Semiconductor device

A semiconductor device having a plurality of semiconductor chips mounted on lead frames is miniaturized by reducing its planar size and thickness. By disposing a rear surface of a first island and a top surface of a second island so as to at least partially overlap each other, a first semiconductor chip on the first island and a second semiconductor chip on a rear surface of the second island are configured so as to overlap each other. Accordingly, a planar occupied area can be set smaller than planar areas of both of the chips. Moreover, thin metal wires to be connected to the second semiconductor chip are extended to a back side. Consequently, a thickness of a semiconductor device can also be reduced.

BACKGROUND OF THE INVENTION

This invention claims priority from Japanese Patent Application Number JP 2007-050583 filed on Feb. 28, 2007, the content of which is incorporated herein by reference in its entirety.

1. Field of the Invention

The present invention relates to a semiconductor device.

2. Description of the Related Art

A semiconductor device using a lead frame generally includes an island, a plurality of leads whose one ends are disposed around the island, and a semiconductor chip provided on the island. Moreover, a bonding pad of the semiconductor chip and the one ends of the leads are connected to each other by use of thin metal wires. This technology is described for instance in Japanese Patent Application Publication No. 2007-5569. Furthermore, the island, the plurality of leads, the semiconductor chip and the plurality of thin metal wires are sealed by an insulating resin so as to expose the other ends of the plurality of leads. Here, in the plurality of leads, portions sealed by the insulating resin are called inner leads, and portions exposed from the insulating resin are called outer leads. By bending the outer leads according to need, the other ends of the leads are mounted on a printed board or the like by use of solder and the like.

Moreover, a stack type semiconductor device is also realized, which is formed by stacking a plurality of chips on an island. In this semiconductor device, a child chip smaller than a parent chip is stacked on the parent chip, and both of the parent chip and the child chip are electrically connected by use of thin metal wires.

The semiconductor device as described above can be miniaturized by a recent high technology. However, a surface of the child chip is disposed at a position higher than that of a surface of the parent chip from a surface of the island. Accordingly, when the child chip is further connected by use of the thin metal wire, a top of the thin metal wire is positioned higher. Consequently, there is a problem that a thickness of the semiconductor device, in other words, a thickness of a package is increased.

SUMMARY OF THE INVENTION

For this reason, it is an object of the present invention to realize a semiconductor device which can mount a plurality of chips thereon without increasing a thickness of a package.

The present invention relates to a semiconductor device into which a first semiconductor chip and a second semiconductor chip are built, both of the chips being disposed so as to be stacked with each other. The first semiconductor chip is fixed on an upper surface of a first island whereas the second semiconductor chip is fixed on a lower surface of a second island. In addition, the first island and the second island are positioned between the first semiconductor chip and the second semiconductor chip, and are disposed so as to be separated from each other.

DESCRIPTION OF THE EMBODIMENTS

With reference to the drawings, embodiments of the present invention will be described.FIG. 1is a cross-sectional view of a semiconductor device.FIG. 2is a plan view of the semiconductor device. Moreover,FIGS. 3 and 4are exploded views of the semiconductor device.

First, before the semiconductor device in each of preferred embodiments of the present invention shown inFIGS. 1 and 2will be explained, description will be given with reference toFIGS. 3 and 4. In the embodiments, the semiconductor device, to be brief, has a configuration in which a first lead frame10shown inFIG. 3and a second lead frame11shown inFIG. 4are disposed so as to overlap with each other. For example, a first island12in the first lead frame10and a second island13in the second lead frame11at least partially overlap each other. Moreover, to put it another way, a first lead14in the first lead frame10and a second lead15in the second lead frame11at least partially overlap each other.

Now, with reference toFIG. 3, the first lead frame10will be specifically described. First, the first lead frame10includes: the first island12; at least one lead16(here, two) which has one end integrated with the first island12and has the other end extended to the left; and at least one first lead14(here, two) which is positioned near the first island12, has one end set close to a right side of the island and has the other end extended to the right. Here, the leads are extended on both sides of the first island12, two on each side. Moreover, the one end of the first lead14is processed to have a rectangular shape and is used as a first bonding part21.

