WO2014177553A1

WO2014177553A1 - Pole-piece for a power semiconductor device

Info

Publication number: WO2014177553A1
Application number: PCT/EP2014/058710
Authority: WO
Inventors: Tobias Wikstroem
Original assignee: Abb Technology Ag
Priority date: 2013-05-02
Filing date: 2014-04-29
Publication date: 2014-11-06

Abstract

A pole-piece 1 5 for a press-pack type power semiconductor device 10 is described. The pole-piece 15 serves to provide an electrical connection to the semiconductor wafer 2 of the power semiconductor device 10 and to convey heat out of the device. A force F is applied between the two pole-pieces 15, 1 6, to ensure good thermal and electrical contact between the pole-pieces 1 5, 16 and the semiconductor wafer 2. The pole-piece 15 transfers heat away from the semiconductor wafer 2 by means of one or more heat-pipes 12, formed between pillars or walls 1 1 in the body section 13 of the pole-piece. The pillars or walls 1 1 serve both to bear the load of applied force F and to define the lateral bounding walls of the heat-pipe(s) 12 within the body section 13 of the pole-piece 15. A second, similar pole-piece may be deployed on the opposite side of the semiconductor wafer 2. A method of fabricating such a pole-piece is also described.

Description

Pole-piece for a power semiconductor device

The present invention relates to pole-pieces for power semiconductor devices and to arrangements for cooling such semiconductor devices during operation.

Power semiconductor devices such as power-diodes, insulated-gate bipolar transistors (IGBT), power MOSFETs or high-voltage thyristors, for example, may be required to block reverse voltages of 1 kV, or even 10 kV or more, and carry a forward current of 1 kA or more. Such devices inevitably generate significant quantities of heat during operation, and this heat must be conveyed away from the semiconductor die or wafer in order to maintain the device within its thermal operating range.

In order to ensure a good electrical contact to the metallization layer across the whole surface of the semiconductor die, slice or wafer (referred to generally in this application as the semiconductor substrate), the semiconductor substrate is typically held in compression between two contact electrodes, which provide electrical contact to the substrate. The contact electrodes are typically cooled, for example using an external evaporative cooling circuit. A heat pipe may be used, for example, to convey heat efficiently away from the contact electrodes to remote heat sinks. Such heat-pipe arrangements for cooled contact electrodes are known from patent documents DD129256 and DE2452922A1 (both CKD Praha), and in GB1471495 (General Electric), for example.

Such external coolers are relatively large, complex systems, and must be adapted to fit different geometries of power semiconductor device. For this reason, it has been common practice to design the housings of power semiconductor devices so that they have simple planar contact surfaces to which external electrodes can be connected and to which general-purpose cooled electrodes can easily be fitted without the need for any adaptation to the specific device geometry. The planar copper surfaces are designed such that they stand proud of the surrounding housing, and can be contacted by a flat connector surface of a general-purpose cooler. The arrangement of a substrate sandwiched under pressure between two solid copper blocks is commonly known as a press-pack. It is known to use pole-pieces to provide a simple, easily-accessible planar surface to which electrodes and a cooler can be connected. Solid copper pole-pieces are favoured because they fulfil a number of functions: the solid copper is strong enough to transfer high applied compression forces from the planar connection surfaces to the semiconductor substrate; they form part of the end caps and provide a hermetic seal for the housing; they act as a thermal connection for transferring heat away from the substrate/wafer to the external cooler, and they provide an electrical connection to the substrate/wafer from outside the device housing.

In this application, the term "pole-piece" is thus used to refer to a physical element or block which is designed to be pressed against a silicon or other semiconductor substrate with a predetermined force, which may be several tonnes. A power semiconductor device fabricated on a 6" (150 mm) diameter wafer, for example, may require a pressing force of nearly 200 kN.

Pole-pieces thus typically provide a mechanical, electrical and thermal connection to the substrate, so that a variety of coolers and connectors can be used with the power device. However, conventional solid copper pole- pieces also present additional thermal resistance between the substrate and the cooler, and additional thermal impedance due to the large thermal mass of the copper in the pole-piece. In order to overcome this problem, it would be possible to dispense with pole-pieces altogether in order to improve the thermal transfer between substrate and cooler. However, this would require a specially- adapted cooler for each different type or geometry of substrate, as discussed above.

