Abstract:
A method of radical-enhanced atomic layer deposition (REALD) involves alternating exposure of a substrate to a first precursor gas and to radicals, such as monatomic oxygen radicals (O.), generated from an oxygen-containing second precursor gas, while maintaining spatial or temporal separation of the radicals and the first precursor gas. Simplified reactor designs and process control are possible when the first and second precursor gases are nonreactive under normal processing conditions and can therefore be allowed to mix after the radicals recombine or otherwise abate. In some embodiments, the second precursor gas is an oxygen-containing compound, such as carbon dioxide (CO 2 ) or nitrous oxide (N 2 O) for example, or a mixture of such oxygen-containing compounds, and does not contain significant amounts of normal oxygen (O 2 ).

Description:
RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 61/290,826, filed Dec. 29, 2009, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The field of the present disclosure relates to thin film deposition, including atomic layer deposition (ALD), and more particularly to radical-enhanced thin film deposition, such as radical-enhanced ALD. 
     BACKGROUND 
     U.S. patent application Ser. No. 11/829,050, filed Jul. 26, 2007 and published as Pub. No. US 2008/0026162 of Dickey et al. (“the &#39;050 application”) describes various methods and systems for radical-enhanced atomic layer deposition (REALD). The specification of the &#39;050 application, which is incorporated herein by reference in its entirety, describes deposition methods involving alternating exposure of a substrate to a first precursor gas and a radical species, wherein the radical species is generated in-situ by an excitation source such as a steady-state direct-current (DC) or radio-frequency (RF) plasma generator. The first precursor gas is introduced at a location spaced apart from and generally downstream from where the radical species is generated, to provide a radical deactivation zone therebetween. In some embodiments disclosed in the &#39;050 application, the plasma generator generates a direct plasma proximal of the substrate surface from a purge gas flowing through the system, wherein the purge gas is substantially nonreactive (inert) with the first precursor gas. In other embodiments, the radical species is generated from a second precursor gas that may be reactive with the first precursor gas. 
     While oxygen radicals are a highly reactive species for oxidation of certain metal precursors, such as trimethylaluminum (TMA) and titanium tetrachloride (TiCl 4 ) for example, the present inventors have discovered that thin films deposited in an REALD process involving oxygen plasma generated from regular oxygen gas (O 2 ) are inferior to thin films deposited by many other ALD methods. The inventors&#39; experiments with ozone (O 3 ) precursors have resulted in even poorer films, which suggests that direct oxygen plasmas formed from O 2  are an inferior precursor for REALD because they contain a relatively high concentration of ozone—a gas that is much more persistent than oxygen radicals (free radicals) and therefore more likely to migrate into the second precursor zone and react with the metal precursor or other precursor introduced there, causing non-ALD deposition to occur. 
     The inventors have recognized these phenomena as an opportunity for improved REALD methods and improved methods of generating oxygen radicals for thin film deposition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic elevation view of a system for web coating according to the prior art; 
         FIG. 2  is a schematic elevation view of a system for REALD web coating according to an embodiment; and 
         FIG. 3  is a schematic top section view of a drum coating system for REALD according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a cross section elevation of a prior system  10  for REALD of the kind described in the &#39;050 application, for deposition of a thin-film coating onto a flexible substrate  12  (shown in profile in  FIG. 1 ), such as a web of plastic film or metal foil, for example.  FIG. 1  illustrates a radical enhanced version of the ALD system for coating flexible substrates described in U.S. patent application Ser. No. 11/691,421, filed Mar. 26, 2007 and published as Pub. No. US 2007/0224348 A1, which is incorporated herein by reference. With reference to  FIG. 1 , system  10  includes a precursor zone  14  and a radicals zone  16 , separated by an intermediate isolation zone  20 . A precursor gas is introduced into precursor zone  14  from a precursor delivery system  24 . A second precursor gas or a purge gas is introduced into radicals zone  16  from a second precursor delivery system  26 . An inert gas delivery system  28  is included for injecting inert gas into isolation zone  20 . Radicals are formed in radicals zone  16  by a radicals generator  29 , which is preferably positioned proximal of the substrate to generate a direct plasma from the second precursor gas (or purge gas) in radicals zone  16 . 
