AVS2017 Session PS+TF-ThA: Plasma Enhanced ALD

We report plasma enhanced atomic layer deposition results for VN using tetrakis(dimethylamido)vanadium (TDMAV) with N₂ plasma. Optimized TDMAV pulsing and N₂ plasma conditions have been established. Analyses include spectroscopic ellipsometry(thickness and optical properties), four point probe(resistivity), XPS(stoichiometry and impurities), XRD(crystallinity), XRR(density and thickness), and sliding wear testing(tribological properties). Depositions were investigated over 150 - 300 °C. Sub-100 μΩ-cm resistivities have been realized at 300 °C.

Wurtzite-structured ZnO thin films have been extensively investigated because of their unique optical, electrical, and piezoelectric properties, making it the material of choice in various applications such as transparent conducting electrodes, surface acoustic wave devices, and as sensors. In engineering ZnO-based devices, the material characteristics have to meet specific requirements in terms of opto-chemical and electrical properties and crystalline structure, together with very high conformality and thickness control. Plasma-assisted atomic layer deposition (PA-ALD) has been shown able to deposit very high quality ZnO thin films, combining the low process temperature with the exceptional atomic-thickness control. Furthermore, the properties of the material can be tuned by varying the plasma characteristics, making it suitable to adapt the material to specific applications.

In this contribution, high quality ZnO thin films have been deposited by PA-ALD optimized at room temperature, adopting diethyl zinc (DEZ) and oxygen plasma. The properties of the material have been investigated as a function of the radio frequency plasma power and plasma exposure time, and characterized by X-ray diffraction (XRD), gracing incidence XRD with synchrotron radiation, spectroscopic ellipsometry (SE), and X-ray photoelectron spectroscopy (XPS). The X-ray diffraction patterns of polycrystalline ZnO thin films showed rather preferred (100) orientation and XPS analysis showed the complete removal of the DEZ carbon ligands, confirming the possibility to obtain high quality crystalline ZnO at room temperature. Furthermore, the effect of the plasma power on the opto-chemical properties, growth, and crystalline structure has been investigated. Finely tuning of the optical properties of the PA-ALD ZnO layers has been achieved, with refractive index ranging between 1.82 and 1.89 at 633 nm. Moreover, the absorption has been found to increase and shift in the visible range at low plasma power. In addition, XRD showed a distortion of the (100) peak at low and high plasma power, indicating the possibility to vary the crystallite size as a function of the plasma parameters.

As an outlook, the optimized PA-ALD process at room temperature allows the deposition of ZnO on thermo-sensitive nanostructured templates, inferring the possibility to adopt it in engineering (flexible) structured devices.

In the recent years ALD Al₂O₃ surface passivation for Si solar cells has gained increasing popularity. The excellent passivation properties of ALD Al₂O₃ are based on a combined effect of chemical passivation and fixed high negative charge density. A bulk of the work has concentrated on thermal ALD, but also plasma enhanced ALD (PEALD) has been considered. However, the effect of plasma parameters, particularly plasma power, on the passivation properties remain uncharted.

In this work, we have studied the effect of plasma power on the properties of PEALD Al₂O₃ deposited at low temperatures with focus on the Si solar cell passivation. The Al₂O₃ was grown from TMA and O₂ plasma. All depositions were done with a Beneq TFS 200 ALD reactor at 90 °C. The lifetime was studied for as deposited and post-annealed samples.

The plasma power significantly affected the film properties. Low plasma power appeared to lead to the lowest film quality in regards of purity, density and refractive index. When the power was increased from 50 to 100–300 W the density increased from 2.6 to 2.8 g/cm^-3. Similarly, the refractive index increased from 1.61 to 1.62 (at 628 nm). The higher plasma power appeared to increase the amount of available O radicals, leading to more efficient reaction completion and improved film optical and structural properties. The same trend was also clear for the passivation properties for Si solar cells. For a 25 nm Al₂O₃ deposited with 50 W plasma power and annealed at 400 °C the lifetime at 10¹⁵ cm^-3 injection level was 1.1 ms, while for Al₂O₃ deposited with 100 or 180 W it was 2.0 ms. The interface properties were also influenced. The 50 W sample had the lowest density of negative charge and the highest interface defect density, which agreed with the lower lifetime of the sample. Interestingly, the 100 W sample had the lowest level of defect density. This can be related to the more moderate level of ultraviolet radiation from the O₂ plasma that the 100 W sample was exposed to as compared to the 180 W sample. Nevertheless, the 180 W sample had the best passivation properties due to its highest negative charge density. In fact, the negative charge density plays a major role in surface passivation when the magnitude of the negative charge density is much greater than that of the defect density.

High quality surface passivation of Si solar cells was achieved with PEALD Al₂O₃ grown at 90 °C. The passivation properties were shown to significantly improve with increasing plasma power.

Monolayer MoS₂ is a direct bandgap semiconductor with promising properties for novel devices. It has been shown that valley polarization can be achieved in MoS₂ monolayers with circularly polarized light, which would allow the realization of novel information processing architectures through manipulation of the valley pseudo-spin. However, current production methods of MoS₂ monolayers are either low yielding, or of relatively poor quality for valleytronic applications. To control the layer number, defectivity, and crystallinity of MoS₂, a novel method for limiting growth through the sulfurization of wafer-scale MoO₃ thin films has been developed.

Thorough characterization of the MoO₃ plasma-enhanced ALD process was performed to understand the effect of MoO₃ process parameters on the resultant MoS₂. MoO₃ films of 20-35 nm were deposited utilizing (NtBu)₂(NMe₂)₂Mo as the organometallic precursor and O₂ plasma for the oxygen source. Variations in Mo precursor dose time and O₂ plasma exposure time show the expected trends, but substrate temperature effects are more significant. Growth per cycle increases with substrate temperature, from 0.88 Å/cycle at 100 ^oC, to 1.32 Å/cycle at 350 ^oC, with the highest GPC of 1.4 Å/cycle at 300 ^oC. Raman spectroscopy shows that films grown at low temperature are amorphous, while polycrystalline film growth occurs above 250 ^oC. These results are consistent with previously demonstrated MoO₃ growth utilizing the same process and precursor¹. Surface roughness, as measured by AFM, also increases with temperature, which is consistent with a transition to polycrystalline film growth. Through XPS analysis, the deposited films were determined to be sub-stoichiometric in all deposits, averaging an O/Mo ratio of 2.6, regardless of substrate temperature.

MoS₂ films were grown on bare sapphire wafers by placing MoO₃ source wafers face-to-face with growth wafers. The wafers were oriented horizontally and enclosed in a graphite susceptor to enable inductive heating. Spacing between the MoO₃ source wafer and sapphire growth wafer was varied from 0 to 1 mm, and wafers were reacted at 700 ^oC in H₂S gas. Initial reactions have shown increasing film deposition with decreased spacing between growth and source wafers. These results suggest that MoS₂ film growth by this method is vapor-phase transport limited. XPS data confirms the formation of MoS₂ on the growth wafer, however several layers of growth are observed. The effect of MoO₃ source wafer crystallinity on MoS₂ film quality is under investigation and will be presented.

[1] Vos, M., Macco, B., Thissen, N., Bol, A., Kessels, W. JVST A, 2015, 34(1), 01A103-1-7.

In this work we used TMA (AlMe₃) and SF₆ plasma for atomic layer deposition (ALD) of aluminum fluoride (AlF₃) films. SF₆ plasma is a novel co-reactant for ALD and we employed quadruple mass spectrometry (QMS) and optical emission spectroscopy (OES) to study the film growth. AlF₃ as well as other metal fluorides such as MgF₂ and CaF₂ generally have a wide bandgap (>10eV) and low refractive index (1.3-1.6). Due to these interesting properties they find use in many applications, including passivation layers in Li-ion batteries, electron transport layers in photovoltaics and protective coatings for optical devices. Previously, ALD of fluorides has been demonstrated using TiF₄ and TaF₅ as the fluorine source for the deposition of MgF₂, CaF₂ and LaF₃ and more recently using HF for AlF₃, ZrF₄, MnF₂, HfF₂, MgF₂ and ZnF_2.^1,2 The novel approach of using SF₆ plasma as a fluorine source is a promising alternative to HF, because of the ease of handling that SF₆ offers. Furthermore, SF₆ plasma provides increased reactivity at lower temperatures and allows for reduced purge times, similar to the benefits of using an O₂ plasma instead of H₂O for metal oxide ALD.

