GB2031223A - Method for bonding a refractory metal contact member to a semiconductor body - Google Patents

Abstract

The invention provides a method for bonding a refractory metal contact 18'/20' to a semiconductor body 12' in which a silver-germanium alloy 44/46 is interposed between the surface of the refractory metal contact member and the surface of the semiconductor body. The assembly is then heated until the alloy enters the slush state. The assembly is thereafter rapidly quenched to freeze the alloy, thereby forming a high-strength bond. Typically, the alloy comprises 85-99% Ag and 15-1% Ge. In one example, the assembly is heated to a temperature of about 835 DEG C. <IMAGE>

Description

SPECIFICATION Method for bonding a refractory metal contact member to a semiconductor body The present invention relates to methods of manufacturing semiconductor devices, such as rectifiers and, more particularly, to a method for joining a contact member to a silicon body, so as to form a high-strength bond therebetween.

Passivated semiconductor devices, such as rectifiers, generally include a semiconductor body composed substantially of silicon, a layer of passivating material such as glass or plastic disposed about the semiconductor body, and at least one metallic contact extending outwardly from the semiconductor body through the passivating layer as an external contact for connection with associated circuitry. In such devices, it is required that the metallic contacts be refractory in nature in order that the coefficients of thermal expansion of the semiconductor body, the passivating layer, and the metallic contact be reasonably matched to avoid breakage during a thermal cycling.

Molybdenum, tungsten, tantalum and various special alloys are typical of the refractory metals which are suitable for use as refractory metal contacts. However, since such metals are both expensive and relatively poor conductors of both heat and electrical current, the refractory metal contacts are generally joined to conventional conductors, such as copper, silver or various special alloys, just beyond the passivating layer in order to form the lead for connection with external circuitry.

A preferred method for making this connection is described in detail in Patent No. 3,930,306, issued January 6, 1976 to Monroe B. Goldberg, et al. and entitled "Process For Attaching a Lead Member To A Semiconductor Device".

It is also necessary to connect the other end of each of the refractory metal contact members to the semiconductor body. However, because of the characteristics of the materials involved, it is impossible by known methods to join the refractory metal contact member directly to the silicon body and obtain a junction having the necessary properties of conductivity and strength. Therefore, a metallic layer, in the form of a coating on the semiconductor body surface, is utilized as a bonding agent in a high temperature brazing process to form the junction.

The semiconductor body surface is coated with a relatively thin layer of aluminum by means of an evaporation process. This process must take place in an inert atmosphere, as the evaporated aluminum layer is required to be virtually free of contaminants in order to form an acceptable bond. Thus, this process requires the use of expensive evaporation equipment and has to be performed in a manner which assures that the evaporated aluminum layer is virtually free of contan inants. Moreover, all subsequent steps in the manufacturing process which requires elevating the assembly to high temperatures (such as brazing the metallic leads onto the refractory contact members) must also take place in an inert atmosphere to protect the aluminum.

Further, the use of an aluminum interlayer as a bonding agent causes the electrical characteristics of the resulting semiconductor device to be significantly down-graded. The semiconductor body is formed of a slice or die of silicon, which has been doped by diffusion or other known doping methods, to produce layers with N-type impurities to form PN junctions at required depths within the body. The presence of the aluminum layer acts as a counterdopant during the subsequent high temperature brazing operations, because the aluminum acts as a source for P-type impurities. The P-type impurities from the aluminum diffuse into the N-type layers of the silicon when the assembly is heated to the elevated temperatures required for the brazing operations.The diffusion of aluminum at brazing temperature can result in aluminum spiking in the area of the PN junction, or serious counter-doping of the N-doped region. Either of these two conditions will significantly degrade the electrical characteristics of the resulting semiconductor device.

To eliminate the deleterious effects associated with the use of an aluminum interlayer, contact metallurgies other than aluminum, such as silverplated tungsten or rhodium-plated molybdenum have been used. However, in such processes, the associated piece part costs are extremely high due to the materials involved and prohibitively expensive process techniques are required to prevent silver contact metallurgies from penetrating into the diffused silicon body during the high temperature assembly operations.

