Characterization of elastic moduli of Cu thin films using nanoindentation technique

Transcription

1 Composites Science and Technology 65 (5) 8 COMPOSITES SCIENCE AND TECHNOLOGY Characterization of elastic moduli of Cu thin films using nanoindentation technique S.H. Hong a, *, K.S. Kim a, Y.-M. Kim b, J.-H. Hahn c, C.-S. Lee d, J.-H. Park e a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373- Guseong-dong, Yuseong-gu, Daejeon 35-7, Republic of Korea b Technology Research Center, Agency for Defense Development, P.O. Box 35, Yuseong-gu, Daejeon 35-, Republic of Korea c Chemical Metrology and Materials Evaluation Division, Korea Research Institute of Standard and Science, Yuseong-gu, Daejeon 35-, Republic of Korea d MEMS Laboratory, Samsung Advanced Institute of Technology, Suwon 4-, Republic of Korea e School of Information Engineering, Tongmyung University of Information Technology, Busan 8-7, Republic of Korea Available online 8 February 5 Abstract The elastic moduli in perpendicular and parallel directions to surface of Cu thin film were investigated by nanoindentation test and micro-cantilever beam bending test and the elastic moduli were compared with the theoretical estimations of elastic moduli based on the texture analysis. The thickness of electroplated Cu thin film, characterized by surface profiler, was varied as 3 2 lm with varying the electroplating condition. The specimens for micro-cantilever beam bending test were fabricated by lithography and isotropic etching process. Elastic modulus in perpendicular direction of Cu thin film, measured by nanoindentation test, was obtained as GPa and decreased with increasing the film thickness. Elastic modulus in parallel direction of Cu thin film, measured by micro-cantilever beam bending test using nanoindentation technique, was obtained as 2 GPa for 2.8 lm thick Cu thin film and 9 GPa for.5 lm thick Cu thin film. Texture of Cu thin film was analyzed from the orientation distribution function calculated from the pole-figures obtained by X-ray diffraction technique. Cu thin film with thickness of 3 lm showed strong h i texture, while h 3i texture increased with increasing the thickness of Cu thin film. The theoretical estimations of elastic moduli in both perpendicular and parallel directions to surface of Cu thin film based on the texture analysis showed a good agreement with experimental measurements based on the nanoindentation technique. Ó 5 Elsevier Ltd. All rights reserved. Keyword: Cu thin film. Introduction It has been known that the mechanical properties of metallic thin films are quite different from those of bulk materials []. Several kinds of high accurate measurement techniques for mechanical properties of thin film have been developed in the past decade such as nanoindentation test, micro-tensile test and micro-cantilever beam bending test [2]. The measurement facility of the * Corresponding author. Tel.: ; fax: address: shhong@kaist.ac.kr (S.H. Hong). mechanical properties is required to have high degree of accuracy for the measurements of the stress strain or strain time relations. The nanoindentation and other techniques have been improved by developing the measurement technology for small load and displacement [3]. The nanoindentation method is one of the most powerful methods to measure mechanical properties of thin film. Various shapes of indenter tips can be used to investigate the yielding behavior of thin films. Furthermore, complicated specimen preparation is not needed to measure the mechanical properties of thin films by nanoindentation technique [4] /$ - see front matter Ó 5 Elsevier Ltd. All rights reserved. doi:.6/j.compscitech.4.2.

