Development of Materials Integration for Laser Gain Media: Single Crystals and Ceramic (Polycrystalline) Materials and Applications

A study of the fundamental issues of materials integration for wafer bonding offers new pathways for achieving photonic materials with unique properties. 

First, the bonding of single crystal oxide crystals (YAG and yttria, for example) with different compositions and even different orientations is shown to critically depend upon several important parameters.  A detectable level of subsurface damage usually accompanies the polishing of these crystals. We studied chemical mechanical polishing (CMP) of these oxide crystals to provide surfaces which exhibit sub-nm roughness, and the absence of subsurface damage.  These surfaces are then activated and we use XPS and other surface sensitive techniques to assess the state of the surface after different processes.  Next, we demonstrate that we can significantly reduce the bonding time and temperature to attach two single crystal pieces based on these improved polishing techniques.  These techniques also apply to polycrystalline (ceramic) materials as well.

Second, we demonstrate wafer bonding for polycrystalline laser gain media.  Two issues are important here.  First, the grain composition – and hence chemistry – may be different than the bulk composition / chemistry.  Under ideal conditions, the grain boundaries may simply be regions of misorientation and be nearly atomically abrupt.  The second issue is that the different grain orientations present on a surface may (i) exhibit different hardnesses and / or (ii) different chem-mech polishing rates.  Therefore, the issues of chemistry are more important than is the case with single crystal materials.  We determined that the best combination of solution chemistry and polishing parameters involves the use of the SUBA 500 (yellow) pad and the same applied force as for the single crystal YAG.  For many combinations of force, chemistry, and pad material, a clear delineation of the grain boundaries was revealed.  However, we were able to achieve a set of polishing conditions for which the boundaries are not preferentially etched.  These surfaces also proved to be damage-free and were able to show a high fraction of bonded interface (>90%) on 50 mm diameter gain media disks.