Patent Publication Number: US-6218218-B1

Title: Method for reducing gate oxide damage caused by charging

Description:
This application claims the benefit of Provisional No. 60/020,002 filed Jun. 21, 1996. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to an improved method for reducing the problems caused by electrostatic charging during the wafer manufacturing process, and in particular, to a method that reduces the failure of gate oxides created when charges within a wafer cause damage to the gate oxide during a subsequent polysilicon deposition process. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with MOS technologies, as an example. 
     As MOS technology advances, increasingly thinner gate oxide layers are required. With thinner layers comes an increased demand for improved efficiency, decreased failure rates, and greater control over the process. One problem that is widely recognized in the wafer manufacturing process is gate oxide charging. Several steps in the process have been implicated in the charging process, including: ion implantation, ashing, surface scrubbing, and etching. 
     Gate oxide damage may also occur during the subsequent deposition of an oxide layer. For example, damage due to a plasma enhanced TEOS or silicate deposition, leading to degradation in hot carrier lifetime, has been reported to vary with the thickness of the oxide layer deposited. Gate oxide damage has been shown to be reduced by a protective layer of polysilicon with a thickness of 150 nm. The 150 nm polysilicon layer was reported to block charging effects during subsequent processing. 
     The conventional theory of plasma-charging damage, on the other hand, relies strongly on the antenna&#39;s ability to collect charges from the plasma. In contrast, a process known as “photoconduction” has been proposed as a mechanism for charging damage that increases as the thickness of the deposition film increases. 
     Both theories, however, fail to explain the continuously increasing damage as more dielectric is deposited. The failure of the photoconduction theory is particularly true since even a thin layer of dielectric can prevent the antennae from further charging. 
     Alternatively, others have proposed that the correlation between charging and oxide deposition indicates that the thickness of the layer causes a “photocurrent” to fall off quickly as the inverse square of the distance that the photons have to travel. In the vertical direction, the fall off would be even faster because less photons would penetrate the thickness of the oxide. 
     Therefore, the current understanding in the field is that thickness dependent damage and the effect of saturation is a combination of the photocurrent fall off and damage saturation, whether the photocurrent does or does not fall off. Furthermore, the photocurrent fall off would also make the photoconductive effect a very local phenomenon. 
     SUMMARY OF THE INVENTION 
     It has been found that gate oxide charging during the wafer manufacturing process leads to high failure rates. The need for eliminating gate oxide failure caused by wafer charging has become increasingly important because gate oxide layers are being made thinner and thinner. What is needed, therefore, is a method of plasma enhanced oxide deposition that prevents circuit failure caused by charging effects. 
     The present invention provides a method of eliminating the conditions that cause gate oxide damage during the post polysilicon gate oxide deposition. In this regard, the present invention comprises maintaining the internal pressure within a production line chamber until the power level within the chamber drops below a specified level. Maintaining the pressure within the plasma enhanced production line during power down remedies the impact of charging, which otherwise creates catastrophic damage impacting on the reliability of the oxide. Furthermore, the present invention reduces the gate oxide damage that occurs during oxide deposition, which is caused by inefficient lateral discharging of the layer following deposition of the polysilicon layer. 
     Although a variety of assembly lines have been used in the art, the invention can be used to solve a problem common to all like-production assembly lines. The present invention can be used with all like-production assembly lines, as they are programmed by the manufacturer to reduce the pressure within the production line chamber following the TEOS deposition step, also known as the power lift step. The post oxide deposition step is used to release the wafer from the electrostatic charging that occurs during the wafer manufacturing process. 
     Another aspect of the present invention is to allow for continued use of any of the processing gases used in the art of plasma deposition. The present invention can use a production line chamber processing gas such as TEOS gas, during the creation of the gate oxide assembly. Other gases, such as silane, may also be used with the present invention. 
     The present invention solves the failure rate caused by charging during the manufacturing of wafers and improves the gate oxide quality of finished products. The present invention was used to discover the mechanism of charging, mainly the drop in pressure during the post oxide deposition step. 
     These and other features of the invention that will be apparent to those skilled in the art from the following detailed description of the invention, taken together with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic drawing of a typical gate assembly, including the deposited layers; 
     FIG. 2 is a schematic drawing of an antenna assembly; 
