Shashikant Hegde

Chip-to-chip optoelectronics SOP on organic boards or packages

In this paper, we demonstrate compatibility of hybrid, large-scale integration of both active and passive devices and components onto standard printed wiring boards in order to address mixed signal system-on-package (SOP)-based systems and applications. Fabrication, integration and characterization of high density passive components are presented, which includes the first time fabrication on FR-4 boards of a polymer buffer layer with nano scale local smoothness, blazed polymer surface relief gratings recorded by incoherent illumination, arrays of polymer micro lenses, and embedded bare die commercial p-i-n photodetectors. These embedded optical components are the essential building blocks toward a highly integrated SOP technology. The effort in this research demonstrates the potential for merging high-performance optical functions with traditional digital and radio frequency (RF) electronics onto large area and low-cost manufacturing methodologies for multifunction applications.

System-level reliability assessment of mixed-signal convergent microsystems

The next-generation convergent microsystems, based on system-on-package (SOP) technology, require up-front system-level design-for-reliability approaches and appropriate reliability assessment methodologies to guarantee the reliability of digital, optical, and radio frequency (RF) functions, as well as their interfaces. Systems approach to reliability requires the development of: i) physics-based reliability models for various failure mechanisms associated with digital, optical, and RF Functions, and their interfaces in the system; ii) design optimization models for the selection of suitable materials and processing conditions for reliability, as well as functionality; and iii) system-level reliability models understanding the component and functional interaction. This paper presents the reliability assessment of digital, optical, and RF functions in SOP-based microsystems. Upfront physics-based design-for-reliability models for various functional failure mechanisms are presented to evaluate various design options and material selection even before the prototypes are made. Advanced modeling methodologies and algorithms to accommodate material length scale effects due to enhanced system integration and miniaturization are presented. System-level mixed-signal reliability is discussed thorough system-level reliability metrics relating component-level failure mechanisms to system-level signal integrity, as well as statistical aspects.

Design, fabrication, and reliability testing of embedded optical interconnects on package

Optical interconnections offer a potential solution to some of the bottlenecks that electrical interconnection systems face. Some of the challenges with optical interconnect design and development are the introduction of new optoelectronic materials and processes, the increasing bandwidth and decreasing loss requirements, and the need for reducing cost without compromising reliability. The goal of this paper is to report our ongoing work on optical interconnects and interfaces for very short-haul backplane, and board/package-level chip-to-chip interconnections. This research fits in with our future proposed integration technology of embedded optoelectronic active devices such as detector arrays, optical amplifiers and laser arrays coupled to the waveguide by alignment tolerant beam turning passive elements. Through testbed design, fabrication, and reliability testing, the waveguide polymer materials and integration process is evaluated, based on a number of factors which include: optical loss, roughness, optical aging, spectral absorption, refractive index and thermal stability.

Enhanced Reliability of High Density Wiring (HDW) Substrates Through New Dielectric and Base Substrate Materials

FR4 has been extensively used as a board material due to its cost effectiveness and overall performance. For high-density wiring (HDW) substrates with microvias and with embedded capacitors, inductors, resistors, RF and optoelectronic waveguides in a single board, there is a need for alternative base substrates to meet the stringent warpage requirements during fabrication. Typically, these base substrate materials should have high modulus, good planarity, in addition to having a CTE that is close to that of silicon so that the flip-chips can be directly attached to the substrate, eliminating the need for an underfill. Although low-CTE, high modulus base substrate materials can eliminate the need for underfill as well as can result in less warpage during fabrication, they can potentially cause delamination and cracking in the interlayer dielectric. This is due to the high CTE mismatch between the base substrate and typical polymer dielectric. This paper aims to explore a combination of base substrate materials and interlayer dielectric materials such that warpage is minimal, dielectric will not crack or delaminate, and flip-chip solder joints, assembled without an underfill, will not crack prematurely during qualification regimes and operating conditions. A non-liner finite element model with Design-of-Simulations approach is used in arriving at optimized thermo-mechanical properties for the base-substrate and dielectric materials to enhance the overall reliability of the integrated substrate with flip chip assembly. The results from the models have been compared with experimental data and a discussion is presented on the various failure modes.Copyright

Thermal-mechanical analysis of Terabus high-speed optoelectronic package

Terabus is a high-speed optoelectronic package designed to run at 10-20 Gb/s/channel over 48 channels of optical/electrical links. The Terabus package consists of an optochip that can be passively attached to an optocard with processes similar to those in microelectronic packaging. Due to the high degree of integration, thermal, mechanical, and assembly challenges arise, and need to be addressed. The thermal requirements for the cooling system include handling a heat flux of 60 W/cm/sup 2/, and keeping the vertical-cavity surface-emitting lasers (VCSELs) at a temperature <85 /spl deg/C. A heat pipe cooling system which satisfies the thermal requirements is designed, assembled and demonstrated. Thermal analysis with finite-element modeling is used to study local temperature distribution in the laser chip. In addition, the model is used to study evolution of stresses in the structure during thermal excursions.

