This PDF file contains the editorial “Editorial: JM3 Impact Factor” for JM3 Vol. 5 Issue 03

Modern steppers and scanners have a projection lens whose numerical aperture (NA) can be varied so as to optimize the image performance for certain lithographic features. Thus a variable fraction of the aberrations is actually involved in the imaging process. In this letter, we present a concise formula for the NA scaling of the Zernike coefficients. In addition, we apply our results to the Strehl ratio.

Various representations of polarization aberrations are described and compared for optical lithography. Polarization aberrations, which are potentially important with hyper-numerical-aperture tools, are a complicated phenomena that refer to induced polarization-dependent wavefront distortions as light propagates through an imaging system. Pupil representations based on the following concepts are discussed: the physical polarization properties, the Mueller matrix, the Jones matrix, and the Jones matrix decomposed into a Pauli spin matrix basis. Although each has its own advantages and disadvantages, it is concluded that the Jones matrix representation decomposed into a Pauli spin matrix basis offers the most useful format for the lithographer due to its compact notation, physically intuitive interpretation, ability to be implemented into standard imaging equations, and its usefulness as an input into a lithographic simulator. Depending on the assumptions that can be made, the pupil specification consists of three to eight independent functions, where a normalization constant is calculated to ensure a physically realizable pupil. An example is shown to illustrate the usefulness of this strategy. Additionally, a simple metric for lens polarization quality based on this representation is proposed.

A production-preferred solution is 193-nm immersion resist without a topcoat. The challenge of 193-nm immersion resist is both low leaching level and high performance. We summarize the screening results of selected 193-nm immersion resists that are designed for use without top coatings. Our evaluation is divided into several phases. Leaching levels of resist samples are first tested. The leaching data are analyzed and compared to our specifications. Both binary intensity mask and alternating phase-shift mask exposures are performed to evaluate the process window, lineedge roughness, and resist pattern profile. Resist films are rinsed by deionized (DI) water prior to or after exposure, and contrast curves are measured to investigate the resist sensitivity change. The results are compared with resist systems that use developer-soluble topcoats.

Various approaches can be used to quantify line width roughness (LWR). One of the most commonly used estimators of LWR is standard deviation σ. However, a substantial amount of information is ignored if only σ is measured. We use an automated approach to investigate LWR, where standard deviation, correlation length, and power spectrum are measured online on critical dimension scanning electron microscopes. This methodology is used to monitor LWR, investigate the effect of LWR on critical dimension precision, and to benchmark new resists for immersion lithography. Our results indicate that online LWR metrology is a critical tool in a variety of applications, including but not restricted to process control.

Dynamic speckle is caused by the finite pulse length and limited spectral linewidth of the partially coherent radiation from the excimer lasers used in optical projection lithography. One effect of the dynamic speckle is that the energy delivered to a certain position at the wafer is a stochastic quantity and cannot be precisely controlled, which fundamentally limits the dose control in the lithographic system. Further, the spatial distribution of the dynamic speckle is shown to depend on illumination conditions, contributing to unwanted effects such as line edge roughness (LER). We present a formulation for the dynamic speckle of the partially coherent optical field, based on a temporal degrees of freedom approach. We show theoretically and by numerical simulation how the choice of the illuminator intensity distribution influences LER. We also point to some pitfalls in the struggle to combat dynamic speckle. It is fundamentally important to realize that instantaneously any combination of uncorrelated fields adds to the total field on a complex amplitude (not intensity) basis just like correlated fields. Therefore, any spatial redistribution of light, e.g., with the help of microlens arrays, does not reduce the dynamic speckle.

EUV lithography has been widely regarded as the lithography technology to succeed optical lithography since the 100-nm era. With the ending of 193-nm immersion lithography in sight, the need for a successor cannot be more urgent. We deal quantitatively with various critical aspects of EUV lithography such as source and power requirement, mask specification, random phase shifting, oblique incidence, reflective projection optics, and resist, to compile a list of the critical issues and to determine the most critical ones to make the technology a success.

The University of Illinois at Urbana-Champaign (UIUC) and several national laboratories are collaborating on an effort to characterize Xe plasma source exposure effects on extreme ultraviolet (EUV) collector optics. A series of mirror samples provided by SEMATECH were exposed for 10 million shots in an Xtreme Technologies XTS 13-35 commercial EUV discharge produced plasma (DPP) source at UIUC and 500,000 shots at the high-power TRW laser produced plasma (LPP) source at Sandia National Laboratory, Livermore (SNLL). Results for both pre- and post-exposure material characterization are presented for samples exposed in both facilities. Surface analysis performed at the Center for Microanalysis of Materials at UIUC investigates mirror degradation mechanisms by measuring changes in surface roughness, texture, and grain sizes as well as analysis of implantation of energetic ions, Xe diffusion, and mixing of multilayers. Materials characterization on samples removed after varying exposure times in the XTS source identify the onset of different degradation mechanisms within each sample over 1 million to 10 million shots. Results for DPP-exposed samples for 10 million shots in the XCEED (Xtreme Commercial EUV Emission Diagnostic) experiment show that samples are eroded and that the surface is roughened with little change to the texture. Atomic force microscopy (AFM) results show an increase in roughness by a factor of 2 to 6 times, with two exceptions. This is confirmed by x-ray reflectivity (XRR) data, which shows similar roughening characteristics and also confirms the smoothening of two samples. Scanning electron microscopy (SEM) pictures showed that erosion is from 5 to 54 nm, depending on the sample material and angle of incidence for debris ions. Finally, microanalysis of the exposed samples indicates that electrode material is implanted at varying depths in the samples. The erosion mechanism is explored using a spherical energy sector analyzer (ESA) to measure fast ion species and their energy spectra. Energy spectra for ions derived from various chamber sources are measured as a function of the argon flow rate and angle from the centerline of the pinch. Results show creation of high-energy ions (up to E=13 keV). Species noted include ions of Xe, Ar (a buffer gas), and various materials from the electrodes and debris tool. The bulk of fast ion ejection from the pinch includes Xe⁺, which maximizes at ~8 keV, followed by Xe²⁺, which maximizes at ~5 keV. Data from samples analysis and ESA measurements combined indicate mechanism and effect for debris-optic interactions and detail the effectiveness of the current debris mitigation schemes.

