Atomic Layer Deposition of Boron‐Doped Al2O3 Dielectric Films

This paper presents preparation of boron‐doped Al2O3 thin films by atomic layer deposition (ALD) using phenylboronic acid (PBA) and trimethylaluminum (TMA) as precursors. Deposition temperatures of 160–300 °C are studied, giving a maximum growth per cycle (GPC) of 0.77 Å at 200 °C. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) are used to study the surface morphology and roughness of the films. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR), Time‐of‐flight elastic recoil detection analysis (ToF‐ERDA), and X‐ray photoelectron spectroscopy (XPS) are used to study the composition of the films. An annealing process is carried out at 450 °C for 1 h to investigate its effect on the elemental composition and electrical properties of the boron‐doped Al2O3 thin films. The boron‐doped Al2O3 70 nm thick film deposited at 200 °C has a boron content of 3.7 at.% with low leakage current density (10−9 to 10−6 A cm−2) when the film thickness is 70 nm. The dielectric constant of this boron doped Al2O3 film is 5.18.


Introduction
With the advancement of semiconductor technology, the size of devices has been continuously reduced to the nanometer level. Due to the reduction in size, parasitic capacitances increase and affect negatively the device performance. [1][2][3] Si 3 N 4 with the dielectric constant k of 7.5 is commonly used as a spacer. [4,5] However, such a high dielectric constant does not effectively reduce parasitic capacitance. [5] It becomes important to DOI: 10.1002/admi.202300173 explore new materials to reduce the capacitance between gate and source/drain. Low-k spacers have been demonstrated to minimize parasitic capacitances, and low-k materials can also reduce crosstalk and propagation delay to enhance the characteristics of device performance. [3,6,7] At present, low-k spacers are widely used in the FinFETs (fin field-effect transistors. Their dielectric constants are usually above 4 of SiO 2 but lower than 7.5 of Si 3 N 4 (e.g., SiCO k = 4.5, SiBCN k = 5.2, SiOCN k = 4.5). [2,[5][6][7][8][9] Alumina (Al 2 O 3 ) thin films have wide potential in a multitude of application areas. [10] In microelectronics, Al 2 O 3 thin films are being explored as a higher-k alternative to SiO 2 due to their excellent dielectric properties, excellent adhesion to a variety of surfaces, and thermal and chemical stability. [11][12][13][14] There are various ways to deposit alumina, such as plasma-enhanced chemical vapor deposition (PECVD), radiofrequency (RF) sputtering, atomic layer deposition (ALD), solgel, etc. [15][16][17][18][19] ALD is a thin film deposition technique based on self-limiting surface reactions. [20] Various processes have been studied for ALD Al 2 O 3 over decades and trimethylaluminum-H 2 O process is a well-established. [21][22][23][24][25][26] ALD Al 2 O 3 is used in several applications, including solar cells, [27] passivation, [28] protective material [29] and as gas diffusion barriers in food packaging. [30][31][32] Al 2 O 3 films obtained by the ALD process are uniform and have leakage current density up to about 1.1 × 10 −6 A cm −2 at top gate electric field E TG = 1 MV cm −1 (15 nm film), [33] while the dielectric constant is generally between 9 and 11. [34][35][36] However, such dielectric constant values are obviously too high for low-k spacers. The dielectric constant can be decreased by boron doping. The boron doped Al 2 O 3 is at the same time higher-k alternative to SiO 2 and lower-k alternative to Si 3 N 4 (k ≈ 7.5). [37,38] Phenylboronic acid (PBA) is a mild Lewis acid that is stable and easy to handle, making it widely used in organic synthesis. [39][40][41] In ALD, PBA can potentially be used for boron-doping of Al 2 O 3 to lower the dielectric constant of Al 2 O 3 . Therefore, we have studied the applicability of boron-doped Al 2 O 3 film by ALD process based on PBA and trimethylaluminum (TMA). In this process, PBA is used as both boron and oxygen source. The low dielectric constant of the boron-doped Al 2 O 3 achieved by a small amount (3%) of boron is excellent for low-k spacer application, and also www.advancedsciencenews.com www.advmatinterfaces.de high breakdown voltage and low leakage current density are measured in nanoscale film.

