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«A Dissertation Presented to The Academic Faculty by Shashikant G. Hegde In Partial Fulfillment of the Requirements for the Degree Doctor of ...»

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INVESTIGATION OF OPTICAL LOSS CHANGES IN SILOXANE

POLYMER WAVEGUIDES DURING THERMAL CURING AND

AGING

A Dissertation

Presented to

The Academic Faculty

by

Shashikant G. Hegde

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy in the

G.W. Woodruff School of Mechanical Engineering

Georgia Institute of Technology

April 2008

COPYRIGHT 2008 BY SHASHIKANT G. HEGDE

INVESTIGATION OF OPTICAL LOSS CHANGES IN SILOXANE

POLYMER WAVEGUIDES DURING THERMAL CURING AND

AGING

Approved by:

Dr. Suresh Sitaraman, Advisor Dr. Gee-Kung Chang School of Mechanical Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Dr. Jonathan Colton Dr. Madhavan Swaminathan School of Mechanical Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Dr. Yogendra Joshi Dr. Patrick Thompson School of Mechanical Engineering Microelectronic Packaging Georgia Institute of Technology Texas Instruments Date Approved: [Dec 5, 2007] ii

ACKNOWLEDGEMENTS

I wish to extend my deepest appreciation to my research advisor, Prof. Suresh Sitaraman who believed in my abilities as an incoming graduate student and introduced me to the field of semiconductor packaging. Over the many years of my graduate study, his constant guidance and encouragement has proved invaluable in my research activities.

Furthermore I would like to thank the members of my thesis committee, Prof. Jonathan Colton, Prof. Yogendra Joshi, Prof. Gee-Kung Chang, Prof. Madhavan Swaminathan and Dr. Patrick Thompson for their valuable feedback.

I am grateful to the Packaging Research Center especially Prof. Rao Tummala, Prof. C.P.

Wong, Dr. Fuhan Liu, Dr. Dan Guidotti, Dr. Mahadevan Iyer for their helpful advice. I also acknowledge fellow graduate students that I had frequent interactions with during my experiments: Fengtao Wang, Yin-Jung Chang, Yangyang Sun, Harley Hayden, and Ramesh Singh. I thank Dr. Brent Wagner for his assistance in the spectroscopy experiments. I thank Swanny Liem for her support. Moreover, I appreciate all the past and current members of the CASPaR lab for their friendship and useful discussions.

Several companies have played a role in this research work. I would like to thank Texas Instruments for supporting this program through Semiconductor Research Corporation. I thank IBM Corporation and Hitachi Chemical for the internship experiences, which greatly improved my research abilities, and provided me relevant industry exposure.

Finally I thank my parents and my sister, Shalu for their encouragement and support during my course of study.

