Dose Latitude Research Articles

Space-charge limitations to throughput in projection electron-beam lithography (SCALPEL)

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.

Control in sub-100 nm lithography in SAL-601

This article focuses on e-beam nanolithography in SAL-601, a negative tone, chemically amplified resist from the Shipley Corporation. It examines the role of exposure conditions and the postexposure bake (PEB) on the ability to define features of critical dimensions of 100 nm and below. Lithography was performed with a 50 kV e beam and 140 nm thick resist, where forward scattering of the e beam is expected to be minimal. Common PEB conditions of 105 °C for 1, 3, and 10 min and 110 °C for 1 min were investigated. Although the different PEB conditions produced a factor of up to 2.4× increase in sensitivity, no change was observed in the contrast, 5.8±0.1, or the minimum linewidths, ∼60 nm. The measured line spread function forward Gaussian widths were characterized by standard deviations, σ=27–33 nm, depending on PEB conditions. These are compared to σ of 19 nm for PMMA and the expected σ of 11 nm for the incident probe (σ=10 nm) convoluted with a Monte Carlo code generated point spread function. The measured backscatter coefficient was 0.51, in agreement with our Monte Carlo simulation results. In the analysis of dose latitude to form critical dimension gaps (±10% precision) between a series of pads of differing sizes, the different PEB conditions produced no observed difference. The feature size, however, had a large effect on the ability to form the critical dimension gap. Furthermore, the dose latitude was much smaller than calculated by integration of Monte Carlo derived line spread functions. A semiempirical model is introduced in which the forward Gaussian width of a measured line spread function (LSF) in developed resist is convoluted with the Monte Carlo code LSF. The calculated exposure profiles based on the semiempirical LSF show much better agreement with the experiment than those based on the expected, σ=11 nm, forward Gaussian width.

X-ray lithography for ⩽100 nm ground rules in complex patterns

Complex patterns with 75–125 nm feature sizes exposed with x-ray lithography are shown. Lithographic results for 75–125 nm lines with varying pitch are compared to simulations of image formation and resist dissolution, showing good qualitative agreement. Exposure dose latitude, nested-to-isolated print bias, image shortening, linewidth change with gap, and linearity of printed linewidth versus mask linewidth are quantified for 11–22.5 μm gaps. Critical dimension control error budgets for resist linewidth uniformity are determined for logic patterns at 75 and 100 nm ground rules. Image shortening is quantified for 75–125 nm ground rule static random access memory-like patterns, indicating hammerheads added to line ends reduce shortening to acceptable levels for ⩾100 nm ground rules at ⩽17.5 μm gaps. With tight gap control and tight mask linewidth control, 100 nm ground rule complex patterns can be printed with good latitude using x-ray lithography.

Extendibility of x-ray lithography to ⩽130 nm ground rules in complex integrated circuit patterns

Previous experimental and theoretical evidence indicates that x-ray lithography can be used to pattern ≤180 nm features. In order to be used in manufacturing, x-ray lithography of complex integrated circuit patterns (i.e., dense two-dimensional patterns) needs to be demonstrated with a practical proximity gap. However, no large body of experimental evidence exists for the extendibility of x-ray lithography for complex patterns with ground rules of ≤130 nm at gaps of 10–20 μm. Simulations of image formation and resist dissolution are shown to have good agreement with experimental results. These simulations are then used to predict exposure latitude and gap latitude for printing one-dimensional 75–125 nm patterns at 10–15 μm gaps. Simulations indicate that at least ±10% exposure dose latitude will exist for simple patterns at these gaps, but significant nested-to-isolated linewidth bias will exist. Gaps must be controlled to ±1 μm for ±10% dose latitude. More complex two-dimensional patterns have been shown to exhibit line end shortening [J. Maldonado, R. Dellaguardia, S. Hector, M. McCord, and L. Liebmann, J. Vac. Sci. Technol. B 13, 3094 (1995)] that simulations qualitatively indicate a rise in part due to image formation. Simple serifs on line ends may be needed to reduce line end shortening.

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.

