Abstract:
A mask comprises a mask substrate and at least one annular equal line space phase shifting pattern on said mask substrate to produce an opaque region on a semiconductor substrate. A method of manufacturing a mask comprises providing a mask substrate; forming a layer of resist material on said substrate; patterning at least one annular equal line space phase shifting pattern on said resist layer; patterning said pattern onto said mask substrate; removing a remaining portion of said resist layer. A method of transferring a pattern onto a semiconductor substrate comprises illuminating a mask comprising at least one annular equal line space phase shifting pattern on the mask to produce an opaque region on a semiconductor substrate.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to integrated circuit fabrication and more particularly to a phase shifting mask used in a photolithography process and a method of manufacturing therefor.  
       DESCRIPTION OF RELATED ART  
       [0002]     In the semiconductor industry, there is a continuing effort to increase device density by scaling the device size. Conventionally, to form an integrated circuit, a resist layer is formed on a wafer and is exposed to radiation through a photomask (“mask”). A mask typically comprises a substantially transparent base material such as quartz with an opaque layer having a desired pattern formed thereon. For example, chrome has long been used to make the opaque layer. When device features are reduced to a dimension below 1 micron, diffraction effects become significant. The blending of two diffraction patterns associated with features which are close to each other has an adverse effect on resolution, because portions of the resist layer underlying the opaque layer near the edges of features will be exposed.  
         [0003]     To minimize effects of diffraction, various kind of phase shifting masks have been used. Typically, a phase shifting mask has a pattern in the opaque layer, corresponding to the pattern to be formed on the underlying resist. In addition, phase-shifters, which transmit the incident radiation and shift the phase of the radiation approximately 180 degrees, are added onto the mask reduce diffraction effects. Alternate aperture phase shifting masks are formed by adding phase-shifters over every other opening. In rim phase shifting masks, phase-shifters are added along or near the outer edges of features. The radiation transmitted through the phase-shifter destructively interferes with radiation transmitted through the feature, thereby reducing the intensity of radiation incident on the resist material underlying the opaque layer near a feature edge to in order to improve image resolution.  
         [0004]     Such phase shifting masks, however, have limitations on their ability to pattern some features and are difficult to fabricate. When two features are placed in close proximity to one another, for example, in rim phase shifting masks, two phase-shifters associated with features which are close to each other would roughly merge into a wide rim resulting in over exposure of the region of resist material between two openings. Further, phase-shifters may be fabricated by a separate step from the formation of the pattern on the opaque layer. To improve resolution by destructive interference, the locations of the phase-shifters must be precisely correlated with the pattern on the opaque layer. For very small features, the alignment tolerance between the opaque layer with pattern and phase-shifters may exceed the capability of the process.  
         [0005]     To resolve these problems, an attenuated phase-shifting mask (“AttPSM”) has been proposed. The AttPSM replaces the opaque layer (which is typically a layer of chrome about 0.1μ thick) with a “leaky” layer which transmits a reduced percentage of the incident radiation. For example, a very thin layer of chrome (approximately 300 angstroms) with approximately 10% transmittance could be used as the leaky layer. In addition, the leaky chrome layer shifts the phase of the transmitted radiation by a certain number of degrees, for example approximately 30 degrees, depending on the thickness and refractive index of the layer. To achieve the required 180 degrees phase shift between radiation transmitted through regions covered by the leaky chrome layer and regions of features, the features are also phase shifted a complementary angle by etching the mask or by placing a phase-shifting material in the regions of features.  
         [0006]     Nonetheless, it is extremely difficult to deposit a thin layer of chrome with uniform thickness across the surface of the mask. Furthermore, physical characteristics such as refractive index fluctuate across the surface of the leaky chrome layer on the mask. The leaky chrome layer itself can not shift the phase of incident radiation 180 degrees. Additional processes needed to achieve this goal increase manufacturing cost and difficulty.  
         [0007]     To overcome these difficulties, an embedded coating material which integrates the property of obtaining the required 180 degrees phase shift into the substrate coating layer which transmits a reduced percentage of the incident radiation, has been used. An embedded coating material such as molybdenum silicide (MoSiO x N y ) is used to achieve AttPSM. However, molybdenum silicide only provides a low transmittance of about 8 percent. A phase shifting mask which can attain high transmittance and without employing an opaque layer such as chrome is needed.  
