Tackya Yammouch, Hirotaka Sugawara, Kenichi Okada, and Kazuya Masu Precision and Intelligence Laboratory, Tokyo Institute of Technology, Japan

A Large Variable Ratio On-Chip Inductor with Spider Legs Shield

Radio Band Spreading Widely

Inductor on Si RF Circuit MEMS Actuator

Comb actuator stationary electrode

movable electrode

Parallel-Plate actuator

stationary electrode

movable electrode

W. C. Tang, M. G. Lim, and R. T. Howe: J. Microelectromechanical Systems 4 (1992) 170.

V. M. Lubecke, and J. -C. Chiao: The IEEE 4th Int. Conf. on Telecommunications in Modern Satellite, Cable and Broadcasting Services, 1999 p. 1

MEMS actuator is used for moving the metalplate above the spiral inductor

A chip can supportonly a narrow band.

DTV

WideBand

WLAN

Blue-tooth

RFID The chip possibleto support wideband is necessary.

↓

→ ←

→ ←

vdd

vctrl

Example of Si RF Circuiton-chipinductor Inductance depends

on the shape.

Inductance valuesare constant.

Impossible tosupport wide band.

If inductors can have variableinductance value, Si RFcircuits that support wideband can be realized.

Conventional Variable Inductor

Metal Plate Metal Plate

Metal plate cancels magnetic flux. But...

Some of fluxes cannot be canceled.

Proposed Variable Inductor

Metal Plate Metal Plate

Shield metals cancel the undesired flux.Φmax/Φmin increases. Thus, variableratio increases.

1. Background

2. Proposed Structure of Variable InductorShielding Magnetic Mechanism

The magnetic fluxpenetrates themetal plate.

Eddy current flowsin the metal plate.

Induced a counter-active magneticfield according toLenz's Law.

The metal plateshields themagnetic flux.

Inductance valueis changed.

Change of themagnetic flux

3. Result of SimulationSimulation Method

outer diameter D = 300 µmline width W = 20 µmline space s = 1.2 µmshield width W = 20 ~ 250 µm

height of metal plate h = 10 µm or

Metal Plate

ShieldMetal

Inductor

h

7.65 µm

0.95 µm

Shield metals consist of 5 layers from M5 to M1.The inductor is made in M5.Simulation is performed by HFSS(Ansoft).

Variable Range of Inductance Value

The variablerange @ 5 GHz

The variableratio @ 5 GHz

conventional

proposed(W = 165 µm)

1.03~2.59 nH

0.83~2.37 nH

2.53

2.8714% up

4. ConclusionThe maximum variable ratio of inductance is 2.87 @ 5 GHz・The variable ratio increases 14% @ 5 GHz・

The proposed variable inductor can be applied to Si CMOS RF circuits,and improves the tuning range of circuits.

Variable ratio r is calculated as follows.r = Lmax/Lmin

Φmax = 11

Φmin = 6

Φmax = 7Shield Metal

Φmin = 2

W

wsD

Metal Plate

It is assumed to use 0.18 µm CMOS process.

8

Magnetic Field around the Inductor

Simulated by HFSS(Ansoft).

InductorInductorShield Metal

Magnetic flux isreduced by theshield metal.

0

1

2

3

4

0.1 10 1

No ShieldShieldNo Shield with Metal PlateShield with Metal Plate

(i)(ii)(iii)(iv)

frequency [GHz]

L [n

H]

W = 165 µm

0

1

2

3

4

5

0.1 1 10

No ShieldShieldNo Shield with Metal PlateShield with Metal Plate

(i)(ii)(iii)(iv)

frequency [GHz]

Q

W = 165 µm

2.4

2.6

2.8

3.0

0 50 100 150 200 250

f = 5 GHz

No Shield

Shield width W [µm]

Varia

ble

Rat

io r

14% up

Hi

Lo

6.0 GHz

400 MHz