Abstract:
The present invention relates to novel baths for generating microstructures which are required in numerous fields of application of the microsystem technique and in microstructuring. It is an object of the invention to provide novel baths for generating microstructures which offer a less expensive and, above all, a solution for their disposal which is more friendly to environment than the previous methods for manufacture of microstructures. The object is realized in that the respective baths are added at least one biogenic catalyst (in particular, an enzyme) which acts upon a preselectable thin layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to novel baths for generating microstructures which are required in numerous fields of application of the microsystem technique and in microstructuring.  
           [0002]    Moreover such baths enable microstructuring of such layers or layer systems the application of which previously has been unusual in the foregoing fields of art. The application of the present invention is of particular advantage in structuring layer systems, the selective etching of which is critical when conventional etching means are used due to their similar chemical behavior, and has been feasible only by employing additional and expensive means such as etching barriers or the like.  
           [0003]    Etching processes and developing processes previously used in microstructuring according to the state of art are generally based on poisonous chemicals and such being harmful to health, as for example strong acids, lyes, and oxidants, or require extremely expensive processes, such as reactive ion etching and plasma etching, respectively, (refer to S. Büttgenbach, Mikromechanik, B. G. Teubner, Stuttgart,  1994 ).  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the present invention to provide novel baths for producing microstructures which offer a less expensive and, above all, a solution for their disposal which is more friendly to environment than the known methods for manufacture of microstructures.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0005]    The object is realized by the characteristic features of the first claim. Advantageous further embodiments are covered by the succeeding claims.  
           [0006]    The invention is based on the discovery that in the field of microstructuring the masking layers which permit structurizing and previously were employed to this end, such as photoresist layers may, by their very presence, locally affect the properties of biopolymer films. The very essence of the invention consists in the intentional selection and in the application of biogenic catalysts, in particular of enzymes for microstructuring of thin layers and, in a particularly advantageous way, of thin layer systems, respectively. When there is in the frame of the invention reference to the term baths, then the same are understood as etching baths according to conventional and known technical solutions as well as solutions which are destined for the formation of a structurized and masked layer.  
           [0007]    Biopolymers which previously have not been utilized in the field of microstructuring are generally enzymatically degradable, just as a great number of organic and inorganic layers and metals. Moreover, there are numerous enzymes disposable which under changed conditions can carry out reactions, which are adverse to their original function. Thus lipases, for example, in organic solutions react as acetylases.  
           [0008]    Enzymes, under suitable conditions, are highly specific biogenic catalysts, which in addition to their proper function can be inhibited by specific inhibitors or competitors. Enzymatic reactions generally take place at ambient temperature and in mild buffer solutions which can be disposed of friendly to environment. Enzymes themselves according to their nature are readily degradable biologically. Very often enzymes are active in narrow temperature ranges. Hence, enzymes may be controlled also thermally as to their activities. It is generally feasible to thermally deactivate common enzymes in a range of from 40 to 120° C., wherein, in most cases, a heating above 60° C. will be sufficient. Thus the requirements are fulfilled to employ solutions as “etching baths” for microstructuring to which at least one additive from enzymes and biogenic catalysts, respectively, are given.  
           [0009]    The invention will be explained in more detail in referring to five embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    In a first embodiment a double layer system from biopolymers, which previously were not used in microsystem techniques and which are coated with a masking layer, for example, of a photoresist, will be structurized. Gelatin will be employed for the double layer system, without limiting the invention thereto. To this end the fact that very often different enzymes may act in similar buffers with high efficiency and selectivity is utilized. A first gelatin layer of about 200 nm thickness is deposited on a substrate, for example, a cleaned silicon wafer and is cross-linked by the addition of glutaraldehyde. A second gelatin layer is deposited thereupon, to which short oligonucleotides (for example, 8 mer) are added and cross-linked with glutaraldehyde. The added oligonucleotides have a sequence which has a specific junction for a restriction enzyme (for example, 5′-GAATTC-3′ for EcoRI). A restriction enzyme of groupII generally acts in buffers which contain metal ions (for example, Mg 2+ ) in millimolar concentrations. It is feasible to degrade the lower and first gelatin layer by a protease (for example, protease K). Very often proteases are inhibited in their function by metal ions, for this reason protease buffers very often include EDTA, EGTA and other chelating agents, the task of which is to eliminate metal contaminations contained in solutions.  
         [0011]    The first and the second gelatin layer will be degraded subsequently in one step. This is achieved by using a buffer solution consisting of 10 mM NaCl, 5 mM MgCl 2 , and 1% glycerol, the second and upper layer being selectively degraded by using a restriction enzyme in those portions which are uncovered by the masking layer. When the degradation is completed, what can be monitored by microscope, the buffer used is added a chelating agent (for example, 10 mM EDTA), which in this manner selectively degrades the first and lower.  
