Patent Description:
The handling of reflected power is a particular challenge in radio frequency (RF) power applications for plasma generation, e. for the etching and deposition of thin films on silicon wafers during the manufacturing of integrated circuits. RF plasmas are highly dynamic loads which can vary quickly. The use of impedance matching networks between an RF power generator and a plasma can compensate for impedance mismatch in steady-state plasma operation. However, such matching networks are in general too slow to immediately adapt to changes in the load impedance. For example, during the ignition phase of a plasma, during power level changes, and in pulse-mode operation, the RF generator is exposed to transient reflected power which has to be dissipated within the generator.

The RF power stage of an RF generator comprises the following circuits.

and are operating at frequencies in the range of <NUM> - <NUM> and at power levels of <NUM> W and higher. For power levels up to approximately <NUM> kW, air cooling is sufficient, higher power levels above <NUM> kW typically require water cooling.

In a combiner, termination resistors are typically used for absorbing and dissipating reflected power. Since reflected power levels can get very high depending on the situation, the termination resistor has to be appropriately cooled such that the resulting heat is quickly transferred to a cold plate or heat sink.

The term heat sink refers to a heat exchanger that transfers heat from an electronic circuit to a fluid medium, e. air or a liquid coolant. One part of the heat sink typically maximizes the contact, and thus heat transfer, to the fluid medium by an enlarged surface area, e. by means of cooling fins or spikes. The term cold plate refers to a heat exchanger through which a liquid coolant is circulated, typically a thick plate with embedded pipes through which the coolant is flowing. Heat sinks and cold plates are typically made from metallic materials, like aluminum, copper, or metallic alloys, but also ceramics and other materials with high thermal conductivity are possible.

A heat spreader is a device with good thermal conductivity, typically a plate, which distributes heat which is generated in a small area to a larger area. A heat spreader is typically used in connection with a heat sink or cold plate by which the heat is transferred to a fluid medium. The distribution of heat in a heat spreader may be enhanced by thermoelectric cooling (e. Peltier elements) or by the use of heat pipes.

In the following, the term "cooling element" is used for all types of air-cooled heat exchangers with fins or similar enlarged surface area, for liquid-cooled cold plates, and for combinations of a heat spreader with an air-cooled heat sink or liquid-cooled cold plate, including possible enhancements like thermoelectric cooling or heat pipes.

A typical power resistor, which allows to clamp it to a heat sink with screws, comprises a heat spreader and a resistor chip soldered to this heat spreader.

However, clamping such a resistor from both ends, i.e. clamping only the heat spreader, leads to a reduced pressure and weaker thermal connection in the center of the device where the heat is generated by the resistor chip. Further, the area necessary for the clamping cannot be used by the resistor chip, thereby reducing the maximum possible size of the resistor chip which increases the total heat load in the center of the resistor.

Typical RF generators have to fit into industry-standard electronic racks, and therefore, the width of the generator is often specified to <NUM>/<NUM> or <NUM> inches, and its height to <NUM> - <NUM> rack units, wherein <NUM> rack unit is <NUM> inches. In consequence, it is difficult to arrange all circuits and components such that they fit the available dimensions, and therefore, the termination resistor has to be placed such that it does not interfere with other circuits or components and consumes minimum space.

Particularly for frequencies below <NUM>, the circuit components, capacitances and inductances have to be dimensioned with decreasing frequency in larger value and, therefore, size. In consequence, e. for the important ISM frequencies of <NUM> and <NUM>, it is difficult to fit all required components of an RF generator into a <NUM>/<NUM>-inch enclosure. Consequently, most RF generators with more than <NUM> kW are forced to use a full <NUM>-inch sized housing.

In modern RF power circuit designs for volume production, in order to achieve low variation from unit to unit, three-dimensional arrangements of wires and ferrites for inductors are avoided because they are difficult to reproduce with the required geometrical precision. Instead, fully planar structures are preferred which can be manufactured with high precision in industrial printed-circuit board (PCB) production. Although, the size of planar circuits is a challenge when <NUM>/<NUM>-inch wide enclosures are to be used, advantageously, planar combiners allow to place the termination resistor in the center of an inductor structure.

