Abstract:
A substrate processing system has a housing that defines a process chamber, a gas-delivery system, a high-density plasma generating system, a substrate holder, and a controller. The housing includes a sidewall and a dome positioned above the sidewall. The dome has physically separated and noncontiguous pieces. The gas-delivery system introduces e a gas into the process chamber through side nozzles positioned between two of the physically separated and noncontiguous pieces of the dome. The high-density plasma generating system is operatively coupled with the process chamber. The substrate holder is disposed within the process chamber and supports a substrate during substrate processing. The controller controls the gas-delivery system and the high-density plasma generating system.

Description:
BACKGROUND OF THE INVENTION 
   One of the primary steps in the fabrication of modern semiconductor devices is the formation of a film, such as a silicon oxide film, on a semiconductor substrate. Silicon oxide is widely used as an insulating layer in the manufacture of semiconductor devices. As is well known, a silicon oxide film can be deposited by a thermal chemical-vapor deposition (“CVD”) process or by a plasma-enhanced chemical-vapor deposition (“PECVD”) process. In a conventional thermal CVD process, reactive gases are supplied to a surface of the substrate, where heat-induced chemical reactions take place to produce a desired film. In a conventional plasma-deposition process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. 
   Semiconductor device geometries have decreased significantly in size since such devices were first introduced several decades ago, and continue to be reduced in size. This continuing reduction in the scale of device geometry has resulted in a dramatic increase in the density of circuit elements and interconnections formed in integrated circuits fabricated on a semiconductor substrate. One persistent challenge faced by semiconductor manufacturers in the design and fabrication of such densely packed integrated circuits is the desire to prevent spurious interactions between circuit elements, a goal that has required ongoing innovation as geometry scales continue to decrease. 
   Unwanted interactions are typically prevented by providing spaces between adjacent elements that are filled with an electrically insulative material to isolate the elements both physically and electrically. Such spaces are sometimes referred to herein as “gaps” or “trenches,” and the processes for filling such spaces are commonly referred to in the art as “gapfill” processes. The ability of a given process to produce a film that completely fills such gaps is thus often referred to as the “gapfill ability” of the process, with the film described as a “gapfill layer” or “gapfill film.” As circuit densities increase with smaller feature sizes, the widths of these gaps decrease, resulting in an increase in their aspect ratio, which is defined by the ratio of the gap&#39;s height to its depth. High-aspect-ratio gaps are difficult to fill completely using conventional CVD techniques, which tend to have relatively poor gapfill abilities. One family of electrically insulating films that is commonly used to fill gaps in intermetal dielectric (“IMD”) applications, premetal dielectric (“PMD”) applications, and shallow-trench-isolation (“STI”) applications, among others, is silicon oxide (sometimes also referred to as “silica glass” or “silicate glass”). 
   Some integrated circuit manufacturers have turned to the use of high-density plasma CVD (“HDP-CVD”) systems in depositing silicon oxide gapfill layers. Such systems form a plasma that has a density greater than about 10 11  ions/cm 3 , which is about two orders of magnitude greater than the plasma density provided by a standard capacitively coupled plasma CVD system. Inductively coupled plasma (“ICP”) systems are examples of HDP-CVD systems. One factor that allows films deposited by such HDP-CVD techniques to have improved gapfill characteristics is the occurrence of sputtering simultaneous with deposition of material. Sputtering is a mechanical process by which material is ejected by impact, and is promoted by the high ionic density of the plasma in HDP-CVD processes. The sputtering component of HDP deposition thus slows deposition on certain features, such as the corners of raised surfaces, thereby contributing to the increased gapfill ability. 
