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Electron inelastic scattering in thin films is briefly discussed. It is found that in thin films of interest the plasmon generation by an energetic electron is the dominant process and the electron energy loss spectra (EELS) can be described by an analytical function of the membrane thickness and the material characteristics. A straightforward procedure using the analytical approximation of EELS and the results obtained from it for the SCALPEL mask-membrane in terms of chromatic aberration evaluation and control are presented.

The global space charge (SC) effect in SCALPEL electron beam lithography system is investigated. First order properties of the SC lensing action (defocus and magnification change) in SCALPEL type projection systems are analyzed using a simple analytical technique. Aberrations induced by the lenses and SC in the projection optics are evaluated numerically using a Monte Carlo code developed to calculate the combined effect of Coulomb interactions and lens aberrations in the charge particle projection systems. We found that the defocus and the magnification change induced by SC are functions of two parameters, the beam perveance and the SCALPEL aperture size, that are critical for the system performance. The strong correlation identified between the best focus plane location and the aberrations induced by SC indicates that the SC lensing action can be effectively compensated by simply adjusting either the wafer plane position or excitations of projection lenses.

A method for optimizing the electron optics of the SCALPEL exposure tool is described. The method uses the SCALPEL mark detection method with a grating mark as an aerial image monitor. The root-mean-square deviation of the recorded backscattered electron signal from an ideal triangle waveform was used as a measure of the image fidelity, scale and orientation. The resolution of the technique is limited only by signal-to-noise and the fidelity of the marks. Experiments were performed using 2 @mm period grating marks that were fabricated in a SiO"2/WSi"2 structure using SCALPEL lithography and plasma processing. The projector lenses and magnification/rotation coils were optimized. For these experiments the measured resolutions for determining focus (@df), magnification (@dM), and rotation (@d@q) of a 250 @mm X 250 @mm field were @df ~ +/-1 @mm, @dM ~ +/-15 ppm and @d@q ~ +/- 0.1 mrad. A straightforward path to improving these results is described.

SCALPEL alignment marks have been fabricated in a SiO 2 /WSi 2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron signal. Single scans of line space patterns yielded mark positions that were repeatable within 30 nm 3σ with a dose of 0.4 μC/cm 2 and signal-to-noise of 16 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response. The mark detection repeatability limit, that can be attributed to SPOL machine factors, was measured to be 20 nm 3σ. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 μm while maintaining 36 nm 3σ precision. The SPOL machine mark detection results are very promising considering that they were measured under electron optical conditions that were not optimized.

Parametric image-forming models were used to demonstrate the imaging of potential mask defects over ranges of descriptive parameters, either isolated or coincident with a critical object. Beyond basic defect printability effects, CD errors due to defects near CD objects cause serious constraint on allowable pattern defects or contaminants, and define post-repair requirements. The models can provide scaling laws based on defect type, size, contrast, and proximity to a CD object, as well as rules describing tolerable defects using a threshold value for the local CD anomaly they cause.

Electrostatic charge generation and accumulation in a:SiN"x thin films irradiated by energetic (100 keV) electrons is investigated and kinetic equations describing the dynamic process are formulated. It is found that the incident electrons, inelastically scattered in the membrane, primarily generate plasmons. The plasmon decay creates electron-hole pairs and secondary electrons (SE). The escape of the SEs from the target leads to a positive electrostatic charge accumulation in the membrane. It is shown that SCALPEL^(R) mask-membrane charging is defined by the balance of the SE escape, hole trapping and transport processes.

Coulomb interactions in high-throughput charged particle lithography systems lead to an uncorrectable image blur, and can also results in image placement errors. Throughput is ultimately limited by the increasing loss of process latitude with increasing beam current, or by the loss of critical dimension or overlay control due to placement errors. We previously developed an analytical model of the stochastic space-charge induced beam blur for our SCALPEL (SCattering with Angular Limitation Projection Electron Lithography) system, and used it to optimize the design of our experimental tool; principally by constraining the column length and optimizing the numerical aperture to achieve the best balance between electron-optical aberrations and the stochastic blur. We have attempted to validate this model with experimental measurements. We have also begun to quantify pattern distortions due to the global space-charge effects. Preliminary measurements show dose latitudes of ∼ 15%, consistent with a total blur of ∼ 150 nm, and a space-charge component no larger than 30 nm. No evidence for current dependent intrafield distortions has been observed.

Simulations of modern, high current electron beam lithography devices may require modeling of optical components and multiple electron sources that are positioned both aligned with, and oblique to, the main device axis. Such devices may include counter streaming regions, where two beams are co-located in space while propagating in opposite directions. Modeling such complex multi-beam systems presents different computational challenges depending on the specific device and regime being modeled. Applications of interest require in some cases the modeling of both global and stochastic space charge, where inclusion of the latter requires direct evaluation of Coulomb interactions.

Simulations of modern, high current electron beam lithography devices may require modeling of optical components and multiple electron sources that are positioned in an oblique fashion with respect to the main device axis and include counter streaming regions, where two beams are co-located in space while propagating in opposite directions. Modeling such complex multi-beam systems presents different computational challenges depending on the specific device and regime being modeled. Applications of interest require in some cases the model of both global and stochastic space charge, where the latter requires direct evaluation of Coulomb interactions. A new approach implemented in MICHELLE-eBEAM is designed to take advantage of the GPU hardware acceleration and novel algorithms to efficiently capture particle dynamics for complex beamlines. In this paper we report on our latest progress and show for a high current electron beam lithography application the achieved accuracy and performance of the new code.

Image blurring as a result of stochastic particle–particle interactions has been investigated for projection electron‐ and ion‐beam lithography systems. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results and experimental data. Large variations in results and serious disagreements between the different theoretical approaches are found. We have formulated a new theory on the basis of a simple, analytical approach that overcomes most of the difficulties experienced by earlier theories with two key concepts: consideration of nearest‐neighbor interactions only, and a randomization length, over which the interactions are correlated. Our model displays satisfactory functional and numerical agreement with Monte Carlo simulation results over a large range of beam currents, as well as with the only available experimental data. The physical basis of our model also enables us to understand the origins of the discrepancies arising from earlier theories.

Summary form only given. Reported on is the progress of the counter streaming, fully self-consistent model development in the MICHELLE Ebeam [1,2,3] code. This GPU-based software module performs high accuracy simulations of electron and ion beams including the effects of stochastic space charge.

Simulation of counter streaming charged particle beams, where two beams are co-located in space while propagating in opposite directions, has many applications ranging from the lower current electron beam lithography regime to the higher current Free Electron Laser (FEL) regime. Modeling such counter streaming beams presents different computational challenges depending on the specific device being modeled. Applications of interest require in some cases the model of both global and stochastic space charge; the latter requires direct evaluation of Coulomb interactions. A new approach implemented in MICHELLE-eBEAM is designed to take advantage of the GPU hardware acceleration and novel algorithms to capture such inter-particle interactions efficiently. In this paper we report on our latest progress and show for a high current electron beam lithography application the achieved accuracy and performance of the new code.

The authors present a new software tool (eBEAM) for high accuracy simulations of electron beams with stochastic space charge effects as a module of the SAIC/NRL MICHELLE particle simulation code. In modeling low current electron beam systems such as in electron beam lithography and electron microscopy, it is necessary to simulate ensembles of individual electrons to account correctly for the statistical effects of interparticle interactions. This article demonstrates the use of CPU/GPU hybrid techniques that run on multiple platforms to accomplish the simulation task within the MICHELLE/eBEAM framework.

A manufacturable process for fabricating alignment marks that are compatible the SCALPEL lithography system is described. The marks were fabricated in a SiO 2 /WSi 2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron (BSE) signal. Scans of 1 μm line-space patterns yielded mark positions that were repeatable within 20 nm 3σ with a dose of 4 μC/cm 2 and signal-to-noise of 32 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response combined with SPOL machine factors. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 μm while maintaining 36 nm 3σ precision. Further improvements in mark detection repeatability are expected when the SCALPEL electron optics is fully optimized.

Charged particle lithography systems face a unique challenge because throughput and resolution are linked through the dependence of beam blur on beam current. Understanding the function from of this dependence is vital, both for understanding the throughput limits of such systems, and also for the purposes of optimizing system design. We have developed a simple model describing the effects of image blur on printed resist feature size. The uncertainty in resist feature measurement enables us to determine the overall image blur to an accuracy of approximately 5 nm. We have also begun to develop an aerial image monitoring scheme that can, in principle, measure the image blur to an accuracy of

We report on a novel technique for tuning the illumination of a lithography tool through the use of variable transmission apertures. In conjunction with this illumination technique, we have developed simulation software capable of identifying the optimum source plane coherence and intensity distribution to increase process latitude. This system approach is capable of analyzing features specific to a given device level, or selected subsets of structure types within a given level. The fabrication of the aperture involves selectively depositing α-C onto a quartz plate that is inserted into the illuminator. Experimental testing has shown this film to be stable in its optical properties with extended exposure to DUV light. A description of the simulation software, aperture fabrication techniques, materials used, and experimental results for several aperture configurations are reported.

