JP7169187B2 - power module - Google Patents

Description

The present invention relates to power modules.

Power modules mounted with power devices (semiconductor elements) such as IGBTs (Insulated Gate Bipolar Transistors), MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), and diodes are used in motor drive inverters and the like.

In order to ensure the reliability of the power module, it is first necessary to efficiently dissipate the heat generated during operation of the power device (simply referred to as "element"). Descriptions relating to such a heat dissipation structure are found in the following patent documents.

JP-A-2008-42039 JP 2008-103623 A

By the way, in order to further improve the reliability of the power module, it is important to ensure heat resistance and durability at the joints of the element electrodes, which are exposed to high temperatures, in addition to ensuring heat dissipation. Therefore, conventionally, a bonding material with a high melting point and high heat resistance (for example, lead-free solder such as SnAgCu, SnSb, etc.) has been used.

However, a power module using such a bonding material has sufficient durability under thermal cycles (temperature cycles), but does not necessarily have sufficient durability under power cycles. In the thermal cycle, the power module is repeatedly exposed to a high-temperature environment and a low-temperature environment with a substantially uniform temperature distribution. In the power cycle, power to the power device is repeatedly turned on and off within a short period of time, and the power module is repeatedly exposed to an environment that causes uneven temperature distribution.

In order to ensure durability under power cycles, bonding materials with low Young's modulus or low yield strength (normal temperature range) have also been used. However, in a power module using such a bonding material, the maximum guaranteed temperature of the bonding temperature (Tj) is limited to a low temperature range (for example, 150° C. or less).

SUMMARY OF THE INVENTION It is an object of the present invention to provide a power module capable of ensuring reliability under power cycles up to a high temperature range.

As a result of intensive research to solve this problem, the present inventors have found that the temperature dependence of the mechanical properties (e.g. yield strength) of the first bonding material that bonds the electrodes on the first surface side (rear surface side/lower surface side) of the power device. and the temperature dependence of the mechanical properties (e.g. yield strength) of the second bonding material that bonds the electrode on the second surface side (surface side/upper surface side) within a predetermined range. It was newly found that the reliability (durability) in the high temperature range can be secured. Developing this discovery led to the completion of the present invention described below.

《Power module》
(1) The present invention provides a power device made of a semiconductor and having at least a first electrode on the first surface side and a second electrode on the second surface side; a first spreader to be bonded; and a terminal to be bonded to the second electrode via a second bonding material, the semiconductor being made of silicon or silicon carbide, and the first electrode being more spread out than the second electrode. The second electrode includes an AlSi layer made of an aluminum alloy containing silicon (referred to as "Al—Si alloy"), and the first spreader and the terminal are copper or a copper alloy The first bonding material has an absolute value (|Δσ ₁ |) of the difference between the 0.2% proof stress at 30° C. and the 0.2% proof stress at 175° C. of 10 MPa or less, The second bonding material has a 0.2% yield strength smaller than the Al—Si alloy in the entire range of 30 to 175 ° C., and has a 0.2% yield strength at 30 ° C. and a 0.2% yield strength at 175 ° C. The absolute value of the difference (|Δσ ₂ |) is 20 MPa or less.

(2) The power module of the present invention has high durability (reliability ) can be demonstrated. For example, even under such an environment, the AlSi layer forming the second electrode is prevented from being cracked.

The reason and mechanism for exhibiting such excellent effects are not necessarily clear. However, the presence or absence of cracks on the second electrode side (especially the AlSi layer) affects the temperature dependence of the mechanical properties (for example, yield strength) of the bonding material, and the first electrode side opposite to the second electrode. It is an unprecedented finding that it is effective to make the temperature dependence of the mechanical properties of the first bonding material smaller than that of the second bonding material on the second electrode side. The present invention has been made based on such discoveries and is epoch-making.

"others"
Unless otherwise specified, "x to y" as used herein includes the lower limit value x and the upper limit value y. A new range such as “a to b” can be established as a new lower or upper limit of any numerical value included in the various numerical values or numerical ranges described herein.

