Initiated Chemical Vapor Deposition of Poly(Ethylhexyl Acrylate) Films

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Materials and Interfaces

Initiated Chemical Vapor Deposition of Poly(Ethylhexyl Acrylate) Films in Large Scale Batch Reactor Kurtulu# Y#lmaz, Huseyin Sakalak, Mehmet Gürsoy, and Mustafa Karaman Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02213 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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Industrial & Engineering Chemistry Research

Initiated Chemical Vapor Deposition of Poly(Ethylhexyl Acrylate) Films in Large Scale Batch Reactor Kurtuluş Yılmaza, Hüseyin Şakalakb, Mehmet Gürsoya, Mustafa Karamana,*

a Chemical b

Engineering Department, Konya Technical University, Konya, 42030, Turkey Advanced Materials and Nanotechnology Department, Selcuk University, 42075, Turkey

*Corresponding

Author:

E-mail: [email protected] Phone: +(90) 332 223 2108 Fax:

+(90) 332 241 0635

Postal address: Department of Chemical Engineering, Konya Technical University, Campus, Konya 42030, Turkey

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The present study demonstrates the successful deposition of poly(ethylhexyl acrylate) thin films in a large-scale closed-batch initiated chemical vapor deposition (iCVD) system. A horizontal cylindrical stainless-steel vacuum tank, which is highly utilized in industrial vacuum applications, was used as iCVD reactor. The effects of substrate temperature, precursor ratio, and pressure on the deposition rates were studied, and the results showed that a deposition rate of 315 nm/min can be achieved in a single run at a reactor pressure of 600 mtorr. At a lower chamber pressure of 400 mtorr, deposition rate decreases, whereas film uniformity increases. By carrying out depositions at successive cycles, thicker films could be obtained, without the need for extensive monomer consumption. The yield percentage was found to be 3.5 for the films deposited in closed-batch system at 400 mtorr, which is 35-fold larger than that of the classical iCVD flow system.

Keywords: iCVD, functional polymer, closed batch, uniformity


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1. Introduction Thin films of functional polymers offer the advantage of designing surface of a material without altering its bulk properties. Antifouling coatings 1, encapsulation of submicrometer particles 2, stimuli responsive thin films 3, superhydrophobic surfaces 4, antibacterial coatings 5 and membrane separations 6 can be given as examples of the functional thin polymer film applications. Functional polymer thin fabrication techniques can be simply categorized into two main parts: liquid-phase techniques and gas-phase techniques. Although the application of liquid-phase techniques for fabrication of thin films is relatively simple and usually does not need any special equipment; the usage of solvents in the application of liquid-phase techniques could cause some damages on the substrate surfaces 7. Furthermore, it is difficult to obtain uniform and conformal thin film on the substrate surfaces due to the surface tension of liquids. On the other hand, as a gas-phase technique, solvent-free chemical vapor deposition (CVD) is advantageous over liquid-phase techniques 8. CVD is a versatile technique convenient for the manufacture of wide range of polymeric thin films. The versatility of CVD offers the main advantage over the control of film structure, morphology, and deposition rates, which is of the many reasons that CVD polymer deposition may be selected as a commercial coating process. By careful design of chamber geometry, selection of activation source, and proper tuning of deposition parameters, any desired property can be put forward by CVD; which can be high or low deposition rates, high structural control, conformality, uniformity, and so on. The deposition of functional polymers on various surfaces can be achieved by a low-temperature CVD processes, such as plasma enhanced CVD (PECVD) 9, hot-filament CVD (HFCVD) 10, oxidative CVD (o-CVD) 11,

and so on. Initiated CVD (iCVD) 12 falls within the borders of HFCVD, in that both

techniques utilize heated-filaments to initiate film-forming reactions. Initially, iCVD was developed to overcome the problems associated by PECVD, such as extensive monomer



