electrical characterisations of new microgap surge absorber fabricated by using conventional...

Electrical characterisations of new microgap surge absorber fabricated by using conventional semiconductor technology

H.Li and H.-Z.Wu

Abstract: The structure of the microgap and the manufacturing processes for a new typc of microgap surge absorber, fabricated by semiconductor technology, arc described. A very stable spark-over voltage with a narrow distribution was obtained by coating the metallic filins in the microgap. As well as the desirable characteristics of common microgap surge absorbers, this new type of microgap surge absorber has other advantages such as small size, low cost and suitability for mass production.

1 Introduction

To protect various low-voltage electrical circuits from transient overvoltage, many types of surge absorbers have been used, such as the transient voltage suppressor (TVS), semiconductor arrester and varistor etc. [l, 21. High response speed is the unique characteiistic of these devices. However, there is high residual voltage in varistors, and their capacitance is also large. The p-n-junction semiconductor devices have the problems of small surge current capacity, large junction capacitance and large leakage current. Old-fashioned gas-discharge tubes that have the largest surge current capacity and very small capacitance are still widely used in communication system, computer, radio and TV equipment.

To improve the performances of the gas discharge tubes, various works have been done. To reduce the dark effcct on the device, the cathodes were coated with barium, stron- tium or calcium carbonate. In the microgap surge absorber (MSA) device, Sn02 semiconductor thin film was deposited on a ceramic tube [3, 41. Then one or several narrow rings (nlicrogaps) with widths of a few tens micrometres were formed on the ceramic tube by laser ablation of SnOz thin film. Thus, the gaseous gap in the gas discharge tube is replaced by the microgap. Fig. I shows the cross-section view of the microgap surge absorber. In this structure, gas discharge occurs along the surface of the semiconductor. Because the microgap can supply initial electrons for gas discharge, the dark effect will be eliminated, the response time of the device will be shortened and impulse spark-over voltage will be reduced [S].MSAs have already replaced old-fashioned gas discharge tubes in some fields. However. the MSA has such disadvantages as unstable spark-over

0 IEE, 2001 TEE Prucwdgs onhe no. 20010248 DOI. 10.1049/ip-cds:20010248 Pap% f i t received 21st February 2000 and in revised fonn 4th Ja11u;lly 2(U1 H. Li is with the Dcpartmenl of Aukmation & Computer Technology, Ningbo Univeixity; Ningbo, 31521 1, People’s Republic of Cxna H.-Z. Wu is with the Department of Physics, Zhejiang Univenily, Hangzhou, 310027. Pcople’s Republic or China

voltage, V,, resulting from random initial discharge. complicated fabrication procedure and thus high cost.

semiconducting f i lm ceromic tube ,g‘ass / .mlcrogoP

u l m m Cross-section qfniicrogq ,surge rrbsorber. M.itli ( I single microgap Fig. 1

In this paper, the fabrication and electrical characterisa- tion of a new type of microgap surge absorbcr (NMSA) is described. Convcntional silicon semiconductor tcchnology was used. To have a stable spark-over voltage a m e l a k film was added betwcen the glass layer and the silicon wafer [6]. The fabricated NMSA has stable spark-over voltage, small size, low cost and good performance.

2 Device structure and fabrication

Fig. 2 shows the cross-section of the NMSA. Its package io DO-35 or DO-41, as commonly used in semiconductor diodes such as Zener diodes and switching diodes. In this structure a special silicon chip with a microgap was placed between two electrodes, and metallic copper was plated on the surface of the electrodes. Lead wires were connected to each electrode. The whole assembly was enclosed in a glass tube, which was filled with inert gas.

electrodes

lead wire

IEE Prot.-Circuits Device.? SJK., Koi. /4R, No. 3, Jurie 2001

The structure of the chip is shown in Fig. 3. It consists of two silicon wafers. metallic filnis and a glass layer. The glass layer was sandwiched between the two sdicon wafers. The thickness of Lhe glass is equal to the width of the microgap. Two silicon wafers with one glass layer fonii one microgap. and more silicon wafers and glass layers can fomi multiple microgaps. The chip with one inicrogap can be used to make NMSAs that have a spark-over voltages of < 500V. Multiple-microgap chips can be uscd to fabricate NMSAs with spark-over voltages of > 50OV.

