The QR codes encode physical position in the plane in binary, as illustrated in Fig. 8.14I. They allow easy navigation to a sample without wires or thermal navigation, while simultaneously allowing the nanoSQUID sensor to stay a safe several microns away from the surface. There are a few engineering challenges associated with fabricating nanoSQUID sensors. I will briefly describe a particularly challenging one in this section. Many of the best elemental superconductors are soft, heavy metals with low melting points like lead and indium. As any person who has spent some time in an experimental physics laboratory knows, solder doesn’t wet too many materials well, and it certainly doesn’t wet glass, so these metals tend to form droplets when deposited onto glass substrates. To form a uniform film, the superconducting metal must freeze instantlyupon landing on the glass micropipette. To make sure this occurs, we must cryogencially cool the glass micropipettes while evaporating the superconducting metal onto them. This process involves specialized machinery that is covered in great depth in other documents and publications, so I won’t discuss it here. However, I do want to discuss the nature of the failure modes of this process. When liquids don’t wet surfaces well, they dewet into droplets, and these droplets tend to get more spherical and less film-like the worse they wet the surface. If this process is allowed to proceed to its conclusion before deposited metal solidifies, growing blackberries in containers the resulting films won’t be connected at all, and your nanoSQUID circuit will be open.
If the substrate is cold enough, the resulting film will at least be continuous, and it is likely that you will get a nanoSQUID. However, the formation of droplets is impossible to completely stop, especially near the edges of films and on the oblique surfaces of the nanoSQUID sensor . These droplets generally won’t short the sensor, but the nanoSQUID sensor is so small that electrons can reach these droplets through tunneling processes. Whenever droplets form between the two superconducting contacts on the nanoSQUID sensor electrons can tunnel between the contacts through the droplet, with the droplet functioning as a quantum dot. The resulting Coulomb blockade phenomenon gives nanoSQUID sensors a very slight electric field sensitivity. Gating exfoliated heterostructures tends to produce large electric fields, and these are detectable as variations in the current through the nanoSQUID as a result of Coulomb blockade in parallel with the SQUID on the tip. Droplets functioning as quantum dots and producing a parasitic Coulomb blockage are so common that we observe them on nearly every nanoSQUID sensor, and we almost always have finite electric field sensitivity . This can be useful for finding the edges of devices in the absence of magnetism, but it is important to remember that not all nanoSQUID signals can be understood as local magnetic fields. Other parasitic contrast mechanisms do exist, but they are rarely dominant over magnetic or electric field sensitivity. At high tuning fork amplitudes, interactions between the nanoSQUID tip and the surface can produce local variations in oscillation amplitude and appear as parasitic signals at the tuning fork frequency.
Of course, the nanoSQUID is highly sensitive to local temperature, so systems with thermal gradients will generally have backgrounds associated with that. But by far the most important parasitic contrast mechanism in the nanoSQUID campaigns discussed here is electric field contrast through parasitic Coulomb blockade.Below I have included a set of instructions for execution of a nanoSQUID magnetic imaging campaign using the instruments in Andrea Young’s lab. It may be useful if you are operating or building a nanoSQUID microscope in a different lab, but I would like to emphasize that the instructions below are merely sufficient for getting the nanoSQUID sensor to a sample, they are almost certainly not optimized for expediency. I’m sure that as the technology matures many steps will be rendered superfluous. Of course, if you’re using the nanoSQUID to study a bulk material and not a microscopic heterostructure, navigation is not necessary and you will be able to skip most of these instructions. A nanoSQUID imaging campaign can begin when the microscope is cooled down and all of the necessary systems are operational. One must check that: -The tuning fork has a good resonance with Q ≥ 1000, the phase-locked loop inside the Zurich lock-in amplifier locks, and you can find an AC tuning fork excitation at which clicking “Set PLL Threshold” produces a 0.25 Hz standard deviation. If you are in Andrea Young’s lab and not some other institution running a nanoSQUID microscope, remember that this custom tuning fork amplifier needs 5 V, not 15 V, unlike most of our custom electronics. -The tip has been characterized and is a SQUID. Sensitivity is good enough that magnetic field noise is ≥25 nT/rtHz . The SQUID interference pattern looks reasonably healthy and corresponds to a diameter that is close to the SEM diameter . It is important to remember that it is possible for the Josephson junctions producing nanoSQUIDs to end up higher on the sensor. These might produce healthy SQUIDs but will not be useful for scanning, and discovery of this failure mode comes dangerously late in the campaign, so SQUIDs high up on the pipette are very destructive failure modes. This failure mode is uncommon but worth remembering. If you have access to a vector magnet, square pot such SQUIDs also usually have large cross sections to in-plane magnetic flux, and this can be useful for identifying them and filtering them out. Damage to the scanners or the associated wiring will appear as deviations from these capacitances. Small variations around these values are fine. After you are done testing these capacitances, reconnect them. Make sure you’re testing the scanner/cryostat side of the wiring, not the outputs of the box- this is a common silly mistake that can lead to unwarranted panic. If you’re working in Andrea Young’s lab, make sure the Z piezo is ungrounded . If for whatever reason current can flow through the circuit while you’re probing the capacitance, you will see the capacitance rise and then saturate above the range of the multi-meter. -Because the nanoSQUID is a sharp piece of metal that will be in close contact with other pieces of metal, it sometimes makes sense to ground the nanoSQUID circuit to the top gate of a device, or metallic contacts to a crystal, to prevent electrostatic discharge while scanning or upon touchdown.