For best heat transfer, the PCB with such an inductor will have a cut-out in its center such that the power resistor can be clamped directly to the cooling element which serves as a ground connection. To avoid the magnetic field of the inductor to couple into the cooling element, typically made from metal, the PCB with the inductor cannot be placed onto the cooling element directly. Instead, a ceramic heat spreader of several millimeters thickness has to be used to enlarge the distance.

Also, for this resistor assembly, non-magnetic materials must be used. Otherwise, eddy currents would be induced by the magnetic field of the inductor which would cause a heating up of the resistor assembly.

In consequence, the flange of the power resistor via which the ground connection of the termination is established is within a cut-out in the ceramic heat spreader and in direct contact with the cooling element, whereas the connection of the resistor to the inductor has to bridge a considerable gap to the PCB on top of the ceramic heat spreader. Moreover, the coefficients of thermal expansion (cTE) of the involved materials are all different. Therefore, in cycling operation when RF is turned on and off frequently, or the RF power level is frequently changed, e. on a scale of seconds, the connections between the top contact of the termination resistor and the inductor experience considerable thermomechanical stress in cycling operation which is common for short semiconductor plasma processes and also during pulsing operation. These brazed or soldered joints can deteriorate rapidly and eventually break at high operating temperatures.

The connection of the flange to the cooling element experiences thermomechanical stress as the flange material, e. a copper molybdenum alloy which has a similar cTE as the resistor material, is not matched to the cTE of the cooling element which is typically made from copper or aluminum or an alloy.

<CIT> describes a resistor assembly to fix a resistor chip by clamping. Other prior art is disclosed in documents <CIT>, <CIT>, <CIT> and <CIT>.

Thus, it is an objective of the present invention to provide a resistor reliably connected to a heat sink or heat spreader in a compact manner.

The problems of the prior art are solved by a resistor according to claim <NUM>.

In a first aspect, a resistor is provided in particular for a power combiner. The resistor comprises a resistor chip. Preferably, the resistor chip comprises an area of between <NUM><NUM> and <NUM><NUM>. Preferably, the size of the resistor chip is between <NUM> x <NUM> and <NUM> x <NUM>. Preferably, the resistor chip has a rated power of <NUM> W or more. Preferably, the resistor chip is built as thick film resistor, in particular based on aluminum nitride (AIN). In particular, the resistor chip has an operating temperature up to <NUM> or more. Preferably, the resistor chip has one terminal on its upper surface, and a second terminal on its lower surface. The lower surface may be fully covered with a metal layer such that the second terminal can be attached to an electrically conducting passive heat spreader that transfers the heat to the cooling element which acts as a ground connection.

According to the present invention, the resistor comprises a clamping element to clamp the resistor chip to a cooling element. Therein, a heat spreader is built to distribute the heat generated by the resistor chip and transfer it to the cooling element. Due to the clamping element, a contact between the resistor chip and the cooling element is established in order to dissipate the heat generated by the resistor chip to the cooling element. Therein, the clamping element has a clamping arm to apply a pressure to the resistor chip. Thus, by application of the pressure directly onto the resistor chip, contact between the resistor chip and the cooling element is improved. Thus, contrary to the prior art, the resistor chip itself is clamped wherein the clamping force is directly applied to the resistor chip instead of to a heat spreading element, e. a flange, of a resistor of the prior art as described above. Thus, the required space for fixing the resistor chip to the cooling element is reduced and consequently, the area of the resistor chip can be increased leading to an improved heat transfer away from the resistor chip. In addition, by clamping (contrary to fixing the resistor by screws as in the prior art) thermal expansion of the resistor chip is allowed, thereby reducing thermally induced stress in the resistor chip. Hence, heat dissipation from the resistor chip to the cooling element is improved and reliability as well as lifetime of the resistor is increased due to a decreased operating temperature.

