Patent Description:
Power semiconductor module arrangements often include a base plate within a housing and at least one substrate is arranged on the base plate. Other power semiconductor module arrangements may solely include a substrate (e.g. Cu-ceramic-Cu substrate) without a base plate. A semiconductor arrangement including a plurality of controllable semiconductor components (e.g., two IGBTs in a half-bridge configuration) is usually arranged on each of the at least one substrates. Each substrate usually comprises a substrate layer (e.g., a ceramic layer), a first metallization layer deposited on a first side of the substrate layer and a second metallization layer deposited on a second side of the substrate layer. The controllable semiconductor components are mounted, for example, on the first metallization layer. The second metallization layer is usually attached to the base plate. Other substrates are known including multiple substrate and metal layers stacked on each other, with buried metal layers acting either as shielding layers or being part of the electronic circuit and electrically connected to other metal layers by means of through contacts. The electrical connections between the semiconductor components and the first metallization layer, between different semiconductor components, between the first metallization layers of different substrates, or between any other elements of the power semiconductor module arrangement often comprise bonding wires. The bonding wire is usually bonded to an electrically conductive surface or bond pad arranged on the component that is to be electrically contacted.

Several different methods for establishing a permanent electrical connection or bond between a bonding wire and an electrically conductive surface are known. Such known method include the so-called wedge bonding, or wedge/wedge bonding. Wedge bonding processes usually utilize ultrasonic energy and pressure to create a bond between the bonding wire, which is usually a thick wire, and the bond pad. Wedge bonding processes are generally low temperature processes which are performed at room temperature. Usually Al-wires (aluminum wires) are used for wedge bonding processes. Bonding wires including other materials such as Cu (copper), for example, are generally preferred with respect to reliability and performance. However, Cu-wires are generally harder than Al-wires. The bond pad usually needs to include other materials than aluminum when using Cu-wires. That is because the hardness of aluminum bond pads is generally not sufficient for the comparably hard Cu-wires, therefore, the bond pad should include a material that is harder than aluminum. Further, for Cu-wires an increased ultrasonic power has to be provided as compared to wedge bonding processes using Al-wires. Even further, the time for performing the wedge bonding process is increased and the Cu-wire needs to be pressed on the bond pad with a larger strength as compared to a wedge bond process using Al-wires and Al-bond pads. This increased strength (or force) may result in severe damages of the bond pad material, for example.

Document <CIT> discloses a wire-bonding process and to a process for producing a bonded joint. A bonding location is heated by means of a laser beam originating from a laser, the arrangement comprising an ultrasonic wedge-wedge bonding unit with a bonding needle, a copper or aluminum bonding wire guide, and a copper or aluminum wire for an ultrasonic wedge-wedge bonding process, and at least one of the bonding locations having a hard-metal coating.

Document "<NPL> discloses a high-temperature thick Al wire bonding technology comprising bonding <NUM>-µm-diameter Al wires to Al pads on an insulated gate bipolar transistor chip at varying substrate temperatures and ultrasonic powers.

Document <CIT> discloses a bonding apparatus and a method of bonding copper bond wires to bond pads on an integrated circuit devices attached to a substrate. A heater block heats the devices and substrate prior to and during wire bonding. A clamp presses the substrate down onto the heater block during wire bonding and thereby forms a region of the substrate isolated from the remainder of the substrate. A bonder head creates ball bonds as it attaches one end of the bond wires to the bond pads on the devices within the isolated region. The bonder head also attaches the other end of the bond wires to substrate pads adjacent the devices being wire bonded. To prevent corrosion of the ball bonds, a gas source floods the substrate and the attached devices that have not yet wire bonded with a purge gas while the heater block heats the substrate and the attached devices.

There is a need for an improved method which provides a stable and durable connection between a bonding wire and the element that is to be electrically contacted.

A method includes heating a first electrically conductive layer that is to be electrically contacted, and that is arranged on a first element, and pressing a first end of a bonding wire on the first electrically conductive layer by exerting pressure to the first end of the bonding wire, and further by exposing the first end of the bonding wire to ultrasonic energy, thereby deforming the first end of the bonding wire and creating a permanent substance-to-substance bond between the first end of the bonding wire and the first electrically conductive layer. The bonding wire either comprises a rounded cross section with a diameter of at least <NUM> or a rectangular cross section with a first width of at least <NUM> and a first height of at least <NUM>. The first layer has a thickness of at least <NUM>.

