JP2006190916A - Method for forming junction electrode in thermionic element and porous thermionic element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a method for forming a junction electrode capable of forming the electrode having a small contact resistance and a high bond strength to a thermoelectric material. <P>SOLUTION: In the method for forming the junction electrode in a porous thermionic element, a plurality of holes 11 are bored to an inorganic material body (a glass body) 10, and a Ti film 21 as a buffer layer is formed on the end face of the inorganic material body (the glass body) 10 closing arrays 13 by an ion plating method to the porous thermionic element 10 with the arrays 13 in which the thermoelectric materials (bismuth) 12 are arranged to the holes 11. In the method, a solder film 22 is formed by a PVD method without an opening to an atmospheric air, and an electrode film is formed as the junction electrode 20. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thermoelectric power generation technology using the Seebeck effect in which an electromotive force is generated when a heterogeneous junction conductor is joined and a temperature difference is given between two junctions, or a current is passed through the heterojunction conductor and the junction The present invention relates to thermoelectric elements used in thermoelectric cooling techniques utilizing the Peltier effect that generates a temperature difference between the parts, particularly in the formation method of bonding electrodes in thermoelectric elements, in the pores formed in inorganic material bodies such as glass The present invention relates to a structure of a bonding electrode in a thermoelectric element (porous thermoelectric element) formed by arranging a thermoelectric material, and a method for forming the bonding electrode.

Thermoelectric power generation technology is attracting attention as one of energy-saving technologies because of the usefulness of energy, but thermoelectric elements that convert thermal energy into electricity (using the Seebeck effect) are based on the heat conduction of the elements themselves. Since the conversion efficiency is 5 to 10% and the efficiency is not effective, research on improving the efficiency is actively conducted.
A thermoelectric element is configured by sandwiching a thermoelectric material between a pair of junction electrodes, and the thermoelectric material portion generally has a structure formed of a bulk or a thin film.
In recent years, a thermoelectric element (porous thermoelectric element) having an array structure in which a porous array is formed in an inorganic material and a thermoelectric material is arranged in the porous array has been proposed as a structure of the thermoelectric element.

  In the above-described thermoelectric element electrode formation, it is required to ensure low resistance and adhesion. Particularly in the case of a porous thermoelectric element, the shape of the porous array is very large when forming a bonding electrode to be bonded to the thermoelectric element. Therefore, it is difficult to join the electrodes, and the resistance at the time of joining becomes as large as several hundred times or more than the assumed value, which is a cause of the efficiency hindrance.

  In the case of forming a bonding electrode using, for example, silver paste in a porous thermoelectric element, a thermoelectric material (bismuth) 32 is arranged on a porous array 31 formed on a glass body 30 as shown in FIG. After polishing the entire end face, a silver paste 33 as a bonding electrode is applied. In this case, since the strength of the thermoelectric material 32 is lower than that of the glass body 30 in the cross-sectional structure at the end face, a dent is generated at the time of polishing. Therefore, the silver particles contained in the silver paste 33 are transferred to the thermoelectric material 32. It cannot enter so as to be in contact with each other, and a conduction failure occurs, which makes it difficult to join the electrodes.

  In this case, even when a low-resistance silver paste is used, the particle size is very large compared to the diameter of the porous array 31 of several tens of μm and does not come into good contact with both ends of the porous array. The resistivity of the thermoelectric material portion is 100 μΩm or more, which is a very large value compared to the resistivity of 1.3 μΩm of the bismuth single crystal.

  In order to eliminate dimples during polishing, glass fluoride was melted with hydrogen fluoride (HF) to expose the thermoelectric material in the porous array, and an electrode was attached with a low resistance silver paste, but the resistivity was 100 μΩm. That was a very large value. This was because glass re-deposited on the surface of the porous array to form an insulating film.

  As shown in FIG. 3B, when the bonding electrode 35 is formed by vapor deposition of silver or gold, the resistivity of the thermoelectric element is 4.3 μΩm (3.3 times that of the single crystal). Further, since the periphery of the thermoelectric material 32 is the glass body 30, the adhesion between the metal vapor deposition film and the glass is poor, and peeling due to insufficient adhesion of the film occurs.

  When solder (tin, lead, etc.) is directly attached to a porous array filled with a thermoelectric material, it is easy to form an intermetallic compound with the thermoelectric material and hinders the movement of carriers. There is.

  Each method for forming a bonding electrode in the porous thermoelectric element described above cannot be used to form an electrode with low contact resistance and high adhesion strength, so that it can be used for thermoelectric generation technology at a practical level. A thermoelectric element having a possible energy conversion efficiency could not be obtained.

