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The report is on the activities how to design, fabrication, characterization and system integration of refractive microlens arrays for sensors and microsystems.
AÂ microlensÂ is a smallÂ lens, generally with aÂ diameterÂ less than aÂ millimetreÂ (mm) and often as small as 10 micrometres (Âµm). The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to opticalÂ diffraction at the small features. A typical microlens may be a single element with one plane surface and one spherical convex surface toÂ refract Â the light. Because microlenses are so small, the substrate that supports them is usually thicker than the lens and this has to be taken into account in the design. More sophisticated lenses may useÂ asphericalÂ surfaces and others may use several layers of optical material to achieve their design performance.
A sensor measures the intensity profileand the wavefront of coherent light in real time andwith high accuracy. The sensor divides an incident wavefrontinto a number of beamlets by the sub-apertures ofa microlens array.Each microlens provides a separate focus on the sub gridof detectors on a CCD camera. The analysis of the resultingpattern is used to measure the optical phase of anincident wavefront.Applications include measurements on astronomicalinstruments, adaptive optics, wavefront compensation,and the eye itself. Jenoptik provides unique microlens arraysolutions with high spatial resolution for Shack Hartmannsensors. These microlens arrays can be designedfor wavefronts with large or small phase depths.
In the 17th century,Â Robert HookeÂ andÂ Antonie van LeeuwenhoekÂ both developed techniques to make small glass lenses for use with theirÂ microscopes. Hooke melted small filaments ofÂ Venetian glassand allowed theÂ surface tensionÂ in the molten glass to form the smooth spherical surfaces required for lenses, then mounting and grinding the lenses using conventional methods.The principle has been repeated by performingÂ photolithographyÂ into materials such asÂ photoresistÂ orÂ UVÂ curableÂ epoxyÂ and melting the polymer to form arrays of multiple lenses.Â More recently microlens arrays have been fabricated using convective assembly of colloidal particles from suspension.
Advances in technology have enabled microlenses to be designed and fabricated to close tolerances by a variety of methods. In most cases multiple copies are required and these can be formed bymouldingÂ orÂ embossingÂ from a master lens array. The master lens array may also be replicated through the generation of anÂ electroformÂ using the master lens array as aÂ mandrel. The ability to fabricate arrays containing thousands or millions of precisely spaced lenses has led to an increased number of applications
In this report I have chosen to explain about the microfabrication technologies such as photolithography and resist processing (reflow) were used to manufacture arrays of refractive microlenses in photoresist. The microlenses were transferred in fused silica by reactive ion etching (RIE).
This is the most common type of lens element.Â It can be used to focus, collect and collimate light.Â It is also useful as a simple imaging lens where image quality requirement is not too critical.A plano-convex microlens is described by the lens diameter Ø, the height at the vertexhL, the radius of curvature R, the refractive index n and the contact angle Î±
Figure . Plano convex Lens
Refractive microlens arrays
The microlens arrays are mastered by Heptagon's proprietary direct laser beam writing technology. Refractive lenses with a profile depth of up to 30 Âµm are offered. The mastering parameters of refractive lenses are optimized for achieving smooth surface profiles. The typical surface roughness values are below 20 nm rms. The diffraction efficiencies are between 90 and 95% (depending on the type of optional anti-reflection coatings).
Refractive microlenses can be packed in quadratic or hexagonal arrays with fill factors close to 100%. Customized solutions with irregular arrangements and even combinations of lenses with different optical and geometrical parameters are available upon request.
PROPERTIES OF PLANO-CONVEX REFRACTIVE MICROLENS ARRAYS:
1.Radius of curvature and focal length.
RADIUS OF CURVATURE AND FOCAL LENGTH:
The Lens profile of an symmetrical Plano-convex lens is,
his the height of the lens as a function of the distance r to the optical axis
R isthe radius of curvature at the vertex and
K is the aspherical constant.
The lens profileh(r) might be spherical (K = 0),
elliptic (âˆ’1 < K <0 or K >0),
parabolic (K = âˆ’1),
hyperbolic (K <âˆ’1) or even more sophisticated.
Radius of curvature:
wherehL is the height at the vertex.
The vertex focal length:
wheren is the refractive index and Î» the wavelength. The focal length f is a function of the
wavelengthÎ» due to material dispersion.
The contact angle Î± at the border of a spherical plano-convex lens (K = 0) :
The diffraction-limited resolution & depth of focus respectively:
Both Î´ x and Î´ z are independent of the lens scale. A downscaling of all length parameters
does not affect the diffraction-limited resolution of a lens. However, a scaling changes the
magnitude of wavefront aberrations which are expressed in fractions of the wavelength.
