Patent Number: 046655417
Section: description

Referring more particularly to the drawing, there is shown a frequency tripled Nd:glass laser system using a Nd:glass laser 10. The laser is operated as a mode locked laser by a Pockels cell controller 12 to produce a single pulse of infrared laser light one ns in duration. The wavelength of this light is about 1.05 microns (.mu.m). The light emanates from the laser in a beam which passes through a tripler 14 to produce a pulse of output light of approximately 0.35 .mu.m in wavelength. The tripler may suitably be of the type described in Pat. No. 4,346,314, issued Aug. 24, 1982 to R. S. Craxton. The one ns pulse of ultraviolet light exits from the tripler 14 in a beam which is focused by a lens 16 to a spot on a flat target 18. In the event that lasers which produce sufficient power in short wavelengths (e.g., the ultraviolet) become available, they may also be used. The light is transmitted through a port in a vacuum chamber 20, which includes the target 14, a mask 22 which defines the pattern, a silicon substrate 24 having a coating 26 of x-ray resist, and a shield 28, suitably of beryllium. The chamber 20 may suitably be evacuated to a pressure of about 10.sup.-6 Torr. The target is suitably of pure iron. Other metals of high atomic number materials may be used. The target may also be a microballoon containing the target material; for example, a material having a strong emission which matches the sensitivity of the photoresist when converted into a plasma by the laser pulse. The microballoon may be supported on a stalk as in laser fusion apparatus. Then the laser beam may be divided into a plurality (two or more) of beams which can implode the target and produce an intense and very small x-ray source. The target material is heated by the laser pulse to x-ray emitting temperatures. A small mass of the target, for example, 50 nanograms is converted into a plasma. Most of the absorbed laser energy goes into kinetic energy of the plasma (for example, seventy-three percent). The rest of the energy is converted into the x-ray flux. Suitably, the Nd:glass laser produces a one nanosecond, 35 Joule (J) laser pulse, after frequency tripling. The total x-ray energy emitted by the iron target 18 is then approximately 5.7 J. The remainder of the energy is converted into the heated plasma. The x-rays radiate as radial rays from the focal spot on the target onto which the laser pulse is focused by the lens 16. This spot may be approximately 100 .mu.m in diameter. The x-rays project towards the shield 28, the pattern 22 and the resist 26. The shield 28, pattern 22 and resist 26 assembly may be positioned at an angle closer to the axis of the laser beam than shown. The inclination of the target 18 may be closer to normal to the laser beam. This alternate arrangement may increase the x-ray flux effective on the resist 26. The plasma or target debris also is projected towards this assembly of shield, pattern and resist. The x-rays are indicated by the lines made up of longer dashes while the plasma/target debris is indicated by the lines made up of short dashes. Consider the arrangement of the shield 28, mask 22, resist 26 and substrate 24. The resist and the substrate may be supported on a heat-sink, for example, of aluminum. It is not believed that the resist is heated by absorbed x-rays, since the weak exposure can only raise the resist temperature by a few degrees. The resist may be any conventional resist such as PBS (poly butyl sulfide), PMMA (poly methyl methacrylate) or COP (poly glyclycidyl methacrylate-co-ethyl acrylate). After exposure, the resist may be developed by known methods, for example, as described in U.S. Pat. No. 4,215,192 issued July 29, 1980, in the case of COP. The resist properties and development techniques are also discussed in L. F. Thompson, et al., J. Electrochem. Soc.: Solid State Sci. Techn., 121, 1500 (1974) and P. D. Lenzo, et al., Appl. Phys. Lett. 24, 289 (1974). The mask is suitably a gold grating which is supported along its edges in a frame. The width of the grating lines and their separations may be approximately 0.45 .mu.m. The mask 22 is suitably spaced in close proximity to the surface of the resist 26; a 25 .mu.m spacing being suitable. The shield 28 is also in close proximity to the resist 26 so as to be thermally coupled thereto. For example, the resist may be 5 mm from the mask 22. The hot plasma/target debris is blocked by the shield 28 and causes heating thereof. Because the shield is in close proximity and thermally coupled to the resist, the resist is heated. Thermal coupling may occur by radiational coupling and conductive coupling, as through the frame or other support structure for the assembly, which is used but not shown to simplify the drawing. The resist may reach a temperature approximately equal to the glass transition temperature of the polymer constituting the resist 26; for example, a temperature of about 100.degree. C. Heating of the resist occurs soon after the exposure of the resist by the x-rays. This is because the target debris arrives at the shield 28 with a delay of approximately one microsecond, which is long after the exposure has taken place; the x-rays travelling at the speed of light and both the x-rays and the plasma being produced essentially simultaneously at the surface of the target. Other shields may be used, depending upon the transmissivity to x-rays which is desired. The shield 28 passes x-rays above about 1 keV. While other materials, such as Mylar also have x-ray transmissive and plasma blocking properties, beryllium is preferred, since it transmits more x-rays for a given plasma blocking capability. As mentioned above, approximately 27% of the laser energy which is absorbed in the target 18, is converted into x-rays. The efficiency of x-ray production by a UV laser light is high, even though some of the laser energy is lost in the tripler 14. The beryllium shield 22, which is suitably 18 .mu.m thick, acts as a filter of the total x-ray energy (5.7 J), and approximately 0.72 J is transmitted through the beryllium shield 28. The x-ray energy density incident on the resist 26, which is located 10 cm from the target 20 is approximately 0.57 mJ per cm.sup.2. The total x-ray energy per unit volume absorbed at the surface of the resist is 0.9 J per cm.sup.3. With conventional x-ray lithography as reported in the above referenced Thompson, et al. and Lenzo, et al. articles, approximately 14 J per cm.sup.3 of laser energy must be absorbed in the same resist in order to obtain an exposure equivalent to that obtained with the 0.9 J per cm.sup.3 energy absorbed in the exemplary apparatus described herein. This is an order of magnitude less x-ray flux (energy) than has heretofore been needed for obtaining a complete exposure. The system is therefore more sensitive by an order of magnitude than systems of x-ray lithography heretofore proposed. Variations and modifications in the herein described method and apparatus, will undoubtedly suggest themselves to those skilled in the art. In particular, heating of the resist upon or following exposure can be applied by any other method of heating, to any other resist and relating to any other radiation source or particles source used for registering a pattern. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.