2 2 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 The elastic modulus is one of the intrinsic properties of a material. However, the elastic modulus of thin film is reported to be quite different from that of bulk material [5]. Electroplated Cu thin films are applied to various fields of MEMS and electronic components. The major applications of Cu thin films are the RF MEMS switch, conductive layer of power inductor coil, and metallization material of integrated circuit [6,7] and their application are rapidly increasing recently. The elastic modulus of Cu thin film is important to estimate the resonance frequency and the residual stress. In thin study, the elastic moduli of Cu thin films, in perpendicular and parallel directions to the film surface, were characterized by nanoindentation test and micro-cantilever beam bending test. The theoretical elastic moduli of Cu thin film, in perpendicular and parallel directions to the film surface, were estimated from the texture analysis. The elastic modulus characterized by nanoindentation test was compared with that calculated value from texture analysis in perpendicular direction to the film surface [8]. The elastic modulus characterized by micro-cantilever bending test was compared with that calculated value from the texture analysis in parallel direction to the film surface. 2. Experimental procedures 2.. Fabrication of electroplated Cu thin film The Cu thin films were fabricated by electroplating process consisted of the following three steps. The first step is to clean the surface of Si wafer, which is used as a substrate. The Si wafer was dipped in acetone for 5 min to remove residual dust on the surface. The surface oxide of the Si wafer was removed by using dilute HF solution. The second step is to make a seed layer on the Si substrate. The electroplating process needs a thin conductive seed layer on the surface of substrate. A very thin Cu film with thickness of nm was deposited on Si wafer by DC magnetron sputtering process under induced voltage of 3 V in vacuum pressure of.5 Pa. The last step is to fabricate Cu thin film above the seed layer on Si wafer substrate by electroplating under a constant current of ma/cm 2 in copper acid sulfate solution. The thicknesses of Cu thin films were controlled from 3 to 2 lm by varying the deposition condition Nanoindentation test of Cu thin film The nanoindentation test was performed by using the nanoindenter II supplied from Nanoinstrument Co. The shape of indenter tip was Berkovich type having the tip radius of nm [9]. The specimen was loaded for s and unloaded for 3 s using the indenter tip. The indentation was performed at a speed of 3 nm/s and the maximum loading rate was less than ln/s. The indentation depth was predetermined less than % of the thickness of Cu thin film. The elastic modulus was measured by continuous stiffness measurement method during the loading with nanoindenter Micro-cantilever beam bending test of Cu thin film The Cu micro-cantilever beam bending test was performed by the Nanoindenter II. Cu micro-cantilever beams were fabricated by lithography and isotropic etching process using the electroplated Cu thin film. Two lithography masks was used to make the microcantilever beams. One mask is needed to make the mold for electroplating of micro-cantilever beams, and the other is needed to pattern etch-window on the microcantilever beams. The micro-cantilever beam specimens with different length from 5 to 25 lm were fabricated. The width of micro-cantilever was fixed as 5 lm, and the space between micro-cantilever beams was kept as lm. The processing steps for fabrication of micro-cantilever beam specimens were shown in Fig., and the fabricated micro-cantilever beam was shown in Fig. 2. The first step for micro-cantilever is to make mold for electroplating on the seed layer. The mold for electroplating is fabricated by lithography process of AZ 92 photoresist with first mask. The second step is electroplating. The electroplating time is from to min. The thickness of electroplated Cu thin films was 2.8 or.5 lm. The third step is to make an etch-window. The photoresist is coated again and the etch-window is fabricated by lithography process with the second mask. The fourth step was to make micro-cantilever by dry etching of the Si substrate under electroplated Cu thin film by using XeF 2 gas. The etch-window is removed at the final step to obtain micro-cantilever beam specimens Texture analysis in Cu thin film The texture of Cu thin film was characterized by using Rigaku D/max-RC X-ray diffraction equipment. The Cu thin film was cut into 8 mm square shape plate and loaded on pole-figure attachment. The pole-figure was measured with reflection method under a condition of 3 kv and ma. The orientation distribution function was calculated from the ( ), (2 ) and (2 2 ) pole-figures obtained from the Cu thin film. The elastic moduli in perpendicular and parallel directions of Cu thin film are calculated from the orientation distribution function by assuming the VoigtÕs model and the HillÕs model [,].

