Selective Area Spatial Atomic Layer Deposition of ZnO, Al2O3, and

Article pubs.acs.org/cm

Selective Area Spatial Atomic Layer Deposition of ZnO, Al2O3, and Aluminum-Doped ZnO Using Poly(vinyl pyrrolidone) Carolyn R. Ellinger* and Shelby F. Nelson Eastman Kodak Company, 1999 Lake Ave., Rochester, New York 14650, United States ABSTRACT: Spatial atomic layer deposition (SALD) is gaining traction in the thin film electronics field because of its ability to produce quality films at a fraction of the time typically associated with ALD processes. Here, we explore the process space for the fabrication of thin film patterned-by-printing electronics using the combination of SALD and selective area patterning. First, a study of SALD growth conditions for the three primary components of our metal oxide thin film electronics, namely alumina (Al2O3) dielectric, zinc oxide (ZnO) semiconductor, and aluminum doped ZnO (AZO) conductor, provides insight into the potential trade-offs in performance, substrate latitude (temperature), and process speed. At constant precursor partial pressures, the precursor exposure times and substrate temperatures were varied from 25 to 400 ms and from 100 to 300 °C, respectively. The very short gas exposure and purge times obtainable only with a spatial implementation of ALD are shown always to be advantageous for throughput and process speed, even though growth is far from the “ideal” ALD condition of saturated monolayer growth. Using the same range of process conditions, we evaluated the ability of very thin layers of poly(vinyl pyrrolidone) (PVP) to inhibit film growth. We demonstrate that PVP sufficiently inhibits the growth of all three materials at temperatures at or above 150 °C to usefully pattern high-quality electronic devices. Additionally, we found that very thin layers of PVP are most effective at higher temperatures and fast ALD cycles. Thus, faster SALD cycles are advantageous from both throughput and patterning performance perspectives.

■

INTRODUCTION We are exploring an approach to printed electronics where the patterning is achieved by printing and active materials are deposited via atomic layer deposition (ALD). In this patternedby-printing approach, an inhibitor material is printed on a substrate, and the globally applied active material only deposits in the areas where the inhibitor is not presentand thus is patterned at the time of deposition.1−3 In our work, the active materials are deposited using an atmospheric pressure, rollcompatible spatial ALD (SALD) process.3 We have previously presented ZnO thin film transistors (TFTs), processed with this method, that had the same device performance as photolithographically patterned devices of the same materials.1,2,4 The use of spatial ALD enables the use of very short cycle times with single gas exposure times between 25 and 400 ms (cycle times of 100−1600 ms). An atmospheric pressure SALD system has no time penalty associated with pumping down a reaction chamber to vacuum levels, and therefore, the process time is approximately the number of cycles required multiplied by the cycle time. Because of the process speed advantage, spatial ALD is being investigated for barrier layers, solar cell passivation, and thin film electronics.4−11 Additional gains in throughput and device design flexibility come from using selective area deposition and printing. In this scheme, there are no inorganic etch steps for the deposited materials and no need for exposure or development of a photoresist. The patterning process time is determined by the © 2014 American Chemical Society

print rate and the time necessary to remove the inhibitor at the end of SALD deposition. For our typical conditions, we can complete the SALD-pattern process cycle in less than 20 min and full circuits in only a few hours.2 Atomic layer deposition can be considered to be sequential self-terminating gas−solid reactions.12 Typically, researchers in the ALD field have preferred ALD conditions where there is saturated, irreversible adsorption for each half cycle of the ALD reaction. However, we have found that devices made from films grown at conditions where the growth is unsaturated, and also where there may be reversible adsorption, have good performance characteristics.3,11 Therefore, as we explored potential trade-offs between cycle time, ability to pattern, and film properties, we have not limited ourselves to “ideal” ALD growth conditions. To understand the process compatibility with low temperature substrates, we examined the SALD growth down to 100 °C; to understand the limits of inhibition and film growth, the highest substrate temperature tested was 300 °C. This phase of our study provides valuable understanding into the growth per cycle (GPC) and associated ALD mechanisms for the various materials as a function of our process conditions. Selective area deposition, alternatively termed area-selective deposition, of ALD films refers to the pattern-wise modification Received: July 22, 2013 Revised: January 13, 2014 Published: January 30, 2014 1514

