Ion beam figuring for precision optics

Abstract

To enhance the capabilities of high-precision optics, the ion beam figuring (IBF) technique has been used for nearly 20 years by a few laboratories and companies over the world. First demonstrated by Wilson et al.1, this type of figuring would later be used on a wider scale to treat various optical materials. The process derives from the creation of local physical etchingsmade through the collision of accelerated ions with target atoms (the so-called sputtering process), and is accomplished through rastering an ion beam with appropriate computed velocities across the workpiece. As a result, a determined profile is etched. Following other mechanical polishing methods, IBF is usually performed as the final step to remove the last long spatial wavelength surface errors (hundreds of nanometers). The main advantages of figuring in this manner are the following: it is a deterministic method (non-iterative and therefore timesaving), it does not require contact when performed (useful for lightweight or ultra-thin substrates), and it allows for figuring exotic shapes. The main constraint is that it requires a vacuum environment for operation. Surface roughening2 and heating3 can also restrict the applicability or performance of the technique. At the Centre Spatial de Liege (CSL), we have been using and developing the IBF technique for more than a decade in order to correct small to medium optics (ranging from 25 to 200 mm in diameter). The facility includes a 1.5 m3 vacuum chamber located in a 10,000-grade clean room.4 The five axes driving the ion source and the substrate mount are software controlled in such a way that the ion beam always scans the workpiece at normal incidence, constant distance, and appropriate velocity. In addition, equipment has been upscaled for a local company to treat optics up to 1-meter diameter. More recently, our ion beam system has also been used to texture surfaces in a deterministic way. Efficient IBF correction of optics requires a beam removal function (the etching profile of the ion beam in the material Figure 1. Typical etching profiles measured on chemical vapor deposited (CVD) silicon carbide for two different ion sources (Kaufman and end-Hall). For the Kaufman ion source, three different profiles are given: unmasked and masked beam with 11 mm and 4 mm aperture (mask 1 and 2, respectively).