Dose Latitude Research Articles

Simulation study of zone-height limit by electron beam lithography for 30 nm Fresnel zone plates in X ray optics

Diffractive Fresnel zone plates with rectangular zones in noble metals are the key components in X-ray microscope as focusing/imaging lenses, which are mainly fabricated by electron beam lithography, followed by Au electroplating. For obtaining high quality focus and/or images in hard-X-rays, the zone height should be high enough to achieve coherent focusing/imaging on the focal plane. However, when the focusing/imaging resolution advances from 50 nm to 30 nm, the required height becomes enormously difficult owing to proximity effect in electron beam lithography, resulting in reduced efficiency. This work carried out systematic simulations based on Monte Carlo model to explore the zone height achievable by electron-beam lithography for 30 nm zone plates under 100 keV in ZEP520A which has higher contrast than PMMA. The designed exposure width for the outermost trench of the zone plate was used as the processing variable to work out the dose latitudes in which the maximum height was replicated. Both the sidewall verticality and the duty ratio were used as criteria to evaluate the lithography quality. By this way, the height limits of electron beam lithography for 30 nm zone plates were theoretically predicted, corresponding to various resist thicknesses from 50 nm up to 600 nm. The simulation results obtained in this work are instructive to the development of nanofabrication process of zone plates with 30 nm resolution.

Effects of lithographic parameters in massively parallel electron-beam systems

The ability of electron-beam (e-beam) lithography to transfer fine features onto a substrate is essential in many applications where high-resolution devices need to be fabricated. However, its low throughput has been the major drawback, especially for transferring large-scale patterns such as optical masks. In order to overcome the drawback, e-beam machines with massively parallel beams were recently developed and their throughput improved by several orders of magnitude has been experimentally demonstrated. In this study, for the optimal use of such parallel-beam systems, the effects of lithographic parameters and faulty beams on the writing quality are analyzed. The metrics of writing quality include the exposure variation and contrast, the total dose required, the dose latitude, and the line edge roughness. The analysis results obtained through an extensive simulation are provided and discussed in this paper.

Natural Radiation Dose and Geomagnetic Effect

Earth keeps getting bombarded by high-energy particles that originate in the outer space and in Sun. Cosmic radiation in Earth’s atmosphere consists of a photon and directly ionizing component and a neutron component. Charged particles of cosmic radiation are deflected by the geomagnetic field that is more expressed at the equator than near the poles. Photon radiation was measured by Radiameter ESM FH 40G-L10 at 26 main meteorological stations in all parts of Croatia. The correlation between the photon dose rate and latitude was examined, as well as multiple correlations for the photon dose, latitude and altitude. The obtained positive correlation coefficients were statistically significant. The neutron component of the cosmic radiation was measured by series of nuclear track-etched detectors, at the same meteorological stations during one year period. The dependence of the neutron dose rate on the altitude was found and the associated equation of the linear regression was derived. Relation between the values of neutron dose N and altitude h, showed significant dependence of the neutron dose on altitude. However, the correlation of neutron dose and geographic (geomagnetic) latitude was not statistically significant, probably due to a small range of geographic latitudes for the measuring stations in Croatia.

Comparison between e-beam direct write and immersion lithography for 20-nm node

E-beam direct write (EBDW) process window (PW) simulations were performed on critical layers in Altera designs of the 20-nm node (minimum metal half-pitch 32 nm). For selected layout clips, a direct comparison is made with 193i simulation results. Local interconnect and Via0 (single patterning) and Metal1 (litho-etch-litho-etch double patterning) layers are considered. The EBDW dose latitude was found to exceed that of the 193i process by a factor of 4. As the electron beam total spot size is of the order of the critical dimension for the considered node, interplay between neighboring features is low. This results in straightforward data preparation with typically two kernels and “clean” PWs. The latter are mainly limited by edge placement errors of line ends. The curves for the various simulation sites roughly overlap, as opposed to the 193i case in which they significantly differ. In EBDW, the performance of square vias equals that of rectangular vias, enabling a denser via packing.