3 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 3 Fig.. Fabrication procedures for micro-cantilever beam bending specimen from the electroplated Cu thin film: (a) fabrication of mold above Cu seed layer on Si substrate; (b) electroplating of Cu thin film; (c) fabrication of window for etching of Si; (d) isotropic etching of silicon by XeF 2 gas; (e) ashing of window for etching of Si. Fig. 2. The shape and size of micro-cantilever beam bending specimen fabricated from the electroplated Cu thin film. 3. Results and discussion 3.. Theoretical calculation of elastic moduli of Cu thin film by texture analysis The X-ray diffraction pole-figure of electroplated Cu thin films on Si wafer was investigated. The (2 ), (2 2 ) and ( ) pole-figures were characterized and the orientation distribution functions were calculated from the pole-figures. The u -axis was degenerated because the specimen showed symmetry parallel to the perpendicular direction to the substrate as shown in Fig. 3. The orientation distribution functions of electroplated Cu thin films showed in Fig. 4. The Cu thin film with thickness of 3. lm showed strong h i texture. Texture of Cu thin film with thickness of 6. lm changed to strong h 3i texture. Cu thin film with thickness of 9. lm showed very strong h3i texture, while Cu thin film with thickness of 2. lm showed h 3i texture tilted to h 23i texture. All Cu thin films showed weak hi texture. The development of preferred orientation of Cu thin film was investigated by Zhang et al. [4]. Although, ( ) plane tends to lie parallel to the plane of thin film in consideration of surface energy, but the ( ), ( ), (5 ), (2 ), (3 ) and ( 3) planes lie parallel to the plane of thin film with increasing the thickness of film due to the strain energy minimization [2 4]. As the electroplated Cu thin film has strong texture, the elastic modulus of Cu thin film is highly anisotropic. The elastic modulus of a single crystal having cubic structure can be obtained from the equation as follows: ¼ S 2 ðs S 2 Þ EðgÞ? 2 S 44 ðh 2 h2 2 þ h2 2 h2 3 þ h2 3 h2 Þ; ðþ where E(g)? is elastic modulus in perpendicular direction to the film surface. S, S 2 and S 4 are the independent elastic compliances of a cubic crystal. S, S 2 and S 44 are obtained as 5., 6.3 and 3.3 TPa, respectively [5]. The h, h 2 and h 3 are direction cosines to transform the crystallographic axis to perpendicular direction to the surface of thin film. The direction cosines can be obtained from the Euler angle. The elastic modulus perpendicular direction to the surface of thin film was predicted by VoigtÕs model [6] as follows: E? ¼ Z EðgÞ 8p 2? F g dg ¼ Z Eðu 8p 2 ; /; u 2 Þ? F g sin / du 2 d/ du ; ð2þ where F g is orientation distribution function at the Euler angle, g: {u /u 2 }, and E? is average elastic modulus in perpendicular direction to textured thin film. The grain shape of Cu thin film was elongated along the perpendicular direction to the surface of thin film. If all the grains have same strain when they are compressed or elongated in perpendicular direction to surface of thin film, the VoigtÕs model can be applied to predict the elastic modulus for electroplated Cu thin film. The elastic modulus in parallel direction to the surface of thin film can be obtained from the equation as follows:

4 4 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 Fig. 3. Experimental pole-figures of ( ), ( ) and ( ) planes of 3. m thick electroplated Cu thin film obtained by X-ray diffraction test. ¼ S 2 ðs S 2 Þ EðgÞ k 2 S 44 l 2 l2 2 þ l2 2 l2 3 þ l2 3 l2 ; ð3þ where E(g) k is elastic modulus in parallel direction to the film surface and l, l 2 and l 3 are direction cosines to transform the crystallographic axis to parallel direction to the surface of thin film. The average elastic modulus in parallel direction to the surface of thin film was predicted by HillÕs model as follows: E k ¼ Z EðgÞ 6p 2 k F g dg þ F EðgÞ g dg ; ð4þ k R 6p 2 where E k is average elastic modulus in parallel direction to the surface of textured thin film Measurement of elastic modulus of Cu thin film by nanoindentation test Elastic modulus of Cu thin film was characterized by using nanoindentation test. The maximum indentation depth was fixed as nm, which is predetermined to be less than % of minimum thickness of electroplated Cu thin film. The elastic modulus was obtained as average value from more than five measurements of a specimen. The result of elastic modulus by nanoindentation test was shown in Fig. 5. The average value of elastic modulus measured by continuous stiffness measurement method at an interval of 5 nm indentation depth during the loading with nanoindenter. Elastic modulus can be characterized by using depth sensing indentation method based on the following equation derived as following equations [4]: S ¼ dp dh ¼ 2 pffiffiffi pffiffiffi E r A p ¼ 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi E r C h 2 c p þ C h c þ C 2 h =2 c þ; ð5þ h c ¼ h :75P=S; ð6þ ¼ m2 i þ m2 m ; ð7þ E r E i E m

