Silicon Photonics taps on the volume manufacturing capability of traditional silicon manufacturing techniques, to
provide dramatic cost reduction for various application domains employing optical communications technology. In
addition, an important new application domain would be the implementation of high bandwidth optical interconnects in
and around CPUs. Besides volume manufacturability, Silicon Photonics also allows the monolithic integration of
multiple optical components on the same wafer to realize highly compact photonic integrated circuits (PICs), in which
functional complexity can be increased for little additional cost. An important pre-requisite for Si PICs is a device library
in which the devices are compatibly developed around a common SOI platform. A device library comprising passive and
active components was built, which includes light guiding components, wavelength-division-multiplexing (WDM)
components, switches, carrier-based Si modulators and electro-absorption based Ge/Si modulators, Ge/Si photodiodes
and avalanche photodiodes, as well as light emitting devices. By integrating various library devices, PIC test vehicles
such as monolithic PON transceivers and DWDM receivers have been demonstrated. A challenge with Si PICs lies with
the coupling of light into and out of the sub-micrometer Si waveguides. The mode size mismatch of optical fibers and Si
waveguides was addressed by developing a monolithically integrated multi-stage mode converter which offers low loss
together with relaxed fiber-to-waveguide alignment tolerances. An active assembly platform using MEMS technology
was also developed to actively align and focus light from bonded lasers into waveguides.
A taper coupler with multimode input and single mode output is presented for coupling between edge emitting laser
diode and silicon waveguide. The tapered coupler structure is optimized and tolerance for laser diode placement is
studied. A typical coupling efficiency of -2dB is achieved from laser diode to silicon waveguide. With tolerance of +/-
4μm laterally or vertically, the variation of the coupling efficiency is about 3dB. The tolerance is large compared with
other methods. Tilting angle at laser diode and the small gap between tapered coupler and silicon waveguide also affect
the overall coupling. From our studies, horizontal and vertical offsets are more critical for laser diode placement in order
to have a good coupling. The new design can be applied to photonics packaging because it will make passive assembly
easier by having larger tolerance for packaging compared with the conventional method with lens.
This paper presents design, simulation and fabrication of a wafer level packaged Microelectromechanical Systems
(MEMS) scanning mirror. In particular we emphasize on the process development and materials characterization of In-
Ag solder for a new wafer level hermetic/vacuum package using low temperature wafer bonding technology. The
micromirror is actuated with an electrostatic comb actuator and operates in resonant torsional mode. The mirror plate
size is 1.0 mm × 1.0 mm. The dynamic vibration characteristics have been analyzed by using FEM tools. With a single
rectangular torsion bar, the scanning frequency is 20 KHz. Besides, the hermetically sealed packaged is favored by
commercial applications. The wafer level package is successfully carried out at process temperature of 180°C. With
proper process design, we may lead the form a single phase of Ag<sub>2</sub>In at the bonding interface, in which it is an
intermetallic compound of high melting temperature. This new wafer level packaging approach allows us to have high
temperature stability of wafer level packaged scanning mirror devices. The wafer level packaged devices are able to
withstand the peak temperature in SMT (surface mount technology) manufacturing lines. It is a promising technology for
commercializing MEMS devices.
A new method of coupling the light from a laser diode to a Single Mode Fiber (SMF) with large alignment tolerances
and without using coupling lenses is presented. A pseudo vertical tapered coupler is designed for light coupling between
laser diode and single mode fiber. It has a large input aperture which is about 100 times the size of the laser waveguide
cross-section. The tapered coupler provides single mode output and matches the mode size with the single mode fiber.
The tapered coupler is fabricated on a silicon optical bench and is located between the laser and the fiber through the
silicon micrfabrication process. The misalignment between the fiber and taper coupler can be very small since this is
controlled by high precision silicon optical bench patterning processes. The coupler relaxes the laser diode placement
accuracies and eliminates the need for a coupling lens. Design Studies showed that the tolerance between the laser diode
and taper coupler can be more than +/-5μm misalignment at x-y, and +/-0.5degree tilting angle tolerance and the
fabricated assembly results are encouraging with good placement tolerances and coupling efficiency. The laser to single
mode fiber coupling tolerances is greatly improved and passive alignment for laser and single mode fiber is realized. The
technology can be useful for multi channel optical assembly where significant device and process cost saving can be
achieved and is suitable for functional integration for silicon photonics.
The notching and stiction problem, which widely exists in silicon on insulator (SOI) microstructure fabrication, were resolved in this study. In this paper, a new plasma trench technique that is based on the deep reactive ion etching (DRIE) process is proposed. In this modified process, the deep reactive ion etching (RIE) was divided into several steps, where conformal plasma enhanced chemical vapor deposition (PECVD) oxide coating, and directional oxide etch back were employed to prevent the notching effect and the reactive ion etching (RIE) lag effect is also improved. Therefore, the microstructures regardless of the feature sizes could be realized. The stiction problem is eliminated by using dry chemical release replacing wet release in this approach, where the notching effect is used. The notching or footing effect was exploited for attaining the lateral etch following the deployment of the anisotropic plasma etching of the inductively coupled plasma (ICP). This method was proven useful for both the uniform and non-uniform feature designs. With this novel process, the high aspect ratio beams can be obtained. The thickness of the silicon layer is 75 μm, while the depth of the beams is 70 μum where the 5 μm silicon was etched to suspend the movable beams. The aspect ratio is as high as 35. Trenches with very different widths of 2.5 μm and 35 μm are also achieved at the same time.
