Effect of Residual Stress on Mechanical Properties and. Interface Adhesion Strength of SiN Thin Films

Transcription

1 Effect of Residual Stress on Mechanical Properties and Interface Adhesion Strength of SiN Thin Films Shou-Yi Chang 1, *, Yi-Chung Huang 1, and Chih-Hsiang Chang 2 1 Department of Materials Science Engineering, National Chung Hsing University, Taichung 402, Taiwan 2 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan Abstract Residual stresses play a significant role in the mechanical reliability of thin films. Thus in this study, the mechanical properties and interface adhesion strengths of SiN thin films containing different residual stresses have been investigated by nanoindentation and nanoscratch tests. With varied residual stresses from compressive to tensile, the penetration depth of nanoindentation test shifted to a higher value, and the variations of indentation depth followed different tendencies. The hardness and elastic modulus decreased from 11.0 and 95 GPa, respectively, for the film containing a compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. During nanoscratch tests, interface delamination occurred and the interface adhesion strengths were accordingly measured. With decreasing compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m 2. Compressive stresses were expected to blunt crack tips and inhibit crack propagation, while tensile stresses enlarged crack opening and facilitated crack propagation. Keywords: residual stress; mechanical property; interface adhesion * Author to whom correspondence should be addressed; shouyi@dragon.nchu.edu.tw 1

2 I. Introduction Thin films have been widely applied to microelectronic, optoelectronic, anti-wear and anti-corrosion coating, and many other industries. However, the mechanical damages of thin films, such as film cracking and interface delamination, severely suppress their processing yield and application reliability [1]. A high resistance to mechanical damages is thus strongly demanded, and it is essentially important to clarify the mechanical properties of thin films before practical applications. During processing and application of thin films, one of the main factors leading to mechanical damages is residual stresses which are basically caused by intrinsic interface coherency, thermal cycling, and/or ion bombardment, etc. Residual stresses play a significant role in the mechanical performance and reliability of thin films. The existence of large residual stresses generally results in film buckling or cracking, and even interface delamination like a typical telephone-cord peeling [1-3]. Therefore, the evaluation of residual stress effect on thin-film mechanical properties is especially important. Nanoindentation and nanoscratch tests have been widely used for the measurement of thin-film mechanical properties [4, 5]. In addition to hardness and elastic modulus, they are also promising to determine interface adhesion strengths through film delamination [6-14]. These two tests are essentially favorable for evaluating the mechanical reliability of thin films because only very small indented areas at a size of a few micrometers will be damaged during tests. Moreover, since the mechanical properties of thin films are reported to be influenced by different types of residual stresses [15-19], to clarify the effect extent of residual stresses on the experimental data measured by nanoindentation and nanoscratch tests, and alternately even to estimate the types and degrees of residual stresses through data variations are very useful for the reliability evaluation of thin films and need to be further performed. The SiN based thin films have been widely used in microelectronics and microsystems. 2

3 Their mechanical performance under the effect of residual stresses is believed to significantly influence the processing yield and application reliability. Thus in this study, SiN thin films have been prepared on glass substrates by magnetron sputtering. Their mechanical properties and interface adhesion strengths have been investigated by nanoindentation and nanoscratch tests. Different residual stresses are introduced to the SiN films to investigate their effect on the mechanical properties and interface adhesion strength of the films. 3

4 II. Experimental Procedures The SiN films of 500 nm thick were deposited on glass substrates by plasma-enhanced chemical vapor deposition. Different types and degrees of residual stresses, including tensile 86 and 45 MPa, and compressive 37, 135 and 235 MPa, were introduced to the deposited films by varying plasma power. Substrate curvatures before and after film deposition were measured to calculate the exact residual stresses by the Stoney equation [20]. From X-ray diffraction analyses (MAC Science MXP3), the crystal structures of all the SiN films were identified as amorphous. Auger electron spectrometry (Microlab 350) was applied to analyze the chemical composition of the deposited SiN films, and these films containing five different residual stresses were all found to be composed of silicon and nitrogen with an atomic ratio of about 2/3. Atomic force microscopy (DI 3100) was applied to characterize the surface roughness of the films. Smooth surfaces of the SiN films with a very small roughness below 2 nm were measured and believed not to affect the measurements of mechanical properties. The mechanical properties of SiN films were measured by a UMIS nanoindenter (Based Model, CSIRO) with a Berkovich diamond indenter (tip radius ~ 100 nm, edge angle ). During each test, the load was applied to a maximum value of 1 mn, hold for 10 seconds, and then released under a loading/unloading rate of mn/sec. The indentation depth was controlled at about 1/10 of film thickness to avoid a substrate effect. At least ten tests were performed for each sample to obtain higher data accuracy. The nanoindenter with a scratch test module was applied to measure the interface adhesion strength between the SiN films and glass substrates. Under the scratch tests, the load was ramped from 0 to 80 mn in 30 seconds, and the scratch length was set as 1500 μm at a moving velocity of 50 μm/sec. A scanning electron microscope (SEM, JEOL JSM-6700F) was used to observe the surface morphologies around indent marks and scratch tracks after nanoindentation and nanoscratch tests. 4