dx.doi.org/10.1021/cm402464z | Chem. Mater. 2014, 26, 1514−1522

Chemistry of Materials

Article

of a substrate surface that selectively enhances or prevents film growth. This has been demonstrated for a variety of precursor systems using chamber-based ALD with patterned surface treatments or application of an inhibitor material to the substrate surface. Most of these chamber-based studies have used self-assembled monolayers as the inhibitor material.13−15 Other groups have also explored polymers as inhibitors but patterned them using photolithography or UV−ozone etching.16−19 We are primarily interested in using printing to pattern the inhibitor and have concentrated on polymers that may be readily formulated into inks for either flexography or inkjet. To this end, we previously presented a survey of polymer inhibitors used in combination with our SALD system and identified poly(vinyl pyrrolidone) (PVP) as a useful polymer inhibitor because of the combination of its inhibition ability, water and alcohol solubility, and the relative ease of formulating PVP inks for multiple printing technologies.1 In that study, we discussed the inhibition behavior of PVP with film thicknesses from 30 to 1000 Å for ZnO precursors and over a variety of SALD cycle times in addition to demonstrating fully patternedby-printing thin film transistors.1 Figure 1a is an SEM image of a PVP printed pattern (dark gray) with 1000 Å of SALD-grown AZO in the open portions

resolution, as the minimum inkjet features in our device patterns are tens of micrometers for both line and space. However, it is important to understand the inhibition limits of very thin polymer films because although PVP inhibition performance improves with film thickness, our inkjet-printed patterns have portions that are very thin.1 Therefore, in this work, we focus on the inhibition properties of the minimum dried PVP film thickness in our inkjet-printed patterns, which is about 50 Å. The three primary materials systems examined in this study, Al2O3, ZnO, and AZO, were independently evaluated using PVP k-30 to inhibit film growth over a range of conditions. The results from this phase of the study, taken in combination with the understanding of SALD growth from the first phase, suggest a working hypothesis for very thin PVP film inhibition failure.

■

EXPERIMENTAL SECTION The experimental SALD system contains two complete ALD cycles and has been previously described in detail.3 Square silicon and glass substrates (62.5 mm square) were used, and attached to a heated backer using vacuum. The substrate was placed on the purge gas flowing from the SALD coating head so that it floated, much as a puck floats on an air hockey table. The close proximity to the head was maintained by the flow of the gases out of and into the SALD head.3 For all of the coatings, the exhaust slot pressure was approximately 2.94 Torr. The purge gas and dilution gas was nitrogen. The metal alkyl precursors used for the data in this paper were dimethylaluminum isopropoxide (DMAI) (Strem, 98+%) and diethyl zinc (DEZ) (Strem, 99.9998%), and the oxygen precursor was water. We use these precursors to grow ZnO (DEZ), Al2O3 (DMAI), and AZO, which is formed by flowing a mixture of DEZ and DMAI. Individual mass flow meters controlled the flow rate of the precursor vapor by bubbling nitrogen through the liquid precursor. These saturated streams were mixed with a dilution flow before being supplied to the coating device. All bubblers were at room temperature. The partial pressure values in Table 1 were calculated assuming a room temperature of 22 Table 1. Reactant Partial Pressures for SALD layer

pDMAI, mTorr

pDEZ, mTorr

pH2O, mTorr

Al2O3 ZnO AZO

48.6 0 7.76

0 529 269.8

566.3 395.5 199.8

°C and a reactor pressure of 760 Torr (atmospheric pressure). The temperature of the coating was established by controlled heating of both the SALD head and the substrate backer. Coating was accomplished by oscillating the substrate relative to the coating head for the number of cycles necessary to build up a deposited film of the desired thickness for the given example (a round trip oscillation has 4 ALD cycles). The precursor exposure time was varied in the experiment from 25 to 400 ms by varying the velocity of the substrate from 101.6 to 6.35 mm/s. The SALD system was designed such that the exposure times for each of the reactive gases and the inert gas (purge) are equivalent.11 Silicon and glass substrates were cleaned using a 100 W, 0.3 Torr oxygen plasma prior to deposition of the inhibitor. Samples were prepared by spin coating a solution of 0.25 wt % PVP k-30 in diacetone alcohol onto the clean substrate at 3000 rpm. After coating, half of the sample was wiped clean using

Figure 1. SEM image and associated EDS spectra of patterned PVP and AZO. (a) SEM image of PVP printed pattern (dark gray) with 1000 Å of SALD grown AZO in the open portions of the PVP pattern (light gray). (b) EDS spectra of the AZO layer taken at point 1 in (a). (c) EDS spectra of the PVP layer taken at point 2 in (a) showing that the printed PVP layer has successfully inhibited the growth of AZO.

of the PVP pattern (light gray). Figure 1b and c are the EDS spectra of spots 1 and 2, respectively, in Figure 1a. Key to note is the presence of Zn and Al peaks in 1b, but not in 1c. These figures illustrate our ability to successfully use printed PVP to pattern SALD inorganic films with no observable growth in the inhibitor area. We are not currently concerned with patterning 1515



