ELECTRICAL PROPERTIES OF RF SPUTTERED NiCr THIN FILM RESISTORS WITH Cu CONTACTS

Investigations on RF sputtered NiCr thin film resistors, fabricated using Cu as conductor metallization, were made. The contact resistance characteristics, resistor film characteristics and TCR of the resistors were measured. The effect of heat treatment on the resistor characteristics was studied. A suitable annealing cycle for the resistor stabilization was studied. A suitable annealing cycle for the resistor stabilization was obtained. The effect of passivation by a thin quartz film on the resistor properties was also examined. The results are presented and discussed in this paper.


INTRODUCTION
NiCr is one of the most commonly used resistive material for fabricating precision thin film resistors.This transition metal alloy exhibits a wide range of resistivity, low tempera- ture coefficient of resistance (TCR) and high stability of electrical properties.Owing to its technological importance NiCr resistors have been the subject of several investigations.[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Conventionally a multilayer Ti-Pd-Au conductor system is used as the contact metalli- zation for NiCr resistors. 11In recent years due to increased cost of the precious metals a need for a low cost replacement metallization for the expensive Ti-Pd-Au has been realized.Different metals 1'1s and metal combinations 16'17 have been investigated for the purpose.We have examined the use of copper as the contact metallization.Copper has a higher conductivity and is less expensive compared to gold.Also, copper is easily solder- able.The limitation on using Cu for contact metallization is that the Cu/NiCr contact cannot be annealed at the high temperature, usually required for resistor stabilization, due to oxidation of Cu and interdiffusion of Cr into Cu. 18he purpose of this paper is to report the results of our investigation on NiCr resistors fabricated with Cu contacts.The contact resistance was measured.The effect of heat treatment on the contact characteristics w.as studied.An optimum thermal annealing cycle for the resistors stabilization was obtained.The resistor characteristics and TCR of the resistors measured before and after the heat treatment are presented.The effect of passivation by a thin quartz film on the resistor properties is also reported in this paper.

EXPERIMENTAL
The resistors were fabricated on Alsimag glazed ceramic substrates.The substrates were cleaned by the standard procedure and were dried by blowing nitrogen gas before loading onto a water cooled anode in the sputtering chamber.An 8620J rf sputtering machine of MRC was employed to sputter the resistor (NiCr) and the conductor (Cu) films.The composition of the NiCr target purchased from MRC was stated to be Ni40/Cr60.The procedure adopted for the films deposition was as follows: The sputtering chamber was first evacuated to a pressure of 8 x 10 -7 torr.Then UHP argon gas was introduced in the chamber through a needle valve.The pressure inside the 44 R.K. NAHAR AND N.M.DEVASHRAYEE chamber was maintained at 5 m torr and the chamber was evacuated, in this state, for 15 minutes to ensure an inert atmosphere in the sputtering chamber.Both the NiCr and Cu targets were presputtered separately for 20 minutes each at a target voltage higher than the actual sputtering voltage.The NiCr and Cu films were sputtered sequentially in the same pump-down.Typical data of the sputtering process are given in Table I.The sheet resistivity of the NiCr film was 200 Ohms/square and thickness of the Cu film was 1.5 microns.
The resistor and conductor patterns were delineated by selectively etching the Cu and NiCr films using a photolithography technique.The copper contacts were protected by Sn Pb 60/40 solder immediately after fabrication.Measurements were taken on the as prepared resistors, after heat treatment of the resistors and after depositing a thin quartz film (0.3 microns) on the resistors.

RESULTS AND DISCUSSION
3.1 Contact Resistance Characteristics.The contact resistance of the Cu-NiCr contact was measured by a simple bridge balance technique.19 The electrical equivalent circuit of the measuring system is shown in Figure l(a).It consists of two symmetrical parts.RI and R2 are the resistances of the NiCr resistors, which are nearly equal in value, and RA, R B and R c are the resistances of the contacts A, B and C respectively, as shown in Figure l(b).Rp, Rq and Rx, Ry are the resistances of the potentiometers used to control the voltage drop in the circuit.E1 and E2 are the potentials of the power supplies.R v is a fixed resistance and a d.c.micro- voltmeter is employed to detect the null position.
In the method used, a source of potential E is connected to provide a current i between points A and B. The compensation current i, between B and C is provided by the source of potential E:.After compensation i.e. for a given E adjusting E: such that i 0, the voltage drop Vp will clearly be given by Vp il, R B By measuring i' and Vp, the contact resistance R B of the contact B can be calculated.
Similarly the resistance of the other contacts can be measured.Employing this technique the current voltage characteristics of the contacts was measured at room temperature (25C) after heating the resistors for an hour at different given in Figure 2. The nature of the characteristics reveal that the Cu-NiCr contact is ohmic in the entire range of measurement.The contact resistances of the as prepared resistors is within 2-3 m Ohm cm. 2 This value increases only marginally, as shown in the figure, when the temperature of the resistor is raised from 25C to 200C.At 225C although the contact I-V characteristics as shown in the figure remain ohmic, the contact resistance increases drastically.It is at this temperature that we believe the interdiffusion of Cr into Cu is substantial.There is also a possibility of some copper film being dissolved in the Sn/Pb solder. 2 The increase in the contact resistance is about 7 times the value obtained after annealing at 200C.
In the light of the above results we decided on 150C as a safe annealing temperature for the Cu contacts protected by Sn/Pb solder.That the 150C temperature would be safe for annealing is evident by the phase diagram 1 of Cr-Cu and Ni.Cu.Any possibility of interdiffusion at this temperature would be negligible.Further the heat treatment at 150C in air rather than at a higher temperature also reduces the possibility of producing an open circuited resistor during the relatively long heat treatment time# The change in the contact resistance during annealing in air ambient at 150C was measured as a function of time.The data is shown in Figure 3. and after annealing for hour (X)at 75C, o )at 150C, ,a )at 200C and )at 225C.
contact resistance measured at room temperature before and after annealing is also included in the figure..2 Resistor Film Characteristics.
Freshly deposited NiCr films shows a tendency for self annealing. 8This results in changes in resistance and TCR of the film.It is therefore necessary to stabilize the film properties.
Usually NiCr films are annealed in air at 300C for 2 hours for stabilization of the resistor film characteristics.The NiCr-Cu resistor system, as discussed in the previous section, cannot be annealed at such high temperatures.The film stabilization temperature was therefore restricted to 150C.As the films were being stabilized in an oven at 150C, the change in the film resis- tance was measured as a function of heat treatment time.Figure 4 shows the increase in the film resistance plotted as a function of the square root of the heat treatment time.
The variation is linear.The data indicates that the increase in the film resistance is due to surface oxidation of the NiCr film and is limited by the diffusion mechanism. 4It may be remembered here that oxidation of the NiCr is preferential.Exposure of the film surface produces CrO3 while Ni remains virtually in the metallic state. 13By oxidizing ANNEALING TIME (HOURS) FIGURE 3 The contact resistance measured as a function of annealing time o at 150 C and X at room temperature.
the top surface of the film the material beneath is protected against further exposure to the ambient gases, thus stabilizing the films.Typical I-V characteristics of the as prepared resistor, after annealing the resistor and after 1000 hours measured at room temperature, are shown in Figure 5.After stabilization the increase in the film resistance measured on a 5 1/2 digit HP 3455A voltmeter was typically 0.35 percent after 1000 hours at room temperature.This shows that the long term stability of these resistors is not good.We believe that the stabilization by the thermal annealing cycle is not adequate.This was confirmed by depositing a quartz thin film on the heat treated resistors.We observed that the stability improves significantly.A plot of the resistance change measured as a function of time at 70C on these resistors is shown in Figure 6.The stability of these resistors improves from 0.35 percentage to 0.04 percentage.TCR of the resistors was measured in the temperature range of 25C to 125C.Typically the TCR of the resistors after annealing was 25 ppm/C.TCR of the resistors measured as a function of annealing time is given in Figure 7.

CONCLUSIONS
The results of our investigation show that it is possible to produce good NiCr resistors using Cu as a conductor with stable electrical properties.The contact between the conductor Cu film and the resistive NiCr film is ohmic.The contact resistance is 2-3 m Ohm cm. 2 The contact I-V characteristics studied as a function of temperature show that above 200C the contact resistance increases drastically.
The resistor stabilization cycle, decided on the basis of the experimental data, for the NiCr resistors with Cu contacts is to anneal at 150C in ambient air for 48 hours.This cycle was found to be safe but the stability of the resistors was not good.The stability   improves significantly after passivation by a thin quartz film.The result o,f our investiga- tion are summarized in Table II.

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FIGUREMeasurement of contact resistance" (a) the fabricated structure o# the NiC resistors with copper contacts; (b) the electrical circuit.

FIGURE 2
FIGURE 2  The contact I-V characteristics measured at room temperature" D before annealing; FIGURE 4   annealing time.

FIGURE 5 FIGURE 6 FIGURE 7
FIGURE5 Current voltage characteristics of the resistors" o as prepared, X after annealing and zX after 1000 hours at room temperature.

TABLE Typical NiCr
/Cu film deposition parameters:

TABLE II Electrical
Properties of NiCr/Cu Resistors: