Contrast transfer function measurements of deep ultraviolet steppers

A 2-dimensional scalar aerial image model was used to computationally study i-line imaging with oblique illumination. Limited comparisons between developed photoresist images and aerial images were made. The effects on CD control, exposure latitude, and bias of varying annular and quadrupole geometry were mapped via simulation. Significant improvements in DOF of isolated lines was achieved with oblique illumination. Isolated line to dense line bias could be adjusted by changing the illumination type. Although oblique illumination improved the aerial image contrast at defocus, it caused degradation in the aerial image contrast at best focus. Long and short range proximity effects degraded the simulated CD control of optimized oblique illumination systems. This was observed in simulations of an SRAM gate cell. The imaging performance at .9 micrometers defocus, of an i-line system (NA .48) with oblique illumination, was judged to be worse than a KrF system (NA .42) with standard illumination. Quadrupole illumination was not found to measurably affect lens distortion.

BackgroundThe transmission electron microscope is used to acquire structural information of macromolecular complexes. However, as any other imaging device, it introduces optical aberrations that must be corrected if high-resolution structural information is to be obtained. The set of all aberrations are usually modeled in Fourier space by the so-called Contrast Transfer Function (CTF). Before correcting for the CTF, we must first estimate it from the electron micrographs. This is usually done by estimating a number of parameters specifying a theoretical model of the CTF. This estimation is performed by minimizing some error measure between the theoretical Power Spectrum Density (PSD) and the experimentally observed PSD. The high noise present in the micrographs, the possible local minima of the error function for estimating the CTF parameters, and the cross-talking between CTF parameters may cause errors in the estimated CTF parameters.ResultsIn this paper, we explore the effect of these estimation errors on the theoretical CTF. For the CTF model proposed in [1] we show which are the most sensitive CTF parameters as well as the most sensitive background parameters. Moreover, we provide a methodology to reveal the internal structure of the CTF model (which parameters influence in which parameters) and to estimate the accuracy of each model parameter. Finally, we explore the effect of the variability in the detection of the CTF for CTF phase and amplitude correction.ConclusionWe show that the estimation errors for the CTF detection methodology proposed in [1] does not show a significant deterioration of the CTF correction capabilities of subsequent algorithms. All together, the methodology described in this paper constitutes a powerful tool for the quantitative analysis of CTF models that can be applied to other models different from the one analyzed here.

This paper describes the exposure technology to manufacture 64M-bit DRAM chips with i-line (365-nm) stepper. The one-shot (conventional) exposure method was tried--a 64 M bit DRAM chip using the i-line lens of numerical aperture (NA)$EG0.50 with or without a phase-shift method--but the process margin was not sufficient in a few layers. The authors have shown that the block exposure method can be used as an exposure technology when the one-shot exposure method cannot be used. The block exposure method divides a step-and-repeat pattern within a single chip into multiple blocks and allows high-resolution exposure of each of these blocks. The stepper with the i-line lens of NAequals0.65 was used. The authors also tried to clarify the effect of coherency caused by the illumination optical system of the i-line stepper. In addition, the alignment accuracy of the block exposure method is discussed.

Aberration measurement using the Ronchigram contrast transfer function

The influence of polarization on the image performance of optical lithography systems has been systematically investigated through comparing the image contrast and the process window with TE-polarization, TM-polarization and un-polarized light, respectively. The simulation results show that the TE-polarization imaging is possible to improve the image performance of optical lithography systems, especially for those systems with high numerical aperture. The effects of polarized-light imaging with a conventional masks and a polarized film mask were studied by carrying out series of experiments under the conditions: 436nm exposure wavelength, numerical aperture of 0.5 and partial-coherence factor of 0.2. It is found that the image quality of the L&S patterns with the polarized film mask is better than that of with the conventional mask and, 0.3 <i>&mu;m</i> L&S patterns are obtained with the polarized film mask.

Artificial Intelligence–Aided Colonoscopy for Characterizing and Detecting Colorectal Polyps: Required, Nice to Have, or Overhyped?

For sub half-micron lithography, control of the Critical Dimension, represents the key to a robust and manufacturable process. The lithographer must understand and tune the exposure system for critical dimension performance using variables such as Numerical Aperture (NA), Partial Coherence (PC), position of the Focal Plane. Exposure tool must be characterized for its lens signature and the tools must be matched for their CD variation within the optical field In a previous study, we demonstrated that the phase-shift focal plane monitor, PSFM accurately measures focal plane variations when appropriate calibrations are employed. That paper also described the development of a model to determine lens classic aberrations such as Coma, Astigmatism and Field Curvature. The publication correlated the results to an approximation of the stepper's Critical Dimension (CD) behavior for a matrix of NA and PC settings. The present study, continues to study CD uniformity through examination of the lens aberrations and field focal signature of optical steppers in a 0.35 um process. We describe a method of measuring the uniformity of numerical aperture (NA) and partial coherence (PC) across the exposure field this new tool is then applied as an aid in optical lens characterization and tool-to-tool matching. The information gathered is thereupon applied to measure lens aberrations and predict CD variation across the field under various settings of the lens focal plane, the predictions are validated by a comparison against CD uniformity as measured by an advanced critical dimension metrology system.

