Metal-ceramic composite

Abstract

To provide a metal-ceramic composite which allows formation of a bond with a high adhesion strength with a polymer molding compound on an exposed ceramic surface of a metallized ceramic substrate containing silicon nitride.SOLUTION: The metal-ceramic composite includes: a ceramic substrate having a front surface and a back surface and containing silicon nitride; and a metal coating on the front surface of the ceramic substrate, the metal coating including at least one recessed part, the surface of the ceramic substrate being exposed by the recessed part. The surface of the ceramic substrate exposed by the recessed part has an extension surface area ratio of at least 7.0% in accordance with the specification ILNAS-EN ISO 25178-2:2022, and Sdr is determined by a confocal microscope technique.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a metal-ceramic composite that can be used as a ceramic circuit carrier in power electronics.

[0002] In power electronics, printed circuit boards should be designed for high current as carriers for power components such as MOSFETs and should be able to dissipate waste heat quickly.

[0003] Ceramic materials such as aluminum oxide, aluminum nitride, and silicon nitride have much higher thermal conductivity than the polymers used to manufacture conventional printed circuit boards, so ceramic circuit carriers are often used in power modules.

[0004] Silicon nitride-based ceramic substrates have very high mechanical strength and high thermal conductivity at the same time, so they are very suitable for applications in power electronics.

[0005] Silicon nitride-based ceramic substrates are described, for example, in the following publications. N. Chasserio et al., “Ceramic Substrates for High-Temperature Electronic Integration,” Journal of Electronic Materials, Volume 38 (2009), pp. 164 - 174; K. Hirao et al., “High Thermal Conductivity Silicon Nitride Ceramics,” Journal of the Korean Ceramic Society, Volume 49 (2012), pp. 380 - 384; Y. Zhou et al.,”Development of high-thermal-conductivity silicon nitride ceramics,” Journal of Asian Ceramic Societies, 3(2015), pp.221-229.

[0006] Silicon nitride-based ceramic substrates exhibit a good balance between high mechanical strength and high thermal conductivity, can be used in electronic components, and are commercially available.

[0007] A ceramic circuit carrier has a ceramic substrate, and metal layers are provided on at least one, usually both, of its surfaces. 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.

[0008] The production of metallized ceramic substrates that function as ceramic circuit boards, which is known to those skilled in the art, is carried out, for example, by bringing the front and back surfaces of the ceramic substrate into contact with metal foils (e.g., copper foil or aluminum foil) and joining them together. The material joining of the metal foils is achieved, for example, by eutectic bonding or the active metal brazing method (AMB). When the metal foil is copper foil, eutectic bonding is also known as the DCB process or the DBC process (DCB: "direct copper bonding"; DBC: "direct bonded copper"). In the case of aluminum foil, the term "DAB" "direct aluminum bonding" is also used for eutectic bonding. Metallized ceramic substrates produced using the DCB process or the AMB process may also be called DCB substrates (or DBC substrates) or AMB substrates.

[0009] The metallization of silicon nitride substrates is usually carried out by the active metal brazing method.

[0010] The active metal brazing filler metal contains, in addition to a main component 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, Chapter 8.2.4.3(“Active metal brazing”), pages 203-204, in Brevier Technische Keramik, Verband der Keramischen Industrie eV, 2003, Fahner Verlag). Reactive elements such as hafnium (Hf), titanium (Ti), zirconium (Zr), niobium (Nb), cerium (Ce), tantalum (Ta), and vanadium (V) are used. In the metallization of a silicon nitride substrate by the active metal brazing method, the reaction layer contains, for example, nitrides, oxynitrides, and / or silicides of the reactive element (A. Ponicke et al., “Active metal brazing of copper with aluminum nitride and silicon nitride ceramics”, Keramische Zeitschrift, 63(5), 2011, 334-342).

[0011] The metal layer carrying the semiconductor components contains one or more recesses and is thus also referred to as a structured metal coating. Due to the structuring, for example, metal conductor tracks are formed on the ceramic substrate. The adjacent conductor tracks are spatially separated by the recesses and are thus electrically insulated from each other. The structuring of the AMB substrate can be carried out, for example, in a two-step process. In the first step, the metal layer is first removed in defined regions (for example, using a first etching solution), and then, in the second step, the adhesion-promoting layer obtained from the active metal brazing process is removed (for example, using a second etching solution). By removing the metal layer and, optionally, the adhesion-promoting layer, the ceramic substrate is exposed again in the defined regions.

