Abstract:
The present invention provides a method for improving the quality of thin oxides formed upon a semiconductor body. The etch and pre-clean processes are performed in situ, taking place in a single apparatus. This reduces the amount of handling of the wafers, their exposure to clean room air, and time delays between clean and oxidation. This results in both a higher yield and greater reliability. In addition, it reduces equipment requirements. The etch, employing a buffered oxide etchant, resist strip, and pre-clean, all occur in a single apparatus without transfer, yielding better results, despite the inherently dirty nature of the resist strip, than the traditional technique of transferring to a new apparatus for each of these steps. The improvements are particularly important for thin oxides such as the tunnel oxides of EEPROMs.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the fabrication of semiconductor devices and in particular to high-quality thin oxides formed on the surface of a semiconductor substrate. 
     The ability to manufacture high-quality oxides is of extreme importance in the production of semiconductor devices. As the need for ever thinner oxides has increased, the requirement for major improvements to obtain reliable ultra-thin oxide films has become vital. This is especially true in the tunnel oxides of Electrically Erasable Programmable Read-Only Memories (EEPROMs), where the oxide may be less than 100 Å thick. In order to improve these oxides, all phases of their manufacture continue to be examined for possible improvements. 
     In the silicon wafer production process, manufacturers use many differing procedures: however, these generically follow a standard series of steps including 
     0. Initial substrate clean 
     1. EEPROM oxidation 
     2. Mask and etch 
     3. Resist strip 
     4. Oxidation pre-clean 
     a) SC 1   
     b) SC 2   
     5. Form (oxidation) thin oxide 
     6. Polysilicon or amorphous silicon deposition and doping. 
     Here SC 1  and SC 2  refer to Standard (also known as RCA) Clean  1  and  2 . SC 1  is used to remove organic materials and traditionally consists of a mixture of NH 4 OH/H 2 O 2 /H 2 O in the concentration ratios of 1:1:5. SC 2  is for removing metallic contaminants, and traditionally is the mixture HCl/H 2 O 2 /H 2 O, again in the concentration ratios of 1:1:5. Various water rinse and drying stages in between these steps are not shown. 
     Again, this is only the outline of the standard procedure. Due to the importance of high-quality thin oxides, many variations in the etch and pre-clean have been explored. The concentrations and compositions, times and temperatures, and order of steps are all parameters that have been studied and modified searching for improvements. Common variations include also having a HF pre-clean stage in addition to, or replacing, the RCA cleans. 
     The mask and etch step is done using a photoresist layer. This forms the patterned mask that determines those areas of oxide removed during the etch. The subsequent need to remove this photoresist and provide the cleanest possible surface for the thin gate oxidation is the purpose of the pre-clean. As the stripping of the photoresist is an inherently dirty process, the wafers are typically transferred to an additional piece of equipment to remove the resist layer, then transferred again, usually to a wet deck, for the last pre-clean step. This transference has been considered necessary for the best result during this important cleaning stage. Although this cleaning then takes place in a cleaner environment, the use of different apparatus in steps 2 through 4b unavoidably increases handling of the wafers, exposure to cleanroom air, and sit time between the clean and oxidation. This handling and exposure are detrimental to the wafers. In addition, this transference requires distinct pieces of equipment. 
     It is the primary object of the present invention to improve the quality of thin oxides by overcoming these limitations resulting from the amount of handling of the wafers, their exposure to cleanroom air, and time delays between clean and oxidation. 
     It is another object to accomplish these results with a reduction in the amount of apparatus required for processing the silicon wafers. 
     SUMMARY OF THE INVENTION 
     These and additional objects are accomplished by the various aspects of the present invention, wherein, briefly and generally, according to one such aspect, the etch and pre-clean processes of forming a high-quality thin oxide are performed in situ. A patterned silicon wafer is placed in the apparatus ready to be etched and is then removed directly to a furnace for a further oxidation step, the wet etch, the photoresist removal, and pre-clean occurring, in a sequential process within a single apparatus. This results in less handling of the wafer, a decrease in processing time, and, consequently, less exposure to cleanroom air. The result is an oxide of higher quality, as expressed in measures such as the charge to break down, Q BD , a measure of the maximum charge which may be placed across the oxide layer. 
     According to another aspect of the present invention, the amount of processing apparatus is reduced. By combining the etch, resist removal, and pre-clean steps, a single apparatus performs these processes in situ, thereby reducing equipment requirements. 
     In a further aspect, by a suitable choice of parameters and the use of a Buffered Oxide Etchant (BOE), higher wafer yields and reliability are obtained. 
    
