Abstract:
A method of depositing or etching a micro- or nano-scale pattern on a work piece is disclosed, including the steps of: (a) placing the work piece in an electrochemical reactor in close proximity to a patterned tool; (b) connecting the work piece such that it is the anode if is to be etched or the cathode if it is to be deposited, and the patterned tool such that it is the counter electrode; (c) pumping electrolytic fluid necessary for the electrolytic operation of the cell formed between the two electrodes; and (d) applying a current across the electrodes to etch or deposit the work piece.

Description:
[0001]    This application is a National stage filing under 35 U.S.C. §371 of International Application No. PCT/GB2005/002795, filed on Jul. 19, 2005, which in turn claims priority to British Application No. 0416600.5, filed Jul. 24, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This invention relates to a process, which can be used to selectively electrochemically deposit, or etch, micro patterns in various substrate materials, preferentially for the fabrication of micro-devices, nano-devices, and the like. 
       BACKGROUND 
       [0003]    In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art. 
         [0004]    Micro- and nano-machined devices are used in a variety of industries including electronics, optical, telecommunications, data storage, medical, chemicals etc. Conventional micro scale electrochemical deposition or etching has led to advances in sensor technologies, optical display technology, and micro-actuators. A simple example is the micro-device used to inflate an automobile air bag, whereby the bag is filled with nitrogen released from a solid compound, wherein the solid compound is a micro resistor, which is heated by an electric current. In the medical field micron sized patterns, on certain substrates, have been shown to promote the growth of certain cells, a particular application being in tissue engineering. 
         [0005]    The main physical attribute of a micro-device is that the scale of its features are measured in microns, that is in millionths of a metre, and that of a nano-device, wherein the scale of the device&#39;s features are measured in nano-metres, that is in thousand-millionths of a metre. Owing to their small size, and their often complex geometries, micro- and nano-devices cannot be manufactured by simple mechanical methods such as cutting, sawing, milling, drilling etc. Under the prior art, methods involve the use of photo-lithography to impose the desired pattern on the substrate of the work-piece followed by chemical etching. The work-piece is first coated with a photo resist. It is then exposed to the image of a photographic mask using visible or ultra-violet light. Unexposed photo-resist is then washed off, and the work-piece etched. The remaining photo resist protects the surface from the etchant. Thus the original photo-mask pattern may be reproduced as a machined surface on the work-piece. The technology of photo-lithography is now well refined, particularly because of its extensive application in the semiconductor industry, and it is for this reason that it has been extended to the fabrication of micro- and nano-devices and featured substrates. 
         [0006]    These conventional photo-lithographic techniques for the fabrication of micro- and nano-devices have a significant drawback in the complexity of the process, use of materials, and their disposal to the environment. Every work-piece must be coated with photo-resist, exposed under the photo-mask, and washed before etching. Following the etch, the residual photo-resist must be removed. 
       SUMMARY OF THE INVENTION 
       [0007]    The current inventive process overcomes this problem by eliminating the need for applying the photo-lithographic process to every work-piece. Instead, it is applied once only, to the tool, which then may be re-used many times to produce a large number of work-pieces by electro-chemical deposition or etching. 
         [0008]    According to the current invention a tool is made such that it is selectively coated by a patterned, electrically insulating, chemically inert, coating, which may be applied by any appropriate method, the preferred method utilising a polymer photo-resist and conventional lithographic techniques known in the art. The tool thus formed is then placed in an electrochemical reactor in close proximity to the work-piece that is to be deposited or etched. The reactor is arranged such that the tool forms the counter electrode, and the work-piece to be deposited or etched, forms the cathode or anode, respectively. The close proximity spacing between the two electrodes is arranged to be dimensionally similar to, and preferably smaller than, the smallest feature that is to be etched in the work-piece. Electrolytic fluid necessary for the electrolytic operation of the cell is continuously pumped through the narrow spacing between the two electrodes to remove reaction products and heat whilst an appropriate electric current is passed through the system. 
         [0009]    Thus the present invention provides a method of depositing or etching a micro- or nano-scale pattern on a work-piece, comprising the steps of:
       a) placing the work piece in an electrochemical reactor in close proximity to a patterned tool;   b) connecting the work piece such that it is the anode if is to be etched or the cathode if it is to be deposited, and the patterned tool such that it is the counter electrode;   c) pumping electrolytic fluid necessary for the electrolytic operation of the cell formed between the two electrodes; and   d) applying a current across the electrodes to etch or deposit the work piece.       
 
