Method to prepare hemi-spherical grain (HSG) silicon using a fluorine based gas mixture and high vacuum anneal
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional to U.S. patent application Ser. No. 08/166,058 filed Dec. 10, 1993, now U.S. Pat. No. 5,407,534. 
    
    
     This application is related to U.S. patent application Ser. No. 08/058,554, filed May 4, 1993, U.S. patent application Ser. No. 08/106,503, filed Aug. 13, 1993. 
     FIELD OF THE INVENTION 
     This invention relates to semiconductor fabrication processing and more particularly to a process for forming Hemi-Spherical Grain (HSG) silicon. 
     BACKGROUND OF THE INVENTION 
     The continuing trend of scaling down integrated circuits has forced the semiconductor industry to consider new techniques for fabricating precise components at submicron levels. Along with the need for smaller components, there has been a growing demand for devices requiring less power consumption. In the manufacture of transistors, these trends have led the industry to refine approaches to achieve thinner cell dielectric and gate oxide layers. 
     In dynamic semiconductor memory storage devices it is essential that storage node capacitor cell plates be large enough to retain an adequate charge (or capacitance) in spite of parasitic capacitances and noise that may be present during circuit operation. As is the case for most semiconductor integrated circuitry, circuit density is continuing to increase at a fairly constant rate. The issue of maintaining storage node capacitance is particularly important as the density of DRAM arrays continues to increase for future generations of memory devices. 
     The ability to densely pack storage cells while maintaining required capacitance levels is a crucial requirement of semiconductor manufacturing technologies if future generations of expanded memory array devices are to be successfully manufactured. 
     One method of maintaining, as well as increasing, storage node size in densely packed memory devices is through the use of &#34;stacked storage cell&#34; design. With this technology, two or more layers of a conductive material such as polycrystalline silicon (polysilicon or poly) are deposited over an access device on a silicon wafer, with dielectric layers sandwiched between each poly layer. 
     Though stacked (capacitor) storage cells are promising candidates to achieve sufficient storage capacitance in a limited area, as the DRAM cell size shrinks, scaling of the stacked capacitor structures is becoming more difficult. 
     Conventionally, it is known that the storage capacitance can be enhanced by using Hemi-Spherical Grain (HSG) silicon to form the storage node electrode without increasing the area required for the cell or the storage electrode height. The available methods include use of Low Pressure Chemical Vapor Deposition (LPCVD), engraving storage electrodes using poly film followed by P-diffusion utilizing POCl 3  source gas, a mixture of spin-on-glass (SOG), coating the polysilicon with resist, and HSG formation by sequent high vacuum annealing. These methods however, are plagued due to formation of HSG silicon only on the inside of the storage node structure and thinning of the electrode itself. 
     SUMMARY OF THE INVENTION 
     The present invention develops a process for forming a hemi-spherical grained silicon by the steps of: 
     forming a silicon layer; and 
     exposing the patterned silicon layer to a fluorine based gas mixture during an annealing period, thereby transforming the silicon layer into said hemi-spherical grained silicon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a process flow diagram of an embodiment of the present invention; 
     FIG. 1.2 is a process flow diagram of a second embodiment of the present invention; 
     FIG. 1.1 is a process flow diagram of the formation of a DRAM capacitor cell plate using the method of the present invention; 
     FIG. 2 is a composite cross-sectional view of an in-process wafer portion depicting the beginning stages of forming a DRAM container storage cell; 
     FIG. 3 is a cross-sectional view of the in-process wafer portion of FIG. 2 after subjecting the container structure to fluorine based gas mixture during a high vacuum anneal; and 
     FIG. 4 is a cross-sectional view of the in-process wafer portion of FIG. 3, depicting a completed DRAM container storage cell. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The present invention is directed to a process for forming hemispherical grain silicon (HSG) as depicted in the process steps of FIG. 1. 
     Referring now to FIG. 1, in step 1A, silicon is formed over a starting substrate in a reaction chamber. The silicon may be either polycrystalline (poly) silicon or amorphous silicon. In step 1B, the formed silicon is typically patterned (although it doesn&#39;t need to be patterned) into a desired shape by methods know in the art, or methods yet to be discovered. In step 1C, the silicon is then transformed into Hemi-Spherical Grain (HSG) silicon by a unique method. During this step, the silicon is subjected to an anneal at a temperature range of at least 450° C. and at a pressure of 1×10 -3  Torr to 1×10 -10  Torr. A diluted fluorine based gas mixture, such as NF 3 , CF 4 , C 2  F 4  Cl 2  gas (diluted with an inert gas, such as Ar or nitrogen) is bled into the reaction chamber. The result is the transformation of a basically smooth silicon surface into HSG silicon. In step 1D, the HSG silicon is conductively doped. 
