Micro-Optics

Download SUSS MicroTec technical publications, white papers and application notes about micro-optics.

Lithographic process window optimization for mask aligner proximity lithography

We introduce a complete methodology for process window optimization in proximity mask aligner lithography. The commercially available lithography simulation software LAB from GenISys GmbH was used for simulation of light propagation and 3D resist development. The methodology was tested for the practical example of lines & spaces, 5 micron half-pitch, printed in a 1 micron thick layer of AZ® 1512HS1 positive photoresist on a silicon wafer. A SUSS MicroTec MA8 mask aligner, equipped with MO Exposure Optics® was used in simulation and experiment. MO Exposure Optics® is the latest generation of illumination systems for mask aligners. MO Exposure Optics® provides telecentric illumination and excellent light uniformity over the full mask field. MO Exposure Optics® allows the lithography engineer to freely shape the angular spectrum of the illumination light (customized illumination), which is a mandatory requirement for process window optimization. Three different illumination settings have been tested for 0 to 100 micron proximity gap. The results obtained prove, that the introduced process window methodology is a major step forward to obtain more robust processes in mask aligner lithography. The most remarkable outcome of the presented study is that a smaller exposure gap does not automatically lead to better print results in proximity lithography - what the “good instinct” of a lithographer would expect. With more than 5'000 mask aligners installed in research and industry worldwide, the proposed process window methodology might have significant impact on yield improvement and cost saving in industry.

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Micro-Optics: Enabling Technology for Illumination Shaping in Optical Lithography

Optical lithography has been the engine that has empowered semiconductor industry to continually reduce the half-pitch for over 50 years. In early mask aligners a simple movie lamp was enough to illuminate the photomask. Illumination started to play a more decisive role when proximity mask aligners appeared in the mid-1970s. Off-axis illumination was introduced to reduce diffraction effects. For early projection lithography systems (wafer steppers), the only challenge was to collect the light efficiently to ensure short exposure time. When projection optics reached highest level of perfection, further improvement was achieved by optimizing illumination. Shaping the illumination light, also referred as pupil shaping, allows the optical path from reticle to wafer to be optimized and thus has a major impact on aberrations and diffraction effects. Highly-efficient micro-optical components are perfectly suited for this task. Micro-optics for illumination evolved from simple flat-top (fly’s-eye) to annular, dipole, quadrupole, multipole and freeform illumination. Today, programmable micro-mirror arrays allow illumination to be changed on the fly. The impact of refractive, diffractive and reflective micro-optics for photolithography will be discussed.

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Talbot Lithography as an Alternative for Contact Lithography for Submicron Features

In this paper we show that using optical photolithography it’s possible to obtain submicron features for periodic structures using the Talbot effect. To use the Talbot effect without the need of an absolute distance measurement between the mask and the wafer we integrate over several exposures for varying wafer mask distances. Here we discuss the salient features of ‘integrated Talbot lithography’. Particularly, we show that to obtain good contrasts an excellent control of the illumination light is essential; for this we use the MO Exposure Optics (MOEO) developed by SUSS MicroOptics (SMO). Finally we show that 1μm and 0.55μm diameter holes can be made using this technique.

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Laser Beam Homogenizing: Limitations and Constraints

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.

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Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

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Wafer-Scale Micro-Optics Fulfill Promise

Early inventions in the field of planar diffractive and refractive micro-optics date back more than a century. In 1891, Gabriel Lippmann invented “interference color photography,” later called Lippmann holograms. This invention was made without lasers and long before Dennis Gabor invented holography in 1948. Lippmann also invented “integral photography,” an autostereoscopic method to display 3-D images for observation with the naked eye.