5 III. Results and Discussion 3.1 Effect of residual stresses on mechanical properties Figure 1 shows the typical load-penetration depth curves of nanoindentation tests of SiN films containing different residual stresses. By the Oliver-Pharr relation [4], the hardness and elastic modulus of the SiN films were obtained as plotted in Figure 2. It was found that the mechanical properties obviously varied with residual stresses, especially the tensile ones. With decreasing compressive stress and increasing tensile stress, the hardness and modulus decreased from 11.0 and 95 GPa, respectively, for the film with a compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. During nanoindentation tests, normal compressive stresses and radial shear stresses are mainly applied to the films. It is known that basically the normal compressive stresses are difficult to result in material failures, but however the radial shear stresses are much easier to [21]. As seen in the indent marks under a high applied load of 200 mn shown in Figure 3, film cracking induced by the radial shear stresses occurred at the sidewalls of the indents. For the brittle SiN films, fracture began as they yielded, and their mechanical performance was dominated by subsequent crack propagation. Residual compressive stresses in the films would retard the work of applied shear stresses and blunt crack tips, thus inhibiting crack propagation and lowering fracture rate. Therefore, a smaller indent mark and fewer cracks were observed as seen in Figure 3 (a) as well as that the SiN film containing the largest compressive stress of 235 MPa exhibited the best measured mechanical properties. In contrary, residual tensile stresses would enhance the effect of applied shear stresses and enlarge crack opening, and thus the film containing the largest tensile stress of 86 MPa showed the lowest mechanical properties [17]. From the viewpoint of atomic bonding force to interatomic spacing [21], the difference in the mechanical properties of SiN films containing different residual stresses can be realized 5

6 as well. It is known that atoms locate at a balanced interatomic separation r 0. Bonds will not break and cracks/defects not initiate until the interatomic separation extends from r 0 to r max under an external force over F max. Although the residual stresses in thin films will not affect the relation between the force and interatomic spacing, however they change the interatomic separation and meanwhile the force required to break bonds. Under residual compressive stresses, the interatomic spacing is compressed, and a larger applied force is needed to break bonds; in contrary under residual tensile stresses, the spacing is extended, and a smaller force is needed. Thus as aforementioned, the mechanical properties of the SiN films containing residual compressive stresses were measured better than those containing tensile stresses. 3.2 Effect of residual stresses on indentation depth Moreover, it was also found from Figure 1 that the load-penetration depth curves of nanoindentation tests of SiN films varied with residual stresses. With decreasing compressive stress and increasing tensile stress, the maximum indentation depth shifted to a higher value under the same applied load of 1 mn. As plotted in Figure 4, it was clearly observed that the penetration depth was lowered by residual compressive stresses since compressive stresses blunted crack tips and inhibited crack propagation. However, it was interestingly found that the variations of indentation depth for the films containing different types of residual stresses followed different tendencies. The ability to inhibit penetration proceeding by compressive stresses was much lower than that to enhance penetration by residual tensile stresses. By fitting data points, the penetration depth h for the films containing residual compressive or tensile stresses σ was found to follow equation (1) or (2), respectively, as the dashed blue or green lines plotted in Figure 4. hc = σ (1) c ht = σ (2) t 6

