Radio frequency generator

Abstract

A radio frequency generator, in particular for plasma applications, i.e. A radio frequency generator, comprises: a cooling element; at least a first radio frequency power stage having a first output and a second radio frequency power stage having a second output, the first radio frequency power stage and the second radio frequency power stage both mounted to a cooling element; a radio frequency combination network mounted to the cooling element, the radio frequency combination network comprising at least a first input and a second input; the radio frequency distribution element comprises a printed circuit board, namely a PCB, with a plurality of conductors; wherein the conductors of the radio frequency distribution element are arranged such that a first output of the first radio frequency power stage is connected to a first input of the radio frequency combination network and a second output of the second radio frequency power stage is connected to a second input of the radio frequency combination network, wherein the printed circuit board of the radio frequency distribution element is arranged perpendicular to the cooling element.

Description

Technical Field

[0001] The present invention relates to a radio frequency (RF) generator, in particular for use in plasma applications or plasma processing tools. Background Art

[0002] Plasma treatment is a very versatile and precise material surface modification technique, particularly in the manufacture of semiconductor chips. Such treatment is employed many times throughout semiconductor wafer processing using plasma treatment tools, which typically operate at frequencies ranging from 0.3 MHz to 300 MHz and radio frequency (RF) power levels of 500 W and higher.

[0003] In such a treatment system, radio frequency power is used to excite gaseous compounds so that free electrons, ions and free radicals are formed. Depending on various treatment parameters, the plasma composition, i.e. the amount of ions, electrons and various free radicals, can be precisely controlled. Such a plasma treatment is preferably used to deposit layers on semiconductor wafers or to etch the surface of semiconductor wafers, both with high uniformity and very precisely over the entire surface.

[0004] RF power generators are typically housed in a 19-inch rack along with other electronics. For high throughput, state-of-the-art plasma processing tools may include multiple plasma processing chambers for parallel processing of multiple wafers. Additionally, typically, a plasma process may be operated with two or more RF generators, such as one for powering the plasma source and a different one for applying RF bias to the wafer. Furthermore, special plasma processes may operate simultaneously using multiple RF frequencies from different RF power generators. Therefore, it may be necessary to fit multiple RF generators into a single 19-inch rack. Since wafer manufacturing space is costly, the size of the RF generator should be as small as possible. For lower RF power levels, such as 1 to 2kW, it is typically necessary to fit two RF generators side by side into a 19-inch rack, while for higher power levels, such as over 5kW, one generator may occupy the entire width of a full 19-inch slot.

[0005] However, even power levels above 2kW require the RF generator to fit into an enclosure compatible with 19 / 2-inch dimensions and 3-5 rack units (U) height (1U = 1.75 inches), i.e. 133-222 mm in height and 450-650 mm in length.

[0006] These size constraints lead to high power densities in modern RF generators and require submodules (e.g., RF power stages or combiners) to be implemented as multi-layer or stacked arrangements. Additionally, when generating RF power, advanced cooling concepts must be used to remove the heat generated by the unavoidable losses (typically in the range of 20% and higher).

[0007] The term "heat sink" refers to a heat exchanger that transfers heat from electronic circuits to a fluid medium (e.g., air or liquid coolant). A portion of a heat sink typically maximizes contact with, and thus heat transfer to, the fluid medium by having a cooling structure with an enlarged surface area (e.g., typically by means of cooling fins or tips or high surface corrugations). The term "cold plate" refers to a heat exchanger through which a liquid coolant circulates, typically a thick plate with embedded tubes through which the coolant flows. Heat sinks and cold plates are typically made of metallic materials such as aluminum, copper, or metal alloys, but ceramics and other materials with good thermal conductivity are also possible.

[0008] 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 heat sinks with air-cooled heat sinks or liquid-cooled cold plates, including possible enhancements like thermoelectric cooling or heat pipes.

[0009] In a high-power RF generator, various parts of the RF section (e.g., power amplifier stages and combiners) are usually arranged on the surface of a cooling element as described above, so that the generated heat is effectively removed and the operating temperature of the electronic circuits and components is kept within a safe operating area. In the case of an air-cooled heat sink with fins or tips, the electronic circuits and components are usually arranged on a surface opposite to the surface with the cooling structure. For example, on the back side of the heat sink relative to the cooling structure.

