Patent Application
20070189918
The invention relates to a device for generating excited and/or ionized particles in a plasma from a process gas, which comprises a generator for generating an electromagnetic wave, a waveguide, and a gas discharge chamber with a gas discharge space, the excited and/or ionized particles being formed in the gas discharge space and the gas discharge chamber comprising a dielectric in which the gas discharge space is formed, the gas discharge chamber being arranged inside the waveguide. The invention also relates to a method for generating excited and/or ionized particles in a plasma from a process gas, in which an electromagnetic wave is generated and is coupled into a dielectric of a gas discharge chamber, there being formed in the dielectric a gas discharge space which comprises a gas inlet and a gas outlet for supplying or removing process gas, and the gas discharge chamber being arranged inside a waveguide.
High-power plasma devices are basically known from the prior art. They are used for example as external plasma sources, as what are known as “remote plasma sources”, for cleaning coating and etching chambers, the plasma being generated in a separate space in order to then convey the excited gas into a reaction chamber through a pipe or other suitable supply means. A further use of the high-power plasma devices lies for example in integrating the devices directly into the coating or etching chamber. It should be noted in this case that, in contrast to remote plasma sources, the excited gas is uniformly distributed over a certain solid angle in the reaction chamber to achieve the desired results.
High-frequency plasma devices which are very efficient are particularly suitable for use in etching and coating processes for semi-conductor components and products from the micromechanics sector. Special plasma devices are required in this case with the process gases being broken down in the smallest space by high-frequency electromagnetic waves and the fractions of which gases are excited further. By using high-power plasma devices etching gases, such as NF3, CF4, C2F6, SF6, O2, etc. are virtually completely broken down into their constituents and as a result are particularly environmentally compatible. As a rule microwaves are used as the electromagnetic waves. By concentrating the microwave energy on the smallest space the materials of the gas discharge chamber are exposed to particularly high thermal loads, with the inner surfaces of the discharge chamber simultaneously being exposed to chemical attack which, as is known, increases exponentially with the temperature of the materials.
A known high-power plasma device is disclosed for example in JP 07029889 A in which the discharge chamber is arranged in a section of a waveguide, into the end of which, remote from the discharge chamber, a microwave generated by a microwave generator is coupled (see
It is therefore the object of the present invention to disclose a device and a method for generating excited and/or ionized particles in a plasma which can be used with high microwave powers and in which the gas discharge chamber also has a long service life. The object is achieved by a device as claimed in the preamble of claim 1, in which the dielectric forms an end base from which side walls, which also consist of dielectrics, extend so as to form the gas discharge space. The device is also constructed in such a way that electromagnetic waves may be coupled into the end base.
The device according to the invention therefore comprises a gas discharge chamber which in turn comprises a dielectric, for example a ceramic, and a cavity, the gas discharge space, formed in the dielectric. The plasma is generated in the gas discharge space. The gas discharge chamber of the present invention is accordingly constructed such that the dielectric forms an end base. This means that at one end face of the gas discharge chamber the dielectric forms a base or bottom. The base can basically have any desired or expedient shape and consists of the dielectric. Side walls which also consist of dielectrics, branch off this base. The base and the side walls together form a cavity which is used as a gas discharge space. The side walls are therefore expediently circumferential in other words. Moreover in the present invention the electromagnetic waves which are generated by the generator are coupled into the end base of the gas discharge chamber. In contrast to the prior art, in the present invention the electromagnetic wave is therefore coupled or lead into the gas discharge chamber only at certain points and in a locally limited manner. The electromagnetic wave then spreads from the end base through the remainder of the gas discharge chamber, i.e. the side walls made from dielectrics and the gas discharge space in which the required gases are located which are excited by the microwave.
The specific embodiment of the present invention, and in particular the only point-wise coupling of the electromagnetic waves into the gas discharge chamber, makes it possible for the gas discharge chamber to comprise a substantially all-over cooling system to dissipate the heat produced in the discharge space as uniformly as possible to the cooling liquid, for example water. This ensures that the uniform introduction of heat generated in the plasma zone can also be removed uniformly by the cooling system and therefore the mechanical stresses in the dielectric, for example a ceramic or glass body, are minimized. An all-over cooling system also keeps the temperature of the discharge chamber as low as possible, so chemical attack on the chamber material is minimized. A further advantage of the invention is that the discharge space of the excitation and discharge chamber can be constructed in such a way that uniform thermal loading of the discharge chamber is achieved, minimizing mechanical stresses. Overall therefore the productivity of high-power plasma devices, in particular of those which are used in semi-conductor production systems, is increased by the present invention and the environmental compatibility of the processes is improved.
