Low-Temperature Molecular Layer Deposition Using Monofunctional Aromatic Precursors and Ozone-Based Ring-Opening Reactions

Molecular Layer Deposition (MLD) is an increasingly used deposition technique for producing thin coatings consisting of purely organic or hybrid inorganic(cid:6)organic materials. When organic materials are prepared, low deposition temperatures are often required to avoid decomposition, thus causing problems with low vapour pressure precursors. Monofunctional compounds have higher vapour pressures than traditional bi(cid:6) or tri(cid:6)functional MLD precursors, but do not offer the required functional groups for continuing the MLD growth in subsequent deposition cycles. In this study, we have used high vapour pressure monofunctional aromatic precursors in combination with ozone(cid:6)triggered ring opening reactions to achieve sustained sequential growth. MLD depositions were carried out by using three different aromatic precursors in an ABC sequence, namely with TMA+phenol+O 3 , TMA+3(cid:6)(trifluoromethyl)phenol+O 3 and TMA+2(cid:6) fluoro(cid:6)4(cid:6)(trifluoromethyl)benzaldehyde+O 3 . Furthermore, the effect of hydrogen peroxide as a fourth step was evaluated for all studied processes resulting in a four(cid:6)precursor ABCD sequence. According to the characterisation results by ellipsometry, infrared spectroscopy and X(cid:6)ray reflectivity, self(cid:6)limiting MLD processes could be obtained between 75 and 150°C with each of the three aromatic precursors. In all cases, the GPC (growth per cycle) decreased with increasing temperature. In(cid:6)situ infrared spectroscopy indicated that a ring opening reactions occurred in each ABC sequence. Compositional analysis using time(cid:6)of(cid:6)flight elastic recoil detection indicated that fluorine could be incorporated into the film when 3(cid:6)(trifluoromethyl)phenol and 2(cid:6) fluoro(cid:6)4(cid:6)(trifluoromethyl)benzaldehyde were used as precursors.


ABSTRACT
Molecular Layer Deposition (MLD) is an increasingly used deposition technique for producing thin coatings consisting of purely organic or hybrid inorganic organic materials.When organic materials are prepared, low deposition temperatures are often required to avoid decomposition, thus causing problems with low vapour pressure precursors.Monofunctional compounds have higher vapour pressures than traditional bi or tri functional MLD precursors, but do not offer the required functional groups for continuing the MLD growth in subsequent deposition cycles.In this study, we have used high vapour pressure monofunctional aromatic precursors in combination with ozone triggered ring opening reactions to achieve sustained sequential growth.
MLD depositions were carried out by using three different aromatic precursors in an ABC sequence, namely with TMA+phenol+O 3 , TMA+3 (trifluoromethyl)phenol+O 3 and TMA+2 fluoro 4 (trifluoromethyl)benzaldehyde+O 3 .Furthermore, the effect of hydrogen peroxide as a fourth step was evaluated for all studied processes resulting in a four precursor ABCD sequence.
According to the characterisation results by ellipsometry, infrared spectroscopy and X ray reflectivity, self limiting MLD processes could be obtained between 75 and 150°C with each of the three aromatic precursors.In all cases, the GPC (growth per cycle) decreased with increasing temperature.In situ infrared spectroscopy indicated that a ring opening reactions occurred in each ABC sequence.Compositional analysis using time of flight elastic recoil detection indicated that fluorine could be incorporated into the film when 3 (trifluoromethyl)phenol and 2 fluoro 4 (trifluoromethyl)benzaldehyde were used as precursors.

