Metal-ceramic composite material

Abstract

The present invention relates to a metal-ceramic composite material comprising: a ceramic substrate comprising a front surface and a back surface and comprising silicon nitride; the metal coating is arranged on the front face of the ceramic substrate, the metal coating comprises at least one concave part, and the surface of the ceramic substrate is exposed through the concave part. Wherein the surface of the ceramic substrate exposed through the recess has a tensile surface area ratio Sdr of at least 7.0% according to standard ILNAS-EN ISO 25178-2: 2022, where Sdr is determined by confocal microscopy.

Description

[0001] The present invention relates to a metal-ceramic composite material which can be used as a ceramic circuit carrier in power electronic devices.

[0002] In power electronic devices, printed circuit boards must be designed for high currents, serve as carriers for power components such as MOSFETs, and be able to dissipate waste heat quickly.

[0003] Since ceramic materials such as alumina, aluminum nitride, and silicon nitride have much higher thermal conductivities than the polymers used to produce conventional printed circuit boards, ceramic circuit carriers are often used in power modules.

[0004] Silicon nitride-based ceramic substrates have very high mechanical strength, along with high thermal conductivity, and are therefore very suitable for applications in power electronic devices.

[0005] Silicon nitride-based ceramic substrates are described, for example, in the following publications:

[0006] N. Chasserio et al., "Ceramic Substrates for High-Temperature Electronic Integration", Journal of Electronic Materials, Vol. 38 (2009), pp. 164-174;

[0007] K. Hirao et al., "High Thermal Conductivity Silicon Nitride Ceramics", Journal of the Korean Ceramic Society, Vol. 49 (2012), pp. 380-384;

[0008] Y. Zhou et al., "Development of high-thermal-conductivity silicon nitride ceramics", Journal of Asian Ceramic Societies, 3 (2015), pp. 221-229.

[0009] Silicon nitride-based ceramic substrates are commercially available, showing a good compromise between high mechanical strength and high thermal conductivity and can be used in electronic components.

[0010] The ceramic circuit carrier contains a ceramic substrate, which is provided with metal layers on at least one side thereof, usually on both sides. In the final module, semiconductor components are applied to one of these metal layers, and the metal layer on the opposite side of the ceramic substrate is thermally connected to a heat sink. The ceramic substrate electrically insulates the metal layers from each other.

[0011] The production of metallized ceramic substrates used as ceramic circuit boards is known to those skilled in the art and is carried out, for example, by bringing the front and back sides of the ceramic substrate into contact with a metal foil (e.g., copper foil or aluminum foil) and bonding them together. The material bonding of the metal foil is achieved, for example, by eutectic bonding or active metal brazing (AMB). If the metal foil is a copper foil, the eutectic bonding is also referred to as the DCB or DBC process (DCB: "direct copper bonding"; DBC: "direct bonded copper"). In the case of an aluminum foil, the term "DAB" ("direct aluminum bonding") is also used for eutectic bonding. Metallized ceramic substrates produced using the DCB or AMB process are sometimes also referred to as DCB substrates (alternatively: DBC substrates) or AMB substrates.

[0012] The metallization of silicon nitride substrates is usually carried out by active metal brazing.

[0013] The active metal solder contains, in addition to the main components such as Cu, Ag, or Au, one or more elements that can react with the ceramic to form an adhesion-promoting reaction layer (see, for example, Technical Ceramics Guide, Chapter 8.2.4.3 ("Active metal brazing") in the Ceramic Industry Association, pages 203 - 204, 2003, Fahner Verlag). Reactive elements such as hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), cerium (Ce), tantalum (Ta), and vanadium (V) are used. When metallizing a silicon nitride substrate by active metal brazing, the reaction layer contains, for example, a nitride, oxynitride, and / or silicide of a reactive element (A. et al., "Active metal brazing of copper with aluminum nitride and silicon nitride ceramics", Keramische Zeitschrift, 63(5), 2011, 334 - 342).