FIG. 2 is a cross-sectional view schematically showing a main part of a double-sided cooling structure type power module; It is the ultrasonic microscope image which observed the 1st electrode surface vicinity after the power cycle test which concerns on each sample. FIG. 3 is a diagram of an energization pattern for one cycle related to a power cycle test; It is the optical microscope image which observed the cross section near the 1st jointing material after the power cycle test concerning each sample. 4 is a graph showing the temperature dependence of proof stress of each metal used as a bonding material.

The components of the present invention may be supplemented with one or more optional components selected from herein. The contents described in this specification may apply not only to the power module of the present invention but also to the manufacturing method thereof. Components related to the "method" can also be components related to the "product".

《Power device》
Power devices are primarily transistors, although they may be diodes. The transistor comprises a first electrode, a second electrode and a control electrode (third electrode). In the case of the present invention, the first electrode has a larger electrode area (or junction area) than the second electrode, the first electrode is on the first surface side of the transistor, and the second electrode is on the second surface side (first surface). on the opposite side). As long as these conditions are satisfied, the transistor may be an IGBT, MOSFET, thyristor, or the like. In the case of an IGBT, for example, the first electrode is the collector electrode and the second electrode is the emitter electrode. For a MOSFET, for example, the first electrode is the drain electrode and the second electrode is the source electrode. In the case of a thyristor, for example, the first electrode is the anode electrode and the second electrode is the cathode electrode. In either case, the control electrode is usually the gate electrode. Incidentally, the power module of the present invention may include devices other than transistors (diodes, etc.) and control circuits.

A power device consists of a semiconductor (Si or SiC). A p-region and an n-region in a semiconductor substrate (chip) are formed by, for example, ion implantation. Also, the first electrode and the second electrode are formed by, for example, metal deposition (metallization), heat treatment (annealing), or the like. Note that the control electrode is formed of, for example, an insulating film (oxide film), a polysilicon film, or the like.

The coefficient of thermal expansion (CTE) of such semiconductors is on the order of 2 to 5 or even 3 to 6 ppm/K. Incidentally, Si has a CTE of about 3 ppm/K, and SiC has a CTE of about 3.7 ppm/K.

The first electrode and the second electrode (especially the second electrode) include an AlSi layer made of an aluminum alloy containing silicon (referred to as "Al--Si alloy"). The AlSi layer is generally provided to ensure adhesion between the semiconductor and the Ni electrode and to obtain good electrical contact characteristics. Cracks are likely to occur in the AlSi layer due to non-uniform temperature distribution, CTE mismatch, etc. that occur in a power cycle environment.

The first electrode and the second electrode may have an extremely thin Al layer or Ti layer (about 50 nm to 500 nm) or the like formed under the AlSi layer (on the semiconductor). Also, a Ni layer or the like may be formed on the AlSi layer. The lower layers such as the Al layer and the Ti layer are provided, for example, to ensure ohmic contact and adhesion. The Ni layer or the like, which is the upper layer, is provided, for example, to ensure bondability with a bonding material.

《Terminal and Spreader》
The first spreader joined to the first electrode and the terminal joined to the second electrode are made of copper (pure copper such as oxygen-free copper) or a copper alloy having excellent electrical and thermal conductivity. These CTEs are typically on the order of 15-19 ppm/K. However, a copper alloy with a small CTE (for example, a Cu--W alloy, a Cu--Mo alloy, etc. with a CTE of about 4 to 10 ppm/K) may be used. Note that these can also be appropriately applied to a second spreader to be described later.

When the power device is a transistor having a control electrode on the second surface side, the terminal is, for example, a spacer that secures a space for a bonding wire that is joined to the control electrode. Its thickness is, for example, about 0.1 mm to 2 mm, further 0.5 mm to 1.5 mm. A second spreader, for example, is joined to the second surface side of the terminal via a third joining material.

The spreader is a heat conducting member that receives heat generated by the power device and dissipates the heat to the outside. The spreader is, for example, a lead frame that also serves as a conductive member, a circuit layer of a DBC (Direct Copper Bond) substrate, or the like.

