Patent Description:
Semiconductor power modules form technology for low, medium and high voltage applications which experience mechanical stress due to fixing to a cooler or during thermal cycling, for example. Thus, there is a risk causing delamination, voids or cracks in module encapsulation and a metal substrate due to mechanical or thermo-mechanical stress, for example. In this respect, it is a challenge generally to prevent damages. Japanese Patent Application <CIT> refers to a metal substrate with circuit metallization layer, metal bottom layer and isolating dielectric layer in-between with fixation areas. Japanese Patent Application <CIT> refers to a metal substrate with circuit metallization layer, metal bottom layer and isolating dielectric layer in-between with fixation areas.

<CIT> refers to a metal substrate with metal bottom layer and isolating dielectric layer with fixation areas.

Embodiments of the disclosure relate to a stable metal substrate structure for a semiconductor power module with a reduced risk of damages occurring. Further embodiments of the disclosure relate to provide a corresponding semiconductor power module and a manufacturing method for such a metal substrate structure.

According to an embodiment, a metal substrate structure for a semiconductor power module comprises a circuit metallization layer for conducting electrical signals, e.g. signal and/or current transfer. The metal substrate structure further comprises a metal bottom layer that is coupled with the circuit metallization layer, and an isolating dielectric layer that is coupled with and arranged between the circuit metallization layer and the metal bottom layer with respect to a stacking direction of the metal substrate structure. The circuit metallization layer or the dielectric layer at least locally forms a top layer of the metal substrate structure, and the top layer and possibly the bottom layer as well comprise a fixation area configured for receiving a fixing element to fix the metal substrate structure to a further component, e.g. a cooler or a heat sink, of the semiconductor power module. At least the top layer comprises a stress relief recess that partially or completely surrounds the respective fixation area in a predetermined distance. The fixation area can also be referred to a fixation section. In particular, the metal substrate structure additionally comprises a circuit area. The circuit area in particular is the area where the electrical components, like semiconductors chips and their electrical interconnections, are arranged. The fixation area and the circuit area are arranged side by side without overlap. The fixation area for example is any area outside the circuit area. The fixation area is mainly intended for mechanical purposes.

By use of the described configuration of the metal substrate structure a reduced risk of damages is feasible when it is mounted to a cooler unit, for example. The one or more stress relief recesses that are associated to a corresponding fixation area can block or stop or at least contribute to hinder a further formation of voids or cracks that my occur during assembling the metal substrate structure by means of fixing elements. Moreover, the described configuration can also contribute to prevent and/or to block delamination of the dielectric layer from the metal bottom layer. A further propagation of damages beyond the stress relief recess is prevented or at least impeded, so that the stress relief recess acts like a firewall.

According to an embodiment of the metal substrate structure, the fixation area includes a penetrating fixation opening configured for receiving the fixing element, e.g. a screw or a bolt. The stress relief recess is formed such that it partially or completely surrounds the fixation opening in a predetermined distance.

According to an embodiment of the metal substrate structure, the stress relief recess is limited by the top layer with a form as a ring- or funnel-shaped groove around the respective fixation area. Alternatively or additionally, the stress relief recess is limited by the top layer with a conical or V-shape with respect to a cross section through the fixation area and the stress relief recess along the stacking direction. Alternatively, the stress relief recess can be limited by the top layer with a rectangular or circular shape with respect to a cross section through the fixation area and the stress relief recess along the stacking direction.

The stress relief recess penetrates the metallization layer and the dielectric layer. The respective stress relief recess can completely or partly penetrate to top layer and hence the metallization layer and/or the dielectric layer. Optionally, the respective stress relief recess can further partly penetrate the metal bottom layer.

According to a further embodiment of the metal substrate structure, the distance between the stress relief recess and the associated fixation opening or fixation area is given in coordination with the dimensions of the fixing element and with respect to the needed clamping force that is designed to be arranged in the fixation area.

According to a further embodiment of the metal substrate structure, the stress relief recess is formed in an area to provide electrical isolation of a local portion of the circuit metallization layer being in contact with the fixation element.

According to a further embodiment of the metal substrate structure, the stress relief recess comprises a predetermined depth along the stacking direction that is equal to or smaller than <NUM>. The formation of the stress relief recess is configured in coordination with the top layer and/or the bottom layer of the metal substrate structure and/or in coordination with the fixing element that is intended to be used and with respect to the needed clamping force.