It is an object of the invention to overcome at least some of the disadvantages inherent in known pole-pieces. Particular advantages of the pole-piece of the invention may include: improved heat transfer characteristics, a more even force distribution across the area of the substrate, a reduction in the amount of metal (eg copper) used in the manufacture of the pole-piece, a reduction in the weight of the assembled power semiconductor device and a wider choice of materials for fabricating the pole-piece. It can also facilitate the omission of an expensive molybdenum layer between the pole-piece and the semiconductor substrate. To this end, the invention envisages a pole-piece as set out in the appended claim 1 , a power semiconductor device comprising such a pole- piece, as set out in claim 8 and a method for manufacturing such a pole-piece as set out in claim 14. Further variants of the invention are set out in the dependent claims 2 to 7, 9 to 12 and 15.

The invention will now be described with reference to the figures, in which:

Figure 1 illustrates a prior art pole-piece arrangement;

Figure 2 shows a first example of a pole-piece according to a first embodiment to the invention;

Figure 3 shows an example of a pole-piece according to a second embodiment of the invention;

Figure 4 shows a second example of a pole-piece according to the first embodiment of the invention.

It should be noted that the figures are provided merely as an aid to understanding the principles underlying the invention, and should not be taken as limiting the scope of protection sought. Where the same reference numbers are used in different figures, these are intended to indicate similar or equivalent features. It should not be assumed, however, that the use of different reference numbers indicates a difference between the features to which they refer.

Figure 1 shows a simplified schematic illustration of a prior-art press- pack power semiconductor device 100 in cross-section, showing two solid copper pole-pieces 4 with a silicon wafer 2 (semiconductor substrate) sandwiched between them and pressed together with applied force F by two cooled contact electrodes indicated by dashed lines 1 . The pole-piece/substrate assembly is mounted inside a housing with insulating walls 7 and end cap flanges 6. The periphery of the substrate 2 is electrically insulated by a ring 5 of a suitable material such as silicone rubber or other insulating elastomer. The space 8 inside the housing is evacuated or filled a gas, exemplarily an inert gas.

Between each pole-piece 4 and the substrate 2 is shown a molybdenum disk 3, 3' (first and/or second layer). This layer of molybdenum serves as a thermo-mechanical buffer to allow for the different thermal expansion coefficients of the copper 4 and the silicon 2 at different

temperatures and for different temperature gradients. As the temperature of the substrate 2 changes, the solid copper pole-piece 4 expands and contracts in the horizontal plane (ie in the plane of the pole-piece/substrate interface) at a different rate from the silicon substrate 2. Furthermore, the thermal expansion and contraction of the copper 4 at the interface is essentially planar, whereas the silicon substrate 2 may experience very strong warping forces due for example to a differences between the thermal expansion coefficients of the silicon body and a passivation layer at or near the surface of the wafer/substrate 2. These warping forces may give rise to localised increases in pressure at the substrate/pole-piece interface, which result in an uneven resistivity of the interface across the wafer, and thereby results in less than optimal operation characteristics.

The minimum height of the device is determined by the voltage at which it will be operated. For example, if the device it to be operated at 10 kV, then the contact surfaces (ie the outer surfaces, facing away from the substrate 2) of the pole-pieces 4, must be at least 18 mm apart in order to prevent an electrical discharge occurring. This minimum distance is occupied by the pole- pieces and the substrate, which means that the pole-pieces 4 must have a combined axial length which has a certain predetermined minimum value (ie the minimum height of the device minus the thicknesses of the substrate and molybdenum disks).

In this application, it is assumed that the housing of the power semiconductor device has a generally cylindrical or prismatic shape, and the terms "axial" and "axis" are used to refer to the principal/main centre axis of the corresponding cylinder or prism, and/or along which the compression force F is exerted on the semiconductor substrate.

The term "power semiconductor device" is in this application to refer to a device comprising a semiconductor substrate which in turn comprises at least one p-n junction and is designed to operate at voltages in the kilovolt range, and to carry currents of several hundreds or even thousands of amperes.

