Patent Publication Number: US-2023147452-A1

Title: Substrate pedestal for improved substrate processing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit to U.S. Non Provisional application Ser. No. 16/814,736, filed Mar. 10, 2020, which is incorporated by reference herein. 
     BACKGROUND 
     Field 
     Embodiments described herein generally relate to semiconductor processing apparatuses that utilize high frequency power devices and, more particularly, to semiconductor processing apparatuses that utilize radio frequency (RF) power generation and/or delivery equipment. 
     Description of the Related Art 
     Semiconductor processing apparatuses typically include a process chamber that is adapted to perform various deposition, etching or thermal processing steps on a wafer, or substrate, within a processing region of the process chamber. To achieve higher deposition rates in a typical plasma-enhanced chemical vapor deposition (PECVD) chamber, plasma radial density is increased by the application of an increased radio frequency (RF) power. The RF power is delivered through a showerhead and a substrate pedestal, over which a wafer is disposed, from an RF generator The substrate pedestal includes a conductive mesh that is brazed to a conductive electrode. 
     However, due to an increased RF current induced by the increased RF power, large Joule heat is generated at a braze joint between the conductive mesh and the conductive electrode, resulting in localized heating at the braze joint and thus non-uniform temperature distribution over the wafer. Small variations in temperature in the wafer during processing can affect the within-wafer (WIW) uniformity of these often temperature dependent processes performed on the wafer. 
     Furthermore, a difference in thermal expansion coefficients of the conductive mesh and the conductive electrode generates thermal stress at an interface, causing a breakage of the substrate pedestal. 
     Accordingly, there is a need in the art to reduce the temperature variation across a wafer by improving the process of delivering RF power to a conductive mesh within a process chamber. Additionally, there is a need to reduce thermal stress at an interface between a conductive mesh and a conductive electrode. 
     SUMMARY 
     One or more embodiments described herein provide a substrate pedestal with an RF mesh connected to a single RF rod or multiple RF rods. 
     In one embodiment, a substrate pedestal includes a thermally conductive substrate support including a mesh, a thermally conductive shaft including a plurality of conductive rods therein, each conductive rod having a first end and a second end, and a sensor. The first end of each conductive rod is electrically coupled to the mesh, and the sensor is disposed between the first and second ends of each conductive rod and configured to detect current flow through each conductive rod. 
     In another embodiment, a substrate pedestal includes a thermally conductive substrate support including a mesh, a thermally conductive shaft including a braided rod therein. The braided rod includes a plurality of conductive rods, each conductive rod having a first end and a second end, and the plurality of conductive rods are braided along a length of the braided rod. The first end of each conductive rod is electrically coupled to the mesh. 
     In yet another embodiment, a substrate pedestal includes a thermally conductive substrate support including a mesh, a thermally conductive shaft including a conductive rod therein, the conductive rod being surrounded by an insulating layer and having a first end and a second end, and a braze joint that connects the mesh and the conductive rod. The braze joint includes a plurality of mesh adapter pieces, each mesh adapter piece having a third end and a fourth end, and a terminal having a fifth end and a sixth end, the third end of each mesh adapter piece is brazed to the mesh, the fourth end of each mesh adapter piece is brazed to the fifth end of the terminal, and the first end of the conductive rod is brazed to the sixth end of the terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG.  1    is a side cross-sectional view of a processing chamber according to a first embodiment. 
         FIG.  2    is a partial side cross-sectional view of a substrate pedestal according to a second embodiment. 
         FIGS.  3 A and  3 B  are cross sectional views of a single RF rod according to the first embodiment and a braided RF rod according to a third embodiment. 
         FIGS.  4 A and  4 B  are partial cross-sectional views of a substrate pedestal according to embodiments of the present disclosure. 
         FIG.  5    is a partial cross-sectional view of a substrate pedestal according to one embodiment. 
         FIG.  6    is a partial cross-sectional view of a substrate pedestal according to one embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a more thorough understanding of the embodiments of the present disclosure. However, it will be apparent to one of skill in the art that one or more of the embodiments of the present disclosure may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring one or more of the embodiments of the present disclosure. 
     Embodiments described herein generally relate to substrate pedestals that are adapted to perform high radio frequency (RF) power processes on a wafer, or substrate, disposed in a processing region of a semiconductor processing chamber. The substrate pedestal includes an RF powered mesh, which is disposed in a substrate supporting element, which is coupled to a RF rod or multiple rods that are adapted to deliver RF energy to the RF powered mesh. 
     The use of multiple RF rods, or a single RF rod that includes multiple braided conductive rods, in the substrate pedestal, allows for spatial distribution of RF current to the RF powered mesh from an RF generator. Thus, localized Joule heating at a braze joint between the RF rod(s) and the RF powered mesh is reduced and more uniform temperature distribution over a wafer, located on the substrate pedestal, can be achieved. Furthermore, the use of multiple mesh adapter pieces interfacing an RF rod and an RF powered mesh in the substrate pedestal reduces thermal stress at the interface, reducing occurrence of breakage of the substrate pedestal. 
       FIG.  1    is a side cross-sectional view of a processing chamber  100  according to a first embodiment. By way of example, the embodiment of the processing chamber  100  in  FIG.  1    is described in terms of a plasma-enhanced chemical vapor deposition (PECVD) system, but any other type of processing chamber may be used, including other plasma deposition, plasma etching, or similar plasma processing chambers, without deviating from the basic scope provided herein. The processing chamber  100  includes walls  102 , a bottom  104 , and a chamber lid  106  that together enclose a substrate pedestal  108  and a processing region  110 . The substrate pedestal  108  may be made of a dielectric material, such as a ceramic material (e.g., AIN, BN, or Al 2 O 3 material). The walls  102  and bottom  104  of the processing chamber  100  may be made of an electrically and thermally conductive material, such as aluminum or stainless steel. As shown in  FIG.  1   , an RF generator  142  is coupled to the substrate pedestal  108 . 
     As shown in  FIG.  1   , a gas source  112  is coupled to the processing chamber  100  via a gas tube  114  that passes through the chamber lid  106 . As shown, the gas tube  114  is coupled to a backing plate  116  so that a processing gas may pass through the backing plate  116  and enter a plenum  118  formed between the backing plate  116  and a gas distribution showerhead  122 . As shown in  FIG.  1   , the gas distribution showerhead  122  is held in place adjacent to the backing plate  116  by a suspension  120 , so that the gas distribution showerhead  122 , the backing plate  116 , and the suspension  120  together form an assembly sometimes referred to as a showerhead assembly. During operation, processing gas introduced to the processing chamber  100  from the gas source  112  can fill the plenum  118  and pass through the gas distribution showerhead  122  to uniformly enter the processing region  110 . In alternative embodiments, process gas may be introduced into processing region  110  via inlets and/or nozzles (not shown) that are attached to one or more of the walls  102  in addition to or in lieu of the gas distribution showerhead  122 . 
     As shown, the substrate pedestal  108  includes a thermally conductive support  130  that has an RF powered mesh, hereafter mesh  132 , embedded inside the thermally conductive support  130 . The thermally conductive support  130  also includes a single electrically conductive rod (referred to as an “RF rod”)  128  disposed within at least a portion of a conductive shaft  126  that is coupled to the thermally conductive support  130 . A substrate  124  (or wafer) may be positioned on top of the thermally conductive support  130  during processing. In some embodiments, the RF generator  142  is coupled to the RF rod  128  via one or more transmission lines  144  (one shown). In some embodiments, the RF generator  142  provides an RF current to the mesh  132  at a frequency of between about 200 kHz and about 81 MHz, such as between about 13.56 MHz and about 40 MHz. The power generated by the RF generator  142  acts to energize (or “excite”) the gas in the processing region  110  into a plasma state to, for example, form a layer on a surface of the substrate  124  during a plasma deposition process. In one embodiment, the RF rod  128  is brazed to the mesh  132  via a braze joint  138 . The RF rod  128  may be made of nickel (Ni) and the mesh  132  may be made of molybdenum (Mo). As thermal expansion coefficients of nickel (Ni) and molybdenum (Mo) are similar (13 μm/(m·K) for Ni and 5 μm/(m·K) for Mo at 25° C.), a break of the braze joint  138  due to thermal stress can be prevented with this selection of materials for the RF rod  128  and the mesh  132 . In other embodiments, the mesh  132  is made of other refractory metal such as tungsten (W). In some embodiments, the RF rod  128  is coupled to the mesh  132  by other joining methods. In some embodiments, an RF filter  150  is provided between the RF rod  128  and the RF generator  142 . The RF filter  150  is generally either one or more low-pass filters or band-stop filters that are configured to block RF energy from reaching the RF generator  142 . 
