Semiconductor apparatus manufacturing method

The semiconductor apparatus includes a semiconductor chip, and a source electrode and a gate electrode which are formed on the semiconductor chip and electrically connected with a lead frame. The source electrode is electrically connected with the lead frame by being laser-welded with a thin-film shaped connecting portion formed at an end of the lead frame. This enables the provision of a semiconductor apparatus with enhanced productivity and yields which exhibits high electrical operability and reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method of a semiconductor apparatus including a semiconductor chip and an electrode which is formed on the semiconductor chip and electrically connected with an external lead and, particularly, to a manufacturing method of a semiconductor apparatus capable of carrying a large current.

2. Description of Related Art

Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), which are used for power supply in consumer-electronics products such as cell phones, personal computers, and digital audiovisual equipments or for driving a motor of vehicles, carry a high current of about 5 to 200 A. It thus requires a thick, heavy wire material. For example, a plurality of large-gauge wires of about 100 to 500 μm are connected to the source electrode of a MOSFET in order to draw a high current.

However, the connection of a bonding wire to the source electrode by ultrasonic bonding or thermocompression bonding requires a bonding area of about 1.5 to 3 times the wire cross-sectional area. The area of the source electrode or the gate electrode, however, is limited to small and thus subject to the limitation by the wire cross-sectional area. Further, high pressure application is required to obtain a large bonding strength, and the mechanical impact causes a decrease in yields. This is described in Japanese Unexamined Patent Application Publication No. 2002-313851 (Oono), for example.

In a semiconductor apparatus disclosed in Oono, a plate-shaped electrical path member is bonded by ultrasonic to thereby obtain a large bonding strength and achieve high yields.FIG. 8is a bird's-eye view of a semiconductor apparatus (Small Out-line Package (SOP)—8 packages) taught by Oono.FIGS. 9A and 9Bare cross sectional views along lines IVA-IVA and IVB-IVB, respectively, inFIG. 8.

Referring toFIG. 8, the MOSFET101is almost entirely fixed and covered by a sealing resin (molding resin)102which is made of epoxy resin or the like. The MOSFET101has eight leads103, such that one end of each lead103is exposed outside the molding resin102.

Referring then toFIG. 9A, the molding resin102includes a semiconductor device (semiconductor chip)105. On the semiconductor device105, a source electrode (source pad)104sand a gate electrode (gate pad)104gare formed on the top surface, and a drain electrode (drain pad), though not shown, is formed at the end of the under surface.

Four terminals arranged on one side out of the terminals of the eight leads103are integrated into one set inside the molding resin102. The four terminals serve as drain-side terminals103dof the leads103which are to be electrically connected with the drain pad. The remaining four terminals out of the terminals of the eight leads103are arranged so as to avoid direct contact with the semiconductor device105and to be electrically disconnected from the drain-side terminals103dand the lead103including a drain-side post107dinside the molding resin102as shown inFIG. 9A. Out of the remaining four terminals of the leads103, three terminals are integrated into one set, and the remaining one terminal is electrically disconnected from those three-in-one terminals of the lead103.

The thee-in-one terminals of the lead103are electrically connected with the source electrode104sof the semiconductor device105by a current path member106to serve as source-side terminals103sof the leads103. The remaining one terminal of the lead103is electrically connected with the gate electrode104gof the semiconductor device105by a single bonding wire108to serve as a gate-side terminal103gof the lead103.

Referring toFIGS. 9A and 9B, in the current path member106, an electrode-side connecting portion106awhich is formed at one end is connected by plane with the source electrode104s, and a lead-side connecting portion106bwhich is formed at the other end is connected by plane with a source-side post107s. The current path member106is directly connected to both the source electrode104sand the source-side post107sof each source-side terminal103sof the leads103at the same time by ultrasonic bonding.

