High Numerical Aperture Lens Research Articles

Sub-Halfmicron Lithography Using a High-Contrast i-Line CEL

A newly developed, metal-free, water soluble i-line CEL and its patterning features are described. A, B and C parameters of CEL are 18 µm-1, 0.12 µm-1, and 0.09 cm2/mJ, respectively. CEL changes the γ value from 3 to 20. This CEL material makes patterns with an aerial image contrast of 0.4 possible. This result suggests that 0.3 µm microfabrication is possible with an i-line lithography that has a high numerical aperture lens.

Optical projection lithography using lenses with numerical apertures greater than unity

We have studied the characteristics and limitations of optical projection lithography using high numerical-aperture (NA) optical microscopy lenses. With oil-immersion, NA's as large as 1.4 were realized, and linewidths as fine as 140 nm were achieved in 70 nm-thick commercial photoresist films using illumination of wavelength λ = 453 nm. Previous researchers have also obtained deep-submicron linewidths using high NA lenses. The new elements in our work are: (1) use of a photoresist-compatible oil to achieve NA = 1.4; (2) use of modern antireflection coatings (ARC) and tri-level processing to circumvent the problem of substrate back-reflection; (3) use of NA 1 optical-projection to make masks for x-ray lithography, and the x-ray replication of such masks. Our motivation in pursuing this work was to explore the limits of high-NA optical projection, and to develop a simple, quick-turn-around method of making sub-quarter-micron-linewidth x-ray masks. Using the formula for minimum feature width W min = kλ/NA, we find that with an ARC and careful focusing one can work at k ∼ 0.43. Focusing and alignment are done by projecting the reticle image onto the resist with yellow light, which does not expose the resist. For exposure, the yellow filter is removed and a narrow-band blue filter (λ = 453 nm) inserted.

Photoluminescence and Optical Beam Induced Current Imaging of Defects

The minority carrier distribution in a semi-infinite semiconductor due to light focussed onto the surface via a high numerical aperture lens is given. The contrast obtained in both Photoluminescence and Optical Beam Induced Current defect imaging is derived as a function of the illuminating lens numerical aperture. It is shown that the maximum resolution attainable in both techniques is comparable.

Materials and Processes At the Leading Edge of Photolithography

ABSTRACTThe evolution of optical lithography to higher resolution patterning with step-and-repeat exposure methods has resulted from the precise fabrication of higher numerical aperture lenses, more precisely engineered and controlled mechanical and electrical subsystems, and new lenses, systems components and light sources which enable efficient printing at shorter wavelengths. Optical resist technology is undergoing a parallel evolution which is sharpening this capability even further. This paper outlines resist materials and processes at the leading edge of photolithography which will enable printing near and perhaps at the theoretical resolution limits. Subtopics discussed include resists sensitive to 248.4 nm excimer laser radiation, planarization methods, multilevel resist materials, and surface-functionalized plasma-developed resists. Finally, some conjecture about the practical resolution limits of optical lithography is presented.

An improved trilevel resist system for submicron optical lithography

A number of spin-on glasses (SOG) have been investigated with respect to their suitability for submicron pattern generation in a trilevel process for optical lithography. Some of them can be significantly improved so as to permit baking of both the bottom polymer and SOG at temperatures below 200 °C without the formation of cracks and allowing the use of inorganic developers. A trilevel resist system has been used in combination with a high numerical aperture lens (Zeiss lens 10-78-02, NA=0.42, λ=436 nm) for generation of 0.5 μm lines and spaces over 1 μm SiO2 and aluminum steps.

In-Line Holographic Lenses of High Numerical Aperture

In-line holographic lenses of numerical aperture of 0.4 and 0.55 were successfully fabricated for the purpose of the application to an optical disk player with a He-Ne laser. They were recorded at 6471 Å in the methylene-blue sensitized dichromated gelatin film of 15 µm in thickness. The special design of the optical recording system and the careful processing of the recording materials were required. The evaluations of the modulation transfer function, the focused beam profile, the average diffraction efficiency and the stability test were performed. Bragg and Raman Nath diffractions occurred in the hologram were also investigated to explain the observed results.

Device Photolithography: Thin Photosensitive Materials

New camera systems, utilizing lenses of high numerical aperture and concomitant shallow depth of focus, require thin recording media. A number of materials potentially fulfilling this requirement are discussed. These include photoresist-coated metal or semitransparent masks, some unconventional photographic processes, and dyed photographic emulsions. The use of dyed photographic emulsions is recommended on the basis of sensitivity and improved resolution and modulation of the recorded image.

Evaluation of Hologram Imaging by Ray Tracing

This paper applies the methods of ray tracing to the evaluation of aberrations produced by Fresnel holograms that are recorded on two dimensional media. Attention is given both to holographic systems that employ lenses to modify the object wave and to lensless systems. Two lensless systems are discussed. The first, a system operating at optical wavelengths, is of numerical aperture 0.025, magnification 25, and unity wavelength ratio, lambda recording/lambda reconstructing. The field size of this device is better than 300 times the resolution; the system is diffraction-limited for points within a depth of 2.5 cm about the nominal object point. The second system, designed to record at x-ray wavelengths and to reconstruct at optical wavelengths, is of numerical aperture 0.0125, magnification 1000, and wavelength ratio 0.001. The field size is better than 3000 times the resolution; the system is diffraction-limited for points within a depth of 0.035 cm about the nominal object point. The performance of a lensless system is compared to that of a holographic system which employs an analytic aplanatic lens of high numerical aperture to form an enlarged real image of the object which is then recorded by a holographic system of unity magnification. The distortions of such a system are those of the lens alone. A method is established to design and ray trace an aplanatic lens. The system with such a lens is found to be in every way superior, at optical wavelengths, to the comparable lensless system.