Patent Number: 056129866
Section: description

DESCRIPTION OF MODES OF PRACTICE OF THE INVENTION In an on-axis embodiment of the invention, shown in FIG. 1, a plane wave X-ray beam, including a representative X-ray 11, is incident on a hologram 15 with an associated opaque frame 13 to be used to produce the desired image or pattern on a chosen wafer or substrate 17. For a given choice of a separation distance d between the hologram 15 and a wafer or substrate 17, an image pixel at a position 19 is contributed to by a localized group of points 21 on the holographic image source 15. The size of the area defined by the group of points 21 is determined by the maximum diffraction angle and by the distance d. In practice, the distance d may be chosen in the range 20 .mu.m.ltoreq.d.ltoreq.3000 .mu.m. The embodiment illustrated in FIG. 1 uses transmission projection of a holographic image. The in-line holographic geometry shown in FIG. 1 is similar to the geometry for contact printing except that the hologram replaces the mask and is located at a larger distance d=20-3000 .mu.m, preferably d.apprxeq.200 .mu.m from the wafer in this example. This on-axis embodiment is implemented by use of an algorithm for designing the hologram, where diffraction effects caused by wavefield propagation over a distance d are accounted for. We seek a hologram 15 that diffracts all incoming plane wave to form the desired image intensity at the wafer 17. This is an inverse diffraction problem, which is always involved in the design of computer-generated holograms and which has been discussed fairly extensively in the literature, for example, by J. R. Fienup, Opt. Eng., vol. 19 (1980) pp. 297-305, and by S. H. Lee, ed., Special Issue on Computer Generated Holograms, Appl. Optics, vol. 26 (1987) pp. 4350-4399. The hologram is assumed to be constructed from an optical medium whose thickness h varies with position and can be chosen by an algorithm, discussed below, and whose complex refractive index n=1-.delta.-i.beta. at the X-ray wavelength .lambda. is known. A constant thickness membrane, or a support membrane that does not further modulate the transmitted wavefield, is assumed to support the hologram. If the thickness h at a certain location in the hologram is chosen to produce a desired value for the phase change (.theta.=2.pi..delta.h/.lambda.), the amplitude attenuation factor r must be r=exp(-2.pi..beta.h/.lambda.). Therefore, the only allowed combinations of amplitude and phase of the beam are those that lie on the logarithmic spiral r=exp(-.theta..beta./.delta.) in the complex plane (r versus .theta. in polar coordinate form). An example of such a curve, for carbon at .lambda.=5 nm, is shown in FIG. 2. Carbon has a refractive index n=1-.delta.-i.beta. with .delta.=0.0039 and .beta.=0.0027 at .lambda.=5 nm. Other attractive choices of the hologram material include germanium (at .lambda.=1 nm), silicon (1.24 nm), GaAs (1.1 nm), Si.sub.3 N.sub.4 (3 mn), SiO.sub.2 (2.5 nm), fused quartz, nickel (1.4 nm), gold (0.56 nm), silver (0.36 nm), platinum (0.44 nm) and palladium (0.38 nm). Because most of the information in diffractive imaging techniques is encoded as phase, we prefer to use materials with a large ratio .delta./.beta. for transmission holographic optical elements. This allows large phase changes for the thickness h to be achieved somewhat independently of changes in amplitude. If large signal attenuation by the hologram 15 in FIG. 1 is required, the signal attenuation can at least approximate this by adding a suitable integer multiple of 2.pi. to the phase angle .theta.. Considerations of resist penetration, resist resolution and organic contaminant contrast suggest that optimum wavelengths for X-ray lithography are either just above the carbon K edge at .lambda.=4.4 nm, or at .lambda.=1-2 nm. Use of .lambda.&gt;10 nm X-rays in many of the published projection lithography schemes is driven by considerations of multilayer technology that are not involved here. FIGS. 3A and 3B show the ratio .delta./.beta. and the thickness required to obtain a 2.pi. phase shift in several materials that have already been successfully used to make high resolution Fresnel zone plates (another type of holographic optical element). All these materials have favorably high ratios .delta./.beta. at .lambda..apprxeq.1 nm wavelength, and .delta./.beta. is also high on the long wavelength sides of the absorption edges. One benefit of considering shorter wavelengths is that, for a desired transverse resolution D, the depth of field DOF.apprxeq.4 D.sup.2 /.lambda. for 100 nm resolution projection lithography increases to even more flavorable values. For the example calculations presented here, we shall consider the use of carbon at .lambda.