Molecular layer deposition of hybrid silphenylene-based dielectric film

Molecular layer deposition (MLD) offers molecular level control in deposition of organic and hybrid thin films. This article describes a new type of inorganic–organic silicon-based MLD process where Aluminium chloride (AlCl3) and 1,4-bis(triethoxysilyl)benzene (BTEB) were used as precursors. Hybrid films were deposited at a temperature range of 300 to 500 °C and high growth per cycle (GPC) up to 1.94 Å was obtained. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to analyze the appearance of the film surface. The hybrid film was amorphous in low-magnification FESEM images but some particulates appeared in high-magnification FESEM images (200 k). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Time-of-flight elastic recoil detection analysis (ToF-ERDA), and X-ray photoelectron spectroscopy (XPS) were employed to analyze the structure and composition of the hybrid film. The ratio of Al/Si in the hybrid film was 0.8. The storage environment of the films affected their capacitance, dielectric constant, leakage performance, and breakdown voltage. A film stored in a high vacuum (10–6 mbar) environment had low leakage current density (< 10–6 A × cm−2 at an applied voltage of 28 V) and a dielectric constant of 4.94, which was much smaller than after storing in a humid ambient environment.


Introduction
In the past ten years, hybrid polymer insulator materials have attracted increasing attention because of their excellent properties.Different from traditional composites, hybrid polymer insulator materials are mixed at the molecular level which reflects the properties of the two components and even inspires some new properties [1][2][3].Some hybrid materials are both highly thermally conductive and insulating [4,5], and can also be used as flexible insulating composite films [5].Hybrid polymers play an important role in optical and color-changing devices [6], ultra-thin porous materials [7][8][9][10], capacitors [11,12] and biomimetic membranes [13].Compared with the traditional dielectric materials, the advantages of dielectric hybrid polymer materials are reflected in high electrical resistance, high breakdown field strength, low dielectric loss, excellent mechanical properties, mild processing conditions, low prices, predictable dielectric properties, and ease of manufacturing [1,14,15].
As the size of electronic devices has continuously decreased, the original planar MOSFETs (metal oxide semiconductor field-effect transistor), which are subject to various challenges, have been replaced by FinFET (fin fieldeffect transistor) structures [16,17].Si 3 N 4 with a dielectric constant of 7.5 is commonly used as a spacer in the FinFET structures.However, such a high dielectric constant contributes to an increased parasitic capacitance upon device size reduction [18].Low dielectric constant materials can decrease the parasitic capacitance and consequent crosstalk.Therefore, materials with a dielectric constant lower than the 7.5 of Si 3 N 4 can be used as low k spacers in FinFET structures [19,20].
Silphenylene silicones have higher melting points and higher thermal stabilities than simple dimethyl silicone, and their dielectric constants are usually less than 3 [21,22].Due to their outstanding properties, they have attracted considerable attention in for example heat-resistant materials and 183 Page 2 of 11 dielectric applications [23].1,4-bis(triethoxysilyl)benzene (BTEB) is a silicon-based nucleophile shown to be reactive in Pd-catalyzed cross-coupling reactions.It is widely used as a bridging precursor to form benzene bridged mesoporous silica nanoparticles with various structural properties and morphologies [24][25][26].It can also be used as a precursor for preparing materials with good dielectric properties.
Molecular layer deposition (MLD) is an ideal vapor phase thin film deposition technology for organic and hybrid organic-inorganic thin films [27,28].Through the self-limiting binary reactions, MLD provides unique advantages for the growth of uniform and conformal hybrid films.MLD uses the same working principle as atomic layer deposition (ALD) [29][30][31].The difference is that while ALD offers precise atomic scale control of film thickness and chemical composition for the deposition of inorganic thin films, MLD operates on the molecular scale and is used for the deposition of organic films and organic-inorganic hybrid thin films [32][33][34][35][36].
The use of organosilicon-based precursors in ALD/MLD has been limited due to their poor reactivity at low temperatures [37,38].1,4-bis(triethoxysilyl)benzene (BTEB) has been used in solution and sol-gel processes [39], but there is no report on using BTEB in MLD.This paper introduces the BTEB and AlCl 3 precursors (Fig. 1) for preparing hybrid thin films by the MLD method at high temperatures (≥ 300 °C).The growth per cycle of the films reaches 1.94 Å, which is significantly higher compared to the other silicon-based hybrid films prepared by MLD [28,40].The films have also excellent dielectric properties.The dielectric constant of this hybrid material is 4.94 after being stored in a vacuum, so it can be used as a low-k spacer material when protected against water absorption from ambient.

