Abstract:
Disclosed is a phase-shifting mask having a pattern comprising a plurality of substantially transparent regions and a plurality of substantially opaque regions wherein the mask pattern phase-shifts at least a portion of incident radiation and wherein the phases are substantially equally spaced, thereby increasing resolution of a given lithographic system. Further disclosed is a semiconductor device fabricated utilizing the phase-shifting mask.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a Continuation Application of U.S. patent application Ser. No. 10/655,050, filed Sep. 4, 2003 now U.S. Pat. No 7,053,405. 

   TECHNICAL FIELD OF THE INVENTION 
   This invention relates to lithographic masks, such as that used for fabricating semiconductor devices. 
   BACKGROUND OF THE INVENTION 
   Lithography is utilized in semiconductor device manufacturing to pattern features on semiconductor workpiece layers for integrated circuit fabrication. 
     FIG. 1  shows a lithographic fabrication system  100  for defining features in a workpiece  120 , in accordance with prior art. Typically, workpiece  120  comprises a semiconductor substrate, together with one or more layers of substances (not shown) such as silicon dioxide and a resist layer  101 , affixed to a surface of workpiece  120 . 
   Typically, radiation of wavelength λ is emitted by an optical source  106 , such as a mercury lamp or a laser. The radiation propagates through an optical collimating lens or lens system  104 , a patterned lithographic mask  103  having a pattern of opaque and transparent features, and an optical projection lens or lens system  102 . The radiation transmitted through mask  103  is imaged by lens  102  onto resist layer  101 , thereby exposing a patterned area corresponding to the mask pattern. If resist layer  101  is positive, exposed areas will be subject to removal after development and if it is negative, exposed areas will remain intact. Thus, the pattern of mask  103  is transferred to (“printed on”) resist layer  101 . “Mask” as used herein means “mask” and/or “reticle”. 
   As known in the prior art, the indicated distances L 1  and L 2  satisfy, in cases of a simple lens  102 , 1/L 1 +1/L 2 =1/F, where F is the focal length of lens  102 . A pattern produced by mask  103  on resist layer  101  will be substantially in focus if resist layer  101  is a distance L 2  from projection lens  102 . This conclusion is based on a geometrical optics analysis which assumes light travels in straight lines. However, when the feature size is comparable to λ/NA, where λ is the illumination wavelength, and NA is the numerical aperture of the projection lens, a physical optics analysis should be considered which includes the wave nature of light. Under this analysis diffraction effects are likely to be produced, decreasing the image resolution even at distance L 2 , thereby reducing resolution of component features. For semiconductor devices it is desirable to maximize the number of circuit components per unit area by minimizing component size. As component size decreases, diffraction effects become more significant, thereby limiting reduction in component size. Decreased sharpness of mask images caused by diffraction effects may reduce product yield and increase device failure rate. 
   Diffraction effects may be severe for conventional or binary masks.  FIG. 2A  depicts a cross-sectional view of a prior art binary mask  10 . Binary mask  10  typically comprises a glass or quartz layer  12  with a patterned chromium layer  40  affixed thereto. The patterned chromium layer comprises a plurality of substantially transparent areas  14 ,  15  and  16  and a plurality of attenuating areas  18 ,  19 ,  20  and  21 . Electromagnetic radiation propagating through areas  14 ,  15  and  16  have electric fields associated therewith. Amplitudes of the electric fields at the mask level are represented with respect to a cross-section of the mask in  FIG. 2B , wherein steps  36 ,  37  and  38  correspond to electric fields from radiation propagating through apertures  14 ,  15  and  16 , respectively. Because of the wave-nature of the radiation it spreads as it propagates. Therefore, even though the electric fields are separated from one another at mask level they may interfere with one another a distance away from the mask, such as at a workpiece surface. This is shown in  FIG. 2C . Due to the diffraction effect, it is clear that the electric field at the workpiece surface spreads wider relative to that at the mask level. The smaller the feature sizes, as represented by transparent areas  14 ,  15 , and  16 , the wider the spread. 
   Solid lines  22  and  24  in  FIG. 2C  represent electric fields from apertures  14  and  16 , respectively, and broken Line  26  represents an electric field from aperture  15 . The amplitudes of the electric fields from adjacent openings ( 14  and  15 , for example) overlap in cross-hatched regions  30  and  32 . As shown in  FIG. 2D , this interference or constructive addition of electric field amplitudes results in an electric field  34  which has a higher intensity at the workpiece surface in regions  30  and  32 , relative to the surrounding areas than at mask level. Therefore, there is less contrast in the light intensity distribution at the workpiece surface than at mask level, thereby reducing the resolution capability of the tool. 
