Mask-aligner lithography is a technology used to transfer patterns with critical dimensions in the micrometer range from below 1 micron for contact printing to a dozen of microns in proximity printing. This technology is widely used in the fabrication of MEMS, micro-optical components, and similar fields. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the UV rang of the spectrum with peaks at 365 nm, 405 nm and 435 nm, respectively the g-, h- and i- lines. These lamps suffer from several disadvantages (inefficient, bulky, dangerous), which makes alternatives interesting. In recent years, high power UV LEDs at the same wavelengths appeared on the market, opening the door to new illumination systems for mask-aligners. We have developed a modular 250 W LED-based illumination system, which can advantageously replace a 1 kW mercury arc lamp illumination. LEDs, arranged in a 7×7 grid array, are placed in the entrance apertures of individual reflectors, which collimate the individual irradiation to an output angle of 10°. A subsequent fly’s eye integrator homogenizes the illumination in the mask plane. It is followed by a Fourier lens, superimposing the individual channels in the mask plane, and a field lens to ensure telecentric illumination. This multisource approach allows the shaping of the source by switching individual illumination channels, determining the illumination angles and the spatial coherence in the mask plane. This concept can be used, for example, to do source-mask optimization. Compared to mercury arc lamp illumination, our system is simultaneously more efficient, compact, versatile, economic and sustainable. In our contribution, we present the design of the system as well as lithographic test prints done with different illumination patterns.
We introduce a novel industrial grade 193nm continuous-wave laser light source for proximity mask-aligner lithography. A diode seed laser in master-oscillator power-amplification configuration is frequency-quadrupled using lithiumtriborate and potassium-uoro-beryllo-borate non-linear crystals. The large coherence-length of this monomodal laser is controlled by static and rotating shaped random diffusers. Beam shaping with imaging and non-imaging homogenizers realized with diffractive and refractive micro-optical elements is compared in simulation and measurement. We demonstrate resolution patterns offering resolutions <2 µm printed with proximity gaps of 20 µm.
We present and discuss Talbot mask-aligner lithography, relying on a continuous wave laser emitting at 193nm for the illumination. In this source, a diode laser at 772nm is amplified by a tapered amplifier in master-oscillator power-amplifier configuration and frequency-quadrupled in two subsequent enhancement cavities using lithium triborate and potassium fluoro-beryllo-borate nonlinear crystals to generate the emission at 193 nm. The high coherence and brilliance of such an illumination source is predestined for plane wave mask-aligner illumination, crucial in particular for high-resolution lithographic techniques such as Talbot lithography and phase-shift masks. Talbot lithography takes advantage of the diffraction effect to image periodic mask features via self-replication in multiples of the Talbot distance behind the photomask when exposed by a plane wave. By placing a photoresistcoated wafer in one of the Talbot planes, the mask pattern is replicated in the resist. Periodic patterns with diverse shapes are required for wire grid polarizers, diffraction gratings, and hole arrays in photonic applications as well as for filters and membranes. Using an amplitude mask with periodic structures, we demonstrate here with such a technique sub-micron feature sizes for various designs at a proximity gap of 20 µm.
We present a novel industrial-grade prototype version of a continuous-wave 193 nm laser system entirely based on solid state pump laser technology. Deep-ultraviolet emission is realized by frequency-quadrupling an amplified diode laser and up to 20 mW of optical power were generated using the nonlinear crystal KBBF. We demonstrate the lifetime of the laser system for different output power levels and environmental conditions. The high stability of our setup was proven in > 500 h measurements on a single spot, a crystal shifter multiplies the lifetime to match industrial requirements. This laser improves the relative intensity noise, brilliance, wall-plug efficiency and maintenance cost significantly. We discuss first lithographic experiments making use of this improvement in photon efficiency.
We present an integrated array imaging system based on a stack of microlens arrays. The microlens arrays are manufactured by melting resist and reactive ion etching (RIE) technology on 8’’ wafers (fused silica) and mounted by wafer-level packaging (WLP)1. The array imaging system is configured for 1X projection (magnification m = +1) of a mask pattern onto a planar wafer. The optical system is based on two symmetric telescopes, thus anti-symmetric wavefront aberrations like coma, distortion, lateral color are minimal. Spherical aberrations are reduced by using microlenses with aspherical lens profiles. In our system design approach, sub-images of individual imaging channels do not overlap to avoid interference. Image superposition is achieved by moving the array imaging system during the exposure time. A tandem Koehler integrator illumination system (MO Exposure Optics) is used for illumination. The angular spectrum of the illumination light underfills the pupils of the imaging channels to avoid crosstalk. We present and discuss results from simulation, mounting and testing of a first prototype of the investigated array imaging system for lithography.
In this paper we present chromatic confocal distance sensors for the parallelized evaluation at several lateral positions.
The multi-point measurements are performed using either one- or two-dimensional detector arrays. The first sensor combines
the concepts of confocal matrix sensing and snapshot hyperspectral imaging to image a two-dimensional array of
laterally separated points with one single shot. In contrast to chromatic confocal matrix sensors which use an RGB detector
our system works independently from the spectral reflectivity of the surface under test and requires no object-specific
calibration. Our discussion of this sensor principle is supported by experimental results. The second sensor is a multipoint line sensor aimed at high speed applications with frame rates of several thousand frames per second. To reach this evaluation speed a one-dimensional detector is employed. We use spectral multiplexing to transfer the information from different measurement points through a single fiber and evaluate the spectral distribution with a conventional spectrometer. The working principle of the second sensor type is demonstrated for the example of a three-point sensor.
In this paper we show that it is possible using optical photolithography to obtain micron and submicron features for
periodic structures in non-contact using the Talbot effect. In order for this effect to work it is important to have good
control of the illumination light and here we show that the MO Exposure Optics (MOEO) developed by SUSS
MicroOptics provides uniform and well collimated illumination light suitable for Talbot lithography. The MOEO can
easily be incorporated into a standard mask aligner. Here we show 1μm and 0.65μm diameter holes in a hexagonal array
in photoresist made in large-gap proximity printing.
Dispersion causes the focal lengths of refractive and diffractive optical elements to vary with wavelength. In our contribution
we show how it can be used for chromatic encoding and decoding of optical signals. We specifically discuss how
these concepts can be applied for the implementation of systems with applications in the growing fields of hyperspectral
imaging and chromatic distance coding. Refractive systems as well as hybrid combinations of diffractive and refractive
elements are used to create specific chromatic aberrations of the sensors. Our design approach enables the tailoring of the
sensor properties to the measurement problem and assists designers in finding optimized solutions for industrial applications.
The focus of our research is on parallelized imaging systems that cover extended objects. In comparison to point
sensors, such systems promise reduced image acquisition times and an increased overall performance. Concepts for
three-dimensional profilometry with chromatic confocal sensor systems as well as spectrally resolved imaging of object
scenes are discussed.
Speckle fields are formed when quasi-monochromatic light is scattered by an optically rough surface. These fields
are usually described by reference to their first and second order statistical properties. In this paper we review
and extend some of these fundamental properties and propose a novel technique for estimating the refractive
index of a smooth sample. Theoretical and experimental results are presented. Separately, we also report on
a preliminary experiment to determine some characteristics of speckle fields formed in free space by a rotating
compound diffuser. Some initial measurements are made where we examine how the speckle intensity pattern in
the output plane changes as a function of the relative rotation angle.