   Even with the use of HDP and ICP processes, there remain a number of persistent challenges in achieving desired deposition properties. These include the need to manage thermal characteristics of the plasma within a processing chamber, particularly with high-energy processes that may result in temperatures that damage structures in the chamber. In addition, there is a general desire to provide deposition processes that are uniform across a wafer. Nonuniformities lead to inconsistencies in device performance and may result from a number of different factors. The deposition characteristics at different points over a wafer result from a complex interplay of a number of different effects. For example, the way in which gas is introduced into the chamber, the level of power used to ionize precursor species, the use of electrical fields to direct ions, and the like, may ultimately affect the uniformity of deposition characteristics across a wafer. In addition, the way in which these effects are manifested may depend on the physical shape and size of the chamber, such as by providing different diffusive effects that affect the distribution of ions in the chamber. 
   There is accordingly a general need in the art for improved systems for improving deposition uniformity across a wafer in HDP and ICP processes. 
   BRIEF SUMMARY OF THE INVENTION 
   Embodiments of the invention provide improved deposition processes under certain processing conditions, particularly under processing conditions where material is deposited with an inductively coupled plasma system under high energy conditions and over extended time periods. Such embodiments provide improved thermal control, a more diffusive gas flow within a process chamber, and improved plasma uniformity under such processing conditions by using a multi-piece dome as part of the process chamber. 
   A substrate processing system is provided with a housing that defines a process chamber, a gas-delivery system, a high-density plasma generating system, a substrate holder, and a controller. The housing includes a sidewall and a dome positioned above the sidewall. The dome has a plurality of physically separated and noncontiguous pieces. The gas-delivery system is configured to introduce a gas into the process chamber through side nozzles positioned between two of the physically separated and noncontiguous pieces of the dome. The high-density plasma generating system is operatively coupled with the process chamber. The substrate holder is disposed within the process chamber and configured to support a substrate during substrate processing. The controller controls the gas-delivery system and the high-density plasma generating system. 
   In some embodiments, the plurality of physically separated and noncontiguous pieces consist of two physically separated and noncontiguous pieces. The side nozzles may be adapted to provide a variable angle for directing the gas into the process chamber. 
   In addition to delivery of gas through side nozzles, the gas-delivery system may be configured to introduce a gas into the process chamber through a top nozzle through a first of the physically separated and noncontiguous pieces. The high-density plasma generating system may comprise an inductively driven RF coil disposed circumferentially about a second of the physically separated and noncontiguous pieces different from the first of the physically separated and noncontiguous pieces. In addition, the high-density plasma generating system may further comprise an inductively driven top RF coil disposed relatively proximate to the first of the physically separated and noncontiguous pieces. 
   In certain instances, the high-density plasma generating system may comprise magneto-dielectric material proximate the side RF coil for concentrating a magnetic field generated by the side RF coil. In one embodiment, the magneto-dielectric material comprises a ferromagnetic material and a dielectric material, the dielectric material provided at greater than 2 wt. % of the magneto-dielectric material, and has a relative permeability greater than 14. 
   In other instances, a magnetic confinement ring having a plurality of magnetic dipoles may be disposed circumferentially around the process chamber. For example, the magnetic confinement ring may be disposed circumferentially about one of the plurality of physically separated and noncontiguous pieces. 
   A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified cross-sectional view of an exemplary ICP reactor system; 
       FIG. 2  is a schematic illustration of certain components of an ICP reactor system; 
       FIG. 3  provides data illustrating a temperature variation of a dome in a single-dome ICP reactor system; 
       FIG. 4  is a schematic illustration illustrating aspects of an ICP reactor according to embodiments of the invention; 
       FIG. 5  provides simulation results of plasma distribution in an ICP reactor according to embodiments of the invention; and 
       FIG. 6  provides SEM views of structures comparing gapfill characteristics of ICP reactors. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   1. Overview 
   Embodiments of the invention provide an ICP reactor that has a multi-piece dome. When the inventors were initially confronted with the problem of improving deposition uniformity in ICP applications, they began by considering a number of different sources for nonuniformities and undertook a variety of studies to understand how these sources contributed to the resultant nonuniformity. These studies included both simulation and experimental studies. In particular, three principal classes of factors were identified as bearing on uniformity characteristics: plasma characteristics, chamber flow distributions, and thermal effects. 
