Material reduced metallic plate on power semiconductor chip

FIELD OF THE INVENTION

The invention relates to a power semiconductor module and to a method for manufacturing a power semiconductor module.

BACKGROUND OF THE INVENTION

To make use of the high-temperature capability and high power density of the newest generations of silicon (Si) and silicon carbide (Sic) power semiconductor devices, advanced packaging and interconnection technologies are needed to provide sufficient reliability and current capability.

Usually, power semiconductor devices, such as rectifiers and inverters, are assembled of one or more power semiconductor modules, which provide the housing and/or electrical interconnection of one or more power semiconductor chips. The topside interconnection of the power semiconductor chips often limit the lifetime of the power semiconductor modules in applications with high cycling requirements.

One possibility is to bond a metallic plate to a top side of the semiconductor chip and to bond the electrical interconnections to the metallic plate. This, for example, is shown in EP 0520 294 Al.

However, due to the mismatch of the coefficient of thermal expansion between the metallic plate and the power semiconductor chip, there may be a considerable stress exerted on the bond interface between the power semiconductor chip and the metallic plate during active or passive thermal cycling. This stress may lead to a degradation of the bond layer and/or chip metallization, which eventually may limit the lifetime of the interconnection. This may become even more severe, when SiC power semiconductor chips are used, due to the different mechanical properties of the SiC material causing increased thermomechanical stress.

In order to minimize the stress caused by the thermal mismatch and maximize the cycling lifetime, one would try to use a metallic plate, which is as thin as possible. However, this may have disadvantages regarding the wire bonding process. With a thin metallic top plate, losses due to damaged chip topside structures may be present, which are caused by high forces and power generated during the wire bonding process. Another disadvantage of very thin metallic top plates may be that these parts become difficult to handle.

US 2007/246 833 Al shows a metal plate, which is bonded to a semiconductor chip. The metal plate has a border, which is thinner than a central part that is used for bonding bond wires.

JP 2000 307 043 A shows a metal plate, which has a border that is thinner than a central part, to which an electrical conductor is attached.

US 2018/053 737 Al shows metal layers on a semiconductor chip, which have a border that is thinner than a central part, where a bond wire is attached.

WO 2017/157 486 Al relates to a semiconductor device and mentions several possibilities, how components on a semiconductor chip can be bonded with each other and machined, such as sintering, etching and stamping.

JP H01 122 129 A shows metal pads with holes for wire bonding.

DESCRIPTION OF THE INVENTION

It is an objective of the invention to provide a power semiconductor module with a highly reliable electric interconnection on the power semiconductor chip, which is easy and economical to manufacture.

This objective is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims and the following description.

An aspect of the invention relates to a power semiconductor module. A power semiconductor module, which also may be called power semiconductor package, may provide the mechanical and/or electrical interconnection of one or more power semiconductor chips. Furthermore, the power semiconductor module may comprise a housing for the one or more power semiconductor chips and/or the electrical interconnection.

It has to be noted that the term power here and in the following may relate to devices (i.e. modules and/or chips), which are designed for processing currents of more than 10 A and/or more than 100 V.

According to an embodiment of the invention, the power semiconductor module comprises a substrate with a metallization layer, a power semiconductor chip bonded to the metallization layer of the substrate, and a metallic plate bonded with a first surface to the power semiconductor chip opposite to the substrate. The power semiconductor chip may be bonded to a substrate, the metallic top plate may be bonded to the topside of the power semiconductor chip.

The metallic plate has a central part and a border, which are both bonded to the power semiconductor chip. The border may be provided at an edge of the metallic plate and/or may surround the central part.

The metallic plate may have two opposite surfaces, i.e. a first, lower surface and a second, upper surface. Both surfaces may be flat. The metallic plate may be bonded with the complete first surface to the power semiconductor chip. In particular, the central part and/or the border may be bonded with its surface facing the semiconductor chip to this chip.

A plurality of metallic interconnection elements are bonded to a second surface of the metallic plate at the central part. The central part may be used for attaching electrical conductors.

The border of the metallic plate is structured in such a way that the metallic plate has less metal material per area at the border as compared to a central part of the metallic plate. The metallic top plate may have a dedicated structure at the border or periphery to reduce the thermomechanical stress on a bond layer between the metallic plate and the power semiconductor chip and/or on a topside of the power semiconductor chip during thermal cycles.

At the border, the metallic plate may be reduced in metal material. In particular, the metal material per area may be reduced. Here, the term“per area” may refer to an area of the metallic plate defined by viewing onto the metallic plate in a direction orthogonal to the metallic plate, the semiconductor chip and/or the substrate. The border of the metallic plate may be the part of the metallic plate outside a central part of the metallic plate, which central part may be used for bonding electrical interconnection elements. The central part may have a larger area as the border.