Experimental section
MLD was carried out by using an ASM Microchemistry F120 reactor.Ultrahigh-purity nitrogen (99.999%) was used as the carrier and purging gas.AlCl 3 (99%, Acros Organics, Morris Plains, NJ, USA) and BTEB (96%, Sigma-Aldrich, Saint Louis, MO, USA) (Fig. 1) were used as precursors for the molecular layer deposition process.The source temperatures were 80 °C for AlCl 3 and 130 °C for BTEB to provide sufficient vapor pressure.The substrates were 5 cm × 5 cm polished Si(100) wafer pieces, 5 cm × 5 cm ITO (indium tin oxide) covered glass, and 5 cm × 5 cm TiO 2 covered (~ 50 nm) polished Si(100).The MLD processes were performed at temperatures ranging from 300 to 500 °C, with the MLD cycle timing being 3/7/3/7 (in seconds) where the AlCl 3 and BTEB pulsing times were 3 s, and the purge times were 7 s.
Thicknesses of the films grown on Si substrates were examined with a FS-1™ Multi-Wavelength Ellipsometer from Film-Sense, while the thicknesses of MLD films on TiO 2 were determined from UV-vis reflectance spectra measured with a Hitachi U-2000 UV Spectrophotometer at wavelengths between 400 and 1100 nm [41].
Field emission scanning electron microscopy (FESEM) characterization was carried out with a Hitachi S-4800 FESEM instrument.In order to study the film morphology and cross-section of the film, the microscope was operated at an accelerating voltage of 5 kV and an emission current of 10 μA.The Oxford INCA 350 spectrometer connected to the Hitachi S-4800 FESEM instrument was used for energy dispersive X-ray spectroscopy (EDS) to study the composition of the films with an accelerating voltage of 20 kV and an emission current of 10 μA.
Atomic force microscope (AFM) images of the surface morphology and roughness of the films were obtained using a Veeco Multimode V instrument.Silicon probes (RTESP-300 from Bruker) with a nominal tip radius of 10 nm and a nominal spring constant of 40 N/m were used to capture tapping mode images in the air.Image artifacts caused by sample tilt and scanner bow were removed by image flattening.The roughness was calculated as the root-mean-square value (R q ) as an average of two images per sample.The final image did not undergo any other image processing and it was obtained from a 500 nm × 500 nm area scanned at a frequency of 0.5 Hz.

Fig. 1 Precursors used in the MLD process
Time-of-flight elastic recoil detection analysis (ToF-ERDA) was used to measure the stoichiometric content of the film with a 5 MV tandem accelerator at the Accelerator Laboratory of the University of Helsinki.Measurements were done using 40 MeV 127 I primary ions and 40° detection angle.
Thermo Fisher iS50 (formerly known as Nicolet) connected with a Harrick VariGATR ATR accessory and a liquid N 2 cooled detector were used to measure attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the hybrid AlCl 3 -BTEB MLD films.Background (only air on the top of the crystal) was measured before each sample, and the background subtraction was done in the Origin Lab software.All spectra were recorded in the range of 700 to 3500 cm −1 and were scanned for 100 times.
X-ray photoelectron spectroscopy (XPS) measurements were performed with Al K-α monochromatized anode (1486.7 eV) in ultrahigh vacuum (10 -10 mbar) using an equipment from PREVAC.The Ag 3d 5/2 peak at 368.21 eV was used to calibrate the energy scale.The individual spectra were measured with 100 eV pass energy and curved slit (slit C) 0.8 mm × 25 mm.The wide scan spectra were taken with 200 eV pass energy and straight slit (slit S) 2.5 mm × 25 mm.All spectra were shifted to C 1 s 284.4 eV and Casa XPS software was used to analyze the data.
To study electrical properties, capacitors were made with the hybrid AlCl 3 -BTEB MLD film as the dielectric and ITO and aluminum films as the electrodes.The hybrid film was deposited on the ITO film on the glass substrate, and then an Electron Beam Evaporator IM9912 was used to evaporate aluminum through a shadow mask to pattern the aluminum electrodes on the surface of the hybrid AlCl 3 -BTEB MLD film.The ITO electrode was contacted by scribing through the hybrid AlCl 3 -BTEB MLD film and soldering indium wires to ITO at the corner of the sample.The capacitance C of the film was measured at 10 kHz/50 mV with a 4284A Precision LCR Meter from Hewlett Packard.The dielectric constant (k, also called relative permittivity ε r ) was calculated from the capacitance C with the following formula: where d is the thickness of the hybrid film, ε 0 the dielectric constant of vacuum, and A the area of the top Al electrode dot.Keithley 2450 Source Meter was used to measure the leakage current density and breakdown point of the films.

