Abstract:
Systems and methods for producing a material of desired thickness. Deposition techniques such as atomic layer deposition are alter to control the thickness of deposited material. A funtionalization species inhibits the deposition reaction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 61/443,579, filed Feb. 16, 2011, the entire contents of which are herein incorporated by reference in its entirety. 
    
    
     The United States Government claims certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and the University of Chicago and/or pursuant to DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a method and system using a surface functionalization step for controlled growth of a product or composition of matter. More particularly, the method and system concern use of alkyl alcohols (ROH), ketones, beta diketonates, and carboxylic acids for the growth of doped materials by controlling the saturation growth rate during the Atomic Layer Deposition (ALD) of metal oxides, metal sulfides, nitrides, noble metals, and other classes of materials. 
     BACKGROUND OF THE INVENTION 
     The properties of doped thin films depend not only on the dopant concentration, but also on the position, bonding, and spatial distribution of the dopants, which control the fraction of active dopants (doping efficiency). Thin film growth techniques such as chemical vapor deposition, sputtering or evaporation lack the ability of actively controlling the local environment of the dopant. 
     Atomic layer deposition (ALD) is a thin film growth method using alternating, self limiting reactions between gaseous precursors and a solid surface to deposit materials monolayer-by-monolayer. Due to its self-limited layer-by-layer nature, ALD provides a partial degree of control of dopant distribution in the direction of growth. The amount of dopant per layer is controlled by the saturation coverage, which is a given value for every precursor/coreactant combination. This leads to strongly inhomogeneous doping profiles for low dopant concentrations. Previous reported strategies to allow more doping control include searching for precursors with lower growth rate per cycle, and operating below the saturation coverage at the expense of losing the homogeneity and conformality of ALD. Consequently, substantial problems currently prevail with use of ALD for growth of metal oxides, metal sulfides, and other materials with prescribed dopant profiles. 
     SUMMARY OF THE INVENTION 
     The process described herein provides enhanced control of the chemical environment of dopants, finer control of the doping profiles in doped materials. It allows the growth of doped thin films with higher doping efficiencies. The dopants are inserted by ALD on a functionalized surface of the matrix host. The introduction of this additional surface functionalization step induces changes in the surface-precursor interaction and controls the growth per cycle in ALD. By providing a way of attaching and removing the surface functional groups in situ, we can carry out this process in a continuous layer by layer fashion, and the control of the amount of material deposited per-cycle allows us to tailor the dopant distribution in the film independent of its composition and also to grow thinner films while maintaining precise dopant concentrations. In addition to allowing a better dopant control, through this method we are able to achieve submonolayer conformal coatings on high aspect ratio features, for instance to decorate the surface with ALD-grown nanoparticles 
     The use of in-situ functionalization species to moderate the growth rate has been reported in the art concerning chemical vapor deposition (CVD), as a way of enhancing the conformality of films. However, the requirements for ALD are very different than in CVD. As mentioned above, due to the pulsed nature of ALD, the interaction of the functionalization species with the surface must be self-limited and must involve the formation of chemically strong bonds with the surface so that the surface functionalization species survive the purge times between dosing steps. The functional groups incorporated on the surface must be chemically compatible with the