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 ...

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