Abstract:
This is a method for masking a structure  12  for patterning micron and submicron features, the method comprises: forming at least one monolayer  32  of adsorbed molecules on the structure; prenucleating portions  46,48  of the adsorbed layer by exposing the portions corresponding to a desired pattern  36  of an energy source  42 ; and selectively forming build-up layers  66,68  over the prenucleated portions to form a mask over the structure to be patterned. Other methods are also disclosed.

Description:
This application is a capital Continuation, of application Ser. No. 07/997,632 now abandoned which is a continuation of Ser. No. 07/662,538 now abandoned, filed Dec. 28, 1992 and Feb. 28, 1991. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     The following coassigned patent applications, now abandoned are hereby incorporated herein by reference: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Serial No. 
                 Filing Date 
                 TI Case No. 
               
               
                   
                   
               
             
             
               
                   
                 07/555,229 
                 07/19/90 
                 TI-10824A NOT ALLOWED 
               
               
                   
                 07/606,231 
                 10/31/90 
                 TI-14237 
               
               
                   
                   
               
             
          
         
       
     
    
    
     NOTICE 
     (C) Copyright, *M* Texas Instruments Incorporated 1991. A portion of the disclosure of this patent document contains material which is subject to copyright and mask work protection. The copyright and mask work owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright and mask work rights whatsoever. 
     FIELD OF THE INVENTION 
     This invention generally relates to semiconductor devices and in particular for masking methods to form submicron features. 
     BACKGROUND OF THE INVENTION 
     The semiconductor industry is constantly striving to achieve higher density electronic devices. As the industry has moved into micron and submicron sized features to achieve higher densities, the need for improved masking methods to create such minute features has increased. 
     One way to achieve high resolution of a submicron sized feature is to increase the numerical aperture of the optical imaging system used to produce a patterned image of the feature. Unfortunately, increasing the numerical aperture of the imaging system to achieve higher resolution results in a drastically reduced depth of field. 
     Other problems are associated with conventional masking techniques, such as a lack of uniformity of exposure of resist through a thick layer of resist and scattered light within the layer of resist due to reflective metallized surfaces under the resist. These problems tend to compound the loss of resolution problem by creating ill-defined patterns at the onset. Thus, there is a need for a method for forming high resolution submicron sized features on a semiconductor device. 
     SUMMARY OF THE INVENTION 
     This is a method for masking a structure for patterning micron and submicron features, the method comprises: forming at least one monolayer of adsorbed molecules on the structure; prenucleating portions of the adsorbed layer by exposing the portions corresponding to a desired pattern of an energy source; and selectively forming build-up layers over the prenucleated portions to form a mask over the structure to be patterned. 
     Preferably the adsorbed layer is removed from the structure, except at prenucleating sites, before the build-up layers are formed; and a portion of the structure is etched to form patterned micron or submicron features. Also, the prenucleated portions and the build-up layers, if not destructive to the performance of the final structure, may be left as a portion of said final structure; the build-up layers may be used to pattern an organic etch resist; the structure may be a single layer substrate or at least one layer over a substrate layer; the energy source may be a radiant energy source or a particle energy source; the prenucleated portions and/or the build-up layers may be formed in a vacuum chamber, may be formed from an energy dissociable gas, and may be metal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional view of the formation of the adsorbed layer; 
     FIG. 2 is a cross-sectional view of the prenucleation formation step; 
     FIG. 3 is a cross-sectional view of the unpatterned energy exposure; 
     FIG. 4 is a cross-sectional view of using an etchant to transfer the pattern; 
    
