Name: high-resolution thermal micro-imaging using europium chelate luminescent coatings
Uploaded: 2017-04-16T13:00:00
Description: Watch this Scientific Journal Video about High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings at JoVE.com

Work at Argonne National Laboratory was funded by the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357, which also funds Argonne's Center for Nanoscale Materials (CNM) where the patterning of the BSCCO mesa was performed. We thank R. Divan and L. Ocola for their help with sample fabrication.

1. Preparation of Sample for Coating
NOTE: If possible, remove all organic contamination from the surface of the sample to be thermally imaged. Any such contamination may react with the deposited EuTFC film and alter its luminescent response, causing position-dependent artifacts in the resulting thermal images. This is of particular importance with samples with Au surface electrodes, which tend to attract organic contamination from the atmosphere. Remove any particles or dust sitting on the samp...

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As demonstrated by our results, the technique described in this article yields high-resolution thermal images of microdevices, with good sensitivity and using only simple optical microscopy equipment. The advantages of this technique relative to alternative methods (which will be discussed below) are strongest at approximately 250 K and below, meaning that its most important applications are for studying the self-heating of devices which are designed to operate at cryogenic bath temperatures. These include superconductin...

Many electronic devices undergo strong self-heating when electrically biased to their normal operating conditions. This is usually due to a combination of low thermal conductivity (such as in semiconductors) and high power dissipation density. Furthermore, in devices with a semiconducting-like electrical resistivity (i.e. with &#8706;&#961;/&#8706;T &#60; 0) it has long been known that there exists the possibility of localized thermal runaway under certain biasing conditions 1,2, in which the bias current flows not uniformly through the device, but rather in narrow filaments which are associated with highly localized self-heating, typically on a scale of microns.
Understanding such self-heating physics may in some cases be essential for optimizing the design of a particular device, meaning that techniques for imaging temperature on micron scales are very useful. There has been a recent resurgence of interest in such techniques from two areas of technology development. The first of these is for imaging quench processes in high-temperature superconducting tapes in which thermal micro-imaging allows quench nucleation sites to be identified and studied 3,4. The second application is for understanding self-heating in stacked intrinsic Josephson junction terahertz sources, which are fabricated from Bi2Sr2CaCu2O8. These have the combination of low thermal conductivity and semiconductor-like electrical conductivity along the relevant direction of current flow (i.e. their crystalline c-axis) described above. Not only do they experimentally show complex inhomogeneous self-heating behavior 5,6,7,8,9,10,11 it has been theoretically predicted that this may be beneficial for THz power emission 12,13.
A number of techniques exist for imaging the temperature of a sample at microscopic length scales. The thermoluminescent technique described here was originally employed for semiconducting devices near room temperature 14,15,16 but has more recently been applied at cryogenic bath temperatures to the superconducting tapes and THz sources described above 3,4,10,11. Improvements in the resolution and signal-to-noise performance of CCD cameras have enabled considerable performance improvements in this technique over the last few decades. The Eu-coordination complex europium thenoyltrifluoroacetonate (EuTFC) has an optical luminescence which is strongly temperature dependent. The organic ligands in this complex effectively absorb UV light in a broad band around 345 nm. The energy is transferred radiation-less via intra-molecular excitations to the Eu3+ ion, which returns the complex to its ground state through the emission of a luminescence photon at 612 nm. The strong temperature dependence arises from the energy transfer process 17 making for a sensitive thermal probe of an object coated with this material. When the coating is excited with a near-ultraviolet source &#8212; such as an Hg short-arc lamp &#8212; regions with lower luminescence intensity correspond to higher local temperature. The resulting images are limited in spatial resolution by the resolution of the microscope optics and the wavelength of the luminescence (in practice, to around 1 micron). Depending on the signal-to-noise ratio required, time resolution is limited only by the shutter speed of the camera, and more fundamentally by the decay time of the luminescence (no more than 500 &#956;s) 15. These characteristics make the technique a very fast probe of device temperature, which yields direct temperature measurements, using comparatively simple and economical equipment.
Variations of this technique published in the past by other groups have employed small concentrations of Eu-chelates dissolved in polymer films and spin-coated on to the sample surface 3,4. This results in a coating which is highly uniform locally, but which has significant thickness variations at steps in the sample topography &#8212; such as commonly occur in microdevices &#8212; resulting in strong spatial variations in the luminescent response which can give artifacts in the images. The technique variation which we describe here employs thermal sublimation in vacuum. Not only does this avoid the macroscopic film thickness variation problem, but the higher EuTFC concentration achieved per unit area significantly improves the sensitivity and reduces the image acquisition time. A related technique employs a coating of SiC granules on the surface instead of the EuTFC 7,8,9. SiC offers temperature sensitivity comparable to the EuTFC coatings described here, but the size of the granules limits the smoothness and resolution of the resulting images.
Several other techniques exist, which offer different combinations of advantages and disadvantages. Direct infrared imaging of blackbody radiation from the sample is simple and has spatial resolution of a few microns, but is only effective when the sample is significantly above room temperature. Scanning probe thermal microscopy techniques (such as scanning thermocouple microscopy or Kelvin probe microscopy) offer excellent sensitivity and spatial resolution, but have slow image acquisition times, necessarily limited by the scanning speed of the tip, as well as requiring highly complex equipment. Scanning laser or scanning electron beam thermal microscopy measures the voltage perturbation when a modulated beam is rastered across the surface of a current-biased device 6,7,18. This offers excellent sensitivity, and is somewhat faster than scanning probe techniques, but once again requires highly complex equipment, and also gives an indirect, qualitative map of the sample temperature.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Europium thenoyltrifluoroacetonate powder</td>
			<td>Sigma-Aldrich</td>
			<td>176494-1G</td>
			<td>Also known as Europium tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate]</td>
		</tr>
		<tr>
			<td>Mercury short-arc lamp with flexible light guide</td>
			<td>Lumen Dynamics</td>
			<td>X-Cite Exacte</td>
			<td>Light source includes internal iris and photosensor for output intensity feedback.</td>
		</tr>
		<tr>
			<td>Peltier-cooled CCD camera</td>
			<td>Princeton Instruments</td>
			<td>PIXIS 1024</td>
			<td>1024 x 1024 pixels, 16-bit resolution</td>
		</tr>
		<tr>
			<td>610 nm band-pass filter</td>
			<td>Edmund Optics</td>
			<td>65-164</td>
			<td>Passband has CWL 610 nm, FWHM 10 nm</td>
		</tr>
		<tr>
			<td>500 nm short-pass filter</td>
			<td>Edmund Optics</td>
			<td>84-706</td>
			<td>OD4 in stopband</td>
		</tr>
		<tr>
			<td>Helium flow cryostat with optical window</td>
			<td>Oxford Instruments</td>
			<td>MicrostatHe2</td>
			<td></td>
		</tr>
		<tr>
			<td>high vacuum grease</td>
			<td>Dow Corning</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Digital Current source</td>
			<td>Keithley</td>
			<td>Model 2400</td>
			<td>Computer-controllable current &#38; voltage source</td>
		</tr>
		<tr>
			<td>Digital Voltmeter</td>
			<td>Hewlett-Packard&#160;</td>
			<td>Model 34420A</td>
			<td>Digital Nanovoltmeter now available as Agilent Model 34420A</td>
		</tr>
	</tbody>
</table>

