Abstract:
A process for forming a semiconductor device having an oxide beanie structure (an oxide cap overhanging an underlying portion of the device). An oxide layer is first provided covering that portion, with the layer having a top surface and a side surface. The top and side surfaces are then exposed to an oxide deposition bath, thereby causing deposition of oxide on those surfaces. Deposition of oxide on the top surface causes growth of the cap layer in a vertical direction and deposition of oxide on the side surface causes growth of the cap layer in a horizontal direction, thereby forming the beanie structure.

Description:
FIELD OF THE INVENTION 
   This invention relates to semiconductor device manufacturing, and in particular to formation of oxide cap structures (known in the art as “beanie” structures) using a process having a minimal number of steps. 
   BACKGROUND OF THE INVENTION 
   In the manufacturing of semiconductor devices, particularly DRAM devices having FETs, the need for ever-increasing density of electrical contacts makes it desirable to fabricate borderless contact structures. The use of cap structures which overhang the sides of the capped region (beanie structures) facilitates processing of borderless contacts. However, the conventional method for forming a beanie structure involves several film deposition and etching steps. Steps in a typical process are shown in  FIGS. 1A-1E . The structure to be capped (e.g. an FET gate structure  2  on a substrate  1 , as in  FIG. 1A ) has an oxide cap layer  2   a  formed at the top of the structure. A layer  3  of sacrificial material is deposited on the substrate, and then planarized and etched so that its top surface is below layer  2   a  ( FIG. 1B ). A conformal layer  4  (typically silicon oxide) is deposited on the sacrificial material and over the gate structure ( FIG. 1C ). This layer is then etched to form structures  5 , resembling spacers used in conventional FET processing, on the upper sidewalls of the gate ( FIG. 1D ). The sacrificial layer  3  is then removed, leaving a beanie structure, comprising structures  5  and cap layer  2   a , on the gate structure  2  ( FIG. 1E ). This process, which requires film deposition, planarization and etching, is cumbersome and costly. 
   Accordingly, there is a need for a process for fabricating a beanie structure with a reduced number of steps, so that the advantages of using beanie structures may be realized in a manufacturing environment. 
   SUMMARY OF THE INVENTION 
   The present invention provides a process for forming a semiconductor device having an oxide beanie structure (that is, an oxide cap covering and overhanging an underlying portion of the device). In accordance with the present invention, this is done by providing an oxide layer (a first layer, typically silicon dioxide) covering that portion, with the layer having a top surface and a side surface, and then depositing an oxide material (typically silicon dioxide) selectively on the top surface and the side surface of the first layer by liquid oxide deposition. The liquid oxide deposition is preferably done by exposing the first layer to an oxide deposition bath having a supersaturated aqueous solution of silica with hydrofluoric acid. 
   Deposition of oxide on the top surface causes growth of the cap layer in a vertical direction and deposition of oxide on the side surface causes growth of the cap layer in a horizontal direction, thereby forming the beanie structure. The liquid oxide deposition may be performed at a temperature less than about 35° C. The completed beanie structure extends vertically and horizontally from the first layer a distance about 5 nm to about 100 nm. It is noteworthy that the beanie structure is formed in a single step (the liquid deposition on the first oxide layer). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1E  illustrate steps in a conventional process for forming a beanie structure on an FET gate. 
       FIGS. 2A ,  2 B- 1 ,  2 C- 1  and  2 D- 2 F schematically illustrate steps in a process for forming a beanie structure on an FET gate during front-end-of-the-line (FEOL) processing, in accordance with a first embodiment of the invention. 
       FIGS. 2B-2  and  2 C- 2  illustrate an alternate process for forming a beanie structure during FEOL processing. 
       FIGS. 2B-3  and  2 C- 3  illustrate another alternate process for forming a beanie structure during FEOL processing. 
       FIGS. 3A ,  3 B,  3 C- 1 ,  3 D 1 ,  3 E and  3 F schematically illustrate steps in a process for forming a beanie structure on regions embedded in a dielectric during back-end-of-the-line (BEOL) processing, in accordance with a further embodiment of the invention. further embodiment of the invention. 
       FIGS. 3C-2  and  3 D- 2  illustrate an alternate process for forming a beanie structure during BEOL processing. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In preferred embodiments of the invention, beanie structures of oxide are formed in a single deposition step (a liquid oxide deposition step), in which silicon oxide is deposited from a supersaturated aqueous solution of silica with hydrofluoric acid onto previously formed oxide cap structures. This oxide deposition selectively forms an oxide layer on oxide surfaces. Accordingly, a beanie structure may easily be obtained by liquid oxide deposition on an appropriately shaped oxide cap. This process may be employed in both FEOL and BEOL situations, as detailed below. 
