Doped silicon structure with impression image on opposing roughened surfaces

摘要

A silicon structure is formed that includes a free-standing wall having opposing roughened inner and outer surfaces using ion implantation and an unimplanted silicon etching process which is selective to implanted silicon. In general, the method provides a recess in a layer of insulating material into which a polysilicon layer is formed. A layer of HSG or CSG polysilicon is subsequently formed on the polysilicon layer, after which ions are implanted into both the layer of HSG or CSG polysilicon and the underlying polysilicon layer. The aforementioned selective etching process is then conducted to result in a relatively unimplanted portion being etched away and a highly implanted portion being left standing to form the free-standing wall. The free-standing wall has an inner surface that is roughened by the layer of HSG or CSG polysilicon. The free-standing wall also has a roughened outer surface to which has been transferred a near-impression image topography of the opposing inner surface. The near-impression image topography of the outer and inner surfaces are due to the grains of the layer of HSG or CSG polysilicon which, during ion implantation and selective etching, transfer the topography of the inner surface to the outer surface so as to also roughen the outer surface. One preferred etching process uses an etchant comprising a selected volume of tetramethyl ammonium hydroxide in solution, which etches unimplanted silicon up to 60 times faster than implanted silicon. A capacitor storage node formed with the method has an increased surface area electrically connected with an underlying silicon substrate.