The space solar cell market is looking for solutions to optimize three important metrics: W/kg, W/m? and ?/W. Both the weight and volume aspects are linked to the launcher payload capacity to get the satellites to orbit. The germanium wafer plays a significant role in both, the cost and the weight elements of these metrics. A solution that allows solar cell manufacturers to use only as much germanium as is strictly necessary for a functioning solar cell, would have a big impact on these metrics. Current approaches rely on backgrinding or etching away the substrate to reduce the amount of superfluous Ge.

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The intended development of the ELLA activity would result in a germanium substrate that contains a weak layer that is designed to both allow epitaxial growth as well as lifting-off the eventual epitaxial structures from the mother substrate.

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The key product features of a wafer with a porous germanium weak layer are as follows:

• • • • • • A re-usable germanium mother substrate ;
          • A porous Ge based weak layer consisting of a stack of porous germanium layers with adjusted porosity to enable lift-off while having an epi-ready front surface ;

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This porous layer must be designed in such a way that the reorganization results in a closed top surface that is epi-ready and an empty cavity underneath that will allow later detachment. In the ELLA project two alternative approaches will be compared for this porous structure:

• • • • • • Electrochemically etched (meso-)porous germanium ;
          • Lithographically defined and dry etched macroporous germanium ;

After successful porosification the following process steps/objectives are targeted:

• • • • • • Epitaxial growth on reorganised Germanium substrates ;
          • Delineation of the foil ;
          • Detachment of the foil (lift-off) ;
          • Reconditioning of parent wafer (for wafer re-use) ;
          • Design for manufacturability: The outcome should be that the designed product can in fact be manufactured with processes that are feasible and can be scaled up and deliver a product at acceptable cost. ;;
          • Eco-Design: demonstrate reduction of Ge use ;
          • Integration with customer process flow ;

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Porosification of semiconductor wafers is not new. This process has been successfully applied to Silicon wafers using the two methods that are intended to be investigated in this activity: electrochemically etched (meso-)porous germanium and lithographically defined and dry etched macroporous germanium. However, attempts to transfer this process to Germanium in the past were not fully successful. In the meantime, major progress has been made in the control of the process steps involved in either manufacturing route. The GeON project demonstrated the principle feasibility of the macroporous Ge while Sherbrooke University has established itself as one the world-wide leaders in the electrochemical etch process. If successful, the mastering of this technology could lead to advantages in many areas: weight reduction of solar cells, cost reduction of wafers, reduction of Ge usage. And this can lead to solar cell products which become a mission enabling technology where very high specific power is required.