Results and discussion
Precursor combinations of a metal chloride and a metal or silicon alkoxide have been used in ALD for depositing metal oxide and silicate films [42,43].Mechanistic studies on the ZrCl 4 -Ti(OiPr) 4 process showed that alkylhalide (1) ε r = C × d∕ε 0 × A elimination is the dominant reaction but also alkoxo ligand decomposition followed by HCl elimination occur [44].On these bases it is proposed that also in the present process ethyl chloride elimination would be the main reaction mechanism (-Et is the abbreviation of -C 2 H 5 ): In reality, the reactions are apparently more complicated.Between a pair of Al and Si atoms there can be more than one Al-O-Si bond.Also, while Si is expected to remain four coordinated, Al can expand its coordination number by bonding with nearby oxygen atoms that thereby also become triply bridging.Therefore, the structure of the film is expected to become more branched than the above simplified equations and imply.In any case, the film is supposed to be made up of (-O-) 3 Si-C 6 H 4 -Si(-O-) 3 units that are linked together with AlO x units.Therefore the film properties, including the dielectric constant are expected to be closer to silphenylene silicones than Al 2 O 3 .For simplicity, the films will be noted as AlCl 3 -BTEB because of the precursors used.

Film deposition
The deposition temperature range of the hybrid AlCl 3 -BTEB MLD film was found to extend from 300 to 500 °C, which is much wider than in other MLD processes [28].AlCl 3 and BTEB seemed to be not fully reacting when the deposition temperature was less than 300 °C because the film was not stable and its color changed in air when taken out of the reactor after the deposition process.The deposition temperature of the hybrid AlCl 3 -BTEB MLD process reached 500 °C and could possibly be even higher but the maximum temperature of the reactor is 500 °C.
After determining the deposition temperature range, the pulsing sequence 1/1/1/1 s (AlCl 3 pulse/N 2 purge/BTEB pulse/N 2 purge) and 1000 cycles were used to determine the deposition temperature giving the highest growth per cycle (GPC) value that is derived from Eq. ( 2): where d is the thickness of the hybrid AlCl 3 -BTEB MLD film and N is the number of deposition cycles.
GPC varies to some extent in the deposition temperature range of 300 to 500 °C as shown in Fig. 2. It first increases with the increase of the deposition temperature, reaches the highest GPC 1.71 Å at 375 °C, and then decreases with the further increase of the temperature.Therefore, the deposition temperature was set to 375 °C in the subsequent (2) GPC = d∕N experiments.As compared to other silicon-based MLD processes, the GPC of the hybrid AlCl 3 -BTEB MLD film was high at all deposition temperatures, varying between 1.32 and 1.71 Å.In order to analyze the samples conveniently, the film thickness was targeted to about 100 nm.Therefore, in the following experiments, 600 cycles were used.
MLD processes are characterized by self-limiting growth behavior [28,45].The pulsing sequence used for studying the effect of the AlCl 3 pulse time was X/7/1/7 s (AlCl 3 pulsing/N 2 purge/BTEB pulsing/N 2 purge).The GPC value increases first rapidly and then slower as the AlCl 3 pulsing time increases as shown in Fig. 3.When the AlCl 3 pulsing time is 3 s, the GPC value saturates to 1.86 Å.Therefore, the pulsing time of AlCl 3 was selected as 3 s and the pulsing sequence used for studying the effect of the BTEB pulse time was 3/7/X/7.When the pulsing time of BTEB is 3 s, the GPC value reaches the saturation value (1.94 Å).Consequently, all further experiments used the pulsing sequence of 3/7/3/7 s (AlCl 3 pulse/N 2 purge/BTEB pulse/N 2 purge).

