The work presented in this thesis covers the essential aspects of MoSi superconducting nanowire single-photon detectors. While the fabrication, characterization, investigation of the intrinsic limits and the detection mechanism constitute the core of this work, the driving motivation behind this thesis was constantly focused on delivering high performance single-photon detectors for demanding in-house quantum communication applications. During the course of this thesis several peer-reviewed articles were published and are presented in two different appendices: Appendix A includes the articles that are relevant and constitute the essential of this thesis, while the ones included in Appendix B concern the applications of SNSPDs in quantum communication experiments.
To present high performance detectors, the nano-fabrication was a substantial part of this thesis and is detailed in Chapter 2, where crucial processes are discussed.
Despite an impressive amount of experimental work to improve the performance of SNSPDs since their invention, a full description of the detection mechanism has not been established yet. This question is addressed with MoSi detectors in Chapter 3 and Appendix A.1. A particularly crucial feature of the detection models is the so-called energy-current relation, which we measured to be non-linear over a large range of energy, contrarily to other studies. In addition, new insights on Fano fluctuations are reported.
The performance achieved during the course of this thesis are discussed in Chapter 4, Chapter 5, and Appendix A.2, where detectors combining high detection efficiency (>85%) and low timing jitter (<30 ps) are presented. Notably, detectors obtained during this work were used in a remarkable long-distance quantum key distribution experiment, presented in Appendix B.1.
The investigation of the intrinsic limits of the timing jitter, partially performed at the NASA Jet Propulsion Laboratory (USA), is included in the second part of Chapter 5 and mentioned in the Appendix A.3. The goal behind this study was not only to improve the system jitter of amorphous MoSi SNSPDs, but also to probe, reach, and quantify the intrinsic component in order to have experimental evidences of the fundamental limits and insights of potential material differences.
Finally, recent and promising results on the maximum count rate are presented in Chapter 6. Furthermore, preliminary results on the parallel design and detectors made out of NbTiN material are discussed, opening up new opportunities and improvements in the SNSPD field.
2 Nano-fabrication
Image credit: CMi, EPFL
Nano-fabrication constitutes a substantial part of this thesis. The first section of the present chapter introduces the basic knowledge of nano-fabrication processes required to make SNSPDs. The second section focuses on the superconducting materials properties, while the following ones correspond to each important step of the fabrication itself.
2.1 The basics of nano-fabrication
Nano-fabrication refers to fabrication processes of objects and structures at the nanometer-scale dimensions. The range of application is endless and numerous modern and everyday applications benefit from nanotechnology advances: from the processors in smartphones, semiconductor electronics, chemistry, medicine, molecular biology, aerospace engineering, to physics and beyond. Research in nano-fabrication requires joint effort across many scientific disciplines, such as collaboration between mechanical engineers, physicists, biologists, chemists, and material scientists. As nanotechnology processes are usually very sensitive to any kind of dust and temperature and humidity fluctuations, the fabrication
is carried out in clean and regulated environment, so-called cleanroom.
Typically used in nano-fabrication and scientific research, a cleanroom is a controlled environment that has a low level of pollutant particles, and a regulated temperature and humidity. The cleanroom has a controlled level of contamination that is specified by the number of particles per cubic meter at a specified particle size. The air is coming from an outside source and filtered to eliminate dust, the inside air is recirculated constantly through filters that remove containments produced from within the cleanroom. Staff enters and leaves cleanrooms through airlocks, and wears protective clothing including face masks, laboratory coats, glove, aprons, hairnets, hoods, sleeves, glasses, and shoe covers.
The equipment inside a cleanroom is designed to generate minimal air contamination, with specialized paper sheets, pencils, etc... Cleanrooms are not a sterile environment as the attention is primarily to controlling dust particles.
The nano-fabrication processes performed during the course of this PhD was carried out mostly in the Centre de MicroNanotechnology (CMi, ISO 7-6) in Ecole Polythechnique Fédérale de Lausanne (EPFL), and partially at the University of Basel. The techniques used during this PhD are listed and briefly explained below.
Photolithography
Photolithography is a technique used in nano-fabrication to pattern 3D structures on a substrate, also called wafer. It uses light, generally in the UV range, to transfer a geometric pattern to a photosensitive chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist (lift-off process). To create complex structures, a wafer may go through many photolithography cycles. The principle is based on the fact that the illuminating light changes the solubility of the photoresist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing step).
