Thermoelectric microdevice fabrication process and evaluation at the Jet Propulsion Laboratory (JPL)

Advances in the microelectronics industry have made it possible to fabricate a multitude of microdevices, such as microprocessors, microsensors, microcontrollers, and microinstruments. These electronic microdevices have significantly reduced power requirements but at the same time require more attention in terms of integrated thermal management and power management and distribution. Micro thermoelectric converters are considered a promising technology approach for meeting some of these new requirements. Thermoelectric microdevices can convert rejected or waste heat into usable electric power, at moderate (200-500K) temperatures and often with small temperature differentials. They can also be easily integrated and provide effective cooling for devices specific in optoelectronics, such as mid-IR lasers, dense-wavelength-division-multiplexing (DWDM) components and charge-coupled-device (CCD) detectors. In the Materials and Device Technology Group at JPL, we have developed a unique fabrication method for a thermoelectric microdevice that utilizes standard integrated circuit techniques in combination with electrochemical deposition of compound semiconductors (Bi2Te3/Bi2.,Sb,Te3). Our fabrication process is innovative in the sense that we are able to electrochemically micro mold different thermoelectric elements, with the flexibility of adjusting geometry, materials composition or batch scalability. Successive layers of photoresist were patterned and electrochemically filled with compound semiconductor materials or metal interconnects (Au or Ni). A thermoelectric microdevice was built on either glass or an oxidized silicon substrate containing 63 couples (63 n-legs/63 p-legs) at approximately 20 microns in structure height and with a device area close to 1700 pm x 1700 pm. In cooling mode, we evaluated device performance using an IR camera and differential thermal imaging software. We were able to detect a maximum cooling effect of about 2K. In power generation mode, a 75 watt light source was illuminated directly above the device while the current generated was measured. A detailed step-by-step overview of the fabrication process will be given, as well as specifics in testing setups, results and future directions. Introduction As a spacecraft travels further away from the sun, for a defined solar panel surface area, solar flux decreases accordingly (inverse square law) and loses effective power. Spacecraft that travel beyond the orbit of Mars or that require longer lasting power systems require a source of electric power other than solar energy. For missions such as Cassini (launched 1997 to study the Saturn system), radioisotope thermoelectric generators (RTGs) are used for power [I]. Thermoelectric devices take advantage of the Seebeck effect for power generation and can also utilize the Peltier effect for active cooling. Thermoelectric coolers have various applications for microprocessors, medical analyzers, portable picnic coolers and many more. In the optoelectronics industry, thermal management is a significant factor in optimizing device performance. For instance, due to excessive heat generated, there are compromises in laser wavelength stability and increased noise levels in detectors In the past few years, advancements in the microelectronics industry have made it possible to miniaturize components, devices, instruments and even spacecraft. With the miniaturization of electronic devices, there has also been a concomitant focus on developing miniaturized power conversion and thermal management systems. Miniaturizing thermoelectric converters will enable milliwatt power at several volts for MEMS devices and other microinstruments [ 1,3]. Additionally, thermoelectric micro coolers offer effective and practical options for precise thermal management in compact optoelectronic devices. A few applications include spot cooling for mid-IR lasers and CCD detectors [4]. An encouraging approach for meeting various power requirements, while simultaneously being able to offer adequate thermal control, are micro thermoelectric converters. These thermoelectric microdevices can operate at moderate (200-500K) temperatures and with small temperature differentials. For the temperature range of 200-500K, alloys based on n-type Bi2Te3 and p-type Bi2-,Sb,Te3, are the best materials suitable for numerous optoelectronic and micro spacecraft applications [ 11. According to scaling laws [ 1,3], the attractive idea behind a thermoelectric microdevice is to increase specific power (W/cm2) by reducing the size of the thermoelectric elements, while maintaining the same aspect ratio of elements in a larger thermoelectric device. Equally important, miniaturization increases maximum cooling and improves cooling densities [5]. A thermoelectric module generally consists of several nand p-type leg elements (couples) connected in series electrically and in parallel thermally. A microdevice will enable potentially thousands of these couples to be connected together in a very small area, leading to open circuit voltages of several volts at even modest temperature gradients [1,3]. At the Jet Propulsion Laboratory (JPL), we have fabricated thermoelectric microdevices using a combination of integrated circuit processing techniques and electrochemical deposition P I . of compound semiconductors (Bi2Te3/ Bi2-xSbxTe3) [6-81. It was possible to construct micro power generators/coolers with leg elements approximately 20 microns tall and approximately 60 microns in diameter (varies somewhat due to conical shape of legs). A thermoelectric microdevice was built on either glass or an oxidized silicon substrate(Si/Si02) containing 63 couples (63 n-legsi63 p-legs) and with a device area close to 1700 pm x 1700 pm. Microdevices were tested and evaluated for power generation and effective cooling performance. Electrochemistry and Materials Properties Electrochemical deposition (ECD) offers an inexpensive and scalable process [9]. Materials can be varied in composition with deposition rates up to several tens of microns per hour. N-type Bi2Te3 and p-type Bi2.,SbxTe3 compounds were deposited at room temperature at constant potential (EG&G PAR 273A) in a standard three electrode configuration. The working electrode was either a metallized glass or metallized oxidized silicon substrate. The cell had a Pt counter electrode and a saturated calomel electrode (SCE) reference. Regions for deposition were defined using a patterned photoresist mask. Thermoelectric leg elements were deposited from solutions containing dissolved elemental metals with a concentration on the order of M in aqueous 1 M HN03 (pH=O). Solutions containing Sb use chelating agents such as citrate, tartrate or ethylene diamine tetraacetate (EDTA) to allow higher concentrations of the less soluble element at pH 0 [1,3]. Leg elements have been electrochemically formed, however with different thermoelectric properties from that of bulk materials. Due to difficulties in obtaining material properties from individual leg elements, we instead measured ECD films (1 cm2, -10 pm thick). As deposited Bi2Te3 films exhibited heavily doped n-type behavior with dense growth. EDX analysis confirmed near Bi2Te3 stoichiometry. Bi2Te3 material properties are as follows: Seebeck = -30 to -60 pV/K, p 1 d c m (in plane), n 1 x lo2' cm-3 and p~ 1525 cm2V-'s-'. ECD p-Bi2.,Sb,Te3 properties have not been fully characterized because of inconsistencies in reproducibility. Material compositions were found to be very sensitive to initial electrolyte concentrations and deposition voltages. At Sb-rich or near Sb2Te3 stoichiometry, desirable dense morphologies were attained but at the sacrifice of Seebeck values. Upon increasing Bi content, both film and leg elements resulted in unfavorable dendritic/columnar growth. Leg morphology is critical to device fabrication and performance. It is extremely difficult to fabricate complete devices if the tops of the electrodeposited legs are too rough (mentioned later). Also, these low density or porous ECD materials are characterized by higher resistivities and reduced mechanical integrity. Leg elements with low mechanical strength are susceptible to stress induced horizontal cracking, which dramatically increases resistivity or ultimately leads to device failure. Nonetheless, even with incomplete materials characterization, preliminary observations indicate that annealing ECD materials at 250°C have promising effects Commercially available gold and nickel bath solutions were used for ECD of bottom base dogbone contacts and top interconnects.