Patent Publication Number: US-6660653-B1

Title: Dual trench alternating phase shift mask fabrication

Description:
FIELD OF THE INVENTION 
     This invention relates generally to semiconductor device fabrication, and more particularly to the use of phase shift masks (PSM&#39;s) in conjunction with such fabrication. 
     BACKGROUND OF THE INVENTION 
     Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC&#39;s with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC&#39;s have begun to be manufactured that have features smaller than the lithographic wavelength. 
     Sub-wavelength lithography, however, places large burdens on lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate in sub-wavelength lithography. The resulting features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. For example, as two mask patterns get closer together, diffraction problems occur. At some point, the normal diffraction of the exposure rays start touching, leaving the patterns unresolved in the resist. The blending of the two diffraction patterns into one results from all the rays being in the same phase. Phase is a term that relates to the relative positions of a wave&#39;s peaks and valleys. In FIG. 1A, the waves  102  and  104  are in phase, whereas in FIG. 1B, the waves  106  and  108  are out of phase. 
     One way to prevent the diffraction patterns from affecting two adjacent mask patterns is to cover one of the openings with a transparent layer that shifts one of the sets of exposing rays out of phase, which in turn nulls the blending. This is shown in FIGS. 2A and 2B. In FIG. 2A, the mask  202  causes an undesirable light intensity as indicated by the line  204 . In FIG. 2B, adding the phase shifter  206  to the mask  202  causes a desirable light intensity as indicated by the line  208 . This mask  202  in FIG. 2B with the phase shifter  206  added is a phase shift mask (PSM), which is a special type of photomask. 
     A typical photomask affects only one of the properties of light, the intensity. Where there is chromium, which is an opaque region, an intensity of zero percent results, whereas where the chromium has been removed, such that there is a clear or transparent region, an intensity of substantially 100 percent results. By comparison, a PSM not only changes the intensity of the light passing through, but its phase as well. By changing the phase of the light by 180 degrees in some areas, the PSM takes advantage of how the original light wave adds to the 180-degree wave to produce zero intensity as a result of destructive interference. 
     PSM&#39;s have gained increased popularity among manufacturers as the feature sizes they are tasked with printing become smaller, and the topography over which these features must be printed becomes more varied. PSM&#39;s offer their customers the opportunity to greatly improve the resolution capability of their steppers. This allows them to print smaller feature sizes using the same equipment and processes. 
     One particular type of PSM is referred to as an alternating PSM. The PSM of FIG. 2B was one example of an alternating PSM. In an alternating PSM, closely spaced apertures are processed so that light passing through any particular aperture is 180 degrees out of phase from the light passing through adjacent apertures. Any light that spills over into the dark region from the two edges that are out of phase will destructively interfere. This reduces the unwanted exposure in the center dark region. 
     FIG. 3 shows another example of an alternating PSM, and more specifically, a dual-trench alternating PSM  300 . The PSM  300  includes two layers, a chromium layer  302 , and a quartz layer  304 . The chromium layer  302  is the same type of layer typically found in other, non-PSM photomasks, in which light is exposed therethrough to an underlying semiconductor wafer. Clear regions within the chromium layer  302  allow light to pass through, whereas opaque regions within the chromium layer  302  prevent light from passing through. The clear and opaque regions are arranged to correspond to a desired semiconductor design, or pattern. In the PSM  300 , there are clear regions  306 A,  306 B,  306 C,  306 D, and  306 E. 
     The quartz layer  304  is more generally a clear or transparent layer, in which different-sized trenches are alternatively added beneath the clear regions of the chromium layer  302  to phase shift light passing through these clear regions. For instance, the alternating clear regions  306 A,  306 C, and  306 E of the chromium layer  302  have shallow trenches beneath them in the quartz layer  304 . Conversely, the alternating regions  306 B and  306 D of the chromium layer  302  have deep trenches beneath them in the quartz layer  304 . The PSM  300  is an alternating PSM in that the PSM design repeats on an alternating basis for clear regions of the chromium layer  302 , such that one clear region has a shallow trench beneath it, whereas the next clear region has a deep trench beneath it, and so on. The PSM  300  is a dual-trench alternating PSM in that there are two trenches that repeat, a shallow trench and a deep trench. 
     The manner by which the PSM  300  of FIG. 3 can be fabricated according to the prior art is summarized with reference to FIGS. 4A-4I. In FIG. 4A, the clear regions within the chromium layer  302  are already present, by a process of photoresist patterning in which the photoresist is first exposed by e-beam writing and then developed, etching the chromium layer  302 , and then stripping the remaining photoresist. A new layer of photoresist  402  has been added, such as by a coating process. In FIG. 4B, the layer of photoresist  402  is exposed to correspond to the deep trenches  306 B and  306 D. The exposure is accomplished by e-beam writing. In FIG. 4C, exposed areas of the photoresist  402  are developed to remove them, and in FIG. 4D, the quartz layer  304  is dry etched to initially form the deep trenches  306 B and  306 D. 
