preparation and characterization of r.f.-sputtered multilayered tinau films

Thin Solid Films, 200 (1991) 341-352

PREPARATION AND CHARACTERIZATION 341

PREPARATION AND CHARACTERIZATION OF R.F.-SPUTTERED

MULTILAYERED TiN-Au FILMS

P. K. BANERJEE, J. S. KIM AND S. S. MITRA

Thin Film Research Laboratory, Department oJ”Eectrica1 Engineering, University of Rhode Island,

Kingston, RI 02881 ( U.S.A.)

(Received September 20, 1990; accepted November 30, 1990)

TiN-Au films with multiple intermediate layers deposited by r.f. sputtering are suggested as a solution to the problem of adhesion of gold films to TIN coatings. Their optical and mechanical properties are investigated and compared with those of conventional electroplated gold films. Various analysis techniques have been used: Auger electron spectroscopy to determine the composition profiles, the direct tensile test for adhesion characteristics, optical rub test for scratch characteristics, salt spray test for corrosion characteristics and the selected ordinate method to identify the color of resultant films. It is shown that poor adhesion of gold films of TiN coatings can be overcome. Resultant multilayered TiN-Au films show the same color as that of commercially available electroplated gold films and their reflectivity reaches 98% of that of gold films over the wavelength range studied. For further applications, electrical properties of multilayered TiN-Au have also been studied. The sheet resistance of these TiN-Au films on n-Si and p-Si is in the range 0.5451/O-6.12 Q/o. Thus these multilayered structures may be used as durable contacts for microelectronic applications.

1. INTRODUCTION

TIN films have been well known to have several useful properties, such as wear resistance’-‘, low electrical resistivity3-‘, chemical stability’, hardness4T6v9, attrac- tive golden yellow color5-6 and high temperature stability2*4,7. Especially, chemical stability together with the golden yellow color makes TiN films promising as a partial replacement of gold films in many decorative applications. There is also demand for gold coatings on top of TiN coatings for electrical connectors, housings etc. However, despite these factors described above, hardly any intensive studies

have been reported on TiN-Au systems. This seems due to the poor adhesion characteristics of gold films to TIN films.

At present electroplating has been a principal technique for large-scale deposition of certain thin films including gold for electronic and decorative applications. Growing concern about the environment has resulted in pressure to reduce discharge of open toxic byproducts of electroplating. Dry process alterna-

0040-6090/9 1 jrS3.50 0 Elsevier Sequoia/Printed in The Netherlands

342 P. K. BANERJEE, J. S. KIM, S. S. MITRA

tives to electroplating (evaporation, sputtering, plasma deposition etc.) have long been known and have been used effectively in the semiconductor industry where relatively small amounts of products require thin film coatings.

The purpose of this study includes exploration of a particular physical vapor deposition technique, namely sputtering, as an alternative to electroplating. This research is focused on multilayered thin film systems, i.e. TiN-Au, which have properties superior to those which can be produced by electroplating. Therefore, in this paper, we have investigated a successful method of producing TiN-Au structures with multiple intermediate layers which are composed of gold and material X (where X = W, Ti, Cr, V, Ni, Cu, NIV, or brass), thus improving the adhesion of gold to TIN. Resulting films have shown superior optical, mechanical and electrical properties.

2. EXPERIMENTAL DETAILS

In order to make the TiN-X-Au structure, first the TIN films were reactively sputtered from a 175 mm diameter titanium target (99.95% pure) in an Ar +N, mixture at a power density of 2.2 W cm-2 with or without a negative substrate bias.

The target-to-substrate distance was kept fixed at 70mm. The depositions were carried out at a background pressure of 6.65 x 10 5 Pa (5.0 x 1 O- ’ Torr). The target was sputter etched at 200 W for 15 min just prior to deposition. The partial pressure of argon was kept fixed at 1.06 Pa (8 mTorr) and the partial pressure of N, was varied from 2.66 x 10m2 Pa (0.2mTorr) to 6.65 x 10m2 Pa (0.5 mTorr). The entire deposition was carried out at room temperature and the surface temperature of the substrates remained lower than 80 “C. The resultant TIN films were stoichiometric and golden yellow in most cases. Nitrogen-rich TIN films whose color was close to brown were also fabricated in order to see the effect of TIN composition on the properties of TiN-X-Au films. The color and properties of TIN films are very sensitive to sputtering parameters. Table I shows some of those properties for different sputtering parameters.