A critical issue for EUV lithography (EUVL) is the minimization of collector degradation from intense plasma erosion, debris deposition, and hydrocarbon/oxide contamination. Collector optics reflectivity and lifetime heavily depend on surface chemistry and interactions between fuels and various mirror materials, such as silicon, in addition to high-energy ion and neutral particle erosion effects. As a continuation of our prior investigations of discharge-produced plasma (DPP) and laser-produced plasma (LPP) Xe plasma interactions with collector optics surfaces, the University of Illinois at Urbana-Champaign (UIUC) has analyzed collector samples before and after exposure in a Sn-upgraded Xtreme Technologies EUV source. Sn DPP postexposure characterization includes multiple samples, Si/Mo multilayer film with normal incidence, 200-nm-thick Ru film with grazing incidence, as well as a Gibbsian segregated (GS) Mo-Au alloy developed on silicon using a dc dual-magnetron cosputtering system at UIUC for enhanced surface roughness properties, erosion resistance, and self-healing characteristics to maintain reflectivity over a longer period of mirror lifetime. Surface analysis draws heavily on the expertise of the Center for Microanalysis of Materials at UIUC, and investigates mirror degradation mechanisms by measuring changes in surface roughness and film thickness as well as analysis of deposition of energetic Sn ions, Sn diffusion, and mixing of multilayer. Results from atomic force microscopy (AFM) and auger electron spectroscopy (AES) measurements show exposure effects on surface roughness and contamination. The best estimates of thickness and the resultant erosion measurements are obtained from scanning electron microscopy (SEM). Deposition, diffusion, and mixing effects are analyzed with depth profiles by AES. Material characterization on samples removed after varying exposure times in the XTS source can identify the onset of different degradation mechanisms within each sample. These samples are the first fully characterized materials to be exposed to a Sn-based DPP EUV source. Several valuable lessons are learned. First, hot mirrors exposed to SnCl₄ gas will cause decomposition of the gas and build up a contamination layer on the surface. Second, erosion is mitigated to some extent by the simultaneous deposition of material. Third, and most important, Gibbsian segregation works and a thin Au layer is maintained during exposure, even though overall erosion is taking place. This phenomenon could be very useful in the design of a collector optics surface. In addition, we present Sn DPP collector erosion mechanisms and contamination and provide insight into plasma-facing optics lifetime as high-volume manufacturing (HVM) tool conditions are approached.

To enhance the quality of Advanced Process Control (APC), the optimization of the sampling plan in critical dimension (CD) metrology is studied through empirical considerations concerning the characteristics of errors and a statistical approach. The metric of the optimization is the accuracy of lot mean estimation. Critical dimension errors are classified into static and dynamic errors. A static error is defined as an error that repeats through lots while keeping its tendency, and a dynamic error as an error whose tendency changes by lot. In the basic concept of our sampling plan, sampling positions and size are determined from the static error and dynamic error, respectively. The balance of the sampling number of the wafer, field, and site under the restriction of total sampling size is optimized by a statistical theory.

A GaN-based optical single-mode rib waveguide with a large cross section is systematically designed and analyzed using the finite-difference vector beam propagation method (FD-VBPM). The method is a combination of numerical aperture and FD-VBPM, which is used to determine the single-mode conditions for a GaN/Al_xGa_1–xN optical rib waveguide. We analyze the optical propagation properties and loss characteristics of the designed waveguides. The analysis illustrates that optical power increases with an increase of the rib width or a decrease of the slab height. Therefore, our analysis results can be helpful in the process of designing a GaN-based rib waveguide for optical communications.

Miniaturized electrodes are one of the most important components of "lab-on-a-chip" devices for separation, pumping, sensing, and other bioanalyses. In microfluidic-based chemical and bioanalytical operations, platinum electrodes are preferred to minimize the interaction with chemicals or biomolecules due to their chemical inertness. Although microfabrication techniques for patterning integrated platinum microelectrodes on silicon, quartz, or glass substrates are available, no techniques have been reported so far for depositing platinum electrodes in soft polymeric microchannels. A novel fabrication scheme is described for forming integrated microelectrodes in a poly-di-methyl-siloxane (PDMS) microchip. The electrode fabrication technique consists of photolithography, thermal processing, sequential sputtering of titanium and platinum, and stripping off photoresist, while soft lithography is used to form the microfluidic channels on PDMS. This approach facilitates precise positioning of the electrodes with a micron-size gap between them, and it can be used for both low and high aspect ratio channels. Platinum electrodes, formed on the PDMS channel surface, demonstrate very good interfacial adhesion with the substrate due to the use of a very thin titanium layer between the platinum and PDMS. The sputtered electrodes have a surface roughness of 50 nm and are able to sense picoA level current through benzene.