Abstract:
A mask combining an alternating phase shift part and an attenuating phase shift part on a single blank and a method of forming said mask. The method involves fewer processing steps, fewer layers of material and is more cost effective than other methods in the current art. A central reason for the simplicity of the method is the use of different intensity levels of E-beam exposure in a single resist layer and achieving phase shifts by transmitting radiation through alternating regions of the same transparent substrate that are etched and not etched.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is a mask and a process for fabricating that mask. More specifically, it is a mask that combines the properties of an attenuated phase shifting mask (Att PSM) and an alternating phase shifting mask (Alt PSM). The method of fabrication achieves simplicity and cost-effectiveness by combining different intensities of electron-beam developing in a single resist medium. 
     2. Description of the Related Art 
     As structural dimensions in microelectronic circuitry become increasingly small, lithographic processes that image the circuit design on semiconductor substrates are called upon to resolve details of 0.35 microns and less. The use of optical lithography in this resolution regime is severely limited by diffraction effects caused by the small openings in the masks which are used to transfer circuit patterns by optical imaging. Diffraction of the transmitted light blurs the edges of small isolated structures and severely limits the ability to resolve spacings between densely packed and closely neighboring structures. Two new technologies have emerged that significantly improve the resolution of the masks used in optical lithography: alternating phase shifting (Alt PSM) and attenuated phase shifting (Att PSM). The concept of phase shifting is central to both technologies. Phase shifting is the translation of an electromagnetic sinusoid by a given angle (the phase angle) relative to a reference wave. When a shifted and un-shifted wave combine on a common image plane, the resulting superposition can produce constructive or destructive interference in the fields depending on the size of the phase shift and concurrent increases or decreases in the field intensities. If the un-shifted wave produces a diffraction pattern by passing through a small aperture in a mask, the unwanted diffraction lobes on either side of the central maximum can be significantly reduced by also passing the wave through an adjacent aperture that is filled with a transparent medium fabricated so as to shift the phase by 180 degrees (π radians). A series of adjacent apertures, with alternate apertures filled with a phase shifting medium, forms what is called an alternating phase shifting mask (Alt PSM). Such masks are discussed in C. Y. Chang and S. M. Sze, “ULSI Technology,” McGraw-Hill Co., N.Y. 1996, pp 284-288. The construction of a mask by combining alternating phase shifted and non-phase shifted structures for bi-directional circuit patterns is discussed by Lin in U.S. Pat. No. 5,472,814, 1995. The application of combined alternating phase shift masks to the reproduction of complex circuit patterns is also discussed by Lin et al in U.S. Pat. No. 5,523,186, 1996. 
     A second technology utilizing the properties of combined shifted and un-shifted electromagnetic radiation, is the attenuated phase shift mask (Att PSM). This design surrounds an isolated opening in a mask with a material such as molybdinum silicide oxynitride (MoSiON) that both phase shifts and attenuates the transmitted radiation on either side of the opening, only where the unwanted diffraction lobes of the non-phase shifted radiation appear. This combination does not affect the central maximum that delineates the opening, but it significantly reduces the intensity of the diffraction lobes on either side. This has the effect of sharpening the image of the opening. The attenuated phase shift mask is discussed by Tzu et al, U.S. Pat. No. 5,783,337, 1998. 
     The Alt PSM technology is effective for improving the resolution of repetitive, closely packed structures, but it provides no benefits when applied to isolated structures. Conversely, the Att PSM technology improves the resolution of small, isolated structures, but offers no advantages where closely packed structures are involved. 
     A way of achieving the improved resolution benefits of phase shifting in its various forms where the circuit design to be imaged has both densely packed and isolated structures, is to combine the alternating and attenuated phase shifting technologies on the same mask. This is the approach taken by Lin et al in U.S. Pat. No. 5,565,286, 1996. Although this combination of technologies is an excellent way to produce high resolution images of complex circuit designs having a variety of structural forms, the fabrication of such a mask involves a large number of separate processing steps as well as a multiplicity of materials, both leading to increased production time and cost. The method of Lin et al (U.S. Pat. No. 5,565,286) cited above, for example, employs separate and multiple layers of different material to act as phase shifters and attenuators of different transmissivities, so that the completed fabrication has as many as five layers of active material. A more cost-effective fabrication process for a combined Alt PSM and Att PSM on the same mask would be of great benefit and is the subject of the invention described herein. 
     SUMMARY OF THE INVENTION 
     This invention is a mask combining both alternating and attenuating phase shift elements and a simplified, cost-effective method for fabricating said mask. Such a mask will improve the resolution of both densely packed and isolated structures in complex circuit designs. 
     A first object of the present invention is to provide a method for forming a mask having both alternating and attenuating phase shifting structures on the same mask substrate. 
     A second object of the present invention is to provide a method for forming such a mask that minimizes the number of processing steps and the number of material layers. 