TABLE I PROPEKTIESOFR.F. REACTIVELYSPUTTERED TIN FILMSFORDIFFERENTSPUTTERING PARAMETERS

Ti 400 7.8:0.2 33 622

7.7:0.3 20 260

7.5:0.5 14 60

7.3:0.7 7.5 130

7.0x1.0 6.1 500

600 7.8:0.2 61 199

7.7:0.3 28 70

7.5:0.5 17 90

7.3:0.7 10.4 150

7.0: 1 .o 8.3 410

Yellow Bronze

Brown 0.15%0.90

Dark brown for wavelengths

Red&brown of 0.4 pm-l .5 pm

Light bronze

Brown

Dark brown

Red-brown Purple

a Total gas pressure of Ar + N, was 1.06Pa (8 mTorr).

R.F.-SPUTTERED MULTILAYERED TiN-Au 343

Once the TiN layer was fabricated, one of interlayer materials X and gold were alternately sputtered one to three times at different power levels for various amounts of time. The partial pressure of argon was kept fixed at 1.2 Pa (9 mTorr) during sputtering of both gold and material X. The diameters of the targets were 175 mm for both titanium and tungsten, 125 mm for brass and 50mm for chromium and vanadium. More than 300 different samples were prepared. For some samples, in order to test adhesion characteristics, gold layers of two different thicknesses were deposited on top of TiN-X-Au structures. In most cases, the thickness of each gold layer was about 1OOOA. The total thickness of TiN-X-Au structures varied from 0.5 urn to 1.0 urn depending on sputtering parameters as well as the number of interlayers. The thickness of the top gold layer for adhesion test was about 0.3 urn.

For the study of optical and mechanical properties, three different types of substrates were used, such as round steel coupons and stainless steel strips, as well as square brass and bronze substrates. Prior to deposition, they were ultrasonically cleaned in toluene, acetone and methanol. The substrates were also sputter etched just prior to deposition. For measurement of sheet resistance, (11 I)-oriented n-type and p-type silicon wafers were used as substrates.

3. RESULTSANDDISCUSSION

3.1. Structure

In order to determine the depth profile of resultant films, an Auger electron spectroscopy (AES) system (Perkin-Elmer PHI 660) at Perkin-Elmer Physical Electronics Laboratory (Edison, NJ) was used. The AES depth profiles for TiN-X-Au films sputtered on steel coupons are shown in Fig. 1. From the profiles in Fig. l(a), it is evident that the interlayer material tungsten and gold were alternately sputtered as expected in the TiN-W-Au film. However, in other structures (Figs. l(b) and l(c)), it is seen that gold and interlayer material X (titanium or brass) exist together. This seems due to the fact that titanium and brass films are so porous that gold could diffuse into titanium (or brass). The Auger profiles show carbon in the films as received. These carbon peaks are thought to be due to hydrocarbons

adsorbed from the air. This is confirmed from the fact that carbon peaks disappeared when a thin layer of the film (50 A) was sputter etched.

3.2. Swfucr topogruphl The surface topographies of TiN-W-Au films and TiN-brass-Au films were studied using a scanning electron microscope. Typical surface topographies are shown in Fig. 2. The TiN-brass-Au films suffer from the presence of macroparticles and are thus considered to be unsuitable for decorative applications”. For the case of TiN-W-Au film, the macroparticles are hardly recognizable. It is believed that the high melting point of tungsten results in very few macroparticles” while the relatively low melting point of brass is responsible for a significantly large macroparticle concentration. Dents in the deposited films originated from the irregularity of the steel substrate itself.

3.3. Adhesion

The adhesion characteristics of TiN-X-Au structures were measured qualita-

(b)

11

10

9

6

t 7

; i 6

i 4

3

2

1

0

(cl

344 P. K. RANERJEE, J. S. KIM, S. S. MITRA

Fig. 1. AES depth profiles of TiN_X-Au films: (a) TiN-W-Au; (b) TiN-Ti-Au; (c) TiN-brass-Au.

