Patent Number: 055725627
Section: summary

TECHNICAL FIELD OF THE INVENTION The present invention relates to techniques for manufacturing semiconductor devices and, more particularly, to techniques for forming patterned features on a semiconductor device. BACKGROUND OF THE INVENTION Photolithography is a common technique employed in the manufacture of semiconductor devices. Typically, a semiconductor wafer is coated with a layer of light sensitive resist material (photoresist). Using a patterned mask or reticle, the wafer is exposed to projected light from an illumination source, typically actinic light, which manifests a photochemical effect on the photoresist, which is ultimately (typically) chemically etched away, leaving a pattern of photoresist "lines" on the wafer corresponding to the pattern on the mask or reticle. The patterned photoresist on the wafer is also referred to as a mask, and the pattern in the photoresist mask replicates the pattern on the image mask (or reticle). As used in the main, hereinafter, with respect to semiconductor lithography, the term "upstream" means towards the illumination or radiation source, and "downstream" means away from the illumination source (or, towards the wafer). For example, a lens in the illumination path of photolithographic apparatus has an upstream side facing the illumination source and a downstream side facing away from the illumination source. FIG. 1 shows a simplified prior-art photolithographic apparatus 110 for exposing a semiconductor wafer (W), more particularly a coating thereon (e.g., photoresist), to light. An optical path is defined from left to right in FIG. 1, as viewed. Prior to exposure, the semiconductor wafer (W) typically receives on its front surface a layer of photoreactive material (not shown), such as photoresist. A light source 112 emits actinic light, and may be backed up by a reflector 114. Light emitted by the light source typically passes through a uniformizer 116, such as a "fly's eye" lens or a light pipe. Light exiting the uniformizer 116 is represented by rays 118a, 118b, and 118c, and passes through a condenser lens 120. The ray 118b represents the optical axis of the photolithographic apparatus. The light source 112, reflector 114, uniformizer 116 and condenser lens 120 form what is termed an "illuminator" which is often detachable as a unit from the photolithographic apparatus. An image mask 122 ("M") is disposed "downstream" of the condenser lens 120, at the focal plane (point) thereof. One type of image mask used in the photolithography process is a chromed glass or quartz plate bearing the pattern to be projected onto the photoresist layer. Light is projected through the image mask, and those areas of the image mask which are not chromed allow the light to expose the photoresist, while those areas of the image mask which are chromed prevent the light from exposing the photoresist. The exposed areas of the photoresist typically resist chemical etching, while the unexposed areas can readily be removed, leaving a pattern of photoresist on the surface of the wafer. Further downstream along the light path, the rays diverge from the mask 122, and pass through a "taking" (imaging) lens 124. Because of its imaging function, the taking lens 124 must be of relatively high quality as compared with the condenser lens 120. The mask 122 is disposed at a common focal point of the two lenses 120 and 124. A semiconductor wafer (W) is disposed at the "downstream" focal plane, or image plane, of the taking lens 124. Those areas of the mask (or reticle) which are not chromed allow the light to expose a photoreactive layer (e.g., photoresist) on the surface of the wafer (W), while those areas of the mask which are chromed (or otherwise opaquely patterned) prevent the light from exposing the photo-reactive layer. The photoreactive layer is typically a photoresist material. The exposed areas of the photoresist resist chemical etching and, in subsequent processing, are used to form defined features on the wafer (such as on a layer of polysilicon underling the photoresist). The resist materials used in photolithography are typically organic. Typical resist materials for visible light photolithography include mixtures of a casting solvent, such as ethyl lactate, and novolac resin (diazoquinone). Inasmuch as the light passing through the image mask (reticle) has an inherent characteristic that induces photochemical activity in the photoresist material, such radiation (e.g., light) is termed "actinic". In current photolithographic apparatus, light having at least a substantial visible content is typically employed. Visible light has a frequency on the order of 10.sup.15 Hz (Hertz), and a wavelength on the order of 10.sup.-6 -10.sup.-7 meters. The following terms are well established: 1 .mu.m (micrometer) is 10.sup.-6 meters; 1 nm (nanometer) is 10.sup.-9 meters; and 1 .ANG. (Angstrom) is 10.sup.