Abstract:
A method for improving thickness uniformity and throughput of a carbon doped oxide deposition process is described. That method comprises removing pre-deposition steps in a deposition phase. Moreover, helium plasma is added to a pre-clean phase to eliminate the production of dummy wafers.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to the field of semiconductor processing. More particularly, the present invention relates to a method to improve wafer to wafer uniformity and throughput in carbon doped oxide film technology. 
     BACKGROUND OF THE INVENTION 
     Semiconductor devices include metal layers that are insulated from each other by dielectric layers. As device features shrink, reducing the distance between the metal layers and between metal lines on each layer increases capacitance. To address this problem, insulating materials that have a relatively low dielectric constant k are being used. Carbon doped oxide (CDO) is one such example of a dielectric film having a low k value. 
     CDO film is typically applied in a deposition process outlined in  FIG. 1 . The deposition process is typically performed within a reactor such as a chemical vapor deposition (CVD) apparatus or chamber. The deposition process begins with setting gas flows and time spacings of the gas flows in operation  110 . The CVD chamber walls are cleaned in operation  112  using a first cleaning plasma. A second cleaning plasma is then struck in operation  114  to clean the CVD spindle which is used for mounting wafers. Next, the CVD chamber is purged of all gasses in operation  116 . Operations  110 ,  112 ,  114 , and  116  comprise a set of operations known as a pre-clean phase. 
     Following the pre-clean phase, gas flows, temperature, and time spacings are set in operation  120 . Radio frequency (RF) power is applied in operation  122  for 20 seconds to energize the gas mixture set in operation  120  for deposition. The RF power applied in operation  122 , however, is only at half power. Full RF power is not applied until operation  124 . Similar to operation  122 , operation  124  is performed for 20 seconds. CDO is then deposited on a wafer for 45 seconds in operation  126 . Because operations  122  and  124  are performed prior to deposition in operation  126 , they are known as pre-deposition operations. Finally, the CVD chamber is purged of all gasses in operation  128 . Operations  120 ,  122 ,  124 ,  126 , and  128  comprise a set of operations known as a deposition phase. 
     Following the deposition phase, gas flows and time spacings are again set in operation  130 . The CVD chamber walls are cleaned in operation  132  using the first cleaning plasma. The second cleaning plasma is then applied in operation  134  to clean the CVD spindle which is used for mounting wafers. Next, the CVD chamber is purged of all gasses in operation  136 . Operations  130 ,  132 ,  134 , and  136  comprise a set of operations known as a post-clean phase. 
     If no other wafer is to be processed as determined in operation  140 , the deposition process is terminated in operation  145 . Otherwise, the process returns to the deposition phase. 
     One or two wafers (dummies) are typically run before the chamber reaches optimum conditions every time the CDO deposition process is initiated. Less than optimal chamber conditions result in poor dielectric thickness uniformity. As a result, dummy wafers are typically used for production. Therefore, a CDO deposition process that helps to eliminate dummy wafer processing and improve dielectric thickness uniformity and throughput is desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the present invention are illustrated by way of example and not in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
         FIG. 1  shows a flowchart of a prior art carbon doped oxide deposition process; 
         FIG. 2  shows one embodiment of deposition processing in accordance with the present invention that does not use pre-deposition to improve throughput; 
         FIG. 3  shows another embodiment of the present invention for a carbon doped oxide deposition process to improve thickness uniformity and throughput; and 
         FIG. 4  shows a graph of carbon doped oxide thickness for a first, second, and third wafer as a function of the helium plasma application time. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
     For one embodiment of the invention, pre-deposition operations  122  and  124  of  FIG. 1  are eliminated to improve processing throughput. As stated above, operations  122  and  124  take up approximately 20 seconds each. Thus, eliminating operations  122  and  124  reduces processing time by 40 seconds if the same dielectric quality can be achieved. Without pre-deposition operations  122  and  124 , however, experiments show that the deposition time of operation  126  needs to be increased by approximately 20 seconds to achieve the same dielectric thickness. 
