Koen D'havé

Antiferroelectric liquid crystals with 45° tilt - a new class of promising electro-optic materials

Abstract Antiferroelectric liquid crystals with a tilt angle of 45 degrees have very interesting optical properties, which seem to have been overlooked so far - perhaps because such materials have hardly been available. We have prepared a four-component mixture of partially fluorinated compounds with a SmCa* phase in the interval between 27.4°C and 121.6°C, in which the tilt angle θ saturates at 45 degrees for T≤80°C, and we investigate the optical properties, theoretically and experimentally. One of the surprising features of 45 degree materials is that they permit a remarkably high contrast by virtue of an excellent dark-state, in spite of the fact that AFLC materials are notoriously difficult to align. This is because a 45° AFLC turns out to be (negatively) uniaxial instead of biaxial. We describe these properties and propose a number of potentially interesting new applications, including a polarizer-free display mode and a three-level “phase-only” modulator.

Progress in EUV lithography towards manufacturing from an exposure tool perspective

EUV lithography is a candidate for device manufacturing for the 16nm node and beyond. To prepare for insertion into manufacturing, the challenges of this new technology need to be addressed. Therefore, the ASML NXE:3100 preproduction tool was installed at imec replacing the ASML EUV Alpha Demo Tool (ADT). Since the technology has moved to a pre-production phase, EUV technology has to mature and it needs to meet the strong requirements of sub 16nm devices. We discuss the CD uniformity and overlay performance of the NXE:3100. We focus on EUV specific contributions to CD and overlay control, that were identified in earlier work on the ADT. The contributions to overlay originate from the use of vacuum technology and reflective optics inside the scanner, which are needed for EUV light transmission and throughput. Because the optical column is in vacuum, both wafer and reticle are held by electrostatic chucks instead of vacuum chucks and this can affect overlay. Because the reticle is reflective, any reticle (clamp) unflatness directly translates into a distortion error on wafer (non-telecentricity). For overlay, the wafer clamping performance is not only determined by the exposure chuck, but also by the wafer type that is used. We will show wafer clamping repeatability with different wafer types and discuss the thermal stability of the wafer during exposure.

Beam steering experiment with two cascaded ferroelectric liquid-crystal spatial light modulators

The design, construction, and evaluation of a laser beam steerer that uses two binary ferroelectric liquid-crystal (FLC) spatial light modulators (SLMs) operated in conjunction are presented. The system is characterized by having few components and is in principle lossless. Experimentally, a throughput of approximately 20% was achieved. The simple system design was achieved because of the high tilt angle FLC material used in the SLMs, which were specifically designed and manufactured for this study. By coherently imaging the first SLM onto the second SLM, pixel by pixel, we obtained an effective four-level phase structure with a phase step of 90 degrees. An appropriate alignment procedure is presented. The beam steering performance of the system is reported and analyzed.

Three-level phase modulator based on orthoconic antiferroelectric liquid crystals

Surface-stabilized orthoconic antiferroelectric liquid crystals (OAFLCs) have a director tilt of theta = 45 degrees and are, with no field applied, negatively uniaxial with the optic axis perpendicular to the cell substrates. We demonstrate that OAFLCs can be utilized to achieve lossless phase modulation with three almost equidistant phase levels. This turns out to be true also for polymer-stabilized OAFLCs, where the polymer network increases the switching speed of the device without affecting the phase modulation appreciably.

Lithography imaging control by enhanced monitoring of light source performance

Reducing lithography pattern variability has become a critical enabler of ArF immersion scaling and is required to ensure consistent lithography process yield for sub-30nm device technologies. As DUV multi-patterning requirements continue to shrink, it is imperative that all sources of lithography variability are controlled throughout the product life-cycle, from technology development to high volume manufacturing. Recent developments of new ArF light-source metrology and monitoring capabilities have been introduced in order to improve lithography patterning control.[1] These technologies enable performance monitoring of new light-source properties, relating to illumination stability, and enable new reporting and analysis of in-line performance.

Surface trapping of ions and symmetric addressing scheme for FLCDs

Abstract It has been considered that applying symmetric addressing schemes can avoid the ionic effect in FLCDs. In this paper, image sticking measurements have been performed to investigate the ion transport process on applying symmetric addressing signals and hence its influence. It has been found that by reversing the polarity of the addressing voltage each frame, image sticking effect has been prominently reduced, nevertheless not completely to zero. Further study has revealed the fact that although accumulation of ions is avoided to a large extent, a number of ions are still trapped at the interface of alignment layer and the liquid crystal layer. These trapped ions cannot be easily released by simply reversing the polarity of the addressing signal and thus influence the electric field distribution in FLC. The number of trapped ions depends on the property of the interface.

Electrical and Optical Measurements of the Image Sticking Effect in Nematic LCD'S

Abstract In this paper we present the results of measurements of the image sticking phenomenon. Simultaneous electric and optical measurements show that the regime leakage current and the optical transmission become asymmetric when a DC voltage is applied. In this way we can measure the change of the effective voltage over the liquid crystal. We believe this change of effective voltage is due to a DC electric field of ions which are separated by the applied DC voltage and trapped at the alignment layers.

Analog low-loss full-range phase modulation by utilizing a V-shaped switched ferroelectric liquid-crystal cell in reflective mode

We have studied the analog (V-shaped switching) mode in ferroelectric liquid crystals in reflective mode for analog phase modulation applications. We have found that several combinations of cell thicknesses and input polarization states exist for which near-lossless analog phase modulation with a range of approximately 2pi rad is obtained, and we demonstrate one such combination experimentally. Despite a slight deviation from the ideal conditions, e.g., the tilt angle was 38 degrees instead of the desired 45 degrees , virtually pure 1.6pi rad phase modulation was obtained; the measured values agree very well with our numerical simulations of the real device.

Focus and dose deconvolution technique for improved CD control of immersion clusters

As critical dimension (CD) control requirements increase and process windows decrease, it is now of even higher importance to be able to determine and separate the sources of CD error in an immersion cluster, in order to correct for them. It has already been reported that the CD error contributors can be attributed to two primary lithographic parameters: effective dose and focus. In this paper, we demonstrate a method to extract effective dose and focus, based on diffraction based optical metrology (scatterometry). A physical model is used to describe the CD variations of a target with controlled focus and dose offsets. This calibrated model enables the extraction of effective dose and focus fingerprints across wafer and across scanner exposure field. We will show how to optimize the target design and the process conditions, in order to achieve an accurate and precise de-convolution over a larger range of focus and dose than the expected variation of the cluster. This technique is implemented on an ASML XT:1900Gi scanner interfaced with a Sokudo RF3S track. The systematic focus and dose fingerprints obtained by this de-convolution technique enable identification of the specific contributions of the track, scanner and reticle. Finally, specific corrections are applied to compensate for these systematic CD variations and a significant improvement in CD uniformity is demonstrated.

Realization of a Four-Electrode Liquid Crystal Device With Full In-Plane Director Rotation

A liquid crystal device with micrometer-scale hexagonal electrodes has been fabricated and characterized. By using weak anchoring at the liquid crystal interfaces, the orientation of the director is completely governed by the applied electric fields. The appropriate voltage waveforms applied to electrodes allow the director in the liquid crystal layer to be rotated in the plane parallel to the substrates over large angles, exceeding 180 deg. This paper is a technological and experimental verification of an earlier proposed device concept