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

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

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.

Lithographic patterning encounters growing challenges to meet the requirements of current and future semiconductor technology nodes. Even e-beam lithography is challenged due to the physical characteristic of the whole transfer process including the e-beam blur, electron scattering, and resist effects. These effects cause an unavoidable blurring of the exposed shapes and are often described as process proximity effect. Besides the correction of this process proximity effect pattern contrast and process window for the lithography step have to be regarded. There are promising approaches for contrast enhancing proximity effect correction concepts. To enable a stable patterning great efforts have to be put into decreasing the errors of all involved technologies. The blurring resulting from the transfer process is usually described by a so-called process proximity function (PPF) and mostly approximated by a superposition of two or more Gaussian functions. All algorithms for proximity effect correction use that PPF to perform their correction. Thus, an accurate determination of that PPF contributes to reducing the error budget of the proximity effect correction scheme. Several methods for PPF calibration were introduced in the past. Some are based on modelling the transfer process and performing Monte Carlo simulations. Another common approach is to design and expose calibration patterns, measure the resulting CDs, and obtain the process proximity function as the result of a simulation based parameter fitting to a model function such as a sum of Gaussian functions. In order to respect the increased accuracy requirements an even more accurate description of the PPF is expected. This paper describes the newly developed PPF-explorer method for the calibration of a pointwise proximity function as a complementary technique, which is based on the exposure and evaluation of new calibration layouts. Following the common assumption that a process proximity function is radial-symmetric, we developed radial-symmetric calibration layouts.

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.

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.

Background: Fast advancing of microwave/terahertz communication constantly demands ultrashort T-shaped gates by innovative nanofabrication techniques. The emerging RE650 resist provides a promising alternative to existing resists owing to its outstanding performance in electron-beam (e-beam) lithography. Aim: To report the exploration of the minimum foot-width achievable by e-beam lithography on RE650/UV5 bilayer resists through both numerical simulation and experimental tests. Approach: Using Monte Carlo method, the spatial charge distributions by e-beam exposure in resists were calculated, followed by modeling the developing process using LAB software to obtain the lithography profiles in resists. By this way, the foot-width was compared for the RE650 thickness varying from 50 to 80 nm with various UV5 thickness fixed. Results: As narrow as 15-nm foot-width has been achieved, but 30-nm foot-width can be routinely replicated in a stable exposure latitude, indicating that the developed process for ultra short T-shaped gates is reliable. Conclusions: The bilayer of RE650/UV5 is capable of replicating 15-30-nm T-shape gates as the minimum width by e-beam lithography, which should give us high prospect for mass production of THz devices.

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

Multiple-electron-beam–direct-write lithography is one of the promising candidates for next-generation lithography because of its high resolution and ability of maskless operation. In order to achieve the thro ughput requirement for high-volume manufacturing, miniaturized electro-optics elements are utilized in order to drive massively parallel beams simultaneously. Electron beam drift problems can become quite serious in multiple-beam syst ems. Periodic recalibration with reference markers on the wafer has been utilized in single-beam systems to achieve beam placement accuracy. This technique becomes impractical with multiple beams. In this work, architecture of a two dimensional beam position monitor system for multiple-electron-beam lithography is proposed. It consists of an array of miniaturized electron detectors placed above the wafer to detect backscattered electrons. The relation between beam drift and distribution of backscattered-electron trajectories is simulated by an in-house Monte Carlo electron-scattering simulator. Simulation results indicate that electron beam drift may be effectively estimated from output signals of detector array with some array signal processing to account for cross-coupling effects between beams. Keywords: Electron beam lithography, Monte Carlo simulation, electron beam drift, beam position monitor system

Electron beam lithography (EBL) is widely used for patterning of sub-micron and nano-scale patterns. Patterns in the order of tens of nano meters have been successfully realized using EBL. There are increasing needs in high aspect ratio structures in sub-micron and nano scales for microelectronics and other applications. Traditionally, high aspect ratio structures in sub-micron and nano scales have been realized by precision lithography techniques and subsequent dry etch techniques. In this work, we present commercially available SU-8 as a potential resist that can be used for direct resist patterning of high aspect ratio structures in sub-micron and nano scales. Such resist pattern can be used as a polymeric mold to create high aspect ratio metallic sub-micron and nano scales structures using electroplating technology. Compared to the most commonly used EBL resist, PMMA (poly methylmethacrylate), SU-8 requires a factor of 100~150 less exposure doses for equal thickness. It results in a significant reduction of EBL processing time. In this paper, characterization results on the patterning of up to 4:1 aspect ratio SU-8 structures with minimum feature size of 500 nm is reported. In addition, preliminary results on high aspect ratio metallic sub-micron structures using electroplating technology are also reported.