It is, therefore, a prime object of the present invention to provide a method for bonding a contact member to a semiconductor body, wherein the electrical characteristics of the resulting device are not degraded.

It is another object of the present invention to provide a method for bonding a contact member to a semiconductor body wherein the resulting joint has a relatively high strength.

It is a further object of the present invention to provide a method for bonding a contact member to a semiconductor body, which does not require evaporation techniques or the associated equipment.

It is a still further object of the present invention to provide a method for bonding a contact member to a semiconductor body, wherein the inter-layer alloy is compatible with the surrounding materials and is inexpensive to manufacture and use.

It is still another object of the present invention to provide a method for bonding a contact member to a semiconductor body, wherein the entire manufacturing operation can take place in a single zone furnace, thereby requiring substantially less expensive equipment than prior art methods.

In accordance with the present invention, a method for bonding a refractory metal contact member to a semiconductor body is provided. A layer of silver-germanium alloy is interposed between the surfaces of the member and the body. The assembly is heated until the alloy enters the slush state. Thereafter, the assembly is rapidly quenched to freeze the alloy, thereby forming the bond.

Preferably, the alloy is composed of 85%-99% silver and 15%-1% germanium, by weight, and, more preferably, approximately 95% silver and 5% germanium, by weight. The assembly is preferably heated to a temperature within the range from 750 C to 9100C and, more preferably, approximately 835 C.

The heating procedure may take place quite rapidly, preferably in the period of approximately 2.5 minutes. The step of quenching preferably takes place prior to the entrance of the alloy into the flow state.

For ease of handling, the alloy layer is preferably provided as a preform.

In one embodiment of the invention, two refractory metal contacts are provided, each of which is joined to a different end surface of the semiconductor body. Leads are then attached to the outer end of each of the refractory metal contact members by soldering, butt-welding, or, preferably, through the use of a brazing alloy composed of copper, silver and phosphorus. Thereafter, a passivating layer comprised of slurry glass or other similar material is formed around the assembly. The passivated assembly can then be encapsulated in plastic or other suitable substance, as disclosed in Patent No.

3,996,602, issued December7, 1976 to Monroe B.

Goldberg, et al., and entitled "Passivated And Encapsulated Semiconductors And Method of Making Same".

The invention will be further described in connection with the accompanying drawings, wherein like numerals refer to like parts, and in which: Figure 1 is a fragmentary exploded plan view of a prior art semiconductor subassembly; Figure 2 is a fragmentary plan view of the subassembly of Figure 1; Figure 3 is a fragmentary plan view of the glass passivated rectifier formed of the subassembly of Figure 2; Figure 4 is an exploded plan view of a semiconductor subassembly, illustrating the first step of a preferred method of the present invention; Figure 5 is a fragmentary plan view of the subassembly of Figure 4 after the metal contacts have been bonded to the semiconductor body; Figure 6 is a fragmentary exploded plan view of a semiconductor subassembly, illustrating the second step of this preferred method;; Figure 7 is a fragmentary plan view of the subassembly of Figure 6 after the leads have been bonded to the contact members; and Figure 8 is a fragmentary plan view of a glass passivated rectifierformed by this preferred method.

Figures 1, 2 and 3 illustrate the manner in which a semiconductor device, such as a glass passivated rectifier, generally designated 10, is formed by prior art methods. The individual components of the subassembly 10 are illustrated in exploded view in Figure 1. The semiconductor body designated by numeral 12, comprises a diffused silicon chip 14 and, at each end thereof, a layer of evaporated aluminum 16 forming a joining surface for the semiconductor body 12. The semiconductor body 12 is formed substantially of silicon which has been doped in a conventional manner so as to form the necessary PN junction. For clarity in illustrating the principles of the present invention, the semiconductor body 12 has been illustrated as a rectifier adapted for connection to only two lead members.However, it is to be understood that the principles of the present invention apply as well to many othertypes of semiconductors, whether formed into a single, thin wafer-like diode (as shown), or relatively long stack of several chips joined in series and brazed together with conventional materials, with each of the various chips having a plurality of leads extending therefrom. The aluminum joining surfaces may be applied to the silicon chip 14 by evaporation deposition techniques or the like.

