEM.

Scanservice beam blanker controller. This box can apply a high voltage to the beam blanker in the SEM, which deflects the beam such that it does not reach the sample stage. When set to EXT, this controller will toggle beam blanking based on an output from the NPGS computer.

FEI integrated ammeter. The SEM has a current measurement panel, which reads the beam current with ~1 pA sensitivity.

A typical lithography procedure involves the following sequence: create pattern files, measure beam current at desired spot size, move SEM to target location, create NPGS “run files” indicating alignment and writing parameters, execute run file. The following sections will describe these steps in greater detail.

OperationsPreliminary steps

1. Log on to Badger.2. Turn on the NPGS computer below the desk, if it’s not already on (this computer should be left

running for DAC stability). The NPGS computer shares a monitor and keyboard with the transfer/Badger computer. You can switch between computers by pressing scroll lock twice.

3. Log in to the NPGS computer as “user”, if you’re not already logged in as such. If you do not log in as user, the NPGS system will not communicate properly with the SEM.

4. Run the NPGS Menu program, if it’s not already running. Frequently, the NPGS program will open a DOS terminal for the DAC calibration to run. You must allow this calibration to complete (~1 minute); cancelling it will throw off the calibration. To manually initiate the DAC calibration, you can click on the “Calibrate DACs” button in the “Commands” dropdown menu in the NPGS program.


Prepare design files1. Before arriving at the SEM, use an external CAD editor (e.g. DesignCAD, Layout Editor, L-Edit) to

design your pattern. Your pattern will typically contain polygons of two varieties: pattern polygons, which are areas to be written, and alignment polygons, which correspond to objects already on your sample (e.g., alignment marks), which NPGS will use for fine pattern alignment.

2. Export the design either as a DesignCAD file (.dc2) or GDSII file (.gds). You should export one file (or more if you do multiple writes) containing your pattern polygons, and one file (or more if you do multiple alignments) containing your alignment polygons.

3. Create a project folder within the NPGS program, if you don’t have one already (Project->Create New Project).

4. Copy the design files into your personal project folder. The desktop of the NPGS computer has a shortcut to the “Projects” folder within which each NPGS project has a folder.

5. On the “Display File Types” drop down menu in the top right corner of the NPGS Menu, select “All pattern files”.

6. If you exported the design as a GDSII file, you’ll need to convert it to a DesignCAD file. Right-click each GDSII file and click “Convert to DC2”.

7. Open DesignCad LT2000 on the converted file by choosing from the menu bar Commands > DesignCAD LT2000.

8. Open your file(s) containing your design. The line style of the polygons as they appear in DesignCad determines how NPGS handles the polygons. A polygon with a simple dashed line is a filled polygon, and a polygon with a solid line is simply a shape

Modifications to pattern files1. In a pattern file (i.e. something that you wish to write), select all polygons. Press the info button,

which is a button with the blue “i” surrounded by a box.2. A window will pop up that contains settings for the selected polygons. To change the line type,

click on the button with the diagonal dashed line.3. In all likelihood, the polygons will be imported as solid lines. In order to tell NPGS to write them,

you need to click the drop down menu next to “Line Type” and choose the second option, a


dashed line.

4. To save this file, use the NPGS menu (not the File menu!) and choose Save.


Modifications to alignment files1. In a similar manner as above, make sure that all polygons in your alignment file are solid lines

corresponding to preexisting features on your sample.

2. There is a drop down menu in the upper-left portion of the window, underneath the word “Layer”, that contains a horizontal line. Open the drop down menu and choose the second option, a dashed line.

3. Using the cube button along the left side of the window, draw squares around each of the alignment marks. These squares will define the windows in which NPGS will scan to look for the alignment marks. If you suspect your initial stage alignment will be poor, you may want to make these boxes large. However, anywhere the SEM scans is likely to be exposed by the electron beam, so beware.

4. Each alignment window (and the alignment mark polygons inside) needs to be in its own layer. Do this by selecting each window-mark group individually, pressing the info button, and choosing the layer number in the drop down menu. Typically people have three or four alignment windows/marks, and will put them in layers 1, 2, 3 and 4.


5. Save this file in the NPGS menu.

Prepare sample1. Load your sample and turn on the beam, as specified in the Nova NanoSEM user manual. It will

improve the accuracy of your current readings to use a holder with a Faraday cup, although this is not necessary.

2. The best settings for writing are 30kV accelerating voltage and a working distance of approximately 5mm. Otherwise, NPGS can write at any magnification and any spot size, though higher beam currents and lower magnifications tend to limit resolution.