metallic film - A

1 n-S i

Fig. 3 C~oss-.section of ciz@ i&i siri@ nricr.ugap

About quarter-millimetre n-type silicon wafers with resis- tivity > l50cm were used to fabricate the NMSAs. Both sides of the siticon wafers were lapped. After cleaning processes, a IOOOA-thick metallic film was evaporated onto the surface of the wafers. A glass powder layer was plated by electrophoresis on the metallic film on the wafers. The thickness of the plated glass powder layer is dependent on the requirement of the width of the microgap, which varies from 20 to 5 0 p . If the width of the microgap is > 50w, the rcsponse speed will dccrease and its impulse spark-over voltage will increase [3]. The next step was placing the two glass-powder plated wafers face to face. Then the structure was sintered in nitrogen ambient at 810°C. At this tenipera- ture, the glass powder melted and changed to a glass layer. After cooling down to room temperature two silicon wafers were bonded by the glass insulator. The whole structure was sprayed by enieiy. Finally. the structure was scribed into 4 5 0 ~ x 4 . 5 0 ~ chips by a diamond-impregnated saw blade, and the glass in the chip was etched to fonii a discharge ring (microgap) of - 10-20pm.

The chips, glass tubes and electrodes were sealed by a glass sealing machine to form the NMSA devices. The steps of the package were as follows: first, the glass tube was evacuated: then an inert gas was filled into the tube and heated to seal the glass; filially the glass tube was cooled down to room temperature. The pressure of the gas contained was dependent on the requirements of spark- over voltage, the width of the niicrogap and the type of gas used. For example, a pressure of 5 x IO4 Pa is required to fabricate the NMSA with r< = 25OV if the width of the microgap is 35 pm and the inert gas is argon.

3 Principle of NMSA devices

We consider NMSA with a discharge configuration shown in Fig. 4. Electrodes A and A' are cathodes with a

<->A cathode

8'

R 'r'

anode

resistance in series. Electrodes B and B' are anodes with a resistance in series. The resistance is of the silicon substrate type. For the configuration, gas discharges occur in electrodes A', B' (microgap) first. Discharge will then change to glow discharge between electrodes A and B when the discharge current is increased.

The discharge in electrodes A' and B' determines the spark-over voltage of the NMSA. This phenomenon can be explained by Townsend's theory. When the electrical field between A' and B is assumed to be approximately homogeneous, the discharge current density inay be expressed as [7]

(1) jD(a - /5') ~ ~ 1 1 ( j a - i9)d)

J' = t r ( l + 7 ) - (ay + 13) exp((n: - ~ ) d ) where j , is the initial current density, /3 is the recombination coefficient, y is the electron secondary emission coefficient, d is the width of the microgap and a! is the Townsend pri- mary ionisation coefficient given by

In eqn. 2, y is the pressure, and A and B are recombination coefficients. Neglecting /3, one can obtain Townsend's brcakdown condition from eqn. 1

y(exp(cud) - 1) = I (3) If yis assumed independent of E and p ? the spark-over voltage can be written as

(4) Bpd

ln(pd) - In(ln(1 + 1/7)/A) 1,; =

The V, as a function ofyd is shown in Fig. 5 .

1001 100 200 300

pd,Po.cm Fig.5 @aik-oiw iioltuge U\ U fmtron qfpd u1 argon

After Townsend's breakdown, the discharge in A" and 3' changes to glow discharge in A and B because the discharge current is limited by R. The behaviour of the glow discharge for NMSA is similar to a DC discharge in flat- plate electrodes which has been studied widely IS-lo]. For the one-dhiensional case, the following equation and boundary conditions are taken from [7]:

The boundary conditions at x = 0 (cathode) are

2 = 0, 3r = ? i J z = Y ( 3 - 3 e )

and at x = d (anode)

x = d , j 2 = 0. j e =.I From eqn. 5 with boundary conditions, the general characteristic of the spatial distribution of the electric field is given

IEE Pvvc -Ciiiuirs D r i i m S J S ~ Pol 14K N o 1, Iiiw Z(Ml 172

in Fig. 6. The plasma potential. cathode fall region and anode fall region are shown in Fig. 6. Cathode fall, V,, and anode fall, V,,, have been determined experimentally for copper electrodes and are given by V, = 130 V and V, = 10V in argon, and V, = 208V and V, = 16V in nitrogen. ~71.

0 d X

Fig. 6 Voltuge cord electric jield mflat-phte glow diiLharfir.

As the glow discharge current increases, the arc discharge occurs suddenly. The voltagc on the NMSA is fixed at -2OV, and a large current can flow across the NMSA.