     Precursor zone  14 , radicals zone  16 , and isolation zone  20  are bordered by an outer reaction chamber housing or vessel  30 , divided by first and second dividers  34 ,  36  into three sub-chambers, namely, a first precursor chamber  44 , a second precursor chamber  46  and an inert gas chamber  50 . A series of first passageways  54  through first divider  34  are spaced apart along a general direction of travel of substrate  12 , and a corresponding series of second passageways  56  are provided through second divider  36 . The passageways  54 ,  56  are arranged and configured for substrate  12  to be threaded therethrough back and forth between precursor and radicals zones  14 ,  16  multiple times, and each time through isolation zone  20 . Isolation zone  20  is, thus, preferably separated (albeit imperfectly) from precursor zone  14  by first divider  34  and from radicals zone  16  by second divider  36 . 
     Passageways  54 ,  56  are configured to restrict the flow of gases between the zones  14 ,  16 ,  20 , to avoid or limit diffusion of precursor gases and radicals into a common zone. Passageways  54 ,  56  may include slits sized only slightly thicker and wider than the thickness and width of substrate  12  passing through them, leaving only a very small amount of headroom and margins to allow substrate  12  to pass therethrough without scraping against the sides of the passageways. For example, headroom and margins may range between microns and millimeters in certain embodiments. Passageways  54 ,  56  may also include elongate tunnels (slit valves) through which substrate  12  passes. 
     To help isolate the precursor gas from the radical species, pressure differentials are preferably established between isolation zone  20  and precursor zone  14  and between isolation zone  20  and radicals zone  16 . In one embodiment, the pressure differentials may be generated by injecting inert gas into isolation zone  20  at a pressure greater than the operating pressure of the precursor and radicals zones  14 ,  16 , and then passively exhausting gases from the zones  14 ,  16 . Pressure differentials may also be generated by pumping from precursor zones via a pump  58  or another source of suction. Exhaust precursors may be reclaimed using a precursor trap  59 , such as a simple inline liquid nitrogen cooled trap. 
     A substrate transport mechanism  60  of system  10  includes a carriage comprising multiple turning guides for guiding flexible substrate  12 , including a set of first turning guides  64  spaced apart along precursor zone  14  and a second set of turning guides  66  spaced apart along radicals zone  16 . Turning guides  64 ,  66  cooperate to define an undulating transport path of substrate  12  as it advances through system  10 . Substrate transport mechanism  60  may include a payout spool  72  for paying out substrate  12  from a first coil (input roll  74 ) for receipt at a first end  76  of isolation zone  20 , vessel  30 , precursor zone  14 , or radicals zone  16 . The substrate transport mechanism  60  may further include a take-up spool  82  for receiving the coated substrate  12  from a second end  84  of isolation zone  20 , vessel  30 , precursor zone  14 , or radicals zone  16  opposite first end  76 , and coiling the substrate  12  into a second coil (take-up roll  86 ). 
     Oxygen Sources for Radical Generation 
     When using oxygen radicals as the oxygen source for ALD processing of metal oxide films, there are some particular chemistries and system configurations that may be preferable to others—particularly configurations different from those that are optimized for the use of water as the oxygen precursor in thermal (non-radical) ALD. 
     In one embodiment of an REALD method according to the present disclosure, monatomic oxygen radicals (O.) are generated from an oxygen-containing second precursor gas and that is not normally reactive with the first precursor (usually a metal-containing precursor), wherein the second precursor gas includes a gaseous oxygen-containing compound and does not contain significant amounts of normal oxygen (O 2 ). These improved REALD methods can allow improved configurations and operation of flexible substrate coating systems, which are described below with reference to  FIG. 2 . Thin films deposited by these methods and systems may be useful as optical coatings, barrier layers for food packaging or electronics, and many other uses. 