precursors involved in the ALD step, and reactive to at least one of the other species, in order to avoid incorporating impurities in the film from residual functionalization species. In at last two examples in the prior art, ligands have been dosed to modify the substrate, and therefore influence the nucleation behavior in ALD. In these cases, the choice of inhibitors was influenced by the chemistry of the growth systems, and stable molecules containing precursor ligands were used. However, it is important to realize that any species that leads to surface functionalization at the temperature of the ALD process can potentially act as a growth moderator. 
     Alkyl alcohols (ROH), carboxylic acids, ketones and beta diketonates are applied herein as surface functionalization species. The key issues are the presence in the functionalization species of a ligand or moiety that establishes stable surface bonds with the surface and also that the functionalization species can be removed by another co-reactant. For instance, ROH species can adsorb on metal oxide surfaces forming alkoxides and carboxylates depending on the temperature and the nature of the surface metal cation. Surface alkoxilation using alkyl alcohols is used herein as an example for the methodology derived for controlling the growth per cycle and the doping distribution in ALD. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1(   a ) illustrates QCM mass signals during alternating EtOH/H 2 O doses on Al 2 O 3  and TiO 2 ;  FIG. 1(   b ) illustrates QCM mass gain during Al 2 O 3  growth using TMA/H 2 O and EtOH/TMA/H 2 O doses; and  FIG. 1(   c ) illustrates influence of EtOH dose time and growth temperature (inset) during Al 2 O 3  growth from EtOH/TMA/H 2 O; 
         FIG. 2  illustrates free carrier concentration as a function of the Al atomic percent in Al:ZnO films; 
         FIG. 3  illustrates mass gain per cycle; 
         FIG. 4(   a ) illustrates normalized growth per cycle for different ALD precursors using EtOH/M/H 2 O pulsing sequences; and  FIG. 4(   b ) illustrates influence of the alkyl group in the growth per cycle of TMA and Ti(O i Pr) 4 (b); 
         FIG. 5  illustrates growth per cycle as a function of the axial position of ZnO films obtained by ALD using a EtOH/DEZ/H 2 O pulse sequence; 
         FIG. 6(   a ) illustrates mass gain during EtOH/H 2 O pulses on an Al 2 O 3  surface grown in-situ from TMA/H 2 O; and  FIG. 6(   b ) illustrates fractional coverage of EtO-surface groups as a function of time; and the dotted line is a fit to a first order desorption kinetics; 
         FIG. 7  illustrates resistivity of 80 nm Al:AnO films obtained by ALD as a function of the TMA/(TMA+DEZ) cycle ratio using both TMA/H 2 O (squares) and EtOH/TMA/H 2 O (circles) sequences for Al deposition; 
         FIG. 8  illustrates an ALD specific method of preparing the metal oxides; and 
         FIG. 9  illustrates a nonspecific growth method to prepare the metal oxides. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The method and system of the invention are based on the modulation of the precursor-surface reactivity by inserting into the ALD process an in-situ surface functionalization step that reduces the density of surface reactive sites, effectively allowing sub-monolayer saturation coverage while preserving the self-limiting aspect of ALD that is crucial to obtaining uniform films. After the self-limited reaction with the precursor, the functional groups are removed in a later step. Both steps are carried out in-situ, so that the process of dopant insertion can be carried out in a continuous mode. Since the reactivity is directed towards not functionalized surface sites, it allows for site-selective deposition. Since one of the central applications is the growth of doped materials, the chemical composition of the surface can be generally different from that of the material grown during the ALD step. 
