Patent Application
20110042815
(1) Field of the Invention
The present invention relates to a semiconductor device and an on-vehicle AC generator.
(2) Description of the Related Art
In power electronics products, as shown in
Newly developed lead-free solders include Sn-0.7Cu, Sn-3.5Ag, Sn-3Ag-0.5Cu, among others, which are widely used for implementing electronic parts onto printed boards. When a semiconductor element having the Ni-based metallization is bonded by using these Sn-based solders, the Ni-based metallization is consumed by the reaction between solder and the Ni-based metallization. In particular, when bonding is carried out under severe conditions such as in the assembly of power semiconductor devices, the Ni-based metallization of the semiconductor element completely disappears.
Pb-free solder materials containing no Pb and having a melting point as high as that of a high-Pb solder include Au-based materials such as Au-20Sn (eutectic, 280° C.), Au-12Ge (eutectic, 356° C.) and Au-3.15Si (eutectic, 363° C.), but they are extremely expensive. Au-20Sn, which has a relatively low Au content, has the disadvantage that it cannot provide sufficient stress buffering in bonding a large area since it is a hard solder and the semiconductor element is easily damaged.
Other Pb-free solder materials include Sn-based medium-temperature solders having a melting point of 200° C. or higher such as Sn-3Ag-0.5Cu. They are widely used for implementing parts on a substrate, and have good bonding reliability at 150° C. or lower. However, when they are retained in use under circumstances of 200° C. or higher for a long period of time, interface reactions proceed at the bonding interface, and bonding reliability is disadvantageously lowered due to the formation of voids, the growth of the intermetallic compound layer and for other causes.
To deal with this problem, for example, Japanese Patent No. 3152945 discloses a technique for suppressing interface reactions of Sn-based solder. Japanese Patent No. 3152945 discloses “a lead-free solder alloy comprising 0.1 to 2% by weight of Cu, 0.002 to 1% by weight of Ni, and the remainder of Sn”. Japanese Patent No. 3152945 reports that the consumption of Cu in the bonded material can be suppressed by adding Cu, and at the same time the growth of the intermetallic compound such as Cu6Sn5 and Cu3Sn at the bonding interface can be suppressed by adding Ni. Moreover, Japanese Unexamined Patent Publication No. 2002-280417 discloses “a semiconductor device having a solder bump comprising an alloy solder on an adhesion layer containing a first metal formed at least on a wiring layer, an intermetallic compound containing a metal which is a main component of the alloy solder and a second metal which is different from the metal being formed between the solder bump and the adhesion layer”.
However, prior art inventions have the problems mentioned below, and they do not have sufficient suppression on interface reactions, and have low bonding reliability. In particular, it was found that suppression of interface reactions in a semiconductor device for on-vehicle AC generators (alternator) used at high temperatures by the prior art is difficult.
That is, in case of the above Japanese Patent No. 3152945, slight suppression of interface reactions can be expected by adding Ni, but interface reactions proceed at a high temperature of 200° C. or higher since the Cu6Sn5 and Cu3Sn compounds are always in contact with Cu and the Sn-based solder. Accordingly, the growth of the Cu—Sn compound continues and voids and other problems are generated at the interface. This results in lowered bonding reliability.
Meanwhile, in case of Japanese Unexamined Patent Publication No. 2002-280417 mentioned above, the intermetallic compound formed closest to the solder becomes a barrier layer between the Sn-based solder and the metal layer, and therefore great effect in suppressing interface reactions can be supposedly obtained. However, it is necessary to provide two layers: a first metal layer and a second metal layer, in advance on the bonded material, entailing the problems that the number of plating steps is increased; costs are increased by carrying out selective local plating; and formation of metal layers is difficult in case of a structure which prevents formation of electrodes. Moreover, the metal layer formed on the outermost surface of the bonding face needs to be reacted with Sn-based solder in bonding to provide a barrier layer. Therefore, when the metal layer formed on the outermost surface is thick, the unreacted metal layer on the outermost surface remains in bonding, which may create the problems that the effect of the barrier layer cannot be sufficiently obtained, and that adjustment of the process such as extending the bonding time to completely allow the metal layer on the outermost surface to react need to be made. On the other hand, when the metal layer on the outermost surface is thin, the barrier layer for suppressing interface reactions becomes thin, and therefore interface reactions may not be sufficiently suppressed at a high temperature of 200° C. or higher. When unreacted portions of the layer formed on the outermost layer of the bonding faces in reactions with the Sn-based solder (e.g., Cu layer) are remaining exposed and, oxidation and corrosion disadvantageously occur from the exposed portions. In contrast, when one tries to locally provide the outermost layer of the bonding faces by local plating or other means in order to avoid the remaining of the outermost layer of the bonding faces, the Sn-based solder may migrate into the metal layer (e.g., Ni layer) lying therebelow this time. In this case, an intermetallic compound (e.g., Ni—Sn compound) is formed between these layers, and interface reactions may proceed in this portion, possibly producing voids due to a change in volume.
