Abrasion resistance of various thin film coatings on thick film resistor materials

Surface and Coatings Technology, 49(1991) 451—456

451

Abrasion resistance of various thin film coatings on thick film resistor materials H.-J. Krokoszinski Corporate Research Laboratory, Asea Brown Boveri, Box 101332, 6900 Heidelberg (F.R.G.)

Abstract The lifetime of resistive heaters in thermal printing heads is mainly determined by the continuous abrasion due to the mechanical contact with the sliding paper. The thick film resistor materials used are not specifically developed for abrasion resistance; therefore protective coatings were applied to the resistor surface in order to enhance the long-term performance of the printing head. The optimum abrasion resistance was achieved with a diamond-like carbon coating but, in this paper, standard coatings are also investigated, since they are interesting from an economical point of view. On two different thick film resistor materials (Tanaka and DuPont), sputtered thin films of Al 201, SiO~and Si3N4 as well as coatings of Si02 (substrate temperatures 250 and 350 °C)obtained by plasma-enhanced chemical vapour deposition were produced (d = 2 j3m) and subjected to the polishing test described elsewhere. In comparison with The uncoated samples the abrasion of the resistors is decelerated by all coatings but to different extents. The criterion of minimum thickness decrease after 20 mm of polishing revealed the chemically vapourdeposited coatings of Si02 as the best in the present test.

1. Introduction

2. Experimental details

The outstanding abrasion resistance of diamond-like carbon coatings which has recently been applied to the surface protection of resistive thick film paste material in thermal printing heads [1] is not likely to be exceeded by any alternative thin film material. Nevertheless, the deposition process required is not very common or widely used. Hence, the cost of large-scale production might be a good reason to investigate the properties of some more materials to see whether their protection effect might be acceptable in the case of lower deposition costs. As known from the literature, both chemical vapour deposition (CVD) and physical vapour deposition (PVD) have been successfully applied to the production of wear-protective coatings [2, 3]. However, the commonly used techniques for quantitative measurement of abrasive wear such as the ball wear jar technique [4] or diamond stylus scratching [5] are not applicable in the present case of non-metallic and curved surfaces such as those of thick film resistors, Instead, as was described in a preview paper [1], the quantitative evaluation of the abrasion resistance in this case was satisfactorily achieved by simultaneously polishing the surfaces of three equally coated test resistors and measuring the thickness after definite periods of time. This leads to very distinct values when comparing bare resistors with resistors coated with thick film glass or diamond-like carbon. In this paper, we apply the same test method to the alternative coating materials deposited using CVD and PVD techniques.

Deposition processes which are applicable to the problem and, additionally, are available in most laboratories are evaporation (electron beam gun), r.f. sputtering and thermally activated CVD or, conveniently, ozone-assisted chemical vapour deposition (OACVD) or plasma-enhanced chemical vapour deposition (PECVD) for lower substrate temperatures. These techniques (except evaporation which is not supposed to provide better results than sputtering) were applied in order to produce well-adhering and abrasion-resistant coatings on resistor paste material. 2.1. Sample preparation Again, two different thick film pastes (DuPont, series 1 7G, labelled D3 and Tanaka, series GZK, labelled T3) were used to print the patterns to be covered with the various coatings for abrasion measurements. We compare three different materials (Al2 03, Si02 and Si3N4) deposited by r.f. magnetron sputtering and different kinds of CVD as shown in Table 1. The deposition parameters (substrate temperature, r.f. power and time) were adjusted so as to produce a layer thickness of 2 j.tm in all cases. As can be inferred from :Table I the deposition of Si3 N4 by thermal CYD failed for both resistor pastes since the substrate temperature nearly reached the burning temperature. This leads to a surface reaction between the coating and the paste as well as to an irreversible change in the resistance.

Elsevier Sequoia, Lausanne

452

H—f. Krokos:inski

/ Abrasion resistance of

the film coatings

TABLE 1. Production processes of the different samples Specimen designation

D3/l/l—3 D3/2/l—3 D3/l/4—6 D3/l/7—9 D3/6/l—9 D3/18/l—9 T3/17/7—9 T3/16/l—3 T3/16/4—6 T3/l6/7—9 T3/18/l—9 T3/6/l —9 T3/8/l—9 T3/9/1 —9

Coating

Al,0

3 Si3N4 SiO, Si Si3N4 3N4

Deposition technique

Substrate temperature (C)

R.f. sputtering R.f. sputtering R.f. sputtering Thermal CVD PECVD”

—

——

R.f. sputtering R.f. sputtering R.f. sputtering Thermal CVD PECVD~ OACVDd OACVDd

Floating Floating Floating 840 300 350 250

500 500 500

III 45 60

70

60 5 5

—

Al,03 Si3N4 SiO, Si3N4 Si3N4 SiO, SiO,

Power (W)

Time

—

—

—

Floating Floating Floating 840 300

500 500 500

III 45 60

—

70

— —

Remark

(mm)

60

OK OK Fail” OK5’ Fail Fail” OK OK OK OK Fail” OK OK OK

“No adhesion. ‘Too hot: “second burning” of resistor pastes. 3 min + NH 3 min ‘ + N, at 200 standard cm3 min I; gas pressure ~Reaction gas mixture, SiH470atW.20 standard cm 3 at 100 standard cm 0.4 mbar; plasma. 450 kHz. dReaction gas mixture. SiH 3 mm -‘ + 0~at 320 standard cm3 min (+ 10— 15/ 03) + N, at 500 standard cm3 min’: gas pressure. 5 mbar. 4 at 10 standard cm

Moreover, both Si 3N4 layers which were deposited at far lower substrate temperatures (sputtered and PEVCD) did not adhere appropriately on DuPont resistors: the sputtered layers were destroyed and removed by the polishing process within the first 10 mm whereas the CVD layers did not reside on the surface from the beginning. On the contrary, on the rougher Tanaka material the CVD of Si1N4 was successful. For the deposition of SiO2 from silane the novel OACVD was used which enables the substrate temperature to be reduced without the need for plasma power [6]. 2.2. Abrasion measurement The abrasion test apparatus and the geometry of the samples (nine square resistors with gold pads on an Al2 03 substrate) were sketched elsewhere [1]. Three samples of one lot were attached to a disc at 120°to each other; this sample holder rotated at 150 rev mm in an eccentric position face to face to the polishing plate of a commercial machine rotating at the same speed. Lubrication was defined by a constantly dripping liquid polishing agent (Gamma Micropolish, A1203 3B; 0.05 j.tm; Buehler). The thickness data acquired on the two paths [1] of the stylus in the Dektak profilorneter (Sloan) were reduced to the values on top of the marginal bulges (front and back bulges) and in the centre of the resistor. All these values decrease with increasing polishing time. The absolute thickness values drawn in the figures are

averaged over 12 values: two tracks, two tips of bulges on one resistor and three different samples simultaneously treated in one abrasion test, thus giving 2 x 2 x 3 = 12 values. Since the tolerances of printing and burning lead to a scatter in the initial thickness values of about ±I ~sm, the error bars depicted in the figures represent a large number of data rather than the accuracy of thickness measurement; this is known to be better than 1%. Of course, a systematic uncertainty arises from the lack of a constant zero line since the abrasion-resistant coating is deposited on the A1203 substrate material in the vicinity of the resistor material and will be removed from there, as well. However, as the polishing process is not likely to reach the foot point of the step (see Fig. 1), the error is considered negligible. The small steps at the edges of the drawings represent the profile of the gold pads.

—

3. Results and discussion All the samples in Table 1 which obtained the cornment OK were subjected to the abrasion test. In Fig. 1, examples for the thickness decrease with increasing polishing time are shown both for bare Tananka resistors (Fig. 1(a)) and for coated resistors (Fig. 1(b) for Si02 and Fig. 1(c) for Si3N4). The bare materials (samples D3/1 and T3/17) served as reference in order to determine the additional effect of the coating. In Fig. 2 the time dependences of the thickness decrease are

11.—f. Krokos:inski / Abrasion resistance of thin film coatings

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this kind of abrasion test (M(20 1) = 4.7 ~sm) than the DuPont paste (Ad(20 I) = 10.7 ~tm). The curves in Fig. 2 are included in the following figures as broken lines for optical reference. —

—

a~ Fig. I. Typical profiles of samples after polishing for I and 20 mm (au., arbitrary units): (a) uncoated Tanaka resistors; (b) Tanaka resistor coated with SiO, using OACVD; (c) Tanaka resistor coated with Si3N4 using r.f. sputtering,

3.1. Coatings produced by r.f. sputtering In Fig. 3, sputtered layers of ‘SiO2 and Al203 on DuPont resistor paste are compared. For both materials the properties appear to be different for the bulge and the centre region. SiO2 (curves a and b) does not provide a significant protection for the bulges; the abrasion within the first 10 mm is very similar to that of

presented for both uncoated resistor pastes. Here, the values for the front and back bulges are drawn separately. Since they do not exhibit any principal difference, we combined these data in all other drawings using common symbols. In order to quantify the abrasion resistance, we measure the thickness decrease from the point after 1 mm to that after 20 mm of polishing. Thus it is obvious that the Tanaka material is considerably more resistant to

unprotected resistors (curve e). However, when the bulges are nearly flat, the abrasion slope decreases significantly, indicating a certain protection due to the coating as long as flat surfaces are concerned. This can be inferred from the values after 1 mm and after 100 mm; for coated samples the Centre curve b drops by only 2.2 jim whereas the thickness of uncoated samples decreases by roughly 15 j.tm during the same period. In contrast with this, sputtered Al2 03 layers provide a small protection effect in the bulge region (curve C;

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Fig. 4. Averaged thickness of Tanaka resistors coated with sputtered oxides and Si3 N4 vs. process time in abrasion test.

