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«A Dissertation Presented to The Academic Faculty by Erdal Uzunlar In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in ...»

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IMPROVEMENTS FOR CHIP-CHIP INTERCONNECTS AND

MEMS PACKAGING THROUGH MATERIALS AND PROCESSING

RESEARCH

A Dissertation

Presented to

The Academic Faculty

by

Erdal Uzunlar

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy in the

School of Chemical & Biomolecular Engineering

Georgia Institute of Technology

May 2015

COPYRIGHT© 2015 BY ERDAL UZUNLAR

IMPROVEMENTS FOR CHIP-CHIP INTERCONNECTS AND

MEMS PACKAGING THROUGH MATERIALS AND PROCESSING

RESEARCH

Approved by:

Dr. Paul A. Kohl, Advisor Dr. Azad Naeemi School of Chemical & Biomolecular School of Electrical and Computer Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Dennis W. Hess Dr. Yogendra Joshi School of Chemical & Biomolecular School of Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Michael A. Filler School of Chemical & Biomolecular Engineering Georgia Institute of Technology Date Approved: January 8, 2015 To my family

ACKNOWLEDGEMENTS

I would first like to express my sincerest gratitude to my advisor, Prof. Paul A.

Kohl, for his precious guidance, encouragement and support during my time in Georgia Tech. I would also like to thank my thesis committee members, Prof. Dennis W. Hess, Prof. Michael A. Filler, Prof. Azad Naeemi and Prof. Yogendra Joshi for their helpful input to my Ph.D. study.

I would also like to give thanks to the current and past members of Kohl group.

Thank you in particular to Dr. Rohit Sharma, Dr. Rajarshi Saha, Dr. Yu-Chun Chen, Dr.

Hyo-Chol Koo, Dr. Mehrsa Raeiszadeh, Dr. Murat Unlu, Zachary T. Wilson, Jared M.

Schwartz and Brennen K. Mueller for their help with the work presented in this thesis, and encouragement throughout my Ph.D. study. Thank you to IEN cleanroom staff, especially Walter Henderson, Gary Spinner and Eric Woods for all their assistance throughout my time in the cleanroom.

This work would not have been possible without the support of Promerus, LLC, Sumitomo Bakelite, Co. Ltd. and Raytheon Company. I would like to acknowledge Dr.

Ed Elce of Promerus, LLC for his insight.

Finally and most importantly, I would like to thank to my parents, Zuhre and Celil Uzunlar, my sister Gulsen Uzunlar Altin, and my wife, Sibel Kalyoncu Uzunlar, for all their encouragement and support throughout the years. Without them, I would have never dreamed of making it to this point. I praise God for endowing me with such a supportive and loving family, and for providing me with enthusiasm and patience to pursue my goals in life.

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4 Size-compatible, Polymer-based Air-gap Formation Processes and Polymer Residue Analysis for Wafer-level MEMS Packaging Application 71

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5 Thermal and Photocatalytic Stability Enhancement Mechanism of Poly(propylene carbonate) due to Copper(I) Impurities 94

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Table 3.1: The average (Ra) and root-mean-square (Rq) surface roughness values measured for samples at each step in electroless deposition process 50

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Table 3.3: C1s peak deconvolution results of as-received PWB sample 58 Table 3.

4: C1s peak deconvolution results of H2SO4 treated PWB sample 59 Table 3.5: C1s peak deconvolution results of H3PO4 treated PWB sample 60

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Table 5.1: XPS analysis results for elements and relative percentages 97 Table 5.

2: UV-vis spectra peak wavelength (nm) and absorbance (AU) values 108

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Figure 3.1: The amount of electroless copper deposited per area on epoxy laminates versus duration of electroless copper deposition 46

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The work presented in this dissertation focuses on improvements for everevolving modern microelectronic technology. As the microelectronics technology progresses, new challenges in materials and processing are faced and need to be resolved.

Improvements in metallization, packaging and the materials used in these processes can further enable reducing cost, and attaining higher performance. Specifically, three topics were investigated in this work: electroless copper deposition on printed wiring boards (PWBs), polymer-based air-gap microelectromechanical systems (MEMS) packaging technology, and thermal stability enhancement in sacrificial polymers, such as poly(propylene carbonate) (PPC). Electroless copper is important in the fabrication of electronic substrates for the formation of dense copper wiring traces on printed wiring boards (PWBs). Electroless copper deposition on PWB materials currently uses costly Pd-based catalysts. The conventional surface pretreatment of PWBs by swell-and-etch method increases the surface roughness dramatically, which leads to high conductor losses that limit the signal propagation speed and integrity between chips on PWB substrates. In that regard, low-cost and equally active Ag-based catalysts, and nonroughening surface treatment methods using mineral acids were investigated for improved electroless copper deposition processes. MEMS packaging plays an important role in protecting MEMS devices, in preserving device performance and in the final product cost. The industrial standard of wafer capping is a heterogeneous process that requires careful alignment of MEMS and capping wafers and bonding at high temperatures which might damage the devices and integrated circuits (ICs). Monolithic,

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developed to reduce cost and improve reliability. In order to address this challenge, a polymer-based air-gap MEMS packaging technology was investigated, which uses a sacrificial polymer, PPC, to create an air-gap by thermal decomposition around the MEMS device, enclosed with a polymeric overcoat, BCB. The air-gap technology relies on the sacrificial polymer, such as PPC, that provides a temporary place-holder for micro cavity creation. The selection of materials and processing conditions in air-gap technology are often limited by the decomposition temperature of PPC, implying the requirement to control or fine-tune PPC thermal stability. For that reason, the thermal stability enhancement of PPC by Cu(I) ions previously reported was investigated to further understand its mechanism.

In the electroless copper deposition study, we investigated an electroless copper deposition technique on an unclad epoxy laminate substrate, composed of a H2SO4 surface pretreatment, Sn/Ag nano-colloidal catalyst seeding, and immersion in a formaldehyde copper electroless bath. The lower cost and similar catalytic activity of Ag compared to Pd for formaldehyde oxidation makes Ag a rational catalyst choice for electroless copper deposition. It was found that the H2SO4 treatment cleaned the substrate of its impurities and provided the adhesion of the catalyst and electroless copper without increasing the surface roughness. Surface pretreatment with H2SO4 was unique in catalyst seeding, compared to other acid treatments including HCl and H3PO4. The sulfate content adsorbed on the epoxy laminate after H2SO4 treatment enabled Sn(II) sensitization by electrostatic attraction. The measured adhesion strength of deposited electroless copper layers indicated that even though H2SO4 treatment did not increase surface roughness,

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ray photoelectron spectroscopy (XPS) results showed Sn(II) oxidation to Sn(IV) and Ag(I) reduction to Ag(0), so as to form a core-shell nano-colloidal catalyst with Ag(0) core and SnO2 shell. It was plausible to consider some residual Sn(II) complexing with chloride ions to form SnCl3ˉ on the outer surface of nano-colloids and preventing agglomeration. Sn/Ag nano-colloids were observed to well catalyze the electroless copper deposition reaction at a practical rate. The chemical adhesion was promoted by H2SO4 treatment rather than mechanical adhesion, and the use of Sn/Ag catalyst with H2SO4 surface treatment facilitated adherent, continuous and uniform electroless copper layers on epoxy laminates.