- meters. Among the problems encountered in photolithography are non-uniformity of source illumination and point-to-point reflectivity variations of photoresist films. Both of these features of current photolithography impose undesirable constraints on further miniaturization of integrated circuits. Small and uniformly sized features are, quite evidently, the object of prolonged endeavor in the field of integrated circuit design. Generally, smaller is faster, and the smaller the features that can be reliably fabricated, the more complex the integrated circuit can be. With regard to uniformity of source illumination, attention is directed to commonly-owned U.S. Pat. No. 5,055,871, issued to Pasch. As noted in that patent, non-uniformities in the illuminating source will result in non-uniformities of critical dimensions (cd) of features (e.g., lines) formed on the semiconductor device, and the illumination uniformity of photolithographic apparatus will often set a limit to how small a feature can be formed. There usually being a small "error budget" associated with any integrated circuit design, even small variations in illumination intensity can be anathema to the design goals. with regard to reflectivity of photoresist films, it has been observed that minor thickness variations in a photoresist film will cause pronounced local variations in how efficiently the illuminating light is absorbed (actinically) by the photoresist film, which consequently can adversely affect the uniformity of critical dimensions (cd) of features (such as polysilicon lines or gates) sought to be formed in a layer underlying the photoresist. This problem is addressed in commonly-owned, copending U.S. patent application Ser. No. 07/906,902, filed Jun. 29, 1992 by Michael D. Rostoker, which discussed techniques for applying a substantially uniform thickness layer of photoresist, and which is incorporated by reference herein. Another, more serious problem with photolithography is one of its inherent resolution. The cd's of the smallest features of today's densest integrated circuits are already at sub-micron level (a "micron" or ".mu.m" is one millionth of a meter). Such features are only slightly larger than a single wavelength of visible light, severely pushing the limits of the ability of visible light techniques to resolve those features. As integrated circuit features become smaller, the demand for more nearly "perfect" optical components increases. At some point, however, such optics become impractical and inordinately expensive, or even impossible to produce. Although the resolving power of light, vis-a-vis submicron semiconductor features is being stretched to its limit, the ability to etch (wet, dry, chemical, plasma) features on a semiconductor wafer is not limited by wavelength. As is well known, ultraviolet light (UV) is slightly higher (in frequency) on the electromagnetic spectrum than visible light. Typically, ultraviolet light has a frequency on the order of 10.sup.15 -10.sup.17 Hz, and has a wavelength on the order of 10.sup.-7 -10.sup.-8 meters. Ultraviolet light is known to be actinic, for example with respect to skin pigmentation. Due to its shorter (than visible light) wavelength, ultraviolet light would seem to hold promise for increased resolution in integrated circuit photolithography. However, it is difficult to find reliable, fluent sources of UV (typically vacuum UV) light. Further, the performance of present day optics begins to degrade substantially at around 190 nm (1.9.times.10.sup.-7 meters; which is towards the top of the visible light spectrum), and is not well suited for focusing UV light. In contrast to visible light, X-rays have a much shorter wavelength. Typically, X-rays have a frequency on the order of 10.sup.17 -10.sup.20 Hz, and have a wavelength on the order of 10.sup.-8 -10.sup.-11 meters. Evidently, due to their shorter wavelength, X-rays have the inherent capability of providing better resolution than visible light. However, as with UV sources, there are some problems with obtaining reliable emission sources that exhibit good fluence. The best (most intense) X-ray sources (e.g., X-ray tubes) produce X-rays in the range of 1-10 .ANG. in wavelength, with a nominal output spectrum between 2 .ANG. and 6 .ANG. in wavelength. Gamma-rays exhibit an even shorter wavelength than X-rays. Typically, Gamma-rays have a frequency on the order of 10.sup.19 -10.sup.22 Hz, and have a wavelength on the order of 10.sup.-10 -10.sup.