=5 nm wavelength, although the method is applicable generally, including extension to the .lambda..apprxeq.1 nm wavelength region. The binary test-object for which specific calculations have been done, shown in FIG. 4, is represented by 256.times.256 0.063 .mu.m.sup.2 pixels, and has black or zero-exposure features ranging in size from 0.063 .mu.m to 1.0 .mu.m on a uniform white background of normalized intensity 1.0. We chose the very narrow minimum linewidth (0.063 .mu.m) to provide a severe test of the algorithm for designing the hologram, although the application of the procedure to pattern minimum feature sizes in the 0.1-0.25 .mu.m range is of immediate interest and is easier to implement. We now describe one procedure that can be used to design a carbon hologram to deliver the X-ray intensity pattern in FIG. 4 to the wafer. As a first step, we backpropagate a .lambda.=5 nm wave field whose initial phase and amplitude at the wafer plane are equal to zero and to the square root of the desired pattern intensity, respectively. This wave field arrives at the hologram plane (located d=200 .mu.m upstream from the wafer) with definite known values of the amplitude and phase at each pixel. We wish to design the hologram so that a unit intensity plane wave transmitted through the hologram would match these values. To do this with carbon, we only have access to complex numbers lying on the spiral in FIG. 2, so we proceed (within a specified upper limit on the pattern thickness) by choosing the correct value for the phase and adding whatever multiple of 2.pi. gets us closest to the correct amplitude. This produces the zero order approximation to the hologram. By forward propagating the wave field that would be transmitted by this hologram to the wafer plane and by taking the square of the magnitude, a zero order approximation to the desired image at the wafer is produced, which image turns out to be a surprisingly good approximation to the desired pattern. We wish to produce a more accurate print of the desired pattern intensity, but we have no interest in controlling the optical phase at the wafer plane. Therefore, for the next iteration we leave the current value of the phase at the wafer plane unchanged. For a desired intensity of 0, we set the wafer-plane magnitude to 0, calculate the average magnitude of the non-zero or "white" pixels, and choose those pixels to have a magnitude of 1.05 times the average magnitude, to drive the algorithm to improving the diffraction efficiency of the hologram while simultaneously producing a uniform "white" intensity. The numerical factor 1.05 here is an example and may be replaced by any number between 1.0 and 1.5 according to convenience. We then combine these magnitude and phase values, backpropagate to the hologram plane, approximate the complex amplitude as before, forward propagate to the wafer plane, and continue this procedure for the desired number of iterations. The carbon thickness or hologram pattern calculated by 100 iterations of this procedure is shown in FIG. 5. This procedure chooses low carbon thickness for most pixels. The wafer plane intensity, produced when this hologram is illuminated by a unit plane wave, is nearly identical with the desired image shown in FIG. 4; the desired dark features remain dark, and the light features have a normalized intensity in the range 0.7-1.5, with an average value close to the target value of 1.0. The good fidelity of this image is also illustrated in FIG. 6, which shows the intensity pattern across a scan line of the wafer-plane image. Ideally, the image relative intensity at each point should be either 1.0 or 0. However, the values illustrated here are easily within the accuracy needed for microlithographic processing. Note that the resolution of the image is essentially the same as that of the hologram. This is true in general for in-line holograms, of which the zone plate is another example. Thus, the manufacturing tolerances and field size required in making the hologram would be essentially the same as for producing a proximity mask for the same test pattern. This result is an important advantage of the present approach, and should be compared to the formidable 0.5-1 nm surface manufacturing tolerances required for the optical reduction projection lithography systems described above. The calculations presented thus far relate to a mostly-white type of test object. We have also tested the applicability