     Chemical species used for surface functionalization in ALD must fulfill the following requirements: i) they must bond strongly enough to survive the purging times between dosing steps under an inert nitrogen or noble gas atmosphere; ii) they must be able to infiltrate into high aspect ratio features to preserve the conformality of ALD; iii) they must not be completely displaced by the precursor molecules during the dosing step; and iv) they must be removed without leaving impurities during the coreactant step. Any chemical precursor able to react with the surface of the matrix host and leave the desired dopant on the surface can be used to generate the dopant layer. This includes, but it is not limited to, ALD precursors. The matrix host can be grown by any thin film grown technique, including Atomic Layer Deposition. The nature of the host is also general, as long as it allows functionalization of its surface and the required reactivity towards the dopant precursor. 
     Herein, as a particular embodiment of our invention, the use of alkyl ROH molecules (R=Me, Et,  i Pr) is described for surface functionalization for the growth of different metal oxides by ALD. By comparing the M/O and ROH/M/O sequences the role is determined for the surface alkoxilation on the surface coverage of the metal precursor as a function of the ROH dosing time and surface temperature. The precursors specifically evaluated include trimethyl aluminum (TMA), diethyl zinc (DEZ), titanium tetrachloride (TiCl 4 ), and titanium tetraisopropoxide [Ti(O i Pr) 4 ], and the preferred oxygen source is H 2 O. In-situ quartz crystal microbalance experiments provide insight into the mechanistic processes taking place on the surface. Doped materials are created by applying this system and method to the growth of Al-doped ZnO films. The use of a growth inhibitor has a noticeable influence on the properties of the materials, with lower resistivities achieved at higher TMA:DEZ ratios. Preliminary results on Al doped TiO 2  and Sn doped In 2 O 3  indicate that the use of surface functionalization to reduce the dopant density is also effective in these systems. 
     To evaluate the adsorption/desorption behavior of alkyl alcohols on metal oxide surfaces, we used an in-situ quartz crystal microbalance (QCM) to monitor the mass changes during alternating, is pulses of alkyl alcohol and H 2 O separated by 5 s purge periods.  FIG. 1(   a ) shows the mass increase/decrease upon insertion of 0.1 Torr pulses of ethanol (EtOH) and H 2 O for Al 2 O 3  and TiO 2  surfaces grown in-situ by ALD using Al(CH 3 ) 3  (TMA) and TiCl 4 , respectively, and H 2 O as coreactant. During the purge times between pulses no mass changes were observed. The oscillating behavior indicates the reversible nature of the adsorption/desorption process. 
     In-situ mass spectrometry (MS) measurements showed an increase of mass fragment m/z=31 (attributed to CH 2 OH + , the dominant crack of EtOH) during the H 2 O pulses after an EtOH pulse. During subsequent H 2 O exposures, the m/z=31 signal intensity dropped to background levels. When D 2 O was used as the coreactant, an increase of mass m/z=19 (DOH + ) was observed during the ethanol pulses and an increase of m/z=32 (CH 2 OD + ) was observed during the subsequent D 2 O pulses. These results are consistent with the dissociative adsorption of EtOH on the Al 2 O 3  and TiO 2  surfaces and the subsequent displacement of the functional groups by H 2 O. 
     The impact of the surface functionalization step on Al 2 O 3  ALD is shown in  FIG. 1(   b ). The QCM data for TMA/H 2 O reproduced the well-known behavior reported in the literature: a mass increase during the TMA pulse and almost no net mass change during the H 2 O pulse. However, in the EtOH/TMA/H 2 O sequence, the mass increase during the TMA dose that followed the EtOH pulse was lower, and during the H 2 O pulse a mass reduction was observed. Moreover, the net mass increase per cycle was smaller in the EtOH/TMA/H 2 O sequence. These results are consistent with EtOH partially blocking the TMA adsorption and subsequently desorbing during the H 2 O pulse. By changing the EtOH dose time we established the self-limiting nature of the EtOH adsorption process (see  FIG. 1(   c )). The inset of  FIG. 1(   c ), shows that the inhibition is almost temperature independent in the range considered. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Effect of alkyl alcohol functionalization on the growth per cycle 
               
