NOVELTY - Etching silicon (m1) involves: electrolessly depositing first metal onto silicon surface (103) to be etched; depositing a second metal that is different from first metal over the surface and the first metal, where a film (107') of deposited second metal covers silicon surface to be etched; removing the metals from regions of the film of deposited second metal that overlie the first metal to leave the second metal partially covering the surface to be etched; and etching silicon by exposing the surface to an aqueous etching composition comprising an oxidant and source of fluoride ions.    USE - For etching silicon which is used for forming an electrode that is useful in a rechargeable metal ion battery (i.e. a lithium ion battery) (claimed). The lithium ion batteries are extensively used in portable electronic devices such as mobile telephones and laptops, and are finding increasing application in electric or hybrid electric vehicles.    ADVANTAGE - The presence of second metal enhances conductivity and connectivity of the anode and disrupts or reduces formation of silicon-electrolyte interphase layer. The removal of the first metal and overlying second metal and the etching of the silicon surface is carried out in a single step. The present method is an efficient process for etching silicon to produce structured silicon. At least two steps of the process are carried out in a single reaction vessel which provides for an efficient process and avoids the potential problem of oxidation of the silicon surface if silicon is exposed to air between stages.    DETAILED DESCRIPTION - Etching (m1) silicon involves: electrolessly depositing a first metal onto a silicon surface (103) to be etched, where the electrolessly deposited first metal partially covers the surface of the silicon to be etched; depositing a second metal that is different from the first metal over the silicon surface and the electrolessly deposited first metal, where a film (107') of the deposited second metal covers the silicon surface to be etched; removing the first metal and the second metal from regions of the film of the deposited second metal that overlie the first metal to leave the second metal partially covering the silicon surface to be etched; and etching the silicon by exposing the silicon surface to an aqueous etching composition comprising an oxidant and a source of fluoride ions. The electrolessly deposited first metal forms islands (105) on the silicon surface to be etched. At least some of the metal islands are connected by bridges of the first metal. At least some of the bridges are removed prior to deposition of the second metal. The electroless deposition of the first metal involves exposing the silicon surface to be etched to an aqueous composition comprising ions of the first metal and a source of fluoride ions or an alkali. The second metal is deposited by electrodeposition in an electrodeposition bath containing an electrolyte comprising a source of the second metal. INDEPENDENT CLAIMS are included for the following:    (1) etched silicon (s1) obtainable by the method (m1);    (2) an electrode comprising an active material of etched silicon (s1), and further comprising a conductive current collector in electrical contact with the active material;    (3) forming the electrode involving depositing a slurry comprising an etched silicon powder formed according to the method (m1) and at least one solvent onto a conductive substrate or current collector, and evaporating the solvent; and    (4) a rechargeable metal ion battery comprising an anode comprising the electrode capable of inserting and releasing metal ions; a cathode formed from a metal-containing compound capable of releasing and reabsorbing the metal ions; and an electrolyte between the anode and the cathode.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of etching bulk silicon.    Silicon substrate (101)    Surface (103)    Islands (105)    Exposed regions of surface (107)    Film (107')    Pillars (109)    Etched surface (111)