Resolution enhancement with source-wavelength optimization according to illumination angle in optical lithography

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.

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.

This paper reports the simulation study of a new phase-shifting mask (PSM), quartz-etch halftone-rim, applied on one and two dimensional patterns, such as contact hole, line/space and polylayer using positive- tone resist (optical parameters focused mainly on (lambda) equals 365 nm, NA equals 0.5, sigma equals 0.6, halftone chrome T equals 4%). When apply this halftone-rim PSM on 0.35 micrometers dense contact holes, the simulations indicated that optimized rim width is 0.15 (lambda) /NA (0.11 micrometers ). Optimized single side width for biased contact is 0.1 micrometers (0.9 X optimized rim width). Optimized biased contact is 0.55 micrometers (0.35 + 0.1 + 0.1 micrometers ) which is 1.57 times larger of 0.35 micrometers design rule. Exposure latitude is 9.9%. Total depth of focus (DOF) is about 1.2 micrometers . Compared with sized-rim PSM (chrome T equals 0%), this halftone-rim PSM has lower operation exposure dose; higher exposure latitude and larger DOF. When apply it on the 0.35 micrometers dense and isolated line/space, optimized rim width is 0.12 (lambda) /NA (0.09 micrometers ). Optimized single side width for biased space is 0.08 micrometers (0.9 X optimized rim width). The total DOF is about 1.0 micrometers for line/space. By using combination of halftone-rim PSM and off-axis illumination (OAI), the total DOF will reach 1.2-1.5 micrometers for 0.35 micrometers dense line/space. However, the improvement for line/space is insignificant if compared with sized-rim PSM. Halftone-rim PSM has advantages on patterning of contact holes but not on line/space in this study.

Generation of needle-shaped beam with super-resolution spot and large depth of focus using diffractive optics

Progress in today's semiconductor industry has been mainly achieved by decreasing the minimal feature size and increasing the complexity and thus the nonplanarity of the devices. Therefore lithography tools have to provide high resolution with a reasonably large depth of focus. Well-established methods to achieve both requirements are off-axis illumination techniques. As topography effects such as nonplanar electromagnetic scattering and notching are critical for line- width control, a rigorous three-dimensional exposure simulation considering both nonplanar surfaces as well as off- axis illumination is of utmost interest. We propose a rigorous method that meets the two challenges of nonplanar substrates and off-axis illumination. Our approach is based on a novel extension of the differential method to the third dimension. It is based on a Fourier expansion of the electromagnetic field in the lateral coordinates and thus belongs to the category of frequency-domain solvers. Due to the moderate computational costs nonplanar topography simulations including off-axis illumination can be performed on common engineering workstations. We will give a survey over the numerical algorithm of the differential method, describe the interface to the imaging and development module, and demonstrate the ability of the overall simulator by comparing simulation results for contact-hole printing over a dielectric and reflective substrates for various illumination apertures.

Photoacoustic microscopy (PAM) achieves high lateral resolution through tight light focusing but suffers from a narrow depth of focus (DOF). Here the enhancement of PAM image quality for 3D microscopy is aimed by achieving high resolution over a large DOF, accurately restoring object sizes, and minimizing artifacts introduced during image acquisition. A novel approach is proposed that initially acquires 3D PAM images with a large DOF but low resolution through loose light focusing, and subsequently enhances resolution and image quality using a deep learning network termed LDHR‐Net. This network is trained on synthetic low‐resolution and high‐resolution image pairs generated using a physical Gaussian beam model, which accounts for depth‐dependent blurring. Application of the proposed model to experimentally acquired phantom data demonstrates significant improvements, achieving lateral resolution of ≈4 µm across an unprecedentedly large DOF of ≈4.5 mm, a 25‐fold increase compared to the intrinsic DOF (≈0.18 mm) of a PAM system with the same lateral resolution. The model's effectiveness and robustness are further validated qualitatively and quantitatively on in vivo microvasculature structures, including those in the mouse ear, back, and brain. This method provides an effective and practical solution for obtaining high‐quality 3D PAM images.

