Superconducting material consideration
The work reported in this thesis has been done using superconducting films of Molybdenum Silicide with a stochiometry of 80%-20% (Mo0.8Si0.2). Usage of MoSi thin films presents several advantages. Unlike crystalline structures commonly used for the fabrication of SNSPDs such as NbN or NbTiN films, MoSi is an amorphous material which makes it easier to deposit in a stable and reproducible way thus enabling fabrication with a high yield. The critical temperature Tc of MoSi is similar than that of WSi [34, 35] (which is another amorphous material commonly used for SNSPDs fabrication)3. The energy bandgap of MoSi is lower than that of NbN or NbTiN, making it a very sensitive material for which the saturated efficiency can be easily reached.
The detector performances of course depend on the superconducting film, and it is possible that some other material lead to better characteristics. In particular, the kinetic inductance of a superconducting nanowire highly depends on the material properties.
Materials with a lower inductivity such as NbN or NbTiN [37] might exhibit faster recovery time after detection and higher latching current values. Although primarily developed to push the limits of our MoSi SNSPDs, the concepts presented in this work could be applied to other sort of material as well and are not restricted to MoSi only. In particular, NbTiN thin films are currently investigated in our group, which have the potential to yield better performances than MoSi on almost all detection aspects. Combining superconducting films of such materials with the solutions proposed in this work could be possible and might improve the performances of commonly used single-meander SNSPDs made out of these materials as well.
1.3 Thesis outline
The work presented in this thesis focuses on improving performances of MoSi supercon-ducting nanowire single photon detectors for quantum applications. In particular, efforts were dedicated to improve the efficiency of the detector at high detection rates (in the order of tens to hundreds of MHz). Similarly, the recovery time of the detectors is reduced, which can be essential for given applications. The first part of the present work briefly explains the design and fabrication process, as well as the measurement setups used for the characterization of our detectors (chapter 2). The second part introduces the main results obtained with multi-nanowire detector designs (chapter 3) and improved readout (chapter 4). Finally, in chapter 5, results obtained in quantum experiments that made
use of the detectors created during this work are presented.
Table 1.1: Comparison of single-photon detectors. The values are indicative of the technology potential, in the sense that the best performances of each category are revealed independently of the other categories. As explained above, this does not mean that one detector based on a specific technology exhibits these performances simultaneously. The data are updated from [2]. For high efficiency SNSPDs, see [4, 5, 6, 7]. For InGaAs SPAD, see [38, 39, 40]. Otherwise, see [2].
Detector Temp. Detection Timing DCR Max. count Spectral
type (K) efficiency jitter rate range
PMT (NUV-NIR) RT 40% at 500 nm 300 ps 100 kHz 10 MHz NUV-NIR
PMT (IR) 200 K 2% at 1550 nm 300 ps 200 kHz 10 MHz IR
MCP-PMT RT 25% at 500 nm 55 ps 100 Hz 10 MHz NUV-NIR
Si SPAD (TJ) RT 65% at 650 nm 400 ps 25 Hz 10 MHz Visible
Si SPAD (SJ) RT 49% at 550 nm 35 ps 25 Hz 10 MHz Visible
CMOS SPAD RT 50% at 550 nm 80 ps 50 Hz 10 MHz Visible
VLPC 6 K 88% at 694 nm 270 ps 20 kHz - Visible
MKID 0.1 K 90% at 285 nm 50 ns 1 Hz 100 kHz X-ray–FIR
TES 0.1 K 98% at 1550 nm 100 ns 3 Hz 100 kHz NIR
InGaAs SPAD 240 K 55% at 1550 nm 50 ps 1 kHz 100 MHz NIR
SNSPD 1-3 K 98% at 1550 nm 3 ps 10−3 Hz 200 MHz X-ray–MIR
2 Design, nano-fabrication, and mea-surement setups
The optimization of existing detector design and creation of new solutions is at the core of the work presented in this document. Although the main results discussed in this thesis concern novel SNSPD designs, a large amount of effort has also been produced to optimize the performances of single-meander SNSPDs fabricated in our group. Hence design, fabrication optimization, and testing of the detectors have been a consequent part of the effort needed to obtain the results presented in this document. In this chapter, the design tools and main nano-fabrication processes will be introduced. The second part of this chapter focuses on the measurement setups used for the characterization of our devices and presents results obtained with Molybdenum-Silicide single-meandered SNSPDs fabricated in our lab.
