Silicon nanomembranes (Si-NM) are typically fabricated by harvesting the top Si layer of the silicon-on-insulator (SOI) wafers. Selective etching of the middle buried oxide layer releases the top Si device layer from the bulk as a nanomembrane. The fragile nanomembrane is susceptible to cracks and wrinkles during transfer to other substrates often resulting in a partial transfer. Furthermore, the scalability of the transfer process is limited. Now M.J. Zablocki and D.W. Prather from the University of Delaware and A. Sharkawy and O. Ebil from EM Photonics, Newark have overcome these challenges by developing a high-fidelity direct transfer process that can be used to transfer electronic and photonic devices without compromising structural integrity and functionality.

A process flow for the fabrication and transferring of a patterned Si-NM from a silicon-on-insulator (SOI) substrate to a SiO² substrate. Reproduced with permission from Optics Letters 36 (1) (2011) DOI:10.1364/OL.36.000058; p. 58. © 2011 Optical Society of America.

The researchers describe their fabrication protocols in the January 1st issue of Optics Letters (DOI:10.1364/OL.36.000058; p. 58). The researchers started with a SOI substrate as a source of the Si-NMs. The recipient silicon dioxide substrate is spin-cast with SU-8 polymer (see Figure). The preprocessed SOI host substrate containing the transferable devices is inverted and contacted with the polymer-cast recipient substrate. Heating the substrates to over 60°C allows the polymer to reflow and any trapped air bubbles are allowed to escape resulting in a uniform transfer. Furthermore, the reflow step allows any fine-tuning of the alignment of the host and recipient substrates. The host and the recipient substrate which are in contact at this point are subjected to a deep reactive ion etch to selectively remove the silicon handle from the host substrate. A buffered oxide wet etch selectively removes the oxide layer from the SOI host leaving the transferred Si-NM on the recipient substrate with the underlying polymer layer. The direct transfer process can be repeated to sequentially stack multiple Si-NMs.

In order to demonstrate the feasibility of transferring a photonic device that retained its functionality, the researchers utilized a photonic crystal directional coupler. Following the transfer, spectral scans between 1550 nm and 1558 nm revealed directional coupling between waveguides at the two wavelengths in the transferred device. The researchers said that to the best of their knowledge “this is the only nanomembrane photonic device which uses the confinement properties of the transferred Si-NM.”