Furthermore, a first semiconductor chip17is provided on a surface of the first island12. Moreover, electrodes (portions indicated by shaded circles inFIG. 3) formed on an upper surface of the first semiconductor chip17and the first bonding parts21of the first leads14are electrically connected to each other. Although, here, thin metal wires18are adopted, metal plates or the like may be used for the electrical connection.

Meanwhile, with reference toFIG. 4, the second lead frame11will be described. A basic shape of the second lead frame11is the same as that of the first lead frame10. The second lead frame11includes: the second island13; at least one lead19which has one end integrated with the second island13and has the other end extended to the left; and at least one second lead15which is positioned near the second island13, has one end set close to a right side of the island and has the other end extended to the right. Here, the leads are extended on both sides of the second island13, two on each side. Furthermore, a second semiconductor chip20is provided on a rear surface of the second island13. Moreover, electrodes (portions indicated by shaded circles inFIG. 4) on the second semiconductor chip20and second bonding parts22of the second leads15are electrically connected to each other. Although, here, thin metal wires18are adopted, metal plates or the like may be used for the electrical connection. Moreover, the one end of the second lead15is processed to have a rectangular shape and is used as the second bonding part22.

First Embodiment

Now, with reference toFIGS. 1 and 2, a first embodiment will be specifically described. A semiconductor device of this embodiment has a structure in which the first and second lead frames10and11described with reference toFIGS. 3 and 4partially overlap each other. Specifically, an island positioned on an upper side inFIG. 1corresponds to the first island12shown inFIG. 3, and an island positioned on a lower side inFIG. 1corresponds to the second island13shown inFIG. 4. Here, since electrical insulation between the first and second islands12and13is required, the islands are separated from each other by a distance L.

This separation distance L is required to be at least about 50 μm. A conventional semiconductor device has a structure in which the lead frames shown inFIGS. 3 and 4are disposed side by side as they are. Accordingly, an area obtained by directly adding areas of the first and second islands12and13is required for mounting. On the other hand, in the first embodiment, as shown inFIG. 2, a rear surface of the first island12and a top surface of the second island13at least partially overlap each other. For this reason, an area occupied by the islands from a planar point of view can also be reduced.

Meanwhile, a rear surface of the first lead14and a top surface of the second lead15are separated from each other by at least about 50 μm, as in the case of the islands. Moreover, with reference toFIGS. 1 and 2, the first and second leads14and15are formed as described below. Specifically, a first slope part30is provided obliquely downward from the first bonding part21, and a first connection part31is horizontally provided at a tip of the slope part30. Moreover, a second slope part32is provided obliquely downward from the second bonding part22, and a second connection part33is horizontally provided at a tip of the slope part32.

Moreover, the leads16and19are similarly formed. Specifically, a slope part34is provided obliquely downward from the lead integrated with the first island12, and a connection part35is provided at a tip of the slope part34. Furthermore, a slope part36is provided obliquely downward from the lead19integrated with the second island13, and a connection part37is provided at a tip of the slope part36.

As described above, in the first embodiment, the first and second islands12and13are separated from each other by the distance L. Moreover, the first and second bonding parts21and22are provided so as to be separated from each other by the distance L or by a distance near the separation distance. Furthermore, the slope parts and connection parts extended from the bonding parts and the slope parts and connection parts extended to the left from the first and second islands12and13may be disposed so as not to come into contact with each other.

In addition, the first and second lead frames10and11are sealed by an insulating resin40. Moreover, rear surfaces of the connection parts31,33,35and37are exposed on a rear surface of the insulating resin40.

