METHOD FOR PRODUCING A SEMICONDUCTOR ASSEMBLY AND DIODE LASER

Abstract

The invention relates to a method for producing a semiconductor assembly, in particular connecting a semiconductor chip to a heat sink. A first metal layer consisting of Pb, Cd, In or Sn is made so thin that it is bonded by means of an opposing second metal layer consisting of another metal, for example gold, in a layer consisting of intermetallic phases. This can prevent migration of the soft metals. The brittle intermetallic layer is prevented from fracturing by a continuous pressing force.

Claims

1. A method for producing a semiconductor assembly, characterized by the following steps: a. providing at least one semiconductor chip having on a first side a first contact face and having on a second side, opposite the first side, a second contact face, b. providing a heat-conducting body having a first connection face, c. providing a cover having a second connection face, d. producing a first metallic layer comprising one or more of the soft metals lead, cadmium, indium, tin, e. producing a second metal layer, where either the first metallic layer is produced on the first connection face and the second metal layer is produced on the first contact face, or vice versa, f. disposing the semiconductor chip between the heat-conducting body and the cover, where the first contact face is facing the first connection face of the heat-conducting body and the second contact face is facing the second connection face of the cover, g. generating at least one force which has a component which presses the cover in the direction of the heat-conducting body, where under the action of the force the first metallic layer is pressed areally onto the second metal layer, h. establishing a mechanical connection of the cover to the heat-conducting body (10) that at least partly maintains the force, i. forming an intermetallic layer by solid-state diffusion of the first metallic layer into the second metal layer and/or vice versa, where the first metallic layer is bound predominantly in intermetallic mixed phases and/or oxidically.

2. The method as claimed in claim 1, further comprising partly oxidizing the first metallic layer before step f).

3. The method as claimed in claim 1, wherein the first metallic layer has a layer thickness of less than 2 μm but at least 200 nm.

4. The method as claimed in claim 1, wherein when the intermetallic layer is formed by solid-state diffusion of the first metallic layer into the second metal layer and/or vice versa, the first metallic layer is bound completely in intermetallic mixed phases and/or oxidically.

5. The method as claimed in claim 1, wherein the yield point of the intermetallic layer under shearing load is at least five times that of the first metallic layer .

6. The method as claimed in claim 1, wherein a fourth metal layer is disposed under the first metallic layer.

7. The method as claimed in claim 1, wherein the first metallic layer per unit area contains a lower amount of substance of the soft metals lead, cadmium, indium and tin than four times the amount of substance of gold contained in total per unit area in the second metal layer and the fourth metal layer.

8. The method as claimed in claim 1, wherein a diffusion barrier layer is disposed under the second metal layer and/or under the fourth metal layer and the metals of the first layer in step i) are bound partly to the metals of the diffusion barrier layer.

9. The method as claimed in claim 1, wherein the first metallic layer is produced in step d) from pure lead, pure indium or pure tin and the connection of the first contact face to the first connection face is free from plastically deformable pure metals Pb, Cd, In and Sn.

10. The method as claimed in claim 1, wherein the semiconductor chip takes the form of a laser bar.

11. The method as claimed in claim 1, further comprising producing a third metallic layer having a nubbed structure on the second connection face or on the second contact face.

12. A diode laser comprising at least one edge-emitting laser bar which comprises one or more emitters, having a first contact face, which takes the form of a p-type contact, and a second contact face , which takes the form of an n-type contact, a heat-conducting body having a first connection face, a cover having a second connection face, where the laser bar is disposed between the heat-conducting body and the cover where the cover is connected mechanically to the heat-conducting body, and the first contact face is areally connected thermally and electrically to the first connection face via an intermetallic layer, and the second contact face is connected electrically to the second connection face, where the intermetallic layer comprises gold (Au) and at least one further metal (ME) from the group lead, cadmium, indium and tin, and the intermetallic layer consists predominantly of one or more mixed phases AuME.sub.3, AuME.sub.2 and/or phases with higher gold fraction.

13. The diode laser as claimed in claim 12, wherein the cover is provided so as to contribute to the diversion of heat from the second contact face and/or the cover is connected thermally and mechanically to the heat-conducting body by means of an electric insulating joining agent.

14. The diode laser as claimed in claim 12, wherein the connection of the first contact face to the first connection face is free from plastically deformable pure metals Pb, Cd, In and Sn.

15. The use of a permanent clamping force for maintaining a securement of a semiconductor component on a heat-conducting body by means of an intermetallic layer, where the intermetallic layer comprises gold (Au) and at least one further metal (ME) from the group lead, cadmium, indium and tin, and the intermetallic layer consists predominantly of one or more mixed phases AuME.sub.3, AuME.sub.2 and/or phases with higher gold fraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0054] FIG. 1 shows a first exemplary embodiment before assembly.

[0055] FIG. 2 shows the first exemplary embodiment after assembly.

[0056] FIG. 3 shows for comparison a diode laser according to the prior art.

[0057] FIG. 4 shows a second exemplary embodiment before assembly.

[0058] FIG. 5 shows the third exemplary embodiment after assembly.

[0059] FIG. 6 shows a fourth exemplary embodiment.

[0060] It should be pointed out that the figures are not drawn to scale. Exaggerated representations, particularly in relation to the respective layer thicknesses, are necessary in order to illustrate the invention.

DETAILED DESCRIPTION

[0061] The invention is to be illustrated on the basis of a first exemplary embodiment in FIG. 1 and FIG. 2. FIG. 1 shows a first exemplary embodiment before the assembly of the diode laser 1. The figure represents a provided laser bar 3 with multiple emitters, which on a first side 6 has a first contact face 7, formed as a p-type contact (anode), and on a second side 8, opposite the first side, has a second contact face 9, which takes the form of an n-type contact (cathode). Likewise indicated is the position of the epitaxial layer 5 near to the first contact face of the laser bar, by means of a dotted line. The n-type contact contacts the substrate 4 of the laser bar.