of the method to mostly-black objects by utilizing the complement of the object in FIG. 4 as the desired image. The method works even better for this case, and one result of the algorithm used is shown in FIG. 7. The overall image fidelity is very similar, but the light intensity in the white image region is about 50 times higher. This indicates that the holographic optical element is capable of concentrating the light in a manner analogous to a zone plate. Both of these example designs show that the carbon relief mask, which is in essence a near-image-plane phase hologram, has the remarkable property that the twin-image, intermodulation arid zero-order terms are all suppressed. A general feature of this type of hologram is localization of the recorded information, corresponding to production of each image pixel by a moderately small region of the hologram defined by the hologram-image distance d and the numerical aperture NA, as indicated in the discussion of FIG. 1 above. Here, NA is equal to .lambda./2s, where s is the minimum feature size. One consequence of this is a great reduction in the required coherence length used in illuminating the hologram to reconstruct the original image (projecting the image onto the wafer in this first embodiment). In the present example, the radius of good mutual coherence would need to be 10 .mu.m transversely, and the monochromaticity .lambda./.DELTA..lambda. would need to be 45 waves for 63 nm (0.063 .mu.m) resolution, or 6 .mu.m and 20 waves for 100 nm (0.1 .mu.m) resolution. These requirements are easy to meet and are consistent with X-ray sources capable of illuminating high wafer throughput systems. Another consequence of the localization of information is that one can readily compute a large wave field by dividing it into smaller computationally tractable parts and stitching the resulting holograms together afterward. Furthermore, in many electronic devices, large regions have repetitive mask patterns; this repetitive feature could be used to ease computation of the hologram. The algorithm for producing a holographic image source rapidly converges to a solution, with most of the optimization occurring during the first few iterations. It may be possible to improve upon this convergence by using an input-output type, rather than an error-reduction type, of algorithm. Uniqueness considerations are eliminated here. We seek any hologram that will produce the desired wafer plane pattern, rather than a unique solution to the inverse source problem. The above simulations assume that we could vary the carbon thickness continuously over the range 0 to 1 .mu.m. FIGS. 8A and 8B illustrate the wafer plane intensity variations calculated with discrete maximum thickness limits of 0.5, 1, 1.5, . . . , 5.0 .mu.m for the white background and black background situations, respectively. For black features on a white background (FIG. 8A), the spread in the range of white intensity values changes little as the thickness limit is varied. In the case of white features on a black background (FIG. 8B), the algorithm seeks to minimize the spread in white intensity values rather than to increase the average white intensity value. If the thickness limit is reduced from 1 .mu.m to 0.5 .mu.m (0.5 .mu.m thickness corresponds to a phase shift of 0.8.pi.), the algorithm is no longer able to produce well-separated white and black intensity levels. The calculations that implement backpropagation are discussed by L. P. Yaroslavskii and N. S. Merzlyakov, Methods of Digital Holography, translated from Russian by Consultants Bureau, New York and London, 1980, pp. 105-111, and incorporated by reference herein. Existing fabrication techniques for binary optics for visible light involve overlays of N binary etch steps (with each etch pattern written by electron beam lithography) to create 2.sup.N allowable thickness values in a refractive media. Because this technology may be directly applicable to our proposed method for X-ray projection lithography, we have also modified our hologram design procedure for the situation where only discrete thickness values are allowed. FIGS. 9A and 9B present water plane intensity limits calculated with discrete thickness increments of 30, 60, 90, and 120 nm, up to a maximum thickness of 1000 nm, for white and black backgrounds, respectively. This allows 34, 17, 12, and 9 thickness levels, respectively. Where white features are printed on a black background (FIG. 9B), the contrast of the pattern remains quite good, even with only nine allowed thickness levels. We judge the case of printing black features on a white background to be acceptable only when the thickness step size is 60 nm or less, corresponding to four etch overlays. We assume that one of the 17 gray levels for 60 nm thickness increments can be safely ignored. Either positive or negative lithographic images, with hologram thicknesses corresponding to about 2.pi. or more of phase shift, and with only four etch overlays, can be produced by this approach. For example, we have done simulations to generate the same test pattern using 2.sup.4 thickness levels in 1500 nm of fused quartz at .lambda.