Assist bar Proximity Correction (OPC) has been demonstrated to increase across pitch performance and depth-of-focus of semi-dense to isolated lines. As the sub-resolution assist feature (SRAF) or assist bar's size increases, so does its desired lithographic effect, as well as its undesired printability. In other words, when large assist features are required at isolated pitches, the assist features may print. A frequency-preserving assist bar solution is the most preferred one, but difficult to realize for opaque assist features due to printability. The concept of frequency-preserving Gray Assist Bar OPC has been introduced as a method to extend imaging performance for small features across a wide rage of duty ratios. In this paper, we will present the experimental validation of this concept. The Gray Assist Bar mask was manufactured using a two-level lithography process, and the optical properties have been characterized using a Woollam VUV VASE system. Additional metrology was performed using an AFM (SNP9000) and CD SEM (KLA8250XR). Exposures on a 0.75NA 193nm scanner clearly show the expected effects. The use of the Gray Assist Bar features reduces the through pitch critical dimension (CD) variations significantly and can hence be regarded as an Optical Proximity Correction. The isofocal inflection point of aerial images is shifted in cases with Gray Assist Bars, resulting in flatter bossung curves and a larger depth of focus (DOF) for the various features through pitch at their target size. This results in larger overlapping process windows. The Gray Assist Bars has also shown a very low printability, even with aggressive off-axis illumination (OAI) settings.

<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.

As critical dimensions continue to shrink in line with the SIA roadmap, the ratio of printed feature size and accepted wavelengths for optical lithography is driving inexorably towards the theoretical limitation of 0.25 for the Raleigh equation constant, k1. With the drive to lower k1 values fundamental limitations start to impact optical lithography. One example is the inability to simultaneously print features at different duty cycles with acceptable process windows. In the k1 regime down to 0.5, dense and isolated features could be printed in one with acceptable process windows. Today advanced lithography is operating at k1 values of 0.42-0.37 using KrF excimer laser light sources at a wavelength ((lambda) ) of 248nm. High lens Numerical Aperture (NA) is required to obtain sufficient aerial image contrast for dense lines, but results in reduced depth of focus which scales proportional to (lambda) /NA2. Using off-axis illumination techniques such as annular illumination can compensate the reduction in depth of focus for dense lines. For isolated lines high NA has only limited impact on the aerial image contrast due to the difference in the diffraction pattern and only serves to reduce the limited depth of focus which, unlike dense lines, does not benefit from the application of off-axis illumination. Use of increasingly strong imaging enhancement techniques will be required at lower k1 values resulting in further trade-offs to be addressed in pattern dependency. For example, quadrupole and di-pole off-axis illumination provides stronger enhancement to the available process window than annular illumination but only for features with specific orientations. In this paper an overview of the different imaging enhancement techniques will be given and examples of the trade-offs between enhancement and techniques, constraints on orientations and duty cycles will have to be applied in the device design. Alternatively, individual device layers will have to be separated by feature type, duty cycle and orientation to allow optimum enhancement techniques to be applied for each feature using multiple exposures. These approaches will be required if optical lithography at k1 values around 0.3 is to be realized. In the paper we will compare the use of two very strong enhancement techniques, dipole illumination and alternating (Levinson type) Phase Shift Mask with respect to process latitude, complexity and aberration sensitivity. To complete the review of low k1 the economic viability of optical lithography utilizing these strong enhancement techniques will be analyzed in terms of Cost of Ownership.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The Large Depth of Focus (hereinafter abbreviated to LDF) mode has been developed. The depth of focus in the LDF mode has been made 3 to 5 times larger than that in a conventional probe forming system by changing the positions of imaging points in the 3-stage lens system. The LDF mode has been applied successfully to observing the specimen with a large height difference (2 mm) without defocus and to analyzing it by EDS.