               
                 of ALD of selected precursors with H 2 O. Values are presented 
               
               
                 normalized to the values obtained without functionalization. 
               
             
          
           
               
                   
                 TMA 
                 Zn(CH 2 CH 3 ) 2   
                 TiCl 4   
                 TTIP 
               
               
                   
                   
               
             
          
           
               
                   
                 MeOH 
                 0.40 
                   
                   
                 0.22 
               
               
                   
                 EtOH 
                 0.48 
                 0.47 
                 0.39 
                 0.26 
               
               
                   
                   i PrOH 
                 0.52 
                   
                   
                 0.26 
               
               
                   
                   
               
             
          
         
       
     
     The ex-situ measured growth rates confirmed the growth modulation due to surface functionalization. Table 1 shows the effect of different alkyl alcohols and ALD precursors during the ALD of metal oxides with H 2 O where the data has been normalized to the growth rate observed in the absence of alkyl alcohol functionalization. A comparison between methanol (MeOH), EtOH and  i PrOH shows a slightly higher inhibition for MeOH. Table 1 also shows that the inhibition strongly depends on precursor chemistry: TTIP being more strongly inhibited than TiCl 4 . No growth reduction whatsoever was observed when the sequence EtOH/H 2 O/TMA/H 2 O was used. 
     Depth-profiling X-ray photoelectron spectroscopic analysis showed carbon impurity levels below the detection limit (0.5%) throughout the thickness of the film. Likewise, the optical properties of the films as determined by ex-situ spectroscopic ellipsometry were not affected by the functionalization step (see supporting info). These measurements confirm negligible incorporation of the functionalization species into the film. 
     This methodology has been applied to the growth of Al doped ZnO (Al:ZnO) by ALD. Al:ZnO is a transparent conducting material and an n-doped degenerate semiconductor upon Al substitutional doping of the Zn sites. We varied the Al doping by executing super-cycles consisting of a number of ZnO ALD cycles followed by 1 Al 2 O 3  ALD cycle. 15  We prepared films with and without EtOH surface functionalization prior to the TMA dose. The compositions were determined by X-ray fluorescence measurements. A 30% reduction in Al incorporation was observed for a given N value due to EtOH surface functionalization. Remarkably, Hall probe measurements revealed that the films prepared using the surface functionalized ALD exhibited higher carrier concentrations of up to 50% for the same Al concentration in the films (see  FIG. 2 ). The increase in doping efficiency is a direct consequence of the more efficient distribution of dopants in the ZnO host. The lower density packing of Al atoms using surface-functionalized ALD reduces the probability of forming Al—O—Al clusters that may not be active as donors. 
     Extending of this methodology to species other than alkyl alcohols provides a wide range of saturation coverages, as well as selective surface site blocking. This methodology has been used to control thin film and nanoparticle nucleation in ALD. We have presently extended this methodology to other compounds including carboxylic acids, ketones and beta-diketones, using water, hydrogen, and hydrogen disulfide as co-reactants and different dopants including metals, transition metals and rare earth materials. This method could also be applied to the doping of non-metals, semiconductors, and semimetals using the appropriate combination of chemical species. We have also utilized this method to reduce the surface concentration of noble metal species deposited on high surface area silica gel supports by ALD resulting in smaller noble metal nanoparticles. 
     The following non-limiting Examples describe illustrative experimental procedures and conditions used in the method of the invention and results for selected products. 
     EXAMPLE 1 
     The ALD films were deposited in a conventional, well known form of viscous flow reactor. The particular design of the deposition reactor is immaterial for the realization of this invention, other than ensuring the delivery of precursor gases to the substrate. Ultrahigh purity nitrogen (99.999%) carrier gas was used at a mass flow rate of 360 sccm and a pressure of 1 Torr. Trimethyl aluminum, diethyl zinc, titanium tetrachloride, titanium tetraisopropoxide, and methanol, absolute dry ethanol, and isopropanol were purchased from Aldrich and used as received. Metal oxide films were deposited by ALD using alternating exposures to TMA/H 2 O, DEZ/H 2 O, and both TiCl 4 /H 2 O and Ti(O i Pr) 4 /H 2 O to deposit films of Al 2 O 3 , ZnO, and TiO 2 , respectively. Ti(O i Pr) 4  was placed in a stainless steel bubbler heated at 45° C. Ultrahigh purity nitrogen at a mass flow of 50 sccm was passed though the bubbler during the Ti(O i Pr) 4  exposures. The ALD timing sequences can be expressed as t 1 -t 2 -t 3 -t 4  where t 1  is the exposure time for the first precursor, t 2  is the purge time following the first precursor exposure, t 3  is the exposure time for the second precursor, and t 4  is the purge time following the second precursor exposure and all units are given in seconds (s). The influence of ROH was determined by using ROH/M/H 2 O sequences, where the timing sequences can be expressed as t 1 -t 2 -t 3 -t 4 -t 5 -t 6 . The typical dosing times used in these studies were 1 s for TMA, DEZ, TiCl 4 , ROH, and H 2 O, and 8 s for Ti(O i Pr) 4 . All purge times were 5 s unless stated otherwise. Unless indicated otherwise, the growth temperature was set to 200 C. 