[0012] Modules based on ceramic circuit carriers used in power electronics can be encapsulated as part of the packaging process, for example, by embedding a power module in a casting compound.

[0013] For example, embedding in a casting compound increases the dielectric breakdown strength. Further, semiconductor components and metal conductor tracks are protected from moisture and mechanically stabilized.

[0014] In the areas exposed by structuring, the ceramic substrate is in direct contact with the casting compound. When operating the power module, significant temperature fluctuations can occur. Since the ceramic material and the casting compound usually have significantly different coefficients of thermal expansion, these temperature fluctuations cause mechanical stress at the interface between the ceramic and the casting compound. And this can result in the casting compound being at least partially detached from the ceramic substrate, forming voids. Moisture entering these voids can damage the power module. Further, at least locally, the dielectric breakdown strength can be significantly reduced.

[0015] As already mentioned 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. To maximize their potential as circuit carriers, it is desirable that there is a high adhesion strength between the casting compound and the silicon nitride surface after embedding in the casting compound.

[0016] Japanese Patent Application Laid-Open No. 2018-046192(A) describes a ceramic-metal composite embedded in a polymer casting compound. Silicon nitride ceramic is used as the ceramic substrate. The exposed area of the ceramic surface of the ceramic-metal composite is treated with a particulate blasting medium in these treatment areas such that the ceramic surface has a maximum profile height Ry of 1.7 μm to 2.7 μm. According to Japanese Patent Application Laid-Open No. 2018-046192(A), this ensures good adhesion of the casting compound to the ceramic substrate.

[0017] An object of the present invention is to provide a metallized silicon nitride-containing ceramic substrate, the exposed ceramic surface of which enables the formation of a joint with high adhesive strength to a polymer casting compound.

[0018] This object is achieved by a ceramic substrate comprising a front surface and a rear surface and containing silicon nitride, a metal coating present on the front surface of the ceramic substrate, including at least one recess, and the surface of the ceramic substrate being exposed by the recess, a metal-ceramic composite including the above, and the surface of the ceramic substrate exposed by the recess has an extended surface area ratio S of at least 7.0% in accordance with the standard ILNAS-EN ISO 25178-2:2022 dr having.

[0019] As is well known, the surface roughness can be determined from a profile (i.e., along a line) or over an area. Various roughness parameters are available for both profile measurement and area measurement. In the ISO 4287 standard, specific roughness parameters such as the arithmetic mean deviation R a (also called the arithmetic mean roughness value) or the maximum height R z etc. are defined for the profile. The roughness parameters determined over an area are defined in the EN ISO 25178 series of standards, for example, the arithmetic mean height S a , the maximum height S z , and the extended surface area ratio Sdr It includes. Compared with the profile parameters, the surface roughness can be determined much more reliably using the surface parameters. Optical measurement methods such as confocal microscopy are used to determine the surface parameters.

[0020] Extended (or "unrolled") surface area ratio S according to ILNAS - EN ISO 25178 - 2:2022 dr is a measure of the surface roughness as it describes the relationship between an ideally flat surface and the actual measured surface. The extended 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. Thus, if the actual surface is completely flat, S dr is 0%. For example, if the actual surface is twice as large as the projected surface, S dr is 100%. When the area increases by 2.5 times, S dr is 150%.

[0021] Parameter S a and S z are pure height parameters, that is, they provide only the z - direction information of the surface being tested, while S dr is a so - called hybrid parameter, and its value depends not only on the height of the surface protrusions but also on the distance between adjacent protrusions.

[0022] Within the scope of the present invention, it has been found that when the exposed surface of the ceramic substrate has an extended surface area ratio S of at least 7.0% dr the adhesion strength of the cast compound on the ceramic substrate can be improved. The determination of the extended surface area ratio S dr is carried out by confocal microscopy.

[0023] Preferably, the extended surface area ratio S of the surface of the ceramic substrate exposed by the recess dr is at least 9.2%.

[0024] In an exemplary embodiment, the extended surface area ratio S of the surface of the ceramic substrate exposed by the recess dr is 7.0% to 20.0%, more preferably 9.2% to 15.0%.

[0025] The extended surface area ratio S dr If it exceeds 20%, the risk of damage to the ceramic substrate increases, which in turn adversely affects the mechanical properties such as the bending strength of the ceramic substrate.

[0026] For example, the surface of the ceramic substrate exposed by the recess has a maximum height S of 8 μm to 20 μm, more preferably 10 μm to 15 μm, in accordance with ISO 25178-2:2022 z The maximum height S z is determined by the confocal microscopy method.