A tunable optical add/drop multiplexer (OADM) is demonstrated byu sing a micromachined 2 × 2 optical switch and a tunable fiber Bragg grating (FBG). The new hybrid OADM can be tuned to add/drop one of the multi input channels dynamically. The insertion loss of the dropped channel and added channel are 2.84 dB and 1.8 dB respectively. The transmission loss is 2.04 dB. These losses are all able to reduce greatlyi f the circulators are modified. The crosstalk between channels is less than -20 dB, and it can be further reduced byi mproving the reflectivityo f FBG. The tuning speed is on the order of millisecond. The tuning range is 3.0nm. The fabricated system is demonstrated by selectively adding/dropping one of the adjacent four channels with the spacing of 100GHz. Systems with multi channels being dropped and / or added can be achieved bycas cading the proposed structure.
This paper describes LISA (Lateral isolated Silicon Accelerometer) technology developed by IME< Singapore and its application on silicon vertical optical switch fabrication. Key processes in LISA technology for optical switch fabrication include deep trench etch and oxide refill to enable insulating anchors in silicon substrate, second deep trench etch to fabricate movable microstructures and metal layer covering for switch surface improvement. In this paper, deep trench (deeper than 35 um) oxide refill process is introduced, the dielectric characteristic of the isolation is evaluated, and more than 100V breakdown voltage is obtained, which is much higher that the requirement in optical switch driving voltage. Some process issues related to high aspect ratio trench etch and release such as notching on silicon beam top and sidewall are shown and discussed, a double spacer process is utilized accordingly to solve the issues. Besides, a mask free metal coating process is presented to improve the mirror surface and light reflectivity. The vertical optical mirrors fabricated by the LISA technology is 35um in height and um in width, the switch displacement is larger than 40um under 35V DC bias, the optical characteristics of the switch is under testing.
Proc. SPIE. 4582, Optical Switching and Optical Interconnection
KEYWORDS: Quantum wells, Signal attenuation, Wavelength division multiplexing, Single mode fibers, Control systems, Micromirrors, Deep reactive ion etching, Optical switching, Variable optical attenuators, Optical interconnects
Variable optical attenuator (VOA) is undergoing to be a mainstream component of wavelength division multiplex (WDM) networks to monitor and control the optical power of wavelength channels. In this paper, a free-space VOA fabricated by micro electromechanical systems (MEMS) technology to operate in the 1.55 micrometers wavelength region is described. It employs a micromirror driven by an electrostatic comb drive to cut partially into the light beam between two single mode fibers (SMFs), enabling the attenuation. The micromirror has a size of 30 micrometers X 30 micrometers and is coated with aluminum to increase the reflectance. The moving fingers of comb drive and the micromirror are supported by folded suspension beams over the substrate. By applying different voltage to the comb drive, the micromirror translates to different position to achieve an attenuation ranging from 0.4dB to 50dB, and even higher. The nonlinear relationship between the position of the micromirror and attenuation is analyzed. The distributions of the light beams at the micromirror and the output fiber end are investigated respectively. And the influence of the separations between the micromirror, the input and output fiber ends is also discussed to obtain different attenuation resolutions. At low attenuation stages, fine tuning of attenuation is obtainable. The largest attenuation is driven by 21voltage. Deep reactive ion etching (DRIE) process is employed to fabricate the VOA and the micro loading effect is remedied by mask design.
High speed, low insertion loss optical add/drop multiplexer (ADM) is designed and fabricated. The optical vertical micromirror is fabricated by deep dry etching, the aspect ratio reaches as high as 20. A thin aluminum film is deposited on the sidewall of the micromirror to increase the reflectivity. The anchors and pads are fabricated firstly, followed by the comb drive, micromirror and fiber grooves. Refilling technique is introduced to electrically insulate the anchors and pads from the substrate while still maintaining the mechanical support. The anchors and pads are strong enough to sustain the floating structures (micromirror and moving comb) and also assure good electrical connection to the electrostatic comb drive so that the external voltage can be applied. By improving dry etching, the finger width is only 2micrometers and the gap is only 2.5micrometers . A typical electrostatic comb drive is fabricated by the deep reactive ion etching (RIE) and underneath releasing. Folded suspension beams of 800micrometers long, 2.0 micrometers wide and 35micrometers deep are employed to support the movable micromirror. The stiffness along the desired lateral direction is 0.21N/m. Comb drive using three electrodes is employed. Its applied voltage is decreased by a ratio of 0.707 compared with that of the two electrodes system, and the switching speed is also increased. To simply the optical fiber assembly, fiber grooves are fabricated along with the other structures. This device has a typical of optical ADM that can be widely used in all optical networks. All of the processes are compatible with IC technology and can be integrated with control circuits in a single chip.