The invention relates to a method of manufacturing a thermoelectric device comprising a flexible foil on which two groups of series-connected, strip-shaped parts are provided, where the materials chosen for the two groups of parts have a different thermoelectric coefficient, and said groups of parts are patterned such that the connections between a part of one group and another part of the other group are alternately situated each time in one of two areas of the foil which are situated at a distance from each other, and where, following the provision of the strip-shaped parts on a substrate, the foil is provided on the strip-shaped parts, after which the substrate is removed. A device obtained by means of such a method constitutes an attractive alternative to a battery power supply. In the case of such a device only a temperature difference present in the vicinity of the device is used to make the device operate as a generator. Replacing or recharging of the device via an electric network, such as the electric mains, is not necessary.
Such a method is known from the publication entitled “Microfabrication of thermoelectric generators on flexible foil substrates as a power source for autonomous Microsystems” by Wenmin Qu et al., published in Journal of Micromechanics and Microengineering (abbreviated to J. Micromech. Microeng), 11 (2001) pp. 146-152. In said publication (see page 147, left-hand column) a description is given of how a 50 μm thick, flexible copper foil is provided, by means of electrodeposition, with groups of strip-shaped parts of antimony (Sb) and bismuth (Bi) in accordance with a pattern which is desirable for the use as a thermoelectric generator. Subsequently, the strip-shaped parts are spin coated with an epoxy film wherein the strip-shaped parts are embedded after curing of the epoxy film. Finally the flexible copper foil is removed by means of etching.
A drawback of the known device resides in that it is less suitable for the use of rigid substrates, such as a semiconductor substrate, instead of the copper foil. Such a semiconductor substrate is comparatively rigid as a result of its thickness, which is often in excess of 100 μm and frequently even above 500 μm. The use of a semiconductor substrate has the advantage that devices, of which one or more of the materials of the strip-shaped parts comprise a semiconductor material, can be formed more readily. Due to the comparatively large thickness of a semiconductor substrate, etching of said substrate is comparatively time-consuming, which is objectionable.
The object of the present invention therefore is to provide a method which is suitable for the use of a comparatively rigid and hence thick substrate, such as a semiconductor substrate, and which nevertheless enables a high speed.
To achieve this, in accordance with the invention, a method of the type mentioned in the opening paragraph is characterized in that for the substrate use is made of a rigid substrate, and prior to the removal of the rigid substrate, a rigid carrier plate is attached onto the foil, which rigid carrier plate is removed again from the flexible foil after the rigid substrate has been removed. The invention is first of all based on the recognition that by providing a rigid carrier plate on the foil before the substrate is removed, the device constantly remains rigid during the gradual removal of the substrate because of the presence of the rigid carrier plate. The invention is further based on the recognition that this enables the use of a grinding technique to remove the rigid substrate. As such a technique is much faster than a chemical etch technique, the substrate, or at least the greater part thereof, can be removed very rapidly. After the removal of the rigid substrate, the rigid carrier plate can be removed again, which results in a flexible (synthetic resin) foil which comprises the groups of strip-shaped parts and can serve as a thermoelectric generator. For the rigid carrier plate use can be made, for example, of a glass or quartz plate. The term rigid as used herein is to be taken to mean “unbendable or at least not easily bendable”. Said term stands in opposition to the term flexible. Using the same material, this means that a rigid body will be substantially thicker than a flexible body (foil) of this material.
In a preferred embodiment of a method, at least the greater part of the rigid substrate is therefore removed by means of grinding. Preferably, a remaining, only comparatively thin part of the rigid substrate is removed solely by means of a chemical etch technique. In spite of this, the method in accordance with the invention remains very fast. In this preferred embodiment use is preferably made of a semiconductor substrate.
Preferably, the rigid carrier plate is attached to the foil by means of an adhesive layer, and after the removal of the substrate, the foil is removed by pulling it loose from the adhesive layer. Such a method is particularly simple and hence attractive. In this method use is made of the recognition that a suitable choice of the adhesive layer enables, on the one hand, the adhesion between adhesive layer, carrier plate and synthetic resin foil to be sufficiently strong to carry out the method in accordance with the invention, while, on the other hand, the adhesion between the adhesive layer and the foil is sufficiently small to remove the foil by puling it loose from the carrier plate provided with the adhesive layer. An adhesive layer that proved to be particulary suitable for this purpose contains a 1,6 hexanedioldiacrylate. Using this adhesive layer in combination with a foil comprising a polyimide and a substrate of glass, good results were achieved.
