Abstract:
A photoresist mask used in the fabrication of integrated circuits, can include a first portion and a second portion. The first portion has a phase shifting material layer and an opaque layer deposed over a transparent layer. The first portion also has trenches in the transparent layer selectively located to provide an alternating phase shifting characteristic. The second portion has the opaque layer deposed over the phase shifting material layer which is deposed over the transparent layer.

Description:
FIELD OF THE INVENTION 
     The present specification relates generally to the field of integrated circuits and to methods of manufacturing integrated circuits. More particularly, the present specification relates to a photoresist mask that combines attenuated and alternating phase shifting masks. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to fabricate millions of devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC. 
     One limitation to the smallness of IC critical dimensions is conventional lithography. In general, projection lithography refers to processes for transferring patterns between various media. According to conventional projection lithography, a silicon slice, the wafer, is coated uniformly with a lithographic coating. The lithographic coating is a radiation-sensitive film or coating (e.g., the photoresist). 
     An exposing source of radiation (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern. 
     Exposure of the coating through a photomask or reticle causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. 
     Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. 
     One alternative to projection lithography is EUV lithography. EUV lithography reduces.feature size of circuit elements by lithographically imaging them with radiation of a shorter wavelength. “Long” or “soft” x-rays (a.k.a, extreme ultraviolet (EUV)), wavelength range of lambda=50 to 700 angstroms are used in an effort to achieve smaller desired feature sizes. 
     In EUV lithography, EUV radiation can be projected onto a resonant-reflective reticle. The resonant-reflective reticle reflects a substantial portion of the EUV radiation which carries an IC pattern formed on the reticle to an all resonant-reflective imaging system (e.g., series of high precision mirrors). A demagnified image of the reticle pattern is projected onto a resist coated wafer. The entire reticle pattern is exposed onto the wafer by synchronously scanning the mask and the wafer (i.e., a step-and-scan exposure). 
     Although EUV lithography provides substantial advantages with respect to achieving high resolution patterning, errors may still result from the EUV lithography process. For instance, the reflective reticle employed in the EUV lithographic process is not completely reflective and consequently will absorb some of the EUV radiation. The absorbed EUV radiation results in heating of the reticle. As the reticle increases in temperature, mechanical distortion of the reticle may result due to thermal expansion of the reticle. 
     Both conventional projection and EUV lithographic processes are limited in their ability to print small features, such as, contacts, trenches, polysilicon lines or gate structures. As such, the critical dimensions of IC device features, and, thus, IC devices, are limited in how small they can be. 
     The ability to reduce the size of structures, such as, shorter IC gate lengths depends, in part, on the wavelength of light used to expose the photoresist. In conventional fabrication processes, optical devices expose the photoresist using light having a wavelength of 248 nm (nanometers), but conventional processes have also used the 193 nm wavelength. Further, next generation lithographic technologies may progress toward a radiation having a wavelength of 157 nm and even shorter wavelengths, such as those used in EUV lithography (e.g., 13 nm). 
     Phase shifting mask technology has been used to improve the resolution and depth of focus of the photolithographic process. Phase shifting mask technology refers to the use of a photolithographic mask which selectively alters the phase of the light passing through certain areas or apertures of the mask to take advantage of destructive interference to improve resolution and depth of focus. The aperture can include a transparent substrate coated by an opaque material, such as, chrome. For example, in a simple case, each aperture in the phase shifting mask transmits light 180 degrees out of phase from light passing through adjacent apertures. This 180 degree phase difference causes any light overlapping from two adjacent apertures to interfere destructively, thereby reducing any exposure in the center. 
     An exemplary phase shifting mask  10  is illustrated in FIG.  1 . Phase shifting mask  10  includes a transparent layer  12  and an opaque layer  14 . Opaque layer  14  provides a printed circuit pattern to selectively block the transmission of light from transparent layer  12  to a layer of resist on a semiconductor wafer. Transparent layer  12  includes trenches  16  which are etched a predetermined depth into transparent layer  12 . The light transmitted through transparent layer  12  at trenches  16  is phase-shifted 180 degrees from the transmission of light through other portions of phase shifting mask, such as portions  18 . As the light travels between phase shifting mask  10  and the resist layer of a semiconductor wafer below (not shown), the light scattered from phase shifting mask  10  at trenches  16  interferes constructively with the light transmitted through phase shifting mask  10  at portions  18 , to provide improved resolution and depth of focus. 