Film structure and morphology
Based on X-ray diffration, the films are amorphous.FESEM was used to examine the morphology of the AlCl 3 -BTEB MLD film grown on silicon at 375 °C with 600 cycles.The thickness of the film was 116 nm as measured by an ellipsometer.The surface morphology of the film can be seen in Fig. 4 at low and high magnifications.At low magnification, the film is featureless as expected for an amorphous film.At high magnification, some particulates can be seen on the surface of the film, however.Two samples were studied with AFM to compare their surfaces.One sample was 116 nm AlCl 3 -BTEB film deposited directly on the polished Si(100) substrate, and the other sample was 126 nm AlCl 3 -BTEB film deposited on the ALD TiO 2 film on the polished Si(100) substrate.The deposition parameters of the MLD AlCl 3 -BTEB films in the two samples were consistent (375 °C, 600 cycles, 3/7/3/7 s).
Different sized scanning areas show essentially the same surface roughness of 1.3 nm for the AlCl 3 -BTEB film deposited directly on the polished Si surface (Fig. 6a, b).Some particulates can be seen on the surface in the 1 μm × 1 μm and 2 μm × 2 μm area images, which is consistent with the high-magnification SEM image in Fig. 4.
For the film deposited on the TiO 2 film, the surface roughness varies greatly from 2.3 nm to 4.5 nm under different scanning areas (1 μm × 1 μm & 5 μm × 5 μm) (Fig. 6c,  d).The surface of the film deposited on the TiO 2 film is rougher than that of the film deposited directly on the polished surface of Si.Because the TiO 2 film surface is rougher than the Si surface, the change in the surface roughness of the AlCl 3 -BTEB film arises from the underlying surface morphology that the MLD film covers conformally.

Film composition
To analyze the composition of the AlCl 3 -BTEB film, a 116 nm film was deposited at 375 °C and measured by ATR-FTIR, ToF-ERDA, and XPS (Figs. 7, 8, 9).ATR-FTIR was used to examine the bonding environment within the MLD AlCl 3 -BTEB film (Fig. 7).Distinctive absorption features are observed in three frequency regions: 800 to 1300 cm −1 , 1400 to 1700 cm −1 , and 2800 to 3000 cm −1 .The absorption bands between 2800 and 3000 cm −1 are attributed to the C − H x stretching vibrations from aromatic carbon in the film [45].The C = C bond from benzene appears at 1624 cm −1 .The absorption peak observed at 1258 cm −1 is consistent with the Si-O bond [46].The shoulder around 1135 cm −1 indicates the peak of Si-C 6 H 4 -Si [47,48].The strong and broad absorption peak of the Si-O-Al bonding at 1030 cm −1 [49] is consistent with a successful reaction between AlCl 3 and BTEB.The All the observed bands in the spectrum are consistent with the proposed deposition scheme (Eqs. 1 and 2) and thereby largely confirm the reaction mechanism.
The elemental depth profiles measured with ToF-ERDA are shown in Fig. 8.To avoid interference from the silicon substrate, 48 nm ALD TiO 2 film was deposited between the 126 nm hybrid film and the Si(100) substrate.Oxygen has the highest content of 52.1 ± 0.7 at.% which implies that the film has been partially oxidized by air after the deposition.9a).The O 1 s peak has three distinct contributions assigned to Al(OH) 3 at 531.6 eV [51], siloxane [52] and C-O bonds at 532.3 eV, and O-Si bonds and OH groups at 533.1 eV (Fig. 9b) [53].The Si 2p peak can be fitted with two peaks, one at 102.3 eV assigned to Al-O-Si [54] and another one at 103.2 eV assigned to siloxane (Fig. 9c) [53].The Al 2p was fitted with two peaks, one at 74.8 eV corresponding to film oxidation with ambient moisture Al(OH) 3 [51] and another one at 75.4 eV for Al-O-Si (Fig. 9d) [54,55].

