Performance of pupil-filtering stepper-lens system

For 100nm-level patterning using optical lithography, high NA system and various RETs such as PSM, off-axis illumination and OPC are obviously required. In particular, assistant feature (AF)-OPC is indispensable to overcome narrow depth of focus (DOF) caused by iso-dense bias and to compensate for linearity difference under the given OAI condition. Previously we reported the application of AF-OPC in DRAM process with 120nm design rule. The extraction of OPC rule and the feasibility of AF-OPC were successfully confirmed by experimental method in real process. In this paper, more comprehensive and aggressive AF-OPC rule is investigated. The old rule is modified in order to obtain larger common DOF. TO avoid dead zone that means discontinuity between dense line and semi-dense line, we apply a comprehensive rule such as insertion of AF between the neighboring main patterns as many as possible. As a result, the discontinuity of OPC application, which is used with or without AF in the boundary region, is effectively minimized. Also, polygon-shaped AF is used to improve DOF of special main pattern. And then, the mask specification and the behavior of isolated line pattern are predicted in case of very high NA KrF and ArF lithography by simulation result. Considering 100nm design rule, the decrease of common DOF is expected to be severer than now. Finally, the optimum AF-OPC rules such as AF size, space and shape are available and shown in case of very high NA KrF and ArF lithography.

Novel diffraction limit implied geometric lens imaging model based on a finite number of pixels for integral imaging display

Markets continuously demand higher resolution and higher productivity in flat panel display (FPD) exposure systems. Our solution to improve resolution and productivity is the use of broadband wavelength illumination. A larger depth of focus (DOF) is also very important to achieve higher productivity because inadequate DOF can cause product defects. To obtain higher resolution and larger DOF, off axis illumination (OAI) conditions have been widely used. OAIs using a narrowband wavelength illumination are well documented and sufficiently studied. On the other hand, Canon FPD exposure tools use a broadband illumination source to achieve higher resolution and productivity. To obtain sufficient OAI effects in broadband exposure lithography, new technology should be developed with consideration of broadband wavelength because OAI effects are different between broadband and narrowband illumination. In this paper we introduce divided spectrum illumination (DSI), a new design concept proposed to achieve both high resolution and large DOF by optimizing the broadband illumination source wavelength band depending on the illumination angle. Experimental imaging results of line and space patterns with a line width of 1.0-μm and pitch of 2.0-μm showed that the DSI design improved resolution. Results showed that test patterns imaged using traditional narrowband OAI could not be resolved at the top of the resist even at the best focus, however resist profiles using DSI were sharp enough to retain pattern fidelity at the top of resist. DOF with DSI also improved 21 % compared to traditional OAI.

We have studied the optical proximity effect, the depth of focus (DOF) and the mask-error-enhancement factor (MEF) of ArF attenuated phase shift masks (att. PSMs) for the application to 100-nm logic gate patterns. In the previous work, we have compared the performances of the binary mask with those of the 6% transmittance ArF att. PSM for printing 100-nm lines, and we obtained the better performances on the att. PSM with the annular illumination. In this paper, we reported the evaluation results of the higher transmittance att. PSMs. We fabricated them with 6-20% transmittance (quartz reference). Those masks have TaSiO bi-layer shifter with two tones. We did not use assist bars. An ArF scanner with NA 0.60 and a positive chemically amplified ArF resist with the thickness of 0.4 um are used. 100 nm semi-dense lines with a pitch of 250 nm are exposed without bias and 100 nm isolated lines are exposed with biases of 5 -15 nm (on wafer). Our results show that the higher transmittance att. PSMs can improve the DOF and the MEF on both semi-dense lines and isolated lines. In our experiments, the highest transmittance of 20% gives the broadest DOF of 0.4 um and the smallest MEF of 1.78 and 1.11 for semi-dense lines and isolated lines respectively. However, some higher transmittance PSMs may create the side lobes on the large patterns. From these results, we demonstrated that that 10% transmittance is suitable for the ArF att. PSMs for the fabrication of 100 nm line patterns.

