Abstract:
A cost-effective method for fabricating MIM capacitors ( 120 ). After metal ( 106 ) deposition, the metal oxide ( 108 ) is formed using an oxidation chemistry that includes CO2 and H2. The CO2/H2 gas ratio is controlled for selective oxidation. Thus, the metal ( 106 ) is effectively protected from oxidation due to the existence of both H2 and CO2 as strong reduction reagent.

Description:
This application claims priority under 35 USC §119(e)(1) of provisional application number 60/102,223 filed Sep. 29, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention is generally related to the field of forming capacitors in integrated circuits and more specifically to a method of forming metal-insulator-metal capacitors. 
     BACKGROUND OF THE INVENTION 
     As semiconductor devices continue to scale to smaller and smaller dimensions, capacitors used in integrated circuits, such as those used in DRAM storage cells, need higher capacitance/unit area. One proposed way to accomplish this is to switch from the conventional polysilicon-insulator-polysilicon capacitors to metal-insulator-metal (MIM) capacitors, such as the one shown in FIG.  1 . In the MIM capacitor  10 , metals used as the top  12  and/or bottom  14  electrodes are those with low oxygen affinity such as W, Ni, Co, Ir, Ru, and Pt. The insulator  16  is a metal oxide using metals that have a high oxygen affinity. Exemplary metal oxides include Ta2O5, Nb2O5, Al2O3, Y2O3, HfO2, TiO2, BaO, SrO and the so-called BST (BaxSr1-xTiO3, 0&lt;×&lt;1). These metal oxides have dielectric constant values ranging from 10 to 1000 (depending on material, microstructure, crystalline phase, and crystal orientation). 
     Currently, Ta2O5 and BST are considered the most promising capacitor materials for 256 Mbit and 1 Gbit DRAMs. Ta2O5 and BST can be deposited through reactions of metal-organic precursors and oxygen and annealed in the O2 or UV-O3 ambient. Unfortunately, due to their low oxidation resistance, W, Ni and Co are easily oxidized in the O2 ambient at the high temperatures required. When trying to deposit Ta2O5 on W, the W oxidizes to form WO3-y which can vaporize and result in a nonuniform dielectric. Moreover, Ta2O5-y can be produced instead of Ta2O5. Ta2O5-y is not stociometric and results in reduced capacitance due to leakage. Although Ir, Ru, and Pt have very high oxidation resistance, they are very expensive and difficult to etch. 
     One prior art method of selective oxidation for poly-metal gate formation was proposed by Kobayashi et al (Proc. of 15 th  Conf. Solid State Devices and Material p. 217 (1983)). In this method, a wet hydrogen oxidation procedure was developed to allow the silicon to oxidize while leaving the tungsten unaffected in a post gate-etch oxidation. The method is based on thermodynamic calculations which show that at, for example, 1000° C. and a P(H2O)/P(H2) ratio (partial pressure ratio of H2O and H2) of 1.0e-05, the equilibrium: 
     
       
         Si+2 H2O←→SiO2+2H2 
       
     
     prefers the right side of the reaction, i.e., oxidation of Si and 
     
       
         W+3H2O←→WO3+3H3 
       
     
     prefers the left side of the reaction, i.e., reduction of WO3 to W. Therefore, under appropriate conditions, it is possible to oxidize silicon again such that the oxidation rate of W will be prevented. 
     Unfortunately, it is difficult to generate a uniform steam of pure H2O without heavy metal contaminants because it is hard to completely remove heavy metal from the clean and deionized water used in steam generators for semiconductor device manufacturing. It is also dangerous to generate and control a proper H2/H2O gas ratio using a burning process of mixed oxygen and hydrogen under an excessive hydrogen environment. 
     The above process has been proposed for light thermal oxidation. In the area of non-selective CVD-SiO2, several CO2-H2 gas chemistries have been proposed. One such gas chemistry is SiH4-CO2-H2 and another is SiH2Cl2-CO2-H2. A CO2-H2 gas chemistry has also been used to produce H2O for H2O addition reactions such as 2AlCl3+3H2O→Al2O3+6HCl. In addition, CO gas has been used in metallurgy as a strong reduction reagent of metal oxides for metal production. 
     SUMMARY OF THE INVENTION 
     A cost-effective method for fabricating MIM capacitors is disclosed herein. After metal deposition, the metal oxide is formed using an oxidation chemistry that includes CO2 and H2. The CO2/H2 gas ratio is controlled for selective oxidation. Thus, the metal is effectively protected from oxidation due to the existence of both H2 and CO2 as strong reduction reagents. 
     An advantage of the invention is providing a cost-effective method for fabricating MIM capacitors using a selective metal-oxide deposition. 
     This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-sectional diagram of a previously proposed MIM capacitor; 
     FIGS. 2A-2C are cross-sectional diagrams of a MIM capacitor according to the invention at various stages of fabrication; and 
     FIG. 3 is a cross-sectional diagram of a MIM capacitor according to the invention having an alternative bottom plate structure. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The invention will now be described in conjunction with a process for forming a MIM capacitor. It will be apparent to those of ordinary skill in the art that the benefits of the invention may be applied to other processes requiring metal oxide deposition over exposed metal. 
     It is known that SiH4-CO2-H2 and SiH2Cl2-CO2-H2 may be used to form SiO2 using a blanket CVD process. A preferred embodiment of the invention uses a CO2/H2 gas ratio in a selective oxidation process to protect the metal electrode during metal-oxide formation. Protection is achieved because H2 and CO are strong reduction reagents. The oxidation-reduction reactions are as follows: 
     
