Abstract:
A method for fabricating a semiconductor device including a silicon substrate includes forming a thin Oxide base film on a substrate, and then annealing the substrate in ammonia. FET gates are then conventionally formed over the gate insulator. The resultant gate insulator is electrically insulative without degrading performance with respect to a conventional gate oxide insulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This continued prosecution application claims the benefit of U.S. Nonprovisional Patent Application Serial No. 09/479,506, entitled “METHOD FOR ESTABLISHING ULTRA-THIN GATE INSULATOR USING ANNEAL IN AMMONIA,” filed on Jan. 7, 2000, in turn, claiming the benefit of U.S. Provisional Patent Application Serial No. 60/169,540, also entitled “METHOD FOR ESTABLISHING ULTRA-THIN GATE INSULATOR USING ANNEAL IN AMMONIA,” filed on Dec. 7, 1999. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the fabrication of semiconductor devices, and more particularly to establishing field effect transistor (FET) gate insulators. 
     BACKGROUND OF THE INVENTION 
     Semiconductor chips or wafers are used in many applications, including as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices. 
     It can readily be appreciated that it is important to electrically isolate various components of an integrated circuit from each other, to ensure proper circuit operation. As one example, in a transistor, a gate is formed on a semiconductor substrate, with the gate being insulated from the substrate by a very thin dielectric layer, referred to as the “gate oxide” or “gate insulator”. As the scale of semiconductor devices decreases, the thickness of the gate insulator layer likewise decreases. 
     As recognized herein, at very small scales, the gate insulator can be become so thin that otherwise relatively small encroachments into the gate insulator layer by sub-oxides from the substrate and from adjacent polysilicon connector electrodes can reduce the insulating ability of the gate insulator layer. This poses severe problems because under these circumstances, even very minor defects in the substrate can create electron leakage paths through the gate insulator, leading to catastrophic failure of the transistor. 
     To circumvent this problem, alternatives to traditional gate oxide materials, such as high-k dielectric materials including nitrides and oxynitrides that can be made very thin and still retain good insulating properties, have been proposed. Unfortunately, it is thought that these materials can degrade the performance of the transistor. Nitride, in particular, has been considered undesirable because it promotes unwanted leakage of electrons through the gate insulator layer. 
     Furthermore, as the gate insulator layer becomes very thin, e.g., on the order of nineteen Angstroms (19 Å), device integration becomes highly complicated. Specifically, it is necessary to etch portions of the polysilicon electrodes down to the substrate, but stopping the etch on a very thin, e.g., 19 Å gate insulator layer without pitting the substrate underneath becomes problematic. Accordingly, the present invention recognizes that it is desirable to provide a gate insulator layer that can be made very thin as appropriate for very small-scale transistors while retaining sufficient electrical insulation properties to adequately function as a gate insulator, and while retaining sufficient physical thickness to facilitate device integration, without degrading performance vis-a-vis oxide insulators. 
     BRIEF SUMMARY OF THE INVENTION 
     A method for making a semiconductor device includes providing a semiconductor substrate, and establishing an oxide base film on the substrate. The substrate is annealed, preferably in ammonia at temperatures up to eleven hundred degrees Celsius (1100° C.), after which FET gates are formed on portions of the film. The preferred base film defines a thickness of no more than twenty four Angstroms (24 Å). However, after annealing, the base film has been converted to a nitrided oxide film having electrically insulating properties which correspond to that of a thinner conventional oxide film having a physical thickness of only 20 Å thick to inhibit undesired tunneling through the nitrided oxide film, thereby resulting in a relatively lower standby current for a relatively higher drive current and capacitance. 
     Other features of the present invention are disclosed or apparent in the section entitled 
     DETAILED DESCRIPTION OF THE INVENTION 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     For understanding of the present invention, reference is made to the accompanying drawing in the following DETAILED DESCRIPTION OF THE INVENTION. In the drawings: 
     FIG. 1 is a flow chart of the manufacturing process; 
     FIG. 2 is a side view of the device after forming the base film on the substrate; 
     FIG. 3 is a side view of the device after annealing the base film; and 
     FIG. 4 is a side view of the device after forming the FET gate stacks on the nitride film. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention are equally applicable to a wide range of semiconductor and integrated circuit design and manufacture regimens, including but not necessarily limited to the production of non-volatile memory devices. All such implementations are specifically contemplated by the principles of the present intention. 
     Referring initially to FIGS. 1 and 2, at block  10  in FIG. 1 a semiconductor substrate  12  (FIG. 2) such as Silicon is provided, and then at block  14  a thin Oxide base film  16  is grown on the substrate  12  in accordance with oxide film formation principles known in the art, in direct contact with the substrate  12 . The thickness “t” of the base film  16  is no more than twenty four Angstroms (24 Å). 
     Moving to block  18  of FIG.  1  and referring to FIG. 3, the substrate  12  with film  16  is annealed in situ in ammonia (NH 3 ) at a temperature of up to eleven hundred degrees Celsius (1100° C.) to establish a Nitrogen concentration in the base film  16 , thereby forming the nitrided oxide film. The Nitrogen is represented by the dots  19 . In accordance with present principles, after annealing, the electrically insulating properties of the nitrided oxide film corresponds to that of a thinner conventional oxide film having a physical thickness of only 20 Å thick (yet having a physical thickness nearly equal to the film  16  prior to annealing) to inhibit undesired tunneling through the nitrided oxide film, thereby resulting in a relatively lower standby current for a relatively higher drive current and capacitance. 
     Next, at block  20  in FIG.  2  and referring now to FIG. 4, a polysilicon-based field effect transistor (FET) stack  28  is formed on the reduced-hydrogen nitrided oxide film, the nitrided oxide film having been converted from the film  16 , in accordance with FET gate stack deposition and patterning principles known in the art. After forming and patterning the FET stacks  28 , the process is completed by forming FET sources and drains  36 ,  38  using conventional principles, and contacts, interconnects, and FET to FET insulation are likewise conventionally undertaken. 
     With the above disclosure in mind, the ammonia anneal of the oxide base film effects the equivalent “electrical thickness” (i.e., electrically insulating properties) of a thinner oxide base film in the formed reduced-hydrogen nitrided oxide film. In other words, for a film that is sufficiently thick for the above-mentioned structural considerations, e.g., 24 Å thick, after annealing, the film advantageously behaves electrically like a film that is only 20 Å thick. This, in turn, advantageously decreases subsequent electron tunneling through the reduced-hydrogen nitrided oxide film, thereby resulting in a lower standby current for higher drive current an capacitance, compared to a film which has not been annealed in ammonia. 
     The present invention has been particularly shown and described with respect to certain preferred embodiments of features thereof. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. In particular, the use of: alternate layer deposition or forming methodologies; etching technologies; masking methods; lithographic methods, passivation and nitridization techniques; as well as alternative semiconductor designs, as well as the application of the technology disclosed herein to alternate electronic components are all contemplated by the principles of the present invention. The invention disclosed herein may be practiced without any element which is not specifically disclosed herein. The use of the singular in the claims does not mean “only one”, but rather “one or more”, unless otherwise stated in the claims.