         [0014]    The tool may be patterned by conventional means to yield a tool which is selectively coated with a patterned, electrically insulating and chemically inert coating. The coating needs to be chemically inert to the conditions in the electrochemical reactor. Typically, the patterning may be carried out on a convention polymer photoresist using known lithographic methods. 
         [0015]    The electrochemical reactor is designed to keep the two electrodes (which are the tool and the work piece) at a constant separation across their faces, within acceptable margins of error. It allows for the electrodes to be connected so as to pass a current between them and for the electrolytic fluid to be pumped between the electrodes. 
         [0016]    The electrolytic fluid is to be selected according to the electrochemical reaction being carried out. For example, in the examples below a copper sulphate solution is used in etching a copper disc. 
         [0017]    The material of the electrodes is selected according to the nature of the final product desired. In the examples below, copper discs are etched. Many micro- and nano-scale patterns are to be found on semiconductor substrates with metals such as gold, aluminium or copper forming the pattern. 
         [0018]    Because of the close spacing between the two electrodes, it being of similar dimension to the required features in the work-piece, the anode is preferentially etched in the areas that face exposed parts of the counter electrode, relative to those areas of the cathode that are masked by the insulating coating. 
         [0019]    The dimensional similarity between the distance between the electrodes and the features to be patterned means that these distances can be in a ratio of about 10:1 or 5:1 to 1:5 or 1:10, preferably about 10:1 to 1:2, and more preferably about 10:1 to 1:1. Thus although in some embodiments it may be preferred that the distance between the electrodes is smaller than the size of feature to be patterned, in other embodiments the converse is true, i.e. the distance between the electrodes is larger than the size of feature to be patterned, by up to 10 times. 
         [0020]    The current applied may be constant or varied, as may the voltage which causes the current to flow. 
         [0021]    After an appropriate period in the electrochemical reactor, the work-piece is etched with a micro- or nano-scale pattern on its surface replicating the pattern imposed on the tool, whereupon it may be removed from the electrochemical reactor. Many work-pieces may be sequentially processed in this way using the one tool. Each work-piece may be subsequently presented to other tools for further complex processing. 
         [0022]    Under the prior art there is a multi-step process to each stage of the fabrication of each micro- or nano-device comprising of:
       (i) coating the work-piece with a photo-resist;   (ii) exposing the work-piece to light through a photo-resist mask;   (iii) removing the photo-resist from the appropriate areas using a solvent;   (iv) exposing the work-piece to an etching (or deposition) solution;   (v) removing the remaining photo-resist using a solvent.       
 
