Abstract:
Device for producing a plasma, in particular for treating surfaces, for chemically reacting gases, or for producing light, by way of microstructure electrode discharges, using a device for producing plasma having at least one guide structure. A microwave generator which can be used to launch microwaves into the guide structure. The guide structure has a locally narrowly limited plasma region in contact with a gas. The guide structure is preferably a metallic waveguide filled with a dielectric material, or an arrangement of strip lines which run on a dielectric plate. The device and the method are particularly suited for processing or activating surfaces or for depositing layers on a substrate.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a device and a method for producing a plasma, in particular for treating surfaces, for chemically reacting gases, or for producing light, by making use of microstructure electrode discharges. 
   BACKGROUND INFORMATION 
   When treating surfaces using a plasma method, it is advantageous for the plasma to be produced as closely as possible to the surface or substrate to be treated, or for a plasma source having a sharply defined or local plasma volume to be introduced in close proximity to the substrate to be treated. This may be achieved by using so-called microstructure electrode discharges, provision being made for dielectric plates having electrodes that are typically disposed at a distance of approximately 100 μm or less from one another. As is generally known, discharges of this kind work within a very broad pressure range and exhibit relatively sharply delimited plasma interfaces, i.e., large-area, but locally narrowly limited, small-volume plasmas are produced. 
   Microstructure electrode discharges have been ignited and operated by d.c. voltage. In this regard, reference is made, for example, to M. Roth et al., “Micro-Structure Electrodes as Electronic Interface Between Solid and Gas Phase: Electrically Steerable Catalysts for Chemical Reaction in the Gas Phase”, 1997, 1st Int. Conf. on Microreaction Technology, Frankfurt/Main and J. W. Frame, “Microdischarge Devices Fabricated in Silicon”, 1997, Appl. Phys. Lett., 71, 9, 1165. High-frequency or microwave excitations have not been implemented under known methods heretofore. 
   It is also known from Kummer, “Grundlagender Mikrowellentechnik” (Fundamentals of Microwave Technology), VEB Publishers-Technology, Berlin, 1986, to direct microwaves via waveguides or strip waveguides (microstrip technology). In the case of the strip waveguides (microstrips), a metallic printed conductor, into which microwaves are launched, is usually applied to a dielectric substrate having a multiply grounded metallic base plate. In the case that there is more than one printed conductor running on the base plate, the metallic base plate can be eliminated. 
   SUMMARY OF THE INVENTION 
   It is believed that the device in accordance with the present invention and its associated method have the advantage over the related art of eliminating the need for the produced plasma to come into direct contact with the device producing the plasma, and, in particular, with the parts of this device being used as electrodes. This may substantially prolong the service life of the entire device in accordance with the present invention and, in particular, of the guide structure being used as microstructure electrodes. Moreover, the device in accordance with the present invention may be easier to service. 
   Moreover, due to the slight penetration depth of currents at high frequencies, the electrode material (i.e., the guide structure (metallic waveguide or strip waveguide) for guiding the launched microwaves in the device producing the plasma) can be kept very thin, which should simplify fabrication. Thus, at a frequency of 2.45 GHz, depending on the material used, the requisite thickness may be merely a few μm. This applies as well for structures or components used for launching the microwaves into the guide structure. In particular, the guide structure can be advantageously vapor-deposited, as well. 
   A locally or spatially narrowly bounded plasma is produced by microwaves in one or preferably in a multiplicity of plasma regions that are isolated from one another, by a supplied gas, which is directed past or through the guide structure, or which acts upon the guide structure. Thus, a gas plasma is produced at the surface of the guide structure, at least on a region by region basis, in the plasma regions and in a plasma volume defined by these regions. 
   Thus, it is quite beneficial for the service life of the device (i.e., of the guide structure functioning as microstructure electrodes) for it to be coated with a protective dielectric layer in the vicinity of the plasma regions. Primarily suited for this are ceramic protective layers. The service life of the microstructure electrodes may be significantly prolonged by this protective layer which cannot be used in a direct voltage operation. 
