A MINIATURE OPTO-ELECTRIC TRANSFORMER

Mitsuteru Kimura,Nobuki Miyakoshi and M a s h Daibou Electrical Engineering, Faculty of Engineering

Tohoku-Gakuin University 1- 13 ,Chuo- 1 ,Tagajo,Miyagi Pref. 985, Japan

ABSTRACT

A miniature opto-electric vansformer consisting of a p-n junction photo-cell ( the active area of 500 x 500 pm2 ) and amultilayerspiral coiltransformer(l.4~1.4 mm2) mono- lithically fabricated on a Si substrate is proposed and its fundamental charactetistics are demaasuaEed. The optical power from a LD is converted into the electric power by the photo-cell, and then the photo-cnrrent is converted into the output voltage by the miniaturized transformer. In this ex- periment each of the plmary and the secondary coil of de- posited AI has double layer of reaangular spiral coil and has sameturnsof 19(i.e. 1:l transformer). TheinducranceL of the double layer coil with a outer core is derived from the simple proposed model. The circuits simulations are per- formed by PSpice simularor,and the experimental results can be well explained by these simulations.

INTRODUCTION

It is desirable to supply the elearic power or the electric signal without wire to moving devices such as rotating de- vices or devices Cperatiag at the special circumstances, such as in the human body or under the lugb voltage.

Recently microfabricated aQuaMs such as the micm m(t tct [ 11, the larerally driven resonatop [2] and the migopump [3] are fabricated on a Si chip using the micromachiolog te- chnology. Their power consumption may be less t h ~ few milliwaa~ because of their small dimensions. If the power is supplied without the wire enough to drive the device itself or the ICs in the movable substance,a wide variety of appli- cations will be possible.

We propose a miniature opto-electric VBasfOpmer (MO- ET) which is composed of a multilayer spval coil vans- fcimer and a monolithically fabricated p-n junction photo-

cell on a Si substrate and demonstrate its fundamental ope- ration with this pmtotype MOET. This MOET may be used as a power LX a signal wireless source embedded in the mo- vable stru- or in the device under the special circum- StllIlCeS.

secondary electrode -

CH2957-9/9110000-0227$01 .OO 0 1991 IEEE 227

I ~~ 77-

Uprimary electrode

Fig. 1. Suvcnw of the miaiature upto-electric transfmer. (a) Top view, (b) Cross sectional view.

STRUCTURE AND OPERATION OF THE MINIATURE OPTO-ELECTRIC TRANSFORMER

U . a U

lo-'!

: : .

10-5

The structure of this MOET is shown in Fig. 1 (a),(b). The p-n junction photo-cell has a active area of 500 x 500

p n z prepared by diffusion of Boron in the n type Si sub- strate and has an antireflection Si& film of about 0.15 pn thick,= shown in Fig. 1. The miniaturized multilayer v~ns-

former is fabricated in following procedure. The soft mag- netic material film of about 1 pm thick as a lowet core of the multilayer tramformer was deposited by RF spuaetylg fmm the target of Fe7s.4 Bi0.9 Zrio.7, and then annealed at 380"- C in the vacuum of 1 x 10-6 Tom. Evaporared AI film of ab- out 3 pm thick was chemically etched for each recrangular coil of about 9.5 turns. Each of the primary and the second- ary coil had two layers of 9.5 turns AI coils (both the prim- ary and the secondary coils have same turns of 19 in this prototype MOET),and each layer was isolated by a poly- imide layer made of polyimide photorrsist ( presented by Toray Co. Ltd) of about 1 iLm thick excep the cmnection regions of double layer coils . The upper core was fabri- cated by the same ways as the lower core,and in the center and the periphery of the transformer the lower and the upper core contacted through only one polymide layer ,which made the outer core transfamer. The area of this miniat- urized transformer is about 1.4 x 1.4 mm2. The end of the primary coil is OhmicaUy contac%ed to the n type Si subst- rate at near center afthe rectaagular coil, and this e a r y coil is diredy driven by the p n junction photo-cell. The SEM micrograph of this finished MOET is shown in Fig.2. The photo-cell is illuminated by the on-off modulated light from the LD OT LED. The current change produced by the photo-cell is converred to the output voltage of the MOET by the miniaturized traasfamer.