   For example, in gapfill applications, better overall gapfill is achieved with higher ion densities in the plasma. Similarly, improved center-to-edge uniformity across a wafer is achieved when the ion distribution in the chamber has better uniformity. One reason that many ICP and HDP chambers have both top and side RF coils has been to try to improve the uniformity of the ion distribution in the chamber. It is generally expected that the effect of a top coil is to produce a plasma density that is greater at the center of the wafer and lower at the edge of the wafer, and that the opposite effect should result from side coils. 
   The chamber flow distributions are generally dictated by the location of gas nozzles that introduce precursor into the chamber, and by the presence of structure within the chamber that may affect flow characteristics, such as baffle structures and the like. In addition, the rates at which precursor gases are provided to the chamber through the nozzles affect the flow characteristics. One measure in which the variation in flow characteristics is evident is in the variation of a deposition/sputter ratio across the wafer. The deposition/sputter ratio is one of several commonly used measures that quantify high-density plasma processes according to the relative contributions of the simultaneous deposition and sputtering components of the process. Characterization of the plasma as a “high-density” plasma means that the average ion density of the plasma is greater than about 10 11  ions/cm 3 , with the deposition/sputter ratio being defined as: 
             D   S     ≡           (     net   ⁢           ⁢   deposition   ⁢           ⁢   rate     )     +     (     blanket   ⁢           ⁢   sputtering   ⁢           ⁢   rate     )         (     blanket   ⁢           ⁢   sputtering   ⁢           ⁢   rate     )       .           
The deposition/sputter rate increases with increased deposition and decreases with increased sputtering. As used in the definition of D/S, the “net deposition rate” refers to the deposition rate that is measured when deposition and sputtering are occurring simultaneously. The “blanket sputter rate” is the sputter rate measured when the process recipe is run without deposition gases; the pressure within the process chamber is adjusted to the pressure during deposition and the sputter rate measured on a blanket thermal oxide.
 
   Other equivalent measures may be used to quantify the relative deposition and sputtering contributions of high-density plasma processes, as is known to those of skill in the art. A common alternative ratio is the “etching/deposition ratio,” 
               E   D     ≡         (     source   ⁢     -     ⁢   only   ⁢           ⁢   deposition   ⁢           ⁢   rate     )     -     (     net   ⁢           ⁢   deposition   ⁢           ⁢   rate     )         (     source   ⁢     -     ⁢   only   ⁢           ⁢   deposition   ⁢           ⁢   rate     )         ,         
which increases with increased sputtering and decreases with increased deposition. As used in the definition of E/D, the “net deposition rate” again refers to the deposition rate measured when deposition and sputtering are occurring simultaneously. The “source-only deposition rate,” however, refers to the deposition rate that is measured when the process recipe is run with no sputtering. Embodiments of the invention are described herein in terms of D/S ratios. While D/S and E/D are not precise reciprocals, they are inversely related and conversion between them will be understood to those of skill in the art.
 
   Thermal effects influence uniformity because they are directly related to the kinetic energy of the ionic species and therefore affect both the deposition/sputter ratio of the plasma and affect diffusive characteristics of the plasma. The temperature within the chamber is often chosen to be consistent with performance criteria defined by the chemical properties of the precursor gases used for the process so that different processes may have different uniformity concerns. For instance, gapfill processes are frequently used for the deposition of silicon oxide by providing precursor flows of monosilane SiH 4  and molecular oxygen O 2  to the chamber with a fluent gas. Depending on the physical structures of the gaps to be filled, including their separations, aspect ratios, and the like, different fluent gases may be preferred. For instance, some processes use a relatively heavy gas like Ar while other processes use lighter gases such as He and/or H 2 , as described in commonly assigned U.S. patent application Ser. No. 10/137,132, entitled “METHOD FOR HIGH ASPECT RATIO HDP CVD GAPFILL,” filed Apr. 30, 2002 by Zhong Qiang Hua et al. and commonly assigned U.S. patent application Ser. No. 10/350,445, entitled “HYDROGEN ASSISTED HDP-CVD DEPOSITION PROCESS FOR AGGRESSIVE GAP-FILL TECHNOLOGY,” filed Jan. 23, 2003 by Bikram Kapoor et al., the entire disclosures of both of which are herein incorporated by reference for all purposes. Processes that use lighter fluent gases like H 2  tend to use higher chamber temperatures, leading to different kinetic characteristics of the plasma ions and affecting the wafer uniformity of the process. 