5 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 5 (a) ϕ 2 (b) ϕ φ φ (c) ϕ 2 (d) ϕ φ φ Fig. 4. Orientation distribution functions of Cu thin films with thickness of: (a) 3. lm; (b) 6. lm; (c) 9. lm; (d) 2. lm, which are calculated from the pole-figures obtained by X-ray diffraction. where E r is reduced elastic modulus, P is indentation load, S is stiffness, h is indentation depth and h c is contact depth. C, C and C 2 are constant according to shape of indentor tip. E m,m m, E i and m i are elastic modulus and PoissonÕs ratio of the specimen and the indenter. The elastic modulus of Cu thin film was measured by nanoindentation test as 23 GPa for 3. lm thick Cu film and 99 GPa for 2. lm thick Cu film. The elastic modulus of electroplated Cu thin film decreased with increasing the thickness of Cu thin film. Elastic modulus of wrought and annealed Cu is measured as 5 GPa. The elastic modulus measured by the nanoindentation test is dependent on surface roughness and thickness of Cu thin film, but is independent on the error from geometric measurement because geometric input is not required. Surface roughness of Cu thin film was 3 nm that was small enough to characterize the elastic modulus accurately in a condition of maximum indentation depth of nm. The theoretical elastic modulus in perpendicular direction calculated from the texture analysis using Eq. (2) was compared to the elastic modulus measured

6 6 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 Elastic Modulus (GPa) 3µ electroplated Cu thin film 6µ electroplated Cu thin film 9µ electroplated Cu thin film 2µ electroplated Cu thin film Depth of Indentation (nm) Fig. 5. The variation of elastic modulus of Cu thin film measured by nanoindentation method with varying the indentation depth. by the nanoindentation method as shown in Fig. 6. The elastic modulus estimated from the texture analysis decreased with increasing the thickness of Cu thin film. The elastic modulus estimated from the texture analysis showed a good agreement with that measured by the nanoindentation test. These results indicate that the elastic modulus measured by the nanoindentation test represents the elastic modulus in perpendicular direction to the surface of Cu thin film Measurement of elastic modulus of Cu thin film by micro-cantilever beam bending test The micro-cantilever beam specimens, fabricated by lithography and isotropic etching process as shown in Fig., were elastically bended on a free end by applying a load by nanoindenter as shown in Fig. 7. The load deflection curves of Cu thin film during micro-cantilever beam bending test was shown in Fig. 8. The elastic modulus measured by micro-cantilever beam bending test was calculated by the following equation [7 9]: E ¼ P 4L 3 ð m 2 Þ ; ð8þ d bt 3 where P is indentation load, d is deflection, L is length of deflected beam, m is PoissonÕs ratio, b is width of microcantilever and t is thickness of micro-cantilever. PoissonÕs ratio of Cu was assumed as.3 as reported by Landolt-Börnstein et al. The elastic modulus, calculated from the slope of load deflection curve and the geometric dimension, was obtained as 2 GPa for 2.8 lm thick Cu micro-cantilever and 99 GPa for.5 lm thick Cu micro-cantilever as shown in Fig. 9. The theoretical elastic modulus in parallel direction to the surface of thin film calculated from the texture analysis using Eq. (4) showed similar value with that measured by the microcantilever beam bending test. These results indicate that the elastic modulus measured by the micro-cantilever Elastic Modulus (GPa) Measured by nanoindentation test (nm) Estimated in perpendicular direction by texture analysis Thickness of Cu Thin Film (µm) 5 Fig. 6. The comparison of elastic modulus in perpendicular direction to the film surface calculated by the texture analysis with that measured by the nanoindentation test of Cu thin film. Fig. 7. A schematic diagram showing the bending of micro-cantilever beam specimen during the micro-cantilever beam bending test.

