Abstract:
A method of producing a free standing structure, the method comprising: providing a substrate having a raised pattern formed on a surface of said substrate, said raised pattern comprising at least one material which forms said surface; depositing material over said raised pattern; and dissolving said substrate to release said deposited material to form said free standing structure from said released deposited material.

Description:
FIELD OF INVENTION 
   The present invention relates to the field of making free standing structures, and in particular, to the field of making free standing nano structures or microstructures. 
   BACKGROUND OF THE INVENTION 
   Custom shaped free-standing nano structures and microstructures have many uses, for example, as support particles in combinatorial chemistry and as labels for molecules. Known methods of creating custom shaped micro particles are described in GB 2306484 and WO 00/16893. 
   In GB 2306484, a method is described of making solid support particles marked with a machine readable code, for use in combinatorial chemistry techniques. The particles are made using deposition, etching and lift-off techniques similar to those used to make micro electronic integrated circuits and micro electromechanical systems (MEMS). A silicon wafer substrate is coated firstly with a soluble sacrificial layer, then with a layer from which the particles will be made, finally with a layer of photosensitive polymer resist. The layer of photosensitive polymer resist is exposed to ultraviolet light through a photolithographic mask which defines the particle shapes. The particles are created by removing unexposed resist and etching away the revealed areas of material around the particles. The particles may then be freed from the silicon wafer substrate by dissolving the sacrificial layer which underlies the particles. The particles may be patterned with text, holes, grooves or notches. This creates a code, which is readable optically to identify each particle. The particles are sized with a longest dimension of between 1 and 500 microns. 
   In WO 00/16893, a method of making micro-barcode labels is described. The labels are made by standard optical lithography and dry etching, and the barcode is stored as a series of holes in the micro labels. The labels are made of aluminium and are anodised to allow the attachment of a wide range of biochemically active agents for use as highly selective probes. Each micro label is typically of a size of about 100 microns long by 10 microns wide by 1 micron thick. 
   Although both of the above methods allow small free standing structures to be fabricated, they both suffer from the problem that they require rather complicated and time consuming fabrication techniques. 
   SUMMARY OF THE INVENTION 
   The present invention at least partially addresses the above problems and, in a first aspect, provides a method of producing a freestanding structure, the method comprising providing a substrate having a raised pattern formed on a surface of said substrate, said raised pattern comprising at least one material which forms said surface; depositing material over said raised pattern; and dissolving said substrate to release said deposited material to form said freestanding structure from said released deposited material. 
   The use of a patterned substrate is advantageous, because it eliminates the need for a masking and etching process to make the free standing structures. This makes the process cheaper, quicker, easier and more reliable than in the prior art, where part of the layer from which the free standing structure is made is etched away to create the shape of the free standing structure. 
   The patterned substrate may be produced from a mould by injection moulding, insert moulding or by pressing. For example the mould may be plate shaped and may be pressed directly onto the substrate to emboss a pattern in it. This allows substrates to be mass produced. Alternatively, the mould may be in the form of a roller and the pressing may be done by rolling the mould across the surface of the substrate. This rolling method has the advantage that it can be done continuously across a large area of substrate. 
   A further alternative is that the substrate may be produced by a solvent replication method. Said method comprises the steps of providing a solvent between a substrate and a mould and pressing the substrate against the mould to allow the solvent to partially dissolves the substrate and adopt a shape complimentary to the mould. The solvent evaporates and the mould is then removed, to leave a substrate of the desired shape. 
   The mould may be made by a number of techniques such as wet etching, reactive ion etching (RIE) or laser processing techniques. In laser processing techniques, a laser is focussed to ablate a small area of the substrate, the depth of the substrate ablated may be controlled by the exposure time of the laser to the substrate. As the laser may be focussed to a small spot size (e.g. to below 1 μm in the far field, or tens of nanometers in the near field) it is possible to etch 3D patterns with the laser by varying the laser exposure time over various areas of the substrate. Wet etching may also be used to create a mould with 3D patterns using so-called preferential etchants which preferentially etch certain crystallographic planes quicker than others. 
