D. L. Windt

IMD—software for modeling the optical properties of multilayer films

A computer program called IMD is described. IMD is used for modeling the optical properties (reflectance, transmittance, electric-field intensities, etc.) of multilayer films, i.e., films consisting of any number of layers of any thickness. IMD includes a full graphical user interface and affords modeling with up to eight simultaneous independent variables, as well as parameter estimation (including confidence interval generation) using nonlinear, least-squares curve fitting to user-supplied experimental optical data. The computation methods and user interface are described, and numerous examples are presented that illustrate some of IMD’s unique modeling, fitting, and visualization capabilities.

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.

XUV optical constants of single-crystal GaAs and sputtered C, Si, Cr 3 C 2 , Mo, and W

The optical constants for sputtered films of C, Si, Mo, and W are presented for photon wavelengths from 13.3 to 304 A, and for single-crystal GaAs and sputtered Cr(3)C(2) from 13.3 to 1302 A. The best-fit values of the optical constants, and also the surface roughness parameter sigma, are obtained from reflectance vs incidence angle measurements using a nonlinear, least-squares curve fitting algorithm. These optical constants are significantly different from previously reported data in portions of this spectral region. The new optical data result in good agreement between theory and reflectance measurements of soft x-ray multilayer coatings made from the sputtered materials.

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.

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.