     As shown in  FIG.  1   , embedded within the thermally conductive support  130  is the mesh  132 , an optional biasing electrode  146 , and a heating element  148 . The biasing electrode  146  can act to separately provide an RF “bias” to the substrate and processing region  110  through a separate RF connection (not shown). The heating element  148  may include one or more resistive heating elements that are configured to provide heat to the substrate  124  during processing by the delivery of AC power therethrough. The biasing electrode  146  and the heating element  148  can be made of conductive materials such as molybdenum (Mo), tungsten (W), or other similar materials. 
     The mesh  132  can also act as an electrostatic chucking electrode, which helps to provide a proper holding force to the substrate  124  against a supporting surface  136  of the thermally conductive support  130  during processing. In some embodiments, the mesh  132  is embedded at a distance DT (shown  FIG.  1   ) from the supporting surface  136 , on which the substrate  124  sits. The distance DT may be very small, such as less than 1 mm. Therefore, variations in temperature across the mesh  132  greatly influence the variations in temperature of the substrate  124  disposed on the supporting surface  136 . The heat transferred from the mesh  132  to the supporting surface  136  is represented by the H arrows in  FIG.  1   . 
     Therefore, spreading out, dividing, or distributing the amount of RF current provided to the mesh  132 , and thus minimizing the added temperature increase created at the mesh  132  junctions results in more uniform temperature distribution across the mesh  132 . A uniform temperature distribution across the mesh  132  creates a uniform temperature distribution across the supporting surface  136  and the substrate  124 . 
     One skilled in the art will appreciate that RF energy is primarily conducted through a surface region of a conductive element, and thus generally the current carrying area of an RF conductor is primarily governed by the surface area of the RF conducting element. The current carrying area of an RF conductor is reduced as the frequency of the delivered RF power increases, due to a decrease in the skin depth the delivered RF power is able to penetrate into the RF conductor as the RF power is delivered through the RF conductor. For example, in an RF rod that has a circular cross-sectional shape and an outer diameter D o , the RF current carrying area between its skin depth and surface (Aca) is equal to the cross-section area (A o =π·D o   2 /4) minus the current carrying area beyond its skin depth (A na =π·D na   2 /4), where D na  is the diameter of the area below its skin depth (i.e., D na =D o −2·δ, where δ is the skin depth). That is, the RF current carrying area is A o −A na =π·(D o   2 /4−D na   2 /4)=π·(D o −δ)δ. Skin depth can be approximated by the equation δ=(ρ/(πfμ r μ o )) 0.5 , where ρ is the resistivity of the medium in Ω·m, f is the driven frequency in Hertz (Hz), μ r  is the relative permittivity of the material, and μ o  is the permittivity of free space. Skin depth refers to the point in which the current density reaches approximately 1/e (about 37%) of its value at the surface of the medium. Therefore, the majority of the current in a medium flows between the surface of the medium and its skin depth. Thus, a single RF rod  128  having a larger diameter D o  and a larger skin depth δ distributes the amount of RF current provided to the mesh  132  in a larger current carrying area, and thus reduces localized heating at the braze joint  138 . In one example, the skin depth for a pure nickel (Ni) material is approximately 1.5 μm and for a pure gold (Au) approximately 20 μm at a frequency of 13.56 MHz, thus a single RF rod  128  made of gold (Au) has a larger RF current carrying area (due to a larger skin depth δ). However, a thermal expansion coefficient of gold (Au) (14.2 μm/(m·K) for Mo and 4.5 μm/(m·K) for W at 25° C.) has a large discrepancy from the thermal expansion coefficients of the material of the braze joint  138  (5 μm/(m·K) for Mo and 4.5 μm/(m·K) for W at 25° C.), and thus the braze joint  138  may not withstand thermal stress, leading to susceptibility for breakages. Therefore, a single RF rod  128  made of nickel (Ni) that has a smaller thermal expansion coefficient (13 μm/(m·K) at 25° C.) may be robust against a breakage caused by thermal stress. 
       FIG.  2    is a partial side cross-sectional view of the substrate pedestal  108 , according to a second embodiment. In the second embodiment, the single RF rod  128  according to the first embodiment is replaced with dual RF rods  228 . The same reference numerals are used for the components that are substantially the same as those of the first embodiment, and the description of repeated components may be omitted. In  FIG.  2   , the dual RF rods  228  are brazed to the mesh  132  at braze joints  238 . The dual RF rod  228  may be made of nickel(Ni). In some embodiments, the dual RF rods  228  may be coupled to the mesh  132  by other joining methods. The dual RF rods  228  divide the RF current provided by the RF generator  142  to the mesh  132  into two RF rods and thus reduces the Joule heating (e.g., I 2 R heating) at each of the dual RF rods  228 , resulting in a more uniform temperature distribution across the thermally conductive support  130 , which translates into, for example, a more uniformly deposited film layer formed across the substrate  124 . 
     In some embodiments, a health check circuit  256  may be inserted on an RF current path of each of the dual RF rods  228  between the mesh  132  and the one or more transmission lines  144 . The health check circuit  256  may be a sensor, such as a voltage/current (V/I) sensor: for use in detecting the current flow through each of the dual RF rods  228  order to detect any damage/degradation to either of the dual RF rods  228 . Such early detection of damage/degradation can be used to identify issues so that the substrate pedestal  108  can be repaired by re-brazing the RF rods  228  before any catastrophic failures occur. 
     The dual RF rods  228  may replace the single RF rod  128  in the processing chamber  100  according to the first embodiment with a few or no modification to the RF filter  150 . In some embodiments, the dual RF rods  228  may be combined using an RF strap (not shown) that is connected to the RF filter  150 . This configuration requires no modification to the RF filter  150  designed for a single RF rod  128 . In some embodiments, one or more RF straps (not shown) may be disposed between the braze joints  238  and the RF filter  150  to compensate for expansions of the RF rods. 
     In the example embodiment described above, dual RF rods  228  are described and shown  FIG.  2   . However, any number of multiple RF rods may be used, including three or more. Current through each RF rod can thus be half (or less) of the current through the single RF rod  128 . Accordingly, current flows into the braze joints  138  at a lower magnitude and at multiple distributed points across the mesh  132 , helping to spread the amount of heat generated across the substrate  124 , creating much less of a heat increase at any one point. Spreading of the generated heat across the substrate  124  acts to improve the uniformity in the film layer deposited on the substrate  124 . As shown, each of the braze joints  138  can be spread relatively far apart from each other, widely distributing the current and the generated heat across the supporting surface  136 , resulting in a more uniform heat spread across the substrate  124 . 