(a) (b) Fig. 2. Scanning electron micrographs of TiN-X-Au films: (a) TiN-W-Au films; (b) TiN-brass-Au film.

tively by tape test as well as quantitatively by direct tensile test using an Instron universal test instrument (Model 1011). The tape test was carried out in two different ways. Before the Instron test, samples from each group were given a type 1 tape test as follows. A 3M Scotch brand #810 tape was applied to a deposited film and pressed firmly to ensure good contact and removal of trapped air. The tape was then pulled off with a firm and quick ~~11’~. For films that passed this test, the direct tensile test with the Instron instrument was carried out with a pulling speed of 0.5mm mini ‘. Because of the maximum limit of the Instron test instrument (280.86 kgf cm 2), quantitative data above the limit are not available. To overcome this limit, another tape test (type 2) which is widely used in electroplating industries was carried out for the sample groups which passed the Instron test. Films deposited on 38 mm diameter steel coupons were used for this type 2 tape test. For this test, a cross mark was engraved with a knife in the center of the coupon and then the coupon was folded in half. Subsequently the type 1 test was performed on the cross-

marked area of the samples. Except for the plain TiN-Au film and the TiN-V-Au films, all other TiN-X-Au

films passed the type 1 tape test when gold and X were alternately sputtered three times each. The adhesion characteristics of TiN-X-Au structures as determined by the direct tensile test are shown in Table II. All films reported in this table were deposited on the steel substrates and gold and material X were sputtered alternately three times each unless otherwise specified.

As shown in Table II, the samples with intermediate layers have shown much better adhesion characteristics than plain TiN-Au films. For the same kinds of multilayered samples, those with triple interlayers of X-Au have shown better adhesion than samples with a single interlayer of X-Au. It is also seen that the substrate itself considerably affects the adhesion strength. Films on steel coupons exhibit better adhesion than those on brass substrates regardless of intermediate layer materials. Nitrogen-rich TiN films showed poor adhesion even in the type 1 tape test and therefore quantitative tests were not carried out on these samples.

Only samples with triple tungsten interlayers passed the type 2 tape test. It may seem possible to correlate the fact that tungsten and gold have almost the same density (19.3 gem- ‘) with the good adhesion characteristics of TiN-W-Au films.


TABLE II

THE ADHESIONCHARACTERISTICSOF TiN-X-Au STRUCTURES

Sample" Strength (kgf cm ‘) Srrmple”

Electroplated Au r 280.86 TiN-Ti-Au

TiN-brass-Au 2 280.86 TiN-Ti-Au”,’

TiN-brasssAub L 280.86 TiNPTiiAud

TiN-W-Au r 280.86 TiN-C-Au

TiN-W-Au’ 136.92 TiN-VAu

TiN-W-Aud 100.55 TiN-Au

a All TIN films were stoichiometric and golden yellow in color.

’ Interlayer X and gold were sputtered once each.

’ Films deposited on brass substrate. d Another gold layer of0.3 urn thickness was sputtered on top.

Srrengrh (kgf cm - 2,

2 280.86

123.09

2 280.86

2 280.86

105.32

57.58

This similarity of gold and tungsten certainly produces less porous multilayers, which improves the adhesion between gold and TIN films.

The effects of the thickness of the interlayers of tungsten as well as that of gold layers on the adhesion characteristics of TiN-W-Au films were studied. TiN-W-Au structures with triple tungsten layers, which have shown the strongest adhesion, are shown in Table III.

TABLE III

THEEFFECTOF THEINTERLAYERTHICKNESSONTHEAI~HESION~HAKATTERISTICSOF TiN-W-Au FILMS

T’hicknrs.ve.s of’interluyvs (A)

w luwr Au /atw

1000 1000

1000 500 1000 250

500 500

250 500

Strength (kgf cm ‘)

2 280.86

2 280.86

r 280.86

L 280.86

2 280.86

From Table III, it is seen that the thickness of interlayers has very little effect on the adhesion of TiN-W-Au structures as long as tungsten and gold layers are alternately sputtered three times each. Thus it is possible to fabricate TiN-W-Au films with good adhesion properties and with total gold layers thinner than 2000 A. With only less than one-half the amount of gold used to make conventional electroplated gold films, it is possible to obtain TiN-W-Au films with the same color and very similar optical properties.