   As illustrated below, the inventors have found that the temperatures desired to implement certain deposition processes result in temperatures sufficiently high to damage portions of an ICP chamber, particularly the ceramic dome at the top of the chamber. This has been observed, for instance, with processes that provide source RF power greater than about 12 kW. One way that the inventors have identified for managing the thermal characteristics within the ICP chamber is to provide a multi-piece dome, with certain embodiments being directed particularly to a two-piece having portions separated by a gas ring having gas nozzles through which gases are introduced to the chamber. Such a configuration also results in providing a more diffusive gas flow in the ICP chamber and a more uniform plasma, improving the overall deposition characteristics. In some instances, a magnetic-field concentrator is additionally included, as may also be a magnetic confinement ring to improve the plasma uniformity further. 
   Embodiments of the invention are described in detail below, beginning with a detailed description of an exemplary ICP chamber. The temperature behavior of the dome when such an exemplary ICP chamber is provided as a conventional single-piece dome are discussed and modifications that introduce a multi-piece dome, together with a magnetic-field concentrator and/or a magnetic confinement ring, are illustrated. Simulation and experimental results illustrating the physical consequences on deposition processes of such a modified configuration are then presented. 
   2. Exemplar ICP Chamber 
   The inventors have implemented embodiments of the invention with the ULTIMA™ system manufactured by APPLIED MATERIALS, INC., of Santa Clara, Calif., a general description of which is provided in commonly assigned U.S. Pat. No. 6,170,428, “SYMMETRIC TUNABLE INDUCTIVELY COUPLED HDP-CVD REACTOR,” filed Jul. 15, 1996 by Fred C. Redeker, Farhad Moghadam, Hirogi Hanawa, Tetsuya Ishikawa, Dan Maydan, Shijian Li, Brian Lue, Robert Steger, Yaxin Wang, Manus Wong and Ashok Sinha, the entire disclosure of which is incorporated herein by reference. An overview of the ICP reactor is provided in connection with  FIG. 1  below. The ICP reactor is part of an HDP-CVD system  110  that includes a chamber  113 , a vacuum system  170 , a source plasma system  180 A, a bias plasma system  180 B, a gas delivery system  133 , and a remote plasma cleaning system  150 . The upper portion of chamber  113  includes a dome  114 , which is made of a ceramic dielectric material, such as aluminum oxide or aluminum nitride. Dome  114  defines an upper boundary of a plasma processing region  116  and is provided as a single-piece dome for conventional systems as illustrated in  FIG. 1 . Plasma processing region  116  is bounded on the bottom by the upper surface of a substrate  117  and a substrate support member  118 . 
   A heater plate  123  and a cold plate  124  surmount, and are thermally coupled to, dome  114 . Heater plate  123  and cold plate  124  allow control of the dome temperature to within about ±10° C. over a range of about 100° C. to 200° C. This allows optimizing the dome temperature for the various processes. For example, it may be desirable to maintain the dome at a higher temperature for cleaning or etching processes than for deposition processes. Accurate control of the dome temperature also reduces the flake or particle counts in the chamber and improves adhesion between the deposited layer and the substrate. 