         [0028]    Under the process described herein, once a tool has been formed using the process under the prior art as described above, there is a single-step process to each stage of the fabrication of each micro- or nano-device comprising of: offering the work-piece to the tool within the electrochemical reactor, and electrochemically depositing or etching micro- or nano-patterns on it. 
         [0029]    The re-useable tool will eventually require replacement, however, a great advantage will be enjoyed in the reduced use of solvents, reduced processing time, and of product reproducibility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0030]    The following description of preferred embodiments can be read in connection with the accompanying drawings in which like numerals designate like elements and in which: 
           [0031]      FIG. 1  shows the flow process of an embodiment of a system according to the invention. 
           [0032]      FIG. 2  shows a cross section in one plane of the electrochemical reactor according to the invention. 
           [0033]      FIG. 3  shows a cross section of the electrochemical reactor according to the invention in a plane orthogonal to that of  FIG. 2 . 
           [0034]      FIG. 4  shows an exploded view of an electrode holder for use in the electrochemical reactor of  FIGS. 2 and 3 . 
           [0035]      FIG. 5  shows a micro-pattern used in the pattern transfer experiments. 
           [0036]      FIG. 6  shows (a) SEM, (b) 2D and (c) 3D profiles of a copper anode etched according to the present invention. 
           [0037]      FIG. 7  shows (a) SEM, (b) 2D and (c) 3D profiles of another copper anode etched according to the present invention. 
           [0038]      FIG. 8  shows (a) SEM, (b) 2D and (c) 3D profiles of a further copper anode etched according to the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0039]    The vertical flow system for the main etching experiments has been described in literature for electrodeposition experiments (Meuleman, W. R. A., et al.,  J. Electrochem. Soc.  149, C479-C486 (2002); Dulal, S. M. S. I., et al.,  Electrochim. Acta,  49, 2041-2049 (2004)). The design of the flow cell, which is shown in  FIGS. 2 and 3 , was based on a model previously constructed by Roy et al. (Roy, S., et al.,  Chem. Eng. Science,  56, 5025 (2001)). The flow system, which is illustrated in  FIG. 1 , consisted of the flow cell  6  with two electrode holders, one for the cathode  7  and one for the anode  8 , a heat exchanger  2 , a filter unit/settling tank  3 , an electrolyte reservoir  4 , magnetically coupled pump  5 , and a flow meter (not shown). The power supply  1  is coupled to the anode and cathode. 
         [0040]    The cross-section of the flow cell as seen in  FIG. 2  was rectangular and the electrolyte circulated upwards through the channel. The electrolyte was stored in a reservoir  4  and the velocity of the electrolyte was controlled by a manual valve and monitored by a digital flow meter. The distance between the channel walls, except at the electrodes, was 3.0 mm. To prevent the formation of any eddy at the inlet and outlet of the flow channel, the entry and exit sections  10  were conically shaped. 
         [0041]    The two electrodes holders  7 ,  8  were placed in the middle of the flow channel, whose positions are adjusted with micro-precision control screws or shims  9 . An interelectrode gap of 0.5 mm between the two electrodes was achieved by using a specific chamfered shape of the electrode holders, which is shown in  FIG. 4 . Copper rods  13 , of diameter of 1.0 cm and 99.99% purity, were segmented into 3 mm thick discs, and inserted into a Teflon cup  12  which fitted into the holder  7 . The back of these electrodes was connected to another copper rod  11  via a spring, as illustrated in  FIG. 4 . In each experiment, the electrodes were loaded in their holders and inserted into the cell. 
         [0042]    Electrolyte was then circulated through for approximately five minutes at a flow rate of 70-90 cm 3 s −1  (&gt;3.5 ms −1  flow velocity) to eliminate air bubbles from the electrode surface. Since there was no reference electrode in these experiments, only the cell potential was monitored or controlled. All experiments, therefore, were galvanostatic. During the course of a pattern transfer experiment, the cathode was plated with copper, which was removed using a 25% HNO 3  solution. 
