Abstract:
Apparatus and method of processing a substrate comprises positioning a substrate within a processing space of a processing chamber, and depositing a material layer, such as a titanium-containing layer, onto the substrate using plasma-enhanced chemical vapor deposition. The substrate is then removed from the processing chamber and the processing space is purged. A gas mixture containing oxygen is then introduced into the processing space and the gas mixture is excited with RF energy to form an oxygen-containing plasma. The oxygen-containing plasma is sustained for a predetermined amount of time in the processing space.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/063,196, filed Apr. 20, 1998 and entitled “Method of Passivating and Stabilizing a TiPECVD/TiN-CVD Process Chamber and Combined Ti-PECVD/TiN-CVD Processing Method and Apparatus.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to plasma-enhanced chemical vapor deposition (PECVD) for applying a coating on a substrate, and more specifically to a method for stabilizing the internal surface of a PECVD process chamber using a plasma. 
     BACKGROUND OF THE INVENTION 
     In the formation of integrated circuits (IC&#39;s), thin coatings or films containing metal and metalloid elements are often deposited upon the surface of a substrate, such as a semiconductor wafer. Such thin films are deposited to provide conductive and ohmic contacts in the circuits formed on the substrate and between the various components of an IC. For example, a conductive thin film might be applied to the exposed surface of a contact or via hole on a semiconductor wafer, with the film passing through the insulative layers on the wafer, to provide plugs of conductive material for the purpose of making electrical interconnections across the insulating layers. 
     One well known process for depositing thin metal films is chemical vapor deposition (CVD), in which a thin film is deposited as a result of chemical reactions between various deposition or reactant gases at the surface of the substrate. In CVD, reactant gases are pumped into proximity with a substrate inside a reaction chamber. The reactant gases subsequently react at the substrate surface, resulting in one or more reaction by-products which form a film on the substrate surface. Any by-products remaining in the chamber after the deposition are removed from the chamber. 
     One variation of the CVD process which is widely utilized is a plasma-enhanced CVD process or PECVD process, in which one or more of the reactant gases is ionized into a gas plasma to provide energy to the reaction process. PECVD may be desirable, for example, for lowering the temperatures which are usually necessary for a proper chemical reaction with standard CVD. In PECVD, electrical energy is delivered to the reactant gas or gases to form and sustain the plasma. For one such PECVD process, the susceptor or support containing the substrate and a planar element in the processing space, such as a planar gas supply element, are electrically biased to operate as opposing RF electrodes for energizing one or more of the reactant gases into an ionized plasma. Such a method is commonly referred to as a parallel plate method because the susceptor and the biased planar element are maintained generally parallel to one another to simulate biased electrical plates with the substrate positioned therebetween and generally parallel to the biased elements. 
     The reactant gases for CVD and PECVD processes are delivered to the processing space and substrate through a gas delivery system which provides the proper flow and distribution of the gases for the CVD process. Generally, such gas delivery systems contain gas-dispersing elements in the reaction chamber, such as gas injector rings or flat showerheads, which spread the entering reactant gases around the processing space to insure a uniform distribution and flow of the gases proximate the substrate. Uniform gas distribution and flow is desirable for a uniform and efficient deposition process, a dense plasma, and a uniformly deposited film. 
     One notable PECVD method involves the deposition of thin films of titanium and titanium-containing layers onto silicon substrates. Generally, for such a method, a plasma comprising TiCl 4 , H 2 , and Ar is utilized. Such a deposition process is described in U.S. Pat. Nos. 5,628,829; 5,665,640; 5,567,243; and 5,716,870, which patents are incorporated herein by reference in their entirety. During the deposition process, TiCl 4 , which is partially reactive, condenses onto the walls of the processing chamber. The partially reacted TiCl 4  may be TiCl 2  or TiCl 3  or hydrogenated versions of these, such as H 2 TiCl 2  or HTiCl 3 . The condensation generally occurs due to the relatively low temperature of the chamber walls with respect to the susceptor and substrate. 
     Specifically, the susceptor and substrate are maintained at a temperature in the range of about 400° C. or above, and usually around 600° C. Such a processing temperature supports a complete reduction of the TiCl 4  gas and the subsequent deposition of titanium metal. However, the processing chamber internal walls are generally not as hot as the substrate and may be maintained at a temperature in the range of 80-200° C., and usually around 175° C. The wall temperature does not support complete decomposition of the TiCl 4 , and thus, titanium sub-chlorides (TiCl x  where x&lt;4) are deposited onto the wall surfaces due to their low vapor pressure. 
     Such sub-chlorides are deposited onto the internal chamber walls as a powder, and they remain in the powder form as long as the processing chamber remains under vacuum pressure. The sub-chloride powder tends to become oily when the processing space inside of the chamber is exposed to atmosphere. This is generally due to moisture absorption, because the powder has a hygroscopic nature. An analysis of the powder has shown that it generally contains a mixture of TiCl 2  and TiCl 3 . 