4 Electrical characterisations

4.7 I ! characteristic Fig. 7 shows typical semilogarithmic plots of the current as a function of voltage measured for the new type of microgap surge absorbers. The samples have a single microgap with a width of 3 5 ~ . Sample A was filled with Ar gas and sample B filled with N2. Both samples have the same pressure of 5 x IO4 Pa.

q. a -{.>-a--

E 10-2- " 3

10-L-

10-6-

100 200 300 4 00 0 voltage, V

Fig. 7 Semilng plot,u o f ~ u n m t ugouat voltugefor. ht:O typ.y of NMSA

From Fig. 7 we can see that I-V characteristics measured from both samples are the characteristic obtained from the discharge in flat-plate electrodes [7]. The glow voltage of sample A is - 140V and that of B is - 220V. The glow voltages are close to V,. + c,, which is 140V in argon and 224V in nitrogen. These voltages are in good agreement with the discharge characteristic of conventional flat-plate electrodes.

4.2 Spark-over voltage The spark-over voltage was measurcd for 300 NMSA samples that were randomly collccted. These samples were packaged by using a 35pm-width single-microgap and an argon-gas pressure of 5 x 104Pa. The distribution of the spark-over voltage is shown in Fig. 8. The measured spark-

TEE Proc.-Cimrits 0evicr.s Syst.. I'ol 148, ,Vo. .$, Juuc 2001

over voltage of 89.6% selected samples is within the range of 250V k 15%). The average V, is - 256V. These results indicate the practicability of the manufacturing method using conventional semiconductor technology.

>. v C RI

3 U @J L L

Fig.8

I 290 330

From Fig. 5 thc theoretical value of b', is 228V at pd = 5 x lo4 Pa x 35pn =175Pa.cm in argon. The error in v, is caused by the inhomogeneous elcctrical field near the microgap and the error in y

Thc effect of the metallic films on the stability of the spark-over voltage is shown in Fig. 9. In sample A, the microgap was coated with a metallic film and thc microgap in sample B was not. Both samples were tested 30 times. It can be seen from Fig. 9 that sample A has very stable spark-over voltage and the undulation of the voltage is * 2V. However: the undulation Tor sample B is about one order higher. These results demonstrate that the metallic film coating in the microgap can stabilise thc spark-over voltage.

1 P 2oo t 1801 , I I

1 6 11 16 21 26 times of test

Fig. 9 Sfubdity of spnrk-over voltuge Sample A has a microgap with metal; sample B has a microgap without metal ....A.... sample A -+- sainplc B

G 200 z t 1001 I

200 300 LOO 500 T,K

@xirk-over voltage V, plotted against tr.~nperutur(, T for the NMSA Fig. 10

From Fig. 10, we can see clcarly that as the temperature changes the spark-over voltage is almost unchanged. This result indicates that the NMSA can work in a wide range of temperatures.

173

4.3 Surge absorption characteristics The surge absorption characteristics of the NMSA were tested by applying a voltage pulse with l000V peak voltage and a 101700p or 11700p waveform. Typical absorption waveforms measured for the sample with a spark-over voltage of 230V are shown in Fig. 11. From the absorption curves, we can see the arc discharge promptly accrues after breakdown of the gas. Both Figs. 4 and 8 show that the voltage drop in the NMSA is limited to - 20V. The impulse spark-over voltage, V,,, is related to the response time. Fig. l l a shows a Vp of 380V, and Fig. l l b a 5, of about NOV. These values are similar to the Vp of MSAs 131.

800 > 0 3 LOO- - P

0

(i)

-

(Ill

(i) Znginal wave'fom: (iij absorption waveform rr 10/700)~s voltage pulse (time scale: 5p'div) h l/700p voltage pdse (time sale: 1 p'div)

The measurements of electrical property changcs of NMSAs after applying surge current (8/20p) for 10 cycles are given in Table 1. The interval of each test was Smin. The data in Table 1 indicate that the surge current capacity for package DO-35 is > 2WA, and DO-41 is > 1500A.

Table 1: Surge current capacity measurements on the NMSAs

Package Peak current*, % Insulation A resistance, MR

100 < 10 > 103 DO-35

200 < 10 > 103 DO-35

500 < 10'(20%) DO-35

Short (60%) DO-35

1000 < 10 > 103 DO-41

1500 < 10 > 103 DO-41

3000 < 102(17%) DO-41

5000 Short (75%) DO-41

* 8/20p waveform of current, applied 10 times with an interval of 5min.

4.4 Capacitance and insulation resistance Fig. 12 shows the capacitance of an NMSA, CO, as a function of width of microgap. The theoretical curve of CO

I74

against d in Fig. 12 was calculated by using the following formula:

where E~ is pennittivity in a vacuum, E is a dielectric con- stant of - 10, S is the chip area of 0.4 x 0.4mm2 and d is the width of the microgap.

