High-frequency generator

Abstract

Particularly an RF generator for plasma applications, comprising a cooling element, at least a first RF power stage having a first output and at least a second RF power stage having a second output, an RF coupling network attached to the cooling element, the RF coupling network having at least a first input and a second input, and an RF distribution element, the RF distribution element being a distribution element composed of a printed circuit board (PCB) having a plurality of conductors, wherein the conductors of the RF distribution element are arranged such that the first output of the first RF power stage is connected to the first input of the RF coupling network and the second output of the second RF power stage is connected to the second input of the RF coupling network, and the PCB of the RF distribution element is arranged perpendicular to the cooling element.

Description

Technical Field

[0001] The present invention relates particularly to a radio frequency (RF) generator for plasma applications or plasma processing equipment.

Background Art

[0002] Plasma processing is a very versatile and precise technique for modifying the surface of materials, particularly in the manufacture of semiconductor chips. Such processes typically employ a plasma processing apparatus operating at a frequency in the range of 0.3 to 300 MHz and with a high-frequency (RF) power of 500 W or more, and are adopted multiple times throughout the entire semiconductor wafer process.

[0003] In such a processing apparatus, RF power is used to excite a gaseous compound so that free electrons, ions, and radicals are formed. Depending on various process parameters, the composition of the plasma, i.e., the amounts of ions, electrons, and various radicals, can be accurately controlled. Such plasma processes are preferably used for the deposition of layers on semiconductor wafers and for the etching of the surfaces of semiconductor wafers, both of which have high uniformity and are carried out very precisely over the entire surface.

Disclosure of the Invention

Problems to be Solved by the Invention

[0004] An RF power generator is usually housed in a 19-inch rack together with other electronic devices. To achieve high throughput, state-of-the-art plasma processing equipment may consist of multiple plasma process chambers for processing multiple wafers in parallel. Even more often, the plasma process may be carried out using two or more RF generators. For example, one is for supplying power to the plasma source and the other is for applying an RF bias to the wafer. Furthermore, special plasma processes may be processed simultaneously at multiple RF frequencies from different RF power generators. As a result, it is necessary to fit multiple RF generators into a single 19-inch rack. Since the space in a wafer fab is expensive, the size of the RF generator should be made as small as possible. At low levels of RF power, such as 1 to 2 kW, it is generally necessary to fit two RF generators side by side in a 19-inch rack. However, at high levels of RF power, such as 5 kW or more, a single generator may occupy the entire width of a 19-inch slot.

[0005] Nevertheless, even when the output level of the RF generator is 2 kW or more, it is required to fit into a housing with a size corresponding to 19 / 2 inches, a height of 3 to 5 rack units (1U = 1.75 inches), that is, a height of 133 to 222 mm and a length of 450 to 650 mm.

[0006] Due to such dimensional constraints, the latest RF generators need to have a high power density and sub-modules such as RF power stages and combiners need to be mounted in a multilayer or stacked arrangement. Furthermore, when RF power is generated, it is necessary to use advanced cooling techniques to remove the heat generated by losses that are usually unavoidable in the range of 20% or more.

[0007] The term "heat sink" refers to a heat exchanger that transfers heat from an electronic circuit to a fluid medium, such as air or a liquid coolant. A portion of the heat sink generally maximizes contact, i.e., heat transfer, with the fluid medium by means of a cooling structure with an enlarged surface area, such as cooling fins, spikes, or high corrugations on the surface. The term "cold plate" refers to a heat exchanger through which a liquid coolant circulates, typically referring to a thick plate in which pipes through which the coolant flows are embedded. Heat sinks and cold plates are generally made of metallic materials such as aluminum, copper, and metal alloys, but materials with good thermal conductivity such as ceramics can also be used.

[0008] In the following, the term "cooling element" is used for all kinds of air-cooled heat exchangers with fins or similar enlarged surface areas, liquid-cooled cold plates, and combinations of heat spreaders and air-cooled heat sinks or liquid-cooled cold plates (including possible expansion functions such as thermoelectric cooling and heat pipes).

