Projection Lithography System Research Articles

Effects of Mo/Si multilayer coatings on the imaging characteristics of an extreme-ultraviolet lithography system

The role of Mo/Si reflective coatings in the imaging performance of an extreme-ultraviolet projection lithography system under polychromatic illumination has been theoretically examined. Using a thin-film computer model, we have explored various multilayer design criteria. Optimum operating conditions, leading to the maximum system transmittance, were found for a tuned multilayer system operating at lambda = 12.7 nm. In this configuration, Mo/Si coatings have been shown to be nondetrimental to the imaging performance of our system with the introduction of only minor modifications to the propagating wave front, which can be adequately described by a simple tilt and defocus term.

A review of ion projection lithography

Although optical lithography has been extended to far smaller dimensions than was predicted 15 years ago, there are definite physical barriers to extending it to the minimum dimensions of 70 nm that are projected to be required 15 years from now. Both focused, point electron beams and ion beams have been used to write dimensions in resist well below 20 nm, albeit at speeds far too slow for production lithography. Projection systems, which employ a mask and, in effect, produce a large array of beams, can provide both small minimum dimensions and high throughput. Ions are particularly well suited for this because they suffer little or no scattering in the resist, the linewidth is not a strong function of dose (good process latitude), and the resist sensitivity is relatively independent to resist thickness or ion energy. IMS in Vienna, Austria has built two generations of ion projection lithography systems which have demonstrated many of the features needed for high throughput lithography. In these systems a stencil mask is irradiated with a uniform beam of light ions, H+, H2+, or He+, and the transmitted pattern is demagnified (by 10× to 3×) and focused on a resist covered wafer at energies in the 70–150 keV range. So far, minimum dimensions down to 70 nm line-space pairs have been demonstrated, drift has been eliminated with a “pattern lock” servo system, and field distortion of less than 0.15 μm over 8×8 mm has been measured in agreement with calculations. Based on these achievements a new generation ion lithography machine has been designed which uses 3× demagnification and will expose a 20×20 mm field at 0.12 μm minimum dimensions with less than 10 nm of distortion introduced by the ion optics. Global and stochastic space charge effects have been modeled and, in some cases, measured with the existing machines. Stochastic space charge effects will not cause unacceptable blur in the new design if the total ion current is kept below 3 μA. At this total current the time to expose one chip is still of order 0.5 s so that the calculated throughput is about 70 wafers (200 mm) per hour. Ion sources with low energy spread (∼2 eV) have been developed and will provide uniform illumination of the mask. Stencil mask fabrication on membranes of 2.5-μm-thick silicon has been developed. Distortion of the pattern cut in the membrane due to stress relief has been modeled, and with proper mask design can be kept below 20 nm (6.7 nm on the wafer). According to calculations and measurements mask distortion due to ion beam heating can be reduced to a negligible level if a radiation cooling cylinder is used. As a result of the building and evaluation of the existing machines and the design of the next generation, significant progress has been made in ion projection lithography which we will review in this article. The next step in the development of ion projection lithography is being conducted by the MEDEA program in Europe, which will develop a full field processing tool.

Stochastic Coulomb interactions in ion projection lithography systems with aberration-broadened crossover

The allowable current in ion beam projection lithography is limited by stochastic Coulomb interactions. Theories for the dependence of these interactions on the parameters of the system, i.e., length, beam energy, print size, crossover diameter, etc. are thus important to guide the design of future machines. All existing theories on blur from stochastic Coulomb interaction assume a homogeneous current distribution in the crossover, where a large part of the blur is produced. However, in actual lithography machines, the crossover may be substantially broadened by the spherical aberration of the ion lenses, thus giving a very inhomogeneous current distribution in the crossover. In most analytical theories, the stochastic blur is independent of the radius of the homogeneously field crossover rc for very small crossovers, and relates to rc as (1/rc)1/2 or (1/rc)1/3 for larger ones. If aberration broadening could do the same, the effect of broadening the crossover from rc=1.5 to 50 μm would decrease the blur by a factor as much as 6, without the negative effect of increasing the curvature-of-field blur. We find physical arguments that such a large factor is impossible. Monte Carlo simulations first of all show that the blur reduction is larger near the edges of the image field than in the center, near the optical axis. For the simulated model system, the reduction on axis can be increased to a factor 2 at very large aberration coefficients.