2.51-

10 20 30 LO 50 d , w

Fig. 12 + experiment

~ theory

Cqiuitunce CO ngautst width Of rtiicwgap of d ?fA'MRr

Table 2 gives the results of measurement of insulation resistance (IR) for the NMSAs. The results indicatc that insulation resistance of NMSAs is very large (> 1000MR).

Table 2: Insulation resistance of the NMSAs

Insulation Testi ng Spark-over resistance, voltage, voltage, Package MR V V

> 1000 50 120-180 DO-35

> 1000 100 160-240 DO-35

> 1000 100 240-360 DO-41

> 1000 250 700-900 DO-41

5 Summary

A comparison of the NMSA with other surge absorber devices is given in Table 3. The NMSA has the smallest CO, thc largcst surge current capacity per unit volume and the highest IR among all the devices.

From the foregoing discussion, we can see that the NMSA has improved electrical characteristics compared to the MSA, in terms of stability of

It used the conventional semiconductor technology, which is suitable for batch processing.

A stable spark-over voltage with a narrow distribution was obtained by the metallic film coatings near the microgap.

Low cost of fabrication, which is comparable to the production of ZnO varistor, was achieved because of the use of semiconductor technology.

The MNSA device is small and its package technique is suitable for surface mount packing such as mini MELF.

The characteristics of response time, surge current capacity, capacitance and insulation resistance are comparable with those of the MSA.

It is expected the MNSAs can be applied to high fre- quency equipment because of the small CO and high IR. Moreover, the MNSA also has good application prospects in the protection of electronic equipment; for example, it protects display driver ICs and transistors from static surge in monitors, and it protects telecommunication equipment such as telephone, fax, etc. from lightning.

size and cost:

I E E Pioc.-Cirmuitr flevices L S ~ ~ ~ t . , Yo/. 148, Yo. 3, June 2001

Table 3: Properties of five types of protective device ___

Size Cost

TVS 20 50 1pA I p s small high

Semiconductor arrester 100 60 1pA I n s small high

Varistor 600 500 IPA I n s big low

MSA (200V) 1500 < I < I n A I p s big high

NMSA (200V) 1500 < I < I n A I p s small low

Surge current Capacitance, Leak Response capacity, A,8/20 ps pF current time

6 References 6 LEBEDEVA, N.N., ORBUKH, V.I., and SALAMOV, B.G.: ‘Inves- tigation of the e f k t of discharge plasma stabilization by a semiconductor’, J. PIivx. III, 1996, 6, pp. 797-805 I

7 YANG, X.J.; and ZHU, D.C.: ‘Gas discharge physics’ (Fudan Uni- versity Press, Shanghai, 1996)

2 GUITA, T.K.: ‘Applicdlion oI“ zincoxide varistors’, .J. An?. Cerflnz. g MEYYAPPAN. M.: and KRESKOVSKY, J.R:: ‘Glow discharge simulation through solutions to the moments of the BolWann trans-

3 port equation’, J AppL I’hys., 1990, 68, (4), pp. 15061510 9 PAI, S.T., and GUO, X.M.: ‘Analytic approach to glow discharge

theory: the physical model’, J. Appl. Phyx, 1992, 71, (12), pp. 582& 5825

10 PAI, S.T., and GUO, X.M.: ‘Analytic approach to glow discharge theory: result and analysis’, J. Appl. I‘hys, 1992, 71, (12), pp. 5826 5833

RADERCHT, P.H.: ‘A revicw of thc shorted emiH‘er principle as applied to pnpn dicon controlled rectifiers’, Int. J. Electron., 1971, 31, (6), pp. 541-545

Soc. 1990, 73, (7), pp. 1817-1840 (in Japanese) TACHIBANA, K., OKUBO, S., and OSHIGE, T.: ‘The discharge or microgaps and their application to surge absorber’, Trms Rst. Eng. Jpn. A , 1980, IOOA, (3) , pp. 169-176

4 TERADA, R., and OSHIGE, T.: ‘On surge absorber’, Prm. Inst. Eleclro.y/u/. Jpn., 1982, 6. pp. 357-362 (in Japanese)

5 ANDO, K., OSHIGE, T., TACHIBANA, K., and H A M , M.: ‘Dk- charge processes in a low-uolVage microgap surgc absorber’, IEEE Tram. Ind Appl, 1985, Li-2 I, (6), pp. 1349-1353

IEE Proc.-Cirircails Devices Sjst.. Vol. 148, !Vu. 3, Julie 2001 175

electrical characterisations of new microgap surge absorber fabricated by using conventional...

Documents