Thinning of gallium arsenide wafers using advanced abrasives and etchants for laser applications
Nojonen, Erika (2021)
2021
Teknis-luonnontieteellinen DI-ohjelma - Master's Programme in Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2021-05-31
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202105255426
https://urn.fi/URN:NBN:fi:tuni-202105255426
Tiivistelmä
Nowadays, lasers are used in many different applications, like telecommunications, radars, and medical treatments. These types of applications use semiconductor lasers which are commonly fabricated on top of GaAs wafers. To produce laser chips or dies with high effciency, the GaAs wafer needs to be thinned. The aim of this thesis is to unify the process of thinning and polishing of GaAs and to improve the durability of a GaAs wafer by removing the stress caused by thinning from the wafer structure. In general, most of the wafer thickness is removed from the semiconductor wafer by thinning using slurry containing a coarse abrasive grain and liquid like water or oil, as well as a lapping or grinding machine. However, this causes various micro-damage to the surface of the wafer, which in turn, cause stress in to the structure of the wafer. The tension in wafer reduce the mechanical strength of the wafer, which however, can be improved by using slurry with smaller grain size that used in thinning.
In this work, double-sided polished 3” GaAs wafers were studied, which had initial thickness of 600 µm. The wafers were frst thinned to 100–150 µm thickness. It was observed that the size of the abrasive grain signifcantly affects to the thinning rate. With larger, 9 µm abrasive grain slurry, the semiconductor material could be removed at a clearly faster rate compared to a smaller abrasive grain slurry with 0.3 µm grain size, which was used in polishing. In addition, it was noted that during polishing, a soft pad is needed as without the polishing pad the surface quality of the wafer was not high enough, i.e., not shiny enough.
Two different polishing pads have been used in the thesis. The polishing results obtained on these pads were contrary, when compared to each other. The frst pad tested, effectively polished the center of the wafer, leaving the edges of the wafer almost intact. In addition, the increased polishing time did not increase the size of the polished area. In turn, the second tested pad polished the wafer more effectively from the edges of the wafer than from the center. With increased polishing time, a better polishing result was observed in contrary to the frst pad. Lastly, few microns of GaAs were wet etched from the polished wafer as a fnishing process. Wet etching removed material evenly from the entire area of wafer, but left the surface of the wafer looking very uneven and unfnished. In addition, atomic force microscopy images were taken from various samples to assess the effect of polishing on surface roughness. As a conclusion, it can be stated that polishing with small grain size slurry can be used as a fnishing process of the GaAs wafers if the polishing time is long enough. Moreover, other polishing parameters need to be optimized to achieve a smooth outcome.
In this work, double-sided polished 3” GaAs wafers were studied, which had initial thickness of 600 µm. The wafers were frst thinned to 100–150 µm thickness. It was observed that the size of the abrasive grain signifcantly affects to the thinning rate. With larger, 9 µm abrasive grain slurry, the semiconductor material could be removed at a clearly faster rate compared to a smaller abrasive grain slurry with 0.3 µm grain size, which was used in polishing. In addition, it was noted that during polishing, a soft pad is needed as without the polishing pad the surface quality of the wafer was not high enough, i.e., not shiny enough.
Two different polishing pads have been used in the thesis. The polishing results obtained on these pads were contrary, when compared to each other. The frst pad tested, effectively polished the center of the wafer, leaving the edges of the wafer almost intact. In addition, the increased polishing time did not increase the size of the polished area. In turn, the second tested pad polished the wafer more effectively from the edges of the wafer than from the center. With increased polishing time, a better polishing result was observed in contrary to the frst pad. Lastly, few microns of GaAs were wet etched from the polished wafer as a fnishing process. Wet etching removed material evenly from the entire area of wafer, but left the surface of the wafer looking very uneven and unfnished. In addition, atomic force microscopy images were taken from various samples to assess the effect of polishing on surface roughness. As a conclusion, it can be stated that polishing with small grain size slurry can be used as a fnishing process of the GaAs wafers if the polishing time is long enough. Moreover, other polishing parameters need to be optimized to achieve a smooth outcome.