Abstract:
An apparatus and the use of such an apparatus and method for producing microcomponents with component structures are presented which are generated in a process chamber on a substrate according to the LIGA method for example and are stripped from the enclosing photoresist with the help of a cooled remote plasma source.

Description:
This application claims priority to and is a continuation of pending U.S. patent application entitled, An Apparatus and Method for Producing Microcomponents and Use Of, filed Dec. 8, 2008, having a Ser. No. 12/330,028, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for producing microcomponents with component structures which are generated in a process chamber on a substrate according to the LIGA method. The invention further relates to a method for producing microcomponents with component structures in which the microcomponents are generated in provided substrates and thereafter are relieved from the enclosing material sections, such that the material sections can be etched away (stripped). The invention further relates to the use of such an apparatus and method for stripping or removing the material sections enclosing the microcomponents. 
     Apparatuses and methods of this kind with which microcomponents are produced for the clock industry are well known from the state of the art. Methods are frequently used here which are based on a combination of lithography and electroplating. These methods are generally well known as LIGA methods. In such a “LIGA” method, a photoresist can be applied at first to a substrate such as a wafer or the like. A negative resist with the designation SU-8 can be used as a photoresist. 
     The substrate can concern a silicon wafer for example which comprises a thin gold layer on its surface on which microcomponents can grow especially well. The photoresist is applied to this silicon wafer. It especially covers the gold layer and can be exposed there selectively by means of a suitable exposure system and a mask. The unexposed places of the photoresist layer are removed in a subsequent development step. 
     In a subsequent electroplating process, a metal such as nickel or gold can be allowed to grow on the thus prepared substrate surface, with the respective metal beginning to grow on the places stripped of the photoresist layer. Depending on the intended application, growth can be stopped when the grown metal structure grows slightly beyond the photoresist layer. 
     Photoresist SU-8 comes with the disadvantage that the removal of the cross-linked SU-8 structures between metal structures deposited by electroplating is currently very complex. SU-8 is based on epoxy resin which in the cross-linked state is very stable against organic and inorganic etchants. Photoresist is removed by means of currently known etchants either very slowly or in such a way that the metallic microcomponents are also attacked and thus become useless. 
     Mechanical removal of the photoresist from the microcomponent is not only laborious and cost-intensive, but also causes a high rejection rate due to the extremely small dimensions of the microcomponents. Some complex structural shapes cannot even be produced at all under these conditions. 
     Conventional plasma methods progress on the one hand very slowly and on the other hand the metal is attacked by ions and electrons. The etching process needs to be interrupted frequently because the substrates otherwise will become too hot. 
     In previous methods, the end of the stripping process, i.e. the removal of the photoresist, is determined on the basis of integral temperature measurement by means of emission spectroscopy, which entails very high costs. When photoresist is removed in an etching chamber in batch operations from several microcomponents on the respective substrates, it was only possible to date to recognize the start of the etching process by means of a rise in the integral process temperature and the global end of the etching process by means of the drop in the integral process temperature. It was not possible to individually monitor the individual substrates which may have differently thick layers of photoresist and thus require differently long stripping periods. It was further not possible to recognize in time whether an individual substrate will become too hot due to a bad contact to the substrate holder for example which rests on a work plate and will thus be damaged. 
     WO 2004/104704 A2 discloses a lithographic method for producing microcomponents. It is tried to form etching chambers in a constructional way, through which removal of SU-8 is facilitated. Several intermediate steps are added as a result of additional bonding layers, support structures, masks and exposure steps, through which construction and production of the microcomponents becomes more laborious. 
     SUMMARY OF THE INVENTION 
     From all the aforementioned disadvantages of the state of the art, the invention is based on the object of ensuring that the photoresist is to be stripped as easily, efficiently and reliably as possible, so that rejects are minimized. 
     The simplicity of production shall be achieved by utilizing and combining already existing techniques and systems with as few as possible, but effective additional steps. Efficiency of production shall be raised by the possibility of simultaneous treatment of several substrates in an etching chamber and by reducing the stripping duration through an increased removal rate of the photoresist. The reliability of the production and thus the reduction in the number of the damaged microcomponents shall be implemented with the help of individual temperature monitoring and lowering the substrate temperature and by avoiding aggressive etchants. 
