Resolution Electron Beam Lithography Research Articles

Proximity effect correction for nanolithography

The resolution of electron-beam lithography is limited by the proximity effect, which is due to the scattering of incident electrons. This scattering leads to exposure of neighboring areas and, therefore, to pattern degradations. It can be modeled by convolving the incident dosage distribution with a point spread function and the subsequent developing process by a pointwise nonlinear function. This article presents an iterative algorithm that exploits this overall model to compute corrections of the proximity effect in the nanometer range. A convex error function is derived that satisfies all physical constraints and the problem is set up as a convex, constrained, and nonlinear minimization problem. The correction algorithm is a hybrid of the conjugate gradient and gradient projection algorithms. Its performance for a test pattern is evaluated and then a detailed analysis of the effect of inaccuracies of the model parameters and of the necessary quantization of the corrected incident dosage distribution is presented.

Resist process to join electron beam lithography and photolithography

An advanced resist process will be introduced connecting the high resolution of electron beam lithography (EBL) and the high productivity of photolithography using a surface imaging technique. A method to overlay these two exposure processes will be discussed. In addition the process was analysed using simulation. A pattern transfer down to 0.2 μm was achieved.

Atomic hydrogen resist process with electron beam lithography for selective Al patterning

The resolution of electron beam lithography using polymer resist is limited to be 10–20 nm because of electron scattering effects and molecular size of polymerized resist. Thinning the resist thickness is a method to improve the resolution. In this article, we discuss a new atomic hydrogen resist process for area-selective Al patterning. The atomic resist is monolayer thick hydrogen terminated on a Si surface, i.e., the thickness of the resist is ultimately thin. The features are (1) patterning of the monolayer thick terminated H on the Si surface by focused electron beam exposure and (2) selective growth of Al on the remaining terminated H area by low pressure chemical vapor deposition using dimethylaluminum hydride [(CH3)2AlH] and H2. In the electron beam exposed area, the terminated H on Si is removed and the Si surface is activated. Then the activated Si surface is oxidized in nanometer thickness in a clean-room environment. The Al is selectively deposited on the remaining terminated H region. We have successfully formed the area-selective Al patterning by the atomic hydrogen resist process.

The resolution of electron beam lithography

We present a model for the primary interaction of the electron beam with bound electrons in the resist. Time-dependent perturbation theory is used to calculate the probability that sites some distance from the electron beam will be exposed by the electromagnetic field around it. We show that, assuming our approximations to be correct, the range of this interaction is adequate to explain the resolution of P.M.M.A.

ZnS micro-Fresnel lens and its uses

A micro-Fresnel lens replication method by inorganic material deposition has been developed. A ZnS micro-Fresnel lens and a completely flat micro-Fresnel lens have been made by this method. The ZnS microFresnel lens stability characteristics are improved for temperature, humidity, and focusing. Furthermore, higher resolution in electron-beam lithography is made possible by lens thickness reduction. The completely flat micro-Fresnel lens is a new device and improves integration performance. This lens can be applied to stacked planar optics devices for use in the construction of 3-D optical circuits.

Electron beam lithography resolution upon exposure of superthick resist layers

Exposure distribution in the thick resist layer upon exposure of submicron lines has been studied. The limitations defined by the resist thickness on the formation of submicron gaps between narrow lines exposed and developed in a positive electron resist have been estimated.

Fabrication of very small devices for the investigation of the one-dimensional transport

Very small metal and semiconductor wires are of great interest for experiments on electron and phonon transport in one-dimensional structures. Methods of fabricating wires with sub 0.1μm linewidth are discussed and the procedures used to make a metallic loop of linewidth 20 × 20nm and 50 by 100nm GaAs wires described in detail. The ultimate resolution of electron beam lithography using polymeric resist is reviewed.

30-nm-Scale Device Fabrication for Electron Transport Studies

ABSTRACTThe study of quantum interference effects in metallic structures requires the lithographic resolution of electron-beam lithography. Resolution and reproducibility can be greatly enhanced by the use of a multilayer resist. We have implemented a polymethylmethacrylate (PMMA) bilayer resist which avoids the typical problem of intermixing of the layers. This is accomplished by an expedient choice of the solvent, xylene, for the upper resist layer. Metal lines 30 nm wide have been fabricated. We also describe an additional deep ultraviolet (DUV) exposure method which facilitates making electrical contact to these ultrasmall structures. Quantum interference, localization effects, and the electron phase-coherence time have been studied.

Theoretical study of the ultimate resolution in electron beam lithography by Monte Carlo simulation, including secondary electron generation: Energy dissipation profile in polymethylmethacrylate

A new Monte Carlo simulation including generation of secondary electrons has been performed to study ultimate resolution in electron beam lithography. With respect to the simulations of inelastic scattering process of the primary electrons we used Gryzinski’s excitation function for single electron excitation processes, generating secondary electrons, while the inelastic mean free path proposed by Seah and Dench for organic material and energy loss spectrum obtained by Ritsko were used for simulating the inelastic scattering processes of the secondaries. The calculations were made to obtain energy dissipation profile for 20-kV electrons, particularly, in the vicinity of the point of incidence on nanometer-scale in 3000-Å-thick films of polymethylmethacrylate (PMMA) on a silicon substrate. The result clearly indicates that the secondary electrons play a most important role in determining the energy dissipation profile in the vicinity of the point of incidence, being the significant source of broadening of the energy dissipation profile. The blurring due to secondary electrons at the surface of the PMMA resist is of the order of 10 nm.

