WO2017114564A1

WO2017114564A1 - Method of etching a metal lead frame

Info

Publication number: WO2017114564A1
Application number: PCT/EP2015/081355
Authority: WO
Inventors: Ismail ITHNAIN; Choo Kean LIM; Weng Heng CHAN
Original assignee: Osram Opto Semiconductors GmbH
Current assignee: Ams Osram International GmbH
Priority date: 2015-12-29
Filing date: 2015-12-29
Publication date: 2017-07-06
Anticipated expiration: 2018-06-29

Abstract

A method of etching a metal lead frame for a semiconductor chip includes the application of an etching agent to a metal plate using a first and a second spray nozzle, which are located on opposite sides of the metal plate. The volumetric flow rate of the etching agent applied to the metal plate via the first spray nozzle is at least 10 % different from the volumetric flow rate of the etching agent applied to the metal plate via the second spray nozzle. A metal plate with an etching trench extending from a first side of the metal plate to a second side of the metal plate is disclosed. The etching trench comprises a first section and a second section, wherein a first side wall and a second side wall of the etching trench are basically perpendicular to the first side of the metal plate in the first section. A third side wall and a fourth side wall limit the etching trench within the second section. The first section of the etching trench extends from the first side of the metal plate to approximately one third of the thickness of the metal plate and the second section extends from the second side of the metal plate to approximately two thirds of the thickness of the metal plate. The first side of the metal plate adjoins the first side wall, which adjoins the third side wall, which adjoins the second side of the metal plate. The first side of the metal plate adjoins the second side wall, which adjoins the fourth side wall, which adjoins the second side of the metal plate. The third and fourth side wall are inclined compared to the first and second side wall in a way that the distance between the first and the second side wall is smaller than the distance between the third and the fourth side wall.

Description

METHOD OF ETCHING A METAL LEAD FRAME

DESCRIPTION Metal lead frames, for example for QFN-type (quad-flat no- leads) semiconductor packages, can be obtained by etching a metal plate from both top side and bottom side. Conventional^¬ ly, the etching agent is applied from the top and from the bottom, with an equal volumetric flow rate from both sides. This etching method leads to metal lead frames, wherein least material is etched away in the middle of the metal plate. This leads to a so-called etching fin, meaning that an etch^¬ ing trench formed by the etching agent within the metal plate has a minimal width in the middle of the metal plate. There- fore, filling up the etching trench with an insulating material, for example silicon, is difficult, as the silicon may be disturbed by this etching fin.

An assignment of the invention is to provide a new etching method for etching metal lead frames, providing a metal plate with corresponding etching trenches, and providing a semiconductor package with the corresponding lead frames.

The solution of this assignment is disclosed in the independ- ent claims of this invention. Preferred embodiments are dis^¬ closed in the dependent claims.

A metal lead frame for a semiconductor chip can be produced with an etching method, wherein an etching agent is applied to a metal plate using a first and a second spray nozzle.

These spray nozzles are located on opposite sides of the met^¬ al plate, and the volumetric flow rate of the etching agent applied to the metal plate via the first spray nozzle is at least 10% different from the volumetric flow rate of the etching agent applied to the metal plate via the second spray nozzle. With an etching method like this, a non-symmetrical etching process can be obtained at the metal plate. Adjusting the ratio of the volumetric flow rates of the two spray noz^¬ zles, the etching of an etching trench within the plate can be tuned in a way that the distance between the two parts, later forming lead frame sections, show a minimal distance at the surface of the metal plate. This leads to improved metal lead frames for semiconductor chips.

In one embodiment of the invention the volumetric flow rate applied by the first spray nozzle is half as much as the vol- umetric flow rate of the etching agent applied via the second spray nozzle. A 1:2 ratio of the volumetric flow rates is ad^¬ vantageous for producing metal lead frames with a reduced etching fin. In one embodiment of the invention the etching process is separated into two etching steps. In a first etching step the etching agent is applied by both the first and the second spray nozzle. In a second etching step, the etching agent is only applied by the second spray nozzle. This leads to a 1:2 ratio of etching agent applied by the two spray nozzles.

In one embodiment of the invention both spray nozzles are driven with the same pressure. The diameter of the second spray nozzle is bigger than the diameter of the first spray nozzle, leading to an increased amount of etching agent reaching the metal plate through the second spray nozzle com^¬ pared to the amount of etching agent applied to the metal plate via the first spray nozzle. A person skilled in the art can choose the diameter of the spray nozzles to obtain any ratio of the volumetric flow rate desired.

In one embodiment of the invention the diameters of the spray nozzles are similar. The second spray nozzle is driven with a higher pressure than the first spray nozzle, leading to an increased volumetric flow rate of etching agent applied to the metal plate via the second spray nozzle compared to the volumetric flow rate of etching agent applied via the first spray nozzle.

In one embodiment of the invention the metal plate and/or the spray nozzles are moved during the application of the etching agent, leading to the formation of etching trenches in the metal. A relative movement between the metal plate and the spray nozzles can be used to achieve etching trenches within the metal plate. These etching trenches can be designed in a way, that the remaining parts of the metal plate then form metal lead frames for a semiconductor chip package.

In one embodiment of the invention the etching trench formed within the metal due to the application of the etching agent via the first spray nozzle has a maximum width of 100 mi^¬ crons. A width below 100 microns is preferable for metal lead frame technology used in semiconductor chip packages. With the etching method as described, it is possible to obtain a width of the etching trench of 100 microns or below for the part of the etching trench formed due to the application of the etching agent via the first spray nozzle and thus getting rid of the etching fin.

In one embodiment of the invention the metal plate comprises copper. Copper is a material which is well-suited for the task of forming metal lead frames.

In one embodiment of the invention the etching agent is hy^¬ drochloric acid, cupric chloride, ferric chloride, ammonium persulfate, nitric acid or a mixture of sulfuric acid and a peroxide. The mentioned etchants are well-known to be capable of etching metals, particularly copper. Therefore, these etchants are well-suited to be used in the etching method as described .

A metal plate comprises an etching trench, which extends from a first side of the metal plate to a second side of the metal plate. The etching trench comprises a first section and a second section with different properties. A first side wall and a second side wall of the etching trench are basically perpendicular to the first side of the metal plate within the first section. A third side wall and a fourth side wall limit the etching trench within the second section. The first section of the etching trench extends from the first side of the metal plate to approximately one third of the thickness of the metal plate. The second section extends from the second side of the metal plate to approximately two thirds of the thickness of the metal plate. The first side of the metal plate adjoins the first side wall, which adjoins the third side wall, which adjoins the second side of the metal plate. The first side of the metal plate also adjoins the second side wall, which adjoins the fourth side wall, which also ad^¬ joins the second side of the metal plate. The third and fourth side wall are inclined compared to the first and sec^¬ ond side wall in a way that the distance between the first and the second side wall is smaller than the distance between the third and the fourth side wall. A metal plate with an etching trench as described is well-suited for the use in a QFN-type semiconductor package.

In one embodiment the thickness of the metal plate ranges from 150 to 300 microns. Metal lead frames with thicknesses from 150 to 300 microns are well-suited for QFN-type semicon^¬ ductor packages.

In one embodiment the distance between the first and the sec- ond side wall is about 100 microns, leading to a distance of more than 100 microns between the third side wall and the fourth side wall.

In one embodiment the metal plate comprises a plurality of etching trenches. This plurality of etching trenches separate lead frame sections formed by the metal of the metal plate. In one embodiment of the invention an etching trench is filled up with an insulating material, particularly silicon or an epoxide compound. Silicon or an epoxide compound are good materials to fill up etching trenches as they are an in- sulator leading to well-structured lead frame sections on the metal plate usable for semiconductor chip packages.

In one embodiment of the invention a semiconductor chip is placed on the first side of the metal plate. An electrical junction of the semiconductor chip is electrically conduc- tively connected to the metal plate and the electrical junc^¬ tions of the semiconductor chip are electronically conduc- tively connected to parts of the metal plate in a way that after a separation process the electrical junctions of the semiconductor chips can be electronically conductively con^¬ nected to individual electronically isolated parts of the metal plate, forming lead frames. The exact design of the etching trenches leading to such a metal plate can be performed by a person of ordinary skill in the art.

The above described properties, features and advantages of this invention as well as the method of obtaining them, will be more clearly and more obviously understandable in the con^¬ text 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 setup with a metal plate and spray nozzles;

Fig. 2 a metal plate with an etching trench;

Fig. 3 a top view of a metal plate with a plurality of etch^¬ ing trenches; and

Fig. 4 a semiconductor chip package. Fig. 1 shows a cross-section of a metal plate 100 with a first spray nozzle 110 on a first side 101 of the metal plate 100 and a second spray nozzle 120 on the opposite second side 102 of the metal plate 100. The two spray nozzles 110, 120 can be identical or similar to each other. They can be used to apply an etching agent 130 to the metal plate 100. The ap^¬ plication of the etching agent 130 leads to the formation of an etching trench 140 which is foreshadowed by a dashed line. The volumetric flow rate of the etching agent 130 applied to the metal plate 100 via the first spray nozzle 110 is at least 10% different from the volumetric flow rate of the etching agent 130 applied to the metal plate 100 via the sec^¬ ond spray nozzle 120. The different amounts of the volumetric flow rate of the etching agent 130 lead to the formation of the etching trench 140 and particularly to its shape.

In one embodiment the volumetric flow rate of the etching agent 130 applied via the first spray nozzle 110 is half as much as the volumetric flow rate of the etching agent 130 ap- plied via the second spray nozzle 120. This 1:2 ratio of the etching agent 130 applied via the two spray nozzles 110, 120 leads to an advantageous shape of the etching trench 140.

In one embodiment of the invention, in a first etching step the etching agent 130 is applied by both the first and the second spray nozzle 110, 120. In a second etching step the etching agent is applied by only the second spray nozzle 120.

In one embodiment both spray nozzles 110, 120 are driven with the same pressure. The diameter of the second spray nozzle 120 is bigger than the diameter of the first spray nozzle 110, leading to an increased volumetric flow rate of the etching agent 130 applied by the second spray nozzle 120. In one embodiment the diameters of the spray nozzles 110, 120 are similar. The second spray nozzle 120 is driven with the higher pressure than the first spray nozzle 110, leading to an increased volumetric flow rate of the etching agent 130 applied by the second spray nozzle 120.

In one embodiment the metal plate 100 or the spray nozzles 110, 120 are moved during the application of the etching agent 130, leading to the formation of etching trenches 140 within the metal. The relative movement of the metal plate 100 to the spray nozzles 110, 120 is used to form etching trenches 140 throughout the metal plate 100. This movement can be carried out in any direction following the surface of the metal plate 100 and thus maintaining the distance between the metal plate 100 and the spray nozzles 110, 120.

In one embodiment the metal plate 100 comprises copper.

In one embodiment the etching agent 130 is hydrochloric acid, cupric chloride, ferric chloride, ammonium persulfate, 25-50% nitric acid or a mixture of sulfuric acid and a peroxide. Fig. 2 shows a metal plate 100 with a trench 140. Using dashed lines the former parts of the metal plate 100, that are now etched away, are indicated. The trench 140 comprises a first section 141 and a second section 142. The metal plate comprises a first side 101 and a second side 102, wherein the first side 101 of the metal plate 100 is in the vicinity of the first section 141 of the etching trench 140. The second side 102 of the metal plate 100 is in the vicinity of the second section 142 of the etching trench 140. At the first section 141, the etching trench 140 is confined by a first side wall 151 and a second side wall 152. The side walls 151, 152 are basically perpendicular to the first side 101 of the metal plate 100. Basically perpendicular in this context means, that the angle between the first side 101 and the first or second side wall 151, 152 in in the range between 85 and 95 degrees. The first side wall 151 and the second side wall 152 also adjoin the first side 101 of the metal plate 100. In the second section 142 of the etching trench 140 a third side wall 153 and a fourth side wall 154 confine the etching trench 140. The third side wall 153 adjoins the first side wall 151 and the second side 102 of the metal plate 100. The fourth side wall 154 adjoins the second side wall 152 and the second side 102 of the metal plate 100 as well. The third side wall 153 and the fourth side wall 154 are inclined com^¬ pared to the first side wall 151 and the second side wall 152. The inclination angle of the third side wall 153 and the fourth side wall 154 can be about 45 degrees, but other an- gles are also possible. It is also possible that the third side wall 153 and the fourth side wall 154 are realized in a faceted or curved shape. The distance between the first and the second side wall 151, 152 is smaller than the distance between the third and the fourth side wall 153, 154.

In one embodiment the distance between the first side wall 151 and the second side wall 152 is 100 microns or below.

In one embodiment this distance between the first side wall 151 and the second side wall 152 is formed via the etching agent 130 applied to the metal plate 100 via the first spray nozzle 110.

In one embodiment the thickness of the metal plate 100 ranges from 150 to 300 microns.

Fig. 3 shows a top view of a metal plate 100 with several different etching trenches 140, particularly a plurality of etching trenches 140. The plurality of the etching trenches 140 separate lead frame sections formed by the metal of the metal plate 100. This can be used to apply a semiconductor chip on the top side of this metal plate, contact electrical junctions of the semiconductor chip with different parts of the metal plate and then separate the metal plate with for example a saw. Then different parts of the metal plate form lead frame sections for this semiconductor chip. The particular shape of the etching trenches 140 of the metal plate 100 can be achieved by a relative movement of the metal plate compared to the spray nozzles. Thereby, either the met^¬ al plate or the spray nozzles 110, 120 can be moved as well as the metal plate 100 and the spray nozzles 110, 120. These etching trenches 140 can be designed in a way, that the re^¬ maining parts of the metal plate then form metal lead frames for a semiconductor chip package. The particular shape of the trenches 140 in Fig. 3 is merely exemplary and other shapes can be designed by a person of ordinary skill in the art.

Fig. 4 shows a semiconductor package consisting of a metal lead frame produced with the method described. A metal plate 100 with an etching trench 140 was used as a starting point. The etching trench 140 is filled up with silicon 143 or an epoxide compound 143 leading to a flat plate consisting of parts of metal and of other parts which are silicon or an epoxide compound. A first metal part forms a first lead frame section 103 wherein a second metal part forms a second lead frame section 104. These lead frame sections 103, 104 are separated by the etching trench 140 filled up with silicon 143 or an epoxide compound 143 and are both part of the ini^¬ tial metal plate 100. A semiconductor chip 160 is placed on top of the metal plate, wherein two electrical junctions 161 are connected to the metal plate 100. One of the electrical junctions 161 is thereby connected to the first lead frame section 103 wherein another electrical junction 161 is connected to a second lead frame section 104. The semiconductor chip 160 in this embodiment is a so-called flip chip with both electrical contacts 161 on the bottom side. It is also possible, that the semiconductor chip 160 is realized in the form of conventional semiconductor chip and the electrical contact between the electrical junctions of the semiconductor chip and the first and the second lead frame 103, 104 are performed by bond wires. To obtain the semiconductor package of Fig. 4, it is possible to take the metal plate described in Fig. 3, fill up the etching trenches 140 at least partly with silicon 143 or an epoxide compound 143, place semiconductor chips 160 on top of the metal plate 100, establish the electrical contacts be^¬ tween the semiconductor chips 160 and parts of the metal plate 100 and then separate the metal plate in a way, that the electrical junctions of the semiconductor chips are elec^¬ tronically conductively connected to individual electronical^¬ ly isolated parts of the metal plate, thus forming the lead frame sections 103 and 104 from Fig. 4.

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 plate

101 first side

102 second side

103 first lead frame section

104 second lead frame section

110 first spray nozzle

120 second spray nozzle

130 etching agent

140 trench

141 first section

142 second section

143 silicon or epoxide compund

151 first side-wall

152 second side-wall

153 third side-wall

154 fourth side-wall

160 semiconductor chip

161 electrical contact

Claims

1. Method of etching a metal lead frame for a semiconductor chip (160), wherein an etching agent (130) is applied to a metal plate (100) using a first and a second spray noz^¬ zle (110, 120), wherein the spray nozzles are located on opposite sides of the metal plate (100), characterized in that the volumetric flow rate of the etching agent (130) applied to the metal plate (100) via the first spray noz^¬ zle (110) is at least 10 % different from the volumetric flow rate of the etching agent (130) applied to the metal plate (100) via the second spray nozzle (120) .

2. Method according to claim 1, wherein the volumetric flow rate of the etching agent (130) applied via the first spray nozzle (110) is half as much as the volumetric flow rate of the etching agent (130) applied via the second spray nozzle (120) .

3. Method according to claim 2, wherein in a first etching step the etching agent (130) is applied by both the first and the second spray nozzle (110, 120), and in a second etching step the etching agent (130) is applied only by the second spray nozzle (120) .

4. Method according to claim 1 or 2, wherein both spray nozzles are driven with the same pressure, and wherein the diameter of the second spray nozzle (120) is bigger than the diameter of the first spray nozzle (110) .

5. Method according to claim 1 or 2, wherein the diameters of the spray nozzles are similar, and wherein the second spray nozzle (120) is driven with a higher pressure than the first spray nozzle (110) .

6. Method according to any of the previous claims, wherein the metal plate (100) and/or the spray nozzles (110, 120) are moved during the application of the etching agent (130), leading to the formation of etching trenches (140) in the metal .

Method according to claim 6, wherein the etching trench (140) formed in the metal due to the application of the etching agent (130) via the first spray nozzle (110) has a maximum width of 100 microns.

Method according to any of the previous claims, wherein the metal is copper.

Method according to any of the previous claims, wherein the etching agent (130) is hydrochloric acid, cupric chloride, ferric chloride, ammonium persulfate, nitric acid or a mixture of sulfuric acid and a peroxide.

Metal plate (100) with an etching trench (140), wherein the etching trench (140) extends from a first side (101) of the metal plate (100) to a second side (102) of the metal plate (100), wherein the etching trench (140) com^¬ prises a first section (141) and a second section (142), wherein a first side wall (151) and a second side wall

(152) of the etching trench (140) are basically perpendicular to the first side (101) of the metal plate (100) in the first section (141), wherein a third side wall

(153) and a fourth side wall (154) limit the etching trench (140) within the second section (142), wherein the first section (141) of the etching trench (140) extends from the first side (101) of the metal plate (100) to ap^¬ proximately one third of the thickness of the metal plate

(100), wherein the second section extends from the second side (102) of the metal plate (100) to approximately two thirds of the thickness of the metal plate (100), wherein the first side (101) of the metal plate (100) adjoins the first side wall (151), wherein the first side wall (151) adjoins the third side wall (153), wherein the third side wall (153) adjoins the second side (102) of the metal plate (100), wherein the first side (101) of the metal plate (100) adjoins the second side wall (152), wherein the second side wall (152) adjoins the fourth side wall (154), wherein the fourth side wall (154) adjoins the second side (102) of the metal plate (100), and wherein the third and fourth side wall (153, 154) are inclined compared to the first and second side wall (151, 152) in a way that the distance between the first and the second side wall (151, 152) is smaller than the distance between the third and the fourth side wall (153, 154) .

11. Metal plate (100) according to claim 10, wherein the

thickness of the metal plate (100) ranges from 150 to 300 microns.

Metal plate (100) according to claim 10 or 11, wherein the distance between the first and the second side wall (151, 152) is about 100 microns.

Metal plate (100) according to any of the claims 10 to 12, wherein the metal plate (100) comprises a plurality of etching trenches (140), wherein the plurality of etch^¬ ing trenches (140) separate lead frame sections formed by the metal of the metal plate (100) .

Metal plate (100) according to any of the claims 10 to 13, wherein an etching trench (140) is filled up with insulating material, particularly silicon (143) or an epoxide compound (143) .

Metal plate (100) according to any of the claims 10 to 14, wherein a semiconductor chip (160) is placed on the first side (101) of the metal plate (100), wherein an electrical junction (161) of the semiconductor chip (160) is electrically conductively connected to the metal plate (100), and wherein the electrical junctions (161) of the semiconductor chip (160) are electronically conductively connected to parts of the metal plate (100) in a way that after a separation process the electrical junctions (161) of the semiconductor chip (160) can be electronically conductively connected to individual, electronically iso^¬ lated parts (103, 104) of the metal plate (100), forming lead frames (103, 104) .