An arrangement is described for establishing a permanent bond connection between a bonding wire and a first electrically conductive surface of a first element. The arrangement includes a bonding chamber, a bonding device arranged within the bonding chamber, wherein the bonding device is configured to press a first end of the bonding wire on the first electrically conductive surface, and a heating device, configured to heat the first layer and the first end of the bonding wire. The arrangement is configured to establish a connection between the first surface having a thickness of at least <NUM>, and a bonding wire, the bonding wire comprising either a rounded cross section with a diameter of at least <NUM>, or a rectangular cross section with a first width of at least <NUM> and a first height of at least <NUM>.

The components in the figures are not necessarily to scale, emphasis is instead being placed upon illustrating the principles of the invention.

In the following detailed description, reference is made to the accompanying drawings. The drawings show specific examples in which the invention may be practiced. It is to be understood that the features and principles described with respect to the various examples may be combined with each other, unless specifically noted otherwise. In the description as well as in the claims, designations of certain elements as "first element", "second element", "third element" etc. are not to be understood as enumerative. Instead, such designations serve solely to address different "elements". That is, e.g., the existence of a "third element" does not require the existence of a "first element" and a "second element". A semiconductor body as described herein may be made from (doped) semiconductor material and may be a semiconductor chip or be included in a semiconductor chip. A semiconductor body has electrically connecting pads and includes at least one semiconductor element with electrodes.

Referring to <FIG>, the general principle of a method for establishing a connection (a bond) between a bonding wire <NUM> and an element <NUM> that is to be electrically contacted is schematically illustrated. Generally, different bonding methods are known such as the so-called ball bonding, ball-wedge bonding, wedge bonding, or wedge-wedge bonding, for example. The present example refers to a wedge-wedge bonding method.

<FIG> schematically illustrates a first element <NUM> that is to be electrically contacted, and a bonding device <NUM>. The first element <NUM> may be or may comprise a semiconductor body, for example. In power semiconductor module arrangements usually one or more semiconductor bodies are arranged on a semiconductor substrate. Each of the semiconductor bodies arranged on a semiconductor substrate may include a diode, an IGBT (Insulated-Gate Bipolar Transistor), a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), a JFET (Junction Field-Effect Transistor), a HEMT (High-Electron-Mobility Transistor), or any other suitable controllable semiconductor element. One or more semiconductor bodies may form a semiconductor arrangement on the semiconductor substrate. This, however, is only an example. The first element <NUM> may be any element, e.g., of a power semiconductor module arrangement, that is to be electrically contacted with a bonding wire <NUM>.

According to another example, the first element <NUM> may be a semiconductor substrate. A semiconductor substrate usually includes a dielectric insulation layer, a (structured) first metallization layer attached to the dielectric insulation layer, and a second (structured) metallization layer attached to the dielectric insulation layer. The dielectric insulation layer is disposed between the first and second metallization layers. Each of the first and second metallization layers may consist of or include one of the following materials: copper; a copper alloy; aluminum; an aluminum alloy; any other metal or alloy that remains solid during the operation of the power semiconductor module arrangement. A semiconductor substrate may be a ceramic substrate, that is, a substrate in which the dielectric insulation layer is a ceramic, e.g., a thin ceramic layer. The ceramic may consist of or include one of the following materials: aluminum oxide; aluminum nitride; zirconium oxide; silicon nitride; boron nitride; or any other dielectric ceramic. For example, the dielectric insulation layer may consist of or include one of the following materials: Al<NUM>O<NUM>, AIN, BN, or Si3N<NUM>. For instance, a substrate may, e.g., be a Direct Copper Bonding (DCB) substrate, a Direct Aluminum Bonding (DAB) substrate, or an Active Metal Brazing (AMB) substrate. A substrate may also be a conventional printed circuit board (PCB) having a non-ceramic dielectric insulation layer. For instance, a non-ceramic dielectric insulation layer may consist of or include a cured resin. The bonding wire <NUM> may be permanently connected to one of the first and second metallization layers, e.g., to the first metallization layer.