     The ALD growth was monitored in situ using a quartz crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS). The QCM utilized a Maxtek BSH-150 bakeable sensor and AT-cut quartz sensor crystals with a polished front surface connected to a Maxtek TM400 film thickness monitor. The QMS (Stanford Research Systems RGA300) was located downstream of the QCM in a differentially-pumped chamber separated from the reactor tube by a 35 micron orifice and evacuated using a 50 1/s turbomolecular pump. Ex-situ thickness and optical properties were measured on flat Si 100 pieces using a J. A. Woolam Co. Alpha-SE spectroscopic ellipsometer. XPS depth profiling of the samples was carried out by Evans Analytical. 
     Quartz crystal microbalance measurements are in agreement with the ex-situ measurements reported above and help to understand the nature of the mechanism. In  FIG. 3  we present the mass gain as a function of the cycle number for TMA/H 2 O and EtOH/TMA/H 2 O sequences. The change of overall slope in the mass gain indicates a lower mass gain per cycle, from 42 to 20 ng/cm 2  at 200° C. For comparison, no inhibition was observed with the EtOH/H 2 O/TMA/H 2 O sequence. As described herein before the evolution of the mass gain during the TMA/H 2 O and EtOH/TMA/H 2 O pulse sequence is presented in  FIG. 1(   b ). During the EtOH pulse, a mass increase is observed due to the adsorption of EtOH on the Al 2 O 3  surface. Then, the mass gain during the TMA pulse is much smaller in the EtOH/TMA/H 2 O than in the TMA/H 2 O. Upon dosing of H 2 O, a decrease in the mass is observed, corresponding to the desorption of the EtOH from the Al 2 O 3  surface. These observations are consistent with a mechanism in which the EtOH adsorbs to the Al 2 O 3  surface and blocks potential adsorption sites for the TMA leading to a lower Al 2 O 3  growth per cycle. During the subsequent H 2 O exposure, the surface becomes rehydroxylated and the EtOH is released from the surface. 
     EXAMPLE 2 
     The behavior presented for TMA/H 2 O is representative of the experimental observations for the DEZ, TiCl 4  and Ti(O i Pr) 4  precursors. In  FIG. 4(   a ) we present a comparison of the growth per cycle for the EtOH/M/H 2 O sequence normalized to that of M/H 2 O (GR/GR0) for 1-5-1-5-1-5 and 1-5-1-5 timings. The growth decrease is approximately 50% for TMA and DEZ and 60% for TiCl 4 , but up to 75% for Ti(O i Pr) 4  (TTIP). The comparison of Ti(O i Pr) 4  and TiCl 4  illustrates both the metal oxide and the specific precursor contribute to the reduction of the growth per cycle. From QCM measurements, the mass gain during the EtOH pulse is the same for both precursors, indicating similar surface coverages of the ethoxide groups. Therefore, the difference in inhibition observed is due to the bigger overlap between the EtOH and precursor adsorption sites in the case of Ti(O i Pr) 4 . 
     Other alkyl alcohols present similar effects. In  FIG. 4(   b ), we compare the effect of MeOH, EtOH, and  i PrOH on the ALD of TMA and Ti(O i Pr) 4 . In both cases the reduction in growth per cycle is greater in the case of MeOH than in EtOH, while both the EtOH and the  i PrOH present similar inhibition effects. This is consistent with the literature on alkyl alcohol adsorption on metal oxides, which suggest that MeOH is less selective upon adsorption than EtOH and therefore can achieve larger surface coverages. 
     The reduction in the growth per cycle was constant along the reactor. In  FIG. 5  the growth per cycle using the 1-5-1-5-1-5 EtOH/DEZ/H 2 O sequence is independent of the axial position along the length of the reactor, thus showing the scalability of this approach to higher substrate areas. 
     EXAMPLE 3 
     We also studied the stability of the EtO— surface groups on an Al 2 O 3  surface. By dosing EtOH/H 2 O pulses on an Al 2 O 3  surface grown in-situ on the QCM using TMA/H 2 O, we determined the mass gain during the EtOH pulse and the mass loss during the H 2 O pulse as a function of the purge time x between the two pulses in 1-x-1-5 sequences. The results obtained, shown in  FIG. 6(   a ) for 1-5-1-5 sequences, confirm the complete displacement of EtOH during the H 2 O pulses. At longer purge times, a difference is observed between the mass gain Δm g  during the EtOH pulse and the mass loss Δm l  during the subsequent H 2 O pulse. This difference can be attributed to EtO— desorption due to surface relaxation or to interaction with residual water during the purge periods. In particular, the Δm l /Δm g  ratio offers a lower boundary of the fractional coverage still present in the surface (assuming that all the loss comes from EtOH molecules leaving the surface). As shown in  FIG. 6(   b ), for purge times below 60 s the difference in mass is below the experimental error (1 ng). After 600 s, the fractional coverage is still above 50%. If a first order desorption kinetics is assumed, the corresponding half-life of the EtO-surface groups is of 630 s at 200° C. 
     From the mass gain observed during the EtOH pulse on Al 2 O 3  and TiO 2  surfaces using QCM it is possible to obtain upper and lower estimates of the surface coverage of the EtO— groups. The mass gain observed using QCM is the net mass adsorbed upon EtOH adsorption. We are interested in two limiting scenarios: if EtOH reacts with a bridging oxygen, then the net mass incorporation on the surface corresponds to a whole EtOH molecule, as shown in Scheme 1(a). If, however, EtOH reacts with a geminal hydroxyl group, then the net mass gain will be that of an ethyl group [Scheme 1(b)].
 