There are three usable regional depth phases, sPg , sPmP , and sPn , and their corresponding reference phases, Pg , PmP , and Pn . The differential time between each depth phase and its reference phase can be used to estimate earthquake focal depth. We have developed a method to determine focal depth for moderate and small earthquakes by using a regional depth-phase modeling (RDPM) method. We used a default focal mechanism to generate the differential times for all earthquakes. To estimate the reliability of the modeled focal depths, we compared our solutions with those obtained by other methods and found the consistency is good. Because the focal depths estimated by RDPM are model dependent, we tested the extent of the dependency and found that a 10% error in the crustal model may generate a 10%–15% error in the modeled depth. The absolute error is determined by the error in the crustal model and the focal depth itself. We found that earthquake location errors have only a small effect on the modeled focal depths. By analyzing synthetic and observed waveforms, we found a distance window within which sPmP and PmP are well developed, and, within the window, the P portion of the waveform is relatively simple and sPmP and PmP are easy to identify. We also demonstrated that the assumptions of sPmP and PmP are correct, that regional depth phases are not developed or not discernible in some regions, and that regional depth phases have special features that can be used to identify those phases.

<sec>A kind of off-axis meta-lens with large focal depth based on a single-layer metasurface is designed and fabricated. Our proposed off-axis focus is realized by combining the two functions of deflection and focus through phase superposition method, and the focal depth can be increased by optimizing the input aperture and off-axis deflection angle. Three-dimensional finite difference time domain (FDTD) method is used for numerical simulation to construct the off-axis meta-lens, then the off-axis meta-lens is fabricated and its focus performance is tested in a microwave anechoic chamber.</sec><sec>Experimental results indicate that at the designed electromagnetic wave frequency (9 GHz), the measured off-axis deflection angle is 27.5° and the focal length is 335.4 mm, which agree with the designed values of 30° and 350 mm. The measured full-wave half-maximum (FWHM) at the focal point is 48.2 mm, however, the simulated FWHM is 40.2 mm, which means that the imaging quality of the measured focus spot is slightly worse than the simulated one. This is mainly due to the fact that the actual parameters of the fabricated meta-lens are inconsistent with simulated parameters. In addition, during the measurement, the large sampling interval in the x- direction also leads to experimental errors.</sec><sec>The focusing efficiency of the off-axis meta-lens at the working frequency of 9 GHz is calculated to be 16.9%. The main reason for the low focusing efficiency is that the plasmonic metasurface works in the transmission mode, which can manipulate only the cross-polarized component of the incident wave, and the maximum efficiency will not exceed 25%. Moreover, the focal depths at 8 GHz, 9 GHz and 10 GHz are 263.2 mm, 278.5 mm and 298.2 mm, respectively, which are 7.02 times, 8.36 times and 9.98 times the corresponding wavelengths, indicating that a larger focal depth off-focus meta-lens is achieved. </sec><sec>This kind of off-axis meta-lens has a simple structure, good off-axis focus ability and large focal depth, which has potential applications in a compact and planar off-axis optical system and large focal depth imaging system. Although the working waveband in this article is the microwave band, according to the size scaling effect of the metasurface, it is also possible to design a large focal depth off-axis meta-lens in other bands such as visible light and terahertz bands by using the same method.</sec>

Photoacoustic microscopy (PAM) is a promising imaging modality because it is able to reveal optical absorption contrast in high resolution on the order of a micrometer. It can be applied in an endoscopic approach by implementing PAM into a miniature probe, termed photoacoustic endoscopy (PAE). Here we develop a miniature focus-adjustable PAE (FA-PAE) probe characterized by both high resolution (in micrometers) and large depth of focus (DOF) via a novel optomechanical design for focus adjustment. To realize high resolution and large DOF in a miniature probe, a 2-mm plano-convex lens is specially adopted, and the mechanical translation of a single-mode fiber is meticulously designed to allow the use of multi-focus image fusion (MIF) for extended DOF. Compared with existing PAE probes, our FA-PAE probe achieves high resolution of 3-5 μm within unprecedentedly large DOF of >3.2 mm, more than 27 times the DOF of the probe without performing focus adjustment for MIF. The superior performance is first demonstrated by imaging both phantoms and animals including mice and zebrafish in vivo by linear scanning. Further, in vivo endoscopic imaging of a rat's rectum by rotary scanning of the probe is conducted to showcase the capability of adjustable focus. Our work opens new perspectives for PAE biomedical applications.