[0014] The metal layer carrying the semiconductor component includes one or more recesses and is thus also referred to as a structured metal coating. For example, metal conductor tracks are formed on a ceramic substrate by structuring. Adjacent conductor tracks are spatially separated by the recesses and are thus electrically insulated from each other. Structuring of the AMB substrate can be carried out, for example, in a two-stage process, in which, in a first step, the metal layer is first removed in a defined area (for example, using a first etching solution), and then, in a second step, the adhesion-promoting layer produced by the active metal soldering process is removed (for example, using a second etching solution). By removing the metal layer and (if applicable) the adhesion-promoting layer, the ceramic substrate is exposed again in the defined area.

[0015] Modules based on ceramic circuit carriers used in power electronics can be encapsulated as part of the packaging process, for example, by embedding the power module in a casting material.

[0016] For example, embedding in a casting material increases the electrical breakdown strength. In addition, the semiconductor component and the metal conductor tracks are protected from moisture and are mechanically stable.

[0017] In the areas exposed by structuring, the ceramic substrate is in direct contact with the casting material. When operating the power module, significant temperature fluctuations can occur. Since the ceramic material and the casting material usually have significantly different coefficients of thermal expansion, these temperature fluctuations cause mechanical stresses to occur at the interface between the ceramic and the casting material. This, in turn, can cause the casting material to become at least partially detached from the ceramic substrate and form cavities. Moisture penetrating these cavities can damage the power module. In addition, the electrical breakdown strength may be significantly reduced, at least locally.

[0018] As described above, silicon nitride-based ceramic substrates are used as circuit carriers in power electronics due to their very high mechanical strength and high thermal conductivity. In order to fully utilize their potential as circuit carriers, it is desirable that, after embedding in a casting material, there is a high adhesion strength between the casting material and the silicon nitride surface.

[0019] JP 2018-046192 A describes a ceramic-metal composite embedded in a polymeric casting material. Silicon nitride ceramics are used as the ceramic substrate. The exposed areas of the ceramic surface of the ceramic-metal composite are treated with a particle jetting medium such that, in these treated areas, the ceramic surface has a maximum profile height Ry of 1.7 μm to 2.7 μm. According to JP2018-046192 A, this should ensure good adhesion of the casting material to the ceramic substrate.

[0020] An object of the present invention is to provide a ceramic substrate containing metallized silicon nitride, the exposed ceramic surface of which allows bonding with a polymeric casting material to form a high adhesion strength.

[0021] This object is achieved by a metal-ceramic composite material, which contains:

[0022] - a ceramic substrate, which includes a front side and a back side and contains silicon nitride,

[0023] - a metal coating, which is present on the front side of the ceramic substrate, wherein the metal coating includes at least one recess, and the surface of the ceramic substrate is exposed through the recess,

[0024] wherein the surface of the ceramic substrate exposed through the recess has a stretched surface area ratio S of at least 7.0% according to the standard ILNAS-EN ISO 25178-2:2022 dr .

[0025] As is well known, the roughness of a surface can be measured from a profile (i.e., along a line) or on the surface. Various roughness parameters can be used for both profile and area measurements. In the ISO 4287 standard, certain roughness parameters are defined for a profile, such as the arithmetic mean deviation R a (also known as the arithmetic mean roughness value) or the maximum height R z . The roughness parameters measured on the surface are defined in the EN ISO 25178 series of standards and include, for example, the arithmetic mean height S a , the maximum height S z and the stretched surface area ratio S dr . Compared with profile parameters, the surface parameters can be used to more reliably measure the roughness of a surface. Optical measurement methods such as confocal microscopy are used to measure the surface parameters.

[0026] The stretched (or "unrolled") surface area ratio S dr according to ILNAS-EN ISO 25178-2:2022 describes the relationship between an ideal plane and the actually measured surface and is thus a measure of the roughness of the surface. The stretched surface area ratio S dr is also known as the "unrolled interface area ratio" and indicates the percentage increase in the actual surface area compared to the projected (and thus completely flat) surface area. Therefore, if the actual surface is completely flat, S dr will be 0%. For example, if the actual surface is twice the projected surface, S dr is 100%. If the area increases to 2.5 times, S dr is 150%.