The term "heat-pipe" is used in this application to refer to a heat- transfer device consisting of a sealed tube or enclosure whose heated and cold ends are made of a material with high thermal conductivity, such as copper or aluminium, and which combines the principles of both thermal

conductivity and phase transition to efficiently manage the transfer of heat across a distance (the length of the heat-pipe) between two solid

interfaces. The heat-pipe contains a working fluid, such as

water, ethanol, acetone, sodium, or mercury, for example, at low pressure, preferably near or below the vapor pressure of the fluid, in which case some of the fluid will be in the liquid phase and some will be in the gas phase. Under these conditions, the speed of the heat transfer is limited only by the rate at which the gaseous fluid can be condensed to a liquid at the cold end. A wick structure may be included inside the heat-pipe. Such a wick serves to carry the condensed working fluid at the cold end back to the heated end, for example by capillary pressure. Gravity may also play a role in returning the working fluid to the heated end of the heat-pipe.

The heat pipe thus comprises an enclosure, a wick and a working fluid. The enclosure is exemplarily evacuated of everything but the wick and the working fluid. The working fluid will partly evaporate, until the saturation pressure is reached at equilibrium. The wick should ensure that all inner surfaces of the enclosure are wet. Any addition of energy to a part of the enclosure will lead to the evaporation (boiling) of the working fluid there.

Normally, this part of the heat pipe is called the evaporator, although it does not have to physically differ from any of the other parts. The gas created at the evaporator is driven by the pressure gradient until it meets a cool surface, where it condenses. This part of the heat pipe is called a condenser. The wick is responsible for pumping the working fluid back to the evaporator.

Figure 2 shows a highly simplified representation of a power semiconductor device comprising pole-pieces 15 and 16 according to the first embodiment of the invention. The pole-pieces 15 and 16 are shown as being similar to each other, but they could also be different in construction. Pole-piece 15 is shown as an electrically conducting structure (made substantially of copper or aluminium, for example) comprising a first end section 9, a second end section 14 and connecting elements 1 1 which extend between the first 9 and second 14 end sections, and whose lateral surfaces bound one or more cavities 1 2 which function as heat-pipes, configured to transfer heat efficiently from their heated ends (the second end sections 14), where they are heated by the semiconductor substrate 2, to their cooled ends (the first end sections 9), where they are cooled by the cooled contact electrodes 1 (external cooler). Exemplarily, the first end sections 9 and the second end sections 14 are arranged parallel to each other. The second end sections 14 are arranged towards the semiconductor substrate 2, whereas the first end sections 9 are arranged farer away from the semiconductor substrate 2, with the heat pipe cavity 12 in between. The heat pipes are completely arranged within the pole- piece such that the evaporators of the heatpipes is arranged towards the second end section 14 and the condenser is arranged towards the first end section 9. Thus, the heat pipes transfer the heat by evaporation of the working fluid at the evaporator from the semiconductor substrate 2 in a vertical direction to the surface, at which the pole-pieces 15, 16, 17, 23, 24 contact the semiconductor substrate 2, to the condenser, at which the gas condenses. Due to the heat pipes being arranged with the evaporator towards the main side of the substrate there is a radial and tangential heat transfer. Due to the heat being moved in a direction away from the wafer main side, there is an axial heat transfer.

The two-dimensional representation in figure 2 shows a power semiconductor device, which may be a cylindrical shape, with cylinder walls 7 and end faces 6. The poles pieces 15 and 16 may also be cylindrical, and the semiconductor substrate may have a circular shape. The pole-piece 15 may comprise a single heat-pipe cavity 1 2, or more than one heat-pipe cavities 12. The separators 1 1 (also referred to as axial-bearing elements, axial load transfer element, or pillars) between the three-dimensional heat-pipes 12 thus formed may be constructed as walls separating individual heat-pipe cavities, or as pillars passing axially through one or more heat-pipe cavities, or as a combination of both walls and pillars.

In the case of multiple heat-pipes, the heat-pipes may be tessellated in a honeycomb or other regular arrangement, for example. Figure 2 shows the heat-pipes 12 as being parallel to each other, and regularly distributed, and all of the same size. However, it should be understood that the size and

distribution and orientation of the heat-pipes can be arranged to suit particular conditions. For example, a semiconductor substrate may generate more heat in some areas than in others, in which case the heat-pipes 1 2 can be arranged for greater heat removal rate in those areas. The walls/pillars 1 1 are

advantageously oriented axially, to maximise their axial compressive load- bearing strength, such that the heat-pipes 12 are also oriented axially. However, the heat-pipes 12 may also be angled away from the main axis of the device, for example in order to transfer heat generated in one small area of the substrate 2 out to a wider area of the cooled end section 9.