According to a further embodiment of the metal substrate structure, the stress relief recess is formed by at least one of laser cutting, etching and computerized numerical control machining.

According to an embodiment, a semiconductor power module comprises a heat sink or cooler unit with a fixation region, e.g. a fixation opening, and an embodiment of the described metal substrate structure. The metal substrate structure is fixed to the cooler unit by a fixing element, e.g. a screw, a bolt, a spring and/or a clamp, that is arranged in the fixation area, e.g. in or close to a fixation opening, of the metal substrate structure and in the fixation region of the cooler unit.

Power semiconductor modules having baseplates, on which substrates with chips are mounted, or insulated metal substrates comprising a relative thick metal plate, an isolating layer, and a circuit metallization, are typically mounted to a cooler by screws or bolts. For this purpose, corresponding screw holes, openings or recesses are provided in the baseplate or in the insulated metal substrate.

It is a recognition in the context of the present disclosure that mechanically critical positions occur in a vicinity of openings or in areas next to a screw or bolt fitting, which realize weak points of a package design for example. Due to the described possible configurations of the metal substrate structure and its one or more stress relief recesses it is possible to counteract and limit an unwanted crack formation and/or delamination in the metal substrate structure resulting from exposed pressure and mechanical stress during assembly, in particular, and/or during operation including thermal cycles, mechanical shocks or vibrations. A further expansion of cracks or delamination beyond the stress relief recess is prevented or at least reduced so that the stress relief recess acts like a barrier.

According to an embodiment, a method for manufacturing an embodiment of the metal substrate structure comprises providing a circuit metallization layer for conducting electrical signals, providing a metal bottom layer, and providing an isolating dielectric layer. The method further comprises coupling the circuit metallization layer, the metal bottom layer and the dielectric layer with each other such that the dielectric layer is coupled with and arranged between the circuit metallization layer and the metal bottom layer with respect to a stacking direction of the metal substrate structure. The circuit metallization layer or the dielectric layer at least locally forms a top layer of the metal substrate structure. The method further comprises forming or specifying a fixation area at the top layer and the bottom layer configured for receiving a fixing element to fix the metal substrate structure to a further component of the semiconductor power module. The method further comprises forming a stress relief recess at least in the top layer that at least partially surrounds the fixation area in a predetermined distance.

The step of forming the stress relief recess can comprise forming the stress relief recess by at least one of laser cutting, etching and computerized numerical control machining or any other applicable method.

The step of providing the dielectric layer can comprise providing a resin sheet or forming the epoxy based resin layer by means of molding, for example. A resin sheet is typically filled with particles of dielectric material.

As a result of that the described semiconductor power module and the described manufacturing method comprise or are related to produce an embodiment of the metal substrate structure, described features and characteristics of the metal substrate structure are also disclosed with respect to the semiconductor power module and the manufacturing method and vice versa.

The fixation element can be realized as a screw or a bolt with an optional washer or spring washer. Alternatively or additionally, the fixation element can comprise a clamp or a spring.

The stress relief recess is formed as a groove and a location of the groove can be given as surrounding the fixation area of the metal substrate partly or completely. The groove can be located next to fixation area or the fixation opening. For example, the groove is located next to a semicircular cut-out or recess at an edge of the insulated metal substrate structure. A distance between the groove and a screw head or a washer or a spring washer or another fixation structure can have a value within <NUM>-<NUM>, for example. Other values are possible as well, in particular dependent on the size of the metal substrate structure. For example, the distance comprises a value between <NUM>-<NUM>, <NUM>-<NUM> or <NUM>-<NUM>.

A depth of the groove is predetermined and can penetrate only the circuit metallization layer partly or completely. Alternatively or additionally, the groove can penetrate the isolating dielectric layer partly or completely. Depending on which layer forms the top layer, the groove may penetrate only the isolation layer partly or completely, assuming there is no circuit metallization layer at that region. A bottom portion of the groove can also penetrate into the bottom metal layer. The groove may also provide electrical isolation of metal pattern being in contact with a screw or bolt or clamp head, for example. A penetration depth can have a value equal to or less than <NUM>, assuming the metallization layer has a thickness up to <NUM> with respect to the stacking direction.

The cross-sectional shape of the stress relief recess defined by the limiting contour of the one or more layers of the metal substrate structure can realize a V-cut or can have a round, rectangular or square shape.

The metal bottom layer of the insulated metal substrate structure can be made of or comprise copper, aluminum, iron, steel, or a corresponding alloy. The isolating dielectric layer can be made of or comprise epoxy resin, typically filled with particles of dielectric material, or another isolating material applicable for a lamination or molding process. The circuit metallization layer can be made of or comprise copper, aluminum, iron, steel, or a corresponding alloy.

The insulated metal substrate structure can comprise a relative thick metal base forming the bottom metal layer, an isolating sheet of epoxy resin with inorganic filler, and a circuit metallization layer. The metal plate can be completely covered by the isolating sheet and the circuit metallization except from isolating grooves for separation of circuit pattern of different electrical potential. In particular, screws or bolts can be used as fixing elements for mounting the insulated metal substrate to the cooler unit or another component of the semiconductor power module.

The screws or bolts are inserted into corresponding holes or attached to recessed portions in peripheral regions, for example. Here a bottom surface of a screw head or of an optional washer gets into contact with the circuit metallization layer or the isolating sheet and exposes pressure and/or torque on the substrate structure, such that also pressure and/or lateral forces at least during the fixation procedure and/or in operation must be considered. In view of conventional power modules, such a pressure and/or torque may result in an uncontrolled delamination or cracking of the layered structure of the insulated metal substrate, which may extend into a circuit area of chips and terminals, where high voltage levels are available. So, the delamination or cracking may significantly reduce the isolation properties and the reliability of the insulated metal substrate. By use of the described configuration of the metal substrate structure including specifically designed and located stress relief recesses in a vicinity of screw holes it is possible to counteract and/or locally limit the aforementioned adverse effects such that critical substrate regions like the circuit area are not affected.

Exemplary embodiments of the semiconductor power package are explained in the following with the aid of schematic drawings and reference numbers. The figures show:.

The accompanying figures are included to provide a further understanding. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. Identical reference numbers designate elements or components with identical functions. In so far as elements or components correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures. For the sake of clarity elements might not appear with corresponding reference symbols in all figures possibly.

The <FIG> illustrate different embodiments of a metal substrate structure <NUM> for a semiconductor power module in respective cross section side views.

The metal substrate structure <NUM> comprises a circuit metallization layer <NUM>, a metal bottom layer <NUM> that coupled with the circuit metallization layer <NUM>, and an isolating dielectric layer <NUM> that is coupled with and arranged between the circuit metallization layer <NUM> and the metal bottom layer <NUM> with respect to a stacking direction A of the metal substrate structure <NUM>. The stacking direction A may be orientated perpendicular to a lateral direction B, C of the metal substrate structure <NUM>.

The circuit metallization layer <NUM> (see <FIG> or the dielectric layer <NUM> (see <FIG> at least locally forms a top layer of the metal substrate structure <NUM>, and the top layer and the bottom layer <NUM> comprise a fixation area <NUM> configured for receiving a fixing element <NUM> to fix the metal substrate structure <NUM> to a further component of the semiconductor power module. The further component can be realized as a heat sink or a cooler unit <NUM> as shown in the <FIG>. The cooler unit <NUM> comprises a fixation region <NUM>, and the metal substrate structure <NUM> is fixed to the cooler unit <NUM> by the fixing element <NUM> that is arranged in the fixation area <NUM> of the metal substrate structure <NUM> and in the fixation region <NUM> of the cooler unit <NUM>. At least the top layer comprises a stress relief recess <NUM> that partially or completely surrounds the respective fixation area <NUM> in a predetermined distance.

By use of the described configuration of the metal substrate structure <NUM> a reduced risk of severe damages is feasible when it is mounted to the cooler unit <NUM>. The one or more stress relief recesses <NUM> that are associated to a corresponding fixation area <NUM>, <NUM> can block or stop or at least contribute to hinder a further formation of voids or cracks that my occur during assembling the metal substrate structure <NUM> by means of the fixing element <NUM>. The progress of damages into a critical region, in particular into a circuit area <NUM>, is blocked by the stress relief recesses <NUM>, such that damages are in particular limited to a certain extension. Moreover, the described configuration can also contribute to prevent delamination of the dielectric layer <NUM> from the metal bottom layer <NUM> and/or the metallization layer <NUM> from the dielectric layer <NUM>.

According to the <FIG> and <FIG> the fixation area <NUM> of the metal substrate structure <NUM> and the fixation region <NUM> of the cooler unit <NUM> include a respective fixation opening <NUM> and the fixing element <NUM> is formed as a screw or bolt that extends into the fixation openings <NUM> and securely fixes the metal substrate structure <NUM> to the cooler unit <NUM>.