   As the quality of the surfaces of the free standing structure will be dictated by the quality of the surfaces of the patterned substrate, it is desirable to form a mould, regardless of the substrate patterning method, which has good quality surfaces. 
   Thus, in a second aspect, the present invention provides a method of producing a mould for making a substrate with a raised pattern, said method comprising:
         providing a mould material having a buried sacrificial layer, said sacrificial layer being parallel to a main surface of said mould material;   masking the main surface of said mould material with a mask pattern;   etching the mould material through said mask pattern with a first etch;   stopping said first etch at an etch stop level within said sacrificial layer; and   etching the mould material with a second etch, to selectively remove the sacrificial layer below said etch stop level.       

   The first etch may be performed by reactive ion etching. This etches through the mould material and into the sacrificial layer. The etching time is chosen so that the etch will stop within the sacrificial layer. 
   The second etch may be a wet etch. The second etch may act more slowly than the first etch. Preferably, it will only etch the sacrificial layer, and not the mould material. However, it is possible that it will etch the mould material at a slower rate than it etches the sacrificial layer. Since the second etch selectively removes the sacrificial layer below the stop level, a good quality surface is formed by the mould material below the sacrificial layer. 
   The second etch may laterally etch the sacrificial layer to form an undercut. The undercut should be small enough to allow the substrate to be removed from the mould without damage. The mould may then be used to make a substrate with pillars or steps which have a slight overhang. The maximum useful size of the undercut on the mould will be dependent on the pliability of the substrate material. 
   Further etching stages may also be performed. These further etches may define patterns with surfaces which are parallel, perpendicular or at some other angle to the main surface. 
   The material used to make the mould should be hard and micromachinable. Some preferred mould materials are silicon, silicon dioxide (including glass or quartz), Si 3 N 4 , sapphire, GaN, Al, and steel. For a silicon mould, the sacrificial layer could be silicon dioxide. For a metal mould, the sacrificial layer could be a different type of metal. 
   The present invention has the advantage over the prior art that a better quality of mould may be produced with a higher resolution of pattern. Once the mould has been made, it is very easy to mass produce high quality substrates in order to make freestanding structures, such as micro-barcodes, small antennas, MEM devices or custom shaped particles. The process of making the substrate, and the process of making the free standing structures do not require the use of reactive ion etching, making these processes easier and cheaper. 
   The pattern which masks the mould material may define a pattern on said substrate comprising a substantially planar upper surface separated from a lower level by vertical sidewalls. Holes may also be provided in said upper surface. The mould may also define a 3D pattern comprising a non-flat upper surface. 
   Generally, the raised pattern will be made from the same material as the substrate, and is integral with the substrate. 
   The substrate may also comprise a second pattern of lower relief than the raised pattern, the second pattern being separated from the raised pattern by a step. Again, preferably, the step is at an angle such that material is not deposited onto it, and yet more preferably, it is substantially vertical. Material may be deposited over the second pattern. The second pattern of lower relief may make up the rest of the surface, or there may be a third, fourth pattern, etc, at other levels. Thus preferably, the mould may have a plurality of different levels such that a substrate is formed having a plurality of different levels. For example, said substrate may be used to produce two or more free standing structures from different levels on said substrate. 
   One preferred example of a pattern for the free standing structure is a barcode pattern. This barcode pattern may comprise a strip with a plurality of holes, where the arrangement of holes determines the unique code of the strip. The barcode may be encoded according to the positions of the holes, the sizes, shapes or orientations of the holes, or some combination of these. The holes may be arranged along the length of the strip in a barcode style pattern. 