EtOH( g )+M-O-M→EtO-M M-OH  (a)
 
EtOH( g )+HO-M→EtO-M+H 2 O ( g )  (b)
 
     Scheme 1 
     Reaction Mechanisms Providing the Highest (a) and the Lowest (b) Net Mass Gain per Ethoxide group formed on a Metal Oxide Surface 
     Schemes 1(a) and 1(b) provide upper an lower estimates of the surface coverage, respectively. In reality, the surface functionalization step may consist of a combination of different reaction pathways with unknown branching ratios, and therefore the net mass gain per functional group will be somewhere in between schemes 1(a) and 1(b). From our QCM data, we obtain a surface density of ethoxide groups between 1.3 and 2.0 EtO—/nm 2  for Al 2 O 3  at 200° C. Similar calculations carried out for a TiO 2  surface yield a value between 1.4 and 2.2 EtO—/nm 2  at 200° C. 
     EXAMPLE 4 
     We have applied growth moderation using alkyl alcohols to the deposition of doped metal oxides by ALD, including Al-doped TiO 2 , Sn-doped In 2 O 3  and Al-doped ZnO. In the latter case, we compared the Nx(DEZ/H 2 O)+(TMA/H 2 O) sequence previously used in the literature with the EtOH moderated sequence: Nx(DEZ/H 2 O)+(EtOH/TMA/H 2 O). Resistivity measurements using four-point probe show that the use of growth moderators broadens the range of TMA/(TMA+DEZ) pulsing ratios in which low resistivity materials can be obtained, due to a lower density of Al dopants deposited per ALD cycle. 
     The results presented in  FIG. 7  correspond to 80 nm thick films. The differences are even more pronounced for nanoscale materials: for a total thickness of 12 nm, the use of moderators reduces the sheet resistance by a factor of three for sequences with a TMA:DEZ ratio of 1:14, when compared with a 40% improvement for 80 nm thick films. These results are promising for the applying this strategy to the growth of functional coatings on nanostructured electrodes to improve the electron collection efficiency in dye sensitized solar cells. 
     Controlled dopant distribution and higher doping efficiencies by surface-functionalized atomic layer deposition. 
     EXAMPLE 5 
     The use of acetone, acetylacetone and acetic acid all resulted in inhibition of the growth. We have showed that ozone and combinations of water and ozone can be used to remove the surface functional groups. The doping has been extended to Lanthanide precursors, for the growth of erbium doped metal oxides. 
     EXAMPLE 6 
       FIGS. 8 and 9  illustrate two example methods of carrying out the method of the invention.  FIG. 7  shows an ALD specific method and  FIG. 9  shows a matrix grown by a non-specific method using the same chemically constructed state as in  FIG. 8 . 
     The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of methods, apparatus, modules and systems.