LIMO's unique production technology is capable to manufacture free form surfaces on monolithic arrays larger than 250
mm with high precision and reproducibility. Different kinds of intensity distributions with best uniformities or
customized profiles have been achieved by using LIMO's refractive optical elements. Recently LIMO pushed the limits
of this lens production technology and was able to manufacture first diffractive optical elements (DOEs) based on
continuous relief's profile.
Beside for the illumination devices in lithography, DOEs find wide use in optical devices for other technological
applications, such as optical communications, laser technologies and data processing. Classic lithographic technologies
lead to quantized (step-like) profiles of diffractive micro-reliefs, which cause a decrease of DOE's diffractive efficiency.
The newest development of LIMO's microlens fabrication technology allows us to make a step from free programmable
microlens profiles to diffractive optical elements with high efficiency. Our first results of this approach are demonstrated
in this paper. Diffractive beam splitters with continuous profile are fabricated and investigated. The results of profile
measurements and intensity distribution of the diffractive beam splitters are given. The comparison between theoretical
simulations and experimental results shows very good correlation.

Novel continuously shaped diffractive optical elements enable high efficiency beam shaping

Diffractive optical elements (DOE) play an important role for laser beam shaping in industry, for example in lithography
or parallel laser material processing. Typically such applications require high damage threshold and low background
illumination (high contrast and efficiency). Usual DOE with binary phase (step-like) profiles are made
microlithographically and suffer from substantial scattering on profile derivative discontinuities. That gives also
tendency to lower damage threshold as compared to intrinsic material values. The LIMO approach is based contrarily on
a proprietary, non etching material processing and is suitable for manufacturing of high-precision free programmable
continuous surface profiles in optical glasses and crystals. We report on linear symmetric diffractive beam splitter 1:11
with high homogeneity and efficiency > 95% and discuss also other DOE designs. The design data, simulations with
measured surface profiles and experimental intensity distributions are in very good agreement.
Furthermore we report on a new type of optical attenuator composed from two DOE gratings. Its dynamic transmission
range is 0.3% to 98%. The required lateral DOE shift is only 5 - 10 μm in the present design, so that the device can be
very fast and applicable for dynamic intensity stabilization.

Efficient diffractive optical elements from glass with continuous surface profiles

Diffractive optical elements (DOEs) are of rising importance for many industrial laser applications, especially for laser
beam shaping and laser beam splitting. Typically, such applications require high damage threshold of the diffractive
optical elements as well as high diffraction efficiency. Usually DOEs with multilevel (step-like) phase profiles are made
microlithographically and suffer from "quantisation" errors and scattering on profile derivative discontinuities. The steplike
structure lowers the DOE damage threshold compared to the intrinsic material values.
LIMO's microoptical technology is suitable for the production of high-precision free programmable continuous surface
profiles in optical glasses, crystals and metals. It can be applied for manufacturing of microlens and micro-mirror arrays
as well as for manufacturing of diffractive optics with continuous reliefs. Both the arrays and DOEs with continuous
relief are suitable for high efficiency laser beam splitting. However, the design approaches to obtain a desirable solution
for the corresponding continuous phase profiles are different.
The results of the wave-optical simulations made by LIMO's own program and by VirtualLab software, and
experimental studies for a 1 to 11 beam splitter with a continuous profile for the wavelength of 532 nm are presented.
Continuous phase profiles for the DOEs were designed by a procedure based on the theory of beam splitting by a phase
grating. Comparative theoretical and experimental studies were also done for splitting with a double-sided microlens
array. For both types of beam splitting the efficiency can be very high (> 98%). The DOEs show especially high
homogeneities of the resulting intensity distribution, however, they are much more sensitive to wavelength variations.
The microlens arrays demonstrate even weaker ghost orders as the DOE splitters and their surface profiles are simpler.
However, the efficiency and homogeneity suffer on interlens gaps.

Efficient beam splitting with continuous relief DOEs and microlens arrays

LIMO's unique production technology based on computer-aided design enables the manufacture of high precision
asphere single lenses and arrays, where every single lens can be individually shaped. These free form micro-optical
cylindrical lens and lens arrays find their application in various types of metrology systems. Due to the high precise
manufacturing of specially designed surface, single lenses can be bond directly onto sensor or sensor arrays, performing
efficient projection of signal onto detector. Optical modules based on micro-lenses arrays enable special intensity
distribution, as well as highly homogeneous illumination with inhomogeneity less then 1% (peak to valley) used in
illumination parts of inspection tools. Due to the special free form profile, a special case of asymmetric lens arrays can
offer extreme uniformity illumination at the target non orthogonal to the illumination path. The feature under inspection
can be uniformly illuminated even if it lies at a specific angle to the illumination. This allows better conditions for
measurement devices arranged orthogonal to the mask or wafer. Furthermore the use of micro-optics enables more
sufficient inspection of laser beam parameters for excimer or 
CO 2 lasers. Additionally very accurate metal patterns can
be applied on the optics and used as alignment marks, apertures or bonding features.

Inspection and metrology tools benefit from free-form refractive micro-lens and micro-lens arrays

Enabling the next technology nodes with optical technologies means further reduced error budgets for optical systems
and new optical approaches for higher precision and increased throughput. This contribution discusses important
aspects and features of laser beam shaping in optical systems for semiconductor manufacturing and inspection. Beam
shaping principles for different types of lasers and illumination requirements are explained.

Beam shaping: top hat and customized intensity distributions for semiconductor manufacturing and inspection

Optical lithography in the deep ultraviolet (DUV) region is being pushed to reach the limits of printing resolution. The
effort required to achieve the 32 nm structure with this technology puts very hard conditions and requests on the
illumination optics. Different kinds of illumination modes are combined to get into a regime of extreme numerical
aperture (hyper NA). Arrays of refractive micro-optics have been and continue to be the ideal solution for high
transmission homogenizing elements for several tool generations.
Illumination of the masks with high numerical aperture is critical for achieving the smallest structure on the
semiconductor material. Exposure tools use different illumination modes to get better imaging of certain mask structures.
The beam shaping necessary to create these illumination modes is achieved mostly with diffractive elements. Most of the
currently used modes can also be created with arrays of refractive micro-optics, manufactured from fused silica and
calcium fluoride. The advantage over the diffractive optical elements lies in efficiency, which comes close to 90%.
An important prerequisite for these special types of optical elements is LIMO's unique production technology which can
manufacture free form surfaces on monolithic arrays exceeding 200 mm edge length with high precision and
reproducibility. These homogenizing elements in the illumination optics can provide a custom designed intensity
distribution, and offer the possibility to correct the failure of other optical elements. Each lens can be designed
individually and can also be shaped asymmetrically. Thus unusual lens sizes and shapes can be produced, and various far
fields such as rectangles, lines, hexagons or multi-poles can be achieved.
In this paper we present novel refractive micro-optical elements which create rectangular dipole illumination. They can
also be combined in such a way as to create a quadrupole with variable intensity ratio between the vertical and horizontal
poles. The huge advantage of such a multipole illumination is polarization control and variable intensity in poles. Working on this combination, the resolution can be enhanced even further.

D. Hauschild

Papers

Novel refractive optics enable multipole off-axis illumination