Showing papers by "Daryoosh Vashaee published in 2001"

High cooling power density SiGe/Si micro-coolers

Abstract: SiGe/Si superlattice micro-coolers are investigated experimentally. They can be monolithically integrated with Si-based microelectronic devices to achieve localised cooling and temperature control. Cooling by as much as 4.2 K at 25/spl deg/C and 12 K at 200/spl deg/C was measured on 3 /spl mu/m thick. 60/spl times/60 /spl mu/m/sup 2/ devices. This corresponds to maximum cooling power densities approaching kW/cm/sup 2/.

Thermoreflectance imaging of superlattice micro refrigerators

Abstract: 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.

Integrated cooling for Si-based microelectronics

Abstract: 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.

Real Time Sub-Micron Thermal Imaging Using Thermoreflectance

Abstract: Thermal measurements on a sub-micron scale are non-trivial, but are important for the characterization of modern semiconductor and opto-electronic devices. In this paper we will discuss the application of the thermoreflectance method for real time sub-micron thermal imaging. By using light in the visible spectrum, the diffraction limit, and thus spatial resolution is improved over a traditional infrared camera based on blackbody emission. With active excitation of the sample and frequency domain filtering, thermal images with 100mK temperature resolution are obtained. Experiments performed on semiconductor micro-coolers and micro-heaters are presented.

Conservation of Lateral Momentum in Heterostructure Integrated Thermionic Coolers

Abstract: Thin film thermionic coolers use selective emission of hot electrons over a heterostructure barrier layer from emitter to collector resulting in evaporative cooling. In this paper a detailed theory of electron transport perpendicular to the multilayer superlattice structures is presented. Using Fermi-Dirac statistics, density-of-states for a finite quantum well and the quantum mechanical reflection coefficient, the currentvoltage characteristics and the cooling power density are calculated. The resulting equations are valid in a wide range of temperatures and electric fields. It is shown that conservation of lateral momentum plays an important role in the device characteristics. If the lateral momentum of the hot electrons is conserved in the thermionic emission process, only carriers with sufficiently large kinetic energy perpendicular to the barrier can pass over it and cool the emitter junction. However, if there is no conservation of lateral momentum, the number of electrons participating in thermionic emission will dramatically increase. The theoretical calculations are compared with the experimental dark current characteristics of quantum well infrared photodetectors and good agreement over a wide temperature range is obtained. Calculations for InGaAs/InGaAsP superlattice structures show that the effective thermoelectric power factor (electrical conductivity times the square of the effective Seebeck coefficient) can be improved comparing to that of bulk material. We will also discuss methods by which the conservation of lateral momentum in thermionic emission process can be altered such as by creating a controlled roughness at the interface of the superlattice barriers. The improvement in the effective power factor through thermionic emission can be combined with the other methods to reduce the phonon thermal conductivity in superlattices and thus obtain higher thermoelectric figure-of-merit ZT.

High-resolution noncontact thermal characterization of semiconductor devices

Abstract: Non-contact optical methods can be used for sub micron surface thermal characterization of active semiconductor devices. Point measurements were first made, and then real time thermal images were acquired with a specialized PIN- array detector. This method of thermal imaging can have spatial resolution better than the diffraction limit of an infrared camera and can work in a wide range of ambient temperatures. The experimentally obtained thermal resolution is on the order of 50 mK.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High Cooling Power Density of SiGe/Si Superlattice Microcoolers

Abstract: : Fabrication and characterization of SiGe/Si superlattice microcoolers integrated with thin film resistors are described. Superlattice structures were used to enhance the device performance by reducing the thermal conductivity, and by providing selective emission of hot carriers through thermionic emission. Thin film metal resistors were integrated on top of the cooler devices and they were used as heat load for cooling power density measurement. Various device sizes were characterized. Net cooling over 4.1 K and a cooling power density of 598 W/cm2 for 40 x 40 mm2 devices were measured at room temperature.

P-type InGaAsP coolers for integrated optic devices

Abstract: Single stage thin film coolers based on thermoelectric and thermionic cooling in p-type InGaAsP superlattice structures have been fabricated. Devices with different sizes and at various ambient temperatures have been characterized. Experimental results showed 0.5 degree centigrade cooling below the ambient temperature at 25C. This cooling over 1 4mu2m thick superlattice barrier corresponds to cooling power densities on the order of 200 W/cm2. The device cools by a factor of two better at higher temperatures (70C). This is due to the reduction of the superlattice thermal conductivity and the broadening of the electronic distribution function at higher temperatures. 150x150 micrometers 2 devices provide largest cooling at room temperature while the optimum device size shrinks as the temperature increases. Simulations results that take into account finite thermal resistance of the InP substrate, the effect of the contact resistance, heat generation in the wire-bonds and metallic pads on top of the device predict accurately the optimum cooling of these micro refrigerators. By eliminating the major parasitic sources of heating (Joule heating in the substrate, heat conduction through the side contact and reducing the contact resistance to 5x7-7 ohm-cm2) simulations show that, ultimately, one can achieve 15 degree(s)C cooling (10's of kW/cm2 cooling power) with single stage p-InGaAsP thin film coolers.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.