7 where the subscripts c and t represent the compressive and tensile conditions, respectively. The predicted penetration depth for the film without residual stress, h 0, was calculated as about nm by extrapolation. A proportional relation between residual stress and penetration depth has been reported, by performing nanoindentation tests of samples containing different residual stresses and measuring depth shifts, as the following equation (3) [19]. 3Er σ cal = ( h0 hc ) (3) ω πa where σ cal is the predicted residual stress, E r the elastic modulus of the samples, and A the contact area. The symbols ω is an indenter tip geometry factor (0.72 for Berkovich tip). By employing the equation (3) to this study, the calculated residual stresses were obtained and plotted in Figure 4 as well. It was found that the calculated residual compressive stresses (fitted with the solid blue line) were rather close to the experimentally measured data (red line) and followed the relation as equation (4). σ c, cal = σ c,exp (4) However, the calculated residual tensile stresses (solid green line) exhibited a much different trend from the experimental ones as equation (5). σ t, cal = σ t,exp (5) That is, the predicted residual tensile stresses (calculated data) required to enlarge penetration depth were much higher than the practical ones (experimental data). Thus, modifications are needed for the above equation (3) approximately as the following equations (6) and (7) in the residual compressive and tensile conditions, respectively. 3Er σ c = B c ( h0 hc ) (6) 1.6ω πa 3Er σ t = B t ( h0 ht ) (7) 24.6ω πa 7

8 where B is a constant. The different trends of calculated residual compressive and tensile stresses were most probably attributed to the different abilities to inhibit or facilitate film deformation. From the relation between atomic bond force and interatomic spacing, it is realized that the changes of interatomic separation under different types of external forces are different and non-linear. That is, the elastic constants of bonds under compressive and tensile forces are different [21]. Especially under higher tensile forces, the elastic constant of bonds more obviously decreases, and subsequently a more significant change in the interatomic spacing is expected. Therefore in this study, under the same altitude but different types of residual stresses, the applied stress further required for film deformation in the residual tensile condition was smaller than that in the compressive condition. Thus, the shift in penetration depth caused by the residual tensile stresses was more obvious than that by the compressive stresses. 3.3 Effect of residual stresses on interface adhesion strength Figure 5 shows the load-penetration depth curves of nanoscratch tests of SiN films with different residual stresses. At the applied loads of mn (penetration depth ~ nm), drastic curve fluctuations were identified. By comparing the surface morphologies of the films along scratch tracks as shown in Figure 6, the fracture behaviors at different scratch stages were realized. Firstly at the early stage of scratch tests, the SiN films were just pressed, and only a slight scratch trace was observed as shown at the right side of Figure 6 (b). Then when the load was applied to mn, the interfaces between the SiN films and substrates began to delaminate where film buckling and peeling occurred as observed at the left side of Figure 6 (b), corresponding to the drastic curve fluctuations. During the scratch tests, shear stresses accumulated at the interfaces when the tip indented and scratched. Under sufficient shear stresses higher than interface adhesion strengths, the interfaces delaminated, and then 8

9 the accumulated stresses were released. As the indenter tip moved forward, the delaminated and buckled films under the tip cracked as shown in Figures 6 (c) and (d). The repeated film delamination, buckling, and cracking thus resulted in the fluctuations of the curves. Figure 7 (a) shows the critical loads P c and scratch track widths d c as the interfaces delaminated which were determined by the beginning points of curve variations. By using the following equation (8), the critical stresses σ c for interface delamination (adhesion strength) between SiN films and glass substrates were obtained as plotted in Figure 7 (b) [9-14]. 2P (4 + ν f )3πμ c σ = c (1 2ν ) 2 f (8) π d c 8 in which μ is the measured friction coefficient of indenter sliding given by the nanoscratch tester as about Afterwards by using the following equation (9), with the thickness t and elastic modulus E f of the SiN films, the fracture energy release rates G c for the interface delamination (adhesion energy) between the films and substrates was then obtained as plotted in Figure 7 (b) as well [9-14]. G c 2 σ c t = (9) 2E f It was found that the variations of the critical stresses for interface delamination and the interface adhesion energy were similar to those of the hardness and elastic modulus of SiN films. With decreasing residual compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m 2. As aforementioned, the residual compressive stresses were expected to blunt crack tips and inhibit crack propagation; whereas the residual tensile stresses would magnify the effect of applied shear stresses during the scratch tests, then enlarging crack opening and facilitating crack propagation. Therefore, the decrease in residual compressive stresses and the increase in tensile stresses would accelerate the failure of interfaces and lower the interface adhesion energy as experimentally measured. 9