[0010] In order to generate a high power radio frequency signal, various architectures can be used to combine the signals of multiple power amplifiers, for example, in the case of four power stages, two pairs can be combined in a pair of first combiners, and then the outputs of the two first combiners are combined into one signal in a second combining network. In this way, by combining four power stages each having 1.25 to 1.5kW via two combining stages, a total output of 5 to 6kW can be generated. The mechanical arrangement of multiple power amplifiers and combining networks is usually carried out in such a way that the power amplifier is directly connected to the input of the first combiner stage, and the output of the first combiner stage is directly connected to the input of the second combiner stage. This means that the two combiner stages are arranged in parallel adjacent to each other and their outputs are directly connected to the appropriately spaced input connectors of the power combiner.

[0011] Unfortunately, the described arrangement, which can be implemented well over a full width of 19 inches, requires too much space to fit into a 19 / 2 inch wide enclosure. Implementing small RF power stages with power levels above 1 kW is difficult, especially at frequencies below 50 MHz, because the size of the circuit structures and components is inversely proportional to the frequency. For example, the dimensions of the 5 to 6 kW arrangement described above only allow two 1.25 kW or 1.5 kW amplifiers to be arranged in a preferred combination with a first combiner stage, but a second power combining stage for combining the two resulting 2.5 kW or 3 kW combinations does not fit into the 19 / 2 inch width of the enclosure.

[0012] Therefore, the second power combination stage must be placed elsewhere on the cooling element, for which, typically at power levels greater than 1kW, a water-cooled cold plate can be used. This requires a longer connection (e.g. more than 10cm) to bridge the distance between the submodules. Therefore, a connection is needed that meets the following requirements:

[0013] Small footprint to fit into the remaining space of a housing with limited dimensions;

[0014] Easy to connect, e.g. using standard connectors and welding or threading;

[0015] Capable of carrying >1kW of power;

[0016] Low losses and proper cooling.

[0017] The standard solution for connecting RF power circuits over a certain distance is to use RF coaxial cables. However, for power levels of several kilowatts, larger cable diameters must be used to keep the cable temperature within reasonable limits. Such cables require a relatively large space due to their minimum allowed bending radius and the need for RF connectors to achieve optimal coupling. Using cables with RF connectors increases costs and requires space for mating connectors on the submodule. In addition, since the cables may become hot during power operation, effective cooling is required, for example, by fans.

[0018] Another state-of-the-art approach is to use metal strips, brackets or bridges, for example made of copper with a silver finish or the like. Such parts can be produced and assembled in a cost-effective and space-reducing manner, for example by using the space above various PCBs. However, such parts are only sufficient for small gaps, such as between submodules arranged directly side by side. In the case of larger distances between submodules, each with 50 ohm input and output impedance, the connection must be impedance matched. This requires that the strip or bridge runs at a certain distance parallel to the ground plane or parallel to a second strip of similar dimensions, forming a structure similar to a transmission line. Therefore, more space and geometrically more complex connection arrangements with specific boundary conditions are ultimately required. In addition, appropriate insulation of the connector and its surroundings must be ensured to avoid corona and arcing effects on components in close proximity. Summary of the invention

[0019] It is therefore an object of the present invention to provide a radio frequency generator which is compact and provides a high power output.

[0020] The given technical problem is solved by a radiofrequency generator according to claim 1 .

[0021] According to the invention, a radio frequency generator, in particular for plasma applications, comprises a cooling element. The cooling element can be cooled by an effective means (e.g., water or air flow) so as to dissipate the heat generated by the radio frequency generator. In addition, the radio frequency generator comprises at least a first radio frequency power stage having a first output and a second radio frequency power stage having a second output. Wherein, the first radio frequency power stage and the second radio frequency power stage are mounted on the surface of the cooling element. In order to increase the output power of the radio frequency generator, additional radio frequency power stages can be adopted and implemented on the same cooling element. Each radio frequency power stage comprises one or more power amplifiers to amplify the radio frequency signal controlled by the driver. In addition, the output of the first radio frequency power stage and the output of the second radio frequency power stage and in particular the output of each additional radio frequency power stage are combined to a common radio frequency output by a radio frequency combining network. The output of the radio frequency combining network can be provided to the radio frequency output of the radio frequency generator via an output sensor (e.g., a directional coupler). Wherein, the radio frequency combining network is mounted on the same surface of the cooling element as the radio frequency power stage.