[0009] High-power RF generators are generally arranged on the surface of the above-mentioned cooling elements such that heat generated by individual components of the RF section, such as power amplification stages and combiners, is efficiently removed and the operating temperature of the electronic circuits and components is kept within a safe operating range. In the case of an air-cooled heat sink with fins or spikes, the electronic circuits and components are usually arranged on the surface opposite to the surface with the cooling structure, e.g., on the back surface of the heat sink with respect to the cooling structure.

[0010] To generate high-power RF signals, the signals of multiple power amplifiers can be combined in various architectures. For example, in the case of four power stages, two pairs can be combined with a first combiner, and the outputs of the two first combiners can be synthesized into one signal by a second combining network. In this way, by combining four power stages of 1.25 - 1.5 kW each with two combiner stages, an output of 5 - 6 kW in total can be generated. The mechanical arrangement of the multiple power amplifiers and the combining network is usually such that the power amplifiers are directly connected to the inputs of the first combiner stage, and the outputs of the first combiner stage are directly connected to the inputs of the second combiner stage. This means that the two combiner stages are arranged adjacent to each other in parallel, and their outputs are directly connected to the input connectors at appropriate intervals of the power combiner.

[0011] However, unfortunately, while the above arrangement can be fully realized within a 19-inch full width, it requires too much space to fit into a 19 / 2-inch-wide enclosure. In particular, at frequencies below 50 MHz, since the circuit configuration and component size are inversely proportional to the frequency, it is difficult to realize a small RF power stage with a power level exceeding 1 kW. For example, the size of the above 5 - 6 kW configuration only allows for arranging two 1.25 kW or 1.5 kW amplifiers in a preferred combination with the first combiner stage, but the second power combining stage for combining the resulting two 2.5 kW or 3 kW combinations does not fit into the 19 / 2-inch width of the enclosure.

[0012] Therefore, the second power combining stage needs to be placed in another location for the cooling element, and usually, when the output level exceeds 1 kW, a water-cooled cold plate is used. In this case, a connection part longer than, for example, 10 cm is required to fill the gap between sub-modules. Therefore, a connection that meets the following requirements is needed. · A small footprint that fits into the available space of the limited-sized enclosure ·Easy connectivity such as standard connectors, soldering, or screw connection ·Capable of supplying power exceeding 1 kW ·Low loss and appropriate cooling

[0013] A standard solution for connecting RF power circuits over long distances is the use of RF coaxial cables. However, at power levels of several kilowatts, larger cable diameters must be used to keep the cable temperature within a reasonable range. Such cables require a relatively large amount of space due to their minimum allowable bend radius and the need for RF connectors for optimal coupling. Using cables with RF connectors increases costs and requires space for mating the connectors to the submodules. Furthermore, since the cables can become hot during the operation of the power supply, efficient cooling by means of fans or the like is required.

[0014] Another new approach is to use metal straps, brackets, and bridges made of, for example, silver-plated copper or similar materials. Such components can be manufactured and assembled in a cost-effective and space-saving manner, for example, by utilizing the space above various printed circuit boards. However, such components are only suitable for relatively small distances, such as the spacing between directly adjacent submodules. When the input and output impedances of both submodules are 50 ohms, a connection that matches the impedances is required when the spacing between the submodules is large. For this purpose, it is necessary to run the strap or bridge parallel to the ground plane at a certain distance or parallel to a second strap of similar dimensions, thereby forming a structure similar to a transmission line. As a result, a wider space and a geometrically more complex arrangement of the connections with specific boundary conditions are required. Furthermore, it is necessary to ensure appropriate insulation around the connectors and avoid corona discharge and arc discharge to nearby components.

[0015] Therefore, an object of the present invention is to provide a compact and high-output RF generator.

Means for Solving the Problem

[0016] Such a technical problem is solved by a radio frequency (RF) generator according to claim 1.

