Abstract:
A method of depositing material on a substrate comprises providing a reactor with a reaction chamber having a first volume, and contacting a surface of a substrate in the reaction chamber with a first precursor at the first chamber volume to react with and deposit a first layer on the substrate. The method further includes enlarging the reaction chamber to a second, larger volume and removing undeposited first precursor and any excess reaction product to end reaction of the first precursor with the substrate.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to reactors used in the semiconductor manufacturing industry and, in particular, to a reactor for pulsed layer deposition of thin films in the fabrication of integrated circuits. 
     2. Description of Related Art 
     A primary goal of the semiconductor manufacturing industry is to reduce the size of integrated circuits in order to them to perform more operations in a shorter time. As integrated circuit device features become smaller, several technical difficulties are presented. One such problem is depositing conformal thin films in holes or trenches having a small diameter or a small width-to-depth ratio. One standard technique for depositing such thin films has been chemical vapor deposition (CVD), which works well for feature sizes on the order of 120 nm and smaller. However, CVD may not be extendable to high aspect ratio features at these dimensions. Pulsed layer deposition (PLD) has been seen as a likely replacement for film deposition and holes below 120 nm in width and for the high aspect ratio features. 
     PLD is useful for depositing thin films having two components. The process generally consists of four steps, which may be repeated to produce films of desired thickness. The steps are normally conducted in a reactor with a controlled environment. The first step consists of saturating a surface with the first precursor or reactant needed to create the film, followed by removing the excess byproducts of the first reaction and any unreacted precursor from the reactor. The next step consists of saturating the surface with a second precursor or reactant in order to form the desired film. The last step is to remove unwanted excess byproducts from the third step and any unreacted precursor. 
     The precursor exposure steps of PLD are said to be self-limiting, that is, the amount of material deposited on the surface stops depositing after a relatively short period of time. However, in order for the deposition reactions of steps one and three to go to completion, the surface must receive a high exposure of the precursor. This is achieved if the precursor is allowed to remain for a long period above the surface or if the concentration of the precursor above the wafer is high. Provided the precursor exposures are high and the purging performed during the second and third steps are sufficient, there results a thin film consisting of the reaction product of the two components. The purging steps are sufficient if concentrations of the un-reacted precursors and reaction byproducts are low enough to minimize or eliminate one precursor or its byproducts from contacting, and possibly reacting with, the second precursor or its byproducts. 
     In addition to the steps outlined above, there are additional practical requirements. First, each step should be as short as possible to fulfill the requirement that the thin film be formed as fast as possible to make the entire process commercially viable for ultra large scale integration (ULSI) of the circuits. Second, the smallest possible amount of precursor should be used because precursor costs must be minimized to make the process commercially viable, and un-reacted precursor and byproducts must be minimized to reduce the need for abatement of environmentally harmful substances. Accordingly, the requirements for a rapid process and use of minimum amount of precursor, coupled with the requirements of high exposure and sufficient purging, necessitate trade offs to be made in the PLD process. 
     SUMMARY OF INVENTION 
     Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method and apparatus for depositing films in a pulsed layer deposition process. 
     It is another object of the present invention to provide a method and apparatus for controlling exposure of precursors or reactants to a substrate during a pulsed layer deposition process. 
     A further object of the present invention is to provide a method and apparatus which reduces the amount of precursor or reactant needed to deposit a film in a pulsed layer deposition process. 
     It is yet another object of the present invention to provide a method and apparatus that reduces the amount of excess byproduct and unreacted precursor or reactant in a pulsed layer deposition process. 
     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification. 
     The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention which is directed to a method of depositing material on a substrate comprising providing a reactor with a reaction chamber having a first volume, and contacting a surface of a substrate in the reaction chamber with a first precursor at the first chamber volume to react with and deposit a first layer on the substrate. The method further includes enlarging the reaction chamber to a second, larger volume and removing undeposited first precursor and any excess reaction product to end reaction of the first precursor with the substrate. 
     The method may further include reducing the reaction chamber to the first chamber volume, and contacting the first layer in the reaction chamber with a second precursor at the first chamber volume to react with and deposit a second layer on the first layer, thereby forming a film. The method then includes enlarging the reaction chamber to the second volume and removing undeposited second precursor and any excess reaction product to end reaction of the second precursor. 
     The removal of undeposited first precursor and any excess reaction product may be by purging the reaction chamber at the second volume with a gas, and/or by exposing the reaction chamber at the second volume to a vacuum. 