high-resolution thermal micro-imaging using europium chelate luminescent coatings

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient optical micro-imaging technique which can be used to map and quantify such behavior. Europium thenoyltrifluoroacetonate (EuTFC) has a 612 nm luminescence line whose activation efficiency drops strongly with increasing temperature, due to T-dependent interactions between the Eu3+ ion and the organic chelating compound. This material may be readily coated on to a sample surface by thermal sublimation in vacuum. When the coating is excited with ultraviolet light (337 nm) an optical micro-image of the 612 nm luminescent response can be converted directly into a map of the sample surface temperature. This technique offers spatial resolution limited only by the microscope optics (about 1 micron) and time resolution limited by the speed of the camera employed. It offers the additional advantages of only requiring comparatively simple and non-specialized equipment, and giving a quantitative probe of sample temperature.

An example of a typical measurement configuration for conducting this experiment at cryogenic bath temperatures is shown in Figure 1a, while a typical curve of 612 nm luminescent response intensity versus temperature is plotted in Figure 1b.
Figure 2 shows an example of typical thermal images of self heating in a Bi2Sr2CaCu2O8 THz sour...

Watch this Scientific Journal Video about High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings at JoVE.com

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

Europium thenoyltrifluoroacetonate (EuTFC) has an optical luminescence line at 612 nm, whose activation efficiency decreases strongly with temperature. If a sample coated with a thin film of this material is micro-imaged, the 612 nm luminescent response intensity may be converted into a direct map of sample surface temperature.

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient ...

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.

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