The top cladding is an important factor when designing photonic devices. It has implications on waveguide properties such as mode guidance and optical loss. Silicon dioxide (SiO2) is a commonly used cladding material for photonic devices as it has low optical loss and also enables a large refractive index contrast between the waveguide core and cladding. We performed an investigation into the material properties of SiO2 thin films deposited by different methods, namely, plasma-enhanced chemical vapor deposition (PECVD) undoped silicate glass (USG), high density plasma (HDP), PECVD tetraethyl orthosilicate (TEOS), and low pressure chemical vapor deposition (LPCVD) TEOS. Material properties including refractive index, surface roughness, absorption, and film stress were characterized for the different SiO2 cladding samples. Our results show that the surface roughness of HDP films was around 55 times better than that of USG films. In terms of film stress however, USG films had the lowest, whereas PECVD TEOS films were found to have the highest film stress, more than twice of that of the USG films. These findings indicate that when deciding on a particular deposition method for the SiO2 cladding, besides considerations of thermal budget and device application, an equally important compromise needs to be made to give a balance among the various material properties.
In this paper we demonstrate the development and optimization of an 800 nm-thick Plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) photonic platform on a 300-mm silicon wafer. The implementation of ArF immersion lithography contributes to superior manufacturing processes, as it provides excellent critical dimension (CD) uniformity inter- and intra-wafers, make it an optimal platform of production of integrated circuits and nanoscale devices.
On-chip spectrometer operating in the mid-infrared (MIR) regime (λ = 2 – 14 μm) enables the miniaturization of a chemical sensing platform that identifies compounds based on their unique molecular fingerprints. Germanium-on-Silicon (Ge-on- Si) material system is a suitable candidate for its transparency in the MIR spectrum and compatibility with silicon processing. As chemical sensing is conducted by having the mode evanescent field interacting with the analyte, the design of Ge-on-Si waveguide for a compact footprint (small bending radius) and large evanescent field coverage is necessary. However, the bending radius of the Ge-on-Si waveguide is limited to hundreds of micrometers due to the low refractive index contrast between germanium and silicon. In this work, we demonstrate a 3 μm thick Ge-on-Si waveguide, with ~89° sidewall angles and a high gap aspect ratio of 10 (resolvable gaps of 300 nm). Different types of Ge-on-Si devices are fabricated including in-plane distributed Bragg grating (DBR) structures, cascaded Fabry-Perot resonators, and polarization splitters. We show that over-etching the Si lower cladding is able to reduce bending loss by ~10x, allowing us to decrease the bending radius to ~50 μm. Designs of 32 waveguide geometries for single mode propagation from 5.5 μm to 11 μm are presented, each of which is integrated with grating couplers operating at specific peak wavelengths. Our measurements show high consistency between the simulated and measured peak wavelengths of the grating couplers, with an inter-chip standard deviation of σλ ⁄ λpeak <1%
Aluminum nitride (AlN) is a promising photonics material contributed by its wide transparency window and remarkable nonlinear optical property. Moreover, its nonlinear effect can be further enhanced by doping Scandium (Sc). Such nonlinear optical property brings potential for high efficiency in nonlinear optical generation processes, such as 2nd harmonic generation and frequency comb generation. Although the nonlinear optical property of Sc-doped AlN looks promising, its waveguide is facing challenge on loss reduction. In this work, we report Sc-doped AlN photonic integrated circuit with reduced waveguide loss of 6 dB/cm around 1550 nm. The waveguide has Sc doping concentration of 10%. Its etching process is tailored through a design of experiment (DoE) approach to achieve smooth surface. An annealing process is also applied to patterned waveguide for optical loss reduction. A loaded Q of 1.41×104 has also been reported from microring resonator on the same wafer. The reported result paves the way towards low-loss Sc-doped AlN for photonic integrated circuits.