     The reaction chamber preferably includes a pedestal adapted to secure the substrate during the deposition and which is movable between first and second positions. A first chamber section is above the pedestal in the first position and defines the first chamber volume. A second chamber section is outside the first chamber section. The reaction chamber is enlarged to the second, larger volume by moving the pedestal to the second position such that the first and second chamber sections together with the pedestal in the second position define the second chamber volume. 
     In another aspect, the present invention is directed to an apparatus for depositing a film on a substrate comprising a reactor having a variable volume reaction chamber for reacting one or more precursors with the substrate to deposit a film thereon, and a pedestal in the reaction chamber adapted to secure the substrate during the deposition. The pedestal is movable between first and second positions, so that a first chamber section is above the pedestal in the first position, and a second chamber section is outside the first chamber section. Volume of the reaction chamber may be varied by moving the pedestal between the first position, where the first chamber section together with the pedestal in the first position define a first chamber volume, and the second position, where the first and second chamber sections together with the pedestal in the second position define a second, larger chamber volume. 
     The pedestal is preferably movable upwards to the first position and downwards to the second position. The second chamber section may be on one or more sides of the pedestal, or below the pedestal. The apparatus may further include a perforated plate, above the pedestal in the first chamber section, which is adapted to diffuse the precursors. It may also include an environmental control for maintaining the first chamber section at a different temperature than the second chamber section. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional elevational view of the preferred reactor apparatus of the present invention, with the pedestal in the raised position providing a smaller reaction chamber volume. 
         FIG. 2  is a cross-sectional elevational view of the preferred reactor apparatus of  FIG. 1 , showing the pedestal in the lowered position providing a larger purging reactor chamber volume. 
         FIG. 3  is a flow chart showing the preferred method of practicing the process of the present invention. 
         FIG. 4  is a cross-sectional elevational view of an alternate embodiment of the reactor apparatus of  FIG. 1 , with the pedestal in the raised position. 
     
    
    
     DETAILED DESCRIPTION 
     In describing the preferred embodiment of the present invention, reference will be made herein to  FIGS. 1-4  of the drawings in which like numerals refer to like features of the invention. 
     The invention is a reactor apparatus and method for pulsed layer deposition of thin films that meets the requirements of high exposure of precursor and sufficient purging while maintaining low precursor usage and fast process time. The reactor consists of an isolated chamber with a pedestal for holding a wafer, a feed through for uniformly introducing precursor and purging fluid over the wafer surface, and a drain or vacuum source to permit the precursor, byproducts and purging fluids egress. The reactor has two zones, a small volume zone, and a large volume zone. The pedestal and wafer is placed in the small volume zone during precursor exposure, and then moved to the high volume zone, maintained at a lower pressure, to allow un-reacted precursor and byproduct an exit. Concurrently with moving the pedestal to the purge position, or immediately following, the purge fluid is turned on so un-reacted precursor and byproducts are both drawn out by the vacuum source and pushed out by the purge fluid. 
     The preferred reactor apparatus for practicing the present is depicted in  FIGS. 1 and 2 . Reactor  10  comprises a reaction chamber housing  100  having internal reactor chamber sections  103 ,  104 . The reactor chamber interior, particularly chamber section  104 , communicates with a passageway  106  leading to a high vacuum source (not shown), with a shut off valve  107  controlling communication therewith. Process fluids enter from a source (not shown) through control valve  108  and passageway  111  into reaction chamber  103 . This source may hold solid, liquid or gaseous compounds, which are then processed by conventional means to provide the precursor in gaseous form through passageway  111 . Perforated diffusion plates  109  and  110  extend across the upper portions of chamber section  103  below passageway  111 , and have different size openings therein in order to diffuse the flow of process fluids evenly into the reaction chamber. 
     Below reactor chamber section  103  is disposed a moveable circular pedestal  102  for securing a wafer substrate  105  on which the film is to be deposited. Pedestal  102  is moveable between an upper position as shown in  FIG. 1 , and a lower position as shown in  FIG. 2 . A columnar pedestal base  115 , on which the pedestal  102  is secured, is slideable in conjunction with a flexible, pleated metal housing section  114  to move between the two positions, while maintaining a sealed environment within the reaction chamber. 
     In the upper position ( FIG. 1 ), an initial, relatively small reactor chamber volume is defined by the upper and side walls  117  of chamber section  103  and the surface of pedestal  102 . In this embodiment, pedestal  102  is of a diameter that is slightly smaller than the spacing between opposite reactor side walls  117 , so that the pedestal may move upward to a position above the lower portion of walls  117 . Reactor chamber section  104  is located below and on at least one side of, preferably completely around, pedestal  102 . When the pedestal is moved down to its lowered position ( FIG. 2 ), a subsequent larger reactor chamber volume is defined by the combined volume of both chamber sections  103  and  104 , as well as pedestal  102 . Pedestal  102  may also be moveable to secure wafer  105  in any intermediate position between the upper and lower positions depicted. 
