- Email: [email protected]
NIST co-operative project
Thin Solid Films 332 (1998) 164±171
Mechanical properties of SiO2 and Si3N4 coatings: a BAM/NIST co-operative project U. Beck a,*, D.T. Smith b, G. Reiners a, S.J. Dapkunas b a
Federal Institute for Materials Research and Testing, Sub-Department VIII.2, Unter den Eichen 87,12205Berlin, Germany b National Institute of Standards and Technology, Ceramics Division, Gaithersburg, 20899, MD, USA
Abstract Mechanical properties, i.e. hardness and elastic modulus, of amorphous SiO2 and Si3N4 PE-CVD coatings have been studied for two coating thicknesses (0.1 and 1.0 mm) and two substrate materials (fused silica, i.e. Herasil, and borosilicate glass, i.e., BK7) using low load instrumented indentation. The coating systems are being considered for possible use as reference materials for thin ®lm mechanical property test methods. Single layers of SiO2 and Si3N4 and a multilayer stack consisting of ®ve double layers of SiO2/Si3N4 (individual layer thickness: 0.1 mm) were investigated on both substrate materials. A special plasma pre-treatment of the substrates prior to deposition ensured that coating adhesion exceeded inner ®lm stress for all systems considered. The applied indentation load ranged from 700 down to 0.1 mN and resulted in indentation depths from more than 1 mm to about 15 nm. The in¯uence of coating thickness and the effect of the substrate on the measurement of hardness and elastic modulus of the ®lms are discussed, with emphasis on the effects of indentation depth vs. coating thickness on the observed coating properties. One coating±substrate system (1.0 mm Si3N4 on Herasil) out of ten was found to be above a critical threshold for tensile cracking of the coating. In this system, termed `overcritical', tensile cracks occurred both prior to, and as a result of, indentation, indicating that Berkovich indentation may also be used to probe residual ®lm stress. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Indentation; Hardness; Modulus; Silica; Silicon nitride; Coatings; Thin ®lms
1. Introduction Reference materials are widely used for calibration of testing equipment. Nowadays, many industrial products are coated or surface-treated. Hence, measurement and testing techniques for quality control of coated and surface modi®ed materials are key issues. Unfortunately, there is a serious lack of reference materials, in particular reference coatings, for mechanical testing of thin ®lms and coatings. This is why the Federal Institute for Materials Research and Testing (BAM) and the National Institute of Standards and Technology (NIST) have begun a 3-year co-operative project to evaluate reference coating systems that might ®ll this gap. The development of such coating±substrate reference systems requires the consideration of substrate (material and surface properties), coating (material properties and process parameters, thickness) and system features (degree of mis®t of substrate and layer properties, ®lm adhesion, residual ®lm stress). Non-destructive ®ngerprints, i.e. those that allow the evaluation and certi®cation of coating± * Corresponding author. Tel.: 149-30-81043560; Fax: 149-3081041827;; e-mail: [email protected].
substrate systems, and destructive ®ngerprints, i.e. those that are closer to the demands on a coating±substrate system under service, are required. There are mechanical quantities that are de®ned for both bulk and coating materials, e.g. hardness, H, and elastic modulus, E. However, ®lm properties are dif®cult to measure, and can appear to be substrateand thickness-dependent. Low load indentation techniques allow the measurement of these coating quantities independent of substrate in¯uence, provided that the indentation depth does not exceed some fraction of the coating thickness, where that fraction depends on the mechanical properties of both the coating and the substrate. Moreover, reference systems should have well-de®ned substrate and coating properties, smooth and defect-free interfaces and surfaces, good coating adhesion, low residual ®lm stress and known coating thickness. In this work, amorphous SiO2 and Si3N4 coatings in two thicknesses and on two different substrates are evaluated as possible reference coating systems. For each of the systems, optical functions measured by spectroscopic ellipsometry (SE) are used as an optical ®ngerprint and load-displacement curves measured by instrumented indentation are used as a mechanical ®ngerprint.