7 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 7 Load (mn) Micro-cantilever beam bending test t =.5µm ; L =29µm ; P/δ = 74.5N/m t = 2.8µm ; L =4µm ; P/δ = 32.4N/m Deflection (nm) Fig. 8. Load-deflection curves of Cu thin film obtained from the micro-cantilever beam bending test. modulus in perpendicular direction to the film surface can be measured by the nanoindentation test, while the elastic modulus in parallel direction to the film surface can be measured by the micro-cantilever beam bending test. (2) The theoretical elastic moduli in perpendicular and parallel direction were estimated from the texture analysis of Cu thin film. The theoretical elastic modulus in perpendicular direction to the film surface can be estimated by VoigtÕs model, while that in parallel direction to the film surface can be estimated by HillÕs model. (3) The theoretical elastic moduli estimated from the texture analysis in perpendicular and parallel direction to the surface of Cu thin film are in good agreement with those measured by the nanoindentation test and the micro-cantilever beam bending test, respectively. These results indicate that the anisotropy in elastic moduli of textured metallic thin film can be characterized quantitatively by the nanoindentation technique. References Elastic Modulus (GPa) Thickness of Cu Thin Film (µm) beam bending test represents the elastic modulus in parallel direction to the surface of Cu thin film. 4. Conclusions Measured by micro-cantilever beam bending test Estimated in parallel direction by texture analysis Fig. 9. The comparison of elastic modulus in parallel direction to the film surface calculated by the texture analysis with that measured by the micro-cantilever beam bending test of Cu thin film. () The elastic moduli in perpendicular and parallel directions to Cu thin film was characterized by using nanoindentation techniques. The elastic 5 [] Hoffman RW. In: Wilsdorf HGF, editor. Thin films. Metals Park (OH): American Society of Metals; 964 [chapter 4]. [2] Brotzen FR. Mechanical testing of thin films. Int Mater Rev 994;39:24. [3] Nix WD. Mechanical properties of thin films. Metall Trans 989;A:227. [4] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res 992;7:564. [5] Suresh S, Nieh TG, Choi BW. Nano-indentation of copper thin films on silicon substrates. Scr Mater 999;4:95. [6] Andrew J-L, Jiang YH, Neves HP, Tien NC. Copper-encapsulated silicon micromachined structures. J Microelectromech Sys ;9:28. [7] Arias F, Oliver SRJ, Xu B, Holmlin RE, Whitesides GM. Fabrication of metallic heat exchangers using sacrificial polymer mandrils. J Microelectromech Sys ;:7. [8] Kim WS, Kim JK, Hwang P. Characterization of nano-wear mechanisms of hard disk coatings. J Electron Mater ;3: 53. [9] Larsson P-L, Giannakopoulos AE, Soderlund E, Rowcliffe JE, Vestergaard R. Analysis of Berkovich indentation. Int J Solids Struct 996;33:22. [] Voigt W. Lehbuch der Kristallphysik, Teubner, Leipzig; 928. p [] Hill R. The elastic behavior of a crystalline aggregate. Proc Phys Soc London 952;A65:349. [2] Harper JME, Cabral Jr C, Andricacos PC, Gignac L, Noyan IC, Rodbell KP, et al. Mechanisms for microstructure evolution in electroplated copper thin films near room temperature. J Appl Phys 999;86:256. [3] Lagrange S, Brongersma SH, Judelewicz M, Saerens A, Vervoort I, Richard E, et al. Self-annealing characterization of electroplated copper films. Microelectron Eng ;5:449.

8 8 S.H. Hong et al. / Composites Science and Technology 65 (5) 8 [4] Zhang J-M, Xu K-W, Ji V. Dependence of strain energy on the grain orientations in an FCC-polycrystalline film on rigid substrate. Appl Surf Sci 2;85:77. [5] Landolt-Börnstein. Numerical data and functional relationships in science and technology, New Series, Group III, vol. 2. Berlin: Springer-Verlag; 979. [6] Bunge HJ, Kiewel R, Reinert Th, Fritsche L. Elastic properties of polycrystals influence of texture and stereology. J Mech Phys Solids ;48:26. [7] Gere JM, Timoshenko SP. Mechanics of materials. 3rd ed.. International Thomson Publishing; 99. [8] Zhang T-Y, Zhao M-H, Qian C-F. The effect of substrate deformation on the microcantilever beam-bending test. J Mater Res ;5:868. [9] Son D, Jeong J, Kwon D. Film-thickness considerations in micro-cantilever-beam test in measuring mechanical properties of metal thin film. Thin Solid Films 3;437:87.