     A third object of the present invention is to provide such a mask that combines both alternating and attenuating phase shifting structures and will improve the resolution of both densely packed and isolated structures in complex circuit designs. 
     These objects are achieved by providing a mask blank comprised of a transparent substrate over which are successively deposited an attenuating absorbing layer, an opaque layer and a resist layer that can be developed following electron beam (E-beam) exposure. The resist layer is exposed to two different depositions of E-beam energy, the higher one for isolated and densely packed circuit structures, the lower one for the rim of attenuated patterns. After development and baking, different resist step heights have been created. The main pattern is then transferred to the opaque layer and attenuating layer by dry etching. An oxygen plasma having high selectivity and anisotropy is then used to dry etch the different resist thicknesses to expose the attenuator patterns. The opaque layer that still exists above the attenuator pattern is then removed by a dry etch. Stripping the remaining resist layer completes the processes and leaves the completed combination of alternating and attenuating phase shift structures. A key element of the simplicity of this design is the use of the optically transparent substrate to provide the necessary phase shifting by means of etched regions alternating with unetched regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a view from above of a completed mask combining an alternating phase shift structure of five parallel rectangular openings and an attenuating phase shift structure consisting of a single, isolated rectangular opening. 
     FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11, FIG.  12  and FIG. 13, are side views depicting the processes by which the mask of FIG. 1 is fabricated, according to a first preferred embodiment of this invention. FIG. 13 is the side view of the completed mask, as depicted from above in FIG.  1 . 
     FIG. 2 shows the mask blank, consisting of a transparent substrate over which there are successively applied absorbing, opaque and resist layers. 
     FIG. 3 shows the resist after two depositions of E-beam energy. 
     FIG. 4 shows the voids created in the resist after developing and baking. 
     FIG. 5 shows the mask after dry etching through the attenuating and opaque layers. 
     FIG. 6 shows the mask after the resist layer has been reduced by the oxygen plasma. 
     FIG. 7 shows the mask after removal of the opaque layer by a dry etch. The dry etch leaves the attenuating layer in place. 
     FIG. 8 shows the mask after removal of the remaining resist layer. 
     FIG. 9 shows the mask covered with a second resist layer. 
     FIG. 10 shows the second resist layer after exposure to E-beam energy over the position of the alternate slits. 
     FIG. 11 shows the removal of the developed resist layer. 
     FIG. 12 shows the result of dry etching the transparent substrate. 
     FIG. 13 shows the mask with the resist stripped away. The original thickness of the transparent substrate relative to the more deeply etched regions provides the necessary 180 degree phase shifting in both parts of the mask. 
     FIG. 14, FIG. 15, FIG.  16  and FIG. 17 are side views depicting the processes by which the mask of FIG. 1 is fabricated, according to a second preferred embodiment of the present invention. 
     FIG. 14 shows the prepared blank of FIG. 2 after exposure of the resist layer to three intensities of E-beam energy. 
     FIG. 15 shows the mask after developing the resist and etching of the opaque layer and attenuating layer. 
     FIG. 16 shows the mask after dry etching of the transparent substrate. 
     FIG. 17 shows the mask after etching by the oxygen plasma. 
     FIG. 18 shows the mask after dry etching of the opaque and attenuating layers. 
     FIG. 19 shows the mask after reducing the resist layer with an oxygen plasma etch. 
     FIG. 13 is the final configuration of the mask after etching the remaining exposed opaque layer and removing the remaining resist layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a mask combining an alternating phase shift portion and an attenuating phase shift portion and a method for forming that mask. A first preferred embodiment will be described by referring to a series of figures labeled FIG.  1  through FIG.  13 . Referring now to FIG. 1, there is shown an overhead view of a simple example of such a mask, combining a single set of five parallel, closely spaced rectangular openings forming the alternating phase shift portion of the mask, together with a single isolated rectangular opening, forming the attenuating phase shift portion of the mask. The three rectangles ( 14 ) are phase shifted by 180 degrees relative to the two rectangles ( 10 ). The nearby attenuating structure has a transparent central portion ( 10 ) surrounded at its rim by an attenuating and phase shifting annulus ( 16 ). All other portions of the mask are optically opaque ( 12 ). 
     FIG. 2 shows a side schematic view of a mask blank, which serves as the starting configuration for the fabrication of the mask. The blank consists of a transparent substrate ( 10 ), which may be quartz, which will ultimately serve as both a phase shifting medium and a non-phase shifting medium. A layer of attenuating material ( 16 ) has been deposited on the substrate. The attenuating material may be the optically attenuating material MoSiON deposited to a thickness of between about 800 angstroms and 1100 angstroms. 
     A layer of opaque material ( 12 ), which may be chromium, is deposited on the layer of attenuating material to a depth of between approximately 700 angstroms and 1000 angstroms. A layer of E-beam sensitive resist ( 18 ) is deposited on the opaque layer, to a thickness of between 5000 angstroms and 7000 angstroms. 
