Room temperature evaporation of titanium isopropoxide [Ti[OCH(CH3)2]4, TTIP] precursor was performed using ultrasonic atomization for TiO2 atomic layer deposition (ALD). Quartz crystal microbalance data show comparable results between room temperature TTIP ultrasonic atomization and conventional thermal evaporation. The TiO2 ALD saturation window is established for room temperature atomized TTIP exposure time and reactor temperatures. Room temperature atomized TTIP grown TiO2 films show smooth surface morphology before/after the annealing treatment. Two-dimensional TiO2 film thickness mappings on a 150 mm diameter Si(100) wafer were performed by spectroscopic ellipsometry. The thickness variation of TiO2 films by the room temperature atomized TTIP is less uniform than that of TiO2 films by thermally evaporated TTIP, probably due to the incomplete evaporation of the TTIP liquid droplets, which is more difficult to transport than its vapor.

The atomic layer deposition (ALD) technique has been developed over several decades to fabricate uniform and conformal thin films with very precise control of the thickness and the stoichiometry.1–5 ALD has been used in a wide range of applications such as microelectronics, catalysts, batteries, optics, medical devices, and energy.6–18 Key to a successful ALD process is the supply of chemical precursor vapor in a timely and uniform manner. For low volatility chemical precursors, a common strategy to increase precursor vapor pressure for ALD is by thermally evaporating the precursor. As described by the Clausius–Clapeyron equation, the vapor pressure of a pure substance exponentially increases with increasing temperature. However, there are two temperature constraints which affect the selection of the precursor evaporation temperature. First, precursor container temperature needs to be lower than the reaction temperature to prevent precursor condensation. Second, the precursor container temperature should be lower than the decomposition temperature. These two constraints limit the application of those compounds, which will decompose before reaching a reasonable vapor pressure for ALD. It also makes it challenging to use low volatility chemicals for low-temperature or room temperature ALD processes. If the chemical precursors can be supplied at low or room temperature, then it will permit the use of a wide range of organometallic compounds that are currently considered inapplicable to ALD due to their low decomposition temperature or low volatility.

The ultrasonic technology has been adapted to many different fields such as industrial, medical, and household applications for the ultrasonic cleaning system, ultrasonic imaging, and the design of room temperature humidifiers.19–25 Ultrasonic spray pyrolysis and direct-liquid chemical vapor deposition (CVD) have been used to apply coatings from liquid chemical precursors because of its simple configurations and low thermal budget on the precursors.26–29 Lowering the precursors’ temperature can preserve precursors for a much longer period and avoid the thermal decomposition associated with high temperatures.30–33 Ultrasonic atomization is particularly attractive to supply low vapor pressure precursors because the precursor evaporation occurs through ultrasonic vibration of the module. Like the household humidifier, ultrasonic atomization generates a mist consisting of saturated vapor and microsized droplets. The microsized droplets continuously evaporate when the mist is delivered to the substrates by a carrier gas. The size distribution of deposited droplets is inversely proportional to the ultrasonic module frequency to the power of 2/3.34,35 For example, an ultrasonic module with megahertz range frequency produces submicrometers/micrometer-size diameter droplets.35 

In this paper, we used room temperature ultrasonic atomization to supply titanium isopropoxide for TiO2 ALD as a proof of concept. The TiO2 ALD using alternating exposure of TTIP and H2O is among the most studied and well-understood ALD processes; it usually uses a precursor temperature of 60–100 °C.36–39 The objective of this work is to compare the TiO2 ALD process using different TTIP delivery techniques: ultrasonic atomizer and conventional thermal evaporation. The growth per cycle (GPC) is comparable to the conventional thermal TiO2 ALD process with low surface roughness. Although the TiO2 film thickness is less uniform than that generated by thermal evaporation, further tuning the atomizer power and carrier gas flow rate may help to improve the uniformity.

GEMStar-6 ALD system (Arradiance, Inc.) with a cross-flow configuration was used to perform TiO2 ALD experiments. A home-made atomizer reservoir (see the supplementary material)50 is attached to the precursor manifold for TTIP evaporation.40 The schematic of the ultrasonic atomizer is shown in Fig. 1(a). The bottom part of the reservoir is made of pyrex glass to observe the operation of the ultrasonic transducer module located inside the reservoir as shown in the supplementary material.50 Power to the ultrasonic module was supplied by electrical wires through electrical feedthrough during the ALD process. The gas manifold temperature gradually increases from 40 to 120 °C.

FIG. 1.

Schematic of the ultrasonic atomizer system (a) and ultrasonic atomization of TTIP (b).

FIG. 1.

Schematic of the ultrasonic atomizer system (a) and ultrasonic atomization of TTIP (b).

