Patent Application
20070074812
The present invention relates generally to plasma, and relates specifically to measuring plasma parameters, for example plasma density, in plasma processing reactors.
An aspect of the present invention is to provide an apparatus for measuring a plasma parameter in a plasma processing reactor. The apparatus comprises a dielectric tube, a sensor disposed in the dielectric tube and a connector disposed in the dielectric tube and coupled to the sensor. The apparatus further comprises a plurality of spacers disposed between the connector and the dielectric tube. The plurality of spacers define a plurality of ducts through which a cooling fluid is adapted to be circulated to control a temperature of the probe.
Plasma is used in material processing reactors because it provides significant benefits in processing rate, accuracy, and processing capabilities in comparison with non-plasma methods. In order to better control the plasma and the processes taking place inside a plasma reactor, it is sometimes useful to introduce measurement devices inside the plasma reactor to measure various parameters of the plasma including, for example, temperature of the plasma and density of the plasma. Plasma density defines, among other things, the radical content in the processing gas and the processing speed. The plasma density is one of the process parameters that a process engineer uses to evaluate a specific plasma process. Plasma density in a processing chamber depends on many factors, including gas composition, gas pressure, flow rate, pumping speed, geometry of the chamber and configurations of various components, such as the electrodes in the chamber, and the materials of the chamber walls.
Plasma density in a processing chamber also depends on the power of ionizing sources, which is typically radio frequency (RF) power applied from various types of coils (i.e., inductively coupled plasma sources, or ICP), RF power applied to electrodes (i.e., capacitive coupled plasma sources, or CCP), microwave power, etc. In addition, plasma density in the processing chamber depends also on the rate of loss of the plasma due to, for example, direct loss to the walls, the electrodes, and various recombination and neutralization processes occurring in the plasma.
In an embodiment of the invention, the coaxial cable 14 is a round, flexible, two-conductor cable consisting of, from the center outwards, a center wire, a dielectric layer, a braided metal mesh sleeve, and an outer shield. The shield prevents signals transmitted on the center wire from affecting nearby components and prevents external interference from affecting the signal carried by the center wire. The center wire extends out beyond the other layers to form open antenna tip 16. The antenna tip 16 can be straight or not straight (for example slightly curved).
The tip 20 of the dielectric tube 18 is located within the area where the plasma density has to be measured. The dielectric tube 18 isolates the coaxial cable 14 and the antenna tip 16 from the plasma environment and prevents direct currents from reaching the coaxial cable 14 and the antenna tip 16. The material of the dielectric tube 18 can be selected to adjust a resonant frequency for the system. The dielectric permittivity of the material of the dielectric tube 18 can thus be chosen to correspond to an expected plasma density range (e.g., quartz has a lower dielectric permittivity than ceramic materials, such as alumina Al2O3). In a chemically active environment, during plasma processing of a material, dielectric depositions on the dielectric tube 18 may occur. However, these chemical depositions on the dielectric tube 18 do not affect the probe data, at least until the thickness of the deposition layer becomes thick enough to be comparable with the thickness of the dielectric tube 18.
The probe 12 includes a base 22. The base 22 closes the dielectric tube 18 and allows the probe 12 to be mounted to a component 24 of the plasma apparatus. The component 24 can be, for example, a wall of the processing chamber of the plasma apparatus. A vacuum seal can be included in base 22 to seal the probe 12 to the component of plasma apparatus (e.g., a wall of the chamber of the plasma apparatus). The base 22 can be made of, for example, metal (e.g., aluminum), ceramic material, etc.
The coaxial cable 14 can be selected to be long enough so that the antenna tip 16 is located at a position remote from the base 22. In this way, the long dielectric tube can reach to a position in the plasma apparatus where the plasma takes places.
In order to measure parameters (e.g., plasma density) of the plasma. The probe 12 is inserted in a plasma environment where temperatures can reach a level which may alter the operation of the probe and may even damage the probe if the probe is left for a long period of time in the plasma environment without appropriate heat protection. This problem may be more acute depending on the shape of the probe 12. For example, a generally slender shape of the probe 12 can prevent effective heat transfer along the dielectric tube 18 towards the base 22. As a result, this may lead to an increase of the temperature of the probe, particularly at the probe end immersed in the plasma. The increasing temperatures can cause differential thermal expansions of the probe parts which can lead to erroneous readings. For example, due to differential thermal expansions of the inner coaxial cable 14 (e.g., the metal wire) and the outer coaxial cable 14 (e.g. the braided metal mesh or the dielectric layer), a length of the antenna tip 16 can change. This may lead to a change in surface wave propagation length and hence may cause a readout of apparent change in plasma properties, e.g. plasma density.