     The object of the invention is achieved in respect of its method in such a way that the photoresist is stripped with the help of a cooled remote plasma source by chemical etching due to the increased density of the radicals, with the temperature of the microcomponents being monitored individually and the progression of the temperature being subjected to the recognition of an end point. 
     The use of a remote plasma source for stripping photoresist offers the advantage that no ions or electrons will reach the substrate and place a thermal load on the same. The radicals strip the photoresist through pure chemical etching. The density of radicals is increased by using a microwave source such as the one from R3T GmbH. A suitable apparatus by means of which the etching process as explained above can be performed especially well is described in the patent specification DE 198 47 848 C1. The apparatus described there comprises a generator for generating electromagnetic waves such as a microwave generator. The electromagnetic waves can be used to form excited and/or ionized particles in a plasma zone. At least the excited particles are conveyed by means of a suitable feed line to the etching chamber in order to trigger there the desired etching process on the surfaces of the substrates. Since the plasma chamber of the source is cooled with a coolant such as water, a cooled beam of radical particles of high radical density is introduced for stripping into the etching chamber, through which a high removal rate is achieved which rises with the density of the radicals on the one hand, and the thermal loading of the substrates, apart from the unavoidable reaction heat, is minimized on the other hand. 
     For an especially advantageous embodiment of the invention, thermal sensors are distributed over the entire work area for individually checking the local substrate temperatures. They are held in a resilient manner in the work plate, so that a mechanical and thus thermal contact is ensured with the bottom side of a substrate holder on which the substrate is located. It is understood that the substrate can also be placed directly on the work plate, and in this case the thermal sensors would have direct contact with the bottom side of the substrate. It has been seen that as a result of the temperature data measured in real time, especially on each of the substrates, it is possible to draw precise conclusions on the progress of the etching process, advantageously on each of the substrates. 
     The term “substrate” describes any structure which can be used as a carrier on which the microstructures can grow. Widely used as silicon wafers which can be provided with a thin gold layer, with said gold layer being used as a starting layer from which the microstructures will grow. In the present connection, the term wafer shall generally be used synonymously for the term “substrate”. 
     “Substrate holders” shall mean such structures which can produce contact between the substrates and the work plate. A substrate is applied to their upper side by means of a contact means and the work plate is located on its bottom side. It is ensured that heat from the substrates which become hot as a result of reaction heat is forwarded via the contact means to the substrate holder and from there to the work plate. Furthermore, a substrate holder is used for horizontally planar positioning of a substrate in an etching chamber. 
     Any devices are suitable as a “work plate” on which the provided substrates can be arranged either with or with a substrate holder within the etching chamber. Ideally, a cooling medium flows through the work plate in order to thus enable the removal of the process heat from the etching chamber. 
     The term “etching chamber” describes any device in which the production of microstructures can be made. In such a chamber, a photoresist layer can be removed from a substrate surface and/or a microstructure surface for example. 
     The term “contact means” describes any means which is suitable of improving the contact between the provided substrates and the substrate holders, so that temperature differences between the provided substrates and the work plate can be compensated in a better way than without such contact means. It is understood that such contact means can be realized in many ways as long as an especially intimate contact is produced between the substrate and the substrate holder. 
     In accordance with the invention, these contact means provide a substantially better contact between the provided substrates and the substrate holders, so that critical thermal energy present in the substrates and/or the microstructures can be dissipated in a substantially better way into the work plate, thus considerably reducing a thermal loading of the substrates, and the microstructures in particular, in an advantageous manner. The likelihood is reduced or ideally excluded that the microstructures to be produced are damaged or even rendered useless as a result of critical temperature conditions. 
     When the contact means comprise wax, it is possible to produce a very intensive contact between the provided substrates and substrate holders by means of the wax in an especially simple constructional way. Moreover, the wax can be removed easily from the substrates or the microstructures that have grown thereon when an etching process has been completed for example and the microstructures are to be relieved of further impurities. 
     Wax as a contact means allows compensating in a very effective way for very small uneven portions on the bottom side of the substrate and/or the upper side of the substrate holder caused by possible production faults or undesirable particles, so that the substrate is aligned in an horizontally planar way and is thus provided with optimal contact to the substrate holder and thus to the work plate, thus producing a favorable thermal contact for heat dissipation. 