=2.5 nm wavelengths and have obtained results of equally good quality as those shown here for 2.sup.4 thickness levels in 1000 nm of carbon at .lambda.=5.0 nm wavelength. Binary optical systems using fused quartz have been fabricated with four etch steps, 500 nm feature width, and 40 nm step thickness, and aspect ratios as high as 20:1 have been achieved in 35 nm linewidth lithography in other contexts. The technological challenges in producing in-line transmission holographic optics for projection X-ray lithography thus appear to be surmountable. As the linewidth used in X-ray lithography is reduced, the problem of contamination becomes more and more serious. Proximity X-ray lithography with .lambda.=1 nm X-rays offers some relief from the problem because some types of contaminant particles present on the mask or wafer will be transparent to the X-rays. This fact is considered to be one of the advantages of X-ray lithography. On the other hand, the existence of contaminant particles between the mask and the wafer places limits on how small the mask-wafer gap can be made, and this in turn leads to a corresponding limit on the diffraction-limited resolution. Furthermore, the use of X-rays does not overcome defects in the mask resulting from contamination during its manufacture. Holography provides a radical new approach to the contamination problem. Because the information relating to each point of the image is distributed over many pixels of the hologram, we expect that it would not matter if a small fraction of that area is blocked by a defect or a contaminant. This will more likely be true if the wafer is in the far field of the defect; i.e., if the defect size is less than about d'=[.lambda.d].sup.1/2 . With a hologram-to-mask separation distance d=200 .mu.m and wavelength .lambda.=5 nm, this threshold defect size becomes 1 .mu.m in the present example. This implies that if a high brightness X-ray source is used for wafer exposure, one may wish to increase the hologram-to-wafer separation distance d above 200 .mu.m to improve immunity to contamination, at the expense of requiring a greater degree of coherence in the illumination of the hologram. As a test of these ideas, we performed simulations where spherical gold particles were assumed to be located on the carbon holograms associated with FIGS. 4 and 7 (one sphere was located at the center, and the other was located near the lower left comer). FIGS. 10A and 10B illustrate an effect of high Z contaminant diameter (0.2-1.0 .mu.m) on wafer plane intensity, for white and black backgrounds, respectively. FIGS. 10A and 10B indicate that, if these high Z contaminants are sufficiently small, of size &lt;0.4 .mu.m for printing black features on a white background, or of size &lt;1 .mu.m for printing white features on a black background, the desired wafer plane image is produced with almost no degradation. This result is also valid for low Z contaminants. These values apply to a choice of d=200 .mu.m for hologram separation distance, but we can obtain immunity to the presence of still larger contaminant particles by using a larger separation distance d. This property, that mask defects or contaminants will often not lead to image defects, is a considerable advantage of the technique, and may have significant impact on the economics of device manufacture. A second embodiment of the invention uses reflection from a hologram to form image features of a given size the wafer or substrate. With reference to the reflection configuration 31 shown in FIG. 11, for a feature size s, illumination wavelength .lambda., hologram width L and a square wafer image field of area w2, the numerical aperture for this arrangement is EQU NA=sin.beta.