Corrections to proximity effects in electron beam lithography. II. Implementation

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.

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.

We report the nanofabrication and characterization of x-ray transmission gratings with a high aspect ratio and a feature size of down to 65 nm. Two nanofabrication methods, the combination of electron beam and optical lithography and the combination of electron beam, x-ray, and optical lithography, are presented in detail. In the former approach, the proximity effect of electron beam lithography based on a thin membrane of low-z material was investigated, and the x-ray transmission gratings with a line density of up to 6666 lines/mm were demonstrated. In the latter approach, which is suitable for low volume production, we investigated the x-ray mask pattern correction during the electron beam lithography process and the diffraction effect between the mask and wafer during the x-ray lithography process, and we demonstrated the precise control ability of line width and vertical side-wall profile. A large number of x-ray transmission gratings with a line density of 5000 lines/mm and Au absorber thickness of up to 580 nm were fabricated. The optical characterization results of the fabricated x-ray transmission gratings were given, suggesting that these two reliable approaches also promote the development of x-ray diffractive optical elements.

A physical method of reducing feature size and proximity effects in sub-quarter-micrometer e-beam lithography is described. A thin layer (50 to 300 nm) of silicon nitride deposited on a semiconductor substrate, prior to resist deposition, has been found to enhance the resist resolution. The samples were patterned with a 50-keV, 15-nm-diam probe generated by a JEOL JBX-5Dll e-beam lithography system. Point spread function measurements in 60-nm-thick SAL-601 on Si are shown to illustrate the resolution enhancement in the nanolithographic regime (sub-100 nm). The technique has been applied to lithography on 400-nm-thick W films, such as would be used in x-ray mask fabrication. The 200 nm of SAL-601 was spun onto W film samples, which were half-coated with 200 nm of silicon nitride. Identical lithographic patterns were written on each half of the sample. On examination of the samples after postexposure processing and development, reduced feature sizes and proximity effects were seen on the sample half with the silicon nitride intermediary layer. For example, in a field effect transistor (FET) type pattern, with a coded 500-nm gap between the source and drain pads, the gate could only be successfully resolved when the intermediary nitride layer was present. Monte Carlo simulations were performed on a CM-200 connection machine. The results show a large number of fast secondary electrons are generated within a 100-nm radius of the incident electron beam. The implications of fast secondary electrons on resolution in e-beam lithography are discussed. The total number of fast secondary electrons entering the resist is reduced by the silicon nitride layer. Simulations compare the thin-layer technique to a bilayer resist technique, used to improve resolution at larger dimensions.

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.

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.

This paper introduces a software toolkit, HNU-EBL, developed at Hunan University (HNU), to simulate and optimize electron beam lithography (EBL). The toolkit consists of three consecutive modules. Firstly, the electron trajectories in EBL resist and substrate layers are simulated by using Monte Carlo (MC), and point spread function (PSF) is obtained by fitting the electron trajectories. Secondly, using PSF, electron dose distribution is optimized using the fast Fourier transform (FFT), to efficiently correct the effects like proximity effect and fogging effect that may jeopardize the lithography resolution and pattern quality. Thirdly, based on the optimized electron dose distribution, the optimized pattern quality is evaluated based on metrics like the edge placement error (EPE). The HNU-EBL toolkit can simulate sub-10 nm EBL, and it has been successfully applied to optimize fabrication of nm-scale devices.

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.

Guaranteed accuracy of the method of ‘simple’ compensation in electron lithography

The constructive procedure for solving the problems of electron transport in the multi-layer specimens for the normally incident electron beam. 1. Solution of the model transport equations for a point beam : energy and charge deposition

High resolution and high density ion beam lithography employing HSQ resist

The SIA roadmap predicts mass production of sub-100 nm resolution circuits by 2006. This not only imposes major constraints on next generation lithographic tools but also requires that new resists capable of accommodating such a high resolution be synthesized and developed concurrently. Except for ion beam lithography, DUV, X-ray, and in particular electron beam lithography suffer significantly from proximity effects, leading to severe degradation of resolution in classical resists. We report a new class of resists based on organic/inorganic nanocomposites having a structure that reduces the proximity effects. Synthetic routes are described for a ZEP520®nano-SiO2 resist where 47nm wide lines have been written with a 40 nm diameter, 20 keV electron beam at no sensitivity cost. Other resist systems based on polyhedral oligosilsesquioxane copolymerized with MMA, TBMA, MMA and a proprietary PAG are also presented. These nanocomposite resists suitable for DUV and electron beam lithography show enhancement in both contrast and RIE resistance in oxygen. Tentative mechanisms responsible for proximity effect reduction are also discussed.

The resolution limit in e-beam lithography is dependent on the resist process, the proximity effect, and the e-beam writer itself. However, it is difficult to distinguish these contributions just by comparing critical dimensions (CDs) of isolated and dense lines. Since the calculation of the parameter α of the point-spread function is usually based on the CD measurement of resist structures, α should include all mentioned contributions. Therefore, the total blur αtotal can be defined as the square root of the squared sums of αprocess, αforwardscattering, and αbeamblur. With keeping αprocess constant, the authors compared Gaussian beam (GB) and variable shaped beam (VSB) writers via the isofocal-dose method and proved that the isofocal dose is independent of the beam profile. For further comparison, they enhanced the method to the linewidth roughness and the total blur. For the latter, a special frequency modulated pattern was applied, which enables the determination of αtotal in dependency on the focus. The analysis of the total blur in best focus and the resolution of dense lines, exposed in a negative chemically amplified resist on GB and VSB writers, indicates their proportionality, which is supported by the same analysis with a nonchemically amplified resist. The resolution of the VSB writer cannot be directly compared to the GB writers’ resolution since the beam blur of the former changes with the shot size.

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

Research at the US Naval Research Laboratory (NRL) in the lithography, patterning, and fabrication of structures of nanometer scale dimensions is reviewed. Electron-beam (e-beam) lithography at high (50 keV) and low (5 eV) energies on resist systems developed at NRL is described. The low energy lithography takes advantage of the spatially confined e-beam available in a scanning tunneling microscope type probe. This instrument allows an in situ exposure and characterization of an e-beam sensitive material. A novel approach for implementing dose correction for proximity effects in e-beam lithography is presented using an error measure based on the physical realities of lithography. The compositional disordering of GaAs-Ga/sub x/Al/sub 1-x/As heterostructures as a technique for pattern replication is described. Introducing silicon into the heterostructure (by implantation or diffusion) enhances the aluminum and gallium interdiffusion which can be used for patterning. A complete bibliography of recently published results is included.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>