Demonstration of electronic design automation flow for massively parallel e-beam lithography

For proximity effect correction in 5 keV e-beam lithography, three elementary building blocks exist: dose modulation, geometry (size) modulation, and background dose addition. Combinations of these three methods are quantitatively compared in terms of throughput impact and process window (PW). In addition, overexposure in combination with negative bias results in PW enhancement at the cost of throughput. In proximity effect correction by over exposure (PEC-OE), the entire layout is set to fixed dose and geometry sizes are adjusted. In PEC-dose to size (DTS) both dose and geometry sizes are locally optimized. In PEC-background (BG), a background is added to correct the long-range part of the point spread function. In single e-beam tools (Gaussian or Shaped-beam), throughput heavily depends on the number of shots. In raster scan tools such as MAPPER Lithography’s FLX 1200 (MATRIX platform) this is not the case and instead of pattern density, the maximum local dose on the wafer is limiting throughput. The smallest considered half-pitch is 28 nm, which may be considered the 14-nm node for Metal-1 and the 10-nm node for the Via-1 layer, achieved in a single exposure with e-beam lithography. For typical 28-nm-hp Metal-1 layouts, it was shown that dose latitudes (size of process window) of around 10% are realizable with available PEC methods. For 28-nm-hp Via-1 layouts this is even higher at 14% and up. When the layouts do not reach the highest densities (up to 10∶1 in this study), PEC-BG and PEC-OE provide the capability to trade throughput for dose latitude. At the highest densities, PEC-DTS is required for proximity correction, as this method adjusts both geometry edges and doses and will reduce the dose at the densest areas. For 28-nm-hp lines critical dimension (CD), hole&dot (CD) and line ends (edge placement error), the data path errors are typically 0.9, 1.0 and 0.7 nm (3σ) and below, respectively. There is not a clear data path performance difference between the investigated PEC methods. After the simulations, the methods were successfully validated in exposures on a MAPPER pre-alpha tool. A 28-nm half pitch Metal-1 and Via-1 layouts show good performance in resist that coincide with the simulation result. Exposures of soft-edge stitched layouts show that beam-to-beam position errors up to ±7 nm specified for FLX 1200 show no noticeable impact on CD. The research leading to these results has been performed in the frame of the industrial collaborative consortium IMAGINE.

Lithographic importance of base diffusion in chemically amplified photoresists

Mesoscopic (i.e., discrete and stochastic) models are applied to study the impact of photoresist processing and material conditions on process performance. The modeling approach includes all exposure, post-exposure bake, and development related parameters for chemically amplified resists. Resist process performance is evaluated in terms of required exposure dose (i.e., sensitivity), dose latitude (i.e., resolution), and line-edge roughness (LER). There exists a well-known trade-off between sensitivity, resolution, and LER. Theoretical models have concluded that optimization of any one or two of these properties go at the expense of the other properties. This paper shows that the effects of base diffusion, which have not been taken into account in previous studies, allow a simultaneous improvement of photoresist sensitivity, resolution, and LER. A putative explanation is given about the mechanism how the coupled diffusion of acid and base can allow a simultaneous improvement of all three process properties.

The effects of molecular weight on the exposure characteristics of poly(methylmethacrylate) developed at low temperatures

Poly(methylmethacrylate) (PMMA) with various molecular weights exposed using 50keV electron beam have been investigated at subzero developer temperatures. Contrast curves for Methyl Isobutyl Ketone (MIBK):Isopropyl Alcohol (IPA)=1:3 and MIBK:IPA=1:7 developers were determined. The contrast curves for different molecular weight PMMA resists converge into a single curve at lower temperatures. At low temperatures, the line edge roughness and resolution improved. However, minimum line pitch shows no significant improvement while dose latitudes for a particular line pitch increased. It is shown that at low temperatures, linewidth resolution and periodicity are invariant for the different PMMA molecular weights (50K to 2.2M) used in this work.

Resist parameter sensitivity analysis based on calibrated simulation for understanding resist limitations in next generation lithography

Simulations for predicting resist effects in the sub 50 nm resolution regime are strongly requested today, as well as for improvement of present resolution and CD control. Therefore this letter reports about a simulative resist parameter sensitivity analysis with help of calibrated resist models, based on Sigma-C’s SOLID software. Target of the study was to learn about the impact of resist parameters on practical resolution limits and to derive specific process and material change proposals. After resist model calibration for 90 nm design rules, process window, mask error enhancement factor, linearity, line end shortening etc. were investigated. The main influencing resist parameters were determined with two independent methodologies: Single-and multiparameter variation, which showed good agreement. Further, the sensitivity analysis was expanded to feature sizes down to 20 nm halfpitch. To decouple all optical influences from the resist, ideal rectangular aerial images were generated and used for simulation. The simulation reveals that an ArF resist might be capable of 40 nm resolution with sufficient exposure dose latitude, comparable to today’s 90 nm design rule. On the other hand even optimized exposure tools can’t provide such ideal rectangular aerial images and there is no commercial resist known today that shows a process window at 40 nm resolution today. Therefore, the main key resist parameters, which are responsible for resolution enhancement, were identified out of this simulation study and proposals for improved processes are given.