3.4. Scratch test The optical rub test which is widely used in electroplating industries was

adopted to obtain scratch resistance characteristics of TiN-W-Au films as well as pure TIN films. Both pure TIN films and TiN-W-Au films successfully passed the test. It is noteworthy that TiN-W-Au films showed scratch resistance properties similar to those of pure TIN films.

R.F.-SPUTTERED MULTILAYERED TiN--Au 347

3.5. Corrosion (salt spray test)

In order to test corrosion characteristics of TiN-W-Au films, the salt spray test was carried out for 2 weeks. Some samples with a relatively thin TIN layer (less than 0.5um) failed the salt spray test and substrates started rusting. Poor salt spray corrosion resistance seems to have its origin in the fact that the TiN films on these samples were not thick enough to compensate for the porosity of the TIN film itself. Further, in our sputtering geometry the edge of the substrates could not be thoroughly covered with TIN films. This may also be another cause for failure in salt spray tests of some samples.

3.6. Opticalproperties

Optical reflectivity measurements were performed using a Cary 17 spectropho- tometer. The spectral range considered was 300 nm I i. I 800 nm. Typical reflectivity curves of TiN-X-Au structures as well as commercially available electroplated pure gold films are shown in Fig. 3. The thickness of the electroplated pure gold film was about 0.5 urn. From Fig. 3, it is evident that all TiN-X-Au structures show similar reflectivity curves, regardless of the interlayer material used. Among them, the TiN-brass-Au structure shows the closest optical characteristics to those of electroplated gold films.

0:;

x .p

‘::

:: c P

0.8 - o : 24k Electroplated gold *:Tii-Brass-Au

0.7 +:TiN-W-Au - x:TiN-Z-Au

0.6 -

0.5 -

0.4 -

0.3 -

0.2 -

0. I t

%o I 300 400 500 600

wavelength

Fig. 3. Reflectivity curves ofTiN-X-Au structures.

700 800 / 9c K)

The effect of interlayer thickness (which is a function of applied r.f. power) on the reflectivity for the TiN-brass-Au films is shown in Fig. 4. It is seen that even a slightly thicker brass layer decreases reflectivity remarkably while the thickness of the gold layer does not have much effect. The effect of the number of interlayers on the reflectivity for TiN-W-Au structures is shown in Fig. 5. Samples with double tungsten layers show much better reflectivity compared with those with triple


1

0.9 -

0.8 - o : thinner brass layer * 0.7 : thicker Au - layer + : standard

A 0.6 - .z z 2 0.5 - Cz e!

0.4 -

0.3 -

0.2 -

0.1 -

%O 300 4& 5&I 6& 7;)o 8&I 9iXI

wavelength

Fig. 4. The effect of interlayer thickness on the reflectivity of TiN-brass-Au films.

0.9 -

0.8 -

0.7 -

0.6 -

0.5 -

0.4 -

0.3 -

0.2 -

0.1 I_

o : double W layers l : triple thicker W layers + : standard triple W layers

L scxl I

300 400 500 600 700 a00 900

wavelen&

Fig. 5. The effect of the tungsten layers on the reflectivity of TiN--W-Au films.

tungsten layers. In contrast to TiN-brass-Au films, however, the effect of the thicker tungsten layer on reflectivity characteristics is hardly recognizable in the case of TiN-W-Au structures. In Fig.6, the effect of interlayer thickness is shown for TiN-Ti-Au films. A thicker titanium layer decreases reflectivity and the magnitude of the reflectivity variation is between those of the TiN-brass-Au and the TiN-W-Au structures.


0.8 -

0.7 - o : standard * : dtickerTi layer

5 0.6 -

‘j: ‘Cl 8 05- .

“r 0.4-

0.3 -

0.2 -

0.1t

1 200 300 400 500 600 700 800 900

wavelcngth(nm)

Fig. 6. The effect of the interlayer thickness on the reflectivity of TiN-Ti-Au films.

The colors of the TiN-X-Au structures were identified from reflectivity curves using the selected ordinate method 13,14 The spectral reflectivities were taken at a . series of selected wavelengths for the source used. These reflectivities were used to calculate the chromaticity coordinates sr, s2, and s3, as follows.

s a = 770 nm

s, = E,S1 ipi di. a = 380 nm

s i = 770 nm

s, = E&p, di. i = 380 nm

s i = 770nm

s, = E,S3Lp, di i = 380 “In

where S1, S,, and S3 are the tristimulus values of the color, E,: is the energy of wavelength 2, S,, S,, S3 are respectively the red, green, and blue tristimulus functions, and pi is the reflection factor at wavelength 1. (the transmission factor z2 at wavelength 2 is substituted for pA when one deals with transmitting media). Then

s1 = S,/(S,+&+S,) s2 = S,/@, + s2 + S,)

s3 = S,/(S, +s2 +S,)

Hence

s,+s,+s, = 1


The color of the film was determined using s1 and s2 coordinates from the Comite International d’Eclairage (CIE) chromaticity diagram13,14. Regardless of interlayer materials, the color of the TiN-X-Au structure was between yellowish orange and orange. Since the color of the electroplated pure gold films was identified as yellowish orange by the CIE diagram, most of the TiN-X-Au films showed a characteristic golden yellow color.

For applications in the IR wavelength region (e.g. IR drying chambers for automotive and other industries), absolute IR reflectivities of the TiN-W-Au films were measured and compared with those of both the pure TIN film and the electroplated pure gold film.

From Fig. 7, it is seen that the IR reflectivity of the TiN-W-Au film reaches about 98”” of that of the electroplated pure film over the wavelength studied. Therefore, such TiN-X-Au films may find a use as durable IR reflector coatings with less usage of gold.

o : 24k Electroplated gold *:TW-W-Auftim

+ : Plain TIN film

0.5 1 1.S 2 2.5

wavelength

Fig. 7. Typical absolute IR reflectivity of a TiN-W-Au film.

3.7. Electriculproperties The electrical properties of TiN-Au films were investigated. The effects of

interlayer materials and the number of interlayers were studied by measuring the sheet resistance of the TiN-X-Au structures. For this purpose, TiN-X-Au structures were fabricated on n-type as well as p-type silicon wafers whose resistivities were in the range 1.00 cm-3.0R cm and the sheet resistance was measured by the four-point probe method.

The sheet resistance value of TiN films alone was in the range 25 0/[7-45 0/o for both n-type and p-type silicon wafers.

As shown in Table IV, the sheet resistance of TiN-X-Au films showed a strong dependency on the number of intermediate layers as well as the interlayer material


TABLE IV

THESHEETRESISTANCEOF TiN-X-Au STRUCTURES

Sample number

Slructure R, (Cl/O) on the,following

n-Si p-Si

I TiN-brass-Au 3 1.53 0.54

3a 1.80 1.08

1 3.24 1.80

2 TiN-W-Au 3 2.79 1.53

I 3.15 2.43

3 TiN-Ti-Au 3 1.80 1.0

1 6.12 I .89

a Gold sputtered at a lower power.

itself and its value was in the range 0.5412/o-6.12Q/n. The effect of interlayer thickness on the electrical characteristics of multilayer TiN-X-Au films was reported elsewhere15.

4. CONCLUSIONS

TiN-X-Au structures prepared by r.f. sputtering presented the following characteristics. First of all, it is seen that poor adhesion of gold film to TiN film can be solved by multilayered TiN-X-Au structures with proper choice of an intermediate lay& material X. Especially, tungsten improves the adhesion between the gold layer and TIN layer remarkably for practical applications of these coatings.

The following results are to be noted. (a) The thickness of an interlayer has less effect on the adhesion properties of

TiN-X-Au films than either the number of interlayers or the interlayer material

itself. (b) Resultant multilayered structure films as well as pure TiN films passed

scratch tests. (c) The resultant films with a thin TIN layer (0.5 pm) show poor salt spray

corrosion resistance. (d) IR reflectivity of TiN-X-Au films is comparable with that of electroplated

gold films over the wavelength range studied. (e) The sheet resistance of TiN-X-Au films is strongly dependent on the number

of intermediate layers.

ACKNOWLEDGMENTS

This work has been supported by Rhode Island Center for Thin Film and Interface Research and National Science Foundation grants. We would like to express our appreciation to the following industries: Tanury Industries of Lincoln, RI, for supplying steel and nickel-coated steel substrates and performing salt spray tests; Perkin-Elmer Physical Electronics Laboratory at Edison, NJ, for performing


AES analysis; EPNER Technology of Brooklyn, NY, for performing scratch tests and absolute reflectivity measurements.

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preparation and characterization of r.f.-sputtered multilayered tinau films

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