With the current path member106, the semiconductor apparatus101taught by Oono allows the cross sectional area of the current path flowing between the source electrode104sof the semiconductor device105and the source-side post107sof each source-side terminal103sof the leads103to be significantly larger than a total of the cross sectional area of the current path flowing through a plurality of bonding wires in a conventional MOSFET. This enables the MOSFET101to have significantly lower resistance between the source electrode104cand the lead103compared with a conventional MOSFET.

As a different bonding technique from the ultrasonic bonding, there is laser bonding which uses laser as disclosed in Japanese Unexamined Patent Application Publication No. 1-310547 (Iino), for example. Further, Japanese Unexamined Patent Application Publication No. 6-244230 (Uemura et al.) discloses a bonding technique using both ultrasonic and laser.

However, the ultrasonic bonding described in Oono and Uemura et al. cause damages to products because it is a mechanical bonding technique and also cause oxidation of wires (connecting member) or electrodes because it uses heat.

The bonding process may be carried out at the temperature of about 300° C., for example, so that an oxide film is formed on metal. Further, the temperature in the bonding process is low compared with the melting point of a metal such as a wire member, which is 800° C. to 1100° C. Thus, a mechanical energy is required in order to break the oxide film formed on a wire member and to enable bonding with the low temperature in the bonding process between the electrode and the wire material. In such a case, mechanical vibration is applied so as to break the oxide film and expose a new surface, which cause damage to the chip. Particularly, a MOSFET has an active cell below the pad, and the damage caused by the ultrasonic vibration can reach the element below the pad, which leads to breakdown of a product. Further, because the process forms an alloy by mechanical contact, the bonding is not appropriate for some types of metal.

Further, the technique of laser bonding can cause damage to a base below the electrode. As described above, large-gauge wires of about 100 to 500 μm or above are used to reduce the resistance in order for the MOSFET to carry a high current. This corresponds to the thickness of 100 to 500 μm. On the other hand, the thickness of the electrode to be connected is as small as about 2 to 6 μm as described in Oono. The adjustment of laser intensity is extremely difficult when laser-welding the members having different thicknesses, and it is impossible to provide bonding by the laser welding. High laser intensity to fuse the wire causes breakdown of the base, and low laser intensity fails to establish connection or obtain desired connection intensity.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a manufacturing method of a semiconductor apparatus including a semiconductor chip and an electrode formed on the semiconductor chip and electrically connected with an external lead. This method laser-welds the electrode with a thin-film shaped connecting area formed partly or entirely on a connecting portion electrically connected with the external lead.

The method makes the laser welding between the semiconductor chip and the external lead through the connecting portion. It uses a thin-film shaped connecting area which is formed partly or entirely on the connecting portion, thereby enabling laser welding with an electrode which is normally significantly thinner than the connecting portion. This achieves the establishment of electrical connection without mechanical vibration.

The present invention provides a semiconductor apparatus manufacturing method which enables an increase in productivity and yields and exhibits high electrical operability and reliability.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary embodiment of the present invention is described hereinafter in detail with reference to the drawings. In the below-described embodiment, the present invention is applied to a semiconductor apparatus for carrying a high current which is used for power supply or switches in cell phones, motors and so on, in which a number of MOSFETs are combined into one MOSFET. While a single transistor normally carries a current of about 0.1 A, such a transistor can carry a current of about 5 to 200 A and can be used for power supply in consumer-electronics products, motor driving in vehicles or the like.

FIG. 1Ais a perspective view of a semiconductor apparatus according to an embodiment of the present invention.FIGS. 1B and 1Care cross-sectional views along line IB-IB inFIG. 1A. As shown inFIGS. 1A and 1B, a semiconductor apparatus1includes a die pad (island)2formed on a lead frame, a semiconductor chip (die)3which is mounted on the die pad2, and a lead frame4which is electrically connected with the semiconductor chip3. Typically, these elements are covered with a molding resin made of epoxy resin or the like and packaged entirely except for one end (outer lead (external lead)4b) of the lead frame4.