     In order to control the temperature of reactor chamber section  103  at a temperature different from the remainder of the reactor chamber, there are provided heating elements  112  along side the sidewalls of chamber section  103 , and cooling coils  113  containing a re-circulating fluid in the overhead wall of chamber section  103 . These electric heaters  112  and cooling coil  113  provide environmental controls for heating or cooling the wafer while in the smaller reactor chamber volume. 
     Operation of the apparatus is depicted in the process steps as shown in  FIG. 3 . At the start of the process  20 , the pedestal is moved to the desired wafer loading position, and the wafer secured to the top of pedestal  102 . The reactor is then pressurized to its base pressure,  22 . The pedestal is then moved upward to the reactor top, compacted position,  24 , and the first precursor or reactant is flowed into chamber section  103  through valve  108  and passageway  111 . The temperature in reactor vessel  103  is also adjusted by environmental controls  112 ,  113 . While in the smaller chamber volume, the wafer surface  105  receives a high exposure of the first precursor. Because of the limited volume, smaller amounts of gaseous precursor may achieve higher desired concentration for reaction. Once the desired reaction is achieved along a layer of the wafer surface, the pedestal is moved to the bottom, expanded position  26 , and the chambers are evacuated through valve  107  and simultaneously purged with a reaction-limiting purge gas through valve  108 . The higher volume within combined reaction chamber sections  103  and  104  rapidly reduces the overpressure and concentration of the first precursor, and limits the reaction on the wafer surface. Once the initial precursor and excess byproducts are removed from the reaction chamber above the wafer substrate  105 , the pedestal is again moved to the top position,  28 , and the second precursor is flowed into the smaller initial chamber volume  103  as described in step  24 . Again, once the reaction with the second precursor is achieved, and a desired film thickness is deposited on wafer  105  surface, the pedestal is moved to the bottom position,  30 , and vacuum and purge gas flow is again initiated in order to stop the reaction and remove excess second precursor and unwanted reaction byproducts from the larger volume reaction chamber. 
     Process steps  24 - 30  may be repeated as necessary to achieve the desired thickness of the deposited layer on the surface of wafer substrate  105 . Once the film deposition is completed, the pedestal is moved back to the desired wafer loading position,  32 , and the reaction chamber is pumped to the wafer transfer pressure. A reactor isolation valve (not shown) is then opened and the wafer may be removed from the reaction chamber. 
     An example of pulse layer deposition utilizing the method and apparatus of the present invention is described below. 
     With the reactor in the compacted position, trimethyl aluminum at a pressure of 12 torr is introduced into the chamber by opening valve  108  for approximately 1 sec, to deposit an approximately monoatomic layer of aluminum oxide on to the wafer surface. The wafer pedestal is then lowered to the reactor expanded position, and an inert purge gas such as nitrogen is introduced at a high rate by opening valve  108  to purge the remaining precursor and reactant gases over the wafer. Simultaneously, valve  107  is opened to evacuate the expanded chamber. Subsequently, the pedestal is moved up, to the reactor compacted position, and a silanol precursor, such as tris-t-pentoxysilanol, is introduced into the chamber by opening valve  108 . The silanol gas is at a pressure of about 1 Torr and the valve is opened for about 5 to 20 seconds, until the polymerization reaction at the wafer surface results in a silicon dioxide layer of about 100-150 angstroms thickness. The aluminum oxide layer serves to catalyze the reaction that deposits the silicon dioxide. The pedestal is then lowered to expand the reaction chamber, and the purge gas is again introduced to stop the reaction and evacuate the chamber. 
       FIG. 4  depicts an alternate embodiment of the reactor apparatus of  FIG. 1 , with the pedestal in the raised position. In this embodiment, pedestal  102  has a diameter larger than the spacing between side walls  117 . Pedestal  102  has chamfered edges  116  that correspond with the lower chamfered corners  118  of reactor walls  117 . If film builds up on the reactor walls, the space between the pedestal and walls as in  FIG. 1  will not become sealed, and the pedestal will not bind to the walls, even if the pedestal contacts chamfered corners  118 . 
     Accordingly, the present invention provides an improved method and apparatus for a pulsed layer deposition in the fabrication of microelectronic circuits. The variable reactor chamber volume makes the process partially viable for ULSI processing. Additionally, the variable reaction chamber volume reduces the amount of precursors used while enabling high concentrations and fast reaction times to be achieved. 
     While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as failing within the true scope and spirit of the present invention.