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-609 0(98)00989-4
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
165
Table 1 Substrates and coatings: optical and mechanical properties
Herasil BK7 PECVD SiO2 PECVD Si3N4
SiO2 content
n (633 nm)
k (633 nm)
H (GPa)
E (GPa)
99.9999 % 70.0% 99.0% 1.0 %
1.458^0.001 1.515^0.001 1.455^0.002 1.905^0.002
,0.001 ,0.001 ,0.001 ,0.005
9.1^0.4 8.2^0.2 7.9^0.5 14.5^0.5
73.1^1.8 93.8^1.5 75.2^4.0 .145
2. Experimental 2.1. Selection of substrate materials Production of reference coatings requires well-de®ned substrate materials. Chemical and physical stability, both for bulk and surface properties over long periods of time, i.e. at least several years, are required. Moreover, surface roughness should be typically less than 1% of coating thickness, however not greater than 1 nm. Surface properties have to be homogenous laterally; amorphous substrate materials are advantageous for this reason. Because of the outstanding quality of their mechanical and optical properties of interest, fused silica (Herasil, Heraeus GmbH) and borosilicate glass (BK7, Schott AG) have been selected as substrate materials (see Table 1). Originally, both materials were developed for optical applications, but they are also widely used in mechanical testing. In particular, fused silica is used as reference material for instrumented indentation (Nano Instruments, Inc.) and BK7 as a reference material for recording hardness measurements (Fischer GmbH). A Round Robin run [1] on BK7 has proven the excellent reproducibility of its mechanical properties. 2.2. Selection of coating materials In addition to the criteria described above for bulk materials, coated systems also require good coating adhesion and low coating stress, i.e. low mismatch between the mechanical properties of the coating and substrate. As a result of these considerations, SiO2 and Si3N4 were selected as coating materials. They were deposited by plasma-enhanced chemical vapor deposition (PECVD) in two different thicknesses (0.1 and 1.0 mm). In addition, a multilayer stack consisting of ®ve double layers of 0.1 mm SiO2 and 0.1 mm Si3N4 (1.0 mm total thickness) was prepared. Process parameters of the deposition process have been presented elsewhere [2]. In order to ensure good coating adhesion on the very smooth substrates, a special plasma pre-treatment prior to deposition and a low-rate PECVD process were
used. Coating±substrate systems with varying degrees of mismatch of mechanical properties of coatings and substrates in terms of hardness, modulus and residual ®lm stress, all with good adhesion, were developed and are given in Table 2. The ranking within one coating material is due to the fact that larger ®lm thickness usually results in higher ®lm stress. 2.3. Non-destructive testing of coatings: SE and GIXRD Non-destructive testing (NDT) is important for the determination of overall coating and substrate properties and to check the reproducibility of the deposition process prior to mechanical testing. Grazing incidence X-ray diffraction (GIXRD) was used to verify that (i) substrate materials are amorphous and (ii) both 0.1 and 1.0 mm thick layers of SiO2 and Si3N4 are also amorphous. SE was applied to derive optical functions of layer materials, i.e. refractive index n and extinction coef®cient k, as a NDT ®ngerprint of the deposition process in the visible wavelength range 300±800 nm. The reproducibility of the deposition process resulted in optical functions deviating from specimen to specimen by only ^0.002 in refractive index for each ®lm material. The extinction coef®cient was below 0.001 and 0.01 for SiO2 and Si3N4, respectively, over the entire wavelength range. Non-destructive thickness determination of both single layers and multilayers with high accuracy (1% of layer thickness or 1 nm) was also possible using SE. 2.4. Destructive testing of ®lms: instrumented indentation Instrumented indentation [3] is a key technique for the determination of mechanical properties of coatings, primarily hardness, H, and elastic modulus, E. There are also efforts for non-destructive determination of the elastic modulus of ®lms and coatings [4]. However, by de®nition, there is no non-destructive technique for the determination of mechanical quantities such as hardness, adhesion and wear resistance of thin ®lms and coatings. Instrumented indentation measurements were performed using a Nano
Table 2 Ranking of mismatch of coating±substrate systems: 1±lowest (no) mismatch, 11±highest mismatch
Herasil BK7
Bare
0.1 mm SiO2
1.0 mm SiO2
Multilayer
0.1 mm Si3N4
1.0 mm Si3N4
1 1
2 3
4 5
6 7
9 8
11 10
166
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
Fig. 1. Crack generation in substrate material due to indentation (700 mN): left Herasil, right: BK7.
Fig. 2. Crack generation in ®lm material due to indentation (90±700 mN): 1 mm Si3N4 on Herasil.
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
167
Fig. 3. Load-displacement curves (low-load range). (a) bare Herasil, (b) 1 mm Si3N4 on Herasil. Curves from six separate indentations are overlaid in each ®gure.
Fig. 4. Load-displacement curves comparing the indentation response of bare Herasil to that of a 1 mm thick Si3N4 coating on Herasil substrate. (a) high-load curves, (b) low-load curves.
Indenter II from Nano Instruments, Inc., equipped with a diamond Berkovich indenter tip (3-sided pyramid). An automatic indentation mode was programmed to place indentations in a 6 £ 6 array at peak loads of 700, 300, 90, 15, 3 and 0.3 mN. Hardness H and elastic modulus E were derived from the unloading segments of the resulting load-displacement curves, following the analysis procedure outlined by Oliver and Pharr that permits the separation of elastic and plastic deformations [3]. Corrections were made to the data for the compliance of the indentation machine, thermal drift of the machine during each indentation procedure, and the deviation of the indenter tip from perfect Berkovich geometry. Partial unloadings at two intermediate loads during each indentation permitted the calculation of H and E for at a total of 18 different loads (and hence depths) on each specimen studied, with each indentation procedure repeated at six different locations. The selected load range resulted in indentation depths from more than 1 mm at a peak load of 700 mN down to less than 15 nm at 0.1 mN. The ability to measure mechanical properties at such shallow depths makes instrumented indentation the technique of choice for the determination of ®lm mechanical properties,
although its application to thin coatings is not without dif®culty (see [5] and references therein). A wide range of peak loads was used in this work to study the coating±substrate system response for indentation depths both smaller and larger than coating thicknesses.
