Wafer Alignment for Raith e-Beam Writer
Roger Robbins 2/24/2014
This is a document describing practical steps to align an un-patterned 4 in dia wafer to the coordinate system of the Raith e-beam writer stage.
In order to align a pattern to the wafer center and flat and thus be able to align multiple levels to each other during a patterning alignment scheme, the wafer itself must be aligned to the Raith wafer platen and to the stage coordinate system. This is similar to defining the u,v coordinates of a small chip in terms of the stage absolute x,y coordinate system to form a local chip-oriented coordinate system for pattern alignment. Alignment markers can be written in the substrate for initial pattern alignment and serve as well for multiple level alignments.
This procedure assumes that there is a file copy of a 4 inch diameter (100 mm) “wafermap” already defined in memory. If it is not there, you will have to make one and define the flat. (I have made one in my workspace that I can transfer to your workspace if you ask.)
- Load a 4 in dia. wafer onto the Raith wafer platen, making sure that all three of the alignment pins touch the edge of the wafer.
- Load the wafer platen onto the Raith stage.
- Activate the electron beam.
- Move the wafer stage to x,y = (-48.779894, +14.90236) mm, which is the location of the first wafer flat locating pin. Set this up as a position in the Stage Control table for one-click position command convenience.
Figure 1. Stage “Go TO” window.
Figure 2. SEM image of alignment at Wafer Flat pin.
- Start the electron beam scan and move the stage a small distance away from the pin using the stage joystick.
- Focus on the surface and fine tune the focus with a “burn spot.”
- Under the File Menu at the top left of the Raith window, choose the menu item “wafermap…” and select “4in Wafer.WLO”. This will bring up a map of the wafer platen showing the wafer location and orientation.
- Then under the “Edit” menu, select “Un-patterned Wafer Adjustment.” This will open a second window with wafer alignment parameters – Fig. 3.Figure 3. Wafer map of 4” wafer with flat and 3 alignment points mapped. Command Menu (Rt).
- Click on the lightning bolt symbol for location P#1 in the alignment menu. This will drive the stage to the last known stage coordinates of the P#1 location – which could be far off the edge. Use the stage video camera and the stage joystick to find your way back to the P#1 wafer edge.
- When you find the edge, click on the “READ” button for the P#1 location.
- Repeat this for P#2 and P#3.
- Next, skip to the “Deskew Marks” section of the alignment window and click on the lightning bolt for location #1.
- Again this might take you into the distant wilderness, but use the video camera and stage joystick to find a spot on the upper portion of the wafer flat.
- Click on the “READ” button for Deskew Marker #1
- Repeat for Deskew marker #2 at a spot below the first one.
- Finally, click on the “Adjust” button at the bottom of the window to accept and enable the locations.
Note: If you had to find the designated locations by long movements of the stage, then repeat the alignment to make sure of the accuracy. Wafer alignment points after second alignment should match wafer map accurately.
Note: Wafer coordinate system origin is set to (0,0) at the center of the wafer.
Also, note the orientation of the (u,v) coordinate system relative to the wafer flat.
(u,v) = (50, 0) is defined perpendicular to the wafer flat edge and is located at the right edge of the depicted wafer in the wafer map.
(u,v) = (0,50) is located at the top of the image in the wafer map.
Note that this coordinate system orientation is rotated by -90 degrees compared to the optical convention of having the top of the pattern away from the wafer flat and the bottom of the pattern at the flat.
The implication of this is that our optical patterning normally orients patterns with the y-axis perpendicular to the flat and the x-axis oriented parallel to the flat. Thus if you just write the Raith e-beam patterns in their default orientation after this alignment, they will be rotated 90 degrees counterclockwise from the optical pattern. Therefore to match optical patterns, the Raith patterns must be rotated by -90 degrees (clockwise) and the array orientation must also be rotated by -90 degrees.