[0027] In an exemplary embodiment, the surface of the ceramic substrate exposed by the recess has an extended surface area ratio S dr of 7.0% to 20.0%, a maximum height S z of 8 μm to 20 μm, and more preferably, an extended surface area ratio S dr of 9.2% to 15.0%, and a maximum height S z of 10 μm to 15 μm.

[0028] The ceramic substrate contains silicon nitride. Silicon nitride-based ceramic substrates suitable for the manufacture of power modules are known to those skilled in the art and are commercially available.

[0029] For example, the ceramic substrate contains silicon nitride at a ratio of at least 70% by weight, more preferably at least 80% by weight.

[0030] Optionally, the ceramic substrate may contain one or more metal oxides. These are added, for example, as sintering aids during the manufacture of the ceramic substrate. Oxide components suitable for silicon nitride ceramics are known to those skilled in the art. For example, the ceramic substrate may contain 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 one or more of silicates. For example, silicon nitride exists as β-silicon nitride.

[0031] The ceramic substrate has a thickness in the range of, for example, 0.1 mm to 1.0 mm.

[0032] On the front surface of the ceramic substrate, there is a metal coating including at least one recess such that the surface of the ceramic substrate is exposed by the recess. This metal coating is also called a structured metal coating. The semiconductor component can be attached to the structured metal coating.

[0033] Optionally, the metal coating can also be used on the rear surface of the ceramic substrate. This rear surface metal coating can also optionally include at least one recess through which the surface of the ceramic substrate is exposed. In order to achieve the most efficient heat dissipation possible, it may be preferable for the rear surface metal coating not to include such recesses.

[0034] The metal coatings present on the front surface and optionally the rear surface of the ceramic substrate are, for example, copper coatings or aluminum coatings. The metal coating has a thickness in the range of, for example, 0.05 mm to 1.5 mm, more preferably 0.2 mm to 0.8 mm.

[0035] When the metal coating is a copper coating, it contains, for example, a copper content of at least 97 wt%, more preferably at least 99 wt%.

[0036] When the metal coating is an aluminum coating, for example, it contains an aluminum content of at least 97% by weight, more preferably at least 99% by weight.

[0037] The metal coating can be applied to the front surface and optionally the rear surface of the ceramic substrate using methods known to those skilled in the art.

[0038] For example, a metal foil (e.g., a copper foil or an aluminum foil) is joined to the front surface of the ceramic substrate by an active metal brazing method.

[0039] In the active metal brazing method, for example, an active metal brazing material is used at a temperature of about 600 to 1,000 °C to establish a connection between the metal foil and the ceramic substrate. Due to their alloy composition, the active metal brazing materials can wet non-metallic inorganic materials such as ceramic substrates. In addition to main components such as copper, silver, and / or gold, the active metal brazing materials also contain one or more active metals such as Hf, Ti, Zr, Nb, V, Ta, or Ce that can react with the ceramic substrate to form a reaction layer.

[0040] Preferably, a reaction layer obtained from the active metal brazing method is present between the metal coating and the front surface of the ceramic substrate. The reaction layer contains, for example, one or more elements E 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. RS including. Element E in the reaction layer RS is particularly preferably titanium. For example, element E RS is present in the reaction layer in the form of nitrides, oxynitrides, and / or silicides. For example, the reaction layer contains element E RSIt contains at least 50% by weight in total. For example, the reaction layer contains nitrides, oxynitrides, and silicides of elemental ESR in a total amount of at least 70% by weight, more preferably at least 85% by weight. In a power electronics semiconductor module, the movement of silver may cause problems. Therefore, in some cases, it is preferable that the reaction layer contains silver at a ratio of 5% by weight or less, more preferably 1% by weight or less, or even does not contain silver at all.

[0041] The exposure of the surface of the ceramic substrate is carried out, for example, in several steps. First, the metal coating is removed, for example, by etching, and the reaction layer formed during the active metal brazing method is exposed. Next, the exposed reaction layer is removed, for example, by etching or laser ablation. Preferably, the exposed reaction layer is removed using an ultrashort pulse laser (for example, an IR picosecond or femtosecond laser).

[0042] In order to electrically insulate the regions of the metal coating separated from each other by the recesses, it is sufficient to perform single-stage or multi-stage removal over a length or under such conditions until the exposed reaction layer is completely removed in the processing region, but the ceramic substrate has not yet been removed by the removal medium (for example, an etching medium or a pulsed laser).