As noted hereinabove, by using a semiconductor substrate it becomes easier to manufacture one or more of the groups of stripshaped parts from a semiconductor material. A particular advantage of a method in accordance with the invention is that the strip-shaped parts can also be readily made of a monocrystalline semiconductor material intead of a polycrystalline semiconductor material. The thermoelectric coefficient of such a monocrystalline semiconductor material is comparatively high, while, on the other hand, the electric resistance may be sufficiently low. The devices thus obtained form suitable thermoelectric generators and, in addition, their manufacture is compatible with the manufacture of the other components of a device wherein the thermoelectric generator will be incorporated as a power supply because these components often comprise semiconductor components such as ICs (=Integrated Circuits). Besides, integration with such components is comparatively easy.
In an attractive modification of the method in accordance with the invention, for the substrate use is made of a monocrystalline silicon substrate which is provided with an isolating layer on which a polycrystalline silicon layer is deposited. For the isolating layer use may be made of a layer of silicon oxide or silicon nitride which may be formed by thermal oxidation or CVD (=Chemical Vapor Deposition). The polycrystalline silicon layer is deposited, for example, by means of CVD. This modification has the advantage that both the method and the device obtained are comparatively inexpensive.
In another, also favorable modification, a buried isolating layer is formed in a monocrystalline silicon substrate by means of an implantation of oxygen ions. The rigid substrate is then formed by the part of the silicon substrate situated below the buried layer, while the superjacent (monocrystalline!) part is used in the formation of the groups of strip-shaped parts. The (last part of the) rigid substrate can be readily removed by means of etching because the buried isolating layer forms a suitable etch-stop layer for, for example, an etchant base on KOH. The buried layer may also (electrically) isolate the strip-shaped parts. In addition, after the removal of the rigid substrate, the isolating layer still present at this stage may be used, if desirable, to carry out technological adaptations to the device.
Preferably, a number of the two series-connected groups of strip-shaped parts are arranged in parallel between two strip-shaped conductors which are formed on the foil. In this manner, the power suppliable by the device manufactured can be substantially increased, leading to greater applicability of the device. The pattern of the strip-shaped parts of the individual groups as well as the pattern of the parallel arrangement thereof are preferably chosen such that, after folding or coiling the foil in a certain way, the connections of all strip-shaped parts are alternately situated in two spaced apart parts or areas of the (folded or coiled) device. The device may thus be compact and can be readily brought into contact with an existing temperature gradient.
The invention also comprises a thermoelectric device, in particular a thermoelectric generator, obtained by means of a method in accordance with the invention.
These and other aspects of the invention are apparent from and elucidated with reference to the embodiment(s) described hereinafter.
The figures are not drawn to scale and some dimensions, such as dimensions in the thickness direction, are exaggerated for clarity. In the different Figures, corresponding areas or parts are represented using the same reference numeral or hatching, whenever possible.
In this example, the device 10 comprises a plurality of zig-zag patterns 100, 101 only two of which are shown in the drawing, and which are arranged in parallel and connected to two strip-shaped conductors 22, 23 of aluminum which are situated near an outer edge of the foil 1. By virtue thereof, the capacitance of the thermoelectric generator 10 is substantially increased. The pattern 101 is mirrored with respect to the pattern 100 with a view to folding the device 10 of this example.
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The invention is not limited to the example described herein, and, within the scope of the invention, many variations and modifications are possible to those skilled in the art. For example, devices having a different geometry and/or different dimensions may be manufactured. Instead of a substrate of Si, use may alternatively be made of a substrate of glass, ceramic or a synthetic resin.
It is further noted that materials other than those mentioned in the examples may be used within the scope of the invention. Also other deposition techniques may be used for the materials mentioned or for other materials, such as sputtering. Instead of wet-chemical etching methods, use may alternatively be made of “dry” techniques, such as plasma etching, and conversely.
It is further noted that the device may comprise frther active and passive semiconductor elements or electronic components, such as diodes and/or transistors and resistors and/or capacitors, whether or not in the form of an integrated circuit. Within the context of the invention, a diode and a capacitor enabling temporary power storage to be realized in a simple manner would be suitable in particular. The manufacture is of course efficiently adapted thereto.
Number | Date | Country | Kind |
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03101497.0 | May 2003 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/50710 | 5/17/2004 | WO | 11/17/2005 |