James Christofferson

Microscale and Nanoscale Thermal Characterization Techniques

In this paper the authors review various microscale and nanoscale thermal characterization techniques that could be applied to active and passive devices. Solid-state micro refrigerators on a chip can provide a uniform and localized temperature profile and they are used as a test vehicle in order to compare the resolution limits of various microscale techniques. After a brief introduction to conventional micro thermocouples and thermistor sensors, various contact and contactless techniques will be reviewed. Infrared microscopy is based on thermal emission and it is a convenient technique that could be used with features tens of microns in size. Resolution limits due to low emissivity and transparency of various materials and issues related to background radiation will be discussed. Liquid crystals that change color due to phase transition have been widely used for hot spot identification in integrated circuit chips. The main problems are related to calibration and aging of the material. Micro Raman is an optical method that can be used to measure absolute temperature. Micron spatial resolution with several degrees temperature resolution has been achieved. Thermoreflectance technique is based on the change of the sample reflection coefficient as a function of temperature. This small change in 10-4-10-5 range per degree is typically detected using lock-in technique when the temperature of the device is cycled. Use of visible and near IR wavelength allows both top surface and through the substrate measurement. Both single point measurements using a scanning laser and imaging with CCD or specialized lock-in cameras have been demonstrated. For ultrafast thermal decay measurement, pump-probe technique using nanosecond or femtosecond lasers have been demonstrated. This is typically used to measure thin film thermal diffusivity and thermal interface resistance. The spatial resolution of various optical techniques can be improved with the use of tapered fibers and near field scanning microscopy. While sub diffraction limit structures have been detected, strong attenuation of the signal reduces the temperature resolution significantly. Scanning thermal microscopy which is based on nanoscale thermocouples at the tip of atomic force microscope has had success in ultra high spatial resolution thermal mapping. Issues related to thermal resistance between the tip and the sample and parasitic heat transfer paths will be discussed

Thermoreflectance based thermal microscope

Thermal images of active semiconductor devices are acquired and processed in real time using visible light thermoreflectance imaging with 34mK sensitivity. By using a 16×16 alternating current coupled photodiode array with synchronous frequency domain filtering a dynamic range of 123dB is achieved for 1s averaging. Thus with a stable and higher power light source, fundamentally the camera can reach 6mK sensitivity over a submicron area. The number of pixels in the image is increased to 160×160 by multiple frame image enhancement and submicron spatial resolution is achieved. The photodiode array system has a maximum 40kHz frame rate and generates a synchronous trigger for recovery of the phase signal. Amplitude and phase images of the thermoreflectance signal for 50×50 micron square active SiGe based microcoolers are presented.

On-chip high speed localized cooling using superlattice microrefrigerators

In this paper, we addressed heating problems in integrated circuits (ICs) and proposed a thin-film thermionic cooling solution using Si/SiGe superlattice microrefrigerators. We compared our technology with the current most common solution, thermoelectric coolers, by strengthening the advantages of its compatible fabrication process as ICs for easy integration, small footprint in the order of ~ 100times100 mum2, high cooling power density, 600W/cm2 and fast transient response less than 40 mus. The thermoreflectance imaging also demonstrated its localized cooling. All these features combined together to make these microrefrigerators a very promising application for on-chip temperature control, removing hot spots inside IC

Characterization of heat transfer along a silicon nanowire using thermoreflectance technique

We studied heat transfer along a silicon nanowire suspended between two thin-film heaters using a thermoreflectance imaging technique. The thermoreflectance imaging system achieved submicrometer spatial resolution and 0.1/spl deg/C temperature resolution using visible light. The temperature difference across the nanowire was measured, and then its thermal resistance was calculated. Knowing the dimension of the nanowire (115 nm in width and 3.9 /spl mu/m in length), we calculated the thermal conductivity of the sample, which is 46 W/mK. Thermal conductivity decreases with decreasing wire size. For a 115-nm-wide silicon nanowire, the thermal conductivity is only one-third of the bulk value. In addition, the transient response of the thin-film heaters was also examined using three-dimensional thermal models by the ANSYS program. The simulated thermal map matches well with the experimental thermoreflectance results.

Statistical detection and imaging of objects hidden in turbid media using ballistic photons.