   Moreover, to fabricate the device, one can revert to existing technologies for generating plasma and, in particular, for guiding and discharging the launched microwaves in the guide structure. Thus, the microwaves are guided very advantageously via a known waveguide hollow conductor arrangement or a known micro-strip arrangement, which is produced and structurally configured using likewise generally known microstructuring methods. 
   The microwaves generated by a microwave generator are advantageously launched into the guide structure via at least one launching structure which communicates electroconductively with the guide structure. The frequency of the supplied microwaves amounts advantageously to 300 MHz to 300 GHz. 
   As part of the device for generating the gas discharge and, respectively, the plasma, the guide structure for the injected microwaves is in an exemplary embodiment a metallic waveguide, which is filled with a puncture-proof, rigid dielectric material, such as silicon dioxide. However, in an alternative exemplary embodiment, the guide structure can be constructed of an arrangement of at least two, preferably parallel spaced metal plates, whose interstitial space is filled in with a dielectric material. Due to its simpler structural design, as compared to closed waveguides, this configuration may offer advantages from a standpoint of production engineering. 
   The waveguides, the metal layers of the waveguides, or the metal plates, advantageously have a thickness, respectively a spacing, that corresponds to the penetration depth of the injected microwaves. Typical values, known, for example from Kummer, “Grundlagender Mikrowellentechnik” Fundamentals of Microwave Technology), VEB Technical Publishers, Berlin, 1986, are within the μm range, given a typical expansion in the length and/or width of the waveguides, i.e., of the metal plates, in the cm range. 
   A particular benefit is derived when the H 10  mode of the launched microwaves is excited and guided in the waveguide, as a guide structure, since, in this case, it is merely the width of the waveguide that is critical for the propagation of the microwaves, and its length, for example, apart from unavoidable attenuation, can be varied substantially freely. 
   Alternatively, the guide structure can advantageously also be an arrangement made up of at least two metallic, in particular parallel conductive strips, which run on a dielectric plate. Here, as well, silicon dioxide is suited, for example, as material for the plate. These conductive strip lines are fabricated with a thickness of a few penetration depths, preferably using known microstructuring methods or microstrip structuring techniques. 
   In addition, provision is made in the vicinity of the guide structure for at least one, but preferably for a multiplicity of, plasma regions, which are advantageously produced by a microstructuring of the guide structure. 
   It is quite beneficial for these plasma regions to be cylindrical holes in the guide structure. Typical cylindrical hole diameters are advantageously about 50 μm to 1000 μm. They are expediently distributed in a regular arrangement in the vicinity of the guide structure. In the case of a waveguide as a guide structure, these cylindrical holes have the considerable advantage, in combination with the excited H 10  mode, that the generated electrical field is aligned within the waveguide in parallel to the cylindrical holes and is substantially homogeneous. As a result, variations in field strength in the direction of the waveguide width are minimal in comparison to higher excitable modes. 
   To avoid or minimize surface stress or material ablation and accompanying gradual destruction of the plasma regions (i.e., of the guide structure) by the generated plasma, the inner wall of the cylindrical holes and, optionally, the entire electrode surfaces as well, are advantageously provided with a dielectric, in particular a ceramic protective layer. This dielectric protective layer only marginally degrades the propagation of the microwaves in the guide structure. 
   The plasma is advantageously produced in the plasma-generation regions at a pressure of 0.01 mbar to 1 bar, a microwave power of approximately 1 mW to 1 watt being advantageously supplied to the plasma regions via the microwave generator and the launching structure. 
   The supplied gas is preferably an inert gas, in particular argon, He or Xe, as well as air, nitrogen, hydrogen, acetylene or methane, that is preferably supplied with a gas flow of about 10 scam to about 1000 scam (standard cubic centimeters per minute). However, in the individual case, these parameters are scaled by the selected dimensional size of the device for producing plasma and are merely to be considered as typical values. Another significant benefit is that the device in accordance with the present invention can be operated while exposed to air, thereby achieving an oxidic surface excitation. Moreover, the broad pressure range within which the work can be done, from atmospheric pressure down to a precision vacuum, makes possible many diverse applications. 
   The device in accordance with the present invention and the method implemented therewith are especially suited for processing or activating the surfaces of a substrate or for depositing layers. Its special advantage lies, in this context, in the spatially narrowly limited extent of the plasma regions and in their immediate vicinity to the substrate surface to be treated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a device including a guide structure having cylindrical holes. 
