Multiple reuse of the parent substrate in a porous silicon based layer transfer process

Considering the rapid increase in world’s energy demands as well as lack of sustainability in conventional sources of energies, increasing the share of renewable energies is assumed to be the most viable solution. Among all renewable energies, photovoltaic technologies can play a vital role in the long term future energy supply. However, for this to happen, continued cost minimization in the photovoltaic industries is imperative. Currently, silicon photovoltaics dominates the other PV technologies by forming 90% of the market share and is expected to be prominent for the foreseeable future. Reducing the thickness of the wafers can be substantially beneficial for further minimizing the module price as the costs for silicon substrate fabrication account for roughly 30-40% of the total module costs. One the promising approaches to reduce the thickness of monocrystalline wafers is the Layer Transfer Process(LTP) based on a porous silicon template. In the porous silicon (PSi) based LTP, a double-layer stack of PSi consisting of a low- and high-porosity layer is anodically etched on the surface of a p+ Czochraslkys silicon substrate. During the annealing at elevated temperatures, the Psi stack reorganizes as such that the low-porosity layer reorganizes into a closed surface containing spherical voids and the the high porosity layer becomes a long lateral void separated by thin silicon pillars. This reorganized porous silicon can act a perfect template for the epitaxial growth. There is no limitation in thickness of the grown mono-crystalline layers. Afterwards, the grown epitaxial layers are detached from the parent substrate at the detachment layer formed in the porous silicon stack(HPL) and then are processed in a free-standing or a bonded approach. To have cost-effective process, the parent substrate has to be reused for multiple times; nonetheless, the surface of the parent possesses different types of defects that can have a negative influence on the quality and detachment yield of subsequent generations of epitaxial layers. As a result, a proper surface reconditioning is required to effectively remove the defects with a minimum silicon removal. The current work offers initial steps into realization of a successful substrate reuse for the Layer Transfer Process. Firstly, the different types of possible defects and undesirable features on the surface of the parent are identified and the challenges at the occurrence of such defects are delineated. Secondly, a proper reconditioning process is then developed to retrieve a suitable surface condition for the subsequent epitaxial cycle. Finally, actual reuse experiments are carried out in order to evaluate the effectiveness of the developed reconditioning processes by monitoring the quality evolution of the fabricated epitaxial foils at each cycle of reuse. In the current work, 4 generations of epitaxial foils with different reconditioning processes are demonstrated.

Uno degli approcci più promettenti per ridurre lo spessore del substrato di silicio monocristallino per applicazioni nel fotovoltaico è l’impiego della tecnica Layer Transfer Process (LTP) su un substrato di silicio poroso. In questo processo, un sistema a due strati a diversa porosità sono prodotti anodicamente sulla superficie di un substrato p+ Czochralsky. A seguito di un trattamento termico a 1130 °C, il silicio poroso riorganizza la propria porosità in uno strato con strutture sferiche vuote separate da piastri in silicio. Tale struttura può essere utilizzata come template per una crescita epitassiale con la possibilità di ottenere elevati spessori dello strato monocristallino. Lo strato epitassiale è poi separato dal template e rpocessato come un normale substrato monocristallino. Per un processo economico, il template deve essere riutilizzato più volte: è quindi necessaria una procedura di ricondizionamento della superficie al fine di eliminare ogni possibile difetto. Il lavoro di tesi mira a studiare tale procedura e a definire le fasi di processo migliori per garantire un numero elevati di riutilizzi. Infine, vengono mostrati i risultati di caratterizzazione e di prestazioni di alcuni substrati prodotti con questa tecnica.