We exploit recent advances in active high-resolution imaging through scattering media with ballistic photons. We derive the fundamental limits on the accuracy of the estimated parameters of a mathematical model that describes such an imaging scenario and compare the performance of ballistic and conventional imaging systems. This model is later used to derive optimal single-pixel statistical tests for detecting objects hidden in turbid media. To improve the detection rate of the aforementioned single-pixel detectors, we develop a multiscale algorithm based on the generalized likelihood ratio test framework. Moreover, considering the effect of diffraction, we derive a lower bound on the achievable spatial resolution of the proposed imaging systems. Furthermore, we present the first experimental ballistic scanner that directly takes advantage of novel adaptive sampling and reconstruction techniques.

MODELING AND OPTIMIZATION OF SINGLE-ELEMENT BULK SiGe THIN-FILM COOLERS

Abstract Modeling and optimization of bulk SiGe thin-film coolers are described. Thin-film coolers can provide large cooling power densities compared to commercial thermoelectrics. Thin-film SiGe coolers have been demonstrated with maximum cooling of 4°C at room temperature and with cooling power density exceeding 500 W/cm2. Important parameters in the design of such coolers are investigated theoretically and are compared with experimental data. Thermoelectric cooling, joule heating, and heat conduction are included in the model as well as non-ideal effects such as contact resistance, geometrical effects, and three-dimensional thermal and electrical spreading resistance of the substrate. Simulations exhibit good agreement with experimental results for bulk Si and SiGe thin-film coolers. It turned out that in many spot cooling applications using two n- and p-elements electrically in series and thermally in parallel does not give significant improvement over single leg elements. This is in contrast to conventional thermoelectric modules and is due to the aspect ratio and special geometry of thin film coolers. With optimization of SiGe thin-film cooler, simulations predict it can provide over 16°C with cooling power density of over 2000 W/cm2.

Integrated cooling for Si-based microelectronics

Thin film thermoelectric coolers are advantageous for their high cooling power density and their potential integrated applications. Si/sub 1-x/Ge/sub x/ is a good thermoelectric material at high temperatures and superlattice structures can further enhance the device performance. Si/sub 1-x/Ge/sub x/ and Si/sub 1-x/Ge/sub x//Si superlattice structures were grown on Si substrates using molecule beam epitaxy. Si/sub 1-x/Ge/sub x/ and Si/sub 1-x/Ge/sub x//Si superlattice thin film microcoolers with film thickness of the order of several microns were fabricated using integrated circuit processing technology. Micro thermocouples and integrated thermistor sensors were used to characterize these coolers. Maximum cooling power density on the order of hundreds of watts per square centimeter was measured at room temperature. It is possible to monolithically integrate these coolers with Si-based microelectronic devices for localized cooling and temperature stabilization.

High power operation of electroabsorption modulators

In this paper we characterize temperature distribution in an electroabsorption modulator. A self-consistent opto-electro-thermal model is developed that accurately predicts thermal runaway in unoptimized EAM structures. Optimized structures with improved thermal design achieve high power operation with a record damage free power dissipation level in excess of 300 mW. In conclusion, using a thermoreflectance imaging technique, surface temperature measurement of active electroabsorption modulators is acquired.

Thermoreflectance imaging of superlattice micro refrigerators

High resolution thermal images of semiconductor micro refrigerators are presented. Using the thermoreflectance method and a high dynamic range PIN array camera, thermal images with 50 mK temperature resolution and high spatial resolution are presented. This general method can be applied to any integrated circuit, and can be used as a tool for identifying fabrication failures. With further optimization of the experimental set-up, we expect to obtain thermal images with sub-micron spatial resolution.

PICOSECOND TRANSIENT THERMAL IMAGING USING A CCD BASED THERMOREFLECTANCE SYSTEM

A new Charge Coupled Device (CCD) based, full-field thermoreflectance thermal imaging technique is demonstrated with 800 picoseconds temporal resolution. Transient thermal images of pulsed heating in single interconnect vias of 350nm and 550nm in diameter are shown. The use of pulsed laser diodes and dedicated synchronization circuits can significantly lower the cost and the image acquisition time compared to the scanning pump-probe laser systems. Also the same set up can be used to study transient thermal phenomena in a wide dynamic range from sub nanoseconds to seconds.© 2010 ASME