An advanced epoxy novolac resist for fast high-resolution electron-beam lithography

Aspects of the formulation of a highly sensitive cresol epoxy novolac-based chemically amplified negative resist (epoxy resist, EPR) and optimization of critical process parameters for high-resolution electron-beam lithography are reported. The bulk resist sensitivity (E80, dose for 80% thickness retention) is 0.9 μC cm−2 at 40 kV. The effect of postapply bake and postexposure bake on resist sensitivity, contrast, and resolution are investigated and optimized for lithography up to the 0.1 μm regime. The resist process is characterized by a good exposure dose latitude and relevant insensitivity to the variation of thermal processing conditions. Postexposure bake temperature variations in the 90–130 °C range cause minimal change in sensitivity but remarkable change in contrast. Due to this behavior the resist process is not described satisfactorily by the reaction kinetic models commonly used to characterize chemically amplified resists of different chemistry.

Evaluation of Incoherent-Type Pupil Filter: Spatial Filter for Incoherent Stream

A new imaging method using an incoherent type filter, spatial filter for incoherent stream (SFINCS) is evaluated by experiment. The experiment was done with an i-line NA0.57 projection lens. The experimental results show the excellent depth-of-focus (DOF) expanding effect of SFINCS for fine hole patterns with good exposure dose latitude and mask manufacturing error latitude even if multiple focus exposure (FLEX) is used.

Positive-tone silylated, dry-developed, deep ultraviolet resist with 0.2 μm resolution

This paper describes the development of a surface-imaging process for a positive-tone silylated, dry-developed bilayer resist which has 0.2 μm resolution and an aspect ratio of 4.5 using deep-UV (248 nm) exposure. The many processing variables such as thermal treatment parameters, silylation conditions, and etching conditions were examined to determine their effects on lithographic performance in terms of resolution, feature size linearity, focus latitude, and sensitivity. Critical to the success of the process are: the bilayer structure which restricts diffusion of the Si, the use of a disilane reagent to increase the Si content of the masking layer, limiting migration of photogenerated acid by the appropriate choice of softbake and post-exposure bake temperatures, initial etching with an Ar/Cl2 mixture to remove the thin layer of silylated resist in the exposed areas, and employing CO2 instead of O2 as the etching gas to eliminate lateral etching of the features. With this process we have obtained good critical dimension linearity down to 0.25 μm for bright-field and dark-field lines and spaces as well as isolated lines and isolated spaces. The dose required is ∼75 mJ/cm2 and the dose latitude is ±6%. Focus latitude is at least ±0.4 μm. We also observe no environmental effects on sensitivity or resolution.

100 kV g h o s t electron beam proximity correction on tungsten x-ray masks

100 kV electron beam exposures of tungsten x-ray masks have been made using ghost proximity correction. 0.15 μm lithography is possible on masks having a 0.3 μm thick tungsten absorber. 0.25 μm features are resolved on masks with a 0.6 μm tungsten film thickness. The linewidth control (±5%) and dose latitude (2.5 nm edge shift per 1% dose change) are as good or better than those obtained by dose modulation, and contrast is sufficiently high to permit dry etch pattern transfer into the tungsten absorber. The process latitude makes 100 kV ghost exposures suitable for patterning 1 Gbit dynamic random access memories on 1m x-ray masks.

Fabrication of quantum semiconductor structures by high energy electron beam lithography of an inorganic epitaxial resist on GaAs

Epitaxially grown calcium strontium fluoride on gallium arsenide has been patterned by high energy electron beam lithography and wet development. Fabrication of lines and dots of 8nm size is reported. Electron diffraction experiments indicate that fluorine is removed under electron bombardment with the formation of a calcium and strontium oxide. The end product of the radiolysis can be selectively etched in an acetic acid solution. A dose requirement of around 2×10 4C/cm 2 was necessary to form these features but a large dose latitude was observed. Possible applications of this epitaxial inorganic resist for the fabrication of quantum semiconductor structures are discussed.

Nanostructure patterning with sor x-ray lithography

Abstract A mask technique using a thin (350 nm) e-beam resist layer and Au electro pulsplating is presented. Isolated absorber lines of 80 nm in size have been successfully fabricated. The absorber lines have been replicated in 300 nm thick PMMA using synchrotron radiation. Mask bias and dose latitude values have been determined for sub-150 nm absorber lines and compared with calculated data.

Advanced i-line lithography: processes for positive and negative patterning using the same acid hardening resist

In this paper, we propose two sub 0.5 μm i-line processes (0.40 NA stepper), for negative and positive patterning, using the same acid hardening resist: the negative patterning is achieved via wet development and the positive patterning via silylation and dry development. We first considere the wet development negative process. Such a negative process is strategic for many lithographic levels, and we here demonstrate its application for sub 0.5 μm gates fabrication [the level of IC fabrication requiring the highest resolution]. The lithographic results obtained over flat and topographic polysilicon wafers are presented and discussed in terms of exposure dose and focus latitudes. The plasma etching behavior of the resist is then studied and discussed as a function of the gas chemistry: the best conditions are finally used for transfering the patterns into a 0.38 μm thick polysilicon layer. In the second part of this paper, we demonstrate a positive dry development process using vapor silylation; different silylation conditions and development modes are evaluated. Such a process could be used where regular wet development processes reach their limits, or where a positive process is better adapted to the geometry of the level in fabrication: only one resist would then be necessary for both polarities.