As described above, the rear surface of the first island12and the top surface of the second island13are vertically separated from each other by L. Moreover, the first and second islands12and13at least partially overlap each other. For this reason, the area occupied by the islands from a planar point of view can be reduced. In addition, the slope part in the second island13is extended obliquely downward, and the second semiconductor chip20and thin metal wires can be disposed on the extended portion. Consequently, it is no longer required to additionally increase a thickness corresponding thereto. As a result, a thickness of the semiconductor device can also be reduced.

Furthermore, as referred toFIG. 1, a distance L11is set longer than the length obtained by adding the thickness of the first island12and the thickness of the second island13, the distance L11being the distance between the lower surface of the first semiconductor chip17and the upper surface of the second semiconductor chip20.

More specifically, the length obtained by adding the thickness of the first island12(L12) and the thickness of the second island13(L13) is, for example, about 1 mm. Accordingly, the distance L11is set longer than 1.05 mm, the distance L11being the distance between the lower surface of the first semiconductor chip17and the upper surface of the second semiconductor chip20. With this setting, it is possible to prevent short-circuiting between the semiconductor chips via the islands therebetween even if a plurality of semiconductor chips mounted on the islands are built into a small space of a semiconductor device.

Second Embodiment

Subsequently, a second embodiment will be described. In the previous embodiment, the description was given of only overlapping of the islands. Meanwhile, in this embodiment, first bonding parts21and second bonding parts22also overlap each other.

Since the bonding part (bonding post) is generally connected by use of a thin metal wire, a head of a bonding tool has to come into contact therewith. Accordingly, at least a predetermined distance is required between two adjacent bonding parts. However, in a semiconductor device of this embodiment, for example, the first bonding part21and the second bonding part22adjacent thereto are in a front-and-back relationship, so that, here, a distance between the first and second bonding parts can be set shorter than the predetermined distance. Consequently, the first and second bonding parts21and22can at least partially overlap each other. In this embodiment, each of second leads15is disposed around a center portion between first leads14.

The second embodiment will be specifically described with reference toFIG. 3. A width of the first lead14is about 0.1 mm, and a size (width) of the first bonding part21is 0.2 to 0.25 mm. Accordingly, for example, when a width of one side of the first bonding part21is 0.25 mm, an end of the first bonding part21protrudes by 0.125 mm from a center of the first lead14, in other words, protrudes by 0.075 mm from upper and lower sides of the first lead14. Moreover, a distance between the first leads14is 0.25 mm, and a distance between the lower side of the first bonding part21and an upper side of a bonding part adjacent thereto is 0.1 mm. The same goes forFIG. 4.

For this reason, back toFIG. 1, the lead frames shown inFIGS. 3 and 4are disposed so as to partially overlap each other as described below. Specifically, the second lead15is disposed between the two first leads14. Accordingly, the first and second bonding parts21and22are disposed so as to partially overlap each other.

As described above, the second embodiment is different from the previous embodiment only in the point that the bonding parts overlap each other. Since the second embodiment is the same as the first embodiment in the other points, description thereof will be omitted below.

Third Embodiment

With reference toFIGS. 5 to 7, this embodiment will be described below.FIG. 5is a plan view showing a semiconductor device of this embodiment.FIG. 6is a plan view showing a first lead frame extracted from the semiconductor device.FIG. 7is a plan view showing a second lead frame extracted from the semiconductor device.

In the embodiments described above, the built-in elements are discrete type transistors. Meanwhile, in this embodiment, an IC having a number of electrodes formed therein is built into the semiconductor device. Accordingly, in this embodiment, lead frames to be used are IC lead frames. Specifically, on the lead frames shown inFIGS. 6 and 7, semiconductor chips having a number of bonding pads thereon are mounted. Accordingly, a number of leads corresponding to the number of the bonding pads are included in the lead frames.