3. Results 3.1. Crack generation For peak loads of 700 mN, the bare substrate materials show different behaviors with respect to crack generation (Fig. 1). Cracks in Herasil occur around the Berkovich indents in an axial or lateral fashion, reaching from one Berkovich corner to the other. For BK7, radial cracks are generated at the corners of indents. The spacing of the indentations, normally 50±75 mm, was increased in some cases to 200 mm to insure that there was no radial crack interaction between neighboring indentations. Cracking within the coatings was found only for one coating± substrate system, i.e. 1 mm Si3N4 on Herasil (Fig. 2). This
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U. Beck et al. / Thin Solid Films 332 (1998) 164±171
approximately 10 s. The partial unloading from a peak load of 0.1 mN is clearly visible in the plots. The earlier partial unloading from a peak load of 0.01±0.02 mN is also present, but the noise at those loads is large; data from peak loads below 0.05 mN are not reported in this work. The scatter from place to place on the coated material is somewhat higher than for bare Herasil, but is quite acceptable given the very shallow indentation depth. Fig. 4 shows the effect that the presence of the 1.0 mm Si3N4 coating has on the load-displacement curves at two different peak loads. At higher load (300 mN, Fig. 4a), the difference in response between the coated system and the bare substrate appears minor. At 3 mN (Fig. 4b), however, it is clear that the indentation depth is much less for the coated system, implying that the hardness is higher. These differences will be quanti®ed in the next section. Even without a quantitative analysis, load-displacement curves might be used as mechanical ®ngerprints of the coating±substrate system. Separate measurements on bare substrate materials should always be made for comparison to the coated system. 3.3. Hardness and elastic modulus The hardness H and elastic modulus E were calculated from power law ®ts to all but the last 10% of the unloading portions of the load-displacement curves for the bare
Fig. 5. (a) Modulus and (b) hardness of the substrate materials as a function of the depth over which the tip and specimen are in contact.
system contained existing tensile cracks (termed `primary cracks' here) prior to indentation, with additional cracks appearing as a result of indentation. In Fig. 2, primary cracks are outside the ®eld of view; all cracks shown resulted from the indentations. Even for peak loads of 90 mN (bottom row), cracks in the ®lm were generated by indentation, if no primary crack or crack generated by a previous indentation passed too closely. Even in this system, however, ®lm adhesion was strong enough to resist the forces arising from the residual stress; no ®lm delamination was observed on this or any other specimen. 3.2. Load-displacement curves Fig. 3a,b each show six overlaid load-displacement curves made at the lowest peak load (0.3 mN) at six different locations on bare Herasil (a) and 1 mm Si3N4 on Herasil (b). The indentation sequence, typical of those at higher loads as well, is as follows: (1) initial contact and surface detection at the origin; (2) loading to 0.02 mN; (3) partial unloading; (4) loading to 0.1 mN; (5) partial unloading; (6) loading to 0.3 mN, (7) 10 s hold; (8) unloading to 0.03 mN); (9) 100 s hold for thermal drift measurement; and (10) ®nal unloading to zero load. Loading and unloading rates varied with peak load such that each loading or unloading segment took
Fig. 6. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick SiO2 coating on each substrate material.
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
169
Average values for H and E of each material are given in Table 1 for depths in the range 50±100 nm, with the cited error being one standard deviation in the observed values. The increase in hardness at high loads for each material is not understood, although the fact that the observed modulus is independent of depth makes an error in calculated contact area an unlikely explanation. For the coated systems, effective modulus and hardness are calculated, again over a range of contact depths, by applying the same analysis used for the bulk substrate materials, i.e. by assuming that the indentations are being made in a homogeneous material rather than a layered system. The result of this analysis is that at contact depths greater than the coating thickness, values of H and E are obtained that approach the bulk values for the substrate material, while at very small contact depths, the effective hardness and modulus approach or reach the correct values for the coating material. At intermediate depths, a transition is observed. Figs. 6±8 show these trends for the three 1.0 mm thick coatings on each substrate material. For the SiO2 coating system (Fig. 6), it is observed that the effective modulus at small contact depths is, within error, equal to the modulus of the Herasil substrate (73 vs. 75 GPa), although its effective hardness is somewhat lower (7.5 vs. 9.1 GPa), indicating that the SiO2 coating is quite similar
Fig. 7. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick Si3N4 coating on each substrate material.
substrate materials and for each of the coated systems, using the method of Oliver and Pharr [3]. (Except at the very lowest loads, the correlation coef®cients of the ®ts were better than 0.9999). Hardness is de®ned as H Pmax =Ac , where Pmax is a maximum load and Ac is the projected contact area between the tip and the specimen at Pmax. Elastic modulus is determined from the stiffness of the contact (i.e. the slope of the unloading curve at Pmax) and the contact area. The modulus determined by this technique, sometimes referred to as the indentation modulus, Ei, is Ei E= 1 2 n2 , where E is Young's modulus and n is Poisson's ratio. In this work, n is taken to be 0.17 for all specimens studied, and the modulus reported is Young's modulus. Although this value for n is exactly correct only for Herasil [3], the correction for other values of n typical of amorphous ceramics (0:15 , n , 0:25) is minor. Each specimen was studied using 18 values of Pmax, ranging from 0.01 to 700 mN, as described above, resulting in values for H and E at 18 different depths. Depths reported are contact depths; they are the depths over which the indenter tip and specimen are in contact at a given Pmax. Fig. 5a,b show the results for the Young's modulus and hardness respectively for the two substrate materials over a wide range of contact depths. It can be seen that the scatter in the data increases signi®cantly for depths less than 50 nm.
Fig. 8. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick multilayer coating on each substrate material.