     The quartz layer  304  is dry etched for 60 degrees, so that if there are defects within the quartz layer  304 , only 60 degrees of such defects will be present. This amount is minimal, and will not affect printing of the semiconductor device using the PSM  300 . The process of FIGS. 4A-D is repeated for a total of three times, so that a total of 180 degrees of phase shift is achieved within the quartz layer  304 . 
     In FIG. 4E, another layer of photoresist  402  is added, such as by a coating process. In FIG. 4F, the layer of photoresist  402  is exposed to correspond to all the trenches  306 A,  306 B,  306 C,  306 D, and  306 E. The exposure is accomplished by e-beam writing. In FIG. 4G, exposed areas of the photoresist  402  are developed to remove them, and in FIG. 4H, the quartz layer  304  is dry etched to completely form the deep trenches  306 B and  306 D that were previously initially formed, as well as to completely form the shallow trenches  306 A,  306 C, and  306 E. 
     The quartz layer  304  is again dry etched for 60 degrees, so that if there are defects within the quartz layer  304 , only 60 degrees of such defects will be present. The process of FIGS. 4E-4H is repeated for a total of four times. Once the fourth time is finished, the remaining photoresist  402  is removed, such that the final PSM  300  remains, as has already been shown in FIG.  3 . 
     This conventional approach to manufacturing the dual-trench alternating PSM  300  has several disadvantages, however. Overlay errors can error between successive exposures of the photoresist, such as between successive e-beam writings. These overlay errors induce anti-reflective layer loss around the etched regions, which can be difficult to discover during inspection. Because photoresist is exposed for a total of eight times, there are eight such successive e-beam writings, and the potential for overlay error inducing anti-reflective layer loss is great. Furthermore, e-beam writing in particular can be a time-consuming and/or costly process, such that fabricating the PSM  300  will be a lengthy and/or costly process since e-beam writing is used eight times. 
     Therefore, there is a need for a process for fabricating a dual-trench alternating PSM that overcomes the problems associated with manufacturing such PSM&#39;s in the prior art. Such a process should minimize the use of e-beam writers as much as possible. Minimizing the use of e-beam writers will allow such a process to avoid as much as possible overlay errors inducing anti-reflective layer loss. Furthermore, such minimization will decrease the time and/or the cost in fabricating dual-trench alternating PSM&#39;s. For these and other reasons, there is a need for the present invention. 
     SUMMARY OF THE INVENTION 
     The invention relates to fabricating a dual-trench alternating phase shift mask (PSM). An opaque layer over a transparent layer of the PSM is patterned according to a semiconductor design. The opaque layer may be a chromium layer, whereas the transparent layer may be a quartz layer. The transparent layer is dry etched a first number of times through a first photoresist layer applied over the opaque layer and patterned according to the deep trenches of the alternating PSM design by using beam writing. This initially forms deep trenches of the PSM. The transparent layer is dry etched a second number of times through a second photoresist layer applied over the opaque layer and patterned according to the deep trenches and the shallow trenches of the alternating PSM design by using backside ultraviolet exposure. This completely forms shallow trenches and the deep trenches of the PSM. The second photoresist layer is then removed. 
     The invention provides for advantages not found within the prior art. E-beam writing is significantly reduced in fabricating a dual-trench alternating PSM, as compared to the prior art, by instead using backside ultraviolet exposure. This significantly decreased overlay error, which means that induced anti-reflective layer loss is also significantly reduced. The reduction in use of e-beam writing also means that the invention generates a dual-trench alternating PSM in a less costly and/or less time-consuming manner as compared to the prior art. Other advantages, embodiments, and aspects of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are diagrams showing the difference between in-phase and out-of-phase waves. 
     FIGS. 2A and 2B are diagrams showing the difference in the resulting light intensity between a photomask without phase shift and a photomask with phase shift. 
     FIG. 3 is a diagram showing an example of an alternating dual-trench phase shift mask (PSM). 
     FIGS. 4A-4H are diagrams showing how the PSM of FIG. 3 can be conventionally manufactured according to the prior art. 
     FIG. 5 is a flowchart of a method to fabricate an alternating dual-trench PSM according to an embodiment of the invention. 
     FIGS. 6A-6N are diagrams showing illustratively the method of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     FIG. 5 shows a method  500  to construct a dual-trench alternating phase shift mask (PSM) according to an embodiment of the invention. A photoresist layer over a chromium layer is first exposed according to a semiconductor design ( 502 ), such as by e-beam writing. The design is so that the PSM can ultimately be used to manufacture semiconductor devices of this design, by, for instance, photolithographic processes that use the PSM as the photomask. The photoresist layer is developed ( 504 ) to remove the exposed parts of the photoresist layer, and the chromium layer is etched ( 506 ) so that the chromium layer is itself patterned according to the semiconductor design. The photoresist layer is then removed ( 508 ). 