-12 meters. Evidently, Gamma-rays provide the potential for even better resolution than X-rays. Furthermore, gamma-ray sources providing intense streams of fluent emission are readily available, such as in the form of Cobalt-60. In the absence of the novel viable gamma-ray and X-ray semiconductor-processing techniques disclosed herein, various techniques for "stretching" the resolution of UV and visible light techniques have been contemplated. One such technique provides a method of forming short-channel polysilicon gates (0.6 .mu.m polysilicon feature size). (See, for example, U.S. Pat. No. 5,139,904, issued Aug. 18, 1992 to Auda et al.) This method employs a technique of laying down a layer of conventional photoresist over a polysilicon layer and patterning the photo-resist to "normal" dimensions (greater than the ultimately desired 0.6 .mu.m dimension). The photo-resist pattern is then uniformly eroded in all dimensions using an isotropic (non-directional) RIE (reactive ion etching) etch process. The size of features in the photo-resist pattern is carefully monitored during the etch process. When the pattern features are eroded to the desired size, the etch process is stopped. An anisotropic (highly directional) etch process is used to etch away portions of the underlying polysilicon outside of the "shadow" of the eroded photo-resist pattern (relative to a generally vertical etch direction). While this technique may be employed to produce small polysilicon structures, it has the same limitations as conventional photolithography with respect to line-to-line spacing. Because the photoresist is initially patterned to "conventional" dimensions, it is not possible with such "stretched" techniques to space pattern features substantially closer with sufficient resolution than is ordinarily possible with conventional photolithography. DISCLOSURE OF THE INVENTION It is therefore an object of the present invention to provide improved techniques for fabricating semiconductor devices. It is another object of the present invention to provide improved techniques for forming ultra-fine features on a semiconductor device. It is another object of the present invention to provide techniques for forming features on a semiconductor device which are not limited by the resolving power of light. It is another object of the present invention to provide wafer processing techniques which yield improved critical dimensions (cd's) in semiconductor features. It is another object of the present invention to provide techniques capable of resolving smaller features (such as polysilicon or metal lines). It is another object of the present invention to provide near-field afocal techniques for processing semiconductor wafers. It is another object of the present invention to provide X-ray lithographic techniques. It is another object of the present invention to provide gamma ray lithographic techniques. It is another object of the present invention to provide means for "shuttering" gamma rays or X-rays. As used herein, the term"lithography" refers to any technique which is employed to define features on a semiconductor wafer, for example patterning photoresist overlying a layer that will subsequently be etched. Generally, all of the lithography techniques discussed hereinbelow employ some form of illumination (or radiating) source. According to the invention, lithography is performed on a semiconductor device using electromagnetic energy of shorter, or of substantially shorter wavelength, than visible or UV light. In one embodiment of the invention, X-rays are used as the illumination (radiation) source. According to an aspect of the invention, Beryllium is used as transparent image mask substrate for imaging X-rays onto a semiconductor wafer. Beryllium has excellent transparency to X-rays, and since it is a metal itself, carriers and opaque masking materials can be readily provided which have similar expansion coefficients, resulting in relatively low distortion of the mask. According to various aspects of the invention, Gold, Tungsten, Platinum, Barium, Lead, Iridium, or Rhodium are used as opaque mask materials to be deposited over a Beryllium substrate (image mask). All of these materials exhibit excellent opacity to X-rays. Further, these materials exhibit adequate adhesion to Beryllium (the image mask substrate) and adequate environmental robustness for utility as lithographic image masks. The resulting image mask (beryllium substrate with a pattern of opaque lines on a surface thereof) is suitably employed for "near field" lithography. By "near field" it is meant that the process is afocal, and by spacing the image mask close to the semiconductor wafer there is limited opportunity for the radiation passing through the image mask to spread. In another embodiment of the invention, Gamma-rays are used as the lithographic illumination source. According to an aspect of the invention, "base" organic resist materials applied to the semiconductor die (wafer) are doped either with organic or with inorganic materials (dopants) which exhibit high absorptivity to gamma-rays, to enhance the sensitivity of the resist material. Preferably, the dopant is inorganic. Examples of organic dopants include polystyrene, phenolformaldehyde, polyurethane, etc. Examples of inorganic dopants include bromine, chromium, tantalum, gold, platinum, palladium, lead, barium, boron, aluminum and magnesium. The dopants are highly reactive to incident gamma radiation, and produce secondary photon emissions of a different wavelength (longer) than that of the incident gamma rays. The organic resist base, which is not ordinarily