Small-pupil versus multifocal strategies for expanding depth of focus of presbyopic eyes

Given a resolution requirement and the imaging wavelength, there is an optimum NA. If the NA is too low, the resolution cannot be achieved, but if the NA is too high, the depth of focus, which is inversely proportional to NA2, becomes unacceptable. There is an optimum NA where the depth of focus is maximum. In this paper the optimum NA given by the aerial image is determined unambiguously by evaluating the normalized depth of focus k2 as a function of the normalized resolution k1, then identifying the k1 at which the function k2/k12 is maximum. The optimum NA is then simply the optimum k1 multiplied by (lambda) /NA. A substantial amount of work is required to evaluate k2 as a function of k1 by means of exposure-defocus trees and windows. In this paper, all k2 and k2/k12 as functions of k1 are given for line-space pairs, isolated line openings, isolated spaces, holes, islands, combination of the 3 long features, and of all the 5 features. A 10% exposure budget is used to simulate the situation of single layer resist systems and 30% exposure budget for multilayer resist systems. The results show optimum NA for individual feature shapes much lower than expectation, gaining insights to the problems occurring in manufacturing and in reducing the usable k1. They also lead to the following observations. The optimum k1 for single resist systems ranges from 0.57 to 0.87 depending on the feature shape. That for multilayer resist systems ranges from 0.42 to 0.7. Opaque spaces have the lowest optimum k1. Line openings have the largest depth of focus. Positive resists and negative masks are preferred to delineate contact holes. Negative resists and negative masks are preferred to delineate gates and metal lines. The opaque island is the limiting feature for the line-space pair, line, space, hole, and island combination at larger k1. That for the combination of line-space pair, line, and space is the line-space pair.

The pupil-filtering lens system for optical lithography experimentally evaluated in this study is based on an i-line stepper lens with a numerical aperture (NA) of 0.5, and its pupil function can be set arbitrarily to change the image characteristics. With the previously proposed super-FLEX filter, the depth of focus was three times that of the system without the filter, and the resolution for window patterns was 20% higher. Image characteristics of the filter, such as the edge-enhancing effect and the proximity effect, are discussed and were experimentally evaluated. The optical and mechanical tolerances required for this system are shown to be within ranges that are attainable in practical stepper systems.

It is known that the resolution possible from a system based on strongly convergent conical wave fronts is capable of transverse and longitudinal resolution of the order of a wave length. Further, this system (“axicon”) should maintain this resolution throughout the region in which the waves converge on an axis. It is, in effect, an imaging device with a very wide aperture and large depth of focus. The latter offers promise for acoustical imaging of certain organs, such as the breast. We have built an axicon designed for a depth of focus of 10 cm, with a convergence half‐angle of 30 degrees. It consisted of a 4‐MHz plane transducer, whose beam was first spread out, then reformed into the conical shape by two axially symmetric, nonspherical mirrors. The system, tested with a fine wire target, gave a beam profile close to theoretical expectation. Wide “skirts” are observed. When used for imaging, it gave the expected resolution, but the skirts gave rise to noise and certain artifacts. The expected depth of focus was observed.

Fan-Out wafer level packaging has seen rapid adoption over the last few years due to its form factor, performance, and cost advantages compared to 3D packaging techniques. Redistribution layers (RDL) are used to route the very high density connections on the chip to the much lower density connections of the substrate. Multiple layers of RDL are required in order to match the line density of the chip. This in turn increases the cost of the total package. Decreasing the metal line width for RDL supports reducing the number of redistribution levels and decreasing the total packaging cost. Reducing the RDL width requires tightening of requirements for the lithography tool and the photoresist process. Also, electroplating and etching processes need to be enhanced. This study investigates creation of RDL structures using a through resist electroplating process. A test vehicle was designed that contains various test structures used to characterize the process. This includes metrology structures for in-line monitoring, CD-SEM measurements, and comb and serpentine electrical test structures. The process starts with the deposition of a Cu seed layer needed for electroplating followed by lithography, electroplating, resist strip and Cu seed removal. The minimum thickness of the seed layer is constrained since the uniformity of the electroplating process also depends on the resistance and thus thickness of this layer. This thickness in turn has a large impact on line fidelity after removal of the Cu seed at the end of the process. In order to fabricate RDL lines with sufficient low resistance the plating thickness needs to be maximized. This translates to maximizing the aspect ratio of the photoresist pattern. In order to minimize the CD loss by Cu seed etching the wet process has to be controlled carefully. The lithography process window must have a large depth of focus in order to accommodate local height variation due to wafer topography in Fan-Out packaging. The optical lithography tool used in this study has a variable numerical aperture (NA) that can be optimized to maximize depth of focus while maintaining resolution performance in photoresist. After completing the development of the 1 μm RDL process the electrical performance was validated with meandercomb electrical test structures. The results show that lithography needs to be performed at the isofocal setpoint in order to obtain the optimal depth of focus of 7.5μm. Seed etch will reduce the Cu linewidth by 300 to 500nm and an offset in lithography CD may be required to compensate this loss. With optimal conditions good electrical yield is obtained for 1.0μm L/S meandercomb test structures.