0 op-ga incident power 0

0913 I m W l 2 0 7 4 [ m W ] m 5 5 5 6 [ m W l

0 8 4 5 9 [ m ~ I . 1083 [ m w ]

. ...... 1 ........ ( . . . . . . . . I . . -

Fig. 2. SEM miamgraph of the MOET.

EXPERIMENTAL RESULTS

The photo-current Ip of the photo-cell as a function of the load resistance RI with a paratneter of the incident power

from a LD (wavelength il : 830 MI )is shown in Fig.3. Fmm this Fig. 3 we can see that Ip has a saturation region in the range of lower load resistance RI. In this saturation re- gion.the photo-cell serves as the constant current source. We should operate the photo-cell in this saturation region. In the decreasing region of Ip due to the larger load resistance R1,the photo-voltage is saturated because of the cMier recombinations in the p-n junction of the photo-cell. The C R time constant due to the combination of the photo-cell with measured junction capacitance Cp of about 500 pF at forward bias voltage of 150 mV and the primary coil with

coil resistance Rp of about 33 0 is about 17 nsec which comesponds to 59 MHz

a ..--.a. I 0 0 0 0 00000 oooooooo 10-2i II ~aa""mn

100 10' io2 103 1 o4 RI in1

Fig. 3. Relations of Photo-current Ip of the p-n junction photo-cell and load resistauce RI as a parameter of the incident light power of 830 nm wavelength from LD.

Fig.4 shows the measured inductance L in (a) and resist- ance R in (b) of double layer coil of 19 turns in total number (number of turns of each layer, n, is about 9.5 )with the magnetic film core of Fm.4 Bio.9 2 ~ 0 . 7 of 2 pn thick as a function of frequency f. The thickness of this AI coil is ab- out 3 pm. Gradual decrease in L may be due to the frequen- cy dependence of the magnetic film.The drastic increase of the resistance R in higher frequency may be due to the reso- nance .because we always measure the effecrive resistance of the L C R circuit. From these curves we see the low fre- quency (f=lOO KHz) inductance L and resistance R are about 3.27 pH and 32.8fl,respeaively

228

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250

c 200

a 1 50

g 100

- I

0 C

m .- m t =

I

I ( 10 4

1 0 5 Frequency f [Hz]

( a )

I

a

0

Q

0 8

Fig. 4. Frequency dependence of inductance L (a) and resistance R (b).

Responses of the input voltage Vp (the upper waveform) .which corresponds to the output of the photn-cell, and the output voltage Vo (the lower waveform) of the multilayer transformer when the LD is driven at 5 MHz Is shown in

Fig. 5. Responses of the input voltage Vp (upper waveform) and the output voltage Vo (lower waveform) of the miniaturized t r " e r . The incident power from LD driven at 5 MHz is about 8 mW (average power : 4mW).

Fig.5 . The received power of the photo-cell is about 8 mW (at @&-on state) of 830 nm wavdeogrb. The output voltage

Vo across the load reSist~ce of 1 K f l is about 330 mV p- p.in rhis case. hmashg the optical power up to 13 mW, the output voltage VO of the MOET became to 510 mV.

In Fig. 6 the dependence of the output voltage Vo on the optical on-off frequency f is shown. We can see a small dip followed by a relatively large peak around 6 MHz in the characwkics. This dip may be due to the frequency de- pendence of the sputtmd magnetic film mger ia l ,d the peak may have a relation with the L-C resonance.

% Q a

a

0

200 ! . . , ;'b6 ' . ' . . ' . ' l b ' ' ".'; ' Os Frequency f [Hz]

8

Fig. 6. Fre~pency dependence of the output voltage VO.

MODEL OF DOUBLE LAYER SPIRAL COIL WITH A OUTER CORE

A. Inductance L

0.0shiro et. al [4] have shown the model of the double layer spiral coil forthe inductance ,capacitance and resist- ance,but it is vay simpwied one and is not for.the closed outer core.