Experimental Section
ASM Microchemistry F120 reactor was used to perform the ALD process. N 2 gas (99.999%) was used as a carrier and purging gas. Phenylboronic acid (95%, Sigma-Aldrich) and trimethylaluminum (Volatec Ltd.) were used as two precursors for the ALD process (Figure 1). The TMA was kept at room temperature during the deposition and PBA was heated to 145°C to provide sufficient vapor doses. For reference experiments, AlCl 3 (99%, Acros Organics, Morris Plains, NJ, USA) evaporated at 80°C was used as an aluminium precursor.
The films were deposited on 5 cm × 5 cm polished Si (100) wafer pieces and 5 cm × 5 cm ITO (indium tin oxide) covered glasses. The deposition temperature range was from 160 to 300°C. The precursor pulsing sequence for TMA/purge/PBA/ purge was 0.5 s/0.5 s/10 s/8 s. When AlCl 3 was used, the sequence was AlCl 3 /N 2 /PBA/N 2 of 1 s/1 s/10 s/8 s and the deposition temperature was 300°C.
Thicknesses of the films were measured by an FS-1 Multi-Wavelength Ellipsometer from Film-Sense (Film Sense LLC, Lincoln, NE, USA). Field emission scanning electron microscopy (FESEM) characterization was performed with a Hitachi S-4800 FESEM instrument (Hitachi High-Technologies Corporation, Tokyo, Japan). An Oxford INCA 350 spectrometer (Oxford Instruments, Abingdon, UK) was used to study the composition of the films by energy dispersive X-ray spectroscopy (EDS) with an accelerating voltage of 20 kV and an emission current of 10 μA.
Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) of the films was performed with a Thermo Fisher iS50 (formerly known as Nicolet) connected to a liquid nitrogen cooled detector and a Harrick VariGATR ATR accessory. Background was measured before each sample and subtracted in Origin Lab software. All spectra were recorded from 700 to 3500 cm −1 and scanned 100 times.
Atomic force microscopy (AFM, Veeco Multimode V instrument) was used to study the surface morphology and roughness of the films. Si probe (RFESP-75 from Bruker) with a nominal tip radius of 10 nm and spring constant of 3 N m −1 was used to capture tapping mode images from the samples in ambient atmosphere. Flattening was done to remove image artifacts from sample tilt and scanner bow. Roughness was calculated as a rootmean-square value (R q ) from flattened images. Final images were obtained from a 500 nm × 500 nm area scanned at 0.5 Hz and without any additional image processing.
Composition of selected films was measured by time-of-flight elastic recoil detection analysis (ToF-ERDA) using a 40 MeV 127 I 8+ primary ion and a 40°detection angle at the Accelerator Laboratory at the University of Helsinki with a 5 MV tandem accelerator.
X-ray photoelectron spectroscopy (XPS) measurement was performed by the equipment from PREVAC with Al Kmonochromatized anode (1486.7 eV). The individual spectra were measured with 100 eV pass energy and the wide scan spectra were taken with 200 eV pass energy. All spectra were shifted to C 1s 284.4 eV. The XPS was conducted under ultrahigh vacuum (10 −10 mbar) and Casa XPS software was used to analyze the XPS data.
An annealing process was carried out in a custom-made tubular oven under N 2 atmosphere. The temperature was set at 450°C and the annealing duration was 1 h.
For electrical measurements, films were deposited on the ITOcovered (indium tin oxide) glass substrates, and the capacitors were completed by evaporating aluminum contacts through a shadow mask using an electron beam evaporator (IM9912 Instrumentti Mattila Oy, Mynämäki, Finland). The ITO electrode was contacted by scribing through the boron-doped Al 2 O 3 film and soldering indium wire to the corner of the sample. Hewlett Packard 4284A Precision LCR Meter (Hewlett Packard, Palo Alto, CA, USA) was used to measure the capacitance C of boron-doped Al 2 O 3 thin films at a frequency of 10 kHz and an applied voltage of 50 mV. After measuring the capacitance of the film, the dielectric constant k (also called relative permittivity r ) was calculated by the following formula: where d is the thickness of the PBA-TMA film, 0 is the dielectric constant of vacuum, and A is the area of the top contact Al electrode dot. The leakage current density and breakdown voltage of the hybrid film was measured by Keithley 2450 Source Meter (Keithley Instruments, Cleveland, OH, USA).