The purpose of the study reported here was to determine the range of material parameters and optical conditions necessary for using light to identify and categorize defects and to measure linewidths in SCALPEL masks. A prototype 4X SCALPEL mask with a 150 nm SiNx membrane and 50/10 nm W/Cr scatterer was used for the measurements. Die to die defect inspections were performed using a KLA 300 Series mask inspection system with 488 nm light in transmission. There was sufficient contrast to detect defects within test features with critical dimensions as small as 0.72 micrometer which would make optical defect inspection feasible for the 0.18 micrometer generation of integrated circuit (IC) reticles. Linewidth measurements were performed with the KMS 310RT mask metrology system in transmission on features ranging from 1.04 to 0.32 micrometer and compared to scanning electron microscope (SEM) measurements. The optically measured linewidths were linear in the range 0.4 to 1.04 micrometer which would be suitable for 0.1 micrometer IC reticles. The optical properties of SCALPEL masks constructed with Si3N4 membranes were calculated as a function of wavelength and membrane thickness. The requirements for extending optical inspection capability to smaller feature sizes and other measurement modes are discussed.

The potential for SCALPEL to provide economically viable production lithography capabilities for post-optical generations depends largely on achieving adequate wafer throughput. We have analyzed throughput-limiting performance attributes of the SCALPEL approach in order to identify critical design issues and develop a process for evaluating its unique parameter space. An important feature of the SCALPEL approach is that small image sub-fields are assembled to form complete device patterns. Further, electron-electron interactions result in a throughput- dependent image blur, which is a governing parameter for many inter-related performance areas of SCALPEL. Error budgets for key issues affecting critical dimension (CD) have been developed to analyze this unique design space, using models of the image-forming process including stitching on sub-field seams. These budgets assist in identifying the most critical design issues and demonstrating their inter-relationships and tradeoffs.

We have designed, constructed, and are now performing experiments with a proof-of-concept projection electron-beam lithography system based upon the SCALPELR (scattering with angular limitation projection electron-beam lithography) principle. This initial design has enabled us to demonstrate the feasibility of not only the electron optics, but also the scattering mask and resist platform. In this paper we report on some preliminary results which indicate the lithographic potential and benefits of this technology for the production of sub-0.18 micrometer features.

A new approach implemented in the MICHELLE-eBEAM code [Ovtchinnikov et al., J. Vac. Sci. Technol. B 28, C6J8 (2010)] is designed to take advantage of the graphics processing unit hardware acceleration using novel algorithms to capture inter-particle interactions accurately and efficiently. This approach is used in the simulation of counter streaming charged particle beams, where two beams are colocated in space while propagating in opposite directions, and has applications in lithographic devices such as in Reflective Electron Beam Lithography (REBL) [Petric et al., J. Vac. Sci. Technol. B 27, 161 (2009)]. Modeling such counter streaming beams presents different computational challenges depending on the specific device being modeled. These applications often require the modeling of both global and stochastic space charge, where the latter calls for accurate evaluation of Coulomb interactions. In this paper, the authors report on our progress and demonstrate, for a high current REBL application, the achieved accuracy and performance of the new code.

We have studied two mark geometries for possible use in a projection e-beam lithography system using SCALPEL (scattering with angular limitation in projection electron lithography). These are V-grooves and vertically etched geometries, pedestals or trenches. We report results of measurements of backscattered electron (BSE) contrast form topographic marks of varying size and as a function of energy up to 100 kV. The marks were fabricated on silicon wafers. The measurements were taken both in a scanning electron microscope and in an experimental SCALPEL machine operating in focused probe mode. The V-grooves ranged from 1.0 to 30 micrometers wide. The vertical etched features ranged from 2 to 30 micrometers wide and 0.6 and 50 micrometers depth. The results depended not only on the feature width and depth, but also on whether the features were isolated or in line and space patterns. Using a BSE ratio of 1.05 as a criterion for acceptable contrast from an alignment mark, V-grooves and vertical etched features had acceptable contrast with exception of the smallest and shallowest features for both geometries.

