Specific Process Knowledge/Lithography/EBeamLithography/JEOLAlignment: Difference between revisions

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+'''Feedback to this page''': '''[mailto:labadviser@nanolab.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.nanolab.dtu.dk/index.php/Specific_Process_Knowledge/Lithography/EBeamLithography/JEOLAlignment click here]'''
+Content and illustration by Thomas Pedersen, DTU Nanolab unless otherwise noted.
 =Aligned exposure on JEOL 9500=
 There is quite a few things to remember in order to align an exposure to an existing pattern. The example below is a step by step guide illustrating global substrate alignment as well as chip alignment. If your job only requires global alignment simply skip the chip alignment part. In the example we assume a layer, L1, is already defined on the substrate and the goal is to align the next layer, L2, to it.
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-Prealignment using the PAMS Metrology Tool and PAMS Microscope view. Image: Thomas Pedersen.
+Prealignment using the PAMS Metrology Tool and PAMS Microscope view.
 |}
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-Setup of the AGCRG subprogram. Image: Thomas Pedersen.
+Setup of the AGCRG subprogram.
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-Result of a successful gain optimisation. Image: Thomas Pedersen.
+Result of a successful gain optimisation.
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-The '''SETWFR''' window and '''Settings''' scan condition windows. Image: Thomas Pedersen.
+The '''SETWFR''' window and '''Settings''' scan condition windows.
 |}
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 *Q mark fine scan
-The rough scan conditions are used to scan over several 100 µm to find the mark while the fine scan conditions are used to accurately determine the center in as small a window as possible. Edit the P mark rough conditions by clicking its '''Setting''' button. Set the scan position x and y to about half the size of the mark, in this case 250 µm. Set the scan width in x and y to 400 µm to scan a large area with a high chance of finding the mark. If the contrast of the mark is not optimal the number of scans or the scan clock (beam dwell time) can be increased. Go to the '''Offset''' pane and make sure all offsets are set to zero. Go to the '''Scan type''' pane and set the mark width and mark length according to the mark, in this case 20 µm and 500 µm, respectively. The mark width is the most important as the system will only acknowledge a feature of the stated dimension to be the mark. Close the window on '''OK'''.
+The rough scan conditions are used to scan over several 100 µm to find the mark while the fine scan conditions are used to accurately determine the center in as small a window as possible. Edit the P mark rough conditions by clicking its '''Setting''' button. Set the scan position x and y to about half the size of the mark, in this case 250 µm. Set the scan width in x and y to 400 µm to scan a large area with a high chance of finding the mark. If the contrast of the mark is not optimal the number of scans or the scan clock (beam dwell time) can be increased. Go to the '''Offset''' pane and make sure all offsets are set to zero. Go to the '''Scan type''' pane and set the mark width and mark length according to the mark, in this case 20 µm and 500 µm, respectively. The mark width is the most important as the system will only acknowledge a feature of the stated dimension to be the mark. With the condition setup, copy the settings to both the Q rough scan and the drift rough scan conditions using the '''Applies to another subprogram''' button. Close the window on '''OK'''.
-Also edit the fine scan conditions of the P mark. The windows are essentially identical, the only difference is the dimensions as we now want to scan a much smaller area in order to pinpoint the mark center with as high accuracy as possible. Ideally the mark should be similar to the one shown below with a wide part for the rough scan and a narrow part at the center for fine scan. In the fine scan define the scan position to be 20-30 µm from the center and set the width to 10 µm or less. The scan is made with 4000 datapoints along the scan axis, thus for a 10 µm wide scan the position resolution is 2.5 nm. The maximum resolution is 1 nm, thus if one defines a scan width less than 4 µm there will simply be less datapoints along the scan axis.
+Also edit the fine scan conditions of the P mark. The windows are essentially identical, the only difference is the dimensions as we now want to scan a much smaller area in order to pinpoint the mark center with as high accuracy as possible. Ideally the mark should be similar to the one shown below with a wide part for the rough scan and a narrow part at the center for fine scan. In the fine scan define the scan position to be 20-30 µm from the center and set the width to 10 µm or less. The scan is made with 4000 datapoints along the scan axis, thus for a 10 µm wide scan the position resolution is 2.5 nm. The maximum resolution is 1 nm, thus if one defines a scan width less than 4 µm there will simply be less datapoints along the scan axis. Again, copy the settings to the Q mark fine condition and the drift fine scan condition via the '''Applies to another subprogram''' button.
 {| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
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-Alignment marks used in this example. Image: Thomas Pedersen.
+Alignment marks used in this example.
 |}
-Once the P mark conditions are set up, setup the Q mark conditions in the same way. From the '''SETWFR''' window click '''Save''' and '''Execute'''. The mark scan will start with the P mark rough scan, if the mark is found it will continue to P mark fine scan, if successful the system will continue with Q mark rough scan and finally Q mark fine scan. Below is a few examples of an mark scan routine. In the left image the system has scanned the mark in the x-direction and found a nice feature, the yellow line indicates the system is currently scanning in y. Observe that in a y-axis scan the beam scans along the y-axis to determine the x-coordinate of the mark and vice versa. The center image shows a completed fine scan in a 10 µm window. The last image shows the data output after successfull mark detection. The important parameter is the P mark observation position offset. In this case the offset is (88,-335), this is entered in the SDF file '''OFFSET''' command to make sure the mark is easily found during job execution.
