METHOD OF PRODUCING A STRUCTURED SURFACE

Abstract: Clusters of an etchable material such as silver are deposited on a silicon substrate (10) to form peaks (12). The peaks (12) are then at least partially etched away by plasma etching to form cones (14) or pillars of silicon on the substrate (10).

Description: METHOD OF PRODUCING A STRUCTURED SURFACE This invention relates to a method of producing a structured surface and is more particularly concerned with a method of producing an array of silicon projections such as cones or pillars on a substrate. An array of silicon cones produced in accordance with the present invention has potential use, for example, in the manufacture of field electron emitters for use in hot emission lithography (emission of electrons from selected regions of a patterned source for parallel electron lithography, see Popeller et al, Appl. Phys. Lett. (73) 1998), multiple tip scanning probe microscopy or cold cathode electron sources for gas sensors or flat panel displays. Arrays of silicon pillars produced in accordance with the present invention have potential use in optoelectronics where they are expected to exhibit photoluminescence and/or electroluminescence (see Nassiopolous et al, Appl. Phys. Lett. (69) 1996). It is known to produce an array of silicon cones by multi-step lithographic techniques which involve the formation of a patterned photoresist mask using an electron or optical beam, followed by etching and photoresist removal. It is also known from EP-B-0083510 to provide a layer of close-packed spheriodal particles, such as polymeric spheres, electrostatically fixed to a substrate, and then to etch the substrate using the spheres as an etch resist to form an ordered array of columnar posts in the substrate. However, it is difficult to use this technique to produce silicon projections of consistently small size and spacing. It is an object of the present invention to provide a method of producing an array of silicon projections in a convenient and economical manner. According to the present invention, there is provided a method of producing an array of silicon projections on a substrate having a silicon surface, comprising the steps of depositing an etchable material on the silicon surface so as to form an array of peaks of the etchable material on the silicon surface, said etchable material being more resistant to etching than the silicon; and subjecting the surface on which the etchable material has been deposited to an etching operation so that an array of projections of silicon corresponding to the arrangement of the peaks is formed in the surface. Also according to the present invention, there is provided a method of producing an ordered array of silicon projections in a silicon substrate, including the steps of coating a surface of the substrate with a layer of particles arranged in a pattern, depositing an etchable material onto the surface in the spaces between the particles to form an ordered arrangement of peaks of the etchable material on the surface, the etchable material being more resistant to etching than the silicon, and removing the particles from the surface. The surface with the ordered array of peaks formed thereon can then be subjected to an etching operation so that an ordered array of silicon projections corresponding to the ordered arrangement of peaks is formed in the substrate. An array of silicon projections produced in accordance with the present invention (or an array of silicon projections produced by any other method) can be used to form an array of cold cathode electron sources by coating the silicon projections (preferably in the form of cones) with an oxide layer which in turn is covered with a layer acting as an electrode. Application of a voltage above a threshold level between the metal and the silicon projections causes the tips of the projections to emit electrons across the oxide layer. Thus, an electron emitter array can be provided in a particularly convenient and economical manner. The etchable material may be silver which is known to form three- dimensional peaks rather than layers when grown by a sputtering or other depositing technique. Alternatively, it is considered possible to use copper, gold or any other metal or other material capable of growing three dimensionally to form peaks. The sputtering or other deposition technique may be of sufficiently short term to produce the peaks in the form of discrete islands on the silicon surface so that portions of the silicon surface remain exposed between the islands. Alternatively, the sputtering technique may result in peaks separated by lands of thinner etchable material which have to be etched away before etching into the silicon surface can take place. The source of the etchable material for the deposition operation is preferably a cluster source with mass selection so as to enable size-selected clusters of atoms of the etchable material to be deposited onto the silicon surface. For example, it is possible to use a mass selector of the type described in WO97/32336 whose disclosure is incorporated herein by reference. The clusters for size selection may be produced in any manner known per se, for example by heating a source of the etchable material in a chamber