Image blurring as a result of stochastic particle-particle interactions has been investigated for a particle projection system. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results. Large variations in results and serious disagreements between the different theoretical approaches are found. The origin of the discrepancies arising from the earlier theories are understood and explained on the basis of our recently developed theory with two key concepts: consideration of nearest-neighbor interactions only, and a randomization length, over which the interactions are correlated.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A proof-of-concept experimental lithography tool, based upon the SCALPELTM principle, is currently being fabricated at AT&T Bell Laboratories. Designed from a lithographic system perspective, we feel that this experimental platform is the minimal configuration necessary to evaluate this specific printing technique, consistent with design rules from 0.18 micrometers to 0.09 micrometers . Details of the machine's electron-optics are presented, including the calculated performance of the electromagnetic lens systems. The critical lithography issues which will be examined using this tool are also discussed.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In projection electron-beam systems resolution and throughput are linked through electron–electron interactions collectively referred to as space-charge effects. Hence, a detailed understanding of these effects is essential to optimizing the lithographic performance of a projection electron-beam lithography system. Although many models have been developed to describe one or more of the various aspects of the Coulomb interactions that occur in the beam, there is minimal experimental data available. We have performed a series of experimental measurements in the scattering with angular limitation projection electron-beam lithography (SCALPEL) proof-of-lithography system to characterize the space-charge effects for such an optical configuration. The results of those measurements have been compared to a combination of computer simulations and analytical models. The agreement between the models and experiments was good, within the limits of experimental error. We determined the exponent in the dependence of blur on beam current to be 1.029±0.16 (1σ), consistent with more recent models. Additionally, we comment on the use of blur modeling for system optimization.

Scattering with angular limitation projection electron lithography (SCALPEL) is a unique charged-particle projection imaging technique that employs a scattering mask with the pattern segmented between supporting struts. An aperture installed in the back-focal plane of the projection lens filters out the electrons scattered at large angles in the patterned area of the mask producing a high contrast aerial image. Various scattering phenomena involved with the energetic (100 keV) electrons carrying the mask pattern information to the wafer through the projection optics are responsible for the aerial image formation in SCALPEL. These phenomena can be grouped into three major categories: (i) electron elastic scattering in the mask responsible for the aerial image intensity and contrast; (ii) electron inelastic scattering in the mask-membrane that might have negative effects, such as membrane charging, beam chromatic blur generation, mask heating, etc.; (iii) Coulomb interactions of electrons in the beam (space charge effect) generating a beam blur that links the system throughput and resolution. Analytical models developed to describe and quantitatively evaluate these phenomena are briefly reviewed. The implication of these models to the design and optimization of the electron projection lithography systems are discussed.

We present an approach to real time direct aerial image monitoring which utilizes the information contained in an alignment signal generated by scanning the image of a mask grating over a corresponding wafer grating and detecting the backscatter electron signal. The basic principles of this measurement technique are described. The effect of noise and other common errors, such as magnification and rotation on the signal quality, are derived and used to set requirements on signal contrast and noise level for obtaining blur values accurate to approximately 1 nm. The application of this approach to measuring space charge blur is described and preliminary data illustrating the concept is presented. The potential for this technique to form the basis of an automated self-calibration system on SCALPEL tools is clear.

Previously, we theoretically investigated the charging of free standing dielectric thin films irradiated by 100 keV electrons and formulated kinetic equations describing the dynamic process [M. Mkrtchyan et al., Microelectron. Eng. 46, 233 (1999)]. It was shown that in the currently used SCALPEL® masks comprising a 1000-A-thick amorphous SiNx film supported by a grillage of Si struts, the membrane charging could be significant and might have an adverse effect on the system performance. The membrane charging, sensitive to both the conductivity and the geometry of conductive path, can be regulated in a straightforward manner by tailoring both of them; for instance, by applying a top surface conductive layer (TSCL) with an appropriate thickness and doping level. Here we discuss the results obtained on the basis of our charging model modified to be applicable to the case of a SiNx membrane with a TSCL (e.g., a 10-nm-thick amorphous Si or poly-Si film doped by boron). The results presented demonstrate that thi...

Electron scattering in thin solid films used for the fabrication of masks for electron projection lithography, e.g., SCALPEL®, is investigated. We have developed an analytical model to calculate electron transmission through the mask membrane and image contrast due to different scattering properties of the patterned area and the membrane. The model utilizes cross sections for electron elastic and inelastic scattering on an atom with exponentially screened Coulomb potential of the nucleus derived in the first Born approximation. The variety and controversy of theoretical and empirical adjustments of the screening parameter are briefly analyzed and attributed to the misinterpretation of experimental data ignoring the effects mostly due to plural scattering of electrons and dense packing of atoms in thin solid films. This model frees us from the computational limitations of Monte Carlo simulations and proves to be effective for the straightforward characterization of various alternative materials for SCALPEL...