Less metal material may be present at the border by removing metal material from the metallic plate, for example by machining and/or etching the metallic plate at the border.

With the metallic plate having less metal material at the border as in a central part, a higher cycling reliability of the topside connection and/or an improved process stability and yield of a wire bonding interconnection may be achieved. This may satisfy an increasing power cycling lifetime demand, which may be needed to satisfy increasing current densities.

The substrate may be made of a plastics plate or a ceramics plate, which is layered at one or both sides with one or more metallization layers. The one or more metallization layers may be made of Cu. The metallization layer may be structured for providing electrical interconnections. For example, an electrical interconnection element bonded to the metallic plate also may be bonded to the metallization layer.

The power semiconductor chip may provide a semiconductor device, such as a diode, transistor and/or thyristor. The power semiconductor chip may be based on Si or SiC as semiconductor material. The power semiconductor chip may have metallic electrodes on one or both sides. With one power electrode, the power semiconductor chip may be bonded to the metallization layer of the substrate. The metallic plate may be bonded to an opposite power electrode.

In the case of a power semiconductor switch, such as a transistor or thyristor, the power semiconductor chip may have a further gate electrode, which may be provided on the same side, to which the metallic plate is bonded.

The metallic plate may be one-piece and/or may be made by machining and/or etching a preform. The metallic plate may be made of metal material.

According to an embodiment of the invention, metal material is removed from a side of the metallic plate providing the second surface. Depressions and/or holes may be provided on a side of the metallic plate (i.e. with the second surface), which is opposite to a side facing the power semiconductor chip (i.e. with the first surface). In such a way, the bonding area can be increased.

According to an embodiment of the invention, the first surface of the metallic plate covers more than 50%, for example more than 90% of an electrode of the power semiconductor chip. There may be one metallic plate for one electrode. The metallic plate may cover the electrode (nearly) completely.

According to an embodiment of the invention, the border surrounds the central part of the metallic plate. All sides of the metallic plate in directions parallel to the maximal extension of the metallic plate may be provided with a border having reduced metal material. In other words, the border may surround the central part completely.

However, it also may be that such a border is present, which surrounds the central part only partially.

According to an embodiment of the invention, the border of the metallic plate is thinner than the central part of the metallic plate and/or has a reduced thickness at the border. One possibility to reduce metal material at the border is to remove metal material over the complete area of the border. The metallic plate may have a thinner region close to its periphery, i.e. its border. A thinner structure at the border may have the advantage that the thermomechanical stress in a bond interface below may be significantly reduced compared to a metallic plate with an overall equal thickness. This advantage also was shown by thermomechanical simulations using a finite element method.

According to an embodiment of the invention, the border has depressions and/or holes, which may be provided in the second surface. A further possibility of reducing metal material is by removing metal material at dedicated areas of the border. The resulting depressions may reach to the edge of the metallic plate and/or may be completely surrounded by a higher area, i.e. may be holes. The depressions and/or holes may be distributed along all of the border.

According to an embodiment of the invention, the border has through holes. All or some of the depressions and/or holes may reach through the thickness of the metallic plate, i.e. the holes may be through holes. However, it also may be possible that all or some of the depressions and/or holes only reach partially through the thickness of the metallic plate. In this case, these holes may be seen as dimples and/or blind holes.

It has to be noted that a reduced thickness of the border may be combined with depressions and/or holes in the border. Furthermore, the border may have multiple different thicknesses. Any solution of a metallic plate with a reduced effective material mass in a region close to the edge of the metallic top plate may be used.

According to an embodiment of the invention, the border has a thickness of less than 100 pm, for example less than 50 pm.

According to an embodiment of the invention, the central part of the metallic plate has a thickness of more than 30 pm, for example more than 100 pm and/or more than 150 pm.

According to an embodiment of the invention, the central part of the metallic plate is flat. The central part may have a constant thickness. No areas with reduced metal material may be present in the central part.

As already mentioned, a plurality of metallic interconnection elements may be bonded to the metallic plate at the central part. The central part may be used for bonding further metal elements. For example, bond wires may be bonded to the central part and/or metallic bands may be bonded to the central part. A wire bond may have a substantially circular cross- section. A metallic band may have an elongated cross-section. For example, the diameter of the cross section in one direction may be 5 times larger than a diameter in another direction. A metallic band also may be called ribbon band.

According to an embodiment of the invention, the metallic plate is made of Cu. Also, the wire bond(s) and/or the metallic band(s) may be made of Cu.

It also may be possible to further reduce stress during thermal cycling by using a low- CTE material, such as molybdenum (Mo), for the metallic plate. However, Mo material may be considerably more expensive as Cu material.