Study of energy broadening of high-brightness ion beams from a surface plasma Penning source and its relevance in ion projection lithography

The energy spread ΔE (Full Width at Half Maximum of the energy distribution) of negative hydrogen ion (H−) beams has been measured for angular beam intensities (IΩ) over a range of about 40 to 100 mA/sr. An IΩ23 dependence of the energy spread is observed. This trend supports the mechanism of energy broadening due to potential energy relaxation. The observed energy spread of ≲ 3 eV at IΩ of about 40 mA/sr projects a favorable scenario toward the demand of a future production-line ion projection lithography system. The maximum value of IΩΔE2 = 4 mA/sr eV2 suggests the possibility of H− beams for ion microscopy and surface analysis applications also. The H− ion source is currently operated in pulsed mode (maximum duty of 1%-- 1 ms, 10 Hz); cw operation can be achieved by improving the cooling system of the electrodes.

Recent Progress in Electron-Beam Cell Projection Technology

NEC's recent progress in the development of electron-beam (EB) cell projection technology is reviewed. To make it practical, not only the development of a high-performance EB direct writing system but also the establishment of its peripheral technologies in the micro-fabrication process are pursued. In order to obtain high lithographic performance in the EB cell projection lithography system HL-800D (Hitachi), the fundamental effects in EB lithography such as the Coulomb interaction effects, the proximity effect and electron scattering by cell projection aperture (EB mask), have been studied. In addition, high-resolution and high-sensitivity resists have been developed. The Coulomb interaction effects are found to be the most critical issue in cell projection lithography because it affects resolution, linewidth accuracy and throughput. For high resolution, the beam current was reduced to suppress the Coulomb interaction. As a result, a resolution of 0.15 µ m, which is sufficient for fabricating a 1G DRAM, was obtained using the high-resolution resist. To achieve high-linewidth accuracy of less than ±5% for 0.2 µ m lines-and-lines (L/S), optimization of the EB mask structure and the development of a proximity effect correction method which includes the Coulomb interaction effect correction were carried out. Inspection technology for devices of 0.2 µ m or less was also investigated in order to accurately measure linewidth, and to detect defects, particularly those caused by shot stitching error. Finally, the cell projection technology has been applied to the device fabrication of a 1G DRAM, and was demonstrated to be feasible for the development of futuristic advanced devices.

Direct aerial image measurements to evaluate the performance of an extreme ultraviolet projection lithography system

We report the application of direct aerial image monitoring to measure the performance of an extreme ultraviolet lithography (EUVL) test bed. There are several issues which might limit the possibility of performing image monitoring experiments on EUVL systems. These issues include: source flux, signal-to-noise ratios, and EUV scanning aperture fabrication. We report the results of initial aerial image scans of equal lines and spaces as well as preliminary measurements of scatter produced by the EUV camera.

High‐power, high‐beam‐quality solid‐state lasers for materials processing applications

The Laser Science and Technology Department at Lawrence Livermore National Laboratory is developing solid‐state lasers with high average power and high beam quality. Specific systems include a laser to generate 1.0–1.4 nm X‐rays for proximity print lithography, a 400‐mJ, 500‐Hz laser for 13.0 nm projection lithography and unique systems for speckle imaging, laser radars, and medical treatments.

Space charge effects in projection charged particle lithography systems

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.

Resolution analysis in electron-beam cell projection lithography system

We have experimentally analyzed the resolution in electron-beam cell projection lithography systems. The results indicated that the Coulomb interaction effect and the proximity effect are critical issues for resolution because of the larger beam current compared with the conventional variable-shaped beam. We achieved a practical resolution of 0.18 μm, which is enough for 1 Gbit dynamic random access memory (DRAM) fabrication, by adjusting the beam current. We have also found that there is some interaction between the individual line beams (streams). Also, the beam profile degradation in cell projection lithography systems depends not only on the beam current but also on the distance between the streams. Therefore, in order to obtain higher resolution, these effects must be considered more carefully than in the case of conventional variable-shaped beam lithography systems. A resolution of less than 0.15 μm, which is required for Gbit level DRAM fabrication, can then be available in cell projection lithography systems.

