WO2018015457A1

WO2018015457A1 - Lead frame

Info

Publication number: WO2018015457A1
Application number: PCT/EP2017/068275
Authority: WO
Inventors: Kah Mun CHOOI; Lay Sin KHOO; Ismail ITHNAIN
Original assignee: Osram Opto Semiconductors GmbH
Current assignee: Ams Osram International GmbH
Priority date: 2016-07-20
Filing date: 2017-07-19
Publication date: 2018-01-25
Anticipated expiration: 2019-01-20

Abstract

The Scope of the invention is a metal lead frame for an opto-electronic device comprising three metal layers, wherein a first metal layer comprises copper, wherein a second metal layer adjacent to the first metal layer comprises nickel and wherein a third metal layer adjacent to the second metal layer comprises silver, characterized in that the second metal layer comprises a polycrystalline structure of nickel with a distribution of grain sizes around a mean grain size, and in that the mean grain size is above 100 nanometres.

Description

LEAD FRAME

DESCRIPTION The invention refers to a metal lead frame for an optoelec^¬ tronic device and an optoelectronic device with such a lead frame. Metal lead frames are usually made of copper, as cop^¬ per is a suitable material for metal lead frames due to its thermal and electrical conductivity.

This application claims priority to and the benefit of German patent application DE 10 2016 133 343.4, which is hereby incorporated herein by reference in its entirety. In optoelectronic devices, such copper lead frames lead to a brown appearance of at least parts of the optoelectronic de^¬ vice. However, a white or bright appearance of the optoelec^¬ tronic device would be desirable. To enhance the appearance of the optoelectronic device, a silver layer can be placed on top of the metal lead frame, thus leading to a bright appear^¬ ance of the metal lead frame and thus of the optoelectronic device. A nickel layer may be applied in between the copper and the silver. Such a metal lead frame is disclosed in DE 10 2012 207 593 Al .

An assignment of the invention is to provide an improved met^¬ al lead frame and an optoelectronic device comprising such a lead frame. The solution of this assignments is disclosed in the inde^¬ pendent claims of this invention. Preferred embodiments are disclosed in the dependent claims.

A metal lead frame for an optoelectronic semiconductor device is proposed that comprises three metal layers. A first metal layer comprises copper. A second metal layer adjacent to the first metal layer comprises nickel. A third metal layer adja^¬ cent to the second layer comprises silver. The second metal layer comprises a polycrystalline structure of nickel with a distribution of grain sizes around a mean grain size. The mean grain size is above 100 nanometres. The nickel grains with a mean grain size of above 100 nanome^¬ tres and grain boundaries in between these grains result in a second metal layer comprising nickel of the metal lead frame with a certain porosity for the copper atoms of the first metal layer. Usually, nickel layers with no porosity for the copper atoms are used to prevent browning of the silver layer due to copper atoms. Due to the mean grain size of above 100 nanometres, some of the copper atoms of the first metal layer can diffuse through the nickel layer and subsequently diffuse through the silver layer. If such a metal lead frame is ar- ranged next to a silicone casting material, the diffused cop^¬ per atoms and the silicon atoms from the silicone casting metal form a copper-silicon-compound. This copper-silicon- compound is white and therefore does not affect the optical properties of the optoelectronic semiconductor device com- prising the lead frame. The diffusion of the copper atoms through the nickel and silver layer may be controlled in a way, that the copper atoms diffusing through the silver layer form the copper-silicon-compound and do not form brownish copper spots on top of the silver layer. On the other hand, the diffusion should be that fast that the silicone casting material does not react with the silver atoms of the silver layer, forming brown silver-silicon-compound at the boundary between the metal lead frame and the silicone casting materi^¬ al. A mean grain size of above 100 nanometres of the poly- crystalline structure of the nickel layer leads to a certain diffusion constant of the copper atoms through the nickel layer allowing for a control of the colour change of the op^¬ toelectronic semiconductor device during operation of the device .

In one embodiment of the invention, the thickness of the sec^¬ ond metal layer is in between 0.4 and 0.6 microns, particu^¬ larly 0.5 microns. The thickness of the third metal layer is in between 2 and 6 microns. These layer thicknesses also lead to a good control of the diffusion of the copper atoms through the second and third metal layer. In one embodiment of the invention, the thickness of the first metal layer is in between 0.1 and 0.3 millimetres. A layer thickness of 0.2 millimetres for the copper layer leads to a mechanically stable metal lead frame. In one embodiment, at least 80%, particularly at least 90% of the nickel grains of the second metal layer comprise a grain size which differs from the mean grain size by 50 nanometres or less. Therefore, the nickel grains within the polycrystal- line second metal layer comprise a uniformly distributed size, which allows for easier control of the diffusion prop^¬ erties of said second metal layer.