     Performance of  502 ,  504 ,  506 , and  508  is shown illustratively with reference to FIGS. 6A-6E. In FIG. 6A, the PSM  600  begins as a quartz layer  602 , a chromium layer  606  over the quartz layer  602 , and a photoresist layer  608  over the chromium layer  606 . The quartz layer  602  is generally a type of transparent or clear layer. The chromium layer  606  is generally a type of opaque layer. In FIG. 6B, the photoresist layer  608  is exposed according to the semiconductor design, as indicated by the regions  610 . Exposure may be accomplished by e-beam or laser writing the semiconductor design in the photoresist layer  608 , to result in the regions  610 . 
     In FIG. 6C, the photoresist is developed to remove those parts of the photoresist layer  608  that were exposed or written on, such that there is no longer photoresist within the regions  610 . In FIG. 6D, the chromium layer  606  is etched down to the quartz layer  602 , to remove those parts of the chromium layer  606  that were exposed by exposure and development of the photoresist layer  608 . Finally, the photoresist layer  608  is removed, such as by stripping, resulting in FIG. 6E, where the patterned chromium layer  606  remains over the layer  602 . 
     Referring back to FIG. 5, a new photoresist layer is next applied over the chromium layer ( 510 ). This can be accomplished by photoresist coating the chromium layer. The new photoresist layer is exposed according to deep trenches an alternating PSM design ( 512 ), and the photoresist layer is developed to remove the exposed parts of the layer ( 514 ). The alternating PSM design is so that a dual-trench alternating PSM results for the semiconductor design when the PSM is finished being fabricated. The quartz layer is next dry etched to initially form deep trenches of the PSM ( 516 ). The dry etching is performed preferably at 60 degrees, and  510 ,  512 ,  514 , and  516  are repeated a number of times, such as three times, each at 60 degrees. This reduces defects in the quartz layer from affecting the ultimate printing of a semiconductor wafer using the PSM to fabricate a semiconductor device on the wafer, since any defect will only exist at 60 degrees, which is minimal enough not to affect the printing. 
     Performance of  510 ,  512 ,  514 , and  516  is shown illustratively with reference to FIGS. 6F-6I. In FIG. 6F, the new photoresist layer  612  has been applied over the chromium layer  606 . In FIG. 6G, the regions  614  result from exposure of the new photoresist layer  612 , such as by e-beam or laser beam writing the deep trenches of alternating PSM design on the new photoresist layer  612 . In FIG. 6H, the new photoresist layer  612  has been developed, such that the parts of the layer  612  that were written on or otherwise exposed are removed. The resulting photoresist layer  612  corresponds to the deep trenches of the alternating PSM design. In FIG. 6I, dry etching has been performed through the quartz layer  602 , to initially form the deep trenches  620  and  622  of the PSM  600 . 
     Referring back to FIG. 5, another photoresist layer is applied over the chromium layer ( 518 ). This can be accomplished by photoresist coating the chromium layer. The photoresist layer is backside exposed through the quartz layer ( 520 ), such as by using ultraviolet rays. The backside exposure reaches the photoresist layer because of the previously removed parts of the chromium layer during dry etching. The photoresist layer is developed to remove the exposed parts of the photoresist layer ( 522 ), and the quartz layer is dry etched to completely form shallow trenches and the deep trenches of the PSM ( 524 ). The dry etching is performed preferably at 60 degrees, and  518 ,  520 ,  522 , and  524  are repeated a number of times, such as four times, each at 60 degrees. This reduces defects in the quartz layer from affecting the ultimate printing of a semiconductor wafer using the PSM to fabricate a semiconductor device on the wafer, since any defect will only exist at 60 degrees, which is minimal enough not to affect the printing. 
     Performance of  518 ,  520 ,  522 , and  524  is shown illustratively with reference to FIGS. 6J-6M. In FIG. 6J, another photoresist layer  615  has been applied over the chromium layer  606 . In FIG. 6K, the photoresist layer  615  is exposed by backside exposure using ultraviolet, x-, or other types of rays  618 . The exposure results in the regions  616  through the removed parts of the chromium layer  606 . Using backside exposure, instead of e-beam or laser beam writing, avoids the overlay errors that can result from beam writing, such that overlay-induced anti-reflective layer losses are also avoided. In FIG. 6L, the photoresist layer  615  has been developed, such that the parts of the layer  615  that were exposed are removed. The resulting photoresist layer  615  corresponds to the deep trenches of the alternating PSM design. In FIG. 6M, dry etching has been performed through the quartz layer  602 . This completely forms the deep trenches  620  and  622  of the PSM  600  that were previously initially formed. This also completely forms the shallow trenches  632 ,  634 , and  636  of the PSM  600 . 
     Referring back to FIG. 5, the photoresist layer that was most recently applied is finally removed ( 526 ), such as via photoresist stripping. Performance of  526  is shown illustratively with reference to FIG.  6 N. In FIG. 6N, the photoresist layer  615  of FIG. 6M has been removed. The resulting PSM  600  in FIG. 6N has deep trenches  620  and  622 , and shallow trenches  632 ,  634 , and  636 . Thus, the PSM  600  is a dual-trench alternating PSM. 
     It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.