@ -185,18 +185,18 @@ Between the two photomasks, there are nine embedded images which can be seen at
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\begin{description}[labelindent=0.5cm]
\item [unidimensional Verniers:] These are the most useful markings for image alignment. For each axis, there are a set of coarse Verniers (bordering the image) and a set of fine Verniers (further from the image) which move 5 times the speed of the coarse Veniers. Every time an image is aligned the white blob will be centered in both the coarse and fine Verniers. These markings essentially amplify and scale the distance between the image (640 microns) and can be used by a motion tracker in a closed-loop alignment system.
\item [unidimensional Verniers:] These are the most useful markings for image alignment. For each axis, there are a set of coarse Verniers (bordering the image) and a set of fine Verniers (further from the image) which move 5 times the speed of the coarse Veniers. Every time an image is aligned the white blob will be centered in both the coarse and fine Verniers. These markings essentially amplify and scale the distance between the image (640 microns) and can be used by a motion tracker in a closed-loop alignment system. Rotational alignment can be achieved by aligning all verniers on the same axis (e.g., both the left and right side at the same horizontal position).
\item [multidimenional Verniers:] These are Verniers that are centered in a two-dimenional space everytime an image is focused.
\item [linear Moire grating:] These gratings can be used to make sure that the plates are aligned rotationally (resulting in a completely monochrome bar without any patterns).
\item [circular Moire grating:] These gratings best represent the grid of the images. When an image is aligned the respective grating on the grid will be dark. The number of fringes denotes the accuracy of the alignment (none means perfectly aligned).
\item [linear Moire grating:] These gratings can be used to make sure that the plates are aligned rotationally (resulting in a completely monochrome bar without any patterns). However, using the unidimensional verniers as explained above is likely more effective.
\item [circular Moire grating:] These gratings best represent the grid of the images. When an image is aligned, the respective grating on the grid will be dark. The number of fringes denotes the accuracy of the alignment (none means perfectly aligned).
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High precision optical stages are used for the alignment of the wafers. Ideally, the wafers are not touching (separated by a few microns). If the wafers touch, they will degrade over time as they move across each other. However, it is very difficult to achieve perfect alignment without the photomasks touching as the they need to be aligned in all 6 degrees of freedom ($x$, $y$, $z$, $\theta x$, $\theta y$, and $\theta z$) in order for the resulting image to be properly produced. Only the $x$ and $y$ axis need to be moved to find the images once all the other degrees of freedom are set accurately.
The original setup is as follows. Each of the photomasks are mounted onto tilt stages to be able to align the masks together rotationally (note that a more ideal setup would use goniometer stages). One of the tilt / goniometer stages is then affixed to a stage with 3 degrees of freedom: $x$, $y$, and $z$. The other is fixed directly to an optical breadboard.
%The original setup is as follows. Each of the photomasks are mounted onto tilt stages to be able to align the masks together rotationally (note that a more ideal setup would use goniometer stages). One of the tilt / goniometer stages is then affixed to a stage with 3 degrees of freedom: $x$, $y$, and $z$. The other is fixed directly to an optical breadboard.
To automate the alignment, high precision motors are used to move one of the photomasks on the $x$ and $y$ axes. In the original setup, the high precision motors are stepper motors. If the photomasks are not touching, an open-loop system can be used to automate alignment. That is, the accuracy of the step count of the motors should be sufficient for alignment. However, if the photomasks need to touch in order to produce the resulting images (as is the often the case with the original setup), the friction between the two photomasks will cause inaccuracies in an open-loop system. To compensate for this, the Vernier markings can be tracked optically (using motion-tracking software or opto-interrupts) in order to create a closed-loop system. The software used to automate the system and control the motors is detailed in the following section.
To automate the alignment, high precision motors are used to move one of the photomasks on the $x$ and $y$ axes. Ideally, two more motors can be implememted: one to compensate for rotational misalignment and the other to separate the plates on the $z$ axis when travelling between images. In the original setup, the high precision motors are stepper motors. If the photomasks are not touching, an open-loop system can be used to automate alignment. That is, the accuracy of the step count of the motors should be sufficient for alignment. However, if the photomasks need to touch in order to produce the resulting images (as is the often the case with the original setup), the friction between the two photomasks will cause inaccuracies in an open-loop system. To compensate for this, the Vernier markings can be tracked optically (using motion-tracking software or opto-interrupts) in order to create a closed-loop system. The software used to automate the system and control the motors is detailed in the following section.