   First Embodiment 
   FEOL Process 
   In this embodiment, a beanie structure is formed on a gate stack during FEOL processing. A substrate  10  has a gate oxide, a gate conductor, and possibly other films deposited thereon, to form a gate stack  15 . A silicon oxide cap layer  20  is deposited on top of the gate stack, as shown in  FIG. 2A . Individual gate structures  21  are then formed by lithographic patterning and etching, using techniques known in the art. The gate structures each have an oxide cap  22 , as shown in  FIG. 2B-1 . 
   The substrate  10  is then placed in a deposition bath  30  for the liquid deposition of silicon oxide. U.S. Pat. No. 5,232,781 (Takemura et al.) describes a type of deposition bath used in this embodiment, namely a supersaturated aqueous solution of silica with hydrofluoric acid. As is understood by those skilled in the art, boric acid is added to the hydrofluoric acid as a scavenger, to induce the supersaturation of the solution with silicon oxide. This process proceeds at approximately room temperature (25° C. to 35° C.). Another significant advantage of this process is that the oxide deposition occurs selectively on oxide surfaces exposed to the bath. Since oxide cap  22  has horizontal and vertical surfaces, the oxide growth is in both the vertical and horizontal directions. This results in formation of an additional oxide cap or beanie  31 , above and on the sides of oxide cap  22  ( FIG. 2C-1 ). The beanie structure extends vertically and horizontally from oxide cap  22  a distance of about 5 nm to about 100 nm. 
   Alternatively, the growth of oxide in the deposition bath may limited to the horizontal direction. This is done by preparing the gate structure  21  with a nitride cap layer  23 , in addition to an oxide cap layer  22  ( FIG. 2B-2 ). When this structure is exposed to the oxide deposition bath  30 , oxide growth will occur only on the exposed oxide surfaces, namely the vertical surfaces on the sidewalls of layer  22  ( FIG. 2C-2 ). The additional oxide  24  formed in the bath extends horizontally from oxide cap  22  a distance of about 5 nm to about 100 nm. In this instance the combination of oxide cap  22 , nitride cap  23  and oxide  24  may be understood as the beanie structure. 
   The substrate is then removed from the oxide deposition bath, and a dielectric layer  32  is formed which overlies the substrate and the gate structures ( FIG. 2D ). 
   In another alternate process, dielectric material  32  is deposited over the structure of  FIG. 2B-1  before exposure to the oxide deposition bath. The dielectric  32  is planarized so that only top surfaces of oxide cap layers  22  are exposed ( FIG. 2B-3 ). In this case the dielectric material should be chosen so that oxide will not be deposited on the surface of the dielectric. When the structure of  FIG. 2B-3  is exposed to the oxide deposition bath  30 , oxide growth occurs vertically from the top surface of cap  22 , and also occurs laterally from the corner  22   c  of cap  22 . The resulting beanie structure  25  is on top of the cap layer  22  and overhangs the structure  21 , but is not present on the vertical surface of layer  22  ( FIG. 2C-3 ). 
   Referring now to  FIG. 2E , the gate contact structures will be formed in the layer of dielectric material  32 . The dielectric material may be SiLK, aerogel oxide or the like. The dielectric material should be etchable in an etch process which is selective with respect to the beanie oxide material, in order to create a borderless contact. Alternatively, an etch stop layer could be formed on top of the beanie structure, so that conventional silicon oxide could be used as the dielectric material. 
   The dielectric layer is then patterned using a resist layer  41 , and source/drain contact openings  42  are etched in the dielectric ( FIG. 2E ), using an etch process that removes the dielectric material selectively with respect to the oxide of the beanie  31  (alternatively, beanie  25 ). As shown in  FIG. 2E , this ensures that the source/drain contact is separated from the gate structure  21  by either the beanie structure or a layer  32   s  of dielectric material on the sidewall of gate structure  21 . The thickness (lateral dimension) of layer  32   s  is determined by the extent of overhang of the beanie structure over the gate structure  21 . It will be appreciated that the beanie structure permits opening  42  to extend laterally over gate structure  21 , so that a borderless contact may be formed. 
