Homo-epitaxial growth of Lithium Niobate by Pulsed-Laser Deposition

International audienceNowadays LiNbO3 single crystals in electro-optics are equivalent to silicon in electronics, and about 70% of radio-frequency (RF) filters, based on acoustic waves (acoustic resonators such as Surface Acoustic Waves (SAW) and Bulk Acoustic Wave (BAW) resonators), are fabricated on these single crystals [1]. LiNbO3-based structures have been mainly obtained by film transfer approaches [2], since obtaining single-phased, stoichiometric, and epitaxial LiNbO3 is challenging by conventional physical and chemical deposition techniques [1]. However, the layers used for devices can be nanometer-level in thickness, which is not always possible with Thin-Film Transfer technics [3]. Homo-epitaxial growth of LiNbO3 thin films by PLD (0 0 1), (1 1 0), and (1 0 0) monocrystalline substrates was demonstrated by L. C. Sauze et al. [4], and the present publication will be the continuation of her work.In this study, LiNbO3 thin films were homo-epitaxially grown by Pulsed Laser Deposition (PLD). Different substrates' orientations ((0 0 1), (1 1 0), and (1 0 4) crystal orientations) were investigated in an attempt to control the LiNbO3 crystalline orientation. In order to control the film crystallinity and chemical composition, growth parameters, such as substrate temperature, oxygen pressure, and target composition, were studied. The physical and chemical properties of the as-deposited LiNbO3 layers were characterized and correlated to the deposition conditions. The surface morphology of films was investigated by Atomic Force Microscopy (AFM). Structural properties of the layers have been characterized by XRD including High-Resolution X-Ray Diffraction (HRXRD). High-resolution reciprocal space mappings were performed to measure the homo-epitaxial deposited layer quality.References[1] A. Bartasyte et al., "Toward High-Quality Epitaxial LiNbO3 and LiTaO3 Thin Films for Acoustic and Optical Applications," Adv. Mater. Interfaces, vol. 4, 2017.[2] J. Shen et al., "A Low-Loss Wideband SAW Filter with Low Drift Using Multilayered Structure," IEEE Electron Device Letters, vol. 43, no. 8, pp. 1371-1374, 2022.[3] Z. Ren et al., "Heterogeneous Wafer Bonding Technology and Thin-Film Transfer Technology-Enabling Platform for the Next Generation Applications beyond 5G," Micromachines, vol. 12, no. 8, p. 946, 2021.[4] L. C. Sauze et al., "Homo-epitaxial growth of LiNbO3 thin films by Pulsed Laser deposition," Journal of Crystal Growth, vol. 601, p. 126950, 2023

Quantitative investigation of SiP and SiGe layers using HAXPES and ToF-SIMS

International audienceNowadays, “more Moore” and “more than Moore” device architectures have strongly increased the importance of novel materials thereby necessitating the availability of adequate characterization and metrology for reliable process control. For instance, the introduction of SiGe or SiP compounds used in Multi Channel Field Effect devices or raised sources and drain leads to the need for the determination of the exact composition of the resulting films. In this work, the quantification of binary materials such as SiP and SiGe has been investigated using mainly non-destructive HAXPES and ToF-SIMS. Indeed, while the main obstacle to the use of RBS is the characterization of thin films, techniques with appropriate quantification capabilities like Atom Probe Tomography and Transmission Electron Microscopy are both time consuming and suffer from a lack of sensitivity due to their highly localized analysis volume. For quantitative characterization, the conventional X-ray Photoelectron Spectroscopy (XPS) is a powerful tool. Yet, its low analysis depth remains a major limiting factor to study buried interfaces and especially in this study, since the obtained Si-based layers are oxidized in ambient conditions (or because they should be protected by metallic layers of a few nanometers). A novel lab-based hard x-ray sources (HAXPES) was used to investigate both the chemical composition at the binary material surface and the in-depth distribution of SiO2 within the layer thanks to the increase of the inelastic mean free path of electrons with increasing photon energy (Chromium Kα, hν = 5414.7 eV) [1]. To confirm the composition of the materials of interest obtained by HAXPES measurements and to calculate the adequate relative sensitive factor (RSF), the same films were characterized by ToF-SIMS. However, such as for HAXPES, Secondary Ion Mass Spectrometry (SIMS) characterization of SiP/SiGe layers often suffers from matrix effects due to the non-linear variation of ionization yields with P/Ge content. This limitation can be surpassed by analyzing reference samples, by following MCs2+ secondary ions or using the full spectrum protocol [2]. Finally, the P and Ge (Si) compositions of the secondary ion beam were calculated and compared with the reference composition as determined by X-ray Diffraction. The repeatability of the measurements and the influence of the layer oxidation were also studied. To conclude, the in-depth composition of the layers and the thickness of surface oxide were accurately evaluated by coupling the HAXPES results with ToF-SIMS.[1] O. Renault et al., Faraday Disc. 236, 288-310 (2022).[2] M. Py, et al., Rapid Commun. Mass Spectrom. 2011, 25, 629–63

Large scale integration of functional radio‐frequency flexible MEMS under large mechanical strain

International audienceA versatile industrial recipe of transferring nitride microelectronic components such as micro-electromechanical systems (MEMS) onto flexible and stretchable substrates is demonstrated. This method bypasses difficulties of temperature-related processing, and is applicable to large-scale and mass production. The technological process of fabrication is presented along with its underlying structural and radio-frequency characterizations. In particular, the Raman strain shifts of aluminum nitride (AlN) thin films are determined for uniaxial and biaxial mechanical deformations. The transferring process onto polymer is also demonstrated by an adhesive bonding of AlN-based MEMS onto a 200 mm silicon (Si) wafer. The devices microstructure is assessed using X-ray before and after transferring, as well as their electrical radio-frequency (RF) features when on Si and polymer substrates. Then, RF measurements are also performed on the transferred and flexible devices; some in their relaxed states, and others in an in situ manner under an increasing macroscopic strain. It is shown that bulk acoustic wave resonator MEMS are fully functional even under 12% uniaxial stretching of the substrate