Small lenses have smaller aberrations than large
The fill factor Î·for both arrays :
Where ris the radius ;pxand pyare the lens pitch. The gap between the lenses is given by Ø âˆ’ px.. The maximum fill factor is 78.5% for rectangular and 90.6% for hexagonal closely packed arrays of circular lenses.
To fabricate a microlens array, the technique used is reflow or resist melting technique. Equipment used are standard semiconductor equipment's which processes resist coating , photolithography , wet processing, etching , etc.,Melting resist technology uses standard semiconductor processes and allows to fabricate microlenses from 10 Âµm to 2 mm diameter of excellent optical quality for wavelength from the DUV to the IR
REFLOW OR RESIST MELTING METHOD:
The main important steps to be followed using resistant molding method are discussed below.
1.A base layer of positive photoresist of about 0.5-1 um thickness is spin-coated on a glass plate.
2.A polymerization bake is used to harden the resist. Another layer of positive photoresist of about typically 1-100 um thickness is coated on top of the base layer .Uniformity on the order of 2% is achieved forthick resist layers.
3.After a prebake at 80-90 C , a chromium-on-glass maskis contact copied in a mask aligner as shown in figure 2.
4.The exposed resist is resolved in a standard developing process. An array of photoresist cylinders is obtained.
5. The resist cylinders are melted at a temperature of 150-200 _C on a hot plate or in an oven.
Fig :Fabrication of refractive microlenses by the reflow or resist-melting method.
FIG: Photolithography; developing and melting of the resist structure.
6. The melting procedure itself is quite simple to perform. A melted-resist structure will
always act like a microlens. Nevertheless, it is not trivial to fabricate microlenses with good
The difference between a suitable lens profile and an unacceptableprofile is only a fraction of a wavelength. Thus, careful optimization of all processing stepsis necessary.
During the melting procedure the edges of the resist structure start melting above thesoftening temperature . Above the glass transition temperature the amorphous resistpolymer changes abruptly from a rubbery state into a glass state system . The surfacetension tries to minimize the surface area by rearranging the liquid masses inside the drop. In the ideal case the resist melts completely, the masses are freely transported andsurface tension forms a spherical microlens. In practice, a complete melting of the resistdrop is not always achievable, especially not in the case of large and flat resist cylinders.
For large resist volumes, the outer part of the liquid drop might already be cross-linked (dueto out-gassing of the solvents), before the inner part is completely melted. For differentlens diameters, height and array types (different packing densities, array size, substratematerial) all process parameters such as exposure energy, developing, prebake, cooling,storing conditions, melting cycle, etc have to be carefully optimized.
Diameter , Numerical aperture and packing density
Height of the cylindrical lens hc which is a resist structure
Height of the lens hL , Volume of the cylinder is Vc ,Volume of the lens is VL
Usually, the resist volume shrinks during the heating due to solvent out-gassing and crosslinking (VL <= VC).
The height of the resist cylinder hC and the lens hL are given by
Usually, the lens is 1.3-1.7 times higher than the resist cylinder before melting
Contact angle is given by :
The contact angle depends on the surface energy, the resist volume and the diameter of thevlens base during the melting step. A minimum contact angle of the order of 10degrees is achieved formelting photoresist on a resist base layer.
To transfer fused silica they have used RIE (reactive ion etching) technique.For a applications in the blue and UV wavelength regions the resist lenses are usually transferred in used silica by reactive ion etching (RIE). This technique helps in transferring silica or Gas to the microlens used in the IR wavelength region
Fig : The RIE transfer process of resist microlenses in fused silica
Melted resist lenses are fabricated on a fused silica wafer. The resist shape is transferredin fused silica by RIE. Atoms from the resist surface and the silica are removed simultaneously by energetic ions until the lens shape is completely etched into the substrate. The etch rate ofthe photoresist and the silica depends strongly on the RIE parameters.The profile of the microlens lightbe slightly deformed after the RIE transfer. Usually the lenses are steeper at the rim andflatter at the vertex. Spherical aberrations are severely enhanced due to the profile change and the lower refractive index of the fused silica.
A profile modification is done by changing the etch rate during the RIE step. The resist is etched more quickly at the beginning and more slowly at the end. The roughness of the lens surfacemight be significantly increased during the RIE transfer. An adequate wafer cooling is mandatory to avoid puncture of the resist surface during the ion bombardment.