    
     All these aspects are important in the manufacturing process of ultra-thin oxide layers, such as those required in the tunnel oxides of Electrically Erasable Programmable Read-Only Memories (EEPROMs) and other semiconductor devices. Additional objects, advantages, and features of the present invention will become apparent from the following( description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  through  1   f  are a cross-sectional view of different steps in the process. 
     FIG. 2 is a flow chart of the process. 
     FIGS. 3 a  through  3   c  present data comparing the process of the present invention with that of the prior art. 
     FIGS. 4 a  through  4   c  show how device yield and reliability vary with variation in etch. 
     FIG. 5 shows the end result of the process for a preferred embodiment as the tunnel oxide in an EEPROM. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention performs in situ etch, resist removal, and pre-clean processes in the manufacture of a high-quality, ultra-thin oxide layer on the surface of a semiconductor. Despite the inherently dirty nature of stripping the photoresist, a higher quality oxide results by executing these steps sequentially within a single apparatus. For specificity, the discussion below is in terms of a preferred embodiment, the tunnel oxide of an Electrically Erasable Programmable Read-Only Memory (EEPROM), but the techniques involved could also be used for Dynamic Random Access Memories (DRAMs), high-speed basic logic functions, or other applications needing such oxides. 
     FIGS. 1 a  through  1   f  show the silicon wafer at the different stages of the process in the preferred embodiment. FIG. 1 a  is the basic silicon substrate  10 , where for an EEPROM the field oxides  11  are already in place. Upon substrate  10  an oxide layer  12 , here about 300 Å thick, is grown in FIG. 1 b . Upon this is placed a patterned photoresist  14  as in FIG. 1 c , which leaves exposed the areas to be etched. The etch is performed using a Buffered Oxide Etchant (BOE, also called buffered HF). FIG. 1 d  is the result of the etch, showing the result oxide layer  12 ′. In FIG. 1 e  the resist is stripped away and the wafer pre-cleaned. Finally, FIG. 1 f  shows the oxide  12 ′ regrown to produce the tunnel oxide  13  of the preferred embodiment. In this embodiment, the tunnel portion of the oxide is less than 100 Å thick and is approximately 0.6 μm wide, the total EEPROM oxide being about 1.8 μm in width. 
     As discussed above, these particular steps, or variations upon them, are fairly standard. The novelty of the current invention is the combination of the etch, resist removal, and pre-clean steps: the wafer is subjected to these processes in situ, within a single apparatus. It goes in as shown in FIG. 1 c  and comes out ready for the tunnel oxide of FIG. 1 f  to be grown. Although it is always desirable to reduce the amount of handling and exposure to cleanroom air that the wafers suffer, it is unexpected that the improvement from this combined process would more than offset the resulting detrimental effects to the pre-clean phase. This is the non-obvious result of the current invention. 
     FIG. 2 is a flow chart to briefly illustrate both the process itself and where this process diverges from the prior art. In step  101 , an oxide layer is grown upon the substrate. Over this is placed the photoresist in step  102 . The end result of this pair of steps corresponds, respectively, to FIGS. 1 b  and  1   c . At this point the wafer is ready to go into the spray machine for processing. Step  103  places the wafer in this machine where steps  104 - 107 , the etch, resist strip, and pre-cleans, all occur. At step  108  the wafer is removed, ready to go to the furnace in step  109  to have the thin oxide grown. The details of these steps will be given below, but the important point here is that steps  104  through  107  all take place in situ within this single apparatus: the wafer goes in as shown in FIG. 1 c  and comes out as in FIG. 1 e . The practice of the prior art is to remove the wafer between steps  105  and  106  in a step, say,  105 . 5 , that would transfer it to a wet deck or other machine in order to continue on to step  106 . It is also common in the prior art to transfer the wafer between steps  104  and  105  in a step  104 . 5 , further increasing the amount of apparatus required. 
     The improvements in the oxides produced by this process are discussed first. The specifics of the process in a preferred embodiment follow. Finally, the thin oxide is placed in a particular environment, an EEPROM, where the requirement of such high-quality ultra-thin layers is vital. 
     FIGS. 3 a ,  3   b , and  3   c  show data for several variations on etch and pre-clean procedures. Clean  1  takes place in situ, with the BOE etch and subsequent pre-clean both occurring within a spray processor. In Clean  2 , after the resist is stripped, the wafers are transferred to a wet deck for cleaning. Table 1 gives the outline of these three variations of the etch and pre-clean phase of fabrication. Cleans  1  and  1   a  are an embodiment of the current invention and are discussed more below, while Clean  2  is an example of the prior art. For comparing data, Clean  1  is used as it differs from Clean  2  only in the transfer to the wet deck. This pair can be considered as a sort of reference embodiment of the present invention and the prior art, respectively, both having a pre-clean phase consisting just SC 1  followed by SC 2 . 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Clean 1 
                 Clean 1a 
                 Clean 2 
               