3. For each spot size at which you plan to write, you need to measure and record the beam current. Do this by moving to a conductive area on the sample stage that has a conducting path to the stage post. A Faraday cup (i.e. small hole in a metal surface that electrons tend to get trapped inside) works best. In the lower right corner of the FEI microscope control window there’s an updating measurement of beam current.

4. Broadly speaking, you will need to have the ability to move the microscope stage with the FEI software reasonably accurately with the beam turned off. You’ll have to move the stage so that the origin of the pattern you want to write is at (or close to) the center of the microscope field of view. Some suggestions:

a. In the FEI software menu, you can choose Stage > xT Alignment [] to rotate the stage to any arbitrary angle. This is useful to align the sample rotation by finding two existing structures on the sample that should lay along a horizontal or vertical line.

b. The FEI software allows you to move in units of millimeters (or fractions thereof), both relative to your current position and to an absolute coordinate system.


c. The stage menu contains the ability to define a user coordinate system by moving to three points and defining what you want each point’s coordinates to be. This is useful if you have alignment marks around the perimeter of your sample that exist at particular coordinates, and your pattern is centered somewhere relative to those coordinates.

5. Blank the beam by moving switch on the beam blanker box to “EXT.”6. Move to a spot close to your sample but not at a sensitive point. You may expose this area.

Zoom far enough in that the microscope field does not expose anything sensitive. Unblank the beam and focus the microscope.

7. Blank the beam.8. Move the stage to the point that you want to be the origin of the coordinate system of your

pattern file.

Create run file1. Go back to NPGS computer. Now you will create the run file that NPGS uses to perform the

actual write operation.2. In the NPGS Menu program, click the button on the left (inside the “Custom Commands” box)

that says “Run File Editor.” This will create a new run file.3. In the drop-down menu next to “Allow Advance Modes” choose “No.”4. We will have two entities inside this run file: an Alignment entity and a Pattern entity. You can

click the “Insert Entity” box to add a new entity. Set it up so the Entity Type of the first is “Alignment” and the second is “Pattern.” At this point, the Run File Editor should look like this:


5. Double-click the inside each of the two boxes that say “PatName.” In the alignment entity, choose your alignment file from your program directory. In the pattern entity, choose your pattern file.

6. Now we need to modify parameters for each of these entities. You can switch back and forth between the two entities by clicking the region below “Entity type” on the left side of the window.

7. In the case of the alignment entity, you will see subsections on the right side of the window corresponding to each layer/alignment mark. Here is a brief description of the important parameters:

a. Magnification: the SEM must be set to this magnification prior to writing. Note that the pattern and alignment magnifications should be set to the same value, unless you want to change the magnification after alignment and before writing (not typical). Upon importing the pattern files Run File Editor automatically gives a recommended value. At the bottom of the Run File Editor window it also tells you allowable ranges. For best resolution, you’ll want to choose a magnification that is close to the maximum allowable. However, if you are just a little misaligned at the maximum magnification, you will be unable to correct the misalignment by beam motion alone. Best to choose a number that is a little bit smaller than the maximum (e.g. 800x if the maximum is 850x).

b. Center-to-center distance and line spacing: these specify the spacing between dots used for the alignment mark scan.

c. Measured beam current: enter the beam current you measured from the SEM.d. Dwell: the number of counts specifies the number of averages at each pixel during the

alignment mark scan. Note that higher numbers will give higher quality images but will expose the resist more.


8. In the pattern entity, the important parameters are:a. Origin Offset: Leave this at 0,0 for easiest use. Remember, the center of the SEM’s total

scanning area should correspond to the origin of your design files.b. Magnification: Same as for the alignment entity.c. Center-to-Center Distance and Line Spacing: These two parameters determine the space

between dots that the SEM writes. Typically smaller spacings improve polygon quality (smoothness, resolution, uniformity), though this has a minimum allowable value, determined by the beam current and clock speed of the deflection electronics.

d. Measured Beam Current: Enter your measured value.e. Configuration Parameter: DO NOT CHANGE THIS. It should remain 1.f. Multiple Pass Mode: Not relevant for e-beam writing. Leave this disabled.g. Dwell: This is the amount of time the SEM spends exposing each dot. It will be

automatically calculated from the beam current and dose.h. Dose: This is the amount of charge that the beam will deposit per unit area, and

depends on your resist and substrate. Click the drop down menu to choose an Area dose, and enter a good dose value. At some point, you should manually write patterns at different doses and look and see which dose works best. Otherwise if you don’t know a good number, ask somebody who does. For 30kV microscopes exposing PMMA, this number is typically 200-400 µC/cm^2.