     Two specific problems are caused by the deposition on the reaction chamber walls. First, the deposition actually adheres very poorly to the walls. This leads to flaking of the residue from the walls and subsequent particle contamination on the substrate. The powder generally cannot be removed by conventional dry etching techniques (with the exception of ClF 3 ). Therefore, the processing chamber generally must be opened approximately every 200 deposition cycles for manual cleaning of the powder. 
     A second problem is that the deposited powder consists largely of titanium sub-chlorides, which have a high vapor pressure relative to the deposition pressure. Therefore, some of the sub-chlorides are volatilized during the deposition process. This volatilized material then diffuses to the substrate, and participates in the film deposition reaction. It has been observed that, for deposition onto silicon substrates, where the product of the deposition reaction is a titanium silicide film, the sub-chlorides from the processing chamber wall lead to a net increase in the amount of film deposited. Therefore, the deposition of the film is somewhat uncontrolled, which is an undesirable characteristic. Using TiCl 2  as an example, the following reaction occurs: 
     
       
         TiCl 2 +2H→2&lt;Ti (s) +2HCl 
       
     
     The deposited titanium quickly reacts with the underlying silicon from the substrate to form titanium silicide, albeit in a somewhat uncontrolled fashion. 
     In order to overcome the effects of the powder residue on the chamber walls, an ammonia plasma treatment has been devised. This treatment is described in greater detail in U.S. Pat. No. 5,593,511 which is incorporated herein by reference in its entirety. The treatment stabilizes the titanium sub-chlorides by reacting them with ammonia and converting them to titanium nitride. The titanium nitride forms as an adherent film on the processing chamber wall. It does not cause particle contamination problems and does not contribute to the film deposition reaction on the substrate. However, it does cause other problems, and thus there is still a need for a suitable solution to address the problems associated with the deposition of titanium sub-chlorides in the processing chamber. 
     The specific problem involving titanium nitride is that the film forms on the process chamber wall and also on the electrical insulator which isolates the RF electrode from the reaction chamber walls. The effect is to create a conducting path from the RF electrode to the grounded reaction chamber walls. This problem is described in U.S. patent application Ser. No. 09/153,128, filed Sep. 15, 1998, entitled “Apparatus and Method for Electrically Isolating an Electrode in a PECVD Process Chamber,” which describes the use of serrations in the insulator to increase the length of the path to ground. Although this method is effective, it only increases the time until a short circuit may occur due to the titanium nitride buildup. 
     Therefore, it is desirable to reduce the effect of the titanium sub-chlorides altogether. To that end, it is one objective of the present invention to reduce, and possibly eliminate, the buildup of titanium sub-chlorides on the processing chamber wall. The invention will become more readily apparent from the Detailed Description and further disclosure below. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes a method which stabilizes the internal surface of a PECVD process chamber, provides a more stabilized deposition process, yields a cleaner chamber, and reduces the possibility of a biased showerhead shorting to ground, such as through the process chamber. 
     The invention comprises a process including the steps of positioning a substrate within the processing space of a processing chamber and depositing a material layer on the substrate using plasma enhanced chemical vapor deposition. In one embodiment of the invention, titanium reactant gas is utilized and a titanium-containing layer is deposited onto a substrate. After deposition, the substrate is removed from the processing chamber and the processing space is purged, such as with a hydrogen/argon mixture. The purging step may be repeated several times. Thereafter, a gas mixture is introduced into the processing space. The gas mixture contains oxygen, and in one embodiment, the mixture is an oxygen/argon mixture. For example, the mixture might be 5% oxygen and 95% argon. However, other mixtures including a greater or lesser percentage of oxygen would be useful as well in the present invention. 
     RF energy is coupled to the processing chamber to excite or maintain an oxygen-containing plasma created from the gas mixture. The oxygen-containing plasma is sustained for a predetermined amount of time within the processing space. For example, a 60 second oxygen-containing plasma has been shown to be useful in achieving results with the present invention. In one aspect of the present invention, the oxygen-containing plasma might be followed by an argon/hydrogen purge of the processing space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given below, serve to explain the principles of the invention. 
     FIG. 1 is an elevational cross-sectional view of a processing system in accordance with the principles of the present invention. 
     FIG. 2 is a graph of various gases and pressures over time for the inventive process. 
     FIG. 3 is a graph of layer thickness and resistivity for various samples of substrates coated according to the principles of the present invention. 
     FIG. 4 is another graph of deposition thickness and resistivity, as shown in FIG. 3, for additional samples. 
     FIG. 5 is a graph of various characteristics of additional samples of substrates coated in accordance with the principles of the present invention. 