Vision-based techniques used in automatic microassembly are limited by inherent problems such as small depth of focus (DOF) and field of view (FOV). Microassembly operations need, however, initially to detect micro parts in a wide FOV and large DOF yet to maintain high resolution for the final state. This paper proposes an active zooming control method that enables adjustment of the FOV and DOF dynamically according to the position and focus measure of micro objects. The proposed method is based on an artificial potential field with the capability to combine different kinds of constraints such as the FOV, focus measure, and joint limits, into the system. The proposed method can ensure the microscopy to maintain a wide FOV and large DOF initially, and high resolution at the end. Simulation and microassembly experimental results are provided to verify feasibility of the proposed approach.

OFF-AXISILLUMINATIONFOR EXTENDINGOPTICAL MICROLITHOGRAPHICSYSTEMPERFORMANCE Steve Brainerd Supervising professor: Raj Solanki Improvements in depth of focus and resolution for microlithographic projection printing systems can be achieved by illuminating the object (mask) with oblique or off axis rays. Three types of illumination as used on a waferstepper with a 0.48 numerical aperture objective lens were characterized for their effect on resolution, depth of focus, and proximity. The illumination types evaluated were partially coherent (sigma =0.64), off-axis quadrupole (inner sigma 0.26, outer sigma 0.70 and opening sigma = 0.26),and off-axisring annular(inner sigma=0.40 and outer sigma =0.70). All projected images were recorded in a modem high contrast positive photoresist. While this type of illumination offers significant performance gains. it is not without limitations. Two shortcomings are the loss of image imKlianceand proximity effects. This project will evaluate original methods to improve the later. The results show that off-axis annular and quadrupole illumination improve the depth of focus for dense 0.36 to 0.50 micron linewidths as compared to partially coherent illumination by 45% and 56% respectively. The defocus proximity effects with off-axis illumination mentioned above is where the depth of focus for an exterior line on a dense line-space group is significantly less than an interior line. Three potential solutions evaluated for improving this exterior linewidth proximity effect were: 1. The use of special sub-resolution compensating structures on the mask (object) features, 2. Contrast enhancement photoresist processing, and 3. Anti-reflective substrate coatings. The use of a modified reticle improved the depth of focus for an exterior line, while contrast enhancement and anti-reflectivecoatings did not offer any improvement. The experimental results were modeled using Silvacos SOLID 3.1 optical lithography simulator (Sjmulation of !!ptical LIthography in three Dimensions). Excellent cOlTelationbetween the SOLID simulation model calibrated to the specific photoresist process used here and the experimental results was achieved.

The trade-off between lateral resolution and depth of focus (DOF) severely limits the capability of endoscopic optical coherence tomography (OCT) for high-resolution deep-tissue imaging. To address this issue, we developed a novel miniature all-fiber axicon OCT probe by inserting a segment of gradient-index (GRIN) fiber between a piece of single-mode fiber (SMF) and an axicon polished from a no-core fiber. The GRIN lens served as a beam expander extending the probe DOF by 5.2 times while maintaining a high lateral resolution of 2 μm. The DOF extension was experimentally verified by measuring the axial profile of the probe output beam and further by imaging multilayered polymer tapes and onion samples. The designed probe with a tight focus over a large DOF holds great potential in endoscopic OCT imaging of deep tissues at the cellular level.