In one embodiment, at least 80%, particularly at least 90% of the nickel grains of the second metal layer comprise a grain size which differs from the mean grain size by 20 nanometres or less. This allows for an even better control of the diffu^¬ sion properties.

In one embodiment of the invention, the mean grain size of the second metal layer is within the range of 150 to 650 na^¬ nometres. Above 650 nanometres, the diffusion of the copper atoms through the nickel layer increases, thus allowing too much copper atoms to diffuse through the metal layer and therefore altering the optical properties of this optoelec- tronic semiconductor device too fast.

In one embodiment of the invention, the mean grain size of the second metal layer is between 190 and 210 nanometres. A mean grain size within this range leads to a good control of the diffusion of the copper atoms through the nickel layer.

In one embodiment of the invention, the mean grain size of the second metal layer is between 290 and 310 nanometres. A mean grain size within this range leads to a good control of the diffusion of the copper atoms through the nickel layer.

In one embodiment of the invention, the mean grain size of the second metal layer is between 390 and 410 nanometres. A mean grain size within this range leads to a good control of the diffusion of the copper atoms through the nickel layer.

In one embodiment of the invention, the mean grain size of the second metal layer is between 490 and 510 nanometres. A mean grain size within this range leads to a good control of the diffusion of the copper atoms through the nickel layer.

In one embodiment of the invention, the mean grain size of the second metal layer is between 590 and 610 nanometres. A mean grain size within this range leads to a good control of the diffusion of the copper atoms through the nickel layer.

An optoelectronic device comprises a metal lead frame accord- ing to the invention and further comprises an optoelectronic semiconductor chip electrically connected to the metal lead frame. Additionally, the optoelectronic device comprises a silicone casting material which is arranged adjacent to the third metal layer of the metal lead frame. During operation of this optoelectronic device, copper atoms from the first metal layer of the metal lead frame diffuse through second and third metal layers of the lead frame and react with sili^¬ con atoms from the silicone casting material. Therefore, the brightness of the optoelectronic device is enhanced.

In one embodiment, the optoelectronic device comprises a cop^¬ per-silicon-compound at the boundary between the third metal layer of the metal lead frame and the silicone casting mate^¬ rial. This copper-silicon-compound is formed due to a diffu- sion of copper atoms through the nickel and silver layers of the metal lead frame during operation of the optoelectronic device. This diffusion can take place due to the thermal heat released during the operation of the optoelectronic device with the semiconductor chip. The formed copper-silicon- compound is white, thus improving the optical properties of the semiconductor device.

In one embodiment of the invention, the copper atoms from the first metal layer of the metal lead frame are capable of dif^¬ fusing through the second and third layer of the metal lead frame. The silicone casting material and the diffused copper atoms are capable of forming a copper-silicon-compound at the boundary between the third metal layer of the metal lead frame and the silicone casting material. This copper-silicon- compound, formed during the operation of the semiconductor device due to the diffusion of the copper atoms through the second metal layer and the third metal layer of the metal lead frame, is white and thus improves the optical properties of the optoelectronic device.

The above described properties, features and advantages of this invention as well as the method of obtaining them, will be more clearly and obviously understandable in the context of the following description of the embodiments, which are explained in more detail in the context of the figures.

In schematic illustration show

Fig. 1 a cross section through a composition of a metal lead frame;

Fig. 2 an amplification of a second metal layer of such a metal lead frame; and

Fig. 3 an optoelectronic device with such a metal lead

frame .

Fig. 1 shows a cross section through a metal lead frame 100, consisting of three metal layers 110, 120, 130. A first metal layer 110 comprises copper and forms the basic structure of the metal lead frame 100, as the first metal layer 110 com- prises the greatest thickness of the three metal layers 110, 120, 130. A second metal layer 120 is arranged adjacent to the first metal layer 110. This second metal layer 120 com^¬ prises nickel and may be galvanically deposited on top of the first metal layer 110. A third metal layer 130 is arranged on top of the second metal layer 120 and comprises silver. The third metal layer 130 may also be galvanically deposited on top of the second metal layer 120. The second metal layer 120 comprises a polycrystalline structure of nickel, wherein the nickel atoms form nickel grains with a mean grain size above 100 nanometres.