   Undesirable diffraction effects become more significant with small dimension pattern features. “Small dimension” as used herein means small size and small spacing between transparent regions relative to λ/NA, where λ is the wavelength of the optical source and NA is the numerical aperture of the projection system. 
   It is known in the art to improve the system resolution by employing phase-shifting masks. The mask imparts a phase-shift to the incident radiation, typically by π radians. Phase-shifting masks generally comprise transparent areas having an optical intensity transmission coefficient T, near 1.0 at the incident radiation wavelength λ, attenuating areas or partially transparent areas having T at λ in the range of about 0.05 to about 0.15, and, optionally, opaque areas, having T less than or equal to about 0.01. 
     FIG. 3A  depicts a cross-sectional view of a prior art π radian-phase-shifting mask  300 . Mask  300  is substantially similar to binary mask  10  but includes a phase-shifter layer  310  over transparent regions  14  and  16 . Phase-shifter layer  310  reverses the direction of the electric field vectors at apertures  14  and  16  relative to aperture  15  as shown in  FIG. 3B  at  320 ,  322  and  330 . The π radian phase-shift is created by employing a phase-shifter layer  310  with a thickness of d=λ/2(n−1) where λ is the wavelength of the optical source and n is the refractive index of layer  310  at λ. The phase-shifter layer modifies the optical distance traveled by incident radiation, thereby producing a phase-shift. As is shown in  FIG. 3C , by peaks  340 ,  345  and  350 , the overlapping regions of adjacent electric fields have opposite amplitudes, and therefore, a destructive interference occurs. The cancellation of the electric field at those locations improves the contrast of the intensity field as shown in  FIG. 3D .  FIG. 3E  depicts a vector diagram of the electric field at a workpiece level produced by radiation propagating through a n radian-phase-shifting mask. Vector  380  represents an electric field from unshifted radiation such as passes through aperture  15 . Vector  390  corresponds to phase-shifted radiation such as that which propagates through aperture  14  and phase-shifter  310 . The amplitude of vector  390  equals the negative of the amplitude of vector  380 , thereby canceling it out upon interference. 
   Phase-shifting masks producing π radian shifts are an improvement over binary masks. However, they do not fully resolve all resolution problems, for example a phase conflict may arise for feature configurations in which a phase transition is generally unavoidable. Whenever a phase transition occurs a dark line will result. 
   Electric field interference has been addressed by using a mask having a π/2 radian shift and a 3/2π radian shift. Liebmann et al, “Alternating Phase Shifted Mask for Logic Gate Levels, Design and Mask Manufacturing”, SPIE vol. 3679 p. 27 (1999). It is also known in the art to use π:⅔ π: ⅓ π:0 radian shifting masks. 
   It is therefore desirable to reduce phase conflict thereby substantially eliminating undesirable lines, and thus facilitating feature size reduction and improving product yield and reliability. 
   SUMMARY OF THE INVENTION 
   The invention relates to a phase-shifting mask having substantially equally spaced phases thereby substantially eliminating zeroth order and reducing first order diffraction frequencies. One embodiment of the invention relates to a three-phase-shifting mask having a pattern composed of substantially transparent regions and substantially opaque regions wherein the mask pattern phase-shifts incident radiation by 0, ⅔π and 4/3π radians. The invention further relates to a semiconductor device fabricated utilizing the phase-shifting mask. In such applications the invention facilitates reduction in component size and improved device reliability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a prior art lithographic system useful in the practice of the invention. 
       FIGS. 2A-2D  depict a prior art binary mask. 
       FIGS. 3A-3E  depict a prior art π radian phase-shifting mask. 
       FIGS. 4A-4E  depict a three-phase-shifting mask of the invention. 
       FIG. 5  depicts Fourier spectra of a three-phase-shift mask, a π radian phase-shift mask and a binary mask. 
       FIG. 6  depicts a mask pattern. 
       FIG. 7  depicts a semiconductor device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   It will be appreciated that the following description is intended to refer to specific embodiments of the invention selected for illustration and is not intended to define or limit the invention, other than in the appended claims. 
   The invention comprises a phase-shifting mask having substantially equally spaced phases such that the zeroth order diffraction frequency is substantially canceled and the first order diffraction frequency is reduced as compared to nonphase-shifting masks or masks having unequally spaced phases. Any number of equally spaced phases may provide substantially similar pattern transferring results and are within the spirit and scope of the invention. However, the phase-shifting mask preferably has three equally spaced phases to simplify manufacturing. Phase-shifting masks having-phase shifts of ⅓π radian multiples can be fabricated by layering readily available ⅓π radian phase-shifting components. 
