Every fabrication step usually needs to be characterized and optimized to reach the desired results. To examples of such tests performed to improve our process are presented in this section. Conformity of the desired patterns is of course one of the principal issues, but many other trouble are encountered. In particular, surface roughness of the substrate before deposition of the superconducting layer plays an important role in the final performance of the devices. The quality of the optical cavity is also critical to maximize photo absorption in the superconducting film.
Surface roughness
Surface roughness of the superconducting thin film is a key parameter to obtain high Tcsuperconductivity. Consequently, the SiO2 surface onto which the film is deposited must exhibit less than 1 nm of RMS roughness. This final value depends on the different deposition steps needed in order to create the cavity mirrors under the superconducting nanowire (see section B.5 for the detail of the fabrication process). In the process used, the superconducting layer is deposited on top of an Ag mirror, protected from oxidation by an Al2O3 layer. An additional deposition of SiO2 permits to precisely adjust the distance between the mirror and the superconducting layer to maximize absorption. The surface roughness of this final SiO2 layer is crucial for the proper functioning of the detector. We used atomic force microscopy to characterize the surface of several samples and optimize
B.4. Fabrication tests and optimization the process to maximize the quality of the final superconducting thin film. The surface roughness of the Cr-Ag-Al2O3 layers alone is not optimized. It is therefore crucial that the SiO2 layer is thick enough to smooth the surface. The characterization done in this work allowed us to optimize the sputtering recipe to ensure a low roughness SiO2 surface. The SiO2 total deposition is usually aimed at 200 nm, which is simultaneously maximizing the intensity of the field at the superconductor position while being thick enough to smooth the Al2O3 roughness, which optimizes the detector performances. As can be seen on fig. B.12, a carefully optimized process can yield sputtered SiO2 with a surface roughness almost equivalent to the original polished thermal oxide present on the wafer.
0.2
Figure B.12: AFM measurements of surface roughness after different growth/deposition processes.
Mirrors stack optimization, reflectivity measurements
Al tough Ag mirrors give yield high reflectivity at 1550 nm, other materials can be considered and have different advantages. In particular, adhesion of the Al2O3 layer on the Ag wasn’t optimal, and the mirrors showed signs of oxidation (see Fig. B.14).
more stable stacks of materials is worth considering. Tests on Ti/Au/Ti stack have been conducted, where the 1 nm Ti layers are used as adhesion layers. The 15 nm Au layer acts as a mirror in the infrared range, and isn’t sensitive to oxidation, thus offering a more reliable process. Moreover the adhesion of the subsequent SiO2 spacer layer on the stack is better than with the Cr/Ag/Al2O3 stack, which enhances the robustness of the process. We also tested Ti/Ag/Ti stacks, which gain from the slightly better reflectivity of the Ag layer, while protecting it with a Titanium layer which maximizes the adhesion and should avoid the formation of bubbles leading to oxidation. Another possibility is to use distributed Bragg mirrors (DBM, i.e periodic stack of layers of alternating materials with varying refractive index). DBM can yield extremely high reflectivity on a narrow wavelength band, which is optimal to maximize detection efficiency in our detector while filtering noise at unwanted frequencies. However, such mirrors are tricky to fabricate as reflectivity is extremely sensitive to process fluctuations. Stacks of SiN/SiO2 layers were fabricated and measured as well. Reflectivity tests of the different layer stacks were performed using a Varian Carry500 spectrophotometer. The reflectivity vs. wavelength is characterized at room temperature and is expected to be consistent at lower temperatures during the detector usage. Results are shown on Fig. B.13.
400 600 800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
Figure B.13: Reflectivity measurement for different mirror types.
The reflectivity at 1550 nm is of course of particular interest in our case. Among metallic mirrors, Cr-Ag-Al2O3 stacks gave the best result with a reflectivity value of 97.2%.
Dielectric stacks of SiO2 and SiN optimized at 1550 nm even show better results, with a measured reflectivity of 98.2%. However such stack is more difficult to fabricate and final results were not yielding to significantly better performances.
B.4. Fabrication tests and optimization
Figure B.14: Optical microscope capture of a badly impacted Ag mirror (light circle).
Bad adhesion of the protective Al2O3 layer yields to bubbles under which the material might oxidize, forming black spots on the image.
Silicon wafer SiO2
Ag mirror SiO2
Au
Ag mirror SiO2
Au
SiO2 MoSi
200 nm
Figure B.15: SEM image of a cross section of the mirror. Bad adhesion between the mirror and the SiO2 layer is clearly visible.
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Office : UNIGE, GAP ; Pinchat22 E-mail :
claire.autebert@unige,ch misael.caloz@unige.ch matthieu.perrenoud@unige.ch Lab : GAP Quantum Technologies, UNIGE
Operators Names :
Claire Autebert, Misael Caloz, Matthieu Perrenoud Supervisor Name : Félix Bussières
Semestral Project Master Project Thesis Other
MoSi Superconducting Nanowire Single-Photon Detectors
Description
Technologies used
!! remove non-used !!
Mask fabrication, negative resist, Dry etching, E-beam lithography, SEM
Photolith masks
Mask #
Critical Dimension
Critical
Alignment Remarks
1 1 nm First Mask Nanowire pattern for MoSi etching
Substrate Type
100/p/DS/1-10 Dry Oxide 100nm