Optical Projection System Research Articles

Optical Materials for Immersion Lithography

193 nm immersion lithography optical projection systems using conventional UV optical materials and water as the immersion fluid, with planar lens/fluid interfaces, have a practical numerical aperture (NA) limit near 1.3. Higher-index resists and immersion fluids are being developed, but the bottleneck for pushing the NA further is the relatively small refractive index of the final lens element material. In this paper we consider the possibility of using novel high-index materials in the last lens element to get around this bottleneck and to push the NA limit to at least 1.5, while containing the lens system size and complexity. We discuss four classes of high-index (n>1.8), wide-band-gap, oxide-based materials that have the potential for being fabricated with optical properties appropriate for lithography optics: group-II oxides, magnesium-aluminum-spinel-related materials, ceramic forms of spinel, and aluminum garnets. We present measurements of the optical properties of these materials, including intrinsic birefringence, and we assess their prospects.

12.2: Distinguished Paper: Super‐short Focus Front Projector with Aspheric Mirror Projection Optical System

AbstractWe have developed a new front projector with a super‐short projection distance of 0.65 m at 100″. We installed a newly developed reflective‐type projection optical system in this projector, which was composed of only four aspheric mirrors that were free of color aberrations. Using this new system and a single‐chip DMD™*, we projected a picture that had excellent sharpness and high contrast from 40″–100″. This paper describes the principles, design, and characteristics of the new WT600™ front projector.

Optical projection systems for simulators

This paper discusses the features of modern displays intended for use in simulators: large viewing angles and the use of several projectors, as well as of devices for eliminating distortions and for ensuring fusion of the images. A description is given of the characteristics of optical projection display systems from the English company SEOS Displays, Ltd.?a world leader in the development and production of simulators for cosmonautics, aviation, and ground and water transport.

10.3: Rear Projection TV: Single Panel LCOS Lumen Throughput

AbstractThe main challenge for single‐panel LCOS liquid crystal on silicon rear projection TV is lumen throughput. Lumen estimates for q single panel LCOS optical rear projection systems are presented. ∼350–450 lumen throughput is predicted using a 120W UHP lamp at 540 color field per second update rates with a WXGA 1280×768 0.82″ LCOS pane for the HDTV color gamut. These predictions correlate with measured results. With the addition of a white “color” field, 850 lumens are possible using a 200W lamp with a single panel LCOS optical engine. Still higher lumen outputs are possible in front projection single panel LCOS systems with a less saturated color‐space, or smaller color gamut, as is typical for front projection systems. The efficacy improvements with single panel LCOS are attributed to 1 improved system F‐number 2 New liquid crystal mode 3 new driving schemes leading excellent 100% duty cycles and 4 engine design to match the attributes of MicroDisplay's 0.82″ diagonal LCOS panel.

Testing extreme ultraviolet optics with visible-light and extreme ultraviolet interferometry

Optics for extreme ultraviolet (EUV) lithography arguably have the most strict fabrication tolerances of any optical systems fabricated to date, and the development of EUV lithography pushes advanced optical fabrication techniques toward never before realized levels of figure accuracy and finish quality. As EUV lithography advances toward viability, the need for ultrahigh-accuracy wave front metrology tools has never been greater. To enable the development of diffraction-limited EUV optical systems, visible-light and EUV interferometries must work in close collaboration. We present a detailed comparison of EUV and visible-light wave front measurements performed across the field of view of a lithographic-quality projection optical system designed for use in the Engineering Test Stand developed by the Virtual National Laboratory and the EUV Limited Liability Company. The comparisons reveal that the present level of root mean square agreement lies in the 0.3–0.4 nm range, with an agreement of 0.15±0.03 nm, excluding astigmatism. Astigmatism is the most significant aberration component for the alignment of this optical system; it is also the dominant term in the discrepancy, and the aberration with the highest measurement uncertainty. With EUV optical systems requiring total wave front quality in the λEUV/50 (0.25 nm) range, and even higher surface-figure quality for the individual mirror elements (∼0.1 nm), improved accuracy through future comparisons, and additional studies, are required.