This paper demonstrates a convenient experimental tech- nique for measuring the resolution of a digital imaging system. The spa- tial frequency response of the system is obtained by deriving the modu- lation transfer function (MTF) from the measured contrast transfer function (CTF). Basic signal-processing techniques are used to establish a relationship between the MTF and the CTF. Experimental methodology is discussed with respect to avoidance of shift variation in the CTF. Dis- crete Fourier transformation is applied for a power-spectrum analysis of the MTF. Using the CTF as a measure of performance, a series of ex- periments were carried out to characterize an imaging system based on an integrated CMOS camera. The experimental results showed that this approach can be applied in general, for the assessment of spatial reso- lution for digital imaging systems. © 1998 Society of Photo-Optical Instrumenta- tion Engineers. (S0091-3286(98)00609-6)

Recently, there has been a resurgence of interest in multimode optical fibers illuminated by a white light source. Largely, in anticipation of many integrated applications in the biomedical domain and spectral sensing benefiting from the broad spectral range and high numerical aperture (NA). Along these lines, the output light from these fibers can be captured by the physics of partially coherent sources. While the Gaussian Schell model has provided a framework for studying partial coherence, to our knowledge, its impact on microstructures remains unexplored. As the sheer complexity arising from the interplay between partial coherence and microstructures transfer function has posed fundamental challenges in deciphering their response. In this work, we introduce a comprehensive numerical model paired with experimental validation to assess the performance of multilayer optical resonators, which are meticulously crafted through high aspect ratio silicon etching under the influence of a partially coherent optical source. The model studies the effects of optical fiber NA, Bragg mirror order, cavity length, and surface roughness of the microstructures on the output of the resonator. The results show that the response under standard multimode fiber (MMF, partial coherent source) has lower insertion loss, more asymmetry versus wavelength, and larger full width at half maximum than the standard single mode fiber (full coherent source). A silicon-on-insulator chip is fabricated using 130 µm deep etching of silicon for Bragg mirrors with 2.25, 3, and 3.25 µm silicon layer widths and a different number of layers. The structures are characterized using a MMF of 62.5 µm core diameter illuminated by an infrared white light source. The theoretical results have been compared with the experimental results and a good agreement has been obtained.

The phase shift focus monitor is a relatively new technique which can be used to determine the optimal imaging plane for a photo-stepper. This determination ofbest focus is accomplished through phase shifting technology developed and patented by IBM. Adapting this concept to an overlay measurement target has advantages of simultaneous x and y measurements, high precision, and automated data collection. Printed on the reticle are phase shifting structures using the box-in-box pattern. These phase shifting structures produce lateral shifting ofimages at the wafer plane when they are imaged out offocus. This is a direct result from utilizing non- 1 80 degree phase shifting material. When the patterns are imaged in focus, no lateral shifting, or overlay error is exhibited by the box-in-box patterns. It has been found that the overlay error of the printed structures increases with the magnitude of defocus. If overlay data is collected from several different levels of defocus, a direct linear function can be found to relate best focus to the overlay error of the printed box-in-box patterns. Additionally, it can be shown that this relation is dependent upon the NA (numerical aperture) and PC (partial coherence) of the imaging system. This technique can be used as a very powerful analysis tool for characterizing photo-steppers, as it provides for the collection ofvast amounts ofdata about a stepper's condition rather quickly. It is laid out in an arrayed pattern with 121 modules in an 1 1 x 11 matrix. The complete matrix is contained in a 22 x 22 mm field (at wafer level on a 5x reduction stepper).Each module contains the phase-shifted box-in box patterns. Several potential applications have been suggested already. One wafer can be used to collect data on current best focus and field tilts in a few minutes. More static characteristics specific to the lens can be determined as well, i.e. astigmatism and field curvature. All of this information can be collected by measuring the overlay error from several of the 121 module positions in the field. This paper will discuss techniques to use the focus monitor to characterize steppers equipped with variable NA and PC. In addition, it can be shown that the monitor can provide quick and accurate measurements to verify and track the stepper's own internal diagnostics for statistical process control. Keywords: phase shift, focus, photostepper, reticle, numerical aperture, coherence, overlay

The effects of mask bias on the Mask Error Enhancement Factor (MEEF) of 180 nm contact holes is studied through lithographic simulation using commercial software and a DUV (248 nm) ESCAP photoresist model. Dense contacts show higher MEEF than isolated or semi-dense contacts. However, dense features exhibit a minimum in MEEF at a single negative mask bias (CD on reticle > 180 nm). Aerial image simulations indicated that low MEEF correlates approximately with high normalized aerial image log-slope (NILS). Hence, factors that affect NILS, such as numerical aperture, partial coherence, and wavelength, also influence MEEF, although without altering the optimum mask bias for minimum dense MEEF. Numerical aperture and wavelength of exposure have the greatest influence on MEEF. For 180 nm contact holes worst case MEEF values below 2 can be achieved by increasing NA to 0.8 at 248 nm or by decreasing (lambda) to 193 nm at 0.6 NA. Resist identity has little influence on the magnitude of MEEF but was the only factor affecting the mask bias setting for minimum dense MEEF.

Automatic CTF correction for single particles based upon multivariate statistical analysis of individual power spectra

Quantifying resolution limiting factors in subtomogram averaged cryo-electron tomography using simulations.

TomoAlign: A novel approach to correcting sample motion and 3D CTF in CryoET

Image formation in phase-contrast electron microscopy is governed by the contrast transfer function (CTF). The key parameter to tune the CTF is the defocus. A precise and unbiased estimate of the defocus is essential to determine the forward model and interpret high resolution images in cryo-TEM. We present an algorithm based on the weak-phase approximation for determination of the defocus and astigmatism of the objective lens from recorded images of an amorphous sample. The algorithm identifies Thon rings in the power spectrum density (PSD) and uses them to estimate astigmatism and defocus together with their uncertainties. For the astigmatism estimation we use a transformation to polar coordinates to fit ellipses to the Thon rings. Averaging the PSD over these ellipses reduces CTF estimation to a 1D problem and enhances the signal-to-noise ratio.