[0022] According to the present invention, a radio frequency generator comprises a radio frequency distribution element, wherein the radio frequency distribution element comprises a printed circuit board (PCB) having a plurality of conductors. The conductors of the radio frequency distribution element are arranged so that the first output end of the first power stage is connected to the first input end of the radio frequency combination network and the second output end of the second power stage is connected to the second input end of the radio frequency combination network. Therefore, through the conductors of the radio frequency distribution element, the radio frequency signal of the first radio frequency power stage and the radio frequency output signal of the second power stage are provided to the radio frequency combination network for combination. Wherein, the printed circuit board of the radio frequency distribution element is arranged perpendicular to the surface of the cooling element on which the radio frequency power stage and the radio frequency combination network are mounted. In other words, the side of the radio frequency distribution element with the shortest length (i.e., the combined thickness of the PCB and the heat sink) is in thermal contact with the surface of the cooling element. Therefore, a compact arrangement of the radio frequency power stage and the radio frequency distribution element can be achieved. In particular, the radio frequency distribution element itself is compact and requires only a small space in the radio frequency generator.

[0023] Preferably, the radio frequency generator provides an output power greater than 2 kW and more preferably greater than 5 kW.

[0024] Preferably, the cooling element of the RF generator is a water-cooled cold plate. Especially for high-power RF generators providing an output power above 5 kW, water cooling is mandatory. If one aims to reduce the size of such a high-power RF generator with water cooling, heat management becomes more challenging due to the dense packing and small distances of the various components generating heat.

[0025] Preferably, each RF power stage provides more than 1 kW.

[0026] Preferably, each RF power stage provides a RF signal having a frequency between 0.3 MHz and 300 MHz.

[0027] Preferably, the length of each conductor in the plurality of conductors provided is greater than 10 cm, wherein the length of the conductor is equal to the distance between the output end of the corresponding radio frequency power stage and the input end of the radio frequency combining network.

[0028] Preferably, the conductors may have different lengths. Alternatively, all conductors have the same length.

[0029] Preferably, the first RF power stage and the second RF power stage provide in-phase RF signals at their outputs. If the conductors have the same length connecting the first RF power stage and the second RF power stage to the RF combining network, then due to the same length, the in-phase signals reach the RF combining network to combine the RF signals.

[0030] Preferably, the RF generator comprises a housing, wherein the RF generator is arranged in the housing. The housing preferably has a standardized 19 / 2 inch size or a 19 inch size. Therefore, the RF generator can be assembled in a 19 / 2 inch rack or a 19 inch rack.

[0031] Preferably, the RF distribution element is arranged parallel to the long side of the housing. Preferably, the RF distribution element is directly adjacent to the long side of the housing, i.e., no further elements are arranged between the RF distribution element and the housing, thereby providing maximum space for other components of the RF generator (such as the RF power stage).

[0032] Preferably, the conductor is constructed as a microstrip or stripline. Microstrip is a type of electrical transmission line that can be manufactured using different technologies and includes a conductor separated from a metal ground plane by a dielectric layer. Stripline uses a flat metal strip as a conductor sandwiched between two metal planes (usually ground planes). The strip and metal planes are separated by an insulating or dielectric material.

[0033] Preferably, the conductor is formed on the surface of the printed circuit board and / or inside the printed circuit board. In particular, the internal conductor can be realized by an internal metal layer.

[0034] Preferably, the printed circuit board of the radio frequency distribution element comprises a metal layer on one surface of the printed circuit board. In particular, if the conductor is constructed as a microstrip, the metal layer is used as ground. Alternatively, the printed circuit board of the radio frequency distribution element comprises a metal layer on both surfaces of the printed circuit board. In this case, the conductor is an inner conductor or is constructed as a stripline.

[0035] Preferably, at least one metal layer of the printed circuit board of the radio frequency distribution element is grounded via a cold plate of the radio frequency generator.

[0036] Preferably, the width of each conductor is less than or equal to the distance between the corresponding conductor and the metal layer. Thus, interference from other modules of the RF generator or other RF signals in the RF distribution element can be minimized, and the influence of metal parts near the RF distribution element on the impedance of the RF conductor can be kept low.

[0037] Preferably, the RF distribution component comprises a heat sink connected to the printed circuit board, wherein the heat sink of the RF distribution component is in contact with a cooling element (usually a cold plate) to transfer heat from the conductor to the cooling element. The metal layer of the printed circuit board can also be grounded through the contact between the heat sink of the RF distribution component and the cold plate.

[0038] Preferably, the heat sink is made of metal such as copper or aluminum or an alloy.

[0039] Preferably, the heat sink is connected to the printed circuit board of the radio frequency distribution component by one or more of an adhesive, clamping screws, a thermally conductive interface material, a solder bond or a bonding pad.

[0040] Preferably, the RF distribution element is connected to the cooling element by one or more of adhesive, clamping screws, thermally conductive interface material, solder joint or bonding pads, thereby enabling sufficient heat transfer from the RF distribution element to the cooling element.

[0041] Preferably, the RF distribution element is connected to the first and second outputs of the power stage and the first and second inputs of the RF combining network by means of brackets and / or screws. The brackets may be welded to pads on the RF distribution element, combiner or power stage, thereby enabling connection to corresponding ports on another circuit by means of screws. Thus, easy assembly is facilitated and a reliable connection is provided between the RF power stage at one end of the RF distribution element and the RF combining network at the other end of the RF distribution element.

[0042] The use of solder pads for attaching brackets is particularly compatible with transmission lines designed as microstrips, since the solder pads can be easily arranged in the top layer of the printed circuit board, which is also provided with the microstrip. In the case of striplines, the realization of contact pads to the signal lines is more complicated. Since the signal lines are located in an inner layer of the printed circuit board, located between the top and bottom metal layers, the contact pads for the signal lines must be formed within the openings of the top metal layer and must be connected to the signal line layer by vias or via arrays.

[0043] Preferably, the conductors include even-mode and odd-mode impedances of approximately 50 ohms with a tolerance of + / -10%. In this way, proper RF energy transfer from the RF power stage to the RF combining network can be achieved and coupling between the conductors can be minimized.

[0044] Hereinafter, the present invention is described in more detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The diagram shows:

[0046] Figure 1 is a schematic diagram of the layout of a radio frequency generator according to the present invention,

[0047] Figure 2 is a detailed view of a radio frequency distribution element according to the present invention, and

[0048] Figure 3 yes Figure 1 Cross-sectional view of the RF generator along direction III. DETAILED DESCRIPTION

[0049] like Figure 1As shown, a radio frequency (RF) generator 10 according to the invention comprises an actively cooled cooling element 30. In particular for RF generators that generate an output power higher than 2 kW, the cooling element 30 can typically be a water-cooled cold plate. In particular, the RF generator does not include air cooling means, such as cooling fins, fans, etc. A driver 11, a first RF power stage 12 and a second RF power stage 14 are arranged on the surface of the cooling element 30. The power stages 12, 14 comprise power amplifiers that provide a radio frequency signal greater than 1 kW at their outputs at a frequency between 0.3 MHz and 300 MHz depending on the specific application. In addition, the RF generator 10 comprises an RF combining network 16, in which the RF signals of the RF power stages 12, 14 are combined into a common signal, which is then fed to the RF output 18 of the RF generator 10. In addition, the RF combining network 16 is arranged on the same surface of the cooling element 30, in which case the cooling element 30 is actively cooled.

