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J. Jpn. Soc. Powder Powder Metallurgy Vol. 61 Supplement, No. S1

Efficiency Increase in On-Chip Buck Converterby Introduction of High Permeability Material to Inductor on Interposer
H. FUKETAa), Y. SHINOZUKAa), K. ISHIDAa), M. TAKAMIYAa), T. FUJIIb), H. SHIMIZUb),
K. KOBAYASHIb), T.SATOc), and T. SAKURAIa)a) Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japanb) SHINKO Electric Industries Co., Ltd., 36 Kita Owaribe, Nagano-shi, Nagano 381-0014, Japanc) Shinshu University, 4-17-1 Wakasato, Nagano-shi, Nagano 380-8553, JapanAbstract
In this paper, an inductor on interposer with a ferrite film is investigated for power supply circuits in order to improve theefficiency of the power supply circuits by introducing a high permeability material to the inductor. A DC-DC buckconverter is used as a power supply circuit in this work. The dependence of the inductance on the location of ferrite film isdiscussed with field electromagnetic simulations. Simulation results indicate that the ferrite film should be located on bothupper and lower sides of the inductor and between metals. By using the inductor with the 10m-thick ferrite film, theefficiency of buck converter is improved by 13% when the efficiency of buck converter with the air core inductor is 60%.
Key Words: Buck Converter, Ferrite, Interposer

I. INTRODUTION
Since mobile devices have been currently smaller and thinner, miniaturization of inductors used for power supply
circuits is required. Although it is one of promising solutions that inductors are implemented in IC package or interposer,
the inductance and Q factor of such inductors are smaller, which results in worsening the efficiency of power supply
circuits. Therefore, introduction of high permeability material to the inductor is expected to improve the efficiency. In thispaper, the efficiency using inductor on interposer with a ferrite film is investigated as shown in Fig. 1. A DC-DC buckconverter is used as a power supply circuit in this work. In order to improve the efficiency of buck converter, the locationof ferrite film is also discussed with electromagnetic field simulations.
II. SIMULATION SETUP
Fig. 2 shows a circuit schematic of buck converter evaluated in this paper. A controller circuit, buffers, and drivers ofbuck converter are implemented in the IC, while an inductor is fabricated in the interposer using metal interconnects of
the interposer. VIN and VOUT are input and output voltages of buck converter, respectively.
According to [1], the efficiency of optimally-designed buck converter is expressed as


Pout
11


Pout  Ploss 1  Ploss
RC11  3 24 0 0 D 1  D PoutLD(1).

As defined in [1], Pout and Ploss represent the output power of buck converter and its power loss, respectively, R 0C0 is thetechnology-dependent parameter of the IC, and D is a duty ratio, which is given by VOUT/VIN.
IC (Buck converter)

VIN

Ferrite film

Si チップInductor
LController

RLInterposer
パッケージ基板

COUT

Metal (inductor)
Fig. 1

VOUT

Inductor on interposer with ferrite film.

ICFig. 2

Interposer

Circuit schematic of buck converter.

Efficiency Increase in On-Chip Buck Converter by Introduction of High Permeability Material to Inductor on Interposer

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 L in (1) is defined as L  L RL

(2)

where L and RL are the inductance and the DC resistance of the inductor, respectively. Equations (1) and (2) indicate thatlarge  L improves  of buck converter and hence  L is evaluated in addition to L in this paper.
Fig. 3 shows the layout of the inductor used for simulations described in Section III. Table I summarize the simulation
setup. The inductor is simulated by a 3D electromagnetic simulator (Agilent EMPro). In this paper, the dependence ofrelative permeability on frequency is not considered. The superimposed DC current characteristics are negligible at theoutput current of 1A, and hence the characteristics are not considered in this paper.
Cross-sectional view

Table I Summary of simulation setup.
ViaInductor
Metal
Ferrite film

Top view(Upper/lower sides)

CopperFerrite film
(Outside)

Diameter

850m

Number of turns

2Ferrite film

850m

Relative permeability
Thickness

80
1m – 100m

Buck converter
Input/output voltage
Output current
Switching frequency

Ferrite film

(Between metals)

850mFig. 3CopperThickness/width of metal 20m / 130m

1.8V / 1.0V
1.0A
10, 100MHz

Layout of inductor used for simulations.

Both sides

Lower side

Thickness of ferrite film (m)Fig. 4 Inductance when ferrite film is located on bothupper and lower sides of inductor and only lower side ofinductor.