       FIGS.  3 A and  3 B  are cross sectional views of the single RF rod  128  according to the first embodiment and a braided RF rod  328  according to a third embodiment. In  FIG.  3 A , the single RF rod  128  includes one conductive rod  302  surrounded by an insulation layer  304 . In  FIG.  3 B , multiple conductive rods  306  ( 7  conductive rods are shown) are braided along a length of the braided RF rod  328  and surrounded by an insulation layer  308 . In the braided RF rod  328 , the sum of the current carrying areas between the surfaces and skin depths of all of the conductive rods  306  combined is larger than the current carrying area between the surface and skin depth of the single RF rod  128 . This provides the advantage of creating a larger area to conduct the majority of RF energy between the braided RF rod  328  and the mesh  132 , which reduces the heat generated at the braze joints  138  and also within the braided RF rod  328  versus a conventional single RF rod configuration shown in  FIG.  3 A , due to Joule heating. For example, a single RF rod  128 , having an outer diameter D R  of 6 mm and skin depth δ of approximately 1.46 μm, has a current carrying area A ca1 =π·(D R −δ) δ of approximately 2.8×10 −2  mm 2 . Comparatively, each of the conductive rods  306  having an outer diameter D C  of 2 mm has a current carrying area A ca2 =π·(D C −δ) δ of approximately 0.9×10 −3  mm 2 . Thus, for a braided RF rod  328  having seven conductive rods  306 , the ratio of the total current carrying area to the current carrying area of the single RF rod  128  (i.e., 7×A ca2 /A ca1 =7×(D C −δ)/(D R −δ)) is about 2.3. Therefore, in the embodiment shown in  FIG.  3 B , the current is distributed over a larger current carrying area, generating less Joule heating at each of the braze joints  138  in the braided RF rod  328  than at the single RF rod  128  shown in  FIG.  3 A . 
     The braided RF rod  328  disclosed herein also provides an advantage over a conventional single RF rod since each of the conductive rods  306 , having a smaller diameter, has a smaller cross-sectional area and thus a smaller contact area at each of the braze joints  138 . The smaller cross-sectional area of the conductive rod  306  reduces the ability of each of the conductive rods  306  to thermally conduct any heat generated in the conductive rods  306  due to the delivery of the RF power therethrough. The reduced ability to conduct heat also spreads the heat more uniformly within the conductive support  130 , helping to create a more uniform temperature distribution across the supporting surface  136  and substrate  124 . Following the prior example above, where the outer diameter D R  of the single RF rod  128  is equal to 6 mm and the outer diameter D C  of each of the conductive rods  306  is equal to 2 mm, the ratio of the thermal conduction areas of the braided RF rod  328  having seven conductive rods  306  to the single RF rod  128  area will be about 0.78. 
       FIG.  4 A  is a partial cross-sectional view of the substrate pedestal  108  according to one embodiment. In the embodiment shown, the braze joint  138  includes a terminal  402  and braze portions  404  and  408 . The RF rod  128 , having a large diameter between about 5 mm and about 12 mm, is brazed to the terminal  402  at the braze portion  404 . The terminal  402  may be made of a ferromagnetic metal, such as iron (Fe), cobalt (Co), nickel (Ni), or other similar materials. In one embodiment, the terminal  402  is a Kovar® Ni—Fe alloy. The terminal  402  is further brazed to an RF terminal  406  (also referred to as an “adapter”) at the braze portion  408 . The RF terminal  406  is brazed to the mesh  132  (shown in  FIG.  1   ) disposed within the substrate pedestal  108 . In one embodiment, the substrate pedestal  108  is made of a ceramic material at temperatures exceeding 1000° C.; thus, the RF terminal  406  may be made of the same refractory metal (i.e., resistant to heat and wear) as the mesh  132 , such as molybdenum (Mo), tungsten (W), or other similar materials. The braze portions  404  and  408  may include one or more transition metals, such as nickel (Ni), or other similar materials. As thermal expansion coefficients of the material that terminal  402  (e.g., 5 μm(m·K) for Mo and 4.5 μm/(m·K) for W at 25° C.) and the material that the RF terminal  406  and the mesh  132  (e.g., 12 μm/(m·K) for Fe, 16 μm/(m·K) for Co,13 μm/(m·K) for Ni at 25° C.) have a large discrepancy, the braze portion  408  is susceptible to stress in the directions indicated by the arrows in  FIG.  4 A , especially at elevated temperatures such as above 500° C., leading to susceptibility for breakages. The magnitude of such stress depends on the size of an interface between the terminal  402  and the RF terminal  406 . A larger area of the interface (e.g., due to a large diameter of the RF rod  128 , such as between about 5 mm and about 12 mm) introduces higher stress at the interface. Specifically, a length L of an area at the interface increases L=L+αLΔT as the temperature rises by ΔT in the area at the interface, where a is the thermal expansion coefficient. 