Methods to test microlenses:
Microlens testing requires in many cases the measurement of surface deviations and the measurement of the wave aberrations of the lens. The lens function is for the multitude of microlenses brought about through curved surfaces of dielectric substrates. Only GRIN lenses rely on the three-dimensional distribution of the refractive index. For the measurement of the surface deviations the Twyman-Green interferometer and for cylinder lenses the grazing incidence test using diffractive beam splitters are best suited. Wave aberrations should be measured in single pass geometry because of systematic errors in case of big aberrations due to double pass arrangements. Therefore, the Mach-Zehnder on the one hand and the shearing interferometer on the other can be recommended for microlens tests. At the time being also wave front sensor as the Shack-Hartmann test are becoming a suitable alternative to the shearing methods because of its simplicity and sufficient sensitivity. A special field is the test of whole lens arrays. This concerns the paraxial as well as the aberration data. Low aperture lens arrays might be illuminated with plane waves indicating the uniformity of the lens data if a whole field is evaluated simultaneously.
Chemical microchips and uTAS.
Off-axis microlenses for optical blood gas sensor.
Microlens array imaging system for photolithography.
Smart mask lithography.
Microlens arrays for optical signal processing.
Chemical microchips and _TAS:
Chemical microchips and miniaturized total analytical systems (_TAS) cover a wide range of disciplines, such as analytical and organic chemistry, biochemistry, electronics,microengineering, solid state physics, laser physics and micro-optics. Usually, a complexarrangement of lasers, detectors, filters, optics and high-precision mechanical stages isrequired for illumination and optical detection. Microlens arrays offer a large potentialto reduce the size and to simplify the architecture of analytical systems.
Fig:uTAS using microlens arrays for illumination and detection in parallel micro capillaries of a chemical chip
Off-axis microlenses for optical blood gas sensor:
Elliptical microlenses were used to compensate the astigmatism for off-axis imaging ,e.g. for chemical sensor heads.
Microlens array imaging system for photolithography:
A microlens array imaging system was developed in connection with a new contactless photolithographic technique called microlens lithography.
Smart mask lithography:
Next generation lithograph tools, such as x-ray, ion or electron beam use thin film membrane windows for the exposure masks. Systematic aberrations due to heat distortions during exposure and other influences are inevitable. The possibility of integrating an active correction system by mechanical stretching with precision actuators is investigated. The basic idea is to measure the placement error of theat a finite number of precision points and use these errors measurements to stretch the membrane with in-plane actuators back to their proper locations. The actuation could be implemented by micro-actuators that are monolithically attached directly to theÂ Â membrane. In order to implement such a system, the following are required: 1) A metrology system that precisly measures the displacement of a set of precision points; 2) Micro-actuators that can be directly attached to theÂ and offer sufficient force and displacement to strain theÂ and 3) A control algorithm and system that correlates 1 with 2.
Smart mask lithography is aimed at the printing of simple patterns, such as a matrix of micro-dots, posts or holes, or other patterns consisting of lines, dots, circles or squares. A smart mask consists of one (or more) layer(s) of micro-optical elements, such as refractive or diffractive microlenses (spherical, cylindrical or elliptical shape), gratings, CGH, apertures, etc.
Conventional mask for contact or proximity printing of a dot matrix.
Smart mask for printing a dot matrix. The mask consists of an array of spherical microlenses
Microlenses can be used in many applications such as:
â€¢ Microlenses for fiber coupling and optical switching;
â€¢ Microlenses for collimation of lasers diodes;
â€¢ Microlenses for imaging systems and sensors;
â€¢ beam homogenizers for lasers und illumination systems;
â€¢ Array optics featuring high precision;
â€¢ For the best imaging characteristics (aspherical lenses);
â€¢ Simplified alignment and assembly (planar substrates
With a large used is refractive microlenses], in special in miniaturization of an optical system and reduction of alignment, being produced in the form of arrays (fields) on planar substrates (wafers). Refractive microlenses are created from synthetic fused silica or silicon in a cleanroom environment using the highly sophisticated processes of the semiconductor industry as photolithography, plasma etching, which assured accuracy production of complex array optics and higher optical quality of microlenses.
Microlens arrays are used for collimating or focusing (laser arrays, detector arrays, fibre optics, sensors, optical interconnects, optical computing, etc), for illumination (flat panel displays, TV projection systems, retro-reflectors, diffusers, etc) and for imaging (photocopiers, 3D-photography, signal and image processing, fibre couplers, microlens lithography, shop testing, astronomy, etc).
Field of Application
fiber coupling and collimation
Shack-Hartmann wavefront sensors
CCD and CMOS sensor optics
LED and laser diode collimation
Investigation is focused on microlens arrays for microsystems and sensor applications. Arrays of refractive lenses (2um-5um diameter) have been fabricated by melting resist technology .Microlens arrays have been transferred in fused silica(reactive ion etching ) and replicated in polycarbonate and polymer(embossing , casting). Lens arrays have been integrated into lithographic systems, sensors and neural networks.