Photolithography method can create extremely small patterns, down to a few hundreds of nanometers in size. It provides precise control of the shape and size of the objects it creates. Photolithography is the standard method of printed circuit board (PCB) and microprocessor fabrication, for example. A standard photolithography process is sketched in the Fig. 2.1.
For the fabrication of SNSPDs, at least three photolithography processes are required: (i) for the mirror deposition, (ii) the electric contact pads, and (iii) the final wafer etching.
Each photolithographic step requires different dimensions and topology, the resist and the process have to be adjusted accordingly.
2.1. The basics of nano-fabrication
substrate
light
mask
(a)
(b)
(c)
(d)
(e)
(f)
photoresist
Figure 2.1: (a) The photoresist is spin coated on the substrate. The photoresist is selected according to the desired specifications, such as positive tone or negative tone, smallest dimensions achievable, exposing light wavelength, etc... (b) A mask is aligned onto the substrate. For direct writing method, no mask is needed, the laser beam moves and prints directly the design. (c) Light is illuminating the photoresist. (d) Patterns are transferred to the photoresist and the mask is removed. (e) Development step, which involves chemical reactions with specific solvents that strip the exposed parts of the photoresist, and harden the non-exposed ones (for positive-tone resist, inversely for negative-tone resist). (f) The substrate is ready for etching process.
Electron-beam lithography
Electron beam lithography (also called e-beam lithography or EBL) uses basically the same principle as the photolithography (see Fig. 2.1), except that direct writing electrons are exposing the resist instead of light, increasing the feature size resolution compared to diffraction-limited photolithography. With optimized process, the typical feature size achievable with EBL is under 10 nm. For the SNSPD fabrication, the nanowire typically ranging from 30 to 200 nm, are patterned with the EBL technique.
Etching
Etching is used in nano-fabrication to remove layers from the surface of a wafer. Etching is a critically important process, and wafers commonly undergoes many etching steps before completion. The term of etching regroups two categories: wet etching and dry etching. Dry etching refers to the removal of material, by exposing it to a bombardment of ions, usually a plasma of reactive gases such as fluorocarbons, oxygen, chlorine, boron trichloride, sometimes with addition of nitrogen, argon, helium and other gases. For wet etching, the wafer is directly immersed in a bath of etchant, usually strong acids that will etch through the materials.
Dry etching technique is used at numerous levels of the SNSPD fabrication process:
(i) nanowires patterning (ion beam etching (IBE) with argon, RIE with CF4, SF6 and Ar gases), (ii) electric contact pads (IBE), and (iii) wafer etching (deep reactive ion etching (DRIE) with Bosch process).
Physical vapor deposition
Physical vapor deposition (PVD) describes a variety of vacuum deposition methods which are used to grow thin films on a substrate. PVD is characterized by a process in which the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase onto the substrate. The most common PVD processes are sputtering and evaporation. As sputtering was a crucial technique to deposit high quality superconducting films during this thesis, this section introduces this technique in detail.
Sputter deposition involves ejecting material from a so-called target onto a substrate.
When multiple targets are used at the same time, it is referred as co-sputtering. A plasma with a inert sputtering gas, usually Argon is maintained closed to the target during the process. Argon atoms will strike the target and eventually eject atoms from the target.
Sputtered atoms ejected from the target have a wide energy distribution. The sputtered ions, typically only a small fraction of the ejected particles, can ballistically fly from the target and impact on the substrates, as shown in Fig 2.2.
2.1. The basics of nano-fabrication
pump target
magnet
gas inlet
Ar O2
N2
gas inlet
pump
heating/cooling system substrat
target
target RF/DC generator
thin film
atom ejected
Ar
Ar Ar
substrate holder
Figure 2.2: Simplified schematic of a sputtering machine. The wafer is placed on a substrate holder, which can be heated up or cooled down during the deposition. The vacuum is maintained to a desired value, sputtering gases and reactive gases can be added in the chamber via inlets. Depending on the target composition, RF or DC generator are set up in order to maintain stable plasma during the process.
For reactive sputtering processes, reactive gases such as O2 and N2 can also be added in the chamber to sputter compounds. The compounds are chemically formed on the target surface, in-flight or on the substrate depending on the process parameters. The composition of the film can be controlled by varying numerous parameters such as the relative pressures of the inert and reactive gases, target power, pre-process target cleaning steps, etc... Film stoichiometry is a crucial parameter for optimizing functional properties of superconducting films. The availability of many parameters that control sputter deposition make it a complex process, but also allow a large degree of control over the growth and microstructure of the film. Those parameters are described in details in the next section.