We present several characterization techniques, which are suitable for small-size microlenses of lens diameters down to 5 μm. For an individual microlens, we apply full characterization for optical performance and surface characteristics.
First, the optical performance is characterized by using a high-resolution interference microscope (HRIM). Second, a
confocal microscope is applied to investigate the surface parameters. Third, the HRIM allows scanning the microlens
array along the optical axis by using a piezo actuator. This leads to a measurement of the 3D intensity distribution near
the focus of the lens. Such 3D intensity maps allow us to characterize the focal properties of each lens in an array. By
studying those characterization techniques, we develop a new method to characterize a large number of microlenses, for instance, over one million lenses, which is already applied to wafer-based manufacturing in a cleanroom fab.
We show a laser beam shaping device made of a deformable continuous reflective membrane fabricated over a scanning
stage. The combination of two actuator schemes enables shaping and smoothing of a laser beam with a unique compact
device. It is designed to shape an input laser beam into a flat top or Gaussian intensity profile, to support high optical load
and to potentially reduce speckle contrast. One single electrode is needed to deform the whole membrane into multiple
sub-reflecting concave elements. The scanning stage is used simultaneously to smooth out the remaining interference
patterns. The fabrication process is based on SOI wafer and parylene refilling to enable the fabrication of a 100 % fill
factor 5 by 5 mm2 deformable membrane. Applications for such device are laser machining and laser display.
Micro-optics is an indispensable key enabling technology (KET) for many applications today. The important role of
micro-optical components is based on three different motivations: miniaturization, high functionality and packaging
aspects. It is obvious that miniaturized systems require micro-optics for light focusing, light shaping and imaging. More
important for industrial applications is the high functionality of micro-optics that allows combining these different
functions in one element. In DUV Lithography Steppers and Scanners an extremely precise beam shaping of the Excimer
laser profile is required. High-precision diffractive optical elements are well suited for this task. For Wafer-Level
Cameras (WLC) and fiber optical systems the packaging aspects are more important. Wafer-Level Micro-Optics
technology allows manufacturing and packaging some thousands of sub-components in parallel. We report on the state
of the art in wafer-based manufacturing, testing and packaging.
Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital
micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and
widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based
tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse
shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for
shaping light in the space and time domain.
We present a dynamic laser beam shaper based on MEMS technology. We show a prototype of a dynamic diffuser made
of single crystal silicon. A linearly deformable silicon micromembrane is used to diffuse a laser beam in one dimension.
Resonance frequencies of the membrane can range from 1 kHz to 20 kHz. Mode shapes of the deformable mirror are
excited using magnetic actuation. Diffusing angle can be tuned by adjusting the driving current in the membrane. We
measured a diffusing angle of 1° for an applied current of 40 mA. The aluminum coated mirror can handle 140 W/cm2 of
visible to infrared optical power. Application to smooth out interference pattern generated by a static diffuser is shown.
Applications may vary with time, and so may the technical realization of these applications. Lasers change and
consequently application wavelengths, due to requirements imposed by material changes, power constraints, scaling of
lateral features, or other parameters and specifications. In applications where diffractive optical elements (DOEs) are
employed a change in laser or wavelength usually demands not only a new fabrication batch but a redesign of the
diffractive structure. This is almost always the case in the special application of laser beam shaping where a DOE is
designed to redistribute the energy of a specific laser beam profile, generally into a flat-top intensity profile (either round
or square).
In this paper we will describe how a mono-mode beam shaping DOE design for 355 nm was adapted to compensate a
wavelength change to 532 nm. A preliminary telescope is necessary to ensure that the input beam retains the initial
properties of beam waist and beam diameter. In this case, the original DOE design can be maintained with some minor
compensation in phase depth for the wavelength change and in positioning of the elements to compensate the scaled
diffraction angles. Further data will be shared showing a side by side comparison of testing results of the 355 nm and 532 nm designs.
