During recent years, microelectronics helped to develop complex and varied technologies. It appears that many of these technologies can be applied successfully to realize Seebeck micro generators: photolithography and deposition methods allow to elaborate thin thermoelectric structures at the micro-scale level. Our goal is to scavenge energy by developing a miniature power source for operating electronic components. First Bi and Sb micro-devices on silicon glass substrate have been manufactured with an area of 1cm2 including more than one hundred junctions. Each step of process fabrication has been optimized: photolithography, deposition process, anneals conditions and metallic connections. Different device structures have been realized with different micro-line dimensions. Each devices performance will be reviewed and discussed in function of their design structure.
Since the last decade, there is a growing interest of wireless sensor nodes with goals of monitoring human environment. Because advances in low power VLSI design and CMOS fabrication have dramatically decreased power requirements of sensors, it is now possible to consider self-powered system in sensor node [1]. In the same time, the interest in producing microelectromechanical systems (MEMS) opens new opportunity in the field of micro power generation. Micro thermoelectric converters are a promising technology due to the high reliability, quiet operation and are usually environmentally friendly. The efficiency of a thermoelectric device is generally limited to its associated Carnot Cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of the materials [2] used in fabrication of the thermoelectric device. Recent developments in micro-thermoelectric devices using thin film deposition technology [3] have shown that energy scavenged from human environment (low thermal gradient) matches energy sensor's power need. The used thermoelectric materials (TE) are here bismuth and antimony. Both Bi and Sb are semimetals, that is, there is an energy overlap between the valence and conduction bands. Near room temperature, the thermoelectric power (or Seebeck coefficient) of both Bi and Sb are enough small: typical values are about -70µV.K -1 for Bi and 40µV.K -1 for Sb [4]. Nevertheless, the acquired experience with these usual thermoelectric materials for our devices will have been very enriching and helpful for our presently device improvement with more competitive thermoelectric materials. Thus, in this paper we review a wafer technology approach to manufacture a thermoelectric device. Design and technological process steps will be identified in order to propose a strategy to manufacture thermoelectric converters (TEC).
We use a four inches glass substrate. 42 chips and 6 test areas are distributed on it. Fig. 1. shows this Bi-Sb prototype.
Dimension of chips are 1x1cm 2 , bismuth and antimony lines widths are 20, 30 or 40µm, spaced by 20µm. This geometry is obtained by photolithography with a serial of 3 masks. G. Savelli, M. Plissonnier, J. Bablet, C. Salvi, J.M. Fournier Energy conversion using new thermoelectric generator
These geometries allow us to obtain 250 lines (125 in bismuth and 125 in antimony in alternation), i.e 125 junctions for the 20x20 chips, 208 lines and 104 junctions for the 30x20 chips and 166 lines and 83 junctions for the 40x30 chips. Moreover Bi and Sb lines are electrically connected in series by using Ti and Au metallic junctions. Fig. 2. shows these lines more in details and an enlargement of the metallic connections.
Bismuth and antimony lines, and titanium and gold connections are deposited by sputtering PVD (Physical Vapor Deposition) system. This deposition choice is explained in the next paragraph. Sputtering process use six inches targets for a best thickness uniformity. Each deposition is realized in Ar atmosphere at 1.2 Pa. In addition, an rf generator provides power supplies from 0 to 300W and operates at a frequency of 13.56MHz. Also it is necessary to add adhesion layers for bismuth and antimony deposition. These sublayers permit Bi and Sb to adhere to the substrate during photolithography operations (resin addition, etching…). A flash Ti adhesion layer (1nm) is used for Bi lines and flash Ti+Au adhesion layers (1nm+1nm) are used for Sb lines. We will check in part 3. that these added layers have no influence on both thermoelectric results and crystallographic structures.
Annealing is the crucial point for this device. Indeed in thermoelectrics, it is important to obtain the lowest electrical resistivity. To succeed in these lowest values high layers quality are necessary. But, for example, bismuth is well known to deposite in the form of grains [5][6] which increases resistivity. Objective is thus to improve layers quality, and so to reduce grains size. For that, in first, sputtering deposition is used because, in PVD, it allows to obtain grain sizes smaller than with evaporation deposition [5]. Furthermore different annealing conditions have been studied to reduce grain sizes. Two kinds of annealings will be compared in section 3: annealing by furnace and by laser. Annealings by furnace are realized at 260°C for bismuth (T m [Bi-Ti]= T m [Bi]=271°C) and 355°C for antimony (T m [Sb-Au]=360°C) with Ar atmosphere for 8 hours (T m are obtained according to Binary Alloy Phase Diagrams).
Annealing by laser are realized using a Xe-Cl excimer laser with 200ns of impulsion length and a wavelength of 308nm.
To analyse our devices performances, we use several characterization tools: Scanning Electron Microscopy (SEM) to check geometry dimensions and photolithography quality, X-rays diffraction to control crystallographic structure of bismuth and antimony, a four probe method is used to measure electrical resistivity, and a thermoelectr
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