     FIG. 4A  depicts a cross-sectional view of a three-phase-shifting mask  400 . Three-phase-shifting mask  400  has a plurality of substantially transparent areas  402 ,  404  and  406  and a plurality of substantially opaque areas  410 ,  412 ,  414  and  416 . Extending across apertures  402  and  404  are phase-shifters,  420  and  422 , respectively. Phase-shifter  420  produces a ⅔π radian shift and phase-shifter  422  produces a 4/3π radian shift.  FIG. 4B  shows the amplitudes of electric fields at mask level wherein the field areas from apertures  402  and  404  are represented by negative steps  430  and  432 , respectively, and the field area from aperture  406  is represented by positive step  434 . Since the electric fields are vectors by nature,  FIG. 4B  should be understood as a snapshot of the fields at a specific moment that will continually change with time.  FIG. 4C  represents electric fields  440 ,  442  and  444  at a workpiece from apertures  402 ,  404  and  406 , respectively. Unlike the binary mask, the electric fields at the overlap region are added destructively. Therefore, where images on the workpiece surface from apertures  402 ,  404  and  406  meet, the intensity is substantially zero as shown in  FIG. 4D  at  450  and  452 . 
   This phenomenon is further depicted in  FIG. 4E .  FIG. 4E  depicts electric field vectors corresponding to an electric field at workpiece level for mask  400 . It should be noted that the amplitudes of the electric fields are the projection of the vectors shown in the figure to the vertical axis. Vector  460  corresponds to an electric field produced by aperture  406  through which unshifted radiation is propagated. Vector  462  represents an electric field at workpiece level produced by radiation propagating through aperture  402  which is phase-shifted ⅔π radians by phase-shifter  420 . Vector  464  defines an electric field at workpiece level of radiation propagating through aperture  404  and 4/3π radian phase-shifter  422 . The amplitude of vectors  462  and  464  are substantially equal when vector  460  is at its maximum amplitude as shown in  FIG. 4E . The vector array rotates clockwise with time at a frequency determined by the frequency of the incident radiation. As vector  460  rotates, its amplitude will decrease. As the amplitude of vector  460  decreases, the amplitude of vector  462  will become more negative and the amplitude of vector  464  will become less negative. However, the sum of the amplitudes of vectors  460 ,  462  and  464  will remain generally equal to zero, thereby substantially eliminating light intensity at the location where the electric fields overlap. For masks having any number of substantially equally spaced phases, corresponding electric field vectors will generally sum to zero. 
   Advantageously, the frequency component of three-phase-shift mask  400  is lower than binary mask  10  or π radian phase-shifting mask  300 . This makes it possible for radiation to pass through the limited numerical aperture of the projection lens, and therefore achieve higher resolution with a given system. This phenomenon will also be present for masks with other numbers of equally spaced phases. 
     FIG. 5  depicts Fourier spectra of a binary mask, a it radian phase-shifting mask and an equally spaced three-phase-shifting mask. The zeroth order diffraction frequency is substantially eliminated and the first order diffraction frequency is reduced with the three-phase-shifting mask as compared to the binary and π radian phase-shifting masks. Binary mask spectrum  510  indicates a first order diffraction frequency centered at C. π radian phase-shifting mask spectrum  520  has a first order diffraction frequency centered at B indicating a lower frequency. Advantageously, three-phase-shifting mask spectrum  530  shows a center of its first order diffraction frequency to be at A indicating an even lower frequency than that of the π radian phase-shifting mask. Lower diffraction frequency corresponds to improved resolution. Therefore, resolution with a three-phase-shifting mask will be better than that with a binary or π radian phase-shifting mask, thereby facilitating formation of smaller features. Other equally spaced phase-shifting masks should produce results similar to those obtained from the three-phase-shifting mask. 
     FIG. 6  depicts one example of a mask pattern in which a phase conflict is likely to occur with a π radian shift.  FIG. 6  shows three opaque mask features  610 ,  620  and  630  surrounded by transparent areas  640 ,  650 ,  660 ,  670  and  680 . If a π phase-shifting mask is employed to shift radiation propagated through transparent areas  640  and  660  by π radians, and radiation propagated through areas  650  and  670  are left unshifted or at zero, the electric field interference produced by diffraction of radiation propagating through transparent areas  640 ,  650  and  660  will be minimized. However transparent area  680  has portions adjacent to transparent areas  640 ,  650  and  660  so that a phase transition is unavoidable between either  680  and  650  or between  680  and  640 / 660 . Where the phase transition occurs, an undesirable dark line will usually be produced. This phenomenon is referred to as “phase conflict”. 