Laser manufacturing of microstructures using a single focus is a well known technology. Multi-spot optics are applied
for process parallelizing if the demand on throughput in mass production rises or large areas of material have to be
processed. Diffractive optical elements (DOEs) are used for parallel laser processing of a repetitive structure. These
elements split the beam into a periodic spot pattern, where each spot shows the same shape and energy. This allows
simultaneous manufacturing of several equal shaped structures at the same time. For patterning a surface this is state of
the art and the appropriate technique to reduce processing time while maintaining a high lateral resolution as well as a
good relative positioning of the structure due to the DOE.
We investigate the usage of microlens arrays as multifunctional elements for forming an arbitrary shaped laser beam into
a spot-, a ring-spot- or a line-array pattern. It can be shown that the intensity distribution of each spot is equal to the
intensity distribution of all other spots in the whole pattern. The shape of the spots is defined by the angular distribution
of the incident beam. We demonstrate that besides other optical properties the output beam profile strongly depends on
the Fresnel-Number and is influenced by diffraction and interference effects. We present important design rules which
consider geometrical and physical optics. The properties of the spot arrays, like spot diameter, Rayleigh length and beam
divergence in dependency of beam and system properties are investigated. Finally we will show some laser micro
structuring and micro drilling results in different materials.
We present a dynamic laser beam shaper based on MEMS technology. We show a prototype of a dynamic diffuser made
of single crystal silicon. A linearly deformable silicon micromembrane is used to diffuse a laser beam in one dimension.
Resonance frequencies of the membrane can range from 1 kHz to 100 kHz. Diffusing angle can be tuned by adjusting the
driving voltage. We measured a diffusing angle of 0.16° for an actuation voltage of 20 V.
Laser beam homogenizing and beam shaping are key enabling technologies for many applications today. Periodic
microlens arrays are widely used to transform Gaussian or non-uniform beam profile into a uniform "flat-top". Each
microlens element samples the input beam and spreads it over a given angular distribution. Incoherent beams that are
either temporally or spatially incoherent can produce very uniform intensity profiles. However, coherent beams will
experience interference effects in the recombination of the beams generated by each individual microlens element.
Rotating or moving elements, such as a rotating diffuser or a vibrating optical fiber, are used to average these
interference patterns. An integration of several different patterns will smooth out the intensity profile. Unfortunately, this
averaging is not always possible. Some applications require a single shot from a pulse laser or work at very high data
rates that do not allow an averaging over 10 to 50 frames. We will discuss the concepts of Köhler illumination and
Köhler integrators and its limitations and constrains for laser beam homogenizing. We will show how micro-optical
elements comprised of a randomly varying component can be used to smooth out interference and speckle effects within
the far-field intensity profile.
Certain high power laser applications require thin homogeneous laser lines. A possible concept to generate
the necessary flat-top profile uses multi-aperture elements followed by a lens to recombine separated beamlets.
Advantages of this concept are the independence from entrance intensity profile and achromaticity. However, the
periodic structure and the overlapping of beamlets produce interference effects especially when highly coherent
light is used. Random optical elements that diffuse only in one direction can reduce the contrast of the interference
pattern. Losses due to undesired diffusion in large angles have to be minimized to maintain a good quality and
high efficiency of beam shaping. We have fabricated diffusers made of fused silica for a wide range of wavelengths
that diffuse only in one direction. Structures are based on an array of concave cylindrical microlenses with locally
varying size and position following a well defined statistical distribution. The scattering angle can be influenced
by process parameters and is typically between 1° and 60°. To predict the influence of process parameters on
the optical properties, a simplified model for the fabrication process and geometrical optics have been used.
Characterization of the fabricated devices was done by stylus measurements for the surface shapes, microinterferometry
to measure phase profiles and high resolution goniometry to obtain far field distribution of light. The simulated data compare very well to measured optical properties. Based on our simulation tool we discuss limits of our fabrication method and optimal fabrication parameters.