       FIG. 2  depicts an alternative specific embodiment of the guide structure. 
       FIG. 3  depicts a first gas guideway in the case of a plasma processing of a substrate using a guide structure. 
       FIG. 4  depicts an alternative specific embodiment including another gas guideway. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a device  1  having a launching structure  10 , a guide structure  11 , and plama regions  12 . In this case, launching structure  10  has the shape of a horn  20 , as is generally known from microwave technology, and is used for launching microwaves into guide structure  11 . The microwaves are generated by a generally known microwave generator (not shown) which is linked to launching structure  10 . Horn  20  passes electroconductively over into guide structure  11 , enabling microwaves to be launched by microwave generator via launching structure  10  into guide structure  11 . 
   In this example, guide structure  11  is designed as waveguide  21  of a metal, such as copper, high-grade steel, gold or silver, which is filled on the inside, for example, with silicon dioxide as rigid, puncture-proof, low-loss dielectric material  22 . Waveguide  21  has a thickness of up to a few mm. Its length is variable, but should amount to one fourth of the wavelength of the injected microwaves. Its width is determined in accordance with the waveguide mode selected. 
   In addition, waveguide  21  is provided with a multiplicity of cylindrical holes  26 , which are configured in a regular arrangement and which define plasma regions  12  located in the vicinity of cylindrical hole  26 . The diameter of individual cylindrical hole  26  amounts to about 50 μm to 1 mm. Thus, device  1  is a microstructure, a plasma being ignited within each plasma region  12  of guide structure  11  subsequent to the supplying of a gas. Inner wall  23  of cylindrical holes  26  and, optionally, the entire electrode surfaces of guide structure  11  are also provided with a dielectric, in particular a ceramic, coating as a protective layer, which is made, for example, of aluminum oxide or silicon dioxide. 
   The frequency of the microwaves launched into guide structure  11  is expediently between 300 MHz to 30 GHz; preferably between 900 MHz and 2.45 GHz are used. In this context, waveguide  21  is preferably dimensionally sized, and the frquency of the microwaves is preferably selected such that the H 10  mode of the launched microwaves is excited in waveguide  21  and propagates. 
   For this, in the individual case, one skilled in the art must match the width of waveguide  21  and the frequency of the microwaves to one another. For excitation of the H 10  mode, merely the width of waveguide  21  is a critical quantity, while it length, for example, is merely relevant to the attenuation of the propagating microwave. The power of the launched microwaves is additionally selected to yield a power of about 1 mW to about 1 watt for each plasma discharge region  12 . 
     FIGS. 3 and 4  elucidate the operation of device  1  for treating the surface of a substrate  30  with a plasma through the microstructure electrode discharges produced using device  1  in plasma regions  12  of guide structure  11 . To this end, in accordance with  FIG. 3 , a gas is directed via a gas supply line  31  from the side facing away from substrate  30  through cylindrical holes  26  of guide structure  11 . Thus, this gas flows past the surface of substrate  30  and then off to the side. As of a minimal injected microwave power, which is essentially a function of the type of supplied gas, the gas flow, the pressure, and the thickness of waveguide  21 , plasma is then generated in plasma regions  12  essentially defined by the dimensions of cylindrical hole  26 . Thus, located between guide structure  11  and substrate  30 , at least on a region by region basis, is a plasma volume  40 , formed by various plasma regions  12 , which are isolated from one another or which merge, depending on the spacing between cylindrical holes  26 . 
   The supplied gas is, for example, an inert gas, respectively a noble inert gas, such as nitrogen or argon, for cleaning or activating the surfaces of substrate  30 . However, in the same way, it can also be a generally known reactive gas, such as oxygen, air, acetylene, hydrogen, or a gaseous or vaporous precursor material, such as an organic silicon or organic titanium compound. Depending on the selection of the supplied gas, chemical reactions can also be induced by device  1  at the surface of the substrate, or a surface coating can be provided, for example in the form of a hard material coating or wear-protection layer. 
