Abstract:
A method of performing a radical-enhanced atomic-layer deposition process on a surface of a substrate that resides within an interior of a reactor chamber is disclosed. The method includes forming plasma from a gas mixture consisting of CF 4  and O 2 , wherein the CF 4  is present in a concentration in the range from 0.1 vol % to 10 vol %. The plasma formed from the gas mixture generates oxygen radicals O* faster than if there were no CF 4  present in the gas mixture. The method also includes feeding the oxygen radicals and a precursor gas sequentially into the interior of the reactor chamber to form an oxide film on the surface of the substrate. A system for performing the radical-enhanced atomic-layer deposition process using the rapidly formed oxygen radicals is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM 
       [0001]    This Application claims priority under 35 USC §119(e) from U.S. Provisional Patent Application Ser. No. 62/042,686, filed on Aug. 27, 2014, and which is incorporated by reference herein. 
     
    
     FIELD 
       [0002]    The present disclosure relates to atomic layer deposition (ALD), and in particular relates to radical-enhanced ALD (RE-ALD), and more particularly to RE-ALD wherein a relatively small amount of CF 4  is used in combination with oxygen to substantially enhance the generation of oxygen radicals to speed up the RE-ALD process. 
         [0003]    The entire disclosure of any publication or patent document mentioned herein is incorporated by reference, including the article by Profijt et al., entitled “Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges,” J. Vac. Sci. Technol. A 29(5), Sep/Oct 2011, pp 050801-1 to 26, and the article by George, entitled “Atomic Layer Deposition: an Overview,” Chem. Rev. 2010, 110, pp 111-113 (published on the Web on 11/20/2009). 
       BACKGROUND 
       [0004]    Atomic layer deposition (ALD) is a method of depositing a thin film on a substrate in a very controlled manner. The deposition process is controlled by using two or more chemicals (“precursors”) in vapor form and reacting them sequentially and in a self-limiting manner on the surface of the substrate. The sequential process is repeated to build up the thin film layer by layer, wherein the layers are atomic scale. 
         [0005]    ALD is used to form a wide variety of films, such as binary, ternary and quaternary oxides for advanced gate and capacitor dielectrics, as well as metal-based compounds for interconnect barriers and capacitor electrodes. 
         [0006]    One type of ALD is called radical-enhanced ALD (RE-ALD). The RE-ALD utilizes radicals generated by a plasma to form one of the precursor gases. Because a radicalized precursor tends to be more reactive than its unradicalized counterpart, it helps induce the reactions when forming the film layers. 
         [0007]    The type of plasma used in the RE-ALD is referred to by the name of the feedgas used to form the plasma. For example, an “oxygen plasma” utilizes oxygen (O 2 ) as the feedgas to produce a plasma that generates oxygen-radical precursors. The oxygen-radical precursors comprise monatomic oxygen, which has two unpaired electrons that make monatomic oxygen very reactive. The oxygen radicals serve as co-reactants that are used in conjunction with a second precursor (say, an Si-based precursor such tris(dimethylamino)silane). The two precursors are feed sequentially into the reactor chamber to produce sequential layers that lead to thin film growth (e.g., SiO 2 ). 
         [0008]    While ALD has many advantages, one of its major disadvantages as compared to other film growth processes (such as chemical-vapor deposition or CVD) is that it is quite slow. For example, a conventional ALD reactor has a growth rates measured in angstroms/minute, while CVD has growth rates measured in microns/minute. Slow growth rates result in excellent film quality but limit the throughput of processed substrates (wafers) in a semiconductor manufacturing line. Thus, while an oxygen plasma is effective in providing oxygen-radical precursors for oxygen-based RE-ALD, there is always a need to increase the speed and/or efficiency of an oxygen-based RE-ALD process. 
       SUMMARY 
       [0009]    An aspect of the disclosure is a method of performing a RE-ALD process on a surface of a substrate that resides within an interior of a reactor chamber. The method includes: providing a gas mixture of CF 4  gas and O 2  gas, wherein the CF 4  gas is present in a concentration in the range from 0.1 vol % to 10 vol %; forming a plasma from the gas mixture to generate oxygen radicals at a rate faster than if there were no CF 4  gas present in the gas mixture; and sequentially feeding the oxygen radicals and a precursor gas into the interior of the reactor chamber to form an oxide film on the surface of the substrate. 
         [0010]    Another aspect of the disclosure is the method as described above, wherein the plasma is preferably formed within a plasma tube that is pneumatically coupled to the interior of the reactor chamber. 
