NOVELTY - A layer of single-crystal silicon is produced by: forming parallel, first trenches in a silicon substrate of (111) orientation; forming a series of narrower second trenches inside the first trenches; etching lateral channels from the second trenches until adjacent and opposing etch fronts meet; and detaching the layer from the substrate.    USE - For producing a layer of single-crystal silicon for a solar cell having a length of 20-50 mm, a width of up to 5 (preferably up to 2) mm, and a thickness of up to 0.1 mm (preferably up to 70 microns) and having a pair of electrical contacts on one face of the cell (claimed).    ADVANTAGE - The invention allows ordinary silicon wafers to be processed to produce thin rectangular strips of silicon or a silicon sheet, where the thickness of the strips or the sheet is smaller than the wafer thickness. The individual strips can be processed into solar cells and then detached from the substrate. Following this process, a layer of substrate is removed which is much thinner than can be achieved by conventional cutting processes, with less waste than using conventional processes. Because the substrate has been reduced in thickness by an amount less than the original substrate thickness, the remainder can be re-used for the production of further silicon strips or sheet. At least two thin layers for use, e.g., in solar cells may be obtained from a single silicon wafer.    DETAILED DESCRIPTION - Producing a layer of single-crystal silicon comprises:    (1) forming parallel, first trenches in a silicon substrate of (111) orientation;    (2) applying an etch-resistant layer to the surface of the substrate;    (3) forming a series of narrower second trenches inside the first trenches;    (4) contacting the second trenches with an etchant to which (111) planes (17) of silicon are resistant, and etching lateral channels (4) from the second trenches until adjacent and opposing etch fronts (16) meet, to define the layer of single-crystal silicon; and    (5) detaching the layer from the substrate.    An INDEPENDENT CLAIM is also included for a sheet of silicon having a thickness of up to 100 microns and including parallel rows of slots arranged endwise and offset with respect to slots in an immediately adjacent row.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a silicon wafer following etching with a wet anisotropic etchant.    Lateral channels (4)    Strips (5)    Etch fronts (16)    (111) planes (17)