=K.lambda./s, (1) where K is a dimensionless empirical number of the order of 1. In FIG. 11, a light beam 33 of wavelength .lambda. is incident at grazing incidence upon an optical substrate 35 containing a hologram. The grazing incidence angle is .alpha. and the diffraction angle of interest is .beta., both being chosen based upon considerations of material reflectance and resolution. A wafer 37 has an area of w.sup.2 and is illuminated by the diffracted image produced by the incident light beam 33, and satisfactory imaging at the wafer 37 is obtained if the coherence length of the light .lambda..sup.2 /.DELTA..lambda. is equal to the largest path length difference among rays arriving at any point in the image. From the geometry of FIG. 11, it is easily verified that a transverse dimension w of the wafer is determined by the relation EQU w=(L/2)sin(.alpha.+.beta.) (2) and that the hologram-to-wafer distance is p=L sin .alpha./sin .beta.. The maximum path difference, which occurs for light rays arriving at the point E on the wafer 37, is found to be the coherence length HBE-AE=BE-GE=(L sin .alpha.).sup.2 /2p and is set equal to .lambda..sup.2 /.DELTA..lambda.. This yields the relation EQU .lambda./.DELTA..lambda.=Kw sin .alpha./[s sin (.alpha.+.beta.)].(3) In order to penetrate at least 0.2 .mu.m into a typical photoresist material, light of wavelength .lambda..ltoreq.13.5 nm must be used. Longer wavelengths have advantages in this situation, because monochrometer technology becomes easier to implement and the coherent output of a light source improves as the wavelength increases. For grazing incidence angles of 0.15 radians (8.59.degree.) or less, gold and carbon each reflect with greater than 60 percent efficiency for all wavelengths .lambda.&gt;9.5 nm. Other attractive choices for the reflective coating include, without limitation, chromium, nickel, rhodium, rhenium, iridium and platinum. Choosing gold with K=1, minimum feature size s=0.1 .mu.m, wafer size w=1 cm and .lambda.&gt;9.5 nm, the configurations illustrated in Table 1 should produce acceptable holographic image patterns with minimum feature size s=0.1 .mu.m or larger on the wafer 37. TABLE 1 ______________________________________ Acceptable Configurations For Hologram Reflection Imaging (s = 0.1 .mu.m) .lambda.(nm) .alpha. .beta. L(cm) p(cm) .lambda./.DELTA..lambda. Beam width (cm) ______________________________________ 13 8.59.degree. 7.47.degree. 7.2 8.3 54,000 1.1 10 8.59.degree. 5.74.degree. 8.1 12.1 60,000 1.2 5 5.degree. 2.87.degree. 14.6 25.4 64,000 1.3 1 2.degree. 0.57.degree. 44.6 156.5 78,000 1.6 ______________________________________ Although all configurations indicated in Table 1 are feasible, the most easily achievable choice for the second embodiment is the configuration with .lambda.=13 nm. This compact systems requires a hologram area of size .apprxeq.8 cm.times.3 cm, with reasonable values of wavelength and feature size, but with synchrotron radiation monochromaticity (.lambda./.DELTA..lambda.) that has not yet been obtained at these low wavelengths. However, the desired radiation monochromaticity appears to be within reach. Using the grating equation (including the conical diffraction for the transverse direction), the fringe period or resolution for "writing" a hologram to deliver a smallest feature size s=0.1 .mu.m for longitudinally and transversely oriented lines, respectively; and for .lambda.=13 nm, the required resolution is 0.31 .mu.m and 0.10 .mu.m for longitudinally and transversely oriented lines, respectively. This second embodiment of the invention requires use of a very high resolution monochromator, and this is best operated from a low emittance, high brightness synchrotron source. An ideal synchrotron source would be a 6-cm-period undulator on a high brightness storage ring, of energy about 1.5 GeV, delivering a single mode beam. Another possibility is use of an X-ray laser having the appropriate emission wavelength. Presently available X-ray lasers have the necessary monochromaticity but do not yet have sufficient flux per mode per pulse to adequately expose 1 cm.sup.2 of photoresist material. In this embodiment, an inverse diffraction problem must also be formulated and solved, in order to determine a hologram required to produce a given image on the wafer. In either embodiment, the wafer or substrate that receives the X-rays from the hologram may be coated with a photoresist material, such as PMMA. The irradiated photoresist material should be developed by procedures well known in the art to bring out the desired image. Although the invention has been illustrated with minimum linewidths as small as 0.06 .mu.m, minimum linewidths as low as 0.01 .mu.m should be achievable with this approach, if the hologram writing device can provide linewidths sufficiently small. Linewidths above a minimum value, such as 0.25 .mu.m, are also easily achieved with this approach.