X-ray microanalysis of optical materials for 157nm photolithography

Next generation microelectronic circuits will have minimum dimensions below 100 nm. It is envisioned that 157 nm laser lithography will be the next step of optical lithography, A.C. Cefalas, E. Sarantopoulou, Microelectronic Engineering, V53, (2000) 465, followed by lithographies at shorter wavelengths e.g. 13 nm. At 157 nn vacuum ultraviolet (VUV) illumination of the mask target lithographic features with dimensions less than 100 nm on the photoresist could be achieved. However, there are problems related with the design of the optical projection system. This is mainly because most of the optical materials in one hand have high absorption coefficient and their optical properties degrade constantly with time under VUV irradiation. Taking into consideration the imaging requirements for this type of application, the refractive index variation over the illuminated volume of the optical material should be better than 10 -6, and hence optical elements should be prepared from ultra high purity materials. Crystals have been examined with the Jeol 2010 F microscope equipped by the energy dispersive X-ray spectroscopy (EDXS), and it has been proved to be an efficient quality control technique for identifying defects and impurities in crystal samples. A non-uniform distribution of concentration of various elements in wide band gap dielectric crystals in confined space regions from 2 to 50 nm was found, and this result sets the limitations in the optical quality of the crystals.

Development of a microslide agglutination assay with the aid of an inexpensive projection microscope

A microslide agglutination assay was developed involving the mixing of 2.5 μl each of antiserum and a cell suspension of Listeria monocytogenes. Cell agglutination in the final volume of 5.0 μl was visually observed at a direct magnification of 22× on the projection screen of an inexpensive US$20 projection microscope. The procedure has the advantage of increasing by a factor of 20 the number of agglutination assays that can be performed with a given volume of antiserum with the use of an inexpensive optical projection system.

Micro-humps formed in excimer laser ablation of polyimide using mask projection system

In this paper we report the results of an investigation into surface deformation caused by thermal effects during excimer laser ablation of polyimide. Obvious surface deformation around hole entrances was observed during the experiment. The surface topology and cross section of the ablated holes were analyzed using topography measurement tool and scanning electron microscopy. It was shown that a micro-hump of 17 to 150 nm in height and 1 to 3 μm in width was formed above the level of the unablated surface. The deformed surface showed rough and color-changed characteristics. An optical diffraction model was employed to explain the cause of this kind of deformation. It was found that the ablating and heating by a near- and under-threshold laser beam became a thermal effect in polyimide material ablation, which was contributed to by a diffraction effect of the optical projection system.

Current Status of the EUV Engineering Test Stand.

The EUV Engineering Test Stand (ETS) has demonstrated the printing of static and scanned 100nm dense features. This milestone was first achieved in 2001 with a developmental set of projection optics (P0 Box 1) and with a low power LPP source (40W drive laser). Since that time, the source has been upgraded to a 1500W (3 chains, 500W per chain) TRW drive laser. Operating with one chain (500W), the printed static images for 100nm features are comparable to those obtained with the low power source, while exposure time was decreased by a factor of 15 to 30. One hundred nanometer dense features printed in step-and-scan mode are of the same image quality as those obtained in static imaging. The stages have demonstrated combined x any y j fitter values of 2 to 4nm RMS over most of the wafer stage travel range, at the designed scanned speed of 10mm/s at the wafer. This value is less than half of the designed value and provides sufficient stability to support printing of 70nm features. EUV specific sensors at the wafer and reticle planes have been demonstrated with the TRW laser and integration will be completed later this year. The developmental projection optics (P0 Box 1) will be replaced later this year with an improved projection optics system (PO Box 2) capable of printing 70nm dense features.

Front‐projection optical‐system design for reflective LCoS technology

Abstract— Optical designs for three‐panel LCoS projection systems are reviewed. The impact of polarization aberrations in prism‐based systems is discussed and a simple model to analyze the sensitivity of contrast to thermal gradients in prisms is presented. To eliminate stress birefringence in LCoS projection systems, we have developed a projection optical system that does not require the use of polarizing prisms. An improved off‐axis design has been designed that simplifies manufacture and reduces cost. The performance of systems based on this architecture will be discussed.