A bonding wire <NUM> is inserted into the bonding device <NUM>. The bonding wire <NUM> may be any suitable electrically conductive wire, e.g., a metal wire. For wedge-wedge bonding methods, usually thick bonding wires are used. Thick bonding wire in this context refers to a bonding wire <NUM> having a rounded cross section (see <FIG>) with a diameter d1 of at least <NUM>, or to a bonding wire <NUM> having a rectangular cross-section (see <FIG>) with a width w1 of at least <NUM> and a height h1 of at least <NUM>, for example. For the purpose of explanation it should be mentioned that in ball bonding methods, one end of the bonding wire is melted using high-voltage discharge, thereby forming a ball of soft material at one end of the bonding wire. This ball is then welded to the element that is to be electrically contacted. As compared to ball bonding methods, the bonding wire <NUM> in wedge bonding methods as described herein is not melted by use of high-voltage discharge to make it softer before pressing the bonding wire <NUM> to the first element <NUM>.

The bonding device <NUM> comprises a guiding element <NUM>. The guiding element is configured to accommodate the bonding wire <NUM> or, in other words, the bonding wire <NUM> may be inserted into the guiding element <NUM>. The bonding device <NUM> is configured to position the bonding wire <NUM> at a desired position with respect to the first element <NUM>. The guiding element <NUM> may include a channel, for example. As is exemplarily illustrated in <FIG>, a first end <NUM> of the bonding wire <NUM> may protrude from the guiding element <NUM>. When lowering the bonding device <NUM> onto the first element <NUM>, the first end <NUM> of the bonding wire <NUM> which protrudes from the guiding element <NUM> is arranged between the bonding device <NUM> and the first element <NUM>. The first end <NUM> of the bonding wire <NUM> protrudes from the guiding element <NUM> at a desired position of the bonding device <NUM>. Therefore, by positioning the bonding device <NUM> with respect to the first element <NUM>, the first end <NUM> of the bonding wire <NUM> may also be precisely positioned with respect to the first element <NUM>. The first end <NUM> of the bonding wire <NUM> is then pressed onto the first element <NUM> by applying a certain pressure with the bonding device <NUM>. This pressure that is applied by the bonding device <NUM> is exemplarily illustrated by means of a bold arrow in <FIG>. As has been described before, no ball is formed at the first end <NUM> of the bonding wire <NUM> before pressing the first end <NUM> to the first element <NUM>. When pressing the first end <NUM> on the first element <NUM> and applying a force to the first end <NUM>, at least the first end <NUM> may additionally be exposed to ultrasonic energy, thereby deforming the first end <NUM> of the bonding wire <NUM> while pressing it to the first element <NUM> and creating a permanent substance-to-substance bond between the first end <NUM> of the bonding wire <NUM> and the first element <NUM>. Additionally, the first element <NUM> may also be exposed, at least partly, to ultrasonic energy.

As is exemplarily illustrated in <FIG>, the first element <NUM> may be electrically connected to a second element <NUM> via the bonding wire <NUM>. A second end <NUM> of the bonding wire <NUM> may be connected (bonded) to the second element <NUM> in essentially the same way as has been described with respect to the first element <NUM> and the first end <NUM> of the bonding wire <NUM> before. The bonding wire <NUM> may be a continuous bonding wire <NUM> which is cut off at a desired location, thereby forming the second end <NUM>, as is exemplarily illustrated in <FIG>. The bonding wire <NUM> may be cut off after establishing the connection with the second element <NUM>, when lifting the bonding device <NUM> from the second element <NUM>, as is exemplarily illustrated by the bold arrow in <FIG>. The second element <NUM> may be another semiconductor body or a semiconductor substrate, for example.