Experimental investigation of stochastic space charge effects on pattern resolution in ion projection lithography systems

Coulomb interactions in charged particle beams are known to blur beam profiles as the total current is increased. While this has been studied in single ion beam columns, similar studies have not been published, so far, for masked ion beam projection systems. Experiments were performed using highly transparent stencil grid masks of 40 mm×40 mm area with resolution test patterns in central and peripheral positions. Such grid test masks have been inserted into the ion projection lithography machine IPLM-02 and into the ‘‘Alpha ion projector’’ operated at 5× ion–optical reduction (Alpha-5×). A standard duoplasmatron ion source was used for both ion projectors. Operating the IPLM-02 at 8.7× demagnification with He+ ions of 73 keV ion energy at the wafer plane (about 50 keV at the crossover), no significant degradation in resolution was found changing the ion beam current through the column from 300 nA to 2 μA. Using the Alpha-5×, the total current through the column was changed either by varying the ion source emission or by blanking parts of the mask, using appropriate apertures. The special design of the Alpha ion projector with an ion energy of 5–7 keV at the crossover region between the imaging lenses is a sort of worst case scenario for space charge effects. Thus, the influence of stochastic space charge effects on resolution could be clearly identified. Using helium ions with 7 keV at the crossover the exposure latitude (change of linewidth in nm with 10% change of exposure dose) increased by 60 nm when changing the current through the column from 3 to 120 nA. Based on this experimental result with the Alpha ion projector and on well-known scaling laws, it can be stated that a newly designed ion projector (ALG-1000) with 200 keV ion energy at the crossover will allow sub-0.15-μm resolution for total ion beam currents through the column of 3 μA, as needed for future high volume production of dense patterns within ≥20 mm×20 mm exposure fields. Furthermore, improved designs of ion optical columns will allow a reduction of system length, thus reducing the influence of stochastic space charge effects. Consequently, ion projection lithography can be extended to the high volume fabrication of sub-0.10-μm feature patterns in large exposure fields.

Physical optics modeling in soft-x-ray projection lithography

We develop a diffraction-based tool that can predict the image quality in projection lithography systems. The effects of partial coherence and source variations and imaging defects in both the condenser and the camera optics can all be determined.

Radiation hardness of molybdenum silicon multilayers designed for use in a soft-x-ray projection lithography system

A molybdenum silicon multilayer is irradiated with 13.4-nm radiation to investigate changes in multilayer performance under simulated soft-x-ray projection lithography (SXPL) conditions. The wiggler-undulator at the Berlin electron storage ring BESSY is used as a quasi-monochromatic source of calculable spectral radiant intensity and is configured to simulate an incident SXPL x-ray spectrum. The test multilayer receives a radiant exposure of 240 J/mm(2) in an exposure lasting 8.9 h. The corresponding average incident power density is 7.5 mW/mm(2). The absorbed dose of 7.8 × 10(10) J/kg (7.8 × 10(12) rad) is equivalent to 1.2 times the dose that would be absorbed by a multilayer coating on the first imaging optic in a hypothetical SXPL system during 1 year of operation. Surface temperature increases do not exceed 2 °C during the exposure. Normal-incidence reflectance measurements at λ(0) = 13.4 nm performed before radiation exposure are in agreement with measurements performed after the exposure, indicating that no sign icant damage had occurred.

Large-area, high-resolution pattern replication by the use of a two-aspherical-mirror system

A soft-x-ray projection lithography system is developed by the use of multilayer mirrors. To determine the feasibility of a high throughput and a large exposure area, we developed a reduction system that consists of two-aspherical-mirror optics. The figure errors of aspherical mirrors are evaluated by a laser interferometer. The rms aspherical figure errors of concave and convex mirrors are 8.8 and 2.0 nm, respectively, which are not enough to yield a resolution of 0.1 µm. The reduction optics is constructed by adjusting the mirror position to compensate for aberrations, and some trial replications are performed. An exposure area of larger than 10 mm × 0.6 mm with a fine pattern of less than a quarter micrometer is achieved.