Additionally, further layers between the first metal layer 110 and the second metal layer 120 as well as between the second metal layer 120 and the third metal layer 130 may be implemented .

Fig. 2 shows a magnification of the structure of the second metal layer 120 of Fig. 1. Within the second metal layer 120 the nickel forms grains 121. The grains 121 have a grain size each. For all grains 121 an arithmetic mean value of the grain size can be calculated. This calculated value is the mean grain size of the grains 121. In between the grains 121, grain boundaries 122 occur. Within the grain boundaries 122, the electronic structure of the nickel of the second metal layer 120 is less defined than within the grains 121. The composition of grains 121 and grain boundaries 122 affects the capability of copper atoms from the first metal layer 110 to diffuse through the second metal layer 120.

In one embodiment, the thickness of the second metal layer 120 is in between 0.4 and 0.6 microns. The thickness of the third metal layer is in between 2 and 6 microns. In one em^¬ bodiment the thickness of the first metal layer is in between 0.1 and 0.3 millimetres.

In one embodiment, at least 80%, particularly at least 90% of the nickel grains 121 of the second metal layer 120 comprise a grain size which differs from the mean grain size by 50 na^¬ nometres or less. In one embodiment, at least 80%, particu^¬ larly at least 90% of the nickel grains 121 of the second metal layer 120 comprise a grain size which differs from the mean grain size by 20 nanometres or less. It is also possi^¬ ble, that all nickel grains 121 comprise a grain size within the specified range.

In one embodiment, the mean grain size of the second metal layer 120 is within the range of 150 to 650 nanometres. This means, that the grains 121 of Fig. 2 have a mean grain size within the range of 150 to 650 nanometres. A mean grain size between 150 and 650 nanometres leads to a good ratio of nick^¬ el atoms within grains 121 and grain boundaries 122, regard- ing the capability of copper atoms from the first metal layer 110 diffusing through the second metal layer 120. With a mean grain size within that range, the diffusion of copper atoms through the second metal layer 120 reaches an optimized val^¬ ue. That means that with the mean grain size within 150 and 650 nanometres, enough diffusion of copper atoms through the second metal layer 120 occurs to prevent silver atoms from the third metal layer 130 to react with a silicone casting material, but on the other hand, the diffusion of copper at^¬ oms through the second metal layer 120 is so low, that no brown copper spots arise on top of the third metal layer 130.

In one embodiment, the mean grain size of the second metal layer 120 is between 190 and 210 nanometres. In one embodi^¬ ment, the mean grain size of the second metal layer 120 is between 290 and 310 nanometres. In one embodiment, the mean grain size of the second metal layer 120 is in between 390 and 410 nanometres. In one embodiment, the mean grain size of the second metal layer 120 is in between 490 and 510 nanome^¬ tres. In one embodiment, the mean grain size of the second metal layer 120 is in between 590 and 610 nanometres.

Fig. 3 shows a cross section through an optoelectronic device 200 with a metal lead frame 100, wherein the metal lead frame 100 comprises a first metal layer 110, a second metal layer 120 and a third metal layer 130 according to the previous em^¬ bodiments. The metal lead frame 100 is placed within a hous^¬ ing 210. An optoelectronic semiconductor chip 220 is placed within the housing 210 and may be electrically connected to the metal lead frame 100. This is achieved in a way that the optoelectronic semiconductor chip 220 is placed on top of the metal lead frame 100, particularly on top of the third metal layer 130 of the metal lead frame 100. A recess of the hous- ing 210, in which the optoelectronic semiconductor chip 220 is placed, is filled with a silicone casting material 230. The silicone casting material 230 is thereby arranged adja^¬ cent to the third metal layer 130 of the metal lead frame 100. In other words, the silicone casting material 230 and the third layer 130 of the metal lead frame 100 touch at a boundary face 231.

In one embodiment, the optoelectronic device 200 comprises a copper-silicon-compound at the boundary face 231 between the third metal layer 130 of the metal lead frame 100 and the silicone casting material 230.

