Optical components such as lenses and mirrors are found in all sorts of devices. There is a great need for high precision optical devices with very flat surfaces (roughness of less than a nanometer) to minimise optical defects and increase detector resolution, especially in scientific instruments. Other areas in which precise polishing is required include the biomedical and semiconductor industries. Current technologies for shaping and polishing ultra smooth (optical) surfaces include: ion plasma etching, which requires high power and expensive equipment; the polishing with ferromagnetic fluids, which is unsuitable for delicate components due to high levels of strain; and chemical mechanical polishing, which uses hazardous chemical etchants.
What it does
We have developed a method combining mechanical and electrochemical polishing. It can be used to polish conductive and semiconducting materials to a specified surface profile down to a roughness of around 20 Angstrom (rms). The material may then be further polished using purely mechanical means down to a roughness of 3 - 4 Angstrom (rms).
How it works
To achieve this, we use a non-conductive polishing lap placed on the surface of a semiconducting/conductive material (see Fig. 1). A layer of electrolytic fluid is sandwiched between the target material and an electrode. The electrolytic fluid is repelled from the electrode when current is supplied to the electrode and so bombards the surface of the material, etching and polishing it in the process. By moving the electrode and altering the applied current, the desired surface profile is created.
Fig. 1: A profile view of the polishing setup. A target material to be polished (100) is placed on top of a polishing lap (32). An electrode (38) is embedded in the lap with a region of electrolyte sandwiched between this and the target (36). A current flows between the electrode and the target, while the polishing lap rotates to create an even, smooth surface of the target material.