  On the other hand, the thermoelectric element module 40 using the Peltier effect used in the thermoelectric cooling technology has a pair of PN elements composed of an N-type semiconductor 43 and a P-type semiconductor 44 between electrodes 41 and 42 as shown in FIG. A plurality of the plates 45 are arranged so that current flows in a meandering manner with respect to each pair of PN elements, so that the plate 45 in close contact with each electrode 41 on one side generates heat, and the plate 46 in close contact with each electrode 42 on the other side is cooled. Is. In the P-type semiconductor element, holes are dominant in charge and heat transport, and in the N-type semiconductor element, electrons are dominant in charge and heat transport. Therefore, current flows in the arrow direction as shown in FIG. When it flows, heat propagates from the top to the bottom of the figure, the one side plate 45 generates heat, and the other side plate 46 is cooled.

  A porous thermoelectric element having the above-described structure is also useful for thermoelectric elements used in thermoelectric cooling technology. In this case, if the resistivity can be kept low, the figure of merit (Seebeck coefficient, resistivity, thermal The value calculated using the conductivity) can be increased, and for example, it can be expected to be applied in the field of thermoelectric cooling in small devices such as those for computer CPU cooling.

  The present invention has been made in view of the above circumstances, and a bonding electrode forming method capable of forming an electrode having low contact resistance and high adhesion strength with respect to a thermoelectric material, and contact resistance in a porous thermoelectric element. An object of the present invention is to provide a structure of a porous thermoelectric element having an electrode having a small and high adhesion strength.

  In order to achieve the above object, a method for forming a bonding electrode in a thermoelectric element of the present invention (Claim 1) is to form a Ti film as a buffer layer on a thermoelectric element material by an ion plating method without opening to the atmosphere. Subsequently, a solder film is formed by a PVD method to form an electrode film to be a bonding electrode.

The method for forming a bonding electrode in a thermoelectric element according to claim 2 is a method in which a plurality of holes 11 are drilled in an inorganic material body (glass body) 10 and a thermoelectric material (bismuth) 12 is disposed in the hole 11. Is a method for forming a bonding electrode in a porous thermoelectric element including the following procedure.
A Ti film 21 as a buffer layer is formed by ion plating on the end face of the inorganic material body (glass body) 10 that closes the array portion 13, and a solder film 22 is continuously formed by PVD without opening to the atmosphere. Thus, an electrode film to be a bonding electrode is formed.

Claim 3 is a porous thermoelectric element comprising an array portion 13 in which a plurality of holes 11 are drilled in an inorganic material body (glass body) 10 and a thermoelectric material (bismuth) 12 is arranged in the hole 11. It is characterized by including the following configuration.
An electrode film having a Ti film 21 as a buffer layer in contact with the array section 13 and a solder film 22 formed on the Ti film 21 on the end face of the inorganic material body (glass body) 10 that closes the array section 13. A joining electrode 20 is formed.

  According to the present invention, a Ti film having excellent adhesion to a thermoelectric material (or thermoelectric material and glass body) is formed on the thermoelectric material by an ion plating method, and the atmosphere is formed so that the solder can easily adhere to the Ti film. Since the solder film is continuously formed by the PVD method without opening, it is possible to prevent contact failure and lower the contact resistance, suppress the decrease in electrical conductivity, and form a bonding electrode with high adhesion strength. it can.

  As a result, since it is possible to obtain an element having a high performance index while suppressing a decrease in electrical conductivity in the porous thermoelectric element, a porous thermoelectric element that can be put into practical use by a thermoelectric power generation technique and a thermoelectric cooling technique is obtained. It becomes possible.

A porous thermoelectric element as an example of an embodiment of the present invention will be described with reference to FIG.
The porous thermoelectric element 1 covers an inorganic material body 10 in which a plurality of holes 11 are perforated, an array part 13 formed by filling the hole 11 with a thermoelectric material 12, and both end faces of the array part 13. A joining electrode 20 made of an electrode film is provided.

The inorganic material body 10 is composed of a glass body (or an alumina body). The inorganic material body (glass body) 10 is formed with a plurality of holes 11 penetrating two opposing surfaces in an array.
Each hole 11 is filled with bismuth (resistivity: 1.3 μΩm), which is an N-type semiconductor, as a thermoelectric material 12 to form an array portion 13.
Instead of bismuth, a bismuth tellurium (Bi—Te), lead tellurium, Bi—Sb, MnSr, Mg ₂ Si, or CoSb material may be used as a thermoelectric material.

