Reminder: Salman Latif PhD Orals Thursday, 2:30pm, CIS-X Auditorium

Salman Latif slatif at
Wed Mar 11 02:14:43 PDT 2009

University Ph.D. Oral Examination

Low Capacitance Silicon CMOS Photodetectors for Optical Interconnects

Salman Latif

Department of Electrical Engineering
Stanford University

CIS-X Auditorium (X-101)

Thursday, March 12, 2009
2:30 PM - 3:30 PM
(Refreshments served at 2:15 PM)


The feasibility of Silicon as a platform for optoelectronics has 
generated intense research efforts and publicity in the last few 
years. One of the areas in which these advances in Silicon photonics 
have the potential to make an impact is the interconnect part of 
electrical systems. As data rates scale to higher speeds, electrical 
interconnects require increased power dissipation and signal 
processing complexity. For example, interconnects take up to 50% of 
microprocessor power, and this portion is expected to rise to 80% in 
the future. Given that power dissipation is a critical parameter in 
today's electronic systems, the use of optics to carry data to and 
from VLSI chips is an attractive option if it can be done at a low 
enough power. To meet future interconnect scaling requirements, 
optical output devices need to have energies of ~10 fJ/bit for I/O. 
This places a huge constraint on the capacitance of photodetectors, 
which form the receiving front end of optical interconnect links. We 
estimate that photodetector capacitances of the order of 1 fF are 
required. These numbers are achievable for small area photodetectors 
fabricated entirely in a Silicon CMOS process that are directly 
integrated with the receiving circuitry at the transistor level.

This talk will present our work on the design, fabrication, 
characterization, and system level demonstrations of various Silicon 
photodetector devices. First we will describe the characterization of 
CMOS compatible detectors fabricated in a commercial Silicon on 
Sapphire (SOS) CMOS process. Detector response times of ~ 35 ps have 
been measured, and devices have capacitance as low as ~ 4 fF. Next, 
these photodetectors are integrated with additional circuitry to 
implement optically triggered sampler circuits on chip. These 
circuits enable us to form a high speed oscilloscope that can measure 
high bandwidth analog signals on-chip. We demonstrate the complete 
capture of a 20 GHz on-chip signal, and precise measurement of skew 
between two separate chip locations. Finally, we present the design 
of nano-scale photodetectors fabricated on a Silicon-on-Oxide 
platform. These detectors have physical dimensions of the order of 
150 nm, and are integrated with optical dipole antennas to resonantly 
enhance responsivity. We measure response times of ~2 ps from 
nano-scale MSM photodetectors fabricated on this platform. Such 
sub-wavelength scale photodetectors offer the promise of 
optoelectronic integration at the scale of transistor dimensions, and 
coupled with resonantly enhanced detection techniques, would result 
in significant power, speed, and area gains.

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