         [0011]    Another aspect of the disclosure is the method as described above, wherein the precursor gas preferably comprises a metal organic precursor. 
         [0012]    Another aspect of the disclosure is the method as described above, wherein the metal organic precursor is preferably selected from the group consisting of: silicon, aluminum, hafnium, titanium, zirconium, tantalum, yttrium and magnesium. 
         [0013]    Another aspect of the disclosure is the method as described above, wherein the oxide film preferably comprises a metal oxide. 
         [0014]    Another aspect of the disclosure is the method as described above, wherein the metal oxide is preferably selected from the group consisting of: Al 2 O 3 , B 2 O 3 , CeO 2 , Co 3 O 4 , Cr 2 O 3 , CuO x , Er 2 O 3 , FeO x , Ga 2 O 3 , Gd 2 O 3 , HfO 2 , IrO 2 , La 2 O 3 , MgO, Nb 2 O 5 , NiO x , PtO 2 , RuO 2 , SiO 2 , SnO 2 , SrO x , Ta 2 O 5 , TiO 2 , Tm 2 O 3 , V 2 O 5 , Y 2 O 3 , ZnO and ZrO 2 . 
         [0015]    Another aspect of the disclosure is the method as described above, wherein the method preferably further includes introducing a purge gas into the interior of the reactor chamber to assist in purging the interior of the reactor chamber of either the oxygen radicals or the precursor gas. 
         [0016]    Another aspect of the disclosure is the method as described above, wherein the substrate preferably comprises a silicon wafer. 
         [0017]    Another aspect of the disclosure is a method of performing a RE-ALD process on a surface of a substrate that resides within an interior of a reactor chamber. The method includes: providing a first precursor gas comprising oxygen radicals O* by forming an oxygen plasma from a gas mixture within a plasma tube, wherein the plasma tube is pneumatically coupled to the interior of the reactor chamber, and wherein the gas mixture consists of CF 4  gas and O 2  gas, wherein the CF 4  gas has a concentration of 0.1 vol % to 10 vol %; providing a second precursor gas from a gas source that is pneumatically coupled to the interior of the reactor chamber; and sequentially introducing the first precursor gas and the second precursor gas into the interior of the reactor chamber to form an oxide film on the surface of the substrate. 
         [0018]    Another aspect of the disclosure is the method as described above, wherein one of the first and second precursor gases preferably comprises at least one of: silicon, aluminum, hafnium, titanium, zirconium, tantalum, yttrium and magnesium. 
         [0019]    Another aspect of the disclosure is the method as described above, wherein the oxide film preferably comprises one of: SiO 2 , Al 2 O 3 , HfO 2 , TiO 2 , ZrO 2 , Ta 2 O 5 , Y 2 O 3  and MgO. 
         [0020]    Another aspect of the disclosure is the method as described above, wherein the method further includes introducing a purge gas into the interior of the reactor chamber to assist in purging the interior of the reactor chamber of either the first precursor gas or the second precursor gas. 
         [0021]    Another aspect of the disclosure is the method as described above, wherein the substrate preferably comprises a silicon wafer. 
         [0022]    Another aspect of the disclosure is the method as described above, wherein the plasma tube is preferably made of quartz. 
         [0023]    Another aspect of the disclosure is the method as described above, wherein forming the oxygen plasma preferably includes subjecting the gas mixture in the plasma tube to inductive coupling. 
         [0024]    Another aspect of the disclosure is a system for performing a RE-ALD process on a surface of a substrate. The system includes: a reactor chamber having an interior configured to support the substrate; a gas source pneumatically coupled to the interior of the reactor chamber and that contains a gas mixture consisting of CF 4  and O 2 , wherein the CF 4  has a concentration in the gas mixture of 0.1 vol % to 10 vol %; a plasma system pneumatically connected to the interior of the reactor chamber and configured to form from the gas mixture a plasma that generates oxygen radicals O* faster than if the gas mixture did not contain CF 4 ; a vacuum pump pneumatically connected to the interior of the reactor chamber; and a controller operably connected to the plasma system, the vacuum pump and the gas source, the controller being configured to introduce the oxygen radicals and a precursor gas sequentially into the interior of the reactor chamber to form an oxide film on the surface of the substrate. 
         [0025]    Another aspect of the disclosure is the system as described above, wherein the precursor gas preferably includes one of: silicon, aluminum, hafnium, titanium, zirconium, tantalum, yttrium and magnesium. 
         [0026]    Another aspect of the disclosure is the system as described above, wherein the plasma system preferably includes an inductively coupled plasma source. 