               
                   
                   
               
             
             
               
                   
                 wet processing: 
                 wet processing: 
                 wet processing: 
               
               
                   
                 BOE 
                 BOE 
                 BOE 
               
               
                   
                 resist strip 
                 resist strip 
                 resist strip 
               
               
                   
                 SC1 
                 SC1 
                 Transfer 
               
               
                   
                 SC2 
                 HF 
                 wet processing: 
               
               
                   
                   
                 SC2 
                 resist strip 
               
               
                   
                   
                   
                 SC1 
               
               
                   
                   
                   
                 SC2 
               
               
                   
                   
               
             
          
         
       
     
     FIGS. 3 a ,  3   b , and  3   c  show breakdown current density, tunnel oxide breakdown voltage, and normalized charge to break down, respectively, for the different cleans. In all three cases, the data points of Clean  1  lie to the right of those from Clean  2 . These higher values for all three of these measurements, as most easily noted in FIG. 3 a , show the invention as an improvement over the prior art. The values for the embodiment of Clean  1   a  are even better than for Clean  1  due to the additional HF stage between SC 1  and Sc 2 , but Clean  1  is the preferred embodiment since it is a simpler process and yields comparable results. 
     Summarizing Q BD  by clean condition, the improved values are seen in Table 2. Again, the most direct comparison is between in situ process Clean  1  and prior art process Clean  1 . 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Clean 1 
                 Breakdown Voltage 
                 Q BD  (C/cm 2 ) 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 11.6 ± 0.1 
                 7.2 ± 0.9 
               
               
                   
                 1a 
                 11.8 ± 0.2 
                 7.2 ± 0.9 
               
               
                   
                 2 
                 11.2 ± 0.0 
                 6.7 ± 0.8 
               
               
                   
                   
               
             
          
         
       