     FIG. 3 is an electronmicrograph of charged induce damage following oxide deposition; 
     FIG. 4 is a graph showing the difference between gate oxide induced failure, with or without the power lift step, and the relative position of the failure on the wafer. 
     Corresponding numerals and symbols in different figures refer to corresponding parts unless otherwise specified. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     In FIG. 1, a typical gate oxide assembly is shown and generally denoted as  10 . The first layer in FIG. 1 is a silicon layer  11  that is, or is part of, a silicon wafer. Over silicon layer  11 , a thin layer of gate oxide  16  is grown. The thickness of the gate oxide layer  16  will depend on the particular application of the gate oxide assembly  10 . For example, depending on the type of transistor for which the gate oxide assembly  10  is designed, the thickness of gate oxide layer  16  can be of any thickness. In one embodiment of the present invention the thickness of the oxide layer  16  is less than 1000 angstroms. The polysilicon gate may be polysilicon or polysilicon overlaid with a layer of silicide. Alternatively, the polysilicon may be doped with phosphorous or boron. The silicide is formed by reacting a metal, for example, Ti or W, with the polysilicon or by depositing the silicide layer on the polysilicon. For most applications for use with the present invention the thickness of the gate oxide layer  16  is less than 1000 angstroms. 
     Prior to the gate oxide growth, a filed oxide  18  is grown to provide electrical isolation. Deposited over field oxide layer  18 , and coming in contact with the gate oxide layer  16 , is a layer of polysilicon  22 . Alternatively, polysilicon layer  18  can be a polysilicon layer overlaid with a silicide layer, known as a polysilicon/silicide stack. Specifically, the polysilicon layer  22  may be a TEOS or PETEOS polysilicon. A layer of tungsten silicide  24  is shown deposited over the polysilicon layer  22 . Alternatively, the tungsten silicide layer  24  can be a polysilicide or a dope polysilicon layer. A further layer of MLO oxide  26  is shown placed over the tungsten silicide layer  24 . Finally, traversing the MLO oxide layer  26  is a metal coupling  28  that comes in contact with the tungsten silicide layer  24 . 
     In FIG. 2, a top view of a typical antenna assembly  27  is depicted. A metal coupling layer  28  in the form of finger-like projections is disposed, in an opposite direction, from tungsten silicide  24  a finger-like sublayer. Connecting the metal coupling layer  28  and the tungsten silicide sublayer  24  are vias  25 . The finger-like tungsten silicide sublayer  24  comes in contact with a gate oxide island at the opposite end of the interconnection of the metal coupling layer  28  and the tungsten silicide sublayer  24 . 
     FIG. 3 is an electronmicrograph of damage to the gate oxide caused by charging in the enhanced plasma deposition chamber without the use of the present invention. The scale shown in the bottom of the figure is 2 μm. These charge generated defects are responsible for the gate oxide integrity defects produced during the power lift in the production chamber. 
     FIG. 4 is a comparison of gate oxide integrity (GOI), produced with and without the method of the present invention. The production line was an Applied Materials P5000 production line using the equipment manufacturer&#39;s specifications and installation. The figure demonstrates the reduction in failures due to a lack of charging by using the method of the present invention during the post oxide deposition power lift step. FIG. 4, therefore, shows that by using the revised high pressure post oxide deposition power lift method of the present invention, the amount of gate oxide integrity breakdown is decreased to about zero in the center of the wafer and greatly reduced damage from charging resulted. 
     Specifically, the improved method of the present invention comprises maintaining the pressure in a production line chamber above a specified pressure. A specified pressure for use with the present invention is about of above 1.2 Torr following the deposition of a TEOS layer. The specified pressure can be used during the power lift step. More specifically, it is an object of the present invention to solve the critical problem of assembly line damage caused by electrostatic charging following the deposition of an oxide layer by maintaining the specified pressure in the production line chamber at a pressure at or above 1.2 Torr. This specified pressure may be used throughout the processing of gate oxide assemblies, like gate oxide assembly  10 . Specifically the specified pressure is maintained during a post oxide deposition step. The post oxide deposition step can be a power lift step. In another embodiment of the present invention the pressure is maintained about or above 1.2 Torr until electrical power is reduced to at least 50 Watts in the production line chamber. 
     Therefore, it is an embodiment of the present invention to maintain pressure within the production line chamber at the specified pressure throughout the oxide deposition process. In one embodiment of the invention the pressure is about 1.2 Torr. Another embodiment of the invention is to maintain a pressure of about 1.5 Torr during the plasma enhanced deposition step in the production line chamber. 