Article

diacetone alcohol in order to measure both film growth and amount of inhibition on a single substrate. After the inhibitor was removed from half of the substrate, the samples were subjected to a one minute hot plate bake either at 200 °C or at the deposition temperature, whichever was higher. This process results in approximately 50 Å-thick PVP films. The thickness of films on the silicon substrates were measured using ellipsometery (J.A. Woollam Alpha-SE spectroscopic ellipsometer, over the range of 450−900 nm.) Measurements were taken after O2 plasma cleaning the substrate, after PVP coating, and after SALD deposition; these data were used to determine the growth per cycle and maximum inhibition thickness. The optical constants of the films were obtained by fitting a Cauchy model to the data, and the reported index of refraction values were taken at 700 nm. The sheet resistance of aluminum-doped zinc oxide (AZO) films was additionally measured using a standard four-point probe setup (Signatone).

■

RESULTS AND DISCUSSION SALD Film Growth for Al2O3, ZnO, and AZO. A typical way to examine the growth behavior in ALD is to graph the growth per cycle (GPC) as a function of time of exposure to one or both reactive precursors. Both precursor dosing times and the subsequent purge times are constrained to be identical in our SALD head and are controlled by how fast the substrate moves over a gas channel. Thus, we plot the GPC versus “residence time”, where residence time denotes the duration that a point on the substrate resides over each channel of the spatial ALD head. The residence time, as used herein, is equivalent to the precursor exposure time and is one-quarter of the total cycle time. The GPC of Al2O3 using DMAI and H2O is shown in Figure 2a for substrate temperatures from 100 to 300 °C, and for residence times from 25 to 400 ms. Within the temperature and residence time regimes evaluated here, the GPC never approaches the limit of monolayer saturation, which would be expected to be between 0.9 and 1.3 Å/cycle, depending on temperature.20 Unlike previously reported growth rates for Al2O3 using heated bubblers of DMAI, the DMAI bubbler here is at room temperature, and therefore, the amount of DMAI introduced to the surface is low because of its low vapor pressure (1.17 Torr at 22 °C; for comparison DEZ is 13.52 Torr at 22 °C).20,21 There is a single point on the graph of Figure 2a, indicating the very low growth per cycle obtained at 100 °C; although growth was measurable, the quality and rate did not warrant full testing. There is also a lack of significant difference between the GPC at 250 °C and that at 300 °C. At 300 °C, the DMAI precursor can decompose20 and may form AlOx during the DMAI exposure.22 Any growth above 250 °C involving DMAI as a precursor, therefore, will likely have a different growth behavior than that at lower temperatures. Although under all conditions the growth per cycle of Al2O3 is low (under 1 Å/cycle), the real-time growth rates are still very reasonable due because of the short residence times of our SALD system. Real time growth rate is calculated by multiplying the GPC and the cycle time. In atmospheric SALD systems, there is little process time prior to the initiation of the first cycle; in our system, the total loading and unloading and gas equilibration time is less than 2 min per sample. Figure 2b shows the calculated real time growth rate for the GPC data in Figure 2a as a function of temperature for each of the residence times explored. For all temperatures, the real time

Figure 2. SALD Al2O3 growth and film properties as a function of temperature and residence time. (a) Growth per cycle (Å/cycle) versus residence time. Nonsaturated growth conditions can be observed from the growth per cycle (Å/cycle) versus residence time (one-fourth of one cycle) for Al2O3 from 100 to 300 °C. (b) Calculated Al2O3 real time growth rate (Å/s) versus temperature. At all temperatures, the highest growth rate occurs at the shortest residence times. (c) Refractive index at 700 nm for Al2O3 films versus temperature.

growth rate is highest for the shortest residence time. A tradeoff can, therefore, be made between real process speed and the degree to which growth has reached the adsorption saturation 1516



