Patent Number: 051877260
Section: description

DETAILED DESCRIPTION OF THE INVENTION The present invention is particularly suited to the utilization of synchrotron X-ray sources which provide a highly collimated and broad band spectrum X-ray flux. The utilization of such an X-ray flux to provide selective exposure of a target photoresist to form structures smaller than that which can ordinarily be formed by absorption X-ray masks is illustrated in FIG. 1. The photon flux illustrated by the lines 20 in FIG. 1 is preferably provided from a synchrotron source and consists of a relatively broad spectrum of spatially coherent, temporally incoherent X-rays. The beam of X-rays 20 is passed through a phase shift mask 22 which includes a carrier 23, having flat top and bottom surfaces 28 and 29, which is substantially transparent to the X-rays, and a phase shifter feature 24 which is preferably transparent to X-rays or which only minimally absorbs the X-rays which are incident on it. The thickness of the phase shifter material 24 is selected such that the photons which are passed through the material 24 are phase shifted by approximately 1/2 of the wavelength (a .pi. shift) of the photons relative to those photons which pass through the region 25 of the mask in which there is no phase shift material. Where a broadband X-ray source, such as a synchrotron, is used, the thickness of the phase shifter material is preferably chosen to provide substantially a 0.5 wavelength phase shift over the region of X-rays which interact most with the photoresist. The phase shifted X-rays are illustrated by the lines labeled 26 in FIG. 1 and the unphase shifted X-rays are indicated by the lines labeled 27 in FIG. 1. The region defining the phase shifter feature 24 intersects the region 25 preferably at a sharp, well-defined intersection 30, wherein the phase shifter material 24 has a substantially upright sidewall 31 as illustrated in FIG. 1. The sidewall 31 is upright in the sense that it is substantially parallel to the direction of propagation of the substantially collimated incoming photons 20 and perpendicular to the surfaces 28 and 29 of the carrier 23. The X-rays 26 and 27 which are passed through the mask 22 are incident on a layer of target photoresist 33 which is supported on a substrate 34. These photons are selectively absorbed by the photoresist 33 and, for purposes of illustration, the line 36 in FIG. 1 shows the relative X-ray energy absorbed by the photoresist from the X-rays 26 and 27 on either side of the line 37 that corresponds to the upright sidewall at the intersection 30 between the phase shift region 24 and the unphase-shift region 25 of the mask. The destructive interference of the spatially coherent X-rays 26 and 27 results in their fields canceling near the boundary line 37 so that the intensity of X-ray energy absorbed by the target photoresist reaches a minimum at a point 38 which lies on the center line 37. Preferably, the energy absorption represented by the point 38 is below the exposure threshold energy level 39 of the resist 33; that is, those areas of the resist which have received photon energy below the level of the line 39 illustrated in FIG. 1 will not, for a positive resist, be dissolvable by a developer solvent. The developer will however, dissolve the regions of the target photoresist to the left of the point 41 at which the threshold 39 intersects the exposure level curve 36 and to the right of the point 42 at which the threshold 39 intersects the curve 36. Of course, in real photoresists, the exposure threshold 39 is not a narrowly defined line as shown in FIG. 1 but varies over a finite transition zone. Thus, when the resist is removed by the developer solvent after exposure, the structure which is defined by the resist remaining on the substrate 34 will not have purely vertical sidewalls at the points 41 and 42 but rather somewhat sloping sidewalls. However, it is preferred that the resist threshold 39 be as sharp as possible to cause the sidewall structures to be as vertical and sharply defined as possible. The phase shift mask 22 may be formed of a variety of materials suited for X-ray lithography processing. For example, the carrier 23 may be a fine membrane of silicon nitride (Si.sub.3 N.sub.4) sufficiently thin not to significantly absorb the incoming X-ray flux (e.g., in the range of 1.mu.m or less). A suitable material for the phase shifter feature 24 is polymethyl methacrylate (PMMA) which provides the desired half wavelength phase shift for a suitable band of X-ray photons if it is formed in a film of a depth between 2.5 and 3 .mu.m. A suitable target resist 33 is also PMMA, which is suited to X-ray lithography processing and has a relatively sharp threshold level 39. A 3.0 .mu.m layer of PMMA as the phase shifter 24 introduces a phase shift of about one-half wavelength (a .pi. phase shift) over the region of the spectra (1,000 to 1,300 eV) where most of the energy in the target resist is deposited. Other materials, such as silicon dioxide, polycrystalline silicon, beryllium, and a variety of other materials, can be utilized as the phase shift material 24 to provide a phase shift mask which is harder and more stable than PMMA. In general, the strength of the interference cancellation depends on the spectral properties of the phase shifting material 24 on the mask but not on the gap distance between the mask 22 and the top surface of the target resist 33. The gap between the mask 22 and the target resist 33 is preferably small, in the range of 15 .mu.m or less, to obtain satisfactory line widths in the exposed target resist. The construction of a phase shift mask 22 in accordance with the present invention having a mask which will produce closed curves on the target resist is shown in FIGS. 2 and 3. In an