James E. Webb

Hyper-numerical aperture imaging challenges for 193 nm

Lithographic methods of imaging in resist can be extended with the addition of immersion fluid. The higher index of refraction fluid can be used to print smaller features by increasing the numerical aperture beyond the limits of dry lithography. Alternately, an immersion optical system can achieve a larger depth of focus at the same numerical aperture as the equivalent dry lithography system. When numerical apertures are significantly greater than 1.0, polarization effects start to impact resolution seriously. Special illumination conditions will be used to extend resolution limits. Additional factors that affect imaging in resist need to be included if we are to achieve new resolution limits using high index of refraction materials to increase numerical apertures. In addition to material inhomogeneities, birefringence and optical surface effects, material absorption, coatings and index differences at boundaries will have a larger impact on image resolution as ray angles in the imaging system continue to increase with numerical aperture. Aerial and resist imaging effects that material characteristics have on polarization, uniformity and aberrations in the lens pupil will be studied.

Contamination and degradation of 157-nm stepper optical components: field experience at International SEMATECH

Significant improvement in 157nm optical components lifetime is required for successful implementation of pilot and production scale 157nm lithography. To date, most of the 157nm optics lifetime data has been collected in controlled laboratory conditions by introducing predetermined concentrations of contaminants and monitoring degradation in terms of transmission loss. This publication compliments prior work by documenting field experience with the 157nm Exitech Microstepper currently in operation at International SEMATECH. Failure mechanisms of various optical components are presented and molecular contamination levels in purge gas, tool enclosure, and clean room are documented. Finally the impacts of contaminant deposition and degradation of components on imaging performance is discussed.

Optical metrology for 193-nm immersion objective characterization

The production of integrated circuits with ever-smaller feature sizes has historically driven the shift to shorter wavelength radiation sources and increases in numerical aperture (the product of the sine of the imaging cone angle and the refractive index of the media at the image plane). When a next-generation design rule demanded a numerical aperture larger than was technically feasible, a move to a shorter wavelength was the only available solution. Immersion imaging is a detour along the path of shorter wavelengths. Here, the resolution improvement is achieved by exceeding the numerical aperture barrier of 1.0 (for optical systems that form an image in air) by placing a liquid between the final element and the image plane. This liquid layer presents numerous challenges to the optical metrologist. Results of testing a 193nm small-field immersion objective will be reported. The immersion fluid for this objective is de-ionized water. The characterization of the optical and physical properties of the water layer and the effect of those properties on the metrology of the objective will be discussed.

Aberration determination in early 157-nm exposure system

Aberrations, aberrations, here there everywhere but how do we collect useful data that can be incorporated into our simulators? Over the past year there have no less than 18 papers published in the literature discussing how to measure aberrations to answering the question if Zernikes are really enough. The ability to accurately measure a Zernike coefficient in a timely cost effective manner can be priceless to device manufacturers. Exposure tool and lens manufacturers are reluctant to provide this information for a host of reasons, however, device manufacturers can use this data to better utilize each tool depending on the level and the type of semiconductors they produce. Dirksen et al. first discussed the ring test as an effective method of determining lens aberrations in a step and repeat system, later in a scanning system. The method is based on two elements; the linear response to the ring test to aberrations and the use of multiple imaging conditions. The authors have been working to further enhance the capability on the test on the first small field 157 nm exposure system at International SEMATECH. This data was generated and analyzed through previously discussed methods for Z5 through Z25 and correlated back to PMI data. Since no 157nm interferemetric systems exist the lens system PMI data was collected at 248nm. Correlation studies have isolated the possible existence of birefringence in the lens systems via the 3-foil aberration which was not seen at 248nm. Imaging experiments have been conducted for various geometrys and structures for critical dimensions ranging from 0.13micrometers down to 0.10micrometers with binary and 0.07micrometers with alternating phase shift mask. The authors will review the results of these experiments and the correlation to imaging data and PMI data.

Optical design forms for DUV and VUV microlithographic processes

Microlithographic objectives have been developed for deep ultraviolet and vacuum ultraviolet wavelengths used for printing and inspection applications related to microlithographic processes. Refractive and catadioptric design solutions using fused silica, calcium fluoride and other crystals are discussed. Several reflective and catadioptric design forms having central obscurations will be compared to refractive forms. Design complexity, performance and limitations are compared.

