Abstract:
A method is provided for performing plasma immersion ion implantation with a highly uniform seasoning film on the interior of a reactor chamber having a ceiling and a cylindrical side wall and a wafer support pedestal facing the ceiling. The method includes providing a gas distribution ring with plural gas injection orifices on a periphery of a wafer support pedestal, the orifices facing radially outwardly from the wafer support pedestal. Silicon-containing gas is introduced through the gas distribution orifices of the ring to establish a radially outward flow pattern of the silicon-containing gas. The reactor includes pairs of conduit ports in the ceiling adjacent the side wall at opposing sides thereof and respective external conduits generally spanning the diameter of the chamber and coupled to respective pairs of the ports. The method further includes injecting oxygen gas through the conduit ports into the chamber to establish an axially downward flow pattern of oxygen gas in the chamber. RF power is coupled into the interior of each of the conduits to generate a toroidal plasma current of Si x O y  species passing through the chamber to deposit a seasoning layer of a Si x O y  material on surfaces within the chamber, while leaving the pedestal without a wafer so as to expose a wafer support surface of the pedestal.

Description:
BACKGROUND 
   Plasma immersion ion implantation of a semiconductor wafer is typically used to form P—N junctions in the wafer surface. The plasma immersion ion implantation (P3i) process is faster or more productive than other implantation processes. In order to attain a requisite implantation or junction depth, ion energy at the wafer surface must relatively high, which can be accomplished by applying a sufficiently high RF bias power to the wafer, or to an electrode within the wafer support pedestal. The P3i reactor chamber is typically constructed of aluminum components whose surfaces are anodized to provide some protection and from plasma in the chamber. One problem is that the high ion energy of the plasma during ion implantation produces ion bombardment of the metallic chamber components, removing metal particles that vaporize into the plasma to spread throughout the chamber and deposit on the wafer. The high ion energy is attained by coupling RF bias power to the wafer at a sufficient level to create a plasma bias voltage on the order of tens or hundreds of kilovolts. Such metal contamination of the wafer can produce defects in the devices formed on the wafer surface. 
   SUMMARY OF THE INVENTION 
   A method is provided for performing plasma immersion ion implantation with a highly uniform seasoning film on the interior of a reactor chamber having a ceiling and a cylindrical side wall and a wafer support pedestal facing the ceiling. The method includes providing a gas distribution ring with plural gas injection orifices on a periphery of a wafer support pedestal, the orifices facing radially outwardly from the wafer support pedestal. Silicon-containing gas is introduced through the gas distribution orifices of the ring to establish a radially outward flow pattern of the silicon-containing gas. The reactor includes pairs of conduit ports in the ceiling adjacent the side wall at opposing sides thereof and respective external conduits generally spanning the diameter of the chamber and coupled to respective pairs of the ports. The method further includes injecting oxygen gas through the conduit ports into the chamber to establish an axially downward flow pattern of oxygen gas in the chamber. RF power is coupled into the interior of each of the conduits to generate a toroidal plasma current of Si x O y  species passing through the chamber to deposit a seasoning layer of a Si x O y  material on surfaces within the chamber, while leaving the pedestal without a wafer so as to expose a wafer support surface of the pedestal. Upon completion of seasoning layer deposition, a wafer is placed on the pedestal. Ion implantation precursor gases are introduced into the chamber through a gas distribution plate that extends across the ceiling. Plasma immersion ion implantation is performed by coupling RF power into the interior of each of the conduits to generate a toroidal plasma of implant species current passing through the chamber so as to implant the implant species into the wafer. The method can further include applying RF bias power to an electrode underlying the wafer to produce a plasma sheath bias voltage on the order of kilovolts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a cut-away side view of a plasma reactor in accordance with one aspect. 
       FIG. 2  is a cut-away top view corresponding to  FIG. 1 . 
       FIG. 3  is a simplified diagram corresponding to  FIG. 1  showing the gas flow pattern by species. 
       FIG. 4  is a graph depicting the variations in stochiometry of a seasoning layer controlled in accordance with the gas flow patterns of  FIG. 3 . 
       FIG. 5  is a diagram depicting a process performed by the reactor of  FIG. 1 . 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale. 