With reference toFIG. 5, the lead frames having a number of leads are laminated. As in the case of the previous embodiment, a first semiconductor chip17is mounted on a surface of a first island12and is connected to first leads14by use of thin metal wires. Moreover, a second semiconductor chip20is mounted on a rear surface of a second island13and is electrically connected to second leads15by use of thin metal wires. Each of the second leads15is disposed between the first leads14. Moreover, parts denoted by reference numerals55and57are suspension leads extended to the outside so as to be continuous with the respective islands. The suspension leads have a function of mechanically supporting the respective islands in the middle of manufacturing steps.

Moreover, here, an insulating resin40is provided but is not shown inFIG. 5.

As is clear fromFIG. 5, a surface of the first island12and a rear surface of the second island13at least partially overlap each other, so that a planar area occupied by the islands can be reduced.

Furthermore, connection points of the leads to the thin metal wires are alternately positioned between a front and a back, so that as in the case of the previous embodiment, the leads can partially overlap each other.

Although not shown in the drawings, slope parts and bonding parts are extended obliquely downward in the respective leads, so that the second semiconductor chip20and the thin metal wires connected thereto are provided in a space defined by those parts. Accordingly, a thickness can also be reduced.

Here, inFIG. 5, the two suspension leads extended integrally from both of the islands are extended to the outside from the respective sides (left sides on the page space). Here, the suspension lead extended to the outside from one of the islands may be provided on the left side and the suspension lead extended to the outside from the other island may be provided on the right side.

Fourth Embodiment

With reference toFIGS. 8A to 8C, a configuration of a semiconductor device50of this embodiment will be described.FIG. 8Ais a cross-sectional view of the semiconductor device50.FIG. 8Bis a plan view of the semiconductor device50shown inFIG. 8Awhen seen from above.FIG. 8Cis a cross-sectional view taken along the line C-C′ inFIG. 8B. In this embodiment, the same parts as those in the other embodiments described above are denoted by the same reference numerals, and description thereof will be omitted.

The configuration of the semiconductor device50is basically the same as that of the semiconductor device shown inFIG. 1. The semiconductor device50is different from that shown inFIG. 1in that a first island12and a second island13are disposed at planarly different positions (seeFIG. 8B). In other words, the first and second islands12and13are not disposed so as to planarly overlap each other. Moreover, with reference toFIG. 8C, the first and second islands12and13are shifted from each other in a thickness direction and disposed so as to at least partially overlap each other. Here, the disposition of the islands at the planarly different positions means that the islands do not planarly overlap each other in the disposition shown inFIG. 8B. Furthermore, overlapping in the thickness direction means that the islands overlap each other in a vertical direction on the page space with reference toFIG. 8C.

Moreover, a positional relationship in the thickness direction between the first and second islands12and13will be described. Both of the islands are not disposed on the same plane. Accordingly, with reference toFIG. 8C, an upper surface of the first island12and a lower surface of the second island13protrude in the thickness direction.

With reference toFIG. 8A, the configuration of the semiconductor device50will be described. The semiconductor device50is formed by resin-sealing a plurality of semiconductor chips. In terms of its external appearance, the semiconductor device50has an approximately cubic shape or an approximately rectangular parallelepiped shape. Moreover, upper and lower surfaces thereof are flat surfaces parallel to each other, and side faces thereof are tilted surfaces, each having its upper part tilted inward than its lower part. Furthermore, a connection part31formed of an end of a lead electrically connected to built-in semiconductor chips is extended to the outside from the lower part of a side face of an insulating resin40which integrally seals the entire device. Moreover, a lower surface of the connection part31and a lower surface of the insulating resin40are positioned on the same plane. The semiconductor device50can be packaged by a reflow step of heating and melting solder cream applied to the connection part31.

As to a specific structure of the semiconductor device50, first, a plurality of semiconductor elements (a first semiconductor chip17and a second semiconductor chip20) are built into the device so as to overlap each other. With reference toFIGS. 8A and 8B, the first semiconductor chip17is fixed to the upper surface of the first island12, and the second semiconductor chip20is fixed to the lower surface of the second island13. For this reason, the first and second semiconductor chips17and20are disposed so as to face opposite directions in the thickness direction of the semiconductor device50.