An object of the present invention is to provide an environmental friendly bonding material of a semiconductor element at low costs, which can maintain bonding reliability even if it is used at a temperature as high as 200° C. or higher for a long period of time, and to provide a semiconductor device and an on-vehicle AC generator using the bonding material.
Among the inventions disclosed in the present application for achieving the above object, a summary of a typical one will be described as follows:
(1) A semiconductor device having a semiconductor element, a supporting electrode body bonded to a first face of the semiconductor element via a first bonding member, and a lead electrode body bonded to a second face of the semiconductor element supported by the supporting electrode body via a second bonding member, the semiconductor device having a Ni-based plating layer and an intermetallic compound layer containing at least one of Cu6Sn5 and (Cu,Ni)6Sn55 compounds at an interface between the supporting electrode body and the first bonding member, and having a Ni-based plating layer and an intermetallic compound layer containing at least one of Cu6Sn5 and (Cu,Ni)6Sn5 intermetallic compounds at an interface between the lead electrode body and the second bonding member, and the intermetallic compound layer having a mean particle diameter of 4.8 μm or larger.
Embodiments of the present invention will be described in detail with reference to the following drawings, wherein:
According to the present invention, an environmental friendly semiconductor device having heat resistance of 200° C. or higher can be provided.
To begin with, the bonding material and bonding mechanism of the present invention will be described with reference to
An example of the bonding material of the present invention an Sn-based solder foil 17 containing a phase 10 of a Cu—Sn compound (e.g., Cu6Sn5) at a temperature from room temperature to 200° C. By bonding bonded materials 12 on which a Ni-based plating 11 is formed using this solder foil 17, Cu6Sn5 phases 10 floating in the solder foil 17 as phases deposit or move onto the Ni-based platings 11, so that compound layers 10 mainly composed of a Cu—Sn compound (Cu6Sn5 phase) are formed. Herein, when the compound layers are formed, the Ni platings 11 are partly fused to form a (Cu,Nu)6Sn5 compound in some cases. In this case, the compound layers 10 become intermetallic compound layers containing at least one of the Cu6Sn5 compound and (Cu,Ni)Sn6 compound. As a result, as shown in
However, the particle diameter of the intermetallic compound layer containing at least one of the Cu6Sn5 compound and (Cu,Ni)Sn6 compound depends on the underlayer, i.e., the Ni-based metallization, and therefore becomes minute.
According to the bonding mechanism of this embodiment, the bonded material need only be provided with at least one layer of Ni plating such as Ni, Ni—P and Ni—B, thereby enabling bonding with less steps. Moreover, according to the bonding mechanism of the present invention, the thickness of the barrier layer formed depends on the amount of the Cu—Sn compound phase contained in the solder foil, whereby the thickness of the barrier can be adjusted by increasing or decreasing the amount of the Cu—Sn compound. Furthermore, as shown in
Herein, as the bonding material of the present invention, the conditions under which that the Cu—Sn compound is contained as phases and the Sn-based solder contains Cu6Sn5 at a temperature from room temperature to 200° C. will be described with reference to
In the composition containing less Cu than Sn-0.9Cu, when the solder is fused and solidified, Sn, which is contained in an amount higher than that of the eutectic composition, is first deposited as a primary phase, and finally Sn and Cu6Sn5 are solidified as a eutectic structure. At that time, since Cu6Sn5 is deposited in a state of being dispersed at the grain boundary and the like inside the bonding portion, it is not deposited on the Ni-based plating in the form of a barrier layer. Accordingly, heat resistance cannot be obtained. In contrast, in the composition containing Cu in an amount higher than Sn-0.9Cu, when the solder is fused and solidified, the Cu6Sn5 phase is first deposited. At this time, since Cu6Sn5 is deposited preferentially on the Ni-based plating, the barrier layer of the Cu—Sn compound is formed. Finally, Sn and Cu6Sn5 are then solidified as the eutectic structure. The barrier layer of the Cu—Sn compound is formed by the mechanism as mentioned above.