[0050] It is desirable to reduce the footprint or manufacturing size of the RF generator 10. However, due to the expected frequencies, especially in the range below 30 MHz, it is not possible to further reduce the specific frequency-related electronic components. Therefore, the space requirements increase as the frequency decreases. At the same time, in the case of higher powers, more heat is generated, which must be dissipated efficiently, wherein at the same time the space available on the cooling element is reduced. Therefore, according to the present invention, the first RF power stage 12 includes a first output terminal 22 and the second RF power stage 14 includes a second output terminal 24. The first output terminals 22 and the second output terminals 24 of the RF power stages 12 and 14 are connected to a first input terminal 26 of the RF combining network 16 and a second input terminal 28 of the RF combining network 16 via an RF distribution element 20. Therein, the RF distribution element 20 is arranged upright or substantially perpendicular to the surface of the cooling element on which the RF power stages 12, 14 and / or the RF combining network 16 are mounted. As shown in FIG. Figure 2 The RF distribution element 20 shown includes a printed circuit board (PCB) 32. The PCB 32 includes a first conductor 34 arranged to connect the first output terminal 22 of the first RF power stage 12 with the first input terminal 26 of the RF combining network 16. In addition, the PCB 32 includes a second conductor 36 connecting the second output terminal 24 of the second RF power stage 14 with the second input terminal 28 of the RF combining network 16. Therein, additional conductors can be implemented in the RF distribution element 20, and the number of conductors can correspond to the number of RF power stages. In particular, the conductors 34, 36 are constructed as microstrips or striplines. Due to the arrangement of the RF power stages 12, 14 and the RF combining network 16, the length of the conductors 34, 36 is equal to the distance between the corresponding output terminals 22, 24 of the RF power stages 12, 14 and the input terminals 26, 28 of the RF combining network 16. Preferably, the length of the conductors 34, 36 of the RF distribution element is 10 cm or more. As shown in FIG. Figure 1 As shown, the RF distribution element 20 extends along the long sides of the housing 40 of the RF generator, thereby providing sufficient space for the RF power stages 12 , 14 .

[0051] like Figure 2 As shown, the conductors 34 and 36 include contact pads 51, 52, 53, 54 at their ends to which connection elements (e.g., brackets) can be attached (e.g., welded). These connection elements are used to connect the conductors 34, 36 to the output terminals 22, 24 of the power stages 12, 14 and the input terminals 26, 28 of the combining network 18.

[0052] Preferably, the housing 40 is constructed as a standardized housing that can be mounted in an electronics rack with a width of 19 inches. More preferably, the housing 40 has a width of 19 / 2 inches so that two RF generators 10 can be mounted side by side in an electronics rack with a width of 19 inches.

[0053] Preferably, the conductors 34, 36 have equal lengths so that signals from power stages with a certain phase difference arrive at the input of the combining network with the same phase difference. In particular, signals with the same phase at the power stages can then be phase combined by the combining network (e.g., a Wilkinson combiner).

[0054] If a combining network is used which requires a different phase difference value between the combined signals, e.g. 90° for a 3-dB hybrid combiner, the desired phase difference present at the output of the power stages is maintained by the RF distribution element 20. Thus, since the RF distribution element 20 does not change the phase difference between the signals, any distance between the RF power stages 12, 14 and the RF combining network 16 can be bridged via the RF distribution element 20, wherein the lengths of the individual transmission lines are equal to each other.

[0055] like Figure 2 and Figure 3 As shown, the RF distribution element 20 and in particular the printed circuit board 32 of the RF distribution element 20 is perpendicular to the cooling element 30, ie, perpendicular to the surface of the cooling element on which the RF power stages 12, 14 and / or the RF combining network 16 are mounted.

[0056] The RF distribution element 20 can be Figure 3 The illustrated contact element 43 is attached to the cooling element 30, which contact element 43 may be configured, for example, as an adhesive, a clamping screw, a thermally conductive interface material, a solder joint or a bonding pad. Figure 3In the example of , the printed circuit board 20 comprises conductors 34, 36 on a surface, wherein the opposite surface of the printed circuit board 32 comprises a metal layer 42. The metal layer 42 is oriented toward the housing 40 of the radio frequency generator. The heat sink 38 is attached to the metal layer 42. The heat sink 38 is connected to the cooling element 30 via the contact element 41. Thus, the metal layer 42 is grounded, i.e., indirectly via the heat sink 38 and the contact element 41 or directly via the contact element 41 to the cooling element. Alternatively, the printed circuit board can be constructed as a metal-clad printed circuit board substrate, such as a copper-clad printed circuit board substrate. Through the heat sink 38, the heat generated in the conductors 34, 36 is effectively transferred to the cooling element 30 and dissipated.

[0057] exist Figure 3 In the example of , the conductors 34, 36 are constructed as microstrips. The conductors 34, 36 are connected to the respective RF power stages 12, 14 via screws and / or brackets that connect the conductors 34, 36 to the respective terminals 46 or RF power stages 12, 14. Figure 3 An example of a contact pad 51 is shown, which is connected to the conductor 34 and to which a 90° angled bracket 45 is attached by a contact element 43 (e.g., welding material). The other end of the bracket 45 is connected to a terminal 46, such as a metal part with internal threads, via a screw 44. The terminal 46 is attached to the output 22 of the power stage 12, such as via welding.

