Abstract:
A photolithography method and apparatus for producing minima of light intensity corresponding to a point in a phase shift mask is described. The phase shift in the light produced by the mask varies in a spiral fashion around the point so that the phase shift measured along lines drawn across the surface of the mask which pass through the point have a 180° jump at the point, and lines passing around the point have no jumps between 130° and 230°, and most preferably no jumps between 100° and 260°.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention is the field of lithographic masks for extremely high resolution imaging of mask features on to various substrates. In particular, the field of the invention is the field of phase shift masks, where the mask pattern is at least in part determined by the change in phase of light interacting with the surface of the mask.  
         BACKGROUND OF THE INVENTION  
       Photolithography  
         [0002]    This invention relates to the field of microlithography for the manufacture of integrated circuits, magnetic devices, and other microdevices such as micromachines. In this field the final product is manufactured in sequential manner in which various patterns are first produced in a “resist” material with each pattern subsequently defining a product attribute. The “resist” materials, generally polymer compositions, are sensitive to light or other forms of radiation. The patterns are formed in the resist by exposing different regions of the resist material to different radiation doses. In the bright (high dose) regions, chemical changes take place in the resist that cause it to dissolve in a chemical bath or be etched away by a gas or plasma more easily (for positive resists) or less easily (negative resists) than in dim (low dose) regions. If the radiation flux on to the resist is too little, the resist is said to be underexposed, and in general the underexposed area of the positive resist will not all dissolve away, and the negative resist will only partially dissolve away when the resist is “developed”, (IE dissolved or etched). With sufficient radiation flux for exposure, the unexposed and exposed areas of the resist will be developed with one area all dissolved away and the other area remaining as a protective coat for the next step in the wafer processing. For a radiation flux that is too great, the resist is overexposed and the areas exposed tend to “bloom” out and etched out lines, for example, are wider than they would be with just a sufficient exposure. For a good resist, a “manufacturing window” exists between such underexposure and overexposure fluxes.  
         Patterned Resist and Process  
         [0003]    The bright and dim regions are formed using an exposure tool which generally transfers corresponding features to the resist from a mask or reticle. The masks or reticles are formed from mask substrates, which are plates of quartz or other material transparent to the radiation used for exposing the resist, coated with an opaque material such as chrome. The chrome is etched away in a pattern to form the mask. The radiation employed may be (but is not limited to) ultraviolet light and x-rays, and the regions of the mask that are opaque and transparent form a pattern of bright and dark when illuminated uniformly. In the most common implementation of this technology, a projection lens forms an image of the mask pattern in the resist film on a planar substrate. That image comprises the high and low dose regions that produce the resist pattern. When some form of light is employed in this process, it is called photolithography.  
         Wavefront Engineering  
         [0004]    The patterns formed in the resist are not identical to those on the mask, and the methods of obtaining the pattern desired for the ultimate manufactured device in spite of deficiencies in the microlithography process is called “wavefront engineering.” Among the various devices used for this purpose are phase shifting masks (PSM)s—which create desired dark regions though interference. Phase shift masks were first published by the inventor of the present invention in a paper entitled “Improving resolution in photolithography with a phase shifting mask,” M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, IEEE Trans. Electron Devices ED-29, 1828-1836 (1982). Since that time, there have been hundreds of patents and thousands of papers issued containing the phrase “phase shift mask”. Phase shift masks allow production of features with smaller linewidths than conventional photolithography, in that the low dose portions of the photoresist are much narrower than the high dose portions. The pitch, however, of such narrow linewidth features is limited to λ/2 N.A., where λ is the wavelength of the light used to expose the resist and N.A. is the numerical aperture of the optical system used to expose the resist.  
           [0005]    There are presently two types of PSMs in use: weak-PSMs such as the Attenuated-PSM and strong-PSMs such as the Alternating-Aperture-PSM. These two differ in that the weak-PSMs have only one type of bright feature, while the strong-PSMs contain two types of bright features identical except for the optical phase, which differs by ˜180°. See, for example, M. Shibuya, Japanese Patent Showa 62-50811, M. D. Levenson et. al. IEEE Trans. Elect. Dev. ED-29, 1828-1836 (1982), and M. D. Levenson, Microlithograpy World 6-12 (March/April 1992).  
           [0006]    Typically, narrow and “dark” lines may produced by phase shift methods, where the phase shift arises from two neighboring areas of the mask which shift the phase of light interacting with the mask by 180°. The two areas of the mask are separated by a border which is a straight line on the mask, and when the mask is imaged on a resist, a very narrow line with little or no illumination between two bright areas results. Such narrow lines are valuable, for example, as gate lines in a semiconductor device. However, the length of the lines is much larger than the width of the line.  
