Patent Publication Number: US-6713233-B2

Title: Multiple pass write method and reticle

Description:
This is a continuation of application Ser. No. 09/571,719, filed May 15, 2000, now U.S. Pat. No. 6,472,123. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the fabrication of photolithography devices such as reticles and semiconductor masks. More particularly, the present invention relates to a method of forming clear fields on a reticle and to reticles formed by electron-beam processing. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of integrated circuits (ICs), microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template, such as a reticle. During the exposure process, radiant energy, such as ultraviolet light, is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes. 
     The manufacture of ICs generally requires the use of numerous reticles or masks. Each individual reticle is expensive and time-consuming to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a typical IC may require 10 to 20 or more reticles. Thus, accurate formation of reticles may yield savings in IC production costs. 
     Reticles and masks typically include an opaque thin film of a metal, such as chromium or molybdenum silicide, deposited in a pattern on a transparent substrate of quartz or glass. Defects in the pattern of chromium or molybdenum silicide may occur as a result of electrostatic charge added to the reticle preform during manufacture of the reticle. In conventional reticle patterning methodologies, a photoresist material overlays the layer of chromium. An electron beam exposes a portion of the photoresist material based upon a predetermined pattern. The exposed portion of the photoresist material is removed leaving uncovered a portion of the chromium. The unexposed photoresist material is then used to block the etch and leave the desired pattern in the metal to create the reticle. 
     Referring to FIGS. 1-3, a reticle preform  10  is shown in various stages of manufacture. The reticle preform  10  includes a substrate  12  located on a base  24 . The substrate  12  is formed from a transparent material, such as quartz or glass. A layer of metal  13 , such as, for example, chromium or molybdenum silicide, overlays the substrate  12  and is located beneath a layer of a photoresist material  14 . The photoresist material  14  is formed of a material which is suitable for exposure by electrons. 
     An electron beam apparatus  16  is schematically shown (FIGS. 1,  2 ) in a position to direct electrons toward the photoresist material  14 . The apparatus  16  includes an electron beam device  18 , such as an electron beam gun, in mechanical and electrical connection with a controller  22 . An actual electron beam gun, such as one manufactured by ETEC systems, is illustrated in FIG.  10 . The electron beam device  18  directs a stream of electrons  26  toward the photoresist material  14  in a predetermined writing pattern  28 , shown by the dashed lines on the photoresist material  14 . The stream of electrons  26  preferably is controlled electrostatically. 
     Conventionally, a single predetermined writing pattern  28  is programmed into the controller  22 , which controls the actions of the electron beam device  18  through the appendage  20 . The writing pattern  28  is followed such that predetermined portions of the photoresist material  14  are exposed by the stream of electrons  26 . The exposed portions of the photoresist material  14  are then removed. The remaining unexposed portions of the photoresist material  14  are used as a mask for etching the now exposed portions of the metal  13  to create a reticle  100  (FIG. 3) having the desired pattern of metal  13 . 
     Specifically, and with reference to FIGS. 1 and 2, the writing pattern  28  separates the photoresist material  14  into a first strip  30 , a second strip  32 , a third strip  34 , a first portion  36 , a second portion  40 , a third portion  70 , and an interlayer portion  68 . In FIG. 1, the writing pattern  28  is shown in dashed lines to indicate that the exposure process has only just started. In FIG. 2, the writing pattern  28  is shown in solid lines to indicate that the exposure process has been completed. 
     In the known process, the stream of electrons  26  exposes the portions  36 ,  40 ,  70 , and  68  allowing for the subsequent removal of the photoresist material  14  resident in the exposed areas. One problem encountered through the conventional methodology is that using an electron beam to expose large photoresist areas, such as the second and third portions  40 ,  70 , sometimes causes a localized build up of electrostatic energy in the reticle preform  10 . The presence of electrostatic energy is detrimental to the accuracy of the stream of electrons  26 , causing the stream  26  to be displaced, or to skew away, from the path intended by the writing pattern  28  (FIG.  2 ), thus altering the pattern of exposed photoresist material  14  from the desired writing pattern  28 . 
