PPF-Explorer: pointwise proximity function calibration using a new radial symmetric calibration structure

One of the major limiting factors in electron beam (e-beam) lithography is the undesired development of circuit area caused by electron scattering (the proximity effect). Since the proximity effect imposes a severe limitation on the ultimate resolution attainable with electron beam lithography, it is important to examine ways in which the proximity effect can be reduced. In order to develop a fast and efficient proximity effect correction scheme it is necessary to have a fast and accurate technique for finding the exposure (energy) deposited at any given point in the circuit pattern. In this paper we present the hierarchical digital signal processing (DSP) model utilized by PYRAMID (our proximity effect correction scheme) to estimate these exposures. Our approach employs a two part hierarchical estimation technique which is several orders of magnitude faster than a direct (spatial domain or FFT-based) convolution yet is able to predict the result of an exposure with a high degree of accuracy. A detailed description of our method of exposure estimation is followed by a thorough analysts of its performance through simulation and experimental results.

In this work, electron beam lithography proximity effect correction (PEC) was experimentally studied for patterning of Si photonic waveguides with a relatively thick resist mask. Beam’s energy density distribution (EDD) was experimentally extracted by the line exposure method; however, exposure lines in this work were developed after cleavage with a high-contrast process to reduce developer-related effects. The measured line spread function was fitted to a 4-Gaussian function to model mid-range energy densities accurately. The extracted EDD showed less proximity effects compared to conventional Monte-Carlo simulation performed by a commercial software. PEC processes with both techniques were experimentally compared for a Si photonic waveguide pattern with different side-cladding trench widths. Microscopic images confirmed that the presented calibration method could achieve better development conditions near the required clearance dosage. Single-mode propagation loss for a 500 × 220 nm Si wire waveguide was reduced from 3.2 to 2.4 dB cm−1 using the presented process.

In recent years, mask critical dimension (CD) linearity and uniformity has become increasingly important. The ITRS roadmap shows the mask CD control requirements exceeding those of the wafer side beyond the 45nm node. Measurements show that there are systematic, uncorrected proximity effects even when a state-of-the-art proximity effect correction (PEC) algorithm is used. The uncorrected proximity effect is predictable with a computational model. The model for e-beam lithography and etch process contains terms to model short-range pattern density effects and plasma shadowing effect in Cr-etch. The model is calibrated using CD measurements on a test mask. The model is valid for arbitrary 2-D patterns. We present a model-based mask process compensation (MPC) method which applies geometric changes to polygons as in OPC. We discuss the goodness of model fit to the calibration data; verification of the calibrated model by SEM images; and the improvement obtained by MPC. The mask writing error, i.e. final inspection CD minus incoming database CD, was reduced by a factor of 2 through the use of MPC.

For current and future semiconductor technology nodes with critical dimensions of 32 nm or below, the e-beam lithography is faced with increasing challenges to achieve a reasonable patterning of structures, especially if a process with a chemically amplified resist is used. The reasons for these limitations are the physical properties of the transfer process used to print a structure onto the resist-coated substrate, which inherently contains an unavoidable blurring of the deposited e-beam energy around the desired shape. This blurring is usually described by a so called process proximity function (PPF) and mostly approximated by a superposition of two or more Gaussian functions. The PPF includes the e-beam blur, electron forward scattering and resist effects (often described altogether by the so called alpha parameter of the PPF [K. Keil et al, "Resolution and total blur: Correlation and focus-dependencies in e-beam lithography," J. Vac. Sci. Technol. B 27, 2722 (2009)]) as well as the backscattering effect (often described by the so called beta parameter of the PPF). When the desired critical dimensions of structures are near or below the alpha parameter of the PPF, depending on their environment it may be just impossible to print the structures because of the vanishing image contrast. The PPF model confirms this well-known behavior but also shows ways and limits for improvements. This paper provides real pattern lithography results - comparing classical and GIDC correction - for exposures done on a Vistec SB3050DW shaped e-beam writer. A performance comparison of the GIDC method and the classical dose correction in terms of data preparation and writing time is presented.

In order to obtain highly accurate patterns for 0.13micrometers design rule and below by electron beam exposure system with high throughput, it is necessary to use block exposure system and to correct proximity effect. We have developed proximity effect correction system for block exposure system using supplementary exposure method. By using this system, line width difference for pattern area density from 5 percent to 40 percent decreased from 70nm to 15nm in 0.13 micrometers line and space patterns. Line width difference between the proximate and isolated region also decreased from 70nm to 20nm. Necessity of proximity effect correction considering coulomb interaction effect is also pointed out.

Easy to adapt electron beam proximity effect correction parameter calibration based on visual inspection of a “Best Dose Sensor”

Recently, the interest in enhancement of critical dimension (CD) accuracy has been significantly increased to satisfy requirements of sub 100nm devices. Proximity effect correction becomes an indispensable choice to improve CD accuracy within local area, and fogging and loading effects compensation has been tried to enhance global CD uniformity. However, proximity effect correction (PEC) parameters obtained without considering additional exposure such as fogging effect and the exposure to compensate it are not appropriate to fabricate real devices. In this paper, we investigated the relation of PEC parameters and various pattern densities and additional exposure experientially, analyzed theoretically using the edge image model to describe absorbed energy. Through evaluations, we could optimize proximity effect correction parameters for EBM-3500 taking additional exposure into account, and realize higher CD accuracy in mask fabrication.