Germanium-on-Silicon (Ge-on-Si) platform has been demonstrated as an excellent candidate for mid-infrared photonics applications, including on-chip mid-infrared spectroscopy and biochemical sensing. However, this platform is often saddled by high propagation loss due to a combination of threading dislocation defects at the Ge/Si interface, absorption in the silicon for λ < 8 μm, and surface scattering due to sidewall roughness. This work investigates the effects on loss reduction through different annealing techniques on Ge-on-Si waveguides fabricated using CMOS-compatible processes. We explore the use of local laser annealing at waveguide sidewalls, whereby the fluence was varied. A non-local annealing technique in hydrogen ambient was also employed as comparison. The propagation losses for wavelengths, ranging from λ = 5 μm to λ = 11 μm, were systematically characterized by fabricating waveguide and grating coupler structures on the same chip. Cutback measurements were performed by varying the waveguide length (of the same width) from L = 1 mm to L = 4 mm. Both hydrogen and laser annealing experiments show marked reduction in the propagation loss, by up to 27% and 46% respectively. This finding paves the way for post-processing techniques to reduce propagation loss in Ge-on-Si platform, which will enable various on-chip mid-IR applications in the future.
We demonstrate a system-level low-power contactless button using MEMS ScAlN-based pyroelectric detector. As pyroelectric detectors can sense instantaneous temperature change, the human finger can act as a thermal source to activate the button. Using our in-house fabricated ScAlN-based pyroelectric detector which does not require any IR source, we package it into a contactless button system designed with electrical read-out circuits and signal processing. This contactless button system could detect the presence of a finger at a center distance measured up to ~4 cm away, ~2 cm radius circle area, suitable for application as contactless elevator button. Our contactless button system using ScAlN-based pyroelectric effect is characterized, tested and compared with a commercial contactless button. The power consumed is measured ~3.5× lower than that of commercial contactless button. The results obtained provide a potential solution towards energy efficient low-power contactless button system.
Microelectromechanical system (MEMS)-based thermal emitter is a key component in an optical sensor to provide broadband emission at mid-infrared wavelengths, where a lot of molecules have their unique absorption profile. However, the thermal emission from a MEMS emitter is typically fixed at a specific spatial coordinate. In this work, a MEMS thermal emitter with piezoelectric actuation to realize active tuning is demonstrated. Thermal emission comes from a doped silicon layer acting as a resistive heater. Piezoelectric actuation is enabled by an aluminum nitride layer on a designed cantilever. The devices are fabricated on a complementary metal-oxide semiconductor (CMOS)-compatible process line. The fabricated thermal emitter at the tip of the cantilever generates broadband MIR thermal emission with spectrum peaked around 10 μm wavelength, and piezoelectric actuation with a displacement of more than 20 μm. The work paves the way towards self-adaptable MEMS directional emitter for various applications including chemical/gas sensing.
We introduce well-developed optical proximity correction (OPC) techniques to the metasurface-based flat optics manufacturing process. Flat optics, formed by subwavelength scale nanostructure pillar (nanopillar) array, so called metasurface, has become promising substitutes for conventional bulky optical components. For its manufacturing, photolithography is preferable rather than the electron beam lithography (EBL) technique because of its time and cost effectiveness for mass manufacturing. However, the required feature size and pitch of the metasurface for the visible light is approaching the process limit of the ArF immersion lithography. It results in critical dimension (CD) errors due to optical proximity effect and could result in efficiency degradation of the flat optics. In the semiconductor manufacturing industry, OPC based on process modelling and numerical computation has been developed for the last few decades to control the CD on the wafer. Here, a machine learning (ML) model is constructed to avoid the time consumption of the conventional OPC method without losing the accuracy. Various pitches of flat optics metalens, from 465 nm to 160 nm, has been studied for the implementation of the ML OPC. The root mean square (RMS) CD errors < 1 nm and the CD accuracies < 6 nm can be achieved. The CD error percentages over the pillar diameters < 6 % is observed and the improvement of CD error and CD accuracy compared to rule based OPC in small pitches of metalens is demonstrated.