   Alternatively the free standing structure may comprise a plurality of notches cut into the sides of the strip, where the arrangement of the notches determine the unique code of the strip. Again, the positions, sizes, shapes or orientations of the notches or some combination of these may determine the unique bar code pattern. The notches may be cut into a single side of the strip, or into alternative sides of the strip to produce a “snake-like” pattern. This has the advantage of eliminating small pieces of waste such as the waste material that will arise from the holes, and may subsequently block the holes of the bar code pattern. Thus, when the barcode patterns are made, there will be no problem with trying to separate small pieces of waste from the desired freestanding bar code structures. 
   The shape of the bar code tag, or another desired pattern shape, may correspond to the shape of the raised pattern on the substrate. The bar code tag will thus be formed on top of pillars or raised areas of substrate. In that case, waste material corresponding to the holes of the bar code, and the area surrounding the bar code, will be deposited onto the second pattern of lower relief on the substrate. When the substrate is exposed to a solvent, the pillars or raised areas will dissolve before the main bulk of the substrate. Thus, the barcode tag will be released from the raised pattern whilst leaving the waste still attached to the substrate. 
   Alternatively the barcode pattern, or other desired pattern, may correspond to the shape of the second pattern of lower relief on the substrate. In this case, the waste material from the holes of the barcode would be deposited on top of the raised pattern of the substrate. Then, when the pillars are dissolved by a solvent, the waste material will be released first, whilst the desired barcode structure is held in position on the substrate. The waste may be removed by being carried away by a suitable flow of liquid. After the waste has all been removed, the desired barcode pattern may then be released from the substrate by a further dissolving stage, releasing it into solution without any contamination from waste particles. This second method may also be particularly useful for making free standing structures with a fairly large area compared with the area of the waste material, e.g. for a micro membrane or mesh with a large number of small holes. This is because the large surface area of the membrane or mesh would mean that it is likely to take longer for the solvent to dissolve the substrate below the membrane or mesh, than to dissolve away the smaller area beneath the waste material. 
   The invention may also be used to make free standing structures for use in micro electromechanical devices (MEMs). Motion sensors or accelerometers may be made with a freestanding metal or metal oxide structure, where external motion will affect the electrical properties of the free standing structure. For example the structure may comprise at least two narrowly separated members each capable of holding an electrical charge such that there is a capacitance between said members, at least one of the members being moveable with respect to the other. Movement of one member relative to another affects their separation, thus changing the capacitance between them. A change in capacitance can be detected to indicate that motion has occurred. The members may be plate shaped, and the member capable of movement may be a freestanding structure suspended on a thin arm. 
   Another possible use of the invention is to make an antenna. The antenna may be of a miniature size, and may comprise a filament or loop of metal extending from a metal contact pad. A semiconductor chip, such as an RF chip, may be bonded to the contact pad. Alternatively, the antenna loop and contact pads may constitute a simple RC circuit, which is itself detectable by scanned RF pulses. 
   A further application of the invention is to make a large number of particles of a desired size or size distribution. These particles may be used as colloidal particles, as chemical tags or encoded carrier particles, or in so called “magnetic wood”. 
   “Magnetic wood” is the name colloquially given to a solid material having a plurality of magnetic particles provided on a surface of said solid material, embedded within said material or interposed between two sheets of solid material. Typically, the solid material is wood, plywood or other building material. The magnetic particles may be ferritic, or they may comprise cobalt or nickel. 
   The magnetic wood blocks out radio signals, for example, it can block out mobile phone frequencies to prevent mobile phones from ringing in restaurants, theatres, etc. However a major difficulty with magnetic wood is that it also blocks out signals of emergency services. By providing magnetic wood with a controlled particle shape and size distribution, it may be possible to tune it to block only certain frequencies. Emergency services radio signals could then pass through whilst ordinary mobile phone frequencies are blocked. The present invention can be used to manufacture a controlled size and shape distribution of magnetic particles for use in magnetic wood. 