With reference toFIG. 8A, the first semiconductor chip17fixed to the upper surface of the first island12is connected to an upper surface of a bonding part14A of a first lead14through a thin metal wire58. Furthermore, the second semiconductor chip20fixed to the lower surface of the second island13is connected to a lower surface of a bonding part of a second lead15through a thin metal wire56. Here, the first and second leads14and15may be disposed so as to partially overlap each other. Accordingly, a planar area of the semiconductor device50can be reduced.

With reference toFIG. 8B, the first semiconductor chip provided on the upper surface of the first island12is disposed not only on the first island12but also over the second island13adjacent thereto. Accordingly, without being limited by a size of the first island12, a large first semiconductor chip17can be disposed. Here, the first semiconductor chip17has a rectangular shape having long sides in a lateral direction (a direction in which the islands are aligned). Furthermore, the same goes for the second semiconductor chip20. The second semiconductor chip20fixed to the lower surface of the second island13is disposed so as to overlap the second island13as well as the first island12. Here, both of the semiconductor chips do not have to be disposed over the two islands. Only either one of the semiconductor chips may be disposed over both of the islands.

Furthermore, with reference toFIG. 8A, the first lead14includes: the bonding part14A having the upper surface connected to the thin metal wire58; a connection part52which is continuous with the bonding part14A and is tilted outward and downward; and the connection part31which is exposed to the outside from the insulating resin40and has the lower surface positioned on the same plane as the lower surface of the insulating resin40. Although the second lead15also has the same configuration, the thin metal wire is connected to the lower surface of the bonding part in the second lead15.

In addition, with reference toFIG. 8B, two leads54extended to the outside from the first island12are provided, and another two leads54extended to the outside from the second island13are also provided. On the page space, the leads54extended from the upper side of the insulating resin40and the first and second leads14and15extended to the outside from the lower side thereof are arranged in a vertically symmetrical manner. Accordingly, a packaging property of the semiconductor device50in the reflow step can be improved.

With reference toFIG. 8C, the first and second islands12and13are disposed so as to be shifted from each other in the thickness direction and to partially overlap each other in the thickness direction. Here, the first island12is disposed slightly closer to the upper side within the insulating resin40. Meanwhile, the second island13is disposed closer to the lower side compared with the first island12. Here, both of the islands may partially overlap each other or may not at all overlap each other.

Here, as an example, a thickness of each of the first and second islands12and13is, for example, about 0.5 mm. In addition, a thickness L1of overlapping thereof is smaller than the thickness of the islands and is, for example, about 0.2 mm. Moreover, a distance L2between the lower surface of the first island12and the upper surface of the second semiconductor chip20is about 0.3 mm. Furthermore, a distance L3between the lower surface of the first semiconductor chip17and the upper surface of the second island13is about 0.3 mm.

By allowing the first and second islands12and13to partially overlap each other in the thickness direction, the thickness of the semiconductor device50can be reduced, and insulation between the islands and the semiconductor chips can be secured. For example, compared with the case where two semiconductor chips are mounted on upper and lower principal surfaces of one island, the thickness of the semiconductor device50is reduced by the length (L1) of overlapping between the islands in the thickness direction.

Moreover, the first semiconductor chip17is disposed on the upper surface of the first island12, and the rear surface and side faces thereof are covered with the insulating resin40which seals the entire device. Furthermore, the second semiconductor chip20is fixed to the lower surface of the second island13, and the upper surface and side faces thereof are covered with the insulating resin40. In addition, by disposing the first and second islands12and13so as to be shifted from each other, the first island12and the second semiconductor chip20are separated from each other, and the second island13and the first semiconductor chip17are separated from each other. Consequently, it is possible to prevent short-circuiting between the semiconductor chips and the islands even if relatively large semiconductor chips are stacked and built into a small semiconductor device50.