   Advantageously, substantially equally spaced phase-shifting masks reduce phase conflict. For example, for the mask pattern depicted in  FIG. 6 , by introducing a third phase and having the phases equally spaced, features  650 ,  660  and  680  can have different phases from one another, thereby substantially eliminating phase conflict. Furthermore, transparent area  640  can have the same phase as transparent area  660  without producing a phase conflict. Because interference of the electric fields from the three features is substantially eliminated, unwanted dark lines will generally be eliminated. 
   The preferred mask thickness will depend on its application and on the mask material. For example, in a photolithographic process used in the fabrication of semiconductor devices the mask thickness is preferably in the range of about 0.22 cm to about 0.64 cm. It will be understood by those skilled in the art that any mask thickness will be suitable that allows the transmission of radiation sufficient to transfer the mask pattern to the workpiece and which has the structural integrity necessary to withstand the process in which it is used. 
   The preferred mask material will also depend on the application for which the mask is used. For example, masks typically comprise glass or quartz when used in photolithographic processes in the manufacture of semiconductor devices. Any material sufficient to withstand the particular lithographic process for which it is used and through which sufficient radiation may be transmitted to transfer the mask pattern to the workpiece may be utilized. Additional examples of mask materials include, but are not limited to, silicon dioxide fluorides, alkaline metals fluorides and alkaline earth fluorides. Calcium fluorides and magnesium fluorides are particularly well suited as mask materials. 
   In a lithographic process radiation is propagated through the mask and focused with a lens onto a workpiece coated with resist. If a negative resist is used, exposed areas will remain intact. If a positive resist is employed, exposed areas will be removed. By this process, the pattern of the mask will be transferred to the workpiece. Areas in which resist has been removed may then undergo additional processes, for example etching and plating, to form features on the workpiece in a desired pattern. 
   The invention further includes a semiconductor device which, when formed using a substantially equally spaced phase-shifting mask, should have better feature definition than that which is formed using a prior art mask, primarily due to improved resolution.  FIG. 7  depicts a schematic of a semiconductor device  200  that may be formed using a substantially equally spaced phase-shifting mask. Those skilled in the art will understand that it shows a simplified drawing of semiconductor device  200  for illustrative purposes only. An actual device may have layers of varying thicknesses and may contain other components. Semiconductor substrate  202  is covered by a first dielectric layer  204 . Above first dielectric layer  204  is a first metal layer  206 . Vias or interconnects  208 ,  210 ,  212  and  214  penetrate layer  204  and conductively connect first metal layer  206  to semiconductor substrate  202 . First metal layer  206  is covered by second dielectric layer  216  which contains vias  218 ,  220  and  222  to connect first metal layer  206  to a second metal layer  224 . This layering sequence may be repeated as necessary as shown in part by layers  226  and  228 , and interconnects  232 ,  234  and  236 . A top passivation layer  230  may be applied to protect device  200  from adverse electrical, chemical or other conditions, and to provide electrical stability. 
   Semiconductor substrate  202  may comprise silicon, for example. Common dielectrics include, but are not limited to, silicon oxides, such as boron phosphorous doped silicate glass (BPSG), those originating from tetraethylorthosilicate (TEOS) and silicon dioxide (SiO 2 ). Common metals include, for example, aluminum, copper and tungsten. In addition, to improve adherence between metal and dielectric layers, thin layers may be introduced between them. Titanium is commonly used for this purpose. Electronic circuitry is defined in the layers by a lithographic technique. 
   In the lithographic process used to form the circuitry in device  200  a resist is deposited over a dielectric layer. The resist is exposed by transmitting radiation through the substantially equally spaced phase-shifting mask onto the dielectric layer surface, thereby defining desired circuitry and substantially eliminating phase conflict. The form of radiation used is dependent on the type of resist and other fabrication parameters. Any form of radiation that may expose the resist without adverse effects to the workpiece may be used. Common examples include, ultraviolet radiation, electron beam radiation and x-rays. If a positive resist is used, the exposed areas will be removed revealing the dielectric layer below. The dielectric layer may then be removed, for example by etching. Any technique that will remove the exposed dielectric layer while leaving the resist covered portions intact may also be used. Negative resists may be used wherein the exposed resist areas are left intact after exposure and the nonexposed areas are removed. For negative resist processes a mask is used that defines the spaces between circuit components rather than the circuitry itself. Lithographic processes using the substantially equally spaced phase shifting mask may also be employed to form other device features, for example interconnects in the dielectric layers. 
   The phase-shifting mask described herein is not limited in use to semiconductor device fabrication and may, within the spirit and scope of the invention, be used for any lithographic process in which it would facilitate transfer of a pattern to a workpiece.