Usually, aluminum (Al) wires are used for wedge bonding processes. However, the use of Al-wires has several disadvantages. Other materials such as Copper (Cu), for example, provide better characteristics with regard to reliability and performance as compared to Al-wires. Copper, however, is generally harder than aluminum. When using Cu-wires, therefore, more pressure needs to be applied to the first end <NUM> (or second end <NUM>) of a Cu-wire as compared to Al-wires. Pressing the comparably hard Cu-wire on the first element <NUM> (or the second element <NUM>) may lead to damages of the first element <NUM> (or the second element12). For example, the first element <NUM> may comprise an electrically conductive layer (e.g., bond pad, not illustrated in <FIG>). Such an electrically conductive layer may comprise an electrically conducting material such as a metal, or a metal alloy, for example. If a Cu-wire is used, the electrically conductive layer often also comprises Copper. The first end <NUM> of the bonding wire <NUM> may be pressed on such an electrically conductive layer. If the first end <NUM> of the bonding wire <NUM> is pressed on the electrically conductive layer with relatively high pressure, however, the electrically conductive layer may be damaged. Therefore, generally ball bonding processes are used instead of wedge bonding processes when bonding Cu-wires.

<FIG> schematically illustrates an exemplary embodiment of a wedge-wedge bonding method that may be used for bonding Cu-wires. The bonding wire <NUM> that is exemplarily illustrated in <FIG> may consist of or may include copper or a copper alloy, for example. The bonding wire <NUM> may be a thick bonding wire having a diameter d1 of at least <NUM>, or at least <NUM>, and at most <NUM>, for example. According to one example, the diameter d1 is between <NUM> and <NUM>. If the bonding wire <NUM> has a rectangular cross section instead of a round cross section, the bonding wire <NUM> may have a width w1 of at least <NUM>, at least <NUM>, at least <NUM>, or at least <NUM>, and at most <NUM>, for example. According to one example, the width w1 is between <NUM> and <NUM>. The bonding wire <NUM> having a rectangular cross section may have a height h1 of at least <NUM>, at least <NUM>, or at least <NUM>, and at most <NUM>, for example. According to one example, the height h1 is between <NUM> and <NUM>.

As is schematically illustrated in <FIG>, the first element <NUM> and the first end <NUM> of the bonding wire <NUM> are heated while bonding the bonding wire <NUM> to the first element <NUM>, that is, while applying pressure to the first end <NUM> of the bonding wire <NUM> and while further exposing the first end <NUM> of the bonding wire to ultrasonic energy. For example, the first end <NUM> of the bonding wire <NUM> may be heated via the first element <NUM>. That is, heat is applied to the first element <NUM> and is subsequently transferred to the first end <NUM> of the bonding wire <NUM>. For example, the first element <NUM> may be arranged on a heating device <NUM>, as schematically illustrated in <FIG>. The heating device <NUM> may be configured to heat the first element <NUM> and further to radiate heat to the surroundings of the element <NUM> and of the bonding wire <NUM>. For example, if the first element <NUM> and the bonding wire <NUM> are arranged in a bonding chamber (not illustrated in <FIG>), the inside of the bonding chamber may be heated by the heating device <NUM>. Heating the first element <NUM> (and an optional electrically conductive layer arranged on the first element <NUM>) and the first end <NUM> of the bonding wire <NUM> supports the bonding process. When heated, the components such as the bonding wire <NUM> or an electrically conductive layer provided on the first element <NUM>, for example, may be formed or reshaped a lot easier than without heating. Heating the components that are to be bonded together further increases the friction value and, therefore, the energy transmission. The solidity (yield strength/tensile strength) of the components decreases with increasing temperature. The energy that is required for forming the bond between the bonding wire <NUM> and the first element <NUM> depends on the temperature of the components. If the temperature increases, the energy required for the bonding process decreases. The total energy that is required to form the bond between the bonding wire <NUM> and the element <NUM> is generally made up of normal force and ultrasonic energy.

However, the probably most important advantage of applying heat to the components is the reduction of mechanical stress affecting the first element <NUM> (or the electrically conductive layer arranged on the first element <NUM>). When pressing the bonding wire <NUM> on an electrically conductive layer, the materials of the bonding wire <NUM> and the electrically conductive layer commingle to a certain degree. A reduced mechanical stress results in a lower commingling of the materials of the different components. It is generally preferable that the materials commingle only to a comparably low degree. The reliability and the quality of the bond connection, however, is at least the same as for conventional wedge-wedge bonding processes without heating, if not better.

