High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

The overall goal of this microscopy technique is to measure self-heating in microelectronic devices operating at cryogenic base temperatures. This method can help answer key questions in the field of high temperature superconductive current tape engineering, such as how quench processes nucleate and propagate. The main advantage of this technique is that it gives a direct measurement of sample surface temperature with spatial resolution of approximately one micron.

Demonstrating the procedure will be Yang Hao, a graduate student in the ANL Superconductivity and Magnetism group. To begin this procedure, make current and voltage connections to the devices on the sample in preparation for thin-film coating, as these steps may introduce contamination that should be removed before coating. Clean the sample in 100%acetone in an ultrasonic bath for 15 seconds.

Without allowing the sample to dry, clean it in 100%isopropyl alcohol in an ultrasonic bath for five seconds. Then, blow the sample dry using a nitrogen gun. If possible, clean any remaining organic residues off the sample surface, using oxygen plasma ashing for 60 seconds.

Next, soak a sublimation source in acetone to remove any melted residues of europium TFC, as these will adversely affect the properties of the new film. Following this, rinse the boat in isopropyl alcohol, allowing it to completely dry before adding europium TFC powder. After removing the europium TFC powder from storage, thoroughly grind it using an agate mortar and pestle to remove any visible lumps.

Next, install the sample holder and sublimation source in a vacuum coating system such that the sample sits approximately 100 millimeters directly above the source boat. Connect the source boat heater leads to their associated vacuum feedthroughs. Fill the source boat approximately two-thirds full with 0.2 grams of ground europium TFC powder.

Mount the sample upside down, directly above the source boat, preferably using sticky dots. To minimize exposure of the sample surface and the europium TFC powder to the atmosphere, begin evacuation of the deposition chamber using a rotary pump as soon as possible. Now, pump the deposition chamber to three times ten to the minus five millibar or less, using a turbomolecular pump.

Following this, program the crystal thickness monitor to read for a film density of 1.50 grams per centimeters cubed. Apply 0.5 watts of power to the source boat heater to gently warm the source until the europium TFC begins to sublimate. Adjust the heater power to maintain a deposition rate of six to seven nanometers per minute.

After 200 nanometers of film deposition, turn off the power to the source. Once the deposition rate indicated by the thickness monitor reaches zero, vent the chamber with dry nitrogen gas. After removing the sample from the chamber, immediately protect it from light and water vapor by storage in a light-proof container in a vacuum desiccator.

At this point, place a blob of vacuum grease on the center cryostat sample stage, approximately one to two millimeters in diameter. If the sample substrate is electrically conducting, isolate it from the stage by placing a 10-micron sheet of mylar on top of the grease and a second similarly-sized blob of grease on top of the mylar sheet. Press the sample down on top of the grease using tweezers to apply force to two diagonally opposite corners simultaneously.

Then, clamp the sample in place at two corners using brass screws and beryllium copper clamps. Make any necessary electrical connections from the sample to the cryostat wiring, taking care not to allow contamination to land on the europium TFC film. Install the microscope's heat shield and optical window.

Cover the optical window of the cryostat with a piece of aluminum foil to prevent bleaching of the europium TFC by ambient lighting in the room. Then, cool the cryostat to the bath temperature of interest and evacuate the sample space with a turbomolecular pump. To collect the thermal image data, install a short-pass filter with a 500 nanometer cutoff wavelength in the illumination optics path.

Then, install a band-pass filter with a 610 nanometer pass band center wavelength and a 10 nanometer full-width at half maximum in the collection optics path. After allowing the light source to warm up and the camera to cool, illuminate the sample, and align and focus the microscope to the region of interest. Following this, collect a reference image with zero current applied to the sample.

Apply electrical bias to the sample and collect an image under the same exposure conditions as the reference. Then, compute the intensity ratio of the sample and reference images. Repeat the previous step for all bias conditions of interest while keeping the bath temperature constant.

Then, repeat the entire process for all bath temperatures of interest. Finally, collect zero applied current reference images sufficient to cover the entire temperature range of interest. Typical thermal images of self-heating in abisco 2212 terahertz source reveal a localized hotspot where local self-heating gives rise to self-sustaining filament of current flowing through the device in the c-axis direction.

The current voltage characteristic for the mesa at a bath temperature of 25 kelvin is displayed here. This contains hysteretic jumps associated with the nucleation of the hotspot at around 11 milliamps and with the jumping of the hotspot from the electrode end of the mesa to the opposite end, between 40 and 60 milliamps. The longitudinal cross-sections of the mesa surface temperature under different bias conditions are shown here.

The lines visible at the edges of the mesa and of the electrode are artifacts due to reflection off near-vertical sidewall surfaces. A 612 nanometer luminescent image in which the film was sublimated using europium TFC that contained millimeter-sized lumps is shown here. A sample whose europium TFC coating has crystallized into domains after 16 hours at 150 kelvin, resulting in uneven and noisy luminescent response, is displayed here.

Once mastered, this technique can be done in three to four hours if it is performed properly. While attempting this procedure, it's important to remember to avoid contaminating the sample surface in any way. Following this procedure, other methods like high-speed thermal imaging can be performed in order to study additional phenomena like hotspot oscillation and breathing modes.

After its development, this technique paved the way for researchers in the field of high temperature superconductive terahertz lasers to explore formation of current filaments in stacked pisco Josephson junctions. After watching this video, you should have a good understanding of how to prepare a microdevice so that its self-heating characteristics can be directly measured. Don't forget that EU(TFC)is a mildly harmful substance and precautions, such as avoiding inhalation of EU(TFC)powder and wearing gloves to avoid skin contact, should always be taken while performing this procedure.