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Polymer micro optics: because size matters

24 Apr 2009

Developments in the manufacture of polymer micro optics are helping optical designers to meet the exacting tolerances that are required for miniaturized optical components and subsystems. Heidi Hall of Jenoptik Polymer Systems takes a closer look.

Micro optics of all types are coming into their own as the relentless push towards compact photonic devices and subsystems continues across markets as diverse as automotive, lighting, biomedical and solar. The advantages are clear. Smaller part diameters allow for shorter focal lengths and smaller, lighter-weight optical systems. What's more, micro optics such as diffractives and microlens arrays enable applications with custom output patterns and enhanced illumination homogeneity.

While commonly used, the term micro optics can mean different things to different people. One common definition covers any optical component with overall dimensions smaller than 2 mm. This includes lenses, prisms, beamsplitters and other typical optical components. To others, micro optics refers to optics of a larger overall dimension (on the multimillimetre scale) but with small optical features within the part. These optics could be diffractive elements with features on the micron scale or refractive lens arrays with the aperture of each array element being less than a millimetre.

While both types of micro optics can be made from various materials (deep UV to IR), the Jenoptik Optoelectronic Systems business unit focuses on plastic micro optics. The ability to create these components in plastic opens up the possibilities for high-volume, low-cost systems with high performance.

Optics with small dimensions
As the size of the optic decreases, mechanical tolerances on parameters such as centre thickness, outer diameter and centre-to-centre displacement (CCD) of the front and back surfaces become even more critical. Consider a lens with a focal length of approximately twice its diameter (approximately f/2). For a 2 mm diameter lens, the focal length is 4 mm. The radii of curvature are likely to be 4 mm or larger with a centre thickness in the range of 1–2 mm.

Compare this with a lens with a diameter of 50 mm, a focal length of 100 mm, radii of curvature of 100 mm or more and a centre thickness of 10 mm. For a centre thickness tolerance of ±20 µm, the error is roughly 1% for the micro optic but only 0.2% for the larger optic. Absolute tolerances that are easily obtainable with the manufacture of larger optics become significant considerations as the lens size decreases. The CCD tolerance also becomes more sensitive, with typical tolerances of the order of 20–50 µm. This is in the range of 2–10% of the semi-diameter of micro optics and leads to a displacement of the optical axis by approximately 10–20 arc min. A centration error of this magnitude is unacceptable in most parts.

Here at Jenoptik Optoelectronic Systems, our engineers have developed proprietary tooling technologies that allow for precise alignment of the front and back optical surfaces to an accuracy of 2–3 µm. This technology can be applied to optics of all sizes, but becomes especially important in the case of micro optics.

The mould insert that creates the optical surface for the part is typically manufactured using a single-point diamond- turning machine. The optical surface of the insert as well as the optical surface of the finished part can be measured by profilometry or interferometry, both of which offer precision measurements with accuracy in the micron to submicron range. A few fringes of error in surface shape can be measured and corrected as easily on a small aperture as a larger aperture. As demonstrated earlier, though, it is not the tolerances on optical surfaces that challenge conventional techniques, but the tolerances when translated down to the micro optic that cause the greatest manufacturing challenges.

For injection moulding, process control is important in maintaining accurate and repeatable part tolerances from run to run, and over the manufacturing lifetime of the part. Control of shot size (the volume of polymer resin that is injected into each moulding cycle) becomes critical when the part is small. A polymer lens with a diameter of 2 mm and a thickness of 1 mm will have a volume of approximately 0.002 cm3 and weigh approximately 2.5 mg. Precise control of the shot size during injection into the mould becomes difficult with a conventional-sized press and injection unit. Also, the percentage of material used per shot for the runner that feeds the mould is significantly greater than the amount of material used for the part, leading to increased part costs owing to material waste.

Microinjection machines suited specifically to the moulding of parts that weigh less than 1 g have become more widely available in recent years. These machines are designed with smaller feeder and injection units to allow increased control for the moulder's processing window, at the same time reducing the amount of material waste during production.

Managing micro optics
The handling of micro optics after the moulding process is nothing if not challenging. Glass optics can be cleaned after handling to remove fingerprints or other contaminants that adversely affect optical performance. In contrast, plastic optics cannot be cleaned economically. What's more, cleaning presents a greater risk of surface damage to the part.