     FIG. 6 is a graph similar to FIG. 5 for selected characteristics of sample substrates. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is an elevational cross sectional view of one suitable CVD processing system for practicing the present invention. Processing system  10  includes a processing chamber  12  formed of a suitable material such as stainless steel, which defines a processing space  14  therein. A substrate support or susceptor  16 , which may be rotating or stationary, supports a substrate  18  within the processing space  14  to receive a material layer or film through CVD and/or PECVD processes. Processing chamber  12  will further comprise other systems, such as a vacuum system including at least one pump  20  for providing a vacuum pressure within processing space  14 , which is generally necessary for PECVD processes. A manifold or showerhead  22  is illustrated supported around its peripheral edges by an insulative support  21  such that the bottom face surface  24  of the showerhead  22  faces into the processing space  14  toward substrate  18 . Insulative support  21  insulates the RF biased showerhead  22  from the electrically grounded chamber  12 . In a preferred embodiment, showerhead  22  is generally parallel to substrate  18 , which is desired for RF parallel plate PECVD processes. An RF power supply  26  is coupled to the susceptor and showerhead for biasing those elements according to well known PECVD principles. For PECVD  10  processes, showerhead  22  may be coupled through an appropriately constructed RF shaft or line (not shown) to RF energy supply  26  for biasing the showerhead and creating a plasma within processing space  14  for plasma-enhanced deposition of PECVD, as understood by a person of ordinary skill in the art. 
     In accordance with the principles of the present invention, separate reactant gases are provided to showerhead  22  by gas delivery system  28  and supplies  30 , respectively. Certain gases are shown as part of supply  30 . However, as will be understood by a person of ordinary skill in the art, the present invention might be utilized for introducing other types of reactant gases through showerhead  22 . The gas delivery system might include one or more rings  34  to direct gas to the showerhead  22  in a uniform manner. Chamber  12  and the processing system  10  will also include an appropriate control system  36  for controlling all of the various sub-systems of the processing system  10 , in accordance with the principles of the invention. As will be understood, system  10  may include other subsystems, not shown, for processing a substrate. 
     The present invention is directed to ensuring uniformly controlled PECVD of titanium and the prevention of the biased showerhead electrode being shorted to ground. Specifically, a unique combination of process steps is utilized in combination with a deposition process for addressing the coating of titanium sub-chlorides throughout the processing space of a processing chamber and specifically, the coating of titanium sub-chloride on processing chamber walls. 
     Referring to FIG. 2, a process is illustrated in accordance with one embodiment of the present invention. For PECVD of titanium, a flow of hydrogen and argon is first initiated, as illustrated by reference numeral  40 . The chamber pressure, indicated by reference numeral  42 , is then dropped, and a plasma is initiated. A flow of TiCl 4  is initiated, as indicated by reference numeral  44 , simultaneously with the H 2 , Argon flow and the plasma. Within the zone defined by reference numeral  45  in FIG. 2, deposition of titanium occurs on a substrate according to the reaction discussed above. Thereafter, the flow of hydrogen and argon and the TiCl 4  is stopped at the end of the deposition process. 
     Thereafter, as indicated by reference numerals  46  and  48 , the processing chamber  12  is purged with a mixture of hydrogen and argon. A first purge occurs, then the processing chamber is pumped down to a base pressure and the purge is repeated, as indicated by reference numeral  48 . The overall pump/purge zone is indicated generally by reference numeral  50 . Thereafter, in the indicated by reference numeral zone  52 , the substrate is unloaded from the processing chamber. The processing chamber  12  is again pumped down to a base pressure and, as indicated at reference numeral  54 , a gas mixture containing oxygen is directed into the processing space of the chamber. Specifically, an oxygen (O 2 ) and argon (Ar) mixture is utilized in accordance with one embodiment of the invention. In a suitable mixture, the mixture is approximately 5% oxygen and 95% argon. The pressure is increased to approximately 500 mTorr. 
     An RF plasma from RF power supply  26  at approximately 250 Watts at 450 KHz is then established in the chamber. In the embodiment illustrated in FIG. 2, the oxygen plasma treatment time is in the range of approximately 60 seconds, as indicated by zone  56 . Thereafter, the processing chamber is again brought down to base pressure and is purged with a hydrogen/argon mixture illustrated by reference numeral  57 . While the embodiment disclosed herein utilizes an oxygen/argon mixture which is 5% oxygen, other mixture percentages would be suitable, in accordance with the principles of the present invention. Furthermore, another inert gas other than argon might also be utilized within such an oxygen mixture. 
     Referring to FIG. 1, supply  30  includes a supply  31  of argon, hydrogen  32 , a supply  33  of TiCl 4 , and a supply  35  of oxygen, which are directed through appropriate valves  39 ,  41  and gas lines associated with gas delivery system  28 . The gases are distributed evenly through the processing chamber due to the operation of the showerhead  22  and gas ring(s)  34 . 
