Atomic Layer Deposition for Quantum Devices
Introduction As the transistor gave rise to the information age, quantum technology has the potential to be the next great leap forward. Quantum technology is the application of quantum physics for real world applications such as quantum computing, sensing, navigation and communication. Here superconducting single photon detectors (SSPDs) and superconducting qubits have been essential building blocks. The SSPDs and superconducting qubits encompass a variety of devices, including microwave resonators, transition edge sensors (TES), superconducting nanowires for single photon detectors (SNSPDs), Josephson junctions and high-Q superconducting resonators. One of the most crucial factors impacting the operation of these quantum devices is the choice and quality of the superconducting material used. It is important that the material of choice exhibits superconducting behaviour at high critical temperatures ( T c ) in order to avoid the additional cryocooler techniques that are needed for low T c material, and that the quantum efficiency (QE) and quantum detection efficiency of these devices is maximised. 1 Furthermore, it is also important that the defects in materials used in tunnel barriers, such as in Josephson junctions, are at a minimum as these could lead to decreased coherence of the qubit. 2 Conventional methods for depositing superconductors include sputtering, pulsed laser deposition (PLD) and chemical vapour deposition (CVD). However, these methods can suffer from drawbacks including a lack of thickness control, poor uniformity and high impurity content. Atomic layer deposition (ALD) is much more beneficial for thin-film deposition due to its ability to produce films with high purity, precise thickness control, conformal coating in high aspect ratio structures and uniformity over large-area substrates. ALD
has already successfully been applied in many fields such as in CMOS technology, high-power transistors, solar cells and many more. An emerging application is for films in devices for quantum applications. Illustrated on Figure 1 are examples where superconducting material has been integrated on to quantum nanophotonic circuitry. Figure 1(a) uses a 4 nm thick NbN nanowire folded in a meander pattern on a Si substrate with a 160 nm layer of SiO 2 . 3 and 1(b) consists of a silicon waveguide where the detection region is covered with NbN. 4 Other nitrides such as TiN may also be a possible candidate for integration in such circuitry. In this whitepaper we focus on the potential of such superconducting nitrides by ALD.
Figure 1: (a) An SSPD consisting of a 4 nm NbN superconducting nanowire folded in a meander pattern. 3 Schematic illustration of (b) SSPD device consisting of an NbN nanowire patterned on a silicon waveguide for single photon detection and (c) 3D integrated superconducting quantum circuit architecture and its various components.
Superconducting Nitrides Eg. NbN, TiN
Silicon Nitride/ Silicon Waveguides
Bump bonds Interposer
Superconducting metal and nitride etch Eg. Nb, Al, Ta, TiN
ALD tunnel barriers Eg. Al 2 O 3 , AlN
TSV etch Deep Si etching
Superconducting Nitrides deposition (conformal) Eg. NbN, TiN
2 ALD for Quantum Devices White Paper © Oxford Instruments
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