     As mentioned, various different wavelengths of light are used in different photolithographic processes. The optimal wavelength of light is based on many factors, such as the composition of the resist, the desired critical dimension (CD) of the integrated circuit, etc. Often, the optimal wavelength of light must be determined by performing a lithography test with photolithographic equipment having different wavelengths. When a phase shifting mask technique is utilized, two different phase shifting masks must be fabricated, each mask having trenches  16  suitable for phase shifting light of the desired wavelength. The fabrication of phase shifting masks is costly. Further, comparison of the effect of the two different wavelengths printing processes is difficult and requires complex software processing to provide a suitable display. 
     Thus, there is a need for an improved phase shifting mask and method of testing photolithographic equipment. Further, there is a need for reducing or eliminating the cost of fabricating multiple phase shifting masks for multiple wavelengths of light. Further still, there is a need for a photoresist mask that combines attenuated and alternating phase shifting masks. 
     SUMMARY OF THE INVENTION 
     An exemplary embodiment relates to a photoresist mask used in the fabrication of integrated circuits. This photoresist mask can include a first portion and a second portion. The first portion has a phase shifting material layer and an opaque layer deposed over a transparent layer, where the first portion has trenches in the transparent layer selectively located to provide an alternating phase shifting characteristic. The second portion has the opaque layer deposed over the phase shifting material layer which is deposed over the transparent layer. 
     Another exemplary embodiment relates to a photolithographic mask which selectively alters the phase of light passing through certain areas of the mask to improve feature resolution and depth of focus in the lithographic process. This mask can include a transparent layer, a first opaque layer deposed over the transparent layer, and a second opaque layer deposed over portions of the first opaque layer. A first portion of the photolithographic mask is defined by an area including apertures in the first and second opaque layers and trenches in the transparent layer beneath every other aperture in the first and second opaque layers. The first portion has an alternating phase shifting characteristic. A second portion of the photolithographic mask is defined by an area including at least one aperture in the first and second opaque layers. The second portion has an attenuating phase shifting characteristic. 
     Another exemplary embodiment relates to a test photolithographic mask having both alternating phase shifting and attenuating phase shifting portions. This test photolithographic mask can include a first section of a transparent layer and a first opaque layer, where the first section is configured to provide alternating phase shifting properties and a second section of the transparent layer, the first opaque layer and a second opaque layer, where the second section is configured to provide attenuating phase shifting properties. 
    
    
     Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiments will hereafter be described with reference to the accompanying drawings, wherein like numerals denote like elements, and: 
     FIG. 1 is an exemplary conventional phase shifting mask; 
     FIG. 2 is a top-down view of a portion of an exemplary layout design of a phase shifting mask in accordance with an exemplary embodiment; 
     FIG. 3 is a cross-sectional view of the portion of the phase shifting mask of FIG. 2 at line  3 — 3 ; 
     FIG. 4 is a cross-sectional view of a portion of a phase shifting mask showing a deposition step; 
     FIG. 5 is a cross-sectional view of a portion of a phase shifting mask showing a step of patterning an opaque layer; 
     FIG. 6 is a cross-sectional view of a portion of a phase shifting mask showing a step of patterning a phase shifting material layer; 
     FIG. 7 is a cross-sectional view of a portion of a phase shifting mask showing a deposition step; 
     FIG. 8 is a cross-sectional view of a portion of a phase shifting mask showing a step of dense line patterning; 
     FIG. 9 is a cross-sectional view of a portion of a phase shifting mask showing a resist deposition step; 
     FIG. 10 is a cross-sectional view of a portion of a phase shifting mask showing a patterning step; and 
     FIG. 11 is a cross-sectional view of a portion of a phase shifting mask showing a trench formation step. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In integrated circuit lithography processes, phase shifting masks can be used to delay or shift the phase of light. This phase shifting can be accomplished by etching regions of quartz on a mask to a precise depth, depending on the wavelength of the light to be used to expose the wafer. Such phase shifting masks are sometimes referred to as alternating phase shifting masks because they use alternating adjacent apertures in the quartz. Another type of phase shifting mask is sometimes referred to as attenuating phase shifting masks. An attenuating phase shifting mask uses an opaque material in the phase shifting mask, such as, molybdenum silicon (MoSi), instead of etched trenches or apertures used in an alternating phase shift mask. The opaque material in an attenuating phase shift mask allows only a small percentage of light to pass through. As a result of the light passing through the opaque material, the phase of the light can be changed by 180 degrees. 