INTRODUCTION MLD (Molecular Layer Deposition
) is fundamentally a modified ALD technique, where pinhole free, uniform nanofilms are produced with self limiting reaction chemistry. 1MLD can be used for manufacturing both purely organic and organic inorganic hybrid films.The first polymer films, polyimides, deposited by the MLD technique were presented in the beginning of the 1990s 2 and since then MLD processes for a vast number of different materials, including polymers, such as polyimides, polyamides and polyureas, in addition to hybrid materials and structures such as metalcones, polymeric fibres and barrier layers have been presented. 3 7mpared to traditional inorganic ALD processes, MLD enables the use of numerous diverse organic compounds, thus opening up the pathway for organic thin film layers with different properties and characteristics.In MLD, properties of both organic and inorganic materials can be mixed. 8910With organic materials, mechanical and physical properties can be greatly varied when compared to brittle ceramic materials.Because of this unprecedented tunability, novel MLD materials are continuously developed, although with certain limitations.Organic materials often possess poorer thermal stability, which restricts the maximum deposition temperatures.In addition, there is an increasing interest to modify MLD material properties by incorporating functional side chains or heteroatoms and to apply MLD coatings onto thermally sensitive substrates.For example in the case of fluorine, enhancement in optical and electronic properties can be expected. 11,12However, additional heteroatoms, multiple functional groups or long hydrocarbon chain lengths increase precursor molecular weight, thus decreasing the vapour pressure and their usability as MLD precursors.Therefore, it is challenging to find suitable precursors with sufficient vapour pressures for low temperature MLD processes.
One possible solution to tackle low volatility is to use monofunctional aromatic compounds, which have higher vapour pressures than heavily substituted compounds.Unfortunately, MLD growth cannot be sustained with only one functional group per precursor molecule, given that after one cycle the growth terminates due to a lack of reactive sites during the next precursor exposure.This phenomenon has been utilised in selective ALD with self assembled monolayers (SAMs) to efficiently block film growth on certain substrate areas. 13 15However, adsorbed monofunctional aromatic cyclic compounds can be opened and activated after they have adsorbed to the substrate, allowing continuous MLD process with consecutive cycles.In this paper, we propose a new way to enable continuous three step processes with monofunctional aromatic precursor and ring opening reactions using phenol, 3 (trifluoromethyl)phenol, 2 fluoro 4 (trifluoromethyl)benzaldehyde together with TMA (trimethylaluminum) and O 3 (Figure 1a).
Previously ozone have been used functionally in MLD processes, when Huang and his colleagues changed alkene terminal groups to carboxylic acids in the process with 7 octenytrichlorosilane, TMA and O 3 . 16In addition, ozone has been used in a MLD process to produce carbosiloxane thin films. 17Furthermore, in conventional organic chemistry, O 3 has been exploited for oxidative cleavage of double bonds in aromatic rings leading to the opening of the aromatic ring structure and finally to the formation of carboxylic acid groups. 18 20In our study, in the case of phenol and 3 (trifluoromethyl)phenol, a single -OH group can react with the substrate bound TMA.With 2 fluoro 4 (trifluoromethyl)benzaldehyde, the reaction happens between a Lewis acid (TMA) and nucleophilic carbonyl oxygen (Figure 1 b). 21,22Exploiting these approaches, a new way of manufacturing novel low temperature processes for MLD coatings is presented.In addition, incorporation of fluorine heteroatoms and the effects of hydrogen peroxide are studied.

EXPERIMENTAL
Deposition processes MLD films were deposited using an ALD Picosun R 200 tool in single wafer mode.N 2 (>99.999%)from LNG (liquid nitrogen gas) was used as a carrier gas.Si (100) (Siltronic Corp.) were used as substrates throughout this study.Three and four step (ABC, ABCD) processes were constructed from TMA (99.999%SAFC) as a metal precursor, ozone (

Characterization
Thicknesses of the deposited films were measured by ex situ ellipsometry (Sentech SE400adv).Thickness and density were also studied with XRR (X Ray Reflectivity, Aalto University).XRR measurements were performed using an X'Pert PRO PANalytical tool with parallel beam conditions, and using Cu Kα radiation.The acceleration voltage was 45 kV and the current was 40 mA.The incidence angle was varied from 0.3 to 3 degrees.XRR results were analysed with X'Pert Reflectivity software.Thickness was determined by fitting a simulated curve to the measured data and density by measuring the critical angle using the TOF ERDA results.Contact angles were measured with the KSV CAM200 Optical Contact Angle Meter.
Angles were measured with deionised water for testing hydrophilicity of the films.Elemental compositions were determined with TOF ERDA (University of Jyväskylä).The beam used in TOF ERDA analysis was 8.515 MeV 63Cu 4+ and the geometry was 20.5+20.5=41degrees. 23e chemical structure of the films was identified after the depositions, with a Fourier transform infrared (Nicolet iS50 FTIR) spectrometer using a single reflection 60 degree germanium attenuated total reflectance (ATR) crystal of the horizontal accessory VariGATR TM .ATR IR spectra were collected by averaging 64 scans at a resolution of 4 cm −1 .Measured Si chips were approximately size of 2×2 cm 2 .A reproducible contact between the sample chip and the Ge ATR crystal was obtained by the built in pressure applicator with slip clutch of VariGATR TM accessory.In situ FTIR (Ghent University, Belgium) depositions were made on regular Si (100) substrates, with a home built ALD tool, which was connected to FTIR measurement equipment in transmission mode (Bruker Tensor 27, globar mid IR source and KBr beamsplitter).

