A need arose for approximately 1 cm2 of a diamond-like-carbon (DLC) concentrator target for the analysis of solar wind nitrogen isotopes. The original target was a circular quadrant with a radius of 3.1 cm; however, the piece did not survive intact when the spacecraft suffered an anomalous landing upon returning to Earth. An estimated 75% of the DLC target was recovered in at least 18 fragments. The largest fragment, Genesis sample 60000, was designated for this allocation, and is the first sample to be subdivided using a laser scribing system. Laser subdivision has associated risks, including thermal diffusion of the implant if heating occurs, and unintended breakage during cleavage. In order to minimize the possibility of unintended breakage of the actual target wafer during subdivision, a careful detailed study involving numerous laser scribing plans was undertaken. The innovation described here involves the results of this study that yielded a cutting plan essentially guaranteeing ~100% cleaving success of this precious space-exposed wafer.
In order to maximize the probability of a successful cleave, the system needed to scribe at least 200 μm into the wafer. Attempts were made to scribe completely through practice wafers by varying the parameters of the laser system holding the laser power fixed at 95% maximum power. The parameters varied were cutting speed, total number of scribe passes, depth advance of the wafer stage per scribing pass, and the hold time between passes. It was determined that if a cut could be made completely through the wafer at 95% power with a given set of parameters, the power could be reduced and only cut to a 200-μm depth, minimizing heating in the wafer. The benefit of these initial tests indicated that a cut roughly 125 μm could be made into a 550-μm-thick wafer. Scribing back and forth along a single line created a scribe cut that was ~10 μm wide. The cutting plan then evolved into scribing multiple lines separated by 5 μm. However, once again, the cut depth seemed to bottom out at just over 125 μm.
The scribing plan began by orienting the wafer on the laser cutting stage such that the 100 and the 010 directions of the wafer were parallel to the corresponding X and Y directions, respectively, of the cutting stage. The laser was programed to scribe 31 lines of the appropriate length along the Y stage direction. The scribe lines were separated by 5 μm in the X direction. The laser parameters were set as follows. The laser power was 0.5 Watt, each line consisted of 50 passes with the Z position being advanced 5 μm per pass, and there was a built-in wait time of 30 seconds before scribing the next line to allow for wafer cool-down from any possible heating via the laser. After the laser finished scribing, the oriented wafer and mounting plate were removed from the cutting stage and placed on the “stage area” of a lighted binocular microscope. This allowed ablated silicon from the laser scribing to be “teased” out of the “scribed” pattern using an ultrasonic-aided, sharpened micro-tool. The loosest Si “fluff” was then removed (vacuumed and or brushed) from the wafer surface. After all of the ablated Si was removed from the scribe channel, the mounted wafer was then repositioned in exactly the same orientation on the laser stage. The laser was focused using the bottom of the wafer channel and the 31-line scribing pattern reprogrammed using the Z position of the groove bottom as the starting Z value instead of the top wafer surface previously used. After the laser completed the second set of scribes, the ablated material was removed from the groove using the described technique.
The wafer was remounted on the stage using exactly the same orientation as before. Again the laser was focused on the bottom of the groove. This time, however, it was programed to scribe only one line down the exact center of the channel. The final scribe line consisted of 100 passes with a Z advance of 5 μm per pass, and the laser power set at 0.5 Watt. Using this scribing plan yielded an ~100% cleaving success rate on non-flight FZ silicon wafers ~550 μm thick with a scribe length of < 4 cm.
This work was done by Howard Lauer Jr., Patti Burkett, Melissa Rodriguez, Keiko Nakamura-Messenger, Simon Clemett, Carla Gonzalez, and Thomas See of Johnson Space Center. For more information, contact the JSC Technology Transfer Office at (281) 483-3809. Refer to MSC-25607-1.