Abstract:
A method for detecting completion of reaction chamber cleaning, comprising monitoring the pressure within the reaction chamber, calculating and tracking voltage versus time gradient of the pressure within the reaction chamber, recognizing when positive and negative gradients of pressure change have occurred and, based on this recognition, terminating the cleaning process of the reaction chamber.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor manufacturing, and more specifically to end-point detection for the cleaning process of cleaning Chemical Vapor Deposition chambers. 
     DESCRIPTION OF THE PRIOR ART 
     The semiconductor industry is continually striving to increase the performance of semiconductor chips while maintaining or striving to decrease the cost of semiconductor chips. These objectives have been successfully addressed by the trend to micro-miniaturization and of the ability to produce chips with sub-micron features. 
     The attainment of micro-miniaturization has been aided by the advances in specific semiconductor fabrication disciplines, basically photolithography and dry etching. The use of more sophisticated exposure cameras, as well as the use of more sensitive photo-resist materials, have allowed sub-micron features to be routinely achieved in photo-resist layers. In addition, the development of dry etching tools and procedures have allowed the successful transfer of the sub-micron images, in an overlying photo-resist layer, to an underlying material that is used in the fabrication of semiconductors. The tools and procedures used during Reactive Ion Etching (RIE) now allow single wafer etching to be performed. This allows each single wafer to be etched individually, with end point detection used for only this single wafer. Thus wafer to wafer uniformity variations, of the layer being patterned using single layer RIE etching, is not as great a problem as encountered with batch RIE etching. Thus large volumes of wafers can be confidently processed using single wafer RIE procedures, with a decreased risk of under or over-etching due to thickness variations of the material being etched. 
     A major limitation of dry etching procedures is the ability to maintain a strong end point detection signal from wafer to wafer. With the use of single wafer RIE tools, the wafer being etched is moved to the etch chamber of the single wafer RIE tool, which also contains a window, which allows the monitoring of the etching sequence. Laser endpoint detection apparatus monitors the chemistry of the reactants and by-products, through this window. At the conclusion of the etching cycle the chemistry of the by-products will change, and the end-point detection process will monitor this change. If however the window through which the endpoint monitoring takes place becomes layered with adhering RIE products, the endpoint detection signal will decrease in intensity which can at times result in erroneous end-point signals. 
     The present invention will describe a process for finding the endpoint for a Chemical Vapor Deposition tool cleaning procedure by monitoring the change in the pressure within the cleaning chamber. The present invention is therefore directed towards the cleaning of sensors or endpoint detection windows. 
     U.S. Pat. No. 5,584,963 (Takahashi) shows a method to clean a CVD reactor. 
     U.S. Pat. No. 5,604,134 (Chang et al.) teaches a particle monitor method for reactors. 
     U.S. Pat. No. 5,753,137 (Ye et al.) shows a CVD chamber cleaning process. 
     U.S. Pat. No. 5,679,214 (Kuo) discloses an endpoint detection for a dry clean process for a RIE tool. 
     U.S. Pat. No. 5,387,777 (Bennett et al.) teaches a process for cleaning plasma tools. 
     U.S. Pat. No. 5,643,364 (Zhao et al.) shows a plasma chamber with a endpoint etch detector. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, it is an objective of the present invention to provide a method for end-point detection for cleaning CVD chambers. 
     Another objective of the present invention is to reduce the usage of cleaning gas during the cleaning of CVD chambers. 
     Yet another objective of the present invention is to prolong the lifetime of CVD processing kits. 
     In accordance with the objective of the present invention, the invention teaches a method to identify the end-point for the CVD cleaning operation by monitoring the pressure within the cleaning chamber. Experiments have shown that the degree of cleaning of a CVD chamber can be predicted by monitoring the pressure within the chamber. 
     If the cleaning cycle is started with a clean CVD chamber, the pressure within the chamber remains constant during a repeat cleaning cycle. If the cleaning cycle is started with a CVD chamber into which first a light coating of the cleaning gas has been deposited, the pressure increases during the cleaning cycle. If the cleaning cycle is started with a CVD chamber into which first a heavier coating of the cleaning gas has been deposited, the pressure again increases during the cleaning cycle but this time at a slower rate than in conditions of a light initial coating. The conclusion from these experiments is that as soon as there are no more impurities within the CVD chamber, the pressure within the chamber takes on a constant value. Inversely than, by monitoring the pressure within the chamber and by detecting the point in time where the pressure is stable, it can be concluded that the chamber cleaning process has been completed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings presented which form an integral part of the present invention show the results obtained during a set of experiments, as follows: 
     FIG. 1 shows the variation in pressure with time for a CVD chamber cleaning process where the cleaning process was started with a previously cleaned CVD chamber. 
     FIG. 2 shows the variation in pressure with time for a CVD chamber cleaning process where the cleaning process was started with a CVD chamber into which a small amount of dopant or impurity gas had been released into the CVD chamber. 
     FIG. 3 shows the variation in pressure with time for a CVD chamber cleaning process where the cleaning process was started with a CVD chamber into which a larger amount of dopant or impurity gas had been released into the CVD chamber. 