[0017] The RF generator for plasma applications according to the present invention is particularly provided with a cooling element. In order to dissipate the heat generated by the RF generator, the cooling element can be cooled by active means such as, for example, a flow of water or air. Further, the RF generator includes a first RF power stage having at least a first output and a second RF power stage having a second output. Among them, the first RF power stage and the second RF power stage are mounted on the surface of the cooling element. In order to increase the output power of the RF generator, additional RF power stages can be employed and implemented on the same cooling element. Each RF power stage is composed of one or more power amplifiers that amplify an RF signal controlled by a driver. Further, by means of an RF combining network, the output of the first RF power stage and the output of the second RF power stage, and in particular the outputs of each additional RF power stage, are combined into a common RF output. The output of the RF combining network may be provided to the RF output of the RF generator via an output sensor such as, for example, a directional coupler. Among them, the RF combining network is installed on the surface of the same cooling element as the RF power stage.

[0018] According to the present invention, the RF generator includes an RF distribution element composed of a printed circuit board (PCB) having a plurality of conductors. The conductors of the RF distribution element are arranged such that the first output of the first power stage is connected to the first input of the RF coupling network, and the second output of the second power stage is connected to the second input of the RF coupling network. Therefore, the RF signals of the first RF power stage and the RF output signals of the second power stage are supplied to the RF coupling network where they are combined by the conductors of the RF distribution element. At this time, the PCB of the RF distribution element is arranged perpendicular to the surface of the cooling element on which the RF power stage and the RF coupling network are mounted. In other words, the side of the RF distribution element with the shortest length, that is, the sum of the thicknesses of the PCB and the heat spreader, is in thermal contact with the surface of the cooling element. This enables a compact arrangement of the RF power stage and the RF distribution element. In particular, the RF distribution element itself is compact and requires little space within the RF generator.

[0019] The RF generator supplies power of 2 kW or more, and more preferably 5 kW or more.

[0020] Preferably, the cooling element of the RF generator is a water-cooled cold plate. In particular, for high-power RF generators that output more than 5 kW of power, water cooling is essential. When attempting to miniaturize such a high-power RF generator equipped with water cooling, heat management becomes more difficult because the individual components that generate heat are densely packed and have a narrow spacing.

[0021] Preferably, each RF power stage supplies 1 kW or more.

[0022] Preferably, each RF power stage outputs an RF signal having a frequency in the range of 0.3 MHz to 300 MHz.

[0023] Preferably, the length of each of the plurality of provided conductors is 10 cm or more. Here, the length of the conductor is equal to the distance between the output of each RF power stage and the input of the RF coupling network.

[0024] Preferably, the conductors may have different lengths. Alternatively, all the conductors may have the same length.

[0025] Preferably, the first RF power stage and the second RF power stage provide RF signals that are in phase at their outputs. If the lengths of the conductors connecting the first RF power stage and the second RF power stage with the RF coupling network are the same, then due to the same length, in-phase signals reach the RF coupling network, facilitating the coupling of these RF signals.

[0026] Preferably, the RF generator includes a housing, and the RF generator is disposed within this housing. Therein, the housing preferably has a standardized 19 / 2-inch standard or 19-inch standard. Therefore, the RF generator can conform to the 19 / 2-inch standard or 19-inch standard.

[0027] Preferably, the RF distribution element is arranged parallel to the long side of the housing. Therein, preferably, the RF distribution element is directly adjacent to the long side of the housing, that is, no other elements are arranged between the RF distribution element and the housing, thereby providing the maximum space for other components of the RF generator such as the RF power stage.

[0028] Preferably, the conductor is configured as a microstrip or a stripline. A microstrip is a type of electrical transmission line that can be manufactured by different techniques and is composed of a conductor separated from a metal ground plane by a dielectric layer. A stripline uses a flat metal strip as the conductor and is usually sandwiched between two metal planes that are usually ground planes. The strip and the metal planes are separated by an insulating material or a dielectric material.