《Bonding material》
(1) A first joint material used for a joint portion between the first electrode and the first spreader (referred to as a “first joint portion”) and a joint portion between the second electrode and the terminal (referred to as a “second joint portion”). Any of the second bonding materials used in the above should preferably have a small change in mechanical properties (strength, creep resistance, ductility, Young's modulus, etc.) with temperature.

The temperature dependence of the mechanical properties of each bonding material is, for example, the difference ( _{Δσ =} σ _L −σ _H ) or its absolute value (|Δσ|). For the first bonding material, for example, the absolute value (|Δσ ₁ |) of the 0.2% proof stress difference should be 10 MPa or less, or 6 MPa or less. For the second bonding material, for example, the absolute value (|Δσ ₂ |) of the 0.2% proof stress difference is preferably 20 MPa or less, or 12 MPa or less. In any case, the 0.2% yield strength difference (absolute value) of the second bonding material on the side of the second electrode (especially the AlSi layer) where cracks are likely to occur It is preferable to reduce the 0.2% proof stress difference (absolute value).

(2) The first bonding material is preferably made of a composite material in which an aluminum layer and an intermetallic compound layer are laminated, for example. Aluminum foil, for example, can be used for the aluminum layer. This aluminum foil is preferably made of pure aluminum, for example, in which the total amount of impurity elements other than Al is 1% by mass or less (less than). Specifically, the purity is preferably 2N to 4N. 2N means 99% purity, 3N means 99.9% purity, and 4N means 99.99% purity. Excessively high-purity aluminum foil is expensive, and the temperature variation of mechanical properties can be large. Excessive impurities can lead to high strength or low ductility in the aluminum layer and reduce its stress relaxation properties.

The thickness of the aluminum layer may be, for example, 60 to 95%, or preferably 75 to 90% of the thickness of the entire composite material. Specifically, the aluminum layer is, for example, about 10 to 500 μm, or even 20 to 200 μm. If the thickness is too small, the reduction of thermal strain occurring in the second electrode (especially the AlSi layer) becomes insufficient. In addition, the stress relaxation property and thermal fatigue resistance of the aluminum layer become insufficient. If the thickness of the aluminum layer becomes excessively large, the reduction in thickness of the power module is hindered.

The intermetallic compound layer is, for example, a metal having a higher melting point than the low melting point metal (layer) formed by vapor deposition or the like on at least one side of the surface to be joined. An intercompound (IMC) is produced. Such bonding by IMC is called solid-liquid interdiffusion bonding or simply "SLID (Solid Liquid Interdiffusion) bonding". In this specification, an aluminum layer (Al layer) and a bonding material (composite material ) is simply referred to as “Al-SLID”.

The combination of low-melting-point metal and high-melting-point metal that produces the intermetallic compound layer (and thus the composition of the intermetallic compound) is selected in consideration of the heat resistance temperature of the power module, the heating temperature during the bonding process, the coefficient of thermal expansion, etc. be. Examples of low-melting-point metals include Sn, In, Ga, Pb, Bi, Zn, and alloys thereof. Refractory metals include Ni, Cu, Ti, Mo, W, Si, Cr, Mn, Co, Zr, Nb, Ta, Ag, Au, Pt, and alloys thereof.

As an example, Sn (melting point: about 230° C.) may be combined with Ni (melting point: about 1450° C.) or Cu (melting point: about 1085° C.). For example, when the Sn layer and the Ni layer are brought into contact and heated at about 350° C. for about 5 minutes, an intermetallic compound layer made of nickel tin (NiSn/melting point: about 795° C.) is obtained. In this way, a composite bonding layer having a high melting point can be obtained while suppressing the bonding temperature. Other high melting point metal/low melting point metal combinations may be Cu/Sn, Ag/Sn, Pt/Sn/, Au/Sn, and the like.

The intermetallic layer is typically much thinner than the aluminum layer. The thickness of the intermetallic compound layer (for one layer) on one side of the aluminum layer is, for example, about 1 to 15 μm, further 3 to 10 μm. The total thickness of the intermetallic compound layer with respect to the thickness of the entire composite material is, for example, about 5 to 40%, further 10 to 25%.