     FIG. 4 shows a flow-chart of the processing steps that are required to determine the end point of the CVD chamber cleaning cycle within the scope of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention relates to the process of cleaning a Chemical Vapor Deposition chamber that has been used for the depositions of WS ix  gases. The deposition sequence for such a chamber is as follows: 
     1) deposit WF 6  or WS ix    
     2) deposit S i ClH 2    
     3) deposit S i H 4 . 
     These gasses are distributed within the CVD chamber using the conventional ‘shower head’ distribution approach. 
     After a sequence of 25 wafer depositions as indicated above, the CVD chamber needs to be cleaned. The cleaning gas uses for this cleaning process is ClF 3 , the typically recommended parameters for this cleaning process are as follows: time of cleaning 200 seconds, 100 SCCM (Standard Cubic Centimeter per Second), temperature 475 degrees C., pressure 3 Torr. Since this cleaning process does not involve a plasma clean, there is no DC bias voltage applied between the plates. 
     Referring now more specifically to FIG. 1, there is shown the variation of pressure within the CVD chamber for the case where the CVD chamber has previously been cleaned. The graph indicates that the pressure within the CVD chamber remains constant during the cleaning process at 2.92 Torr. 
     Referring now to FIG. 2, there is show the variation of pressure within the CVD chamber for the case where the CVD chamber has previously been thin coated, that is a one time deposition had been applied, with ClF 3 . The figure indicates that the pressure within the CVD chamber increases during the cleaning process. The pressure within the CVD chamber increases to 3.16 Torr, point 2, after which the pressure decreases to 2.92 Torr, point 3, and remains stable after that. At the point where the pressure within the CVD chamber has decreased to 2.92 Torr and where it remains stable after that, the cleaning process of the CVD chamber is complete. 
     Referring now to FIG. 3, there is show the variation of pressure within the CVD chamber for the case where the CVD chamber has previously been thick-coated, that is three depositions of ClF 3  have been applied. The figure indicates that, similar to the phenomenon that was observed under FIG. 2, the pressure within the CVD chamber increases during the cleaning process. The pressure within the CVD chamber again starts, point 1, a rapid increase to 3.16 Torr, point 2, after which the pressure decreases to 2.92 Torr, point 3, and remains constant after that. The only difference between the phenomena of FIGS. 2 and 3 is that the variation in pressure occurs at longer at a slower rate for the case where the cleaning process was started with a heavier coated CVD chamber. At the point where the pressure within the CVD chamber has decreased to 2.92 Torr and where it remains stable after that, the cleaning process of the CVD chamber is complete. 
     It is to be noted that, where the CVD chamber has not yet reached the state of being clean, the pressure versus time curve exhibits three points of interest. These three points are where there is a sharp change in the pressure versus time curve. The first point, see 1, FIG. 3, is where the pressure stops increasing and, for a short period of time, levels of to being constant. This point has a negative coefficient of pressure variation. The second point, see 2, FIG. 3, is where the pressure has reached its highest value and starts to decrease. This point also has a negative coefficient of pressure variation. The third point, see 3, FIG. 3, is where the pressure changes from decreasing to a constant value. This point has a positive coefficient of pressure variation. It is clear that, between points 1 and 2, FIG. 3, the pressure increases with time. Initially this pressure increase is very gradual, it reaches the point where the pressure increases at a rapid rate which is the point where the reaction for Cl 3 F 3  and WSix is starting to take place. 
     From the above it is clear that by monitoring the pressure within the CVD chamber it can be determined when the cleaning process of the CVD chamber is complete. The following flowchart shows this procedure. 
     FIG. 4 shows the sequence of events that occur and which enable detection of completion of the CVD chamber cleaning process. It is to be noted that these events can be traced real-time by computer monitoring and analysis. Time-delay in measuring the various parameters or in implementing the conclusions derived from the measured parameters can therefore be considered to be non-existent. 
     FIG. 4, step  1 , shows the start of the cleaning process by the release of the ClF 3  cleaning gas into the CVD chamber. 
     FIG. 4, step  2 , initiates the monitoring pressure within the CVD chamber. 
     FIG. 4, step  3 , is the first point where the curve of pressure versus time undergoes a rapid change, that is where the pressure stops increasing and levels of to being constant. Refer to point 1, FIG.  3 . This point has a negative coefficient of pressure variation. Computer monitoring can detect this point. 
     FIG. 4, step  4  is the second point where the curve of pressure versus time undergoes a rapid change, that is where the pressure has reached its highest value and start decreasing. Refer to point 2, FIG.  3 . This point has a negative coefficient of pressure variation. Computer monitoring can detect this point. 
     FIG. 4, step  5  is the third point where the curve of pressure versus time undergoes a rapid change, that is where the pressure changes from decreasing to a constant value. Refer to point 3, FIG.  3 . This point has a positive coefficient of pressure variation. Computer monitoring can detect this point. 
     FIG. 4, step  6  indicates that the CVD chamber monitoring apparatus has detected and identified the conditions where the cleaning of the CVD has been completed. The monitoring apparatus therefore terminates the cleaning process. 
     Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims.