GB2235642A - Method of joining components. - Google Patents

Method of joining components. Download PDF

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Publication number
GB2235642A
GB2235642A GB9019212A GB9019212A GB2235642A GB 2235642 A GB2235642 A GB 2235642A GB 9019212 A GB9019212 A GB 9019212A GB 9019212 A GB9019212 A GB 9019212A GB 2235642 A GB2235642 A GB 2235642A
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United Kingdom
Prior art keywords
silver
layers
components
another metal
temperature
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Granted
Application number
GB9019212A
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GB9019212D0 (en
GB2235642B (en
Inventor
Giles Humpston
David Michael Jacobson
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Marconi Electronic Devices Ltd
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Marconi Electronic Devices Ltd
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Publication of GB9019212D0 publication Critical patent/GB9019212D0/en
Publication of GB2235642A publication Critical patent/GB2235642A/en
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Publication of GB2235642B publication Critical patent/GB2235642B/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L24/00—Arrangements for connecting or disconnecting semiconductor or solid-state bodies; Methods or apparatus related thereto
    • H01L24/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
    • H01L24/83—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected using a layer connector
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
    • B23K35/24—Selection of soldering or welding materials proper
    • B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
    • B23K35/3006—Ag as the principal constituent
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    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
    • H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
    • H01L23/492—Bases or plates or solder therefor
    • H01L23/4924—Bases or plates or solder therefor characterised by the materials
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    • H01L24/26—Layer connectors, e.g. plate connectors, solder or adhesive layers; Manufacturing methods related thereto
    • H01L24/28—Structure, shape, material or disposition of the layer connectors prior to the connecting process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/001—Interlayers, transition pieces for metallurgical bonding of workpieces
    • B23K2035/008—Interlayers, transition pieces for metallurgical bonding of workpieces at least one of the workpieces being of silicium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
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    • B23K35/001—Interlayers, transition pieces for metallurgical bonding of workpieces
    • B23K35/005—Interlayers, transition pieces for metallurgical bonding of workpieces at least one of the workpieces being of a refractory metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
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    • B23K35/007—Interlayers, transition pieces for metallurgical bonding of workpieces at least one of the workpieces being of copper or another noble metal
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
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    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
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    • B23K35/262—Sn as the principal constituent
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Description

: 1 i!- -, SCC/3798 Methods of Joining Components This invention relates
to methods of joining components.
More particularly the invention relates to methods of joining components by soldering, suitable for use in joining a silicon wafer to a component of molybdenum or tungsten.
Such methods find application in the manufacture of high power silicon semiconductor devices. In such devices the electrical connections to the silicon wafer in which the device is formed are conventionally made by way of relatively massive copper electrodes, one on each side, which also serve to conduct heat away from the wafer. Ideally the wafer would be bonded directly to these copper electrodes. In practice the difference between the thermal expansion coefficients of copper and silicon renders this impracticable. Instead, one main face of the wafer is conventionally brazed to a molybdenum or tungsten backing plate, normally in the form of a disc, and a dry sliding contact between one copper electrode and the disc is made. The other copper electrode makes a dry sliding contact with another molybdenum disc which in turn makes a dry sliding contact with the main face of the wafer remote from the brazed disc, the wafer being unable to withstand the stress associated with two brazed joints.
In conventional high-power silicon semiconductor device manufacturing processes the wafer is brazed to the disc using an Al-12%wtSi braze the melting point of which is 5770C. The use of a braze of such a high melting point introduces stresses in the brazed assembly which give rise to bowing. This can be largely removed by clamping under pressure in the final encapsulated package, but the stresses remain and these can impair performance of the device.
It is an object of the present invention to provide a method of joining components which alleviates the above described problem.
According to the present invention there is provided a method of joining components comprising: forming between the components discrete layers each consisting either substantially of silver or substantially of another metal, there being at least one layer of each metal, the silver and another metal in said layers having total volumes in a ratio different from that in a eutectic alloy formed by silver and said another metal; and raising the temperature of said layers above the melting point of said eutectic alloy for a period sufficient to cause initially formation of said eutectic alloy and then, by reaction between said eutectic alloy and one or other of silver and said another metal, formation of a material having a melting point higher than said temperature.
In one particular embodiment of the invention said another metal is tin or a tin based material.
In another particular embodiment of the invention said another metal is indium or an indium based material.
It will be appreciated that the advantage of the present invention lies in that it enables a joint to be made at a temperature lower than that required to remelt the joint after being formed.
Several methods in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:Figure 1 is a diagram illustrating first and second methods; and h Figure 2 is a diagram illustrating a third method.
The methods to be described by way of example essentially comprise joining a first component, e.g. of silicon, to a second component e.g. of molybdenum or tungsten either directly, or via an intermediate component of a metal e.g. an iron-nickel alloy which is readily plated with the jointing metals, i.e. silver and another metal which forms a eutectic with silver e.g. indium or tin. Prior to forming the joint the relevant face of the first component is provided with metallisations which are nonreactive with silver at the temperatures employed in forming the joint and subsequent service of the joint and are adherent to, and where the first component is of silicon, form an ohmic contact with, silicon. The relevant face of the second component and the third component, if present, is similarly provided, prior to forming the joint, with metallisations which are non-reactive with silver and adherent to the material of the component. Layers of the jointing metals are then provided between the metallised surfaces of the components to be joined, and the components assembled and heated under pressure for a time which is a function of the thicknesses of the layers of jointing metals employed.
Referring to Figure 1, in the first method to be described one main face of a 9mm diameter silicon wafer 1 containing a diode structure is joined to one main face of a molybdenum disc 3 of the same diameter whilst the other main face of the disc 3 is simultaneously joined to a copper electrode 5.
Prior to the joining operation the relevant main face of the wafer 1 is first provided with a metallisation comprising a vapour deposited aluminium layer 7 of thickness about 1.0Am which is heat treated to form a sintered ohmic contact with the silicon wafer 1. Layers 9 and 11 of titanium and platinum are then applied in turn over the aluminium layer 7, by vapour deposition, to provide a metallic layer for application of a jointing metal layer 13 of silver by electroplating. The layers 9 and 11 typically together have a thickness of about 0.5ALm and the layer 13 a thickness of from 5 to 101M.
The molybdenum disc 3 in its as supplied condition has on its surface a thin rhodium layer 15. To the rhodium layer 15 on each main face of the disc 3 there are applied, in turn, by electroplating, layers 17 and 19 of jointing metal consisting of silver and tin respectively, the layers 17 and 19 having thicknesses of about 10p-m and 5Am respectively. The exposed faces of the tin layers 19 may, if desired, be provided with a protective gold flash 21.
The copper electrode 5 is provided on one main face with an electroplated silver layer 23 of thickness the same as the silver layer 13 carried by the silicon wafer 1.
To join the components 1, 3 and 5 together the components 1, 3 and 5 are assembled as shown ir, Figure 1 and maintained at a temperature of 2750C in a vacuum furnace for a period of about one hour under an applied pressure of 1 MPa.
The resulting joints between the silicon wafer 1 and the disc 3 and between the disc 3 and the copper electrode 5 are found to be capable of surviving a tensile load up to a temperature of about 6000C. Furthermore the joints are found to be capable of thermal cycling between 250C and 1750C for more than 2500 cycles and heat treatment at 3000C for 2,300 hours without any sign of failure.
The mechanism of joint formation is as follows. On heating to 2750C, i.e. above the melting point of the eutectic binary alloy formed by silver and tin of 2210C, the silver and tin in layers 13, 17, 23 and 19 first form the-eutectic binary alloy in liquid form. The compound A93Sn then forms by reaction between silver and the liquid phase Ag-Sn at the liquid- silver interface. By ensuring that the silver layers 13, 17 23 are sufficiently thick relative to the tin layers 19 to constitute 70% by volume of the total volume of the silver and tin layers, 13, 17, 23 and 19, i.e. much more than the volume proportion (3.5%) of silver in the eutectic alloy, and maintaining the joint temperature above 2210C for a sufficient length of time to cause all the tin in the layers 19 to react to form A93Sn and leave a surplus of silver in the joints, the resulting joints will have a re-melt temperature of more than 4800C, the melting point of A93Sn. It has been found that the presence of A93Sn does not embrittle the joints. It will be appreciated that the theoretical maximum possible melting point of a joint is 980% the melting point of silver.
It will be appreciated that in the above method the thickness of the gold flashes 21 is critical. If they are too thick then the brittle compound AuSn4 tends to form.
In the second method to be described, the procedure is as described above with reference to Figure 1 except that the tin layers 19 are replaced by layers of indium about 6JAm thick, the gold flashes 21, if present, are replaced by silver flashes, and heating takes place at about 1750C, i.e. about 350C above the melting point of the eutectic binary alloy formed by silver and indium, for about two hours with an applied pressure of 0.3 MPa. The resulting joint is found to survive the application of a tensile load up to a temperature of about 9000C. The volume of silver is substantially 77% of the total volume of silver and indium in this particular method.
It will be understood that when the assemblies made as described above are packaged into a device the copper electrode 5 forms one heat sink electrode of the device whilst the other heat sink electrode (not shown) makes a dry sliding contact with the exposed surface of the silicon wafer 1.
Referring now to Figure 2, in the third method to be described by way of example, one main face of a silicon wafer 25 of diameter 50mm is joined to one main face of a molybdenum disc 27 of the same diameter. In view of the relatively large diameter of the disc 27 joining of the other face of the disc 27 to a copper electrode is not practicable. Hence, dry sliding contact is made with the exposed faces of wafer 25 and disc 27 by copper electrodes (not shown) in the completed device. Consequently the disc 27 is initially provided with any conventional metallisation (not shown) suitable when dry sliding contact with a copper electrode is required.
The silicon wafer 25 is provided on one main face with layers 29, 31, 33 and 35 of aluminium, titanium, platinum and silver corresponding to layers 7, 9, 11 and 13 described above with reference to Figure 1.
The molybdenum disc 27 is provided on one main face over an as supplied rhodium layer 37 with an electroplated silver layer 39 of thickness equal to that of -the silver layer 35 carried by the silicon wafer 25.
The other required jointing metal e.g. tin or indium, is introduced into the joint on a carrier in the form of a disc 41 of appropriate diameter of a metal that is readily plated with the jointing metal and is reasonably well expansion -matched to silicon over the working temperature range of the completed device, e.g. -500C to +2500C. An iron- nickel alloy such as Invar 42 is suitable.
The carrier disc 41 is thus provided on each main face with coatings 43, 45 and 47 of silver, indium and silver respectively of thicknesses 11, 3 and 0.1 Lxm respectively.
To join the components 25 and 27 together the components 25, 27 and 41 are assembled as shown in Figure 2.
After heating at about 1WC for about two hours under a pressure of 0.3 MPa joints are formed between carrier disc 41 and wafer 25 and carrier disc 41 and molybdenum disc 27 typically of thickness of only about 10 m with an edge-to-edge bow of less than 6im. This compares with a bow typically of 60 on for a corresponding assembly using a conventional A1-12%wtSi braze.
It will be appreciated that in the method of Figure 2 the indium layers 45 may be replaced by tin layers, in which case the silver flashing layers 47 may be replaced by gold flashings.
In addition to the relatively low temperature required to make a joint by a method according to the invention, a method according to the invention has the advantage that even with relatively low pressure applied during formation of joints, the joints exhibit low incidence of voids in joints. This arises partly 1 from the fact that only a small fraction of the total material within the zone of reaction, typically less than 25%, is liquid at any time. Hence the relative quantity of molten alloy expelled from a joint is smaller than in conventional soldering processes. The components joined are also clean and flat, unlike when a solder foil is used, and void levels due to trapped gases are reduced.