       
         CO2+H2→H2O+CO (basic CO2-H2 reaction) 
       
     
     
       
         Si+2H2O→SiO2+2H2 (oxidation) 
       
     
     
       
         M (metal)+H2O→MO (metal oxide)+by-products (oxidation) 
       
     
     
       
         MO+H2→M+H2O (H2 reduction reaction) 
       
     
     
       
         MO+CO→M+CO2 (CO reduction reaction) 
       
     
     By controlling the CO2/H2 gas ratio, the metal will be effectively protected from oxidation due to the existence of both H2 and CO as strong reduction reagents. The CO2/H2 ratio depends on the material and selective oxidation temperature, which can be chosen based on thermodynamic calculation. For the case with W as the bottom electrode and TaCl5 as the precursor for a Ta2O5 film, a typical CO2/H2 ratio is 1E-5 to1E-1 at a temperature of 727° C. 
     Metal organic precursors and metal chlorides can react with H2O in either a wet hydrogen or a dry CO2/H2 oxidation procedure to form metal oxides. For example, 
      Ta(OC2H5)5+H2O→Ta2O5+by-products 
     
       
         2TaCl5+5H2O→2Ta2O5+10HCl 
       
     
     
       
         M (metal)+H2O→MO (metal-oxide)+by-products (oxidation) 
       
     
     
       
         MO+H2→M+H2O (H2 reduction) 
       
     
     
       
         MO+CO→M+CO2 (CO reduction) 
       
     
     In this example, M refers to the underlying metal electrode for which protection from oxidation is desired. H2 and/or CO are strong reduction reagents and protect the metal from oxidation. Although the above example uses a Ta-organic precursor or a Ta-chloride, the invention can be easily applied with similar reactions to other metal-organic precursors and metal-chlorides having a strong oxygen affinity such as Nb, Al, Y, Hf, Ba, and Sr. 
     Like Si, the oxygen affinity of Ta, Nb, Al, Y, Hf, Ba, and Sr are stronger than that of W, Ni, and Co. Therefore, under the appropriate process conditions (temperature, gas pressure, ratio, gas flow, etc.) it is possible to oxidize Ta, Nb, Al, Y, Hf, Ba, and/or Sr to form their oxides (Ta2O5, Nb2O5), Al2O3, etc.) such that the oxidation rate of W, Ni, and/or Co is very small. Accordingly, W, Ni, and/or Co may be used for the metal electrodes of a MIM capacitor with oxides of Ta, Nb, Al, Y, Hf, Ba and/or Sr as the capacitor insulator. Alternatively or additionally, metal-nitrides such as TiN, WN, WSiN, or TiSiN may be used for the capacitor electrodes. 
     A process for forming a MIM capacitor according to a preferred embodiment of the invention will now be described with reference to FIGS. 2A-2C. Referring to FIG. 2A, a semiconductor body  102  is processed through the formation of bottom metal electrode  106 . In typical DRAM applications, metal electrode  106  is in electrical contact (through at least a diffusion barrier  105 ) with semiconductor body  102  and partially extends over dielectric  104 . Dielectric  104  is sometimes referred to as an interlevel dielectric (ILD) or a poly-metal dielectric (PMD). Dielectric  104  typically comprises silicon dioxide. 
     Bottom metal electrode  106  may be formed by conformal metal deposition to a thickness in the range of 100 Å-1000 Å followed by pattern and etch as shown in FIG.  2 A. Alternatively, bottom metal electrode  106  may fill the hole in dielectric  104  as shown in FIG.  3 . In that case, the thickness of bottom metal electrode  106  may be in the range of 1000 Å-10000 Å. In the preferred embodiment, bottom metal electrode  106  comprises W. Alternatively, bottom metal electrode  106  may comprise other metals with low oxygen affinity such as Ni or Co or metal nitrides such as TiN, WN, WSiN, or TiSiN. 
     Referring to FIG. 2B, metal-oxide  108  is deposited over bottom metal  106 . Metal oxide  108  contains a metal of high oxygen affinity such as Ta, Nb, Al, Y, Hf, Ba or Sr. In the preferred embodiment, metal-oxide  108  comprises Ta2O5. Metal-oxide  108  is deposited used a metal-organic precursor or a metal-chloride and a dry CO2/H2 gas chemistry. The ratio of CO2/H2 is controlled to minimize the oxidation of bottom metal  106 . For example, Ta(OC2H5)5 or TaCl5 may be combined with CO2 and H2 to form Ta2O5 with minimal oxidation of bottom metal  106 . An example is the growth of Ta2O5 with TaCl5 as a precursor and W as the bottom electrode  106 . A typical process condition can be a CO2/H2 ratio between 1E-5 to 1E-1 at a process temperature of 727. The range of the ratio and temperature can be chosen based on thermodynamic calculation. A typical temperature range is between 400-1000° C. The time duration depends on the thickness of the TaO5 film thickness required. The thicker the film, the longer the time needed. For Ta2O5, the thickness of metal oxide  108  may be in the range of 50 Å-300 Å. 
     After metal-oxide  108  deposition, the metal oxide  108  is preferably annealed in either a wet hydrogen oxidation procedure or a dry CO2/H2 oxidation procedure under appropriate conditions to protect the metal electrode  106  from oxidation. 
     After annealing, the top metal  110  is deposited as shown in FIG.  2 C. In the preferred embodiment, top metal  110  also comprises W. Alternatively, top metal  110  may comprise other metals with low oxygen affinity such as Ni or Co or metal nitrides such as TiN, WN, WSiN, or TiSiN. 
     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.