RESULTS AND DISCUSSION
Based on the ellipsometry measurements, the highest GPC of 5.4 Å/cycle was achieved by using TMA+phenol+O 3 at 80 o C (Specific values in supportive information, Table S1).GPC decreased with increasing deposition temperature from 80°C to 200°C (Figure 2 and 3).A similar trend was observed for the other evaluated processes; the GPC decreased for increasing substrate temperatures.The GPC of the TMA+3F+O 3 process reached 3.6 Å/cycle and with TMA+4F+O 3 the GPC got up to 4.6 Å/cycle.The typical decrease in MLD/ALD growth rate at higher temperatures is often explained by the decrease of reactive sites (-OH, -CH 3 ) and precursors, decomposition of organic moieties or different orientations of the long chain molecules. 24 27drogen peroxide was added to obtain an ABCD type pulsing sequence (TMA+organic molecule+O 3 +H 2 O 2 ).According to conventional chemistry, O 3 opens the aromatic ring and an additional oxidizing step is needed to obtain -OH groups (oxidation of the aldehydes to carboxylic acids). 19However, it seems that H 2 O 2 decreases the GPC, except for phenol process at temperatures from 100°C to 200°C.When hydrogen peroxide was used as a fourth precursor, the highest growth rate was approximately 80% of the value measured for the ABC process, but there was much more variation in the obtained values.
Linearity of the film growth as a function of number of MLD cycles was tested for all aromatic precursors at 100°C without hydrogen peroxide.According to the thickness results, all processes  S2).Furthermore, saturation was tested as a function of pulse length of the aromatic precursor.Relatively long pulse times were needed for fluorinated organic precursors, but eventually processes started to saturate (supportive information, Figures S2 and   S3).The process with phenol precursor was saturated already with pulse length of 0.2 seconds (Figure 4).When the organic precursor was not used, growth rate decreased near to the growth rate of Al 2 O 3 .The effect of ozone on the film growth was also tested.It was discovered, that GPC increased as a function of length of O 3 pulse until stabilisation (approximately 2s, depending on the process) and finally decreased slightly in the limits of measured pulse length (5s, Figure 5).A similar trend was observed also for 3F process (supportive information, Figure S4).XRR   exhibited the highest relative aluminium and oxygen content and a relatively low amount of the hydrocarbons.In the case of TMA+4F+O 3 , a reaction happens between carbonyl oxygen and aluminium, and coverage stays smaller, whereas in the case of phenol and 3F, the reaction occurs between a hydroxyl group and TMA, and the coverage grows bigger due to the more favourable reaction.Furthermore, the coverage of 4F is probably smaller because of the bigger molecular size resulting in steric hindrance.Differences in coverage are directly proportional to the amount of organic structures and free carboxylic acids.It is also likely that the amount of incorporated fluorine atoms affect the hydrophilicity, which explains the difference of contact angles between phenol and 3F processes.Films deposited with 3F had the smallest contact angles (58° and 51°), i.e., the biggest hydrophilicity.The use of hydrogen peroxide caused the decrease of the angles in the cases of phenol and 3F.This is likely due to the increasing amount of carboxylic acids, produced by hydrogen peroxide.Hydrogen peroxide had no clear effect on the process with 2 fluoro 4 (trifluoromethyl)benzaldehyde.
TOF ERDA measurements were performed for the films deposited by TMA+4F+O 3 at 75°C and by TMA+phenol+O 3 and TMA+3F+O 3 at 100°C (For TOF energy histograms and depth profiles, see supportive information, Figures S5 S16).The oxygen content was the highest in the case of TMA+4F+O 3 and the lowest in the films deposited from TMA+3F+O 3 , whereas carbon content had a just opposite trend.According to the TOF ERDA results, the Al:O ratio in the films is lower (0.4), than the Al:O ratio in pure Al 2 O 3 suggests (0.7) (Table 3).This can be explained with the carboxylic acid formation during O 3 pulse and the ring opening reaction.It is probable that a subsequent TMA pulse reacts with carboxyl groups forming bidentate and bridging complexes, resulting in an excess of oxygen in every cycle.Ratios of aluminium and carbon vary between 0.7 and 1.6.results, more fluorine was incorporated into the films when using 3F, which has less fluorine atoms in its molecule compared to the 4F (Figure 1a).It is unlikely, that the ozone would not affect on the double bonds still remaining in the structure after the ring opening reaction.It is possible, that after aromatic ring opening, the subsequent ozone pulse breaks a second double bond from the carbon chain, thus forming a new carboxylic acid structure and decreasing the amount of carbon of the original aromatic ring.Differences between