A demonstration of an on-chip CO2 gas sensor is reported. It is constructed by the integration of a MEMS-based thermal emitter, a scandium-doped aluminum nitride (ScAlN) based pyroelectric detector, and a sensing channel built on Si substrate. The integrated sensor has a small footprint of 13mm × 3mm (L×W), achieved by the replacement of bulky bench-top mid-IR source and detectors with MEMS-based thermal emitter and ScAlN-based pyroelectric detector, with their footprints occupying 3.15 mm × 3 mm and 3.45 mm × 3 mm, respectively. In addition, the performance of the integrated sensor in detecting CO2 of various concentrations in N2 ambient is also studied. The results indicate that the pyroelectric detector responds linearly to the CO2 concentration. The integration of MEMS emitter, thermal pathway substrate, and pyroelectric detector, realized through CMOS compatible process, shows the potential for massdeployment of gas sensors in environmental sensing networks.
Lithium niobate (LN) has experienced significant developments during past decades due to its versatile properties, especially its large electro-optic (EO) coefficient. For example, bulk LN-based modulators with high speeds and a superior linearity are widely used in typical fiber-optic communication systems. However, with ever-increasing demands for signal transmission capacity, the high power and large size of bulk LN-based devices pose great challenges, especially when one of its counterparts, integrated silicon photonics, has experienced dramatic developments in recent decades. Not long ago, high-quality thin-film LN on insulator (LNOI) became commercially available, which has paved the way for integrated LN photonics and opened a hot research area of LN photonics devices. LNOI allows a large refractive index contrast, thus light can be confined within a more compact structure. Together with other properties of LN, such as nonlinear/acousto-optic/pyroelectric effects, various kinds of high-performance integrated LN devices can be demonstrated. A comprehensive summary of advances in LN photonics is provided. As LN photonics has experienced several decades of development, our review includes some of the typical bulk LN devices as well as recently developed thin film LN devices. In this way, readers may be inspired by a complete picture of the evolution of this technology. We first introduce the basic material properties of LN and several key processing technologies for fabricating photonics devices. After that, various kinds of functional devices based on different effects are summarized. Finally, we give a short summary and perspective of LN photonics. We hope this review can give readers more insight into recent advances in LN photonics and contribute to the further development of LN related research.
A thermal emitter fabricated on complementary metal-oxide-semiconductor (CMOS)-compatible facilities is a key component for low-cost mid-infrared gas sensing. While conventional thermal emitters have broad spectrum and wide emission angle, which limit the sensing performance. In this work, a microelectromechanical system (MEMS)-based thermal emitter with photonic crystal has been designed and fabricated using CMOS-compatible technology. The photonic crystal enables the emission wavelength selectivity within mid-infrared regime. By engineering photonic crystal dimension, the emission enhancement wavelength can be matched to the fingerprint wavelength of chemical gas for efficient chemical gas sensing purpose.
We present the optical and electrical properties of AlN-based and 12% doped ScAlN-based pyroelectric detectors fabricated on 8-inch wafers respectively. Both AlN and ScAlN materials are deposited at a temperature of ~200oC, making them potential candidates for CMOS compatible MEMS pyroelectric detectors. FTIR spectroscopy is used to measure the absorption of these pyroelectric detectors over the wavelength range of ~2–14 μm and the results show absorption improvement up to ~75% for ScAlN-based pyroelectric detectors compared to that of AlN-based pyroelectric detectors at the wavelength of 4.26 μm where CO2 gas absorption of IR radiation is anticipated. Higher output current (~3-fold increase) is also observed from ScAlN-based pyroelectric detectors. Other than pyroelectric coefficient that contributes to improved performance for ScAlN-based pyroelectric detectors, we believe that absorptivity of the device also plays a major role in the performance of pyroelectric IR detectors. The results obtained from the study of the electrical and optical properties of AlN-based and ScAlN-based CMOS compatible MEMS pyroelectric detectors will bring forth potential applications of these detectors onto multi-functional integrable and monolithic platforms.
Gas sensors have wide applications including industrial process control, environment monitoring, safety control, etc. The distribution of these sensors enables data generation for the emerging trend of big data and internet of things. In this work, chip-based non-dispersive infrared (NDIR) gas sensors are demonstrated. Silicon substrate-integrated hollow waveguide (Si-iHWG), which is formed through silicon wafer etching and bonding, is used as optical channel and gas cell. A high sensitivity of 50 ppm for CO2 sensing is demonstrated. The Si-iHWG chip-based sensor with compactness, low cost, versatility, and robustness provides a promising platform for miniaturized gas sensing in various application scenarios.
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