         [0012]    Alternatively, it is feasible to add the chelating agent to the upper and second gelatin-oligonucleotide layer already while the latter is manufactured. Said chelating agent will be set free with progressing degradation of the oligonucleotides and in this way the inactivation of the buffer by the metal ions contained in the former is eliminated. The protease will then be capable to degrade the lower and first layer as well as the restriction enzyme. The allosteric regulation of the foregoing enzymes by way of metal ions is counteractive, permitting a successive application. There are also proteases which are only able to function by use of metals, which would permit a simultaneous application (for example, the family of zinc proteases, such as carpoxypeptidase A). Furthermore and according to the present state of biotechnological research, it is feasible without any problems to produce metal resistant or metal sensitive mutations of the enzymes. In a second embodiment the application of a bath according to the invention for producing a selfsupporting membrane will be disclosed. A thin biopolymeric layer of about 200 nm thickness is applied to a cleaned silicon wafer by spin-coating. In the present example, said layer is formed by 10% gelatin dissolved in water to which 5% glutaric dialdehyde is added. Said layer is not water-soluble and is resistant to conventional photoresist coating developers. In the present example a subsequent coating is carried out under use of a photoreversal resist as commonly on sale. Said photoresist layer is treated according to the instructions, masked according to a desired subsequent structure, exposed, and structurized. The entire sandwich assembly consisting of the silicon wafer, the cross-linked gelatin layer, and the structurized photoresist layer is inserted into an enzymic bath. Provided that gelatin is utilized for the biopolymer layer the enzymic bath preferably consists of a protease K-buffer substantially constituted of 10% SDS, 10 mM NaCl, 10 mM EDTA and 10 mM Tris-HCl, to which 10 mg/ml protease K is added. The pH-value of said bath is set to 8.5. When such an enzymic bath is employed a gelatin layer of about 200 nm thickness is entirely degraded at ambient temperature within about 8 h. The result of the “etching” process is a selfsupporting photoresist membrane.  
         [0013]    In a third embodiment the action of a bath according to the invention will be described which provides a degradation stop (in analogy to known etching stop layers).  
         [0014]    When a selected layer has to be protected against external influences and at the same time there has to be the possibility to definitely set free said layer, for example, to produce thin membranes what, in turn, will be described by example of gelatin, then this is feasible without any problems, provided that the gelatin layer which later forms the membrane is doped by metals or specific inhibitors for degradation of the used enzyme. Such inhibitors are generally artificial analogues of the transition states of the substrate which has to be enzymatically decomposed and have bonds which are not decomposable by the enzyme; for example, ether bonds by proteases which decompose peptide bonds. A thin layer prepared in this manner will be provided with a second thin layer of agarose having a thickness of about 1 μm. After applying a respective masking layer to the second thin layer the latter is provided with a desired recess by employing a bath consisting of 0.1 g/ml agarase, 5 mM EDTA, 10 mM NaCl, and 10 mM Na 2 HPO 4 . When the degradation has progressed to the prepared first thin layer, the agarase is inhibited to progress with degrading by the inhibitors provided in the first layer. In a fourth embodiment the application of a bath according to the invention to structurize a double layer system formed of gelatin and agarose is realized in that, in the present case, two enzymes are added to the bath consisting of 5 mM NaCl, 5 mM Na 2 HPO 4 , 0.1 g/ml agarase, and 1 mg/ml thermoresistant proteinase, recovered from a thermophilic organism (for example, thermos thermophilous). The agarase is characterized by being active at ambient temperature, at which the thermoresistant proteinase does not exhibit any detectable activity. In this first step of degradation, only the desired structure of the upper layer consisting of agarose is removed. Then the temperature of the bath is increased to 72° C. and thus the proteinase is activated, whereas the agarase does not show any detectable activity any longer. Only the degradation of the lower gelatin layer of the double layer system is carried out.  
         [0015]    In a fifth embodiment the structurizing of a thin layer system constituted of iron-gelatin-lipid by virtue of at least one bath according to the invention will be disclosed. A solution is used to which a siderophor such as aerobactin or enterobactin is added by about 10 mg/ml. In the unmasked portions the iron layer of about 100 nm thickness is degraded after about 1 h at a temperature of 20° C. In the buffer solution according to the second embodiment including a proteinase, it is feasible to remove the gelatin layer under the mask which has been predetermined by the generated Fe-structure through which the lipid layer is set free. Said lipid layer can be structurized by way of an aqueous solution consisting of 1% glycerol and 1 mg/ml lipase which is suited for degrading the lipid layer.  
         [0016]    The invention is not limited to the baths as disclosed in the foregoing embodiments. The selection of a special relevant biogenic catalyst, in particular an enzyme, with respect to an actual case of a thin layer to be structurized can be carried out without any further inventional activity and under use of biotechnological skill.