Fast and accurate proximity effect correction algorithm based on pattern edge shape adjustment for electron beam lithography

Automatic determination of spatial dose distribution for improved accuracy in e-beam proximity effect correction

Direct write electron-beam (e-beam) lithography, which has the maskless patterning capability and the quick turnaround for new device designs and design changes, has been applied to making the engineering samples for the development of the System on Chip products (SoC). Using the e-beam lithography to the multilevel interconnect metal was known to be evaluate in view of cost and throughput. In the case of the high-energy e-beam lithography, however, the backscattered electron from the metal caused a significant proximity effect. Authors evaluated the e-beam proximity effect using the accelerating voltage 50keV on some multi-level interconnect metal structures which consist in tungsten wiring, or Cu wiring. It was found that the backscattering range and the ratio of the backscattering energy to the incident energy depend on the thickness of metal, but also on the distance from the resist to the metal. Therefore authors propose a new method of evaluating e-beam lithography property, concept of "EB-tree". That indicates the wafer backscatter property that has heavy metal wiring using e-beam lithography. EB-tree shows the relations of wafer backscatter range and heavy metal thickness, ratio of the backscattering energy and heavy metal thickness. EB-tree could show wafer property cause of lower levels layout, understructure metal wiring, that must be taken into account when e-beam lithography.

In electron-beam lithography the proximity effect limits achieved resolution. A possible approach for proximity effect correction is the transform based method which uses deconvolution. This method requires high computing power and data reduction algorithms because of the small grid size. The fine grid is mainly needed for preserving pattern, not for the resolution of the correction. The new method called PROXECCO avoids this disadvantage by separating the calculation into correction related and pattern related steps. The grid size is spatially independent from pattern and can be as coarse as needed for correction. This results in a quadratical gain in computing time for the transformation. The pattern dimensions are kept exactly maintained and data reduction is not needed anymore.

In cell projection lithography, critical dimension (CD) control is one of the important issues for device fabrication as well as resolution. Because plural patterns are exposed in one shot under the same dose, the proximity effect correction is more difficult than in the conventional variable-shaped beam (VSB) lithography. We have analyzed the CD deviation in order to obtain high CD accuracy of less than 0.02 µ m (range) which is sufficient for manufacturing 1 G dynamic randam access memory (DRAM). We have found that the Coulomb interaction effect plays an important role in CD deviation. We have proposed a new exposure intensity distribution (EID) function which contains a factor introduced for the first time to compensate the proximity effect and the Coulomb interaction effect simultaneously. The results indicate that the new EID function is very effective to compensate the Coulomb interaction effect and improve the CD deviation from 12% (0.03 µ m) to 6% (0.015 µ m) for 0.25 µ m lines-and-spaces (L/S) patterns.

A new method for the measurement of acid diffusion parameters in negative-tone e-beam chemically amplified resists and its application for proximity effect correction is presented. It is based on the measurement of the diameter of lithographic features (pillars) obtained from single-pixel e-beam exposures. Generally, long post exposure bake (PEB) times or low post apply bake (PAB) temperatures lead to wider features compared with short PEB times and high PAB temperatures, due to enhanced acid diffusion. The acid diffusion coefficient was found to depend exponentially on cross-linking density. Additionally, proximity effect correction parameters (triple Gaussian parameters) are easily calculated. These parameters are t PEB dependent and in the limit (t PEB→0) are almost equal to the corresponding values for non-amplified resists. The method has been applied to SAL-601 (Shipley), and the calculated proximity parameters were used successfully to correct the proximity effect of complicated layouts with a 0.15 µ m critical dimension.

The EBES4 electron beam lithography system was designed to optimize overlay and edge placement accuracy. Unique features such as 1/64 micrometer vector scan addressing, figure by figure dose modulation and a high brightness thermal field emission (TFE) gun support advanced application requirements such as electron proximity effect correction (PEC) and optical proximity effect correction (OPC). An EBES4 system has been installed, completed acceptance testing at Diamon Images, Inc. and is currently writing leading edge reticles in production mode. This paper describes the system performance and stability from both acceptance test data and production evaluations of the prototype EBES4 system installed for production. Data describing pattern placement, overlay, CD uniformity, dose modulation, butting, line edge roughness and reliability of performance is presented. A major advantage of EBES4 is the implementation of electron proximity effect correction by dose modulation of individual figures. Spatial dose corrections are calculated by commercially available software packages such as PROXECCO that refracture data and calculate dose corrections. CATS TM pattern preparation software retains the dose information and formats the writing of frequency multipliers within the EBES4 pattern files. Improvements in CD linearity using PROXECCO PEC correction software are described.

Mathematical formulations for the correction of proximity effects in electron beam lithography have been developed in the preceding paper. The implementation of these techniques to practical electron lithographic patterns involves (1) an efficient evaluation of the proximity interaction between every pair of shapes in the pattern. This interaction can be computed explicitly in limited cases; an algorithm for general cases is described. (2) The solution of the linear equations that yield the corrections to proximity effect. This is accomplished via a series of algorithms that involve dissecting the data into overlapping zones of different types. Pattern data manipulation is reduced by an algorithm involving generation of additional data, tagging, and sorting.