A pair of refractory metal contact members 18,20, generally referred to as "slugs", include joining surfaces 22. 24 and 26, 28, respectively, at opposite ends thereof. The contact members 18 and 20 are formed of refractory metal materials which are preferably composed substantially of molybdenum, tungsten, tantalum and alloys thereof. Such alloys are selected according to their known coefficients of expansion to insure that the coefficients of the semiconductor body 12, the contact members 18 and 20, and any materials to use to passivate the semiconductor body are compatible.

A pair of thermally and electrically conductive axial lead members 30, 32 are provided for bonding to surfaces 24,28 of metal contacts 18 and 20, respectively. Leads 30 and 32 may be bonded to the contact members 18 and 20 by soldering or buttwelding. However, it is preferable to form the necessary joints by brazing with an alloy preform composed, on a weight basis, of about 80-89% copper, about 5-15% silver, and about 4-6% phosphorus, which is a commercially available 8011515 silver solder or high temperature brazing alloy marketed by Engelhard Industries Division of Engel hard Materials & Chemicals Corp., Murryhill, New Jersey, underthetrademark"SiLVALOY 15" and by Handy & Harmon, Inc., under the trademark "SIL FOS".The brazing alloy preforms 38,40 are interposed between surfaces 24, 34 and 28, 36, respectively.

The refractory metal contact members 18 and 20 are joined to semiconductor body 12 by placing surfaces 22 and 26 against aluminum surfaces 16 at either end of the semiconductor body 12 and conventionally brazing the aluminum surface 16 to surfaces 22 and 26 to form a refractory metall aluminum/silicon brazed joint. The aluminum and silicon form a "hard contact" eutectic having a melting point of about 575 C which forms the joint.

Due to the nature of the aluminum and silicon materials involved, it is essential that this joint be formed by brazing in an inert atmosphere and is typically performed in a controlled environment of one atmosphere, or slightly higher, of dry nitrogen, argon or similar inert gas.

Lead members 30 and 32 are thereafter joined to contact members 18 and 20, respectively, through the use of a brazing alloy preforms 38, 40, respectively. The joining surfaces are placed in contact with the preforms which are then heated for about 15 minutes at a temperature of approximately 7000C and are held at that temperature for a period of time sufficient to melt the alloy (generally about 5 mi nutes) and thereafter the molten alloy preforms are allowed to cool and solidity in contact with the joining surfaces of the respective contact members and lead members, such that a unitary structure is formed.This brazing operation may be performed under atmospheric conditions, that is, in a reducing, inert or oxidizing environment; however, care must be taken to insure that the non-inert gases of the atmosphere do not come in contact with the aluminum joining surface 16, which is inevitably heated as part of the heating of the brazing alloy preforms 38, 40. The resulting unitary structure appears as it is illustrated in Figure 2.

After the brazing operations, the exposed surface of the semiconductor body 14 is preferably etched to remove contaminants and a layer of passivating material 42 is applied thereto to prevent recontamination. The passivating material 42 is applied over the exposed surface of semiconductor body 14 and a portion of the contact members 18,20 between the end surfaces 24, 28, respectively, thereof to protect the device from exposure to contamination. Passivating material 42 is typically glass which has been finely ground into a slurry, applied by a conventional technique onto the exposed surface of the semiconductor body 14 and finally heated, in situ, to a temperature sufficient to fuse the passivating material 42. The completed glass passivated rectifier appears as illustrated in Figure 3.It should be noted that for some applications, it is desirable to encapsu late the passivated rectifier in a layer of nonconductive plastic. However, this encapsulation procedure is optional and, therefore, not illustrated.