       SUMMARY OF THE INVENTION  
       [0008]     A mask comprises a mask substrate and at least one annular equal line space phase shifting pattern on said mask substrate to produce an opaque region on a semiconductor substrate. A method of manufacturing a mask comprises providing a mask substrate; forming a layer of resist material on said substrate; patterning at least one annular equal line space phase shifting pattern on said resist layer; patterning said pattern onto said mask substrate; removing a remaining portion of said resist layer. A method of transferring a pattern onto a semiconductor substrate comprises illuminating a mask comprising at least one annular equal line space phase shifting pattern on the mask to produce an opaque region on a semiconductor substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     A more complete understanding of the present invention can be obtained by reference to the detailed description of embodiments in conjunction with the accompanying drawing, in which:  
         [0010]      FIG. 1A  illustrates a top view of a mask with an annular equal line space phase shifting pattern;  
         [0011]      FIG. 1B  illustrates a cross sectional view of the mask shown in  FIG. 1A ;  
         [0012]      FIG. 2A  illustrates a top view of a mask with another embodiment of annular equal line space phase shifting pattern;  
         [0013]      FIG. 2B . illustrates a cross sectional view of the mask shown in  FIG. 2A ;  
         [0014]      FIGS. 3A-3H  illustrate processes of manufacturing a mask shown in  FIG. 1A ;  
         [0015]      FIG. 4  illustrates an off-axis illumination of a mask during a photolithographic process;  
         [0016]      FIG. 5  illustrates a circuit pattern to be formed in a semiconductor substrate;  
         [0017]      FIG. 6  illustrates a corresponding pattern on a phase-shifting mask to form the circuit pattern shown in  FIG. 5 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     As shown in  FIG. 1A , an exemplary embodiment is a mask  100  containing an annular equal line space phase shifting pattern  110  and other features, such as a line  160 , on a mask substrate  105 . In this embodiment, the mask substrate  105  is transparent to incident radiation because no chrome is used to form the pattern  110  and other features. The incident radiation can be, for example, I-line (365 nm) or deep ultraviolet radiation (193 nm). A phase difference may be generated if the incident radiation travel paths of different length in the mask substrate.  
         [0019]     The pattern  110  is an annular equal line space phase shifting structure that comprises annular rings  120 ,  130 ,  140 , and a central portion  150 . The outermost annular ring  120  has a phase shift of approximately 180 degrees from the mask substrate  105 . The inner annular ring  130  has a phase shift of approximately 180 degrees from the outermost ring  120 . Likewise, the innermost annular ring  140  has a phase shift of approximately 180 degrees from the inner annular ring  130  and the central portion  150  has a phase shift of approximately 180 degrees from the innermost annular ring  140 . That is to say, phases of adjacent annular rings shift 180 degrees and phases of alternate annular rings are the same. In summary, annular rings  120  and  140  have the same phase, for example π (180 degrees). The mask substrate  105 , annular ring  130 , and the central portion  150  have the same phase, for example 0, that is 180 degrees different from that of annular rings  120  and  140 .  
         [0020]     Referring to  FIG. 1B , which is a cross sectional view from line AA′ of  FIG. 1 , the widths  120   a  and  120   b  of the annular ring  120 , the widths  130   a  and  130   b  of the annular ring  130 , the widths  140   a  and  140   b  of the annular ring  140 , and the width  150   a  of the central portion  150  are approximately the same. Accordingly, the pattern  110  is characterized as an annular equal line space structure. Although pattern  110  is transparent to an incident radiations, it creates a corresponding dark region on a semiconductor substrate through a known photolithographic process, resulting from the diffraction of its annular equal line space phase shifting structure.  
         [0021]     In other embodiments, number of annular rings may vary; as long as an outermost annular ring has a phase shift of approximately 180 degrees from the mask substrate, each inner annular ring has a phase shift of approximately 180 degrees from its outer adjacent annular ring, and the central portion has a phase shift of approximately 180 degrees from its adjacent innermost ring. In an alternate embodiment as shown in  FIGS. 2A and 2B , patterns  210  and  240  respectively have only one annular ring and a central portion. The pattern  210  has an annular ring  220  and a central portion  230 . The pattern  240  also has an annular ring  250  and a central portion  260 . The annular rings  220  and  250  have a phase shift of approximately 180 degrees from that of a mask substrate  205  and of the central portion  230  and  260 . In addition, to form equal line space structure, the widths  220   a  and  220   b  of the annular ring  220 , and the width  230   a  of the central portion  230  are the same; the widths  250   a  and  250   b  of the annular ring  250  is the same as the width  260   a  of the central portion  260 .  