   The plasma is produced in plasma region  12  with the aid of microwaves launched into guide structure  11  and with the supplying of a gas, and depends on the dimensional design of guide structure  11 , the type of supplied gas, the diameter of cylindrical holes  26 , the width of waveguide  21 , and the desired treatment of the surface at a pressure of about 0.01 mbar up to about 1 bar. Each variable is to be determined in the individual case by one skilled in the art based on simple preliminary tests. A preferred pressure is from 10 mbar up to 200 mbar, with plasma gas being supplied with a typical gas flow of a few sccm up to about 1000 sccm. However, this value is likewise to be adapted by one skilled in the art to the particular process parameters for each case, after performing preliminary tests. 
   As a second exemplary embodiment,  FIG. 4  depicts an alternative routing of the supplied gas via gas supply line  31 . In this context, the gas flows past, in between the surface of substrate  30  and guide structure  11 , and is not fed through cylindrical holes  26 . Apart from that, however, the parameters for producing the plasma in plasma regions  12  are completely analogous to the exemplary embodiment elucidated with the aid of  FIGS. 1 and 3 . 
   In a third exemplary embodiment, as a slight variation of waveguide  21 , guide structure  11  is made of two parallel spaced metal plates, whose interstitial space is filled with silicon dioxide. Apart from that, guide structure  21  is constructed substantially similar to the first examplary embodiment and  FIG. 1 , especially with respect to dimensional design, cylindrical holes, and material. The advantage of using two parallel metal plates in place of waveguide  21  is that, from a standpoint of production engineering, they are simpler and less expensive to fabricate than a closed, integrated, waveguide  21 . In this case, the guidance and propagation of the launched microwaves is carried out by way of a capacitive coupling of the two plates. Analogously to the preceding exemplary embodiments, the gas is supplied in this exemplary embodiment in the manner explained with respect to  FIG. 3  or  4 . 
   As a further exemplary embodiment,  FIG. 2  clarifies an alternative specific embodiment of guide structure  11 , the launched microwaves being guided via strip lines  24  using microstrip technology. In this case, horn  20  is not necessary since the microwaves generated by the microwave generator are injected via coaxial plug connectors (not shown). 
   In detail, in this example at least two, but preferably a multiplicity of, metallic strip lines  24  are applied to a dielectric plate  25 , which is made of a puncture-proof, rigid dielectric material, such as silicon dioxide. These strip lines  24  expediently run in parallel to one another at a distance that is a function of the frequency and the dielectric material used, and are preferably made of copper or gold, which is optionally applied to a galvanic reinforcement, such as nickel. The optimal spacing of strip lines  24  for igniting and sustaining a plasma in plasma regions  12  is additionally a function of the type of gas supplied and of the prevailing pressure and must, therefore, be determined in simple preliminary tests. 
   Furthermore, analogously to  FIG. 1 , cylindrical holes  26  are provided in dielectric plate  25  between strip lines  24 . With respect to the dimensional design of guide structure  11  and of cylindrical holes  26 , reference is made to the preceding explanations regarding the first exemplary embodiment. In particular, in this case as well, cylindrical bores  26  can be provided with a dielectric coating, for example in the form of a ceramic protective layer, on inner wall  23 . Cylindrical bores  26 , in turn, define locally limited plasma regions  12 , in which microstructure electrode discharges are ignited via the injected microwaves directed via strip lines  24  in response to the supplying of a gas or on exposure to air. When cylindrical holes  26  are arranged in a dense enough configuration, the plasmas produced in plasma regions  12  merge, and a laterally homogeneous plasma develops. 
   In the case of a guide structure  11  in accordance with  FIG. 2 , the gas guidance is completely analogous to the exemplary embodiments already explained and can be carried out in the manner explained with respect to  FIG. 3  or  4 , in that the gas is directed through cylindrical holes  26  or conveyed between substrate  30  and guide structure  11 . 
   REFERENCE SYMBOL LIST 
   
       
         1  device 
         10  launching structure 
         11  guide structure 
         12  plasma region 
         20  horn 
         21  waveguide 
         22  dielectric material 
         23  inner wall 
         24  strip line 
         25  dielectric plate 
         26  cylindrical hole 
         30  substrate 
         31  gas supply line 
         40  plasma volumes