–  –  –

ACKNOWLEDGEMENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS

LIST OF ABBREVIATIONS

SUMMARY

CHAPTER 1. INTRODUCTION

1.1 Optoelectronic Package Technologies

1.2 Polymer Optical Waveguide Materials

1.3 Factors affecting Optical Loss in Waveguides

CHAPTER 2. LITERATURE REVIEW

2.1 Optical Loss Mechanisms

2.2 Effect of Cure Conditions

2.3 Stress-Birefringence Effects

2.4 Effect of Thermal Aging

CHAPTER 3. RESEARCH GAPS, OBJECTIVES AND APPROACH

3.1 Research Gaps in Existing Body of Literature

3.2 Research Objectives and Approach

3.3 Thesis Outline

CHAPTER 4. OPTICAL LOSS MECHANISMS IN WAVEGUIDES

4.1 Siloxane polymer waveguides

4.2 Optical propagation in waveguides

4.3 Vibration Overtones

4.4 Electronic Transition Absorption

4.5 Rayleigh Scattering

–  –  –

CHAPTER 5. EFFECT OF THERMAL CURING ON OPTICAL LOSS

5.1 Waveguide Fabrication

5.2 Cure Kinetics Experiments

5.3 Dielectric Cure Monitoring

5.4 Spectroscopy Analysis

5.5 Optical Loss Changes during Waveguide Curing

CHAPTER 6. THERMAL AGING INDUCED OPTICAL LOSS

6.1 Spectroscopy Analysis during Thermal Aging

6.2 Accelerated Reliability Testing on Waveguides

6.3 Optical Loss Changes in Waveguides during Thermal Aging

CHAPTER 7. STRESS-INDUCED BIREFRINGENCE

7.1 Birefringence Measurements

7.2 Stress-Optical Coefficients

7.3 Minimizing Stress-Birefringence

CHAPTER 8. FIELD-USE CONDITIONS

8.1 Numerical Model of the Embedded Optoelectronic System

8.2 Temperature Distribution Results

CHAPTER 9. RESEARCH CONTRIBUTIONS AND FUTURE WORK

9.1 Research Contributions

9.2 Overall Approach

9.3 Future Work

REFERENCES

–  –  –

Table 1.1 Different types of polymers used as optical waveguides





Table 2.1 Telcordia GR1209 reliability tests

Table 2.2 Telcordia GR1221 reliability tests

Table 4.1 Loss mechanisms in polymer waveguides

Table 4.2 Position of the fundamental vibration absorption

Table 6.1 Urbach coefficients for different aging conditions

Table 6.2 Changes in absorption coefficient

Table 8.1 Material properties in Thermal Model

–  –  –

Figure 1.1 Future of Optical Interconnects [3]

Figure 1.2 SMT package from NTT Corp.

[4]

Figure 1.3 Embedded optoelectronic package at PRC

Figure 2.1 Calculated and experimental values of absorption loss [28]

Figure 4.1 Generic representation of organo-siloxane structure

Figure 4.2 Optical transmission through a waveguide

Figure 4.3 Optical loss mechanisms shown along wavelength spectrum

Figure 4.4 Absorption overtones of C-H in PMMA [47]

Figure 4.5 Sellmeier model for the siloxane core and cladding

Figure 5.1 Surface topography of the FR4 board

Figure 5.2 Fabrication steps for waveguides on silicon substrate

Figure 5.3 Array of waveguides on FR4

Figure 5.4 Cross-section of illuminated waveguides

Figure 5.5 Condensation cure reaction in siloxanes

Figure 5.6 Isothermal TGA

Figure 5.7 Dynamic DSC scans at different prebaking conditions (baseline corrected).

54 Figure 5.8 Subsequent Dynamic DSC scan showing total heat

Figure 5.9 Subsequent Dynamic DSC scan showing reversible heat

Figure 5.10 Isothermal DSC scan at 150ºC (baseline corrected)

Figure 5.11 Evolution of DOC during isothermal cure

Figure 5.12 Autocatalytic cure kinetics model

Figure 5.13 Dielectric permittivity during isothermal curing

Figure 5.14 Dielectric loss factor during isothermal curing

Figure 5.15 Ionic conductivity during isothermal curing

Figure 5.16 Evolution of DOC and cure index

–  –  –

Figure 5.18 TMA temperature scan on fully cured sample

Figure 5.19 Spectroscopy sample

Figure 5.20 Absorption spectrum changes during curing

Figure 5.21 Reduction of hydroxyl overtones during curing

Figure 5.22 Comparison of measured and modeled refractive index

Figure 5.23 Optical loss due to Rayleigh scattering

Figure 5.24 Schematics of uncladded and cladded samples

Figure 5.25 Optical loss change for uncladded waveguides during isothermal cure.

.....77 Figure 5.26 Optical loss change for cladded waveguides during isothermal cure..........77 Figure 5.27 Image of scattered light through an uncladded waveguide

Figure 6.1 Absorption overtones

Figure 6.2 Spectroscopy measurements during thermal aging at 175ºC

Figure 6.3 Spectroscopy measurements during thermal aging at 150ºC

Figure 6.4 Spectroscopy measurements during thermal aging at 125ºC

Figure 6.5 Spectroscopy measurements during thermal aging at 100ºC

Figure 6.6 Spectroscopy measurements during thermal aging at 70ºC

Figure 6.7 Absorption peak changes during thermal aging at 125ºC

Figure 6.8 Accelerated thermal cycling experiment on waveguides

Figure 6.9 Accelerated thermal aging experiment on waveguides

Figure 6.10 RI variation with (a) Thermal Cycling (b) Thermal and Humidity Aging.