The lead frame4includes a portion for mounting the semiconductor chip3(which is referred to as the die pad or the internal lead) and a portion projecting outside of the package and soldered to a substrate (which is referred to as the external lead). Because a different feature is required for each portion, the die pad and the internal lead may be nickel-plated or non-plated, and the external lead may be solder- or tin-plated, for example.

The semiconductor chip3may be formed of a vertical MOSFET, for example. A drain electrode, though not shown, is formed on its under surface and connected to the die pad2. An external lead, also not shown, which is connected with the die pad2serves as a drain-side terminal. On the upper surface of the semiconductor chip3are formed a source electrode (electrode pad)3aand a gate electrode (electrode pad)3b. A connecting portion4ahaving a thin-film shape (flat-plate or ribbon-like shape) which is formed at the end of the internal lead of the lead frame4is electrically connected with the source electrode3aand the gate electrode3b. In the semiconductor apparatus1illustrated inFIG. 1B, the lead frame4is not plated. In a semiconductor apparatus40illustrated inFIG. 1C, on the other hand, the lead frame4includes a base41such as Cu and a plating layer42such as Ni which is formed on the base41.

In the semiconductor apparatus1according to this embodiment, the connecting portion4aof the lead frame4is bonded by laser welding with the source electrode3aand the gate electrode3b, and one or a plurality of weld-bonding portions5are provided depending on the bonding area. The use of laser welding for connection facilitates the positioning between the lead frame4and the semiconductor chip3and thus enables easier manufacturing such as one-time connection using a galvanometer mirror or the like.

As described earlier, the connecting member such as the internal lead or the wire and the electrodes on the semiconductor chip3cannot be laser-welded simply. While the thickness of the electrode on the semiconductor chip3is typically about 4 to 5 μm, the thickness of the connecting member such the wire is about several tens to hundreds times greater. If a thickness ratio of the members to be welded differs largely, the use of high laser power with the intent to weld those members can result in breakdown of the base underneath. On the other hand, the use of low laser power fails to establish connection or obtain desired connection intensity. Thus, the breakdown of the elements below the electrode can occur unless optimizing the thickness of the connecting portion with respect to the thickness of the electrode.

On the other hand, the technique of using the ribbon-like or flat-plate connecting member and performing the ultrasonic bonding as disclosed in Oono causes the element to be subject to breakdown due to the mechanical vibration, and the ultrasonic bonding of a large area requires a large-scale machine to increase manufacturing costs.

The inventor of the present invention has found that a connecting member and electrodes on the semiconductor chip3can be laser-welded by forming at least apart of the connecting member to be electrically connected with the electrode, which is referred to hereinafter as the connecting portion, into a thin-film shape (flat-plate or ribbon-like shape) and minimizing a difference (ratio) in thickness between the connecting portion and the electrodes. Accordingly, the lead frame4of this embodiment has the connecting portion4ahaving a thin-film shape which is thinner than the other part. The laser welding is applied between the connecting portion4aand the electrodes3aand3b. As described later, the connecting portion4ais not necessarily entirely thin-film shaped. The laser welding is possible with equal effect as long as at least an area to be laser-welded, which is a part of the connecting portion4a, is thin-film shaped to serve as a welding area.

Specifically, the inventor of the present invention has found that suitable connection intensity can be obtained by laser welding without damaging the base below the electrodes by setting the thickness of the connecting portion4ato be less than thirty times the thickness of the electrodes3aand3bon the semiconductor chip3. The thickness of the connecting portion4ais preferably 5 to 30 times, more preferably 10 to 20 times, the thickness of the electrodes3aand3b.

As described earlier, when using laser welding, it is preferred to set an appropriate thickness ratio between the connecting portion4aand the electrodes3aand3b. If the thickness of the connecting portion4ais more than thirty times the thickness of the electrodes3aand3b, high laser power is required for the laser welding. This can cause damage underneath the electrode, which leads to breakdown of the element. Specifically, the laser fusing of the thick connecting portion4acan cause the laser to penetrate through the thin electrodes3aand3bto break down the base underneath. For example, if the wire is about 500 μm thick and the electrode is about 5 μm thick, the laser can undesirably reach beyond the depth of 5 μm, or, even if the laser does not reach beyond the electrode thickness range, undesirable variation occurs in the bonding strength or the like. It is thus preferred that the thickness of the connecting member is less than 30 times the thickness of the electrode.

