10-07-2012, 10:58 AM
High speed smart pixel arrays (SPAs)
-SMART-PIXEL-ARRAY nitin.doc (Size: 2.02 MB / Downloads: 54)
INTRODUCTION
High speed smart pixel arrays (SPAs) hold great promise as an enabling technology for board-to-board interconnections in digital systems. SPAs may be considered an extension of a class of optoelectronic components that have existed for over a decade, that of optoelectronic integrated circuits (OEICs).
The vast majority of development in OEICs has involved the integration of electronic receivers with optical detectors and electronic drivers with optical sources or modulators. In addition, very little of this development has involved more than a single optical channel. But OEICs have underpinned much of the advancement in serial fiber links. SPAs encompass an extension of these optoelectronic components into arrays in which each element of the array has a signal processing capability. Thus, a SPA may be described as an array of optoelectronic circuits for which each circuit possesses the property of signal processing and, at a minimum, optical input or optical output (most SPAs will have both optical input and output).
The name smart pixel is combination of two ideas, "pixel" is an image processing term denoting a small part, or quantized fragment of an image, the word "smart" is coined from standard electronics and reflects the presence of logic circuits. Together they describe a myriad of devices.
These smart pixels can be almost entirely optical in nature, perhaps using the non-linear optical properties of a material to manipulate optical data, or they can be mainly electronic, for instance a photoreceiver coupled with some electronic switching.
Smart pixel arrays for board-to-board optical interconnects may be used for either backplane communications or for distributed board-to-board communications, the latter known as 3-D packaging. The former is seen as the more near-term of the two,
employing free-space optical beams connecting SPAs located on the ends of printed circuit boards in place of the current state-of-the-art, multi-level electrical interconnected boards. 3-D systems, on the other hand, are distributed board-to-board optical interconnects, exploiting the third dimension and possibly employing holographic interconnect elements to achieve global connectivity (very difficult with electrical interconnects).
Most work in high speed SPAs has involved the use of either multiple-quantum-well (MQW) modulators or vertical-cavity surface-emitting lasers (VCSELs) as the optical source, and each of these has taken one of two approaches, monolithic and hybrid (e.g., monolithic VCSELs/GaAs and hybrid VCSELs/Si). The hybrid approaches are rapidly gaining popularity since they can take advantage of mainstream silicon microelectronics for the pixel logic circuitry, thereby leveraging the 30 billion dollar silicon semiconductor industry.
LIGHT SOURCE MODULATION USING VCSELS
The figure shows a very simple depiction of a VCSEL showing the substrate, layers of GaAs and AlAs that form the Bragg planes, the quantum well region where gain occurs, the p and n doped regions that make the p.n. diode junction
In a discussion of light source modulated smart pixels, it is necessary to understand the devices that produce the light. The Vertical Cavity Surface Emitting Laser (VCSEL) is a very important and useful light source.
VCSELS utilize a quantum well structure to confine charges to an active region much like edge emitting lasers. The main difference between VCSELS and other semiconductor lasers is the vertical structure. Most semiconductor lasers are planar and emit out of the edge facet on all sides. This configuration allows more active region than in VCSELS. The vertical lasers are constructed from the same planar epitaxy method as the edge emitting lasers, then etch back is used to produce a cylindrical structure. Because the light spends a relatively small amount of time in the gain region, it is necessary to optimize the cavity. Layers are grown such that they form Bragg planes so that light with the desired wavelength is preferentially propagated. This structure is illustrated in figure. The VCSEL is crucial to smart pixel applications because of the ability of VCSELS to form two dimensional arrays. They are constructed out of material that is convenient for fabrication of photodetectors and in some cases logic, so, devices like VCSELS and FCSELS can be utilized in monolithic smart pixels.
THE VCSEL/SI SMART PIXEL ARRAYS
The VCSEL-based SPAs that will be discussed are hybrid components involving GaAs optoelectronic chips and Si electronic chips. Creating a hybrid Si/GaAs structure involves epitaxially growing GaAs on Si or bonding the two together. Although the former is likely to lead to faster SPAs, it has proven to be a low yield process because of the large lattice mismatch that exists between GaAs and Si, leading to unacceptable GaAs defect levels for fabricating laser diodes.
One way to combine the GaAs and Si chips is to mount both onto a common base substrate which can support electrical microstrips between the two chips. The conventional way of doing this is to bond both to the base substrate with their device sides up and then to electrically connect them by wire bonding both chips to the microstrips and their associated bonding pads on the base substrate.
For large array sizes, an unrealistic amount of space on the chips and on the base substrate will be devoted to bonding pads, and the length of the electrical connections between the chips will defeat much, if not all, of the advantage of the optical interconnects. Borrowing a technique from the emerging technology of multi-chip module (MCM) fabrication, the chips can be placed device-side down (called flip-chip) and bump bonded to the carrier.