       FIG.  4 B  is a partial cross-sectional view of the substrate pedestal  108  according to one embodiment, in which the RF terminal (adapter)  406  shown in  FIG.  4 A  is replaced by an RF terminal (adapter)  410  that includes multiple pieces (also referred to as “mesh adapter pieces”)  412 . While a total cross-sectional area of the multiple pieces  412  is kept equal to the cross-sectional area of the RF terminal  406 , each of the multiple pieces  412  has a smaller diameter than the RF terminal  406 . Thus, the RF terminal  410  having smaller diameter multiple pieces  412  reduces the local stress induced due to the difference in the thermal expansion coefficients. 
       FIG.  5    is a partial cross-sectional view of the substrate pedestal  108  according to one embodiment. In the embodiment shown, the braze joint  238  includes terminals  502  and braze portions  504 . The multiple RF rods  228 , which has multiple individual rods circularly arranged, are brazed to the terminals  502  at the braze portions  504 . The terminals  502  may be made of a ferromagnetic metal, such as iron (Fe), cobalt (Co), nickel (Ni), or other similar materials. In one embodiment, the terminal  502  is a Kovar® Ni—Fe alloy. The terminals  502  are further connected to a tube  506  that replaces the adapters (the RF terminal  406  in  FIG.  4 A  and the RF terminal in  FIG.  4 B ). The tube (adapter)  506  has a hollow ring shape with a large diameter in a plane perpendicular to the length of the multiple RF rods  228 . The tube  506  is connected to all individual RF rods in the multiple RF rod  228 , and may be made of a ferromagnetic metal, such as nickel (Ni), or other similar materials. Since the tube  506  is hollow, the tube  506  can bend and absorb stress caused by thermal expansion of the multiple RF rods  228 . 
     Due to the hollowness and a large diameter, a surface area of the tube  506  can be significantly increased as compared with a solid tube having a smaller diameter. For example, a solid tube with a diameter D ST  of 5 mm (which is approximately a diameter of a typical RF rod currently used), a perimeter of the solid tube is πD ST  ˜15.7 mm. For a hollow tube with a diameter D HT  of 45 mm and thickness t of 2 mm, a total perimeter (a sum of an outer perimeter and an inner perimeter of the hollow tube) is π·D HT  +π·(D HT -t)˜276 mm. Thus, a surface area of the hollow tube is about 17 times larger than the solid tube. This increased surface area reduces localized heating in the tube  506 . 
       FIG.  6    is a partial cross-sectional view of the substrate pedestal  108  according to one embodiment, in which the one-level mesh  132  shown in  FIG.  2    is replaced by a two-level mesh  632 . The structure of the two-level mesh  632  reduces heat generation at positions of the braze joints  238  on the substrate  124  (referred to as RF hotspots). Specifically, the structure of the two level-mesh  632  helps moving the hotspots down within the substrate pedestal  108 , and thus reducing the hotspots on the substrate  124 . 
     In the example embodiments described herein, substrate pedestals that are adapted to perform high radio frequency (RF) power processes on a wafer, or substrate, disposed in a processing region of a semiconductor processing chamber include multiple RF rods or multiple braided conductive rods, such that RF current to a RF powered mesh via the multiple RF rods or multiple braided conductive rods provided by an RF generator is spatially distributed. Thus, localized Joule heating at a braze joint between the multiple RF rods or multiple braided conductive rods and the RF powered mesh is reduced and more uniform temperature distribution over a wafer, located on the substrate pedestal, can be achieved. Furthermore, the use of multiple mesh adapter pieces interfacing an RF rod and an RF powered mesh in the substrate pedestal reduces thermal stress at the interface, reducing occurrence of breakage of the substrate pedestal. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.