           [0007]    There is a demand for features which are holes in the resist with as small an area or as small a diameter as possible. Such holes are used, for example, to produce contacts to underlying conducting lines or other parts of semiconductor devices. FIG. 1 shows a sketch of the intensity of the light produced by three prior art methods for producing such holes. A T-mask is just a hole in the chrome covering of a normal mask substrate which produces the intensity given by curve  10 . The minimum diameter of the light pattern is determined by diffraction, and does not get smaller as the hole in the chrome is reduced beyond a certain point. The Attenuating PSM (curve  14 ) and Rimshift PSM (curve  12 ) technologies result in somewhat smaller diameter bright areas.  
           [0008]    U.S. Pat. No. 5,807,649 teaches a double exposure system for exposing a photoresist using a phase shift mask and with a second mask to expose unwanted dark areas left by the phase shift mask. U. S. Pat. No. 5,620,816 teaches a double exposure system where a chromeless phase-edge shift mask is used to expose all of the photoresist except on lines running in rows and/or columns, and then a customized mask is used to expose unwanted portions of the lines and/or columns. If the same or another chromeless phase-edge mask is used to expose the same resist wherein dark lines run perpendicular to those dark lines left by the first exposure, some of the unexposed areas of the resist left by the first exposure with be exposed, and an array of unexposed spots with very small diameter will result. The prior art does not show any method of producing very small regions of unexposed photoresist using a single exposure.  
         PSM Design  
         [0009]    Various Electronic Design Automation (EDA) tools are known for preparing the patterns used in conventional and phase-shifting masks. In addition, OPC tools alter those patterns to account for the realities of the exposure systems. It is also known that the pattern of apertures on the phase-shifting mask need not correspond closely to the ultimate circuit pattern, at least not when a conventional block-out mask is employed for a second exposure on the resist film in concert with a first exposure made using an alternating-aperture PSM. Such second exposures erase anomalies due to phase-conflicts. Numerical Technologies, Inc., in U.S. Pat. No. 5,858,580, in particular, has demonstrated the In-Phase design system which employs a block-out mask similar in geometry to the ultimate circuit feature along with an alternating-aperture PSM composed of pairs of small apertures (shifters), one of which has 0° phase, while the other has 180°—which define the narrowest dark features between them.  
           [0010]    Related U.S. patents issued to the inventor of the present invention are U.S. Pat. Nos. 6,287,732 and 6,251,549 issued Sep. 11, 2001 and Jun. 26, 2001 respectively. Related U.S. patent applications by the inventor of the present invention are applications entitled “Generic phase shift mask” filed Sep. 6, 2001 as U.S. patent application Ser. No. 09/947,336 and “Photolithography method and apparatus” filed Feb. 25, 2002 as U.S. patent application Ser. No. 10/083,049. The above identified references, U.S. patents and U.S. patent applications are hereby incorporated by reference.  
         OBJECTS OF THE INVENTION  
         [0011]    It is an object of the invention to provide apparatus, methods, and systems for the production of very small area features on a device.  
           [0012]    It is an object of the invention to provide a phase shift mask substrate having features which provide for production of very small area features on a device.  
           [0013]    It is an object of the invention to provide a phase shift mask substrate having features which provide for production of very small area features on a device.  
           [0014]    It is an object of the invention to provide a phase shift mask having features which provide for production of very small area features on a device.  
           [0015]    It is an object of the invention to provide a method of making a phase shift mask substrate having features which provide for production of very small area features on a device.  
           [0016]    It is an object of the invention to provide a method of using a generic phase shift mask substrate having features which provide for production of very small area features on a device.  
           [0017]    It is an object of the invention to provide a method of using a generic phase shift mask substrate having features which provide for production of very small area features on a device.  