9. Click the Save button at the bottom of the window to save your run file.

Prepare microscope settings for writing1) The SEM should still be blanked, and the stage should be positioned so that the center of the

viewing area is the origin of the pattern/alignment designs.2) Set the microscope magnification to match the values in the run file.3) Set the microscope spot size to match the beam current values put in the run file.4) In the Microscope Control software menu bar, choose Scan > External. This cedes control of the

beam to the NPGS computer.5) Buckle your seatbelt – we’re about to write!

Execute the run file1) On the NPGS computer, right click on your run file and choose “Process Run File.” From here on

out, you’ll need to move fast!2) Occasionally a DOS terminal will pop up and NPGS will perform a DAC calibration. Do not

interrupt this process.3) A screen will remind you of the magnification your SEM should be set (check it on the SEM to be

sure), among other parameters for the write. If all of this is okay and you’re ready to go, press spacebar and be ready to do your alignment step.

4) Immediately type shift-S. This limits NPGS to perform a single scan in each window and then stopping.


5) Once all the alignment windows have scanned, type “a” to automatically adjust brightness and contrast. You can manually adjust this as well by dragging up or down the green horizontal lines on the far right side of the screen.

6) Each alignment window should contain its alignment mark. (If not, you should consider making the alignment windows bigger in the alignment pattern file!) In each case, drag the overlay to line it up with the image of the alignment mark.

7) You can get a more precise alignment by repeating this process a few times. If you wish to go another round, press the space bar and it will re-scan each window. However, the more times you scan, the more heavily you will expose the alignment mark area, so beware!

8) Once you are confident with your alignment (i.e. when scanning the windows, the alignment marks line up with the overlays), press enter.

9) NPGS will say “Recalculate matrix before saving?” at the top of the screen. Press enter or ‘y’ to apply the alignment correction that was just calculated from dragging your alignment marks to match the SEM image.

10) You’ll be prompted as to whether you want to repeat the alignment. Press enter to decline and continue on.

11) You will be reminded again of the magnification for your pattern write. If you’ve been doing things as suggested above, this will be the same as for the alignment step, so no action is required. Press the spacebar to begin writing the pattern. You will see a rendering of what the program is writing. If something is wrong, press escape to interrupt the write.

12) Once your pattern is written, you’ll hear a soothing jingle. Hit any key to continue. 13) NPGS will ask whether you want to repeat the write. Press enter to decline. This will terminate

the pattern writing program.

Tidy up1) At this point you are finished writing (unless you wish to write the pattern again somewhere

else, in which case you can move the stage on the microscope computer and execute the run file again). However, since most Nova Nano users are only interested in using it as an SEM, you need to leave the microscope settings in a reasonable state. THIS IS VERY IMPORTANT.

2) Move the stage a far distance so that once you turn the beam back on you will not expose your sample.

3) Unblank the beam by setting the beam blanker box to “ON.” Don’t forget to this, or else non-NPGS user will be very confused.

4) On the microscope computer, choose Scan > Full frame. 5) Zoom all the way out.6) Change the accelerating voltage to 5 kV and the spot size to 3.7) Turn off the beam and remove your sample, in accordance with your microscope training.8) You’re done!

Preparing a dose arrayWhen using new resist, a new substrate, or a new pattern, it’s standard practice to write an array of your pattern at various e-beam doses to figure out the optimal dose. This section will show you how to do this.

1) Open the run file editor. You might want to refer to the screenshot below as you follow these instructions.

2) Set number of entities to process to 1.3) Set entity type to array.


4) Enter the pattern name.5) Number of rows/columns specifies the size of the array.6) Set enable exposure steps to yes. A dialog box will pop up asking you to specify how the doses

should vary across your array. Note that this dialog works in percentage of the base dose, specified later.

7) Specify the spacing between the cells of the array (array spacing field) and the origin position (initial XY move field).

8) Entity 1 (the array) should be highlighted after that last modification, and you should see on the right hand side of the screen the various write parameters for your various layers. Fill these out (instructions are given earlier in this manual), and remember that the dose specified is the base dose for the array.

9) Save and process the run file. Note that you could also include an alignment entity (simply increase the number of entities to 2, and move an alignment entity before the array entity) if you wanted to align each element of the array.


Trouble ShootingIf encountering a problem, report it on Badger and send an email to the user list.

Contact:Cliff Knollenberg [email protected] list [email protected]


About this document1.0 10/22/2012 Andrew Bestwick, Patrick

GallagherInitial Document

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Overviewsnsf.stanford.edu/equipment/fab/snp/SOP_Nabity.docx · Web viewThis manual describes how to perform basic e-beam lithography with the Nabity Pattern Generation System (NPGS)