Article

limit on the surface. Shorter cycle times allow for reasonable process times for the layer thicknesses required for thin film electronics. For example, at 200 °C and a residence time of 50 ms, a 1000 Å layer of Al2O3 can be grown in about 10 min. Process temperature is known to impact the film quality of Al 2 O3 , and it is expected that short residence times (unsaturated growth) may also have an impact. For Al2O3 films grown using different precursors, it has been reported that film density increases as the refractive index increases, suggesting lower refractive index films have more voids or even different stoichiometry.23,8 However, it was found that there was not a strong correlation between the film density and the dielectric constant or breakdown voltage,23 which is consistent with our ability to make good devices from these layers. For all process conditions used above, the films grown using our SALD system have a refractive index at 700 nm between 1.55 and 1.65 (Figure 2c), consistent with literature values for aluminum oxide grown with both TMA or DMAI precursors.21,23 These measured refractive index values, and presumably the associated film properties, are much more dependent on growth temperature than on residence time. The oxygen to aluminum ratios were determined by XPS for the films grown at 50 ms residence time and were also found to be consistent with films grown in vacuum ALD systems, again regardless of metal precursor.20,21,23 (In specific, the oxygen/ aluminum ratio was 1.9 at 100 °C, 1.7 at 150 °C, and 1.6 for films grown at 200 °C and above.) Additionally, although alumina grown at 100 °C had a small amount of carbon contamination (less than 1%), the XPS results did not indicate any contaminants in the films grown at higher temperatures. For growth of ZnO, the semiconductor material, we find a different phase space than was seen in Al2O3. Figure 3a shows that at temperatures below 200 °C, the GPC for ZnO has the classic ALD shape, namely GPC increases with increasing residence time, indicating incomplete surface reactions (submonolayer adsorption) at short residence times. However, at growth temperatures of 200 °C and greater, the GPC decreases as the residence time is increased, as was seen previously for different partial pressures of these reactants11 and as is known in other precursor systems.12 In our SALD system, the purge time varies with the exposure time, meaning that as the residence time gets longer, the purge time is getting longer as well. This is in contrast to a typical GPC versus precursor exposure time plot from chamber-based ALD, where the purge time is determined by flow dynamics of the chamber and so is fixed. We have previously demonstrated that it is the increasing purge portion of the SALD cycle that reduces the reactivity of the surface,11 either through a desorption mechanism or (more likely) as a result of further surface reactions, in which less reactive Zn−O bonds form on the surface with a corresponding reduction of hydroxyls. In either mechanism, the available reactive sites of the surface decrease as a function of residence time. For long residence times, a decrease in GPC (from 2 to 1 Å/cycle) with increasing temperature (from 150 to 300 °C) is also observed in Figure 3a and is similar to growth rates reported by other groups.24 Interestingly, in all cases, the calculated real time growth rate for ZnO is significantly higher at the shorter residence times, as can be seen in Figure 3b. Over the range of temperatures and residence times evaluated, the range of GPC is small, whereas the residence time varies by a factor of 16. Relatively large changes in residence times (doubling from 25 to 50 ms) result in relatively small changes in GPC, resulting in a strong

Figure 3. ZnO growth and refractive index as a function of temperature and cycle time. (a) ZnO growth per cycle (Å/cycle) versus residence time. A transition in curve shape is observed between 150 and 200 °C. (b) Calculated ZnO real time growth rate (Å/s) versus temperature. The highest ZnO real time growth rate is observed at 200 °C and 25 ms. (c) Refractive index at 700 nm for ZnO films versus temperature.

dependency of the real time growth rate on residence time. Under the conditions tested, the real time growth rate of ZnO is fastest at 200 °C. In previous work, it was shown that shorter residence times and lower temperatures are correlated to more resistive ZnO.11 In this study, the refractive index of the ZnO 1517



Article

films was measured as shown in Figure 3c. Generally, as the process temperature increases, the refractive index increases and becomes less influenced by residence time. Figure 4a shows the GPC for the third material studied, AZO, grown at the same process temperatures used for ZnO

system, as for the previous two, shorter residence times yield the fastest real time growth rates, with the peak of 15.9 Å/s found at 200 °C and 25 ms (not shown.) Figure 4b shows the refractive index of the AZO films, with data for ZnO and Al2O3 films grown with a residence time of 50 ms included for comparison. As expected, the properties of the AZO films vary with different growth conditions. At 100 °C, the AZO refractive index is high, pushing toward the ∼2 of undoped ZnO, whereas at higher deposition temperatures, the AZO index moves toward that of aluminum oxide. For temperatures between 150 and 250 °C, the refractive index decreases at longer residence times, suggesting better Al incorporation. Aluminum incorporation should determine the conductivity of the AZO, so for conductivity assessment, 1000 Å films were grown at a subset of the process conditions and measured via four point probe. The 250 °C growth temperature and 100 ms residence time gave the lowest resistivity, of 4.56 × 10−4 Ohmcm. There is a strong inverse correlation of sheet resistance to refractive index as shown in Figure 5. The aluminum

Figure 5. Inverse sheet resistance versus refractive index for AZO. Plotted here is the inverse measured sheet resistance of 1000 Å AZO films grown on glass versus the refractive index extracted from films grown using the same conditions on silicon. The data were obtained by SALD growth at 150, 200, and 250 °C.