In one embodiment, the copper atoms from the first metal lay^¬ er 110 of the metal lead frame 100 are capable of diffusing through the second and third metal layers 120, 130 of the metal lead frame 100. The silicone casting material 230 and diffused copper atoms from the first metal layer 110 are ca^¬ pable of forming a copper-silicon-compound at the boundary face 231 between the third metal layer 130 of the metal lead frame 100 and the silicone casting material 230. The heat oc^¬ curring during the operation of the optoelectronic semiconductor device 200 can trigger the diffusion of the copper atoms of the first metal layer 110 through the second and third layers 120, 130. Due to the diffusion, on the one hand side a browning of the semiconductor device 200 due to a reaction of the silver atoms from the third metal layer 130 with the sil^¬ icone casting material 230 is suppressed, wherein on the oth^¬ er hand the diffusion of the copper atoms through the second and third metal layer 120, 130 is so slow, that no browning of the optoelectronic semiconductor device 200 due to copper atoms at the boundary face 231 occurs. Therefore, the adjust^¬ ing of the diffusion of the copper atoms from the first metal layer 110 through the second metal layer 120 and the third metal layer 130 lead to an optimized semiconductor device 200 within proved optical properties and less browning than opto^¬ electronic semiconductor devices known. Although the invention was described and illustrated in more detail using preferred embodiments, the invention is not lim^¬ ited to these. Variants of the invention may be derived by a person skilled in the art from the described embodiments without leaving the scope of the invention.

REFERENCE NUMERALS

100 metal lead frame

110 first metal layer

120 second metal layer

121 grain

122 grain boundary

130 third metal layer

200 optoelectronic device

210 housing

220 optoelectronic chip

230 silicone casting material

231 boundary face

Claims

1. A metal lead frame (100) for an optoelectronic device

(200) comprising three metal layers (110, 120, 130), wherein a first metal layer (110) comprises copper, wherein a second metal layer (120) adjacent to the first metal layer (110) comprises nickel and wherein a third metal layer (130) adjacent to the second metal layer

(120) comprises silver, characterized in that the second metal layer (120) comprises a polycrystalline structure of nickel with a distribution of grain sizes around a mean grain size, and in that the mean grain size is above 100 nanometres.

2. The metal lead frame (100) according to claim 1, wherein the thickness of the second metal layer (120) is in be^¬ tween 0.4 and 0.6 microns and wherein the thickness of the third metal layer (130) is in between 2 and 6 mi^¬ crons .

3. The metal lead frame (100) according to any of claims 1 or 2, wherein the thickness of the first metal layer (110) is in between 0.1 and 0.3 millimetres.

4. The metal lead frame (100) according to any of claims 1 to 3, wherein at least 80%, particularly at least 90% of the nickel grains (121) of the second metal layer (120) comprise a grain size which differs from the mean grain size by 50 nanometres or less.

5. The metal lead frame (100) of claim 4, wherein the at

least 80%, particularly at least 90% of the nickel grains

(121) of the second metal layer (120) comprise a grain size which differs from the mean grain size by 20 nanome- tres or less.

6. The metal lead frame (100) according to any of claims 1 to 5, wherein the mean grain size of the second metal layer (120) is within the range of 150 to 650 nanometres.

7. The metal lead frame (100) according to claim 6, wherein the mean grain size of the second metal layer (120) is within the range of 190 to 210 nanometres.

8. The metal lead frame (100) according to claim 6, wherein the mean grain size of the second metal layer (120) is within the range of 290 to 310 nanometres.

9. The metal lead frame (100) according to claim 5, wherein the mean grain size of the second metal layer (120) is within the range of 390 to 410 nanometres.

10. The metal lead frame (100) according to claim 6, wherein the mean grain size of the second metal layer (120) is within the range of 490 to 510 nanometres.

11. The metal lead frame (100) according to claim 6, wherein the mean grain size of the second metal layer (120) is within the range of 590 to 610 nanometres.

12. An optoelectronic device (200) with a metal lead frame (100) according to any of claims 1 to 11, comprising an optoelectronic semiconductor chip (220) electrically con^¬ nected to the metal lead frame (100), comprising a sili^¬ cone casting material (230), wherein the silicone casting material (230) is adjacent to the third layer (130) of the metal lead frame (100) .

13. An optoelectronic device (200) according to claim 12,

comprising a copper-silicon-compound at the boundary face (231) between the third metal layer (130) of the metal lead frame (100) and the silicone casting material (230) . An optoelectronic device (200) according to any of the claims 12 or 13, wherein the copper atoms of the first metal layer (110) of the metal lead frame (100) are capa^¬ ble of diffusing through the second and third metal lay^¬ ers (120, 130) of the metal lead frame (100), and wherein the silicone casting material (230) and the diffused cop^¬ per atoms are capable of forming a copper-silicon- compound at the boundary face (231) between the third metal layer (130) of the metal lead frame (100) and the silicone casting material (230).