[0029] Preferably, the conductor is formed on the surface and / or inside of the PCB. In particular, the internal conductor can be realized by an internal metal layer.

[0030] Preferably, the PCB of the RF power distribution element includes a metal layer on one surface of the PCB. In particular, when the conductor is configured as a microstrip, the metal layer is used as a ground. Alternatively, the PCB of the RF distribution element has metal layers on both surfaces of the PCB. In this case, the conductor is configured as an internal conductor or a stripline.

[0031] Preferably, at least one metal layer of the PCB of the RF distribution element is connected to the ground via the cold plate of the RF generator.

[0032] Preferably, the width of each conductor is smaller than or equal to the distance between each conductor and the metal layer. Thereby, 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 components near the RF distribution element on the impedance of the RF conductor can be kept low.

[0033] Preferably, the RF distribution element consists of a heat sink connected to the PCB. The heat sink of the RF distribution element contacts a cooling element, typically a cold plate, and transfers heat from the conductor to the cooling element. There, due to the contact between the heat sink of the RF distribution element and the cold plate, the metal layer of the PCB can also be connected to the ground.

[0034] Preferably, the heat sink is made of a metal such as copper, aluminum, alloy, etc.

[0035] Preferably, the heat sink is connected to the PCB of the RF distribution element by one or more of an adhesive, a clamp screw, a conductive thermal interface material, soldering, or an adhesive sheet.

[0036] Preferably, the RF distribution element is connected to the cooling element by one or more of an adhesive, a clamp screw, a conductive thermal interface material, a solder joint, or an adhesive sheet. This enables sufficient heat transfer from the RF distribution element to the cooling element.

[0037] Preferably, the RF distribution element is connected to the first output and the second output of the power stage, and the first input and the second input of the RF coupling network by brackets and / or screws. The brackets can be soldered to pads of the RF distribution element, a combiner, or a power stage, whereby the connection to the corresponding ports of other circuits is made by screws. This facilitates assembly and provides a reliable connection between the RF distribution element and one end of the RF power stage, and the RF coupling network at the other end of the RF distribution element.

[0038] The use of solder pads for attaching brackets is particularly compatible with transmission lines designed as microstrips. In the case of striplines, the pads for connecting to the signal lines become more complex. Since the signal lines are located between the upper metal layer and the lower metal layer in the internal layer of the PCB, the contact pads to the signal lines are formed within the openings of the upper metal layer and must be connected to the signal line layer by via holes or via arrays.

[0039] Preferably, the conductors constitute an even-mode impedance and an odd-mode impedance of about 50 ohms with an error of ±10%. By doing so, appropriate RF energy transfer from the RF power stage to the RF coupling network is achieved, and the coupling between the conductors is minimized.

[0040] Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

Brief Description of the Drawings

[0041]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0042] As shown in FIG. 1, the RF generator (Radio Frequency (RF) generator) 10 according to the present invention is composed of a cooling element 30 that is actively cooled. In particular, in an RF generator that generates an output exceeding 2 kW, usually, the cooling element 30 may be a water-cooled cold plate. In particular, the RF generator does not include air cooling such as cooling fins and fans. 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 and 14 are composed of power amplifiers that output an RF signal of 1 kW or more at a frequency of 0.3 MHz to 300 MHz according to a specific application. Further, the RF generator 10 includes an RF coupling network 16 to which the RF signals of the RF power stages 12 and 14 are coupled to a common signal supplied to the RF output 18 of the RF generator 10. Also, in this case, the RF coupling network 16 is arranged on the same surface of the cooling element 30 that is actively cooled.