         [0027]    Another aspect of the disclosure is the system as described above, wherein the system preferably further includes the substrate. The substrate preferably comprises silicon. 
         [0028]    Another aspect of the disclosure is the system as described above, wherein the gas source preferably includes operably coupled first and second gas sources. The first gas source preferably contains the O 2  gas and the second gas source preferably contains the CF 4  gas. 
         [0029]    Another aspect of the disclosure is a RE-ALD method that includes exposing a substrate to a first precursor chemical that reacts with the surface of the substrate to form a chemisorbed monolayer followed by a first purging step, which removes excess precursor and reaction products. Next, oxygen radicals O* produced from a plasma source are delivered to the surface of substrate, reacting with the chemisorbed monolayer, creating an atomic scale layer of the desired material. The method also includes preparing the surface such that it will again be reactive with the first precursor chemical. This can include performing a second purge step. The above process is repeated to build up atomic-scale layer of the desired material to a desired final thickness. The method also includes forming the oxygen radicals O* using a feedgas gas mixture of CF 4  and O 2  to form an oxygen plasma as described below, with the CF 4  having a concentration in the range from 0.1 vol % to 10 vol %. In an example, the desired material is an oxide such as a metal oxide. 
         [0030]    Additional features and advantages are set forth in the Detailed Description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
           [0032]      FIG. 1  is a schematic diagram of an example RE-ALD system used to carry out the RE-ALD methods disclosed herein that employ a select mixture of O 2  and CF 4  gases to generate a precursor gas having monatomic oxygen radicals O*; and 
           [0033]      FIG. 2  is a close-up cross-sectional view of the plasma system of the RE-ALD system of  FIG. 1  illustrating how the oxygen molecule O 2  in the O 2  and CF 4  gas mixture is dissociated into two monatomic oxygen radicals (O*+O*) within the plasma system, along with residual species denoted R. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
         [0035]    The claims as set forth below are incorporated into and constitute part of this Detailed Description. 
         [0036]    In the discussion herein, an oxygen radical is denoted as O* and represents monatomic oxygen, which has a total of six electrons, two of which are unpaired. The unpaired electrons make O* very reactive, i.e., more reactive than diatomic oxygen (O 2 ). 
         [0037]    In the discussion below, the oxygen radicals O* constitute a first precursor gas that serves as an oxidizing precursor, while a second precursor gas is a non-oxide gas that forms, in combination with the first precursor gas, a thin-film oxide compound on a surface of substrate. 
         [0038]      FIG. 1  is a schematic diagram of an example RE-ALD system  10 . Various configurations for the RE-ALD system  10  are possible, and the RE-ALD system  10  of  FIG. 1  shows one basic configuration that can be employed. The RE-ALD system  10  includes a reactor chamber  20  having a top wall  22 , a bottom wall  23  and a cylindrical sidewall  24  that define a reactor chamber interior  26 . A stage  30  resides within the reactor chamber interior  26 . The stage  30  supports a substrate  40  that has an upper surface  42  on which an oxide film  142  is formed via a RE-ALD process as discussed below. An example substrate  40  is a silicon wafer used in semiconductor manufacturing. Other example substrates  40  include GaAs, GaN, glass, and polymers, such as polyimide. 
         [0039]    The RE-ALD system  10  also includes a vacuum pump  46  that is pneumatically connected to the reactor chamber interior  26  and serves to control the pressure within the reactor chamber interior  26  (e.g., in the range of about 10 mTorr to about 500 mTorr). 