           [0018]    It is an object of the invention to provide a method for making photolithographic exposures of two dimensional arrays of small area features having pitch &lt;λ/2 NA, where λ is the wavelength of light used by the photolithographic exposure system and NA is the numerical aperture of the photolithographic system.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention is a system, apparatus and method to produce very small areas features on a device. A phase shift mask substrate is produced where an area on the mask substrate has a phase shift variation around a point, such that light interacting with the area around the point has the phase of the light shifted so that there is a sharp and deep minimum of the light intensity in a first area corresponding to the point, and an annular second area surrounding the first area where the light intensity is much greater than the minimum through the entire annular area. The phase shift varies in a spiral pattern about the point so that the phase shifts suddenly by approximately 180° when measured along lines crossing the point, and the phase shifts slowly or suddenly by an amount less than 130° or greater than 230° when measured along any other line traversing an area on the mask corresponding to the second annular area. In particular, the most preferred embodiment has a substrate with a one turn spiral staircase like set of “steps” etched down from the surface into the substrate, with the height of each step producing a phase shift from step to step of less than or much less than 130°, and the phase shift from the bottom step back up to the first step of greater than approximately 230°. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 shows intensity of light produced by prior art photolithography masks.  
         [0021]    [0021]FIG. 2 shows a perspective sketch of a preferred embodiment.  
         [0022]    [0022]FIG. 3A shows the phase shift of FIG. 2 as a function of the polar angle θ.  
         [0023]    [0023]FIG. 3B shows the phase shift of FIG. 2 as a function distance along line  27 .  
         [0024]    [0024]FIG. 4 shows a plan view of a phase shift mask substrate.  
         [0025]    [0025]FIG. 5 shows the phase shift produced about a point as a function of the polar angle θ.  
         [0026]    [0026]FIG. 6A shows the most preferred embodiment of the invention.  
         [0027]    [0027]FIG. 6B shows a perspective view of four steps of the spiral staircase around an area  60 .  
         [0028]    [0028]FIG. 7 shows the phase shift produced about an opaque area  60 .  
         [0029]    [0029]FIG. 8 shows the results of a calculation of the intensity profiles.  
         [0030]    [0030]FIG. 9 shows an alternative embodiment pattern.  
         [0031]    [0031]FIG. 10 shows the phase shift produced about an opaque area  90 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    [0032]FIG. 2 shows a perspective sketch of a preferred embodiment of the invention. A flat surface  20  of a phase shift mask substrate has a feature  21  etched into it. The circular edge  22  of the feature  21  is shown cutting into the surface  20 . The sketch shows a spiral ramp  23  circulating about a center line  24 . The ramp starts at the original surface  20  of the phase shift mask substrate at a line  24  where the phase shift is 0° with respect to the phase shift produced at the original surface  20 , and proceeds clockwise around the center line  25 . The ramp imparts a phase shift on light impinging upon the phase shift mask according to the depth of the cut. Phase shifts of 90°, 180° 270° and 260° with respect to the original surface  20  are shown at the positions marked on the figure. The phase shifts produce a pattern of light corresponding to the feature  21  where the light has a deep minimum of intensity at the point corresponding to the point of intersection of the center line  25  with the surface of the phase shift mask substrate  20 . There is no sharp change in the phase shift from one area of the spiral ramp to another, so no dark line is produced in the field surrounding the point of intersection, and the sharp change of phase between the bottom  26  of the spiral ramp  23  and the top of the spiral ramp  24  is 360°, which also produces no dark line feature. The edge  22  produces a dark line in the illumination pattern when the phase shift between the original surface  20  and the ramp  23  is near 180°. The resulting unexposed photoresist may be exposed in a later step.  
         [0033]    The feature of FIG. 2 produces an isolated area of minimum intensity surrounded by an annular area where the intensity is much greater throughout the annular area than the exposure at the minimum of the exposure.  
         [0034]    [0034]FIG. 3A shows the phase shift produced by the feature of FIG. 2 as a function of the polar angle θ measured around the center line  25 . FIG. 3B shows an equivalent figure of the phase shift measured along a line  27  which nearly encircles center line  25 . As shown in FIG. 3B, the phase of the light changes without jumps and the phase change totals nearly 360° from one end of the line  27  to the other. In preferred embodiments, the spiral shown in FIG. 2 will have a phase change from one end of line  27  to the other of nearly 720°, so that the intensity change when passing across line  26  is still very small. In general, the spiral shown in FIG. 2 will have a phase change from one end of line  27  to the other of nearly m times 360°, (where m is an integer) so that the intensity change when passing across line  26  is still very small.  
         [0035]    A spiral pattern such as shown and described in FIG. 2 and FIG. 3 is pressed into a phase shift mask substrate by methods similar to those used to reproduce CD recordings, or other methods as known in the art of producing surface features.  