fragmentation and undesired ion bombardment on delicate substrate surfaces 13, 14. Different from classical HFCVD, vapor-phase molecules called “initiator” are fed to the reaction chamber together with the monomer vapors in iCVD. Such initiator molecules have thermally liable chemical bonds, which are fragmented into radicals upon contact with the heated filament surface. The radicals generated in the vapor phase then diffuse through the substrate surface, which is cooled intentionally to allow for monomer adsorption. With the help of radicals originated from initiator molecules, a classical free radical polymerization occurs on the substrate surface, which results in the formation of a polymer thin film 15. In this way, highly conformal, uniform and defect-free thin polymeric films can be deposited on virtually any substrates; since substrates remain free from heat, plasma and solvents. In literature, many different types of polymeric thin films having intended functionalities, such as, hydrophobic 16 , hydrogel 17, functionalizable 18, zwitterionic 19, nanoadhesive 20, antimicrobial 21, etc. have been deposited by iCVD on various surfaces. Classical laboratory scale iCVD reactors resemble flow reactors, namely precursor vapors constantly flow into the reactor, at the same time unreacted vapors and vapor-phase byproducts are continuously exhausted by a vacuum pump. The operation of batch reactors (Figure 1-a) is intrinsically unsteady, which are well suited for the production of valuable products, whereas flow reactors are run under near steady-state conditions and they are typical for large-scale productions. The advantages of the batch reactor lie with its versatility. A single reactor can carry out a sequence of different operations without the need to break containment. Although classical iCVD reactors are flow reactors; the iCVD process can be considered to be batch or semi-batch because of the requirement of unloading the coatedsubstrates and loading new substrates between different runs. After each run, flow of precursors is ceased, vacuum is broken, and chamber is opened to atmosphere to carry out substrate exchange procedure; which rule out the advantages of a flow reactor. However, it


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was shown that more chemicals were used in classical flow iCVD process as compared to batch iCVD process to produce the same amount of material 22. This difference is mainly due to high levels of monomer consumption during the monomer flow rate calibration in classical flow iCVD process. High level of monomer consumption in classical flow iCVD process, which is one of the largest operating expenses in iCVD thin film fabrication, poses a major disadvantage for classical flow iCVD. In this study, it is aimed to deposit poly(ethylhexyl acrylate) (PEHA) films using iCVD process in a large-scale vessel, which is operated under closed-batch conditions. PEHA is an important member of acrylates family, which is used extensively in coating applications due to its suitably low glass transition temperature. It can be used in combination with some other acrylates, such as PMMA and PAA, to produce pressure sensitive adhesives. A horizontal cylindrical stainless-steel vacuum tank was used as iCVD reactor (Figure 1-c), which is highly utilized in industrial vacuum applications because of the requirement of easy access for loading and unloading, easy cleaning, and lower manufacturing costs as compared with that of rectangular geometry chambers. In such a large-scale system, the consumption of monomer would be very if the system was operated in flow mode considering the high fillingup time to reach the deposition pressure during both flow-adjustment and deposition periods. The advantages of operating under closed-batch mode is outlined; by comparing the costs associated with monomer and energy consumptions between the laboratory scale flow reactor and large-scale closed-batch reactor.



Figure 1. (a) Classical batch reactor, (b) Classical laboratory scale iCVD reactor, (c) Horizontal cylindrical reactor used in this study 2. Experimental 2.1.

Materials

Silicon wafer (100, p-type) was used as a substrate. The monomer 2-ethylhexylacrylate (EHA, 98%) and the initiator di-tert butyl peroxide (TBPO, 98%) were purchased from Sigma– Aldrich. They were used without any further purification and modification. 2.2.

iCVD of PEHA in large-scale batch-type and classical flow-type reactors

The schematic representation of large-scale horizontal cylindrical vacuum reactor used in this study is given in Figure 1c. The reactor is made of stainless-steel; having dimensions of 75 cm width and 50 cm diameter. The reactor has stainless-steel doors on both ends, which allow for front and back-loading of the substrates. The reactor has a small quartz window at the top allowing visual access and in‐situ film thickness monitoring via interferometry during the deposition. Chamber is equipped with vacuum ports (KF 25) for mounting the pressure gauge, electrical feedthroughs, and vacuum pump connections. Vacuum was achieved using an oil-sealed rotary vane vacuum pump (2XZ-15C, EVP). Substrates were placed onto a cooled stage connected to a recirculating chiller/heater (Lab. Companion RW-0525G, South Korea) to control the substrate temperature. It was set at three different temperatures (15, 25 and 30 °C). In order to measure the real substrate temperature, a K-type thermocouple was directly attached to substrate surface. A nichrome (Ni–Cr 80/20 wt.%, 0,3 mm diameter) filament array was located 2.5 cm above the substrate to provide thermal energy for initiating the polymerization. Photographs of the large-scale batch-type iCVD reactor used in this study is given in Figure S1 and Figure S2. The whole reactor body was covered by heating wires, and temperature of the reactor body was adjusted using a PID temperature controller. The filament temperature and reactor wall temperature were maintained constant at 200 and 36 °C,


