Electro-optic Platform for Free Space CMOS Photonics.

Abstract

Lab-on-chip (LOC) systems are becoming increasingly popular for biomedical science as they present the opportunity to combine compact and efficient microelectronics together with microfluidics enabling new applications in point-of-care diagnostics. This system integration however, poses significant challenges in the assembly of such devices for mass manufacture. Specifically, to achieve a robust fluidic isolation, an insulating material must be deposited to seal the chip and wire bonds but allow fluid to access the sensing surface. This is typically achieved by using an insulating epoxy for encapsulation but requires several processing steps in order to become planar and reliably interface with the microfluidics; a technique with many limitations.

Towards addressing these challenges, this thesis proposes to develop a non-galvanic means of achieving both power transfer to the chip and bi-directional data communication such that the system requires no bond pads (or delicate bond wires). The aim is to achieve this specifically via a free space optical link (i.e. to external discrete optoelectronic devices) with the additional constraint that any structures designed on chip are implementable in a commercially available, unmodified CMOS technology. Furthermore, in order to maintain the desired benefits of a “bondpad-less” chip, the platform must utilise no off-chip components. This thesis develops the underlying devices required towards achieving the aim whilst satisfying all constraints. Specifically devices tasked with optical energy harvesting, optical data input and optical data output are tackled.

The thesis begins by outlining the motivations for this research (Chapter 1) and reviewing the relevant state-of-the-art (Chapter 2) including a concise overview of alternative methods for achieving the underlying aims. The relevant theory, pertinent to electro-optical phenomena at semiconductor junctions is then developed within the context of CMOS technology (Chapter 3). More fundamental, background theory is also included in appendix A. That pertains to the propagation of light in Silicon and mechanisms of photon absorption in doped Silicon. Original contributions within the domain of theory include developing the phenomenon of free-carrier absorption (FCA) applied to a realistic, CMOS-based junction, identifying key variables, expressions and analyzing the expected level of performance.

Then, for the first time, this thesis demonstrates free-space optical modulation in a standard CMOS technology. A large portion of this work (Chapter 4 - for design repository see appendix B) is thus devoted to the design, implementation and testing of prototype devices for use as data read-out elements. A variety of modulator devices featuring differing geometries, created by distinct doping procedures and implemented in different CMOS technological nodes (UMC 0.13 [mu]m, IBM 0.18 [mu]m and AMS 0.35 [mu]m) are presented, tested and compared. This allows for modulator performance to be examined in relation to key design choices made at the physical device layout and technology choice levels.

This thesis then develops the common data read-in and power scavenging mechanism, along with associated circuits (Chapter 5). Once again structures designed with different geometries and created by different manufacturing processes in different technological nodes are presented, tested and compared, yielding an indication towards underlying trends. Key contributions here include extracted photodiode model parameters that express the contributions of vertical and lateral junction components to photocurrent generation, by junction 'family' and by CMOS process. This provides a powerful resource to circuit designers requiring a first estimate to phototransduction efficiency in technologies with unspecified optoelectronic devices.