Robert Pownall

Characterization of CMOS compatible waveguide-coupled leaky-mode photodetectors

Near-field scanning optical microscopy has been employed for the first time to analyze integrated photodetectors. Waveguide-coupled leaky-mode polysilicon metal-semiconductor-metal photodiodes fabricated in commercial complementary metal-oxide-semiconductor technology for on-chip optical interconnects exhibit a measured effective absorption coefficient of 0.67 dB/mum allowing a 10-mum-long detector to absorb 83% of the light in the waveguide with an estimated responsivity of 0.35 A/W at 654 nm. The measured effective absorption coefficient is in good agreement with effective index mode overlap calculations

Geometry Dependence of CMOS-Compatible, Polysilicon, Leaky-Mode Photodetectors

Complementary metal-oxide-semiconductor-compatible metal-semiconductor-metal polysilicon photodiodes fabricated in a commercial 0.35-mum technology offer estimated responsivities of up to 0.35 A/W at 654 nm. An effective absorption coefficient of 0.63 dB/mum was extracted from responsivities for 5- to 10-mum-long waveguide-coupled detectors. Increasing responsivity at smaller contact spacing indicated a two-part photocurrent response, with secondary photocurrent dominating at small contact spacings and high electric fields

Fully CMOS-Compatible On-Chip Optical Clock Distribution and Recovery

Clock distribution in the multi-gigahertz range is getting increasingly difficult due to more stringent requirements for skew and jitter on one hand and the deteriorating supply voltage integrity and process variation on the other hand. Global clock network, especially in nanometer CMOS designs with ever increasing die sizes, has become a prominent performance limiter. A potential alternative to traditional interconnect technology for achieving clock distribution beyond 10 GHz while maintaining required skew and jitter budgets is using on-chip optical interconnects. A practical on-chip optical clocking system must be CMOS compatible in order to provide attractive cost effectiveness for system level integration and ease of manufacturing. This paper presents the design of a fully CMOS compatible optical clock distribution and recovery system in a 3.3 V, 0.35-μm CMOS process. Experimental results from the test chip prove the feasibility of providing optical-electrical interface in devices and circuits in a fully CMOS compatible manufacturing environment. Although the test chips were designed in a mature CMOS process technology and the measured performance is low, the test chips demonstrated the feasibility of on-chip optoelectronic integration with fully CMOS compatible process. On-chip optical clock distribution is one of the natural applications of fully CMOS compatible on-chip optical interconnect technology.

A novel low-loss y-type splitter with adjustable branching ratio

Low loss splitters based on two mode waveguides are evaluated using near-field scanning optical microscopy and computer simulations. The branching ratio is a temperature sensitive function of the relative mode phases at the splitter.

A Waveguide Biosensors Local Evanescent Field Response to an Immunoassay Complex

A compact photonic immunoassay biosensor that can simultaneously sense multiple analytes has been implemented. NSOM results indicate 8% modulation of the local evanescent field due to an 18 nm biological adlayer on the waveguides surface.

DC and AC performance of leaky-mode metal-semiconductor-metal polysilicon photodetectors

Metal-semiconductor-metal (MSM) polysilicon photodetectors which are compatible with all standard complementary metal-oxide-semiconductor (CMOS) processes and which were made in a commercial 0.35 ìm process have demonstrated DC responsivities up to 1.3 A/W at 690 nm. An effective absorption coefficient of 0.63 dB/ìm was found from a comparison of responsivities of 5- and 10-μm long detectors. For a constant bias voltage, responsivity varies as the inverse square of the contact spacing, with responsivity continuing to increase for the smallest available contact spacing devices. Responsivities corresponding to quantum efficiencies over 200% were observed, implying a gain mechanism. For AC performance, electrical pulse full-width at half-maximum (FWHM) as low as 0.81 ns and 10% - 90% rise times as low as 0.39 ns have been measured in response to ~0.65 ns FWHM optical input pulses. The ability to modulate the source laser diode limits the measured pulse performance of the detectors. Observed DC and pulse results are well explained by an analytic expression which incorporates the effects of bulk and contact recombination. Possible means of improving the detector speed are proposed.2

AC Performance of Polysilicon Leaky-Mode MSM Photodetectors

Pulse response of polysilicon metal-semiconductor-metal (MSM) photodetectors fabricated in a standard CMOS processes is described, including demonstration of pulse full-width at half-max (FWHM) of 1.32 ns. Pulse FWHM as low as 0.81 ns has been measured, as have 10%-90% rise times of 0.39 ns. Measured detector performance is limited by laser diode modulation capabilities. An analytic expression for the time domain response in the presence of body and contact recombination is reported.

Optical characterization of a leaky-mode polysilicon photodetector using near-field scaning optical microscopy

Near-field scanning optical microscopy was used to characterize the light absorption capability of a leaky-mode coupled polysilicon photodetector fabricated for CMOS on-chip optical interconnects. The observed results are in good agreement with modal calculations.