The figures illustrate examples of pole-pieces in which significantly more of the volume of the pole-piece body section 1 3 is occupied by the heat- pipes than by the walls/pillars between the heat-pipes. However, this ratio may be reversed, and the body section may be constructed, with correspondingly increased strength under axial compression, such that the volume occupied by the heat-pipes is significantly smaller than that of the walls/pillars. Or the volumes may be similar. Generally, the total volume occupied by the one or more heat-pipes is advantageously in a ratio of between 0.2 and 5 to the total volume occupied by the one or more walls/pillars in the heat-pipe cavity.

Since the thermal transfer of heat between the heated end and the cooled end of the pole-piece can mainly be performed by the heat-pipes 12, the thermal transfer characteristics of the pillars/walls 1 1 becomes less important, and it is possible to consider materials other than copper for the pillars/walls 1 1 . It is possible to use glass, or glass-like substances, for example, or even a semiconductor material such as silicon. This has the advantage that its thermal coefficient of expansion is the same or very similar to that of the semiconductor substrate, and its presence can therefore help to reduce the thermally-induced stressed at the substrate/pole-piece interface. The pole-pieces, e.g. 15, 16, 17, 23, 24, are electrically conductive so that an electrical connection between the semiconductor substrate 2 and the cooled contact electrodes 1 is achieved.

On the pole-piece, exemplarily an external cooler 1 is arranged, which transport the heat away from the semiconductor substrate 2. The cooler 1 may be made of an electrically conductive material, exemplarily of copper, so that it can function as an electrode for the semiconductor substrate 2. The cooler 1 comprises surface enhancing means like holes and/or protuberances like cooling fins, which improve the cooling. The force to press the pole-pieces 15, 1 6, 1 7, 23, 24 with the semiconductor substrate 2 in between together is applied to the external cooler 1 so that also the cooler 1 is pressed together with the pole-pieces 15, 1 6, 17, 23, 24 and the semiconductor substrate 2. Such a cooler 1 may be arranged on one pole-piece or on both pole-pieces. Each of the pole-pieces 15 and 16 is shown in figure 2 as having a molybdenum disc 3, 3' (first and/or second layer) placed between the second end section 14 of the first and second pole-piece 15, 16 and the semiconductor substrate 2ar its planar or substantially planar surfaces, for reasons explained above. However, it may in some instances be possible to omit the molybdenum disc, because the structure of the pole-piece 15 and/or 16. Planar or substantially planar shall mean that the surfaces at which the main electrodes are arranged, are perpendicular to the main axis. The second end section 14 (heated end) may comprise a relatively thin sheet of copper, for example, which has a lower thermal resistance and a much lower thermal impedance than a solid copper pole-piece. The structure of the pole-piece 1 5 is almost as strong in the axial direction as a solid pole-piece, but is less rigid in the plane of the substrate/pole-piece interface, which means that it can bear large loads in the axial direction, but can also deform in the plane of the substrate (pole-piece interface to accommodate small deformations in the semiconductor substrate. Such deformations may be in the plane (eg thermal expansion and contraction in the plane), or they may include a warping or swelling of the semiconductor substrate against the contact surface of the pole-piece. Thus, one or both of the molybdenum discs 3 shown in figure 2 could be omitted.

Figure 3 shows a simplified representation of a power semiconductor device 20 which uses a pole-piece according to the second embodiment of the invention. In this case, the two pole-pieces are significantly different from each other. The pole-piece 17 is an axially elongated version of one of the pole- pieces 15 and 16 shown in figure 2, which the other pole-piece, 19, is a much thinner conducting plate, which contains no heat-pipes.

As mentioned earlier, the pole-pieces 15 and 1 6 of figure 2 may advantageously have the same construction, since it is easier to manufacture two pole-pieces with heat-pipes which are the same, than to manufacture two different pole-pieces with heat-pipes. However, it is simpler still to manufacture just one pole-piece having heat-pipes, and a simple solid pole-piece as the other. It has conventionally been proposed to position the substrate 2 at a midpoint along the central axis of the device, equidistant from the surfaces facing the contact electrodes 1 (external cooler). However, the substrate 2 can also be placed nearer to one electrode 1 than the other, in which case the second embodiment arrangement shown in figure 3 may be used. The device shown in figure 3 is shown with a molybdenum disc or layer 3 between the substrate and the elongated pole-piece 17. However, the molybdenum disc could be omitted, as discussed above. Similarly, a molybdenum disc could be fitted between the simple solid pole-piece 19 and the substrate 2, although this is not shown in figure 3.