[0058] As in Figure 2 In the embodiment of the present invention, the thermal and electrical ground connections of the RF distribution element 20 are made via contact elements 41, which may be thermally conductive interface materials, solder joints, or bonding pads. Alternatively or additionally, screws passing through the top of the heat sink 38 and through the contact elements 41 into the cooling element 30 may be used to securely attach the RF distribution element 20 to the cooling element at several locations along the long side of the RF generator.

[0059] Figure 3 The advantages of the arrangement shown are that it is easy to assemble and disassemble with screws and that all parts can be mounted with high precision, minimizing performance variations between units.

[0060] By means of the present invention, a compact arrangement of submodules of an RF generator can be achieved, wherein the RF distribution element provides increased flexibility for the arrangement and provides a compact and efficient connection between the submodules in order to transmit the RF signals from the respective RF power stages to the RF combining network in a reliable manner.

Claims

1. A radio frequency generator, i.e. a radio frequency generator, particularly for plasma applications, comprising: Cooling elements; at least a first RF power stage and a second RF power stage, the first RF power stage having a first output terminal, the second RF power stage having a second output terminal, the first RF power stage and the second RF power stage both being mounted to a surface of the cooling element; a radio frequency combining network mounted to the surface of the cooling element, the radio frequency combining network comprising at least a first input and a second input; a radio frequency distribution element, wherein the radio frequency distribution element comprises a printed circuit board, i.e., a PCB, having a plurality of conductors, wherein the conductors of the radio frequency distribution element are arranged such that the first output of the first radio frequency power stage is connected to the first input of the radio frequency combining network and the second output of the second radio frequency power stage is connected to the second input of the radio frequency combining network, respectively, The printed circuit board of the radio frequency distribution element is arranged perpendicular to the surface of the cooling element.

2. The radio frequency generator according to claim 1, characterized in that: The length of each of the plurality of conductors is greater than or equal to 10 cm.

3. The radio frequency generator according to claim 1 or 2, characterized in that: The conductors have the same length.

4. The radio frequency generator according to any one of claims 1 to 3, characterized in that: The first RF power stage and the second RF power stage provide in-phase RF signals at their outputs.

5. The radio frequency generator according to any one of claims 1 to 4, which is arranged in a housing, wherein the housing preferably has a standardized 19 / 2 inch size or a 19 inch size.

6. The radio frequency generator according to claim 5, wherein: The radio frequency distribution element is arranged parallel to the long side of the housing, and preferably directly adjacent to the long side of the housing.

7. The radio frequency generator according to any one of claims 1 to 6, characterized in that: The conductor is designed as a microstrip or a stripline.

8. The radio frequency generator according to any one of claims 1 to 7, characterized in that: The conductor is formed on a surface and / or inside the printed circuit board.

9. The radio frequency generator according to any one of claims 1 to 8, characterized in that: The printed circuit board of the radio frequency distribution element includes a metal layer on one surface of the printed circuit board or on both surfaces of the printed circuit board.

10. The radio frequency generator according to claim 9, characterized in that: The width of the conductor is less than or equal to the distance between the conductor and the metal layer.

11. The radio frequency generator according to any one of claims 1 to 10, characterized in that: The RF distribution component includes a heat sink connected to the printed circuit board, wherein the heat sink of the RF distribution component is in contact with the cooling element to transfer heat from the conductor to the cooling element.

12. The radio frequency generator according to any one of claims 1 to 11, characterized in that: The RF distribution element is connected to the cooling element by one or more of an adhesive, a clamping screw, a thermally conductive interface material, a solder bond, or a bonding pad.

13. The radio frequency generator according to any one of claims 1 to 12, characterized in that: The RF distribution element is connected to the first output end of the first RF power stage, the second output end of the second RF power stage, the first input end and the second input end of the RF combining network through brackets or screws.

14. The radio frequency generator according to any one of claims 1 to 13, characterized in that: The even mode impedance and odd mode impedance of the conductor are 50 ohms + / - 10%.

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