Figure 4. Growth of AZO as a function of temperature and cycle time. (a) AZO growth per cycle (Å/cycle) versus residence time. AZO was grown by supplying the zinc and aluminum precursors simultaneously to the substrate, resulting in complex growth conditions as can be observed from the GPC data shown. (b) Refractive index at 700 nm for AZO films versus temperature (°C). There is a strong dependence of the refractive index of AZO on the SALD process conditions, as seen here, with ZnO and Al2O3 data included for reference.

incorporation was measured by XPS for samples grown with residence time of 50 ms, and indeed, the atomic percent of aluminum increases from 0.33% for substrate temperature of 150 °C to 2.45% at 300 °C. In a separate experiment, the ratio of DEZ and DMAI was varied to try to improve the conductivity of the AZO films deposited at low temperatures. Although there were small changes in the measured sheet resistance of the samples, the film properties were primarily determined by the process temperature and not the relative amounts of precursor. Inhibition of Film Growth for Al2O3, ZnO, and AZO Using PVP k-30. Thus far, we have shown that using our SALD system, we can obtain reasonable growth rates for the three layers used to form electronic devices at growth temperature ranging from 150 to 300 °C. Each of these conditions can result in good quality thin-film transistors (not shown here), suggesting that the materials properties are useful for thin film electrical, optical, or electro-optical devices. For ease of patterning, it is desirable to be able to selectively inhibit

and Al2O3. The aluminum and zinc precursors are supplied to the substrate simultaneously, resulting in a complex reaction space. This method inherently couples the growth per cycle with the amount of aluminum incorporation into the AZO matrix. Thus, although the overall growth rate (GPC) is dominated by the kinetics of the ZnO reaction, the amount of aluminum incorporated into the matrix is determined by the competition between the DMAI and DEZ precursors at the surface. For these experiments, we used DMAI and DEZ flows that resulted in a minimum film resistivity at 200 °C and 50 ms residence time (1.83 × 10−3 Ohm-cm). The AZO GPC data is similar to the data shown in Figure 3a for ZnO for long residence times (100 ms and greater), even though the partial pressures of DEZ and H2O were much lower. For this material 1518



Article

film growth at any or all of these growth conditions. To that end, we examined how well the growth of these materials can be inhibited by 50 Å-thick layers of poly(vinyl pyrrolidone) k30 (PVP). Previous studies of ALD inhibition using polymers have used thicker layers of polymer, typically more than 10× the 50 Åthick layers used in this study. For example, the inhibition of noble metals and metal oxides was investigated in two related studies; the first used PMMA films from 700 to 1000 Å thick and the second used PVP k80 films ranging from 1000 to 2000 Å.16,17 Although both polymers exhibited complete inhibition for the noble metal growth conditions, they showed mixed results when growing aluminum oxide from either AlCl3 or trimethylaluminum (TMA) and water for 500 cycles at 250 °C.16,17 In all cases, the thicker layers of polymer used exhibited some level of inhibition failure, resulting in Al2O3 growth either on the polymer surface or within the polymer films.16,17 These ALD films could still be patterned using lift-off, and the difficulty of removing the inhibitor-plus-Al2O3 layer was found to be dependent on the precursor chemistry and the choice of polymer.16,17 Other studies have examined the role of diffusion in films of polymers, including PMMA, at thicknesses above 4000 Å.18,19,25 In our work, we are primary interested in conditions of full inhibition, where there is little or no oxide growth on the thin inhibitor. The polymer thickness used in this experiment, near 50 Å, is very thin, and full surface coverage of the polymer is not assured. Additionally, although the bulk Tg of PVP is roughly 160 °C, a significant suppression of Tg at these film thickness can be expected.26 A potential further complication is the known PVP plasticization with humidity and temperature, which may change the structure of the polymer surface.27,28 To our knowledge, no studies of the properties of PVP films have been done at this thickness, at these temperatures, or with the rapid water cycles that are used in our SALD system. We can assume, however, that at our typical operating conditions of 200 °C, 50 Å PVP films are soft and may change with time in the SALD system because of the temperature and exposure to water. Additionally, we know that the PVP films, though soft, are not mobile on the substrate surface at the temperatures tested; other experimentation has shown that polymers that are liquid (can flow) at the SALD process conditions are visibly deformed by the pressure fields created by SALD process gases. To successfully inhibit, the PVP must both inhibit the initiation of growth on the surface of the PVP and also prevent the precursors from reacting with the substrate surface below the PVP film. Because 50 Å PVP polymer films are too thin to usefully talk about diffusion into the layer, we consider reaction with the PVP to be a surface effect. Failure begins when a reactant molecule adsorbed (physisorbed or chemisorbed) on the PVP remains long enough to react with the other half cycle and form an island of growth. We have found that GPC is fairly constant as a function of cycles from the initial time on clean substrates and from shortly after the inhibition failure time on the polymer inhibitor films. This observation is consistent with an island growth mechanism of failure, where once islands merge, standard ALD growth occurs.29 Figure 6 illustrates that at all process conditions evaluated for the growth of Al2O3 from DMAI and water, some level of inhibition is possible using the thin PVP layer. Inhibition of Al2O3 growth peaks around 250 °C. Again, this is likely attributable to DMAI decomposition at 300 °C. Shorter residence times are better at all temperatures. Although the