   When producing small particles for “magnetic wood” or other purposes, it is desirable to use a patterned substrate where the pattern comprises a plurality of levels. This allows particles to be closely packed as particles may be provided laterally adjacent to one another, but provided on different levels. For example, to make a plurality of hexagonal shaped particles in the most efficient way, it is desirable to have three different levels formed on the substrate. Every hexagon is surrounded by six other hexagons, each of which may be at one of the two different levels to the central hexagon. The surrounding six hexagons may alternate between these two levels so that neighbouring hexagons are always at different levels. This means that the complete plane can be tiled with hexagon shapes, without any neighbouring hexagons being on the same level. It is then very simple to release all the hexagons together by dissolving the substrate. 
   The material which is deposited to form the free standing structure may be a metal, for example aluminium. The metal may be anodised or otherwise treated to allow a wide range of biochemically active agents to bind to it. Alternatively, the material which is deposited to form the free standing structure may be a non-metal. Some preferred materials for making the free standing structure are Al, aluminium silicon alloy, Al 2 O 3 , Sb, As, Ba, BaTiO 3 , Be, Bi, B, Cd, CdO, CdSe, CdS, Ca, C, CeO 2 , Cr, Co, Cu, Ga, GaAs, GaP, Ge, Germanium oxide, Au, In, indium oxide, Fe, Fe 2 O 3 , Fe 3 O 4 , Pb, Li, LiF, Mg, MgF 2 , Mn, Mo, Ni, nichrome, Nb, Pd, Pt, Se, Si, SiO 2 , SiO, Si 3 N 4 , Ag, Ta, Te, Sn, tin oxide, Ti, TiO 2 , TiO, W, Zn, ZnSe, ZnS and Zr. Combinations of the above materials are also possible. The material chosen will depend to a certain extent on the intended use of the resultant free standing structure. 
   Preferably, the thickness of an aluminium layer deposited onto the substrate would be between 10 nm and 10 μm, and even more preferably, around 1 μm. This preferred thickness range may vary if a different material was used. 
   The deposited material may be deposited onto the substrate by electron beam evaporation. It is also possible to use thermal deposition, whereby the material is heated so that it evaporates over the substrate. However, this will tend to be less useful for producing thicker layers because the amount of material available for deposition is limited. It will also tend to heat the substrate more than in the case of electron beam deposition, and it is undesirable to get the substrate too hot, as it may scorch or burn. 
   The substrate is preferably made of a material which is injection mouldable. Some preferred substrate materials are cellulose acetate (CA), ethyl cellulose, cellulose acetate butyrate, cellulose acetate propionate, acrylonitrile-Butadiene-Styrene (ABS), Nylon (PA), polycarbonate (PC), polypropylene (PP) and polystyrene (PS). Various solvents could be used to dissolve the substrate. 
   A further aspect of the invention provides an apparatus for producing a free standing structure, the apparatus comprising: means for providing a substrate having a raised pattern formed on a surface of said substrate, said raised pattern comprising at least one material which forms said surface; means for depositing material over said raised pattern; and means for dissolving said substrate to release said deposited material to form said free standing structure from said released deposited material. 
   The means for providing a substrate may be a machine to mass produce substrates from a mould, e.g. an injection moulding machine or a machine comprising a mould in the shape of a roller, which continually presses out substrates of the required shape. 
   The means for depositing may comprise an evaporator which can deposit layers of metal film. The evaporator may be an electron beam or thermal evaporator. The means for dissolving said substrate may comprise a spray for washing the substrate with solvent. The apparatus may further comprise a centrifuge or filtration media to remove the deposited material from the solvent. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which 
       FIGS. 1A and 1B  show a cross sectional view of the production of a freestanding aluminium layer to make a barcode structure, according to a first embodiment of the invention. 
       FIGS. 2A and 2B  show a perspective view of the process shown in  FIGS. 1A and 1B . 
       FIGS. 3A-D  are micrographs of real materials made by the process shown in  FIGS. 1A and 1B . 
       FIGS. 4A-D  show a cross-sectional view of the production of a freestanding aluminium layer according to a second embodiment of the invention. 
       FIG. 5  shows a stage in the manufacture of a metal barcode with notches instead of holes. 