   The lower portion of chamber  113  includes a body member  122 , which joins the chamber to the vacuum system. A base portion  121  of substrate support member  118  is mounted on, and forms a continuous inner surface with, body member  122 . Substrates are transferred into and out of chamber  113  by a robot blade (not shown) through an insertion/removal opening (not shown) in the side of chamber  113 . Lift pins (not shown) are raised and then lowered under the control of a motor (also not shown) to move the substrate from the robot blade at an upper loading position  157  to a lower processing position  156  in which the substrate is placed on a substrate receiving portion  119  of substrate support member  118 . Substrate receiving portion  119  includes an electrostatic chuck  120  that secures the substrate to substrate support member  118  during substrate processing. In a preferred embodiment, substrate support member  118  is made from an aluminum oxide or aluminum ceramic material. 
   Vacuum system  170  includes throttle body  125 , which houses twin-blade throttle valve  126  and is attached to gate valve  127  and small-molecule-enhanced turbomolecular pump  128 . As described in detail below, the turbomolecular pump  128  has the modified performance characteristics making it suitable for efficient exhaustion of low-mass molecular species. It should be noted that throttle body  125  offers minimum obstruction to gas flow, and allows symmetric pumping. Gate valve  127  can isolate pump  128  from throttle body  125 , and can also control chamber pressure by restricting the exhaust flow capacity when throttle valve  126  is fully open. The arrangement of the throttle valve, gate valve, and small-molecule-enhanced turbomolecular pump allow accurate and stable control of chamber pressures from between about 2 millitorr to about 2 torr. 
   The source plasma system  180 A includes a top coil  129  and side coil  130 , mounted on dome  114 . A symmetrical ground shield (not shown) reduces electrical coupling between the coils. Top coil  129  is powered by top source RF (SRF) generator  131 A, whereas side coil  130  is powered by side SRF generator  131 B, allowing independent power levels and frequencies of operation for each coil. This dual coil system allows control of the radial ion density in chamber  113 , thereby improving plasma uniformity. Side coil  130  and top coil  129  are typically inductively driven, which does not require a complimentary electrode. In a specific embodiment, the top source RF generator  131 A provides up to 2,500 watts of RF power at nominally 2 MHz and the side source RF generator  131 B provides up to 5,000 watts of RF power at nominally 2 MHz. The operating frequencies of the top and side RF generators may be offset from the nominal operating frequency (e.g. to 1.7-1.9 MHz and 1.9-2.1 MHz, respectively) to improve plasma-generation efficiency. 
   A bias plasma system  180 B includes a bias RF (“BRF”) generator  131 C and a bias matching network  132 C. The bias plasma system  180 B capacitively couples substrate portion  117  to body member  122 , which act as complimentary electrodes. The bias plasma system  180 B serves to enhance the transport of plasma species (e.g., ions) created by the source plasma system  180 A to the surface of the substrate. In a specific embodiment, bias RF generator provides up to 5,000 watts of RF power at 13.56 MHz. 
   RF generators  131 A and  131 B include digitally controlled synthesizers and operate over a frequency range between about 1.8 to about 2.1 MHz. Each generator includes an RF control circuit (not shown) that measures reflected power from the chamber and coil back to the generator and adjusts the frequency of operation to obtain the lowest reflected power, as understood by a person of ordinary skill in the art. RF generators are typically designed to operate into a load with a characteristic impedance of 50 ohms. RF power may be reflected from loads that have a different characteristic impedance than the generator. This can reduce power transferred to the load. Additionally, power reflected from the load back to the generator may overload and damage the generator. Because the impedance of a plasma may range from less than 5 ohms to over 900 ohms, depending on the plasma ion density, among other factors, and because reflected power may be a function of frequency, adjusting the generator frequency according to the reflected power increases the power transferred from the RF generator to the plasma and protects the generator. Another way to reduce reflected power and improve efficiency is with a matching network. 
