Abstract:
A mechanical particle filter comprises a membrane having a plurality of pores. At least one partial region of the surface of the membrane, that is accessible for the medium to be filtered, includes a carbon material having a diamond structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to German Patent Application No. 10 2008 035 772.3, filed in Germany on Jul. 31, 2008, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a mechanical particle filter comprising a membrane having a multiplicity of pores, and to a method for manufacturing such a particle filter. 
     2. Background Information 
     Particle filters of this type are used to filter particles, for example bacteria, from a fluid. The particles filtered out can be analyzed in order to ascertain the pollution of the fluid with specific particles. 
     US 2003/0150791 A1 discloses a particle filter of the type mentioned in the introduction, wherein the membrane is formed from a silicon-based material. In order to form the pores, a mask material is applied to the silicon and small spheres are pressed into said mask material and displace the mask material at points. Afterward, the locations of the silicon membrane that have been uncovered in this way are etched open to create pores. Finally, the mask material is removed. 
     By contrast, U.S. Pat. No. 5,753,014 discloses a method for manufacturing a membrane filter, wherein a mask can be lithographically applied with the aid of a photosensitive layer. After exposure, the pores of the membrane are produced by etching. 
     DE 10 2006 026 559 A1 discloses the porosification of a substrate, for example composed of silicon, proceeding from the surface thereof, such that it is pervaded by thin channels or holes. This process can be adapted for example by electrochemical etching with light irradiation. As soon as the desired membrane thickness has been reached, the porosification process is ended. 
     SUMMARY 
     The invention is based on the object of providing an, in particular mechanical, particle filter of the type mentioned in the introduction which has an improved mechanical and chemical stability in comparison with known particle filters. Moreover, the intention is to provide a method for manufacturing such an improved particle filter. 
     In order to achieve said object, a particle filter is proposed wherein at least one partial region of a surface of the membrane which is accessible for the medium to be filtered is produced from and/or coated with a carbon material having a diamond structure. 
     An advantageous manufacturing method for the particle filter is the subject matter of the alternative independent claim. 
     Advantageous configurations of the invention are the subject matter of the dependent claims. 
     The particle filter according to the invention has the advantage that the carbon material having a diamond structure is almost completely inert chemically. As a result, simple purification, that is to say removal of the particles accumulated by the filter, can be realized in a simple manner since the particles scarcely form stable bonds with the membrane. Furthermore, a carbon material having a diamond structure is very stable mechanically, such that, when the filter is used, a high differential pressure between the two sides of the membrane can be used. The flow rate through the filter is thereby increased. 
     The membrane can be produced completely from the carbon material. Since the carbon material is transparent on account of its diamond structure, a membrane constructed in this way makes it possible, by means of simple translumination of the membrane, to identify residual contaminants after purification or structural defects in the membrane in a simple manner. 
     The membrane can be produced completely from diamond. 
     The membrane is advantageously supported by a carrier, to which it is fixed. This further increases the loading capacity of the particle filter. 
     The carrier can be formed from a material which can be patterned by lithographic methods. This makes it possible to use the frame material during the manufacture of the membrane as a support and subsequently to remove it in a gentle manner from the porous region of the membrane. 
     In an advantageous configuration, the material of the carrier has a crystal structure which predetermines the direction of an anisotropic etching process. The form of the carrier can be determined reliably in such a material. 
     The carrier can be formed from silicon. Silicon has the advantage that it is available inexpensively, can be subjected to lithography in industrially known methods and is mechanically stable. 
     The silicon advantageously has a (110) orientation. By virtue of this orientation, sidewalls of the carrier which are almost completely planar and perpendicular to the surface of the membrane are achieved in the course of etching after the lithography. 
     In the method proposed for advantageous manufacture, firstly an etching mask is applied on one side of a carrier and patterned, then a layer composed of a carbon material having a diamond structure is applied on the other side, wherein an etching mask is applied to the layer composed of carbon material and is patterned, then the layer composed of carbon material is patterned by etching, and, finally, the carrier is patterned by etching. 
     Such a method has the advantage that it produces a membrane which has a high loading capacity and which is adapted to the carrier in such a way that it has no prestresses. Furthermore, the thickness of the layer composed of carbon material and also the arrangement and form of the pores can be defined in a simple manner. 
     In an advantageous configuration, the layer composed of carbon material is patterned by plasma etching. This method allows a reliable definition of the pore size and produces pore walls having low roughness. 
     The carrier can be patterned by wet-chemical anisotropic etching. This allows the excess carrier material to be removed, without the membrane being attacked. 
     The etching masks are advantageously removed after patterning. This avoids a situation in which the material of the etching masks comes into contact with the fluid to be filtered and possibly enters into chemical or physical interactions which can destroy the particle filter or influence the result of analyses. 