Low-voltage electron beam lithography

The use of low-voltage electron beam lithography to reduce proximity effects, improve throughput, and reduce substrate damage caused to underlying materials has been investigated. Various films of PMMA were exposed with a field emission scanning electron microscope adapted with blanking capability and a 16 bit resolution beam control package. The exposure voltages used were from 1 to 15 keV with probe sizes of ≤150 to ≤70 Å, respectively. The dose latitude or working dose range was determined for each voltage on film thicknesses of 0.05, 0.18, and 0.38 μm poly(methylmethacrylate). Optimum beam voltage for a particular thickness which maximizes the energy deposited within the resist has been approximated using Monte Carlo modeling and verified experimentally. Atomic force microscopy showed that at lower voltages the dose required to properly expose a feature is relatively low and as beam voltage increases, the dose required to expose a given area increases. This data also verifies the fact that if the exposing voltage is too low, the resist will not be fully exposed. Because low energy electrons have a smaller interaction volume, there is a significant decrease in the proximity effect that is evident when high energy electrons are used for exposure. Comparison of low energy and higher energy electron beam exposures in this study reveal this decrease. Results indicate that by using low energy electrons for resist exposure the dose can be lowered and thus the throughput increased; the proximity effect is significantly decreased; and because of the limited penetration depth of low energy electrons, the substrate damage is potentially minimized.

Fabrication of submicron microwave bipolar transistors by direct write electron beam lithography

To study the effects of reducing the lateral dimensions of bipolar transistors to values below 1 ?m, a production process based on optical lithography, is adapted to fabricate submicron sized interdigitated microwave transistors. The emitter, base contact, and metallization patterns are defined by direct write electron beam lithography. For alignment of the patterns, special registration marks on (111) Si-wafers, compatible with further silicon processing have been developed. With a Philips EBPG/03 Beamwriter the resulting overlay accuracy is between 50 and 100 nm. The emitter and base contact resist patterns are transferred to a SiO 2 layer by reactive ion etching. The exposure dose latitude for the definition of the Ti/Pt/Au metallization pattern is severely limited by the proximity effect and a high contrast electron beam resist is needed. To tansfer by sputter etching, the resist pattern into a high quality submicron metallization pattern, Ti is used as an etch mask.

Effect of EB Acceleration Voltage and Beam Sharpness on Process Latitude of 0.2 µm Lines

The 0.2µm line delineation capability of electron beam direct-writing is estimated by computer simulation. The effects of acceleration voltage and beam sharpness are evaluated by looking at process latitudes. The process latitudes refer to dose and development time latitudes, where proper resist profiles are obtained. The process latitudes are compared for acceleration voltages of 30 and 50 keV; and beam blurs of 0.0, 0.05, and 0.1 µm. For patterning on a bare Si substrate, an acceleration voltage higher than 30 keV with beam blur less than 0.1 µm can fabrlcate 0.2 µm lines. However, a 50 keV acceleration voltage is required for patterning on a W layer, because 30 kev, even with 0.0 µm beam blur, has only a small dose latitude.

Application tests of an image reversal resist suitable for G- and I-line exposure

Abstract AZ RAYPO, an image reversal resist operating via an acid- catalysed thermal cross-linking mechanism, has been evaluated with respect to resolution, dose and focus latitudes, thermal stability and etch resistance. The experiments using high numerical aperture g- and i-line steppers for the imaging exposure confirmed suitability for both exposure wave- lenghts as suggested by spectroscopic data. Preliminary tests using topography wafers demonstrated resolution of minimum dimensions down to 0.55 μm.

Search for the optimum numerical aperture

Results on the dependences of focus and exposure dose latitudes on the numerical aperture of lithographic g-line, i-line, and KrF exposure systems are given for 0.6, 0.8 and 1.0 um lines and spaces. This is done for two different resist systems by both SAMPLE 1.7 simulations and g-line experiments. The data are analysed by the use of k-factors and show some unique behavior that is strongly resist dependent.