     With respect to precursor source gases used for generating oxygen radicals, there are many possibilities, particularly with respect to precursors that are gases at room temperature. For example O 2  (normal oxygen), CO 2 , CO, NO, N 2 O, NO 2 , air, etc. may all be used for REALD processes in which the primary reaction is oxygen based. Example oxygen-based REALD reactions include formation of aluminum oxide (Al 2 O 3 ) thin films at low temperatures (substrate and precursors heated to less than 150° C. and preferably less than 80° C.) using TMA and O. oxygen radicals), and formation of titania (TiO 2 ) thin films at low temperature from TiCl 4  and O.. These REALD reactions can be performed in various systems described in the &#39;050 application, and particularly the systems shown in FIGS. 1-3 and 6 thereof. 
     For attaining the highest concentration of oxygen in REALD, O 2  would be a logical choice. However, the present inventors have discovered that alternative oxygen-containing precursor gases, such as CO 2  and N 2 O may be preferable for several reasons. First, CO 2 , N 2 O, and many other gaseous compounds are not flammable and not highly reactive, and therefore may be safer for some methods, systems, applications and installations. Oxygen gas is highly reactive and must be handled with care. More importantly, in comparison with O 2 , plasmas generated from many oxygen-containing precursor compounds like CO 2  and N 2 O are less prone to the formation of ozone (O 3 ) in or near the plasma. An oxygen plasma generated from O 2  typically forms O 3  as a byproduct by the recombination of O. with O 2 . And while plasmas generated from gaseous oxygen-containing compounds such as CO 2  also include O., they are much less likely to form O 3  because there is far less O 2  present to facilitate such a reaction. 
     While O 3  may be somewhat active in ALD film growth with certain precursors, it may form an inferior oxide film compared to that formed with O.. Such is the case with TMA+O 3 , compared to TMA+O., for example. Al 2 O 3  films 200 Å thick made at low temperature with O 3  as the co-reactant with TMA had almost no barrier properties—i.e., they exhibited a very high water vapor transmission rate (WVTR). However, Al 2 O 3  films made at room temperature with TMA and O. (direct plasma) exhibit barrier properties at least as good as films made by thermal ALD with water as the co-reactant, but the growth rate with TMA+O. is more than double per ALD cycle. 
     Further, when a plasma forms both O. and substantial amounts of O 3 , the film resulting from the combination of reactions (e.g., TMA+O 3  and TMA+O.) may be inferior to a film formed mostly or entirely from reactions with O. in the substantial absence of O 3 . 
     Moreover, O 3  has a relatively long lifetime compared with O., which is highly unstable. As such, it is more difficult to isolate O 3  from the second (metal) precursor zone. On the other hand, O. recombines extremely efficiently and quickly. As such, migration of O. into the metal-containing precursor zone can be prevented simply by adequate spatial separation or by interposing flow restricting devices between the radical zone and the metal-containing precursor zone. 
     In some embodiments, a substantially pure oxide may be formed through the use of a second precursor gas consisting essentially of a gaseous oxygen-containing compound such as CO 2  or N 2 O, because the non-oxygen constituents of the second precursor gas (e.g., carbon, nitrogen) do not react with the metal-containing first precursor, or at least not with the chemisorbed species of the first precursor. In certain other embodiments, the reaction of the non-oxygen constituents of the second precursor with the metal-containing first precursor (or the chemisorbed species thereof) may be very minor compared to the reaction of the metal-containing precursor (or its chemisorbed species) with the oxygen radicals, so that a mostly pure oxide is formed. 