Such Al-SLID has stable mechanical properties (0.2% yield strength, etc.) within the above temperature range (30 to 175° C.), and its temperature dependence is sufficiently small. Therefore, Al-SLID (composite material) may be used as the first bonding material or the second bonding material. It can be considered that the mechanical properties of Al-SLID almost depend on the mechanical properties of the aluminum layer that occupies most of it.

(3) Various materials having relatively low temperature dependence of mechanical properties can be used for the second bonding material. For example, in addition to using the Al-SLID described above, conventional tin-copper solder (simply referred to as “SnCu solder”) can also be used. SnCu solder, for example, is made of a tin alloy containing 0.4 to 1.5% by mass, further 0.5 to 1% by mass of Cu, with the balance being Sn and impurities when the whole is 100% by mass. In addition, as long as the absolute value of the 0.2% proof stress difference (|Δσ ₂ |) is within a predetermined range, a solder made of an alloy such as Ag or Ni may be used.

(4) Various types of solder, Al-SLID, etc. can be used as the third bonding material for bonding the second surface side of the terminal and the second spreader. Between the terminal and the second spreader, the temperature is lower and the temperature distribution is substantially more uniform than between the power device and the terminal or the first spreader. Therefore, the temperature dependence of the mechanical properties of the third bonding material does not necessarily matter.

Various power modules were manufactured by changing the combination of the first joint material and the second joint material. They were subjected to a power cycle test to observe whether or not cracks occurred in the second joint portion (especially the AlSi layer of the second electrode) to evaluate the durability (reliability) of the power module. The present invention will be described in more detail while citing such specific examples.

《Summary of power module》
FIG. 1 shows a schematic cross-sectional view of a double-sided cooling structure type power module M that is an embodiment of the present invention. The power module M includes a transistor 1 that is an IGBT, a terminal 2 that is a spacer, and a first spreader 31 and a second spreader 32 that are lead frames.

The transistor 1 includes an IGBT substrate 10, a first electrode 11 as its collector electrode, a second electrode 12 as its emitter electrode, and a control electrode 13 (third electrode) as its gate electrode.

The first electrode 11 is joined to the first spreader 31 via the first joining material 41 . The second electrode 12 is joined to the first surface side of the terminal 2 via the second joining material 42 . The control electrode 13 is joined to the signal terminal 52 by a bonding wire 51 . The second surface side of the terminal 2 is joined to the second spreader 32 via the third joint material 43 .

Note that FIG. 1 shows, as an example, the case where the first bonding material 41 is an Al-SLID (composite material) composed of an aluminum layer 413 and intermetallic compound layers 411 and 412 formed on both sides thereof. rice field.

《Power cycle test》
(1) Samples Various power modules M (samples) to be subjected to the power cycle test were manufactured. First, the substrate 10 of the transistor 1 is composed of a silicon (Si single crystal) chip (12.4 mm.times.12.4 mm.times.135 .mu.m thick). The electrode surface (bonding surface) of the first electrode 11 is 12.4 mm×12.4 mm (area: 154 mm ² ), and the electrode surface (bonding surface) of the second electrode 12 is 11 mm×10 mm (area: 110 mm ² ).

The first electrode of the transistor 1 is formed by sequentially stacking an AlSi layer (5 μm thick), a Ti layer (250 nm thick), and a Ni layer (3 μm thick) on the lower silicon surface of the substrate 10 . The second electrode of the transistor 1 is formed by sequentially stacking an AlSi layer (5 μm thick) and a Ni layer (3 to 5 μm thick) on the silicon upper surface of the substrate 10 . Each layer is formed by sputtering a raw material metal (target). The AlSi layer was made of an aluminum alloy (referred to as "Al-1% Si") with Si: 1% by mass and Al: the balance as the raw material metal.

Oxygen-free copper (pure copper) foil was used for terminal 2 (thickness 1.15 mm) and spreaders 31 and 32 (thickness 2 mm).

Various bonding materials shown in FIG. 2A were used for the first bonding material 41 and the second bonding material 42 . SnCu is a tin alloy solder (thickness: 100 μm) containing 0.7% by mass of Cu and the balance of Sn. SnSb is a tin alloy solder (thickness: 100 μm) containing 5% by mass of Sb and the balance of Sn. The alloy composition referred to in this specification is a mass ratio (% by mass) to the entire alloy unless otherwise specified.

The Al-SLID is formed by stacking an aluminum layer 413 made of pure aluminum (2N) foil (thickness 100 μm) and intermetallic compound layers 411 and 412 made of Sn and Ni (thickness 5 μm each). The thickness of the aluminum layer is 91% of the entire Al-SLID.

In addition, Al-SLID is performed by heating (270 ° C.) in a state in which a bonding sheet in which Ni and Sn are metallized in order on both sides of a pure aluminum foil is interposed between the surfaces to be bonded where Ni is metallized, and the SLID reaction is performed. is formed by giving rise to The above-described SnCu (Sn-0.7 mass % Cu) solder (thickness: 150 μm) was used as the third bonding material.

(2) Test A power cycle test was performed by applying the energization pattern shown in FIG. 2B to each sample (power module M) for 50,000 cycles. The energization was turned on and off so that the temperature at the center of the device varied between 30° C. (Tj _min ) and 150° C. or 175° C. (Tj _max ). The element center temperature was measured by a temperature sensor using a diode.

"observation"
(1) For each sample, the second joint portion (near the second joint member 42) where the second electrode 12 and the terminal 2 (first surface side) are joined was observed with an ultrasonic microscope. The observed image and the presence or absence of cracks in the second joint (in particular, the AlSi layer of the second electrode 12) are also shown in FIG. 2A.

(2) The cross section of the second joint of each sample was observed with an optical microscope. The observed images are collectively shown in FIG.

"evaluation"
The following can be understood from FIG. 2A. When SnCu solder is used for the first bonding material 41 and the second bonding material 42 as in sample 11, cracks are not observed in the second bonding portion if the upper limit value (Tj _max ) of the element center temperature is up to 150°C. I didn't. However, even when SnCu solder is used for the first bonding material 41 and the second bonding material 42, as in the sample 12, when the upper limit value (Tj _max ) of the element center temperature reaches 175° C., cracks occur in the second bonding portion. There has occurred.

Even when SnSb solder, which has a higher strength (higher melting point) than SnCu solder, was used for the first bonding material 41 and the second bonding material 42, as in Sample 2, cracks occurred in the second bonding portion. Furthermore, as in Sample 4, when Al-SLID was used as the first bonding material 41 and SnSb was used as the second bonding material 42, cracks occurred in the second bonding portion.

On the other hand, when the first bonding material 41 is Al-SLID and the second bonding material 42 is SnCu solder or Al-SLID as in Sample 3 or Sample 5, cracks do not occur in the second bonding portion. rice field.

Furthermore, as can be seen from FIG. 3, it was also found that cracks were generated in the AlSi layer of the second electrode 12 in the case of the cracked sample. In addition, when there were no cracks in the second joint portion as in Sample 3, no cracks or undulations were observed in the second electrode 12 (especially the AlSi layer).

《Consideration》
Based on the results described above, the presence or absence of cracks occurring in the AlSi layer of the second electrode (and thus the durability and reliability of the power module) can be determined by the difference (combination) in the materials of the first and second bonding materials. The reason for the difference is currently presumed as follows.

(1) FIG. 4 shows the temperature dependence of the 0.2% proof stress of each metal material used for the first bonding material 41 and the second bonding material 42 . This temperature dependence is data obtained by performing a tensile test on each metal material under conditions of a strain rate of 0.01 to 0.06%/sec using a tensile tester with a constant temperature bath. Al-SLID showed a temperature dependence of proof stress of 0.2 for Al (2N), which occupies most of the Al-SLID. Incidentally, the 0.2% proof stress is the stress value when the plastic strain after unloading is 0.2% on the stress-strain diagram obtained by the tensile test performed at each temperature.

