LiNbO3 Wafers Research Articles

The development of laser cutting and drilling processes for thick Lithium Niobate wafers for electronic sensor applications

Lithium Niobate (LiNbO3) is a CVD grown synthetic salt having a trigonal crystal structure consisting of niobium, lithium, and oxygen. Due to its variety of mechanical and optical properties, single crystal LiNbO3 finds end use applications in optical waveguides, piezoelectric sensors, optical modulators, and electronic oscillators.LiNbO3 wafers are traditionally mechanically processed using abrasive disks and wire saws, and as a result of the inherently low fracture toughness cut-edge chipping and fracture occur lowering process yields. This study aimed to develop a laser process for the cutting, profiling and drilling of thick whole LiNbO3 wafers, using a femtosecond pulsed 515 nm wavelength source. The stresses induced during processing were controlled through a combination of wafer annealing, crystallographic (012) and (010) cleavage plane orientation. Process generated thermal gradients were controlled using beam manipulation and a water spray liquid flow process. The developed processes substantially reduced material fracture and significantly enhanced cut-edge quality, elevating yields above 80%.

Surface acoustic wave digital microfluidics with surface wettability gradient.

This paper reports a digital microfluidic technology that combines surface wettability gradient and surface acoustic waves. The technology enables selection of the driven object, facilitating reactions among multiple droplets and improving the precision of droplet control. Octagonal patterns with a wetting gradient and orthogonally distributed interdigital transducers were created on the surface of a LiNbO3 wafer by photolithography. Leveraging the propagation characteristics of surface acoustic waves on different wetting models, the latter serve as a switch for microfluidic motion, successfully achieving selection of the driven object and demonstrating sequential reactions among multiple droplets. Also, under the excitation of standing surface acoustic waves, droplets on the wetting gradient surface move slightly in the direction of wetting gradient descent, significantly enhancing the positional accuracy of the droplets to the micrometer level. With the advantages of surface acoustic wave digital microfluidics, this technology addresses the challenges of multi-droplet digital manipulation and improved droplet positional accuracy.

Direct Wafer Bonding of 128° Y-Cut LiNbO3: A Pathway to High Frequency (50 GHz) Filter Applications

In this work, room temperature direct wafer bonding of complementarily oriented 128° Y-cut LiNbO3 substrates is achieved with an interfacial layer less than 1 nm thick. This compares well to commercially available structures with thickness of 3 nm and this interfacial width is particularly important for eventual multilayer bonding sequences with each layer on the order of only tens of nm. The (0114) indices describe the surface plane of 128° Y-cut LiNbO3 wafers; and the wafers were bonded such that their in-plane [055.12] directions were antiparallel. Due to its trigonal crystal structure, this zone axis has 1-fold symmetry and therefore is crystallographically distinguishable from either side of the bonded interface. The wafers were cleaned with standard cleaning protocols and subsequently brought in contact face-to-face and bonded. The bonding was performed in ambient atmosphere and the applied pressure was on the order of ~kPa. The bonded pair was then annealed at 200 °C for 10 h in order to strengthen the bond, which is fully compatible with exfoliation of He-implanted LiNbO3 layers. Limited literature has been reported for direct bonding of LiNbO3 to LiNbO3 and even fewer investigate the atomic structure of the bonded interface.1-3 Tomita et al., observe structurally disordered bonded interfaces after annealing at high temperatures (up to 1000 °C). High resolution transmission electron microscopy of the interface presented here reveal an abrupt interface (≤ ~1 nm) and a φ X-ray scan has confirmed a relative twist of ~180° between substrates. This is a follow-up work for what has previously been achieved by our group demonstrating direct wafer bonding and layer transfer of LiNbO3 to substrates such as sapphire and oxidized silicon.4 Combining these latest results will enable the fabrication of a multilayer structure of complementarily oriented LiNbO3 thin films on a sapphire substrate, which has applications for high frequency (50 GHz) filters. Similar structures using a different LiNbO3 orientation (X-cut) have been demonstrated to maintain high quality factor (Q) at 15.8 GHz.5 The authors would like to acknowledge the support from DARPA through the Compact Front-end Filters at the ElEment-level (COFFEE) program.[1] Tulli, D., Janner, D. and Pruneri, V., “Room temperature direct bonding of LiNbO3 crystal layers and its application to high-voltage optical sensing.” Journal of Micromechanics and Microengineering, 21(8), p.085025 (2011).[2] Tomita, Y., Sugimoto, M., & Eda, K., “Direct bonding of LiNbO3 single crystals for optical waveguides.” Applied physics letters, 66(12), 1484-1485 (1995).[3] Berini, P., Mattiussi, G., Lahoud, N. and Charbonneau, R., “Wafer-bonded surface plasmon waveguides.” Applied physics letters, 90(6), p.061108 (2007).[4] Liao, M. E., Matto, L., Huynh, K., and Goorsky, M. S., “Direct Wafer Bonding of 128° Y-Cut LiNbO3 to Sapphire”. WaferBond ’22, Presentation (2022).[5] Lu, R., and Gong, S., 2021 Joint Conf. EFTF/IFCS, 1 (2021). Figure 1