[0027] Although the parameters S a and S z are pure height parameters, i.e., they only provide information in the z-direction of the tested surface, S dris a so-called hybrid parameter whose value depends not only on the height of the surface elevation but also on the distance between adjacent elevations.

[0028] Within the scope of the present invention, it has been recognized that if the exposed surface of the ceramic substrate has a tensile surface area ratio S of at least 7.0%. dr , then the adhesion strength of the casting material on the ceramic substrate can be improved. The tensile surface area ratio S dr is determined by confocal microscopy.

[0029] Preferably, the tensile surface area ratio S of the surface of the ceramic substrate exposed through the recesses is at least 9.2%. dr

[0030] In an exemplary embodiment, the tensile surface area ratio S of the surface of the ceramic substrate exposed through the recesses is 7.0% to 20.0%, more preferably 9.2% to 15.0%. dr

[0031] If the tensile surface area ratio S dr exceeds 20%, the risk of damage to the ceramic substrate increases, which in turn adversely affects mechanical properties such as the flexural strength of the ceramic substrate.

[0032] For example, the surface of the ceramic substrate exposed through the recesses has a maximum height S of 8 μm to 20 μm, more preferably 10 μm to 15 μm, according to standard ILNAS-EN ISO 25178-2:2022. z . The maximum height S z is determined by confocal microscopy.

[0033] In an exemplary embodiment, the surface of the ceramic substrate exposed through the recesses has a tensile surface area ratio S of 7.0% to 20.0% dr and a maximum height S of 8 μm to 20 μm z , more preferably a tensile surface area ratio S of 9.2% to 15.0% dr and a maximum height S of 10 μm to 15 μm z .

[0034] The ceramic substrate contains silicon nitride. Silicon nitride-based ceramic substrates suitable for producing power modules are known to those skilled in the art and are commercially available.

[0035] For example, the ceramic substrate contains silicon nitride in a proportion of at least 70% by weight, more preferably at least 80% by weight.

[0036] ​​Optionally, the ceramic substrate may contain one or more metal oxides. These oxides are added, for example, as sintering aids during the production of the ceramic substrate. Oxide components suitable for silicon nitride ceramics are known to those skilled in the art. For example, the ceramic substrate contains one or more of the following oxides: one or more alkaline earth metal oxides, such as magnesium oxide, one or more transition metal oxides (e.g., one or more rare earth oxides, such as yttrium oxide or erbium oxide); silicon oxide (e.g., SiO 2 ) or silicate. For example, silicon nitride is present as β-silicon nitride.

[0037] The ceramic substrate has, for example, a thickness in the range of 0.1 mm to 1.0 mm.

[0038] A metal coating is present on the front side of the ceramic substrate, and the metal coating includes at least one recess such that the surface of the ceramic substrate is exposed through the recess. The metal coating is also referred to as a structured metal coating. A semiconductor component can be attached to the structured metal coating.

[0039] Optionally, the metal coating can also be used on the back side of the ceramic substrate. The back side metal coating may also optionally include at least one recess through which the surface of the ceramic substrate is exposed. To achieve the most efficient heat dissipation possible, it may be preferred that the back side metal coating does not include such recesses.

[0040] The metal coating present on the front side and optionally the back side of the ceramic substrate is, for example, a copper coating or an aluminum coating. The metal coating has a thickness, for example, in the range of 0.05 mm to 1.5 mm, more preferably 0.2 mm to 0.8 mm.

[0041] If the metal coating is a copper coating, it includes, for example, at least 97 wt%, more preferably at least 99 wt% of copper content.

[0042] If the metal coating is an aluminum coating, it includes, for example, at least 97 wt%, more preferably at least 99 wt% of aluminum content.

[0043] Methods known to those skilled in the art can be used to apply the metal coating to the front side and optionally the back side of the ceramic substrate.

[0044] For example, a metal foil (e.g., a copper foil or an aluminum foil) is bonded to the front side of the ceramic substrate by active metal brazing.

[0045] In active metal brazing, for example, an active metal solder is used at a temperature of about 600 °C to 1000 °C to form a connection between a metal foil and a ceramic substrate. Due to their alloy composition, active metal solders are able to wet non-metallic inorganic materials such as ceramic substrates. In addition to the main components such as copper, silver, and / or gold, active metal solders also contain one or more active metals such as Hf, Ti, Zr, Nb, V, Ta, or Ce, which can react with the ceramic substrate to form a reaction layer.

[0046] Preferably, the reaction layer produced by active metal brazing is present between the metal coating and the front side of the ceramic substrate. The reaction layer contains, for example, one or more elements E RS , selected from Hf, Ti, Zr, Nb, V, Ta, and Ce, preferably selected from Hf, Ti, Zr, Nb, and Ce, more preferably selected from Hf, Ti, and Zr. The element E in the reaction layer RS is particularly preferably titanium. For example, the element E RS is present in the reaction layer in the form of nitrides, oxynitrides, and / or silicides. For example, the reaction layer contains a total of at least 50 wt% of the element E RS . For example, the reaction layer contains a total of at least 70 wt%, more preferably at least 85 wt%, of nitrides, oxynitrides, and silicides of the element ESR. In a semiconductor module for power electronics devices, the migration of silver can cause problems. Therefore, it may be preferable that the reaction layer contains silver in a proportion of not more than 5 wt%, more preferably not more than 1 wt%, or even does not contain silver.

[0047] The surface of the exposed ceramic substrate is carried out, for example, in multiple steps. First, the metal coating is removed, for example, by etching, and the reaction layer formed during active metal brazing is exposed. Then, the exposed reaction layer is removed, for example, by etching or laser ablation. Preferably, an ultrashort pulse laser (e.g., an IR picosecond or femtosecond laser) is used to remove the exposed reaction layer.

[0048] In order to electrically insulate the regions of the metal coating separated from each other by the recesses, the single-stage or multi-stage removal is carried out only for such a long time or under such conditions until the exposed reaction layer has been completely removed in the treated area, but it will be sufficient that the ceramic substrate has not been removed by the removal medium (e.g., the etching medium or the pulsed laser).

[0049] However, within the scope of the present invention, the exposure of the surface of the ceramic substrate is preferably carried out such that the removal medium (preferably an ultrashort pulse laser, e.g., a picosecond or femtosecond laser) not only completely removes the exposed reaction layer, but also ablates the ceramic material until the exposed surface of the ceramic substrate has a tensile surface area ratio S of at least 7.0% dr .

[0050] The appropriate processing duration and the parameters of the appropriate removal medium can be easily determined by those skilled in the art through a series of tests.

[0051] In order to set the tensile surface area ratio S dr within the scope according to the present invention, it has been proven that it is advantageous if the laser beam moves on the surface in parallel scan lines during the removal of the reaction layer and the roughening of the exposed surface of the ceramic substrate subsequently, and if intersecting scan lines are avoided as much as possible. The pulse energy of the laser pulse is selected to be high enough to cause material removal along the scan line.

[0052] For example, at least 50%, preferably at least 70% or even substantially the entire exposed surface of the ceramic substrate exposed through the recesses can have the tensile surface area ratio S dr according to the present invention and optionally the above-mentioned maximum height S z .

[0053] An exposed surface of a ceramic substrate having a tensile surface area ratio S dr of at least 7.0% results in an improved adhesion strength of the casting material to the ceramic substrate.

[0054] The present invention also relates to a semiconductor module, which contains:

[0055] the above-mentioned metal-ceramic composite material, and

[0056] one or more semiconductor components.

[0057] Preferably, the semiconductor module further contains a casting material, wherein the casting material contacts the exposed surface of the ceramic substrate of the metal-ceramic composite material through the recesses.

[0058] The casting materials for electronic components are known to those skilled in the art. The casting material contains, for example, a polymer (e.g., a thermoplastic polymer or a thermosetting polymer). For example, the casting material contains an optionally cured epoxy resin or silicone resin, polyurethane or an inorganic cement (e.g., phosphate cement).