Although it is ideal for the laser welding to equalize the thickness of the electrodes and the thickness of the connecting portion4a, if the thickness of the connecting portion4ais less than 5 times the thickness of the electrodes3aand3b, it is unable to secure enough strength of the connecting portion4a, which is not practical, Thus, the thickness of the connecting portion4ais preferably equal to or more than 5 times the thickness of the electrodes3aand3b, or more preferably equal to or more than 10 times to obtain more stable intensity. For example, if the thickness of the electrodes3aand3bis 5 μm, the thickness of the connecting portion4aof the lead frame4is preferably about 25 to 150 μm.

Further, it is preferred to determine the contact area of the connecting portion4awith the electrodes3aand3bbased on the thickness of the connecting portion4aand the current flowing through the electrodes3aand3b. It is further preferred to determine the contact area based on a prescribed electrical resistance in the connecting portion between the connecting portion4aand the electrodes3aand3b.

As described above, the semiconductor apparatus according to this embodiment may carry a current of 5 to 200 A, and it is preferred to design the sizes of the connecting portion4aand the electrodes3aand3bsuch that the contact area is in accordance with desired current and resistance values. The contact area indicates the area where the connecting portion4ais actually in contact with the electrodes3aand3b, not the area of the entire connecting portion4a. Specifically, the connecting portion4ais laser-welded at a plurality of points, and the part surrounded by the outermost weld-bonding portions5serves as the contact area.

The electrical resistance of the contacting portion4ais preferably low for smaller current loss. However, because the semiconductor apparatus (MOSFET)1controls ON/OFF of a switch by a prescribed resistance value, the resistance to enable the switching function is required at least. It is thus preferred to determine the thickness of the connecting portion4aand the contact area based on the resistance value necessary to serve as a switch.

The area of the connecting portion4amay be larger than the area of the electrodes3aand3b.FIG. 2is a cross sectional view along line II-II inFIG. 1A. As shown inFIG. 2, the electrode3ais formed on the semiconductor chip3which is mounted on the die pad2. A nonconductive cover layer6which may be made of polymide (PI), phosphate glass (PSG), an oxide film or the like is formed in the vicinity of the electrode3ato cover the elements around the electrode3a. This prevents the connecting portion4afrom being electrically continuous with other elements even if the connecting portion4ais larger than the actual area of the electrode3a. This further facilitates the positioning between the connecting portion4aand the electrodes3aand3bbecause of the connecting portion4abeing large.

Because the part of the connecting portion4awhich extends off the electrodes3aand3bis placed on the insulating cover material6, no electrical continuity is established when the contacting portion4acontacts the elements in the vicinity of the electrode3a. Although a sight gap can be present between the connecting portion4aand the electrode3ain this case, the connecting portion4aand the electrode3aare fused by the laser welding into a bonding portion5to thereby allow them to be electrically bonded.FIG. 2schematically illustrates that.

The connecting portion4a(lead frame4) is typically formed of aluminum, copper, or an alloy of those. The use of gold, silver, palladium, or an alloy of those is also possible. Further, a metal material may be formed on the surface of such a connecting member.

The copper is particularly suitable for use as a connecting member because of its high conductivity; however, it has low laser absorbency. Therefore, for easier laser welding, it is possible to provide the plating of nickel (Ni) having high laser absorbency on the surface of copper or form an alloy of Ni and copper.

In a MOSFET, for example, a gate electrode is formed below a source electrode (lead electrode) with an insulating film interposed therebetween. The insulating film may be formed of oxide silicon glass (BPSG: borophosphorous silicate glass) added with phosphorous and boron, PSG (phosphorous silicate glass) or the like. If damage is applied to the insulating film by laser welding, the MOSFET becomes inoperable. It is therefore preferred that the insulating film as the base is formed of a heat resistant material.

