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Micro/Nano Lithography

Integrating porous silicon with microsystems for chip-to-chip communications

Micro-optical components—including reflective mirrors and diffractive optical elements—made by new fabrication approaches enable optical interconnects to speed data between chips.
1 August 2008, SPIE Newsroom. DOI: 10.1117/2.1200807.1189

The rapid development of integrated circuit (IC) technologies has relied on many technological developments, including the miniaturization of transistors. This greatly increased the complexity of traditional metal interconnect architectures, leading to larger parasitic capacitance and signal propagation delay. Subsequently, the chip-to-chip communication delay known as the interconnect bottleneck1,2 is becoming the primary roadblock to computation speed. One possible solution to this problem is to use free-space optical interconnects (FSOIs), which have numerous advantages over metal wiring, including higher signal transmission speed, larger signal capacity, lower noise, and lower power expenditure.

Various FSOIs based on vertical-cavity-surface-emitting lasers (VCSELs) have been developed at chip and board levels. However, most existing FSOI systems—for example, as developed by Rong Wang3 and Marc Châteauneuf4—use large packages to integrate the discrete FSOI components with the optoelectronic and microelectronic modules. To create mass-producible microscale FSOI components that can be fully integrated with these modules, we propose new microfabrication approaches that combine porous silicon (PSi) with micro-optoelectromechanical systems (MOEMS). We developed several types of FSOI components based on this method.

It is well known that PSi multilayer structures can be created and separated from the substrate by electrochemical etching of silicon in an electrolyte containing hydrofluoric acid and ethanol.5–7 By incorporating the PSi optical structure fabrication steps into the MOEMS component production process, two types of devices have been created to address various interconnect application needs: membrane structures for light-beam focusing and redirecting, and freestanding structures for light beam steering, filtering, and splitting. Both designs contain a PSi part that determines the optical properties of the component and a mechanical part that is responsible for the movement of the optical component.

Figure 1. Partial cross-section scanning electron microscope (SEM) image of a membrane device. PSi: Porous silicon. DBR: Distributed Bragg grating.

For both types of components, the mechanical part incorporates a a gold/silicon nitride (Au/Si3N4) thermal bimorph actuator that generates drive forces to move the optical component to specific deflection angles at a low drive voltage. Also, in both cases, the properties of the optical part are determined by the design of the PSi multilayer structure. Specially designed PSi optical structures such as distributed Bragg reflectors (DBRs) and Fabry – Perot interferometers can be easily realized for specific wavelength ranges by adjusting the applied current during the electrochemical etching for different applications.8–10

Figure 1 shows the fabrication result of one typical 480×480μm membrane component.11 The PSi multilayer DBR membrane is suspended over an air gap and supported by the Au/Si3N4 actuator. When the drive voltage is ramped from 5 to 30V, the PSi membrane is deformed and the corresponding radius of curvature changes from 91.2 to 11.6mm. This type of device can serve either as an adjustable reflector or a concave mirror with focusing ability for dynamic misalignment compensation.

Figure 2. SEM micrographs of freestanding structures with double-arm support in a 4×4 array. (inset) Magnified image of two devices from this array.

Figure 2 shows the freestanding components with a 200×200μm optical part arranged in a 4×4 array.11 Each single device can be addressed and controlled individually. Both the direct current (DC) and alternating current (AC) performances of the devices have been evaluated. A 5V DC drive voltage applied to the device results in a tilting angle of 7°, while the AC response of such a device demonstrates a resonant frequency of 6.3kHz. These components can serve either as optical switches or optical filters for a chip-to-chip interconnect depending on PSi structure. Moreover, we created a new type of freestanding diffractive optical component (DOE) by using a layer of patterned photoresist as a protective mask during the creation of PSi multilayer structures.12,13 Figure 3(a) and (b) shows one such DOE and its corresponding diffraction pattern.11 This type of component can be used as an adjustable beam splitter to redistribute the single beam input signal into multichannel outputs.

Figure 3. SEM micrograph of a 200×200μm freestanding PSi diffractive optical element with 10μm periodic spacing. (b) Diffraction pattern from the same component at an incident laser wavelength of 543.4nm.

In summary, both membrane and freestanding structures of PSi-based MOEMS adjustable micromirrors have been designed and fabricated. PSi DOEs as beam splitters have also been created using a photoresist mask during electrochemical etching. In the near future, we will further improve the mechanical and optical performance of the components and develop fabrication processes. This will allow us to integrate the proposed components with optoelectronic and microelectronic modules on the wafer level, to realize chip-to-chip free-space optical interconnects.

This work was funded by the Semiconductor Research Corporation (contract KC1292).

Da Song, Natalya Tokranova, Alison Gracias, James Castracane
College of Nanoscale Science and Engineering
University at Albany
State University of New York (SUNY)
Albany, NY 

Da Song is currently a PhD candidate. His research focuses on the design and fabrication of porous silicon optoelectronic devices using MOEMS technology. His present work is supported by the Semiconductor Research Corporation.

Natalya Tokranova has 18 years of experience in the design, fabrication, and characterization of silicon semiconductor devices, including position-sensitive detectors and microelectromechanical systems (MEMS) devices. She has more than 28 papers published in international peer-reviewed journals and three patents. Since 2000 she has been working at the College of Nanoscale Science and Engineering.

Alison Gracias joined the College of Nanoscale Science and Engineering as a process engineer in 2003, and is currently pursuing a master's degree.

James Castracane is currently the director of the Center for Advanced Technology in Nanomaterials and Nanoelectronics, and a professor at the College of Nanoscale Science and Engineering. His research interests include MEMS, electro-optics, and biomedical instrumentation.