Requirements and comments
Users are expected to have performed the following steps, following the SOP, before starting Mark Alignment:
- Loading sample
- Global UVW alignment (origin and angle correction using the chip's edges)
- SEM column alignment (focus, aperture alignment, and stigmation correction)
- Manual/automatic writefield alignment using a particle on the sample.
Beam current measurement and patterning parameter calculation can be performed after 3-Point Alignment is completed, but has to be done before starting Manual/Semi-automatic/Automatic Mark Alignment through the positionlist.
Also, keep in mind that alignment procedures require a finite tool operation time, hence your patterning procedure will take longer to complete, compared to exposures that do not require alignment.
Alignment mark
This is a reference mark whose position is well defined with respect to your main structures. The standard structure mark suggested by Raith looks like the image below.
Notice the different colours? They mean that these different elements are in different layers of the GDSII file, and that means that each element has a different purpose. Even though the elements in this structure are in different layers, it is recommended that a single structure with all the elements is used and referenced throughout the design to be patterned. In this way one avoids ambiguity between adjacent marks. The arrangement of these elements (which is displayed in front/back) is unimportant, as EBL is intrinsically a 2D process.
We recommend these marks to be present on every writefield of your design, allowing for precise alignment after every stage motion. You must have at least 3 of these per writefield, but we recommend that you include 4, one per corner, for best results.
For details on the dimensions of the elements and how to construct this structure, you can take a look in the Demo.csf file in the RAITH150 Two NanoSuite software and look for the WF_Mark structure.
Blue cross
This element is the reference mark. It can (and we recommend it to) be in the same layer as the first structures to be patterned. This guarantees that it will be there for the subsequent exposure steps.
It is important that, once the first layer of fabrication is finished, this mark is visible in the SEM under the resist and any deposited layer on top of the mark. Hence, one wants it to be of a material that gives good contrast with the surroundings (e.g., metal on Si or Si with air trenches around it). Furthermore, it should be easily found, which means that it cannot be too small, while not being so large as to needlessly increase writetime and/or occupy otherwise usable space in the writefield.
The wider arms on the outside make it easier to perform automatic alignment, while the thinner links in the middle increase precision when performing manual alignment.
For (coarse) 3-point alignment, these marks can be about 50 µm wide and should be located closer to the corners of the chip. In the Demo.csf file these marks are named Global_mark.
For manual and automatic writefield alignment, on the other hand, these marks can be around 10 µm wide. As a general rule, you can make these marks about 1/10 of the writefield size.
Brown square
This element is used for manual writefield alignment and must be in layer 63. Whenever this layer is present in any writefield of an item in the positionlist, the tool performs a manual alignment procedure before patterning the device layer(s).
If you are producing your design with a software different than the RAITH NanoSuite software, make sure that these are POLYGON elements. In NanoSuite, you can add these using the menu Add → Manual Mark Scan.
This square tells the software where to scan to obtain an image of the alignment mark. Because the software is looking for the centre of the mark, this square must be centred with the cross in both X and Y directions.
It needs to be large enough such that the whole cross is within it. Even though you will be interested only on the centre of the cross, it is recommended that the square span the whole mark. That is because the stage positioning has some imprecision—which we are trying to correct with this procedure—and the cross will likely not be centred in the scanned image. If the square is not big enough, it may happen that the centre of the cross is not in the image, which will hinder the alignment procedure.
This element must not be present on marks for 3-point alignment (see Global_Mark in Demo.csf file).
Green rectangles
These elements are used for automatic writefield alignment and must be in layer 61. Whenever this layer is present in any writefield of an item in the positionlist, the tool performs an automatic alignment procedure before patterning the device layers.
If you are producing you design with a software different than the RAITH NanoSuite software, make sure that these are PATH elements with a given width (standard is 500 nm). In NanoSuite, you can add these using the menu Add → Auto Mark Scan.
These paths tell the software where to scan lines and average them, producing a signal used to determine the centre of the cross. These paths must also be centred with the arms of the cross. The scan direction is inferred from the path orientation.
These paths need to be long enough to cross both edges of the arm of the cross. The width is used to average the signal of several scanned lines.
These elements must not be present on marks for 3-point alignment (see Global_Mark in Demo.csf file).
Chip Layout
This is an example of a good design, using the aforementioned guidelines:
The black line represents the edges of the entire specimen (e.g., a 10 mm × 10 mm chip); the three global marks and the pattern at the chip centre are not to scale. The green dashed line represents one 100 µm × 100 µm writefield, shown in detail below, and is only a guide to the eye, not part of the actual pattern. And the labels P1, P2, and P3 are also not part of the actual pattern, but a reference for the following step-by-step section.
Note that the brown squares (layer 63) don't go all the way to the edge of the writefield. This helps avoid alignment errors because distortion increases as the beam is deflected closer to the writefield boundary.
As mentioned before, you can see that the crosses' colour matches the colour of some structures of the main pattern, which is the first layer to be exposed. After this layer is exposed, the crosses will not be patterned again, but will be scanned for the alignment procedure.