     The carrier and/or the membrane can finally be coated with a layer composed of carbon material having a diamond structure. The entire particle filter is thus reliably separated from the fluid to be filtered. 
     The layer composed of carbon material can be deposited by means of chemical vapor deposition in a methane-hydrogen atmosphere. This constitutes a particularly uniform and reliable deposition of diamond-like carbon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details and further advantages of the particle filter according to the invention and of the method according to the invention will become apparent from the following description of a preferred exemplary embodiment. In the drawings, which only schematically illustrate the exemplary embodiment, specifically: 
         FIG. 1  illustrates a plan view of a particle filter; 
         FIG. 2  illustrates a cross section through a particle filter along the line II-II in  FIG. 1 ; 
         FIG. 3  illustrates a cross section through a particle filter as in  FIG. 2  during a production step; 
         FIG. 4  illustrates a section through a particle filter as in  FIG. 2  with an alternative orientation of the lattice structure of the carrier, and 
         FIG. 5  shows a section as in  FIG. 2  through a diamond-coated particle filter. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The particle filter  214  shown in  FIG. 1  and  FIG. 2  has a membrane  312  and a carrier  314 . Pores  316  arranged in a grid are introduced into the membrane  312 . The pores  316  have a round or square cross section. 
     The carrier  314  supports the membrane  312  in an edge region  318 . A through-flow region  320  is provided in the region of the pores  316 . 
     The manufacture of the particle filter  214  will be described below with reference to the figures. 
     As shown in  FIG. 3 , a silicon wafer  322  having a (110) crystal orientation is provided as the starting material. 
     The silicon  323  is thermally oxidized, such that, for example, SiO 2    324  having a thickness of approximately 500 nm is produced. The SiO 2    324  formed is subsequently removed from the front side  330 . The SiO 2    324  on the rear side  332  is patterned in order later as etching mask  326 . 
     On the front side, diamond  328  or DLC (diamond-like carbon) is deposited for example with a thickness of approximately 1 μm. A chromium layer is applied with a thickness of approximately 100 nm, for example, and patterned. It serves as an etching mask for the subsequent patterning of the diamond  328 . 
     The diamond  328  is preferably patterned by plasma etching and the chromium mask is subsequently removed.  FIG. 3  shows the particle filter after this step. 
     The front side  330  is then protected in an etching holder and the silicon is etched wet-chemically anisotropically starting from the rear side  332 . By way of example, TMAH or potassium hydroxide is appropriate as etchant. In this case, the SiO 2    324  on the rear side  32  serves as an etching mask  326 . After the conclusion of the etching process, this layer is removed. The particle filter  214  then appears as in  FIG. 2 . 
     Finally, the complete particle filter  214  can be coated with a diamond layer  334 , as a result of which an extremely stable particle filter  214  that is both chemically and mechanically resistant arises. Even the silicon is protected and the entire particle filter  214  is enveloped with diamond  328 . The only exception to this is possible outer areas that are uncovered when the particle filters  214  are sawn apart (separated). However, the outer areas are generally separated anyway by sealing rings from the fluid to be filtered. 
     If such outer areas are also intended to be protected, the individual chips or particle filters  214  can be coated with a diamond layer  334  after the separation of the wafer. 
     The diameter of the pores  316  decreases as a result of the additional diamond layer  334 . This should already be taken into account during the patterning of the chromium mask, particularly if a desired diameter of the pores of approximately 450 nm, for example, is intended to be obtained. 
     The particle filter  214  illustrated in  FIG. 5  thus acquires a diamond layer  334  which protects it against chemical and mechanical influences. 
     Alternatively, the silicon can be completely removed, as a result of which individual thin filter membranes are obtained. 
     The use of silicon having a (110) orientation has the advantage that perpendicular walls arise during etching, as a result of which a high packing density of particle filters  214  on a silicon wafer  322  is achieved. This can also be obtained by dry etching of the silicon, although this process is more cost-intensive. In addition, it should be ensured in this case that the etching process is ended upon reaching the diamond  328 . 
     However, the silicon wafer  322  can also consist of silicon having a (100) orientation. During the wet-chemical anisotropic etching of such a silicon wafer  322 , however, oblique edges rather than perpendicular edges are produced, as a result of which the packing density is reduced. 
     As an alternative to thermally oxidized silicon (SiO 2    324 ), it is also possible to use other etching masks, for example differently deposited SiO 2    324  or Si 3 N 4 . A use of SOI wafers or the utilization of further methods is likewise conceivable. A particle filter  214  with use of SOI wafers having a (100) orientation is shown in  FIG. 4 . 
     The particle filters  214  completed by such an alternative process can subsequently be provided with a diamond layer  334 , as a result of which a particle filter  214  completely protected by a diamond  328  once again arises. This method involves greater outlay in terms of processing, but affords the advantage that the diamond layer  334  does not have to be patterned. 