[0062] The first contact face bears a coating comprising a thick-gold sublayer 20, covered by a second diffusion barrier layer 21 (represented as a thick line). An external second metal layer of gold 15 is disposed on the second diffusion barrier layer 21.

[0063] Additionally represented is a provided heat-conducting body 10 with a first connection face 11. The heat-conducting body 10 is provided with an Ni/Au coating which has an Ni sublayer as first diffusion barrier layer 19 and an external fourth metal layer of gold 18. The first connection face is coated with a first metallic layer 14 of lead, indium or tin. Additionally represented is a provided cover 12 with a second connection face 13. The second connection face carries an applied third metallic layer 17 of indium or tin. In an alternative embodiment this layer has a nubbed structure (not represented). The third metallic layer may therefore be provided together with the cover. The laser bar is disposed between the heat-conducting body 10 and the cover 12, with the first contact face 7 facing the first connection face 11 of the heat-conducting body and the second contact face 9 facing the second connection face 13 of the cover. The cover may have an Ni/Au coating (not represented), in the same way as the heat-conducting body.

[0064] Likewise represented is a curable joining agent 23 which may be applied, for example, in the form of an as yet uncured viscous epoxy resin adhesive to the corresponding face of the heat-conducting body or of the cover.

[0065] FIG. 2 shows the diode laser 1 during/after assembly. At least one force 22 is generated which has a component that presses the cover 12 in the direction of the heat-conducting body 10. Under the action of the force, the first contact face 7 is pressed areally onto the first connection face 11, and the first metallic layer 14 is pressed areally onto the second metal layer of gold 15.

[0066] A mechanical connection is established between the cover 12 and the heat-conducting body 10 by means of the electrically insulating joining agent 23. The cured joining agent at least partly maintains the force 22. The completed diode laser emits laser radiation 2 in direction z.

[0067] In the method, an intermetallic layer 16 is formed by solid-state diffusion of the first metallic layer 14 into the second metal layer of gold 15 and/or vice versa, with the first metallic layer 14 being bound into intermetallic mixed phases. An oxidic binding of some of the atoms of the first metallic layer may optionally be present additionally. The second diffusion barrier layer 21 prevents atoms of the first metallic layer penetrating into the thick-gold sublayer 20. As a result the latter layer remains intact. The intermetallic layer 16 is generated between the first diffusion barrier layer 19 and the second diffusion barrier layer 21.

[0068] FIG. 3 shows for comparison a diode laser according to the prior art. The diode laser is mounted with an indium layer 14 as first metallic layer. The indium layer is made with a thickness such that it is permanently retained as a layer of pure metal. Only in a zone near to the interface is it possible for intermetallic phases 16 to form, which according to the existing teaching are undesirable.

[0069] FIG. 4 shows a second exemplary embodiment before the assembly of the diode laser 1. Here the second metal layer of gold 15 takes the form of a thick-gold layer. A second diffusion barrier layer covering the thick-gold layer is absent in this embodiment.

[0070] FIG. 5 shows the third exemplary embodiment after assembly. The thick-gold layer mounted on the semiconductor chip is utilized here as a reservoir of gold for forming the intermetallic layer 16. The fourth metal layer of gold also contributes to the reservoir.

[0071] Error! Reference source not found. It represents the schematic drawing after an electron micrograph of a polished section of a completed semiconductor assembly, here a diode laser, in an enlarged detail representation. The laser bar has a substrate 4 and an epitaxial layer 5. The first contact face of the laser bar bears a coating comprising a metallic sublayer 20 (buffer layer), about 1 μm to 5 μm thick, which is covered by a second diffusion barrier 21 less than 0.5 μm thick. In the figure, the diffusion barrier layer 21 is visible between the metallic sublayer 20 and the intermetallic layer 16. The metallic sublayer may consist, for example, predominantly of gold, of copper or of tin. Because of the second diffusion barrier layer 21, the metallic sublayer 20 is retained even after the formation of the intermetallic layer 16 in step i) of the method. Before the establishment of the mechanical connection, a second metal layer, preferably of gold, is applied (not visible in the figure) on the second diffusion barrier layer 21. This second metal layer is absorbed completely in the intermetallic layer 16 when the intermetallic layer 16 is formed in step i).

[0072] The heat-conducting body 10 is provided with a metallic coating. This coating comprises a first diffusion barrier layer 19, around 0.5 μm to 10 μm thick, which may also be referred to as a buffer layer. Before the establishment of the mechanical connection, a fourth metal layer, preferably of gold, is applied (not visible in the figure) on the first diffusion barrier layer 19. This fourth metal layer is absorbed completely in the intermetallic layer 16 when the intermetallic layer 16 is formed in step i).

[0073] Before the establishment of the mechanical connection, the first metallic layer with a thickness of around 0.5 μm to 2 μm (not visible in the figure) is applied to the second metal layer and/or to the fourth metal layer. This first metallic layer is absorbed completely in the intermetallic layer 16 when the intermetallic layer 16 is formed in step i).

[0074] The intermetallic layer has a thickness of around 0.5 μm to 2.5 μm. The intermetallic layer 16 comprises mixed phases of the metals of the first, second and fourth metallic layers and optionally oxides of the material of the first metallic layer. The intermetallic layer is free from pure metallic phase of the material of the first metallic layer. It preferably comprises predominantly the respectively least gold-rich intermetallic phase of the metal of the first metallic layer with gold. In one modification of the first exemplary embodiment, the intermetallic layer comprises intermetallic phases of up to 50% gold fraction, measured as the amount of substance, which can be expressed in moles.

[0075] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.