Often the clear aperture (or functional area) of the lens is a large percentage of the overall diameter, leaving a negligible working area for handling. On a micro optic, this working area could be as small as 100 µm. Not surprisingly, manual handling of such small parts is very difficult and not helped by the fact that injection-moulded components inherently pick up static charge if the environment around the moulding machine is not controlled.

A solution to this problem is to use custom automation. For the manufacture of injection-moulded micro optics, automated equipment is designed and manufactured to pick the part from the mould face and separate ("degate") the part from the runner, eliminating manual handling at the component level. The trade-off in the use of automation for volume manufacture is higher initial capital investment versus the long-term advantage of lower unit costs due to reduced labour, repeatable processes, greater capacity and higher yields.

Another advantage of injection-moulded optics over glass is the ability for the optical designer to add mounting or assembly features (such as a flange or tab) to the optical component. This advantage can be realized at the microlens level as well. One option for handling micro optics is to leave the component on the runner, essentially providing a "handle", until the component is ready for the final assembly operation. Another option is to mould an array of micro optics versus single lenses, handle the parts in the array package for all required secondary operations, and then cut or dice the array prior to final assembly. The disadvantage of this approach is maintaining the cleanliness of the array during the final cutting operation, while the physical shape of the optic is typically limited to square or rectangular.

For a designer of small optics, it is vital to understand the full process chain that will be used in component manufacture and test. Design for manufacturing is a "must-have". The metrology equipment capabilities and the test and measurement techniques that will be used to verify the part parameters should be considered in specifying tolerances and indicating critical-to-function dimensions. The more the designer can incorporate the natural advantages of the processes being used, the more successful the outcome. Taking advantage of the ability to design mounting and assembly features into the part, the designer should work closely with the moulder to design in features that not only aid with assembly but also improve the quality of the finished part.

For instance, the addition of a flange around the perimeter of the lens can be designed to locate the lens relative to other optics or within the barrel, and eliminate an additional part such as a spacer. With injection-moulded parts there is a small area of internal stress near the area of the gate that can negatively affect the function of the lens if it protrudes into the working aperture. (The stress can give rise to birefringence effects, caused by molecules of the plastic "freezing" in a stretched conformation where the plastic is injected into the gate cavity.) With a correctly designed flange, however, this internal stress can be drawn outside of this region, thereby eliminating this moulding effect on performance.

Optics with small features
For the class of micro optics with small features, such as microlens arrays and diffractives, the accuracy of the optical surface becomes the critical detail. Microlens arrays are arrays of refractive elements with the aperture of each array element in the range of tens of microns up to 1 mm. The individual optical elements can be spheres, aspheres or freeform surfaces and the individual apertures are typically circular, rectangular or hexagonal. Diffractive micro optics employ patterns of features on the micron scale that cause the input light to be diffracted into a specific output pattern or in a specific direction.

Successful replication of micron-scale features in injection-moulded components begins with the creation of very accurate tooling. Greyscale photolithography, for example, can be used to generate the diffractive or refractive features in photoresist. An etching process then transfers the pattern into a glass substrate to produce either the finished optical component in glass or a tool master that is used to manufacture the optical inserts for the mould. The process is well controlled and alignment issues are avoided by creating all optical features on the surface with one process.

Alternatively, a mould tool can be created by single-point diamond-turning the diffractive pattern directly onto the tool insert. The diffractive pattern can be cut into a flat surface or on top of a base curve, either aspheric or spherical.

Depending on the size and aspect ratio of the features, the diffractive elements can be replicated through either injection-moulding or injection-compression-moulding techniques. For smaller, sharper features, injection-compression techniques achieve greater fill of submicron features by applying additional pressure to the plates of the mould after the injection of the polymer material. The trade-off with this dual technology is typically a slightly increased part cost, primarily driven by increased cycle times.

Micro optics and microprojectors

There are many applications for micro-lens arrays, with a lot of current interest in the illumination path for microprojectors. A double-sided lens array functions to homogenize the light, increasing spatial uniformity of the illumination on a spatial light modulator, liquid-crystal-on-silicon panel or other image-generating device, which in turn gives better uniformity in the final image.

When the light source is a laser, the microlens array also serves to reduce coherence effects and thereby improve overall image quality. For double-sided microlens arrays, the alignment of the elements on the front side of the array to the back side of the array is performance-critical. Proprietary tooling technology developed here at Jenoptik can achieve alignment within 2–3 µm.

• This article originally appeared in the May 2009 issue of Optics & Laser Europe magazine.

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