10 IV. Conclusions In this study, the mechanical properties and interface adhesion energy of SiN thin films under the effect of residual stresses were investigated by nanoindentation and nanoscratch tests. The hardness and modulus decreased from 11.0 and 95 GPa, respectively, for the film containing a residual compressive stress of 235 MPa to 9.6 and 84 GPa for the film with a tensile stress of 86 MPa. Residual compressive stresses were expected to blunt crack tips and inhibit crack propagation, while tensile stresses enlarged crack opening and facilitate crack propagation, leading to the changes of mechanical properties. Moreover, the penetration depth of nanoindentation tests was found to shift to higher values with decreasing residual compressive stress and increasing tensile stress. The calculated residual compressive stresses were close to practical values, but however the calculated tensile stresses exhibited a much different trend from experimental data. During nanoscratch tests, interfaces delaminated with film buckling and peeling under the accumulation of sufficient shear stresses higher than interface adhesion strengths. With decreasing residual compressive stress and increasing tensile stress, the interface adhesion energy decreased from 1.8 to 1.5 J/m 2 since the tensile stresses accelerated the failure of interfaces. 10

11 Acknowledgements The authors gratefully acknowledge the financial supports for this research by the National Science Council, Taiwan, under Grant No. NSC E , and in part by the Ministry of Education, Taiwan, under the ATU plan. 11

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14 Figure Captions Figure 1 Typical load-penetration depth curves of nanoindentation tests of SiN films with different residual stresses. Figure 2 Hardness and elastic modulus of SiN films with different residual stresses measured by nanoindentation tests. Figure 3 SEM surface morphologies around indent marks on SiN films with different residual stresses of (a) compressive 235 and (b) tensile 86 MPa after nanoindentation tests at a maximum applied load of 200 mn. Figure 4 Penetration depths of nanoindentation tests and calculated residual stresses of SiN films with different residual stresses. Figure 5 Typical load-penetration depth curves of nanoscratch tests of SiN films with different residual stresses. Figure 6 SEM surface morphologies along scratch tracks on SiN films with a residual compressive stress of 235 MPa after nanoscratch tests at a maximum applied load of 80 mn; (a) low-magnification view of scratch track; (b) beginning, (c) middle, and (d) late stages of film cracking and interface delamination. Figure 7 (a) Critical loads and scratch track widths and (b) critical stresses and adhesion energy for interface delamination of SiN films with different residual stresses measured by nanoscratch tests. 14

15 Load (mn) MPa 45 MPa -37 MPa -135 MPa -235 MPa Penetration Depth (nm) Figure 1 Typical load-penetration depth curves of nanoindentation tests of SiN films with different residual stresses. 15

16 Hardness (GPa) Modulus (GPa) 8 Hardness Modulus Residual Stress (MPa) Figure 2 Hardness and elastic modulus of SiN films with different residual stresses measured by nanoindentation tests. 16

17 Figure 3 SEM surface morphologies around indent marks on SiN films with different residual stresses of (a) compressive 235 and (b) tensile 86 MPa after nanoindentation tests at a maximum applied load of 200 mn. 17

18 Penetration Depth (nm) Calculated Stress Penetration Depth Residual Stress (MPa, Cal.) Residual Stress (MPa, Exp.) Figure 4 Penetration depths of nanoindentation tests and calculated residual stresses of SiN films with different residual stresses. 18

19 Load (mn) MPa 45 MPa -37 MPa -135 MPa -235 MPa Penetration Depth (nm) Figure 5 Typical load-penetration depth curves of nanoscratch tests of SiN films with different residual stresses. 19

20 Figure 6 SEM surface morphologies along scratch tracks on SiN films with a residual compressive stress of 235 MPa after nanoscratch tests at a maximum applied load of 80 mn; (a) low-magnification view of scratch track; (b) beginning, (c) middle, and (d) late stages of film cracking and interface delamination. 20

21 Critical Load (mn) (a) Load Width Critical Track Width (μm) Residual Stress (MPa) 1.0 Critical Stress (GPa) (b) Stress Energy Adhesion Energy (J/m 2 ) Residual Stress (MPa) 1.2 Figure 7 (a) Critical loads and scratch track widths and (b) critical stresses and adhesion energy for interface delamination of SiN films with different residual stresses measured by nanoscratch tests. 21