Close modal

TiO2 ALD with thermally evaporated TTIP used a timing sequence 1-10-0.75-12 s, where 1 and 0.75 s are the exposure time for TTIP and H2O, respectively, 10 and 12 s are used as the purge time between exposures, respectively. TiO2 ALD using room temperature atomized TTIP employed similar operating conditions. Except in the atomized ALD, the bubbler was kept at room temperature and carrier gas was used to back-filled the precursor container for additional 10 s. The timing sequence is (t1-10)-10-0.75-12. The parenthesis includes the two steps depicted in Fig. 1(b). t1 represents the TTIP actual exposure time and 10 s are used to backfill the TTIP container with the carrier gas N2.

To observe the detailed TiO2 growth characteristics, an in situ QCM (quartz crystal microbalance) system (INFICON) was installed and monitored in real-time during the ALD growth process with/without atomizer. TiO2 films were grown on Si(100) wafers with native SiO2 on the surface. Spectroscopic ellipsometry (SE) thickness measurements were performed using an alpha-SE ellipsometer (J. A. Woollam) to confirm the thickness and GPC of TiO2. The thickness values of native SiO2 were measured before TiO2 growth at each point to avoid errors for calculating TiO2 thickness. To compare the thickness uniformity of TiO2 produced with atomizer evaporated TTIP to that of TiO2 with thermally evaporated TTIP, TiO2 films were grown using the conventional thermal ALD method with a thermally heated TTIP reservoir at 70 °C for 100 cycles. TTIP exposure was fixed to 1 s while H2O exposure was maintained to 0.75 s. Two-dimensional TiO2 thickness mapping was performed by measuring up to 70 different spots on 150 mm Si wafers after TiO2 growth. After measuring points, the data were converted into a two-dimensional matrix for the thickness mapping. X-ray diffraction (XRD) of TiO2 thin films produced with the atomized and conventional TTIP were performed before/after annealing in Ar for 90 min using MiniFlex powder diffraction equipment (Rigaku) with Cu kα x-ray gun. The surface morphology of atomized TTIP grown TiO2 films for 500 cycles at 180 °C was observed by Brucker Dimension Icon atomic force microscope (AFM) before/after the annealing treatment at 400 °C for 90 min.

Typical TiO2 ALD processes deliver TTIP at 60–100 °C, which corresponds to a saturated vapor pressure between 0.59 and 8.4 Torr as shown in Fig. 2(a).41 The saturated vapor pressure of TTIP at 25 °C is as low as 0.02 Torr. As shown in Fig. 2(b), when TTIP is kept at 25 °C, no obvious mass uptake was observed by using in situ QCM experiments during TiO2 ALD at 140 °C. With the ultrasonic atomizer, the linear growth of TiO2 was observed with a GPC of ca. 9.0 ng/cm2 per ALD cycle or 0.21 Å/cycle. It is feasible for ultrasonic atomization to provide TTIP precursors at room temperature for TiO2 ALD. The ultrasonic atomizer generates TTIP droplets (mist visible in the supplementary material50), which is likely to saturate the gas phase much faster than a planer interface with a bulk liquid. However, care must be taken to prevent precursor condensation and droplets from reaching the reactor chamber, as nonuniform deposits could occur. In our experiments, the partial pressure of TTIP during each dose is less than 2 Torr, much less than the saturation pressure of TTIP at the deposition temperature of a minimum 125 °C. Therefore, no condensation is expected.

FIG. 2.

Saturated vapor pressure of TTIP as a function of temperature (a) and mass uptake during TiO2 ALD at 140 °C when TTIP container was kept at room temperature.

FIG. 2.

Saturated vapor pressure of TTIP as a function of temperature (a) and mass uptake during TiO2 ALD at 140 °C when TTIP container was kept at room temperature.

Close modal

GPC as a function of atomized TTIP exposure duration is shown in Fig. 3(a). The thickness of TiO2 films on Si(100) wafer was obtained using SE. One second exposure of atomized TTIP gives a GPC of 0.14 Å/cycle at 180 °C. After increasing exposure duration to 2 s, GPC increased and stayed at about 0.23 Å/cycle up to 5 s of exposure. Therefore, the self-limiting growth of the TiO2 using atomized TTIP is confirmed. It indicates the self-limiting growth of the TiO2 layer, which is the most unique character for the ALD process compared to the CVD process.42 That is, unlike ultrasonic spray pyrolysis, atomized TTIP vapor can be adsorbed on the substrate surface as a mono/submono layer instead of forming micrometer-size droplets. In Fig. 3(b), thickness and GPC as a function of the number of cycles are shown. TTIP exposure was maintained as 5 s with the atomizer. GPC shows a gradual increase up to 200 cycles and remains almost at the same level. Due to the limited density of the hydroxyl group on the surface, the total number of adsorbed TTIP can be affected at the initial stage of TiO2 growth.43 

FIG. 3.