When using the lead frame4as a connecting material, a beam to bridge the lead frame4and the electrodes3aand3bis formed by bending or the like. In this case, the shape of the end of the lead frame4(connecting portion4a) may be adjusted appropriately according to the chip size.

It the lead frame4and the electrodes3aand3bare connected electrically, not by the wire bonding through a different connecting member (wire) as in a conventional technique, the connecting part decreases to enable the provision of more reliable semiconductor apparatus.

If the connecting portion4aand the electrodes3aand3bare connected by ultrasonic bonding, it is necessary to make oscillation at a bonding portion. This requires the positioning between the connecting portion4aand the electrodes3aand3bwith an accuracy of about ±25 μm. On the other hand, the laser welding, which is employed in this embodiment, assures good bonding with the positioning accuracy of about ±100 μm. Because of such a rough positioning accuracy required, the number of times to check the lead or chip position for the positioning can be reduced to thereby lower the manufacturing costs.

Further, the laser welding fuses the metal of the connecting portion4aof the lead frame4and the electrodes3aand3bcompletely for connection, thus enabling the bonding of dissimilar metals. This eliminates the need for the process of forming electroless nickel/displacement gold plating or electroless copper plating suitably adhered to an aluminum electrode of a semiconductor wafer, which is called the UBM (underbump metal) process, or rewiring, thereby enabling the provision of low cost semiconductor apparatus.

Furthermore, the bonding by the laser welding does not apply any mechanical stress, thus not causing damage to the semiconductor chip3to thereby increase yields. In addition, because the laser welding is a normal temperature process in which heat is generated only in the welding portion, it does not cause oxidation of the lead frame4or the semiconductor chip3.

A manufacturing method of the semiconductor apparatus according to this embodiment is described hereinafter. This embodiment uses an external lead (lead frame)4whose end is shaped into the thin-film connecting portion4a. The lead frame4of such a shape can be obtained by forming the thin-film shaped connecting portion4asimultaneously during molding. This enables the formation of the lead frame4with the thin-film shaped connecting portion4ato enable laser welding without any additional step. The connecting portion4ais formed at the end of the lead frame4so as to have a given thickness and the size and shape according to the electrodes3aand3bof the semiconductor chip3.

Then, the semiconductor chip3is mounted on the die pad2. The die pad2and the semiconductor chip3may be electrically connected by soldering, silver paste, or the like. The lead frame4is placed thereon so that the connecting portion4aof the lead frame4and the electrodes3aand3bon the semiconductor chip3are placed on one another. As described above, the positioning accuracy may be about ±100 μm because they are to be bonded by laser welding.

Referring now toFIGS. 3A and 3B, laser is applied to partly fuse the connecting portion4aof the lead frame4and the electrodes3aand3bto thereby weld the metals. For example, the connecting portion4aand the source electrode3amay be welded at a plurality of positions with a prescribed interval because the electrode3ais large.

The use of a galvanometer for the laser welding enables one-time connection to enhance the productivity. The semiconductor chip3and the lead frame4can be connected easily as described above.

The connection between the lead frame4and the electrodes3aand3bis not necessarily made by directly connecting the end of the lead frame4which is processed as above. Alternatively, the lead frame4and the electrodes3aand3bmay be connected through a different connecting member. In this case, the ratio of the thickness of the electrodes and the thickness of the connecting member should be 30 or lower, or preferably in the range of 5 to 30. The connecting member may be a wire or a ribbon-like connecting member, for example.