     FIG. 1.2 demonstrates, in steps 1.2A-1.2D, the basic concept depicted in FIG. 1 except that the formed silicon need not be patterned prior to forming HSG. 
     The temperature anneals can be implemented in at least two ways. A first way entails fixing the temperature at a constant temperature, say 600° C., for the duration of the anneal. The second way entails slowly ramping up the temperature from room temperature to say 650° C. (560°-620° C. is preferred). It is preferred that the pressure be in the range of 1×10 -3  Torr to 1×10 -10  Torr as the ambient during the anneal is the key to transforming the silicon into HSG silicon. 
     As an example, one of the many uses of the present invention is depicted in the construction of a DRAM container storage cell developed in FIGS. 1.1 and 2-4. 
     Referring now to FIG. 1.1, in step 1.1A, a bottom silicon capacitor plate is formed over a starting substrate in a reaction chamber. The silicon plate may be either polycrystalline (poly) silicon or amorphous silicon. In step 1.1B, the formed silicon plate is patterned into a desired shape by methods know in the art, or methods yet to be discovered. An optional step 1.1E may be performed before or after step 1.1B which entails the steps of forming a layer of disilane based silicon (again, either polycrystalline (poly) silicon or amorphous silicon). In step 1.1C, the silicon is then transformed into Hemi-Spherical Grain (HSG) silicon by subjecting the forming silicon an anneal at a temperature range of at least 450° C. and at a pressure of 1×10 -3  Torr to 1×10 -10  Torr. A diluted fluorine based gas mixture, such as NF 3 , CF 4 , C 2  F 4  Cl 2  gas (diluted with an inert gas, such as Ar or nitrogen) is bled into the reaction chamber. During this time, nucleation begins at the silicon surface as the gas is bled in and the silicon is exposed to heat and vacuum anneal and the native oxide is replaced by the nucleation of the silicon particles. The result is the transformation of a basically smooth silicon surface into HSG silicon. The HSG silicon can be further enhanced by performing an optional etch step that will cause the rugged surface to become more pronounced. In step 1.1D, the HSG silicon is then conductively doped if it hadn&#39;t been previously doped during steps 1.1A and 1.1E, to form the bottom plate of a DRAM capacitor cell. The methods depicted in FIGS. 1, 1.1 and 1.2 can be used in such processes as rapid thermal processing chemical vapor deposition, molecular beam epitaxy or conventional low pressure chemical deposition systems. 
     Referring now to FIG. 2, a starting substrate 20 is processed with diffusion region 24 formed therein, which is formed between word lines 21. A planarized insulating layer 22 (typically borophosphosilicate glass (BPSG)) is formed over substrate 20, word lines 21 and diffusion region 24. An opening in layer 22 is formed to provide access to diffusion region 24. A container structure 23, formed from either amorphous silicon or polysilicon, makes contact to diffusion region 24 and extends above layer 22. 
     Referring now to FIG. 3, silicon structure 23 is subjected to a high vacuum anneal at a temperature of at least 450° C. and at a pressure of 1×10 -3  Torr to 1×10 -10  Torr or less. During the anneal, a NF 3  gas (diluted to approximately 50% with Ar or nitrogen) is bled into the reaction chamber. The result is the transformation of the basically smooth silicon surface of structure 23 (in FIG. 2) into HSG silicon (the rugged structure 23 in FIG. 3). Rugged structure 23 is then conductively doped to serve as a storage node cell plate of an DRAM storage cell. It is preferred to conductively dope structure 23 prior to the anneal step so that problems of doping after HSG formation are avoided. However, if so desired the structure could be conductively doped prior to anneal. 
     Referring now to FIG. 4, the DRAM storage cell is completed by forming cell dielectric layer 41 over structure 23, followed by the formation of a second cell plate 42 (typically a conductively doped polysilicon or metal based layer). From this point on, the wafer is completed using conventional fabrication process steps. 
     It is to be understood that although the present invention has been described with reference to a preferred embodiment, various modifications, known to those skilled in the art, may be made to the structures and process steps presented herein without departing from the invention as recited in the several claims appended hereto.