Thin Solid Films, I73 ( 1989) 2 1 l-224

PREPARATION AND CHARACTERIZATION 217

REACTIVE SPUTTERING OF RuOz FILMS

E. KOLAWA, F. C. T. SO*, W. FLICK, X.-A. ZHAOj’, E. T-S. PAN AND M-A. NICOLET

Cal$ornia Institute of Technology Pasadena, 91125 (U.S.A.j

(Received December 21, 1987; revised July 18, 1988; accepted April 19, 1989)

The growth rate, resistivity and intrinsic stress of RuO, films deposited by reactive sputtering with an O,-Ne or an O,-Ar gas mixture were investigated as a function of the following sputtering parameters: gas composition, gas pressure, sputtering power and substrate bias. We found that stoichiometric RuO, films could be obtained over a wider range of sputtering conditions in the O,-Ne mixture than in the O,-Ar mixture.

1. INTRODUCTION

Ruthenium dioxide, like other dioxides of the platinum group, e.g. Rho,, 0~0, and IrO,, exhibits metallic conductivity and possesses the rutile structure. Among this group of conducting oxides, RuO, is unique in that it is the only stable oxide of the metal at elevated temperatures . 1*2 This distinguishing feature is one of the reasons why RuO, was selected for this study.

It has been reported that RuO, films can be prepared by metal-organic chemical vapor deposition (MOCVD) which requires high deposition temperatures (about 600°C) and produces films significantly contaminated by carbon3. An advantage of the sputtering process is that it does not require substrate heating and thus is compatible with photolithography. In previous studies we have shown that RuO, films can easily be obtained by conventional reactive r.f. sputtering4-6. A recent paper by Krusin-Elbaum et al.’ also reports on RuO, formation by reactive sputtering in O,-Ar gas. We show here that an O,-Ne gas mixture is more effective than O,-Ar for the formation of RuO, films. When an O,-Ar ambient is used in our sputtering system, RuO, films can be obtained over a more restricted range of sputtering parameters.

* Present address: Optoelectronics Division, Hewlett-Packard Company. San Jose, CA 95131, U.S.A.

‘/‘Permanent address: Shanghai Institute of Metallurgy, Academy of Sciences ofChina, Shanghai, China.

0040~6090/89/$3.50 0 Elsevier Sequoia/Printed in The Netherlands

218 E. KOLAWA et al.

2. EXPERIMENTAL PROCEDURE

Silicon wafers (n-type, (111) oriented), silicon wafers covered with thermally grown SiO, (3OOOA), carbon tape and thin cover glass slides

(18 mm x 18 mm x 0.175 mm) were used as substrates. Prior to loading into the chamber, the silicon waters were passed through organic cleaning steps in ultrasonic baths of trichloroethylene, acetone and methanol and were then etched with dilute HF. The SiO, substrates and glass slides were cleaned in the same ultrasonic baths.

All films of this study were deposited by reactive r.f. sputtering using a planar magnetron cathode 7.5 cm in diameter. The substrate plate was placed about 7 cm below the target and was neither cooled nor heated externally. The sputtering system is equipped with an oil diffusion pump and a cold trap that yields a background pressure below 1 x 10e6 Torr prior to sputter deposition. The total gas pressure of the premixed 02-Ne or O,-Ar mixture was adjusted with a variable leak valve and monitored with a capacitive manometer prior to striking the discharge (subsequently referred to as an “initial” total gas pressure). Films were sputtered with an initial total gas pressure of 5,10 or 15 mTorr in both O,-Ne and O,-Ar gas mixtures. The relative initial partial pressure of 0, in neon, defined as the ratio of the initial partial pressure of oxygen to the initial total gas pressure of the gas, p(O,)/p(O, +Ne), was varied from 0% to 50%. Films deposited in O,-Ar discharge were sputtered in the 0°A-600/0 range of the initial partial pressure of oxygen.

A Sloan Dectak profilometer was used to measure the thicknesses of films. The thickness of all films was in the 800-1OOOA range. Their resistivities were then determined from sheet resistance measurements using a four-point probe. A Read camera was used to investigate the crystallographic structure of the films by X- ray diffraction. The atomic composition of the films was determined by backscatter- ing spectrometry (BS). Carbon substrates were used for some of these measurements to increase the accuracy of the oxygen-to-ruthenium ratio derived from backscatter- ing spectra.

The film stress was determined using the profilometer by measuring the resulting deflection profiles of samples with thin cover glass substrates. Original deflections of the glass substrates were taken into account. Details on the procedure are given in ref. 8. To determine the stability and resistivity of RuO, films as a function of annealing temperature, RuO, films were deposited onto SiO, (oxidized silicon) substrates with two different sputtering powers and were annealed in a vacuum at pressures of better than 5 x 10e7 Torr.