[0059] Measurement method

[0060] Determination of the stretched surface area ratio S dr and the maximum height S z ​

[0061] In order to determine the tensile surface area ratio S dr and the maximum height S z, a 3D image of the surface of the ceramic substrate was obtained using a confocal microscope μsurf custom (NanoFocus AG, Germany), which imaged at least 800 μm × 800 μm of the exposed surface of the ceramic substrate. The microscopic 3D image was analyzed with the aid of software μSoft Analysis Premium (7.4.8872; NanoFocus AG, Germany). For this purpose, any deflection of the bonded substrate in the 3D image was first corrected (using a quadratic polynomial). Then, the tensile surface area ratio S dr and the maximum height S z .

[0062] Composition of the adhesion-promoting layer

[0063] The composition of the adhesion-promoting layer was determined by combining energy-dispersive X-ray spectroscopy (EDX) with scanning electron microscopy (SEM-EDX).

[0064] In REM-EDX, a focused primary electron beam is rastered (scanned) point by point over the sample surface. Scattered electrons are detected using a detector, and the number of electrons per pixel results in a microscopic image of the sample surface displayed in grayscale. In addition, the primary electron beam excites the sample to emit characteristic X-ray radiation, and the elements in the sample and their weight ratios can be determined by analyzing the energy spectrum using an EDX detector. For inspection, for example, a scanning electron microscope (JSM-6060SEM, JEOL, Ltd., Japan) with a silicon drift EDX detector (NORAN, Thermo Scientific, Inc.) and analysis software (Pathfinder Mountaineer EDS System, e.g., version 2.8, Thermo Scientific, Inc.) was used. For scanning electron microscopy, the following settings were used: magnification: 1,000×, acceleration voltage = 15 kV, working distance = 10 mm, spot size (50 - 60) (adjusted to achieve 25% + / - 5% of the dwell time of the EDX detector). The following settings of the EDX detector were used to detect the EDX spectrum: live time = 30 s, rate = automatic, low energy cut-off = 100 keV, high energy cut-off = automatic (per SEM acceleration voltage).

[0065] With the help of SEM-EDX, the composition of the adhesion-promoting layer can be determined both qualitatively (detecting certain elements and phases, such as the metal nitride phase present in the adhesion-promoting layer) and quantitatively. For example, the measurements were made at at least 10 points on the adhesion-promoting layer. Examples

[0066] Four individual silicon nitride substrates are separated from a starting silicon nitride substrate having a predetermined breaking point for separation. The ceramic substrates S1 - S4 have matching dimensions (174 mm × 139 mm × 0.32 mm).

[0067] For each of the silicon nitride substrates in the individual silicon nitride substrates, S z and S dr are measured in the area of the front side that is again exposed later after metallization. The substrates show substantially matching S dr values. The S z values of the ceramic substrates also substantially match.

[0068] The S dr of the starting substrate: 5.0% + / - 0.5%

[0069] S z : 5.9 + / - 0.9 μm

[0070] The silicon nitride substrates are metallized by the same active metal brazing process under the following conditions.

[0071] On one of the side faces of the ceramic substrate, an active metal brazing paste is applied by screen printing over an area measured at 168 mm × 130 mm and pre - dried at 125°C for 15 minutes. The active metal brazing paste consists of 67 wt% copper powder, 19.8 wt% tin powder, 3.7 wt% titanium hydride, and 9.5 wt% organic carrier. The thickness of the paste after pre - drying is 25 μm + / - 5 μm. Subsequently, a copper foil made of oxygen - free high - conductivity copper with a purity of 99.99% and dimensions of 170 mm × 132 mm × 0.3 mm is placed on the pre - dried paste. Then the resulting arrangement is flipped, the paste is similarly applied to the opposite side of the ceramic substrate by screen printing, pre - dried, and assembled with the copper foil to obtain a sandwich arrangement. The sandwich arrangement is weighted with a 1 kg weight, fired at a maximum temperature of 910°C for 20 minutes, and then cooled to room temperature to obtain an unstructured metal - ceramic composite. Due to the production by active metal brazing, an adhesion - promoting reaction layer exists between the metal coating and the ceramic substrate. The reaction layer contains titanium (e.g., in the form of nitride).

