Abstract:
A method of forming a field emission device for a flat panel display includes operating a projection exposure apparatus. This comprises placing three layers of exposure sensitive material on a device in succession, with steps of exposure and removal of material between deposition of subsequent layers of exposure sensitive material. Furthermore, a field emission device is formed by exposing a third layer of exposure sensitive material, wherein a tip on the field emission device or plurality of tips on the field emission devices can be obtained with differing sharpness characteristics by varying the depth and diameter of holes in a mask used during exposure of exposure sensitive material.

Description:
This is a division of parent application Ser. No. 09/326,031, filed Jun. 4, 1999, now U.S. Pat. No. 6,064,145. The contents of this parent application is being relied upon and incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of fabricating a field emitting tip and a field emitting tip made by such a method. The field emitting tip of the present invention can be employed in a flat panel display. 
     2. Description of the Prior Art 
     A flat panel display (FPD) can include a number of field emission devices (FED). Various fabrication techniques for manufacturing a cathode used in a FED have been proposed when manufacturing a FPD including a FED. 
     FIG. 1 illustrates the structure of a conventional FED. An anode plate  10  is provided to receive electrons  20  from a cathode  30 . A metal ring  40 , which focuses the electron emission, is provided. 
     FIGS.  2 ( a )-( f ) illustrate a conventional process of manufacturing cathode  30  as shown in FIG.  1 . The process begins with a bare silicon substrate  50 . To form a cap  60 , a small layer of SiO 2  is deposited on the silicon substrate and then a layer of Si 3 N 4  is provided thereon. As shown in FIG.  2 ( a ), the layers of SiO 2  and Si 3 N 4  are patterned and subjected to a reactive ion etch (RI E) to form cap  60  for the field emission tip of the device. 
     An isotropic silicon etch is performed to remove a portion of silicon from the surface of substrate  50 . As illustrated in FIG.  2 ( b ), cap  60  protects part of silicon substrate  50  from the etch and a pyramid structure  70 , which is a characteristic of the prior art field emission tip, is produced. 
     As shown in FIG.  2 ( c ), after pyramid structure  70  is formed, portions of silicon surrounding the pyramid are etched. Thermal oxidation is then performed to sharpen the field emission tip, which leaves an oxidation layer  80 . 
     As shown in FIG.  2 ( d ), an insulating layer  90  is then deposited to a desired thickness by evaporation. After insulating layer  90  is formed, a mask material  120  is laid down and patterned to define the array edges and metal  100  is deposited to form a metal ring  110 . 
     Finally, as shown in FIG.  2 ( f ), mask  120  and cap  60  are removed along with the material formed on cap  60  and mask  120  during the process to form the field emission device having metal ring  110 . 
     Hyung Soo Uh and Jong Duk Lee, “New Fabrication Method of Silicon Field Emitter Arrays Using Thermal Oxidation,” J. Vac. Sci. Tech. B 13(2) pages 456-60 (1995) describes a similar process using a well region formed in a semiconductor substrate. FIGS.  3 ( a )- 3 ( f ) illustrate this process, where FIGS.  3 ( e ) to  3 ( f ) illustrate the overlying structure being lifted off by etching the oxide surrounding the tip using a buffered hydrofluoric acid (HF) solution. 
     These conventional processes to produce a field emitting tip, however, suffer certain deficiencies. For example, because the conventional processes use a thermal oxidation process to sharpen the tapered profile of the silicon field emitting tip, the processes are time consuming. Further, the thermal oxidation process is not easy to control and, thus, the degree of sharpness of a tip may be degraded due to uncertainties based on the degree of oxidation. Also, cap  60 , which functions as a hard mask during the processes, may fracture if processing is not carefully controlled. As described, for example, in U.S. Pat. No. 5,753,130, a special kind of micro-sphere hard mask can be used in the formation of sharp tips in an attempt to alleviate this problem, but providing a special mask is undesirable. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method of forming a semiconductor device, a method of operating a projection exposure apparatus, and a semiconductor device that substantially obviate one or more of the problems due to limitations and disadvantages of the prior art. To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention can comprise a method of forming a semiconductor device including forming a hole in a surface of a substrate, covering the surface with a layer of semiconductor material so that a valley is formed in the layer of semiconductor material above the hole, providing masking material in the valley, and etching the masking material and the layer of semiconductor material to form a portion of the layer into a structure having a peak above the hole. 