Endoholography or endoscopic holography combines the features of endoscopy and holography. The purpose of endoholographic imaging is to provide the physician with a unique means of extending diagnosis by providing a life-like record of tissue. It is our contention that endoholographic recording will provide means for microscopic examination of tissue and, in some cases, may obviate the need to excise specimens for biopsy.In this method, holograms, which have the unique properties of three-dimensionality, large focal depth, and high resolution, are made with a newly designed endoscope. The endoscope uses a single-mode optical fiber for illumination and single-beam reflection holograms are recorded in close contact with the tissue at the distal end of the endoscope. The holograms are viewed under a microscope.By using the proper combinations of dyes for staining specific tissue types with various wavelengths of laser illumination, we have seen increased contrast on the cellular level. Using dyes such as rose bengal in combination with the 514.5 nm line of an argon ion laser, and trypan blue and methylene blue with the 647.1 nm line of a krypton ion laser, holograms of the stained colon of a dog, showed the architecture of the colon’s columnar epithelial cells.It is hoped through chronological study using this method in-vivo, an increased understanding of the etiology and pathology of diseases such as Crohn’s diseases, colitis, proctitis, and several different forms of cancer, will help lead to their control.Endoholography or endoscopic holography combines the features of endoscopy and holography. The purpose of endoholographic imaging is to provide the physician with a unique means of extending diagnosis by providing a life-like record of tissue. It is our contention that endoholographic recording will provide means for microscopic examination of tissue and, in some cases, may obviate the need to excise specimens for biopsy.In this method, holograms, which have the unique properties of three-dimensionality, large focal depth, and high resolution, are made with a newly designed endoscope. The endoscope uses a single-mode optical fiber for illumination and single-beam reflection holograms are recorded in close contact with the tissue at the distal end of the endoscope. The holograms are viewed under a microscope.By using the proper combinations of dyes for staining specific tissue types with various wavelengths of laser illumination, we have seen increased contrast on the cellular level. Using dyes such as ros...

Next generation devices demand for higher bandwidths and increasing functionalities which requires integration technologies to overcome limitations in Moore’s Law. Approaches for enhancing device capabilities, in particular power consumption, form factor, performance and functionality, drive integration in the third dimension. One popular example for 3D Integration is Package-on-Package (PoP), where memory stacks are mounted above the processor. Heterogeneous Integration (HI) through System in Package (SiP) is one of the key technologies to enable high density system integration for next generation fan-out (FO) application standards, using higher bandwidth and higher chip-to-chip interconnection density. High IO density applications have started to use packages with fan-out solutions. Future applications will be more complex including FPGA, high level processors and high demanding CPU/GPU and the memories associated. High performance heterogeneous systems, especially those with embedded large dies will require very large package sizes, greater than 50x50mm². This creates performance pressure on materials and equipment, to overcome challenges and provide solutions to eliminate design rules while maintaining fine resolution, large depth of focus (DOF) and high overlay accuracy. New and future device packaging technologies demand for low cost of ownership lithography applications, providing fundamental requirements in advanced packaging for next generation fan-out application. Future devices require high resolution for fine line redistribution layers (RDL), enabling high density heterogeneous packages, as well as large depth of focus (DOF) for process stability. To reduce production cost for larger packages, lithography systems need to provide large die scalability and production of non-repeated, heterogeneous patterns without stitching. The elimination of stitching will reduce the production process and related yield loss. Furthermore, lithography tools need to support the trend of fan-out wafer-level packaging and provide new features for die shift compensation which occurs during pic and place of the die to the carrier wafer and during the reconstitution by molding. Therefore, die shift compensation is essential for fan-out wafer-level packaging applications allowing high accuracy overlay. This paper presents technical challenges and provides solutions for future high density heterogeneous fan-out applications, using a full-field UV projection scanning system for large package integration. Furthermore, high resolution for fine line RDL down to 1.5/1.5μm line and space is demonstrated. Aspect ratios as high as 10:1 are shown for square vias with 1:1 pitch for tall copper pillars in thick resist applications. Full process window for 2/2μm L/S RDL is determined. Solutions to enable limitless design for large heterogeneous packages without the need of stitching as well as alignment for high overlay accuracy is presented. In addition, unique capabilities and features of the UV projection scanning technology as well for production as R&D purpose is demonstrated. The extendibility to large panel packaging integration is discussed.

In lithography, forbidden pitch refers to pitch that suffers degradation in the process window due to the application of off-axis illumination (OAI). Destructive light field interference of diffracted light from a mask at forbidden pitch causes reduction in image contrast and depth of focus (DOF) and limits the pitch range to be patterned. In this paper, a modification to conventional OAI shape is proposed to minimize the effect of forbidden pitch. The modification is based on the interaction of illumination source with pattern density. The modified source employs double annular illumination. Simulation is carried out to investigate the effect of the modified source for one dimensional line and space pattern with a pitch varying from 130to500nm. Results shows that the maximum critical dimension fluctuation is around 3% compared to 13% in conventional annular illumination. Furthermore, the degradation in DOF is within 21% of DOF compared to 49% in conventional annular illumination.

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