Title page for ETD etd-0725102-114518

Type of Document Dissertation
Author Chomnawang, Nimit
Author's Email Address nimitch@ccs.sut.ac.th
URN etd-0725102-114518
Title Three-Dimensional Micromachined On-Chip Inductors for High Frequency Applications
Degree Doctor of Philosophy (Ph.D.)
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Pratul K. Ajmera Committee Chair
Jeong-Bong Lee Committee Co-Chair
Jagannathan Ramanujam Committee Member
John M. Tyler Committee Member
Martin Feldman Committee Member
Phillip Sprunger Dean's Representative
  • high frequency
  • micromachining
  • photoresist mound
  • dome-shaped inductor
  • arch-like inductor
  • inductor
  • mems
  • rf
  • photoresist dome
  • on-chip
  • photoresist mesa
Date of Defense 2002-07-19
Availability unrestricted
Demands for wireless communication are ever-escalating for consumer and military communication applications. The requirements of portability, more functionality and lower cost have been driving forces toward smaller, more sophisticated and flexible wireless devices with lower power consumption. To meet these requirements, monolithically integrated passive inductors with high Q-factors and high self-resonant frequencies are desirable. Q-factor and self-resonant frequency of an inductor are significantly degraded at high frequencies due to conductor ohmic loss, magnetically induced eddy current in the conductive substrate, and lower self-resonant frequency from capacitance between conductive substrate and conductors.

In this dissertation, novel three-dimensional arch-like solenoid and dome-shaped spiral inductors are designed, fabricated, and characterized. MEMS-based fabrication techniques such as copper electroplating through voids in thick SU-8 photoresist molds and EAGLE2100 conformal photoresist molds on sacrificial arch-like or dome-shape SJR5740 photoresist mounds are utilized. An air gap between the inductor and the silicon substrate is used to reduce the degradations of inductor performance. According to the Sonnet electromagnetic simulations, 30 μm air-gap suspension over the substrate is an adequate choice for these inductors. Suspended arch-like solenoid copper inductor has flat bottom conductor connected to arch-like top conductor with an air core in between. This design has only 2 contact points per inductor turn to minimize series resistance. Suspended domeshaped spiral copper inductor is fabricated on a sacrificial photoresist dome with the outer end connected to one probe pad, and the inner end connected to the other probe pad through vias and an air-bridge. The sidewalls of spiral turns in this design overlap less with each other thereby reducing inter-turn capacitances.

Fabricated inductors are characterized and modeled at high frequencies from Sparameter measurements. ABCD-parameters, derived from the S-parameters are translated into a simplified physical π-model. The resulting arch-like suspended inductors with 2-5 turns have inductances between 0.62 to 0.79 nH, peak Q-factor values between 15.42 to 17 at peak-Q frequencies between 4.7 GHz to 7.0 GHz, and self-resonant frequencies between 47.6 GHz to 88.6 GHz. The 3-turn dome-shaped spiral inductor has inductance of 3.37 nH, peak Q-factor of 35.9 at 1.65 GHz, and self-resonant frequency at 18.74 GHz.

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