March, 2014

Inductance (nH)

Inductance (nH)III. SIMULTATION RESULTS
A. Dependence of Inductance on Location of Ferrite Film
In order to investigate the dependence of inductance on the location of the ferrite film, the following two conditions areevaluated; 1) the ferrite film is located on both upper and lower sides of the inductor or only lower side of inductor, and 2)
the ferrite film is located between metals or not.
Fig.4 shows the inductance when the ferrite film is located on both upper and lower sides of the inductor and only lower
side of the inductor. The inductance with the ferrite film located on both upper and lower sides of the inductor is muchhigher than that with the ferrite film located on only lower side of inductor. Fig. 5 illustrates the inductances with andWith ferrite film
between metals

Without ferrite film
between metals

Thickness of ferrite film (m)Fig. 5 Inductance with and without ferrite film
between metals.

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H. FUKETA, Y. SHINOZUKA, K. ISHIDA, M. TAKAMIYA, T. FUJII, H. SHIMIZU, K. KOBAYASHI,T.SATO, and T. SAKURAI

without the ferrite film between metals (the ferrite film between metals is indicated in Fig. 3). In both cases, the ferrite
film is located on the upper and lower sides of the inductor (the thickness of the ferrite film is 10m) and the outside ofthe inductor (the width of ferrite film is 10m). The ferrite film between metals improves inductance by 20-60%.
The increase in  L improves the efficiency of buck converter as indicated in (1). In order to increase  L , Figs. 4 and 5
indicate that the ferrite film should be located on both upper and lower sides of the inductor as well as between metals,
because  L depends on L as shown in (2).
B. Efficiency Improvement of Buck Converter by Using Inductor with Ferrite Film
In order to investigate the efficiency of buck converter improved by using the inductor with the ferrite film,  L , which
is derived in (2), is evaluated. Fig. 6 shows the dependence of  L on the thickness of the ferrite film.  L (ferrite) and L (air) are  L of the inductor with the ferrite film and the air core inductor, respectively. When the thickness of theferrite film is 10m,  L (ferrite) increases by 5.7 times compared with  L (air) , as indicated in Fig. 6.
Fig. 7 shows the efficiency improvement of buck converter by using the inductor with the ferrite film compared with theefficiency with the air core inductor when the switching frequency is 100MHz. By using the inductor with the 10m-thickferrite film, the efficiency of buck converter is improved by 13% and 8% when the efficiencies of buck converter with theair core inductor are 60 and 80%, respectively.
100

10MHz
100MHzEfficiency () ofbuck converter (%)
Buckコンバータの効率[%]L(ferrite) (nH/W)

10000

1000

5.7x100

L of air core inductor (L(air))(= 70nH/W)
10

1

10Thickness of ferrite film (m)100

Fig. 6 Dependence of L of inductor with ferrite film
and air core inductor.

90+8%80(air)

70+13%60

5090%80%
70%60% of buck converter with air core inductor
1

5.7

10

L(ferr τte)/ L(air)
1/τ0100Fig. 7 Efficiency() improvement of buck converterby using inductor with ferrite film. (air) is  of buckconverter with air core inductor.

IV. CONCLUSION
In this paper, an inductor on interposer with a ferrite film was investigated for power supply circuits. A DC-DC buckconverter was used as a power supply circuit in this work. The inductance and  L improved by the ferrite film wasevaluated with field electromagnetic simulations. First, the dependence of the inductance on the location of the ferrite film
was discussed. Simulation results indicate that inductance with the ferrite film located on both upper and lower sides of
the inductor is much higher than that with the ferrite film located on only lower side of inductor. Furthermore, the ferrite
film between metals improves inductance by 20-60%. These results indicate that the ferrite film should be located on both
upper and lower sides of the inductor as well as between metals. Then, the efficiency improvement of buck converter byusing the inductor with the ferrite film was evaluated. By using the inductor with the 10m-thick ferrite film, theefficiency of buck converter is improved by 13% when the efficiency of buck converter with the air core inductor is 60%.
An investigation considering the dependence of relative permeability on frequency is future work.
REFERENCES[1] G. Schrom, P. Hazucha, F. Paillet, D. S. Gardner, S. T. Moon, and T. Karnik, “Optimal Design of Monolithic Integrated DC-DCConverters,” Proc. IEEE International Conference on IC Design and Technology, pp.1-3, 2006.

J. Jpn. Soc. Powder Powder Metallurgy Vol. 61 Supplement, No. S1