JPH1041443A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the thermal contact resistance of a Cu post electrode with a Mo disk. SOLUTION: This device has a flat pressure-welding structure, and is constituted so that sandwiching a silicon element between Cu post electrodes on both its surfaces via respective Mo disks, a pressure is applied across the Cu post electrodes to form electric junctions. In this case, both on a surface 61 of the Cu post electrode and on a surface portion 21 of the Mo disk, such thin films 41 , 42 as Ag and Al ones are disposed respectively by sputtering, depositing, plating and the like. Even when irregularities are present both on the surface 61 of Cu and on the surface 21 of Mo, there is generated no clearance both between the surface 61 and the thin film 41 and between the surface 21 and the thin film 42 . Also, since on a pressure-welding surface (b), the pressure welding of such soft thin film surface as Al and Ag ones to each other is performed, there is generated no clearance on the surface b, either. Therefore, the thermal contact resistance of the Cu post electrode with the Mo disk is made small.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-power semiconductor device having a flat pressure contact structure.

[0002]

2. Description of the Related Art FIG. 3 shows a flat type pressure contact structure of a power semiconductor device. In the figure, 1 is a silicon element, 6 is a copper cathode post electrode, 7 is a copper anode post electrode, and 8 is a gate electrode portion.

[0003] Copper post electrodes 6 and 7 and ceramic case 1
1 constitutes a case.

In general, molybdenum (Mo) disks 2 and 3 serving as an external lead electrode and a high heat conduction / stress relieving material are provided on both surfaces of a silicon (Si) element 1 on which impurity diffusion, electrode pattern formation, etc. have been performed. Pressure is applied from both sides of the copper post electrodes 6 and 7 to make them adhere to each other. At this time, silver foils 4 and 5 are interposed in order to reduce stress and the like due to a difference in thermal expansion between copper and molybdenum.

FIG. 4 shows a structure of the semiconductor device in an actual operation state. The inside of the semiconductor package has a structure in which the Si element 1 is sandwiched between copper post electrodes 6 and 7 and Mo disks 2 and 3. Heat sinks (cooling fins) 17 and 18 are attached to both sides of the semiconductor package.

The heat sinks 17 and 18 have a role of releasing heat generated from the Si element 1 to the outside (surroundings) in an actual operation state. The heat sinks are made of conductive material such as aluminum or copper. It is used not only for releasing heat but also as an electrode.

[0007]

FIG. 5 shows, as a heat dissipation equivalent circuit, a state in which heat generated by current flowing through the element is transmitted to the ambient temperature. Here, Tj: element junction temperature, Tc: case (Cu post electrode)
Temperature, Tf: Fin temperature, Ta: Ambient temperature Rth (Si), Rth (Mo), Rth (Cu), R
th (Fin): thermal resistance of the component itself Rth (Si-Mo), Rth (Mo-Cu), Rth
(Cu-Fin): thermal contact resistance between components Rth (Fin-a): thermal resistance when heat escapes from the fin to the surrounding air Thermal resistance Rth... The cooling rate is worse as the value is larger, and if not cooled excessively, the temperature of the element rises above the rating, leading to destruction.

[0008] Tj (junction temperature) in the semiconductor package
The thermal resistance Rth (j−c) up to Tc (case temperature) is determined by the semiconductor package material and structure. Next, the thermal resistance Rth (c-fi) from Tc to Tfin
n) is the thermal resistance generated between the semiconductor package and the cooling fin, and this value becomes extremely large depending on the mounting method, so that the heat may be prevented from escaping.

As semiconductor devices have become larger in diameter and larger in capacity, heat loss generated from the devices increases. Therefore, reducing the thermal resistance of the semiconductor case is a very serious problem.

FIG. 6 shows an example of the result of calculating the ratio of the thermal resistance of the current case to each component. In this example, the silver foil interposed between the Mo disk and the copper post electrode among the constituent materials was omitted, but the contact resistance generated between the Mo disk copper post and the electrode accounted for the largest proportion, followed by the Mo disk Own electrode and Mo
The ratio of the contact thermal resistance with the disk is large. Since the thickness and the like of the Si element itself are determined by the element performance, this value is immovable.

[0011]

[Table 1]

Table 1 shows the results of calculating the effect on the thermal resistance due to the presence or absence of the silver foil interposed between the Mo disk and the copper post electrode. In the table, model [1] has no silver foil. The contact thermal resistance between the Mo disk and the copper post electrode is affected by the hardness and roughness of the copper material surface, but is not affected by the copper surface in the model [2] including the silver foil, and is approximately 700%
It has been obtained that the thermal resistance can be reduced to a certain degree.