   Matching networks  132 A and  132 B match the output impedance of generators  131 A and  131 B with their respective coils  129  and  130 . The RF control circuit may tune both matching networks by changing the value of capacitors within the matching networks to match the generator to the load as the load changes. The RF control circuit may tune a matching network when the power reflected from the load back to the generator exceeds a certain limit. One way to provide a constant match, and effectively disable the RF control circuit from tuning the matching network, is to set the reflected power limit above any expected value of reflected power. This may help stabilize a plasma under some conditions by holding the matching network constant at its most recent condition. 
   Other measures may also help stabilize a plasma. For example, the RF control circuit can be used to determine the power delivered to the load (plasma) and may increase or decrease the generator output power to keep the delivered power substantially constant during deposition of a layer. 
   A gas delivery system  133  provides gases from several sources,  134 A- 134 E chamber for processing the substrate via gas delivery lines  138  (only some of which are shown). As would be understood by a person of skill in the art, the actual sources used for sources  134 A- 134 E and the actual connection of delivery lines  138  to chamber  113  varies depending on the deposition and cleaning processes executed within chamber  113 . Gases are introduced into chamber  113  through a gas ring  137  and/or a top nozzle  145 . A plurality of source gas nozzles  139  (only one of which is shown in the illustration) provide a uniform flow of gas over the substrate. Nozzle length and nozzle angle may be changed to allow tailoring of the uniformity profile and gas utilization efficiency for a particular process within an individual chamber. In one embodiment, twelve source gas nozzles made from an aluminum oxide ceramic are provided. 
   In addition, a plurality of oxidizer gas nozzles  140  (only one of which is shown), which in a preferred embodiment are co-planar with and shorter than source gas nozzles  139 . In some embodiments it is desirable not to mix source gases and oxidizer gases before injecting the gases into chamber  113 . In other embodiments, oxidizer gas and source gas may be mixed prior to injecting the gases into chamber  113 . In one embodiment, third, fourth, and fifth gas sources,  134 C,  134 D, and  134 D′, and third and fourth gas flow controllers,  135 C and  135 D′, provide gas to body plenum via gas delivery lines  138 . Additional valves, such as  143 B (other valves not shown), may shut off gas from the flow controllers to the chamber. 
   In embodiments where flammable, toxic, or corrosive gases are used, it may be desirable to eliminate gas remaining in the gas delivery lines after a deposition. This may be accomplished using a 3-way valve, such as valve  143 B, to isolate chamber  113  from the delivery lines and to vent the delivery lines to vacuum foreline  144 , for example. As shown in  FIG. 1 , other similar valves, such as  143 A and  143 C, may be incorporated on other gas delivery lines. Such three-way valves may be placed as close to chamber  113  as practical, to minimize the volume of the unvented gas delivery line (between the three-way valve and the chamber). Additionally, two-way (on-off) valves (not shown) may be placed between a mass flow controller (“MFC”) and the chamber or between a gas source and an MFC. 
   The chamber  113  also has top nozzle  145  and top vent  146 . Top nozzle  145  and top vent  146  allow independent control of top and side flows of the gases, which improves film uniformity and allows fine adjustment of the film&#39;s deposition and doping parameters. Top vent  146  is an annular opening around top nozzle  145 . In one embodiment, first gas source  134 A supplies source gas nozzles  139  and top nozzle  145 . Source nozzle MFC  135 A′ controls the amount of gas delivered to source gas nozzles  139  and top nozzle MFC  135 A controls the amount of gas delivered to top gas nozzle  145 . Similarly, two MFCs  135 B and  135 B′ may be used to control the flow of oxygen to both top vent  146  and oxidizer gas nozzles  140  from a single source of oxygen, such as source  134 B. In some embodiments, oxygen is not supplied to the chamber from any side nozzles. The gases supplied to top nozzle  145  and top vent  146  may be kept separate prior to flowing the gases into chamber  113 , or the gases may be mixed in top plenum  148  before they flow into chamber  113 . Separate sources of the same gas may be used to supply various portions of the chamber. 