The method of the present invention and the resulting semiconductor device are illustrated in Figures 4-8. Since the parts of the semiconductor device utilized in the method of the present invention and illustrated in Figures 4-8 correspond closely to the parts of the semiconductor device 10 of the prior art illustrated in Figures 1-3, same are designated with the identical numerals, which have been primed to indicate that same are portions of a device manufactured in accordance with the present invention.

As illustrated in Figure 4, the first step in the manufacture of a semiconductor device 10' by the method of the present invention is to bond refractory metal contact members 18', 20' to semiconductor body 12'. Semiconductor body 12' is a conventional silicon chip which may be identical to semiconductor body 12, discussed above, except that body 12' has no evaporated aluminum surfaces 16 on the ends thereof. Joining of surface 22' of refractory metal contact member 18' to the upper surface of semicon ductor die 12' and of joining surface 26' of refractory metal contact member 20' to the lower surface of semiconductor body 12' takes place through the use of a layer of a binary, non-eutectic alloy, preferably provided in the form of preforms 44 and 46, respectively.

Preforms 44 and 46 are formed of a silvergermanium alloy composed of from 85%-99% silver and 15%-1% germanium, by weight, preferably approximately 95% silver and 5% germanium, by weight. The high temperature brazing technique which is utilized to bond the refractory metal contact members 18' and 20' to semiconductor body 12' is accomplished by placing one side of preform 44 against surface 22' of member 18' and the other surface against the top surface of silicon body 12'.

Similarly, preform 46 is placed against surface 26' of member 20' and the lower surface of silicon body 12'. The assembly is then rapidly heated, over the course of approximately 2.5 minutes, to a temperature within the range of 7500C - 91 0"C and, preferably, to a temperature of approximately 835 C, such that the alloy enters the slush state. However, before the alloy enters into the flow state, the assembly is rapidly quenched to freeze the alloy and create the bonds. After the first step of the manufacturing process, the assembly appears as illustrated in Figure 5.

The second step in the process is to connect the assembly with the thermal and electrical conductive leads 30' and 32', as illustrated in Figure 6. This second step takes place precisely as described above. The bonds between the contact members 18' and 20' and the respective lead members 30' and 32' can be formed either by soldering, butt-welding or, preferably, by high temperature brazing techniques utilizing preforms 38' and 40' composed of copper, silver and phosphorus. The materials and techniques involved in this process are disclosed in detail in Patent No. 3,930,306 referred to above.

In general, the lead members 30', 32' are joined to surfaces 24'-, 28' of contact members 18' and 20', respectively, by interposing preforms 38' and 40', respectively, therebetween and maintaining contact between all the surfaces. The preforms are then heated for about 15 minutes, at least to their wetting point of about 7050C and held at that temperature for a period of time sufficient to melt the alloy, generally about 5 minutes, and thereafter, the molten alloy of the preforms is allowed to cool and solidify in contact with the joining surfaces, such that a unitary structure is perfomed. The brazing operation may be performed under atmospheric conditions, that is, in a reducing or oxidizing environment.However, since there are no aluminum joining surfaces, it is not necessary to insure that the non-inert gases of the atmosphere do not come in contact with the assembly during the process. The resulting structure is illustrated in Figure 7.

After the final brazing operation, the exposed surface of semiconductor body 12' is preferably etched to remove contaminants with a solution of nitric and hydrofluoric acids and a layer of passivating material 42' is applied thereto to prevent recontamination. The passivating material 42' is applied over the exposed surface of semiconductor body 12' and the portion of contact member 18' and contact member 20' to completely encapsulate the semiconductor device 12' and protect it from exposure to contamination. This is illustrated in Figure 8. The passivating material 42' is typically glass which has been finely ground into a slurry, applied by a conventional technique onto the exposed surface of the semiconductor body 12' and finally heated, in situ, to a temperature sufficient to fuse the passivating materials 42'.As an optional feature (not shown) the passivated rectifier may be encapsulated with a coating of plastic material as disclosed in Patent No.

3,996,602 referred to above.