reactive to gamma radiation, is however highly absorptive of these secondary emissions (from the dopants), which are actinic with respect to the organic resist base, thereby causing the resist base to become chemically converted. The high cross-section (absorptivity) of the organic resist base to the secondary emissions also limits the amount of "blooming" (spreading) inherent in the secondary emissions. According to another aspect of the invention, an organic resist material has an absorptive (to gamma radiation) film of material disposed on a surface thereof. The film atop the photoresist is organic or inorganic, preferably inorganic, and provides secondary emissions (photons) which convert the underlying photoresist. The film is termed a "secondary resist layer". Examples of organic resist materials suitable for the secondary resist layer include polystyrene, phenolformaldehyde, polyurethane, etc. Examples of inorganic secondary resist materials suitable for the secondary resist layer include bromine, chromium, tantalum, gold, platinum, palladium, lead, barium, boron, aluminum and magnesium. The secondary resist layer, when exposed to gamma radiation, produces secondary photon emissions of a different wavelength (longer) than that of the incident gamma rays. The underlying organic resist material is highly absorptive of these secondary emissions, which are actinic with respect to the organic resist, causing it to become chemically converted. The high cross-section (absorptivity) of the underlying organic resist to the secondary emissions, and its close juxtaposition to the overlying secondary resist film, limit the amount of "blooming" (spreading) that would otherwise be expected to be experienced. Other combinations of organic resist bases (or layers) either doped with high cross-section (to gamma radiation) dopants or underlying more absorptive (to gamma radiation) layers are disclosed and otherwise contemplated. Other aspects of the invention are directed to direct-write, afocal, lithography techniques and to means for directing, concentrating, collimating and shuttering beams of radiative energy. According to the invention, a broad incident beam of radiation can be concentrated and collimated, providing a very narrow, very intense beam of radiation (such as X-ray or gamma radiation) useful over a short range of distances as by means of a hollow, horn-shaped (e.g., conical) afocal concentrator (described extensively hereinafter). The afocal concentrator has a tapered section and a cylindrical section. The tapered section has a broad mouth at one end and a narrow opening at an opposite end. The cylindrical section has a diameter equal to that of the narrow opening, and is formed continuously therewith. A broad incident beam of radiation enters the mouth of-the tapered portion and is concentrated in the tapered portion and is collimated in the cylindrical portion to provide a collimated, intense output beam that can be directed onto a semiconductor wafer. In order to produce patterns on the wafer, either the collimator or the wafer is moved (in two axes). Preferably, the wafer would moved and the concentrator would be fixed in position. According to various aspects of the invention, the concentrator may have any of various tapered forms, including a linear, cone-shaped taper, an exponential taper, or some combination thereof. In any case, the inner surface of the afocal concentrator is highly reflective of the incident radiation. According to various other aspects of the invention, the afocal concentrator may be used to collimate (thereby intensify) any of various forms of radiation, including gamma radiation, X-ray radiation, UV light, and visible light. In the main hereinafter, the utility of the collimator for very short wavelength radiation that cannot be focused by conventional optics is discussed. According to other aspects of the invention, the reflective inner surface (bore) of the afocal concentrator is formed of aluminum, nickel, or chromium. The entire collimator can be formed of a single material, or its bore can be plated. According to the invention, a surface acoustic wave (SAW) device operating as a shallow angle scattering surface, can act as a shutter for X-ray or gamma-ray radiations. In the context of the present invention, such a shutter would controllably allow/prohibit the downstream (towards the wafer, or towards the concentrator) passage of radiation from a fluent, continuous source of radiation. A thin, reflective film of, for example, aluminum, nickel, or chromium, is disposed over the surface of a Surface Acoustic Wave (SAW) device. When the SAW device is not activated, the reflective surface is substantially planar, and reflects incident radiation at an angle equal and opposite to its angle of incidence. This beam, the position of which is highly predictable, can be used to pattern a layer (e.g., photoresist) on a semiconductor wafer. A tightly collimated beam approaching the surface of the SAW (such as from the