       FIG. 6A  shows an example design for an accelerometer, and  FIG. 6B  shows an example design for a motion sensor made according to the method of the invention.  FIG. 6C  shows a SEM micrograph of a metal layer deposited on cellulose acetate, to form a motion detector according to the invention. 
       FIGS. 7A-C  show three example designs for a micro antenna made using the methods of the invention. 
       FIGS. 8A and 8B  show the production of particles using the methods according to the first embodiment of the invention. 
       FIGS. 9A and 9B  are micrographs of the production of disk shaped particles.  FIG. 9C  is a micrograph of a particle of 150 nm diameter, made using the method of the invention. 
       FIGS. 10A-E  are micrographs of the production of 4 μm diameter rings and 2 μm particles. 
       FIGS. 11A , B and C show three stages in the preparation of a mould from which a substrate can be made. 
       FIG. 12  shows the use of the mould to injection mould substrate. 
       FIG. 13  shows the use of a roller type mould to continuously roll out a pattern on a substrate. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A and 1B  are cross sectional views, showing two stages in the production of a freestanding aluminium structure according to the first embodiment of the invention. 
   In  FIG. 1A , a soluble substrate  100  has a raised pattern  102  provided on lower substrate level  104 . The pattern  102  defines an upper level  106  which is separated from the lower level  104  by substantially vertical side walls  108 . 
     FIG. 1A  is a cross section through pattern  102 , such that the pattern  102  appears as a plurality of vertical pillars. In reality the pillars are elongated into the plane of the paper and are connected at their ends to form, in this embodiment, a continuous structure as indicated by the dotted lines. 
   In  FIG. 1A , an aluminium layer has been deposited onto the substrate  100  such that there is an upper aluminium coating  112  formed on upper level  106  and a lower aluminium coating  116  formed on lower level  104 . The aluminium deposition is directional such that no aluminium is provided on the side walls  108 . The side walls  108  cause the upper aluminium coating  112  to be discontinuous from the lower aluminium coating  116 . Due to the overall shape of pattern  102 , the sections of upper aluminium coating  112  are all connected to each other to form an essentially planar structure. 
   In the method of the embodiment described with reference to  FIGS. 1A and 1B , the substrate  100  is dissolved in order to release the upper coating  112  of aluminium. 
     FIG. 1B  shows the stage of the process after the substrate  100  has been partially dissolved to release upper aluminium coating  112 . The substrate is dissolved by placing it in a flowing solvent. The flow direction of the solvent is indicated by the arrow. 
   The raised pattern  102 , on which the upper aluminium coating  112  was deposited, is dissolved away, releasing upper aluminium coating  112 . This leaves areas of exposed substrate  118  remaining together with the lower aluminium coating  116 . 
   The released upper aluminium coating  112  is swept away by the flowing solvent. In this specific embodiment, the upper aluminium coating forms a free standing structure which may be used as a bar code. The structure being essentially planar with a plurality of parallel substantially elongate holes  114  formed therein. 
   In  FIGS. 2A and 2B ,  3 D perspective views are shown, corresponding to the cross sectional views of  FIGS. 1A and 1B  respectively. To avoid unnecessary repetition, like reference numerals will be used to denote like features. 
   In  FIGS. 2A and 2B  the barcode structure is clearly shown, with holes  114  arranged at different spacings along the length of the barcode  112 . When the top layer of substrate is dissolved, the barcodes  112  are released, leaving waste material  116  in place on the substrate. 
   An example of this process in a real material is shown in  FIG. 3A to 3C .  FIG. 3A  shows a SEM photograph of metal strips deposited onto a pattern comprising a plurality of raised ridges  212  formed in the surface of a soluble substrate. The ridges  212  have a width of less than 1 μm. The top of each of the ridges and the surrounding area  216  of lower-lying flat substrate are coated with metal, but the steps  220  on the sides of the ridges are not coated with metal, showing the exposed substrate. 
