Owing to the demand from fast growing optical and opto-electronic industry, optical design has advanced from merely planar/spherical to aspheric and to free-form (non-axial symmetrical) in shapes. On top of the increasing complexity in shape, the requirement for form accuracy is getting higher. Micro-lens array (MLA) is one of the typical examples of these difficult-to-produce optical components which are in great demand. MLA has now been widely used in wafer level optics, lighting system, optical fiber coupling devices, brightness enhancement system of LCD, pico-projector, lens of CIS module scanner and etc. Several processes, namely, thermal reflow process, energy beam processing, ultra-precision machining, LIGA/LIGA-like, have been successfully developed to fabricate MLAs. Amongst those processes, ultra-precision machining is considered to be able to achieve higher form accuracy, better surface roughness, and to offer greater flexibility in material/shape selection.
Three ultra-precision machining processes namely fast tool servo, slow tool servo and diamond milling, are frequently used to produce MLA. Though slow tool servo has the advantages of no extra attachment and fast setting-up, the complicated three dimensional tool shape compensation and tool-path generation are major reasons for resulting in poor form accuracy, pre-matured tool failure and terrible surface finish. This research aimed to develop a model of three dimensional tool shape compensation for generating 3D tool path in slow tool servo diamond turning of symmetrically arranged MLAs such as those in wafer level optics.
An aspheric MLA of 100% filling factor with form accuracy (p-v) and surface roughness (Ra) better than 0.2 μm and 5nm respectively was successfully produced in the present study. The uniformity of each micro lens was less than 0.05μm. Apart from turning tests, 1D/2D slow tool servo diamond shaping tests were also investigated in this research. The MLA of scanner with form accuracy of 0.2 μm, Ra of 5nm, uniformity of each micro lens ≤ 0.05 μm and sagittal error ≤ 2 μm was generated by the developed tool generation algorithm and 2D slow tool servo diamond shaping process.