   A remote microwave-generated plasma cleaning system  150  is provided to periodically clean deposition residues from chamber components. The cleaning system includes a remote microwave generator  151  that creates a plasma from a cleaning gas source  134 E (e.g., molecular fluorine, nitrogen trifluoride, other fluorocarbons or equivalents) in reactor cavity  153 . The reactive species resulting from this plasma are conveyed to chamber  113  through cleaning gas feed port  154  via applicator tube  155 . The materials used to contain the cleaning plasma (e.g., cavity  153  and applicator tube  155 ) must be resistant to attack by the plasma. The distance between reactor cavity  153  and feed port  154  should be kept as short as practical, since the concentration of desirable plasma species may decline with distance from reactor cavity  153 . Generating the cleaning plasma in a remote cavity allows the use of an efficient microwave generator and does not subject chamber components to the temperature, radiation, or bombardment of the glow discharge that may be present in a plasma formed in situ. Consequently, relatively sensitive components, such as electrostatic chuck  120 , do not need to be covered with a dummy wafer or otherwise protected, as may be required with an in situ plasma cleaning process. In  FIG. 1 , the plasma-cleaning system  150  is shown disposed above the chamber  113 , although other positions may alternatively be used. 
   A baffle  161  may be provided proximate the top nozzle to direct flows of source gases supplied through the top nozzle into the chamber and to direct flows of remotely generated plasma. Source gases provided through top nozzle  145  are directed through a central passage  162  into the chamber, while remotely generated plasma species provided through the cleaning gas feed port  154  are directed to the sides of the chamber  113  by the baffle  161 . 
   3. Dome Characteristics 
   The relevant structural characteristics of the ICP system of  FIG. 1  in discussing dome characteristics are presented in simplified form in  FIG. 2 . In this drawing the ICP chamber is defined generally by chamber walls  212  and chamber dome  228 . A substrate  204  is disposed on a substrate support and may have material deposited on it from a plasma formed by inductive coupling of energy from side coil  220  and top coil  224  to gas supplied from side nozzles  216  and from a top nozzle through baffle  226 . In response to finding that that the dome  228  in such a structure is subject to fracture for high-energy processes, such as where the side and top coils  220  and  224  collectively provide RF energy greater than about 12 kW, the inventors hypothesized that such fracture may be a consequence of thermal effects, particularly for relatively long-time processes. 
   Results of an experimental investigation are provided in  FIG. 3 . This graph plots the temperature of the top and side portions of the dome  228  over time during a deposition process. The process has a certain periodicity evident from the data collected for the top (curve  304 ) and for the side (curve  308 ), with a generally increasing trend superimposed over this periodicity. The superimposed trend is illustrated for the top of the dome  228  by curve  312  and for the side of the dome  228  by curve  316 . This generally increasing trend is stronger for the side portion of the dome than for the top, illustrating a clear lack of cooling at the side of the dome. These results provide confirmation that thermal effects for long-time processes contribute to the observed dome fracture during high-power processes. 
   Improved thermal management, and therefore a decreased risk of fracture during such processes, may be achieved in embodiments of the invention by using a multi-piece dome, such as the two-piece dome illustrated in  FIG. 4 . In this embodiment, the ICP chamber has a substrate support  408  for supporting a substrate  404  within chamber walls  412 . The two parts of the dome are denoted by reference numbers  432  and  436 , and are separated by a gas ring  414  that is used to supply gases to the chamber through nozzles  416 . The nozzles  416  may advantageously be configured to provide a variable angle so that flows into the process chamber may be directed differently for different processes. Gas may also be provided to the chamber via a top nozzle that directs gas into the chamber through a baffle  428 . A plasma is generated inductively from the gas flows by coupling RF power from a side coil  420  and/or from a top coil  424 . 