We propose the simplified model for the rectangular spud coil of double layer with the outer core (see Fig. 7 (a),(b)) on assumpuons of that as follows: 1 .The upper coil and lower one of double layer planar coils are completely overlapped ady for the calculation of the indudance L.

2.The distance between adjacent coils is equal to each coil widthw.

3.There are two paths for the magnetic flux produced by the i th coil current. One is the path (Route 1) through closed cores, and the &er (Route 2) is the closed path h u g h the route ofthe upper core+airgap betweenthe inner shunt c m and thei th coil + the lower coce air gap between the outes shunt core and the i rb coil 3 the upper core.

229

The reluaance Rm in the Route 1 can be writren BS,

where 1 s is the relative magnetic permeability ,& is the thickness of the magnetic core film. a is the distance bet- ween the center of the coil and the outer shunt core, and c is a half length of a side of the rectangular inner shunt core.

The relucrance Rij for the magnetic flux Qij through the j th coil due to the current I of thei th coil inthe Route 2 can be given as, for is j ,

(2) for i > j ,

(3) ,where li is the distance from the i th coil to the center of the coil,i.e. li = l i + 2 ( i - I)w,.anddisthedistancebetween the upper add the lower core

The magnetic flux @m in the Route 1 due to the current I flowiag 2 n turns coil, which imerlinks h u g h every coil, is given as,

(4)

I 1 I . &

inner shunt core

- - i-th coil

outer shunt core -

i-th coil f 1

1

ou.-- shi

j-th coil ( b )

Fig. 7. Schematic diagram of the double layertramdormer with a outer core for model of the inductance. (a) Top view, (b) Cross s e c r i d view of the main part.

The summation of the magnetic flux Qm fa r each turn of

coil in Route 1, @mt, can be written as,

The inductance in Route 1 ,Lm, is give as , (5) Qmt = 2 n Q m = L m I . . . .

Lm=4nz . .. Rm

The summation of the magnetic flux Qij for each turn of coil

in Route 2, @gt, can be written as,

The inductance in Route 2,Lg,'can be given as, m i

The internal self-inductance of the coil, L,is written as ,

where s is the total length of the double layer coil [5].

with each layer of n turns as a "mation of these induct- ance,i.e.,

For n=9.5, a=750 pm, =I15 pm, d=5 pm, tm=2 pm, w=20 pm , 1i=237 pm, and ps=500, calculated values of Lm,Lg,LiandLareLm=t.22(LH,Lg=2.75pH,Li=O.Ol pH and L =3.98 pH. This theoretical value .3.98 pH,of the inductance L of the double layer coil of outer core type multilayer transformer is in relatively good agreement with the experimental one of 3.2 pH at the low frequency.

We can get the total inductance L of the double layer coil

L = L m + L g + L i :.. (10)

B. Capacitance C

We made a simple model to evaluate the stray capacitance C between the double layer coil and the magnetic film layer through the insulating layer of polyunide on assumptions as follows, 1. The diman= di between the lower coil and the lower

core is equal tothat betweentheupper coil sad the upper core. The distaclce Q between the upper d and the lower core is equal to that between the lower coil and the

upper -. 2.The upper coil is @laced without overlap with the

lower core. 3 The distaae between tbe upper anSthe lower core .d, is

written as, d =d i + d2 + t. where t isthetltickness ofthe Coil. The srray capsciunCe C cm be written as, C = 1 6 w n (11 +(n-1 ) w }%e, (L+ 1) ....

di dz (11)

We have obtained the m y capacitance C of 46 pF from above eqn.(ll) for practical values of &=3.2. t=3 pm, di =I pm and d2=4 pm. The measured stray capacitance bet- ween the secondary coil and the magnetic core was about 72

230

1 I T "

pF. The discrepancy between theoretical value and exper- mental one may arise from that we have not taken account of the pad electrodes,&. in the theory.