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U. Beck et al. / Thin Solid Films 332 (1998) 164±171
In order to highlight the differences between the mechanical response of the coated and uncoated substrates, it is useful to plot the effective modulus and hardness, not as absolute values, but as deviations from the values observed for the uncoated substrates. In Figs. 9 and 10, the effective hardness and modulus, respectively, of the 1.0 mm coating systems are shown normalized by the H and E of their substrates at each depth independently; i.e. the values of H and E from Figs. 6±8 are divided by the data in Fig. 5. This presentation makes clear several of the points made above: the similarity of the SiO2 coating properties to those of Herasil, the high values of H and E for the Si3N4, the intermediate result for the multilayer system, and the asymptotic approach of all effective H and E values to those of the substrate materials for large contact depths.
4. Discussion and conclusions It is believed that after further optimization the selected layer and substrate materials will be appropriate for reference applications in mechanical testing. The approach used in this work, that is, to combine two substrate materials with
Fig. 9. Normalized effective hardness of the three 1.0 mm thick coatings on (a) Herasil and (b) BK7.
mechanically to bulk SiO2. The 1.0 mm Si3N4 coating system (Fig. 7) has both higher hardness and higher modulus at small contact depths than either substrate material. It is interesting to note here that for contact depths less than 300 nm, the measured hardness for the coated system is approximately independent of load, at 14.5 GPa. It is reasonable to assume that this value represents the true hardness of the Si3N4 coating. The observed modulus, however, does not level off at small depths, even below 100 nm, indicating that the modulus measurement is more sensitive to substrate effects than is the hardness measurement. The value of 145 GPa for the modulus of the Si3N4 coating reported in the Table 1 is the value observed at the shallowest depths; the actual value may be higher. In Fig. 8 it is shown that the modulus and hardness for the 1.0 mm SiO2/ Si3N4 multilayer are intermediate in value between SiO2 and Si3N4 coatings, as expected. Neither H nor E reach a constant value at small indentation depths in this system, however, but that is not unexpected as the top layer is 0.1 mm thick Si3N4 compared to a minimum indentation depth of 15 nm, which is more than 10 % of layer thickness.
Fig. 10. Normalized effective modulus of the three 1.0 mm thick coatings on (a) Herasil and (b) BK7.
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
two coating materials and two different thicknesses for a total of eight coating±substrate systems, along with an additional check of the experimental results by means of a multilayer system using common coating materials and layer thickness, was judged to be very useful. Moreover, a low hardness and modulus material (SiO2) was contrasted with an high hardness and modulus material (Si3N4), see also [6]. Coating±substrate systems with degrees of mechanical mismatch from almost none (0.1 mm SiO2 on Herasil) to very high (1 mm Si3N4 on Herasil) were investigated. The latter system is an interesting threshold system for study of residual coating stress. Lowering the thickness or changing the substrate material results in a completely crack-free coating even after indentation. In a system like this, nanoindentation techniques might also be used as stress sensing technique. One rule of thin ®lm technology has been proven again, i.e. that the mechanical properties of a coating can approach the bulk values for the same material, given similar microstructures, as has been shown for hardness and modulus of deposited SiO2 and bulk Herasil. Fingerprints, both optical and mechanical, may become of greater interest in the near future as they include information on possible method- and material-related errors in processing or deposition. Given the high sensitivity and wide range of measurement inherent in both the optical functions and load-displacement curves, it is unlikely that even slight variations in coating properties will escape detection, and highly unlikely that any combination of systematic or random errors could lead to a self-compensation effect that could mask a problem in coating deposition. It must be noted that the Si3N4 substrate±coating systems investigated showed no clear low-load plateau in modulus that could be clearly identi®ed as the correct modulus for the coating, even for the 1.0 mm thick coating at indentation depths of 50 nm. Scatter, on the other hand, increased significantly at depths less than 50 nm, making it impossible to
171
determine conclusively the Si3N4 modulus on the basis of the indentation data alone. The reported data combined with a model of the elastic response of a layered system, could possibly determine the Si3N4 modulus with some con®dence. This analysis is the subject of future work.
Acknowledgements This work was supported by Ceramics Division of NIST and Sub-Department Surface Technology of BAM as a starting point for co-operation of both institutes in the ®eld of reference coatings within a BAM-NIST project. Certain trade names and products of companies are identi®ed in this paper to adequately specify the materials and equipment used in this research. In no case does such identi®cation imply that the products are necessarily the best available for the purpose or that they are recommended by BAM or NIST. We would like to gratefully acknowledge the work of Mrs M. MaÈnn, deposition and SE measurement, BAM, and Dr M. Vaudin, GIXRD analysis, NIST.
References [1] Chr. Ullner, G.D. Quinn, VAMAS TWA 3 Round Robin Report, 1997, in press. [2] U. Beck, G. Reiners, Th. Wirth, V. Hoffmann, F. PraÈûler, Thin Solid Films 290±291 (1996) 57. [3] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 [6] (1992) 1564. [4] D. Schneider, Th. Schwarz, B. Schultrich, Thin Solid Films 219 (1992) 92. [5] T. Page, S. Hainsworth, in: D.T. Smith (Ed.), NIST Special Publication 896, Conference Proceedings: International Workshop on Instrumented Indentation, 1996, p. 21. [6] S.R.J. Saunders, D.T. Smith, T. Yoshida, VAMAS Bulletin No. 21, Dec. 1997, pp. 11±19..
