X-RAY STUDY OF RESIDUAL STRESSES IN THIN CHROMIUM METALLIZATIONS ON GLASS SUBSTRATES

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1 Copyright 0 JCPDS-International Centre for Diffraction Data 1997 X-RAY STUDY OF RESIDUAL STRESSES IN THIN CHROMIUM METALLIZATIONS ON GLASS SUBSTRATES L. s. suominen American Stress Technologies, Inc. 61 McMurray Road, Pittsburgh, PA C. Zhou, MA. Korhonen, and C.-Y. Li Department of Materials Science and Engineering Cornell University, Ithaca, NY ABSTRACT The adhesion of Cr on glass substrates is of great interest for glass-silicon circuit applications. Because of the nature of deposition process and differences in thermal expansion coefficients, very large residual stresses can arise which may lead to debonding of the Cr film. In this paper, effects of deposition type, glass type, thermal anneal, and film thickness on the residual stress are studied. We found that room temperature deposition of Cr results in high inherent tensile stresses up to 1400 MPa. E-beam deposition leads to higher stresses at the same film thickness. Both E-beam and sputter deposition result in high, and roughly similar, intetiacial fracture energies, Gf > 5-8 J/m*. Therefore, the inherent tensile stresses in the Cr on glass system are the cause for the apparent loss of adhesive strength at larger Cr-layer thicknesses. INTRODUCTION Thin metallic coatings on ceramic substrates are widely used in various microelectronics as well as optical and optoelectronics devices. In this study we are concerned with the Cr-metallization on glass which is of great technological interest for optical coatings, flat panel display applications, and thin film transistors (TFT). Because of the nature of deposition process and/or differences in thermal expansion coefficient, during deposition and/or subsequent manufacturing steps very large residual stresses arise in the Cr-metallization bonded to glass, which can lead to lateral cracking, peeling, or complete delamination, see e.g. [l] for a recent review of the integrity of thin films. The problem of practical adhesion of Cr-metalhzation to a glass substrate is actually quite a complicated one: The real inter-facial adhesion is usually understood to mean the chemical adhesion. The chemical adhesion energy can be determined, e.g. by measuring the equilibrium contact angles of the phases meeting at the interface [2]. However, the practical adhesion energy, as measured by any of the multitude of interface cracking experiments, usually turns out be many times larger than that due to the chemical adhesion alone. This stems fi-om the inelastic deformation, including frictional effects, accompanying the propagation of interfacial cracks. These dissipative mechanisms depend intricately and sensitively on the specimen geometry, interface roughness, and the microstructure of the interface regime. Worse still, the apparent fracture energy depends also on the cracking mode induced by the

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3 Cowriaht 0 JCPDS-international Centre for Diffraction Data 1997 particular experiment used to probe the adhesive strength. Finally, the residual stress in the Cr-glass system will help the applied stress to induce cracking, and in an extreme case, may be sticiently high to induce spontaneous cracking with no externally applied load at all. To gain basic understanding of the physics underlying the practical adhesion it is necessary to address the individual components of the apparent adhesive strength. X-ray stress measurement is able to yield the stress state directly after deposition, after post-deposition treatments, as well as the change of the stress as due to plastic deformation or cracking. Nanoindentation testing of thin metalliiations residing on top of rigid substrates is capable of displaying the apparent hardness change due to proximity of the interface, the more sensitively the closer the interface is to the indentor tip [3]. Therefore, based on the indentation hardness readings versus indentation depth, it becomes possible to deduce the effect of inter-facial adhesion, as separated fi-om the bulk plasticity or residual stress effects. An important goal of this study is to gain better understanding how inherent stresses arise during deposition of Cr on glass substrates. Of course we are also interested in means to reduce the residual stress levels in order to enhance the practical adhesion. We shall compare fracture strength values derived fi-om the stress measurement, as discussed below, to the hardness values obtained by nanoindentation testing to probe the diierent facets of practical adhesion. In this first account of our Cr-glass adhesion research we will restrict ourselves in reporting the measured X-ray stresses for Crfilms of different thickness deposited on glass by two methods of evaporation. EXPERIMENT Two commercially available glass types, denoted briefly as A and B below, were selected for the study: A, an alkaline earth aluminosilicate, with CTE of 3.8 ppm/k B, a barium borosilicate, with CTE of 4.7 ppmk For glass surface preparation for deposition, in order to evaporate organic contaminants, we used 30 min vacuum anneal at 550 C which was previously found to promote good adhesion [4]. Cr-films of 0.25, 0.50, 1.00, and 2.00 urn thickness, were vacuum deposited (<loe-6 torr) at room temperature by E-beam (EB) and sputtering using the standard practices. The residual stress in Cr-metallizations was determined by X-ray diffraction after the deposition, and after an additional 15 minutes anneal at 450 C. The motivation for the post-deposition anneal is that, during manufacturing the metallization may have to experience temperatures of about 350 C or more, as in typical amorphous Si TFT processing, e.g. [5]. A solid state X-ray stress analyzer, AST X2001 *), was used for the stress measurement. The principle of this solid state X-ray camera technique has been detailed elsewhere [6]. The AST X2001 *) AST X2001 is a trademark of American Stress Technologies, Inc.