       Electrode Preparation 
       [0043]    Each copper disc, which served as an anode, was polished to a mirror finish using #1200, #2400, and #4000 grit emery paper. The measured surface roughness of the polished copper discs was about 20-40 nm, but larger machining damage remained—however, these did not influence the results. After polishing, the copper discs were slightly convex; the copper discs were found to be approximately 60 μm thicker in the middle than at the edges. 
         [0044]    The cathodes were gold coated glass discs with a diameter of 1.0 cm. Electrical contact between the gold surface and the back of the glass disc was made by painting the back and side wall with conductive silver paint (RS Components). The cathode was patterned using photolithography by modifying a standard photolithographic process for 100 mm wafers. 
         [0045]    In the photolithography experiments, each glass disc was cleaned with acetone and glued at the centre of a clean silicon wafer with double-sided adhesive tape. Then, the glass discs were individually coated with photoresist (Shipley, SPR 220-7.0) using a EV 101 Spin Coater. A few drops of resist were added manually to the middle of the glass disc at a spin rate of 500 rpm. After spin coating the samples were baked at 95° C. for one hour to remove any excess solvent. Four coated glass discs were then placed on a silicon wafer and the glass photomask with the micro-pattern was placed onto the four glass discs. The photoresist was then exposed to UV light through the photomask by using the EV 620 Contact Aligner. The exposure time was 35 seconds. The samples were then developed for two minutes using a developer (Shipley, MF-26A). After cleaning with deionised water, the cathodes finally were hard-baked for one hour at 105° C. These photolithography procedures produced a resist thickness of 7-8 μm. 
         [0046]    The micro-pattern used for the primary etching experiments was previously used in a work about a novel gold electrodeposition process for microelectronic applications (Theory and Practice of Pulse Plating, Ed. J-C. Puippe and F. Leaman, Published by American Electroplaters and Surface Finishers Society, Orlando, Fla., USA, ISBN 0-936569-02-6 (1986)). The mask pattern consisted of large squares, which were delineated by lines ABCD, as illustrated in  FIG. 5 . When this pattern is transferred to a glass disc, the grey regions represent the resist covered areas and the white regions denote exposed areas. As shown, the uncovered areas consist of lines with 100 μm thickness (t 1 ) and 3.0 mm length (t 4 ). Within each large square, 81 smaller squares of 100 μm (t 3 ) sides are placed. These squares were separated from each other by a distance of 200 μm (t 4 ). The advantage of using this pattern is that the replication of both 1 and 2 dimensional features can be investigated. 
         [0047]    Other micro-patterns were designed to test the pattern transfer performance of the technique. One of these was a pattern consisting of straight lines with varying width and spacing. These pattern designs allowed examination of the reproduction of one-dimensional structures of small widths—as small as 10 μm. Since the width of the lines and line spacing were changed in these experiments, the current density and the feature width could be changed independently. This allowed observation if either of these two factors had any effect on the pattern transfer. 
       Current and Potential Control 
       [0048]    A variety of current and potential controls were used in the pattern transfer experiments. Etching experiments at constant current between 0.3 Acm −2  and 1.0 Acm −2  were performed with a DC power supply (PL 310, Thurlby Thandar). Etching experiments at a constant cell voltage were carried out by using voltage control on the same instrument. 
         [0049]    The applied current and cell voltage as well as the corresponding time to obtain the same total etch depth are listed in Table 1. The table also shows the different electrolytes and conductivities used in the etching experiments. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Constant 
                 Constant 
                 Etching time 
               