In the device according to the invention the electromagnetic wave, in particular a microwave with the conventional and officially permitted frequencies of 915 MHz, 2.45 GHz or 5.8 GHz, is advantageously coupled into the gas discharge chamber by means of a coupling pin, which is part of a coaxial conductor, through which the wave is guided from the microwave generator to the gas discharge chamber. Coupling or decoupling of microwave energy from a coaxial system into a waveguide system by means of a coupling pin is basically generally known. In the device according to the invention the arrangement of the coupling pins is selected such that all of the microwave energy may be coupled into the gas discharge chamber without some of the energy being reflected. It should be noted in this connection that the propagation velocity of the wave in the dielectric decreases at ε1/2 (root of relative dielectric constant) and consequently the dimensions of the coupling pins and their spacing from the reflection planes must be adjusted accordingly. If for example aluminum oxide with a relative dielectric constant of approx. 9 is used as the dielectric, the propagation velocity of the microwave in the dielectric is reduced to a third of the value in air or under vacuum and by appropriate dimensioning of the dielectric and the pin coupling is adjusted accordingly, so the waves are not reflected.
The coupling pins are also expediently dimensioned and fitted into the dielectric of the gas discharge chamber such that there is no impedance jump during the transition of the coaxial conductor to the dielectric of the gas discharge chamber and consequently all of the energy is coupled into the dielectric, without reflection losses occurring. It is particularly advantageous if by appropriate dimensioning of the coupling pin in the region of the dielectric of the gas discharge chamber and leading through into the waveguide, the waveguide is constructed as an electromagnetic oscillating circuit, so the microwave can be fed into the gas discharge chamber particularly effectively. This is achieved for example in that, with respect to its diameter and its length, the coupling pin in the region of the dielectric and waveguide lead-through is constructed so it acts as an oscillating circuit at the given microwave frequency. Consequently a wide variety of device operating conditions, such as pressure difference of 3 powers of ten, and also different process gases now only have a negligible effect on the reflected power of the device.
It is also advantageous for the end of the coupling pin to be fitted into the dielectric so as to be directly adjacent, without there being a gap between coupling pin and dielectric. Consequently the microwave can be fed into the dielectric particularly effectively and without reflection. The heat generated in the coaxial conductor by surface currents may also be dissipated into the dielectric via the coupling pin. This embodiment is particularly suitable for devices with high microwave powers. A further advantage of coupling of the microwaves by way of the coupling pin lies in the fact that, despite a wide variety of device operating conditions with respect to power and pressure, no subsequent adjustments are required.
In addition to capacitive coupling of the microwave energy into the dielectric by means of the coupling pin, inductive coupling by means of a coil is possible. This method is particularly effective with lower frequencies. The microwave can, moreover, also be supplied to the dielectric by waveguide supply lines, in particular in the case of very high frequencies.
In a preferred embodiment of the invention the gas discharge chamber is constructed in such a way that it substantially fills the waveguide. This is taken to mean that the gas discharge chamber is dimensioned such that the interior of the waveguide is substantially completely occupied by the gas discharge chamber. With its outer surface the dielectric therefore adjoins the inner surface of the waveguide and the gas discharge space is in turn formed inside the dielectric. This embodiment is advantageous since on the one hand a compact arrangement may be produced which saves space and on the other hand all of the energy of the microwave remains concentrated on the gas discharge chamber, where it is then consumed in the gas discharge space. The waveguide is conventionally made of metal and is preferably, in particular in this embodiment, constructed as a heat sink, i.e. a cooling system, in particular a water cooling system, is provided in the waveguide. It surrounds the gas discharge chamber from its end base through to the gas outlet. Since the microwave is only coupled into the gas discharge chamber at certain points and otherwise only the relatively small and locally limited inlet for the process gas runs into the gas discharge chamber, the cooling system can adjoin the surface of the gas discharge chamber over a large area and thus an optimum cooling result may be achieved.