     FIG. 3B  shows an optical micrograph corresponding to  FIG. 3A . The ridges  212  are shown from above at a lower magnification. It can clearly be seen how different thicknesses of line and different spacings of line may be used to build up a barcode pattern to uniquely identify a tag. 
     FIG. 3C  shows an optical micrograph of the metal strips floating free in solution after the substrate has been dissolved. As described with reference to  FIGS. 1A ,  1 B,  2 A and  2 B, the metal which has been deposited on top of the ridges  212  is released by dissolving the substrate in the solvent. It is these metal strips which are seen floating in the solvent in the micrograph of  FIG. 3C . 
     FIG. 3D  is a micrograph showing a further substrate having diagonal ridges  230  separated by narrow diagonal trenches  232 . The width of each trench  232  is just 360 nm. The total width of a trench  232  plus ridge  230  is 2 microns. An aluminium layer has been deposited on top of the silicon substrate, producing metallic strips. These strips may be extracted from the substrate by dissolving the substrate as previously explained. 
   A second embodiment of the invention is demonstrated in  FIGS. 4A-D .  FIG. 4A  shows a cross-sectional view of the stage of the process before any of the substrate  100  has been dissolved, and corresponds to  FIG. 1A . Again, the substrate  100  has a raised pattern  102  provided on a lower level  104  of substrate  100 . 
   A cross section of the pattern is illustrated in  FIG. 4A . The pattern  102  comprises a plurality of ridges. Unlike the pattern described with reference to  FIGS. 1A ,  1 B,  2 A and  2 B, ridges  130  are not connected to each other. 
   An aluminium layer is deposited onto the substrate, creating upper aluminium coating  130  on top of pattern  102  and lower aluminium coating  132  on lower level  104 . The upper aluminium coating is in the form of a plurality of islands and the lower aluminium coating  132  forms a generally planar structure with elongated holes defined by the ridges that constitute pattern  102 . 
   In this embodiment the higher level of aluminium is the waste material, and the lower level of aluminium is the desired pattern for the free standing structure. 
     FIG. 4B  shows the stage of the process after part of the substrate has been dissolved. In the same manner as  FIG. 1B , the upper aluminium coating  130  is released from the substrate. As this coating is waste, it is washed away. 
   The remaining substrate  100  continues to hold the lower aluminium coating  132 , with the desired pattern, in place. 
     FIG. 4C  shows the stage of the process where upper aluminium coating  130  and the pattern  102  has been removed. The lower aluminium coating  132  with the desired pattern is still attached to the substrate  100 , after the debris has been removed. The holes  138  in the free standing structure  132  can be seen in cross section. 
     FIG. 4D  shows the situation after a further part of the substrate  100  has been dissolved, to release the lower aluminium coating  132  and hence provide the desired free standing structure. The process shown in  FIGS. 4A-D  may be more useful than that shown in  FIG. 1  in the case where the desired structure has a fairly large area compared to the area of waste is removed from it. 
   For example, to produce micro-porous membranes or micro-meshes, it may be much easier to get rid of the waste material first, leaving the remaining aluminium fixed to the substrate and ready to release when necessary. The process shown in  FIGS. 4A-D  may be less useful in the situation where the desired part of the aluminium structure has a small area, and the area of the waste material is large. 
   An alternative design for a bar code structures is shown in  FIG. 5 . The barcode shown here is made from a strip with notches  142  along its sides instead of holes. This eliminates the presence of small pieces of debris when the barcode structure is made. When the substrate is dissolved, waste material from the notches remains attached to the waste material surrounding the strip, therefore is held in place more securely while the barcode itself is released. Some of the notches  142  adjoin a first edge of the strip  140 , and other notches  144  adjoin the opposite edge of the strip. Thus, the barcode forms a “snake-like” structure. 