               
                   
                   
                 Current 
                 Cell 
                 to obtain 
               
               
                 Electolyte 
                 Conductivity 
                 density 
                 Potential 
                 same total 
               
               
                 Composition 
                 [Sm −1 ] 
                 [Acm −2 ] 
                 [V] 
                 depth [s] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.1 M CuSO 4  &amp; 
                 46.0 
                 0.3-1.0 
                   
                 60-678 
               
               
                 0.5 M H 2 SO 4   
               
               
                 0.1 M CuSO 4   
                 2.7 
                 0.3-1.0 
                 5.0 
                 60-180 
               
               
                   
               
             
          
         
       
     
         [0050]    The electrolyte flow rate was varied between stagnant and 150 cm 3 s −1  (which corresponds to a fluid velocity of 7.5 ms −1 ) to see if it had any effect on the etching performance. Pulsed etching experiments were performed by using a pulse current power supply (CAPP-25/20-K, Axel Akerman). Pulsing cell voltage was applied. For a square wave pulse with peak potential V p , pulse-on time t p , and pulse period t pp, (so that t p /t pp  is the duty cycle), the “average” cell potential V a  for the current waveform is given by: 
         [0000]        V   a   =V   p ×( t   p   /t   pp )  (1) 
         [0051]    The “average” cell potential includes ohmic drop within the electrolyte and potential changes due to non-Faradaic processes (Hoar, T. P., “The Anodic Behaviour of Metals”,  Modern Aspects of Electrochemistry, Vol.  2, The University Press, Glasgow (1959)). Table 2 shows the parameters used during pulsed voltage etching experiments. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 V p   
                 t p   
                   
                   
               
               
                   
                 [V] 
                 [ms] 
                 t p /t pp   
                 Cycles 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 10 
                 1.0 
                 0.02 
                 4000 
               
               
                   
                 10 
                 1.0 
                 0.01 
                 4000 
               
               
                   
                 10 
                 10.0 
                 0.1 
                 4000 
               
               
                   
                 20 
                 1.0 
                 0.02 
                 4000 
               
               
                   
                 20 
                 1.0 
                 0.02 
                 8000 
               
               
                   
                 20 
                 1.0 
                 0.02 
                 12000 
               
               
                   
                 20 
                 1.0 
                 0.02 
                 20000 
               
               
                   
                 20 
                 1.0 
                 0.01 
                 4000 
               
               
                   
                 20 
                 10.0 
                 0.1 
                 4000 
               
               
                   
                   
               
             
          
         
       