so as to evaporate atoms of the source material into a stream of a cold inert gas (e.g. helium) where the material starts to form particle clusters. A self-assembled ordered array of clusters may be used to give an ordered array of projections such as cones. The use of size-selected clusters is particularly advantageous in that this enables a very narrow size distribution of extremely small peaks to be formed on the silicon substrate, thereby making it possible to form an array of extremely narrow projections of even size on the substrate. For example, in the case of silver clusters, control of the cluster size to 4000 atoms per cluster enable projections in the form of narrow pillars ( ie with a height-to-width aspect ratio of greater than 1:1) to be produced. It is considered, however, that such pillars can be produced using as few as 3000 silver atoms per cluster. Additionally, clusters produced by evaporation into a gas stream can be provided on the substrate without any passivation layer thereon. This layer is formed when clusters are produced by growing them in solution, because the growth is stopped chemically using a passivating agent (typically an organic material). With such a solution-growth procedure, not every cluster has exactly the same size. Also, the presence of the passivating layer means that, as the passivating layer is eroded, there is an undesirable change in the etching characteristics of the clusters as the etching step proceeds. The etching operation is anisotropic in nature and is preferably a reactive (chemical) etching operation, and is more preferably effected using a plasma. It is possible to perform this operation using commercially available plasma etching equipment. The etching may be conducted using the gases CF 4 and SF 6 which may be in a ratio of about 1 : 1 by volume. The etching may be effected at a pressure of about 6 mTorr at room temperature. The etching time depends inter alia upon the maximum height or thickness of the islands. The etching conditions can be chosen (e.g. by appropriate selection of rf- power during plasma etching) to produce projections in the form of pillars or cones. By appropriate choice of materials and the etching conditions in the present invention, it is possible to produce projections (eg pillars or cones) which are narrow and have a high height-to-width aspect ratio. For example, it is possible to produce silicon projections less than 10 nm, and preferably less than 5 nm wide, i.e. of a size suitable for obtaining electroluminescence. A high height-to-width aspect ratio of 10:1 to 20:1 is possible which can further enhance electroluminescent properties of the projections. This is in contrast to the much lower aspect ratio of the posts disclosed in EP-B-0083510. It is within the scope of the present invention to perform the etching step in such a way that the tips of the peaks of the etchable material remain unetched, thereby to provide a structure in which the tops of the projections are defined by small regions of the etchable material. Preferably, contact between the substrate and an electrode of the plasma etching equipment is enhanced by the provision of a contact-enhancing material, for example vacuum grease, between these two parts. In the case where an ordered array of silicon projections is required using a layer of particles arranged in a pattern, the layer may be a monolayer of colloidal particles, preferably polymeric spheres, 50 nm to 30 μm in diameter, coated over substantially the entire surface of the substrate, which particles become fixed electrostatically to the substrate arranged in said pattern. Methods of forming an ordered pattern of particles on such a substrate and subsequent removal of the particles are disclosed in EP 0083510 Bl and EP 0270212 Bl, which are incorporated herein by reference. The particles are conveniently carried onto the surface in a liquid carrier medium, and the surface of the silicon may be pre-treated to promote wetting thereof by the liquid carrier medium. Where the medium is an aqueous solution, the surface of the silicon is conveniently pre-treated to render it hydrophilic. A suitable hydrophilic surface may be prepared by growing a layer of oxygen on the surface of the silicon. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figs. 1 to 3 are schematic side views showing various stages in a method according to the present invention for forming an array of silicon cones in a substrate; Fig. 4 is a picture taken with a scanning electron microscope of the array of silicon cones; Fig. 5 is a scanning electron micrograph showing a silicon sample with polystyrene balls arranged in a pattern; Fig. 6 is a scanning electron micrograph showing the sample following the sputtering of silver between the balls and subsequent removal of the balls; and Fig. 7 is a scanning electron micrograph showing hexagonal arrays of silicon pillars formed by etching the sample of Fig. 6. Referring now to Fig. 1, there is illustrated a substrate 10 fabricated from single crystal silicon 111. The upper surface of the substrate 10 has a multiplicity of islands 12 defined by silver atoms grown thereon by plasma magnetron sputtering. The silver islands 12 increase in height and width as the sputtering