Preferably, the clamping element comprises a base element connected to the clamping arm in an L-shape. Therein, in particular, the base element has a first end connected to the clamping arm and an opposite second end directly connected to or in contact with the cooling element in the mounted state. In particular, the base element and the clamping arm are one piece and integrally built. Therein, in particular, the angle between the base element and the clamping arm might be <NUM> ° or less. Thus, by the base element a contact point/line is created, wherein the clamping element pivots around this contact point/line in order to enable the clamp arm to apply a pressure to the resistor chip.

Preferably, the second end of the base element is inclined to allow rotation movement of the clamping element around the edge which is in a direction away from the resistor chip. Thus, the edge in a distance larger away from the resistor chip of the second end of the base element becomes the pivot point/line of the clamping element. Thereby, the pressure applied by the clamping element can be increased by up to <NUM> % versus the situation with a pivot line at the edge closer to the resistor chip, corresponding to a flat second end of the base element. With a flat second end of the base element, the clamping element would rotate around a pivot line on the second end of the base element which is close to the resistor chip. As an alternative to an inclined second end, the second end may comprise a protrusion providing a pivot line at a position of the second end away from the resistor chip.

Preferably, the clamping arm and the base element are made from a rigid non-magnetic material, e. from an aluminum alloy, brass, or an austenitic steel.

According to the invention, between the clamping arm and the resistor chip, a pressure distribution plate is arranged. By the pressure distribution plate, the pressure applied by the clamping arm is distributed over the entire area of the resistor chip. Preferably, the pressure distribution plate is a rigid element built from a metal or plastic. Preferably, the pressure distribution plate is a separate element or may be integrally built with the clamping arm. Preferably, the thickness of the pressure distribution plate is between <NUM> and <NUM>, wherein the area of the pressure distribution plate is preferably between <NUM><NUM> and <NUM><NUM>.

According to the invention, the lower side of the clamping arm connected to the pressure distribution plate comprises an alignment feature and an upper surface of the pressure distribution plate comprises a corresponding alignment feature engaging the alignment feature of the clamping arm and configured to align the position of the clamping arm and the pressure distribution plate. Therein, the alignment feature may be a ball, detent, semispherical protrusion, or the like. Therein, the alignment feature of the clamping arm may be integrally built with the clamping arm or may be an additional element. Therein, a semispherical alignment feature provides a beneficial contact surface in order to evenly transfer the pressure of the clamping arm to the pressure distribution plate. Therein, the corresponding alignment feature of the pressure distribution plate has a shape corresponding to the alignment feature of the clamping arm in order to engage or receive the alignment feature of the clamping arm. Therein, the corresponding alignment feature may be built as groove, recess, dent or the like. Of course, the shapes of the alignment feature of the clamping arm and the corresponding alignment feature of the pressure distribution plate can be switched. By the alignment feature of the clamping arm and the corresponding alignment feature of the pressure distribution plate, an orientation of the pressure distribution plate is maintained.

Preferably, an insulating layer is arranged between the resistor chip and the clamping arm and preferably arranged directly between the resistor chip and the pressure distribution plate. Therein, the insulating layer may be arranged at an upper surface of the resistor chip. The insulating layer is electrically insulating. Preferably, the insulating layer is made from a high temperature elastic material like perfluoroelastomere, polyimide or polyamide. By the insulating layer, a short circuit between the in particular metallic pressure distribution plate and the resistor chip is avoided. Therein, the insulating layer may be integrally built with the pressure distribution plate, if the pressure distribution plate is built from a plastic material. Thereby, the insulating effect is inherently provided by the pressure distribution plate itself and thus, combines the element of the pressure distribution plate and the insulating layer. Alternatively, insulating layer and pressure distribution plate are separate elements. By building the insulating layer as an elastic layer, pressure distribution from the pressure distribution plate to the resistor chip is improved and slight manufacturing imperfections of the resistor chip and / or the pressure distribution plate can be compensated by the elastic insulating layer without creating a short-circuit.