     
     Another advantage of the in situ etch and pre-clean is a reduction in the amount of apparatus needed to process wafers. The purpose of the transfer to a wet deck or other second device in the prior art, such as Clean  2 , was to exploit the better pre-clean obtained in the wet deck. But by looking at the etch/resist strip/pre-clean process as a whole, the improved quality of the combined in situ processing removes the motivation for this transfer and, hence, the need for the additional apparatus. 
     Clean  1  is now considered in more detail. This embodiment is based on the basic SC 1  followed by SC 2  pre-clean and, as mentioned above, for that reason can be treated as a sort of reference embodiment. It is also the preferred embodiment. Approximate values of the parameters are given are given in Table 3. The values given in this preferred embodiment are all with respect to the apparatus used, an FSI Mercury MP Surface Conditioning System. Given the importance of the pre-clean stage, many of the variations found in the literature are possible, either in times for the rinse and clean stages or in the inclusion of other clean stages in addition to, or instead of, the given SC 1 /SC 2  combination. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Step 
                 Time (seconds) 
               
               
                   
                   
               
             
             
               
                   
                 BOE dispense (50:1) 
                 217-257 
               
               
                   
                 Rinse (deionized filtered water) 
                 225 
               
               
                   
                 resist strip (H 2 SO 4 —H 2 O 2 ) 
                 550 
               
               
                   
                 Rinse 
                 480 
               
               
                   
                 SC1 (NH 4 OH/H 2 O 2 /H 2 O) 
                 235 
               
               
                   
                 Rinse 
                 310 
               
               
                   
                 SC2 (HCl/H 2 O 2 /H 2 O) 
                 110 
               
               
                   
                 Final Rinse 
                 460 
               
               
                   
                 Dry 
                 440 
               
               
                   
                   
               
             
          
         
       
     
     BOE is used due to its more stable etch characteristics. Unbuffered HF causes both excessive undercutting at the resist oxide interface and lifting of the resist. It is important not only to produce a good etch at a given window, but also that the etch result is uniform, both across a given wafer and from one wafer to another. Better uniformity is obtained by use of a highly dilute BOE concentration dispensed at high rotation speed and low flow rate. In particular, the respective values of approximately 50:1, 500 rpm, and 1200 sccm are preferred, with the atomization pressure fixed at 45 psi. The time window on the dispense of BOE is to account for differing etch rates over time. 
     The BOE concentration of 50:1 is a far more dilute chemistry than is common, almost an order of magnitude so, when compared to the more usual 6:1 or 7:1 concentrations. A concentration range of 30:1 to 150:1 is preferred, with the ratios of approximately 45 to 60 favored as 50:1 is employed in the preferred embodiment. The values of 500 rpm for rotation speed and 1200 sccm for flow rate are, respectively, at the extreme high and low edges of the parameter space available in the FSI. By use of these parameter values well beyond the common range normally applied, the process produces a much better yield by making the etch more uniform across the wafer. It is believed this is because these values help ameliorate etch rate disparities due to the geometry of both the FSI device and the photoresist upon the wafer. Within the photoresist, more concentrated BOE produces a faster etch, but at the cost of less uniformity between dense and isolated regions. A higher flow rate also produces a faster etch, but the combination of a low flow rate and high rpm helps to offset inherent variations in the device due to the geometry of how the BOE is dispensed. This produces a more uniform result both across a given wafer and also between different wafers. In an alternate embodiment which did not employ a spray tool, such as the FSI, but instead a bench, dilute BOE with surfactant could be used, however, the surfactant, used to wet the surface, is considered an added contaminant and would then have to be removed during the remaining clean operations. 
     Device yield and reliability depend upon both the amount of etch and its uniformity. Although it is desirable to maximize both device properties, these may be contradictory aims. In practice, the processing uses a trade off where reliability is maximized while maintaining yield above a prescribed minimum. This is shown in FIGS. 4 a  through  4   c.    
     FIG. 4 a  shows how yield varies with the amount of filed oxide removed during the etching process. Yield is measured in terms of “hard fails” per wafer, that is, the number of device transistors which fail to function at time zero: the lower the number of hard fails, the higher the yield. The extent of etching which occurs is in terms of the mean amount of field oxide removed as measured in angstroms. This mean field oxide is a reference oxide included on the wafer to monitor process steps. In particular, it is employed here to quantify etch properties. The use of a reference oxide to monitor the etch process is needed since, in tunnel formation, the actual EEPROM oxide is etched back to the substrate, and therefore can not provide an accurate indication of overetch. As shown in FIG. 4 a , the number of hard fails decreases, and yield therefore increases, monotonically with the amount of etch. The error bars are included to model the amount of variation about a given modelled point. 
     The variation of reliability with etch is shown in FIGS. 4 b  and  4   c . The X-axes are again the amount of reference oxide removed. The Y-axes are now the rolloff values for the threshold voltages, V roll . V roll  indicates the inability of a device to maintain a charge, and is thus a measure of reliability. The two graphs of FIGS. 4 b  and  4   c  are for two different devices on the chip whose reliability is particularly important. Both show a minimum in the etch range of 380-520 Å, centered at approximately 460 Å, all values beyond the approximately 300 Å etched from EEPROM oxide to form the tunnel area. These values give the best reliability, both in terms of the curve and in terms of the spread of data about the curve. 
     That the graphs for reliability, FIGS. 4 b  and  4   c , have a minimum, while the graph for yield, FIG. 4 c , continues downward, requires a compromise between the aims of maximizing both reliability and yield. A certain amount of loss for the yield, as long as below a maximum value, must be offset against improved reliability. Therefore, the amount of etch is preferred to lie somewhat to the right of the minima of FIGS. 4 b  and  4   c , tending to err on the side of more etch. A range of 515 Å to 615 Å (i.e., 215 Å to 315 Å in excess of the amount of EEPROM oxide removed to form the tunnel area), centered about the preferred value of 565 Å, is favored. 
     Using the 50.1 concentration of BOE, Table 4 compares the uniformity of etch for several values of flow rate and rotation speed. The quantity σ represents the standard deviation of the mean field oxide removed. This value of a gives variations in uniformity of the etch process, as measured in the amount of mean field oxide removed at selected reference points on the wafers. The values of 500 rpm and 1200 sccm yield the lowest value of σ which is increased if either flow rate is increased or rotation rate decreased. The values of 500 rpm and 1200 sccm are at the limit of the parameter space available in the preferred embodiment employing the FSI machine, with the implication that going beyond this range could further improve uniformity. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 flow rate (sccm) 
                 rotation rate (rpm) 
                 σ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1200 
                 250 
                 12.587 
               
