Process and design techniques for low loss integrated silicon photonics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.Includes bibliographical references (p. 256-260).Microprocessors have truly revolutionized the efficiency of the world due to the high-volume and low-cost of complimentary metal oxide semiconductor (CMOS) process technology. However, the traditional scaling methods by which chips improve are soon to end. The continued drive towards smaller circuit elements and dense chip architecture has yielded to power consumption, heat production, and electromagnetic interference (RC-delay) limitations. A logical solution to surmounting this electronic interconnect bottleneck is to utilize photonic interconnects. Photonic interconnects (waveguides) offer high data bandwidths with low signal attenuation and virtually zero heat dissipation. Strategic replacement of RC speed-limited electronic interconnects with photonic interconnects is a logical step to improving data processing performance in future microprocessors. Integration of photonic circuits onto electronic chips also enables sought after networking technologies that have higher complexity and unique functionality. Similar to the integrated microchip, the employment of CMOS technology in the fabrication of integrated photonic chips enables high yield, low cost, and increased performance. Essentially, the development of an integrated CMOS compatible photonic circuit technology is an enabler of improved communication.(cont.) However, there are many challenges in realizing a viable, integrated photonic circuit technology. The constraints associated with fabrication of CMOS compatible, high-index-contrast, planar, thin-film photonic devices add difficulty in realizing the necessary components for a complete photonic circuit. Of these components: light source, waveguide, modulator, splitter, filter, and detector; all are limited in performance and functionality by optical transmission loss. As a result, this thesis has focused on diagnosing and addressing the various loss mechanisms that exist in fabricating CMOS compatible channel waveguides. As the building block of higher order photonic devices, waveguides are useful as diagnostic tools with which one can characterize photonic loss mechanisms. Waveguide test methodologies are developed to accurately diagnose the waveguide loss mechanisms (e.g. bulk absorption and interface roughness-scattering) by analyzing transmission loss (T) as a function of signal wavelength (x), waveguide width (w), waveguide height (h), effective index (neff), number of bends (N), and optical power (P).(cont.) Four high index waveguide materials are investigated: silicon on insulator (SOI), amorphous silicon (a-Si), polycrystalline silicon (poly-Si), and Silicon Nitride. The dominant loss mechanism for each material system is different and as a result, unique process and design techniques are developed for each. For SOI waveguides, the loss is dominated by sidewall roughness. As a result, a novel post-etch wet chemical oxidation smoothing method is developed to reduced sidewall roughness and improve waveguide transmission. The employment of a hybrid waveguide design further reduces SOI waveguide losses to 0.35 dB/cm. For a-Si waveguides, loss is dominated by bulk absorption arising from dangling bonds. Loss reduction is achieved by increasing the H-content in the films, thereby satisfying the dangling bonds and reducing the number of absorption sites. Amorphous silicon bulk losses are reduced from 15.2 ± 2 dB/cm to < 1 dB/cm, representing a tractable path for integrating high index contrast waveguides onto multiple chip levels. For SiN waveguides, N-H bond absorption at %=1510 nm is the dominant loss mechanism. Here the use of low H-content precursors is investigated to reduce the number of N-H bond absorption sites.(cont.) A total of six SiN materials are compared with losses as low as 1.5 dB/cm. Ring resonator devices, comprised of channel waveguides, are also investigated. Ring resonators serve as filters for multiplexing and demultiplexing broadband optical signals, dispersion compensators for accurately controlling phase, lasers, and ultrafast all-optical switches. In realizing these devices a ring trimming method is developed to compensate for non-deterministic pattern transfer errors which limit dimensional precision and preclude the fabrication of identical devices across an entire wafer. In this work, a novel photo-oxidation trimming method, using a UV-sensitive, polysilane top cladding material, is employed. The UV-induced refractive index decrease of polysilane (4%) enables accurate and localized trimming of ring resonators. Ring modulator devices are modeled as well. The employment of integrated SiGe ring modulators that utilize the fast Franz-Keyldish effect is discussed. The design constraints involved in monolithically integrating photonic and electronic components are discussed. In particular, the CMOS process challenges: material limitations, epitaxial compatibilities, thermal-budget imposed process order, and device communication requirements are utilized in arriving at an optimal application specific, electronic-photonic integrated chip (AS-EPIC) architecture.by Daniel Knight Sparacin.Ph.D