[0043] It is desirable to reduce the installation area of the RF generator 10 or the size of the building. However, due to the intended frequency, especially in the range below 30 MHz, it is not possible to further reduce the electronic components that depend on a specific frequency. Therefore, the spatial requirements increase as the frequency decreases. At the same time, at higher outputs, more heat must be effectively dissipated, and at the same time, the available space on the cooling element decreases. Therefore, according to the present invention, the first RF power stage 12 constitutes the first output 22, and the second RF power stage 14 constitutes the second output 24. The first output 22 and the second output 24 of the RF power stages 12, 14 are connected to the first input 26 of the RF coupling network 16 and the second input 28 of the RF coupling network 16 by the RF distribution element 20. There, 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 coupling network 16 are mounted. As shown in FIG. 2, the RF distribution element 20 is composed of a printed circuit board (PCB) 32. The PCB 32 includes a first conductor 34 arranged to connect the first output 22 of the first RF power stage 12 and the first input 26 of the RF coupling network 16. Further, the PCB 32 includes a second conductor 36 that connects the second output 24 of the second RF power stage 14 and the second input 28 of the RF coupling network 16. There, additional conductors may be mounted on the RF distribution element 20, but 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 coupling network 16, the lengths of the conductors 34, 36 are equal to the distances between the respective outputs 22, 24 of the RF power stages 12, 14 and the inputs 26, 28 of the RF coupling network 16. Preferably, the lengths of the conductors 34, 36 of the RF distribution element are 10 cm or more. As shown in FIG. 1, the RF distribution element 20 extends along the long side of the housing 40 of the RF generator that provides sufficient space for the RF power stages 12, 14.

[0044] As shown in FIG. 2, the conductors 34, 36 have contact pads 51, 52, 53, 54 at their ends, and connection elements, such as brackets, can be attached to these contact pads 51, 52, 53, 54, for example, by soldering. These connection elements are used to connect the conductors 34, 36 to the outputs 22, 24 of the power stages 12, 14 and the inputs 26, 28 of the coupling network 18.

[0045] Preferably, the housing 40 is constructed as a standardized housing that can be attached to a 19-inch-wide electronic equipment rack. More preferably, the housing 40 has a width of 19 / 2 inches so that two RF generators 10 can be attached side by side to a 19-inch-wide electronic equipment rack.

[0046] Preferably, the conductors 34, 36 are of equal length such that signals from power stages having a certain phase difference reach the inputs of the coupling network with the same phase difference. In particular, signals in phase at the power stages are combined in phase by a coupling network such as a Wilkinson combiner.

[0047] When a coupling network that requires different values of the phase difference between the combined signals (for example, 90° in the case of a 3dB hybrid combiner) is used, the required phase difference present at the outputs of the power stages is maintained by the RF distribution element 20. As a result, since the phase difference between the signals is not changed by the RF distribution element 20, any distance between the RF power stages 12, 14 and the RF coupling network 16 can be bridged via the RF distribution element 20 where the lengths of the individual transmission lines are equal to each other.

[0048] As shown in FIGS. 2 and 3, the RF distribution element 20, particularly the PCB 32 of the RF distribution element 20, is perpendicular to the surface of the cooling element 30, that is, the cooling element on which the RF power stages 12, 14 and / or the RF coupling network 16 are mounted.

[0049] As illustrated in FIG. 3, the RF distribution element 20 can be attached to the cooling element 30 by a contact element 43 configured, for example, as an adhesive, a clamp screw, a conductive thermal interface material, a solder joint, or a bonding sheet. In the example of FIG. 3, the PCB 20 includes conductors 34 and 36 on a surface of the opposite side of the PCB 32 that includes a metal layer 42. Therein, the metal layer 42 is directed toward the housing 40 side of the RF generator. A heat sink 38 is attached to the metal layer 42. The heat sink 38 is connected to the cooling element 30 via a contact element 41. Thereby, the metal layer 42 is connected to ground, i.e., the cooling element, indirectly via the heat sink 38 and the contact element 41, or directly via the contact element 41. Alternatively, the PCB can be configured as a metal-clad PCB substrate, for example, a copper-clad PCB substrate. By the heat sink 38, the heat generated in the conductors 34 and 36 is efficiently transferred to and dissipated by the cooling element 30.

