Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y15/00—Nanotechnology for interacting, sensing or actuating, e.g. quantum dots as markers in protein assays or molecular motors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
  • H01J9/02—Manufacture of electrodes or electrode systems
  • H01J9/022—Manufacture of electrodes or electrode systems of cold cathodes
  • H01J9/025—Manufacture of electrodes or electrode systems of cold cathodes of field emission cathodes

Definitions

• 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).
• EP-B-0083510 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.
• spheriodal particles such as polymeric spheres
• 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.
• 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 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.
• 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.
• 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.
• 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.
• 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 cold inert gas e.g. helium
• a self-assembled ordered array of clusters may be used to give an ordered array of projections such as cones.
• 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.
• 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.
• 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.
• projections eg pillars or cones
• 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.
• 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.
• a contact-enhancing material for example vacuum grease
• the layer may be a monolayer of colloidal particles, preferably polymeric spheres, 50 nm to 30 ⁇ m in diameter, coated over substantially the entire surface of the substrate, which particles become fixed electrostatically to the substrate arranged in said pattern.
• 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.
• 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.
• 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.
• Fig. 7 is a scanning electron micrograph showing hexagonal arrays of silicon pillars formed by etching the sample of Fig. 6.
• 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.
• 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.
• an electrically insulating oxide eg silicon dioxide
• a metal electrode layer eg silver
• 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°.
• 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.
• 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.
• 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.
• 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.
• 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.

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• 1999
  • 1999-05-20 EP EP99922377A patent/EP1088341A2/de not_active Withdrawn
  • 1999-05-20 JP JP2000551425A patent/JP2002517087A/ja active Pending
  • 1999-05-20 WO PCT/GB1999/001606 patent/WO1999062106A2/en not_active Application Discontinuation

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