     FIG. 3 shows the mask blank after two depositions of 50 kev E-beam energy to the resist layer. The more deeply penetrating deposition ( 20 ), of approximately 27 μC/cm 2 , penetrates to the opaque layer ( 12 ). The less deeply penetrating deposition, of approximately 7 μC/cm 2  forms a shallow region ( 22 ). 
     FIG. 4 shows the exposed areas of the resist having been removed ( 20 ), ( 22 ) after developing and baking. 
     FIG. 5 shows the condition of the mask after being subjected to a dry etch of the exposed opaque layer ( 12 ). This etch also removes the attenuating layer ( 16 ) immediately below the opaque layer, leaving rectangular openings ( 26 ) that terminate at the surface of the transparent substrate ( 10 ). 
     FIG. 6 shows the results of applying a highly selective and anisotropic oxygen plasma to the surface of the resist in its condition as indicated in FIG.  5 . The plasma selectively etches down the shallow rim region ( 28 ) to expose the opaque layer ( 12 ) beneath it. 
     FIG. 7 shows the results of etching away the opaque layer ( 12 ), exposed by the plasma etch of the previous figure. The resulting opening ( 30 ), now has an exposed layer of attenuating material ( 16 ) surrounding its rim. 
     FIG. 8 shows the mask after the remaining resist layer ( 18 ) is removed. 
     FIG. 9 shows a second resist layer now having been deposited on the mask just illustrated in FIG.  8 . 
     FIG. 10 shows this resist layer exposed by a 50 kev E-beam at a deposition intensity of 27 μC/cm 2 . The exposed regions ( 34 ) will ultimately form the non-phase shifting portions of the transparent substrate ( 10 ). 
     FIG. 11 illustrates the result of developing and baking the E-beam exposed regions of the resist layer to expose the surface of the transparent substrate below ( 36 ). 
     FIG. 12 shows the results of subjecting the three exposed regions of the transparent substrate to a dry etch to a depth of approximately 2700 angstroms below the surface of the substrate ( 38 ). Light passing through the optically transparent substrate at its original thickness will acquire a phase shift of 180 degrees relative to the light that passes through the thinner, etched regions of the substrate. The depth of etch is determined by the difference in optical pathlengths required to produce a 180 degree phase shift in the transmitted light. 
     FIG. 13 shows the finished mask, the remaining resist layer having been removed. The side view of the mask in FIG. 13 corresponds exactly to the top view of FIG.  1 . 
     A second preferred embodiment of the present invention will be explained by reference to FIG. 1, FIG. 2, FIG.  14  through FIG.  17  and FIG.  13 . FIG. 1, again, shows an overhead view of the mask to be fabricated and FIG. 13, shows the side view of the same mask, now having been fabricated according to the processes depicted in FIG.  14  through FIG. 17, according to a second preferred embodiment. FIG. 2 shows the same mask blank used in the first preferred embodiment, now being used as the starting point for the second preferred embodiment. 
     Referring now to FIG. 14, we see the results of exposing the resist layer ( 12 ) to three different exposure depositions of 50 kev E-beam energy. The highest amount, approximately 27 μC/cm 2 , produces the three deepest exposure depths ( 20 ). The intermediate E-beam deposition amount, approximately 10 μC/cm 2 , produces three intermediate depth exposures ( 22 ). The lowest E-beam deposition amount, approximately 7 μC/cm 2 , is used to create the rim ( 24 ). 
     FIG. 15 shows the openings ( 26 ), ( 28 ), ( 30 ) produced by developing and baking the exposed resist layer and the result of etching away the opaque layer ( 12 ) exposed by the deepest of those openings ( 28 ). 
     FIG. 16 shows the results of a dry etch of the transparent substrate ( 10 ) beneath the openings ( 28 ) to a depth of 2700 angstroms, that will ultimately define the regions transmitting light that is not phase shifted. 
     FIG. 17 shows the results of applying a selective, anisotropic oxygen plasma etch to the resist layer ( 18 ). The intermediate depth openings ( 34 ) are etched back to expose the opaque layer. 
     FIG. 18 shows the results of a dry etch that removes the layers of opaque material ( 12 ) and the attenuating material ( 16 ) previously exposed in openings ( 36 ). 
     FIG. 19 shows the result of a final etch by the selective, anisotropic oxygen plasma, etching back the resist layer in the rim region ( 38 ), to expose the opaque layer beneath it. A final wet etch of the exposed opaque layer, leaving now the attenuating layer exposed, followed by a removal of the remaining resist, produces the final mask configuration of FIG.  13 . 
     As is understood by a person skilled in the art, the preferred embodiment and examples of the present invention are illustrative of the present invention rather than limiting of it. Revisions and modifications may be made to processes, structures and dimensions through which is fabricated, using a cost effective method, a combined attenuated-alternating phase shift mask in accord with the preferred embodiment and examples of the present invention while still providing such an attenuated-alternating phase shift mask in accord with the present invention and appended claims.