         [0022]     The pitch (Pcs) of critical dimension (two times of a critical dimension) of a pattern that can be exposed on a semiconductor substrate under a specific environment, is calculated as follows: 
 
 Pcs =λ/((1+δ) NA ) 
 
 where Pcs is the pitch of critical dimension; λ is the wavelength of an incident radiation for patterning a semiconductor substrate; δ is the degree of coherence; and NA is the numerical aperture of a photolithography equipment. 
 
         [0023]     The pitch (Pm) on a mask substrate is N times of the corresponding pitch (Ps) on a semiconductor substrate where N can be an integer equal to or larger than one. For example, a four times (4×) mask is used in a stepper for photolithography processes, i.e. Pm=4Ps. In order to form a large opaque region on a semiconductor substrate, there is no requirement of minimum mask pitch (Pm) for the annular equal line space phase shifting pattern as long as photolithography technology allows. As a result, a mask pitch (Pm) smaller than the corresponding critical dimension pitch on a semiconductor substrate (N×Pcs, for example 4 Pcs) can result to a large opaque region on a semiconductor substrate. However, the mask pitch (Pm) of an annular equal line space phase shifting pattern has to be smaller than two times of the corresponding critical dimension pitch on a semiconductor substrate (N×2 Pcs, for example 8 Pcs), in order to form a large opaque region on a semiconductor substrate. That is to say, 0&lt;Pm&lt;N×2 Pcs.  
         [0024]     Those skilled in the art can calculate an appropriate mask pitch (Pm), such as 2 times of  120   a , in order for an annular equal line space structure, such as pattern  110 , to produce a corresponding dark region on a semiconductor substrate. In one embodiment using a 4× stepper, the mask pitch (Pm) of 960 nm, which corresponds to pitch (Ps) of 240 nm on the semiconductor substrate, is used in an equal line space structure to generate a corresponding dark region on a semiconductor substrate under the photolithography environment. For example, where the wavelength (λ) of an incident radiation is 248 nm; the degree of coherence (δ) is 0.85; the numerical aperture (NA) is 0.75; and off axis illumination is applied, the mask pitch (Pm) is 960 nm. Accordingly, for a specific pattern on a mask substrate that intends to generate a corresponding dark region on a semiconductor substrate, number of annular rings needed can be determined by the appropriate mask pitch (Pm).  
         [0025]     As illustrated in  FIG. 3A-3D , an exemplary embodiment of manufacturing the mask  100  is to form a conductive layer  320  and a resist layer  330  above a mask substrate  310  and to pattern an annular equal line space phase shifting pattern  110  onto the mask by photolithography and etching. In  FIG. 3A , the layer of conductive material  320  such as chrome is formed over the mask substrate  310 , such as a quartz. Chemical vapor deposition can be used to form the chrome layer  320 . A layer of resist material  330  is formed over the chrome layer  320 , for example, by sputtering the resist material over the chrome layer  320 . An exposure source (not shown), for example a laser with a wavelength of 364 nm (I-line) or electron beam, is used to transfer the desired pattern  110  onto the resist layer  330 . Exposed portions of the resist layer  330  and their underlying portions of the chrome layer  320 —for example, the portions corresponding to annular rings  120  and  140  in  FIG. 1A —are etched away. In some embodiments, a wet etching or an anisotropic dry etching can be used. After etching, as illustrated in  FIG. 3B , trench-like shapes  340  are formed. The remaining part of the resist layer is then removed by for example ashing. The pattern left on the chrome layer  320  as shown in  FIG. 3C  is used to etch the mask substrate  310  to a predetermined thickness. The predetermined thickness is designed to create a phase difference of 180 degrees as to an incident radiation employed to pattern a semiconductor substrate, such as a silicon wafer. After etching the mask substrate  310 , trench-like shapes  350  are formed as shown in  FIG. 3C . The remaining portion of the chrome layer  320  is then removed. As shown in  FIG. 3D , a phase shifting mask with the pattern  110  is fabricated. Trench-like shapes  350  correspond to the cross sectional view of annular rings  120  and  140  in  FIG. 1A .  FIG. 3E-3H  demonstrate another embodiment of manufacturing the mask  100 . When a radiation source is used for exposure such as a laser writer with a wave length of 193 nm, the resist layer  330  is formed over the mask substrate  310  without a conductive layer. The annular equal line space phase shifting pattern  110  is formed on the resist layer  330  and then transfer onto the mask substrate  310  by etching.  