Photon tunneling microscopy (PTM) is an optical microscopy technique that can, by using frustrated total internal reflection, be sensitive to changes in surface topography as small as 1 nm. We have developed a simple calibration technique that generates plots of sample reflectivity as a function of topography. This empirical data is then used to convert a gray-scale image of a specimen into an accurate topographic image. Errors, such as accurate magnification calibration and detector nonlinearity, lead to only small differences between the empirical data and our theoretical prediction. We have used PTM to study the evolution of latent images in a chemically amplified resist, ARCH2, as a function of dose, both after exposure and after postexposure bake, and have obtained good agreement between the topography measured in the PTM with thickness changes determined by ellipsometry from large exposed areas. Comparison of the results of the PTM with those obtained by near-field scanning optical microscopy demonstrate that changes in topography on the order of 3–5 nm are visible in the PTM. The topography that appears to develop in a latent image is a function of feature size. This effect occurs before the lateral resolution limit of the microscope is reached, and is a result of the mechanical constraint arising from unexposed resist material around the exposed features. Techniques such as adjusting the polarization and wavelength of the illumination can improve the sensitivity of the technique to small changes in topography, while in the case of materials that undergo changes in refractive index, contrast can be seen even in the absence of topography.

The ability to examine latent resist images can provide a means of separating out the influences of the exposure itself from the subsequent postexposure bake and from the development process in determining the form of the final developed image. We describe the technique of photon tunneling microscopy (PTM) and its application to examining latent resist features. The PTM is based upon the concept of frustrated total internal reflection, and can give information on latent images that is highly sensitive to both changes in topography as small as 1 nm and refractive index changes on the order of 10−2–10−3. Preliminary modeling shows how this technique can give quantitative information about the progress of the radiation induced chemistry in resist. Experimental data for two positive-tone, chemically amplified materials, ARCH2 and CAMP1 illustrate the potential of the technique.

In recent literature the partial coherence uniformity of a lithography tool has been suspected to be a cause of critical dimension control and pattern placement problems. A direct measurement of the uniformity of partial coherence has been developed to determine the magnitude and distribution of this problem. Apertures have been fabricated that block illumination everywhere except for a small angular range and in one quadrant. The illumination reaching the wafer plane of the stepper is measured with a photodetector on the wafer chuck designed to measure exposure uniformity. The intensity distribution can vary across a chip field by as much as ±20% for large off-axis illumination (OAI) angles. However, if no aperture is inserted, the field is uniform to within ±1.5%. The consequences of this nonuniformity are surprisingly subtle; at best focus, there is no difference between the aerial image of a feature with or without the uneven angular distribution. The combination of defocus with the nonuniform OAI leads not only to image shifts, but to feature dimension variations as well.

Charged particle image projection lithography systems have been proposed and are currently under development for design rules of 0.18 μm and below. Although charged particle projection lithography systems do not suffer from diffraction as a limit to spatial resolution as in photolithography, image degradation due to the effects of mutual repulsion of particles in the beam, space charge, will ultimately limit the performance of these systems. Space charge effects increase with increasing beam current. The uncorrectable image blur caused by space charge effectively reduces the dose latitude in projection charged particle lithography and therefore limits the ultimate throughput of such systems. We will describe the effects of space charge in charged particle projection lithography systems using a model we have previously developed. We will compare the predictions of the model with experimental data for an ion projection system and predict the performance of electron and ion beam systems under development. The predictions for image blur caused by space charge are used to calculate the dose latitude as a function of beam current and thus throughput for the SCALPEL(R) electron beam lithography system.

Image blurring as a result of stochastic particle interactions has been investigated for projection electron‐ and ion‐beam lithography systems. The investigation was made on the basis of a simple, nearest‐neighbor, analytical model, proposed and developed here, for stochastic particle–particle interactions. The results obtained using this model are in close agreement with those given by Jansen [Coulomb Interactions in Particle Beams (Academic, Boston, 1990)] for an extended parallel beam segment in the Holtsmark regime; at the same time they are extendable over a wide range of conditions, unlike Jansen’s results. The results obtained for a parallel beam are applied to more realistic systems by dividing the beam into nearly cylindrical, uncorrelated slices. This method is used to determine the dependence of the image blur on beam parameters for a doublet. Our results correlate well with those obtained by Monte Carlo calculations.