     
       Characterization 
       [0052]    For the characterization of the patterned cathodes and the etched copper anodes different measurement systems were used. For feature lengths and widths measurements, an Olympus MX50 microscope, equipped with a BRSL ‘DAVID’ system was used. An Alpha-Step 200 stylus profilometer was used to determine the etch depth and surface roughness. Non-contact 3D measurements were carried out with a ZYGO NewView 5020 optical profiler to measure depth and length scales. Scanning electron microscopy was used to determine the surface morphology as well as defects before and after a pattern transfer experiment. The scales in  FIGS. 6 to 8  is presented using the optical profiler, because it shows both the feature length, depth as well as roughness—other scaling has not been shown for brevity. 
       Experiments 
       [0053]    The effect of fluid flow on pattern transfer was first determined. This experiment was carried out first because at the high electrode overpotentials attained during transpassive etching, oxygen evolution is expected. The evolving oxygen could block the electrode surface, thereby preventing further etching, caused by the high localized resistance offered by a gas bubble. Pattern transfer experiments at constant or pulsed current and voltage revealed that this could seriously impair the etching performance. 
         [0054]    When a gas bubble was trapped within the resist, it resulted in local circular areas (the shape of a bubble) remaining un-etched. In addition, the photoresist on the cathode (counter electrode) was often detached due to the turbulence generated by gas evolution. 
         [0055]    As the electrolyte flow rate was increased, the gas bubbles detached from the surface more easily and electrochemical dissolution could proceed. The etching performance for electrolyte flow rate of 70 cm 3 s −1  (3.5 ms −1 ) were found to give satisfactory performance, and therefore, this flow rate was used for all further experiments described below. 
         [0056]    The next parameter to be investigated was the electrolyte conductivity. The effect on pattern transfer was examined by direct current experiments using electrolytes of different conductivity. The applied current density was fixed at 1.0 Acm −1  and the etching time was 180 seconds in these experiments. 
         [0057]    The etched features for acidified electrolytes, such as 0.1 M CuSO 4  with 0.5 M H 2 SO 4  electrolyte, were found to be a ‘derivative’ of the tool pattern; for example a square shape, such as the small squares of  FIG. 5 , produced sine-wave like features on the substrate. 
         [0058]    Etching experiments with non-acidified electrolytes produced accurate pattern transfer. An example of this is illustrated in  FIG. 6 ; this pattern was etched using tool patterned as in  FIG. 4  using a 0.1 M CuSO 4  solution with an applied current density of 1.0 Acm −1  and an etching time of 180 seconds. The small squares in that pattern, with 100 μm×100 μm, are reproduced as a square with a flat bottom, as shown in the SEM ( FIG. 6   a ) and the 3D optical profile ( FIG. 6   b ). The length and depth scales are resolved in the 2D optical profile ( FIG. 6   c ) etched copper sample; the feature length is 120 μm and the etch depth is 1.5 μm. Since best etching results were achieved into a 0.1 M CuSO 4  electrolyte with a conductivity of 2.7 Sm −1 , all etching experiment described below are reported for this specific electrolyte, unless stated otherwise. 
         [0059]    From the above, it can be seen that varying the nature of the electrolyte whist keeping the tool pattern the same, can alter the pattern etched on the work piece. 
         [0060]    The next parameter to be investigated was the effect of applied current density or cell voltage on pattern transfer characteristics. Etching experiments in the current density range between 0.3 Acm −2  and 1.0 Acm −2  were carried out to determine the performance at higher currents, where pre-passive or transpassive dissolution is expected to occur. Overall, the etching experiments at high current densities showed better pattern transfer than the experiments in the active dissolution region. Etch depths up to 1.5 μm were reached for applied current densities of 1.0 Acm −2  and an etching time of 180 s; however, when the etching time was increased beyond 180 seconds, the etch depth did not increase. This showed that that the substrate was dissolving at the same rate everywhere and that etching selectivity was lost. 
         [0061]    Pattern transfer experiments were also carried out using a constant cell voltage between 1.0 V and 2.0 V. For applied cell potentials of 1.0 V the resulting current density rose up to a steady value between 3.5-7.0 Acm −2 . A current density rise to such high values could indicate dissolution in the transpassive region, and some of the experiments showed periodic oscillations with an amplitude of around 0.2 Acm −2  and a frequency of 0.2-0.5 Hz. These periodic oscillations may be induced by sequential periods of film growth, oxidation, and partial dissolution and removal of salt and oxide layer (Lee, H. P., et al.,  J. Electrochem. Soc.,  132, 1031 (1985)). 
         [0062]    As shown by the SEM micrograph of a linear pattern in  FIG. 7   a , which was obtained by applying a constant potential 1.0 V for 180 seconds in a 1.0M CuSO 4  electrolyte, the etched area is relatively rough. The tool pattern was lines covered with photoresist which were 70 μm in width separated by an exposed area of 70 μm. The 3D optical profiles in  FIG. 7   b  show the smooth top surface and a rough etched bottom surface, as observed in the SEM. The length and depth scales, as resolved in the 2D optical profile of  FIG. 7   c , show a line width of 70 μm and an etch depth of 1.5 μm. The profile of the etched lines shows relative vertical walls at the top but a curved bottom. 
         [0063]    However, using pulsed cell voltages with a peak potential of either 10 V or 20 V were found to be more successful. The pulse-on time t p  was varied between 1.0 ms and 10.0 ms with duty cycles between 0.01 and 0.1.  FIG. 8   a  shows the scanning electron micrograph,  FIG. 8   b  the 2D optical profile and  FIG. 8   c  the 3D optical profiles of an etched copper sample using pulsed voltages. The original micropattern consisted of exposed linear features of 10 μm separated by a resist covered area of 50 μm. This was obtained using 4000 pulse cycles of 20 V voltage pulses and 1 ms on time and a duty cycle of 0.02. The 2D scale resolution shows an etch depth of 1.0 μm, a feature width of about 10  82  m, with relative vertical walls and a flat bottom. In contrast to the active dissolution experiments, as the cycle numbers (hence etching time) were increased, the etch depth increased. For 20,000 pulse cycles, an average etch depth of 3.3 μm was obtained. 
         [0064]    Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.