A novel cryogenic scanning laser microscope with a spatial resolution of less than 5 μm has been designed for on-chip in situ investigations of the working properties of normal and superconducting circuits and devices. The instrument relies on the detection of the electrical response of the circuit to a very localized heating induced by irradiation with 675 nm wavelength light from a semiconductor laser. The hot spot is moved by a specially designed piezoelectric scanner sweeping the tip of a single-mode optical fiber a few μm above the circuit. Depending on the scanner design the scanning area can be as large as 50×500 μm2 at 4.2 K. The microscope can be operated in the temperature range 2–300 K using a standard temperature controller. The central microscope body is mounted inside the vacuum can of a dip-stick-type cryoprobe. A damped spring system is used to reduce interference from extraneous mechanical vibrations. The microscope is integrated in a personal-computer-based data acquisition and control setup.
A novel cryogenic scanning laser microscope with a spatial resolution of less than 5 μm has been designed for on-chip in situ investigations of the working properties of normal and superconducting circuits and devices. The instrument relies on the detection of the electrical response of the circuit to a very localized heating induced by irradiation with 675 nm wavelength light from a semiconductor laser. The hot spot is moved by a specially designed piezoelectric scanner sweeping the tip of a single-mode optical fiber a few μm above the circuit. Depending on the scanner design the scanning area can be as large as 50×500 μm2 at 4.2 K. The microscope can be operated in the temperature range 2–300 K using a standard temperature controller. The central microscope body is mounted inside the vacuum can of a dip-stick-type cryoprobe. A damped spring system is used to reduce interference from extraneous mechanical vibrations. The microscope is integrated in a personal-computer-based data acquisition and control setup.
The first local oscillators based on moving magnetic flux quanta in long Josephson junctions are being developed for superconducting integrated quasi-optical SIS receivers. In order to further refine these oscillators one has to understand the complex dynamics of these devices. Since the local tunnel current is one of the most important internal junction parameters which together with the boundary conditions determine the dynamics, it is of vital importance to experimentally determine the current density throughout the entire junction with high spatial resolution. Here we report on measurements on different oscillator samples, performed with a novel Cryogenic Scanning Laser Microscope (CSLM) having a spatial resolution of less than /spl plusmn/2.5 /spl mu/m over a 500 /spl mu/m/spl times/50 /spl mu/m wide scanning area in the temperature range 2 K-300 K. Even though the dynamical states are extremely sensitive to external noise this microscope enables us to make stable in-situ measurements on operating Josephson junctions. Recent results are presented and discussed.>
Low temperature scanning electron microscopy (LTSEM) offers the possibility for spatially resolved investigations of microwave properties of single Josephson junctions, superconducting striplines, resonators, and complex cryoelectric circuits during their operation. The spatial resolution of about 1 μm is at least two orders of magnitude better than the typical wavelength (≳100 μm in the elements) in the frequency range of interest (50–500 GHz). We demonstrate that LTSEM is a well suited tool for the experimental investigation of superconducting microwave devices and cryoelectronic circuits.
We have developed a low temperature (LT), ultra-high-vacuum (UHV) system that combines two complementary techniques, scanning probe microscopy (SPM) and electrical transport measurements, within a single platform. By providing simultaneous access to the atomic-scale surface landscape and the macroscopic device response of the same sample, the setup enables direct correlations between local structural/spectroscopic signatures and global electronic transport behavior in two-dimensional (2D) devices. The system allows experiments where atomic-scale modifications or controlled manipulations are performed while continuously monitoring their impact on device-scale performance. The setup consists of two interconnected UHV chambers: a dedicated preparation chamber and a separate measurement chamber that houses a liquid-helium cryostat and the SPM/transport stage. Base pressure is 1x10^-11 Torr. A key feature is direct optical access to the sample, enabling rapid and reliable tip positioning with an accuracy of 5 microns x 5 microns within 10 minutes. The SPM, operated using custom-built electronics, can track the exact same sample region across a temperature range from 2.9 K to 400 K, with mechanical stability below 1 pm. System performance is demonstrated on graphene devices, and bulk Pb is used to determine energy resolution. Using superconducting tips, scanning tunneling spectroscopy measures the superconducting gap with an energy resolution of 30 microV. Transport measurements track the temperature dependence of both resistivity and critical current across the superconducting transition of an in-situ prepared Pb nanowire.
We report on the design and performance of a cryogenic (300 mK) near-field scanning microwave microscope. It uses a microwave resonator as the near-field sensor, operating at a frequency of 6 GHz and microwave probing amplitudes down to 100 μV, approaching low enough photon population (N ∼ 1000) of the resonator such that coherent quantum manipulation becomes feasible. The resonator is made out of a miniaturized distributed fractal superconducting circuit that is integrated with the probing tip, micromachined to be compact enough such that it can be mounted directly on a quartz tuning-fork, and used for parallel operation as an atomic force microscope (AFM). The resonator is magnetically coupled to a transmission line for readout, and to achieve enhanced sensitivity we employ a Pound-Drever-Hall measurement scheme to lock to the resonance frequency. We achieve a well localized near-field around the tip such that the microwave resolution is comparable to the AFM resolution, and a capacitive sensitivity down to 6.4 × 10(-20) F/Hz, limited by mechanical noise. We believe that the results presented here are a significant step towards probing quantum systems at the nanoscale using near-field scanning microwave microscopy.
Integrated semiconductor circuits operating at cryogenic temperatures can play a crucial role in scaling quantum computing architectures based on trapped-ion and superconducting qubits. Other technologies, such as low-temperature detectors, can also benefit significantly from these circuits. These applications demand high-frequency, cryogenic temperature measurement systems for the thorough characterization of semiconductor components and circuits. This work presents a customizable, high-frequency, fast, and reliable cryogenic measurement setup for measurement temperatures ranging from room temperature to 4.2 K. It features up to two dc probes or two ground-signal-ground probes for frequencies of up to 67 GHz, which can be configured in a 180° or 90° configuration. In addition, up to 48 BeCu wires configured as twisted pairs provide supply and control signals. Furthermore, up to four rf connections can be mounted directly to a printed circuit board. The setup features an optical microscope in the vacuum chamber to position the probes, which is further utilized by a machine vision algorithm, allowing the detection of pads and automatic positioning of the probes over the pads. The hardware is located in a 550 × 500 × 500 mm3 large vacuum chamber with two independent pulse tube cryocoolers with cooling powers of up to 0.9 and 0.4 W at 4.2 K. Exemplary room temperature and cryogenic S-parameter, transient frequency (fT), time-domain, capacitance-voltage, and dc measurements of single transistors and integrated circuits fabricated by Infineon are presented, demonstrating the system's measurement capability for quantum computing and other applications.
A thermal annealing technique for Nb∕Al–AlOx∕Nb Josephson devices based on laser heating is presented. This technique allows “locally” modifying the Josephson critical current density, which is not possible using standard procedures based on oven or hot plate heating. In fact, the heating of a single circuit element with a good spatial resolution is possible. At room temperature, the selected junction is exposed to a focused Ar+ laser beam aligned by an optical system. A thermographic imaging allows controlling the temperature distribution on the whole chip. Experimental results on high quality Josephson junction measured in liquid helium have shown a continuous reduction of the critical current density up to about 40%. Neighboring junctions have not exhibited any measurable change ensuring the capability to locally modify the Josephson critical current density. As first application, the present technique has been employed to recover noisy dc superconducting quantum interference device magnetometers with nonoptimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30to2.5fT∕Hz1∕2.