     Applicant has determined that where electrostatic energy has caused a displacement of the electron stream  26  the photoresist material  14  exposed may not be consistent with the amount intended to be exposed according to the writing pattern  28 . Instead, the exposed photoresist material  14  which is subsequently removed will leave first, second, third, and fourth uncovered areas of metal  56 ,  42 ,  71 , and  69  which respectively were beneath the portions  36 ,  40 ,  70 , and  68 . Since the exposed portions  36 ,  40 ,  70 , and  68  did not exactly correspond with the writing pattern  28 , the underlying metal areas  56 ,  42 ,  71 , and  69  also will not match the desired metal areas according to the writing pattern  28 . In addition, the remaining unexposed portions of photoresist material  14 , namely a first strip  50 , a second strip  52  and a third strip  72  do not match with the unexposed strips that were to be formed according to the intended writing pattern  28 . 
     After removing the exposed photoresist material  14  (as described above), the exposed areas of metal, namely the first, second, third and fourth uncovered areas of metal  56 ,  42 ,  71 ,  69  are etched. The remaining unexposed portions of photoresist, namely the first, second, and third strips  50 ,  52 ,  72  are washed away by a known method to form the reticle  100  (FIG.  3 ), including metal strips  62 ,  64 ,  66  positioned on the substrate  12 . 
     Since the exposed and unexposed portions of the photoresist material  14  did not match the writing pattern  28 , the metal strips  62 ,  64 ,  66  will likewise differ from the desired strips. The discrepancy between the actual metal strips  62 ,  64 ,  66  and the desired strips may be substantial enough to cause the reticle  100  to form defective semiconductor devices. Alternatively, additional measures may be required to compensate for the discrepancy. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of forming a reticle including exposing a first portion of the photoresist layer in accordance with a first writing pattern and exposing a second portion of the photoresist layer in accordance with a second writing pattern. 
     The present invention also provides a photolithography device for forming a semiconductor device that has a transparent substrate and a pattern of conductive material overlaying the substrate. The conductive material pattern is formed utilizing multiple write passes of electron beam energy. 
     The present invention also provides an apparatus for forming a photolithography device. The apparatus includes a device for projecting electrons at a layer of photoresist material and a controller for controlling the device such that a multiple of write passes based upon corresponding patterns sequentially expose portions of the photoresist material. 
     These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view showing a reticle being fabricated from a reticle preform by way of an electron beam apparatus. 
     FIG. 2 is another partial perspective view of the reticle preform of FIG. 1 showing a skewed stream of electrons from the electron beam apparatus. 
     FIG. 3 is another partial perspective view showing a reticle formed from the reticle preform of FIGS. 1 and 2. 
     FIG. 4 is a partial perspective view of a reticle preform illustrating a first write pass in accordance with an embodiment of the present invention. 
     FIG. 5 is another partial perspective view of the reticle preform of FIG. 5 illustrating a second write pass. 
     FIG. 6 is another partial perspective view of the reticle preform of FIG.  5 . 
     FIG. 7 is another partial perspective view showing a reticle formed from the reticle preform of FIG.  5 . 
     FIG. 8 is a schematic view of a controller and database constructed in accordance with an embodiment of the present invention. 
     FIG. 9 is a flow diagram of a method of forming a reticle in accordance with an embodiment of the present invention. 
     FIG. 10 is a perspective view of an electron beam apparatus for use in the formation of a reticle in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 4-7 depict a reticle preform  10  and a reticle  200  in various stages of formation. The reticle preform  10  includes a transparent substrate  12 , formed of quartz, glass, or another suitable material. A layer of metal  13  is deposited over the substrate  12 , and a photoresist material  14  is deposited on the metal layer  13 . The metal layer  13  may comprise any suitable conductive material, such as, for example, chromium, molybenum silicide, chromium fluoride, titanium nitride, or other like material. 
     Instead of exposing the photoresist material  14  through the use of a single write pass strategy, the present invention utilizes a multiple write pass strategy. The multiple write pass strategy minimizes build up of electrostatic charge on the reticle preform  10 , which may cause inaccurate exposure of the photoresist material  14 , leading to etched metallic strips which are improperly positioned, pitted, or uneven. The multiple write pass strategy leads to registration improvement for clear field reticles, such as the reticle  200 , between the desired pattern and the actual pattern by minimizing displacement errors on live geometries. 
     As shown in FIG. 4, a portion of the photoresist material  14  along the writing pattern  28  has been exposed by the electron beam apparatus  16  during a first write pass. The portions of the photoresist material  14  that will be exposed during the first write pass are along the writing pattern  28 . 
     Specifically, the first write pass exposes photoresist material  14  in the first portion  36 , the interlayer portion  68 , a second portion  74 , along the edge of a third portion  38 , and along an edge of a fourth portion  160  facing the third strip  72 . As shown in FIG. 4, the stream of electrons  26  has begun exposing the photoresist material  14  along the edge of the fourth portion  160  facing the third strip  72 . 