processes can be explained with the cleavages of different double bonds in aromatic structures (Figure 6).It is expected, that in the case of 3F, the majority of the cleavages focus on the C3 C4 double bond, whereas in the case of 4F, the majority focus on the C1 C2 double bond, thus diminishing the amount of fluorine atoms substantially.The trends for Al and C compositions in the TOF ERDA data can be understood based on these considerations.Furthermore, the relative amounts of fluorine are explained with these second cleavages induced by ozone.It is also probable, as indicated by the lower deposition rate, that the surface coverage of the bulky 4F might not be as high as it is in the case of other organic precursors, hence the bigger Al:C ratio in the TOF ERDA results.
However, as there are several possibilities for reactions during the second O 3 pulse and following the TMA pulse, more detailed research via modelling is needed.Densities measured by XRR were 2.1 2.4 (g/cm 3 ) for the films deposited by three different processes (Table 4).Films deposited by the TMA+4F+O 3 process had the highest density, 2.4 g/cm 3 , which is explained with the lowest content of the organic compounds, compared to other processes, hence the highest ratio between aluminium and carbon.ATR IR spectra for all processes are presented in Figures 7 9. Hydroxyl groups of carboxylic acid (3100 3600 cm 1 ), CO 2 and CO 2 Al structures (1389 1397 cm 1 , 1606 1616 cm 1 and 1420 1390 cm 1 ), and Al O stretches (704 879 cm 1 ) were observed regardless of the process used (Table 5).In addition, the carbonyl group band (1720 cm 1 ) of carboxylic acid can be seen as a shoulder in all three spectra.The incorporation of a fluorinated aromatic precursor was evidenced by the strong band of C F structure for films deposited by the TMA+3F+O 3 process seen at 1123 cm 1 and 1124 cm 1 .Furthermore, in films deposited by the TMA+3F+O 3 and TMA+4F+O 3 processes, a band around 1320 1340 cm 1 (CF 3 attached to benzene ring) was observed.These bands are missing in the spectra of non fluorinated phenol derived films.When phenol or 3F was used as the organic precursor in the ABCD type reaction, H 2 O 2 did not induce significant changes to the ATR IR spectra obtained.In the case of 4F with H 2 O 2 , there is an additional band at 1336 cm 1 , probably originating from the stretch of CF 3 attached to a benzene ring.In situ transmission FTIR spectra were collected during 200 deposition cycles and the sum of the differential spectra were averaged out from all the events.Examples of in situ FTIR spectra are presented in Figures 10 and 11.According to these results, it seems evident that the ring opening reaction is occurring between aromatic precursors and O 3 .In the TMA spectra, there is a clear stretching vibration band from deformation of Al CH 3 (1205 cm 1 ) and antisymmetric CH 3 stretching band (2936 cm 1 ). 31After an aromatic precursor pulse, there is a negative absorbance in the area of CH 3 stretching (2936 cm 1 ), when the CH 3 structures disappear.Furthermore, after the aromatic precursor pulse, the absorption bands at 1495 cm 1 , 1595 cm 1 and 3030 3070 cm 1 can be seen and attributed to the aromatic structure. 32With 3F and 4F pulses, also bands originating from the CF 3 stretch at 1328 cm 1 and 1330 cm 1 , as well as the C F stretch at 1000 1200 cm 1 can be distinguished. 30,32In the case of phenol, aromatic C O stretching band is observed (1295 cm 1 ). 29The subsequent third precursor, the O 3 pulse, removes aromatic species as evidenced by the negative absorbance (1495 cm 1 , 3020 3070 cm 1 ) in the difference spectra near the aromatic band regions.At the same time carbonyl group (C=O) stretching vibration from carboxylic acid and carboxylate ion (CO 2 ) bands appear at 1726 cm 1 , 1610 cm 1 and 1465 1590 cm 1 (Figure 11). 33 35The O 3 pulse affects the fluorine bonding as well, since band originating from the CF 3 structure attached to benzene ring is substituted with band of aliphatic C F at 1200 1300 cm 1 , as detected in processes with 3F or 4F.In addition, after O 3 pulse observed hydroxyl band above 3200 cm 1 is originating from carboxyl acids further supporting the ring opening theory.As expected, the bands originating from -OH groups disappear during the subsequent TMA pulse and the reaction forms the CO 2 Al structure.The obtained band frequencies from in situ FTIR ( to mention, that the reactions in the films analysed by ATR IR where not as completed as during the in situ FTIR, since there were still some aromatic structures left in the deposited films used in the ATR IR analysis in the case of 3F and 4F (CF 3 structure attached to benzene ring, 1320 1340 cm 1 ).To be able to make further conclusions about the role of H 2 O 2 , more detailed research with in situ FTIR is needed.Temperature in all processes 100°C, 200 cycles. 29 35ecursor pulse Observed frequency (cm