3. RESULTS AND DISCUSSIONS

Figure 1 presents the growth rate of films deposited in the O,-Ne gas as a function of the initial oxygen partial pressure and the forward power (reflected power was taken into account). The forward sputtering power was varied from 100 to 450 W for O,-Ne and from 100 to 500 W for O,-Ar gas mixtures. The initial total gas pressure was maintained constant (10 mTorr) and the substrate bias was zero. Films sputtered in pure neon and 10% O2 consist of metallic ruthenium over the whole range investigated. Sputter deposition with 20% O2 and a power exceeding

REACTIVE SPUTTERING OF RuO, FILMS 219

Fig. 1. The growth rate of RuO, (or ruthenium) films as a function of the forward sputtering power and

the initial partial pressure of 0, in the O,-Ne sputtering gas. The initial total gas pressure is always

1OmTorr and the substrate bias is zero. The dotted and shaded areas show sputtering conditions for the

formation of RuO, and ruthenium films respectively. The overlap of these areas indicates a transition

from ruthenium to RuO, whose exact position and nature are not known.

350 W also yields ruthenium films. All films deposited at oxygen partial pressures higher than 20% are polycrystalline stochiometric RuO, films of the rutile structure regardless of the sputtering power. To obtain RuO, films at 20% 0, partial pressure, the sputtering power needs to be lower than 350 W. These facts were deduced from BS spectra of the films deposited onto carbon substrates, and from X-ray diffraction using a Read camera. As can be seen in Fig. 1, the growth rate of sputtered films at fixed forward powers decreases slightly as the initial O2 partial pressure increases from 0% to 20x, and rises again beyond that value. For samples sputtered at 50% initial 0, partial pressure, the growth rate of RuO, films is almost the same as the growth rate of ruthenium films sputtered in pure neon.

Figure 2 shows the corresponding plot for the growth rate of films sputtered in the O,-Ar ambient. RuO, films are obtained only for low forward powers (100 and 200 W) in combination with initial partial pressures equal to and higher than 10% and 20% for 100 W and 200 W respectively.

RuO, films can be obtained in our sputtering system over the wide ranges of initial 0, partial pressure and of forward r.f. power only in an Ne-0, discharge. In an Ar-0, gas, RuO, films are obtained only in a narrow range of deposition conditions. One possible explanation is that the sputtering rate of ruthenium is lower in neon than in argon for the same sputtering conditions. Assuming that oxygen atoms arrive at the substrate at a fixed rate, the ratio of oxygen to ruthenium at the substrate will be higher when neon rather than argon is used. The second explanation is based on the fact that our results concur with those described by Aita and Trang*” for r.f. sputtering of platinum in O,-Ne and O,-Ar discharges. They reported that, during sputtering of platinum and gold targets in O,-Ne and O,-Ar gas mixtures, the formation of an oxide film on the substrate is controlled by a

E. KOLAWA et al.

Fig. 2. The growth rate of RuO, (or ruthenium) films as a function of the forward sputtering power and

the initial partial pressure of O2 in the O,-Ar gas. Sputtering conditions and presentation are the same as in Fig. 1.

reaction at the target surface 1o,11 The difference between the two discharges arises . from the fact that metastable neon atoms with energy levels of 16.6 and 16.7 eV can ionize an 0, molecule in the ground state (ionization potential, 12.1 eV) by Penning ionization, whereas metastable argon atoms cannot (metastable levels at 11.5 and 11.7 eV). Another process, the quasiresonant transfer of excitation, is possible in both O,-Ne and O,-Ar discharges, but it produces oxygen atoms at higher energy levels in an O,-Ne discharge. In general, oxygen species produced by interactions with neon are more energetic than those produced by interactions with argoni2. As a consequence, the use of neon rather than argon as the carrier gas facilitates the deposition process greatly. This model properly explains our observations for reactive r.f. sputtering of Ru02 also.

Figures 3 and 4 show the film resistivity as a function of the forward sputtering power and the 0, initial partial pressure for samples sputtered in O,-Ar respectively. The initial total gas pressure is 10 mTorr. The resistivity of ruthenium films is in the range 2&30 ulL cm, and increases slightly with an increase in the initial partial pressure of 0,. This tendency is similar for the ruthenium films sputtered in both ambients. The relatively high resistivity value of the RuO, films (150-180 @cm) sputtered in both the O,-Ne and the O,-Ar ambients in comparison to the bulk value (46 uLn cm) is possibly due to the microcrystalline structure of the RuO,. A transmission electron microscopy analysis showed that the grain sizes are of the order of 50 8, and 120 A for samples sputtered in O,-Ne gas at low (100-200 W) and high (400-500 W) forward power respectively. In Fig. 5, the resistivities of samples deposited in a gas containing 30% O2 and 70%Ne at 10mTorr initial total gas pressure, at 200 W or 450 W forward power and at zero bias are shown as a function of the temperature of annealing in vacuum for 15 min. After annealing at 800 “C, the resistivity is about one third of its initial value. X-ray data show that the films still possess the RuO, structure. The decrease in resistivity is

REACTIVE SPUTTERING OF RuO, FILMS 221

Fig. 3. Resistivity of RuO, (or ruthenium) films as a function of forward sputtering power and of the

initial partial pressure of 0, in the 02-Ne sputtering gas. Sputtering conditions and presentation are the

same as in Fig. 1.

0

Fig. 4. Resistivity of RuO, films as a function of forward sputtering power and of the initial partial

pressure of 0, in the O,-Ar sputtering gas. Sputtering conditions and presentation are the same as in

Fig. 1.

associated with a growth of the grains of the RuO, films, as was established by transmission electron microscopy.

To investigate the effect of total gas pressure and power, films were deposited at initial total pressures of 7.5, 10 and 15mTorr and a fixed 40% initial 0, partial pressure in both O,-Ne and O,-Ar gas at various forward powers and zero bias. The power was varied from 100 to 475 W. Figures 6 and 7 present the results for O,-Ne and O,-Ar respectively. Samples deposited in O,--Ne ambient were all RuO, films (as Fig. 1 already shows for an initial total pressure of 10mTorr). The

E. KOLAWA et al.

p(02+Ne)=l 0 mTorr-

50 bu:r: HuO, fiims

0 1’ ” ” ’ “. ” ’ “‘I 0 100 200 300 400 500 600 700 800 TEMPERATURE ICI

Fig. 5. Resistivity of RuO, films deposited in O,-Ne gas as a function of annealing temperatures in

vacuum. Annealing duration is 15 min for each temperature

2 250 E

2

: 200

d I150 I-

z g 100

p(O.$/p(02+Nek40%

p(Oz+Ne), o - 15 mTorr A - 10 mTorr 0 - 7.5 mTorr bias=0

1 * OO

I 1 I I I

100 200 300 400 500

POWER [WI

Fig. 6. Growth rate of RuO, films deposited in O,-Ne gas as a function of forward power for three

different initial total gas pressures. The initial partial pressure of 0, is 40% and the bias is zero.

growth rate of these films increases slightly as the initial total gas pressure rises (Fig. 6). All samples prepared in O,-Ar ambients with 7.5 mTorr initial total gas pressure were ruthenium films. The RuO, films were obtained only when sputtering was performed with 10 or 15mTorr total gas pressure combined with the low 100 or 200 W forward sputtering power. A step-like increase in the growth rate is observed when the forward power rises and a transition from RuO, to ruthenium films takes place. The lower growth rate of RuO, films may be attributed to oxidation of the ruthenium target. Similar trends have been observed for reactive sputtering of TiN13 but not for W,_,N,r4 and Mo~_,O,~~, argon always being the carrier gas for the reactive sputtering. Figure 8 presents the intrinsic stress and resistivity of RuO, reactively sputtered in O,-Ne ambient at an initial total pressure of 10 mTorr and an initial partial oxygen pressure of 30% with a negative d.c. substrate bias ranging from 0 to - 120 V. The intrinsic stress of all films is compressive. The stress depends

REACTIVE SPUTTERING OF RuO, FILMS 223

700 t p(02Vp(02+Ar)=&O%

1600 .c d d 500

: d 400

E 3 300 s c) 200

100

1 OO

p(02+Ar),

o - 15 mTorr A - 10 mTorr 0 - 7.5 mTorr bias=0

200 300 400 POWER IWI

_l 500

Fig. 7. Growth rate of RuO, films deposited in O,-Ar gas as a function of forward power for three

different initial total gas pressures. The initial partial pressure of 0, is 40% and bias is zero.

6.0 -

‘ij 5.0 -

k 3 4.0 -

P z 3.0 - 2 I

_

g 2.0 - B _ 8

1.0 -

RuO, films

300 w p(02Vp(02+Nek30%

p(02+Nek 10 mTorr

- 190

- ,604

z z - 14or

_ Y Y

- 1200

- 100

L1. I * I. I. I Il. II O 0 20 40 60 60 100 120 BIAS [-VI

Fig. 8. Variation in stress and resistivity of RuO, films deposited in an O,-Ne ambient with negative

substrate bias at an initial total gas pressure of 1OmTorr and an initial partial oxygen pressure of 30%.

on the bias in a way that is again also characteristic of TiN8*i3. In the range from 0 to - 30 V the resistivity drops and the stress increases.

4. CONCLUSIONS

RuO, films can be prepared by reactive r.f. sputtering in an O,-Ne gas mixture over a wider range of sputtering conditions than by sputtering in O,-Ar. This property makes reactive sputter deposition in an O,-Ne ambient a process that is easy to control. Obvious similarities with the reactive sputtering characteristics of Pt-0 and Au-O systems sputtered in O,-Ne and O,--Ar suggest that the formation of RuO, is controlled by a reaction which takes place on the target. Reactive sputtering of TiN falls in the same general category, while that of W,_,N, and Mo,_,O, does not.

224 E. KOLAWA et al.

ACKNOWLEDGMENTS

The authors would like to express their appreciation to Dr. C. R. Aita for helpful discussions and insights. Technical assistance from Rob Gorris is gratefully acknowledged. We thank Intel Corporation for a grant. The main financial support for this work was provided by the Army Research Office under Contract DAAG29- 85-K-0192.

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