   DETAILED DESCRIPTION OF THE INVENTION 
   In order to minimize or prevent metal contamination from occurring during the P3i process, the chamber interior surfaces can be coated with a non-metallic “seasoning” film prior to the plasma immersion ion implantation and prior to introduction of the wafer into the chamber. The ideal thickness of the seasoning film, at which metal contamination is reduced below specified limits, is readily determined using empirical methods. Typically, the minimum thickness is on the order of 1000 Å, although better result are obtained at more ideal thicknesses, such as 2000 Å. The seasoning film must be removed following the P3i process and thereafter replaced because some of the film is removed—or its thickness reduced—in a non-uniform manner during the P3i process, so that it may not offer adequate protection from metal contamination during a subsequent P3i step. This is particularly true of cases in which the ion implantation plasma is formed of a gas including a fluoride compound of the ion implantation species. The seasoning film removal step can be carried out by filling the chamber with seasoning-removal gas species obtained from an external (“downstream”) plasma source. Such gases may be corrosive species such as fluorine-containing compounds, for example. 
   The seasoning film is deposited using a high density plasma enhanced chemical vapor deposition (HDPCVD) process by introducing a silicon-containing gas (e.g., silane) and oxygen gas into the reactor chamber and igniting a plasma. Radicals, neutrals and/or ions of silicon-oxygen compounds are formed in the plasma, which deposit on the interior chamber surfaces to form a thin film or coating of SiO 2  and/or Si x O y , for example. The problem is that the thickness of the seasoning film is highly non-uniform because of non-uniformities in gas flow, non-uniform RF power or field distribution throughout the chamber and obstructions offered by some mechanical features in the chamber interior. For chamber surfaces in areas of lower gas flow, where the seasoning film deposition rate is slowest, the minimum required seasoning film thickness (e.g., 1000 Å) is not reached until an excessive seasoning film thickness (e.g., 12,000 Å) is reached in other areas of high gas flow and the highest deposition rates. The result is that the seasoning deposition step takes longer than it should—depressing productivity. Moreover, the post-implant seasoning removal process exposes the chamber interior surfaces where the seasoning film was thinnest (1000 Å) well before removal of the thicker (12,000 Å) portions of the seasoning film. The chamber surfaces first exposed during the post-implant seasoning removal step are therefore attacked by the cleaning gases during the remainder of the cleaning step, shortening the life of those components and increasing the operating cost of the reactor. 
   A related problem arises from the non-uniform distribution of the removal rate of the seasoning film during the P3i process and during the post-implant seasoning removal (clean) process. The removal rate during the P3i step is not uniform, because the P3i plasma is concentrated primarily in the wafer-ceiling gap or process zone. In cases, for example, where the seasoning precursor gas is introduced from the chamber bottom, bottom-facing surfaces of some chamber components, such as radial struts supporting the wafer pedestal, receive the thickest coating. Unfortunately, these surfaces face away from the main plasma (i.e., away from the wafer-ceiling gap) and therefore receive little ion bombardment, and are therefore the least reduced in thickness during the P3i step. Having started out with the greatest seasoning thickness and then having been attacked the least during the P3i step, such surfaces bear a disproportionately thick seasoning film and are therefore difficult to clean without undue exposure of other chamber surfaces to corrosive cleaning gases. 
     FIG. 1  illustrates a toroidal source plasma reactor for plasma immersion ion implantation having gas distribution features that solve the foregoing problems. These gas distribution features are used during the pre-implant seasoning deposition step to form a seasoning film throughout the chamber interior that is highly uniform. Some of the features may be used to control the stochiometry of the seasoning film to achieve desired characteristics, as will be discussed herein. 
   The reactor of  FIG. 1  includes a cylindrical chamber  100  defined by a cylindrical side wall  102 , a ceiling  104  and a floor  106 . A wafer support pedestal  108  includes an electrostatic chuck  110  having a wafer support surface for holding a semiconductor wafer  112 . The ceiling  104  has two pairs of openings  114  to which respective mutually orthogonal external reentrant conduits  116 ,  118  are coupled. Each conduit  116 ,  118  completes a closed reentrant path for an oscillating plasma current passing through the process region defined by the gap between the pedestal  108  and the ceiling  104 . The ceiling  104  is a gas distribution plate having an array of gas injection orifices  120  facing the chamber interior and an interior gas distribution manifold  122 . Optionally, the manifold  122  may be divided into radially inner and outer portions  122   a ,  122   b , to establish independent inner and outer gas injection zones (or groups)  124   a ,  124   b  of the orifices  122 . In this case, a pair of separately controlled gas supplies  126   a ,  126   b  are coupled to the inner and outer manifolds  122   a ,  122   b . A pair of plasma RF source power generators  128 ,  130  are coupled to apply RF power to the interiors of respective ones of the conduits  116 ,  118  via respective impedance matches  132 ,  134  and power applicators  136 ,  138 . Each power applicator  136 ,  138  may be of the same structure which consists of a magnetically permeable core or ring  140  wrapped around the respective conduit  116  or  118 , and a conductive coil  142  wrapped around the ring  140 . The electrostatic chuck (ESC)  110  consists of a conductive electrode  110   a  and an insulator layer  110   b  in which the electrode  110   a  is contained. An RF bias power generator  142  is coupled to the ESC electrode  110   a  through an impedance match  144 . A D.C. chuck voltage supply  146  is coupled to the ESC electrode  110   a . The pedestal  108  is supported on three radial struts  150 - 1 ,  150 - 2 ,  150 - 3 , best shown in  FIG. 2 , that extend inwardly from the side wall  102  and underneath the pedestal  108 . 
