W. K. Waskiewicz

Interface imperfections in metal/Si multilayers

The structural and optical properties of Mo/Si and Ru/Si x‐ray multilayers prepared by sputter deposition in argon have been examined using high‐resolution transmission electron microscopy, optical profilometry, and x‐ray and soft x‐ray reflectance. We find that for Ru/Si, similar to previous results for Mo/Si, lower argon pressure during deposition results in smoother layers and higher reflectance. For low‐pressure deposited multilayers, interfacial roughness is negligible compared to interfacial diffuseness; the presence of amorphous interlayer regions in both of these systems is the major cause of reduced reflectance.

Diffraction-limited soft-x-ray projection imaging using a laser plasma source

Projection imaging of 0.1-microm lines and spaces is demonstrated with a Mo/Si multilayer coated Schwarzschild objective and 14-nm illumination from a laser plasma source. This structure has been etched into a silicon wafer by using a trilevel resist and reactive ion etching. Low-contrast modulation at 0.05-microm lines and spaces is observed in polymethylmethacrylate.

Surface finish requirements for soft x-ray mirrors

We have examined the correlations between direct surface-finish metrology techniques and normalincidence, soft x-ray reflectance measurements of highly polished x-ray multilayer mirrors. We find that, to maintain high reflectance, the rms surface roughness of these mirrors must be less than ~ 1 Å over the range of spatial frequencies extending approximately from 1 to 100 µm(-1)1 (i.e., spatial wavelengths from 1 µm to 10 nm). This range of spatial frequencies is accessible directly only through scanning-probe metrology. Because the surface-finish Fourier spectrum of such highly polished mirrors is described approximately by an inverse power law (unlike a conventional surface), bandwidth-limited rms roughness values measured with instruments that are sensitive to only lower spatial frequencies (i.e., optical or stylus profileres) are generally uncorrelated with the soft x-ray reflectance and can lead to erroneous conclusions regarding the expected performance of substrates for x-ray mirrors.

Variation in stress with background pressure in sputtered Mo/Si multilayer films

We have measured the stress in Mo/Si multilayer films deposited by magnetron sputtering, using the wafer‐curvature technique, and find a strong dependence on background pressure. We find that for multilayers containing 40 bilayers of ∼4.3 nm Si layers and ∼2.6 nm Mo layers, the stress increases from approximately −280 MPa (compressive) to −450 MPa as the background pressure in the deposition chamber (i.e., measured just prior to deposition) decreases from 1.0×10−5 to 6.0×10−8 Torr. For multilayers of the same period but with thicker Mo layers, the dependence on background pressure is even stronger. X‐ray (λ=0.154 nm) diffraction measurements reveal only a slight increase in interfacial roughness for films deposited at high background pressure, but no evidence was found for any differences in the microstructure of the polycrystalline Mo layers that comprise these structures. The peak soft x‐ray (λ=13 nm) reflectance, which is sensitive to interfacial roughness at longer spatial wavelengths, also shows no c...

Use of trilevel resists for high-resolution soft-x-ray projection lithography

A projection optical system with 20:1 reduction has been used with radiation at ∼36 nm to evaluate resists for use in soft‐x‐ray projection lithography. The high absorption of soft x rays by carbon‐based polymers requires that an imaging resist layer be very thin. The sensitivities and contrasts of several such resists are reported. By incorporating a thin imaging layer into a trilayer resist scheme, we have exposed, developed, and transferred features as small as 0.2 μm into silicon.

The SCattering with Angular Limitation in Projection Electron-Beam Lithography (SCALPEL) System

A SCALPEL\circledR (SCattering with Angular Limitation in Projection Electron-beam Lithography) proof-of-concept lithography system, comprising a tool, a reticle and a resist, has been designed to address the critical issues that must be investigated to determine if this approach is viable as a practical lithographic technology.

Roughness study of a positive tone high performance SCALPEL resist

In this article we discuss the line edge roughness of positive chemically amplified resists exposed on the SCALPEL exposure system in terms of the image formation process. The image formation process for a SCALPEL exposure on a positive chemically amplified resist has been simulated using discrete models from exposure through development. Key parameters have been identified that enable image formation simulations without the need of detailed molecular models. Molecular models are needed though to obtain several of these parameters. Surface and line edge roughness, as measured by scanning electron microscopy and atomic force microscopy, have been simulated and compared to experimental results. Results are consistent with a “percolation network formation for diffusion-reaction development” model for chemically amplified resists.

Diffraction‐limited soft x‐ray projection lithography with a laser plasma source

A laser plasma source of extreme ultraviolet and soft x‐ray radiation has been used to print diffraction‐limited features using soft x‐ray projection lithography. A spherical condenser optic, a Si/Ge transmissive mask and a Mo/Si multilayer‐coated Schwarzschild objective having 20:1 reduction ratio were employed to pattern selected single‐layer and trilevel resists. At a numerical aperture of 0.12, a 0.1‐μm line and space pattern is clearly delineated and weak modulation is observed for the analogous 0.05‐μm pattern.

Soft-x-ray projection lithography : printing of 0.2-μm features using a 20 : 1 reduction

We demonstrate nearly diffraction-limited printing of 0.2-microm features, using soft x rays of approximately 36-nm wavelength. An open-stencil transmission mask with minimum features of 4 microm was imaged by a twentyfold-reduction Schwarzschild-type objective onto silicon wafers coated with various e-beam resists. Implications for soft-x-ray projection lithography are discussed.

Electron scattering and transmission through SCALPEL masks

Electron scattering in thin solid films used for the fabrication of masks for electron projection lithography, e.g., SCALPEL®, is investigated. We have developed an analytical model to calculate electron transmission through the mask membrane and image contrast due to different scattering properties of the patterned area and the membrane. The model utilizes cross sections for electron elastic and inelastic scattering on an atom with exponentially screened Coulomb potential of the nucleus derived in the first Born approximation. The variety and controversy of theoretical and empirical adjustments of the screening parameter are briefly analyzed and attributed to the misinterpretation of experimental data ignoring the effects mostly due to plural scattering of electrons and dense packing of atoms in thin solid films. This model frees us from the computational limitations of Monte Carlo simulations and proves to be effective for the straightforward characterization of various alternative materials for SCALPEL...