A further aspect of the invention relates to a method of manufacturing a power semiconductor module. It has to be understood that features of the method as described in the above and in the following may be features of the power semiconductor module as described in the above and in the following, and vice versa.

According to an embodiment of the invention, the method comprises: bonding a power semiconductor chip to a metallization layer of a substrate; structuring a border of a metallic plate, such that the metallic plate has less metal material per area at the border as compared to a central part of the metallic plate; and bonding the metallic plate with a first surface to the power semiconductor chip opposite to the substrate. The central part and the border are bonded to the power semiconductor chip. In other words, the metal plate may be structured and bonded to the top of the power semiconductor chip, which is bonded with the other side to the substrate.

The method further comprises: bonding a plurality of interconnection elements to a second surface of the metallic plate at the central part of the metallic plate. In particular, the interconnection element, such as a wire bond or metallic band, may be welded to the metallic plate. Due to the reduced border structure, the central part may have an increased thickness, which may enable the bonding of thicker wire bonds or metallic bands. In such a way, a current capability of the topside connection may be increased.

According to an embodiment of the invention, the border of the metallic plate is structured with electrochemical etching. In particular, a structure with a thinner border may be implemented with a two-step etching process, which may be feasible at little additional cost. Also, a depression and/or hole structure may be implemented with etching.

According to an embodiment of the invention, the border of the metallic plate is structured by stamping. In particular, depressions and/or holes may be implemented with stamping.

According to an embodiment of the invention, the metallic plate is sintered to the power semiconductor chip. One possibility of bonding the metallic plate to an electrode of the power semiconductor chip is sintering. For example, the metallic plate, which may be made of Cu, may be sintered to the power semiconductor chip using a micro- or nanoparticle paste, which may comprise silver (Ag) and/or copper (Cu) particles.

However, it also may be possible to bond the metallic plate with another process, such as soldering.

According to an embodiment of the invention, the interconnection element is ultrasonic welded to the central part. For example, wire bonds and/or metallic bands may be ultrasonic welded to the central part.

According to an embodiment of the invention, the interconnection element is laser welded to the central part. A metal band may be laser welded to the central part. In the case of laser welding, it may be that the central part has a thickness of 100 pm or more.

In summary, a power semiconductor module with a highly reliable chip topside interconnection may be provided. A metallic top plate may be bonded to a topside of the power semiconductor chip, which may enable more reliable electric interconnections, such as Cu wire bonds. A thinner border structure and/or dimple border structure may be provided in the metallic top plate, which may improve the reliability of the bond interface between the metallic top plate and the power semiconductor chip. This may enable a use of thicker metallic top plates, which may be advantageous in terms of Cu wire bonding process stability and yield and may enable increased current capability of the topside connection by the use of thicker wire bonds or metal bands, such as ribbons.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

Fig. 1 schematically shows a cross-sectional view of a power semiconductor module according to an embodiment of the invention.

Fig. 2 schematically shows a perspective view of a power semiconductor module according to a further embodiment of the invention.

Fig. 3 schematically shows a perspective view of a power semiconductor module according to a further embodiment of the invention. Fig. 4 schematically shows a cross-sectional view of a power semiconductor module according to a further embodiment of the invention.

Fig. 5 schematically shows a cross-sectional view of a power semiconductor module according to a further embodiment of the invention.

Fig. 6 schematically shows a top view of a power semiconductor module according to a further embodiment of the invention.

Fig. 7 shows a flow diagram for a method for manufacturing a power semiconductor module according to an embodiment of the invention.

The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Fig. 1 shows a power semiconductor module 10 comprising a substrate 12, to which a power semiconductor chip 14 is bonded via a first (bottom) power electrode 16. For example, the power semiconductor chip 14 may provide a diode, thyristor and/or transistor. The electrode 16 may be a source or emitter electrode. The power semiconductor chip 14 may be based on Si or SiC.

The substrate 12 may have a (top side) metallization layer 18, to which the power semiconductor chip 14 is bonded, and may have a (bottom side) metallization layer 20 at an opposite side. With the metallization layer 20, the substrate 12 may be bonded to a baseplate 22 of the power semiconductor module 10. The metallization layer 18 may be structured to provide electrical connections of the power semiconductor chip 14 to other components of the power semiconductor module 10. The metallization layers 18, 20 may be made of Cu.

To the top side of the power semiconductor chip 14, a metallic plate 24 is bonded, which may be made of Cu or Mo. On the side opposite to the side, which is bonded to the metallization layer 18, the power semiconductor chip 14 has a further (top) power electrode 26, to which the metallic plate 24 is bonded. It also may be that the power semiconductor chip 14 has a gate electrode 28 at the same side as the power electrode 26. The electrodes 16, 26 and 28 may be made of Cu. The electrode 26 and optionally 28 may be thinner as the metallic plate 24, for example at least 5 times. The gate electrode may be connected with a wire bond 30 to a part of the metallization layer 18. Also, the metallic plate 24 and therefore the power electrode 26 may be connected to a part of the metallization layer 18 with one or more further wire bonds 32, which may have a larger diameter as the wire bond 30.

