Increasing robustness of EpiWafer transfer process leading to carrier lifetimes of 1.6 ms using large scale production feasible substrate wafers

In the layer transfer process, the appropriate tuning of the porosity and the thickness of the porous silicon layer stack is crucial for the reorganization process und thus for the crystal quality of the epitaxially grown silicon wafer (EpiWafer). In anodic electrochemical etching of silicon, the layer thickness is controlled by the etch duration. For a substrate with a given doping density etched in a given electrolyte, the porosity can be adjusted solely through the applied current density. As this may be a limitation to obtain very different porosities for the layers of the stack, we used different electrolytes to etch the different layers of the stack. This way, higher porosity values can be achieved for the detachment layer while keeping low porosity for the seed layer. By doing so, the range of applicable process parameters is increased. Using a very thick stack of three porous layers, we demonstrate a minority carrier lifetime of about 1.6 ms for an n-type (1 Ωcm) EpiWafer grown on non-polished and not shiny-etched but KOH saw damage etched substrate wafer.