     A specific discovery by the inventors, was that the oxygen plasma step of the invention leaves the inside of the reaction chamber very clean, and generally free of significant deposition. The exposure of the titanium sub-chlorides on the processing chamber wall to the plasma which contains oxygen forms a volatile titanium oxychloride (TiO x Cl y ) by-product. The general reaction can be expressed as follows: 
     
       
         TiCl y ( s )+O x ( g )→TiO x Cl y   
       
     
     The volatile reaction by-product may then be pumped away by the vacuum system and pump  20 . Furthermore, to some extent, the oxygen plasma also reacts with the titanium sub-chlorides on the reaction chamber wall to create a non-conducting layer of titanium oxide. Therefore, the formation of a conducting titanium nitride (TiN) layer on the insulative support  21 , which would eventually cause a short circuit, is reduced and/or prevented. The present invention provided desirable results in several different ways. First, the deposition and resulting thickness of the film on the substrates is more controlled and relatively stable over the processing of multiple wafers. 
     Referring to FIG. 3, deposition and resistivity characteristics are shown graphed over a sample of 8 substrates within a 200 substrate run (e.g. every 25 th  substrate was measured). The deposition thickness in angstroms (Å) is indicated by reference numeral  60 , wherein resistivity in μOhms-cm is indicated by reference numeral  62 . The samples reflect a process where the formation of titanium nitride occurs on the processing chamber walls and showerhead support. FIG. 4 also illustrates deposition thickness  64  and resistivity  66  for 8 samples within a 200 wafer run wherein titanium nitride is formed on the walls of the processing chamber, and on the insulative showerhead support  21 . As illustrated in the plots  60  and  64 , the deposited film thickness shows a distinct increasing trend in subsequent samples. Such a trend indicates that the plasma conditions inside of the processing chamber are changing for subsequent samples as the conductive TiN filmed is formed on the insulative support  21  within the chamber. That is, the deposition rates are not as consistent as the number of samples progresses. 
     FIGS. 5 and 6 illustrate graphs of thickness and resistivity for a series of substrate samples which were coated in a process using the oxygen-containing plasma of the present invention between each sample. While there was an unexplained aberration in the data for the second substrate sample, the titanium deposition thickness as indicated by plot  68 , is relatively stable over the various samples and does not show an increasing trend for each subsequent sample. FIG. 5 illustrates plots of average deposition thickness  68 , resistivity  70 , average sheet resistance  72 , and non-uniformity  74 , which is the standard deviation of resistivity as measured at 49 points around a substrate by a four-point probe. 
     FIG. 6 illustrates average deposition thickness and resistivity for the various samples as illustrated in FIG. 5 on a more expanded scale on the Y-axis. FIGS. 3,  4 ,  5 , and  6  illustrate that greater stability in the thickness of the material deposited onto a substrate is obtained utilizing the present invention, rather than a technique wherein titanium nitride is formed within the processing space. 
     Further observation of sample processes indicated an additional advantage of the present invention as evidenced by a visual inspection of the inside wall of the processing chamber. Two hundred substrate samples were run when an ammonia plasma was utilized to form titanium nitride and another 200 substrate samples were run utilizing the oxygen plasma in accordance with the principles of the present invention. When utilizing an ammonia plasma for titanium nitride, the showerhead electrode  22  and the susceptor  16  exhibited a golden TiN deposition. The TiN film extended onto the insulative support  21  thereby forming a conductive layer on the insulative support, as discussed above. The inside walls of the processing chamber  12  were covered and discolored by a bluish film which was predominantly TiN. 
     Alternatively, following the 200 runs utilizing an oxygen-containing plasma in accordance with the principles of the present invention, the inside of the reaction chamber, including the walls thereof and various components therein, were significantly cleaner. There was no significant change to the insulative support  21 , although the walls of that support were coated with a thin white film. An additional observation distinguishing the present invention from a method utilizing an ammonia plasma to form titanium nitride was an electrical difference between various films formed in each process. Resistance measurements were made on the films that had been formed on the insulative support  21 . When utilizing an ammonia plasma, a golden TiN film was formed on the insulative support and the resistance was found to be less than approximately 1 Ohm. However, when an oxygen-containing plasma was utilized in accordance with the principles of the present invention, the measured resistance of the film was always greater than 4000 Ohms. 
     Accordingly, the present invention provides significant advantages in creating a more stable deposition process while maintaining a cleaner processing chamber. The reduction of TiN film buildup within the processing chamber further reduces the probability of contamination based upon particles from the TiN film flaking from the processing chamber wall and other covered components, such as the insulative support  21 . Furthermore, the reduction of the buildup of a conductive TiN film on the insulative support  21  reduces the probability of the RF biased showerhead  22  shorting to the grounded chamber  12 . 
     While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant&#39;s general inventive concept.