It will be appreciated that whilst in the examples described above the relative volumes of silver and tin or indium are such that all the tin or indium alloys with the silver leaving a surplus of silver in the joint, in other methods according to the invention a surplus of tin or indium may result. However, much higher joint remelt temperatures result where there is a surplus of silver in the joint since higher melting point phases form by the reaction between silver and tin or indium. When there is a surplus of tin or indium in the joint lower re-melt temperatures result since tin and indium have melting points close to the silver-tin and silver-eutectic melting points.
It is further pointed out that metals other than tin and indium, for example gallium, may be used with silver in a method according to the invention. The required features of the metal used with silver are of course that it forms with silver a eutectic of suitably low melting point and that it will react with the eutectic alloy or the eutectic alloy will react with silver without forming brittle compounds. Furthermore, the metal used with silver may be an alloy or combination of two or more metal elements i.e. a separate discrete layer of an alloy of two or more metal elements may be used or two or more separate discrete layers each consisting of a metal element or a metal alloy may be used. Similarly, a silver based discrete layer may be used instead of a substantially pure discrete silver layer.
It will further be understood that whilst the method according to the invention is particularly suitable for joining a silicon component to a component of molybdenum or tungsten, it finds application wherever it is desired to form a joint between two components which has a re-melt temperature higher than the temperature of formation of the joint.
1 -g-

Claims (16)

1. A method of joining components comprising: forming between the components discrete layers each consisting either substantially of silver or substantially of another metal there being at least one layer of each metal, the silver and another metal in said layers having total volumes in a ratio different from that in a eutectic alloy formed by silver and said another metal; and raising the temperature of said layers above the melting point of said eutectic alloy for a period sufficient to cause initially formation of said eutectic alloy and then, by reaction between said eutectic alloy and one or other of silver and said another metal, formation of a material having a melting point higher than said temperature.
2. A method according to Claim 1 wherein the volume fraction of silver in said layers is greater than the volume fraction of silver in said eutectic alloy.
3. A method according to Claim 1 or Claim 2 wherein said another metal is tin or a tin based material.
4. A method according to Claim 3 wherein said temperature is substantially 2750C.
5. A method according to Claim 3 or Claim 4 when dependent on Claim 2 wherein the volume of silver in said layers is substantially 70% of the total volume of all said discrete layers together.
6. A method according to Claim 1 or Claim 2 wherein said another metal is indium or an indium based material.
7. A method according to Claim 6 wherein said temperature is substantially 1750C.
8. A method according to Claim 6 or Claim 7 when dependent on Claim 2 wherein the volume of silver in said layers is substantially 77% of the total volume of all said discrete layers together.
9. A method according to any one of Claims 3 to 8 wherein said period is between one and two hours.
10. A method according to any one of the preceding claims wherein both said components carry a said layer consisting substantially of silver.
11. A method according to any one of the preceding claims wherein a said layer of said another metal is carried on one only of said components.
12. A method-according to any one of the preceding claims wherein one of said components consists substantially of silicon.
13. A method according to Claim 12 wherein the surface of the silicon component to be joined is provided with a sintered aluminium layer overlaid by titanium and platinum layers.
14. A method according to Claim 12 or Claim 13 wherein the other of said components consists substantially of molybdenum or tungsten.
15. A method according to Claim 12 or Claim 13 wherein the other of said components consists substantially of an iron-nickel alloy.
16. A method of joining components according to Claim 1 substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.
Published 1991 at7be Patent Office. State House. 66171 High Holborn. londonWCIR4'IP. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point ",mielinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.
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GB8920101D0 (en) 1989-10-18
GB9019212D0 (en) 1990-10-17
US5106009A (en) 1992-04-21
EP0416847A2 (en) 1991-03-13
GB2235642B (en) 1993-12-08
EP0416847A3 (en) 1992-03-11

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