A wide range of lasers from the UV to the IR are selected based on their optical power and spectral characteristics to
match the particular absorption behavior for the material to be processed. Periodic microlens arrays are often used as
multi-aperture integrators to transform the Gaussian or non-uniform beam profile into a homogenized intensity profile
either in 1-D or 2-D distribution. Each microlens element samples the input inhomogeneous beam and spreads it over a
given angular distribution. Incoherent beams that are either temporally or spatially incoherent can produce very uniform
intensity profiles. However, coherent beams will experience interference effects in the recombination of the beams
generated by each individual microlens element. For many applications, for example pulsed laser sources, it is not
possible to use a rotating or moving element, such as a rotating diffuser, to circumvent the interferences resulting from
the beam coherence. Micro-optical elements comprised of a randomly varying component can be used to help smooth
out the interference effects within the far-field intensity profile.
Today there exist different commercial micro-optics measurement instruments for the characterization of micro-optical
components and microlenses in particular. However there is often a lack of a complete quantitative optical
characterization of the latter components. Therefore we will focus in this paper on the optical characterization of all
types of refractive microlenses, more in particular spherical and aspherical microlenses. Moreover the results of the
performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to
select the most appropriate instrumentation tools for characterizing refractive microlenses.
Refractive, diffractive and reflective micro-optical elements for laser beam shaping and homogenizing have been manufactured and tested. The presented multifunctional optical elements are used for shaping arbitrary laser beam profiles into a variety of geometries like, a homogeneous spot array or line pattern, a laser light sheet or flat-top intensity profiles. The resulting profiles are strongly influenced by the beam properties of the laser and by diffraction and interference effects at the micro-optical elements. We present general design rules for beam shaping and homogenizing. We demonstrate the application of such multifunctional micro-optical elements for a variety of applications from micro-laser machining to laser diagnostic systems.
Many applications in laser manufacturing like semiconductor lithography, micro-machining, micro-structuring or
material-analysis require a homogeneous intensity distribution of the laser beam over its complete profile. Refractive and
diffractive beam homogenizer solutions have been developed for this challenge, but their applicability strongly depends
on the physics of the individual laser beam. This paper investigates the influence of laser beam properties like spatial
coherence for microlens beam homogenizers. Diffraction at the small lens apertures and interference effects of periodic
arrays are explained by using diffraction theory. Different microlens beam homogenizer configurations are presented.
Design considerations that might be helpful for the layout of a specific microlens beam homogenizer system are
discussed. It is shown that, among other factors, the Fresnel number is the most important quantity to characterize the
influence of diffraction effects on microlens laser beam homogenizers. The influence of the spatial partial coherence will
be explained for the example of a Fly's eye condenser. For cw laser sources, the influence of a rotating diffuser plate on
grating interference and speckles effects is investigated. Finally, the theory will be compared to some practical examples
in planar laser measurement techniques, in combustion diagnostics and micromachining with Excimer lasers.
Today there exist different commercial and proprietary micro-optics measurement instruments for the characterization of micro-optical components and microlenses in particular. However there is often a lack of a complete quantitative optical characterization of the latter components. Therefore we will focus in this paper on the optical characterization of spherical microlenses. Moreover the results of the performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to select the most appropriate instrumentation tools for characterizing refractive spherical microlenses.
The optical properties of plano-convex refractive microlenses with low Fresnel Number (Typically FN < 10) are investigated. Diffraction effects at the lens stop limit the range of the effective focal length. The upper limit of the focal length is determined by the diffraction pattern of a pinhole with equal diameter. Refraction and diffraction have antagonist effects on the focal length when changing the wavelength of illumination. Diffraction effects at the lens stop are used to balance dispersion and to design microlens achromats. Gaussian beam propagation method has been used for simulation. The presented results are of relevance for applications like Shack Hartmann wavefront sensors or confocal microscopes, where microlenses with small apertures and long focal lengths are widely used.
