Chemical Mechanical Polishing of III-V Materials for Wafer Bonding Applications

ABSTRACT

                Wafer bonding of III-V cells offers opportunities to integrate materials with different bandgaps and different lattice parameters without forming high defect densities.  However, growth conditions that produce the best cells do not necessarily produce epitaxial layer surfaces that are suitable for wafer bonding.  The chemical mechanical polishing of III-V materials including GaAs, InP, InAs, and alloy layers used in high efficiency III-V solar cell structures is investigated with sodium hypochlorite and citric acid solutions.  It is found that the surfaces can be polished to below 0.5 nm RMS surface roughness without the induction of crystalline damage using similar abrasive-free polishing solutions for all the materials with controlled polishing rates of 10 nm / min.  The low removal rate is key for applications that include epitaxial layers that must be subsequently polished without l=excessive loss of material.  The actual composition of the slurry is adjusted for the particular alloy composition.  A balance between reduced sub-surface mechanical damage and smooth surface morphology is obtained by adjusting the amount of citric acid similar to Br-methanol polishing.  However, the damage-free planarization in these cases may be aided by an oxide formation as predicted by Pourbaix diagrams.  Subsequent bonding suing sulfur passivation on the CMP surface shows excellent electrical characteristics with low resistivity as well as high strength bonding after annealing at 400 °C.  Additionally, triple axis x-ray diffraction is found to be an extremely sensitive, non-destructive method for evaluating damage induced by the CMP process.