Switchable LTCC/Polyimide Based Thin Film Coils

1
Switchable LTCC/Polyimide Based Thin Film Coils
Loren Rieth 1,2, Siddharth Chakravarty 1, Jui-Mei
Hsu 1, Richard Normann 3, Florian Solzbacher
1,2,3, Matthias Klein 4, Michael Töpper 4, Sohee
Kim 5
1 Materials Science and Engineering, Univ. of Utah, Salt Lake City, UT 2 Electrical and Computer Engineering, Univ. of Utah, Salt Lake City, UT
3 Bioengineering, Univ. of Utah, Salt Lake City, UT 4 Fraunhofer Institute for Reliability and Microintegration IZM, Berlin, Germany
5 Fraunhofer Institute for Biomedical Technology IBMT, St. Ingbert, Germany 5 Fraunhofer Institute for Biomedical Technology IBMT, St. Ingbert, Germany

• IV. Polyimide Based Coil Design and Simulation
• Two separate coil designs are simulated
• Type I coil with a single winding layer (fewer
  turns)
• Type II coil with two winding layers (more
  turns)
• Initial coils will have two winding layers with
  a switch to disable one winding layer for
  controlled inductance and capacitance
• Coils will be Cu or Au metal traces
  electrodeposited on polyimide/Kapton substrates
• Metal lines will be from 10 to 20 µm thick for
  low resistance (lt 1 k?) and high fabrication
  yields
• Metal line height, the spacing between lines
  will be from 10 to 20 µm wide to maximize
  inductance and minimize parasitics
• Polyimide based coils will be adhered to a LTCC
  ferrite platelet to enhance voltage gain and
  Q-factor, and to reduce the magnetic field on the
  signal processing IC below

Type I Coil
Type II Coil

• I. Objectives and Introduction
• Develop a chronically implantable inductive
  power coupling for in vivo microelectronics
• Use electromagnetic simulations to design the
  coil
• Optimize coil design for 2.64 MHz operation
• Maximize inductance and minimize parasitic
  losses (e.g. capacitance resistance loss
  tangent)
• Microfabricate a thin coil with a high Q-factor
  (quality factor) to efficiently receive power
  from an external supply
• Supply 3 V to power signal/telemetry IC
• Receive command signals encoded on the power
  waveform
• 5 ? 5 mm2 to match UEA dimensions
• Two approaches have been followed including
• An LTCC ferrite device with an integrated metal
  coil
• A microfabricated gold (Au) coil on a polyimide
  substrate bonded to a LTCC ferrite platelet

Q-factor and inductance as a function of the coil
line height. Line spacing is 10 µm and 75 areal
coverage.
Inductance versus the distance between the coil
and ferrite platelet. Single layer coil with
line height and width of 20 µm, line spacing of
10 µm, and 62 turns for 75 areal density.
Quality factor and inductance versus number of
turns for Type I and II coils. Maximum Q occurs
when windings fill 80 of internal spiral area
line spacing 10 µm and line height 20 µm.

• II. LTCC Coil Design and Testing
• Low temperature co-fired ceramic (LTCC)
  technology to form thin (200 µm) ferrite
  platelet
• Ferrite material based on Fe2O3 has high
  permeability (µr200) and high resistance (R)
• Ferrite concentrates the magnetic flux in the
  coil
• The coil is formed with screen printed Ag paste
  in a square geometry planar coil (20 µm thick)
• Coil performance measurements
• 22 mm diameter external (supply) coil with 17
  turns on two layers
• Supply coil inductance 6.03 µH
• 10 mm separation between supply and test coils
• 2.64 MHz supply frequency with 13.8 V
  peak-to-peak sine wave

Table II. Simulation results for Au coils on
polyimide substrates
Q-factor and inductance as a function of coil
line width. Line spacing is 10 µm and line
height is 20 µm and 75 areal coverage.
Type I (single-layer thin-film Au coil on ferrite) Type I (single-layer thin-film Au coil on ferrite) Type I (single-layer thin-film Au coil on ferrite) Type II (double-layer thin-film Au coil on ferrite) Type II (double-layer thin-film Au coil on ferrite) Type II (double-layer thin-film Au coil on ferrite)
ws20µm 15µm 10µm 20µm 15µm 10µm
Inductance (µH) 12 20 46 45 80 180
Series resistance (?) 63 112 251 126 224 502
Q at 2.64 MHz 3.0 3.0 3.0 5.9 5.9 5.9
Parasitic capacitance (pF) 0.13 0.12 0.12 13.2 13.9 14.6
Self-resonance (MHz) 127 103 68 6.5 4.8 3.1
Bandwidth (MHz) 0.43 0.43 0.43 0.32 0.33 0.34
Voltage gain, w/o tuning cap. 0.017 0.022 0.034 0.033 0.043 0.066
Voltage gain, with tuning cap. 0.051 0.066 0.099 0.18 0.24 0.33
Optical micrographs of initial Au coil samples on
polyimide substrates with a design optimized by
simulation results.
Coil parasitic capacitance between metal traces
and between the spiral and a conducting layer.

• V. Conclusions, Future Work, and
  Acknowledgements
• Measured inductance and voltage gain on LTCC
  coils (screen printed Ag traces) are too low
• A new coil design based on metal traces
  fabricated on a polyimide substrate and bonded to
  a LTCC ferrite platelet have been investigated
• Simulated the polyimide coil designs results
  suggest it will achieve sufficient inductance and
  voltage gain for integrated neural prosthetic
  devices (and in vivo microelectronics)
• Fabricated initial Au coil on polyimide
  substrates, and achieved more than an order of
  magnitude more turns than the LTCC based coil
  architecture
• Test Type I and Type II coil performance with
  the reference coil to measure inductance, voltage
  gain, resistance, capacitance and Q-factor
• Refine the coil design to most efficiently
  supply the signal processor IC power requirements
  and minimize coil size
• Investigate biocompatibility of the polyimide
  based architecture with encapsulation layer
• We gratefully acknowledge NIH support
  (HHSN265200423621C / N01-NS-4-2362) and
  discussions with Prof. Normanns, Prof.
  Harrisons, and Prof. Solzbachers students

• III. LTCC Coil Results and Discussion
• Coil inductances are very low and would require
  large capacitors (10 nF) for 2.64 MHz operation
  with high Q
• Large capacitors are not available in Surface
  Mount Device (SMD) component sizes compatible
  with device integration
• A higher inductance is needed, which requires
  more turns
• The voltage gain is very low, which would require
  driving the supply coil at dangerous voltages (gt
  100 Vrms) to achieve a 3 Vrms supply

Optical pictures of the initial LTCC ferrite coil
samples shown with a dime for scale.