Figure 6. Maximum inhibition thickness for Al2O3 as a function of temperature. The different curves correspond to different residence times. The maximum inhibition thickness of ∼850 Å is highest for 25 ms residence times and 250 °C.

difference in maximum inhibition thickness at 150 °C is small, it is worth noting that the number of cycles prior to inhibition failure does depend strongly on residence time, with about 100 cycles of inhibition at 400 ms and over 1000 cycles of inhibited growth at 25 ms. At low temperatures, excess water adsorption is likely to play a detrimental role.8 Putting the data in context, our typical ZnO-based thin film transistors have an Al2O3 dielectric layer thickness between 150 and 750 Å and, thus, can be only be patterned selectively with ease at the higher temperatures. Furthermore, thicker dielectrics require shorter residence times with these very thin PVP films. Generally, we find that the process conditions that lead to greater growth per cycle on untreated surfaces (i.e., higher temperatures and longer residence times) correlate with a diminished ability of PVP to inhibit growth. This likely results from both an increase in the number of nucleation sites per cycle as well as an increase in the growth rate at any already nucleated site. Whatever the failure mechanism, we have found that we are able to fully remove patterned PVP inhibitor using a low power oxygen plasma even after film growth up to the inhibition failure point. For ZnO growth, as shown in Figure 7, the maximum inhibition thickness was obtained at the highest temperature and shortest cycle time evaluated and was measured to be 3000 Å (where for Al2O3, the maximum in any condition was ∼850 Å). At all process conditions evaluated for ZnO growth, more than 150 Å of inhibition is possible using these very thin PVP layers, which is sufficient to pattern the semiconductor in a typical ZnO thin film transistor. For low temperature ZnO growth conditions, where the GPC of ZnO increases with increasing residence time, we see the same signature as for Al2O3, namely that conditions leading to higher GPC lead to a reduction in the maximum inhibition thickness. However, above 150 °C, where the GPC decreases with increasing residence time (Figure 3a), the PVP surface presumably continues to experience an increase in adsorption with increasing residence time, resulting in an increase in nucleation sites. Both the inhibition performance for our very thin PVP films and the real time growth rate of ZnO are, thus, optimized by using short residence times. As with ZnO and Al2O3, AZO film growth can be inhibited by very thin PVP layers for all process conditions evaluated, as 1519



Article

°C using a fixed 50 ms residence time for each exposure step. In this experiment, the dilution flow and bubbler temperatures were kept constant, and the partial pressures of DEZ and H2O in the reactor were varied using the reactant flow rate. The data in Figure 9a show the typical response to concentration, where

Figure 7. Maximum inhibition thickness for ZnO as a function of temperature. The different curves correspond to different residence times. PVP k-30 is most effective at inhibiting the growth of ZnO at residence times of 25 ms.

can be seen from the data in Figure 8. Consistent with data for the other two materials, AZO films grown using shorter

Figure 9. Impact of reactant partial pressure on GPC and inhibition of ZnO. (a) GPC of ZnO versus reactant partial pressure for 50 ms residence time at 200 °C. (b) Maximum ZnO inhibition thickness versus reactant partial pressure. For each curve, the partial pressure of one precursor is held constant. Lower doses of precursor per exposure, for both DEZ and H2O, result in a higher maximum inhibition thickness of ZnO grown using SALD at 200 °C with a 50 ms residence time. Circled data in each plot are identical.