The bonding process may be performed in an ambient air environment. As is exemplarily illustrated in <FIG>, the wedge-wedge bonding process that has been described with respect to <FIG> above, may also be carried out in a protective atmosphere (inert atmosphere), for example. That is, an inert protective gas <NUM> may be provided. The inert protective gas <NUM> may surround the bonding device <NUM>, the bonding wire <NUM> as well as the first element <NUM>. The inert protective gas <NUM> further assists the bonding process and further enhances the advantages that have been discussed above. Further, the inert protective gas <NUM> functions as protection against oxidation of the metallic components. Heating the metallic components such as the bonding wire <NUM> and the electrically conductive layer may result in an oxidation of the surfaces of the metallic components if an oxidation protection is not provided. An oxidation of the surfaces of the bonding partners reduces the reliability of the bond that is formed between the bonding partners. At worst, an oxidation of the surfaces of the bonding partners may result in unwanted lift offs of the bonding wires.

Performing the bonding process in an inert gas atmosphere, however, is only an example. Instead of an inert gas, a reducing protection gas may be provided. A reducing protection gas may comprise, e.g., Ar, SF<NUM> or H2, for example. Alternatively, any other suitable gases may be provided. Both, an inert gas and a reducing protective gas, may alter the tribological stress exerted on the bonding partners. In particular, such gases may positively alter the tribological stress such that the abrasion of the bonding partners may be reduced. As is exemplarily illustrated in <FIG> and as has been explained above, the first element <NUM> may comprise a first layer <NUM> of electrically conducting material. The first layer <NUM> may function as a bond pad and the bonding wire <NUM> may be permanently connected to the first layer <NUM>. As has been described before, the first layer <NUM> may comprise a metal such as Copper, for example. The first layer <NUM> may comprise the same material as the bonding wire <NUM> and may have a thickness d2 of less than <NUM> and at least <NUM> (not claimed). The first layer <NUM> may completely cover a surface of the first element, as is exemplarily illustrated in <FIG>. This, however, is only an example. It is also possible that the first layer <NUM> only partly covers one of the surfaces of the first element <NUM>.

As has been described above, the material of the bonding wire <NUM> and the material of the first layer <NUM> may at least partly commingle during the bonding process, because of the hardness of the thick Cu-wire. By heating the first layer <NUM> and the bonding wire <NUM>, e.g., with the heating arrangement that has been described with respect to <FIG> above, the commingling of the materials of the first layer <NUM> and the bonding wire <NUM> may be reduced. Even for first layers <NUM> having a thickness d2 of at least <NUM> and less than <NUM> (not claimed), at least <NUM>% of the thickness d2 below the first end <NUM> of the bonding wire <NUM> may remain intact after bonding the bonding wire <NUM> to the first layer <NUM>. Remaining intact in this context means that the material does not commingle with the material of the bonding wire <NUM>. This reduced commingling may be further supported by providing a protective gas atmosphere, as has been described with respect to <FIG> above. The temperature of the first layer <NUM> and the bonding wire <NUM> may be, e.g., at least <NUM>, at least <NUM>, at least <NUM> or at least <NUM>, and at most <NUM>.

According to an even further example, the bonding process is performed in a vacuum. For example, the bonding partners (e.g., first element <NUM>, bonding wire <NUM>) and the bonding device <NUM> may be arranged in a vacuum chamber while performing the bonding process. The vacuum, together with the increased temperature, has a similarly positive effect as the inert gas and the reduced protective gas that have been described before.

According to an even further example, the thickness of the first layer <NUM> may be increased as compared to the arrangement that has been described with respect to <FIG> before. This is exemplarily illustrated in <FIG>. The thickness d3 of the first layer <NUM> may be greater than <NUM>, for example. If the thickness d2, d3 of the first layer <NUM> is increased, a larger percentage of the material of the first layer <NUM> that is arranged below the first end <NUM> of the bonding wire <NUM> remains intact after the bonding process. This means that the commingling of the material of the bonding wire <NUM> and the first layer <NUM> does not affect the whole thickness d2, d3 of the first layer <NUM>. For example, if the thickness of the first layer <NUM> is increased to more than <NUM>, at least <NUM>% or at least <NUM>% of the thickness d2, d3 below the first end <NUM> may remain intact. However, it is also possible that more or less of the thickness d2, d3 below the first end <NUM> remain intact.