process proceeds. The sputtering is stopped when the average thickness of the sputtered silver is about 20 nm. Referring now to Fig. 2, the structure illustrated in Fig. 1 is then mounted in a commercial plasma etching machine (Oxford Plasma Technology - type 80 plus) with the underside of the substrate being adhered to the electrode of the machine using a small amount of vacuum grease which ensures intimate contact between the substrate 10 and the electrode. Plasma etching is effected at 10 °C under a pressure of 6 mTorr using a rf power of 150 W in an atmosphere consisting of a 1:1 by volume mixture of CF 4 and SF 6 at a total gas flow rate of 50 seem. The plasma etching is continued for about 1.5 minutes until the silver islands 12 have only just been completely sputtered away, after which etching is immediately stopped. The exposed areas of the silicon layer are etched away during this procedure at a greater rate than the rate at which the silver is etched away, with the result that an array of silicon cones 14 is formed in the surface of the substrate 10 (see Figs. 3 and 4). The heights of the silicon cones is thus determined by the time taken to etch the silver islands away. The tips of the cones 14, ie those areas which were under the highest points of the silver islands 12, have a radius of about 10 nm in this embodiment. The cones 14 have an average base width of 50 nm and an average height of 150 nm. The resultant structure may then be coated with a thin layer of an electrically insulating oxide, ( eg silicon dioxide), followed by a metal electrode layer (eg silver) to produce an array of cold cathode electron sources. The shape of the silicon material remaining after etching can be controlled by control of the rf-power during the plasma etching. Effecting plasma etching at rf-powers in the range of from 80W up to 200W, with the other conditions being as described above. Etching using a rf-power in the range of about 80W to HOW resulted in the formation of silicon pillars. Above HOW, the resultant silicon projections remaining after etching had a conical shape with the angle between the side walls increasing as the etching power increases. At a power of about 200W, the cone angle is about 30°. Various ways of producing ordered arrays of projections will now be described. In one method embodying the invention, an aqueous solution containing polymer (e.g.polystyrene) balls about 500 nm in diameter is dropped onto an oxygen terminated silicon surface of a silicon sample. Oxygen terminated in this context means a surface on which a layer of oxygen has been chemically grown to thereby render the surface hydrophilic, promoting an even spread of the solution over the surface. The solution with the balls is then dried very slowly by carefully evaporating off the water. This method produces a slightly imperfect monolayer of balls. It is, however, possible to achieve more perfect monolayers when desired using alternative methods. For example, one such alternative method of layering the balls involves adding a surfactant to the solution and spin coating the balls onto the surface, as disclosed in "Nanosphere Lithography - A Material Generals Fabrication Process For Periodic Particle Array Surfaces", J. Vac , Sci. , Technol., A, 103:1553, 1995 by J. C. Hulteen and R. P. van Duyne. Another alternative method is disclosed in "Colloid Monolayers As Versatile Lithographic Masks", Langmuir, 13:2983, 1997 by F. Burmeister et al, involving production of a monolayer on a glass slide and, after annealing, floating off the layer in order to transfer it onto the surface. Alternative methods suitable for drying the solution coated on the silicon surface are disclosed in "From Mesoscopic To Nanoscopic Surface Structures: Lithography With Colloid Monolayers", Adv. , Mater., 10:495, 1998 by F. Burmeister et al. The dried balls are arranged in close packed hexagonal patterns, as clearly seen in Fig. 5. Silver is then deposited onto the sample by sputtering performed at 50 W, 226 N, for 11 minutes at a pressure of 7 x 10 2 mbar. Another etchable material capable of growing three dimensionally to form peaks, for example gold, could be used instead of silver. Also, the material could be deposited in some other fashion, for example by evaporation. The silver reaches the surface of the silicon sample only at the gaps between the balls. A cluster source with mass selection may be used as described in above to enable size-selected clusters of etchable material to be deposited in the gaps. The balls are then dissolved in chloroform in an ultrasonic bath to leave silver peaks arranged in hexagonal patterns on the surface of the silicon sample, as shown in Fig. 6. Alternatively, the balls can be cleaved to remove them from the surface. The desired regular hexagonal arrays of silicon projections, shown as pillars in the sample of Fig. 7, are then formed in the silicon substrate by reactive ion etching at 100 W for 2 minutes 45 seconds with SF 6 and CF 4 (25 seem and 25 seem). This etching process involves both chemical and physical (sputtering) etching effects. As can be seen from Fig.7 in particular, the pillars are very narrow and have a very high height-to-diameter aspect ratio which is the range of 10:1 to 20:1.