We report on an array of atomic force microscopes (AFM) based on a simple optical set-up using heterodyne detection. The deflection of AFM cantilevers is given by the path differences between the reference and the measuring wave in a Michelson interferometer. A matrix of micro-lenses is placed just above the cantilevers, in such a way that the deflected light from each cantilever is collected by one micro-lens. Both the micro-lenses and the cantilever chips are previously glued to increase the robustness of the system. The interference between the light from each micro-lenses and the
reference light is selected by a diaphragm and subsequently detected by a photodetector. This procedure is repeated for each cantilever. In order to validate our instrument we measure the profile of a binary grating having a step height of 19.66 nm. By a piezoelectric platform a lateral range of 10 μm was scanned with a speed of 1 μm/s and an integration time of 10 ms, which leads to a lateral resolution of 10 nm. The profiles measured by the cantilevers are in good agreement with the profile of the sample grating.
Wafer-based manufacturing of Micro-Optics is based on standard technologies from Semiconductor Industry, like resist coating, lithography, reactive ion etching, deposition, sputtering, and lift-off. These well-established technologies allow the manufacturing of almost any Micro-Optics' structure shape. The excellence of the Micro-Optics component depends much on the proper choice of the manufacturing equipment and the process control. As all processes are standard Semiconductor technology, the quality is merely a question of the budget and the optimization effort. For characterization and testing, the current situation is different. Neither the test equipment from Semiconductor industry nor the test equipment from classical optics manufacturing is suitable to for Micro-Optics. Most of test instruments Micro-Optics industry is using today have been developed by research institutes or by the manufacturing companies
themselves. As Micro-Optics is still a niche market, all instruments are built in small series. This lack of suitable test equipment is a major problem for the Micro-Optics industry today. All process optimization in manufacturing is closely related to the capability to measure the quality of the products. We report on the state of the art in wafer-based manufacturing and summarize the standard characterization tools for Micro-Optics.
Digital holographic Microscopy (DHM) is an imaging modality reconstructing the wavefront in a numerical form,
directly from a single digitalized hologram. It brings quantitative data derived simultaneously from the amplitude and
phase of the complex reconstructed wavefront diffracted by the object and it is used to determine the refractive index
and/or shape of the object with accuracy in the nanometer range along the optical axis. DHM comprises a microscope
objective to adapt the sampling capacity of the camera to the information content of the hologram.
This paper illustrates some of the possibilities offered by DHM for micro-optics quality control. Actual results obtained
by DHM, yielding an axial precision up to 3.7 nm, will be compared with measurements performed with interferometers
by SUSS MicroOptics SA and with the profiles measured with a mechanical scanning probe instrument (Alpha step 200
from Tencor Instrument). Two different micro-lenses arrays where tested: a quartz refractive lenses array (observed with
transmission DHM) and a Silicon refractive lens array (observed with reflection DHM).
The metrology of refractive microlens arrays is analyzed using Twyman-Green, Mach-Zehnder, and white light interferometers. The advantages and limitations of each are discussed in their application to the measurement of spherical and aspherical microlens arrays.
We report on the fabrication of aspherical refractive microlens arrays on 8'' fused silica and silicon wafers at Suss Neuchatel, Switzerland. Refractive, plano-convex microlenses are fabricated by using photolithography, a reflow or melting resist technique and reactive ion etching. Diffraction-limited optical performance of the microlenses is achieved for refractive microlenses from 100 microns to 1.5 mm diameter and 2 to 50 microns sag. Aspherical lens profiles (aspherical constant from k equals -0.5 to -5.2) are obtained by varying the etch parameters during the reactive ion etching transfer. Microlens arrays in fused silica and silicon are fabricated for high-efficient fiber coupling and telecommunication. Densely packed arrays of cylindrical lenses (packing density > 98%, parabolic profile) are fabricated for flattop illumination at UV-wavelengths. Excellent array uniformity of is required for microlenses used within Microlens Projection Lithography systems.