   During plasma immersion ion implantation, an implant species precursor gas, such as a boron fluoride or a boron hydride in the case of a boron implantation step, is injected through the ceiling gas distribution plate  104  while plasma source power is applied by the generators  128 ,  130  to produce an oscillating closed plasma current in the reentrant path through the process region overlying the wafer. For this purpose, an ion implantation process gas supply  180  is coupled to the inner and outer gas manifolds  122   a ,  122   b  of the gas distribution plate  104 . Optionally, the RE generator  143  applies bias power to the ESC  110  to control ion energy and (hence) implant depth. The gas distribution plate  104  is optimized for uniform gas distribution across the surface of the wafer or wafer support surface of the ESC  110 , but is not structured for uniform gas distribution throughout the chamber interior. Therefore, the gas distribution plate  104  is not, by itself, suitable for use in depositing the seasoning film throughout the chamber. 
   The gas distribution features that provide for a uniform seasoning film in the chamber include a center array of gas injection orifices  202  along the side wall of the pedestal  108  for injecting the silicon-containing gas. The center array of orifices  202  is formed in a hollow gas distribution ring  200  supported on the side wall of the pedestal  108  and extending around the periphery of the pedestal. In the illustrated embodiment, the pedestal  108  supports an electrostatic chuck (ESC)  110 . The ESC includes a conductive base  204  underlying the insulating layer  110 . The base  204  may include internal features for utilities such as coolant passages and backside gas flow passages (not shown). In the illustrated embodiment, the gas distribution ring  200  is attached to the outer periphery of the base  204 . The gas injection orifices  202  on the ring  200  may be oriented at an angle A relative to the horizontal wafer plane so as to inject the silicon-containing gas at an upward direction. This promotes better deposition on upward facing surfaces of interior chamber features, such as the wafer support surface of the ESC  110  and the top surfaces of the radial struts  150 , for example. A silicon-containing (e.g., silane) gas supply  206  is connected to the hollow interior of the gas distribution ring  200 . 
   The oxygen gas is injected during the seasoning film deposition step through the four conduit ports  114  in the ceiling  104 . For this purpose, conduit injection orifices  210  inject gas into the conduits  116 ,  118  near each of the conduit ports  114 . An oxygen gas supply  212  is coupled to each of the conduit injection orifices. Oxygen injection through the conduit ports  114  promotes a more oxygen-rich gas mixture near the sides of the chamber and, therefore, a more silicon-rich gas mixture over the center of the chamber, i.e., over the wafer support surface of the ESC  110 . This is because the conduit ports  114  are all located near the periphery of the ceiling  104 . 
   During the pre-implant seasoning film deposition step, the wafer  112  is absent and no implant process gases are supplied to the gas distribution plate  104 . Therefore, the gas distribution plate  104  is available for use during the seasoning film deposition step. Optionally, the gas distribution plate  104  may be exploited during the seasoning film deposition to control the thickness distribution and the stochiometry distribution of the seasoning layer. For example, a further increase in the silicon content of the gases over the center of the chamber (over the pedestal  108 ) can be realized by injecting the silicon-containing gas (silane) through the center (inner) gas distribution zone  124  of the gas distribution plate  104 . For this purpose, the inner zone gas supply  126   a  stores silane, for example. To promote a thicker film on upward facing surface without detracting from the predominance of silicon-containing gas at the center, oxygen gas could be injected through the outer gas injection zone  124   b  of the gas distribution plate  104 . In this case, the outer zone gas supply  126   b  stores oxygen gas. The proportion of silicon to oxygen in the center and periphery of the chamber is controlled or affected by the different gas flow rates to the inner and outer zones  122   a ,  122   b  as well as the gas flow rates to the conduit injection orifices  210  and to the gas distribution ring  200 .  FIG. 3  summarizes the foregoing gas flows by species from each of the gas injection elements of the reactor of  FIG. 1 . 