The bonding device <NUM> illustrated in <FIG> is only one example. The bonding device <NUM> may be implemented in any other suitable way. One further possible example is schematically illustrated in <FIG>. As is illustrated in <FIG>, the bonding device <NUM> may comprise a so-called wedge <NUM>. The wedge <NUM> is mainly configured to apply a pressure to a bonding wire <NUM>, thereby pressing the bonding wire <NUM> on the first element <NUM> to form the bonding connection. The bonding wire <NUM> may be held in place by a so-called wire guide <NUM>. In particular, the bonding wire <NUM> may be led through the wire guide <NUM> and protrude from an opening in the wire guide <NUM>. The wire guide <NUM> may be positioned in relation to the wedge <NUM>. For example, the wire guide <NUM> may be permanently coupled to the wedge <NUM> such that the wire guide <NUM> and the wedge <NUM> always perform the same movements and the wire guide <NUM> is always positioned in the same position with regard to the wedge <NUM>. The bonding device <NUM> may further comprise a blade <NUM> for cutting the bonding wire <NUM>, where required. Any other suitable implementations of the bonding device <NUM> are also possible.

The commingling of the materials of different bonding partners is exemplarily illustrated in <FIG> illustrates a cross-sectional view of a section of the first layer <NUM> and a first end <NUM> of a bonding wire <NUM> that is bonded to the first layer <NUM>. The first layer has a thickness d3, as has been described with respect to <FIG> before. The commingling of the materials is indicated with the dotted area. The commingling may occur in the first layer <NUM> as well as in the bonding wire <NUM>. In the area below the first end <NUM> of the bonding wire <NUM>, the first layer <NUM> may be seen as having a first sub-layer including only material of the first layer <NUM> and a second sub-layer including commingled material (material of the first layer <NUM> and the bonding wire <NUM> intermixed). The first sub-layer have a thickness d31 and the second sub-layer may have a thickness d32. As has been described above, the thickness d31 of the first sub-layer may be at least <NUM>%, at least <NUM>% or at least <NUM>% of the thickness d3 of the first layer <NUM>.

According to a further example, the hardness of the first layer <NUM> may be chosen such that the wear of the first layer <NUM> is reduced. For example, the hardness of the first layer <NUM> may be chosen depending on the hardness of the bonding wire <NUM>. The bonding wire <NUM> may have a first hardness and the first layer <NUM> may have a second hardness, wherein the second hardness is greater than the first hardness. For example, the second hardness may be ≥<NUM>% of the first hardness. A greater hardness of the first layer <NUM> may be attained by alloying the material of the first layer <NUM>, e.g., the copper of the first layer <NUM>. Increasing the hardness of the first layer <NUM> with respect to the hardness of the bonding wire <NUM> reduces the tribological stress on the first layer <NUM>.

According to another example, a greater hardness of the first layer <NUM> may be attained by adjusting the fine crystalline copper structure of the first layer <NUM>. For example, the grain size of the microstructure within the first layer <NUM> may be less than the thickness d2, d3 of the first layer <NUM>.

Now referring to <FIG>, an arrangement that may be used to perform a bonding process as has been described with respect to <FIG> above is exemplarily illustrated. For example, the arrangement may comprise a bonding chamber <NUM>. The first element <NUM> may be arranged within the bonding chamber <NUM>. A heating device <NUM> may form a bottom of the bonding chamber <NUM>. This, however, is only an example. According to another example, the heating device <NUM> may be arranged within the bonding chamber <NUM>. The first element <NUM> may be arranged on the bottom of the bonding chamber <NUM>, e.g., on the heating device <NUM>. The bonding device <NUM> that is configured to receive the bonding wire <NUM>, to position the bonding wire <NUM> with respect to the first element <NUM>, and to press the bonding wire to the first element <NUM> is arranged within the bonding chamber <NUM>. The bonding device <NUM> may be coupled to a control unit <NUM>. The bonding chamber <NUM> may comprise an opening <NUM>. The bonding device <NUM> may protrude through the opening <NUM> to the outside of the bonding chamber <NUM>. The opening <NUM> may be covered by a flexible cover <NUM>, which may be a sort of bellows, for example. The flexible cover <NUM> may prevent a significant gas exchange between the inside and the outside of the bonding chamber <NUM>. In this way, an inert gas or a reduced protective gas could be prevented from escaping the bonding chamber <NUM>. Further, the flexible cover <NUM> which closes the bonding chamber <NUM> allows to create a vacuum within the bonding chamber <NUM>.