Figure 8. Maximum inhibition thickness for AZO as a function of temperature. PVP k-30 is most effective at inhibiting the growth of AZO at residence times of 25 ms for any deposition temperature.

exposure times or at higher temperatures are more easily inhibited. The maximum inhibition thickness of AZO is significantly higher than that for Al2O3 and slightly higher than that of ZnO at most conditions. The inhibition failure is likely due to nucleation of ZnO from the DEZ precursor. The reduced partial pressure of DEZ presented during AZO growth relative to that used for ZnO growth may account for the differences in maximum inhibition thickness. We will see in the next section how the relative amounts of precursor influence the growth per cycle and inhibition ability of thin PVP layers. Regardless the reason, the ability to inhibit thicker AZO layers is fortunate because low sheet resistance conductors can best be achieved by conductor layer thicknesses around or above 1000 Å and are preferably grown at higher temperature (as can be seen in Figure 5). Further Exploration of ZnO Inhibition and Film Growth. To better understand impact of precursor exposure on the effectiveness of PVP to inhibit ZnO film growth, the DEZ and H2O precursor flows were varied individually at 200

the growth per cycle plateaus, indicating a complete surface reaction for the 50 ms exposure time and 200 °C. The data also indicate that GPC has a stronger dependence on the amount of DEZ than on the amount of water. We see further evidence that the conditions that give rise to greater GPC also lead to poor inhibition in the comparison of the data in Figure 9a and b. At higher reactant partial pressures, where there is a higher GPC, the ability to inhibit film growth using PVP is diminished. Taken as a whole, there is a clear trend toward poorer inhibition as a function of GPC. Interestingly, the lowest concentration of H2O on the graph (which also uses the highest concentration of DEZ) has both a reasonable GPC and a reasonable level of inhibition. The short residence times of SALD provide a rich process space for thin film device fabrication, and the data presented 1520



Article

indicating that a full four layer TFT circuit can easily be completed, patterning and all, with less than two hours of processing time. Taken together, these data represent the first complete look at the primary materials and processes useful in the patterned-by-printing approach to electronics. Additionally, the ability to digitally pattern the PVP inhibitor opens a clear path for novel device architectures and processing schemes and for optimizing these processes for the fabrication of high-quality printed electronics.

suggest materials optimization at short cycle times would be advantaged. Figure 10 recasts data shown earlier, highlighting

■

AUTHOR INFORMATION

Corresponding Author

*C. R. Ellinger. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■

Figure 10. Maximum patterned layer thickness versus the SALD growth time required. Decreasing the residence time simultaneously increases the patternable layer thickness and decreases the time that it takes to grow the thicker layer. Maximum patterned layer thickness (maximum inhibition thickness) for Al2O3, ZnO, and AZO grown at 250 °C are plotted versus the time necessary to growth a layer of that thickness via SALD (minutes) at various residence times.

ACKNOWLEDGMENTS The authors would like to thank Janet Barilla and Andrew Thompson for their technical support, Paul Fellinger for XPS, Stephen Stoker for SEM and EDS, and Lee Tutt for his lively discussion of ALD and inhibition mechanisms. Additionally, the authors thank David Levy for inspiration.

■

the relationship not typically present in materials patterning and processing. As seen, the shorter cycle times of SALD simultaneously increase the patternable layer thickness and decrease the time that it takes to grow the thicker layer. Although the data in Figure 10 is for growth at 250 °C, this general relationship holds true for all deposition temperatures. Ultimately, any optimization will depend on the requirements of a given application and associated devices. With the broad applicability of printed electronics, and more generally of patterned thin films, there are likely to be multiple “optimum” solutions even within the space explored to date. We have made, for example, functional TFTs using the patterned-byprinting technique at 150 and 200 °C on glass and flexible supports,1,2,4 which show that the active materials had decent properties over that range.