2.1 SNSPD design and nanofabrication
The first step needed before starting the fabrication process is of course the design of the different elements of the detector. The geometry of the nanowire, integrated elements (namely, resistors and superconducting kinetic inductances), contact pads and external shape of the detector are defined using kLayout. The .gds file format is an industry standard for computer aided design (CAD) and is used by all the cleanroom tools used during the fabrication process presented hereafter. GDS files can be produced via the software’s user interface, however this approach is highly time consuming. Producing design files via Python scripts allows the creation of multiple designs with different dimensions and shapes (2.1).
kLayout
The KLayout software works with the GDS standard for CAD conception. The software allows the creation, visualization, and edition of GDS design file with sub-nanometer resolution. The tool is particularly useful to visualize GDS files created via Python.
Figure 2.1: Screenshot of KLayout’s user interface. Each layer (i.e the colors listed on the right hand side) corresponds to the geometry of a specific fabrication process. Each device can be visualized individually via the ’cell’ list on the left-hand side of the interface.
gdsCAD for Python
Creating hundreds of different designs with various dimensions cannot be done manually.
The python packagegdsCAD provides tools to generate.gds files using python scripts.
An extensive amount of work during this thesis has been dedicated to develop a python software tool to create various SNSPD designs elements and connect them together to generate the full .gds file needed for fabrication. This software has been used to produce
2.1. SNSPD design and nanofabrication all the advanced designs presented in this document. It allows for different connection of multi-nanowires detectors, with additional components such as integrated resistors and inductors. The dimensions of every component can be modified by the user. This permits the creation of multiple designs of similar constructions with varying dimensions. Several hundreds of detector design variations were produced and fabricated.
Nano-fabrication
Nano-fabrication refers to the different processes used to create micro and nano structures into various materials. These techniques are extensively used in the manufacturing of integrated electronics and allow for high precision, high yield, and reproducibility.
Extensive work has been performed during this thesis to improve the fabrication process of the SNSPDs fabricated in the lab, giving higher yield, reproducibility, and improved overall performances. The fabrication was performed at center for micro and nanotechnologies at the Swiss Institute of Technology, Lausanne (EPFL - CMi). Figure 2.2a shows the typical stack of patterned materials used for the fabrication of a detector, while figure 2.2b shows an actual SEM image of the top layer once connected to contact electrodes.
(a) Scaled representation of the layer stack ob-tained after the SNSPD patterning process.
The z-axis was scaled by a factor 100 for a better view of the different layers of the optical cavity.
(b) Colored SEM image of the electrodes (yel-low) made of 10 nm Ti and 90 nm Au layers deposited on top of the patterned supercon-ducting material (red).
Figure 2.2: Illustration of the fabrication stack of patterned layers of different materials.
The optical cavity is closed by a final layer of 10 nm SiO2. See appendix B.5 for the detailed process flow.
The different nanofabrication processes used to fabricate the devices are described in greater details in appendix B. And some of the process optimizations are presented in
Packaging
Once the wafer fabrication is done, the devices are individually separated by etching through the wafer using a deep reactive ion etching (DRIE) process (details in appendix B).
The final devices “lollipop” shape enable the use of a self-aligned packaging technique, with the head of the lollipop matching the diameter of zirconium fiber optic sleeves [41].
The electrodes are accessible outside the sleeve and are wire-bonded to a coaxial cable SMP connector (fig. 2.3). Once the devices are packaged on their holder, they are placed in a cryostat and cooled down to 0.8 Kelvin.
Figure 2.3: Packaged SNSPDs. The optical fibers are self-aligned with the center of the meandered nanowire thanks to the matching diameter between the detector head and zirkonium ferrule.