Direct bonding of LiNbO3 and GaAs at room temperature by using activated Si atom layer

Heterogeneous integration of LiNbO3 and GaAs at room temperature has been achieved by using an activated Si atom interlayer, which demonstrate effectiveness in the fabrication of GaAs based LiNbO3 wafer. By using the activated Si atom interlayer as the adhesive layer, 4-inch LiNbO3 wafer is successfully directly bonded to the surface of GaAs wafer. The wafer bonding rate reaches over 95%. After bonding, the LiNbO3/GaAs bonding pair is cut into 10 × 10 mm2 square bonded chips. After annealing, the maximum bonding strength of square bonded chips exceeds 22 MPa. This means that the bonding strength of LiNbO3/GaAs wafer can be comparable to that of bulk materials. The characterization results of the bonding interface confirm that the intermediate amorphous Si layer is effective in improving the bonding strength and quality of LiNbO3/GaAs heterojunction. Heterogeneous integration of LiNbO3 with GaAs and other Ⅲ-Ⅴ compound semiconductor materials is conducive to reducing the area occupied by optical communication platforms and improving the overall efficiency of devices.

Scalable optical-to-terahertz converter with a prism-coupled plane-parallel lithium niobate plate.

A nonlinear optical converter of femtosecond laser pulses to terahertz radiation, which combines the tilted-pulse-front pumping and prism coupling techniques, is proposed and experimentally tested. In contrast to the conventional tilted-pulse-front scheme with a prism-shaped LiNbO3 crystal, the converter consists of a plane-parallel LiNbO3 plate sandwiched between two dielectric prisms. One prism is used to couple the pump beam into the LiNbO3 plate, another prism couples the generated terahertz radiation out of the plate. The proposed scheme enables scaling to large-diameter LiNbO3 wafers and large-aperture high-energy pump laser beams resulting in generation of strong terahertz fields. In a proof-of-principle experiment with a 1-mm thick, small size (1 × 2 cm2) LiNbO3 plate pumped by a mJ-class laser, the conversion efficiency comparable to that of the conventional scheme (∼0.1%) was demonstrated.

Effects of sample-aperture cone distance on the environmental charge compensation in near-ambient pressure hard X-ray photoemission spectroscopy

Charging effect of insulating samples is a major problem in photoemission spectroscopy. Herein, the charging of a LiNbO3 wafer and glass plate was compensated by introducing nitrogen gas using a near-ambient pressure hard X-ray photoemission spectroscopy apparatus. This is because the secondary electrons created by electron scattering in the gas neutralized the charge on the sample surface. Moreover, the pressure required to compensate for the charging was found to strongly depend on the distance (d) between the sample surface and the aperture cone of the electron energy analyzer. At a normal distance of d= 0.3 mm, a pressure of 2500 Pa was required to compensate for the charging of the insulating samples. However, at d= 2.1 mm, the required pressure was reduced to only 250 Pa. The large d dependence of distance can be attributed to that electron scattering is largely disturbed by the aperture cone at small d values.