exemplary technique for forming the mask, the X-rays 20 from the synchrotron beam are also utilized to provide an exposure through a master mask 44 formed of a carrier 45 (e.g., one .mu.m or less silicon nitride) with metal absorbers 46 and 47 (e.g., gold) formed on the surface of the carrier 45. The absorbers 46 have sharply defined upright sidewalls so that when the substantially collimated X-rays in the beam 20 are passed therethrough, they expose the layer 49 of photoresist (e.g., PMMA) in a pattern corresponding to the metal absorber pattern 46 and 47, wherein areas 50 shielded by the absorbers 46 and 47 have unexposed resist whereas the areas 51 in which the X-rays in the beam 20 are not impeded by the metal absorber have fully exposed resist. The exposed PMMA is then removed utilizing a suitable PMMA developer bath, e.g., a mixture such as 2-(2-butoxyethoxy) ethanol, tetrahydro-1-4 oxazin (morpholine), and 2-aminoethanol (ethanolamine) and de-ionized distilled water. After development, the resist areas 24 left on the carrier 23 have the sharply defined vertical upright sidewalls 31 which are desirable for obtaining the maximum phase shift interference when the X-ray beam passes through the mask 22. X-ray lithography is particularly advantageous when forming the mask 22 since relatively thick layers of resist 49 can be plated onto the carrier 23 and exposed leaving very well-defined smooth, upright sidewalls 31. Of course, the X-ray mask can be formed utilizing the exposure of the resist to form a mold, with the phase shift material then being plated onto the carrier 23 around the structures 24 so that the deposited material assumes the shape of the sidewalls 31. The PMMA which defines the mold may then be removed by exposing it to X-rays and then developing it with a developer bath to remove the PMMA in its entirety. The X-ray phase shift mask 22 shown in FIG. 3 has islands of the phase shifter features 24 surrounded by regions 25 on the carrier which have no phase shifter. Thus, when the mask 22 of FIG. 3 is used to expose a target layer of resist, the results after the resist is developed will be structure remaining which consists of a closed curve corresponding to the periphery of the phase shift features 24 on the mask. However, the present invention is not limited to production of closed curves of thin wall-like structures. FIGS. 4 and 5 illustrate a mask 54 which can produce an example of a structure other than a closed thin wall. In the mask 54 shown in these figures, the carrier 23 has a layer of a metal absorber 55 (e.g., gold, typically in the range of 0.8 .mu.m thick) having a terminating wall 56. A phase shifter 57 is then deposited on the carrier 23 and partially over the absorber 55 in the manner described above. The phase shifter 57 includes a lower section 58 which is of a thickness selected to introduce a half wavelength phase shift of photons passing therethrough with respect to the areas of the carrier which have no phase shifter or absorber formed thereon. The phase shifter 57 also has a higher level 59 which is plated on over the absorber 55. A transition region 60 joins the top surfaces of the regions 58 and 59 in a laterally sloped pattern rather than in a sharply defined vertically upright wall. The sloped section 60 will generally be naturally formed as the layer of PMMA resist (e.g., 2.5-3.0 .mu.m PMMA) is deposited on the carrier and over the absorber 55 which stands up above the surface of the carrier. When the mask 54 is used to expose a layer of target photoresist in the X-ray beam, a structure such as shown in FIG. 6 is formed on the substrate 34. A well-defined feature 65 is left in the developed target photoresist underneath the absorber 55, and a closed thin wall structure 66 is left in the developed resist under the boundary sidewall 61 of the phase shifter 57 where such sidewalls are not over the mask 55. Since the X-rays which pass through the phase shifter in the region 59 which is over the mask 55 are essentially completely absorbed by the absorber 55, there is no corresponding structure formed on the substrate 34 underneath the sidewalls 62 of the phase shifter. It is also possible to make totally isolated open thin wall structures utilizing a phase shift mask in accordance with the present invention. This may be accomplished by creating a phase shifter having upright sidewalls, where phase shift effects are desired, and sloping sidewalls where phase shift effects are not wanted. The sloping walls result in X-rays of strongly different phases not interacting so that the fields do not substantially cancel, thus resulting in a relatively uniform high intensity X-ray flux across the interface. Such X-ray mask structures can be formed by providing an initial exposure of the X-ray resist 49 as illustrated in FIG. 7. The X-ray photons come in at an angle to a master (absorber) X-ray mask comprised of a carrier 70 having a metal absorber 71 formed thereon in the desired pattern. The X-rays passed through the mask intersect the resist layer (e.g., 2.75 .mu.m PMMA) 49 at an angle, resulting in exposed regions 73 and an unexposed region 74 which meet at slanting surfaces 75. The result, after development, is a phase shifter 77 as illustrated in FIGS. 8 and 9 having substantially upright or vertical sidewalls 78 and sloping sidewalls 79. The amount of phase shift which will be provided to X-rays passing normally to the carrier 23 at the areas of the sidewalls 79 will vary continuously over the whole length of the sidewall, thus essentially eliminating any interference effects at the position under the sidewalls 79. Utilization of the mask of FIG. 9 to expose a target layer of photoresist results, after development, in the structure shown in FIG. 10 in which two thin walls 80 of photoresist structure