REFERENCES

(1) Levy, D. H.; Ellinger, C. R.; Nelson, S. F. Appl. Phys. Lett. 2013, 103 (4), 043505. (2) Ellinger, C. R.; Levy, D. H.; Nelson, S. F. Proc. NIP28/DF2012 2012, 304. (3) Levy, D. H.; Nelson, S. F.; Freeman, D. J. Display Technol. 2009, 5, 484. (4) Levy, D. H.; Nelson, S. F. J. Vac. Sci. Technol., A 2012, 30, 1. (5) Sun, J.; Mourey, D.; Zhao, D.; Park, S. K.; Nelson, S. F.; Levy, D. H.; Freeman, D.; Cowdery-Corvan, P.; Tutt, L.; Jackson, T. N. IEEE EDL 2008, 29, 721. (6) Levy, D. H.; Nelson, S. F.; Freeman, D.; Cowdery-Corvan, P. J. Appl. Phys. Lett. 2008, 92, 192101. (7) Poodt, P.; Lankhorst, A.; Roozeboom, F.; Spee, K.; Maas, D.; Vermeer, A. Adv. Mater. 2010, 22, 3564. (8) Poodt, P.; Knaapen, R.; Illiberi, A.; Roozeboom, F.; van Asten, A. J. Vac. Sci. Technol., A. 2012, 30 (1), 01A142. (9) Illiberi, A.; Roozeboom, F.; Poodt, P. Appl. Mater. Interfaces 2012, 4 (10), 268−272. (10) Maydannik, P. S.; Käar̈ iäinen, T. O.; Cameron, D. C. Chem. Eng. J. 2011, 171, 345. (11) Nelson, S. F.; Levy, D. H.; Tutt, L. W.; Burberry, M. J. Vac. Sci. Technol., A. 2012, 30 (1), 01A154−1−01A154−6. (12) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. (13) Park, K. J.; Doub, J. M.; Gougousi, T.; Parsons, G. N. Appl. Phys. Lett. 2005, 86, 051903. (14) Jiang, X.; Bent, S. F. J. Electrochem. Soc. 2007, 154, D648−D656. (15) Yan, M.; Koide, Y.; Babcock, J. R.; Markworth, P. R.; Belot, J. A.; Marks, T. J.; Chang, R. P. H. Appl. Phys. Lett. 2001, 79, 1709. (16) Färm, E.; Kemell, M.; Santala, E.; Ritala, M.; Leskelä, M. J. Electrochem. Soc. 2010, 157 (1), K10−K14. (17) Färm, E.; Kemell, M.; Ritala, M.; Leskelä, M. J. Phys. Chem. C 2008, 112, 15791−15795. (18) Sinha, A.; Hess, D. W.; Henderson, C. L. J. Vac. Sci. Technol., B 2007, 25, 1721. (19) Sinha, A.; Henderson, C.; Hess, D. W. ECS Transactions 2007, 3 (15), 233−241.

■

CONCLUDING REMARKS We have shown that it is possible to deposit the materials necessary to form ZnO thin film electronics from 100 to 300 °C using spatial atomic layer deposition, with the likely lower limit lying between 100 and 150 °C for the precursors and partial pressures used. Under these conditions, this process should be compatible with plastic substrates and, therefore, useful for the fabrication of printed flexible electronics. We have also shown that very thin films of the water-soluble inhibitor, PVP k-30, can effectively inhibit growth over the range of SALD process conditions and materials evaluated. Both inhibition and real time growth rate were shown to benefit from short cycle times, making the patterned-byprinting methodology attractive to rapid manufacturing. The examined PVP films are comparable to those of the thinnest portions of our inkjet patterns, indicating SALD can be easily patterned using inkjet printing techniques. For a midrange temperature deposition of 200 °C, each of the four layers necessary for building a TFT can deposited in less than 10 min, 1521



Article

(20) Cho, W.; Sung, K.; An, K.-S.; Lee, S. S.; Chung, T.-M.; Kim, Y. J. Vac. Sci. Technol., A. 2003, 21 (4), 1366−1370. (21) Potts, S. E.; Dingemans, G.; Lachaud, C.; Kessels, W. M. M. J. Vac. Sci. Technol., A. 2012, 30 (2), 021505. (22) Lee, S. S.; Lee, E.-S.; Kim, S. H.; Lee, B. K.; Jeong, S. J.; Hwang, J. H.; Kim, C. G.; Chung, T.-M.; An, K.-S. Bull. Korean Chem. Soc. 2012, 33 (7), 2207−2212. (23) Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. Chem. Mater. 2004, 16, 639−645. (24) Min, Y.-S.; An, C. J.; Kim, S. K.; Song, J.; Hwang, C. S. Bull. Korean Chem. Soc. 2010, 31 (9), 2503−2508. (25) Wilson, C. A.; Grubbs, R. K.; George, S. M. Chem. Mater. 2005, 17, 5625−5634. (26) Paeng, K.; Ediger, M. D. Macromolecules 2011, 44, 7034−7042. (27) Bhattacharya, S.; Sharma, D. K.; Saurabh, S.; De, S.; Sain, A.; Nandi, A.; Chowdhury, A. J. Phys. Chem. B 2013, 117 (25), 7771− 7782. (28) Fitzpatrick, S.; McCabe, J. F.; Petts, C. R.; Booth, S. W. Int. J. Pharm. 2002, 246, 143−151. (29) Alam, M.; Green, M. J. Appl. Phys. 2003, 94 (5), 3403.

■

NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on February 7, 2014, with minor errors in the References section. The corrected version was published ASAP on February 14, 2014.

1522


Selective Area Spatial Atomic Layer Deposition of ZnO, Al2O3, and

Recommend Documents