     Referring now to FIG. 2, a mask portion  20  can include opaque portions  22 , a transparent portion  24 , and trenches  26 . Opaque portions  22  can be a patterned layer of an opaque material, such as, chrome. Patterns of opaque portions  22  can be configured to form a dense line region  27  and an isolated line region  29 . Transparent portion  24  can be a layer of transparent material, such as, quartz which is located beneath opaque portions  22 . Trenches  26  are trenches located within transparent portion  24 . Trenches  26  are formed by etching transparent portion  24 . 
     In an exemplary embodiment, trenches  26  are configured to have a depth which provides a 180 degree phase shifting effect. Generally, the depth of trenches  26  depends on the wavelength of light being used. As discussed, phase shifting masks can be utilized to improve mask resolution and depth of focus by phase shifting light at certain portions such that the light waves passing through the mask interfere constructively instead of destructively with proximate or adjacent transmitted light. 
     FIG. 3 illustrates a mask portion  30  which includes an opaque layer  32 , a transparent layer  34 , and trenches  36 . Mask portion  30  illustrates mask portion  20  viewed in cross-section about line  3 — 3 . In an exemplary embodiment, opaque layer  32  is chrome (Cr) and transparent layer  34  is quartz (Qz). In operation, mask portion  30  allows light to pass through portions of transparent layer  34  not covered by opaque layer  32 . Light passing through transparent layer  34  at trenches  36  is shifted in phase by 180 degrees compared to light passing through transparent layer  34  at portions  38 . Advantageously, mask portion  30  can include a dense line region  37  and an isolated line region  39 . In an exemplary embodiment, dense line region  37  is an alternating phase shifting mask area and isolated line region  39  is an attenuating phase shifting area. 
     FIGS. 4-11 describe exemplary steps in a method of forming a mask portion having an alternating phase shifting mask area portion and attenuating phase shifting mask area portion. Advantageously, an alternating phase shifting mask portion can be very effective in improving the depth of focus and resolution limit for dense line areas whereas attenuating phase shifting mask area portions can be very effective in improving the depth of focus and resolution limit for isolated space or contact patterns. 
     Referring to FIG. 4, a mask portion  40  can include a transparent layer  42 , a phase shifting mask material layer  44 , and an opaque layer  46 . In an exemplary embodiment, transparent layer  42  can include a transparent material, such as, quartz and can have a thickness of 0.25 inches. In an exemplary embodiment, phase shifting mask material  44  is deposited over transparent layer  42 . Phase shifting mask material layer  44  can include molybdenum silicon (MoSi) and can have a thickness which is approximately equal to: 
     (wavelength of the lithographic stepper)/2 (n−1), where n is the refraction index of the phase shifting material. In an exemplary embodiment, opaque layer  46  can be chrome and can have a thickness of between 600 and 1000 Angstroms. 
     FIG. 5 illustrates a mask portion  50  which can include a transparent layer  52 , a phase shifting material layer  54 , an opaque layer  56 , and a resist layer  58 . Mask portion  50  can represent mask portion  40  described with reference to FIG. 4 after a patterning step. In an exemplary embodiment, resist layer  58  can include a photoresist or an e-beam resist material which is coated over opaque layer  56  and patterned to form an aperture  59 . A portion of opaque layer  56  is then etched according to aperture  59 . 
     FIG. 6 illustrates a mask portion  60  which can include a transparent layer  62 , a phase shifting material layer  64 , an opaque layer  66 , and a resist layer  68 . Mask portion  60  can represent mask portion  50  described with reference to FIG. 5 after a patterning step. In an exemplary embodiment, resist layer  68  is coated or deposited over opaque layer  66  and a portion of phase shifting material layer  64 , which is exposed by an aperture in opaque layer  66 . Resist layer  68  is patterned to etch an aperture  69  in phase shifting material layer  64 . Aperture  69  will be used in an isolated line feature of an attenuating phase shifting area, as described below. 