Growth of large size near-stoichiometric lithium niobate single crystals with low coercive field for manufacturing high quality periodically poled lithium niobate

Periodically poled lithium niobate is an important component in mid-infrared lasers due to its perfect nonlinear optical property. However, fabrication of high-power laser based on periodically poled lithium niobate is still a great challenge, because lithium niobate crystal wafer with large thickness for large clear aperture periodically poled lithium niobate is hard to be completely poled due to the high coercive field of commercial congruent lithium niobate (LiNbO3). In this work, we have successfully grown 3-inch near-stoichiometric LiNbO3 by efficiently regulation of temperature gradient with the aid of thermal field simulation. Characteristics on the as-grown large size near-stoichiometric LiNbO3 proved that the [Li]/[Nb] of crystal is around 1:1, and the crystal possesses a typical single domain with high optical homogeneity. Compared with congruent and Mg-doped LiNbO3, the near-stoichiometric LiNbO3 crystal has better piezoelectric performance. The forward and backward coercive fields of the near-stoichiometric LiNbO3 crystal were 2.90 kV/mm and 2.43 kV/mm, respectively, which were approximately 8 times lower than that for CLN (21 kV/mm). The periodically polarization experiments proved that only 1.36 kV can realize the complete polarization for a 1 mm thick near-stoichiometric LiNbO3 wafer. The mass-productive near-stoichiometric LiNbO3 crystals grown in this work will have great applications in high-power mid-infrared solid-state laser.

Dynamically taming focal fields of femtosecond lasers for fabricating microstructures

Micromachining based on femtosecond lasers usually requires accurate control of the sample movement, which may be very complex and costly. Therefore, the exploration of micromachining without sample movement is valuable. Herein, we have illustrated the manipulation of optical fields by controlling the polarization or phase to vary periodically and then realized certain focal traces by real-time loading of the computer-generated holograms (CGHs) on the spatial light modulator. The focal trace is composed of many discrete focal spots, which are generated experimentally by using the real-time dynamically controlled CGHs. With the designed focal traces, various microstructures such as an ellipse, a Chinese character “Nan”, and an irregular quadrilateral grid structure are fabricated in the z-cut LiNbO3 wafers, showing good qualities in terms of continuity and homogeneity. Our method proposes a movement free solution for micromachining samples and completely abandons the high precision stage and complex movement control, making microstructure fabrication more flexible, stable, and cheaper.

Bonding of LiNbO3 and Si wafers at room temperature using Si nanolayers

We report the room temperature bonding of LiNbO3 and Si wafers based on the use of Si nanolayers. The proposed method employs physical sputtering, which simultaneously activates the surface of an etched Si wafer and forms a Si nanolayer on the surface of a LiNbO3 wafer. Following sputtering, both wafers are immediately brought into contact and the newly formed Si nanolayer acts as a nanoadhesive. The data presented herein demonstrate that this technique is more effective at directly bonding LiNbO3 and Si than the conventional surface-activated bonding method. Following activation, the bonded surface energy, which reflects the bond strength, was estimated to be approximately 2.2 J m−2. This result indicates that the bonding was strong enough to withstand the processes associated with the fabrication of microelectronics devices, including wafer thinning.

Mo/Ti multilayer Bragg reflector for LiNbO3 film bulk acoustic wave resonators

A 43° rotated Y-cut LiNbO3 (LN) single-crystal thin film coated with a bottom electrode and a Mo/Ti Bragg reflector is successfully transferred onto the 4-in. LN wafer substrate by means of crystal-ion-slicing (CIS) technique and benzocyclobutene bonding method. The multilayer film is crack-free, and the Mo/Ti reflector shows good acoustic reflection efficiency. A thin film bulk acoustic wave resonator based on the Mo/Ti Bragg reflector and the LN thin film (LN-BAW) is prepared, and the properties are further analyzed. The results indicate that the LN-BAW provides a large Kt2 of ∼16.63% and the Q-factor and figure-of-merit of the LN-BAW reach 183 and 30.4, respectively.

Demonstration of GaN/LiNbO3 Hybrid Wafer Using Room-Temperature Surface Activated Bonding

In this study, a GaN/LiNbO3 hybrid wafer was demonstrated using room-temperature bonding based on a surface activated bonding (SAB) method. The SAB using Fe-containing Ar ion beam bombardment achieved a strong bond between GaN and LiNbO3 wafers. The bonded wafer made using the modified SAB method was successfully cut into 1 × 1 mm2 dies using a dicing saw without interfacial debonding, and the measured tensile strength was estimated to be greater than approximately 26 MPa. These results show the presence of a strong bond that may be sufficient for device applications. In addition, TEM observation clearly indicated that the Fe-containing nanolayer deposited during ion beam bombardment appears to work well as an adhesive and form a strong bond between the negative surface of LiNbO3 and Ga-face of GaN. These results show the potential of this room-temperature bonding method to achieve a future GaN/LiNbO3 hybrid platform that can fully exploit the unique properties of each material.

