The fabrication of aspherical microlenses using focused ion-beam techniques

•We describe a new method for ion-milling bespoke profile geometry; using it to produce parabolic lens moulds.•We discuss a technique for removing ion-beam damage using chemical etching.•Using ion-range modelling we demonstrate how increasing the dose when ion milling enhances the damage removal properties of wet etching.•We visibly show the massive extent of focused ion-beam tails, seen to be up to 50μm in radius.•We manufacture parabolic micro-lenses using our technique. Aspheric lenses are the most common method for correcting for spherical aberrations but, in microlens production, highly-controlled lens profiles are hard to achieve. We demonstrate a technique for creating bespoke, highly-accurate aspheric or spherical profile silicon microlens moulds, of almost any footprint, using focused ion-beam milling. Along with this, we present a method of removing induced ion-beam damage in silicon, via a hydrofluoric acid etch, helping to recover the surface's optical and chemical properties. In this paper, we demonstrate that our milled and etched moulds have a roughness of 4.0–4.1nm, meaning they scatter less than 1% of light, down to wavelengths of 51nm, showing that the moulds are suitable to make lenses that are able to handle light from UV up to infra-red. Using empirical experiments and computer simulations, we show that increasing the ion-dose when milling increases the amount of gallium a hydrofluoric acid etch can remove, by increasing the degree of amorphisation within the surface. For doses above 3000μC/cm2 this restores previous surface properties, reducing adhesion to the mould, allowing for a cleaner release and enabling higher quality lenses to be made. Our technique is used to make aspheric microlenses of down to 3μm in size, but with a potential to make lenses smaller than 1μm.