[0043] However, within the scope of the present invention, the exposure of the ceramic substrate surface is preferably such that the removal medium (preferably an ultrashort pulse laser, for example, a picosecond or femtosecond laser) not only completely removes the exposed reaction layer, but also the exposed surface of the ceramic substrate has an extended surface area ratio S of at least 7.0% dr until it has. The ablation of the ceramic material is carried out.

[0044] The appropriate processing time and appropriate parameters of the removal medium can be easily determined by those skilled in the art through a series of tests.

[0045] Extended surface area ratio S drTo set within the scope according to the present invention, when moving the laser beam across the surface with parallel scanning lines during the removal of the reaction layer and subsequent roughening of the exposed surface of the ceramic substrate, and when avoiding intersecting scanning lines as much as possible, it has been found to be advantageous. The pulse energy of the laser pulse is selected to be high enough to cause material removal along the scanning line.

[0046] For example, at least 50%, preferably at least 70%, or even substantially the entire surface of the ceramic substrate exposed by the recesses has an extended surface area ratio S according to the present invention dr , and optionally the maximum height S described above z may have.

[0047] An exposed surface of a ceramic substrate having an extended surface area ratio S of at least 7.0% dr results in improved adhesion strength of the casting compound to the ceramic substrate.

[0048] The present invention relates to a semiconductor module including the metal-ceramic composite described above and one or more semiconductor components.

[0049] Preferably, the semiconductor module also includes a casting compound, and the casting compound contacts the surface of the ceramic substrate of the metal-ceramic composite exposed by the recesses.

[0050] Casting compounds for electronic components are known to those skilled in the art. The casting compound contains, for example, a polymer (e.g., a thermoplastic polymer or a thermosetting polymer). For example, the casting compound optionally contains a cured epoxy resin or silicone resin, polyurethane, or an inorganic cement (e.g., a phosphate cement).

[0051] Measurement method Determination of the extended surface area ratio S dr and the maximum height S z ​ Extended surface area ratio S dr and maximum height S z To determine the extended surface area ratio S dr and maximum height S z were determined in accordance with the standard ILNAS-EN ISO 25178-2:2022.

[0052] Composition of the adhesion promoting layer The composition of the adhesion promoting layer is determined by energy-dispersive X-ray spectroscopy (EDX) combined with scanning electron microscopy (SEM-EDX).

[0053] In REM-EDX, a focused primary electron beam is induced (screened) point by point on the sample surface. Scattered electrons are detected using a detector, and the number of electrons per pixel results in a grayscale microscopic image of the sample surface. Furthermore, 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 the inspection, for example, a scanning electron microscope (JSM-6060 SEM, JEOL, Ltd.) equipped with a silicon drift EDX detector (NORAN, Thermo Scientific, Inc.) and analysis software (Pathfinder Mountaineer EDS System, for example version 2.8, Thermo Scientific, Inc.) are used. In the case of scanning electron microscopy, the following settings are used: magnification: 1000 times, acceleration voltage = 15 kV, working distance = 10 mm, spot size (50 - 60) (adjusted to reach 25% + / - 5% of the dead time of the EDX detector). The following settings of the EDX detector were used to detect the EDX spectrum: live time = 30 seconds, speed = automatic, low energy cut-off = 100 keV, high energy cut-off = automatic (following the SEM acceleration voltage).

[0054] The composition of the adhesion promoting layer can be determined both qualitatively (detection of specific elements and phases, for example, metal nitride phases present in the adhesion promoting layer) and quantitatively by SEM-EDX. For example, the measurement is performed at at least 10 points on the adhesion promoting layer.

Example

[0055] Four individual silicon nitride substrates were separated from a silicon nitride starting substrate having a predetermined breaking point for separation. The ceramic substrates S1 - S4 had matching dimensions (174 mm × 139 mm × 0.32 mm).

[0056] For each of the individual silicon nitride substrates, S z and S drwas determined in the area of the front surface that would be exposed again after metallization. The substrates showed substantially matching S dr values. The ceramic substrates also substantially matched in their S z values. The S of the starting substrate dr : 5.0% + / − 0.5% S z : 5.9 + / − 0.9 μm

[0057] Silicon nitride substrates were metallized by the same active metal brazing process under the conditions described below.

[0058] On one side of the ceramic substrate, an active metal brazing paste was applied by screen printing to an area with dimensions of 168 mm × 130 mm and pre-dried at 125° C. for 15 minutes. The active metal brazing paste consisted of 67 wt% copper powder, 19.8 wt% tin powder, 3.7 wt% titanium hydride, and 9.5 wt% organic vehicle. The thickness of the paste after pre-drying was 25 + / − 5 μm. Subsequently, an oxygen-free highly conductive copper foil made of copper with a purity of 99.99% and dimensions of 170 mm × 132 mm × 0.3 mm was placed on the pre-dried paste. Next, the obtained arrangement was turned over, the paste was similarly applied to the opposite side of the ceramic substrate by screen printing, pre-dried, and the copper foil was attached to obtain a sandwich arrangement. The sandwich arrangement was loaded with a 1 kg weight and fired at a maximum temperature of 910° C. for 20 minutes and then cooled to room temperature to obtain an unstructured metal-ceramic composite. By manufacturing by the active metal brazing method, an adhesion promoting reaction layer exists between the metal coating and the ceramic substrate. The above reaction layer contains titanium (for example, in the form of a nitride).

[0059] Each of the three metal-ceramic composites was subjected to a first structuring process using an etching solution containing CuCl 2 . The metal coating in the etched area of the front surface of the ceramic substrate was removed substantially completely. However, for the reaction layer obtained from the active metal brazing process, CuCl 2It was not removed by the etching solution.

[0060] In Comparative Example VB1, the exposed reaction layer was removed using an etching solution containing ammonium fluoride, fluoroboric acid, and hydrogen peroxide.

[0061] 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, since a laser beam having a relatively low pulse energy was used, 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, resulting in higher material removal. In EB1, the laser beam was induced on the scanning lines intersecting, while in EB2, the laser was induced on the parallel scanning lines.

[0062] For the exposed ceramic surfaces of Examples EB1 and EB2 according to the present invention and Comparative Examples VB1 and VB2, the stretched surface area ratio S dr and the maximum height Sz were measured. Subsequently, a casting compound was applied to each of the exposed ceramic surfaces, and the adhesive strength was determined.

[0063] The adhesive strength was determined as follows. To determine the adhesiveness of the casting compound (silicone), two plates (plate size: 20×20×0.32 mm) were cut out from each of the exposed areas of the respective ceramic substrates. Then, two plates taken from the same ceramic substrate were joined using Sylgard 527 silicone to form a test piece. Since the overlap of both plates was 1 cm, the adhesive surface was always 2 cm 2 in area. The silicone was cured at 125°C in air for 2 hours. Each test piece was compressed with a load of 50 g to produce a uniformly thin silicone layer.

[0064] The test pieces thus manufactured were tested for shear strength (testing machine: model zwicki500, ZwickRoell GmbH&Co.KG). In each case, the maximum shear force was determined.

[0065] The results are summarized in Table 1 below.

Table 1

[0066] The examples demonstrate that when the elongation surface area ratio S dr is within the scope of the present invention, a significant improvement in the adhesion strength of the casting compound to the exposed ceramic surface is achieved.

Claims

1. a ceramic substrate comprising silicon nitride, the ceramic substrate having a front surface and a rear surface; a metal coating on the front surface of the ceramic substrate and including at least one recess, a surface of the ceramic substrate being exposed by the recess; The surface of the ceramic substrate exposed by the recess has an extended surface area ratio S of at least 7.0% according to the standard ILNAS-EN ISO 25178-2:2022. ｄｒ and S ｄｒ is the metal-ceramic composite as determined by confocal microscopy.

2. The extension surface area ratio S of the surface of the ceramic substrate exposed by the recess ｄｒ The metal-ceramic composite of claim 1, wherein is between 7.0% and 20.0%, more preferably between 9.2% and 15.0%.

3. The surface of the ceramic substrate exposed by the recess has a maximum height S of 8 μm to 20 μm in accordance with the standard ILNAS-EN ISO 25178-2:2022. ｚ and S ｚ The metal-ceramic composite of claim 1 , wherein:

4. The metal-ceramic composite of claim 1, wherein said metallic coating is a copper or aluminum coating.

5. A reaction layer is present between the ceramic substrate and the metal coating, the reaction layer containing one or more elements E selected from Ti, Hf, Zr, Nb, V, Ta, and Ce. ＲＳ The metal-ceramic composite of claim 1, comprising:

6. The metal-ceramic composite of claim 1 ; and one or more semiconductor components.

7. 7. The semiconductor module of claim 6, further comprising a casting compound, said casting compound contacting a surface of said ceramic substrate of said metal-ceramic composite exposed by said recess.

Images