4 CoavriahtO JCPDS-International Cenke for Diffraction Data 1997 using a modified ~-inclination technique [6]. Stresses were measured with Cr-Kct radiation yielding Cr 21 l-reflection at an angle 20 m The irradiated area was about 3 mm in diameter and the exposure times ranged from 30 to 120 seconds After verifying that the measured strain vs. sin2v-plot yielded straight lines, the line profiles were recorded using the inclinations of w = 0 and 45. The lie shifts were evaluated by the cross-correlation technique [e.g. 71 and the biaxial stresses of the Crcoating were calculated by using the Young s modulus E = 279 GPa and Poisson s ratio of u = The standard deviation of the stress readings was estimated to be +/- 60 MPa. RESULTS AND DISCUSSION The X-ray stress results after E-beam and sputtering deposition, and after an additional 450 C anneal, are shown in Table I for both glass types. The after deposition stresses (denoted depos. in Table I) were recorded one month after the deposition, while the after anneal stresses (denoted anneal in Table I) were measured immediately following the annealing treatment. It can be seen that, generally, E-beam deposition results in higher stresses, and consequent lateral cracking, peeling, and decohesion. Although the Cr-metallization in all samples appeared intact from the metallization side, careful inspection of the interface from the glass side displayed faint lateral cracks shortly tier deposition in all 2 urn and some 1 ym thick samples. With time (up to a month) the cracks propagated through the film thickness, increased in number, and led to partial peeling (sputtered samples) or near-complete decohesion (E-beam deposited samples). Of course, cracking was reflected in the reduction of the stress level with time. Note, however, that the residual stress in the untracked samples did not change with time. As to the effect of the glass type, in the case of the 0.25 pm film glass A has smaller stress than glass B, but the situation reverses at 0.50 urn. Also, while glass A develops a larger stress at 0.25 urn during sputtering, the stresses become practically the same at 0.50 urn. The trend appears to be that the inherent stress is small at small thicknesses, grows through a maximum between urn, and falls afler this. That thicker films eventually develop cracks, despite the falling stress level, is explained by the thickness dependence of the inter-facial fracture energy, see Eq. (1). The main difference between TABLE I: X-ray stresses after deposition and an additional anneal (MPa) thick. (pm) E-BEAM SPUTTERING Glass A Glass B Glass A Glass B depos. anneal depos. anneal depos anneal depos anneal *) ) *) *) 116*) ::I*) *) 266 ) 689 ) 479*) 562 ) ) lateral cracking at the interface followed by debonding

