A. M. Hawryluk

Energy dissipation in a thin polymer film by electron beam scattering

Monte Carlo calculations have been performed to determine the spatial distribution of energy dissipated in a 4000‐A‐thick film of polymethyl methacrylate (PMMA), due to an incident electron beam. The calculations were performed for 5‐, 10‐, and 20‐keV electrons on a silicon substrate and also for 20‐keV electrons on copper and gold substrates. The effect of varying the beam diameter from 250 to 3000 A was evaluated. A detailed comparison is made between the Monte Carlo results and analytic models used to predict the energy dissipated. The plural scattering model is found to be in good agreement with the Monte Carlo calculations, whereas discrepancies are found with the multiple scattering model. The large‐angle backscattering model is found to have several important limitations. Energy dissipation is calculated for the exposure of dots, isolated lines, and arrays of closely spaced lines—geometries that are of significance in electron beam lithography.

Spatial period division—A new technique for exposing submicrometer‐linewidth periodic and quasiperiodic patterns

We describe a technique for exposing patterns of spatial period p/n using near‐field diffraction from masks of spatial period p. The technique, which we propose to call ’’spatial‐period‐division’’, can be used with visible, UV, or x‐ray radiation. With soft x rays, it is possible to produce gratings of much finer spatial period than can be achieved by current laser holographic methods. A simple model predicts that a mask pattern of period p, with an opening or slit of width ?p/2n, will give rise to an intensity pattern of period p/n at a distance from the mask, S=p2/nλ, where λ is the wavelength of the incident radiation and l≪p. The model was confirmed at visible wavelengths using a scaled simulation of a laboratory x‐ray lithography unit. The feasibility of the technique was demonstrated at the 4.5 nm carbon K x‐ray wavelength by ’’doubling’’ a 196.8 nm period grating‐pattern x‐ray mask to produce a 98.4 nm period pattern in PMMA. Exposure of higher spatial‐frequency‐multiples appears feasible, especial...

Energy dissipation in a thin polymer film by electron beam scattering: Experiment

A series of experiments of scanning electron beam exposure of PMMA films on silicon, copper, and gold substrates is described, and compared with the calculations of Monte Carlo and analytic models of energy dissipation per unit volume. Film thickness was varied from 1000 to 10 000 A, accelerating voltage from 10 to 20 kV, and linear charge density from 1×10−9 to 1×10−5 C/cm. Good agreement was obtained in the comparison with the predictions of Monte Carlo calculations, whereas some discrepancies were observed in the comparison with the analytic models. The assumption inherent in the theoretical approaches that PMMA acts like a linear recording medium, in the sense that exposure is additive, is experimentally evaluated by means of an Abel inversion. The assumption that developed profiles represent surfaces of equal energy dissipation was examined by means of a computer simulation of the development process.A series of experiments of scanning electron beam exposure of PMMA films on silicon, copper, and gold substrates is described, and compared with the calculations of Monte Carlo and analytic models of energy dissipation per unit volume. Film thickness was varied from 1000 to 10 000 A, accelerating voltage from 10 to 20 kV, and linear charge density from 1×10−9 to 1×10−5 C/cm. Good agreement was obtained in the comparison with the predictions of Monte Carlo calculations, whereas some discrepancies were observed in the comparison with the analytic models. The assumption inherent in the theoretical approaches that PMMA acts like a linear recording medium, in the sense that exposure is additive, is experimentally evaluated by means of an Abel inversion. The assumption that developed profiles represent surfaces of equal energy dissipation was examined by means of a computer simulation of the development process.

Gold transmission gratings with submicrometer periods and thicknesses ≳0.5 μm

Gold gratings with spatial periods of 0.3 and 0.2 μm have been fabricated in thicknesses of 0.6 and 0.25 μm, respectively, and used in x‐ray spectroscopy and spatial‐period‐division. Fabrication techniques included: holographic lithography, shadowing, x‐ray lithography, and gold microplating. Control of linewidth to tolerance of the order of 10 nm has been demonstrated for gratings of 0.2 μm period. A high resolution imaging spectrometer, composed of a 22× Wolter x‐ray microscope in conjunction with a gold transmission grating, was tested. At a wavelength of 0.69 nm, a resolving power, λ/Δλ, of 200 was demonstrated. Resolution in this case was source‐size limited. Gratings of 99.5 nm period were exposed in PMMA by x‐ray (λ = 4.5 nm) spatial‐period‐division.

Fabrication of x‐ray masks using anisotropic etching of (110) Si and shadowing techniques

We describe a process for producing x‐ray masks of grating patterns with extremely smooth line edges. The technique developed by Flanders, in which a square‐wave profile relief grating in polyimide is obliquely shadowed with an x‐ray absorber, is followed, except that the original square‐wave structure is produced in (110) silicon by anisotropic chemical etching rather than in SiO2 by reactive ion etching. In this way, significant improvements in edge acuity are achieved because relief grating sidewalls are defined by atomic (111) planes. Holographic lithography is used to expose grating patterns in AZ 1350 over a thin Si3N4 layer on the (110) Si. The Si3N4 is patterned by reactive ion etching and serves as the mask for anisotropically etching the square‐wave‐profile grooves. At the proper crystallographic orientation the groove sidewalls are defined by (111) planes, and groove bottoms are approximately flat. The structure in Si is then transferred to polyimide which is obliquely shadowed and forms the x‐...

Deep-ultraviolet spatial-period division using an excimer laser

We have used the deep-UV output from an ArF laser and a grating mask with 199-nm spatial period to fabricate a 99.5-nm-period grating pattern in polymethyl methacrylate resist by spatial-period division. For sub 100 nm, lithography of periodic and quasi-periodic patterns (including Fresnel zone plates) by spatial-period division, deep-UV radiation offers a number of advantages over soft x rays.

X‐ray phase lens design and fabrication

We have fabricated the first high resolution phase lens for use at x‐ray wavelengths. The lens is a Fresnel phase plate made of silver approximately 5500 A thick. The structure has 100 zones with a minimum zone width of 3200 A. The lens is designed for use at the Al Kα line (approximately 1.5 keV) with an anticipated first order diffraction efficiency of ≂25%, and a spatial resolution of 4000 A. Details of phase lens design and fabrication are presented.

Addendum: New model of electron free path in multiple layers for Monte Carlo simulation

In this note, it is shown that the previous Monte Carlo simulations of energy dissipation in polymer films by electron beam scattering [R. J. Hawryluk, A. M. Hawryluk, and H. I. Smith, J. Appl. Phys. 45, 2551 (1974)] are consistent with the recent calculation of S. Horiguchi et al. [Appl. Phys. Lett. 15, 512 (1981)]. Such Monte Carlo simulations have been widely used in electron beam lithography.

Planar Techniques for Fabricating X-Ray Diffraction Gratings and Zone Plates

Planar techniques employed in fabricating Fresnel zone plates and diffraction gratings are reviewed briefly, with emphasis on recent developments.

Deep‐UV spatial‐frequency doubling by combining multilayer mirrors with diffraction gratings

A multilayer dielectric mirror was deposited over an aluminum grating of 198 nm period on a quartz substrate. This was then used as a parent mask to expose a grating of 99 nm period in PMMA by deep‐UV spatial‐frequency doubling with an ArF laser (λ=193 nm). The multilayer mirror suppressed the zero order permitting the doubled pattern to be exposed over a depth of field of several micrometers. Holographic lithography and spatial‐frequency doubling are compared. It is shown that the latter is preferred if λ/Δλ<105.