     Instead of silicon, it is also possible to use other materials as carrier for the membrane  312  composed of diamond  328 . In particular, hard metal, titanium or refractory metals such as, for example, W, Ta, Mo and the carbides thereof are appropriate in this case. SiC and Si 3 N 4  are likewise particularly suitable. 
     The diamond deposition takes place, in particular, by means of CVD (chemical vapor deposition) in a methane-hydrogen atmosphere. The energy required for the dissociation of the gases is advantageously made available by a hot filament. However, microwave plasma or impulse discharge excitation (arc jet) is also possible. 
     In order to detect the particles, the latter can be marked with fluorescent dyes. These dyes are excited by a laser and the emitted light is measured by a detector. 
     Since diamond is transparent, the use of the particle filters  214  described here enables the illumination and the detection to be effected from different sides. This is advantageous when detecting the particles. 
     The particle filters  214  comprising a membrane  312  composed of diamond  328  are particularly suitable in particular for determining and measuring viruses in media such as blood and saliva. Relatively fine pores  316 , for example having a diameter of 50 nm, are used for this purpose. Pores  316  having a very small diameter beyond the resolution limit of conventional exposure and patterning methods can be manufactured reproducibly by a finished particle filter, or one in which at least the diamond  328  has already been patterned, being coated with a further diamond layer  334 . Pores  316  are narrowed as a result. 
     Direct detection without fluorescence can be used, particularly in the case of spatially resolved illumination, in order to be able to identify structural defects in the particle filter  214  or inadequate purification. This information can furthermore be evaluated in such a way that a warning indication is issued or the particle filter  214  is exchanged. 
     In order to detect bacteria in drinking water, the hole diameter can be 450 nm. In this case, the membrane thickness is approximately 1 μm. 
     The pores  316  are intended to have a high verticality with respect to the surface of the membrane  312 . 
     The roughness of the perforation on the inner side of the pores  316  is rms&lt;2 μm, preferably rms&lt;100 nm, and particularly preferably &lt;50 nm. 
     The grain size of the diamond layer is intended to be less than 1 μm, preferably less than 50 nm, and particularly preferably less than 20 nm. 
     The flexural bending stress of the diamond layer is intended to be more than 1 GPa, preferably more than 4 GPa, and particularly preferably more than 7 GPa. The modulus of elasticity is intended to be above 500 GPa, preferably above 700 GPa, and particularly preferably above 1000 GPa. 
     The particle filters  214  can be used not only for detection or analysis, but also for the targeted purification of media (filtering), for example for the purification of drinking water. 
     The particle filter  214  allows accumulation of bacteria in water or air through a micromechanical surface filter, for example in order to improve a detection limit of an analysis device. By virtue of the use of diamond  328  in the membrane  312 , the particle filter  214  has a high chemical and mechanical robustness. This brings about a high degree of recycling and hence a high degree of automation. 
     As is described in greater detail in DE 10 2006 026 559 A1, to which reference is expressly made for further details, the particle filter can be used in a detection method in which, in order to detect specific particles in media (e.g. bacteria in drinking water), the medium is pumped through thin filters. The particle filter  214  has pores  316  having a diameter adapted in such a way that the particles to be detected and all particles which are just as large or larger remain on the filter surface, i.e. are accumulated there. 
     As described here, diamond or a diamond-like material will be used as material for such a filter, in order to achieve very high mechanical and chemical stability. 
     The high mechanical stability makes it possible to generate a high differential pressure between the two sides of the membranes, as a result of which the flow rate through the filter can be increased. Alternatively or additionally, the pore density can be increased in order to increase the percentage proportion of the total area of the filter that is constituted by the pore area. This is of interest particularly with regard to a miniaturization of the overall system. 
     Both liquids and gases can be appropriate as media to be filtered.  FIGS. 1 and 2  show a plan view and a cross section through the particle filter used as filter element. The pores are preferably round, but can also have some other form. 
     After the accumulation of the particles on the filter surface, they are detected directly or e.g. after marking with dyes. In particular, the particles, e.g. bacteria, viruses or toxins, can be specifically provided with fluorescent dyes, e.g. fluorescence-marked antibodies, in order to detect them after excitation with light having a suitable wavelength by means of a detector, e.g. photomultiplier or CCD camera. This principle can also be applied to other marking and detection methods. 
     In order to enable fully automatic operation in a detection system, the fluidic system and in particular the filter is cleaned after each sample examined. In this case, all previously added substances (sample to be examined, marking substances, auxiliary reagents, dirt and impurities) are removed by the use of aggressive chemicals such as e.g. acids, alkaline solutions or solvents for cleaning purposes.