GPC of TiO2 as a function of atomizer exposure time (a) and TiO2 thickness and GPC as a function of the number of ALD cycles at TALD = 180 °C (b).

FIG. 3.

GPC of TiO2 as a function of atomizer exposure time (a) and TiO2 thickness and GPC as a function of the number of ALD cycles at TALD = 180 °C (b).

Close modal

Detailed QCM data were plotted for TiO2 films with atomized TTIP at the reactor temperature of 125, 140, and 180 °C in Figs. 4(a)4(c), respectively. The QCM results are similar to our previous studies of TiO2 ALD using thermally evaporated TTIP.44 The growth of TiO2 is linear. The minor difference between each cycle may be caused by the thermal drift of the QCM crystal. In Fig. 4(a), after TTIP exposure, the mass gain was observed because of TTIP chemisorption on the substrate. Pressurizing [step 1 in Fig. 1(b)] occurs after step 3 so that the change of pressure in the ALD reactor can be noted from the triangular peak in each cycle. However, it is a false reading from QCM due to the pressure change, which is brought back to the same level after finishing this process. Subsequent H2O exposure results in mass loss due to the loss of isopropoxide ligands and the replenishment of hydroxyl groups. Interestingly, at different reactor temperatures, similar mass change patents were observed by the in situ QCM. In Fig. 4, the mass gain/loss ratios are 2.2, 2.2, and 2.1 at 125, 140, and 180 °C, respectively. The surface chemistry of TiO2 ALD was studied using in situ infrared and Raman spectroscopy.36,45 In the first half cycle, TTIP reacts with two surface hydroxyl groups forming surface species Ti[OCH(CH3)2]2 (molecular weight: 165.9 g/mol). During the H2O exposure, the surface species loses –OCH(CH3)2 ligands and forms Ti(OH)2 (molecular weight: 81.9 g/mol), representing a mass loss of 84 g/mol. Hence, the mass gain/loss ratio in one TiO2 ALD cycle is ca. 2.0. It suggests that the surface chemistry of TiO2 ALD at these temperatures are similar.

FIG. 4.

Detailed QCM signal from 14 to 17 cycles at different reactor temperatures of 125 (a), 140 (b), and 180 °C (c).

FIG. 4.

Detailed QCM signal from 14 to 17 cycles at different reactor temperatures of 125 (a), 140 (b), and 180 °C (c).

Close modal

To identify the growth characteristics as a function of the reactor temperature, GPC values from SE measurements were obtained as functions of reactor temperature with and without atomizer in Fig. 5(a). As was observed previously, the growth rates without atomizer (still N2 pressurized) were almost negligible throughout the reactor temperature up to 200 °C. With the atomizer, the GPCs of TiO2 ALD were almost maintained as 0.22–0.23 Å/cycle at the reactor temperature from 125 to 180 °C and 0.31 Å/cycle at 200 °C. The GPCs in this temperature range are within the ALD windows and consistent with the literature.42,46

FIG. 5.

GPC of TiO2 films by atomized TTIP and H2O in ALD as a function of the reactor temperature (a) and XRD patterns of TiO2 thin films (b): as-deposited TiO2 from thermally evaporated TTIP (1), after annealing (1) at 400 °C (2), as-deposited TiO2 from atomized TTIP (3), after annealing (3) at 400 °C (4).

FIG. 5.

GPC of TiO2 films by atomized TTIP and H2O in ALD as a function of the reactor temperature (a) and XRD patterns of TiO2 thin films (b): as-deposited TiO2 from thermally evaporated TTIP (1), after annealing (1) at 400 °C (2), as-deposited TiO2 from atomized TTIP (3), after annealing (3) at 400 °C (4).

Close modal

In Fig. 5(b), XRD patterns of atomized TTIP and thermally evaporated TTIP grown TiO2 films produced with 500 cycles at 180 °C were compared before/after the annealing treatment at 400 °C under Ar for 90 min. Anatase (101) peaks appear after the annealing treatment in both cases. It indicates that the as-deposited TiO2 films at 180 °C were mostly in the amorphous phase.

AFM morphology measurements with root mean squared (RMS) roughness of atomized TTIP grown TiO2 films produced with 500 cycles at 180 °C (thickness: 158 Å) were performed before/after the annealing treatment under Ar atmosphere at 400 °C for 90 min as shown in Figs. 6(a) and 6(b), respectively. The overall size of grains became larger after the annealing treatment because of the coalescence of TiO2 grains. RMS roughness values of TiO2 films before/after the annealing treatment were 1.0 and 3.4 Å, respectively. This roughness is similar to what is reported on the conventional TiO2 ALD47,48 and smaller than the TiO2 generated from pulsed direct-liquid injection ALD.49 

FIG. 6.

AFM images of atomized TTIP grown TiO2 thin films for 500 cycles (thickness = 158 Å) before (a) and after (b) annealing at 400 °C in Ar for 90 min. Each image is 1 × 1 μm2.

FIG. 6.

AFM images of atomized TTIP grown TiO2 thin films for 500 cycles (thickness = 158 Å) before (a) and after (b) annealing at 400 °C in Ar for 90 min. Each image is 1 × 1 μm2.

Close modal

100 cycles of TiO2 ALD using atomized TTIP and thermally evaporated TTIP, respectively, were carried out on 150 mm diameter Si(100) wafers at 180 °C. The uniformity of the TiO2 thin films was investigated by using spectroscopy ellipsometry and the results are shown in Fig. 7. Maps show the thickness distributions from thicker to thinner thickness as those are indicated in the scale bars to the right of each plot. The TTIP and H2O inlets were to the left of the Si(100) wafer and labeled in Fig. 7. The exhaust for evacuation was located in the center-right to the wafer. The maximum and minimum thicknesses for the atomized TTIP grown TiO2 in Fig. 7(a) were 34.4 and 24.6 Å, respectively, and were 38.7 and 34.4 Å for the thermally evaporated TTIP grown TiO2 in Fig. 7(b), respectively. The uniformity of TiO2 using thermally evaporated TTIP is better than that using atomized TTIP vapor. In both cases, the thickness values tend to decrease near evacuation exhaust. With the atomized TTIP, the maximum thickness of TiO2 was less than what was obtained from the thermally evaporated TTIP. It excluded the possibility of CVD due to the condensation of TTIP from the atomized TTIP. For the TiO2 films produced using the atomized TTIP, the thinner downstream TiO2 thickness suggested that it likely suffered from a lack of TTIP precursor. While further experiments are needed to validate this point, we suggest that the uniformity of TiO2 produced using the atomized TTIP could be potentially improved by adjusting TTIP exposure time, ultrasonic atomizer input energy, and carrier gas flow rate. In addition, the time for the TTIP atomization could probably be shortened by adding an additional gas line for the pressurization. With that, the pressurization of the TTIP container could occur simultaneously during the H2O exposure and purge, saving 10 s in each TiO2 ALD cycle.

FIG. 7.

Two-dimensional thickness distribution of TiO2 on a 150 mm diameter Si(100) wafer. TiO2 ALD was carried out at 140 °C with atomized TTIP at room temperature with time sequence (5-10)-10-0.75-12 s (a) and thermally evaporated TTIP at 70 °C with time sequence 1-10-0.75-12 s (b).

FIG. 7.

Two-dimensional thickness distribution of TiO2 on a 150 mm diameter Si(100) wafer. TiO2 ALD was carried out at 140 °C with atomized TTIP at room temperature with time sequence (5-10)-10-0.75-12 s (a) and thermally evaporated TTIP at 70 °C with time sequence 1-10-0.75-12 s (b).

Close modal

Room temperature ultrasonic atomization was used to supply TTIP precursors for TiO2 ALD. TiO2 GPCs using atomized TTIP were comparable to those obtained from thermally evaporated TTIP. With room temperature atomized TTIP, a saturation growth of TiO2 was observed in the temperature window of 125–200 °C. Atomized TTIP grown TiO2 films of 15.8 nm was amorphous and became anatase phase after the annealing treatment. The surface morphology of the films remained smooth before/after the annealing treatment. Two-dimensional SE TiO2 thickness mapping on 150 mm Si(100) wafer shows that the TiO2 films generated from atomized TTIP vapor are less uniform than the TiO2 films produced from the 70 °C thermally evaporated TTIP. It is most likely caused by a lack of sufficient TTIP precursor in the downstream. It could be improved by tuning the ALD operation conditions and/or the ALD reactor geometry.

TiO2 ALD using TTIP and H2O was used as the probing ALD process in this work, as its surface chemistry and reaction engineering have been well-studied. While it is not necessarily important to evaporate TTIP at room temperature for TiO2 ALD as TTIP is considered thermally stable in the commonly used 60–100 °C heating range, the finding in this study is useful for the application of chemical precursors that are not volatile and thermally stable. By thermal evaporation, these precursors could have decomposed before reaching a sufficient vapor pressure for ALD. Using room temperature ultrasonic atomization, new ALD processes could be developed using low volatility and unstable precursors.

Research at UAH has been supported by the U.S. Department of Defense (DOD), by the U.S. National Science Foundation (NSF) (Grant No. CBET-1511820), and by the National Aeronautics and Space Administration (NASA) Established Program to Stimulate Competitive Research (EPSCoR) Research Infrastructure Development Award (Grant No. 80NSSC19M0051).

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See supplementary material at https://doi.org/10.1116/6.0000464 for the video of the ultrasonic atomized TTIP evaporation.

Supplementary Material