   Resist layer  41  is then removed and the dielectric layer is again patterned to form the gate contacts. The gate contact openings  43  are formed by using another etch process (or a combination of etch processes) that removes the dielectric material overlying the gate structure, and etches through the beanie oxide and the oxide of the cap  22 . The resulting structure is shown in  FIG. 2F . Contact opening  43  is said to be a fully landed contact on gate structure  21 . It is also possible to build a partially landed contact to the gate, in which the bottom of opening  43  is partly on the top of structure  21 , and partly in dielectric  32 . In this case the etch process must be controlled so that opening  43  does not reach substrate  10 . 
   It will be appreciated that, provided the etch process for dielectric layer  32  is selective with respect to the nitride in layer  23  and the oxide in structure  24 , the same results may be obtained using the beanie structures of  FIG. 2C-2 . 
   The contacts are then metallized by depositing metal in openings  42  and  43 . Processing then continues using techniques known in the art. 
   It is noteworthy that, after the oxide caps  22  are formed on the gate structures, the beanie structures are formed in a single process step. This permits the advantages of beanie structures to be realized at considerably lower cost than with conventional processes. 
   Second Embodiment 
   BEOL Process 
   Beanie structures may also be formed in a single deposition process and used to advantage at the back end of the line (BEOL). One typical BEOL arrangement is shown in  FIG. 3A , where two metal lines  51  are embedded in an interlayer dielectric  50  (the top surfaces of the metal lines and dielectric layer being coplanar), while another metallized region  501  is encased in the dielectric. It is often desired to form contacts to both regions  51  and  501 . 
   In this embodiment, silicon dioxide caps  52  are first formed at the tops of regions  51 , as shown in  FIG. 3B . This may be done (for example) by etching or polishing regions  51  to form a shallow recess therein, depositing a blanket layer of oxide, and subsequently polishing away the blanket layer so that only oxide  52  in the recesses remains. At this point, the top surface of oxide  52  is coplanar with the top surface of dielectric  50 . The surface of layer  50  is then etched or polished, so that oxide caps  52  protrude from the surface ( FIG. 3C-1 ). 
   As in the first embodiment, the entire structure is then placed in deposition bath  30  for the liquid deposition of silicon oxide. As shown in  FIG. 3D-1  (compare  FIG. 2C-1 ), oxide then grows on oxide surfaces exposed to the bath; the oxide growth is in both the vertical and horizontal directions. This results in formation of an additional oxide cap or beanie  61 , above and on the sides of oxide cap  52 . Alternatively, the structure of  FIG. 3B  may be left unchanged until after the liquid deposition of oxide. In that instance, an oxide cap or beanie structure will be formed similar to that shown in  FIG. 2C-3 . 
   In another alternative, a nitride cap layer  53  may be formed on top of oxide cap  52  ( FIG. 3C-2 ). When this structure is exposed to deposition bath  30 , oxide  54  grows only horizontally from the vertical surface of cap  52  ( FIG. 3D-2 ). In this instance, a combination of oxide cap  52 , nitride cap  53  and oxide  54  may be understood as the beanie structure, similar to the first embodiment. 
   The beanie structures may then be used in forming openings for borderless contacts, as in the first embodiment. An additional dielectric layer  62  is formed which overlies layer  52  and the beanie structures ( FIG. 3E ). The dielectric material of layers  50  and  62  (SiLK, aerogel oxide or the like) should be etchable in an etch process which is selective with respect to the beanie oxide material. The dielectric layer  62  is then patterned using a resist layer  71 , and a contact opening  72  to buried region  501  is etched in the dielectric, using an etch process that removes the dielectric material selectively with respect to the oxide of the beanie structures  61  (alternatively, oxide  54 ). As shown in  FIG. 3E , this ensures that the contact to region  501  is isolated from region  51  by both the beanie structure and a layer  50   s  of dielectric material adjacent to region  51 . Resist layer  71  is then removed and the dielectric layer  62  is again patterned to define the contact openings to regions  51 . These contact openings  73  are formed by using another etch process (or a combination of etch processes) that removes the dielectric  62 , and etches through the beanie oxide and the oxide of the cap  52 . The resulting structure is shown in  FIG. 3F . As in the first embodiment, it will be appreciated that the overhang of the beanie structure permits a borderless contact to be formed. 
   The contacts are then formed by depositing metal in openings  72  and  73 . Processing may then continue, using techniques known in the art. 
   While the present invention has been described in conjunction with specific preferred embodiments, it would be apparent to those skilled in the art that many alternatives, modifications and variations can be made without departing from the scope and spirit of the invention. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.