are left on the carrier 34, corresponding to positions underneath the sidewalls 78 of the phase shifter 77. Utilizing the phase shift masks of the present invention, it is possible to select the width of the thin wall structures which are ultimately formed. An example of the manner in which this may be accomplished is illustrated in FIGS. 11 and 12. The phase shift mask 84 shown in FIG. 11 includes a region of phase shift feature material 85 formed directly on the surface of the carrier 23 and another phase shift feature 86 which is formed on top of a spacer 88 which spaces the phase shifter 86 further away from the target material than the phase shifter feature 85. The spacer 88 may be formed of a very low loss material that provides substantially no phase shift of the X-rays passing through it (so that there is no interference effects at the sidewalls of the spacer 88), or the spacer 88 can comprise a material that will result in a phase shift of the X-rays but with no interference effects by selecting the thickness of the spacer 88 so that it is a multiple of a full wavelength phase shift (a 2.pi. phase shift or multiple thereof). Thus, when X-rays 20 pass through the mask 84, interference effects will only take place at the sidewalls 89 of the phase shifter feature 85 and the sidewalls 90 of the phase shifter feature 86 resulting in thin wall structures as shown in FIG. 12 after development of the target photoresist. The thin wall structures 93 are formed under the sidewalls 89 of the feature 85, and the wider wall structures 94 are formed under the sidewalls 90 of the feature 86. In general, the larger the gap between the phase shifter and the target resist material, the broader the diffraction pattern, increasing the size of the interference region and thereby the width of the wall structures that are formed. Of course, the gap between the mask and the target structure can be varied in total to change the width of all of the lines that are formed in the target photoresist. In the present invention, it is preferred that the X-ray flux 20 which is passed through the phase shift mask, have a fairly broad band of wavelengths therein since it is found that the sidelobes of the X-ray energy intensity in the interference pattern under the sidewalls of the phase shifter is more moderate where a broad band of X-rays is utilized rather than a substantially monochromatic X-ray source. It is also preferable that the phase shifter material have some absorption of the X-rays that are passed therethrough. The reason for this is illustrated with respect to FIGS. 13 and 14. FIG. 13 shows a phase shift mask having a carrier 34 and two phase shifter features 24 formed thereon separated by a region 25 of no phase shift. If the phase shifter regions 24 are formed of a material which is a perfect phase shifting material with no absorption, an X-ray intensity pattern at the target photoresist for X-rays passed through the mask of FIG. 13 is illustrated by the graph 98 in FIG. 14. The X-ray flux intensity reaches a minimum at the points 99 which lie under the sidewalls 31 of the phase shifters 24. The relative level of the intensity, illustrated by the graph 101, in the areas underlying the phase shifters 24 is substantially the same level as the intensity, illustrated by the graph 102, in the area underlying the region of no phase shift 25. However, as illustrated in FIG. 14, there is a substantial oscillation in the X-ray intensity in positions adjacent to the minima 99. The effect of utilizing a practical phase shifter mask material is illustrated by the graph 104 in FIG. 15, which corresponds to the pattern of X-ray flux passed through a mask having a central region 24 of a real phase shifting material and surrounded by areas 25 of no phase shift. The minima 106 of the intensity lie under the positions of the sidewalls 31, but in this case the X-ray intensity in the areas 107 outside of the sidewalls which underlie the regions 25 is higher than the X-ray energy 109 in the region which underlies the phase shifter feature 24. Nonetheless, the minima 106 are sharply defined and differ substantially in energy from the somewhat lower energy level 109 under the phase shifter, and the oscillations in the X-ray intensity are significantly damped out. The graph 105 of FIG. 15 was computed for an absorbing mask formed of 2.5 .mu.m PMMA on a carrier of 0.8 .mu.m silicon nitride, with a gap between the X-ray mask and the target photoresist of 10 .mu.m. The graph I05 was calculated assuming a monochromatic X-ray beam of 1200 eV and a 13 .mu.m beryllium filter for an exposure lasting one second. A carrier intensity of 0.033 and a mask contrast of 1.328 were utilized. Some of these high frequency oscillations from the monochromatic case can be smoothed out by blurring the light source of the system. This would add spatial incoherence which would help smooth the image. The present invention thus provides phase shift masks which are capable of achieving exposed structures on targets that have widths below 50 nm. Thin wall structures in photoresists can thus be produced by this technique. However, the technique can also be utilized to provide selective exposures of resists for other purposes commonly used in micro-electronic processing, such as removal of photoresist in selected areas to allow doping, oxidation, and deposit of other layers of materials. In accordance with the present invention, proximity X-ray lithography can be extended into the nanometer range, thus making it unnecessary to utilize projection systems for X-rays to achieve such small structures. It is understood that the invention is not confined to the particular embodiments set forth hereon as illustrative, but embraces such modified forms thereof as come within the scope of the following claims.