It will now be appreciated that the present invention relates to a method for bonding a refractory metal contact member to a semiconductor body to form a passivated rectifier, or the like. The technique involves the physics of contact adhesion principally in regard to the elastic deformation of materials at high temperatures. To achieve adhesion between the molybdenum slugs and the plated silicon body, a non-eutectic alloy preform of silver and germanium is placed between each semiconductor body surface and the molybdenum slug to be bonded thereto. The assembly is rapidly heated until the alloy enters a slush state and thereafter rapid quenching freezes it in this condition. The high stresses involved in this process create plastic deformation of the contact metals and adhesion occurs when the region of contact enters the slush state and is suddenly frozen.

This method results in strong bonds between the molybdenum slugs and the silicon body. Subsequent high temperature exposure in the 700 C range for lead attachment and glass firing does not adversely affect the brazed joint.

The glass passivated rectifier which is the end product of the above method exhibits higher power handling capability than conventional aluminum alloy glass passivated rectifiers and matches the power capability of more expensive glass passivated rectifiers made with contact metallurgies other than aluminum.

While only a single preferred embodiment of the present invention has been disclosed herein for purposes of illustration, many variations and modifications could be made thereto. It is intended to cover all of these variations and modifications which fall within the scope of the present invention, as defined by the following claims:

Claims (18)

1. A method for bonding a refractory metal contact member to a semiconductor body compris ing the steps of: interposing a layer of silver-germanium alloy between the surfaces of the member and the body; heating the assembly until the alloy enters the slush state; and rapidly quenching the assembly to freeze the alloy.

2. A method for manufacturing a semiconductor device of the type having a semiconductor body with first and second surfaces, first and second refractory metal contact members, each having a first and a second surface, and first and second electrically conductive leads, the method comprising the steps of: interposing a first layer of silver-germanium alloy between the first surface of one of the members and the first surface of the body; interposing a second layer of silver-germanium alloy between the first surfact of the other member and the second surface of the body; heating the assembly until the alloy enters the slush state; rapidly quenching the assembly to freeze the alloy; and bonding the first and second leads to the second surface of each of the members, respectively.

3. A method according to Claim 1 or Claim 1, wherein the alloy is comprised of 85% to 99% silver, and 15% to 1% germanium, by weight.

4. A method according to Claim 3, wherein the alloy is composed of approximately 95% silver and 5% germanium, by weight.

5. A method according to any preceding claim, wherein the step of heating comprises heating the alloy to a temperature above 750 C.

6. A method according to any preceding claim, wherein the step of heating comprises heating the alloy to a temperature below 91 00C.

7. A method according to Claim 6 and Claim 7, wherein the step of heating comprises the heating the alloy to a temperature of approximately 835 C.

8. A method according to any preceding claim wherein the heating step takes place over a period of approximately 2.5 minutes.

9. A method according to any preceding claim, wherein the step of quenching takes place priorto the entrance of the alloy into the flow state.

10. A method according to any preceding claim, wherein the alloy layer is a preform.

11. A method according to Claim 2, or any one of Claims 3 to 10 when appendant thereto, including the additional step of coating the assembly with a passivating layer.

12. A method according to Claim 2 or any one of Claims 3-11 when appendant thereto, wherein the device is a rectifier.

13. A semiconductor device comprising a silicon body having first and second surfaces, first and second refractory metal contact members, each having a first and a second surface, first and second electrically conductive leads, a first silvergermanium alloy layer interposed between the first surface of one of said members and the first surface of said body, a second silver-germanium alloy layer interposed between the first surface of the other of said members and the second surface of said body, and means for joining each of said leads to the second surface of each of said members, respectively.

14. A device according to Claim 13, wherein each of said layers is a preform.

15. A device according to Claim 13 or Claim 14, wherein the alloy is comprised of 85% to 99% silver and 15% to 1% germanium, by weight.

16. A device according to Claim 15, wherein the alloy is composed of approximately 95% silver and 5% germanium, by weight.

17. A device according to any one of Claims 13-16 and further comprising a passivating layer surrounding said device.

18. A device according to any one of Claims 13-17, wherein the device is a rectifier.