Page 7 of 31 ACS
Paragon Plus Environment Langmuir produced ALD type films with linear growth as a function of the number of cycles (Supportive information, Figure S1, Table

28 Page 8 of 31 ACS Paragon Plus Environment Langmuir 9 Figure 2 .
Figure 2. GPC of ABC processes as a function of deposition temperature.The thicknesses were

Figure 3 Figure 4 . 10 Figure 5 .
Figure 3 GPC of ABCD processes as a function of deposition temperature.The thicknesses were

Figure 6 .
Figure 6.Possible bond cleavages during the second ozone pulse.In the case of 3F, the majority

Page 15 of 31 ACS Paragon Plus Environment Langmuir 16 Figure 7 . 17 Figure 8 .
Figure 7. ATR FTIR spectra of 72 nm and 80 nm thick films deposited by TMA+phenol+O 3 and

Table 1 .
).In all processes, the terminal MLD step was ozone or hydrogen peroxide, depending on the process.Aluminium oxide (Al 2 O 3 ) was manufactured from TMA and O 3 as reference material.Films deposited with 4F had the highest contact angles, 73° and 74° (with H 2 O 2 ).Furthermore, these angles corresponded to the utmost with the Al 2 O 3 reference material (70°).Films deposited with 3F had notably smaller contact angles of 58° and 51° (with H 2 O 2 ).Contact angles for the films with phenol were 60° and 58° (H 2 O 2 ).Contact angles with deionised water for MLD films deposited on Si at 100°C.The process based on 4F had the highest contact angles and angles closest to Al 2 O 3 .Similarity between the 4F process and pure Al 2 O 3 is probably due to the fact that films produced with 4F

Table 2 .
TOF ERDA measurements of the elemental compositions (at %) of all processes.+H 2 O 2 processes, respectively.Similarly, 4F processes, with and without H 2 O 2 , resulted in 1.9% and 2.9% of fluorine, respectively.According to the TOF ERDA When using 3F as an organic precursor, 8% and 7.4% of fluorine was detected in the films from

Table 3 .
Ratios between elements according to the TOF ERDA measurements.

Table 5 .
ATR IR band assignments for deposited MLD films.Processes with H 2 O 2 are

Table 6 )
and ex situ ATR IR correspond well to each other, indicating similarity of the ALD processes despite the different equipment.Although, it is good