Solid-Stare Necrronics Vol. 33, No. 8, pp. 1 I IO-I 112, 1990 Printed in Great Britain. All rights reserved

0038-l lOI; $3.00 + 0.00 Copyright c 1990 Pergamon Press plc

THE EFFECTS OF CURRENT SPREADING IN THE SEMICONDUCTOR ON THE DETERMINATION OF CONTACT RESISTANCE

(Received 20 October 1989; in revised Yom 19 January 1990)

INTRODUCTION

Ohmic contacts to 111~ V compound semiconductors such as GaAs, InP and InSb are crucial in the operation of MES- FETs, HBTs, LASERS and LEDs. Device performance and reliability are affected by the contact resistance and adhesion of the metallisation to the semiconductor. For contact resistance determination, the transmission line method is frequently used[l]. The method consists of measuring the voltage drops from a number of equally spaced contact strips between two large contact pads when a current is passed between them (Fig. I). The usual analysis assumes homogeneous sheet resistivity and uniform current flow in the semiconductor between the contact pads A and B. Such an assumption leads to a prediction of linear voltage drops between the pads which is not always observed. The separa- tion between the contacts can be as large as 1 cm and the current may not flow uniformly over that distance. Where current Row is confined to the semiconductor between the pads through mesa etching, a nonlinear voltage drop with distance can again result from sheet resistivity variation in the semiconductor. Errors in the determination of low contact resistance values can be quite large if these effects are not taken into account. In this paper, we take into account current spreading. The contact resistances can be measured accurately without performing the mesa etch, thus simplifying test pattern fabrication. Variation in sheet resis- tivity can be similarly treated.

EXPERIMENTAL DETAILS

Experimentally, the voltage drops are measured from the contact strips 1, 2, shown in Fig. 1 when a current, it,, flows between the contact pads A and B. The contact pads and strips are obtained through photolithographic masking followed by etching after the metal has been e-beam evapo- rated and sintered on the GaAs substrate. Distance T, between the edge of the contact pad and that of the mesa is indicated on Fig. I. With current spreading in the semiconductor, the potential drops do not vary linearly as shown in Fig. 2. The data points in Fig. 2 are the measured voltages at the contact strips with the contact pad edges being at the right and left hand margins of the graph. A current source is used to force a constant current in the range of l_lOmA through the contact pads while the voltage at the strips is measured with a high impedance voltmeter. Five sets of data are taken before the same sample is mesa-etched to vary the distance T,. In this way, the same contact resistance is being measured and a reliable method of obtaining the contact resistance should predict the same value independent of the mesa width 227 + Z, indicated in Fig. 1.

THEORY

The potential distribution V(s) under the contact pad using the TLM mode1 including non-zero metal sheet

resistivity[2] is given by

V(x) = ; AL (e“L {

- 1) + BL(1 -e-“L)+Ey L’i,.v 1

(1) 0 rc

where i0 is the current flowing between the pads: R,, is the metal sheet resistivity; R, is the semiconductor sheet resistivity; Z is the width of the contact pad; TU is an effective width for current flow in the semiconductor below the pad which takes into account the current flow via its lateral edges; W is the length of the contact pad: rc is the contact resistivity,

L= ~ J rc R: + R;

R;=R,: ‘0

With current spreading and also if there is resistivity varia- tions, the voltage on the contact strip located at x is given

by

V(x)= s ’ &(x-l ”

-TxiOdx +K (2)

where K is the voltage at X = W and T(x) is the effective width for current flow given by

T dV(x) dJ’(x-) R,( W a dx ,cw

= T(x)x--.R(x). (3)

To compute the contact resistivity, rc, the intercept L, in Fig. 1 is determined from the gradient of the voltage plot in Fig. 2 at strip distance equal 0 mm. This gradient which is equal to i0 R,(W)/T, from eqn (2), is calculated from the polynomial fit, rather than the linear fit as is normally done, so as to take into account non-uniform current flow. By measuring the voltage at the edge of the contact pad A and with the value of L,, R: and R, can be computed from which the contact resistivity, r, is calculated from rc = L*(R: + R,). In this paper, the method of using the polynomial fit is refered to as the “Modified Transmission Line Method” or MTLM.

RESULTS AND CONCLUSION

Figures 3 and 4 shows the contact resistance of Au/Ge/Ni/Au metallization contact of thickness 100/50/50/600 nm sintered at 380°C for 15 min as a function of the mesa width (2T, + Z). Figure 3 shows the contact resistance obtained for the contact at the higher potential

1110

Notes 1111

7 8 9 10 11 12

Fig. 1. Structure for measurement of contact resistance using the transmission-line method (TLM). The lower graph shows the plot of voltages at the contact strips as a function of position.

whereas Fig. 4 shows the result for the contact at ground potential. As five sets of readings (with different currents) are taken for each sample, the spread in value of the contact resistance as experimentally obtained is represented by the vertical bar. The dashed line indicates the results of the specific contact resistance obtained using the linear fit while the full line is obtained using the polynomial fit for three different mesa widths (2T, +Z) on the same sample. At a mesa width of 2.5 mm the current does not spread but flows virtually in the semiconductor between contact pads A and B. The following may be noted:

(i) The contact resistance as measured using the MTLM is approximately constant independent of the mesa width. This indicates a more reliable method of obtaining the contact resistance.

(ii) The same value of contact resistance of 3.16 x 10m5 Qcm-* is obtained by MTLM for both the contact pads A and B on the same sample (see Figs 3 and 4). This is not observed for the linear fit even at a mesa width of 2.5 mm where there is no current spreading. In Fig. 3, the contact resistance of pad B is measured to be 1.413 x 10-5Rcn-2 whereas that deduced for pad A is 5.010 x 10-5fIcm-2. This distivity of the semiconductor which distorts the linear fit. The poly- nomial fit however takes into account this variation.

I -3.0

-3.2 Au/Ge/Ni/Au-100/50/50/600 /+

Contact pod E / /

I-7

X 10/20/30/60 Ge/Au/Ni/Au

12- i=lOmA I

*/’

5: 10 -

*’ *’

*’

E 6-

% X’ *’

0 6-

2 X’

*’ *’

--_

’ 4- Lineor flt

*’ - Polynomiol fit

-7 (degree 7)

2%

0 I I I I I I I I I I I.

0123456769 10 11

Strip distance (mm)

Fig. 2. Voltage at the contact strips versus distance from edge of contact pad A. The data points are fitted by a

polynomial (-) and a linear equation (---).

k -3.4 - - MTLM /. /

r 0 -3.6- ---TLM

* 5 -3.6 - / 2 / - .!? -4.0 /

? /

-4.2 - / /

t; /

0 -4.4 / -

/ z

T T 8 -4.6 - / .I. /

0” - 4.6 -1

-5.0 I I

01 2 3 4 5 6 7 0 91

Width of mesa etch (mm)

0

Fig. 3. Contact resistance of contact pad B for three mesa widths 2T, + Z. The polynomial fit, designated as MTLM, gives an approximately constant contact resistance. The linear fit used in TLM, gives a decreasing value of contact resistance with mesa width 2i”, + Z. Five sets of data with different current, iO, are obtained for each mesa width. The spread in value is denoted by the vertical bar at each data

point. Z = 2.5 mm.

1112 Notes

_ -2.6. NE -2.6 - Au/Ge/Nl/Au-100/50/50/6~

Contact pod A k -3.0 - /’ HI - MTLM ” = 0 -3.2 - --- TLM

- 3.4 -

/ /’ r /

z.x /

5 -3.6 - 5 .E -3.6 I L -4.0

5

‘;

r::’

-4.4 - 8

0”

...l.//“’ ! r

- A -4.6

-*n. 1 1 1 1 I I 1 I I I.”

0 1 2 3 4 5 6 ? 6 9 10

Width of meso etch (mm)

Fig. 4. Contact resistance of contact pad A for three mesa widths 2T, + 2. The same sample is used for obtaining the data in Fig. 3. Contact pads A and B are on opposite sides

of the same sample.

The determination of contact resistance can therefore be subject to serious error if non uniform current flow is not taken into account. Hence using the polynomial fit, the mesa etch step required to limit current spreading can be omitted.

Centre for Optoelectronics Department of Electrical Engineering National University of Singapore IO Kent Ridge Crescent Singapore MI 1

S. J. CHUA T. C. CHONG

S. H. LEE

Harris Semiconductor (S) Pte Lid 105 Boon Keng Road, No. 03-01 Singapore 1233

Y. S. WANG

REFERENCES

1. H. H. Berger, Solid-Sr. Electron. 15, 145 (1972). 2. G. S. Marlow and M. B. Das, Solid-St. Electron. 25,91

(1982).