   A further use of the invention is to make planar test devices, such as motion sensors and accelerometers.  FIG. 6A  shows an example of an accelerometer design. The grey areas show deposited metal and the white areas show spaces between the areas of metal. The accelerometer is made up of a central mass  160  fixed by suspension means  162  to allow it to move from side to side along direction A. The holes  168  allow the substrate to be dissolved immediately underneath the central mass  160 . The central mass  160  has a comb structure  164  attached to it. This is interleaved with a stationery comb structure  166 . When the device is subjected to an acceleration, the distance separating the arms of the two comb structures changes. Thus the capacitance changes, and this can be measured to determine the acceleration. The measurement of acceleration is made more accurate by the fact that the stationary comb structure is divided into four separate parts, each part with a separate electrical contact. Thus, four separate measurements of capacitance can be made. 
   Two parts of the comb structure are on one side of the central mass  160 , and the other two parts are on the other side. This allows the effects of a small sideways movement (perpendicular to direction A of  FIG. 6A ) of the central mass  160  to be compensated for. On each side of the central mass  160 , the two comb structure parts have different positionings with respect to the arms of the central mass  160 . One of the comb structure parts is arranged to increase in separation from the arms of the central weight  160  when a “forward” acceleration occurs along direction A, and decrease when a “reverse” acceleration occurs along direction A. The other is arranged to increase in separation when a “reverse” acceleration occurs, and decrease when a “forward” acceleration occurs. The results of the two measurements can be averaged to give a more accurate acceleration reading. In a device using the structure of  FIG. 6A , means may be provided to measure and compare all four capacitance readings, and use them to calculate an accurate value of the acceleration. 
     FIG. 6B  shows an example of a motion sensor design. The motion sensor has two parallel masses  176  suspended on thin arms  177 . Any accelerating forces will cause the plates to move relative to one another, thereby changing the capacitance. This type of sensor would be useful for detecting whether excessive motion had occurred during shipping of fragile goods, which could be very useful for insurance companies who were insuring the goods. 
     FIG. 6C  is a SEM micrograph showing a real example of a motion sensor made according to the invention. A metal layer has been deposited onto a cellulose acetate substrate. Within the metal layer, two masses have been formed, each of which is suspended on a thin arm. The substrate has not yet been dissolved in the structure shown, but this can be done so that the masses become freestanding. 
   A further device which could be made using the invention is a miniature antenna.  FIGS. 7A  to C show three example of such an antenna. 
   The antenna of  FIG. 7A  comprises a spiral track  251  which generally follows a square pattern. The end of the spiral at its centre is joined to a contact pad  253 . A silicon chip, such as an RF chip, may be attached to the contact pad. 
   The antenna of  FIG. 7B  again comprises a spiral track  251  which follows a generally square pattern spiral shape. The outer end of the spiral is joined to a contact pad  255 . Again, a silicon chip, such as an RF chip, may be attached to the contact pad. 
   The antenna of  FIG. 7C  comprises two tracks  261 ,  263  wound together in a complementary spiral formation. The two tracks are joined at the centre of the spiral such that together the two tracks form a loop. 
   The outermost end of one of the tracks is connected to a first contact pad  265  and the outermost end of the other track is connected to a second contact pad  267 . 
   The antenna may be useful for applications such as smart labels and bluetooth devices. It is also possible to make fractal shaped antennas. These occupy space more efficiently than more traditional designs, and they can be useful for multi-band operation. 
   Yet another use of the method of the invention is for making large numbers of particles of custom size and shape.  FIG. 8A and 8B  show the method of the invention in use to produce of a plurality of micro particles. The metal layer left behind on the substrate forms a micro-porous membrane or a nano-porous membrane. This membrane may be flexible, rigid or semi-rigid. 
     FIG. 8A  shows an example of a substrate  100  shaped with cylindrical pillars  180  for making such particles. When the metal layer  110  is deposited onto the substrate, circular metal particles  182  are formed on top of the pillars.  FIG. 8B  shows the particles being released after the substrate has been dissolved. The remaining sheet of metal left on the surface of the substrate  100  forms a mesh or membrane structure  184 . A mesh made in this way could be used for filtering on a micron scale, or for providing a support platform on which to grow cells. A support platform can encourage growth of cells, enabling them to knit together with higher spatial precision, as required for (e.g.) nerve fibre regrowth or bone regrowth. 