Exposure and development simulations for nanometer electron beam lithography

A new Monte Carlo simulation including generation of secondary electrons combined with development process was applied to study ultimate resolution of electron beam lithography. With respect to the simulation of inelastic scattering processes of the primary electrons we used Gryzinski’s excitation function for single electron excitation processes, and for generating secondary electrons, while the inelastic mean free path proposed by Seah and Dench for organic materials, and the energy loss spectrum obtained by Ritsko were used for simulating the inelastic scattering processes of the secondaries. The etching process which we used is the time evolution development proposed by Neureuther et al., using the model of Hatzakis et al. for polymethylmethacrylate (PMMA). The calculations were made to assess the line‐profile resist development of 30 nm‐thick films of PMMA on 60 nm‐thick silicon substrate for line exposure by a parallel electron beam 6 nm wide at 50 keV. The result shows a developed profile given by the contour representing the shortest evolution time which intersects the substrate is ∼18 nm wide at the surface. The theoretical result was compared with the experimental study of ultimate resolution of electron beam lithography, done by Broers.

Resolution, overlay, and field size for lithography systems

Resolution, overlay, and field size limits for UV, X-ray, electron beam, and ion beam lithography are described. The following conclusions emerge in the discussion. 1) At 1-µm linewidth, contrast for optical projection can be higher than that for electron beam. 2) Optical cameras using mirror optics and deep UV radiation can potentially produce linewidths approaching 0.5 µm. 3) For the purpose of comparing the resolution of electron beam and optical exposure, it is useful to define the minimum linewidth as twice the linewidth at which the contrast of the exposure system has fallen to 30 percent. 4) X-ray lithography offers the highest contrast and resist aspect ratio for linewidths above about 0.1 µm, but for dimensions below 0.1 µm, highest aspect ratio is obtained with electron beam. 5) With electron beam exposure on a bulk sample, contrast for a 50-nm linewidth is the same as that for 1-µm linewidth, provided the resist is thin. Higher accelerating voltages make it easier to correct for proximity effects and to maintain resolution with thick resist. 6) Ultimately the range of secondary electrons limits resolution in electron beam lithography, just as the range of photoelectrons limits resolution in X-ray lithography. In both cases, minimum linewidth and spacing in dense patterns is about 20 nm. Resolution with ion beams will probably be about the same because the interaction range of the ions will be similar to the electrons.

An approach to correct the proximity effect in electron beam proximity printing

Electron beam proximity printing employs shadow projection for submicron lithography. The pattern mask has physical holes for the transmitted electrons. The mask stencil problem (ring shaped apertures would fall out of these masks) is overcome by distributing the pattern to be printed on two complementary masks. The use of two complementary masks provides means to correct the proximity effect, known to impede micron resolution in electron beam lithography. To that end, for a shape in the first complementary mask a smaller correction shape in the second is introduced. The double dose deposited in the central position of the shape modifies the development process so that the proximity effect can be corrected. Experiments are reported which demonstrate the feasibility of this proximity correction method.

A theoretical estimate of the voltage dependence of the pattern resolution in electron-beam lithography

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Substrate thickness considerations in electron beam lithography

A comprehensive theoretical and experimental study of the spatial distribution of electron energy dissipation in a very thin polymer film for dot, line, and parallel line exposures over a wide range of substrate thickness and exposure dosage is reported. The two Monte Carlo models used in the theoretical calculations are reviewed and the theoretical results obtained are discussed. Experimental fabrication of thin substrates and effective techniques of electron beam lithography on such substrates are described. The concept of equienergy dissipation contours is discussed and then used to compare experimental data with theory. Good agreement between experiment and theory has been obtained. With substrate thickness as a variable, the fundamental influence of electron scattering on the resolution of electron beam lithography has been verified.

High resolution electron-beam lithography on thin films

The influence of electron scattering on the resolution of electron-beam lithography has been studied. Theoretically, we have used two different Monte-Carlo approaches to study the spatial extent of energy dissipation in a thin film of electron sensitive polymer film coated on various thicknesses of silicon substrates. The two Monte-Carlo approaches are the conventional continuous-slowing-down approximation approach and the direct simulation approach in which individual inelastic scattering is taken into account. Experimentally, we have exposed lithographic patterns on the structures mentioned above. Agreement between both Monte-Carlo approaches and experiment is satisfactory. Results show that higher resolution in electron beam lithography can be achieved by using thin electron sensitive resist layers and thin substrates. Improvement in proximity effect is also obtained for thin structures.