     Consequently, in one embodiment, a method of forming a thin film involves alternately exposing a substrate to: (1) a first precursor that chemisorbs to the substrate surface, leaving a chemisorbed species at the surface that is reactive with oxygen and oxygen radicals, and (2) an oxygen radical species, such as monatomic oxygen radicals (O.), that is generated in a plasma formed from a second precursor including a gaseous oxygen-containing compound (or mixture) and not including substantial amounts of O 2 . For example, a suitable gaseous compound or mixture devoid of a significant amount of O 2  may include less than 3% (mole fraction) O 2 . In some embodiments, a suitable gaseous compound or mixture may include less than 2%, less than 1%, less than 0.1%, or less than 0.01% mole fraction O 2 . In some embodiments, a suitable compound or mixture is said to be substantially devoid of O 2  when it contains less than 0.001% mole fraction O 2 . In some embodiments, a suitable gaseous compound or mixture contains less than 10 ppm, or less than 1 ppm of O 2 . 
     Example precursors reactive with oxygen radicals include diethylzinc (DEZ), which is reactive with oxygen radicals to form ZnO, and Tris[dimethylamino]silane (aka TDMAS), which is reactive with oxygen radicals to form SiO 2 . In an ALD reaction involving TDMAS precursor and oxygen radicals, a good quality film can be deposited at temperatures below which even water does not work as the oxidizer in a normal thermal ALD process—for example at temperatures below 130° C. Another example precursor that may be used with a REALD process of the kind disclosed herein is tin tetrachloride (SnCl4), which reacts with oxygen radicals to form tin dioxide (SnO 2 ). 
     In other embodiments, a method of forming a thin oxide film by REALD involves oxygen radicals formed from O 2  and a precursor that is not reactive with O 3 . Examples of precursors not reactive with O 3 , at least at processing temperatures under 100° C., include titanium tetrachloride (TiCl 4 ), hexachlorosilane (Si 2 Cl 6 ), and tetrachlorosilane (SiCl 4 ). 
     In summary, the processes described above provide a substantial improvement in oxidation capability over other methods and chemistries. Improved reactivity of oxygen radicals without the presence of ozone enables the use of a wider range of metal-containing precursors and other precursors, including ones with acceptable volatility and chemisorption (or adsorption) qualities but poor reactivity with non-radical oxygen and oxygen-containing compounds. 
     The foregoing discoveries enable new configurations of systems and methods for ALD coating of flexible substrates, such as the flexible substrate deposition system  210  illustrated in  FIG. 2 , which is a variation on the system  10  illustrated in  FIG. 1 . With reference to  FIG. 2 , a first precursor  224 , such as TMA, is introduced into a first precursor zone  214 . A second precursor consisting essentially of an oxygen-containing precursor gas  226  that is not reactive with first precursor  224  is injected into an oxidation zone  216  (second precursor zone). A plasma or other radical-generating mechanism  229  is operably associated with oxidation zone  216  of chamber  230 , wherein the radicals generator  229  generates atomic oxygen from the oxygen-containing precursor gas  226 . Radicals generator  229  may include a radio-frequency (RF) plasma generator, microwave plasma generator; direct-current (DC) plasma generator, or UV light source, and preferably continuously generates a population of oxygen radical species (illustrated by a cloud in  FIG. 2 ) in-situ within oxidation zone  216  by means of a plasma, for example. Radicals generator  229  may be operated in a continuous or steady-state mode without incurring the penalties of plasma ramp times and build-up of undesirable films or deposits on radicals generator  229  and walls of chamber  230 . Oxygen-containing precursor gas  226  may consist of any of the aforementioned gaseous oxygen-containing compounds or mixtures thereof. 
