Applications of LIGA technology to precision manufacturing of high-aspect-ratio micro-components and -systems: a review

An extreme high fill-factor microlens array mold insert in photoresist fabrication using a thermal reflow process is presented. The experimental results proved that a square microlens array could be produced without a peripheral gap. A square microlens array with an extreme high fill-factor (almost 100%) was successfully fabricated. In this experiment, square photoresist columns were formed on a silicon substrate using a lithographic process. The square pattern was laid out in an ortho-square on a polyethylene terephthalate (PET) based mask. Precise temperature and time control was used during the thermal reflow process. The square microlens array was formed from the uniformly flowing melted photoresist. The photoresist column surface transforms into a spherical profile due to the surface tension effect. The error was within ±8% between the fabricated microlens characteristics and the theoretical model used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized high fill-factor square microlens arrays.

https://iopscience.iop.org/article/10.1088/0960-1317/14/8/012/pdf

High fill-factor microlens array mold insert fabrication using a thermal reflow process

Micro-machined tools and component holders useful for the manufacture, assembly (196), and testing of precision micro-miniature parts are provided, particularly embodiments well adapted for precision micro-miniature systems incorporating optical and/or electronic components including lenses (233, 397), optical fibers (197), and the like. Structure and method find particular applicability in the precision optical systems of the read, write, or read/write heads of optically assisted storage devices for digital computers. Removable tools are used to temporarily position while components are glued or otherwise fixed into place during assembly, and then completely removed for use with another head assembly. Break-away tools or holders (326) typically include three major portions: an element holder, a handle (313, 314), and a break-away neck portion connecting the holder with the handle. The element holder provides a mounting block (395) for an optical, electrical, or mechanical element. Once the elements are attached to the mounting blocks, they are moved into position within the larger assembly via the handle. The mounting block (395) is glued to the assembly and a small predictable force is applied to the handle to break the neck and separate the handle from the holder which is at that time attached to the assembly. Tools may be formed by photolithographically patterned and etching a substrate such as silicon to selectively carve the substrate. Structures formed from plated metal, either on a base material, or made entirely of metal are also provided by the invention. Several embodiments of process for forming the inventive structures are also provided.

Structure for micro-machine optical tooling and method for making and using

The LIGA process, which is based on deep-etch X-ray lithography, electroplating, and moulding allows the commercial fabrication of three-dimensional microstructures. The actual state of the art of the LIGA process is summarized. This contains new process developments, like the possibility of realizing movable, stepped or inclined microstructures as well as the availability of new materials like nickel-cobalt alloys or ceramics. In the second part, applications of LIGA structures demonstrating the characteristic LIGA properties are presented. Examples are given for the fields of sensors, actuators and communication technologies.

http://iopscience.iop.org/article/10.1088/0960-1317/2/3/004/pdf

The LIGA technique-what are the new opportunities

This invention concerns a process for producing a fiber optic circuit switch with at least two light guides in one plane and a moving optical switching element (7) made out of a sandwich wafer (39). In a first operation, the covering layer (43) is masked in such a way that after this step preferably movable partial areas (45) exposed transversally, particularly of the switching element (7), have a narrow width and switching partial areas (47) affixed to the base substrate (25) have a second width larger than the first width. In a second operation, the open areas of the masked covering layer (43) are etched in a dry-etching process with a deep anisotropic reactive ion etching forming nearly vertical etching walls up to an amorphic intermediate layer (41). In a third operation, an etching agent which only attacks the intermediate layer (41) is used and the etching time selected so that the intermediate layer (41) is only etched clear underneath the partial areas (45) with the first width, but not underneath the partial areas (47) of the second width. The optic fiber ends are arranged so closely together that the switching element (7) can still just be inserted between them, yet upon removal of the switching element (7), radiation coupling from one optic conductor point into the one located opposite takes place without any assistance of additional optical imaging elements.

A fiber optic circuit switch and a process for its production

Deep-etch x-ray lithography allows the fabrication of micro-optical components with critical lateral dimensions of a few micrometers and structural heights up to several hundred micrometers. Using interferometric methods, the structural tolerances of the side walls of fabricated microstructures are shown to be about 0.3 micrometers per 300 micrometers of structural height. A parallel HeNe laser beam is easily passed through several microprisms, which is evidence for the high precision achievable in positioning micro-optical components on a baseplate. Focussing a light beam by cylinder lenses is also possible. The free two- dimensional shape and the high precision achievable in positioning allow fabrication of micro- optical components with integrated fiber fixing grooves in which multi-mode fibers are precisely positioned. Simple fiber connectors, e.g., fiber forks or Y-couplers, can be achieved. By using a light-guiding resist system, divergence losses can be diminished.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Characterization of micro-optical components fabricated by deep-etch x-ray lithography

The LIGA process is used to fabricate micro-optical benches which allow to mount hybridically active and passive optical components with very high precision and without active alignment. Moreover, also micro mechanical structures like electro-mechanical actuators are fabricated on the same substrate. To avoid any lateral misalignment al fixing structures in the optical bench are produced in the lithography step. Due to the high precision of x-ray lithography lateral tolerances are in the range of 0.1 to 0.2 micrometers depending on thermal distortions. Thus, optical losses for these components are rather small. The potential of the free space concept based on LIGA technology for the fabrication of devices for optical telecommunication has been demonstrated by a bi-directional transceiver module as well as an optical bypass. In the case of the optical bypass element, a movable mirror is fabricated on the substrate together with the fixing elements. This movable mirror is the end face of an electro-static actuator which allows to move the mirror into the collimated light beam between two fibers and thus, change the direction of the light. For the first prototypes the losses in the beam without mirror are about 1.7 dB, whereas the losses in the deflected beam are about 4.5 dB.

Micro-optical and optomechanical systems fabricated by the LIGA technique

Microtechnology offers the possibility to fabricate photonic devices. Especially the LIGA technique allows the fabrication of micro-optical benches for free-space optical set-ups, with mounts for inserting hybrid components structured to micrometer accuracy well aligned to each other. These optical benches are not only used to build up passive optical devices but also opto-mechanical systems have been realized. Also concepts to integrate active optical components like laser and photo diodes have been tested. In the paper the concept and the technology to fabricate hybrid micro-optical systems is described. Two systems, a bi-directional transceiver module and an opto- mechanical by pass switch have been fabricated and evaluated. Their performance data prove the ability of the concept of micro-optical functioning modules based on free- space micro-optical systems fabricated by LIGA technique and assembly.

Micro-optical devices based on free-space optics with LIGA micro-optical benches: examples and perspectives

Deep etch lithography of PMMA by the LIGA-technique or H+ lithography is a powerful tool for the fabrication of very high refractive microprisms. We have realized a microintegrated optical imaging system using microprisms fabricated by the LIGA-process and microlenses with a diameter of 250 micrometers . Thus the size of the imaging system is less than 2 mm+.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Jost Goettert

Papers

Characterization of micro-optical components fabricated by deep-etch x-ray lithography

Micro-optical and optomechanical systems fabricated by the LIGA technique

Micro-optical devices based on free-space optics with LIGA micro-optical benches: examples and perspectives

Micro-optical setup with microlenses and microprisms based on refractive optics