     In another aspect of the invention, a method of operating a projection exposure apparatus can include placing a first layer of exposure sensitive material on a first layer of material of a device, exposing the first layer of exposure sensitive material using a mask, removing portions of the first layer of material in accordance with the exposure of the first layer of exposure sensitive material, placing a second layer of exposure sensitive material on a second layer of material of the device, exposing the second layer of exposure sensitive material using the mask, removing portions of the second layer of material in accordance with the exposure of the second layer of exposure sensitive material, placing a third layer of exposure sensitive material on a third layer of material of the device, and exposing the third layer of exposure sensitive material using the mask. 
     In a further aspect of the present invention a semiconductor device includes a first layer of material having a hole in its upper surface, and a second layer of material covering the hole, wherein the second layer of material includes a peak structure that is disposed in correspondence with the hole in the upper surface of the first layer of material. 
     Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the embodiments of the invention and together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a conventional field emission device. 
     FIGS.  2 ( a )- 2 ( f ) are sectional views of structures at steps in a manufacturing process according to a conventional process of forming a FED. 
     FIGS.  3 ( a )- 3 ( f ) are sectional views of structures at steps in a manufacturing process according to another conventional process of forming a FED. 
     FIGS.  4 ( a )- 4 ( k ) are sectional views of structures at steps in a manufacturing process of forming a FED consistent with the present invention. 
     FIG. 5 is an schematic illustration of a projection exposure apparatus consistent with the present invention. 
     FIG. 6 is an SEM photograph of actual tips of FEDs formed by a process consistent with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     A process flow for making tip and metal ring components of a field emission device (FED) consistent with the present invention is described next. 
     FIG.  4 ( a ) illustrates a state of a device during initial processing of a bare substrate  400 . For a FED, substrate  400  is preferably conductive and may include, for example, silicon. 
     A projection exposure apparatus is then used to form shallow holes  405  on substrate  400 . The projection exposure apparatus can be a scanning- or stepping-type projection exposure apparatus that employs any type of radiation. The process of forming holes  405  depends on the type of projection exposure apparatus used. For example, when a projection exposure apparatus employing irradiation based on a light beam is used, a photoresist, e.g., a positive photoresist, is applied to substrate  400 . FIG. 5 schematically illustrates a projection exposure apparatus employing irradiation based on a light beam, which includes functions associated with conventional projection exposure apparatus.. The projection exposure apparatus has a light source  550 , which illuminates a photomask  560  placed on reticle stage  570 . Reticle stage  570  can move along the X-axis synchronously with stage  600  during exposure, if a scanning-type exposure is employed. Photomask  560 , which has a patterned plate that selectively transmits light, is disposed on an object side of a projection optical lens system  580  of the projection exposure apparatus and a wafer  590 , including substrate  400  having the photoresist thereon, is placed at the image side of projection optical lens system  580 , on stage  600 . Exposure is carried out to transfer the pattern of photomask  560  onto the photoresist. 
     After the pattern is formed, etching removes portions of the substrate based on the pattern of photomask  560 . The pattern of photomask  560  and the etching process is configured based on the sharpness of the tips to be formed. For example, by varying the diameter and depth of the holes, tips with differing sharpness characteristics can be obtained. A silicon layer  410  of epitaxially-grown or deposited silicon (dependent on the structure of substrate  400 ) is then formed on substrate  400  to cover holes  405 . Because silicon layer  410  is of a uniform thickness, a valley is formed in portions above the holes  405 . Subsequently, silicon layer  410  is covered by an oxide masking layer  420 , which can be composed of, for example, SiO 2 . 
     As shown in FIG.  4 ( b ), a planarizing process, such as chemical-mechanical polishing (CMP), is performed thereafter to remove oxide masking layer  420  except in the valley above hole  405 . The remaining portion of masking layer  420  is a cap  430  having a downwardly extending tip. 
     As shown in FIG.  4 ( c ), following the CMP process, a silicon tip  440  is formed as an upwardly extending peak. FIG. 6 shows an SEM photograph of actual tips formed by the process of the present invention. Notably, when silicon tip  440  is formed, cap  430  serving as an oxide hard mask is also consumed. Therefore, no extra step is required for removing a residual hard mask as in other conventional methods. Tip  440  is formed and cap  430  is consumed during a reactive ion etch (RIE). 
     The following is one example of an etching recipe that can produce a sharp silicon tip  440 . Fluorocarbon is introduced into the chamber at a flow rate in the range of approximately 25 to 75 sccm and a temperature in the range of approximately 20 to 60° C. The etching proceeds for approximately fifty seconds. Because of the differing properties of silicon layer  410  and cap  430 , the gas removes cap  430  and gives a controlled slope to silicon tip  440 . Of course, other gases or mixtures of gases, for example including argon, and parameters are within the scope of the invention. Adjusting these variable parameters can help to achieve a desired sharpness of tip  440 . 