The gas discharge chamber is expediently constructed in such a way that provided at the end of the gas discharge space, which opposes the end base, is the gas outlet for the gas discharge space. It is also expedient to provide the gas inlet at the end of the gas discharge space which adjoins the end base. This means that uniform propagation of the plasma over the entire discharge space is established. The thermal loads, viewed over the entire gas discharge chamber, are kept relatively constant thereby, and this in turn contributes to the prevention of damage in the dielectric. It is also ensured that all of the process gas introduced into the gas discharge space is captured by the microwaves and thus a high level of efficiency is established.
The gas discharge chamber is advantageously symmetrical with respect to the longitudinal axis of the waveguide. This embodiment contributes to uniform distribution of the thermal load and simplifies production of the device according to the invention.
The end base is advantageously constructed as a solid, cylindrical or hemispherical body. The inner shape of the waveguide should be adapted accordingly, so it advantageously rests directly on the end base. This results in an advantageous shape for forming the discharge space in cooperation with the side walls, and the microwaves coupled into the end base can be distributed even more uniformly through the entire gas discharge chamber.
In a preferred embodiment of the invention the side walls of the gas discharge chamber comprise at least one cross-sectional taper. This cross-sectional taper is particularly preferably circumferential. As a result of this at least one purposeful cross-sectional taper or reduction in the cross-section of the dielectric, which is particularly preferably provided in the region of the gas outlet, the microwave coupled into the gas discharge chamber can issue into the gas discharge space from the dielectric in the region of the cross-sectional taper in an augmented manner, so a discharge maximum is prevented at the end of the gas discharge space or the gas discharge chamber, i.e. in the region of the gas outlet. Such a discharge maximum could lead to damage to the gas discharge chamber at this location.
A plurality of cross-sectional tapers are advantageously provided, it being particularly advantageous to provide the reduction in dielectric cross-section from the gas inlet to the gas outlet gradually since the microwave energy fed into the end base at the end face of the gas discharge chamber can be gradually supplied to the gas discharge space as a result. Particularly uniform and, as a result, advantageous distribution of the microwave energy can be attained in this connection if the size of the taper increases in the direction of the gas outlet, i.e. in the region of the taper the dielectric cross-section decreases in the direction of the gas outlet.
The at least one cross-sectional taper is expediently in the form of a circumferential recess and in particular a circumferential annular groove. The annular groove can for example comprise a U-shaped cross-sectional profile and is preferably formed on the inner side of the dielectric.
In a further preferred embodiment of the invention the side walls of the gas discharge chamber are constructed such that their cross-section continually tapers in the direction of the gas outlet. This means that their cross-section is continually reduced from the start of the side walls at the end base to their end at the gas outlet. This continual reduction in the cross-section can be provided with a constant degree of reduction or with different degrees of reduction in certain sections. The degree of cross-sectional taper is preferably constant. This may be achieved for example by a conical formation of the discharge space, with the outer sides of the side walls being formed parallel to each other. Consequently the microwave uniformly exits the tapering dielectric cross-section, so the process gas is uniformly excited over the entire conical space. It is also advantageous in this embodiment for the steepness of the cone to predefine the solid angle at which the excited gases issue from the gas discharge space if the gas outlet is constructed such that it runs over the entire width of the cone base. The solid angle can be pre-defined such that the excited process gases are distributed optimally uniformly over a respective workpiece.