Sub-70 nm extreme ultraviolet lithography at the Advanced Light Source static microfield exposure station using the engineering test stand set-2 optic

Static microfield printing capabilities have recently been integrated into the extreme ultraviolet interferometer operating at the Advanced Light Source synchrotron radiation facility at Lawrence Berkeley National Laboratory. The static printing capabilities include a fully programmable scanning illumination system enabling the synthesis of arbitrary illumination coherence (pupil fill). This new exposure station has been used to lithographically characterize the static imaging performance of the Engineering Test Stand Set-2 optic. Excellent performance has been demonstrated down to the 70 nm equal line/space level with focus latitude exceeding 1 μm and dose latitude of approximately 10%. Moreover, equal line/space printing down to a resolution of 50 nm has been demonstrated using resolution-enhancing pupil fills.

Comparison of MIBK/IPA and water/IPA as PMMA developers for electron beam nanolithography

In electron beam lithography, resist behavior, such as sensitivity, contrast, exposure dose latitude, roughness and resolution, are influenced by the nature of the resist, the developer type and composition, and the development technique. In earlier work, ultrasonically assisted development was used to improve resolution and line edge roughness. Here we investigate the influence on resist behavior of an unconventional developer consisting of IPA and water. We observe improvements in sensitivity (∼40%), in contrast (∼20%), in exposure dose latitude (∼40%) and in roughness (nearly an order of magnitude) compared to the use of a conventional 1:3 MIBK/IPA developer. These improvements assisted in the fabrication of gratings of minimum size 16 nm equal line spacing in PMMA resist with 3:7 water/IPA developer.

Resist process development for sub-100-nm ion projection lithography

Ion projection lithography (IPL) is designed to realize resist structures on the wafer plane well below 100 nm. Patterns from a stencilmask are printed with 4× reduction using an electrostatic lens system. First exposures with the IPL Process Development Tool [Proc. SPIE 3997 (2000)] will start within the fourth quarter of this year. In the meantime, a baseline resist process for i-line, DUV and e-beam resists has been established with exposures on a 1× masked ion beam lithography (MIBL) tool at IMS-Vienna . The results as obtained with 75-keV He + ions are compared with experiments using a 30-keV e-beam writer at IMS-Chips, Stuttgart. Resists were provided by Shipley and Clariant. For the first time, through close interaction with industrial resist companies, ion beam resist materials are available and characterized with well tailored sensitivities in order to vary the IPL exposure dose between 0.4 and 3 μC/cm 2 (double the dose to clear large areas value). These materials will be essential to study and establish the required IPL resist exposure dose for sub-100-nm structures. We measured the sensitivities (dose to clear large areas), contrast curves as well as MIBL dose latitudes. With higher precision than previously done [J. Vac. Sci. Technol. B15 (1997) 2355] the contrast curves have to be established as input data for IPL simulations. The IPL sensitivity is lower by more than an order of magnitude, compared to 30-keV e-beam exposure. Our paper presents and discusses the results. For IPL resist simulations the Monte Carlo simulator SRIM 2000 [Ziegler et al., The Stopping and Ranges of Ions in Solids, Pergamon, New York (1985)] is used to obtain energy density distributions in C x H y O z resist materials. We are presenting and comparing point spread functions of 60 and 75 keV H + ions, He + ions and electrons.

Impact of exposure induced refractive index changes of photoresists on the photolithographic process

In many commercial and noncommercial photoresists the real and the imaginary parts of the refractive index are changed during exposure. Using a finite-difference beam-propagation algorithm, we analyze the impact of these nonlinear optical effects on the photolithographic process. Changes of the real part of the refractive index have a considerable impact on dose latitudes, sidewalls, swing curves, iso-dense bias and other process parameters. These effects become more dominant as the thickness of the resist layer increases.

Fabrication of <5 nm width lines in poly(methylmethacrylate) resist using a water:isopropyl alcohol developer and ultrasonically-assisted development

We report on the fabrication of 3–4 nm wide continuous lines in a positive tone electron beam resist poly(methylmethacrylate) on a solid substrate. This narrow linewidth was made possible through the use of a nonsolvent-based developer system, water:isopropyl alcohol, together with ultrasonically-assisted development, which reduced the effective development time thus limiting the swelling of the unexposed resist. This combination of solvent system and development technique results in a smaller radius of gyration in the developing polymer molecules and in a wider exposure dose latitude compared to conventional processing and so allows ultrasmall features to be reproduced.

Nanolithography using ultrasonically assisted development of calixarene negative electron beam resist

Negative resist image distortion is caused by resist swelling during development and is a major problem in ultrahigh resolution electron beam lithography. This problem has been overcome through the use of ultrasonically assisted development. In addition, exposure dose latitude is increased by 50% compared to conventional dip development, due to the improvement in contrast. These advantages have been exploited in order to realize ∼6 nm wide lines in calixarene resist.