   An example of particle formation, as described in  FIG. 8A  and B, is shown in  FIGS. 9A  and B.  FIG. 9A  shows a substrate with six cylindrical pillars  180 . The substrate and the tops of the pillars have been coated with a metal layer  110 , forming particles  182  on top of the pillars.  FIG. 9B  shows an optical micrograph of the remaining sheet of metal after the substrate has been dissolved. It now has a regular array of holes corresponding to the removed particles  182 , thus forming a net type structure  184 . 
     FIG. 9C  is a micrograph of a particle with a diameter of 150 nm, formed using the soluble substrate technique according to the invention. It is not trivial to make particles of this size. 
   A more complex example of particle formation is shown in  FIG. 10 .  FIGS. 10A and 10B  show an optical photograph and a SEM photograph respectively of a substrate before dissolution. The substrate is shaped with a large number of ring shapes of raised relief, arranged in a grid pattern. The rings have a 4 μm outer diameter and a 2 μm inner diameter. A metal layer is deposited onto the substrate, forming metal rings, metal particles and a metal membrane or mesh with an array of holes. 
     FIG. 10C  shows freestanding 2 μm particles and 4 μm diameter rings after the substrate has been dissolved.  FIGS. 10D  and E show the substrate after the rings and particles have been removed, leaving a membrane with 4 μm holes. 
   Other shapes are also possible for the particles, in addition to the circular shape, and the ring shape. 
     FIG. 11  shows a method of making a mould which is suitable for making the soluble substrates used in the invention. The mould must be accurately shaped, and of very good surface quality. Reactive ion etching, which is a standard method of etching moulds of this type, tends to give rounded corners and irregular surfaces. To avoid this problem, a two stage etching process is used. As shown in  FIG. 11A , a section of mould material  300  is provided, with a sacrificial layer  304  below the surface of the mould material  300  and running parallel to the surface of the mould material. 
   The depth of the sacrificial layer determines the height of features such as pillars which will be formed on a substrate made from the mould. A masking layer  306  is deposited on top of the moulding material, and this masking layer is patterned to leave holes  308 . Reactive ion etching is then carried out on the masked and patterned moulding material to produce etch pits which extend as deep as the sacrificial layer  304 , but are prevented by the sacrificial layer  304  from being etched any deeper. This is shown in  FIG. 11B . Due to the nature of reactive ion etching, the surface quality of the inside of the pit  314  is not good. The surface is rough and the corners  312  may be rounded. A second etching stage is then carried out using a wet etch. The wet etch etches out part of the sacrificial layer directly at the bottom of the pit  310 . Slight undercutting  319  of the mould material surrounding the pit may also occur. A good quality surface is produced at the bottom of the pit and any rounded corners are tidied. 
   This method allows moulds to be produced which have structures on a 10 nm scale. It is not trivial to produce such small structures. The sacrificial layer may be a buried doped layer or may be a layer of a different material to the rest to the mould material. An alternative to using a wet etch is to use another type of edge which is selective in etching the buried doped sacrificial layer. 
   Once the mould has been made, a substrate may be formed by injection moulding or by rolling. 
     FIG. 12  shows the arrangement for injection moulding of a substrate. A mould  350  is shown. The fluid substrate  352  is injected below the patterned surface of the mould, and allowed to harden. The mould is then remove to leave a patterned substrate, having a complementary pattern to that of the mould. 
     FIG. 13  shows production of a patterned substrate by rolling. The mould is shaped as a cylinder with pattern surface  356 . The cylinder is then rolled across the surface of substrate  358 , pressing into it and thus imprinting a pattern of ridges and holes on the substrate. The advantage of this rolling process is that it continuous and the substrate can be produced in a continuous flow.