In a further preferred embodiment of the invention the side walls of the discharge chamber comprise at least one, in particular circumferential, projection. The projection protrudes from the side of the side walls facing the waveguide and the extent of the, as a rule, U-shaped cross-section of the projection beyond the side wall corresponds to half the wavelength of the electromagnetic wave (λ/2) in the dielectric. In the case of a U-shaped cross-section the extent is therefore composed of the added-together lengths of the two U-legs and the connecting piece between the U-legs. In principle the cross-section of the projection can also have any other desired shape, wherein care should always be taken that the cross-sectional extent corresponds to λ/2. The at least one projection acts as a blocking element for microwaves and is used to limit the propagation of the microwaves. The at least one projection is therefore expediently provided at the gas outlet, and thus in the gas flow direction, at the end of the gas discharge chamber. Propagation of the microwave at the end of the gas discharge chamber, i.e. in the region of the gas outlet, can be limited thereby. This prevents the microwaves from being able to issue from the gas discharge chamber and enter into the processing or reaction space connected downstream, and in which a workpiece is to be arranged for processing by the excited process gases, which would have an adverse effect on the processing operation. The at least one projection is particularly preferably constructed as a circumferential bead and in particular as a dielectric ring with a relatively small width. The dielectric ring is placed onto the dielectric side walls. As a result of the fact that the at least one, advantageously U-shaped, projection has a cross-sectional extent of half the wavelength of the microwave, a positive half-wave is generated on one side of the projection and a negative half-wave on the other, which half-waves overlie each other and thus compensate to zero. This means that the electromagnetic waves are prevented from reaching the end of the discharge chamber opposing the end base and being able to cause damage there. With particularly high microwave powers it is particularly preferred for blocking elements to be combined with cross-sectional tapers of the side walls, so the energy is reliably consumed in the gas discharge space up to the gas outlet. The at least one projection can likewise be provided in a device according to the invention of which the side walls are constructed with a cross-section that continually tapers in the direction of the gas outlet.
In a further preferred embodiment at least one, in particular circumferential, shoulder is provided on the inner sides of the side walls. A plurality of shoulders is particularly preferably arranged one after the other, so a pyramid-like graduation of the gas discharge space results. The graduations are preferably provided such that the gas discharge space widens in the direction of the gas outlet. The length of the shoulders corresponds to a distance of the electromagnetic wave of λ/4 in the dielectric. A large number of maxima are produced at the shoulders thereby which are reflected from the preceding electromagnetic wave and the succeeding electromagnetic wave reflected at the shoulders of the side walls, it being possible to purposefully improve the effectiveness of the gas discharge in the maxima and it also being possible to distribute the microwave energy throughout the gas discharge space at uniform intervals. With a discharge space graduated in a pyramid-like manner the solid angle, at which the gases issue from this space, can be predefined and corresponds to the slope of the pyramid. For this purpose the gas outlet should be formed such that it extends over the entire base of the graduated pyramid. The solid angle can therefore be adapted to the dimensions of the workpieces to be treated. Instead of λ/4 the length of the shoulders may also be λ/4+nλ, where n=1, 2, 3, etc.
The waveguide is preferably substantially cuboidal, cylindrical, elliptical or conical. If the wave has the shape of a circular cylinder the diameter of the waveguide in the region of the end base is expediently selected such that it is greater than the cut-off wavelength of the electromagnetic wave and thus propagation of the electromagnetic wave is possible in at least the basic mode. The field configuration of the electromagnetic wave in the cylindrical waveguides is best illustrated in cylindrical coordinates. In cylindrical coordinates the solution to the wave equation provides the Bessel function. In the region of the discharge space of the gas discharge chamber the diameter of the waveguide should also be selected such that it is greater than the cut-off wavelength of the microwave. If the waveguide is cuboidal, the width of the waveguide should expediently be predefined such that it is greater than λ/2 of the electromagnetic wave. Appropriate selection of the diameter of the round waveguide or the width of the cuboidal waveguide allows the formation of an advantageous number of electromagnetic wave modes.
The waveguide may also be constructed such that it has different shapes in certain regions. According to a further preferred embodiment the waveguide is cylindrical in the region of the end base and widens conically in the further region of the gas discharge space to allow a gas discharge chamber with a combustion space or gas discharge space with a particularly large solid angle. As a result large-area workpieces, such as semi-conductor wafers with a diameter of 300 mm, can be processed very uniformly with excited gases. In a particularly advantageous embodiment of the invention the gas discharge space of the gas discharge chamber is also conical and the cross-section of the side walls of the dielectric tapers uniformly in the direction of the gas outlet, so the microwave can issue uniformly. At the end of the gas discharge space there is provided a microwave blocking element in the form a circumferential annular bead. It is also possible in this embodiment to provide shoulders on the inner side of the side walls. The cross-section of the waveguide surrounding the gas discharge chamber is advantageously round, elliptical or rectangular.