               
                 1700 
                 250 
                 11.080 
               
               
                 2200 
                 250 
                 7.322 
               
               
                 1200 
                 330 
                 7.193 
               
               
                 1700 
                 330 
                 6.526 
               
               
                 2200 
                 330 
                 7.887 
               
               
                 1200 
                 500 
                 6.132 
               
               
                 1700 
                 500 
                 6.994 
               
               
                 2200 
                 500 
                 11.312 
               
               
                   
               
             
          
         
       
     
     The preferred embodiment of the current invention is in the production of the tunnel oxide of an EEPROM memory cell. Once the etch, resist removal, and preclean is complete the tunnel oxide may be grown as shown in FIG. 1 f . A preferred arrangement is shown in FIG. 5, where the tunnel oxide  13  is in the center portion of the EEPROM oxide  12 ″. The dimensions of the preferred embodiment are a width of 1.6-2.0 μm for the entire oxide with a thickness of 300-400 Å, with respective values of 1.8 μm and 350 Å preferred, and a tunnel portion with a thickness of 60 to 110 Å and width of 0.4-0.8 μm, with respective values of 85 Å and 0.6 μm preferred. The other labelled portions of FIG. 5 are the P-well oxides  11 , the arsenic implant layer  20 , and an amorphous silicon layer  40  grown over the tunnel oxide. The slight thinning of the EEPROM oxide  12 ″ near the tunnel oxide  13  is a result of the arsenic implant procedure and is not discussed here. 
     It is to be understood that while the invention has been described above in conjunction with preferred specific embodiments, the description and examples are intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims.