We use diffraction-limited infrared microscopy to capture temperature fields in a variety of structures relevant for electronic packaging, energy conversion systems (including automotive waste heat recovery), and novel nanostructured materials. The basic tool for these measurements is an Infrascope (Quantum Focus Instruments) IR microscope using a 256×256 InSb focal plane array for detecting wavelengths ranging from 3 to 5 μm. The IR microscope has a temperature sensitivity up to 0.1 K and spatial resolution of up to 2 μm. Prior to measuring spatially varying temperature maps, the emissivity for each point within the sample must be calibrated by heating the sample to a uniform temperature and calibrating the radiance. The measurements taking place with the instrument fall into essentially two categories: Device Characterization and Materials Characterization.
Device Characterization – Hot Spot Detection: Infrared microscopy is a useful technique for obtaining the spatially-resolved temperature profile of fully operational microprocessors. An accurate power map of the chip can be calculated from these in situ temperature measurements, which in turn can allow for a priori predictions of hot spot location and intensity. Thus, infrared microscopy can be used as a design tool that can greatly aid in circuit design and optimization of cooling solutions. The Nanoscale Heat Transfer Laboratory, in collaboration with local semiconductor companies, has built up an infrastructure for simultaneously cooling and optically probing microprocessors. In addition, lab has developed a detailed calibration methodology to account for the attenuation in the microprocessor silicon die and IR-transparent heat sink in order to maximize temperature map fidelity and power map accuracy.
Materials Characterization – Comparative Infrared Thermal Microscopy: The thermal conductivity of thin film samples (~25 um to ~2 mm thick) is typically measured using a comparative method based on ASTM standard E1225, using the infrared (IR) microscope instead of thermocouples. A one-dimensional heat flux is generated across a stack consisting of the sample film sandwiched between two reference layers. A heater is attached to one reference layer, and the other reference layer is affixed to a heat sink. Typically quartz (k ~ 1.4 W m-1 K-1), Pyroceram 9606 (Corning Glass, k ~ 4 W m-1 K-1), or PDMS (k ~ 0.2 W m-1 K-1) are used as the reference layers depending on the expected thermal conductivity and thickness of the sample to ensure that the temperature gradients in the sample and in the reference layers are comparable.
Fourier’s law describes heat transport for each layer in the sample. The samples are prepared such that all layers in the stack have the same cross-section and the heat flux is constant across the sample and two reference layers. Thus, the ratio of the thermal conductivity of the sample and the reference material can be determined from a ratio of the temperature gradient in the sample and the reference layer. Additionally, temperature jumps at interfaces result from thermal boundary resistances and the intrinisic film thermal conductivity and boundary resistances can be directly determined from the two-dimensional temperature maps. The use of two reference layers allows quantification of the heat flux through the sample and esitmation of the relative importance of any other heat transfer mechanisms, such as convective and radiative loss, compared to the conduction through the sample stack to be determined.
Breaking the Diffraction Barrier (well, not actually) using a Solid Immersion Lens (SIL): To achieve resolution below about 3 micrometers, a limit governed by diffraction, it is necessary to restrict or modify the photons emitted by the sample surface. We explored a modification to infrared imaging using a solid immersion lens made from silicon, which is essentially transparent to infrared radiation. The lens was fabricated using the silicon overlayer in a silicon-on-insulator (SOI) substrate by means of reactive ion etching, which transferred a lens shaped resist volume into single crystal silicon. The silicon SIL improves spatial resolution by allowing photons to interact with a higher refractive index as they depart from the sample surface, thereby reducing the effective spatial limit due to diffraction. Our SIL was used to demonstrate resolution below 1 micrometer for laser radiation near 10 microns, and we also demonstrated thermal scans from heated microstructures.