According to the respective requirements with respect to the size of the workpieces to be processed and the microwave power required, excitation chambers of different sizes can be built and the correspondingly suitable microwave frequencies (for example 915 MHz, 2.45 GHz or 5.8 GHz) selected therefor. This means for example that, with an identical design, the 915 MHz device is approx. six times larger than the 5.8 GHz device.
In a further preferred embodiment the gas discharge chamber is fitted in the waveguide by means of interference fit. Consequently the heat produced during gas discharge may be particularly effectively onwardly conveyed to the cooling liquid by means of the interference fit of the cooling jacket on the gas discharge chamber, thus allowing very effective cooling of the gas discharge chamber.
Some typical application examples of high-power plasma devices constructed according to the present invention will be given below.
Remote Plasma Sources
Processes for Cleaning Coating Chambers and Etching Chambers:
Microwave power: 2 to 30 kW, preferably 2 to 6 kW
Frequency: 2.45 GHz or 915 MHz
Pressure: 0.5 to 5 torr
Gases: NF3, C2F6+O2, SF6+O2, Cl2+NF3
Plasma Sources with Conical Gas Egress and Relatively Large Solid Angle
Stripping of Photoresists and Etching of Workpieces:
Microwave power: 0.5 to 30 kW, preferably 0.5 to 4 kW
Frequency: 2.45 GHz or 915 MHz
Pressure: 0.1 to 5 torr
Gases: O2, N2, forming gas, NF3, CF4
Activation and Cleaning of Surfaces:
Microwave power: 0.5 to 30 kW, preferably 0.5 to 4 kW
Frequency: 2.45 GHz
Pressure: 0.05 to 5 torr
Gases: O2, N2, H2, forming gas, CF4, Ar
The object is also achieved by a method for generating excited and/or ionized particles in a plasma from a process gas in which an electromagnetic wave is generated and is coupled into a dielectric of a gas discharge chamber, there being formed in the dielectric a gas discharge chamber which comprises a gas inlet and a gas outlet for supplying or removing process gas. The electromagnetic wave is also coupled into an end base of the dielectric, the gas discharge space being arranged between the end base and the gas outlet. By appropriate configuration of the dielectric the energy of the electromagnetic wave coupled into the gas discharge chamber in the end base is also preferably consumed in the method in the gas discharge space up until the gas outlet is reached.
The invention will be described in more detail hereinafter with reference to embodiments illustrated in the drawings, in which schematically:
In the various embodiments described hereinafter identical reference numerals are used for identical parts. All illustrations shown in the figures are longitudinal sections through the device according to the invention.
An opening is provided in the waveguide 11 through which a coaxial conductor 18 runs which ends in the end base 13a of the dielectric 13. At its end the coaxial conductor 18 has a coupling pin 18a which is fitted into the end base 13a in such a way that it rests on the dielectric on all sides. This coaxial conductor 18 is connected to a microwave generator 19. The generated microwaves are coupled from the microwave generator 19 into the end base 13a via the coaxial conductor 18 and by means of the coupling pin 18a thereof. From the base the microwaves propagate through the entire gas discharge chamber 12. The coaxial conductor 18 is introduced into the plasma-generating device from oblique top left. In the waveguide 11, in both its end region and in the side walls, there are cooling lines 20, along which cooling liquid, in particular water, flows to cool the gas discharge chamber 11. The cooling lines 20 substantially cover the entire surface region of the gas discharge chamber 12 and are locally interrupted by only the coaxial conductor 18 and the gas supply 17. Two circumferential annular grooves 21 are provided on the inner side of the side walls 13b of the dielectric 13. The annular grooves 21 have a rectangular cross-section and, viewed in the longitudinal direction, are arranged in the region of the side walls 13b facing the gas outlet 16. The upper annular groove has a shallower depth than the lower one, i.e. the dielectric is less thick in the region of the lower annular groove than in the region of the upper annular groove.
The plasma generating device 10 illustrated in
In the case of the plasma generating device 10 shown in
In the case of the device 10 illustrated in
The device illustrated in
The device in
In
In the devices in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 006 289.2 | Feb 2006 | DE | national |