<FIG> shows a different embodiment with a fixing element <NUM> that is formed as a clamp and the metal substrate structure <NUM> is fixed to the cooler unit <NUM> by means of the clamp. Accordingly, there is no need for a fixation opening in the fixation area <NUM> of the metal substrate structure <NUM> and the fixation region <NUM> of the cooler unit <NUM>.

The stress relief recess <NUM> can comprise a V-cut cross-sectional shape (see <FIG> and <FIG>) that can be formed in the metal substrate structure <NUM> by means of laser processing, for example. The stress relief recess <NUM> can be limited alternatively by the corresponding layers <NUM>, <NUM> and/or <NUM> of the metal substrate structure <NUM> and can comprise a conical shape (see <FIG>), a round or circular shape (see <FIG>) or a rectangular shape (see <FIG>.

Exemplary top views are illustrated in <FIG> and <FIG>.

The stress relief recess <NUM> can be formed to be limited by one or more of the metallization layer <NUM>, the dielectric layer <NUM> and the bottom layer <NUM> of the metal substrate structure <NUM>. According to the <FIG> the stress relief recess <NUM> is formed as a V-cut or conical groove that penetrates completely the metallization layer <NUM> and the dielectric layer <NUM> and partly the bottom layer <NUM>.

According to <FIG> the stress relief recess <NUM> is formed as a ring-shaped groove that penetrates completely the metallization layer <NUM> and partly the dielectric layer <NUM> and that does not extend into the bottom layer <NUM>. According to <FIG> the stress relief recess <NUM> is formed as a rectangular groove as well but the groove penetrates completely the metallization layer <NUM> and the dielectric layer <NUM> and extends deeply into the bottom layer <NUM>.

Thus, a depth of the stress relief recess <NUM> can vary depending on its intended location in the metal substrate structure <NUM> and/or depending on a respective thickness of the layers <NUM>, <NUM> and/or <NUM> of the metal substrate structure <NUM> and/or depending on the dimensions of the fixing element <NUM>, for example, and depending on the applied force or pressure. This also applies to the other formations of the stress relief recess <NUM>.

The distance between the stress relief recess <NUM> and the fixing element <NUM> is given along the lateral direction B and can be within <NUM>-<NUM>, for example. The distance can refer to respective center axes of the stress relief recess <NUM> and the fixing element <NUM>. Alternatively, the distance can refer to points of the stress relief recess <NUM> and the fixing element <NUM> closest to each other along the lateral direction B or alternatively refer to an average distance. Accordingly, with reference to <FIG> the distance can be given with a predetermined value between the inner upper edge of the V-cut stress relief recess <NUM> and the outer surface of the screw or bolt head.

Insofar the stress relief recess <NUM> at least surrounds a half of the fixing element <NUM>, the stress relief recess <NUM> can be formed symmetrically around the fixing element <NUM> as indicated in the <FIG> and <FIG>. Alternatively, the stress relief recess <NUM> can be formed asymmetrically with respect to the fixing element <NUM> as indicated in <FIG>.

Thus, the positioning of stress relief recess <NUM> and the distance between the stress relief recess <NUM> and the fixing element <NUM> can vary depending on its intended location in the metal substrate structure <NUM> and/or depending on a respective thickness of the layers <NUM>, <NUM> and/or <NUM> of the metal substrate structure <NUM> and/or depending on the dimensions of the fixing element <NUM>, for example. This also applies to the other formations of the stress relief recess <NUM> and the fixing element <NUM>.

As shown in the top view of <FIG> the metal substrate structure <NUM> comprises the circuit area <NUM> which for example is arranged in a middle region of the metal substrate structure <NUM>. The fixation area <NUM> is arranged outside of the circuit area <NUM>. In a ready-to-operate condition, electric components like semiconductor chips or devices are arranged and electrically connected in the circuit area <NUM>. As exemplarily shown in connection with the top left fixation area, the fixation opening <NUM> can be surrounded by a plurality of stress relief recesses <NUM>. This is of course possible at the other fixation openings <NUM>, too. It is possible to arrange any feasible number of stress relief recesses <NUM>, for example two, three or four stress relief recesses <NUM>, which are concentrically arranged or arranged in other arrangements to surround the fixation opening.