   Providing the dome in multiple pieces in the manner illustrated in  FIG. 4  advantageously improves thermal management of processes in the chamber. Experiments performed by the inventors have confirmed that even with source RF powers provided by the side and top coils  420  and  424  to exceed 12 kW, fracture of the dome may be avoided. The positioning of the side nozzles  416  in this fashion is generally farther from the substrate  404  than in the design illustrated in  FIG. 2 , with a consequence that the multi-piece dome configuration provides a more diffusive flow within the processing chamber. In addition, the plasma uniformity is improved, resulting in generally improved deposition characteristics. In addition, the configuration shown in  FIG. 4  provides the side RF coils  420  at a position that is closer to the substrate  404  surface when compared with the configuration of  FIG. 2 ; as illustrated in the discussion below, this provides improved plasma uniformity at the substrate  404  surface. 
   In some embodiments, use the multi-piece dome may be coupled with other mechanisms that may also improve plasma uniformity and/or the diffusivity of the gas flow within the processing chamber. For instance, the inductive RF coupling may be enhanced by providing a magnetic-field concentrator  420  disposed proximate the side coil  420 . The use of such a magnetic-field concentrator  420  is described in further detail in copending, commonly assigned U.S. patent application Ser. No. 10/963,030, entitled “MAGNETIC-FIELD CONCENTRATION IN INDUCTIVELY COUPLED PLASMA REACTORS,” filed Oct. 12, 2004 by Siqing Lu et al., the entire disclosure of which is incorporated herein by reference. Briefly, such a magnetic-field concentrator  420  may comprise a magneto-dielectric material that acts to concentrate the magnetic field generated by the side coil  420 . In some embodiments, the magneto-dielectric material comprises a ferromagnetic material and a dielectric material, with the dielectric material being provided at greater than 2 wt. % of the magneto-dielectric material. 
   The ICP system may also include a magnetic confinement ring  440  disposed circumferentially about one of the pieces of the dome, preferably around the lower piece of the dome. The use of such a magnetic-confinement ring is described in detail in U.S. patent application Ser. No. 11/053,363, entitled “INDUCTIVE PLASMA SYSTEM WITH SIDEWALL MAGNET,” filed Feb. 8, 2005 by Siqing Lu et al., the entire disclosure of which is incorporated herein by reference for all purposes. In some embodiments, the magnetic confinement ring may have a plurality of magnetic dipoles disposed circumferentially around a symmetry axis orthogonal to a plane of the substrate, and may provide magnetic field with a net dipole moment substantially nonparallel with the substrate plane. 
   The combination of these different mechanisms may improve both the diffusivity of the flow within the process chamber and may improve the plasma uniformity, leading to improvements in deposition quality. The inventors have evaluated these factors through simulations and through experimental measurements, with some results being presented in  FIGS. 5 and 6 .  FIG. 5A  shows the results of a simulation of RF power density within the plasma as inductively coupled within the chamber. Power levels are identified by the legend at the right of the plot and by numerical values indicated for contours on the plot. The top and side sources are evident in the results of the simulation as high-power origins at the top and right side of the result. Diffusion characteristics resulting from the plasma-power distribution are indicated with arrows, including both ion and radical diffusion as well as ambipolar diffusion. As previously intimated, the change in position of the side coils  420  in the two-piece dome configuration to be closer to the substrate  404  surface results in improved plasma uniformity at the substrate  404  surface. 
   Deposition performance using the one-piece and two-piece domes was compared by performing a gapfill test on a substrate. A silicon oxide layer was deposited using flows of SiH 4 , O 2 , and H 2  under conditions that provided a D/S ratio of 9.5 and a net deposition rate of 670 Å/min. In  FIG. 6 , the top panels show SEM photographs for the one-piece dome and the bottom panels show SEM photographs for the two-piece dome; the left panels provide SEM photographs taken at the center of the substrate while the right panels provide photographs taken at the edge of the substrate. Clipping is observed for the one-piece dome chamber at the center of the substrate, but no such clipping is observed for the two-piece dome, confirming that improved gapfill uniformity may be achieved with the multi-piece dome configuration. 
   Having fully described several embodiments of the present invention, many other equivalents or alternative embodiments of the present invention will be apparent to those skilled in the art. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.