     After tip  440  is formed, a thick oxide layer  450  of SiO 2  is deposited to cover tip  440 . 
     As shown in FIG.  4 ( d ), RIE is then used to planarize the oxide surface so that only a thin layer of oxide layer  450  with a thickness of approximately 100 Å remains above tip  440 . Alternatively, CMP can be used to polish oxide layer  450  to leave a thickness of oxide layer  450  above tip  400  of approximately 100 Å. Thus, the CMP should stop when tip  440  is nearly exposed, i.e., such that the thin layer of oxide layer  450  remains above tip  440 . 
     As shown in FIG.  4 ( e ), planarized oxide layer  450  is partially etched to form a step  460  around tip  440 . Preferably, this partial etch can use the same projection exposure apparatus and the same photomask that was used to form holes  405  in substrate  400 . For example, for use with the projection exposure apparatus shown in FIG. 5, a photoresist having an opposite reaction to that of the photoresist used to form holes  405 , e.g., a negative photoresist when a positive photoresist is used to form holes  405 , is applied to planarized oxide layer  450 . Photomask  560  is placed on the object side of projection optical lens system  580  of the projection exposure apparatus and substrate  400  including the structure having planarized oxide layer  450  with the negative photoresist is placed on the image side of projection optical lens system  580 . Exposure is carried out to transfer the pattern of photomask  560  onto the photoresist. After the pattern is formed, etching removes portions of oxide layer  450  based on the pattern of photomask  560 . 
     Then, as shown in FIG.  4 ( f ), a metal  470  is formed on portions of the oxide layer  450  other than step  460 . This metal can be deposited uniformly and then etched away, by, for example, CMP to remove the metal deposited on step  460 . A silicon nitride layer  480  is then formed on step  460  and metal  470 . Preferably, a thin oxide layer  490  is also formed between silicon nitride layer  480  and metal  470  to relieve stress between metal and silicon nitride. 
     RIE is then used to etch away the portion of silicon nitride layer  480  above step  460 . As shown in FIG.  4 ( g ), preferably, the RIE stops etching when it reaches step  460 . Preferably, this partial etch can use the same projection exposure apparatus and the same photomask that was used to form holes  405  in substrate  400  and step  460  in oxide layer  450 . For example, for use with the projection exposure apparatus in FIG. 5, a photoresist having the same reaction as that of the photoresist used to form holes  405 , e.g., a positive photoresist when a positive photoresist is used to form holes  405 , is applied to silicon nitride layer  480 . The same photomask is placed on the object side of projection optical lens system  580  of the projection exposure apparatus and substrate  400  including the structure having nitride layer  480  with positive photoresist is placed on the image side of projection optical lens system  580 . Exposure is carried out to transfer the pattern of photomask  560  onto the photoresist. After the pattern is formed, etching removes portions of silicon nitride layer  480  based on the pattern of photomask  560 . While the same photomask is used repeatedly in three of the exposure operations of the present invention, any other combination of using the same photomask as in previous steps is also possible. 
     As shown in FIG.  4 ( h ), an additional layer of silicon nitride  480 ′ is then uniformly deposited on the device. As shown in FIG.  4 ( i ), layer  480 ′ is uniformly etched. The uniform etch will preferably, stop when it reaches step  460 , thereby forming silicon nitride spacers  500  which overlap step  460 . 
     As illustrated in FIG.  4 ( j ), a wet etch using, for example, a buffered HF solution, removes step  460  and thereby exposes tip  440 . This process can result in the removal of some of metal  470  to form a metal ring  510 . 
     Finally, silicon nitride layer  500  is removed by, for example, a wet chemical etch as shown in FIG.  4 ( k ). Hot, concentrated phosphoric acid can be used to perform this wet chemical etch. A FED  550  is the structure that results from the process. 
     Field emission devices manufactured in accordance with the present invention can be used during manufacturing and operating a flat panel display (FPD). A FPD having FEDs of the present invention can provide a sharper picture, consume less power, and occupy less space, especially when compared to a cathode ray tube display. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the context of the present invention and in its practice without departing from the scope or spirit of the invention. 
     As an example, the processes involved in this invention can also be applied to manufacturing of other types of semiconductor devices. Depending on process requirements, combinations of several of the steps can be used to achieve formation of special structures in other devices. 
     Also, those skilled in the art will recognize that the level of conductivity of the materials used in the process can be varied, as demanded by the final product sought to be produced. Further, the invention is not limited to silicon-based embodiments, and other suitable materials may be used. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.