As is clear from FIG. 4, the 0.2% yield strength of SnSb solder is considerably large at low temperature (25° C.), but it rapidly decreases at high temperature (200° C.). In other words, the mechanical properties of SnSb solder tend to change greatly with temperature (high temperature dependence).

On the other hand, the 0.2% proof stress of SnCu solder is not so large at low temperature (25° C.), but the change with respect to temperature is small (the temperature dependence is small). Furthermore, Al-SLID (Al(2N)) has very little temperature dependence, with almost no change in 0.2% proof stress in the range of 25 to 200°C.

Also, as is clear from FIG. 4, looking at the absolute value (|Δσ|) of the 0.2% yield strength difference between 30° C. and 175° C., SnSb solder |Δσ|=24 MPa (>20 MPa )Met. On the other hand, the SnCu solder was |Δσ|=13 MPa (10 MPa<|Δσ|<20 MPa), and the Al-SLID was |Δσ|=3 MPa (|Δσ|<10 MPa). Furthermore, SnCu solder and Al-SLID had lower 0.2% proof stress than Al-1%Si over the entire temperature range of 30 to 175°C.

(2) Under the power cycle, the temperature of the transistor 1 (especially the substrate 10) becomes non-uniform, resulting in non-uniform thermal stress and deformation/strain occurring in each electrode and each junction. Such non-uniformity is considered to be a major factor causing cracks in each electrode (especially the AlSi layer of the second electrode). Such a tendency tends to become more pronounced as the element temperature (upper limit) increases.

However, as in the present invention, a bonding material with low temperature dependence of mechanical properties is used for the bonding portion of the power device, and the second bonding material on the side of the second electrode, which is prone to cracking, is used on the opposite side of the second bonding material. If the temperature dependence of the first bonding material on the side of a certain first electrode is further reduced, the above-described non-uniform strain and the like occurring in each bonding portion and each electrode can be remarkably suppressed, although the reason is not clear. It is thought that

Regardless of the detailed mechanism, by adopting the power module of the present invention, it is certain that durability and reliability can be ensured or improved.

M power module 1 transistor 10 substrate 11 first electrode 12 second electrode 13 control electrode 2 terminal 31 first spreader 32 second spreader 41 first joint material 42 second joint material 43 third joint material

Claims (7)

a power device made of a semiconductor and having at least a first electrode on the first surface side and a second electrode on the second surface side;
a first spreader bonded to the first electrode via a first bonding material;
a terminal joined to the second electrode via a second joint material;
The semiconductor consists of silicon or silicon carbide,
The first electrode has a larger electrode area to be joined than the second electrode,
The second electrode includes an AlSi layer made of an aluminum alloy containing silicon (referred to as "Al-Si alloy"),
the first spreader and the terminal are made of copper or a copper alloy;
The first bonding material has an absolute value (|Δσ ₁ |) of the difference between the 0.2% yield strength at 30° C. and the 0.2% yield strength at 175° C. of 10 MPa or less,
The second bonding material has a 0.2% yield strength smaller than the Al—Si alloy in the entire range of 30 to 175 ° C., 0.2% yield strength at 30 ° C. and 0.2% yield strength at 175 ° C. The absolute value of the difference (|Δσ ₂ |) is 20 MPa or less ,
The first bonding material is a power module made of a composite material in which an aluminum layer and an intermetallic compound layer are laminated . 2. The power module according to claim 1, wherein the second bonding material is a tin alloy solder containing copper. 2. The power module according to claim 1, wherein the second bonding material comprises a composite material in which an aluminum layer and an intermetallic compound layer are laminated. 4. The power module according to claim 2, wherein said aluminum layer is made of aluminum foil with a purity of 2N to 4N. The power module according to any one of claims 2 to 4, wherein the aluminum layer has a thickness of 60 to 95% of the total thickness of the composite material. The power module according to any one of claims 2 to 5, wherein the intermetallic compound layer is made of tin and nickel. The power device is a transistor further having a control electrode on the second surface side,
the terminal is a spacer that secures a space for a bonding wire that is joined to the control electrode;
The power module according to any one of claims 1 to 6, further comprising a second spreader joined to the second surface side of the spacer.

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