aforementioned collimator) at a known shallow angle, will be reflected off of the reflective surface of the unactivated SAW device at a predictable angle. When the SAW device is activated, however, the surface of the SAW device becomes distorted and deflects or scatters the incident beam. By providing a beam stop or an aperture and positioning it such that radiation from the incident beam will pass the beam stop (or aperture) only when reflected at an angle corresponding to its reflection off of the planar surface of the unactivated SAW device, an effective shutter is formed. Hence, the planar and distorted surface of the SAW device, in combination with a knife-edge, opaque beam stop or aperture, effectively functions as a shutter, turning an incident beam ON and OFF, respectively, particularly for very short wavelength radiation (e.g., X-rays or Gamma rays). It is not necessary, according to the invention that the incident beam be "cleanly" reflected in any particular direction when the SAW device is activated (distorted surface). It is only necessary that the reflected beam be reflected from the SAW device anywhere other than past the beam stop or aperture when the SAW device is activated. In a similar manner, a magnetostrictive device may be employed instead of a SAW device, in combination with a beam stop or aperture, to form an effective shutter mechanism. Again, the surface of the magnetostrictive device can selectively be made planar, to reflect incident radiation past a beam stop or aperture, or it can be made non-planar, to divert incident radiation from passing the beam stop or aperture. As with the SAW device, the magnetostrictive device is coated with a material that is highly reflective vis-a-vis the incident radiation. In either case, namely employing a SAW device or a magnetostrictive device, the reflective element acts as a "surface distortion device" for the purposes of the present invention. Other devices whose surfaces may selectively be distorted may be employed, in combination with a beam stop or aperture, to achieve a similar shuttering function. According to various other aspects of the invention, the Surface Acoustic Wave or magnetostrictive shutter may be used to shutter radiation of a variety of wavelengths, including gamma-rays, X-rays, UV light, etc. In the main hereinafter, the utility of these surface distortion devices in conjunction with non-visible radiation is discussed. Further, according to the invention, direct-write gamma-ray lithographic apparatus is provided. An omni-directional radiation source provides a source of intense gamma-ray radiation. A suitable radiation source is a Cobalt-60 pellet which passively (without any external power) radiates intense, fluent (e.g., steady, not varying or intermittent) gamma-ray radiation. A reflector (similar to the reflector 114 discussed with respect to FIG. 1, above) may be employed behind the Cobalt-60 pellet to improve the directionality and intensity of the emissions from the pellet. Gamma-ray radiation from the gamma-ray radiation source enters (is incident to) a shutter device, such as the SAW or magnetostrictive-based shutter devices described above. The shutter device serves to selectively gate (block or pass) the incident beam, resulting in a controlled gamma-ray beam. The controlled gamma-ray beam enters the mouth of an afocal concentrator, such as that described above and in greater detail with respect to FIG. 4 et seq. The afocal concentrator narrows, intensifies and collimates the controlled beam to provide a collimated beam. A semiconductor wafer is positioned a distance from the output of the afocal concentrator such that the collimated beam impinges upon the surface thereof. The surface of the wafer is coated with a layer of gamma-sensitive resist, such as that described above. Preferably, the wafer is mounted to a movable carriage, by which means the wafer may be positioned such that the collimated beam may be caused to impinge on any point on the resist layer, to form a pattern in the resist layer for further processing (e.g., chemical etching). This is referred to as "direct write" lithography. The on/off state of the collimated beam may be effectively controlled by selectively activating and de-activating the shutter device. Preferably, the distance between the wafer and the output of the afocal concentrator is approximately 5 .mu.m. Even if the collimated beam of gamma radiation is not perfectly collimated, by positioning the wafer so close to the output of the collimator, there is not much opportunity for the collimated beam to spread out. In an alternate embodiment of the direct-write gamma-ray lithography apparatus described hereinabove, the positions of the shutter device and the afocal concentrator are reversed. In other words, the gamma radiation would be collimated, then shuttered, then caused to impinge on a semiconductor wafer. Other objects, features and advantages of the invention will become apparent in light of the following description thereof.