The one or more further wire bonds 32 are bonded to a central part 34 of the metallic plate 24.

At a border 36, i.e. outside of the central part 34, the metallic plate 24 is structured in such a way that it has less metal material per area as compared to the central part 34. At the border, the metallic plate 24 has reduced metal material. In the case shown in Fig. 1, the border 36 is thinner as the central part 34 in a direction orthogonal to the extension direction of the metallic plate 24 along the power semiconductor chip 14.

With such a border 36, i.e. a border 36 with reduced metal material, stress caused by thermic cycling may be reduced.

Fig. 2 shows a further embodiment of a power semiconductor module 10 with a border 36 with reduced metal material. Analogously to Fig. 1, the border 36 is thinner as the central part 34. The border 36 has an overall constant thickness. As shown, also the central part 34 may have a constant overall thickness. A rim 38 may be provided between the border 36 and the central part 34.

However, it also may be that a border 36 with reduced thickness is oblique and/or has varying thickness.

Fig. 2 also shows that the border 36 surrounds the central part 34 completely. Furthermore, a width of the border 36 may be the same on all lateral sides of the metallic plate 24.

Fig. 3 shows that the border 36 may have the same thickness as the central part 34, but that the metal material of the metallic plate 24 is reduced at the border 36 by providing the border with dimples and/or holes 40. The dimples and/or holes 40 may surround the central part 34 in one or more rows.

Fig. 4 shows that the dimples and/or holes 40 may reach only partially through the thickness of the border 36 or may reach completely through the thickness of the border 36, i.e. may be through holes. Additionally, as shown in Fig. 4, a reduced border thickness may be combined with dimples and/or holes 40. Fig. 4 also shows that a metallic band 42 may be bonded to the central part 34. It has to be noted that such a metallic band 42 may also be used in the other embodiments shown in Fig. 1 to 6 instead or additionally to wire bonds 32.

In Fig. 5, other types of depressions 40’ are shown, which may reach only partially through the thickness of the border 36 or may reach completely through the thickness of the border 36. Again, the thickness of the border 36 may be equal or smaller compared with the thickness of the central part 34.

As shown in Fig. 6, the depressions 40’ may be arranged in a row around the central part 34, which row may completely surround the central part. Contrary to holes 40, depressions 40’ may extend to the edge of the metallic plate 24.

Fig. 7 shows a flow diagram for a method for manufacturing a power semiconductor module 10 as shown in Fig. 1 to 6.

In step S10, the power semiconductor chip 14 is bonded with its power electrode 16 to the metallization layer 18 of the substrate 12. This bonding may be performed by soldering or sintering. The same sintering process as described with respect to step S14 may be used.

In step S 12, the border 36 of the metallic plate 24 is structured, such that the metallic plate 24 has less metal material per area at a border 36 as compared to the central part 34 of the metallic plate 24.

The border 36 of the metallic plate 24 may be structured with electrochemical etching. A border 36 of different thickness, as shown in Fig. 1 to 4, may be produced with electrochemical etching. Depressions 40’ and/or holes 40, as shown with respect to Fig. 3 to 6, may be produced with electrochemical etching.

The border 36 of the metallic plate 24 additionally or alternatively is structured by stamping. Depressions 40’ and/or holes 40, as shown with respect to Fig. 3 to 6, may be produced with electrochemical etching.

In step S14, the metallic plate 24 is bonded to the power semiconductor chip 14 opposite to the substrate 12.

The metallic plate 24 is sintered to the power semiconductor chip 14. For example, the metallic plate 24 may be sintered to the power semiconductor chip 14 with nanoparticles, for example made of Ag or Cu.

In step S16, one or more of the interconnection elements 32, 42 are bonded to the central part 34 of the metallic plate 24.

For example, an interconnection element 32, 42, such as a wire bond 32, may be ultrasonic welded to the central part 34. It also may be that an interconnection element 32, 42, such as a metal band 42, is laser welded to the central part 34.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word“comprising” does not exclude other elements or steps, and the indefinite article“a” or“an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

10 power semiconductor module

12 substrate

14 power semiconductor chip

16 power electrode

18 metallization layer

20 metallization layer

22 baseplate

24 metallic plate

26 power electrode

28 gate electrode

30 wire bond

32 wire bond

34 central part

36 border

38 rim

40 hole

40’ depression

42 metallic band