Dielectric properties
To investigate the dielectric properties of the MLD AlCl 3 -BTEB film, a 116 nm thick film was measured in a metal/insulator/conductive layer device structure (Fig. 10).The ITO film and Al dot serve as the two electrodes.The capacitance of the MLD film was measured under a constant voltage of 50 mV at 10 kHz.Table 1 shows the capacitance and the dielectric constant of hybrid AlCl 3 -BTEB MLD films after storing in different environments.The high vacuum (HV) environment was in the e-beam evaporator which reaches 10 -6 mbar, while the humid environment is the laboratory environment.The capacitance measurement was carried out in a laboratory air at room temperature.
Storage environments have a substantial influence on the dielectric constant and capacitance of the film.The dielectric constant of the film stored in a high vacuum environment can reach as low as 4.94, whereas the dielectric constant of the film stored in a humid environment can reach as high as 7.45.The film thickness remained constant at 116 nm in the different storage environments.When measured at 1 MHz, the capacitance and dielectric constant were   the same as at 10 kHz for the film stored in vacuum, but for the sample stored in a humid environment there was 20% decreases while going from 10 kHz to 1 MHz.The likely reason for the high permittivity (dielectric constant) after the storage in the humid environment is that water molecules incorporate to the hybrid AlCl 3 -BTEB film structure and increase the polarity of the film.In high vacuum, water molecules are removed.The film stored in the humid environment was also annealed in a tubular oven.Annealing at 100 and 150 °C had almost no effect on the capacitance that stayed above 100 pF.Therefore, somewhat surprisingly, the high vacuum environment seems to be more effective in removing moisture than the annealing.Leakage current densities of the AlCl 3 -BTEB films are shown in Fig. 11.Different storage environments have an influence also on the leakage performance and the breakdown voltage of the film.The breakdown voltage of the AlCl 3 -BTEB film stored in the high vacuum environment is 37 V, while it is 28 V for the film stored in the humid environment.The leakage current density of the sample after the high vacuum storage environment is also lower at all voltages as compared with the sample stored in the humid environment.

Conclusion
In this study, hybrid inorganic-organic AlCl 3 -BTEB films were successfully deposited on the Si(100) surface by a MLD process.Compared with other silicon-based ALD or MLD processes, the deposition process of AlCl 3 -BTEB requires a relatively high deposition temperature, from 300 to 500 °C.The GPC value which reached 1.94 Å was also much higher than in other silicon-based ALD and MLD processes.The surface of the AlCl 3 -BTEB hybrid film in the lowmagnification SEM image was very uniform and smooth, but many particulates appeared in the high-magnification SEM image.The FTIR, ToF-ERDA and XPS spectra clearly demonstrate the elements and functionalities contained within the films but also indicated that the films had been partially oxidized in the air.The ratio of Al/Si in the film was 0.8.The dielectric constant (4.94) and leakage current density (< 10 -6 A × cm −2 at 28 V) of samples stored in a high vacuum were smaller than those stored in a humid environment, and also the breakdown voltage was greater for the sample stored in a high vacuum.Therefore, the AlCl 3 -BTEB films need to be stored in a high vacuum environment or coated with a protective film to achieve low dielectric constant, good leakage performance and high breakdown voltage.Since the dielectric constant of the hybrid film stored under vacuum conditions is 4.94, it can be used as a low-k spacer material instead of Si 3 N 4 in the FinFET structures provided that the water absorption can be prevented with either a protective layer or proper manufacturing process flow; once embedded into the device structure, the upper layers provide the protection.

Fig. 2
Fig. 2 GPC of the hybrid AlCl 3 -BTEB MLD films at different deposition temperature

Fig. 9 X
Fig. 9 X-ray photoelectron spectra of the 116 nm MLD AlCl 3 -BTEB film.a C 1 s region, b O 1 s region, c Si 2p region, and d Al 2p region

Fig. 10
Fig. 10 Schematics of the capacitance measurement of the 116 nm MLD AlCl 3 -BTEB film

Fig. 11
Fig. 11 Leakage current densities and breakdown voltages (a, b) of 116 nm MLD AlCl 3 -BTEB films after being stored in different storage environments

Table 1
The capacitance and dielectric constant of 116 nm MLD AlCl 3 -BTEB film measured after storing in different environments