Injection Moulded Diffraction Gratings on Curved Surfaces. A Breakthrough for a Cost-Effective Industrial Mass-Production
Diffraction gratings and other high-precision optical elements are widely used in sensors, analytic systems, laser-applications and various other fields. However, due the very small structural sizes, even below the µm range, and the high accuracy requirements, their mass-production possibilities are limited to few techniques such as hot embossing or nanoimprint lithography. NIL also has the restriction that such a device is fabricated from different materials: A resist is required for the structures, the macroscopic geometry is normally defined by a blank, and an additional coating might be required.
To date, cheap mass-production of gratings is realised by replication via roll-to-roll on foils or hot embossing on thin substrates. Hence, the structures are on flat and/or elastic surfaces, which limits their application. To obtain rigid optical components, even with curved surfaces, such micro-/nanostructures are normally manufactured on top of a blank substrate. However, thereby two more points have to be considered: firstly, the surface geometry of the blank must be fabricated very precisely, e.g. by cutting glas, and secondly, the macro-/nanostructures have to be added on top without interrupting the surface’s geometry and the grating’s structure. This elaborate process chain causes the prices of such elements to remain relatively high.
Microstructures can also be replicated into plastic by injection moulding as known from the process of optical storage media production like CDs. However, to produce structured optical components, the fabrication of the tool inserts is critical because those micro- and nanostructures cannot be fabricated directly in hard metals, which are required for long lifetimes of the tools. In addition, the demands on the surface fitting, which are often below 1 λ (632 nm), are an exacting piece of work to obtain.
In the presented process chain, a high precision 2.5 dimensional tool insert is made by nickel electroplating of a master structure in the cm range, which bears a concave blazed grating. The nanostructure and macroscopic geometry could further be replicated in plastic by injection moulding and the component was further metallised to act as a mirror. SEM, AFM, and wavefront interferometry determined the accuracy of the replication.
Finally, this highly efficient process chain gives access to micro- and nanostructured plastic components, which can replace expensive glass wares in various applications.