         [0026]     In another embodiment, a layer of phase shifting material can be formed on the mask substrate to produce a phase difference of approximately 180 degrees in order to generate an annular equal line space phase shifting pattern.  
         [0027]     As shown in  FIG. 4 , a mask  430  with an annular equal line space phase shifting pattern can be illuminated to produce a corresponding dark region on a resist layer  450  in order to pattern the underlying semiconductor substrate  460 . In one embodiment, a single point off-axis illumination (OAI) is used in a photolithographic process. Light from a radiation resource is blocked by  420  and can only pass through an aperture  410  to form incident radiation at an angle  475  away from an axis  425 . In other embodiments of off-axis illumination, an annular or quadrupole aperture can be employed to illuminate the mask. By using an off-axis illumination, both the resolution and the depth of focus (“DOE”) of a photolithographic process are increased. As a result of using an off-axis illumination, normally after an incident light  470  passes through a feature other than an annular equal line space phase shifting pattern on the mask  430 , only 0 order  480  and +1 order  482  of the diffraction resulting from an incident radiation  470  are collected by a projection lens  440  to form an image on a resist layer  450  which is deposited on a semiconductor substrate  460 .  
         [0028]     However, when an incident radiation  470  passes an annular equal line space phase shifting pattern where N×Pcs&lt;Pm&lt;N×2 Pcs, 0 order  480  of the diffraction disappears and only +1 order  482  of the diffraction enters a projection lens  440  to form an image. Because the intensity of +1 order  482  of the diffraction alone is much lower than a threshold exposure intensity, the portion of resist underlying an annular equal line space phase shifting pattern is not exposed. In another embodiment, when a mask pitch (Pm) is smaller than the corresponding critical dimension pitch on a semiconductor substrate (N×Pcs, for example 4 Pcs), i.e. Pm&lt;N×Pcs, not only 0 order of the diffraction disappears but +1 order of the diffraction is also not collected by a projection lens. As a result, a large opaque region on a semiconductor substrate can be obtained. An opaque region that corresponds to the annular equal line space phase shifting pattern is then formed on the resist layer  450  and further transferred to the semiconductor substrate  460 . Without employing an annular equal line space phase shifting pattern, a large feature such as a pad or an interconnect would be exposed to a ring-like shape with a hollow inside rather than a solid shape that the feature is designed to be. Thus, when a sufficiently large feature, depending on the photolithography environment, begins to be exposed as a hollow ring rather than a solid dark region on a semiconductor substrate, an equal line space phase shifting pattern can be applied to the large interconnect to improve the result of exposure.  
         [0029]     As shown in  FIG. 5 , an integrated circuit design on a semiconductor substrate usually contains a larger interconnect area  510  and thinner interconnect lines  520  to  570 . A phase shifting mask capable of transferring a pattern containing both large opaque areas  510  and features with critical small dimension  520  to  570  is necessary. As mentioned above, a chromeless phase shifting mask with an annular equal line space phase shifting pattern thereon can be employed to transfer an opaque region onto a semiconductor substrate such as a wafer. Thus, to form a larger interconnect area  510  on a wafer, an annular equal line space phase shifting pattern  605  on a mask comprising annular rings  610 ,  620  and a central portion  630  as shown in  FIG. 6  is used. The annular ring  610  and the central portion  630  are at the same phase, which is approximately 180 degrees different from that of the annular ring  620  and of the mask substrate  600 .  
         [0030]     On the other hand, in order to form equal-line-space interconnect lines  520 ,  530 , and  540  on a semiconductor substrate such as a wafer, corresponding lines  640 ,  650 , and  660  on a mask have to be positively biased. Lines  640 ,  650 , and  660  are of a phase approximately 180 degrees different from the mask substrate  600 .  
         [0031]     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended Claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.