Preferably, the resistor comprises pressure means connected to the clamping arm and connectable to a heat sink or heat spreader and configured to create the pressure of the clamping arm. Preferably, the pressure means are built as screw.

In particular, the pressure means extend through the clamping arm and preferably through the pressure distribution plate. Therein, in particular the through hole of the clamping arm might conform the outer diameter of the pressure means, wherein the through hole of the pressure distribution plate may be larger than the pressure means in order to allow movability of the pressure distribution plate relative to the pressure means. In addition, by the pressure means, orientation of the pressure distribution plate is maintained. In combination with the alignment feature of the clamping arm and the corresponding alignment feature of the pressure distribution plate, the position of the pressure distribution plate is determined and fixed.

Preferably, the pressure means do not extend through the resistor chip. In particular, the pressure means are distant and apart from the resistor chip. By avoiding contact between the pressure means and the resistor chip, the possibility of a short-circuit is prevented.

Preferably, at a lower surface of the resistor chip, opposite to the clamping arm, a conduction layer is arranged, wherein preferably the conduction layer is thermally conductive and / or electrically conductive. Therein, preferably the thermal conduction is larger than <NUM> W/m/K, more preferably larger than <NUM> W/m/K and most preferably larger than <NUM> W/m/K, thereby exceeding the heat conduction of a heat sink made for example from aluminum having a heat conduction of <NUM> W/m/K and / or the heat conduction of a heat spreader for example made from Al<NUM>O<NUM> having a heat conduction of <NUM> W/m/K.

Preferably, the conduction layer is compressible. Preferably, the conduction layer has a compressibility of <NUM>% or larger, more preferably <NUM>% or larger. Preferably the conduction layer has a bulk modulus of less than 1000kPa, preferably 600kPa. By the compressibility of the conduction layer, small manufacturing imperfections of the resistor chip and / or the underlying surface provided by the cooling element can be compensated, thereby providing a full contact between the respective surfaces in order to enable optimum heat dissipation from the resistor chip to the cooling element.

Preferably, the conduction layer is provided by a heat transfer paste or a thermal pad or a compressible graphite thermal interface material, preferably with a compressibility of <NUM>% or larger, more preferably of <NUM>% or larger.

Preferably, the conduction layer provides an electrical contact between ground and the resistor chip.

Preferably, the resistor comprises a connecting element, preferably built as ribbon, connected to the resistor chip and, in particular connected at a side of the resistor chip opposite to the base element of the clamping element. Thus, in particular the connecting element is connected to a short side of the resistor chip. Therein, the connecting element may be connected to the resistor chip on a top surface thereof. Thus, by the connecting element, the resistor is electrically connected to the respective circuit. As mentioned above, the second terminal of the resistor chip may be provided by the lower surface of the resistor chip by means of a metal layer.

Preferably, the connecting element is S-shaped. By the S-shape thermal expansion can be compensated and thus cannot lead to a damage of the connecting element.

The invention is described in more detail with reference to the accompanying figures.