[0050] In the example shown in FIG. 3, the conductors 34 and 36 are configured as microstrips. The conductors 34 and 36 are connected to the respective RF power stages 12 and 14 via screws and / or brackets that connect the conductors 34 and 36 to the respective terminals 46 or the RF power stages 12 and 14. FIG. 3 shows an example of a contact pad 51 connected to the conductor 34 and to which a 90°-angle bracket 45 is attached by a contact element 43, for example, a solder material. The other end of the bracket 45 is connected to the terminal 46, for example, a metal part with an internal thread, via a screw 44. The terminal 46 is attached to the output 22 of the power stage 12, for example, by soldering.

[0051] Similar to FIG. 2, the thermal connection and the electrical ground connection of the RF distribution element 20 are made via a contact element 41 that can be a conductive thermal interface material, a solder joint, or a bonding sheet. In order to firmly attach the RF distribution element 20 to the cooling element at several locations along the long side of the RF generator, screws can also be used that pass through the upper end of the heat sink 38 and through the contact element 41 into the cooling element 30.

[0052] The arrangement shown in Fig. 3 has the advantage that it can be easily assembled and disassembled by screws, and all components can be attached with high precision, resulting in little variation in performance from unit to unit.

[0053] According to the present invention, a compact arrangement of sub - modules of an RF generator can be achieved, and the RF distribution element improves the flexibility for this arrangement and provides a compact and efficient connection between sub - modules in order to transfer RF signals from each RF power stage to the RF coupling network in a reliable manner.

Claims

1. A high-frequency, RF generator for plasma applications, comprising: a cooling element; a first RF power stage having a first output and a second RF power stage having a second output, at least attached to the surface of the cooling element; an RF coupling network including at least a first input and a second input, attached to the surface of the cooling element; an RF distribution element; and wherein the RF distribution element includes a printed circuit board (PCB) having a plurality of conductors, and the conductors of the RF distribution element are arranged such that the first output of the first RF power stage is connected to the first input of the RF coupling network, and the second output of the second RF power stage is connected to the second input of the RF coupling network respectively; the PCB of the RF distribution element is arranged perpendicular to the surface of the cooling element. An RF generator characterized by the above.

2. The RF generator according to claim 1, wherein the length of each of the plurality of conductors is 10 cm or more.

3. The RF generator according to claim 1 or 2, wherein the lengths of the conductors are the same.

4. The RF generator according to any one of claims 1 to 3, wherein the first RF power stage and the second RF power stage supply RF signals in phase at their outputs.

5. The RF generator according to any one of claims 1 to 4, arranged within a housing, the housing preferably having a standardized 19 / 2-inch format or 19-inch format.

6. The RF generator according to claim 5, wherein the RF distribution element is arranged parallel to the long side of the housing, preferably directly beside the long side of the housing.

7. The RF generator according to any one of claims 1 to 6, wherein the conductors constitute microstrips or striplines.

8. The RF generator according to any one of claims 1 to 6, wherein the conductors are formed on the surface and / or inside of the PCB.

9. The RF generator according to any one of claims 1 to 8, wherein the PCB of the RF distribution element includes metal layers on one side or both sides of the PCB.

10. The RF generator according to claim 9, wherein the width of the conductor is smaller than or equal to the distance between the conductor and the metal layer.

11. The RF distribution element includes a heat sink connected to the PCB, and the heat sink of the RF distribution element contacts the cooling element to transfer heat from the conductor to the cooling element. The RF generator according to any one of claims 1 to 10.

12. The RF distribution element is connected to the cooling element by one or more of an adhesive, a clamp screw, a conductive heat conductive material, soldering, or an adhesive sheet. The RF generator according to any one of claims 1 to 11.

13. The RF distribution element is connected to the first output of the power stage, the second output, the first input of the RF coupling network, and the second input by brackets or screws. The RF generator according to any one of claims 1 to 12.

14. The even-mode impedance and odd-mode impedance of the conductor are 50 Ω ± 10%. The RF generator according to any one of claims 1 to 13.

Images