     In FIG. 5, all of the photoresist material  14  to be exposed during the first write pass has been exposed, as shown by a solid line along the border of the write pattern  28 . Further, the second write pass has begun along the write pattern  128 , as shown by the partial solid line along the edge of the fourth portion  160  facing the third strip  72 . Both of the write passes have been completed as shown by the solid lines along the write patterns  28 ,  128  in FIG.  6 . 
     The sequential write passes leave unexposed some portions of the photoresist material  14 , namely the three strips  50 ,  52 ,  72  in the illustrated embodiment. The second write pass exposes a larger portion of photoresist material  14  than the first write pass. The exposed portions, as well as the underlying metal areas, are etched away, and then the unexposed strips  50 ,  52 ,  72  are removed, leaving metal strips  162 ,  164 ,  166  on the substrate  12  to form the reticle  200 . Unlike the reticle  100 , the metal strips  162 ,  164 ,  166  are accurately formed due to the multiple writing strategy. 
     FIG. 8 illustrates the interconnection between the electron beam device  18 , the controller  22 , a database  50 , and an input device  60 . The controller  22  may be mechanically and electrically connected to the electron beam device  18 . The controller  22  is further electrically connected to the database  50  and the input device  60 . The input device  60  may be any mechanism or system capable of inputting commands to the controller  22 , such as a keypad, keyboard, touch pad, mouse, write pad, or other suitable input device. 
     In use, an operator inputs a command through the input device  60  to the controller  22  for the electron beam device  18  to perform a first write pass which follows a first pattern. The controller  22  accesses the database  50  to retrieve and upload the first pattern. With the first pattern uploaded, the controller  22  moves the electron beam device  18  in accordance with the first pattern, thereby accurately positioning the electron beam device  18  to expose photoresist material  14  consistent with the first pattern. In the illustrated embodiment, the first pattern exposes photoresist material  14  in areas immediately adjacent to portions of the reticle preform  10  which will be formed into metallic strips. The present invention should not be limited, however, to the embodiments described and illustrated herein. In particular, sequential write pass patterns other than those shown in the drawings may be employed. 
     After the electron beam device  18  has exposed all the photoresist material  14  to be exposed during the first write pass, an operator inputs a command through the input device  60  to the controller  22  for the electron beam device  18  to perform a second write pass following a second pattern. The controller  22  accesses the database  50  to retrieve and upload the second pattern. With the second pattern uploaded, the controller  22  moves the electron beam device  18  in accordance with the second pattern, thereby accurately positioning the electron beam device  18  to expose photoresist material  14  consistent with the second pattern. 
     The second pattern exposes photoresist material  14  in large areas adjacent to areas of the reticle preform  10  which will be formed into the metallic strips. If electrostatic energy builds up due to the exposure of such large areas, the stream of electrons  26  may become displaced from the path it should take consistent with the second pattern. However, since the first pattern exposed photoresist material  14  immediately adjacent to the areas of the reticle preform  10  which will be etched into metallic strips, adverse effects stemming from a less accurate electron stream  26  in the second write pass are minimized. 
     Instead of an operator inputting two separate commands to the controller  22 , an operator may select a pair of write patterns, and send a single command to the controller  22  to perform the first of the pair of write patterns, and then perform the second of the pair of write patterns. 
     FIG. 9 illustrates steps for preparing a reticle in accordance with one embodiment of the present invention. A reticle preform  10  is positioned on a base structure  24  at step  300 . A first portion of photoresist material is exposed in a first write pass at step  305 . A second portion of photoresist material is exposed in a second write pass at step  310 . Then, the exposed photoresist material is developed at step  315  and the underlying metal is etched to form a reticle  200 . 
     As can be seen by the embodiments described herein, the present invention encompasses a method of inhibiting adverse effects stemming from electrostatic charge in a localized area of a photoresist material during the formation of a reticle. The method utilizes a multiple (two or more) write pass technique in which the first write pass of electron beam energy is directed at a small area of the photoresist immediately adjacent to where conductive strips will be etched, and the second or subsequent write pass of electron beam energy is directed at the clear field locations. 
     While the invention has been described in detail in connection with the preferred embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while the foregoing description has focussed on the fabrication of reticles, it is also applicable for the fabrication of semiconductor masks. Further, while a dual write pass strategy has been described, a multiple write pass strategy may be employed. Accordingly, this invention is not seen as limited by the foregoing description, but is only limited by the scope of the appended claims.