Film Deposition
A key characteristic of ALD is the self-limiting film growth behavior that derives from saturative surface reactions of the precursors. [42] Since TMA starts to decompose at around 330°C, [43] deposition temperature range of 160-300°C was studied for the PBA-TMA ALD process. The deposition sequence 0.5 s/0.5 s/10 s/8 s (TMA pulse/N 2 purge/PBA pulse/N 2 purge) and 1000 cycles were used to determine the best deposition temperature. The growth per cycle (GPC) value is derived from equation (2): where d is the thickness of the film and N is the number of deposition cycles. Figure 2 shows the GPC values versus deposition temperature. When the deposition temperature is 160-200°C, the borondoped Al 2 O 3 film has the highest GPC value of 0.77 Å, that is slightly lower than in the traditional TMA-H 2 O ALD process (0.88 Å). [33] At higher temperatures the GPC decreases linearly being 0.65 Å and 0.60 Å at 250 and 300°C, respectively. In comparison, the PBA-AlCl 3 process had a GPC of 0.34 Å at 300°C using the same deposition parameters as the PBA-TMA process, which is less than in the corresponding Al 2 O 3 process based on AlCl 3 and H 2 O (0.4 Å 500°C). [44] Further experiments were carried out at 200°C with 1000 deposition cycles of PBA-TMA. The saturation of the PMA and TMA reactions were studied by varying their pulse times one at a time. The timing used for the PMA saturation test was 0.5 s /0.5 s/X s/8 s (TMA pulse/N 2 purge/PBA pulse/N 2 purge), while the timing used for the TMA saturation test was Y s /0.5 s/10 s/8 s, where X and Y are the varied pulse times of PBA and TMA, respectively. Figure 3 shows the GPC curve while varying the TMA pulse time. The GPC is high already with 0.2 s TMA pulse and saturates with 0.5 s pulses. With PBA the saturation takes much longer,  10 s, than with TMA (Figure 4). Therefore, in the subsequent experiments, the deposition parameters of the PBA-TMA process were 0.5 s/0.5 s/10 s/8 s.
The reactor used in this study is of the cross-flow type. The films were not so uniform as typically deposited by a well behaving ALD process. Figure 5 shows thickness gradients of the films deposited with the PBA-TMA process and the PBA-AlCl 3 process. The point of 0 mm corresponds to the precursor inlet edge of the substrate and 50 mm is the outlet edge. Film thicknesses close to the precursor inlet are significantly higher than close to the outlet edge of the substrate. The thickness gradient of the PBA-TMA films seemed to be independent of the precursor pulse times, thus excluding CVD-like growth. Clarification of this behavior would require additional mechanistic studies, but since the obtained GPC is significantly lower than in the conventional TMA + H 2 O process, possible cause might be blocking of downstream reactive sites by reaction byproducts originating from PBA. With this kind of nonideal processes different ALD  reactor geometries, like showerhead tools, should be beneficial for improving the film uniformity.

Film Morphology
According to the XRD results (not shown), the boron-doped Al 2 O 3 films were amorphous. FESEM shows that a 70 nm thick boron-doped Al 2 O 3 film is smooth (Figure 6). The films deposited at 200 and 300°C were measured by AFM to compare their surface morphological characteristics (Figure 7). The surface of the film obtained at a deposition temperature of 200°C has higher roughness of 0.7 nm, while the film deposited at 300°C (0.6 nm) is slightly smoother. The film deposited by the PBA-AlCl 3 process has slightly lower rms roughness of 0.5 nm.

Film Composition
ToF-ERDA was used to analyze the composition of the films before and after annealing at 450°C in nitrogen. Films were deposited with the PBA-TMA process at 200 and 300°C, and with the PBA-AlCl 3 process at 300°C. In all cases, 1000 cycles were used. According to the depth profiles (Figure 8) and Table 1, the Al:O ratio was dependent on the deposition temperature and process used. The hydrogen contents in the film deposited with the PBA-TMA process at 200°C and in the PBA-AlCl 3 film are significant, but only ≈6.5 at.% in the film deposited with the PBA-TMA process at 300°C. Boron is summed from the two boron isotopes ( 10 B and 11 B) that were detected separately. The content of boron in the film deposited with the PBA-AlCl 3 process (8.1 at.%) is much higher than that in the films deposited with the PBA-TMA process (≈3.5 at.%). The carbon contents in all three films were equal to or less than the boron content, implying that the phenyl groups were not incorporated as such with boron into the films. On the other hand, carbon was detected also in the film deposited with AlCl 3 . Therefore, PBA rather than TMA appears to be the carbon source.
The annealing process has only a minor effect on the boron content in the films, regardless of the process used. The oxygen content in the film deposited with the PBA-AlCl 3 process increased significantly after annealing compared with the film deposited with the PBA-TMA process, from 47.4 to 57.5 at.%. On the contrary, the carbon content in the film deposited with the PBA-AlCl 3 process decreased significantly after annealing, from 4.2 to 0.8 at.%. In the film deposited with the PBA-AlCl 3 process there was 3.4 at.% chlorine which comes from the AlCl 3 precursor. Annealing decreased the chlorine content below the detection limit of ToF-ERDA. Al contents of all three films were not significantly affected by the annealing. Also, no crystallization upon the annealing could be detected with XRD.  ATR-FTIR was used to measure the 70 nm film deposited with the PBA-TMA process at 200°C (Figure 9). The weak peak at 1498 cm −1 corresponds to the Al-OH. [45] The B-O stretching vibration peak is at 1348 cm −1 . [46] The Al-O bonds formed by the PBA-TMA process show two strong peaks at 770 and 870 cm −1 . [47] XPS data were obtained by measuring a 70 nm film deposited with the PBA-TMA process at 200°C (Figure 10). The Al 2p region had two peaks where Al 2 O 3 is at 74.1 eV and Al(OH) 3 at 74.5 eV. [48] The B 1s region has one peak at 192 eV as B 2 O 3 . [49] The C 1s region had three peaks with C-C bond at 284.4 eV, C-OH bond at 285.8 eV, and C=O bond at 289.15 eV. [48] The O 1s had contribution of three peaks where Al 2 O 3 is at 530.4 eV, Al(OH) 3 at 531.4 eV, and B-O bond at 532.4 eV. [48]