       FIG. 2  shows a modified deposition phase without the pre-deposition operations  122  and  124 . In operation  220 , the gas flows, temperature, and time spacings are set as was the case for operation  120  of  FIG. 1 . The wafer then undergoes a CDO deposition process in operation  226  for approximately 65 seconds. Instead of being ramped up prior to deposition as in operations  122  and  124 , the RF power is turned on to full power during deposition. In other words, the RF power is set to a single predetermined power level without sustained intermediate power levels. The RF power may be set to the range of 200–4000 watts for full power depending on the CVD chamber configuration. The chamber is then purged of all gases in operation  228 . The pre-clean and post-clean phases remain unchanged for this embodiment. Because approximately 40 seconds are saved by eliminating operations  122  and  124  and the new deposition operation  226  now requires an additional 20 seconds over the former deposition operation  126 , each wafer achieves a total time gain of approximately 20 seconds per wafer. 
     For another embodiment of the invention,  FIG. 3  combines the deposition phase of  FIG. 2  with a modified pre-clean phase to help further improve throughput. The pre-clean phase of  FIG. 3  comprises first setting the gas flows and time spacing in operation  310 . A helium plasma is then applied to the CVD chamber in operation  311 . Experiments have shown that the operation  311  helps to improve CDO thickness uniformity in the first two wafers after the CDO deposition process is initiated. The desired CDO thickness may range from 2000–20000 angstroms. 
       FIG. 4  depicts a graph of CDO thickness on a first, second, and third wafer after the CDO deposition process is initiated. X-axis  405  represents the wafer being processed, while y-axis  410  represents the thickness of the wafer. Curve  420  shows a first, second, and third wafer where the chamber is not treated with a helium plasma. The first wafer has a CDO thickness of approximately 6750 angstroms. The second wafer has a CDO thickness of approximately 6860 angstroms. The third wafer has a CDO thickness of approximately 6900 angstroms. 
     In contrast, curves  430 ,  440 , and  450  shows wafers where the chamber is first treated with a helium plasma in operation  311  for periods of one minute, two minutes, and three minutes respectively during the pre-clean phase. Each of curves  430 ,  440  and  450  are more linear than curve  420  with respect to the first, second, and third wafers. In other words, there is less variation between the thickness of the first wafer and the second wafer of each deposition run of each of curves  430 ,  440  and  450  than curve  420 . Moreover, there is little variation between the thickness of the second wafer and the third wafer of each deposition run of curves  430 ,  440 , and  450 . With minimal variations between wafers, no dummy wafers are needed. This saves considerable time that more than makes up for the helium conditioning. 
     Choosing whether to apply a helium plasma for one, two, or three minutes in operation  311  involves a tradeoff between efficiency and quality. On the one hand, reducing the helium plasma application time increases throughput. On the other hand, increasing the helium plasma application time decreases variation in thickness uniformity. 
     The CVD chamber walls are cleaned in operation  312  using a first cleaning plasma after the helium plasma treatment of operation  311 . A second cleaning plasma is then applied in operation  314  to clean the CVD spindle which is used for mounting wafers. Next, the CVD chamber is purged of all gasses in operation  316 . Operations  312 ,  314 , and  316  correspond to operations  112 ,  114 , and  116  of  FIG. 1 . 
     Following the pre-clean phase, the deposition phase comprises setting gas flows, temperature and spacing in operation  220 , depositing CDO for a period of approximately 65 seconds in operation  226 , and purging the CVD chamber of gasses in operation  228 . The post-clean phase comprises setting gas flows and time spacing in operation  330 , cleaning the CVD chamber walls in operation  332 , cleaning the spindle in operation  334 , and purging the chamber in operation  336 . 
     Operation  340  then determines if another wafer is to be processed. If additional wafers are to be processed, the CDO deposition process returns to the deposition phase. Otherwise, the process is terminated in operation  345 . 
     In the foregoing specification the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modification and changes may be made thereto without departure from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.