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Specific Process Knowledge/Lithography/EBeamLithography/2D detection system

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Introduction

In addition to alignment by beam scanning the JEOL 9500 system also has a 2D alignment feature. The working principle of this is that the system will take a SEM image of a specified location and compare the SEM image to a reference image. By comparison of these the system will determine an offset and use this for alignment.

Possible advantages of this is

  • Alignment can be done using either the Secondary Electron or the Back Scatter detector or a combination of them
  • Alignment might be possible to features with very low contrast
  • Alignment can be done to an arbitrary shape

The following will illustrate how to set this up. It is a bit more complicated than regular beam scan alignment and knowledge of beam scan alignment is a prerequisite for understanding the following. Please contact us if you would like to try this alignment mode rather than trying on your own. For reference, the JEOL provided manual for 2D alignment can be found here: File:2D mark detecting 20171212.pdf

Reference images

During alignment the system will take SEM images and compare these to reference images of the alignment marks. These reference images can in principle be SEM images obtained from the system but it is simpler to use black and white drawings of the alignment marks. In this example we will use a cross as illustrated below.

Alignment mark used in this example. The mark is 1000 µm in both directions.

The system must know the scale of the mark it is looking for, hence one must assign a physical dimension to the reference mark. The reference image must also be converted to a TIFF image. Both dimension assignment and conversion to TIFF is done with a command in a terminal window. In this example the actual mark is 1000 x 1000 µm and the input image is a PNG file called "PQRef1000mu.png". The command is:

ebimg setsize inputimage.png outputimage.tiff dimension

The dimension is given in nanometer. In the example the actual command is

ebimg setsize PQRef1000mu.png PQRef1000mu.tiff 1000000

This will generate a TIFF file with a scale of 1000 µm. Next, this must be converted with yet another command to a .ref.gz file. The command is

ebimg mkref inputimage.tiff outputimage.ref.gz

In our case it will be

ebimg mkref PQRef1000mu.tiff PQRef1000mu.ref.gz

After entering this command a window will open, showing the reference mark with a green cross indicating the center. For a symmetric mark this green cross will be in the center. It is possible to change the center point of the mark by moving the green mark with the mouse, this could be useful if an asymmetric mark is used. However, for a symmetric mark simply press "ESC" to close the window. This will generate the actual reference image file with the correct scaling. This file should be moved to

/home/eb0/jeoleb/prm/mark

SETWFR and CHIPAL setup

The alignment procedure is setup using the same subprograms as usual. The only difference is that we select a different scan type. Thus, in the "Scan Condition Settings" window, in the "Scan Type" pane, we select "Arbitrary shape" instead of the usual "Cross mark". This enables the "2D" pane of the settings window. In the "2D" pane we can now set a scan size, which should be similar to the feature size or slightly larger. With the "Reference" button we can select the reference image file previously generated. In this example we use a cross but as the "Arbitrary shape" scan type suggests, an arbitrary shape can be used.

Settings of the "Scan type" and "2D" panes.

Finally, in the "Gain" pane we can setup the gain parameters. Notice that it is possible to switch between "BE" and "SE", i.e. backscattered electrons and secondary electrons. Depending on the layer stack and materials there can be a big difference in the contrast and this is the only way on the system to use secondary electrons for alignment.

It can be difficult to get these gain settings right. The best option is to move the stage to the mark location, turn on SEM mode, find working settings manually and copy these to the scan conditions using the "Applies to another subprogram..." feature.

Gain settings used for image scan. Unlike the usual "Cross mark" scan type, the "SE" detector is now available for alignment.

Setup for CHIPAL scan conditions is exactly the same as for SETWFR and hence we will skip it here.

Execution

Execution is like usual by either running SETWFR or CHIPAL or the usual commands for automatic execution. During execution the system will make imagescans with the chosen detector and compare the resulting image with the reference file. If the match is accepted, the system will continue to the next mark. Below is the result of our example.

Imagescans obtained during alignment. Rough alignment scan (left) is 1000 µm while fine scan (right) is 10 µm. The green cross indicates the center as determined by the system.

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