X-ray or gamma ray systems or devices – Specific application – Lithography
Reexamination Certificate
1998-11-06
2001-03-06
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Lithography
C250S492200
Reexamination Certificate
active
06198792
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to projection lithography employing soft x-rays and in particular to methods of preventing contaminants generated by the wafer from depositing onto the surfaces of mirrors and lens. The invention is especially suited for systems that use a camera that images with acuity along a narrow arc or ringfield. The camera uses the ringfield to scan a reflective mask and translate a pattern onto the surface of the wafer.
BACKGROUND OF THE INVENTION
In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a “transparency” of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the “projecting” radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. “Long” or “soft”, x-rays (a.k.a. Extreme UV) (wavelength range of &lgr;=100 to 200 Å (“Angstrom”)) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning, e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015, available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration (“VLSI”) involves chips with circuitry built to design rules of 0.25 &mgr;m. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (“UV”) delineating radiation. “Deep UV” (wavelength range of &lgr;=0.3 &mgr;m to 0.1 &mgr;m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 &mgr;m or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Two x-ray radiation sources are under consideration. One source, a plasma x-ray source, depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (“YAG”) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 &mgr;m to 250 &mgr;m spot, thereby heating a source material to, for example, 250,000° C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Another source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays.
Electrons, accelerated to relativistic velocity, follow their magnetic-field-constrained orbit inside a vacuum enclosure of the synchrotron and emit electromagnetic radiation as they are bent by a magnetic field used to define their path of travel. Radiation, in the wavelength range of consequence for lithography, is reliably produced. The synchrotron produces precisely defined radiation to meet the demands of extremely sophisticated experimentation. The electromagnetic radiation emitted by the electrons is an unavoidable consequence of changing the direction of travel of the electrons and is typically referred to as synchrotron radiation. Synchrotron radiation is comprised of electromagnetic waves of very strong directivity emitted when electron or positron particles, traveling within the synchrotron at near light velocity, are deflected from their orbits by a magnetic field.
Synchrotron radiation is emitted in a continuous spectrum or fan of “light”, referred to as synchrotron emission light, ranging from radio and infrared wavelengths upwards through the spectrum, without the intense, narrow peaks associated with other sources. Synchrotron emission light has characteristics such that the beam intensity is high, and the divergence is small so that it becomes possible to accurately and deeply sensitize a photolithographic mask pattern into a thickly applied resist. Generally, all synchrotrons have spectral curves similar to the shape shown in FIG. 1 of Cerrina et al. (U.S. Pat. No. 5,371,774) that define their spectra. The particular intensity and critical photon energy will vary among different synchrotrons depending upon the machine parameters.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object:image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term “proximity”), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength &lgr;≦16 Å is required for 0.25 &mgr;m or smaller patterning to limit diffraction at feature edges on the mask.
Use has been made of the synchrotron source in proximity printing. Consistent with traditional, highly demanding, scientific usage, proximity printing has been based on the usual small collection arc. Relatively small power resulting from the 10 mrad to 20 mrad arc of collection, together with the high-aspect ratio of the synchrotron emission light, has led to use of a scanning high-aspect ratio illumination field (rather than the use of a full-field imaging field).
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavel
Kanouff Michael P.
Ray-Chaudhuri Avijit K.
Burns Doane Swecker & Mathis L.L.P.
Church Craig E.
EUV LLC
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