The bonding chamber <NUM> may comprise one or more gas inlets <NUM> that are configured to direct a gas such as an inert gas or a reducing protective gas into the bonding chamber <NUM>. The flow of gas inside the chamber <NUM> is exemplarily illustrated in <FIG> by bold arrows. The flexible cover <NUM> may not completely seal the opening <NUM> of the bonding chamber <NUM>. Therefore, some gas that is led into the chamber <NUM> though the gas inlets <NUM> may exit the chamber <NUM> through the flexible cover <NUM> or through gaps between the flexible cover <NUM> and the bonding chamber <NUM>. The control unit <NUM> may protrude through an opening in the flexible cover <NUM>, thereby closing the opening in the flexible cover <NUM>. The control unit <NUM> and the bonding device <NUM> that is connected to the control unit <NUM> need to be moved in order to position the bonding wire <NUM> in relation to the first element <NUM>. The flexible cover <NUM> allows such movements of the control unit <NUM> and the bonding device <NUM>.

An optional vacuum unit <NUM> may be arranged in the bottom of the bonding chamber <NUM>. The vacuum unit <NUM> may be configured to create a vacuum, thereby drawing the first element <NUM> which is arranged above the vacuum unit <NUM> towards the bottom of the bonding chamber <NUM>. In this way, the first element <NUM> may be firmly held in place during the bonding process when comparably large forces are exerted on the first element <NUM>. The first element <NUM>, however, may be held in place in any other suitable way. The bonding chamber <NUM> mainly functions as a gas cover. However, the bonding chamber <NUM> may be further configured to keep heat that is generated by the heating device inside a specified volume (inside the bonding chamber <NUM>).

Now referring to <FIG>, according to another example the opening <NUM> of the bonding chamber <NUM> may be covered with a brush protection cap <NUM>. <FIG> schematically illustrate top views of different implementations of a brush protection cap <NUM>. According to a first example, a multiplicity of bristles <NUM> protrudes from a first side into the opening <NUM>, thereby essentially covering the opening. A brush protection cap <NUM> generally does not provide a gas tight cover for the opening <NUM>. Therefore, gas that is led into the chamber <NUM> may exit the chamber <NUM> though openings between the bristles <NUM> of the brush protection cap36. This is schematically illustrated with bold arrows in <FIG>. The control unit <NUM> may be arranged outside the bonding chamber <NUM>. The bonding device <NUM> may protrude through the bristles <NUM> into the bonding chamber <NUM>. The flexible bristles <NUM> allow the bonding device <NUM> to move in order to position the bonding wire <NUM>. As is schematically illustrated in <FIG>, bristles <NUM> may protrude into the opening <NUM> from two different sides. In the example of <FIG>, the bristles do not overlap. This, however, is only an example. According to another example, the bristles <NUM> protruding from different sides of the opening <NUM> may overlap. Further, the bristles <NUM> may be arranged in any other suitable way, thereby covering the opening <NUM> and allowing a movement of the bonding device <NUM>.

Claim 1:
A method comprising:
heating a first electrically conductive layer (<NUM>) that is to be electrically contacted, and that is arranged on a first element (<NUM>); and
pressing a first end (<NUM>) of a bonding wire (<NUM>) on the first electrically conductive layer (<NUM>) by exerting pressure to the first end (<NUM>) of the bonding wire (<NUM>), and further by exposing the first end (<NUM>) of the bonding wire (<NUM>) to ultrasonic energy, thereby deforming the first end (<NUM>) of the bonding wire (<NUM>) and creating a permanent substance-to-substance bond between the first end (<NUM>) of the bonding wire (<NUM>) and the first electrically conductive layer (<NUM>); wherein
the bonding wire (<NUM>) either comprises a rounded cross section with a diameter (d1) of at least <NUM> or a rectangular cross section with a first width (w1) of at least <NUM> and a first height (h1) of at least <NUM>, and
the first layer (<NUM>) has a thickness (d2, d3) of at least <NUM>.