Therefore, in order to reduce the temperature of the probe 12 to a temperature at which damage of the probe does not occur and in order to maintain the temperature of the probe relatively constant, to reduce thermal expansions, a temperature control system 26 is used. The temperature control system 26 comprises two spacers 28a and 28b. In an embodiment of the invention, the spacers 28a and 28b wrap around the coaxial cable 14 in a spiral-like configuration.
The spacers 28a and 28b serve at least two purposes. One purpose is to maintain a space between the coaxial cable 14 and the tube 18 so as to prevent possible contact of the coaxial cable 14 with a relatively hot surface of the tube 18 when the probe 12 is inserted in a plasma environment. Another purpose is to divide the space between the coaxial cable 14 and the tube 18 into two ducts 30a and 30b (shown in
The two ducts 30a and 30b allow a cooling fluid to circulate around the coaxial cable 14 and around the interior surface of tube 18. The cooling fluid circulates in duct 30a from the base 22 of the probe 12 toward the tip 20 of the probe and returns through duct 30b towards the base 22 of the probe 12. The ducts 30a and 30b formed by the spacers 28a and 28b are connected at the base 22 of the probe (probe mounting block), respectively, to the inlet channel 32a and outlet channel 32b. The cooling fluid is supplied by a cooling fluid source (not shown) through the inlet channel 32a. The cooling fluid enters the duct 30a through the inlet 32a and travels, for example spirally, guided by the spacers 28a and 28b, towards the tip 20 of the probe 12. When the cooling fluid reaches the tip 20 of the probe 12, the fluid passes through a perforated end-piece 34 which serves to mechanically hold the spacers 28a and 28b to an end of coaxial cable 14 at the probe tip 20. The arrows in
During assembly, the coaxial cable 14 is mounted in the base (probe mounting block) 22. Then the spacers 28a and 28b are wrapped around the coaxial cable 14 and held at the end of the coaxial cable using the end-piece 34. The dielectric tube or sheath 18 is slipped over the entire assembly coaxial cable-spacers. The dielectric tube 18 is then sealed at the base (mounting block) side to prevent fluid leakage. A seal may be achieved using soldering, brazing, or various types of adhesives.
The spacers 28a and 28b are wrapped around the coaxial cable 14 and the diameter of the spacers 28a and 28b is approximately equal half of a difference between an internal diameter of the dielectric tube 18 and an external diameter of the coaxial cable 14. In this way, by slipping the tube 18 over the coaxial cable-spacers assembly, the coaxial cable 14 is substantially centered in the dielectric tube or sheath 18.
The spacers 28a and 28b do not require any additional sealing against either the coaxial cable 14 external surface or the dielectric tube 18 internal surface. A contact between the spacers 28a and 28b and the external surface of the coaxial cable 14 and the internal surface of the tube 18 forms adequate channels for the cooling fluid to be guided therethrough without additional sealing. Of course such sealing can be employed. Furthermore, even in the case where the spacers 28a and 28b are not in intimate contact with the coaxial cable exterior surface and the sheath interior surface, that is the ducts 28a and 28b are not “perfectly” isolated from each other which may result in some fluid seeping from one duct (for example 30a) to the other (for example 30b), the bulk of the cooling fluid will circulate from the base 22 of the probe 12 to the tip 20 of probe and returns back to the base 22 of the probe 12 and thus provide adequate temperature control.
The spacers 28a and 28b can be made of any suitable material including, but not limited to, metal, plastic, ceramic materials or a combination thereof. In an embodiment of the invention, the spacers 28a and 28b are made of a plastic material such as Polytetrafluoroethylene (PTFE), commercialized by Dupont Corporation under the trademark TEFLON. PTFE is suitable for use as a spacer material as it is resistant to high temperatures such as the temperatures reached during plasma processing. In addition, PTFE has a low friction coefficient and is reasonably malleable which allows the spacers 28a and 28b to be made with a slightly larger cross-section dimensions (e.g., oversize diameter) than the dimensions of the space between the coaxial cable 14 and the dielectric tube 18. This enhances sealing of the spacers against the coaxial cable 14 and interior surface of the dielectric tube 18, while the low friction coefficient of PTFE facilitates insertion of the assembly coaxial cable-spacers inside the dielectric tube 18. Hence, even when the spacers 28a and 28b are slightly oversized, i.e., the spacers 28a and 28b have a larger diameter than the dimension of the space between the interior surface of the coaxial cable and the interior surface of the sheath 18, the low friction coefficient and malleability of PTFE would facilitate mounting the assembly coaxial cable-spacers and thus prevent potential damage of the probe components.