Low-energy e-beam proximity lithography (LEEPL) is proposed as the simplest integrated circuit lithography for minimum feature sizes ≤0.1 µm. This new e-beam lithography is similar to 1× X-ray proximity lithography except that the X-ray beam is replaced with a beam of low-energy electrons of 2 kV. This low e-beam energy permits the use of single-crystal 0.5-µm-thick silicon stencil masks without an absorbing metal layer of high atomic number. This membrane mask is thick enough for good heat conduction and thin enough for feature sizes ≤0.1 µm. Mask distortion caused by fabrication can be corrected by a fine-tuning deflector. Therefore, a mask with a residual distortion of more than 100 nm is acceptable. This eliminates the main difficulty of X-ray proximity lithography. The proposed system is not affected by a space-charge effect in the electron optics column, and a proximity effect with respect to both wafer and mask writings, and it is fundamentally low-power lithography which needs no special cooling system. The analysis shows that the e-beam column can be made entirely of electrostatic components to achieve sufficient resolution. For an appropriate resist process for this low-energy e-beam, we propose a bilayer process such as the chemical amplification of resist lines (CARL) process which consists of a chemically amplified thin deep ultraviolet (DUV) photoresist and a thick planarizing layer as a starting point. We estimated a throughput of about 40 12 inch wafers per hour and a resolution of a significantly less than 50 nm.

Periodic sub-wavelength electron beam lithography defined photonic crystals for mode control in semiconductor lasers

Density-controlled selective growth of well-aligned ZnO nanorod arrays by a hybrid approach

BackgroundThe nest flea index of Meriones unguiculatus is a critical indicator for the prevention and control of plague, which can be used not only to detect the spatial and temporal distributions of Meriones unguiculatus, but also to reveal its cluster rule. This research detected the temporal and spatial distribution characteristics of the plague natural foci of Mongolian gerbils by body flea index from 2005 to 2014, in order to predict plague outbreaks.MethodsGlobal spatial autocorrelation was used to describe the entire spatial distribution pattern of the body flea index in the natural plague foci of typical Chinese Mongolian gerbils. Cluster and outlier analysis and hot spot analysis were also used to detect the intensity of clusters based on geographic information system methods. The quantity of M. unguiculatus nest fleas in the sentinel surveillance sites from 2005 to 2014 and host density data of the study area from 2005 to 2010 used in this study were provided by Chinese Center for Disease Control and Prevention.ResultsThe epidemic focus regions of the Mongolian gerbils remain the same as the hot spot regions relating to the body flea index. High clustering areas possess a similar pattern as the distribution pattern of the body flea index indicating that the transmission risk of plague is relatively high. In terms of time series, the area of the epidemic focus gradually increased from 2005 to 2007, declined rapidly in 2008 and 2009, and then decreased slowly and began trending towards stability from 2009 to 2014. For the spatial change, the epidemic focus regions began moving northward from the southwest epidemic focus of the Mongolian gerbils from 2005 to 2007, and then moved from north to south in 2007 and 2008.ConclusionsThe body flea index of Chinese gerbil foci reveals significant spatial and temporal aggregation characteristics through the employing of spatial autocorrelation. The diversity of temporary and spatial distribution is mainly affected by seasonal variation, the human activity and natural factors.

Fresnel zone plates (FZPs) for soft X-ray microscopy with an energy range of 284 eV to 540 eV are designed and fabricated in a simple method. An adequate aspect ratio of the resist mold for electroplating was obtained by the proximity effect correction technology for an incident electron beam on a single thick layer resist. Without additional complicated reactive ion etching, a sufficient electro plating mold for nickel structures was fabricated. The overall fabrication procedures which involve a mix-and-match overlay technique for electron beam lithography and an optic exposure system that centers the membrane on the nanostructures, and hybrid silicon etching technology in junction with deep anisotropy and a KOH wet method in order to release the backside Si substrates of the Si3N4 membranes with no deformation of FZPs are introduced. High quality nanostructures with minimum outermost zone widths of 50 nm and diameters of 120 microm were fabricated with simplified fabrication process and with cost-effective.

A nanometer electron beam lithography system has been developed and used for fabricating sub-0.1μm gate MOSFETs. The system uses a Zr/O/W thermal field emitter (TFE) and has a 5-nm- diameter beam at a current of 100pA, and an acceleration voltage of 50kV. A 10-nm line in PMMA resist on a thick Si substrate was demonstrated. We develop an inorganic resist, LiXAl1-XF, which shows a potential for high resolution lithography less than 10nm. A chemically amplified negative resist was used as a single layer mask for MOS FET gate fabrication, and showed high resolution less than 0.1μm width. Proximity effect correction was applied to the gate lithography, resulting in excellent line width control even less than 0.1μm. Operation of a 40-nm-poly-silicon gate NMOSFET was confirmed.