FIGS. 4A and 4Billustrate the case of using a ribbon-like connecting member, andFIGS. 5A and 5Billustrate the case of using a wire.FIGS. 4A and 4Bare a perspective view and a cross-sectional view, respectively. In the following description, the same elements as inFIGS. 1 and 2are denoted by the same reference numerals and not described in detail herein. This embodiment uses a ribbon-like connecting member15for the connection between the lead frame14and the electrodes on the semiconductor chip3. Specifically, one end of the lead frame14and one end of the ribbon-like connecting member15are electrically connected, and the other end of the ribbon-like connecting member15and the electrodes3aand3bof the semiconductor chip3are electrically connected. The connection of the ribbon-like connecting member15with the semiconductor chip3and the lead frame14is made by the laser welding. The ribbon-like connecting member15is thinner than the lead frame14, and a thickness ratio of the ribbon-like connecting member15with respect to the electrodes3aand3bof the semiconductor chip3is set to 30 or lower, or preferably 5 to 30. The lead frame14is thicker than the ribbon-like connecting member15, and a thickness ratio of the lead frame14with respect to the ribbon-like connecting member15is set to 30 or lower for stable laser welding. In the laser welding between the lead frame14and the ribbon-like connecting member15, the thickness ratio is not necessarily 30 or lower because there is no risk of damaging a lower-layer element.

FIGS. 5A and 5Bare a top view and a cross-sectional view, respectively. They show the case of using a wire25for the connection of the lead frame14and the electrodes of the semiconductor chip3. As shown inFIGS. 5A and 5B, the lead frame14and a connecting portion25bformed at one end of the wire25are electrically connected, and a connecting portion25aformed at the other end of the wire25and the electrodes of the semiconductor chip3are electrically connected. Although the wire25normally has a circular cross section, the ends (connecting portions25aand25b) of the wire25are thin-plate shaped in this embodiment for the laser welding with the lead frame14and the electrodes, respectively.

In this case also, a thickness ratio of the flat-plate connecting portion25aof the wire25and the electrodes3aand3bis preferably within the range of 5 to 30 for the same reasons as described above. The connecting portion25bto be welded with the lead frame14is also preferably shaped into a flat plate for easier laser welding.

As described above, it is possible to perform laser welding with the electrodes on the semiconductor chip by forming a thin-film connecting area in a part of the connecting portion.FIG. 6illustrates such a case.FIG. 6Ashows the connecting portion after dimpling, andFIG. 6Bshows the process of laser welding using the dimples.

As shown inFIGS. 6A and 6B, a dimple35ais formed at one end of the connecting member35to serve as a connecting portion by molding or the like. The dimple35ais thinner than the other part, which allows optimization of the thickness ratio with the electrodes3aand3b. The same effect is obtained when the dimple35aand the electrodes on the semiconductor chip3are welded by laser10. Although the dimpling is performed on the connecting member35to form the dimple35ain this example, the dimpling may be performed to make connection between the lead frame14and the semiconductor chip3. Specifically, the dimpling may be performed at one end of the lead frame14to be connected with the semiconductor chip so as to form a dimple to serve as a welding area.

Further, as a semiconductor apparatus50shown inFIG. 7, the connecting area of the connecting portion4ato become the bonding portion5may have an opening which is filled with metal, as a bonding material51, that is highly absorbent of laser light. For example, if the connecting portion4a(lead frame4) is made of Cu, the bonding material51may be metal having higher absorbency of laser light than Cu, such as aluminum, Ni, aluminum alloy, and Ni alloy. Expressly the bonding material51and the connecting area of the connecting portion4acontain aluminum and copper, respectively. Lesser laser light power may be applied to laser-weld the bonding material51because a melting point of aluminum or aluminum alloy is lower than a melting point of Cu or Cu alloy. The bonding material51maybe placed in the position of each bonding portion5to which laser light is applied as shown inFIG. 7, or may be plate-shaped to cover the positions of a plurality of bonding portions5.

When applying laser light to the bonding material51, the laser light may be applied to the bonding material51only or to the bonding material51and the surrounding connection portion4a. If the laser light is applied to both of the bonding material51and the surrounding connecting portion4aat the same time, it is possible to monitor the progress to control the laser light so as to melt the bonding material51without allowing the connecting portion4ato melt. This prevents overheating of the bonding portion5, thereby avoiding the breakdown of elements and increasing yields.