..93 Figure 6.11 Waveguide optical loss changes during thermal aging at 150ºC.................95 Figure 6.12 Arrhenius model for uncladded waveguides

Figure 6.13 Arrhenius model for cladded waveguides

Figure 6.14 Optical loss rate fitted to WLF model

Figure 6.15 Optical loss rate for uncladded waveguides

Figure 6.16 Optical loss rate for cladded waveguides

–  –  –

Figure 7.2 Critical angle detection

Figure 7.3 Finite-element model of a board-level optoelectronic system

Figure 7.4 Stress, σy distribution in waveguide core cross-section

Figure 7.5 Stress-induced birefringence in waveguide core cross-section

Figure 7.6 TE mode in fabricated waveguide

Figure 7.7 Effect of planarization layer CTE on the stress-birefringence

Figure 8.1 Embedded optoelectronic package at PRC

Figure 8.2 Schematic of the finite-element thermal model

Figure 8.3 Modeled temperature distribution around laser

Figure 8.4 Top view of temperature distribution as measured by a IR camera.

...........120 Figure 8.5 Maximum temperature of laser at different power levels

Figure 9.1 Design approach for thermal curing of optical polymer waveguides.

.........126 Figure 9.2 Design approach for lifetime of optical polymer waveguides

–  –  –

In high performance electronic systems, with increasing chip speed and larger number of processors, the system performance is being limited by off-chip metal interconnects. In such systems, optical interconnects are being considered to replace electrical interconnects because of their high capacity for bandwidth and due to having less constraints on link length. Polymer waveguides are attractive as an optical interconnect medium because they can enable low-cost and high-density interconnects, and can be relatively easily integrated into organic board-level processes. The optical loss in the polymer optical waveguides is the key criterion used to evaluate their performance.

Although thermal curing and aging factors can change the optical loss of polymer waveguides significantly, there is limited study on the effect of those factors. Also the field-use conditions of an embedded optoelectronic package with these polymer waveguides have not been reported in existing literature.

In this research, the relationship between optical loss variations in polymer waveguides with polymer properties during thermal curing is investigated. The evolution of degreeof-cure is determined from differential scanning calorimetry and compared to optical absorption from spectroscopy. Optical loss due to scattering mechanisms is related to local density fluctuations, which is studied using dielectric analysis. Optical loss measurements are conducted on both uncladded and cladded waveguides during isothermal cure, which result in certain optical loss trends. Based on the trends, the underlying mechanisms for the optical loss variations are proposed and a cure process schedule to realize the lowest optical loss is recommended. Process-induced thermal stresses can also affect the polymer waveguide by introducing stress birefringence. In this work, the stress-optical coefficients of the siloxane polymer are extracted and employed in a numerical modeling method to determine the stress-induced birefringence in an optical waveguide system. The outlined methodology is generic in nature and can

–  –  –

substrate/board materials to assess how stress-induced birefringence can be minimized for a given polymer core material.

This research also seeks to understand the effect of thermal aging in polymer waveguides, and develop an optical loss model to ensure that the optical performance over extended field-use conditions is within the optical loss budget. The thermal-aging dependent optical loss is determined for waveguide samples at several different accelerated temperature conditions. To determine the field-use conditions, the temperature distribution in the vicinity of the embedded laser and the polymer waveguide is determined. Using such thermal experimental data, the analytical reliability models were employed to determine the optical loss with time, and provide a practical way of determining whether the optical waveguides would perform within the optical loss budget during field-use conditions.

–  –  –

For more than three decades, semiconductor manufacturers have been able to keep pace with Moore's Law, which postulates that as transistor size shrinks, the number of transistors on a chip doubles every 18 months. The reduction in length scale of the transistor helps to reduce capacitance, leading to faster switching speeds and higher operating frequencies for the integrated chip (IC). Currently microprocessor ICs are being manufactured with transistors measuring 65 nm in node length and are projected to go down to 45 nm by the year 2010 [1]. However it is becoming apparent that the strategy of shrinking of length scales of transistors to improve device performance and reduce manufacturing costs is not yielding comparable improvements in computer system performance. The reduction of the length scale of the transistors is also hitting several physical barriers. Transistors containing features measuring 45 nanometers will have substantial leakage current, increased heat dissipation and difficulties in lithography.



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