SU-8 microgrid strain measurement

Kelly & I finally succeeded in getting a measurement of the SU-8 scaffold change in size upon liftoff.  Kelly, lifted off a sample (~0.5 x 2 cm) using the XeF2 dry etch and transferred the sample to a glass slide with pdms bonded to it. The slide was then placed on top of the printed mask used to pattern the SU-8 microgrid (SU-8 scaffold was facing down so that it and the mask were close to same plane of focus).  We went over to Dave Borkholder’s lab and used his Leica upright fluorescence microscope to take images down the length of the sample using a 5x objective in bright field (visible light only).  Note in the figure that we put a series of dots with a magic marker to have a reference for how far to advance the sample for each image.  Kelly then stitched the micrographs together in Photoshop, and we measured the change in offset between the mask grid and the SU-8 grid.  We counted the number of grids going across (total of 189), keeping things aligned with the mask grid.  There was a very slight misalignment such that the SU-8 grid is approximately 0.1 degrees off from the mask.  We measured the grid size at several places along the length; it is consistently 60 pixels.  The SU-8 is 7.5 pixel to the right of the mask grid at the right most image and 34 pixels to the right of the left most mask grid in the second to last image (see figure).  Therefore, the SU-8 shrank by 34-7.5 = 26.5 pixels over the span of 189 x 60 = 11,340 pixels.  Thus, the SU-8’s strain is 0.23%.

For reference, the SiN layer that it supports is predicted to shrink by approximately 0.1 %. (film stress of 250 MPa, E = 260 GPa).  The two strains are close to each other and quite low in magnitude, so this matches well with the observation that the lifted off samples would lay reasonably flat with no visible wrinkling of the SiN membrane.