         [0040]    The RE-ALD system  10  also includes a precursor gas source  50  that is pneumatically connected to the reactor chamber interior  26  and that provides a precursor gas  52  to the reactor chamber interior  26  as part of the RE-ALD process. In an example, the precursor gas  52  is any gas that combines with oxygen to form an oxide film  142  on the upper surface  42  of substrate  40 . Example first precursor gases include: silicon-based precursors (e.g., to form SiO 2 ), aluminum-based precursors (e.g., to form Al 2 O 3 ), hafnium-based precursors (e.g., to form HfO 2 ), titanium-based precursors (e.g., to form TiO 2 ), zirconium-based precursors (e.g., to form ZrO 2 ), tantalum-based precursors (e.g., to form Ta 2 O 5 ), Yttrium-based precursors (e.g., to form Y 2 O 3 ) and magnesium-based precursors (e.g., to form Mg 2 O 4  or MgO). Examples precursor gasses  52  therefore include:
   Si—tris(dimethylamino)silane   Al—trimethyl aluminum   Hf—tetrakis(dimethylamido)hafnium   Ti—titanium tetrachloride   Zr—tetrakis(dimethylamido)zirconium   Ta—pentakis(dimethylamido)tantalum   Yt—Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)yttrium   Mg—Ethylcyclopentadienyl(magnesium)   
 
         [0049]    In an example, the oxide film  142  is a metal oxide, and further in an example is a metal oxide selected from the following group of metal oxides and formed using a suitable precursor gas  52 : Al 2 O 3 , B 2 O 3 , CeO 2 , Co 3 O 4 , Cr 2 O 3 , CuO x , Er 2 O 3 , FeO x , Ga 2 O 3 , Gd 2 O 3 , HfO 2 , IrO 2 , La 2 O 3 , MgO, Nb 2 O 5 , NiO x , PtO 2 , RuO 2 , SiO 2 , SnO 2 , SrO x , Ta 2 O 5 , TiO 2 , Tm 2 O 3 , V 2 O 5 , Y 2 O 3 , ZnO and ZrO 2 . 
         [0050]    The RE-ALD system  10  further includes an optional second gas source  60  that is pneumatically connected to the reactor chamber interior  26  and that provides an inert gas  62  to the reactor chamber interior  26 . The inert gas  62  serves as a purge gas between introducing the different precursor gasses to speed up the sequential layering processes when forming the oxide film  142 . Note that the second gas source  60  can be combined with the precursor gas source  50  so that the precursor gas  52  and the inert gas  62  can flow into the reactor chamber interior  26  through the same conduit. 
         [0051]    The RE-ALD system  10  also includes an oxygen plasma system  100  that is pneumatically coupled to the reactor chamber interior  26 .  FIG. 2  is a close-up cross-sectional view of a portion of an example oxygen plasma system  100 . The oxygen plasma system  100  is configured to receive a gas mixture  112  of O 2  and CF 4  gasses as a feedgas to form a plasma  114 . The production of plasma  114  is a complex process that produces many species. For example, in the present case, the O 2  and CF 4  of gas mixture  112  dissociate into O 2   + , O − , O, O2 − , O 3 , O*, CF 3 , CF 3   − , CF 3   + , CF 2 , F and F − . For simplicity, the plasma  114  is referred to herein as “oxygen plasma”  114  for ease of discussion and because oxygen is the main feedgas used. The species other than the particular oxygen radicals O* species of interest are considered herein as residual species and are collectively denoted as R in  FIG. 2 . 
         [0052]    As noted above, the oxygen plasma  114  generates the oxygen radicals O* that enter the reactor chamber interior  26  and that constitute an oxidizing precursor gas  116 . In an example, the gas mixture  112  has 0.1 vol % to 10 vol % CF 4 , with the rest of the gas mixture  112  being O 2 . In another example, the gas mixture  112  has 1% vol to 10% vol CF 4 , with the rest of the gas mixture  112  being O 2 . In other examples, small amounts of other gasses may be present in the gas mixture  112  without substantially reducing the improved oxygen radicals O* generation provided by the presence of the CF 4  gas. 
         [0053]    An example oxygen plasma system  100  includes a gas source  110  configured to deliver the gas mixture  112 . In an example, the gas source  110  includes an O 2  gas source  110 A and a CF 4  gas source  110 B that are operably connected so that the two gases can be mixed to form the gas mixture  112 . In another example, the gas source  110  includes a single gas source that includes the gas mixture  112 . 
         [0054]    The gas source  110  is pneumatically coupled to an input end  122  of a plasma tube  120 . The plasma tube  120  also has an output end  123 , an outer surface  124  and an interior  126 . An example plasma tube  120  is made of quartz and is substantially cylindrical. Other known materials can be used for plasma tube  120 . A radio-frequency (RF) coil  130  resides around the outer surface  124  of plasma tube  120  and is operably connected to an RF source  134  that includes an RF-matching network. 
         [0055]    The RE-ALD system  10  includes a number of valves  150  to control the flow of the precursor gas  52 , the inert gas  62  as a purge gas, the gas mixture  112  (including the formation of gas mixture  112  from the gas sources  110 A and  110 B), as well as the pneumatic connection of vacuum pump  46 , to the reactor chamber interior  26 . The valve  150  at the output end of gas source  110  adjacent the reactor chamber  20  is optional and may not be required given the low pressure in the reactor chamber interior  26 . 