I) = 6.0 jim compared with 10.5 jim for uncoated samples) but nearly no deceleration of the abrasion is observed when the bulges are flattened (t > 50 mm). Similar effects are observed on Tanaka resistors (Fig. 4); sputtered Si07 layers (curves a and b) exhibit poor protection quality on the bulges (i.~d(20 1) = 4.0 jim compared with 4.7 jim for bare resistors) whereas Al203 layers are superior in this region (curves c and d; i~d(20 1) = 3.0 jim) but for t > 50 mm the thickness decreases strongly to the curve of uncoated samples. Sputtered Si3N4 on Tanaka resistors (Fig. 4, curves e and f) do not improve the abrasion resistance of the thick film material significantly. For t <20 mm the thickness curves of the bulges (curve e) as well as of the centre (curve f) follow the respective reference lines (curves g and h) in an almost parallel manner. Slight deceleration is observed only in the region above 20 mm.

intended to use a thickness of 2 jim in both cases the cross-sectional scanning electron microscopy (SEM) micrographs reveal somewhat smaller thicknesses: for a substrate temperature of 350 “C, about 1.0 jim (sample T3/8/5 in Fig. 5(a)) and, for a substrate temperature of 250 “C, about 1.4 jim (sample T3/9/5 in Fig. 5(b)). This difference might be the reason for the slightly larger abrasion resistance of the 250 °Csample (Fig. 6, curves a and b) compared with the 350 “C sample (curves c and d). In the case of equal thicknesses the same properties of both SiO, layers can be anticipated as long as the adhesion to the substrate is comparable. Very similar results are obtained with the PECVD layers of Si3N4 (Fig. 6, curve e); for t <20 mm the thickness curve e follows that of the thin SiO2 curve c and for t > 20 mm a higher deceleration is achieved than by the oxide layers. In all cases the CVD coatings exhibit the smallest M(20 1) values as is demonstrated by the data in Table 2. Additionally, we listed the Ad( 100 1) values which represent (in most cases) the abrasion resistance of a coating on a large flat area rather than on the curved side wall of a bulge.

M(20

—

—

—

3.2. Coatings obtained by chemical vapour deposition OACVD of Si02 leads to properly adhering layers on the rough surface of Tanaka resistors. Although it was

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11.-f. Krokosinski / Abrasion resistance of thin film coatings

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SiO,, OACVD, substrate temperature of 350 ~C; curve e, Si3N4, PECVD.

Fig. 5. SEM micrograph of SiO, layers produced on Tanaka resistor material by OACVD at different substrate temperatures; (a) 350 T, average thickness of about 1.0 tIm; (b) 250 ~C average thickness of about l.4jim. (Magnifications. 10000 x .) TABLE 2. Decrease in bulge thicknesses of all samples

4. Summary The abrasion test data collected in Table 2 show that the smooth surface of DuPont resistor paste can only be coated with sputtered oxide layers. Since the paste material itself is more severely eroded by our polishing method (by a factor of more than 2) compared with the Tanaka material, the absolute deceleration effect of the oxide layers is also smaller (roughly to the same extent: a factor of 2). Coatings deposited on Tanaka material by CVD provide a larger protection effect than do sputtered layers of the same material. SiO2 appears to be the most promising candidate for curved surfaces whereas for flat surfaces the protection quality of Si3N4 is at least comparable. However, the properties of diamond-like

Resistor paste

Coating

Technique

DuPont DuPont DuPont

AI,03 SiO,

Tanaka Tanaka Tanaka Tanaka Tanaka

Al,03 SiO, Si3N4 Si3N4

—

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Tanaka Tanaka

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Tanaka

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3.3 2.2

14.0 10.5

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Sputtering Sputtering

-

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“Extrapolated from Fig. 5, curve e. ending at 60 “Substrate temperature, 350 ~C. cSubstrate temperature. 250 °C. dData from ref. I for comparison.

1.0”

mm.

456

H-f. Krokos:inski / Abrasion resistance of the film coatings

carbon cannot be reproduced by any of the alternative coating materials. Acknowledgments The author obtained the thick film samples from T. Strálman from ASEA Brown Boveri—HAFO in Sweden. He is grateful for the CVD depositions done by R. Weber and acknowledges the great care and pains taken by K. Tetzlaff with the polishing work and thickness measurements.

References I H.-J. Krokoszinski, Proc. mt. Conf on Diamond Films, CransMontana, September 17—19, 1990, in Surf Coat. Technol., 47 (1991) 761. 2 H. E. Hintermann, Wear, /00(1984) 381. 3 A. Matthews, Surf Eng., 1(1985) 93. 4 P. K. Mehrotra and D. T. Quinto, f. Vac. Sci. Technol. A, 3 2401.and D. S. Rickerby, Thin Solid Films, 18/ (1989), 5 (1985) S. J. Bull

~

6 R. Weber, 0. WahI and M. Baier, German Patent DE 4035951C1, Asea Brown Boverei Corporate Research Heidelberg.