         [0036]    [0036]FIG. 4 shows a plan view of a phase shift mask substrate where the surface is formed into a number of flat parallel areas in the form of hexagons. In FIG. 4, there are three levels which cause phase shifts of 0°, 120°, and 240°. Each intersection  40  of three of the borders of the hexagon areas will produce a deep minimum in the radiation pattern of light interacting with the surface. Instead of the spiral ramp of FIG. 2, a spiral staircase having 3 treads is formed about each point. Note that adjacent points share two treads in common, and that the sense of the spiral (descending clockwise or counterclockwise) is opposite for two adjacent points. The phase boundaries  42  between two adjacent phase shift areas noted in FIG. 4 produce dark lines in the pattern having a theoretical intensity of about 25% of the full intensity for fully coherent light, while the theoretical intensity corresponding to the point  40  is zero. In practice, the minimum intensity is about 2% because of scattering in the optical system. Thus, the resist must be overexposed to ensure that the dark lines corresponding to the boundaries  42  between the phase shift areas are sufficiently exposed with respect to the areas corresponding to the points  40  so that the dark lines do not print and the dark points do print.  
         [0037]    Specifications for resolving power and focusing of a spot or line of light in optical systems often are given in terms of full width at half maximum intensity. In the present case, the intensity is a minimum I min  at a point, and a maximum intensity I max  occurs in an annular region surrounding the point. The intensity varies throughout the annular region. For example, along a circular line drawn in the annular region which has the point in the center, the intensity drops to an intensity I′ min  at points on the circular line corresponding to a phase change on the mask. A specification for the “full width” of the intensity minimum may still be drawn with reference to either the maximum intensity or the intensity of the minimum measured along the defined line. In the sense of the present specification, the full width of the intensity minimum is defined as the full width of the intensity minimum measured at half the minimum intensity measured along the line (I′ min /2). The full width of the intensity minimum of the present invention is preferably less than λ/2 N.A.  
         [0038]    [0038]FIG. 5 shows the phase shift produced about a point  40  as a function of the polar angle θ measured around the point  40 . The phase shift when measured along a line nearly encircling the point  40  will appear similar to FIG. 5 when plotted against distance along the line. The spiral shown in FIG. 2 will have a phase change from one end of line  27  to the other of nearly 720°, so that the intensity change when passing across line  26  is still very small.  
         [0039]    [0039]FIG. 6A shows the most preferred embodiment of the invention, where the phase shift mask substrate is formed into a number of flat parallel areas in the form of squares. In FIG. 6A, there are four levels which cause phase shifts of 0°, 90°, 180° and 270°. The intersections where four corners of the squares meet will cause phase shifts which cancel the intensity projected on to photoresist, and result in a deep minimum in the intensity at points corresponding to the corners of the squares. The phase shift at boundaries  62  is only 90°, in contrast to the 120° phase shift produced at the boundaries  42  of FIG. 4, so the intensity in the lines corresponding to the boundaries  62  is much greater than the 25% intensity for the lines of FIG. 4. FIG. 6A also shows optional opaque areas  60  covering the corners of the squares shown in FIG. 6A. The opaque areas reduce the intensity of the light in areas of the resist corresponding to the corners of the squares of the mask substrate of FIG. 6A. Note that adjacent opaque areas  60  have two steps of the spiral staircases in common, and that the sense of the spiral is again opposite for each pair of adjacent areas  60 .  
         [0040]    [0040]FIG. 6B shows a perspective view of four steps of the spiral staircase around an area  60 .  
         [0041]    [0041]FIG. 7 shows the phase shift produced about an opaque area  60  as a function of the polar angle θ measured around the center of the opaque area  60 .  
         [0042]    [0042]FIG. 8 shows the results of a calculation of the intensity profiles of the light produced by the mask of FIG. 6. The dark lines running horizontally and vertically are an artifact and should be ignored. Note that the minima  80  corresponding to the areas  60  of FIG. 6 are surrounded by annular areas  82  where the intensity is substantially greater than the intensity of the minima  80 .  
         [0043]    [0043]FIG. 9 shows an alternative embodiment pattern used to produce intensity minima on a hexagonal grid using 4 different phase shift levels. The opaque areas  90  show the hexagonal grid points of the phase shift mask substrate corresponding to intensity minima desired on the resist. The areas corresponding to intersection points  92  will also have intensity minima, and must be exposed using a second mask and a second exposure in order to produce the desired hexagonal pattern of unexposed resist.  
         [0044]    [0044]FIG. 10 shows the phase shift produced about an opaque area  90  as a function of the polar angle θ measured around the center of the opaque area  90  for the embodiment of FIG. 9.  
         [0045]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.