Method of interface state reduction in MNOS capacitors
CROSS-REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned U.S. patent application Ser. No. 191,526 filed May 9, 1988 entitled &#34;Method of Noise Reduction in CCD Solid State Imagers&#34; to Sheu and Hawkins. 
     FIELD OF THE INVENTION 
     This invention relates to MNOS (Conductor-Nitride-Oxide-Semiconductor) capacitors, and more particularly to a method to reduce the interface state density in such capacitors. 
     BACKGROUND OF THE INVENTION 
     MNOS capacitors are often advantageously used. M represents a conductor such as metal but more frequently is polysilicon. N is a nitride layer. O is an oxide layer preferably SiO 2  and S is the silicon substrate. A problem with MNOS capacitors is that the interface states density in MNOS capacitors is higher than that in MOS (Metal-Oxide-Semiconductor) capacitors due to the fact tha nitride is a diffusion barrier to molecular hydrogen which passivate the interface states. Alternative ways to introduce hydrogen into the active area in the MNOS devices is performed by removing the nitride in the field oxide in the vicinity of device active area followed by annealing in hydrogen at 1000° C. to laterally drive the hydrogen to the active Si/SiO 2  interface, or, by implanting hydrogen into the field oxide area followed by annealing at 450° C. also to laterally drive the hydrogen to the device active area. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new method to introduce atomic hydrogen into the MNOS capacitors to reduce the Si/SiO 2  interface state density. This method involves deposition of a CVD oxide layer on top of the MNOS capacitors followed by the deposition of an Al layer. After annealing the wafer, the Al and CVD oxide are etched off by standard wet etch. Al react with trace water at the Al/oxide interface and release atomic hydrogen. One importance difference between molecular and atomic hydrogen is that atomic hydrogen can diffuse through the nitride layer at 450° C. while the other cannot. As a result, interface state density in MNOS capacitors can be reduced significantly after this treatment while no effect is observed after conventional sintering in a hydrogen containing ambient. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an MNOS capacitor during the sintering step of this invention; and 
     FIG. 2 is a graph that compares the interface states density with and without using the method of this invention. 
    
    
     MODE OF CARRYING OUT INVENTION 
     Referring to FIG. 1 a MNOS capacitor 10 is shown just prior to a sintering step. It has six layers. Layer 12 is a silicon substrate. A gate oxide layer 14 SiO 2  is thermally grown from layer 12; layer 16 is CVD nitride which can be formed by the reaction of SiH 2  Cl 2  with NH 3  at 800° C.; layer 18 is a patterned polysilicon gate electrode. The term M in MNOS stands for metal, but any other conductor can be used. In fact, polysilicon is preferred. 
     In accordance with the method of this invention, a Chemical Vapor Deposition (CVD) oxide layer or layer 20, preferably having a thickness of about 5000 Å is deposited on top of a series of MNOS capacitors. An Al film or layer 22 is then deposited on top of the CVD oxide layer. The Al layer is preferably 5000 Å to 1 μm thick. The CVD oxide film is exposed to atmospheric moisture during the transferring from the oxide deposition equipment to the Al deposition equipment. Trace water may be incorporated in the CVD oxide during this time. The capacitors 10 are sintered at 450° C. for 30 minutes. The sintering ambient can be N 2  or 10% H 2  in N 2 . The Al and CVD oxide are then completely etched off by standard wet etch. The result of the above process is that the gate oxide/silicon interface is passivated by atomic hydrogen. 
     As shown in FIG. 2 actual data was obtained using low-frequency capacitance versus voltage measurements. The resultant minimum interface state density D it  on Si/SiO 2  interface was measured from a capacitor using the prior art process discussed above was about: 
     
         15×10.sup.10 /(cm.sup.2.e.V) 
    
     At the same position at the X- axis D it  using the present invention was about: 
     
         2×10.sup.10 /(cm.sup.2.e.V) 
    
     These data show that in accordance with this invention the reduction of interface density is substantial. The interface state density reduction is due to the production of atomic hydrogen released from the Al/oxide interface during reaction between A`and trace water in the oxide. The atomic hydrogen diffuses through the polysilicon gates electrodes 18 and nitride layer 16 to the Si/SiO 2  interface to annihilate interface states. 
     Advantage 
     In order to passivate interface state, the invention not only require minor changes in a conventional process. The invention involves a low temperature process which is required in VLSI processes. The invention is also free from the radiation damage which is inherent in the method using hydrogen implantation. 
     The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.