Development and characterisation of acoustofluidic devices using detachable electrodes made from PCB.

Acoustofluidics has been increasingly applied in biology, medicine and chemistry due to its versatility in manipulating fluids, cells and nano-/micro-particles. In this paper, we develop a novel and simple technology to fabricate a surface acoustic wave (SAW)-based acoustofluidic device by clamping electrodes made using a printed circuit board (PCB) with a piezoelectric substrate. The PCB-based SAW (PCB-SAW) device is systematically characterised and benchmarked with a SAW device made using the conventional photolithography process with the same specifications. Microparticle manipulations such as streaming in droplets and patterning in microchannels were demonstrated in the PCB-SAW device. In addition, the PCB-SAW device was applied as an acoustic tweezer to pattern lung cancer cells to form three or four traces inside the microchannel in a controllable manner. Cell viability of ∼97% was achieved after acoustic manipulation using the PCB-SAW device, which proved its ability as a suitable tool for acoustophoretic applications.

Good-Quality and High-Efficiency Dicing for Thick LiNbO3 Wafers Using Picosecond Laser Pulses.

Lithium niobate (LiNbO3) has become popular with applications in electronics and communication industries due to its excellent electro-optical and nonlinear properties. This paper presents the influence of laser power, repetition frequency, number of subpulses, depth of each pass, and scanning velocity in picosecond laser dicing on multiple characteristics of LiNbO3 using the Taguchi method. By means of analysis of variance and analysis of relations between the characteristics, the optimal ps-laser-dicing parameter is obtained with good quality and high efficiency, which is applied to LiNbO3 products. The result indicates that picosecond laser dicing provides an alternative to machine thick LiNbO3 wafers with narrow kerf width, micro chipping, smooth surface, and high productivity.

Focusing surface acoustic waves assisted electrochemical detector in microfluidics.

This article demonstrates a novel electrochemical detection device. The device is composed by two focusing interdigital transducers for exciting focused surface acoustic waves by applying an AC signal, a three-electrode system for electrochemical measurement, and a liquid pool for holding liquid on a LiNbO3 wafer. The amperometry current of ferrocenecarboxylic acid and potassium phosphate buffer solution is used to characterize the detection sensitivity. Two experiments are carried out to optimize the device design. The result shows that the two focusing interdigital transducers with arc degree 30° and distance 5mm can remarkably enhance the liquid mixing rate. Under this condition, the oxidation current is about 27 times larger than that without surface acoustic wave stirring.

Mid-Infrared Electro-Optical Modulation Using Monolithically Integrated Titanium Dioxide on Lithium Niobate Optical Waveguides

Tunable photonic circuits were demonstrated in the mid-Infrared (mid-IR) regime using integrated TiO2-on-LiNbO3 (ToL) waveguides. The upper waveguide ridge was made by a sputtered TiO2 thin film with broad transparency at λ = 0.4–8 µm and an optimized refractive index n = 2.39. The waveguide substrate is a z-cut single crystalline LiNbO3 (LN) wafer that has strong Pockels effect, thus enabling the tunability of the device through electro-optical (E-O) modulation. A sharp waveguide mode was obtained at λ = 2.5 µm without scattering or mode distortion found. The measured E-O coefficient γeff was 5.9 pm/V approaching γ31 of 8.6 pm/V of LN. The ToL waveguide showed a hybrid mode profile where its optical field can be modified by adjusting the TiO2 ridge height. Our monolithically integrated ToL modulator is an efficient and small footprint optical switch critical for the development of reconfigurable photonic chips.

Direct bonding of LiNbO3 and SiC wafers at room temperature

Application of the surface activated bonding (SAB) method to direct bonding of LiNbO3 and SiC was investigated. SAB using argon fast atom beam bombardment resulted in the successful formation of a room-temperature bonded LiNbO3/SiC wafer for the first time. Cross-sectional transmission electron microscopy (TEM) observations showed a void-free bonded interface at the atomic level. The bonded interface had an amorphous-like layer that was a few nanometers thick, which was considered to be formed during the bombardment process. The resulting LiNbO3/SiC structure can be utilized to realize a new configuration of various devices including surface acoustic wave filters for use in high-power applications that can satisfy both temperature compensation and heat dissipation requirements.