That is, the composition containing the Cu6Sn5 phase in an amount higher than the eutectic composition may be selected as the bonding material of the present invention. In the Sn—Cu two-phase system, Cu need only be contained in an amount of 0.9 wt. % or higher, but the eutectic composition varies depending on the alloy system when other elements are contained. Therefore, in either case, a bonding material having the composition containing the Cu6Sn5 phase in an amount higher than that in the eutectic composition may be selected. In case of Sn-3Ag-0.5Cu and Sn-0.7Cu normally used in this composition, the amount of Cu6Sn5 phase is lower than that in the eutectic composition, and therefore no barrier layer is formed on the Ni-based plating.
Although the bonding material of the present invention and its bonding mechanism have been described above, the form of providing of the bonding material is not critical on foil, and as shown in
Since the Sn-based solder containing the Cu6Sn5 phase at a temperature from room temperature to 200° C. has good wettability, the composition having a liquid phase linear temperature which is preferably the bonding temperature or lower may be selected.
When the mean particle diameter is smaller than 4.8 μm, the (Cu,Ni)6Sn5 compound containing a large amount of Ni largely occupies the intermetallic compound layer. Therefore, even if the intermetallic compound layer containing at least one of the (Cu,Ni)6Sn5 compounds exists on the Ni-based metallization in bonding, Ni likely diffuses through the intermetallic compound, and the disappearance of the Ni-based metallization in bonding the solder cannot be sufficiently suppressed. Moreover, when the crystal grains are minute, the proportion of the boundaries of crystal grains increases. Since the rate of diffusion is higher at the boundaries of crystal grains than in the grains, the more minute the crystal grains, the more likely Ni diffuses.
On the other hand, in
Moreover, it is desirable that the intermetallic compound layer containing at least one of Cu6Sn5 and (Cu,Ni)6Sn5 compounds having a mean crystal particle diameter of 4.8 μm or larger does not contain Cu3Sn. In case where Cu3Sn is present in the intermetallic compound layer, when heat is generated when the power semiconductor device is energized, or when the semiconductor device is used under high heat circumstances of 150° C. or higher, Cu3Sn is transformed into Cu6Sn5 or (Cu,Ni)6Sn5. Therefore, Kirkendall voids and voids associated with the change in volume are produced in the vicinity of the bonding interface, whereby bonding reliability cannot be obtained. Due to the reaction between the Ni-based metallization and the Sn-based solder, forming the intermetallic compound layer containing at least one of the Cu6Sn5 and (Cu,Ni)6Sn5 compounds on the Ni-based metallization very likely causes unreacted Cu and Cu3Sn compounds to remain locally.
It is desirable that the amount of Cu contained in the intermetallic compound and the solder portion in total is 4 mass % or higher. In case of the bonding portion having the intermetallic compound with a mean crystal particle diameter smaller than 4.8 μm and the amount of Cu contained in the intermetallic compound and the solder portion in total lower than 4 mass %, the disappearance of the Ni-based metallization of the semiconductor element in bonding cannot be suppressed.
Next, an embodiment of a semiconductor device using the bonding material of the present invention and a method for manufacturing the same will be described with reference to
The semiconductor device shown in
By conducting bonding using the bonding material of the present invention, reactions at the interface can be suppressed even during use at high temperatures, thereby providing a semiconductor device having bonding reliability. Although other materials can be partially used without using the bonding material of the present invention in all the bonding portions, it is preferable that the bonding material of the present invention is used in all the bonding portions from the perspective of bonding reliability. At this time, any material can be used as long as it is the bonding material having the composition containing the Cu6Sn5 phase in an amount higher than that in the eutectic composition and/or the (Cu,Ni)Sn6 compound, and it may be different from each other in the bonding portions.
Herein, any one of Al, Mg, Ag, Zn, Cu and Ni can be used as the coefficient of thermal expansion difference buffer 9. These are metals with small yield stress, and are easily deformed by inertia. To this end, by applying these metals to the bonding portions, the stress generated in the bonding portions by the coefficient of thermal expansion difference in the bonded material during cooling after being bonded and during heat cycle can be buffered. At this time, as shown in
As the coefficient of thermal expansion difference buffer 9, any one of Cu/invar alloy/Cu composite material, Cu/Cu2O composite material Cu—Mo alloy, Ti, Mo and W can be used. Due to this coefficient of thermal expansion difference buffer 9, the stress caused by the coefficient of thermal expansion difference between the semiconductor element and the Cu electrode generated in bonding during heat cycle and during cooling after being bonded can be buffered. At this time, when the thickness is too small, the stress cannot be sufficiently buffered, and cracks may be generated in the semiconductor element and intermetallic compound. Therefore, the thickness is preferably 30 μm or more.