As a reference for the steps detailed below, we will follow the model above with the following colourmap:
| Layer | Colour |
|---|---|
| 1 |
|
| 2 |
|
| 3 |
|
| 61 |
|
| 63 |
|
Note that when patterning a sample for the first time, only layer 1 above is included in the positionlist. Layers 61 and/or 63 are only included from the second patterning onwards.
Step-by-step
1. Global alignment or 3-point alignment
Before starting your second patterning procedure, it is necessary to give to the software more precise reference points for origin and angle correction than the edges of the sample, which are typically very irregular (even if you diced it with the dicing saw). The 3-point alignment is then very important to obtain good results on multilayer patterning.
Below we detail the steps for performing 3-point alignment of your sample position/orientation.
i. After loading the sample for the second time, follow the SOP up to the point when you perform beam current measurement. This means that you still find the chip corners and do origin and angle correction for the Global UV coordinates, as well as writefield alignment.
ii. After measuring the beam current, drive back to the first Global Mark closer to the bottom left corner of the chip.
iii. Go to the 3-Points tab in the Adjust UVW window. Make sure all checkboxes are unchecked.
NOTE: If you are only performing this alignment once, you can stay in the Global mode of the transformations. If you intend to perform this step for various chips in the same sample, e.g. a wafer, you can use Global for the wafer and switch to Local for each chip. Note, however, that Global overwrites all transformations in Local; this means that you must do Local always after Global and never the opposite, unless you explicitly want to make both the same.
iv. Bring the crosshairs up and, using the joystick, align the centre of your mark to the crosshairs.
v. Assign appropriate UV coordinates of this mark (based on your design) to P1.
vi. Click the eyedropper button to read in the XY coordinates to P1.
vii. Check the P1 checkbox and click ADJUST.
viii. Move to the next Global Mark and repeat steps iv to viii, updating P2 and P3, one at a time.
Note that UV coordinates of each mark can be anything, as long as they are consistent with your design. For instance, if the markers are equidistant from the centre of your chip, you can assign the coordinates such that the centre of you chip is at (U,V) = (0,0).
This step makes sure that the stage motion is precise enough such that whenever it moves to one of the writefields, the respective marks are within the field-of-view of the SEM at your patterning magnification.
2. Manual writefield alignment marks
In some cases it may not be necessary or even possible to use automated writefield alignment. For instance, if you only have a few writefields to align and expose it may be simpler to just use the manual procedure. Also, if the contrast and/or signal-to-noise ratio of the images of your marks are poor, it may be impossible for the automated alignment to succeed. In that case there is no other option but sticking to the manual procedure.
Below we detail the steps for performing manual writefield alignment.
i. Drag your design to a new positionlist, select both the layer you want to pattern (layer 2 in the example above) and layer 63 (Manual Markscan).
NOTE 1: If desired, you can select only layer 63 and perform the alignment procedure without patterning anything. In this manner, you can test the procedure without any risk to your sample, as the device region will not be exposed.
NOTE 2: If you are opening a positionlist file, make sure you included layer 63 in the appropriate items.
ii. Set the first writefield's UV coordinates such that it matches the patterned structures already in the sample.
NOTE: If you are testing alignment without patterning (NOTE 1 above), you may want to adjust the working area such that it only covers a single writefield. To do so, Right-click the positionlist item, select Properties, and adjust the lower-left and upper-right coordinates of the working area appropriately. Click OK to accept the changes and Double-click the item in the positionlist to verify. Remember to reset the working area when you decide to pattern the sample.
iii. Scan the positionlist (or preferred items)
iv. When layer 63 is present in the executed item, the tool will open another positionlist named Align.pls and scan an image of a mark.
v. Once a mark image shows up, hold CTRL + Left-click and drag to the centre of the mark—exactly as in the manual writefield alignment procedure detailed in the SOP—and release CTRL + Left-click once in the correct position.
vi. Click Continue.
vii. Repeat steps v and vi until all marks have been scanned.
A "Calculated Writefield Correction" window will appear when the process is finished.
The tool will proceed to patterning your devices in that writefield once all marks have been scanned.
If more writefields have layer 63 active, this procedure will be repeated for each one of them. We recommend that this procedure is performed only for the first writefield, while the others should use the automatic markscan, unless the mark signal makes it impossible for the automatic process to succeed.
3. Automatic writefield alignment marks
If you have a large number of writefields to align and pattern, you most likely will want to use the automated procedure to avoid spending hours in front of the tool. However, we recommend that all users perform manual alignment at least once before moving on to the automatic option—this enables users to determine whether their marks are reliable for automatic alignment based on the image quality (mainly contrast and signal-to-noise ratio) and to check if the UV coordinates and working area of the positionlist are correct.
Below we detail the steps for performing automatic writefield alignment.
i. Drag your design to a new positionlist, select both the layer you want to pattern (layer 2 in the example above) and layer 61 (Automatic Markscan).
NOTE 1: If desired, you can select only layer 61 and perform the alignment procedure without patterning anything. In this manner, you can test the procedure without any risk to your sample, as the device region will not be exposed.