+From the '''SETWFR''' window click '''Save''' and '''Execute'''. The mark scan will start with the P mark rough scan, if the mark is found it will continue to P mark fine scan, if successful the system will continue with Q mark rough scan and finally Q mark fine scan. Below is a few examples of an mark scan routine. In the left image the system has scanned the mark in the x-direction and found a nice feature, the yellow line indicates the system is currently scanning in y. Observe that in a y-axis scan the beam scans along the y-axis to determine the x-coordinate of the mark and vice versa. The center image shows a completed fine scan in a 10 µm window. The last image shows the data output after successfull mark detection. The important parameter is the P mark observation position offset. In this case the offset is (88,-335), this is entered in the SDF file '''OFFSET''' command to make sure the mark is easily found during job execution.
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-Examples of PQ mark detection scans. Left: P mark rough scan. Center: P mak fine scan. Right: Output in the '''Calib''' window. Image: Thomas Pedersen.
+Examples of PQ mark detection scans. Left: P mark rough scan. Center: P mak fine scan. Right: Output in the '''Calib''' window.
 |}
 =Chip alignment - CHIPAL=
-If the exposure uses chip alignment it should be setup and tested with the CHIPAL subprogram.
+If the exposure uses chip alignment it should be setup and tested with the CHIPAL subprogram. Any chip with a scan mark can be used to test the chip alignment settings, one must of course know the center position of the chip as defined with the '''ARRAY''' command in the JDF.
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-CHIPAL parameter window and CHIPAL scan conditions. Image: Thomas Pedersen.
+CHIPAL parameter window and CHIPAL scan conditions.
+|}
+Select the '''CHIPAL''' subprogram and edit the parameters. In the '''Measurement mode''' choose '''Mode 1''' or '''Mode 4''' for one or four mark detection, respectively. Enter the chip center coordinates in the '''Chip center coordinate position''' (global coordinates). Enter the chip alignment marks (local chip coordinate system) in the '''Chip mark Mx design position''' fields. This will determine the position of the test scan. The scan parameters with the '''Settings''' button. In the settings window set the scan position close to the center of the mark, typically 10 µm. Make the scan width fairly small for best accuracy, just like for the rough scan the scan will consist of 4000 datapoints. Make sure the offsets are set to zero in the '''Offset''' pane and that the correct mark dimensions are set in the '''Scan type''' pane. Accept the parameters with '''OK''' and click '''Save''' and '''Execute''' in the '''CHIPAL''' window.
+{| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
+|-
+| [[image:Chipscan.png|400px]] || [[image:ChipResult.png|400px]]
+|-
+| colspan="2" style="text-align:center;|
+Chip mark scan result in the SSP window and the '''Calib''' window.
+|}
+The chip mark is now scanned and the resulting backscatter signal can be seen in the SSP window. After successfull mark scan the result will also be shown in the '''Calib''' window.
+=Job execution=
+After successful execution of '''SETWFR''' and '''CHIPAL''' the condition file can now be saved and the exposure job can be executed, just as in the case of an unaligned exposure.
+=Alignment troubleshooting=
+Alignment on the JEOL system can be tricky, in this section we will briefly try and explain the cause and remedy for the most common problems encountered during the alignment process or job execution.
+==Nothing detected in P mark rough scan==
+===Possible cause===
+#The gain settings are not correct.
+#The scan width is not big enough, i.e. the scan does not hit the mark.
+#Scan width is already 500 µm but still there is no signal.
+===Solutions===
+#If the gain settings are not correct they can be adjusted via the automatic gain correction program '''AGCRG''' as described in the procedure above.
+#Increase scan width to 500 µm. There is cassette dependent offset between the prealigner stage coordinate and the actual system coordinate, increasing the scan width to 500 µm should however always overcome this offset.
+#If the mark is made with a very thin line it can be hard to detect in a wide scan. The scan will consist of 4000 datapoints, if for instance the scan width is 500 µm the resolution of the scan is 125 nm. If the mark is only 1 µm wide it would show up in 8 points out of 4000 datapoints, that is not enough for the system to accept it as a feature.
+==Using the SEM for position verification==
+If marks can not be found it can be necessary to use the SEM mode to manually verify the stage coordinate of the mark. Using the SEM mode should be kept at a minimum as the SEM mode will expose the area you are looking at and, more importantly to the tool, it will evaporate resist off your sample and into the system. Thus SEM use should be kept at a minimum and SEM should not be used on resist covered areas at beam currents higher than 6 nA.
+= 2D alignment option =
+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.
+{| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
+|-
+| [[image:PQRef1000mu.png|300px]]
+|-
+| colspan="1" style="text-align:enter;|
+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.
+{| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
+|-
+| [[image:scantype.png|400px]] || [[image:2Dpane.png|400px]]
+|-
+| colspan="2" style="text-align:enter;|
+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.
+{| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
+|-
+| [[image:gainwindow.png|400px]]
+|-
+| colspan="1" style="text-align:enter;|
+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.
+{| style="border: none; border-spacing: 0; margin: 1em auto; text-align: center;"
+|-
+| [[image:2Droughscan.png|400px]] || [[image:2Dfinescan.png|400px]]
+|-
+| colspan="2" style="text-align:enter;|
+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.
 |}