     Oxidation zone  216  is not pumped—i.e. an exhaust line is not directly connected to the oxidation chamber  246 . When system  210  is in use, oxygen-containing precursor gas  226  flows out of oxidation zone  216  through slits  256  into a central buffer zone  220 , and then through slits  254  into first precursor zone  214 , each time experiencing a pressure drop due to throttling at slits  254 ,  256  (aka slit valves or passageways) in the partitions  234 ,  236  between the zones  214 ,  220 ,  216 . Finally a mixture of the first precursor  224  and the oxygen-containing precursor gas  226  is exhausted from first precursor zone  214  via an exhaust port and drawn away by pump  258 , optionally through a precursor recovery trap  259 . 
     By the time the oxygen-containing precursor gas  226  migrates from oxidation zone  216  into first precursor zone  214 , all or substantially all of the atomic oxygen (O.) generated in the plasma at oxidation zone  216  has re-combined with other species in the plasma, thus becoming inactive with respect to the first precursor  224 . The oxygen-containing precursor gas  226  thus serves both as a precursor source for reactive oxygen radicals (O.), when excited by the plasma, and as a purge gas or isolation gas. Pressure differentials between the zones  214  and  220 , and  220  and  216  (both differentials in the same direction) provide twice the resistance to migration of first precursor  224  into oxidation zone  216  as compared to the embodiment shown in  FIG. 1 , in which the purge gas is introduced into isolation zone  20  at a pressure slightly higher than the two precursor zones  14 ,  16 , which are both pumped. 
     In one embodiment, radicals generator  229  includes an RF plasma generator or microwave plasma generator that generates a plasma within oxidation zone  216  by exciting the oxygen-containing precursor gas  226  with RF or microwave energy. Passageways  256  may be sufficiently narrow so as to confine the plasma within oxidation zone  216 . 
     A substrate carriage or other substrate transport mechanism  260  moves the substrate quickly through the plasma such that the REALD processes described herein can be performed while an internal temperature of the substrate is maintained below 150° C. throughout the deposition process, and in some embodiments below 80° C. throughout the deposition process. 
     If the oxygen-containing precursor gas  226  is O 2 , or contains a significant amount of O 2 , some amount of ozone would likely enter first precursor zone  214 . In this case, it would be desirable to use a precursor that reacts with atomic oxygen (O.), or at least that leaves chemisorbed species at the substrate surface that are reactive with atomic oxygen, but that is not reactive with O 3 . If, alternatively, a gaseous oxygen-containing compound is used (such as CO 2 ), the levels of O 3  entering the chamber will be small to negligible. In that case, a first precursor that does react with O 3  could be used, without the detrimental effects of non-ALD film growth in the first precursor zone of the chamber. 
     The reaction chamber configuration  230  shown in  FIG. 2  eliminates or reduces the need to pump from oxidation zone  216 , and facilitates the introduction of a large amount of compound oxygen-containing precursor gas  226  directly into oxidation zone  216 , where a plasma or other radical-generating means  229  is provided. The oxygen-containing precursor gas  226  then also provides some or all of the purge-based isolation, given that the key radicals of oxygen recombine or become de-activated prior to entering first precursor zone  214 . 
     The improved methods described above may be utilized with virtually any of the reactor configurations and process methodologies described in the &#39;050 application. For example, the use of compound oxygen-containing gases discussed herein may facilitate operation of the system shown in FIG. 4 of the &#39;050 application for oxidation reactions. In an embodiment, a gaseous compound containing oxygen, such as CO 2 , is utilized as the purge gas from which oxygen radicals are generated. A precursor reactive with oxygen radicals (but not with CO 2 ) is injected at a location in the reciprocating path of the substrate of FIG. 4 generally downstream of the radical generator. The radical generator is spaced apart from the precursor injection site a distance sufficient to provide a radical deactivation zone therebetween, as further described in the &#39;050 application. 