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respectively. Depositions were carried out using three different reactor pressures (200, 400 and 600 mtorr), which was measured by a capacitance type pressure gauge (MKS, Baratron), and two different initiator/monomer (I/M) ratios (1/1 and 1/2) while keeping all other parameters constant. At the start-up, the iCVD chamber was first evacuated down to ultimate system pressure of 5 mtorr; then the system was isolated from the vacuum pump. The system was then filled with the precursors at the beginning by successive feeding of first monomer and then the initiator molecules until the desired deposition pressure was reached. I/M ratios were adjusted by monitoring the partial pressures of precursors during the filling-up period. Schematic illustration of large-scale batch-type iCVD protocol is given in Figure 2. For comparison purposes, PEHA thin films were also deposited in a classical flow-type iCVD system, details of which is given elsewhere 23. The rectangular vacuum chamber of the system is made of stainless-steel, having dimensions of 20 cm in width, 30 cm in length, and 5 cm in height. Substrate was placed on the back-side cooled reactor floor connected to a recirculating chiller/heater (Thermo Neslab). A tungsten (99.95%, 0.375 mm diameter) filament array was located 2 cm above the substrate surface. Vacuum was achieved using Edwards RV-12 vacuum pump. Monomer EHA and initiator TBPO were vaporized in separate stainless-steel jars, then they were metered into the reactor using needle valves. Monomer jar and manifold pipeline were kept constant at 70 °C and 85 °C, respectively, with the help of PID temperature controllers. The deposition conditions applied were as follows: 600 mtorr reactor pressure, 280 °C filament temperature, 25 °C substrate temperature, 0.9 sccm EHA flowrate, and 0.6 sccm TBPO flowrate.



Figure 2. (a)-(c)Schematic illustration of large-scale batch-type iCVD protocol

2.3.

Characterizations

Fourier transform infrared spectroscopy (FTIR) was employed to characterize the chemical structure of as-deposited PEHA thin films using a Bruker Vertex 70 FTIR spectrophotometer. The spectra were obtained between 400 and 4000 cm-1 wavenumbers at a resolution of 4 cm-1. Ex-situ film thickness measurement was carried out using Avantes Reflectometer (Avaspec-ULS2048L spectrometer with AvaLight-DH-S BAL light source) to verify the interferometric thickness measurements of PEHA thin films. The water contact angles of as-deposited PEHA thin films were measured by microliter sessile drop contact angle analysis with a video capture system (Kruss Easy Drop) using 2.0 μL de-ionised water.


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3. Results and discussion 3.1.

Chemical Structure of PEHA thin films

The chemical structure of as-deposited PEHA films was revealed using FTIR. Figure 3 shows the FTIR spectrum of iCVD PEHA thin film in comparison with the spectrum of EHA monomer. It is clearly seen that, FTIR bands between polymer and monomer spectra match well, indicating the large-scale closed-batch iCVD’s ability to produce clean and well-defined polymers. C=C stretching peak, which is visible in the spectrum of EHA monomer at 1635 cm-1, is absent in the polymer spectrum, indicating that polymerization proceeded through unsaturated C=C bond. The as-deposited PEHA film is free from incorporated monomer molecules, due to the proper adjustment of deposition conditions in the large batch system used in this study, which don’t allow condensation of monomer on the substrate surface. The observation of narrow, sharp and strong absorption bands in the polymer spectrum confirms that the processing method used in this study does not alter the chemical functionality of the monomer; typical for iCVD polymers deposited in classical laboratory scale flow reactors 2428.

Especially, the strong vibrations observed at 1724 cm-1 and between 2850-3000 cm-1,

which belong to C=O and CH2 stretchings, respectively, prove the retention of carbonyl and pendant ethylhexyl functionalities.



Figure 3. FTIR spectra of monomer EHA and iCVD polymer PEHA (600 mTorr reactor pressure, 200 oC filament temperature and 45 oC substrate temperature)

3.2.