     The invention can be used with any of the production lines available for wafer production. In one embodiment of the present invention is the use of an Applied Materials P5000 production line. However, all like-production lines can be used with the invention. What is meant by all like-production assembly lines within the scope of the present invention, are production lines that use plasma enhanced deposition. Furthermore, the method of the present invention can be used following an oxide deposition step that produces an oxide layer having a thickness less than 1000 angstroms. 
     In another embodiment of the present invention a processing gas may be used in the polysilicon plasma deposition step. In one embodiment of the present invention, the gas used in the processing chamber is TEOS gas. In yet another embodiment, of the present invention the processing gas is silane. 
     Yet another embodiment of the present invention is a method of reducing charging damage that involves maintaining the pressure within the chamber of a production line, such as an Applied Materials P5000 production line, about or above 1.2 Torr wherein the steps performed within the production chamber comprise: a setup step, where for example, a gate oxide assembly is created; a polysilicon deposition step; and a power lift step. In one embodiment, the pressure in the production line chamber is maintained about or above 1.2 Torr during, specifically, the power lift step. 
     For example, a variety of tools to detect damage by charging have been developed over the last decade. Large area capacitors (approx. 0.15 cm 2 ) were initially used, allowing for the gate oxide integrity (GOI) breakdown distribution to be effectively determined. Depending upon the monitor sensitivity (polysilicon terminating on moat or on field oxide), cleanup, furnace issues and LOCOS processing can be effectively monitored. The large area capacitor can be used to monitor QBD (breakdown charge). Alternatively, another detector for use with the present invention is a small capacitor. A quasistatic evaluation of a capacitor can also be monitored for DIT (density of interface traps) values or for changes in DIT. The monitor used herein for detecting charging has been the antenna structure. 
     The antenna monitor can have one or more capacitors that vary in antenna (collector) to moat (discharge) area ratios. This antenna monitor is a highly effective tool for detecting process/equipment charging. Under these conditions a charge on an isolated gate can be monitored for indications of damage. The MOS transistor can also be monitored for increases in the substrate current. The following example demonstrates the use of the antenna structure. 
     Using the present invention the process parameters and their contributions toward gate oxide damage were detailed, and an explanation of the charging mechanism was found. The identification of the mechanism of charging lead to the identification of the improved methods of the present invention for improving processing efficiency, decreasing failure rates and allows for greater control over the device the processing and equipment used for processing wafers. 
     In operation, N-100 wafers having a resistivity of 15 ohm-cm were processed through a pad oxide of 20 nm and a Silicon Nitride deposition of 140 nm. After a photolithography pattern is placed, the Silicon Nitride was removed by plasma etching. The wafers were cleaned in a piranha solution and an SC2 clean preceded a field oxidation of 0.60 μm. Following excess Silicon Nitride removal and a dummy gate oxidation of 30 nm, the oxide was removed and an SC2 clean preceded the growth of the gate oxide of 13 nm. The gate oxide was grown using a dry-wet-dry routine (the dry segments were 02 and 4% HCl). The photolithography patterns of a charging monitor were used (capacitor antenna structures having ratios of 100/1 up to 7500/1). A polysilicon layer of 0.2 μm and a tungsten silicide layer of 0.25 μm were deposited for the gate electrode and a plasma etch removed the excess gate material after the patterning operation. The wafers were ashed to remove the photoresist. A 0.30 μm TEOS film was deposited and then etched to form sidewalls. The wafers were probed at post sidewall etch to insure the integrity of the gate oxide at that point. 
     A 30 nm plasma enhanced oxide was deposited in an Applied Materials P5000 reactor. In the normal CMOS routing this oxide is used as a screening oxide for the ion implant of the source and drain. The thickness of the deposited oxide was varied along with process parameters for each of the deposition thicknesses to determine the sensitivity to charging. The recipe for the oxide deposition generally consists of three major steps: 1) setup, 2) deposition and 3) power lift. 
     The power lift is a step designed to insure that the wafer releases from any electrostatic bonding to the substrate, and is standard in the industry. Variations in RF power, temperature of substrate, were made along with variations in the thickness of the deposited film (0.03-1.1 μm). Some splits included removal of selected steps in the process. 
     Additional details of the process are given in Table 1. After the oxide was deposited, the wafers were processed through a contact oxide removal. The contacts were wet etched so as not to confuse any dry etching issues with the deposition under investigation. Wafers were then probed on a 450 Keithley tester for GOI values. 16 sets of antenna ratio capacitors were evaluated. The ratios of polysilicon moat to oxide gate ranged from 100/1 to 7500/1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Deposition Process 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Parameter 
                 Setup 
                 Deposition 
                 Power-Lift 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Time (Sec) 
                 15 
                 Variable 
                 20 
               