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Advanced Mask Aligner Lithography (AMALITH)

Mask aligners were the dominating lithography tool for the first 20 years of semiconductor industry. In the 1980s industry changed over to projection lithography. However, mask aligners were never sorted out and still today hundreds of new mask aligners are sold each year. This continuing success of mask aligner lithography is due to two basic trends in lithography: (a) Costs for leading-edge lithography tools double approximately every 4.4 years; and (b) the number of lithography steps per wafer was increasing from a few litho layers to more than 35 layers now. This explains why mask aligners, a very cost-effective solution for uncritical litho layers, are still widely used today. In over 50 years of semiconductor industry the mask aligner system has changed tremendously. However, only little effort was undertaken to improve the shadow printing process itself. We now present a new illumination system for mask aligners, the MO Exposure Optics (MOEO), which is based on two microlens-type Köhler integrators located in Fourier-conjugated planes. The optics stabilizes the illumination against misalignment of the lamp-to-ellipsoid position. It provides improved light uniformity, telecentric illumination and allows freely shaping the angular spectrum of the illumination light by spatial filtering. It significantly improves the CD uniformity, the yield in production and opens the door to a new era of Advanced Mask Aligner Lithography (AMALITH), where customized illumination, optical proximity correction (OPC), Talbot-lithography, phase shift masks (AAPSM) and source mask optimization (SMO) are introduced to mask aligner lithography.

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Wafer-Scale Micro-Optics Fabrication

Micro-optics is an indispensable key enabling technology for many products and applications today. Probably the most prestigious examples are the diffractive light shaping elements used in high-end DUV lithography steppers. Highly-efficient refractive and diffractive micro-optical elements are used for precise beam and pupil shaping. Micro-optics had a major impact on the reduction of aberrations and diffraction effects in projection lithography, allowing a resolution enhancement from 250 nm to 45 nm within the past decade. Micro-optics also plays a decisive role in medical devices (endoscopes, ophthalmology), in all laser-based devices and fiber communication networks, bringing high-speed internet to our homes. Even our modern smart phones contain a variety of micro-optical elements. For example, LED flash light shaping elements, the secondary camera, ambient light and proximity sensors. Wherever light is involved, micro-optics offers the chance to further miniaturize a device, to improve its performance, or to reduce manufacturing and packaging costs. Wafer-scale micro-optics fabrication is based on technology established by the semiconductor industry. Thousands of components are fabricated in parallel on a wafer. This review paper recapitulates major steps and inventions in wafer-scale micro-optics technology. The state-of-the-art of fabrication, testing and packaging technology is summarized.

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Facettenreiche Alleskönner von morgen (germany only)

Die MIKRO- UND NANOOPTIK hat sich zu einer Schlüsseltechnologie der modernen Photonik entwickelt. Die Bandbreite aktueller Entwicklungen reicht beispielsweise von adaptiven Mikrooptiken über die 3D-Laserlithografie bis hin zur Subwellenlängen-Mikrooptik auf Basis von Metamaterialien.

Published in Mikroproduktion 02/11

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Homogenous monochromatic irradiance fields generated by microlens arrays

Microlens array homogenizers are an attractive choice in the field of radiometry and photometry to generate highly uniform beams with high efficiency. In the present paper a microlens array homogenizer used to determine the spectral responsivity of large size, partially filtered photometer is presented. The effect of non-uniformity of the field is shown to be smaller than 0.2% in the whole visible spectrum.

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Laser Beam Homogenizing: Limitations and Constraints

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.

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Conformal Photoresist Coatings for High Aspect Ratio Features

The paper will emphasize recent improvements to both hardware and process methodology in an effort to broaden the scope of structures and materials suitable for spray coating. Practical extensions of this new technology will be explored and discussed, along with an assortment of new structures and applications that spray coating has enabled.

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Micro-Optics: From High-End to Mass-Market

Photonics is said to be the most important key technology in the 21st century, some even call the 21st century the “photon century”. It might be a bit too early to name a whole century after it, but indeed, photon-based technology has much impact on our everyday life at the beginning of the new century. Chip manufacturing, lighting, health care and life-sciences, space, defense, and the  transport and automotive sector rely on photon-based technology. Photonics isalso supposed to offer novel solutions where today’s conventional technologiesreach their limits in terms of velocity, capacity and accuracy.

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High numerical aperture silicon collimating lens for mid-infrared quantum cascade lasers manufactured using wafer-level techniques

We present an aspheric collimating lens for mid-infrared (4-14 µm) quantum cascade lasers. The lenses were etched into silicon by an inductively coupled plasma reactive ion etching system on wafer level. The high refractive index of silicon reduces the height of the lens profile resulting in a simple element working at high numerical aperture (up to 0.82). Wafer level processes enable the fabrication of about 5000 lenses in parallel. Such cost-effective collimating lens is a step towards the adoption of quantum cascade lasers for all its potential applications.

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