In the MEMS packaging study, a polymer-based air-gap MEMS packaging approach was investigated to reduce the cost, to simplify the packaging process and to analyze the polymeric residue. The idea was to achieve wide (~2.5x3.2 mm) and tall (10μm) air-gaps to provide size-compatible packaging to MEMS devices, such as accelerometers and gyroscopes. The study provides a framework for size-compatible and clean air-gap formation by selecting the type of PPC, optimizing thermal treatment steps, identifying air-gap formation options, assessing air-gap formation performance, and analyzing the chemical composition of residue. BCB was identified as a PPC-compatible overcoat material providing excellent mechanical stability to air-gap structures. A kinetic comparison was done between PPC decomposition and BCB curing processes to determine the optimal thermal treatment recipe. It was found that the optimal treatment is a two-step heating process that includes a BCB curing at a lower temperature in the first step, and PPC decomposition at a higher temperature. Two air-gap formation processes,

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photosensitive PPC), were proposed depending on how the PPC was patterned. Less residue and more reliable air-gap structures were obtained using the RIE process. The direct photopatterning process was simpler, however a greater amount of residue was observed due to photoacid generator (PAG), at least twice the residue in the RIE process.

Comparison of two-layer PPC structures with both photosensitive and non-photosensitive PPC layers revealed that it was possible to obtain reliable air-gap structures only if the photosensitive PPC was the bottom layer. Nanoindentation measurements indicated that the mechanical strength of BCB caps was due to prestressing in BCB because of its tensile stress on silicon substrates. The polymer-based air-gaps provide a monolithic, low-cost, IC-compatible MEMS packaging option.

In the study of thermal stability of PPC, the thermal stability enhancement of PPC by Cu(I) ions was investigated to determine its mechanism. An increased thermal stability of PAG-containing PPC film was previously reported by our group (PAG was Rhodorsil-FABA, an iodonium-based PAG). XPS analysis done on the surface of PPC/PAG films prepared on Cu-sputtered substrates showed the presence of copper in Cu(I) oxidation state. The amount of copper obtained in the film was found to be similar to the amount of PAG (Rhodorsil-FABA). Cu(I) ion can be formed through oxidation of copper surface by dissolved oxygen in the PPC solution. The thermal stability of PPC/PAG films was studied for a number of iodonium and sulfonium-based PAGs.

Every PAG/PPC film including iodonium-based PAGs showed thermal stability increase in the UV exposed cases. No thermal stability increase was observed using any sulfonium-based PAG. Based on these observations, a mechanism was postulated for the

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iodonium in the cation of the PAG and Cu(I) disturbs acid creation mechanism of PAG.

In the absence of acid, the PPC decomposition is not catalyzed, so the thermal stability of PPC increases on Cu surfaces.

Overall, the work presented in this dissertation endeavored to improve the chipchip interconnection and MEMS packaging through metallization, processing and materials research. Ag-based catalysts can reduce cost, and non-roughening H2SO4 treatment can ensure fast and reliable signal propagation between chips through electroless copper layers on PWBs. MEMS packaging using polymer-based air-gaps can enable simpler packaging process and produce low-cost MEMS devices. Control of thermal decomposition temperature of PPC can allow a wider range of processes and applications.

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Modern microelectronic technology is based on digital components such as integrated circuits (ICs), and on analog components such as microelectromechanical systems (MEMS). While the improvement in IC performance has been conventionally obtained by increasing transistor speed through transistor scaling and high frequency operation, the performance of ICs is currently limited by the delay in the interconnect lines connecting the ICs due to high loss (conductor loss and dielectric loss), rather than transistor speed. For MEMS, improvements have been focused more on the functionality and sensitivity of devices, while packaging of devices has become a bottleneck due to a wide range of device dimensions and packages being device-specific. The overarching goal of this thesis study is to improve IC interconnection and MEMS packaging through materials and processing research, technology development and reliability advancement.

For IC interconnection improvement, we have investigated electroless copper deposition on epoxy laminate substrates that could be used for chip-chip interconnection. For MEMS packaging improvement, we have pursued an air gap-based MEMS packaging technique utilizing a sacrificial polymer, poly(propylene carbonate) (PPC) to achieve low-cost, monolithic, IC-compatible packaging option. For materials improvement, we have studied the thermal stability enhancement mechanism of PPC through Cu incorporation. The mechanism could allow control or fine-tune of thermal decomposition of PPC, and could widen PPC’s processing and application space.

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