Synchrotron radiation sources and condensers for projection x-ray lithography

The design requirements for a compact electron storage ring that could be used as a soft-x-ray source for projection lithography are discussed. The design concepts of the x-ray optics that are required for collecting and conditioning the radiation in divergence, uniformity, and direction to illuminate the mask correctly and the particular x-ray projection camera used are discussed. Preliminary designs for an entire soft-x-ray projection lithography system that uses an electron storage ring as a soft-x-ray source are presented. It is shown that, by combining the existing technology of storage rings with large collection angle condensers, a powerful and reliable source of 130-A photons for production line projection x-ray lithography is possible.

Laser driver for soft-x-ray projection lithography

A design of a diode-pumped Nd:YAG laser for use as the driver for a soft-x-ray projection lithography system is described. This laser will output up to 1 J per pulse with a 2- to 5-ns pulse duration and a 400-Hz pulse repetition rate. The design employs microchannel-cooled diode laser arrays, zigzag slab energy storage, a regenerative amplifier cavity that uses phase conjugator beam correction for near-diffraction-limited beam quality, and stimulated Brillouin scattering pulse compression to achieve the required pulse length.

Condenser optics, partial coherence, and imaging for soft-x-ray projection lithography

A condenser system couples the radiation source to an imaging system, controlling the uniformity and partial coherence at the object, which ultimately affects the characteristics of the aerial image. A soft-x-ray projection lithography system based on a ring-field imaging system and a laser-produced plasma x-ray source places considerable constraints on the design of a condenser system. Two designs are proposed, critical illumination and Köhler illumination, each of which requires three mirrors and scanning for covering the entire ring field with the required uniformity and partial coherence. Images based on Hopkins' formulation of partially coherent imaging are simulated.

Wavelength considerations in soft-x-ray projection lithography

The choice of the operational wavelength for a soft-x-ray projection lithography system affects a wide variety of system parameters such as optical design, sources, resists, and multilayer mirrors. Several system constraints limit the choice for the operational wavelength. In particular, optical imaging requirements place an upper limit and throughput issues place a lower limit on the wavelength selection. We have determined that there are several discrete wavelength regions between 10 and 25 nm that satisfy the system-imposed constraints of high resolution, large depth of focus, and high throughput.

Particle–particle interaction effects in image projection lithography systems

Using commercially available software (Discrete Coulomb Interactions Software from Munro’s Electron Beam Software Ltd.) we have investigated image broadening as a result of stochastic interactions for projection systems. We have derived empirically, design constraints applicable to ion and electron projection systems and used them to analyze system designs suggested in the literature. We conclude that for many of the suggested designs stochastic interactions will prevent useful throughputs from being achieved. Finally, we discuss system design approaches which are necessary for a successful high-throughput, high-resolution image projection lithography system.

Differential equation design of finite-conjugate reflective systems

Based on the conditions of aplanatism, a differential equation design method has been applied to the optimization of finite-conjugate, multimirror systems for soft x-ray applications. The shapes of two surfaces in a multimirror system are determined by numerically solving differential equations for the mirror surfaces. The shapes of other mirrors in the system are used to control the remaining aberrations of those systems, such as astigmatism, field curvature, and distortion. Applications include a three-minor projection-lithography system, which has been optimized by this method and compared with results obtained by conventional optical design, and a two-mirror soft x-ray imaging microscopy system, which is predicted to yield a diffraction-limited performance for a large N.A.

High performance optical lithography using a separated light source

In projection lithography systems with fly’s-eye elements, a virtual source is created as an array of approximately mutually incoherent point sources. This article describes light amplitude and phase simulations for an image of a mask illuminated by a point source. This article discusses the dependence on the point-source location for a plane perpendicular to the optical axis. An image projected by a separated light source, rather than a ring-shaped light source, improves due to interference effects between multiple apertures is shown. Resolution of 0.25 μm equal lines and spaces was improved experimentally at a wavelength of 365 nm and a numerical aperture of 0.54. Also, how an isolated pattern image can be improved theoretically using an optimally separated light source is shown.