However, as is used in the current process, it is not possible to fill the micro gaps existing on the copper and molybdenum surfaces as indicated by the arrow g in FIG. 7 simply by sandwiching the silver foil. Actually, the finishing accuracy of the copper surface is (▽▽
▽), with irregularities of about 6 μm. The actual copper post electrode covers the surface mainly using nickel plating (~ 5μ).
m), and the surface irregularities are thought to be small, but the nickel material itself is a hard material and has poor adhesion to silver foil.

Although the surface of the molybdenum disk is mirror-finished, it is considered that the surface has irregularities of about 1 μm or less.
Further, the surface may be damaged during the production, and it is considered that a considerable gap is generated. Silver foil is a soft material by nature and can fill some gaps by deformation, but does not adhere completely.

[0015] The increase in contact thermal resistance leads to poor heat transfer due to the presence of the micro gaps described above, resulting in an increase in thermal resistance.

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem, and has as its object to provide a copper post electrode and a large heat resistance with low contact thermal resistance between the Mo disk, the Si element, and the cooling fin. An object is to provide a power semiconductor device.

[0017]

SUMMARY OF THE INVENTION The present invention is directed to a semiconductor device in which a copper post electrode is sandwiched between both surfaces of a silicon element and a copper post electrode is applied therebetween to electrically connect the silicon element to the copper post electrode and the molybdenum disk. Each of the surfaces to be coated is coated with a soft metal thin film having high thermal conductivity.

Alternatively, in a semiconductor device in which both surfaces of a silicon element are sandwiched between copper post electrodes and electrical connection is made by applying pressure, the pressure contact surface of the copper post electrode with the silicon element is sequentially thermally expanded with a plurality of low thermal expansion metal materials. Cladding is performed so that the rate becomes low.

Further, the surfaces of the copper post electrodes and the cooling fins provided on the outside thereof and facing each other are preferably coated with a thin film of a soft metal having high thermal conductivity.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 In the semiconductor device shown in FIGS. 3 and 4, as shown in FIG. 1, opposing surfaces 2 ₁ , 3 ₁ and 6 _{1 of} molybdenum (Mo) disks 2 and 3 and copper post electrodes 6 and 7 are arranged. by coating a soft metal with high thermal conductivity, such as respective silver or aluminum to ₇₁ to form a thin film 4 _1, 4 _2, 5 _1, 5 _2.

As a method for forming the thin film, techniques such as sputtering, vapor deposition, and plating are used. By using these methods, the gaps between the Mo disks 2 and 3 and the thin films 4 ₁ and 4 ₂ are completely filled and adhered as indicated by the arrow a.

If the thickness of the thin film is specified to be equal to or greater than the surface roughness of the molybdenum disk, the surface is completely covered with a soft material such as silver or aluminum. It adheres perfectly and the heat transfer to the copper post electrode is no longer affected by the surface roughness of the Mo disk. The surface finish is defined, for example, as shown in Table 2.

[0023]

[Table 2]

Conventionally, the surface of a copper post electrode is coated with a hard material such as nickel. By forming a softer soft thin film, the surface of the copper post is no longer affected by the copper post surface roughness. The thin film is completely brought into contact with the Mo disk on which the thin film is formed, so that the Mo disk is completely in close contact.

As described above, by forming a thin film of silver or the like on the surface of the Mo disk or copper post electrode, the contact surface with the copper post contacts with the thin film interposed therebetween. Due to the flatness of the formed thin film, the thin film is completely adhered to the contact surface, and the thermal conductivity is improved.

In addition, the contact surface with the silicon (Si) element has conventionally been a problem of contact between the Mo disk and the aluminum electrode, and it is impossible to bring the Mo disk into complete contact with the surface roughness and hardness of the Mo disk. there were. However, by interposing a soft thin film, the contact between the thin film and the aluminum electrode is changed, so that the thin film is deformed and can be brought into close contact.

As a synergistic effect, since the surface is coated with a thin film, the surface roughness of the Mo disk can be reduced to some extent. In the past, the contact surface had to be mirror-finished (more than ▽▽▽) to improve the contact. However, the surface of the molybdenum disk was completely covered with a thin film of silver, aluminum, etc. Buried in Therefore, the surface finish can be reduced to (▽▽▽) finish or (▽▽) finish, and significant cost reduction can be achieved.

In the copper post electrodes 6 and 7, the same effect can be obtained even when the above-mentioned coating thin film is adopted on the contact surfaces with the cooling fins 17 and 18, and the contact thermal resistance generated between the case and the cooling fins is reduced. Can be done. As a soft thin film material, in addition to silver and aluminum, gold, tin, an aluminum-gold alloy, and an aluminum alloy (Al-10.2Zn-0.
9Mg-0.4Zr) and a zinc alloy (Zn-22Al) are suitable.