Mechanical properties of SiO2 and Si3N4 coatings: a BAM/NIST co-operative project U. Beck a,*, D.T. Smith b, G. Reiners a, S.J. Dapkunas b a
Federal Institute for Materials Research and Testing, Sub-Department VIII.2, Unter den Eichen 87,12205Berlin, Germany b National Institute of Standards and Technology, Ceramics Division, Gaithersburg, 20899, MD, USA
Abstract Mechanical properties, i.e. hardness and elastic modulus, of amorphous SiO2 and Si3N4 PE-CVD coatings have been studied for two coating thicknesses (0.1 and 1.0 mm) and two substrate materials (fused silica, i.e. Herasil, and borosilicate glass, i.e., BK7) using low load instrumented indentation. The coating systems are being considered for possible use as reference materials for thin ®lm mechanical property test methods. Single layers of SiO2 and Si3N4 and a multilayer stack consisting of ®ve double layers of SiO2/Si3N4 (individual layer thickness: 0.1 mm) were investigated on both substrate materials. A special plasma pre-treatment of the substrates prior to deposition ensured that coating adhesion exceeded inner ®lm stress for all systems considered. The applied indentation load ranged from 700 down to 0.1 mN and resulted in indentation depths from more than 1 mm to about 15 nm. The in¯uence of coating thickness and the effect of the substrate on the measurement of hardness and elastic modulus of the ®lms are discussed, with emphasis on the effects of indentation depth vs. coating thickness on the observed coating properties. One coating±substrate system (1.0 mm Si3N4 on Herasil) out of ten was found to be above a critical threshold for tensile cracking of the coating. In this system, termed `overcritical', tensile cracks occurred both prior to, and as a result of, indentation, indicating that Berkovich indentation may also be used to probe residual ®lm stress. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Indentation; Hardness; Modulus; Silica; Silicon nitride; Coatings; Thin ®lms
1. Introduction Reference materials are widely used for calibration of testing equipment. Nowadays, many industrial products are coated or surface-treated. Hence, measurement and testing techniques for quality control of coated and surface modi®ed materials are key issues. Unfortunately, there is a serious lack of reference materials, in particular reference coatings, for mechanical testing of thin ®lms and coatings. This is why the Federal Institute for Materials Research and Testing (BAM) and the National Institute of Standards and Technology (NIST) have begun a 3-year co-operative project to evaluate reference coating systems that might ®ll this gap. The development of such coating±substrate reference systems requires the consideration of substrate (material and surface properties), coating (material properties and process parameters, thickness) and system features (degree of mis®t of substrate and layer properties, ®lm adhesion, residual ®lm stress). Non-destructive ®ngerprints, i.e. those that allow the evaluation and certi®cation of coating± * Corresponding author. Tel.: 149-30-81043560; Fax: 149-3081041827;; e-mail: [email protected].
substrate systems, and destructive ®ngerprints, i.e. those that are closer to the demands on a coating±substrate system under service, are required. There are mechanical quantities that are de®ned for both bulk and coating materials, e.g. hardness, H, and elastic modulus, E. However, ®lm properties are dif®cult to measure, and can appear to be substrateand thickness-dependent. Low load indentation techniques allow the measurement of these coating quantities independent of substrate in¯uence, provided that the indentation depth does not exceed some fraction of the coating thickness, where that fraction depends on the mechanical properties of both the coating and the substrate. Moreover, reference systems should have well-de®ned substrate and coating properties, smooth and defect-free interfaces and surfaces, good coating adhesion, low residual ®lm stress and known coating thickness. In this work, amorphous SiO2 and Si3N4 coatings in two thicknesses and on two different substrates are evaluated as possible reference coating systems. For each of the systems, optical functions measured by spectroscopic ellipsometry (SE) are used as an optical ®ngerprint and load-displacement curves measured by instrumented indentation are used as a mechanical ®ngerprint.