     It is further especially advantageous to use the individually transmitted temperature curves as evaluation criteria for individual recognition of the end points. The transmitted temperatures rise at the beginning of the stripping process from a starting temperature such as the temperature of the work plate to the respective temperature maximum as a result of reaction heat and remain stable as long as the reaction progresses. With the end of the stripping process, the temperatures start to decrease rapidly. The start, middle, end or other suitable places of the decreasing flank of the individual temperature curve is recognized as the individual end point of the stripping process. 
     Ideally, a monitoring device which can be a computer or the like for example and displays and monitors the temperatures individually sent by the thermal sensors signalizes the end of the stripping process either acoustically and/or visually, so that the operator optionally removes the finished stripped microcomponent from the etching chamber or waits until further microcomponents have been completely stripped. 
     It would also be possible to arrange the monitoring device in such a way that the stripping process is controlled automatically. 
     The object of the invention is achieved by means of the apparatus in such a way that a microwave source is arranged outside of the etching chamber to increase the density of the radicals. 
     The arrangement of a plasma chamber outside of the etching chamber is further advantageous. Ideally, said plasma chamber is cooled with a cooling medium. Water or other agents are considered as cooling medium for example which are able to dissipate heat from the plasma chamber. 
     A remote plasma method can preferably be used in this connection in which the plasma is generated advantageously outside of an etching chamber and ideally no ions and/or electrons reach a substrate within the etching chamber. The substrate is advantageously not additionally thermally loaded by these ions or electrons. In particular, an already cooled radical particle beam with a density of radicals can be provided favorably especially with the apparatus as described above from the patent specification DE 198 47 848 C1. 
     The arrangement of the spring-supported thermal sensors in the work place beneath the substrate positions is especially advantageous. The thermal sensors have contact with the bottom side of the substrate holder or with the bottom side of the substrate when no substrate holders are used. Ideally, a spring-supported thermal sensor is associated with each employed substrate or substrate holder. Individual temperature measurement and monitoring of each substrate in real time are thus ensured. 
     Ideally, a monitoring device will be used which will individually display the temperatures in real time determined by the thermal sensors and will monitor them by means of suitable criteria. The monitoring comprises the determination of the end points and signaling the termination of the stripping processes. It shall further be monitored whether a stripping process will run according to a predetermined scheme. If malfunctions occur such as the overheating of a substrate, the monitoring device will signalize this malfunction and will initiate suitable measures so that the substrate will not be damaged. 
     The arrangement of a monitoring device at a location which need not necessarily be close to the plasma chamber is further advantageous. The operator can conveniently observe the stripping process in real time from his/her office. When a completed stripping process is signalized via the monitoring device, the operator can decide whether or not this completed microcomponent is removed from the etching chamber. 
     As a result of the advantages explained above, the application of the described apparatus and method is well-suited for stripping photoresist for microcomponents with component structures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained below in closer detail by reference to an embodiment shown in the drawings, which show schematically: 
         FIG. 1  shows a sectional enlarged view of a microcomponent on a substrate produced according to the LIGA method; 
         FIG. 2  shows a sectional view of an apparatus for producing microcomponents; 
         FIG. 3  shows a detailed view of the work plate of  FIG. 2 , and 
         FIG. 4  shows an exemplary visualization of three temperature curves of three substrates. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a microcomponent on a substrate  14  produced according to the LIGA method. Photoresist  1  encloses the metal  2  which was electroplated to a gold layer  3  which is used as a starter layer. 
     The apparatus  10  for producing microstructures (not shown here) as shown in  FIG. 2  comprises an etching chamber  12  in which a thermostatic work plate  13  is arranged. 
     A microwave source  18  is arranged outside of the etching chamber  12 . Furthermore, a water-cooled plasma chamber  11  is shown which is also located outside of the etching chamber  12 . The microwave source  18  can be used to produce cooled remote plasma  19  in the water-cooled plasma chamber  11 . A detailed description of how the present remote plasma process works in detail shall be omitted in this case because the remote plasma process concerns a well-known method. A respectively suitable apparatus and respectively suitable methods are also described in detail in the initially mentioned patent specification DE 198 47 848 C1. 
     In the present case, the generated remote plasma  19  is advantageously already cooled by means of the water-cooled plasma chamber  11 , so that in this way an advantageous reduction in temperature can already be realized concerning the etching process yet to be explained. 
     The present remote plasma method can be controlled by means of the apparatus  10  shown here in such a way that a cooled radical particle beam  20  will reach the etching chamber  12  with an especially high density of radicals. 
     As is also mainly shown in the illustration according to  FIG. 3 , substrates  14  are arranged on a thermostatic work plate  13 . In the sectional views of this specific embodiment, two substrates  14  are placed by means of two substrate holders  15  on the work plate  13 . Reaction heat which is produced during an etching process for example in which areas of material (not shown here) such as the photoresist layer SU-8 are etched away from the substrates  14  can be dissipated away from the substrates  14  in an especially advantageous manner. This dissipation of the reaction heat is achieved especially well when the two substrate holders  15  are made of thermally well-conducting material. To ensure that the heat transmission can be achieved over the full surface area from the illustrated substrates  14  to the substrate holders  15 , a contact means  16  (see  FIG. 3  in particular) is each arranged between the substrates  14  and the respective substrate holders  15 . These layers of contact means are also used for horizontally planar positioning of the substrates  14  on the substrate holders  15  in the etching chamber  12 . The contact means  16  in this embodiment is made of wax. 
     The wax layers are used to position the substrates  14  in a fixed but detachable way and individually above the respective substrate holders  15 . Both the wax layers as well as the substrate holders  15  especially represent means within the terms of the invention for placing in a planar way the provided substrates  14  on the thermostatic work plate  13 . 
     The wax layers ensure very advantageously that the already mentioned reaction heat is dissipated from the substrates  14  and thus also from the microstructures (not shown here explicitly) to be produced, so that especially the microstructures are also thermally loaded in a lesser way, as a result of which the rejection of microstructures can be reduced advantageously. 
     Moreover, spring-supported thermal sensors  17  are provided in the thermostatic work plate  13  which can be used to detect the current temperatures directly on the substrate holders  15 . 
     This ensures advantageously to realize a monitoring of the actually existing temperatures directly on the substrate holders  15  and thus also on the substrates  14  and the respective microstructures. 
     A thermal sensor  17  is preferably associated with each substrate holder  15  and each substrate  14  which is arranged on the work plate  13 . It can thus reliably and permanently be ensured that each of the existing thermal sensors  17  is in operative contact with the associated substrate holder  15 . 
     When the thermal sensors  17  are held in a resilient or spring-supported manner, as shown in  FIG. 3 , it can be ensured especially well that the thermal sensors  17  are always pressed with sufficient force against the substrate holders  15 . 
     Since there is a direct connection between the temperatures applied to the substrates  14  and the presence of photoresist that still needs to be etched away, it is possible to draw conclusions on the basis of the determined temperature data on how far the etching process has already progressed on one of the substrates  14 , as already described above. In particular, due to a rapidly falling temperature during the etching process it is possible to recognize whether an etching process has already finished on one of the substrates  14  because no reaction heat is then produced on this substrate. 
     In order to enable a respective evaluation of the temperature data detected by means of the thermal sensors  17  it is advantageous when the present apparatus  10  comprises a suitable monitoring device which offers the possibility for visualizing the detected temperature data by means of display means  24 . 
     The present example visually shows in the display means  24  a first temperature curve  21  of a substrate  14 , a second temperature curve  22  of another substrate  14  and a third temperature curve  23  of a third substrate (not shown here). 
     The temperature curves  21 ,  22  and  23  show the respective temperatures on the respective substrates  14  over a specific period of time. It is described on the basis of the first temperature curve  21  in which phase the first substrate  14  is located. At the beginning of the etching process, the substrate  14  on which the first spring-supported thermal sensor  17  is arranged has an initial temperature  21   a  which is close to the temperature of the thermostatic work plate. With the start of the stripping process, the temperature on the substrate  14  rises to an equilibrium temperature  21   b . The equilibrium temperature will occur when the temperature gradient occurring towards the work plate  13  is dissipating the reaction heat from the substrate  14 . The equilibrium temperature remains virtually constant as long as the stripping process occurs in this substrate  14 . Once the substrate  14  has been stripped of photoresist, the temperature will start to drop. The start, middle, end or other suitable places of the decreasing flank of the temperature curve can be recognized as the end point of the stripping process  21   c . The possibility for setting an optimal etching duration is given by suitable setting of the end point recognition. 
     The present apparatus can be used to display different temperatures and different temperature curves  21 ,  22 ,  23  in connection with each of the substrates  14 , so that an evaluation of the respectively occurring etching process on the respective substrate  14  can be made rapidly and securely. Reasons for the different temperature curves can be differently running reactions on the respective substrate  14  which may indicate differently thick layers of resist.