Synchrotron radiation sources and condensers for projection x-ray lithography.

The design requirements for a compact electron storage ring that could be used as a soft-x-ray source for projection lithography are discussed. The design concepts of the x-ray optics that are required for collecting and conditioning the radiation in divergence, uniformity, and direction to illuminate the mask correctly and the particular x-ray projection camera used are discussed. Preliminary designs for an entire soft-x-ray projection lithography system that uses an electron storage ring as a soft-x-ray source are presented. It is shown that, by combining the existing technology of storage rings with large collection angle condensers, a powerful and reliable source of 130-A photons for production line projection x-ray lithography is possible.

Mask technologies for soft-x-ray projection lithography at 13 nm.

We describe a variety of technologies for patterning transmissive and reflective soft x-ray projectionlithography masks containing features as small as 0.1 µm. The transmission masks fabricated for use at 13 nm are of one type, a Ge-absorbing layer patterned on a boron-doped Si membrane. Reflective masks were patterned by various methods that included absorbing layers formed on top of multilayer reflectors, multilayer-reflector-coating removal by reactive ion etching, and ion damage of multilayer regions by ion implantation. For the first time, we believe, a process for absorber repair that does not significantly damage the reflectance of the multilayer coating on the reflection mask is demonstrated.

Characterization of AZ PN114 resist for soft-x-ray projection lithography.

Using 14-nm wavelength illumination, we have imaged 0.1-µm-wide lines and spaces in single-layer thin films of the highy sensitive, negative, chemically amplified resist AZ PN114 by usingboth a Schwarzschild 20× camera and an Offner ring field 1× optical system. For soft-x-ray projection lithography the approximate 0.2-µm absorption length in resists at 14-nm wavelength necessitates a multilayer resist system. To explore further the requirements of the imaging layer of such a system, we have transferred patterns, exposed by a high-resolution electron beam in a 60-nm-thick layer of AZ PN114, into the underlying layers of a trilevel structure. Significant pattern edge noise and resist granularity were found. It remains to be determined whether the observed noise is dominated by statistical fluctuations in dose or by resist chemistry. We also investigated pinhole densities in these films and found them to increase from 0.2 cm(-2) for 380-mm-thick films to 15 cm(-2) for 50-nm-thick films.

Soft-x-ray projection imaging with a 1:1 ring-field optic.

A molybdenum/silicon multilayer-coated 1:1 ring-field optic with a numerical aperture of 0.0835 is used to carry out soft-x-ray projection imaging with undulator radiation at 12.9 nm. An ideal optic of this type should be able to image 0.1-µm features with a contrast exceeding 90% at this wavelength. The useful resolution of our ring-field optic is experimentally found to be approximately 0.2 µm, probably because of the presence of substrate figuring errors.

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.

Soft-x-ray projection lithography: printing of 0.2-microm 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.

Simple efficient travelin-wave excitation of short-wavelength lasers using a conical pumping geometry.

A conically shaped pumping geometry can produce an efficient burst of laser radiation, without the need for an optical cavity, by restricting amplified spontaneous emission losses to a small region near the apex of the cone. Requirements on the active medium and on the size and intensity of the pumping source to make such a burst laser are derived. We calculate that a 15-mJ pulse of energy at 37.2 nm at an efficiency of 0.15% can be extracted from sodium vapor photoionized with radiation from a 1.06-microm-laser-produced plasma using this pumping geometry.

Population inversions in C IV in a recombing theta-pinch plasma.

Population inversions on the 5g-4f and 4f-3d transitions of C IV at 253.0 and 116.9 nm have been observed in the recombination phase of a theta-pinch plasma initially containing 14 mTorr of acetylene and 0.5 mTorr of hydrogen gas. Population ratios of 17:11:1 for the 5g, 4f, and 3d levels were deduced from measured absolute line intensities of the 5g-4f and 4f-3d, and 3d-2p transitions at 253.0, 116.9, and 38.4 nm. The resulting gain-length product for a single pass through the 23-cm-long plasma column, based on the measured Doppler-broadened linewidth, is 2.3% at 116.9 nm and 5.8% at 253.0 nm.

Stimulated emission in the ultraviolet by optical pumping from photoionization-produced inner-shell states in Cd(+).

Stimulated emission on three UV transitions in Cd(+) has been observed by transferring population from inner-shell d-electron states, populated by photoionization, to outer-shell p-electron states using the output from a narrow-frequency dye laser. The use of similar techniques in other elements could eventually result in a number of new lasers in the UV and VUV.

Gain scaling of short-wavelength plasma-recombination lasers.

The theoretical Z(7.5) dependence of the small-signal gain of plasma-recombination lasers on the ionization stage Z of the laser species implies a straightforward development of short-wavelength lasers but appears to be significantly larger than the experimentally achieved gain scaling. A new analysis, which incorporates a simple experimental parameter representing the efficiency of populating the upper level at a given plasma electron density, results in a gain scaling with ionization state (Z(4.5)) that is more consistent with experiments and also reveals a need for better experimental control of the ion-production and electron-ion-recombination processes when one attempts to make such lasers.

High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source.

A soft-x-ray-pumped inner-shell photoionization laser has been produced in Cd vapor at 4416 and 3250 A. A gain of 5.6 cm(-1) has been measured at 4416 A, and a reasonably high-energy storage of 0.2 mJ/cm(3) in the upper laser states has been obtained.

Recombination laser transitions in expanding plasmas of Mg, Ca, Cu, Zn, Ag, Cd, In, Sn, Pb, and Bi.

Fifty new laser transitions in the visible and near infrared in ten metal vapors have been observed in the recombination phase of the expanding plasmas produced by a segmented-plasma laser device. Several of the strong visible transitions might be attractive for applications requiring simple lasers that operate at high repetition rates with pulse energies of the order of 0.1 mJ. All the transitions reported here suggest candidates for short-wavelength recombination lasers when isoelectronically scaled to higher ion stages.

Recombination lasers in Nd and CO(2) laser-produced cadmium plasmas.

This is the first report of a recombination laser that has been produced in laser-vaporized target material. Inputlaser-pulse energies as low as 0.5 mJ at 1.06 microm and 5 mJ at 10.6 microm have resulted in laser oscillation in Cd I at 1.40, 1.43, 1.45, and 1.64 microm in approximately 1-cm-radius cadmium plasmas. This laser appears to have the potential for high efficiency. In addition, scaling of this plasma-recombination process to higher-ionization stages should yield shortwavelength lasers.