Deposition rates of PEHA thin films

Effect of substrate temperature on deposition rates at different initiator to monomer (I/M) ratios is given in Figure 4. The deposition rates are higher at higher I/M ratio at substrate temperatures of 37 and 45 oC. It is important to note that the substrate temperatures reported in Fig. 3 were measured by attaching a thermocouple on the substrate surface. There is nearly 20 oC difference between the measured temperatures and chiller set temperatures. Such a large difference in temperatures originates from the high heat load on the substrate surface due to the radiation from the hot filaments. The high deposition rates at high initiator concentration implies that the kinetics of the deposition is limited by the decomposition of the initiator and thus the radical formation. For both studied I/M ratios, the deposition rate increases when substrate temperature increases from 37 to 45 oC. In literature, it has been


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shown for iCVD that the rate of monomer adsorption, which is related to the deposition rate, is lower at higher substrate temperatures. In contrast, reaction kinetics is faster at higher substrate temperature because of the Arrhenius dependence of the rate coefficients 29-33. Therefore, the observed enhancement of deposition rate with substrate temperature indicates that iCVD of PEHA is not adsorption-limited but mainly reaction kinetics-limited. As the substrate temperature is further increased to 49 oC, the deposition rate decrease, due to the fact that at such a high substrate temperature the saturation vapor pressure of monomer (PM,sat) is quite high; causing a decrease in rate of adsorption, which makes the deposition mechanism adsorption-limited. Figure 5 shows the dependence of deposition rates on pressure. As the pressure increase from 200 to 600 mtorr, there is nearly a linear increase in deposition rates. After 600 mtorr, at which highest deposition rates were observed, no depositions could be carried out because of the extensive condensation of monomer on the substrate surface, as well as on the all other inner surfaces of the vacuum chamber. The saturation vapor pressure of EHA monomer under a stage temperature of 30oC is around 300 mtorr, which makes the saturation ratio (the ratio of monomer’s partial pressure to its saturation vapor pressure) close to unity under a deposition pressure of 600 mtorr. Hence monomer condensation is expected at higher deposition pressures due to the fact that the saturation ratio becomes higher than unity. The highest deposition rate (315 nm/min) was obtained at a chamber pressure of 600 mtorr and at a substrate temperature of 45 oC, when there are equal molar amounts of monomer and initiator molecules inside the vacuum reactor.



Figure 4. Central Deposition rate in a single run at different PM/PI at different substrate temperatures (Reactor pressure of 600 mtorr) 350

300

Deposition Rate (nm/min)

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250

200 150

100

50

0 200

250

300

350

400

450

500

550

600

650

Pressure (mtorr)

Figure 5. Central Deposition rate in a single run at different pressures (Ts=45oC, M/I=1/1)


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450 400 350

Deposited Thickness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300 250 200 150 100 50 0 1

2

3

4

5

Number of Cycle

Figure 6. Change of central film thickness with cycled runs (600 mTorr reactor pressure, 200 oC

filament temperature and 45 oC substrate temperature)

Classical flow-type iCVD reactor runs under steady-state conditions, where constant deposition rates are observed throughout the deposition process. The deposition can be stopped when the desired film thickness is reached by ceasing the flow of precursors and turning off the filament power. The batch-type system used in this study works under dynamic conditions, where the rate of deposition is time dependent. During a deposition cycle, the deposition rate decreases because of the depletion of precursors in time. The reported deposition rate values in Figures 4 and 5, are the average deposition rates, which were found by dividing the film thickness by total deposition time. For the batch-type iCVD system used in this study; it was possible to obtain thicker films without breaking the vacuum, by running cycled depositions. After the first run; the throttle valve between the chamber and pump was opened to evacuate the system, then the valve was set to close, and flow of precursors were allowed to fill the system up to desired pressure. Next, the filament power was turned-on to start the deposition of second run. Figure 6 shows the average deposition rates achieved in each cycle for a 5-cycle deposition experiment. After the first cycle, the



average deposition rates were observed to be slightly higher (375 nm), most probably because of active sites on the surface of as-deposited PEHA film formed after the first cycle. After the end of the last cycle, a PEHA film with a thickness of 1.8 m was obtained. It is possible to obtain much thicker films by running more successive deposition cycles.

3.3.