Bonding electrodes 20 are respectively formed on both end faces of the inorganic material body (glass body) 10 facing the array portion 13. The bonding electrode 20 is composed of an electrode film having a Ti (titanium) film 21 as a buffer layer for improving adhesion to the thermoelectric material 12 and a solder film 22 formed on the Ti film 21. . The Ti film 21 interposed between the thermoelectric material 12 and the solder film 22 is a Ti film 21 formed by an ion plating method because it needs to be a material that can ensure adhesion between the thermoelectric material 12 and the glass body. . Further, the Ti film 21 is a material that can also ensure adhesion with the solder film 22.
The solder film 22 is continuously formed by the PVD method without opening to the atmosphere in order to improve the adhesion with the Ti film 21.
Alternatively, a two-layer structure with a Ti film in which a Cu film or a Ni film is laminated on the Ti film 21 may be used.

Since the Ti film 21 described above is formed by the ion plating method, the contact resistance can be reduced by increasing the adhesion strength and ensuring the adhesion as compared with the case of forming by the vacuum deposition. In other words, while vacuum deposition forms a film only when a substance evaporates in a vacuum, in the ion plating method, the evaporated particles are ionized and radicalized by RF (radio frequency) power (positively charged). This is because, by negatively biasing the substrate, the particles are accelerated using the Coulomb force to adhere to the substrate.
Further, since RF ion plating can form a film at a relatively low temperature without increasing the substrate temperature, it is suitable for a low melting point material such as bismuth used as the thermoelectric material 12.

According to the above structure, since the Ti (titanium) film 21 having excellent adhesion to the thermoelectric material 12 and the glass body 10 is interposed, it is possible to improve adhesion and suppress a decrease in electrical conductivity. it can.
Moreover, since the formation of intermetallic compounds can also be prevented, it is possible to improve the adhesiveness of the solder.

Next, a method for forming a bonding electrode in the above-described porous thermoelectric element will be described with reference to FIG.
An inorganic material body (glass body) 10 (FIG. 2A) in which a plurality of through-holes 11 are perforated is placed in bismuth melted at high temperature (270 ° C. or higher) in a high vacuum, and argon gas is used. The thermoelectric material 12 is arranged in the hole 11 by press-fitting bismuth and solidified to form the array part 13. The length L of the glass body 10 is 0.1 to 5 mm, and the diameter φ of the hole 11 is 10 nm to 1000 μm. Alternatively, bismuth powder is put into each hole 11 of the inorganic material body (glass body) 10, bismuth is melted by hot pressing, and bismuth is disposed in each hole 11.

  Next, both end surfaces of the inorganic material body (glass body) 10 on which bismuth is disposed are polished for the purpose of surface treatment. At this time, since the strength of bismuth (thermoelectric material) is lower than that of glass, a dent is generated in the thermoelectric material 12 corresponding to each hole 11 during polishing (FIG. 2B).

Subsequently, the bonding electrode 20 is formed so as to cover the array portion 13 on one end face. In forming the bonding electrode 20, (1) Ar bombardment is performed as a surface cleaning operation, (2) Ti (titanium) film is formed as a buffer layer by RF ion plating, and (3) Solder film is formed by PVD. Each formation takes place.
Instead of the solder film in the above (3), a solder (a solder used for soldering at the time of wiring connection) and a Cu film or a Ni film having good wettability may be laminated.

  Hereinafter, details of specific conditions and the like in the steps (1) to (3) will be described.

Ar bombardment process A small amount of argon gas is introduced into a vacuum vessel, a high frequency (13.56 MHz) is generated from an electrode installed in the vacuum deposition vessel, and a substrate is cleaned by generating a discharge (plasma).
The Ar bombardment process was performed under the following conditions.
Final vacuum: 3.5 × 10 ⁻⁴ Pa or less Partial pressure after introducing argon: 3.5 × 10 ⁻² Pa
Substrate temperature: room temperature
RF (high frequency) power: 100W
Substrate bias voltage: 100V
Processing time: 10 minutes

-RF ion plating process of Ti (titanium) A high frequency (13.56 MHz) is generated from an electrode installed in a vacuum deposition vessel, and an Ar (argon) gas is introduced as an assist gas to deposit a Ti film 21 while causing discharge. (FIG. 2 (c)).
The RF ion plating process of Ti (titanium) was performed under the following conditions.
The final vacuum and the partial pressure after introducing argon are the same as in the Ar bombardment process.
Deposition rate: 10 Å / sec Film thickness: 1000 Å Substrate distance: about 500 mm
Substrate temperature: room temperature

-Forming process of solder film The solder film is formed as lead-free solder by a film formed by sequentially performing vacuum deposition of tin (Sn) and vacuum deposition of silver (Ag).