               
                   
                 Pressure (Torr) 
                 9 
                 9 
                 &lt;1 
               
               
                   
                 RF Power (W) 
                 0 
                 500 
                 50 
               
               
                   
                 Temp (C) 
                 390 
                 390 
                 390 
               
               
                   
                 Spacing (cm) 
                 0.508 
                 0.508 
                 1.98 
               
               
                   
                   
               
            
           
         
       
     
     The voltage needed to produce 1 nA of current, Vlf, was used as a criteria for determining oxide failure. A drop of 5% in Vlf was deemed a failure. Some of the legs of the test matrix produced charging, with the centers of the wafers being 2s predominantly affected. The current-voltage characteristics of the failed capacitors were found to be near shorts, as expected since the starting material was N-type. The charging on the capacitors having antenna ratios of 2500/1 and greater. 
     Table 2 demonstrates that charging occurred on all sections of the test, except, when the deposited oxide was thick or the power lift step was omitted. Note that in Table 2 there is a correlation of failures to deposited oxide thickness. The data show that there is less damage due to charging as the deposited oxide becomes thicker. This indicates that the charging occurs at the end of the process, after the oxide had been deposited. The failures typically occurred in the center of the wafer on the ratio capacitors of 2500/1 and above. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Number of Capacitors Showing Vlf Failures 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 30 nm TEOS 
               
               
                 Antenna 
                   
                   
                   
                 (w/o Power 
               
               
                 Ratio 
                 30 nm TEOS 
                 100 nm TEOS 
                 1.1 μm TEOS 
                 lift) 
               
               
                   
               
               
                 7500/1 
                 5 
                 1 
                 0 
                 0 
               
               
                 5000/1 
                 3 
                 0 
                 0 
                 0 
               
               
                 2500/1 
                 1 
                 0 
                 0 
                 0 
               
               
                 1000/1 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     Both the removal of the Power Lift step and the oxide thickness sensitivity indicated that the charging was occurring during the power lift step of the depositions. Additional studies were undertaken to determine the mechanism underlying the oxide deposition damage. Variations were made to the spacing, pressure and temperature at the Power Lift step. Table 3 demonstrates the results. To control for the thickness of the deposited film, this thickness was held at 30 nm. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Number of Capacitors Showing Vlf Failure 
               
            
           
           
               
               
               
               
            
               
                 Antenna 
                 Pressure (Torr) 
                 Temp (C.) 
                 Spacing (w/o Power Lift) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Ratio 
                 0.6 
                 1.5 
                 7.0 
                 390 
                 430 
                 1.79 
                 1.98 
               
               
                   
               
               
                 7500/1 
                 5 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 5000/1 
                 3 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
            
           
         
       
     
     For use with the present invention the pressure conditions were shown to cause the charging following deposition of an oxide layer, in particular during the power lift step. Additional studies were performed to better define the pressure above which no charging could be produced. At pressures below 1.2 Torr charging could be found. To understand the source of the charging, a study was conducted in which the Power Lift step was used but the wattage was reduced to zero and the pressure was dropped to 0.9 Torr. Under these conditions there was no degradation in Vlf. 
     Charges are created at the surface of the deposited oxide during the power lift step even though the power is only 50 watts. When the pressure within the chamber is high enough, the charges are bled off to the edge of the wafer and no damage is done. If the pressure drops below a specified level, however, there are insufficient molecules to make the charge transport efficient and the charge is not drained away. Under these conditions a charge buildup occurs at the oxide surface, especially at the center of the wafer where the charge drain off is at a minimum. The charge is reflected through the thin deposited oxide to the polysilicon electrode. When the accumulated charge produces a voltage that results in the dielectric strength of the oxide being exceeded, an oxide rupture occurs as the current is discharged to the substrate. 
     The deposition of a thin plasma oxide following sidewall etch was shown to be capable of causing charging sufficient to damage the gate oxide beneath the polysilicon electrode. The thickness of the deposited oxide, the pressure in the chamber, and the power level are all important factors in the charging mechanism. The tendency to create charging grew less as the deposited oxide thickness was increased. For thin oxides (30 nm or less) and power levels of 50 watts, a parameter found to be important during the post deposition step was pressure. The charge bleed off at low pressure (&lt;1.2 Torr) was insufficient to affect the center of the wafer and resulted in damaged gate oxides. This study demonstrates that as the gate oxides become thinner charging conditions may be brought to surface which did not exist on prior oxide thicknesses. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.