Claims: CLAIMS 1. A method of producing an array of silicon projections on a substrate having a silicon surface, comprising the steps of depositing an etchable material on the silicon surface so as to form an array of peaks of the etchable material on the silicon surface, said etchable material being more resistant to etching than the silicon; and subjecting the surface on which the etchable material has been deposited to an etching operation so that an array of projections of silicon corresponding to the arrangement of the peaks is formed in the surface. 2. A method as claimed in claim 1, wherein the etchable material is selected from silver, copper, gold or any other metal capable of growing three dimensionally to form peaks. 3. A method as claimed in claim 1 or 2, wherein the etchable material is provided by evaporation of the etchable material into a gas stream to form clusters. 4. A method as claimed in claim 1, 2 or 3, comprising the step of providing the etchable material for the deposition operation using a cluster source with mass selection so as to enable size-selected clusters of atoms of the etchable material to be deposited onto the silicon surface. 5. A method as claimed in any preceding claim, wherein the etching operation is a plasma etching operation. 6. A method as claimed in any preceding claim, wherein the etching conditions are chosen to produce projections in the form of pillars or cones having a height-to- width aspect ratio of 10: 1 to 20:1. 7. A method of producing an ordered array of silicon projections in a silicon substrate, including the steps of coating a surface of the substrate with a layer of particles arranged in a pattern, depositing an etchable material onto the surface in the spaces between the particles to form an ordered arrangement of peaks of the etchable material on the surface, the etchable material being more resistant to etching than the silicon, and removing the particles from the surface, and then subjecting the ordered arrangement of peaks to an etching operation so that an ordered array of silicon projections corresponding to the ordered arrangement of peaks is formed in the substrate. 8. A method as claimed in claim 7, wherein the etchable material is provided by evaporation of the etchable material into a gas stream to form clusters. 9. A method as claimed in claim 7 or 8, further comprising the step of providing the etchable material for the deposition operation by a cluster source with mass selection so as to enable size-selected clusters of atoms of the etchable material to be deposited onto the silicon surface. 10. A method as claimed in claim 7, 8 or 9, wherein the layer of particles is a monolayer of colloidal particles coated over substantially the entire surface of the substrate. 11. A method as claimed in any one of claims 7 to 10, wherein the particles are carried onto the surface in a liquid carrier medium, and the surface of the silicon is pre-treated to promote wetting thereof by the liquid carrier medium. 12. A method as claimed in claim 11, wherein the liquid carrier medium is an aqueous solution, and the surface of the silicon is pre-treated to render it hydrophilic. 13. A method as claimed in claim 12, wherein the surface is rendered hydrophilic by growing a layer of oxygen on the surface of the silicon. 14. A method of producing an array of cold cathode electron sources comprising the steps of forming an array of silicon projections on a substrate; coating the projections with an oxide layer, and covering the oxide layer with a layer acting as an electrode. 15. A method as claimed in claim 14, wherein the array of silicon projections is formed by a method as claimed in any one of claims 1 to 13.

Inventors: PALMER RICHARD EDWARD, SEEGER KATRIN, PALMER, RICHARD, EDWARD, SEEGER, KATRIN
Applicants: PALMER RICHARD EDWARD, SEEGER KATRIN, UNIV BIRMINGHAM, PALMER, RICHARD, EDWARD, SEEGER, KATRIN, THE UNIVERSITY OF BIRMINGHAM
Application year: 1999
Main classification: H01L21
IPC codes: H01J9, H01L21