AlF₃ films were prepared on Si samples over a temperature window of 50°C to 300°C. Since SF₆ plasma etches Si and SiO₂, a thin Al₂O₃ layer was deposited prior to AlF₃ growth. Using in situ spectroscopic ellipsometry (SE) the growth per cycle (GPC) was determined to decrease from 1.5Å at 50°C to 0.5Å at 300°C. Interestingly, no significant impurity levels of S, C and O were detected in the bulk of the AlF₃ films using X-ray photo-electron spectroscopy (XPS), even for low deposition temperatures. Furthermore, XPS measurements showed a F/Al ratio of 3.0±0.2. The low impurity content and the stoichiometric F/Al ratio are in line with a refractive index of 1.35 at 633nm as determined by SE.

The reaction mechanism of the ALD process was addressed based on a combination of OES and QMS. These measurements suggest that CH₄ is released during the TMA dosing, and that CH₄, C₂H₂, HF, and CH_xF_y–species are formed during the plasma exposure. Furthermore, the reaction products during the plasma exposure show different trends in their release (e.g., mostly directly after striking the plasma or peaking after a few seconds of plasma exposure), which will be used to suggest a possible reaction mechanism. In addition, consumption of F can be observed similar to what is found in etching using SF₆ plasma. Overall this work shows that SF₆ plasma is a promising co-reactant which can inspire the ALD of a wide range of metal fluorides.

¹Pilvi et al., Chem. Mater. 20 (2008)

²Lee et al., Chem. Mater. 28 (2016)

Recently, it has been shown that the ion energy can play a significant role on the physical and chemical properties of thin films grown using plasma-enhanced atomic layer deposition (PEALD).¹ In this work, we demonstrate the impact of ion energy control during PEALD of transition metal nitrides (e.g., TiN_x, HfN_x, etc.) which are of great interest for nanoelectronic device applications owing to their low electrical resistivity and excellent diffusion barrier properties.² Ion energy control during plasma exposure was carried out in a commercial 200 mm remote plasma ALD system (Oxford Instruments FlexAL) equipped with radio-frequency (RF) substrate biasing (13.56 MHz, up to 100 W power, -350 V resulting DC bias voltage). In such low pressure, remote inductively-coupled-plasma reactors, the ion energy can be controlled independently of the ion flux by applying an RF bias signal on the substrate table during the plasma exposure step.

Depositions performed under no bias conditions for TiN_x (at 200°C) and HfN_x (at 450°C) films using a 10 s H₂ (+Ar) plasma yielded electrical resistivities of 1960±60 and (900±0.7)x10³ μΩcm and mass densities of 3.8±0.2 and 10.1±0.2 g/cm³, respectively. Enhancing ion energies with substrate biasing during PEALD was observed to have pronounced effects on the chemical composition, microstructure and material properties of these transition metal nitrides. Energetic ion bombardment through application of bias voltages lowered film resistivity by one order of magnitude for TiN_x (139±10 μΩcm at -187V bias) and by two orders of magnitude for HfN_x ((330±70)x10¹ μΩcm at -130V bias) while also increasing their respective mass densities (4.9±0.2 and 10.5±0.2 g/cm³). The residual stress of these films were also observed to change from tensile under no bias to compressive under bias conditions. The oxygen impurity content for films deposited without substrate biasing (~20 to 30%) was observed to be significantly reduced (≤ 4%) in films grown with bias voltages applied during plasma exposure. Furthermore, it will be discussed how the use of substrate biasing enhances PEALD process capability by providing several additional knobs (magnitude, duration and duty-cycle of bias, etc.) for tuning a wide range of material properties.

¹ Profijt et al., J. Vac. Sci. Technol. A, 31, 01A106 (2013)

² Karwal et al., J. Vac. Sci. Technol. A, 35, 01B129 (2017)

The rapid shrinking of semiconductor devices has created a need for the low-temperature (≤400 °C) atomic layer deposition (ALD) of highly-conformal silicon nitride (SiN_x) and C‑containing SiN_x films. However, to date, the ALD of these films remains challenging. In this work, we report the surface reaction mechanisms during the ALD of SiN_x and C‑containing SiN_x for several ALD processes. Initially, our research focused on a baseline SiN_x ALD process that used alternating exposures of Si₂Cl₆ and NH₃ plasma. This process was subsequently modified by replacing the NH₃ plasma half-cycle with a CH₃NH₂ plasma to simultaneously incorporate both C and N. Finally, to overcome the limitations of SiN_x films deposited using H-containing plasmas, a three-step ALD process was developed that used Si₂Cl₆, CH₃NH₂, and N₂ plasma. The film composition, reactive surface sites, and adsorbed surface species were monitored using in situ attenuated total reflection Fourier transform infrared spectroscopy, which allowed us to elucidate the surface reaction mechanisms. In addition, in situ four-wavelength ellipsometry was used to obtain the growth per cycle (GPC). Ex situ analysis was used to obtain the conformality and elemental composition.

