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integrated circuits An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny M ...
, optical interconnects refers to any system of transmitting signals from one part of an integrated circuit to another using light. Optical interconnects have been the topic of study due to the high latency and power consumption incurred by conventional metal interconnects in transmitting electrical signals over long distances, such as in interconnects classed as '' global interconnects''. The
International Technology Roadmap for Semiconductors The International Technology Roadmap for Semiconductors (ITRS) is a set of documents produced by a group of semiconductor industry experts. These experts are representative of the sponsoring organisations which include the Semiconductor Industry ...
(ITRS) has highlighted interconnect scaling as a problem for the semiconductor industry. In electrical interconnects, nonlinear
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
s (e.g. digital signals) are transmitted by copper wires conventionally, and these electrical wires all have resistance and
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
which severely limits the rise time of signals when the dimension of the wires are scaled down. Optical solution are used to transmit signals through long distances to substitute interconnection between dies within the
integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny M ...
(IC) package. In order to control the optical signals inside the small IC package properly,
microelectromechanical system Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
(MEMS) technology can be used to integrate the optical components (i.e.
optical waveguide An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light g ...
s,
optical fibers An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...
,
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
,
mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the ima ...
s, optical
actuators An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) a ...
, optical
sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
etc.) and the electronic parts together effectively.


Problems of the current interconnect in the package

Conventional physical metal wires possess both resistance and
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
, limiting the rise time of signals. Bits of information will overlap with each other when the frequency of signal is increased to a certain level.David A. B. Miller, ‘Rationale and Challenges for Optical Interconnects to Electronic Chips’, Proceedings of the IEEE, Vol. 88, No. 6, June 2000


Benefits of using optical interconnection

Optical interconnections can provide benefits over conventional metal wires which include: # More predictable timing # Reduction of power and area for clock distribution # Distance independence of performance of optical interconnects # No frequency-dependent Cross-talk # Architectural advantages # Reducing
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may ...
dissipation in interconnects # Voltage isolation # Density of interconnects # Reducing the wiring layers # Chips could be tested in a non-contact optical test set # Benefits of short optical pulses


Challenges for optical interconnect

However, there are still many technical challenges in implementing dense optical interconnects to silicon CMOS chips. These challenges are listed as below: R.K. Dokania and A.B. Apsel,"Analysis of Challenges for On-Chip Optical Interconnects", ACM Proceedings of Great Lakes Symposium on VLSI, May 10-12 , 2009, Boston # Receiver circuits and low-capacitance integration of
photodetectors Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation. There is a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by ...
# Evolutionary improvement in optoelectronic devices # Absence of appropriate practical optomechanical technology # Integration technologies # Polarization control # Temperature dependencies and process variation # Losses and errors # Testability # Packaging


See also

*
Organic light-emitting transistor An organic light-emitting transistor (OLET) is a form of transistor that emits light. These transistors have potential for digital displays and on-chip optical interconnects. OLET is a new light-emission concept, providing planar light sources that ...
*
Silicon photonics Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic components. These operate in the infrared, most commo ...


References

{{reflist Integrated circuits