    
     Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a structure  12  is placed in a pressure chamber  22  which is made air tight by appropriate seals, not shown, and is provided with an inlet  24  and an outlet  26 . Inlet  24  and outlet  26  each have a flow valve  28  and  30 , respectively, to control access to the chamber  22 . The structure  12  may be a single layer of material, or multiple layers of different materials, depending on the application. The point of importance is that there is a need to pattern a portion of the structure  12 . 
     A photosensitive gas is pumped into the chamber  22  via inlet  24 . A photosensitive gas such as, for example, trimethyl aluminum (Al(CH 3 ) 3 ), will photodissociate (i.e. break up into its more simple constituents upon exposure to a specified wavelength of light) and deposit one or more of the constituents (e.g. aluminum) on the structure  12  surface. The photosensitive gas is pressurized, for example, to 20 torr, until a monolayer  32  of adsorbed gas molecules forms on the structure  12 . After the required time, the unadsorbed photosensitive gas is evacuated through outlet  26 . 
     The structure  12  is then placed in an imaging system  34  such as, for example, a projection printer, as shown in FIG.  2 . The system  34  comprises a chamber  35  for receiving the structure  12  and a patterning mask  36 . The mask  36  is removably fixed to chamber  35  above the structure  12  and is provided with cut-outs, for example, cut-outs  38  and  40 , corresponding to the desired pattern to be transferred to the structure  12 . An energy source  42  is installed into the chamber  35  to provide the energy needed to photodissociate the adsorbed monolayer  32 . 
     The energy source  42  is selected to be compatible with the adsorbed layer  32 . The energy rays, as indicated by arrows  44 , have an energy sufficient to dissociate the gas in the adsorbed layer  32 . For example only, a wavelength of approximately 193 nanometers should be sufficient to photodissociate most gases that may be selected, in an optical imaging system. 
     The adsorbed monolayer  32  is exposed to the energy source  42  through cut-outs  38  and  40  in the mask  36 . As previously discussed, the energy source  42  causes the adsorbed monolayer  32  to photodissociate and create prenucleation sites  46  and  48  on structure  12 . Prenucleation sites  46  and  48  are comprised of one of the simpler constituents of the photosensitive gas of adsorbed layer  32 . For example, if the photosensitive gas comprises trimethyl aluminum, the photodissociation will cause the methyl (CH 3 ) to break from the surface of the structure  12  leaving prenucleation sites comprised of aluminum (Al). 
     The adsorbed layer  32  surrounding the prenucleated sites  46  and  48  remain unaffected by the energy source  42  since mask  36  provides exposure only to the prenucleated sites. It may be desirable at this point to remove the remaining non-nucleated regions of the adsorbed layer  32  by any appropriate method, such as heating, dissolving or etching, which will provide a clean surface for subsequent steps. 
     The prenucleation sites are then used to initiate further deposition which joins sites in the region to form a mask of the desired pattern. This can be done by selective deposition (e.g. tungsten from WF 6  by chemical vapor deposition (CVD) assisted by directed energy) nucleated by the prenucleation sites. By this method, build-up layers  66  and  68  may be selectively deposited on the prenucleated sites  46  and  48 , this may or may not be done in a vacuum, depending on the process and materials used. 
     Referring to FIG. 3, the structure  12  with the prenucleated sites  46  and  48  ready for build-up may alternately be placed in an unpatterned energy exposure apparatus  50  (as an alternative to the build-up by the selective deposition of tungsten, for example). The apparatus  50  is appropriately sealed, not shown, to provide a vacuum chamber  52  with an inlet  54  and an outlet  56 . Inlet  54  and outlet  56  are each provided with flow valves  58  and  60 , respectively, to control access to chamber  52 . An energy source  62  is installed in the top of the chamber  52  to provide the energy needed to produce photodissociation during the energy exposure. The energy source  62  must be compatible with the photosensitive gas selected for energy exposure. A photosensitive gas is allowed to fill chamber  52  through inlet  54  as indicated by arrow  64 . The energy source  62  is activated to produce energy rays as indicated by arrows  65  which pass through the photosensitive gas filling the chamber  52 . The energy rays photodissociate the gas into simpler constituents thereof creating condensable and non-condensable atoms. The condensable atoms will selectively nucleate on the prenucleated sites  46  and  48 . Therefore, the prenucleated sites  46  and  48  will build up layers of the condensable atoms to form build-up layers  66  and  68 . After a predesignated amount of time, the remaining gas is evacuated through outlet  56  as indicated by arrow  70 . Thus, if the photosensitive gas were trimethyl aluminum , the energy rays  66  might dissociate the gas into partially dissociated product. One or two of the three methyl molecules would remain as a non-condensable gas, whereas the aluminum atoms and remaining methyl molecules would be condensable. The condensable atoms would collect upon the prenucleated sites  46  and  48  and further dissociate to form the build-up layers  66  and  68 . 