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-609 0(98)00989-4
U. Beck et al. / Thin Solid Films 332 (1998) 164±171
165
Table 1 Substrates and coatings: optical and mechanical properties
Herasil BK7 PECVD SiO2 PECVD Si3N4
SiO2 content
n (633 nm)
k (633 nm)
H (GPa)
E (GPa)
99.9999 % 70.0% 99.0% 1.0 %
1.458^0.001 1.515^0.001 1.455^0.002 1.905^0.002
,0.001 ,0.001 ,0.001 ,0.005
9.1^0.4 8.2^0.2 7.9^0.5 14.5^0.5
73.1^1.8 93.8^1.5 75.2^4.0 .145
2. Experimental 2.1. Selection of substrate materials Production of reference coatings requires well-de®ned substrate materials. Chemical and physical stability, both for bulk and surface properties over long periods of time, i.e. at least several years, are required. Moreover, surface roughness should be typically less than 1% of coating thickness, however not greater than 1 nm. Surface properties have to be homogenous laterally; amorphous substrate materials are advantageous for this reason. Because of the outstanding quality of their mechanical and optical properties of interest, fused silica (Herasil, Heraeus GmbH) and borosilicate glass (BK7, Schott AG) have been selected as substrate materials (see Table 1). Originally, both materials were developed for optical applications, but they are also widely used in mechanical testing. In particular, fused silica is used as reference material for instrumented indentation (Nano Instruments, Inc.) and BK7 as a reference material for recording hardness measurements (Fischer GmbH). A Round Robin run [1] on BK7 has proven the excellent reproducibility of its mechanical properties. 2.2. Selection of coating materials In addition to the criteria described above for bulk materials, coated systems also require good coating adhesion and low coating stress, i.e. low mismatch between the mechanical properties of the coating and substrate. As a result of these considerations, SiO2 and Si3N4 were selected as coating materials. They were deposited by plasma-enhanced chemical vapor deposition (PECVD) in two different thicknesses (0.1 and 1.0 mm). In addition, a multilayer stack consisting of ®ve double layers of 0.1 mm SiO2 and 0.1 mm Si3N4 (1.0 mm total thickness) was prepared. Process parameters of the deposition process have been presented elsewhere [2]. In order to ensure good coating adhesion on the very smooth substrates, a special plasma pre-treatment prior to deposition and a low-rate PECVD process were
used. Coating±substrate systems with varying degrees of mismatch of mechanical properties of coatings and substrates in terms of hardness, modulus and residual ®lm stress, all with good adhesion, were developed and are given in Table 2. The ranking within one coating material is due to the fact that larger ®lm thickness usually results in higher ®lm stress. 2.3. Non-destructive testing of coatings: SE and GIXRD Non-destructive testing (NDT) is important for the determination of overall coating and substrate properties and to check the reproducibility of the deposition process prior to mechanical testing. Grazing incidence X-ray diffraction (GIXRD) was used to verify that (i) substrate materials are amorphous and (ii) both 0.1 and 1.0 mm thick layers of SiO2 and Si3N4 are also amorphous. SE was applied to derive optical functions of layer materials, i.e. refractive index n and extinction coef®cient k, as a NDT ®ngerprint of the deposition process in the visible wavelength range 300±800 nm. The reproducibility of the deposition process resulted in optical functions deviating from specimen to specimen by only ^0.002 in refractive index for each ®lm material. The extinction coef®cient was below 0.001 and 0.01 for SiO2 and Si3N4, respectively, over the entire wavelength range. Non-destructive thickness determination of both single layers and multilayers with high accuracy (1% of layer thickness or 1 nm) was also possible using SE. 2.4. Destructive testing of ®lms: instrumented indentation Instrumented indentation [3] is a key technique for the determination of mechanical properties of coatings, primarily hardness, H, and elastic modulus, E. There are also efforts for non-destructive determination of the elastic modulus of ®lms and coatings [4]. However, by de®nition, there is no non-destructive technique for the determination of mechanical quantities such as hardness, adhesion and wear resistance of thin ®lms and coatings. Instrumented indentation measurements were performed using a Nano
Table 2 Ranking of mismatch of coating±substrate systems: 1±lowest (no) mismatch, 11±highest mismatch
Herasil BK7
Bare
0.1 mm SiO2
1.0 mm SiO2
Multilayer
0.1 mm Si3N4
1.0 mm Si3N4
1 1
2 3
4 5
6 7
9 8
11 10
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Fig. 1. Crack generation in substrate material due to indentation (700 mN): left Herasil, right: BK7.
Fig. 2. Crack generation in ®lm material due to indentation (90±700 mN): 1 mm Si3N4 on Herasil.
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Fig. 3. Load-displacement curves (low-load range). (a) bare Herasil, (b) 1 mm Si3N4 on Herasil. Curves from six separate indentations are overlaid in each ®gure.
Fig. 4. Load-displacement curves comparing the indentation response of bare Herasil to that of a 1 mm thick Si3N4 coating on Herasil substrate. (a) high-load curves, (b) low-load curves.
Indenter II from Nano Instruments, Inc., equipped with a diamond Berkovich indenter tip (3-sided pyramid). An automatic indentation mode was programmed to place indentations in a 6 £ 6 array at peak loads of 700, 300, 90, 15, 3 and 0.3 mN. Hardness H and elastic modulus E were derived from the unloading segments of the resulting load-displacement curves, following the analysis procedure outlined by Oliver and Pharr that permits the separation of elastic and plastic deformations [3]. Corrections were made to the data for the compliance of the indentation machine, thermal drift of the machine during each indentation procedure, and the deviation of the indenter tip from perfect Berkovich geometry. Partial unloadings at two intermediate loads during each indentation permitted the calculation of H and E for at a total of 18 different loads (and hence depths) on each specimen studied, with each indentation procedure repeated at six different locations. The selected load range resulted in indentation depths from more than 1 mm at a peak load of 700 mN down to less than 15 nm at 0.1 mN. The ability to measure mechanical properties at such shallow depths makes instrumented indentation the technique of choice for the determination of ®lm mechanical properties,
although its application to thin coatings is not without dif®culty (see [5] and references therein). A wide range of peak loads was used in this work to study the coating±substrate system response for indentation depths both smaller and larger than coating thicknesses.