For the baseline Si₂Cl₆ and NH₃ plasma ALD process, our infrared spectra show that on a post‑NH₃-plasma-treated SiN_x growth surface, Si₂Cl₆ reacts with surface –NH₂ species to form –NH and –Si_xCl_2x-1 (x = 1, 2) surface species. In the subsequent NH₃ plasma step, the –Si_xCl_2x-1 surface species are removed and the –NH₂ surface species are restored, allowing for the continuation of the ALD process. Film growth during the Si₂Cl₆ and CH₃NH₂ plasma ALD process occurs via an almost identical reaction mechanism, with the exception that C is incorporated in the form of -N=C=N- species during the CH₃NH₂ plasma step. In the three-step ALD process, Si₂Cl₆ again reacts with surface –NH₂ species, while in the CH₃NH₂ step, the CH₃NH₂ reacts with –Si_xCl_2x-1 surface species via the formation of Si-N linkages to form Si₂N-CH₃ surface species. During the N₂ plasma step, the Si₂N‑CH₃ surface species are removed and the –NH₂ species are restored. When we compare the GPC and conformality (see Fig. 1) of the three-step ALD process to an aminosilane and N₂ plasma ALD process, we observe that the three-step ALD process has a higher conformality (~90%) and a higher GPC (~0.9 Å). However, these values are less than those reported for NH₃- or CH₃NH₂‑plasma-based ALD processes. This suggests that the three-step ALD process behaves as an intermediate between an NH₃- or CH₃NH₂‑plasma-based ALD process and an aminosilane and N₂ plasma ALD process.

Due to continuous feature size scaling down and change to the 3D structures new process innovations are now required more than ever. Conformal film formation of Al compounds such as AlN is one of the key technologies. AlN is widely used in thermal management applications and due to its compatibility with III–V compounds it has shown growing interest e.g. as interface material. There is also huge potential for AlN in MEMS manufacturing. PEALD at low temperature is one of the suitable solutions for these applications.

We have earlier reported the superiority of low pressure microwave ECR(M-ECR) plasma for Si substrate nitridation at low temperature [1]. In present study, AlN film properties were evaluated for demonstrating the advantage of the newly combined tool with the M-ECR plasma and the leading ALD system from Picosun. TMA (Trimethylaluminum) was used as Al precursor while nitrogen plasma was generated with M-ECR plasma generator to form AlN. Film properties such as density, crystallinity and conformality were studied. Composition of film was analyzed by XPS with Ar sputter.

AlN film properties were investigated as a function of deposition temperature as shown in Fig. 1. The film density increases with deposition temperature and at 200°C the density is 3.09g/cm³, which is consistent for literature values for bulk AlN [2]. It was shown that longer N₂ plasma exposure time improved film density. The temperature series in Fig. 1 shows that the growth rate was 0.57Å/c and 0.54Å/c at 100 and 250ºC, respectively. Interestingly, it was found a thickness dependent crystallization. For 20nm films deposited between 100 and 250ºC showed an amorphous structure, whereas 30nm film thickness and above show crystalline structure (verified by XRD) and higher roughness, see Fig. 2. Good quality films with conformality of < 0.5% (1σ non-uniformity) for 300mm wafer and high purity (C<1%, O<3%) were achieved. Efficient generation of the radicals and ions by M-ECR plasma at low pressure [3] is supposed to improve the film properties.

The step coverage obtained in this study at AR 1:20 is >90% as shown in Fig. 3. This is promising for 3D device fabrication where conformal coating of high aspect ratios is crucial. From these results, PEALD with M-ECR plasma can be one of the most advantageous solutions for next generation devices and opens the possibilities for beyond-silicon CMOS devices.

[1] H.Hamamura et al., 16th International Conference on Atomic Layer Deposition (ALD2016)

[2] JCPDF 00-003-1144 (AlN)

[3] H.Enami et al., submitted to ALD2017