Embodiment 2 In the power semiconductor having the flat pressure contact structure shown in FIGS. 3 and 4, pressure contact is performed in the order of copper post electrode 6 (7) -silver foil 4 (5) -molybdenum electrode 2 (3). The part being performed is clad (composite) as shown in FIG. That is, a structure in which a plurality of low thermal expansion metals are bonded to the surface of the copper post electrode 6 (7) so that the coefficient of thermal expansion of the portion pressed against the silicon element is close to the coefficient of thermal expansion of silicon. .

Ideally, the coefficient of thermal expansion changes gently as shown by the solid line (a) in FIG. 2 (b). However, as shown in FIG. 2 (a), from the copper surface 6 ₁ (7 ₁ ) By bonding a plurality of low thermal expansion metals (1 to N) as the cladding material CM so as to have a low thermal expansion coefficient in order, as shown by a dotted line (b) in FIG. Can be changed.

The method of cladding is to completely eliminate the contact thermal resistance by using rolling, explosion bonding, etc., and by bringing them into close contact (the joining surfaces are joined so that the two main materials are entangled). .

When one kind of intermediate material is used between Cu and Mo / W, the materials (1) to (3) in Table 3 are suitable. Also,
When the number of intermediate materials between Cu and Mo / W is two or three or more, the copper content of the molybdenum / copper alloy is changed as shown in (1) of Table 4 or as shown in (2) of Table 4 Combine 3 ingredients.

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

Since the present invention is configured as described above, the following effects can be obtained.

(1) By coating the surface of the molybdenum disk and the copper post electrode with a soft material, it is possible to completely fill the micro gaps generated at the time of pressure welding and to eliminate the contact resistance.

(2) By coating a soft thin film also on the press contact surface between the copper post electrode and the cooling fin, the contact thermal resistance between the case and the fin can be reduced.

(3) By cladding the copper post electrode, a molybdenum disk as a heat buffer plate interposed between the copper post electrode and the silicon element can be eliminated. Further, since the gap between each of the low thermal expansion materials and copper is completely adhered, there is no contact thermal resistance.

[Brief description of the drawings]

FIG. 1 is an explanatory cross-sectional view of a copper and molybdenum pressure contact portion according to a first embodiment;

FIGS. 2A and 2B are explanatory diagrams of a cross-sectional structure of a clad copper post surface according to the second embodiment, and FIG. 2B is a diagram showing a thermal expansion coefficient of a clad material.

FIG. 3 is an exploded cross-sectional view of a power semiconductor flat pressure welding structure.

FIG. 4 is a cross-sectional view showing a pressure contact state of the power semiconductor.

FIG. 5 is a heat dissipation equivalent circuit diagram of the power semiconductor.

FIG. 6 is a graph showing a ratio of a thermal resistance in a case structural material.

FIG. 7 is an explanatory view of a cross-sectional state of a copper-molybdenum bonding surface according to a conventional example.

[Explanation of symbols]

1 ... Silicon (Si) element (wafer) 1 ₁ ... aluminum cathode electrode 1 ₂ ... oxide layer 1 _3: anode electrodes 2 ... surface 4,5 ... silver foil 4 ₁ of molybdenum (Mo) disk 2 ₁ ... molybdenum disc, 4 _2, 5 _1, 5 ₂ ... coated silver thin 6,7 ... copper post electrode 6 ₁ of aluminum or the like, 7 ₁ ... the surface of Dopo electrode 8 ... gate electrode portions 8 ₁ ... copper gate ring 8 ₂ ... Gate lead wire 8 ₃ PPS resin cup 11 Ceramic case 12 15 Weld ring 13 PPS resin ring 14 Rubber 17 18 Heat sink (cooling fin) CM Clad material.

Claims (3)

[Claims] 1. A semiconductor device in which both surfaces of a silicon element are electrically sandwiched by a copper post electrode via a molybdenum disk and pressure is applied to the silicon element, wherein the copper post electrode and the surface of the molybdenum disk facing each other are soft and high heat conductive. A semiconductor device characterized by being coated with a metal thin film. 2. A semiconductor device in which both surfaces of a silicon element are sandwiched between copper post electrodes and pressure is applied to electrically connect the silicon element, wherein a pressure contact surface of the copper post electrode with the silicon element is sequentially thermally expanded with a plurality of low thermal expansion metal materials. A semiconductor device characterized by being clad so as to have a low efficiency. 3. The cooling fin according to claim 1, wherein a cooling fin is provided on the outer side of the copper post electrode by pressing.
A semiconductor device characterized in that opposite surfaces of a copper post electrode and a cooling fin are coated with a soft metal thin film having high thermal conductivity.