The length of a heat-pipe (in axial direction) has very little influence on the rate at which it can transfer heat, so the elongated pole-piece 1 7 can be expected to convey away heat as fast as the shorter pole-piece 15 or 16.

Heat-pipes are usually affected by gravity - they work more efficiently when the return of the working fluid to the heated end is aided by gravity. In the example shown in figure 2, when the device is oriented with its main axis vertical, as shown in the figure, the upper one of the two pole-pieces, 15, would function more efficiently, because gravity would assist the flow of the working fluid to the heated end of the pole-piece 15, whereas the flow would be hindered by gravity in the lower pole-piece, 16. In the variant shown in figure 3, where only one 17 of the two pole-pieces 17, 18 comprises heat-pipes, the imbalance due to gravity can therefore be eliminated. For a horizontal arrangement of the silicon wafer 2, the heat-pipe(s) could be arranged in that pole-piece, which is arranged on top of the silicon wafer 2 (either anode or cathode pole-piece).

Note that the devices illustrated in the figures are greatly simplified, and omit many details which would be necessary in a real device. A passage through the pole-piece 15 for a gate lead, for example, is not shown.

Figure 4 shows a power semiconductor device according to a variant of the first embodiment of the invention. In this variant, the heat-pipes 12 and walls/pillars 1 1 of each pole-piece 23, 24 are tapered. The taper may be in either direction, or mixed (for example, some heat-pipes 12 may be narrower at the heated end and wider at the cooled end, while others are narrower at the cooled end and wider at the heated end). The tapered walls or pillars 1 1 may be configured to have a larger heat-transfer area at one end of the heat-pipe 12 (the cooled end in the illustrated example), while offering improved mechanical load-distribution at the other end of the heat-pipe 12 (the heated end in the illustrated example, thus avoiding or reducing localised areas if increased pressure on the substrate 2). The various embodiments and variants of the pole-pieces described above can be manufactured by casting or otherwise forming a tub assembly or element comprising the body section of the pole-piece and one of the end sections of the pole-piece, with the heat-pipe cavities formed in the body section (by boring, milling, etching, casting, for example). The heat-pipe working liquid, and a wick if a wick is used, can then be introduced into the heat-pipe cavity, and the assembly can then be heated to boil the working fluid until a

predetermined amount of liquid remains, while the remainder of the heat-pipe cavity is filled with the working fluid in gaseous phase, whereupon the other end section is fitted to the body section as a lid, and tightly sealed. During the boil and before sealing the boiling working fluid drives the atmosphere out of the heat pipe. The gas exit during this step is exemplarily chosen small in order to minimise reverse-flow of atmosphere into the heat pipe.

Claims

1 . A pole-piece (15, 1 6, 17, 23, 24) for a power semiconductor device (10, 20, 30), the pole-piece comprising:

a first end section (9) for thermally connecting, at an outer surface of the power semiconductor device (1 0, 20, 30), to an external cooler (1 ),

a second end section (14), for thermally connecting to a

semiconductor substrate (2) of the power semiconductor device (10, 20, 30), and a body section (13), extending between the first (9) and second (14) end sections,

characterized in that

the body section (1 3) encloses a heat-pipe cavity comprising one or more heat-pipes (12) for transferring heat from the second end section (14) to the first end section (9), wherein the heat-pipe cavity is completely embedded in the pole-piece (15, 16, 17, 23, 24).

2. A pole-piece (15, 1 6, 17, 23, 24) according to claim 1 , wherein the pole-piece (15, 1 6, 17, 23, 24) is adapted for transferring a force (F) applied to the first end section (9), through the body section (13), to the second end section (14) so as to press the second section (14) of the pole-piece (15, 16, 1 7, 23, 24) against the semiconductor substrate (2), and wherein the pole-piece (15, 1 6, 17, 23, 24) comprises at least one axial load transfer element (1 1 , 22) extending through the heat-pipe cavity and between the first end section (9) and the second end section (14) so as to transfer the said force (F) from the first end section (9) to the second end section (14).