     FIG. 4 illustrate the results of the final stages of the preferred embodiment of the present invention. The desired pattern under the build-up layers  66  and  68  may now be transferred to the structure  12  by any appropriate etch process, such as plasma dry etching with a halogen based etchant, to etch the structure  12  that is not under the build-up layers  66  and  68 . The resulting products are layered columns comprising a portion of the structure  12 , prenucleated sites  46  and  48 , respectively, and build-up layers  66  and  68 , respectively. 
     Once the plasma etch is completed, the prenucleated sites  46  and  48 , and the build-up layers may be removed, for example, by an etching process, or they may be left as they are, depending on the material used and the effect on subsequent processing steps and the completed device. The final result is the desired submicron pattern formed on the structure  12 . 
     The method described above may be used in many applications and with many processes and materials. The energy used to produce a patterned image on the structure  12  may encompass a broad spectrum of sources. These include those commonly used, such as the g-line or the i-line from a Hg arc discharge lamp, or, for example, deep ultra-violet light, x-rays, e-beams, or ion sources may be used. Therefore, limitations due to specific energy sources is overcome. The terms photodissociable and photosensitive are used for convenience in the above description, however, the term photo may be replaced with radiant energy or electromagnetic energy, or particle energy, for example, throughout the discussion (e.g. photodissociable may be replaced with radiant energy dissociable). 
     The deposition processes that may be prenucleated to produce a masking layer are also broad in scope. For example, any fluid used for chemical vapor deposition (CVD), photo-activated or plasma-enhanced CVD, or electron cyclotron resonance deposition may be used (e.g. generally metal-organic or metal halides may be used). The material that is deposited may be metal, insulator, or semiconductor. The choice of material is not limited to the low temperature domain of organic material (which is commonly used in conventional masking techniques), but is driven by the application. As a result, the applications for the masking scheme described above are very diverse. For example, it may be used to mask high resolution ion implants, oxidation of silicon, or etching of underlying films. The masking may be used to pattern another masking material and thus be used to indirectly pattern permanent features on the final structure (e.g. to mask an organic etch resist). 
     In Table 1 below, a nonexhaustive list of possible materials which may be used in the aforementioned method is provided. The table contains potential combinations of masking materials (e.g. the Al prenucleated regions and/or build-up layers), the carrier species (e.g. Al(CH 3 ) 3  as a photodissociable gas or WF 6  as an energy dissociable gas), and the etched material (e.g. the top layer of the structure  12 , in which the submicron features are to be formed). The table is meant for clarification and example purposes only and should not be considered in a limiting sense. 
     A preferred embodiment has been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. Words of inclusion are to be interpreted as nonexhaustive in considering the scope of the invention. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 MASKING MATERIAL 
                 CARRIER SPECIES 
                 ETCHED MATERIAL 
               
               
                   
               
             
             
               
                 Al 
                 Al(CH 3 ) 3   
                 SiO 2   
               
               
                 Al 
                 Al(CH 3 ) 3   
                 Si 3 N 4   
               
               
                 Al 
                 Al(CH 3 ) 3   
                 Si 
               
               
                 Al 
                 Al(CH 3 ) 3   
                 W 
               
               
                 Pb 
                 Pb(CH 3 ) 4   
                 SiO 2   
               
               
                 Pb 
                 Pb(CH 3 ) 4   
                 Si 3 N 4   
               
               
                 Pb 
                 Pb(CH 3 ) 4   
                 Si 
               
               
                 Pb 
                 Pb(CH 3 ) 4   
                 W 
               
               
                 Mo 
                 Mo(CO) 6   
                 Al 
               
               
                 Mo 
                 Mo(CO) 6   
                 Si 
               
               
                 Mo 
                 Mo(CO) 6   
                 TiN 
               
               
                 Mo 
                 Mo(CO) 6   
                 Ta 2 O 5   
               
               
                 W 
                 WF 6   
                 Si 
               
               
                 W 
                 WF 6   
                 SiO 2   
               
               
                 W 
                 WF 6   
                 Al