Four-channel WDM transmitter with heterogeneously integrated III-V/Si photonics and low power 32 nm CMOS drivers

Artículo científicoWe experimentally demonstrate a novel four-channel wavelength division multiplexing transmitter operating at 1.3 μm wavelength employing heterogeneously integrated III-V/Si photonic circuit copackaged with low-power 32-nm SOI CMOS driver integrated circuits (ICs). Error-free operation (BER < 10−12 ) has been achieved across all four channels for back-to-back, 2 and 10 km single-mode fiber transmission at 25 Gb/s per each channel, targeting intra- and inter-datacenter interconnect applications. Power consumption as low as 19.2 mW for four CMOS driver ICs has been recorded, which yields 0.19 pJ/bit energy efficiency

A WDM 4x28Gbps integrated silicon photonic transmitter driven by 32nm CMOS driver ICs

A four-channel WDM silicon photonic transmitter with integrated lasers and modulators driven by low-power 32nm CMOS drivers, is demonstrated to operate at a data rate of 4×28Gb/s with BER<10-12 and power consumption of 10.0pJ/bit.IEEE Communications Society,IEEE Photonics Society,The Optical Societ

Thermal Annealing Effects on the Mechanical Properties of Bio-based 3D Printed Thermosets

3D printing technologies can address many sustainability aspects of creating new materials, such as reduced waste and on demand production, which reduces the carbon footprint of transport and storage. Additionally, creating bio-based resins for 3D printing is a viable way of improving the sustainability of polymeric materials. Coupled with this, by using dynamic covalent chemistry (DCC), we can provide materials with smart properties like self-healing or reprocessability to either extend their usable lifetime or provide an alternative to the materials ending up in a landfill. Here, we report a series of completely bio-based aromatic resins for digital light projection (DLP) printing. By incorporating β-hydroxyesters and a zinc catalyst, the polymer networks can participate in transesterification reactions to provide self-healing capabilities or reprocessability. The self-healing abilities of these materials were characterized using optical microscopy, and the reprocessability using a hot-press. Additionally, by subjecting the printed thermosets to thermal annealing, considerable changes in the mechanical performance were observed leading to more than a 2000% increase in the Young’s modulus. The thermal behavior after annealing was also studied and a discussion on the effect of the structural differences between the aromatic monomers is proposed. These resin formulations address two of the key goals of sustainable materials: using renewable resources and obtaining recyclable materials while remaining competitive through their mechanical performance and compatibility with 3D printing technologies

Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer

We have experimentally demonstrated single mode amorphous silicon channel waveguides with low optical transmission loss of 2.7±0.4 dB/cm for TE mode in the 1550 nm range. This result was achieved by using hydrogen passivation of a-Si dangling bonds and a thin, low loss silicon nitride intercladding layer prepared by plasma enhanced chemical vapor deposition between the waveguide core and the oxide cladding layer. The silicon nitride intercladding layer reduces waveguide sidewall roughness scattering and preserves the hydrogen passivation

Recyclable, Biobased Photoresins for 3D Printing through Dynamic Imine Exchange

Transimination reactions are highly effective dynamic covalent reactions to enable reprocessability in thermosets, as they can undergo exchange without the need for catalysts, by exposing the materials to external stimuli such as heat. In this work, a series of five biobased vanillin derived resin formulations consisting of vanillin acrylate with vanillin methacrylate functionalized Jeffamines® were synthesized, and 3D printed using digital light projection (DLP). The resulting thermosets produced, displayed a range of mechanical properties (Young’s modulus 2.05 – 332 MPa) which allow for an array of applications. The materials we obtained have self-healing abilities which were characterized by scratch healing tests. Additionally, dynamic transimination reactions enable these thermosets to be reprocessed when thermally treated above their glass transition temperatures under high pressures using a hot-press. Due to the simple synthetic procedures and the readily available commercial Jeffamines®, these materials will aid in promoting a shift to materials with predominantly biobased content and help drift away from polymers made from non-renewable resources