Structure origination and replication

The origination of master microstructures on large areas is still not very well established. Within the field of mechanical, electrical and optical microsystems origination techniques such as e-beam writing, laser writing, focused ion beam etching, photo or x-ray lithography are widely used and mature. Unfortunately, many of them are not suited to originate well defined continuous surface-relief profiles and are especially not suited to originate the mi­crostructures on large areas homogeneously. So far, mainly ultra-precision machining and interference lithography are used as origination techniques for homogeneous large-area master structures.

Ultra-precision machining is a technique where the classical machining techniques such as turning, drilling, milling, and cutting are performed by using ultra-precision machines, dia­monds as tools and metals as material. The typical dimensions of microstructures which are made by ultra-precision machining are in the range of 10 pm to 500 pm.

Interference lithography makes use of the interference pattern which is formed when two or more coherent light waves are superposed. In a typical optical set-up, a laser is used as a source for ultra-violet (UV) radiation. The laser beam is split into two beams. Each of the beams is directed by mirrors towards a substrate coated with photoresist where the beams are superposed after being expanded. In Fig. 2, a photo of one of the interference lithog­raphy laboratories at Fraunhofer ISE is shown. When the process is sufficiently controlled also very demanding surface-relief structures can be originated by single or multiple expo­sures (Fig. 3). Of course, origination of such structures on large areas is still a technologi­cal challenge and not every exposure gives the required result.

The master structures cannot be used as embossing tools directly in the case of photore­sist and are not often used for cost reasons in the case of machined metal masters. The standard process chain includes therefore the replication of the master structures by elec­troforming into nickel. When photoresist master structures are used, a thin conducting layer is deposited by evaporation, sputtering or by the wet chemical reduction of silver firstly. Then, nickel is grown with a thickness in the range of 50 pm to 3 mm by using nickel sulphamate solutions on top of the master structures. This first nickel replica is then sepa­rated from the orginal. After passivation, the first nickel replica is copied by electroforming again. By applying the process repeatedly, several generations of so-called nickel shims can be produced without too much loss in the structural details.

The daughter generations of the nickel shims are used for replicating the micro structures. For polymers a large variety of mature replication techniques exists, e. g. hot compression molding, injection molding, and reactive processes including radiation curing. The latter are especially suited for high-volume large-area applications.

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