C. Resistance R

The resistance R of the two layers spiral coil can be simply expressed by a following formula considering the total length of spiral coils,

where p is the resistivity of the coil marerial. For the resistivity p of evaporated Al film of about 3 pa cm , the

calculated value of resistance R is about 32 n. Comparing with experimental d t s of the low fquency resistance R of 32.8 fl shown in Fig.4 @), we can see the theoretical result agrees well with the experimental one ,

R = 16 n p { I i + (n -1)w ) I w c , (12)

EQUIVALENT CIRCUITS AND SIMULATION

The equivalent circuits of this MOET is shown in Fig.8. The circuits simulations are performed by P Spice simulat- or. One of results of simulation is shown in Fig. 9 for the photo-cell ament Ip=3 mA which is corresponding to the received optical power of 8 mW, the primary and the secondary coil resistance Rp=Rs=33 0 , secondary load

resistance Rl is 1 Kn, the primary and the secondary coil inductance Lp=Ls=3.3 pH (experimentally obtained lower frequency value ), estimated values of stray capacitances ,Cps=30 pF, Cp=Cs=72 pF,coupling constant between the primary and the secondary coil k=O.9.and taking account of the p-n junction capacitance of about 500 pF of the photo- cell . From these close results to the experimental one as shown in Fig.5. we can see the equivalent circuits will be valid for the MOET with multilayer spiral coils.

I I R P " Rs

'P I R I

p-n j ! Load Photo Gel( Thin Film Transformer Resistance

Fig. 8. Equivalent cirmirs of the M o l 3 for circuit simulations.

23 1

- 7 - l - r ~___ 7 -

g -200 .__._ J! ....._...~........... --.- .-..... ---- ..... -------~ . --------* 3 0 0.5 1.0 1.5 2 . 0

i; Time [p]

Fig. 9. Result of circuit simulation by P Spice simulator for

Cps=30 pF, Lp=Ls=2 pH, junction capacitance of the 5MHz. Ip=3mA. Rp=Rs=33 a , Rl=l Kn, Cp=Cs=?2 pF,

photo-cell Cd=500 pF, coupling c~lu~uLllt k~0 .9 .

CONCLUSIONS

A miniatwe opto-electric traosformer(M0ET) ,which consists of a monolithically fabricated p-n junction photo- cell and a miniaturized multilayer transformer with spiral coils on a Si chip,is proposed and demoantnued. In this expaiment the photocell is directly illuminated by a LD of 830 nm wavelength driven by on-off signal of frequency up to 10 MHz.and the corresponding output voltage Vo of the VBLlSformer with turns ratio of 1: 1 (the p ~ a r y and the secondary coil have same turns of 19) is measured. The simplified model fcr the inductance L of the double layer spiral coil with a outer core is proposed and compamd with experimental one.The t h e d c a l value of L is in good agreement with the experimental one. The stray capacitance C and the coil resistance R are also evaluated.

The equivalent Circuits of MOET is presented and circuit simulations are performed by P Spice simulator. We can find that simulated d t s is almost same as the experimen- tal one. Since this MOET can be driven by the on-off light

through a optical fiber, this can be used as a electrical source under special circumstances, such as in a human body,to drive ICs oc actuators.

ACKNOWLEDGEMENTS

The authors would like to thaak Mr.Yuichi Sat0 for assistance in device fabrications.

. ..

REFERENCES

[l] L.-%Fan .Y.-C.Tai and R.S.Muller, "IC-Processed miaomotns: design,technology and testing," Senscw &Am-, ZO(1989) pp.4147

[2] L.P.P-0 and Y.-H.Cho, "Mechanical Des@ Issues in Lataally-Driven MicroJtrurmre~," .5k"md

[3] M. Esashi,S.Eob,A.Masudn,F.Fhquist,T.MPwo,I.Lun dstciim and S.Choi, "The fabrication of a Micra Valve byMeansofMiaamachiaiag," Aac. &&Seaar S+nprmum, ( 1986) pp. 269-2'22

Mioianve Planar Inductor," JEEETrrar: M w & k

[5] S.Takeyams, ?BtxwyofBtxznmzgtz&cP?enmemt,

A", A21-A23(1990) pp.loM)-1064

[4] O.Oshko,H.Tsujimoto md K.Shime, "A Novel

MAG-23(1987) pp.3759-3761

M ~ r ~ e ( l 9 6 3 ) ~p.384-385

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