NOTE 2: If you are opening a positionlist file, make sure you included layer 61 in the appropriate items.
ii. Set the first writefield UV coordinates such that it matches the patterned structures already in the sample.
NOTE: If you are testing alignment without patterning (NOTE 1 above), you may want to adjust the working area such that it only covers a single writefield. To do so, Right-click the positionlist item, select Properties, and adjust the lower-left and upper-right coordinates of the working area appropriately. Click OK to accept the changes and Double-click the item in the positionlist to verify. Remember to reset the working area when you decide to pattern the sample.
iii. Scan the positionlist (or preferred items)
iv. When layer 61 is present in the executed item, the tool will open another positionlist named Align.pls and scan the marks in the writefields. A linescan window will be shown for each scan and quickly vanish when it is done.
No information about the quality of the alignment will be shown in this case. You can, nonetheless, follow the progress of the procedure in the Align.pls positionlist table. A green dot to the left means that the step somewhat succeeded and a red dot means that it failed. However, keep in mind that a green dot not necessarily mean that the mark was found properly (see the troubleshooting section below): it only means that the edge-finding algorithm didn't fail.
Once all marks have been scanned, the tool will proceed to patterning your devices in that writefield.
If more writefields have layer 61 active, this procedure will be repeated for each one of them. We recommend that this procedure is performed for all writefields, providing maximum precision in your second layer of patterning. If you chose to skip every other writefield (or any number of them) by not including layer 61 (or 63) in them, the tool will use the most recent parameters.
Troubleshooting
Mark is not in image for manual alignment
If one or more marks do not show up in the scanned image during manual writefield alignment, it means that the positions of the physical marks do not match the scanned areas in your design (layer 63). There are two main reasons for this to happen:
a. The position of the first writefield, as input in the positionlist, is offset.
To solve this, check the position of the first writefield with respect to your origin. Remember that you reassigned the origin by performing the 3-point alignment—if necessary, you can drive to the global marks and redo the 3-point alignment. Also, remember that the UV coordinates inserted in the positionlist are of the centre of the first (bottom-left) writefield in the design.
b. The working area is not defined properly, causing one or more of the scanning areas to be out of place.
To solve this, double-click the item in the positionlist and check where the structures are in the working area and in the writefields. Fix if needed by opening the Properties window of the item and adjusting the working area accordingly. Remember that the bottom-left corners of the working area and the array of writefields are aligned.
Automatic alignment fails to find one or more marks
If the linescans show only a noise-like signal, with no clear edges, it means that either the scanning lines in layer 61 do not match the marks on the sample, or the contrast between the mark and the substrate is too low. For the former, check and adjust the positionlist item UV coordinates and working area to solve the issue. For the latter, this means that you may have to rely on the manual alignment procedure instead of the automatic one.
If you think there should be enough contrast but the edge-finding algorithm still does not work properly, you can follow the steps below to calibrate it.
Because this procedure requires good image contrast, it may fail for one or more of the marks because the algorithm was not properly set. The following steps are intended for calibrating the system such that the alignment succeeds, provided there is enough contrast. The values used below assume the mark used is the standard mark in the Demo.csf file. Adjust as needed for your specific sample, especially when optimizing values in steps viii and xiv.
i. Drag your design to a new positionlist and select only layer 61 (Automatic Markscan).
ii. Right-click the item in the positionlist and select Properties.
iii. Adjust the Working Area coordinates such that it spans only one writefield containing alignment marks.
iv. Make sure that the position of the item in the positionlist matches your previous pattern.
v. Execute the positionlist. A series of linescans will be shown.
vi. Go to File → Open Positionlist and open the file Align.pls—this file contains the most recent alignment procedure executed. Some of the items may have a red mark to their left—these are the failed linescans.
vii. Double-click one of the items with an error and a linescan window should open.
viii. Select Threshold Algorithm from the dropdown menu for Filter and click the Apply button 
. This will open a configuration window.
ix. In the Parameter set section, select Align write field from the dropdown menu.
x. In the Threshold section set 50 for Lower and 70 for Upper. Mode should be set to relative and be greyed out.
xi. In the Edge definition section:
Left: Set 1 in the first textbox, select from left, and set the last textbox to 50.
Right: Set 1 in the first textbox, select from right, and set the last textbox to 50.
xii. In the Structure section set Type to maximum and Width range from 500 nm to 2500 nm.
xiii. In the Display section, leave the default option Position selected.
xiv. Click OK. The linescan window should now show vertical bars indicating that it found the mark. Now we need to optimize the parameters to improve the results.
NOTE: You can (and we recommend you to) further adjust these parameters to try and further improve the algorithm. Nevertheless, these should be good initial working values.
xv. Once finished with the optimization, close the linescan window, but leave the Align.pls window open.
xvi. To test the parameters select the Align.pls window and go to Scan → All to run the entire list again. All items should end with a green mark to their left, indicating that the software was now able to find all the alignment marks properly.
NOTE: If some of the linescans fail again, you will have to go through steps vii to xiv again and look for parameters that allow for those linescans to succeed.
xvii. If all items in the Align.pls positionlist succeeded, you can close that window and start patterning your devices.