The cooling fluid introduced through the inlet 32a and transported through ducts 30a and 30b can be any suitable fluid including gases and liquids or a mixture thereof. In an embodiment of the invention, fluorinated cooling liquids such as FLUORINERT made by 3M corporation and GALDEN made by Solvay Solexis. corporation can be used. Other liquids that may be used for such application include super-cooled gases for demanding applications (for example, applications where a long probe is used to reach inside a very hot plasma), such as liquid nitrogen or liquid carbon dioxide. Generally, liquids provide a higher heat transfer coefficient than gases. However, in certain circumstances liquids may be impractical for use. For example, in the case where the probe has a relatively small cross-section dimension (e.g., small diameter) resulting in relatively narrow ducts, a high inlet liquid pressure may be needed to drive the liquid through the narrow ducts. In such cases, gas cooling may be more suitable. Examples of gases that may be used for cooling or for temperature control in the probe 12 include air, argon and helium (in general, noble gases), nitrogen, etc.
A plasma apparatus is usually provided with a source of pressurized cooling fluid in order to control a temperature or temperatures of various components of the plasma apparatus as needed. Therefore, it is also possible to tap into this readily available source of pressurized cooling fluid and use it to control the temperature of the probe 12.
In an embodiment of the invention, the apparatus for measuring plasma density 10 further comprises one or more temperature sensors 40 inserted at a number of points along the probe 12 to provide temperature signals via wires or optical fibers 63 to a feedback control system (not shown).
For example, as illustrated in
By providing a temperature signal feedback to the temperature control apparatus, the temperature of the probe 12 can be controlled. For example, the temperature can be maintained below a certain limit so as to prevent differential thermal expansions of the probe parts which can lead to erroneous reading of the plasma density or in certain circumstances may lead to damage of the probe. In some applications, it may be desirable to maintain the temperature within a certain range of temperatures so as to prevent temperature fluctuations and thus provide a more stable reading of the plasma parameters, e.g., plasma density.
The apparatus 50 further includes a base or feed block 60. The base 60 closes the dielectric tube 58 and allows the connector 54 with sensor 52 to be mounted to a component 62 of the plasma apparatus (not shown). The base 60 can be made of, for example, metal (e.g. aluminum), dielectric material or the like. The connector 54 can be selected to be long enough so that the sensor 52 disposed at an end of the connector 54 is located at a position remote from the base 60. In this way, the long dielectric. tube can reach to a position in the plasma apparatus where the plasma takes places. This allows for a more accurate measurement of the plasma parameter while at the same time preventing overheating of the base 60.
Similarly, to the embodiment shown in
The two ducts 70a and 70b allow a cooling fluid to circulate around the connector 54 and around an interior surface of tube 58. The cooling fluid circulates in duct 70a from the base 60 toward tip 72 of tube 58 and return through duct 30b from the tip 72 to the base 60. The ducts 30a and 30b formed by the spacers 28a and 28b are connected at the base 60 to the inlet channel 68a and outlet channel 68b. The cooling fluid enters the duct 70a through the inlet 68a and travels, for example spirally, guided by the spacers 66a and 66b, towards the tip 72. When the cooling fluid reaches the tip 72, the fluid passes through a perforated end-piece 74 which serves to mechanically hold the spacers 66a and 66b to the outer surface 56 of connector 54 at the tip 72. After passing the perforated end-piece 74, the cooling fluid returns via the duct 70b formed by the two spacers 66a and 66b to be evacuated through outlet 68b.
In addition, similarly to the previous embodiments, the apparatus for measuring plasma parameter may further include one or more temperature sensors 76 inserted at a number of points along the connector 54, for example attached to outer surface 56 of connector 54, to provide temperature signals via wires or optical fibers 63 to a feedback control system 600 (shown in
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the present invention. In fact, after reading the above description, it will be apparent to one skilled in the relevant art(s) how to implement the invention in alternative embodiments. Thus, the present invention should not be limited by any of the above-described exemplary embodiments.
Moreover, the method and apparatus of the present invention, like related apparatus and methods used in the plasma arts are complex in nature, are often best practiced by empirically determining the appropriate values of the operating parameters, or by conducting computer simulations to arrive at best design for a given application. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.
In addition, it should be understood that the figures, are presented for example purposes only. The architecture of the present invention is sufficiently flexible and configurable, such that it may be utilized in ways other than that shown in the accompanying figures.
Further, the purpose of the Abstract of the Disclosure is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract of the Disclosure is not intended to be limiting as to the scope of the present invention in any way.