     A further variant on the embodiment of FIG. 4 of the &#39;050 application used with a gaseous oxygen-containing compound as the purge gas is illustrated in  FIG. 3  of the present application, which is a top cross-section view of a drum reactor system  300 . With reference to  FIG. 3 , one or more substrates (not shown) are mounted on a cylindrical drum  310  that spins about its axis within a reaction chamber  312 . Purge gas  320  that includes a gaseous oxygen-containing compound and that is devoid of substantial amounts of normal oxygen (O 2 ) is injected at or upstream of a radical generator  330 . As illustrated, the radical generator  330  may include a series of baffles  340  to assist with deactivation and recombination of radicals and to prevent radicals from migrating to the site where a precursor is introduced (in this example the injection site  350  for TMA). However, in other embodiments, simpler baffles may suffice or no baffles may be needed. Precursor, such as TMA chemisorbs to the surface of the substrate at the precursor injection site  350  in the form of a chemisorbed species before the substrate is transported by rotation of drum  310  to radical activation zone  360 . Oxygen radicals are generated from the purge gas  320  at radical activation zone  360  where they react with the chemisorbed precursor species at the substrate surface to complete the ALD cycle and form a monolayer. The oxygen radicals are preferably generated with a plasma, although other sources for excitation may be utilized, as noted in the &#39;050 application. Because the oxygen radicals recombine so quickly and easily, no substantial amount of the oxygen radical species is carried into precursor injection site  350 . Moreover, because the direction of purge gas flow is from radical zone  360  to an exhaust pump  370  on the opposite side of drum  310  and reaction chamber  312 , downstream of precursor injection site  350 , little or no precursor migrates to radical generation site  360 . Consequently, the system  300  of  FIG. 3  enables ALD-type deposition of oxide thin films without complicated flow control, baffling or substrate transport mechanisms. 
     EXAMPLE 1 
     TiO 2  Thin Film from TiCl 4  and Oxygen Radicals 
     Substrate: 2.2 meter band (loop) of PET wrapped around roller guides to circulate through the precursor zone and oxidation zone repeatedly, in a 3-zone reactor system of the kind shown in  FIG. 2  but with only two passageways  254  and two passageways  256 . 
     Oxidation temperature: 70° C. 
     Working pressure: 1.2 Torr, nominal 
     Radicals generator: DC plasma, power of approximately 200 W, with electrode placed within 1 cm of the substrate. 
     Oxygen-containing gas: clean dry compressed air injected into oxidation zone. (Note: TiCl 4  does not readily react with ozone to form TiO 2  on a polymer surface at 70° C.) 
     Growth rate: approximately 1 Å/cycle. 
     EXAMPLE 2 
     Al 2 O 3  Thin Film from TMA and Oxygen Radicals 
     Substrate: 2.2 meter band (loop) of PET wrapped around roller guides to circulate through the precursor zone and oxidation zone repeatedly, in a 3-zone reactor system of the kind shown in  FIG. 2  but with only two passageways  254  and two passageways  256 . 
     Substrate temperature: 90° C. 
     Working pressure: 1.2 Torr, nominal 
     Radicals generator: DC plasma, power of approximately 200 W, with electrode placed within 1 cm of the substrate. 
     Oxygen-containing gas: CO 2 . (Note: TMA is slightly reactive with high concentrations of O 2 , and is reasonably reactive with O 3 .) 
     Growth rate: approximately 1.6 Å/cycle. 
     EXAMPLE 3 
     ZnO Thin Film from DEZ and Oxygen Radicals 
     Substrate: 2.2 meter band (loop) of PET wrapped around roller guides to circulate through the precursor zone and oxidation zone repeatedly, in a 3-zone reactor system of the kind shown in  FIG. 2  but with only two passageways  254  and two passageways  256 . 
     Substrate temperature: 90° C. 
     Working pressure: 1.2 Torr, nominal 
     Radicals generator: DC plasma, power of approximately 200 W, with electrode placed within 1 cm of the substrate. 
     Oxygen-containing gas: high purity CO 2 . (Note: DEZ is highly reactive with O 2  and O 3 .) 
     Expected growth rate: 1.2 Å/cycle. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.