MagScale Calibration

Absolute Pattern Size

<<<< First, confirm that the NPGS Y output range is set to the maximum allowed input range for the SEM (see "y_mode" and "Y_range" in Pg.sys which could be found in options > system files on the NPGS software). Do not connect NPGS to the SEM unless the maximum X and Y voltage output ranges from NPGS are below the safe input voltage ranges for the SEM as described in the documentation for the microscope. >>>

  • Mount one of your pre-patterned samples and put it into the microscope for viewing with the patterns aligned horizontally on the microscope display screen and optimize the SEM at the typical settings for lithography (usually 30 kV, 12.5 mm WD, and 120 pA of current, the values for the working distance and current are to be verified during the focusing steps as explained in this user manual).

<<<Focus on the edge of one of you patterns. This is essential in order to get the correct size calibration. If using a PC based SEM, check the "Magnification Calibration" setting in the SEM software, since the value specified must be used for all subsequent lithography. Basically, it does not matter what the SEM magnification calibration is set to, i.e., Display, Polaroid, or Video Printer, as long as the setting used during the following calibration is always used for lithography.>>>

  • Change the system to pattern writing mode* and process the run file: "scale1" (found in the "Samples" project) with the calibration standard in view at the desired magnification (for example 500 X). This alignment pattern has a single window in the form of a square frame and the overlay consists of a square with side lengths suitable for the user. See the "Introduction to NPGS (NPGS: System Installation) System Installation: Summary Checklist" section of the NPGS Manual under "Help" in the NPGS Menu program for color images. A frame is used rather than a filled square for two reasons: 1) To reduce the time needed to acquire the image; 2) To make the scan direction parallel to each side of the structure that will be imaged.

*Setting the system to pattern writing mode normally involves selecting external control for the XY scanning of the microscope. This is often done using a manual NPGS/SEM switch box, through software on the microscope, or with the NPGS Remote Enable Switch through software on the NPGS PC. Also, the microscope's blanker may need to be set to external control, usually with a switch on the blanker electronics.

  • Use the keyboard command 'a' to auto contrast the image, 'p' to set the pixel size to 1, and use 'c' to set the center-to-center spacing to a high number for coarse alignment and a low number for fine alignment. The command '?' will display a brief explanation for all the keystroke commands that are available. (Hovering over the keystroke commands displayed in the upper left part of the screen will display a brief description and clicking will run the command.)

 

  • While viewing the pattern on the NPGS PC screen, hit "s" on the NPGS keyboard so that "SEM Mode (Overwrite)" is displayed on the NPGS PC screen. This mode causes NPGS to overwrite each scan of the image. It is then useful to change the SEM parameters to see which ones affect the image. For example, the scan rotation should rotate the image and the electronic image shift controls should offset the image. Any SEM parameter that changes the display during the calibration will also have an effect during pattern writing, so this is a good chance to learn which SEM settings are important
  • If necessary, use the scan rotation to make the pattern parallel to the overlay lines. (The command 'o' should be used to overwrite the previous image after any change in the scan rotation or sample position.) Use the 'Ins' key to select between "Window=ALL" and "Window=MAG". When in the "ALL" mode, the arrow keys will change the position of the overlay, and in the "MAG" mode, the arrow keys will change the size of the overlay. Use the arrow keys while switching between "Window=ALL" and "Window=MAG" until the top and bottom lines of the overlay are aligned such that the pattern will fit between the horizontal and vertical overlay lines (to within one screen pixel). Note that there is a minimum and maximum allowed change for the overlay magnification (0.5x to 2x). Consequently, if the overlay cannot be made to match the grid size, stop at the allowed magnification limit, and continue with the next step. Subsequently, when the process is repeated, it will be possible to make the overlay match the pattern.
  • Hit SPACE BAR to calculate the new magnification. If an error message is displayed when the space bar is hit, it means that the "mag scale" value presently being used too far from the correct value to allow the system to be calibrated in a single step. In this case, repeat the previous step, but position the overlay bars such that an error is not displayed, then continue to the next step. When the entire process is repeated, an accurate alignment of pattern as explained in the previous step should be possible. If nothing seems to happen, it means the new "magscale" factor has been calculated.
  • Hit "Ctrl-E" and a message should appear at the top left corner of the screen displaying the new "magscale" factor. Write down this new number and go to your working directory \NPGS\Project\YOURDIR and go to menu > Options > Systems Files and edit  Pg.sys to modify the "mag scale". If the standard feature originally appeared larger than the designed distance, "mag scale" will decrease, and if the standard feature originally appeared smaller, "mag scale" will increase. Note that changing the "mag scale" parameter does not change the actual point spacing on the sample, but only tells NPGS what the spacing actually is.
  • Process the run file "scale1" again and verify that the vertical size of the highlighted box matches the known feature size. If the match is not satisfactory, repeat the steps 3 to 6. To restart, follow the instructions that are displayed after Pg_Edit is closed, or return to step 3.
  • If the "mag scale" parameter has been changed, it should be updated in all "Pg.sys" files. They are found in "\NPGS\Projects", "\NPGS\Projects\Samples", and in each project directory created by the user. Note that from within Pg_Edit, clicking "File -Save as Default" will copy the currently displayed system file to the master set of system files in "\NPGS\Projects" and clicking "File -Load Defaults" will copy the corresponding master file to the current project.

Also, the "mag scale" value must be copied to the projects in any "Office Installations" of NPGS, so that run files made in the office installations will have the correct calibration for use on the SEM

The mag scale is a number that correlates magnification and field size. For example, a mag scale of 95000 means that at 1000x magnification, the field size will be 95x95 um^2. This is why this value must be modified when the height of the sample is changed. By changing the height of the sample, the field will change and thus the mag scale needs to be changed. If you fracture and you have holes between your fractures, this is most probably your problem.