Furthermore, in this embodiment, a space between the lower surface of the first island12and the upper surface of the second semiconductor chip20and a space between the upper surface of the second island13and the lower surface of the first semiconductor chip17are filled with the insulating resin40. Here, if it is difficult to fill the spaces with the insulating resin40containing fillers, the spaces may be filled with a resin material having an excellent fluidity (for example, a resin having a relatively small filler content) prior to a resin sealing step. Moreover, the spaces may be filled with an adhesive made of an epoxy resin and the like.

With reference toFIG. 8A, the first semiconductor chip17is, for example, a MOSFET. A gate electrode and a source electrode are provided on the upper surface thereof, and the rear surface thereof is set to be a drain electrode. Moreover, the electrodes (the gate electrode and the source electrode) on the upper surface are individually connected to the first leads14through the thin metal wires58, and the rear surface is fixed to the upper surface of the first island12. Meanwhile, the second semiconductor chip20is, for example, a MOSFET. A drain electrode on the upper surface thereof on the page space is fixed to the lower surface of the second island13. Furthermore, electrodes on the lower surface of the second semiconductor chip20are connected to the lower surfaces of the bonding parts of the second leads15through the thin metal wires56. Moreover, if the rear surfaces of the semiconductor chips transmit currents therethrough, the semiconductor chips are fixed to the principal surfaces of the islands, on which the respective semiconductor chips are mounted, by use of a conductive adhesive or by eutectic bonding. In addition, if the rear surfaces of the semiconductor chips are not required to be conductive, the semiconductor chips may be mounted on the islands by use of an insulating adhesive.

With reference toFIG. 8B, in this embodiment, bonding pads (electrodes) on the semiconductor chips are provided so as to overlap the islands on which the semiconductor chips are mounted. Specifically, an electrode60(a spot where the thin metal wire58is connected) on the first semiconductor chip17is positioned within the first island12from a planar point of view. Accordingly, even if bonding energy (pressing force, vibration force and heat) is applied to the electrode60by a bonding tool when the thin metal wire58is formed, the first semiconductor chip17in this portion is strongly supported by the first island12. For this reason, the first semiconductor chip17can be prevented from being destroyed by the bonding energy applied by the bonding tool. The same goes for the second semiconductor chip20. An electrode62provided on the second semiconductor chip20is provided so as to overlap the second island13.

As a method for stacking a plurality of semiconductor chips inside a semiconductor device, there is a method for stacking two semiconductor chips back to back on both of upper and lower surfaces of one island. However, this method has a risk that the stacked semiconductor chips are short-circuited through the island if rear surfaces of the semiconductor chips are required to be conductive. Meanwhile, in this embodiment, with reference toFIG. 8C, the two islands separated from each other are provided and are disposed so as to be shifted from each other in the thickness direction. Accordingly, the rear surface of one of the islands, through which the current passes, can be insulated from the semiconductor chip mounted on the other island.

Furthermore, in general, when two semiconductor chips are built into a semiconductor device, the respective semiconductor chips are mounted on separate islands and the chips and the islands are laminated. Accordingly, since the two islands and the two semiconductor chips are laminated in a thickness direction, an increase in the thickness due to lamination is unavoidable. Meanwhile, in this embodiment, with reference toFIG. 8C, the first and second islands12and13, on which the respective semiconductor chips are fixed, are set to overlap each other in the thickness direction. For this reason, a package thickness can be reduced by suppressing an increase in the thickness due to lamination of the semiconductor chips.

In the present invention, the rear surface of the first island and the top surface of the second island are disposed so as to at least partially overlap each other. Accordingly, the first semiconductor chip on the first island and the second semiconductor chip on the rear surface of the second island are formed so as to overlap each other.

Consequently, a planar occupied area can be reduced to be smaller than plane areas of both of the chips. Moreover, since thin metal wires connected to the second semiconductor chip are extended to a rear side, a thickness of the semiconductor device can also be reduced.