If you have decided to set up such a circuit, make sure that the sample, the gates, and the nanoSQUID circuit are all simultaneously grounded. If you forget to float one of these circuits and bias the SQUID or gate the device, you can accidentally pump destructive amounts of current through the nanoSQUID or device. However, you must make sure that the z piezoelectric scanner is not grounded. You can now begin your approach to the surface. You should ground the nanoSQUID and the device. If you are in Andrea’s lab, verify that the three high current DB-9 cables going from the coarse positioner controller box to the box-to-cable adapter are plugged in in the correct positions. The cables for each channel all have the same connectors, so it is possible to mix up the x, y, and z axes of the coarse positioners. This is a very destructive mistake, because you will not be advancing to the surface and will likely crash the nanoSQUID into a wirebond, or some other feature away from the device. The remaining instructions assume you are using the nanoSQUID control software developed in Andrea’s lab, primarily by Marec Serlin and Trevor Arp. The software is a complete and self-contained scanning probe microscopy control system and user interface based on Python 3and PyQT. Open the coarse positioner control module. Click the small capacitor symbol. You should hear a little click and see 200 nF next to the symbol . The system has sent a pulse of AC voltage to the coarse positioners; the click comes from the piezoelectric crystal moving in response. Check that you see a number around 1000 µm in the resistive encoder window for axis 3 . Note whether you see a number around 2000-3000 µm in the windows for axis 1 and axis 2. If you are in Andrea’s lab, it is possible that you will not for axis 2. Axis 2 has had problems with its resistive encoder calibration curve at low temperature. The issue seems to be an inaccurate LUT file in the firmware; new firmware can be uploaded using Attocube’s Daisy software. It is not a significant issue if you cannot use the axis 1 and 2 resistive encoders; however, it is critical that there be an accurate number for axis 3. Set the output voltage frequency to be somewhere in the range 5-25 Hz . Set the output voltage to 50 V to start . Make sure that the check box next to Output is checked. Move 10 µm toward the sample . If Axis 3 doesn’t move, don’t panic! It’s usually the case that the coarse positioners are sticky after cooling down the probe before they’ve been used. Try moving backwards and forwards, then increase the voltage to 55 V, then 60 V. Once they’re moving, decrease the voltage back to 50 V. Note the PLL behavior- if there’s a software issue and pulses aren’t being sent, you won’t see activity in the PLL associated with the coarse positioners. Under normal circumstances you should see considerable crosstalk between the PLL and the coarse positioners while the coarse positioners are firing. There are significant transients in the resistive encoder readings after firing the coarse positioners; this is likely a result of heating, but could also have a contribution from mechanical settling and creep. We have observed that the decay times of transients are significantly longer in the 300 mK system than in the 1.5 K or 4 K systems, likely indicating that these transients are largely limited by heat dissipation, at least at very low temperatures. Go into the General Approach Settings of the Approach Control window. There’s a setting in there for coarse positioner step size- set that to 4 µm or so. This is the amount the coarse positioners will attempt to move between fine scanner extensions. They always overshoot this number . Overshooting is of course dangerous because it can produce crashes if it is too egregious. In the Approach Control window, click Set PLL Threshold, verify that standard deviation of frequency is 0.25 Hz. Enter 5 µm into the height window. Verify that Z is ungrounded . Click Constant Height. Check that the PID is producing an approach speed of 100 nm/s. It is important that you sit and watch the first few rounds of coarse positioner approach. This is boring, but it is important the first few coarse positioning steps often cause the tuning fork to settle and change, which can cause the approach to accelerate or fail. Also by observing this part of the process you can often find simple, obvious issues that you’ve overlooked while setting up the approach. Getting to the surface will take several hours. Typically you’ll want to leave during this time. When you return, the tip should be at constant height. I’d recommend clicking constant height again and approaching to contact again to verify that you’re at the surface. You should be between 10 µm and 20 µm from the surface. It may be necessary to withdraw, approach with the coarse positioners a few µm, and then approach again to ensure you have enough scanner range in the z direction. Click withdraw until you’re fully withdrawn. Click Frustrate Feedback to enable scanning with tip withdrawn. I will present instructions as if you are attempting to navigate to a device through which you can flow current. This will generate gradients in temperature from dissipation and ambient magnetic fields through the Biot-Savart law, both of which the nanoSQUID sensor can detect. I strongly recommend that you navigate with thermal gradients if at all possible.