-Sn (tin) vacuum deposition process The introduction of Ar (argon) gas is stopped, the degree of vacuum is raised again, and the Sn film is formed with the high frequency power and the substrate bias voltage turned off.
The vacuum deposition process of Sn (tin) was performed under the following conditions.
Degree of vacuum: 3.5 × 10 ⁻⁴ Pa or less Deposition rate: 200 Å / sec Film thickness: 25000 Å (2.5 μm)
Substrate temperature: room temperature

Ag (silver) vacuum deposition step After the Sn film is formed, the Ag film is formed to form the solder film 22 (FIG. 2D).
The vacuum deposition process of Ag (silver) was performed under the following conditions.
Degree of vacuum: 3.5 × 10 ⁻⁴ Pa or less Deposition rate: 20 Å / sec Film thickness: 650 Å Substrate temperature: room temperature

  The above-described Ar bombardment process, Ti (titanium) RF ion plating process, Sn (tin) vacuum deposition process, and Ag (silver) vacuum deposition process are provided with an interval of 10 minutes to cool the substrate. To be done.

  After forming the bonding electrode 20 on the surface opposite to the surface formed in the same process, the resistivity of the porous thermoelectric element bonded with solder or low-resistance silver paste between the two electrodes was measured to find 1.4 μΩm. Thus, a value close to the resistivity of 1.3 μΩm of the bismuth single crystal could be secured. This value is very small compared to the resistivity of 100 μΩm or more in each method described in the conventional example, and it becomes possible to produce an electrode in a porous thermoelectric element, which has been impossible in the past, and can be put to practical use. A highly efficient porous thermoelectric element can be obtained.

  In the above example, the method of forming the bonding electrode in the porous thermoelectric element has been described. However, the method of forming the bonding electrode when the thermoelectric material (thermoelectric element material) part is formed of a bulk or a thin film has low resistance and good adhesion. It is effective for forming an electrode.

  For example, when a thermoelectric material is formed in a bulk that is a cubic block made of bismuth, if a junction electrode is formed on the bulk surface and the back surface by the above-described process, the bulk (thermoelectric material) Thus, it is possible to obtain a thermoelectric element including a bonding electrode having a low contact resistance and a high adhesion strength.

  In addition, a thermoelectric material layer is formed on a substrate by a thin film process, and bonding electrodes are formed on both ends of the surface of the thermoelectric material layer, thereby providing a bonding electrode having low contact resistance and high adhesion strength to the thin film thermoelectric material. A thermoelectric element can be obtained.

It is a section explanatory view of a porous thermoelectric device as an example of an embodiment concerning the present invention. (A)-(d) is process explanatory drawing which shows the formation method of the joining electrode in a porous body thermoelectric element. (A) And (b) is sectional explanatory drawing of the thermoelectric element for demonstrating the prior art example of the formation method of the joining electrode in a porous body thermoelectric element. It is structure explanatory drawing of the module which uses a thermoelectric element.

Explanation of symbols

1 Porous Thermoelectric Element 10 Glass Body (Inorganic Material Body)
11 Hole 12 Thermoelectric Material 13 Array 20 Bonding Electrode 21 Ti Film (Buffer Layer)
22 Solder film

Claims (3)

A Ti film as a buffer layer is formed on the thermoelectric element material by an ion plating method, and a solder film is continuously formed by the PVD method without opening to the atmosphere to form an electrode film to be a bonding electrode A method for forming a bonding electrode in a thermoelectric element. For a porous thermoelectric element comprising a plurality of holes in an inorganic material body and an array part in which a thermoelectric material is arranged in the hole,
An electrode film that forms a Ti film as a buffer layer by an ion plating method on the end face of the inorganic material body that closes the array portion, and subsequently forms a solder film by the PVD method without opening to the atmosphere. Forming a bonding electrode in a thermoelectric element. In a porous thermoelectric element comprising an array part in which a plurality of holes are formed in an inorganic material body, and a thermoelectric material is disposed in the hole,
A porous film characterized in that an electrode film having a Ti film as a buffer layer in contact with the array section and a solder film formed on the Ti film is formed on the end face of the inorganic material body that closes the array section. Body thermoelectric element.

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