[0072] Each of the three metal - ceramic composites is subjected to a first structuring process using an etching solution containing CuCl 2 . The metal coating in the etched area on the front side of the ceramic substrate is substantially completely removed. However, the reaction layer produced by the active metal brazing process is not removed by the etching solution containing CuCl 2 .

[0073] In Comparative Example VB1, an etchant containing ammonium fluoride, fluoboric acid, and hydrogen peroxide was used to remove the exposed reaction layer.

[0074] In Examples EB1 and EB2 according to the present invention and Comparative Example VB2, the exposed adhesion-promoting layer was removed by laser treatment with a pulsed laser beam. In Comparative Example VB2, a laser beam with a relatively low pulse energy was used, such that only a small amount of material was removed. In Examples EB1 and EB2 according to the present invention, the same pulse energy was used, which was increased compared to the pulse energy used in Comparative Example VB1, and thus resulted in a higher material removal. In EB1, the laser beam was directed along intersecting scan lines, while in EB2, the laser was directed along parallel scan lines.

[0075] For the exposed ceramic surfaces of Examples EB1 and EB2 according to the present invention and Comparative Examples VB1 and VB2, the tensile surface area ratio S dr and the maximum height Sz were measured. Subsequently, a casting material was applied to each of the exposed ceramic surfaces, and the adhesion strength was determined.

[0076] The adhesion strength was determined as follows:

[0077] To determine the adhesion of the casting material (silicone), two plates (plate size: 20 mm × 20 mm × 0.32 mm) were cut out from each of the exposed areas of the corresponding ceramic substrates. Then, the two plates obtained from the same ceramic substrate were bonded with Sylgard 527 silicone to form a specimen. The overlap of the two plates was 1 cm, such that the adhesive surface was always 2 cm 2 . The silicone was cured in air at 125 °C for 2 hours. Each specimen was compressed with a weight of 50 g to produce a uniformly thin silicone layer.

[0078] The shear strength of the specimens thus produced was tested (testing machine: model zwicki500, ZwickRoell GmbH & Co. KG). The maximum shear force was determined in each case.

[0079] The results are summarized in Table 1 below.

[0080] Table 1: S on the exposed ceramic surface dr and S z values and the adhesion strength between the exposed ceramic surface and the mold material

[0081]

[0082] The examples show that when the tensile surface area ratio S dr is within the scope of the present invention, a significant improvement in the adhesion strength of the casting material to the exposed ceramic surface is achieved.

Claims

1. A metal-ceramic composite material, comprising: a ceramic substrate including a front side and a back side and containing silicon nitride, a metal coating present on the front side of the ceramic substrate, wherein the metal coating includes at least one recess and the surface of the ceramic substrate is exposed through the recess, wherein the surface of the ceramic substrate exposed through the recess has a tensile surface area ratio S of at least 7.0% according to standard ILNAS-EN ISO 25178-2:2022 dr , where S dr Assayed by confocal microscopy.

2. The metal-ceramic composite material according to claim 1, wherein the tensile surface area of ​​the surface of the ceramic substrate exposed by the recess is greater than S dr It is 7.0% to 20.0%, more preferably 9.2% to 15.0%.

3. The metal-ceramic composite material according to claim 1 or 2, wherein the surface of the ceramic substrate exposed by the recess has a maximum height S of 8 μm to 20 μm according to the standard ILNAS-EN ISO 25178-2:2022. z , where S z Assayed by confocal microscopy.

4. The metal-ceramic composite material according to any one of the preceding claims, wherein the metal coating is a copper or aluminum coating.

5. The metal-ceramic composite material according to any one of the preceding claims, wherein there is a reaction layer between the ceramic substrate and the metal coating, wherein the reaction layer contains one or more elements selected from Ti, Hf, Zr, Nb, V, Ta and Ce. RS .

6. A semiconductor module, comprising: The metal-ceramic composite material according to any one of claims 1 to 5, One or more semiconductor components. 7 . The semiconductor module according to claim 6 , further comprising a mold material, wherein the mold material contacts the surface of the ceramic substrate of the metal-ceramic composite material exposed through the recess.

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