We report on the fabrication of high quality microlens arrays on 4', 6' and 8'-fused silica wafers. Refractive, plano-convex microlenses are fabricated by using photolithography; a reflow or melting resist technique and reactive ion etching. A diffraction-limited optical performance (p-v wave aberrations of < (lambda) /8, Strehl ratio $GTR 0.97) is achieved. Aspherical lens profiles are obtained by varying the etch parameters during the reactive ion etching transfer. The microlens arrays are used for Microlens Projection Lithography (MPL) and within UV-light illumination systems. Microlens Projection Lithography is an innovative technique using KARL SUSS Mask Aligners equipped with an ultra-flat microlens-based projection system. The projection system consists of 500.000 identical micro-objectives side- by-side. Each micro-objective consists of 3 to 4 microlenses. A fully symmetrical optical design eliminates coma, distortion and lateral color. The lens system is frontal- and backside telecentric to provide a unit magnification (+1) over the whole depth of focus. Each micro- objective images a small part of the photomask pattern onto the wafer. The partial images from different channels overlap consistently and form a complete aerial image of the photomask. Microlens Projection Lithography provides an increased depth of focus ($GTR 50 microns) at a larger working distance ($GTR 1 mm)than standard proximity printing. Microlens Projection Lithography allows photolithography on curved on non-planar substrates, in V-grooves, holes, etc. using a KARL SUSS Mask Aligner.
We report on recent progress in the fabrication of fused silica micro-optical elements, such as blazed gratings, refractive microlenses and microprisms. The elements are first made in photoresist and then they are transferred into fused silica by reactive ion etching. High selectivity etching is needed to realize structures with a high aspect ratio. Results are shown using various metallic etch masks. The shaping of optimized profiles is also presented to generate microlenses which are aspheric, or which have a low numerical aperture.
Diffractive optical elements (DOEs) are characterized by a large negative dispersion. This property can be exploited to correct chromatic aberration in hybrid optical systems. The diffraction efficiency of phase DOEs decreases when the wavelength deviates from the design wavelength. Consequently, DOEs are usually applied only in optical systems where the spectral bandwidth is limited. Thus the dilemma that the chromatic correction capability of the DOE can not be fully exploited in optical systems with large spectral bandwidths where the correction could be most useful. It has been shown that by careful selection of the dispersion properties of two different optical materials it is possible to achromatize the diffraction efficiency of the DOE in a sandwich configuration, Fig. la.1 The dispersion properties of the two materials must be matched such that refractive index change at the interface, An(2), remains proportional to the wavelength, A, i.e.AIzn() = constant. The introduction of a second surface profile removes the above constraint on the dispersion properties of the materials. Such an approach has been described where an air gap has been inserted between the two materials and the depth of the surface profile for each material are optimized to minimize the variation of the phase modulation with wavelength, Fig. lb.2 Better performance can be achieved by placing the second surface relief on the exterior of one of the sandwiched materials, Fig. ic. The advantages are reduced shadowing effect, less Fresnel reflection losses, and much better fabrication tolerances. Furthermore, the second surface profile has removed the materials limitation. Materials that are much better suited to fabrication can be used, thus allowing for mass fabrication at reduced costs.
We report on our activities in the design, fabrication, characterization and system integration of planar micro- optical elements. Microlens arrays, gratings, diffusers, beam shapers and beam splitters have been fabricated, tested and integrated in chemical analysis systems ((mu) TAS, fluorescence detection), tracking sensors for satellites, displacement sensors, optical lightpipes, LCD projector illumination photospectrometers, neural networks and multiple channel imaging systems for photolithography. Packaging and alignment strategies for sensors and optical microsystems were investigated.
The potential of diffractive optical elements for advanced laser communication terminals has been investigated. Applications include beam shaping of high- power laser diode arrays, optical filter elements for position detection and hybrid (refractive/diffractive) elements. In addition, we present a design example of a miniaturized terminal including diffractive optics.
KEYWORDS: Neurons, Photodetectors, Optoelectronics, Very large scale integration, Analog electronics, Optical computing, Silicon, Chemical elements, Tolerancing, Optical interconnects
A systems is described which finds solutions to the 6-city TSP using a Kohonen-type network. The system shows robustness with regard to the light intensity fluctuations and weight discretization which have been simulated. Scalability to larger size problems appears straightforward.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.