   The position (e.g., axial height) of the gas distribution ring  200  and the angle A of the orifices  202  of the gas distribution ring  200  can be adjusted to achieve a desired uniformity of the coating of Si x O y . Oxygen and SiH4 can be supplied at a controlled rate to the distribution ring  200  for injection through the orifices  202  in order to control the stochiometry of the coating or seasoning deposition. 
   We have found that the foregoing features solve the problem non-uniform seasoning deposition. Whereas prior to the invention the seasoning thickness varied from a minimum of about 1000 Å on some upward facing surfaces to a maximum of about 12,000 Å on some downward facing surfaces, the invention produces a much smaller variation in seasoning thickness, permitting us to establish a much greater minimum thickness (of about 2000 Å) without exceeding a maximum of about 3000-4000 Å. This greater minimum thickness is achieved in a much shorter deposition time, while the removal step is performed very quickly, thus increasing throughput. 
     FIG. 4  is a graph depicting a desired radial distribution of Si—O stochiometry of the seasoning layer that can be controlled with the foregoing gas distribution features. Specifically, by providing more silane (silicon-containing) gas over the wafer pedestal  108  and more oxygen gas at the periphery, the stochiometry of the Si x O y  seasoning film (i.e., the ratio x:y) is distributed so as to have a silicon-rich proportion at the center and a silicon-lean proportion at the periphery. The advantage is that a material with a higher conductivity is provided where it is needed, i.e., on the ESC  110 . The higher conductivity of the silicon-rich mixture covering the ESC  110  enables the electric charge on the wafer to be removed more rapidly during de-chucking of the wafer, leading the better throughput or productivity. By providing a more conductive path for charge on the wafer to dissipate, the electrostatic clamping force holding the wafer to the ESC  110  is more rapidly removed when the D.C. chucking voltage source is switched off to dechuck the wafer. In  FIG. 4 , the proportion (x) of silicon is maximum over the pedestal  108  while the proportion (y) of oxygen is minimum in the same area. The relationship is reversed at the periphery, as indicated in the graph of  FIG. 4 . A nominal value for x is 1 while a nominal value for y is 2. The stochiometric variation illustrated in the graph of  FIG. 4  between the radially inner and outer regions of the deposited seasoning film is increased by increasing the flow of the silicon-containing gas to the center (e.g., through the gas distribution plate  104  or through the gas distribution ring  200 ) relative to the oxygen gas flow to the periphery (e.g., through the conduit ports  114 ). 
     FIG. 5  depicts a process involving pre-implant seasoning deposition, plasma immersion ion implantation and post-implant cleaning or seasoning removal. The entire cycle begins with no wafer on the pedestal  108  (block  250  of  FIG. 5 ). For pre-implant seasoning deposition, a silicon-containing gas (e.g., silane) is injected through gas injection orifices  202  around the side of the pedestal  108  (block  252 ) and oxygen gas is injected through the ceiling ports  114  of the external reentrant conduits  116 ,  118  (block  254 ). An optional step (block  256 ) is to inject either silane or oxygen through ceiling gas distribution plate  104 . In the case of silane, the flow rate is adjusted to achieve about a 2-10% enhancement of the silicon content of the seasoning layer deposited on the ESC  110  over the nominal 1:2 silicon-to-oxygen ratio of silicon dioxide. A plasma is generated (by applying RF power either to the applicators  136 ,  138  or to the ESC electrode  110   a , to deposit a Si x O y  seasoning film on chamber interior surfaces (block  258 ). An optional step (block  260 ) is to adjust gas flow (e.g., either silane or oxygen) through ceiling gas distribution plate  104  to achieve desired enrichment of silicon proportion (x:y) of seasoning film on the wafer support surface of the electrostatic chuck, in accordance with  FIG. 4 . Then, the seasoning deposition process is stopped after desired seasoning film thickness (e.g., 2000 Å) has been reached (block  262 ). A wafer is placed onto the pedestal  108  (block  264 ). Plasma immersion ion implantation is performed (block  266 ) by introducing an implant species-containing process gas and applying RF source power to the conduit RF power applicators  136 ,  138 . During this step, ion energy (implant depth) may be controlled by applying RF bias power to the ESC electrode  110   a  from the generator  142 . Upon completion of the implant step, wafer is removed (block  268 ). The seasoning film is removed (block  270 ) by introducing a seasoning layer etch gas from a downstream plasma source, for example. 
   While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.