Dielectric Properties
In order to analyze and compare the electrical properties of the boron-doped Al 2 O 3 films, 70 nm thick films were deposited with the PBA-TMA process on ITO-covered glass substrates at 200 and 300°C. In addition, about 40 nm film was deposited on ITO with the PBA-AlCl 3 process at 300°C. Al/Al 2 O 3 :B/ITO capacitor www.advancedsciencenews.com www.advmatinterfaces.de structures were made by evaporating Al electrodes through a shadow. The area of each Al electrode is 2.04 ×10 −7 m 2 . Capacitance and leakage properties of the films were measured by applying voltages to the Al and ITO electrodes as shown in Figure 11.
Capacitance measurements were performed at room temperature. Table 2 shows the dielectric constants of the films before  and after the annealing at 450°C in N 2 atmosphere. Among the as-deposited films, those deposited with the PBA-TMA process at 200°C had the lowest dielectric constant (5.18), which is slightly lower than that of the film deposited with the same process at 300°C (5.73). The film deposited with the PBA-AlCl 3 process at 300°C had the largest dielectric constant of 6.77.  The annealing increased the dielectric constants of all the films to a range of 7.30-8.27 but their relative order remained the same as before the annealing. Figure 12 shows the leakage current densities and breakdown points of the same films before (left) and after the annealing (right). The leakage current densities of the films after the annealing were generally worse than before the annealing, and the breakdown points were also lower than before the annealing. As no crystallization was detected upon annealing, the increase in leakage after the annealing must be ascribed to the change of film composition (Table 1) and consequent changes in defects controlling the leakage. Before the annealing, the film deposited with TMA at 200°C had a low leakage current density (10 −9 to 10 −6 A cm −2 ) and high breakdown point (5.6 MV cm −1 ) compared to the corresponding film deposited at 300°C. The film deposited using AlCl 3 had the highest breakdown points (7.1 MV cm −1 before the annealing, 4.6 MV cm −1 after the annealing). Overall, the leakage current density of the film deposited using TMA at 200°C was an order of magnitude lower than that of the film deposited using AlCl 3 , but with lower breakdown point. After the annealing, the leakage current density and breakdown point of the film deposited using TMA at 300°C were both worse than those of the film deposited using AlCl 3 . The dielectric constants and leakage current densities of the Al 2 O 3 :B films before the annealing were lower than those reported for the Al 2 O 3 films deposited by the ALD AlCl 3 -H 2 O and TMA-H 2 O processes. [50,51]

Conclusions
Boron-doped Al 2 O 3 films were successfully deposited on Si substrates by ALD using PBA and TMA as the precursors. The maximum GPC of 0.77 Å was obtained with relatively long PBA pulse of 10 s at a deposition temperature of 200°C. Although the films had significant thickness gradients, deposition process was still well controlled. According to SEM and AFM, the films were smooth and amorphous with RMS values below 0.8 nm. Based on the composition analysis by ATR-FTIR, XPS, and TOF-ERDA, the boron content in the film deposited with the PBA-TMA process at 200°C was 3.7 at.%. The deposition temperature has a significant effect on the electrical properties of the boron-doped Al 2 O 3 films. The 70 nm PBA-TMA film obtained at a deposition temperature of 200°C had the lowest dielectric constant of 5.18 and the lowest leakage current density (10 −9 to 10 −6 A cm −2 ). The annealing at 450°C decreased leakage current densities of the films and reduced breakdown voltages, but it also increased the permittivities.