In the following it is referred to <FIG>. The resistor according to the present invention comprises a resistor chip <NUM> having an upper surface <NUM> and a lower surface <NUM>. The resistor chip <NUM> comprises a connecting element <NUM> built as ribbon. The connecting element <NUM> may be bent in a S-shape. The second terminal of the resistor chip <NUM> is provided by the lower surface <NUM> which is connected usually to ground. This second terminal may be a metal layer <NUM> which covers the lower surface <NUM>. Further, the resistor comprises a clamping element <NUM> having a clamping arm <NUM> and a base element <NUM>. Therein, the base element <NUM> comprises a first end <NUM> connected to the clamping arm <NUM> and a second end <NUM> which is directly connected to a cooling element <NUM> of a radiofrequency (RF) generator. The clamping element <NUM> is connected to the heat spreader <NUM> by a screw <NUM>. The second end <NUM> of the base element <NUM>, as shown in <FIG>, comprises an inclined surface <NUM> having an angle α. By the inclined surface <NUM> a pivot line <NUM> is built at the outer edge of the base element <NUM>, i.e. the edge farer away from the resistor chip <NUM>. By this angle α provided by the inclined surface <NUM>, the clamping force generated by the clamping element <NUM> can be increased by <NUM> % compared to a flat surface which would rotate around a pivot line at the edge closer to the resistor chip <NUM>. When turning the screw <NUM>, a pressure is applied by the clamping arm <NUM> to a pressure distribution plate <NUM>. Therein, the screw <NUM> may extend through a through hole <NUM> of the pressure distribution plate <NUM>. Further, the clamping arm <NUM> comprises a semispherical alignment feature <NUM>, wherein the pressure distribution plate <NUM> comprises a semispherical recess <NUM> in order to receive the alignment feature <NUM> of the clamping arm <NUM>.

Thereby, the position of the pressure distribution plate <NUM> is defined by the alignment feature <NUM> and the corresponding recess <NUM> together with the through hole <NUM> receiving the screw. The pressure distribution plate <NUM> distributes the pressure applied by the clamping arm <NUM> and is built from a rigid material such as rigid plastic or metal. Between the pressure distribution plate <NUM> and the upper surface <NUM> of the resistor chip <NUM> an insulating layer <NUM> is provided in order to electrically insulate the resistor chip <NUM> from the pressure distribution plate <NUM>. Therein, the screw <NUM> may extend through a through hole of the insulating layer <NUM>. The insulating layer <NUM> may be compressible in order to evenly distribute the pressure provided by the pressure distribution plate <NUM> onto the resistor chip <NUM> and allow thermal expansion without introducing thermal stress into the resistor chip <NUM>. The insulating layer <NUM> may be made from a high temperature elastic material like perfluoroelastomere, polyimide or polyamide.

At the lower surface <NUM> of the resistor chip <NUM>, between the cooling element <NUM> and the resistor chip <NUM>, a conduction layer is arranged being thermally and electrically conductive. Thus, by the conduction layer contact of the lower surface <NUM> of the resistor chip <NUM> to ground is established. At the same time heat generated by the resistor chip is transferred to the cooling element <NUM>.

Referring to <FIG> showing a power not being part of the present invention which might be part of a high-power RF generator. The power combiner comprises at least one inductor <NUM>, wherein the resistor according to the present invention is connected to the inductor <NUM> by its connecting element <NUM> as termination resistor.

Preferably, the resistor according to the present invention has an operating temperature of <NUM> or above, preferably <NUM> or above.

Wherein in the prior art, a flanged resistor connected to the cooling element by two screws allows to handle the maximum absorbed power of <NUM> W, the present invention increases the resistor chip area and at the same time improves thermal contact between the resistor chip and the cooling element thereby allowing up to <NUM> W or more of absorbed power. At the same time, the operating temperature of the resistor can be reduced by <NUM> or more.

Claim 1:
Resistor assembly comprising
a resistor chip (<NUM>) and
a clamping element (<NUM>) to clamp the resistor chip (<NUM>) to a cooling element (<NUM>),
wherein the clamping element (<NUM>) has a clamping arm (<NUM>) to apply a pressure to the resistor chip (<NUM>),
wherein between the clamping arm (<NUM>) and the resistor chip (<NUM>) a pressure distribution plate (<NUM>) is arranged such that the pressure applied by the clamping arm (<NUM>) is distributed over the entire area of the resistor chip (<NUM>),
characterized in that
the lower side of the clamping arm connected to the pressure distribution plate (<NUM>) comprises an alignment feature (<NUM>), and the upper surface of the pressure distribution plate (<NUM>) comprises a corresponding alignment feature (<NUM>) engaging the alignment feature (<NUM>) of the clamping arm (<NUM>), and is configured to align the position of the clamping arm (<NUM>) and the pressure distribution plate (<NUM>).