Comparative study of the ion-slicing mechanism of Y-cut LiNbO3

Ion-cutting of piezoelectric LiNbO3 (LN) thin film provides a material platform for the design and fabrication of novel integrated photonics and RF MEMS devices. In this paper, the ion-slicing mechanisms of He-implanted LN with different orientations are investigated. The anisotropy of film exfoliation is observed on LN wafers with different orientations. The Z-cut LN shows regular surface blistering and “plate-like” exfoliation, while the Y-cut LN shows the unique “rolled-up” exfoliation. Two types of defect, i.e. the pressure-related plateau defect and the stress-related crack defect, are observed to contribute to the film exfoliation. Moreover, the defect evolution in H-implanted LN is investigated. In comparison with the He-implanted LN, implanted H ions are mainly trapped by O-H bond and the implantation-induced strain is not strong enough, which are inadequate to form the continuous crack. Therefore the H ions are not favorable for the mass production of LNOI substrates.

Ultrathin adhesive layer between LiNbO3 and SiO2 for bonded LNOI waveguide applications

A RT wafer bonding method for waveguide applications was investigated using an ultrathin Fe intermediate adhesive layer between LiNbO3 (LN) and SiO2 wafers. Here we focus on the optimal amounts of Fe in this layer to minimize the propagation losses of the resultant LN on-insulator (LNOI) waveguide. A sub-nanometer-thick Fe-containing intermediate layer exhibited strong bonding strength (surface energy: >1 J m−2) at RT, which may be sufficient for device applications. The influence of the Fe intermediate layer on the propagation loss of light through the LNOI waveguide was also investigated using numerical calculations. The present study is expected to be a significant contribution to the development of fabrication techniques for waveguides composed of various materials to be bonded by this RT bonding method using a metal intermediate adhesive layer.

Excitation and detection of evanescent acoustic waves in piezoelectric plates: Theoretical and 2D FEM modeling

This paper presents the results of the theoretical and 2D FEM modeling excitation and detection of evanescent acoustic waves in piezoelectric plates. By application of 2D ordinary differential equations derived by Auld, we obtained the dispersion curves of A1 and SH1 waves in YX LiNbO3 and YX KNbO3 plates in proximity to a zero group velocity point. The branches corresponding to evanescent acoustic waves are distinguished. A frequency range where real part of evanescent wave velocity is more than imaginary one was found. In this region evanescent mode is characterized by opposite directions of the phase and group velocities, i.e. this is backward wave. The theoretical analysis was verified in commercial 2D FEM software COMSOL 5.3. By modeling a set of interdigital transducers placed on the surface of Y-cut LiNbO3 wafer with various values of the spatial period we found the resonant frequencies corresponding to evanescent A1 mode. Due to proximity of this wave to a zero group velocity point its properties should be extremely sensitive to change of waveguide quality and ambient air. This is open the possibility to use these waves for development of high sensitive sensors and for nondestructive waveguide analysis.

Lithium Niobate Crystal with Embedded Au Nanoparticles: A New Saturable Absorber for Efficient Mode‐Locking of Ultrafast Laser Pulses at 1 µm

AbstractPlasmonic Au nanoparticles embedded in LiNbO3 crystals as efficient saturable absorbers to realize 74.1 ps mode‐locked laser pulse generation at 1 µm are reported. The system is fabricated by Au ion implantation and subsequent annealing, a well‐developed chip technology. The strong optical absorption band peaking at 640 nm is observed due to the localized surface plasmon resonance. Z‐scan investigation shows that the LiNbO3 crystals with embedded Au nanoparticles possess ultrafast saturable absorption properties at near‐infrared 1 µm wavelength. With this feature the Au nanoparticles embedded LiNbO3 wafer is applied as saturable absorber into a laser‐written Nd:YVO4 waveguide platform. Stable laser pulses at 1064 nm based on an efficient passive Q‐switched mode‐locking process, reaching a fundamental repetition rate of 6.4 GHz and a pulse duration of 74.1 ps, are obtained. Since LiNbO3 has broadband applications in various optical systems, this work opens the way to develop intriguing devices in LiNbO3‐based photonic circuits by using embedded metallic nanoparticles.