Nanofabrication and diffractive optics for high-resolution x-ray applications

Short wavelength x-ray radiation microscopy is well suited for a number of material and life science studies. The x-ray microscope (XM1) at the Advanced Light Source Synchrotron in Berkeley, California uses two diffractive Fresnel zone plate lenses. The first is a large condenser lens, which collects soft x-ray radiation from a bending magnet, focuses it, and serves as a linear monochromator. The second is the objective zone plate lens, which magnifies the image of the specimen onto a high-efficiency charge coupled device detector. The objective lens determines the numerical aperture and ultimate resolution. New objective lens zone plates with a minimum linewidth of 25 nm and excellent linewidth control have been fabricated using Berkeley Lab’s 100 keV Nanowriter electron beam lithography tool, a calixarene high-resolution negative resist, and gold electroplating. Although the condenser zone plate is less critical to the resolution of the instrument, its efficiency determines the flux on the sample and ultimately the exposure time. A new condenser zone plate was fabricated and has a 9 mm diameter, 44 000 zones, and a minimum zone width of 54 nm (optimally the condenser and objective should have the same zone width). It is also fabricated with the Nanowriter at 100 keV using poly(methylmethacrylate) resist and nickel electroplating. The phase shift through the nickel absorber material enhances the diffraction efficiency over an amplitude only zone plate. To evaluate the microscope’s performance transmission test patterns have been made and imaged. Lineout data show modulation for 30 nm lines and 60 (1:2) spaces to be almost 100%. These new diffractive optical elements represent a significant advancement in the field of high-resolution soft x-ray microscopy. Diffractive optical elements have been used to measure the wave front error of an extreme ultraviolet projection optical system. The reference wave is generated by the spherical wave generated by diffraction from a small freestanding pinhole.

Aberration measurement from specific photolithographic images: a different approach

Techniques for measurement of higher-order aberrations of a projection optical system in photolithographic exposure tools have been established. Even-type and odd-type aberrations are independently obtained from printed grating patterns on a wafer by three-beam interference under highly coherent illumination. Even-type aberrations, i.e., spherical aberration and astigmatism, are derived from the best focus positions of vertical, horizontal, and oblique grating patterns by an optical microscope. Odd-type aberrations, i.e., coma and three-foil, are obtained by detection of relative shifts of a fine grating pattern to a large pattern by an overlay inspection tool. Quantitative diagnosis of lens aberrations with a krypton fluoride (KrF) excimer laser scanner is demonstrated.

Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy

The phase-shifting point-diffraction interferometer (PS/PDI) was recently developed and implemented at Lawrence Berkeley National Laboratory to characterize extreme-ultraviolet (EUV) projection optical systems for lithography. Here we quantitatively characterize the accuracy and precision of the PS/PDI. Experimental measurements are compared with theoretical results. Two major classes of errors affect the accuracy of the interferometer: systematic effects arising from measurement geometry and systematic and random errors due to an imperfect reference wave. To characterize these effects, and hence to calibrate the interferometer, a null test is used. This null test also serves as a measure of the accuracy of the interferometer. We show the EUV PS/PDI, as currently implemented, to have a systematic error-limited reference-wave accuracy of 0.0028 waves (lambda/357 or 0.038 nm at lambda = 13.5 nm) within a numerical aperture of 0.082.

CMOS device fabrication and the evolution of optical lithographic exposure tools

Advances in the performance of integrated circuits have long been fuelled by continuing progress in microlithography. For at least thirty years, the minimum feature size of leading edge semiconductor devices have been reduced to 70% of the previous generation every 3 years. In recent years, this extraordinary shrink rate in feature size has accelerated, so that production of 250nm generation microprocessors and DRAMs started in 1997, a year before the 1994 National Technology Roadmap for Semiconductors forecast, and initial production of 180nm generation devices is now forecast for 1999, a full two years before the 1994 roadmap predicted. In this paper, aspects of optical lithography being developed to address the requirements of advanced CMOS device production will be outlined. The extraordinary synergy between the requirements of IC fabrication and the evolution of optical lithographic projection systems will be explored. The 180nm and 150nm device generations will use 248nm lithography on high NA, KrF excimer laser steppers and scanners and DUV chemically amplified resists. It is likely that the 130 and 100nm generations will be supported by 193nm lithography using ArF scanning tools and VDUV CA resists. A further extension of optical lithography, to 157nm, using the F2 excimer laser as a source, is possible to address production at 100 and 70nm. Beyond the 70nm generation node, conventional optical lithography will become difficult, if not impossible, to use to fabricate the most critical masking layers and other, post-optical, lithography technologies may start to find their niches in IC manufacturing.

Distorted wave front produced by a high-resolution projection optical system having rotationally symmetric birefringence

The influence of birefringence caused by rotationally symmetric stress distribution in a high-resolution projection optical system is investigated. The general form of the pupil function is derived based on the Jones matrix calculation, expressing the wave front as a combination of the two orthogonal polarization components. Assuming a linearly polarized incident beam, it is found that the main polarization portion of the wave front exiting the projection lens has astigmatic aberration in the Seidel region and shows phase singularity at four pupil points at which the amplitude transmittance becomes zero.