         [0056]    The RE-ALD system  10  also includes a controller  200  operably connected to the oxygen plasma system  100  and the valves  150 . The controller  200  is configured to control the operation of the RE-ALD system  10  to form the oxide film  142  on the upper surface  42  of substrate  40 . In particular, the controller  200  is configured to control the opening and closing of the valves  150  as needed to control the operation of RE-ALD system  10 . In an example, the controller  200  performs the sequential introduction of precursor gases  116  and  52  into the reactor chamber interior  26 , including the removal of one precursor gas  116  or  52  before introducing the other precursor gas  116  or  52 . In an example, the controller  200  controls the introduction of inert gas  62  as a purge gas to assist in removing one of the precursor gases  116  or  52  from the reactor chamber interior  26  prior to introducing the other precursor gas  116  or  52  into the reactor chamber interior  26 . In another example, the controller  200  controls the mixture of O 2  and CF 4  and gases from respective gas sources  110 A and  1106  to form the desired gas mixture  112 . 
         [0057]    With reference to  FIG. 2 , the gas mixture  112  enters the input end  122  of plasma tube  120  and travels into the interior  126  of plasma tube  120 . In the meantime, the RF source  134  provides the RF coil  130  with an RF-frequency signal that inductively forms the oxygen plasma  114  within the interior  126  of plasma tube  120 . More specifically, as the gas mixture  112  flows towards the output end  123 , the RF energy from the RF coil  130  drives azimuthal electrical currents in the (rarified) gas mixture  112 , which initiates the formation of oxygen plasma  114  by the dissociation of diatomic oxygen O 2  into two oxygen radicals O*, along with the formation of residual species R. Because the output end  123  of plasma tube  120  is pneumatically connected to the reactor chamber interior  26 , and because the reactor chamber interior  26  has a relatively low pressure, the oxygen plasma  114  is drawn into the reactor chamber interior  26  as the precursor gas  116 . 
         [0058]    In another example embodiment, the RE-ALD system  10  is configured for capacitive coupling to form the oxygen plasma  114 , and the inductive coupling example is shown for the sake of illustration. The methods disclosed herein are not limited by the means by which the oxygen plasma  114  is formed. 
         [0059]    Using a concentration of 0.1 vol % to 10 vol % of CF 4  gas with O 2  gas (e.g., the CF 4  gas makes up 1 to 10 vol % of the gas mixture  112  while the O 2  makes up the rest of the gas mixture  112 ) serves to increase the production of oxygen radicals O* in the oxygen plasma  114  as compared to when there is no CF 4  gas present. The precise mechanism for the CF 4  addition enhancing the O* generation is unknown. However, without being bound by theory, evidence suggests that the CF 4  acts to increases in the plasma density or electron temperature, which results in higher O* densities in the oxygen plasma  114  as compared to not using CF 4  in the gas mixture  112 . 
         [0060]    The use of greater amounts (concentrations) of CF 4  gas in the gas mixture  112  beyond 10 vol % can be problematic because the CF 4  (and in particular its constituent F) can start to participate the RE-ALD process when in fact the only role of the CF 4  in the gas mixture  112  is to enhance (i.e., increase) the production of oxygen radicals O* to improve the rate at which the RE-ALD process can be carried out in the RE-ALD system  10 . 
         [0061]    Thus, with continuing reference to  FIGS. 1 and 2 , in an example method, the RE-ALD is performed using the RE-ALD system  10  wherein the oxygen plasma  114  is formed from the gas mixture  112  of CF 4  and O 2  as disclosed above. The oxygen plasma  114  in turn generates oxygen radicals O* that constitute first precursor gas  116 , wherein in an example the oxygen radicals O* are formed at a rate faster than the rate as compared to using only oxygen in the gas mixture  112 . In example, the rate is between 1.1× and 2× the rate as compared to using only oxygen in the gas mixture  112 . 
         [0062]    The method then includes sequentially introducing the first precursor gas  116  and the second precursor gas  52  into the reactor chamber interior  26  to form the oxide film  142  on the upper surface  42  of a wafer. The sequential introduction of the precursor gases  116  and  52  is repeated to the extent needed to obtain a desired thickness for the oxide film  142 . As noted above, the inert gas  62  as a purge gas can be employed to assist in purging the reactor chamber interior  26  of one precursor gas  116  or  52  before introducing the other precursor gas  116  or  52 . It is noted that in  FIG. 1 , both precursor gases  116  and  52  as well as the inert gas  62  as purge gas are shown as being introduced at the same time into the reactor chamber interior  26  for ease of illustration. 
         [0063]    It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.