Since the Sn-based solder has a thermal conductivity higher than a high-lead solder, the resistance of the semiconductor device can be lowered and its heat radiation can be increased. As in
As the Ni-based plating to be provided on the bonded materials, Ni, Ni—P, Ni—B and the like may be used as mentioned above, and Au plating, Ag plating and Pd plating may be further provided on the platings. This can improve wettability. In that case, the plating layers such as Au and Ag are all diffused within the solder during bonding, whereby the barrier layer of the Cu—Sn compound can be formed on the Ni-based plating of the underlayer. Moreover, at least one metallization layer of Ti, Pt, Cr and V may be provided beneath the Ni-based metallization layer. Even when at least one metallization layer of Ti, Pt, Cr and V is provided beneath the Ni-based metallization layer, providing the Ni-based metallization layer thereon forms the intermetallic compound layer having at least one of the stable Cu6Sn5 and (Cu,Ni)6Sn5 compounds at the bonding interface.
Next, the method for manufacturing the semiconductor device will be described. The components and bonding members are layered as shown in
Another form of a method for manufacturing a power semiconductor module using the bonding material of the present invention will be now described with reference to
Using a fixture which prevents a shift in position as shown in
The state of the Ni-based metallization remaining in the semiconductor element after this semiconductor device was assembled was examined by observing its cross section and by nondestructive testing with ultrasound wave. The results are shown in Table 1. The case where 90% or more of Sn and the unreacted Ni-based metallization is remaining relative to the area of the bonding portions is indicated by o, while the case where less than 90% is remaining is indicated by x. In all of Examples 1 to 16, it was confirmed that 90% or more of the Ni-based metallization was remaining relative to the area of the bonding portion.
Although the process of producing the overall structure is divided in two separate processes: bonding the semiconductor device and the substrate; and bonding the substrate and the supporting member in the description above, the bonding can be conducted by one process after the semiconductor element, solder foils, substrate, solder foil and supporting member are laminated.
The bonding structure is the same as in Examples 1-16. The state of the Ni-based metallization remaining in the semiconductor element after this semiconductor device was assembled was examined by observing its cross section and by nondestructive testing with ultrasound wave. The results are shown in Table 1. The case where 90% or more of Sn and the unreacted Ni-based metallization was remaining relative to the area of the bonding portions is indicated by o, while the case where less than 90% was remaining was indicated by x. In all of Comparative Examples 1-7, the proportions of the areas of the bonding portions of the Ni-based metallization remaining in the semiconductor element were less than 90%. The areas of the Ni-based metallization remaining in all the materials were about 20%.
Although the invention made by the inventors of the present invention has been specifically described above with reference to embodiments, the present invention is not limited to the above embodiments, and it would be obvious that various changes can be made without departing from the spirit of the invention.
That is, although the application of the present invention is described by taking a semiconductor element of a power module as an example in the above, applicable semiconductor devices are not necessarily limited to alternators, and it can be applied to all bonding using a Sn-based solder as well.
Another form of a semiconductor device using the bonding material of the present invention will be now described with reference to
When bonding is carried out using a substrate with the semiconductor element mounted thereon as a base, the bonding portions of the semiconductor element are re-fused. The disappearance of the Ni-based metallization can be suppressed by similar effects also at this time.
Although the surface mount device 108, chip part 103 and through-hole mount device 111 are all implemented in
Another form of a semiconductor device using the bonding material of the present invention will be now described with reference to
The semiconductor device shown in
Another form of the semiconductor device will be described with reference to
The semiconductor device shown in
Although some embodiments of the semiconductor device have been described above, the present invention is not limited to these forms, and it would be obvious that various changes may be made without departing from the spirit of the invention. For example, it may be used for front-end modules such as power transistors, power ICs, IGBT substrates and RF modules, die bonding of power modules for automobiles, among others. Moreover, the bonding material of the present invention used for bonding may be provided in any form as long as it is a Sn-based solder with the composition containing the Cu6Sn5 phase in an amount higher than that in the eutectic composition, and may be provided in the leveling process of printed boards, dipping to parts, printing, and as foils and wires, among others.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-192720 | Aug 2009 | JP | national |
