1

PARC’s on-chip microcoil demonstrates world-record performanceKoenraad Van Schuylenbergh, Christopher L. Chua, David K. Fork, Jeng-Ping Lu and Bernie Griffiths¹

Palo Alto Research Center

¹ Mixed Signal Systems Inc., Scotts Valley

OUT-OF-PLANE INDUCTORS

• Inductor losses:– Magnetic field in substrate ⇒ eddy currents– Skin and proximity effects– Capacitive substrate coupling

• Solution = out of plane and parallel to substrate

2

STRESSEDMETAL™ TECHNOLOGY

σ∆Y

hr =Pressure [mTorr]

Stress [Gpa]

0 1 2 3 4 5

2

1

0

-1

-2 compressivecompressive

tensiletensile

neutralneutral

Film stress varies with ambient sputter pressure

STRESSEDMETAL™ MICROCOIL

• 3-dimensional MoCr scaffolds

• electroplated after self assembly, electroforms seam

3

INDUCTOR SELF ASSEMBLY

beforebefore afterafter

life video at www.parc.com/solutions/oopcoil

Video recording of an actual assembly, NOT a simulation.

WAFER-SCALE SELF ASSEMBLY

• unmodified 0.6µm BiCMOS wafer (X-FAB xb06).

• Each coil contacts the ESD unprotected base of a RF BJT.

• The StressedMetal™ process did not degrade the BJTs’ RF performance.

4

MICROCOIL CHARACTERIZATION

• One-port RF-IV impedance measurement: Z = R + j.ω.L

• Convert to equiv. Ls-Rs-Cs model for better physical understanding

Example:Ls = 10nHfres = 750MHzRs = 0.5Ω (skin effect ignored)

1

10

100

10 100 1000frequency [MHz]

QS = ωLS/RS

LS L

Q = ωL/RL

R

LS

RS

CS⇔

MICROCOIL ON 10..20Ω.cm SILICON

• 1µm aluminum GND (53mΩ/sq)mimicking CMOS top metallization

• Some minor substrate coupling

quality factor Qs

0

40

80

1 10 100 1000frequency [MHz]

6 turns 5 turns 4 turns

inductance Ls [nH]

0

5

10

15

1 10 100 1000frequency [MHz]

6 turns, f0 = 2.1 GHz 5 turns, f0 = 2.5 GHz 4 turns, f0 = 3 GHz

5

MICROCOIL ON 10..20Ω.cm SILICON

• 5µm copper GND

• No lossy substrate coupling

• Quality factor as high as onglass substrates

inductance Ls [nH]

0

5

10

15

1 10 100 1000frequency [MHz]

6 turns, f0 = 2.2 GHz 5 turns, f0 = 2.7GHz 3 turns, f0 = 4.2GHz

quality factor Qs

0

40

80

1 10 100 1000frequency [MHz]

6 turns 5 turns 3 turns

FOR COMPARISON: MICROCOIL ON GLASSinductance Ls [nH]

0

5

10

15

1 10 100 1000frequency [MHz]

6 turns, f0 = 4.0 GHz5 turns, f0 = 6.4 GHz4 turns, f0 = 8 GHz

quality factor Qs

0

40

80

1 10 100 1000frequency [MHz]

6 turns5 turns4 turns

6

BALANCED L-C OSCILLATORS

planar or 3-D coil

VCC

oscbuffer

outputbuffer

outputstage

Vref Vref

off-chipbias R’s

bandgapreference

(*)

(*)

(*) shorted with planar L

S11S11

-9Ω at 1GHz-9Ω at 1GHz

-21Ω at 1GHz-21Ω at 1GHz

BALANCED L-C OSCILLATORS

• Each optimized to its LC tank:– Planar: Qs ≈ 8– OOP: Qs ≈ 40

• Zin simulations:– Planar: -21Ω (1GHz)– OOP: -9Ω (1GHz)

• Vtank simulations:– Planar: 0.44Vpp

– OOP: 3Vpp

7

OSCILLATOR PROTOTYPE

• Single ended and balanced

• Planar & OOP side by side

• X-FAB xb06:– 0.6µm BiCMOS– 2 metals– 15GHz fTmax for RF NPN

BALANCED L-C OSCILLATORS

fosc 966.6 1215 MHzVCC 3.3 3.3 VICC (all/osc. only) 20/3..4 21/3..4 mA Pout in 50Ω -1.2 -1.5 dBm PN @ 100kHz -98.6 -110.9 dBc/Hz

-98.77 dB/Hz100.0 kHz

MKR

RBW 3.0kHz VBW 90Hz SWP 3.80secSPAN 400.0kHzCENTER 966.6450MHz

RL -1.2dBmATTEN 10dB

10dB/MKR -98.60dB/Hz

100.0kHz

D

in-plane spiralsinin--plane spiralsplane spirals

-111.6 dB/Hz100.0 kHz

MKR

RBW 3.0kHz VBW 90Hz SWP 3.80secSPAN 400.0kHzCENTER 1.2154447GHz

RL -1.5dBmATTEN 10dB

10dB/MKR -110.9dB/Hz

100.0kHz

D

3D CT coil3D CT coil3D CT coil

12.3 dB12.3 dB

normalizednormalized14.6 dB14.6 dB

8

SUMMARY

• High-Q out-of-plane 3D microcoils

• Integration on various substrates incl. active circuit wafers

• StressedMetal™ coils on 15..20Ω.cm silicon:– Q = 40..50 with thin aluminum GND– Q = 60..80+ with thick copper GND

– Both values are the highest published at around 1GHz for unaltered low resistance silicon. The numbers on thick copper are very close to the theoretical maximum based on skin effect alone.

• Monolithic balanced BiCMOS oscillators: StressedMetal™ coils on Al yield 12.3dB PN improvement @ 100kHz over in-plane spirals (normalized 14.6dB)

• Up to 6dB better phase noise possible with copper GND

• Compatible with injection molding