Thin specialty glass for reliable thin film PV modules

Glass has long been used for photovoltaic (PV) module covers and thin-film (TF) module substrates and superstrates. These applications typically use float glass of soda-lime-silica composition and thickness ≥ 3.2 mm. Thin specialty glass is being considered as a replacement for substrates and superstrates for dual-glass laminated TF PV modules. This study focuses on module in-service stresses and their impact on mechanical reliability when using thin specialty glass. Substrate and superstrate thicknesses as low as 0.7 mm were analyzed using various support conditions. Hail impact tests were conducted according to IEC 61646 with standard ice balls (25mm diameter at 23 m/sec). Wind and snow loads as described in IEC 61646 were studied by finite element analysis. Reliability analyses were performed using experimental results, modeled stress levels, initial glass strength, and strength decrease over time (fatigue). This study concludes that thin specialty glass is reliable in the variety of configurations analyzed. Wind and snow load stresses increased systematically as glass thickness decreased. However, support structure configuration showed a greater influence on stress and reliability levels than glass thickness. High speed wind tests on full sized laminates with complete support structures provided validation of performance under realistic conditions. Thin specialty glasses are shown to be a reliable option for thin-film photovoltaic module substrates and superstrates.

Performance enhancement of 157-nm Newtonian catadioptric objectives

Newtonian design forms have been developed to explore higher numerical aperture imaging systems at a wavelength of 157 nm with elements made of CaF2 crystal. First-generation systems working at 0.60 NA are currently printing features smaller than 130 nm for resist-process-development. Second-generation design forms, working with variable numerical apertures above 0.75 NA, will push feature sizes significantly below 100 nm. Several aspects of second-generation designs have been improved to accommodate the need for characterizing and enhancing imaging performance. Closed-loop methods of optimization to reduce aberrations have been developed to characterize and control the effects of crystal-related birefringence on imagery. In addition these systems are learning vehicles to enhance knowledge of aberration-image performance dependence at high numerical apertures.

Accelerated testing of liquid crystal components for use in optical networking devices

Liquid crystal components developed for optical networking devices were exposed to accelerated environments. These components survived the equivalent of 22dBm/channel. Contrast ratio was stable at the 40dB level after 1250 hours of 85/85 exposure.

Na-free glass for elimination of potential induced degradation

Na-free glass was investigated as a means of reducing or eliminating potential-induced degradation (PID) in c-Si PV modules. Multi-cell and single cell modules, manufactured with glass on both the front and back sides (glass/glass), were biased at -1000V while being subjected to PID-accelerating conditions - 85°C and 85%RH, and 85°C while wrapped in Al foil. Under these conditions, all modules having conventional soda lime glass (SLG) on the front side failed. The use of sodium-free (Na-free) glass on both sides of glass/glass modules eliminated both moisture ingress and PID. Modules with Na-free glass on the front and SLG glass on the back exhibited significant PID resistance compared to modules with SLG on the front. Multicell modules with either glass type on the front, but with conventional backsheets, failed due to moisture ingress under 85°C/85% RH conditions. During the course of the testing, it was observed that shunt resistance was a sensitive leading indicator of PID, decreasing by almost two orders of magnitude before a decrease in efficiency became detectable.

The capability of a 1.3-NA μstepper using 3D EMF mask simulations

Lithographic methods of imaging in resist can be extended with the addition of immersion fluid. The higher index of refraction fluid can be used to print smaller features by increasing the numerical aperture beyond the limits of dry lithography. Alternately, an immersion optical system can achieve a larger depth of focus at the same numerical aperture as the equivalent dry lithography system. When numerical apertures are significantly greater than 1.0, polarization effects start to impact resolution seriously. Special illumination conditions will be used to extend resolution limits. Additional factors that affect imaging in resist need to be included if we are to achieve new resolution limits using high index of refraction materials to increase numerical apertures. In addition to material inhomogeneities, birefringence and optical surface effects, material absorption, coatings and index differences at boundaries will have a larger impact on image resolution as ray angles in the imaging system continue to increase with numerical aperture. Aerial and resist imaging effects that material characteristics have on polarization, uniformity and aberrations in the lens pupil will be studied.