Projection display throughput: Efficiency of optical transmission and light-source collection

The optical system for a projection display based on three miniature reflective spatial light modulators (SLMs) is described. The total projection display light throughput is a function not only of the optical system efficiency but also of the light-collection and light-coupling efficiency referred to here as the lamp-SLM coupling. The optical system efficiency is the transmission of the optical components in the projection display. These are examined in detail through measurements and estimates of the components in the system. The various optical components include UV-IR filtering, illumination optics, polarization optics, color separation and recombination optics, SLM efficiency, and projection optics. The lamp-SLM coupling, which is the amount of usable light that can be collected from a particular lamp coupled to the projection optical system, is determined by the light-source luminance, the efficiency of the light-collection optics, and the optical system etendue. For small SLMs, less than 50 mm diagonal, for example, the lamp-SLM coupling efficiency falls off rapidly with SLM size and optical system f-number. The dependence of this coupling efficiency on SLM size is determined from measurements of the light-collection efficiency as a function of aperture size, where the apertures are used to simulate SLMs of the same dimensions. A variety of arc lamps were investigated for use in the projection display based on IBM reflective SLM devices. The lamp-SLM coupling dependence on arc gap was determined. The measurements are used to compare various lamps and to estimate directly the throughput for the complete projection system. The SLMs used in the projection display are liquid crystal devices which utilize only one polarization of light while discarding the second. Converting the discarded polarization into useful light can in principle double the throughput of the projector. However, polarization conversion results in doubling of the size of the light source and thus produces less efficient lamp-SLM coupling, particularly for long-arc-gap lamps. Measurements and analysis of throughput enhancement by polarization conversion are presented, and the dependence on arc gap and optical system etendue is discussed.

Polarization analyses of aerial images produced by an optical lithography system

The influence of polarization on the image formation of one-dimensional periodic patterns in a projection optical lithography system has been investigated. Assuming a linear polarizer at thelens pupil, I derived a simple expression representing the image intensity as the summation over spatial-frequency harmonics as well as three orthogonal polarizations. I calculated the coefficient for each image component as a function of the pattern frequency by independently varying the degree of partial coherence such that the image qualities of the two extreme polarization cases could be thoroughly compared.

At-wavelength characterization of an extreme ultraviolet camera from low to mid-spatial frequencies with a compact laser plasma source

The final alignment and characterization of an extreme ultraviolet (EUV) lithographic projection optics system must be performed at the operating wavelength in order to account for errors associated with the combined effect of multilayer coating and substrate fabrication error. Two complementary metrology techniques have been developed which can operate on a compact laser plasma source: EUV lateral shear interferometry (LSI) and aerial image monitoring (AIM). LSI quantifies residual low spatial frequency wave front errors associated with the “figure” of each optical element. To characterize the effect of mid-spatial frequency errors that lead to scatter in the image plane, an EUV aerial image monitor was employed. EUV interferometry was compared to visible light interferometry measurements of the optical system and was found to be in reasonable agreement for low spatial frequency errors. The contrast as measured by AIM and photoresist exposures indicates an additional reduction of contrast associated with scatter due to large mid-spatial frequency errors.

Monochromatic Projection Optical System for ArF Excimer Laser Lithography

We have developed an ArF excimer laser exposure system that combines a monochromatic quartz projection lens system and a spectrally-narrowed ArF excimer laser. The bandwidth of the laser was narrowed to 0.7 pm by using three prisms and an etalon. We delineated 0.2 µm lines-and-spaces using a chemically amplified resist and 0.16 µm lines-and-spaces using a top surface imaging resist.

Process Technology for Micromachines. Precise Hole Array Drilling of Ceramics by Excimer Laser Photo-ablation Process.

A precise drilling technology with cross section control for use on thin ceramic materials has been developed using Excimer laser photo-ablation process. An homogenous illumination optical system and projection system of high resolution optical elements are applied to a newly designed mass production system. Homogeneity of laser energy under±5% on the mask surface has been obtained by the illumination system, making a final hole array with a distribution of exit hole diameter of 3.5±0.7μm in one processing field of 1.2×1.2mm square area. If a crack occurs in the process because of high repetition rate or high energy from the laser, productivity declines. Designing a specimen holder and He-gas assist enabled cracks to be avolded even at high repetition rate of 200Hz.This paper describes hole array drilling technique with cross section control of ceramics by multi musk method, production system and an application.