     FIG. 7 illustrates a mask portion  70  which can include a transparent layer  72 , a phase shifting material layer  74 , an opaque layer  76 , and a resist layer  78 . Mask portion  70  can represent mask portion  60  described with reference to FIG. 6 after a deposition step. In an exemplary embodiment, resist layer  78  is deposited over opaque layer  76 , exposed portions of phase shifting material layer  74 , and exposed portions of transparent layer  72 . In an exemplary embodiment, resist layer  78  is used in the patterning of densely spaced features in a portion  77  of mask portion  70 . A portion  79  of mask portion  70  can include an isolated feature formed in the patterning steps described with reference to FIGS. 4-6. 
     Referring now to FIG. 8, a mask portion  80  can include a transparent layer  82 , a phase shifting mask layer  84 , an opaque layer  86 , and a resist layer  88 . Mask portion  80  can represent mask portion  70  described with reference to FIG. 7 after a step of dense line patterning. In an exemplary embodiment, trenches or apertures  83  can be formed in phase shifting material layer  84  and opaque layer  86 . Trenches  83  are located in a portion  87  of mask portion  80 . Portion  87  corresponds to an alternating phase shifting mask portion of mask portion  80 . 
     Referring now to FIG. 9, a mask portion  90  can include a transparent layer  92 , a phase shifting material layer  94 , an opaque layer  96 , and a resist layer  98 . Mask portion  90  can represent mask portion  80  described with reference to FIG. 8 after a resist deposition step. In an exemplary embodiment, resist layer  98  can be coated over mask portion  90  to form trenches in transparent layer  92  to have an alternating phase shifting mask effect. Referring now to FIG. 10, resist layer  98  is patterned in a portion  97  of mask portion  90  to form trenches  93 , which are used in the formation of trenches  95  (FIG. 11) in transparent layer  92 . 
     As illustrated in FIG. 11, trenches  95  are etched to a depth that is approximately equal to: 
     (the wavelength of lithographic stepper)/2 (n−1), where n is the refraction index of the phase shifting material. In an exemplary embodiment, portion  97  of mask portion  90  can include alternating phase shifting mask characteristics and a portion  99  of mask portion  90  can include attenuated phase shifting mask characteristics. In an exemplary embodiment, the transmittance of the attenuated phase shifting mask characteristic of portion  99  can be 5-30 percent. 
     Advantageously, mask portion  90  can help a lithography engineer decide which kind of phase shifting mask to choose in the integrated circuit (IC) fabrication design process. A lithography engineer can run experiments for different patterns using mask portion  90  and collect engineering data before ordering a mask for fabrication. One advantage of mask portion  90  is that it has both attenuating and alternating phase shifting mask characteristics, allowing engineers to forego the time and expense of running two different tests to get design results. 
     Alternating portion  97  and attenuating portion  99  of mask portion  90  divide the mask in half. That is, alternating portion  97  comprises one half of mask portion  90  and attenuating portion  99  comprises the other half of mask portion  90 . Alternating portion  97  and attenuating portion  99  each provide a large image associated with an image to be projected. The image to be projected can represent an entire integrated circuit wafer or an integrated circuit chip in a direct lithographic tool. As such, mask portion  90  provides both attenuation and alternating phase shifting qualities. Each portion of mask portion  90  provides an identical image to the other portion so that qualities of each portion can be compared with each other. For example, using mask portion  90 , a lithography engineer can decide weather an image is best projected using an attenuated phase shifting mask or an alternating phase shifting mask by using mask portion  90  to see the characteristics and attributes of the image. In an alternative embodiment, mask portion  90  is utilized where areas in an integrated circuit design include densely located features and isolated located features. Densely located features would be patterned using portion  97  and isolated located features would be patterned using portion  99 . 
     While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. Other embodiments may include, for example, different arrangements of alternating and attenuating portions of photoresist phase shifting masks. The invention is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations that nevertheless fall within the scope and spirit of the appended claims.