Stress Engineering in Germanium-Silicon Heterostructure Using Surface Activated Hot Bonding

International audienceThe manufacturing of heterostructures is interesting in many fields such as photonics, solar energy production and quantum technologies. This paper, dedicated to germanium on silicon heterostructure manufacturing and stress engineering, builds up on LETI and EVGroup’s hot bonding technology (1). The coefficients of thermal expansion (CTE) mismatch between germanium and silicon is used to induce some in-plane tensile stress in a thin germanium layer transferred by the Smart Cut TM technique onto a silicon substrate. In this approach, a bulk germanium wafer is directly bonded on a bulk silicon wafer, using surface activated hot bonding (SAHB). Process integration advantages are the low defect density of bulk germanium and the tensile stress that can be tuned using the bonding temperature. According to X-Ray diffraction measurements, for a bonding performed at 250°C, the lattice parameter deformation reached +0.06%, resulting in a 82 MPa tensile stress in a 370 nm thick germanium layer

Strain characterization in SiGe epitaxial samples by Tip Enhanced Raman Spectroscopy

The progressive downsizing of semiconductors is driving information processing technology into a broader spectrum of new applications and capabilities. Strained silicon has become one of the best solutions for integrated circuits thanks to the advantages in terms of miniaturization. Indeed, a biaxial tensile stress applied to the silicon in the channel region of a MOSFET increases the mobility of carriers. This stress can be imposed by doping the silicon underneath with germanium, causing a mismatch between the lattice constant thus improving the electrons’ mobility [1]. Over the years, there has been an increasing need, especially in the industrial sector, to develop faster and non-destructive characterization techniques to monitor strain during the manufacturing phases of semiconductor devices. Currently, Tip-Enhanced Raman Spectroscopy (TERS) is one of most powerful methods for strain characterization, as it is a non-contact and non-destructive technique with a lateral resolution of a few nanometers and the capability of analyzing and collecting signals from the most superficial layer of a sample. The enhanced field is strongly restricted to the surface plasmons region, just a few nanometers deep [2], thanks to the simultaneous use of a nanometric tip of an Atomic Force Microscope (AFM) and a laser from a Raman spectrometer [3]. The analyzed sample was provided by CEA-Leti (Laboratoire d'électronique des technologies de l'information, Grenoble) and consists of a (001) silicon substrate where an epitaxial layer of Si0.7Ge0.3 with thickness of 17 nm is grown following several patterns. The AFM probe employed is characterized by an innovative coating which enables its implementation in the clean room for in-line characterization. TERS is used to map the variation in the position of the silicon peak in the local Raman spectrum (≈520.5 cm-1) along the sample pattern in order to identify the strain profile with a resolution of a few nanometers. The results confirm that TERS represents a powerful tool in monitoring the quality of production lines in the semiconductor industry and currently provides the best resolution among the Raman techniques for the strain characterization. References [1] P. Dobrosz et all, Surface and Coatings Technology, 2005, 200, 1755–1760. [2] F. Shao, R. Zenobi, Analytical and Bioanalytical Chemistry, 2019, 411, 37–61. [3] N.Hayazawa et al., Nanosensing Materials Devices, and Systems III, 2007, Proc. of SPIE Vol. 6769, 67690P

Impact of strain on Si and Sn incorporation in (Si)GeSn alloys by STEM analyses

International audienceThe structural properties of CVD-grown (Si)GeSn heterostructures were assessed thanks to scanning transmission electron microscopy at the nanometer scale. Quantitative energy dispersive x-ray (EDX) spectroscopy together with precession electron diffraction and geometrical phase analysis (GPA) were performed to probe the chemical and structural properties of the different layers. Results presented in this paper demonstrated the advantages of a multilayer structure, with successive layers grown at decreasing temperatures in order to gradually accommodate the in-plane lattice parameter and incorporate more and more Sn into the stack. It was shown how the GeSn emissive layer could be manufactured with low plastic deformation and a high relaxation rate, necessary for better light emission performances. SiGeSn alloys used as confinement barriers around the emissive layer were also investigated. For such thin layers, we showed the importance of the starting lattice parameter (SLP) prior to the growth on their composition. Indeed, higher SLPs resulted, for the very same process conditions, into higher Sn contents and lower Si contents. The interest in combining EDX, which was accurate enough to detect slight chemical concentration variations, and GPA, for local strain analyses, was clearly demonstrated. Present results will be very useful to predict and control the bandgap and structural quality of (Si)GeSn materials and, in turn, device properties

Computational model for predicting structural stability and stress transfer of a new SiGe stressor technique for NMOS devices

International audienceA new method to induce tensile stress in a PDSOI NMOS device for RF applications is proposed, which is based on relaxing a SiGe layer built underneath silicon. By means of TCAD simulations, we demonstrate that stress transfer from SiGe to Si occurs by means of at least two different mechanisms: SiGe relaxation due to amorphization and the formation of Stacking Faults during recrystallization. By considering both phenomena, a tensile stress of 0.5 GPa can be injected into the silicon channel. Moreover, the impact of annealing steps on the detrimental out-of-SiGe Ge diffusion has been simulated by considering an inter-diffusion model, showing the importance of adapting the PDSOI process flow to account for the presence of the new stressor