Large scale uniformity

In order to evaluate the large are thickness and contact angle uniformities of the asdeposited PEHA films, 200-nm-thick films (based on in-situ interferometer measurement) were deposited on 2 400 cm2 area at reactor pressures of 400 mtorr and 600 mtorr. The coating zone was divided into three different equal areas, each of which is 20 cm in width by 40 cm in length. Ten pieces of silicon wafers (1cm x 1cm) were placed on these areas. Both the thickness and contact angle values of PEHA thin films were measured from these silicon wafers. The obtained average thicknesses and contact angle values were written regarding areas in Figure 7. As seen in Figure 7 a and c, more uniform film thickness (~±5%) was observed at reactor pressure of 400 mtorr as compared to that (~±11%) obtained at reactor pressure of 600 mtorr. The reason of this observation could be attributed to that spatial concentration gradients are reduced at low deposition pressure due to rapid gas phase diffusion, which results in improved uniformity 34. Although slight differences were observed in the contact angle results of as-deposited PEHA thin films on different points at both reactor pressures (Figure 7 b and d), the values are considered to be the same within experimental error. This observed similar PEHA thin film wettability behavior from different points indicates that chemically homogenous coating.


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Figure 7. Large area thickness (a-c), and water contact angle (b-d) uniformities of asdeposited films at different pressures

3.4.

Cost and yield analysis

The cost and reaction yield analysis were done to figure out the economic benefits of using a batch-type reactor in iCVD rather than a classical flow reactor. In order to calculate reaction yield, thin film weight was determined by multiplying the obtained thin film volume by density of polymer. Then, the reaction yield was calculated as the thin film weight divided by the amount of monomer consumed during deposition process. Table 1 shows the comparison between a classical iCVD and batch type iCVD. The major cost items were monomer price and electricity price required to run the vacuum pump and power the filament



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arrays. The electricity consumption of large-scale system is higher because of the larger pump and larger filament array as compared to the small reactor. The consumption values for the classical iCVD reactor were obtained by mimicking the PEHA deposition conditions in a laboratory scale iCVD system. In large batch system, the monomer consumption is much higher at higher deposition pressure, because of the requirement to have more monomer vapor molecules to fill the system up to desired pressure. In classical flow system; the monomer is not only consumed during the deposition, but also consumed during the flowrate calibration procedure. Therefore, the monomer consumption value in the flow system is comparable to that of large system, even if it’s almost 50-fold lower volume. The total coating cost was calculated by choosing a 100 nm-thick film as a basis and dividing the cost per 100 nm film by the deposition area. From the calculated values, it is apparent that the total cost is much lower in a large-scale batch system, and the cost is even lowered when the deposition is carried out at low pressures. The yield percentage is 3.5 for the batch system run at 400 mtorr, which is 35-fold larger than that of the small-scale flow system.

Table 1. The cost and yield analysis comparison between classical iCVD and large-scale batch-type iCVD Laboratory scale

Large scale batch

Large scale batch

continuous iCVD

iCVD (400 mtorr)

iCVD (600 mtorr)

Consumed Monomer (g/100 nm)

0.89

0.62

2.04

Power delivered to filament array (watts)

28.70

121.66

121.66

450

1500

1500

1.0235

0.0328

0.1077

0.10

3.50

1.06

Pumping Power (watts) Total cost (＄/100 nm.cm2. 10-3) Yield (%)

*dPolymer = 0.903 g/mL, Monomer cost= 79.05 ＄/L, Electricity unit cost=0,075 ＄/ kWh


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4. Conclusions Based on results of this study, it was shown that the versatility of iCVD process and the inherent advantages of batch operation mode can be combined to enable rapid (315 nm/min), solvent-free, environmentally friendly, and uniform large scale (2 400 cm2) deposition of PEHA thin films. As compared to classical iCVD flow reactor, reaction yield was improved by up to 35-fold with more than 30 times lower production cost in large scale batch-type iCVD system. Batch operation approach in iCVD reduces energy consumption and usage of chemicals per the amount of produced thin film, which can help to minimize environmental footprint of iCVD deposition process. Furthermore, batch-type iCVD process intrinsically allows for much easier operation control by eliminating flow rate calibration procedure. Large scale batch-type iCVD can be extended to fabricate many other types of functional polymeric thin films.

5. Acknowledgment This study was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK) with a grant number of 118M041.

Supporting Information: The photographs of large scale iCVD system used in this study are given in Supporting Information.



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Initiated Chemical Vapor Deposition of Poly(Ethylhexyl Acrylate) Films

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