High-density, end-to-end optoelectronic integration and packaging for digital-optical interconnect systems (Invited Paper)

Recent progress toward implementing high-density, optical-digital building blocks necessary to accomplish efficient, end-to-end optical interconnect architecture on low cost FR-4 boards has been demonstrated. The optical interconnect system consists of fabricating an optical buffer layer separating board metallurgy from the optical lightwave circuit layer, and implementing optical links between embedded lasers and detectors. We will show an example of 1310 nm light from an edge emitting distributed-feedback or Fabry-Perot laser operating at 10 Gb/s being guided to the photo-detector by a polymer waveguide. Both lasers and detector are embedded in the waveguide and all construction is built on a low-cost FR-4 board with 3 levels of metallurgy.

Materials, processes and reliability of mixed-signal substrates for SOP technology

Materials, processes and reliability challenges in mixed-signal (Digital, Optical and RF) microvia substrates for System-on-a-package (SOP) technology are presented. Models and methodologies to thermo-mechanically evaluate the microvia substrate reliability are discussed. Upfront process mechanics models with design of simulations approach are presented to evaluate various dielectrics and substrate materials with respect to warpage, dielectric cracking and microvia reliability in multi-layered microvia boards. Systematic optimization studies are conducted to arrive at appropriate set of material and geometry parameters to minimize the inelastic strain in the microvias, the film stress in the dielectric, and the warpage in the substrate. The test vehicles are subjected to liquid-to-liquid thermal shock cycles between -55/spl deg/C to 125/spl deg/C to assess reliability and model validation. Material length scale effects due to reduced feature sizes of microvias (10 microns or less) are addressed through computational algorithms to simulate the increased plastic strain hardening effects due to spatial strain gradients. System-level mixed-signal reliability is also discussed taking into consideration component-level reliability as well as statistical implications.

Thermal Aging Reliability of Package-Level Polymer Optical Waveguides

Polymer optical waveguides are viewed as a potential interconnect solution in board-level optoelectronic systems. In this paper, the optical loss changes in siloxane polymer waveguides during thermal aging conditions are studied for the wavelengths of 850 and 1310 nm. The optical loss in waveguides during intended operation and temperature exposure can increase due to factors such as oxidation of waveguides, increased absorption, and scattering. In addition to these inherent changes in the optical properties of the waveguides, physical failures such as delamination and cracking of waveguides will also increase the optical loss. This paper focuses on the first set of parameters that affects the optical loss and as a first step; the optical absorption of the polymer material is characterized through spectroscopy experiments. The thermal-aging dependent optical loss is determined for waveguide samples at several different accelerated temperature conditions. The temperature contours in a polymer waveguide with an embedded laser are determined from experiments as well as finite-element modeling. Using experimental data, analytical models have been developed that relate the optical loss with temperature and time, and provide a practical way of determining the reliability of the optical waveguides during field-use conditions.

Stress-induced birefringence in siloxane polymer waveguides

With the increasing use of siloxane polymers as optical waveguide material, there is a need to understand and thus to reduce the stress-induced birefringence in siloxane polymer due to thermal processing and material property mismatch. In this letter, stress-optical coefficients of a siloxane polymer material are experimentally determined by measuring the refractive indices of the uncured and cured siloxane polymer material in two orthogonal directions. Employing such coefficients and temperature- and direction-dependent material properties, a numerical modeling method is presented to determine the stress-induced birefringence in an optical waveguide system. In the case study that is presented in this work, it is seen that the coefficient of thermal expansion of the planarization layer has the maximum effect on the birefringence, and it is possible to reduce the stress-induced birefringence by reducing the property mismatch between the planarization layer and the core layer. The outlined methodology is ge...

Design-for-Reliability Tools for Highlyintegrated System-on-Package Technology

With the dramatic advances made in Microsystems industry, System-on-a-Package (SOP) technology holds promise in terms of reduction in size, cost, and improved performance. To be able to achieve such benefits in an integrated system, it is necessary not to compromise the overall reliability of the system. Therefore, the SOP technology will require up-front system-level design-for-reliability approaches and appropriate reliability assessment methodologies to ensure the reliability of digital, optical, and RF functions as well as their interfaces. Design-for-reliability requires (i) Mechanics-based reliability prediction models for various failure mechanisms associated with Digital, Optical, and RF Functions, and their interfaces in the system (ii) Design optimization models for the selection of suitable materials and processing conditions, for reliability as well as functionality and (iii) System-level reliability models understanding the component and functional interaction. This presentation will focus on the reliability assessment of digital, optical, and RF functions in SOP-based microsystems [1]. Upfront physics-based design-for-reliability models for various functional failure mechanisms are presented to evaluate various design options and material selection even before the prototypes are made. Advanced modeling methodologies and algorithms to accommodate material length scale effects, due to enhanced system integration and miniaturization are presented. System-level mixed-signal reliability is discussed thorough system-level reliability metrics relating component level failure mechanisms to system-level signal integrity as well as statistical aspects.