3. Results 3.1. Crack generation For peak loads of 700 mN, the bare substrate materials show different behaviors with respect to crack generation (Fig. 1). Cracks in Herasil occur around the Berkovich indents in an axial or lateral fashion, reaching from one Berkovich corner to the other. For BK7, radial cracks are generated at the corners of indents. The spacing of the indentations, normally 50±75 mm, was increased in some cases to 200 mm to insure that there was no radial crack interaction between neighboring indentations. Cracking within the coatings was found only for one coating± substrate system, i.e. 1 mm Si3N4 on Herasil (Fig. 2). This
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approximately 10 s. The partial unloading from a peak load of 0.1 mN is clearly visible in the plots. The earlier partial unloading from a peak load of 0.01±0.02 mN is also present, but the noise at those loads is large; data from peak loads below 0.05 mN are not reported in this work. The scatter from place to place on the coated material is somewhat higher than for bare Herasil, but is quite acceptable given the very shallow indentation depth. Fig. 4 shows the effect that the presence of the 1.0 mm Si3N4 coating has on the load-displacement curves at two different peak loads. At higher load (300 mN, Fig. 4a), the difference in response between the coated system and the bare substrate appears minor. At 3 mN (Fig. 4b), however, it is clear that the indentation depth is much less for the coated system, implying that the hardness is higher. These differences will be quanti®ed in the next section. Even without a quantitative analysis, load-displacement curves might be used as mechanical ®ngerprints of the coating±substrate system. Separate measurements on bare substrate materials should always be made for comparison to the coated system. 3.3. Hardness and elastic modulus The hardness H and elastic modulus E were calculated from power law ®ts to all but the last 10% of the unloading portions of the load-displacement curves for the bare
Fig. 5. (a) Modulus and (b) hardness of the substrate materials as a function of the depth over which the tip and specimen are in contact.
system contained existing tensile cracks (termed `primary cracks' here) prior to indentation, with additional cracks appearing as a result of indentation. In Fig. 2, primary cracks are outside the ®eld of view; all cracks shown resulted from the indentations. Even for peak loads of 90 mN (bottom row), cracks in the ®lm were generated by indentation, if no primary crack or crack generated by a previous indentation passed too closely. Even in this system, however, ®lm adhesion was strong enough to resist the forces arising from the residual stress; no ®lm delamination was observed on this or any other specimen. 3.2. Load-displacement curves Fig. 3a,b each show six overlaid load-displacement curves made at the lowest peak load (0.3 mN) at six different locations on bare Herasil (a) and 1 mm Si3N4 on Herasil (b). The indentation sequence, typical of those at higher loads as well, is as follows: (1) initial contact and surface detection at the origin; (2) loading to 0.02 mN; (3) partial unloading; (4) loading to 0.1 mN; (5) partial unloading; (6) loading to 0.3 mN, (7) 10 s hold; (8) unloading to 0.03 mN); (9) 100 s hold for thermal drift measurement; and (10) ®nal unloading to zero load. Loading and unloading rates varied with peak load such that each loading or unloading segment took
Fig. 6. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick SiO2 coating on each substrate material.
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Average values for H and E of each material are given in Table 1 for depths in the range 50±100 nm, with the cited error being one standard deviation in the observed values. The increase in hardness at high loads for each material is not understood, although the fact that the observed modulus is independent of depth makes an error in calculated contact area an unlikely explanation. For the coated systems, effective modulus and hardness are calculated, again over a range of contact depths, by applying the same analysis used for the bulk substrate materials, i.e. by assuming that the indentations are being made in a homogeneous material rather than a layered system. The result of this analysis is that at contact depths greater than the coating thickness, values of H and E are obtained that approach the bulk values for the substrate material, while at very small contact depths, the effective hardness and modulus approach or reach the correct values for the coating material. At intermediate depths, a transition is observed. Figs. 6±8 show these trends for the three 1.0 mm thick coatings on each substrate material. For the SiO2 coating system (Fig. 6), it is observed that the effective modulus at small contact depths is, within error, equal to the modulus of the Herasil substrate (73 vs. 75 GPa), although its effective hardness is somewhat lower (7.5 vs. 9.1 GPa), indicating that the SiO2 coating is quite similar
Fig. 7. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick Si3N4 coating on each substrate material.
substrate materials and for each of the coated systems, using the method of Oliver and Pharr [3]. (Except at the very lowest loads, the correlation coef®cients of the ®ts were better than 0.9999). Hardness is de®ned as H Pmax =Ac , where Pmax is a maximum load and Ac is the projected contact area between the tip and the specimen at Pmax. Elastic modulus is determined from the stiffness of the contact (i.e. the slope of the unloading curve at Pmax) and the contact area. The modulus determined by this technique, sometimes referred to as the indentation modulus, Ei, is Ei E= 1 2 n2 , where E is Young's modulus and n is Poisson's ratio. In this work, n is taken to be 0.17 for all specimens studied, and the modulus reported is Young's modulus. Although this value for n is exactly correct only for Herasil [3], the correction for other values of n typical of amorphous ceramics (0:15 , n , 0:25) is minor. Each specimen was studied using 18 values of Pmax, ranging from 0.01 to 700 mN, as described above, resulting in values for H and E at 18 different depths. Depths reported are contact depths; they are the depths over which the indenter tip and specimen are in contact at a given Pmax. Fig. 5a,b show the results for the Young's modulus and hardness respectively for the two substrate materials over a wide range of contact depths. It can be seen that the scatter in the data increases signi®cantly for depths less than 50 nm.
Fig. 8. Effective (a) modulus and (b) hardness of composite systems consisting of a 1.0 mm thick multilayer coating on each substrate material.
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In order to highlight the differences between the mechanical response of the coated and uncoated substrates, it is useful to plot the effective modulus and hardness, not as absolute values, but as deviations from the values observed for the uncoated substrates. In Figs. 9 and 10, the effective hardness and modulus, respectively, of the 1.0 mm coating systems are shown normalized by the H and E of their substrates at each depth independently; i.e. the values of H and E from Figs. 6±8 are divided by the data in Fig. 5. This presentation makes clear several of the points made above: the similarity of the SiO2 coating properties to those of Herasil, the high values of H and E for the Si3N4, the intermediate result for the multilayer system, and the asymptotic approach of all effective H and E values to those of the substrate materials for large contact depths.
4. Discussion and conclusions It is believed that after further optimization the selected layer and substrate materials will be appropriate for reference applications in mechanical testing. The approach used in this work, that is, to combine two substrate materials with
Fig. 9. Normalized effective hardness of the three 1.0 mm thick coatings on (a) Herasil and (b) BK7.
mechanically to bulk SiO2. The 1.0 mm Si3N4 coating system (Fig. 7) has both higher hardness and higher modulus at small contact depths than either substrate material. It is interesting to note here that for contact depths less than 300 nm, the measured hardness for the coated system is approximately independent of load, at 14.5 GPa. It is reasonable to assume that this value represents the true hardness of the Si3N4 coating. The observed modulus, however, does not level off at small depths, even below 100 nm, indicating that the modulus measurement is more sensitive to substrate effects than is the hardness measurement. The value of 145 GPa for the modulus of the Si3N4 coating reported in the Table 1 is the value observed at the shallowest depths; the actual value may be higher. In Fig. 8 it is shown that the modulus and hardness for the 1.0 mm SiO2/ Si3N4 multilayer are intermediate in value between SiO2 and Si3N4 coatings, as expected. Neither H nor E reach a constant value at small indentation depths in this system, however, but that is not unexpected as the top layer is 0.1 mm thick Si3N4 compared to a minimum indentation depth of 15 nm, which is more than 10 % of layer thickness.
Fig. 10. Normalized effective modulus of the three 1.0 mm thick coatings on (a) Herasil and (b) BK7.
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two coating materials and two different thicknesses for a total of eight coating±substrate systems, along with an additional check of the experimental results by means of a multilayer system using common coating materials and layer thickness, was judged to be very useful. Moreover, a low hardness and modulus material (SiO2) was contrasted with an high hardness and modulus material (Si3N4), see also [6]. Coating±substrate systems with degrees of mechanical mismatch from almost none (0.1 mm SiO2 on Herasil) to very high (1 mm Si3N4 on Herasil) were investigated. The latter system is an interesting threshold system for study of residual coating stress. Lowering the thickness or changing the substrate material results in a completely crack-free coating even after indentation. In a system like this, nanoindentation techniques might also be used as stress sensing technique. One rule of thin ®lm technology has been proven again, i.e. that the mechanical properties of a coating can approach the bulk values for the same material, given similar microstructures, as has been shown for hardness and modulus of deposited SiO2 and bulk Herasil. Fingerprints, both optical and mechanical, may become of greater interest in the near future as they include information on possible method- and material-related errors in processing or deposition. Given the high sensitivity and wide range of measurement inherent in both the optical functions and load-displacement curves, it is unlikely that even slight variations in coating properties will escape detection, and highly unlikely that any combination of systematic or random errors could lead to a self-compensation effect that could mask a problem in coating deposition. It must be noted that the Si3N4 substrate±coating systems investigated showed no clear low-load plateau in modulus that could be clearly identi®ed as the correct modulus for the coating, even for the 1.0 mm thick coating at indentation depths of 50 nm. Scatter, on the other hand, increased significantly at depths less than 50 nm, making it impossible to
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determine conclusively the Si3N4 modulus on the basis of the indentation data alone. The reported data combined with a model of the elastic response of a layered system, could possibly determine the Si3N4 modulus with some con®dence. This analysis is the subject of future work.
Acknowledgements This work was supported by Ceramics Division of NIST and Sub-Department Surface Technology of BAM as a starting point for co-operation of both institutes in the ®eld of reference coatings within a BAM-NIST project. Certain trade names and products of companies are identi®ed in this paper to adequately specify the materials and equipment used in this research. In no case does such identi®cation imply that the products are necessarily the best available for the purpose or that they are recommended by BAM or NIST. We would like to gratefully acknowledge the work of Mrs M. MaÈnn, deposition and SE measurement, BAM, and Dr M. Vaudin, GIXRD analysis, NIST.
References [1] Chr. Ullner, G.D. Quinn, VAMAS TWA 3 Round Robin Report, 1997, in press. [2] U. Beck, G. Reiners, Th. Wirth, V. Hoffmann, F. PraÈûler, Thin Solid Films 290±291 (1996) 57. [3] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 [6] (1992) 1564. [4] D. Schneider, Th. Schwarz, B. Schultrich, Thin Solid Films 219 (1992) 92. [5] T. Page, S. Hainsworth, in: D.T. Smith (Ed.), NIST Special Publication 896, Conference Proceedings: International Workshop on Instrumented Indentation, 1996, p. 21. [6] S.R.J. Saunders, D.T. Smith, T. Yoshida, VAMAS Bulletin No. 21, Dec. 1997, pp. 11±19..