3. A pole-piece (15, 1 6, 17, 23, 24) according to one of the preceding claims, wherein the total volume of the one or more heat-pipes (12) is in a ratio of between 1 :0.2 and 1 :5 to the total volume of the one or more axial load transfer elements (1 1 , 12).

4. A pole-piece (15, 1 6, 17, 23, 24) according to one of the preceding claims, wherein one or more of the or each axial load transfer element (12) and/or one or more of the or each heat-pipe (12) is tapered.

5. A pole-piece (15, 1 6, 17, 23, 24) according to claim 4, at least one of wherein the or each tapered axial load transfer element (12) is wider near the second end section (14) than the first end section (9) or wherein the or each tapered heat-pipe (12) is wider near the first end section (19) than the second end section (14)

6. A pole-piece (15, 1 6, 17, 23, 24) according to one of the preceding claims, wherein the body section (13) and/or the or each axial load transfer element (1 1 , 12) comprises an electrically insulating or semiconducting material.

7. A pole-piece (15, 1 6, 17, 23, 24) according to claim 6, wherein the electrically insulating or semiconductor material is a glass or silicon.

8. A power semiconductor device (10, 20, 30) comprising a first pole- pole-piece (15, 17) in form of a pole-piece according to one of claims 1 to 9, and a second pole-pole-piece (16, 18), wherein the semiconductor substrate (2) is held in compression between the second end sections (14) of the first and second pole-pole-pieces (16, 18).

9. A power semiconductor device (10) according to claim 8, wherein the second pole-pole-piece (15, 16, 17, 23, 24) is a pole-piece (15, 1 6, 17, 23, 24) according to one of claims 1 to 9.

10. A power semiconductor device (20) according to claim 8, wherein the second pole-piece (18) does not comprise a heat-pipe.

1 1 . A power semiconductor device (10, 20, 30) according to one of claims 8 to 10, wherein the axial length of the first pole-piece (15, 17) is greater than the axial length of the second pole-piece (16, 18) by a predetermined factor, preferably by a predetermined factor, which is greater than 5.

12. A power semiconductor device (10, 20, 30) according to one of claims 8 to 13, which comprises at least one external cooler (1 ) which is arranged on the first end section (9) of the pole-piece (15, 1 6, 17, 23, 24), wherein the external cooler (1 ) is made of electrically conductive material and wherein the external cooler (1 ) functions as an electrode for the semiconductor substrate (2).

13. A power semiconductor device (10, 20, 30) according to one of claims 8 to 12, wherein at least one of - a first layer (3) of a first electrically conducting material is disposed between the second end section (14) of the first pole-piece (15, 17, 23) and a substantially planar surface of the semiconductor substrate (2) and wherein the first electrically conducting material has a thermal coefficient of expansion which is more similar to that of the semiconductor substrate (2) than that of the first pole-piece (15, 1 7, 23), or

- a second layer (3') of a second electrically conducting material is disposed between the second end section of the second pole-piece (16, 18) and a second substantially planar surface of the semiconductor substrate (2) and wherein the second electrically conducting material has a thermal coefficient of expansion which is more similar to that of the semiconductor substrate than that of the second pole-piece (1 5, 16, 1 7, 23, 24).

14. Method of manufacturing a pole-piece (15, 16, 17, 23, 24) for a power semiconductor device (10, 20, 30) according to one of claims 1 to 13, the method comprising the steps of:

fabricating a tub element comprising the body section (13) of the pole-piece (15, 16, 17, 23, 24) and a first one of the first (9) or second (14) end sections of the pole-piece (15, 16, 17, 23, 24),

fabricating a heat-pipe cavity comprising one or more heat-pipes (12) in the body section (13), such that the heat-pipe cavity is completely embedded in the pole-piece (15, 1 6, 17, 23, 24),

introducing a heat-pipe working liquid into the one or more heat-pipe cavities (12),

partially evacuating the one or more heat-pipe cavities (12), attaching the other of the first (9) and second (14) end sections to the body section (13) so as to seal the one or more heat-pipe cavities (12).

15. Method according to claim 14, comprising the step of introducing a heat-pipe wick material into the one or more heat-pipe cavities (12).