<FIG> shows an embodiment, in which the stress relief recess <NUM> surrounds the circuit area <NUM>. For example, the embodiments of <FIG> and <FIG> can be combined, such that the stress relief recesses <NUM> surround the fixation openings <NUM> and a further stress relief recess <NUM> surrounds the circuit area <NUM>. It is also possible that just the stress relief recesses <NUM> surrounding the fixation openings <NUM> or just the stress relief recess <NUM> surrounding the circuit area <NUM> is arranged. It is possible to arrange any feasible number of stress relief recesses <NUM> which surround the circuit area <NUM> (partially or completely), for example two, three or four stress relief recesses <NUM>, which are arranged in layers around the circuit area <NUM>.

<FIG> illustrates a flow chart for a method for manufacturing an embodiment of the metal substrate structure <NUM>. In a step S1 the circuit metallization layer <NUM>, the metal bottom layer <NUM> and the isolating dielectric layer <NUM> can be provided. For example, these layers <NUM>-<NUM> are provided with a given fixation opening <NUM> for receiving a screw or a bolt.

In a step S2 the circuit metallization layer <NUM>, the metal bottom layer <NUM> and the dielectric layer <NUM> are coupled with each other such that the dielectric layer <NUM> is coupled with and arranged between the circuit metallization layer <NUM> and the metal bottom layer <NUM>.

In a step S3 one or more stress relief recesses <NUM> are formed in the circuit metallization layer <NUM> and/or the dielectric layer <NUM> and optionally in the metal bottom layer <NUM> partly as well. The step of forming a stress relief recess <NUM> at least in the top layer can be done by means of laser cutting and/or etching and/or computerized numerical control machining.

Due to the described manufacturing process a stable and resistant metal substrate structure <NUM> is feasible and a risk of damage formation during mounting, in particular, and/or in operation including thermal cycling and mechanical shocks or vibrations can be reduced. Thus, the described embodiments can contribute to reduce a further formation of voids or cracks to at least limited possible damages and/or delamination of the dielectric layer <NUM> to a certain extension. A further propagation of damages beyond the stress relief recess <NUM> into the circuit area <NUM> is prevented or at least impeded, so that the stress relief recess <NUM> acts like a firewall to protect the circuit area <NUM>.

A sufficient pressure, which is exposed on a surface of the mounted insulated metal substrate structure <NUM> by a screw head or a clamp is needed to achieve a proper thermal interface between a backside of the metal substrate structure <NUM> and the cooler unit <NUM>. The screw head can be in direct contact with the top metallization layer <NUM>, or an optional washer and/or a spring washer can be arranged between the screw head and the top metallization layer <NUM>. So, a mechanical stress on the metal substrate structure <NUM> cannot be avoided, especially in the vicinity of the fixation elements.

The described embodiments of the metal substrate structure <NUM> and its stress relief recess <NUM> can enable limitation and control of the mechanical stress or a progress of damages to a certain extend. The stress relief recess <NUM> forms a recessed portion or a groove applied on a top surface of the insulated metal substrate structure <NUM>, which at least partly surrounds the screw hole or the location of the screw head or the optional washer in a certain relative distance. If the pressure and/or torque applied by the head of the screw or bolt or clamp causes delamination and/or cracking, the propagation of the delamination or of cracks is stopped or at least hindered at the stress relief recess <NUM>. A further propagation beyond the stress relief recess <NUM> is prevented or at least impeded, so that the stress relief recess <NUM> acts like a firewall.

Claim 1:
Metal substrate structure (<NUM>) for a semiconductor power module, comprising:
- a circuit metallization layer (<NUM>) for conducting electrical signals,
- a metal bottom layer (<NUM>) that coupled with the circuit metallization layer (<NUM>), and
- an isolating dielectric layer (<NUM>) that is coupled with and arranged between the circuit metallization layer (<NUM>) and the metal bottom layer (<NUM>) with respect to a stacking direction (A) of the metal substrate structure (<NUM>), wherein the circuit metallization layer (<NUM>) or the dielectric layer (<NUM>) at least locally forms a top layer of the metal substrate structure (<NUM>),
wherein the top layer and the bottom layer (<NUM>) comprise a fixation area (<NUM>) configured for receiving a fixing element (<NUM>) to fix the metal substrate structure (<NUM>) to a further component (<NUM>) of the semiconductor power module, and wherein a stress relief recess (<NUM>) penetrates the metallization layer (<NUM>) and the dielectric layer (<NUM>).