Comparison of PEC approaches for SCALPEL

SCALPEL is a projection electron lithography technology with resolution limited by aberrations and electron-electron interactions, not diffraction. The main CD control issue is adequate dose latitude, for image blur approaching the feature size in a high throughput scenario. This includes the effect of the unwanted electron back-scatter (EBS) proximity dose. Due to operation at 100KeV, the back-scatter range is large (∼ 25 microns) and resulting CD changes depend only on the density surroundings of a critical feature. We compare mask pattern data biasing (adjustment of binary mask object sizes) with our proposed GHOST scheme; both available as methods of proximity effect correction (PEC). We demonstrate data bias for a gate-level test pattern with realistic features and density, using a very simple model. These CD control results are confirmed with CAPROX™, a complete device-pattern PEC calculation tool from Sigma-C.

Critical tool performance analysis for SCALPEL extensibility

An important feature of any advanced lithography technology is its cost-effective extension over several device generations beyond its introduction. Since SCALPEL is an electron imaging system with resolution not limited by diffraction, a unique extensibility analysis is needed to establish development pathways for the evolution of SCALPEL to the 50 nm generation and beyond. A combination of modest progress in four performance areas—effective optical field width, aberrations, acceptable dose latitude, and stage performance—will allow SCALPEL to maintain good throughput over several lithography generations without aggressive optical design scaling.

Comparison of sensitivity and exposure latitude for polymethylmethacrylate, UVIII, and calixarene using conventional dip and ultrasonically assisted development

Experiments are reported on the characteristics of electron-beam exposed polymethylmethacrylate (PMMA), UVIII, and calixarene resists for ultrasonically assisted development and conventional dip development. Ultrasonically assisted development provides improvements in sensitivity and contrast compared to conventional dip development, for the same development time in the case of 10 μm wide features in PMMA and UVIII, whereas no appreciable change is seen for calixarene. Also, we observe a substantial improvement in exposure dose latitude for smaller linewidths with ultrasonically assisted development compared to dip development for PMMA, UVIII, and calixarene. This improvement decreases somewhat as the linewidth increases. The increases in sensitivity and exposure dose latitude may be significant also in improving ultimate resolution. Isolated lines of 36 nm width have been obtained with UVIII using ultrasonically assisted development.

An Evaluation of High Acceleration Voltage Electron Beam Writing on X-Ray Masks

High acceleration voltage electron beam (EB) writing on X-ray masks was evaluated. Normalized dose latitudes of 100 keV for the 0.15 µm line and space (L&S), isolated lines and isolated space patterns were 3.5, 2.7 and 7.6 times larger than those of 25 keV, respectively. It was also found that the back-scattered electrons extend over a range of 10 µm in 100 keV writing. The maximum mesh size in the pattern area density method for proximity effect correction was evaluated by Monte Carlo simulation. When a ±10% pattern size error was permitted for a 0.1 µm line pattern, the 0.3 µm mesh size in 50 keV writing was markedly improved to 5 µm at a higher voltage of 100 keV. Reducing the beam size by half from 60 nm (2σ) in 100 keV writing also increases the mesh size from 0.4 to 5 µm. Moreover, the fine W–Ti patterns of 0.06–0.14 µm were formed using 100 keV Gaussian EB writing and electron cyclotron resonance (ECR) etching systems, where good linearity and linewidth deviations within 9 nm (3σ) were obtained.

Patterning Characteristics of Hole Patterns in Synchrotron Radiation Lithography

The patterning characteristics of synchrotron radiation lithography for hole patterns using a low-contrast mask (contrast: 3.5) are investigated. A resolution of 110 nm provides a large exposure dose latitude of more than 10% when the proximity gap is less than 15 µm, and the replicated minimum hole size is 70 nm with a 10-µm gap. Resist pattern sizes from 110 to 360 nm are kept within ±10% with proximity gaps of less than 20 µm. However, the exposure dose margin to guarantee mask linearity decreases to several percent because of the different optimum exposure dose for each pattern. Pattern size correction using mask bias to improve the mask linearity and exposure dose margin is proposed.

The printing conditions for 0.13μ m features in x-ray lithography using Pohang light Source

The optimal conditions to print the 0.13 μ m line and space are presented for newly constructed x-ray proximity lithography system at Postech Advanced Lithography Center (PALC). The PALC uses the synchrotron light source which is in Pohang Accelerator Laboratory (PAL). The optimal values of the absorber thickness, the bias-width and the spatial blur of the source are investigated with other conditions such as the mask to wafer gap remaining fixed. As a criterion for the optimal aerial image, we introduce hybrid figure of merit(h-FOM) which combines the dose latitude and contrast of the aerial image.