5 glass A and B in this respect appears to be that the maximum stress occurs at a slightly different Cr thickness. Fig. 1 depicts the X-ray stresses for the sputtered fihns after the deposition and after the thermal anneal. It is immediately obvious that the stresses in the metalliiations on both glass types behave roughly similarly \ 0 Glass A: deposition _ anneal Glass B: deposition anneal I._ I I I I 0.0 0, ,o 2 THICKNESS, pm Fig. 1: X-ray stresses for sputtered Cr-metallizations directly after deposition and after an additional anneal at 450 C During the 450 C anneal, the additional thermal stress due to CTE difference is about 360 MPa compression for glass A and 225 MPa compression for glass B. Therefore, the actual tensile stresses during the anneal should be smaller than directly after the deposition. In the absence of thermally induced structural changes, including yielding, we expect the stress to return back to the after deposition value after the anneal. Obviously, this is the case for the 0.25 urn sputtered metallization only; the stress afler the anneal is the same as before within the accuracy of the measurement. Therefore, the purely elastic behavior of the sputtered 0.25 urn films indicates a stable grain boundary or defect structure. For the other films the stress tends to increase as a result of the 450 C anneal. This suggests that the inherent stress level is, indeed, governed by the relaxation of the defect structure, most notably columnar grain boundaries, as suggested by Pulker [S]. The measured X-ray line widths appear to confirm this hypothesis: every time the anneal results in an appreciable stress increase, there also is a corresponding decrease in linewidth, indicative of structural relaxation. Fig. 2 depicts the line widths of the sputtered metallizations afler the deposition and after the thermal anneal. For the sputtered fihns it is obvious that the 450 C anneal induces clear structural changes in Cr metallization on glass B, while glass A appears to be promote thermal stability in the metallization. Copyright@ JCPDS-International Centre for Diffraction Data 1997

6 Gloss A: deposition - anneal Glass 8: deposition anneal I-_ I I I I,o on ,o 2!.5 THICKNESS, pm Fig. 2: X-ray line widths for sputtered Cr-metallizations directly after deposition and after an additional anneal at 450 C TABLE II: X-ray line width at half-maximum (arb. units) E-BEAM SPUTTEFUNG thick. Glass A Glass B Glass A Glass B W) depos. anneal depos. anneal depos. anneal depos. anneal *) ) ) *) 5.53 ),41*) ---*) 2.79*) 2.98 ) 3.08*) 2.90 ) *) lateral cracking at the interface followed by debonding Both after the deposition and the subsequent anneal, the maximum stress observed in the Crmetallization, without inducing cracking, is remarkably high, on the order of 1400 MPa. Cracking started at the interface where lateral cracks were first apparent on the glass side. Subsequently they propagated through the Cr film resulting ultimately in a fish-net type of debonding pattern. The cracking pattern was clearly visible on glass surface indicating that cracks propagated at least partially on the glass side. Based on the measured stresses we can obtain an estimate for the interfacial fracture energy. As detailed by Evans and Hutch&on [l], for thin films residing on a thick substrate the fracture strength is given by Gf=adm, (1) Copyright8 JCPDS-International Cente for Diffraction Data 1997

7 where h = l-u for debonding and h denotes the film thickness. This formula can be used to estimate the lower bounds for interfacial fracture energy by using the highest stress values from Table I (observed for 0.5 l_trn films): We obtain for E-beam deposition G&lass A) > 8.1 J/m* and Gt(glass B) > 5.2 J/m* and for sputtering Gdglass A) = 6.9 J/m* and Gt(glass B)= 5.3 J/m2. Although these must be considered only rough estimates, we decide that the inter-facial strength of Cr-glass system resulting fi-om E-beam and sputtering deposition is so high that crack initiation is determined by the fracture strength of the glass. The sign&ant difference found in the cracking behavior between the E-beam and sputter deposited Cr on glass appears to derive from the different inherent stress level. At the same film thickness, the fish-net pattern was much denser for glass B, which suggests that glass A has higher fracture strength. Recent literature suggests that increasing the substrate temperature to about 200 C during Crdeposition and/or controlling the argon pressure during sputtering [9], it may be possible to reduce the inherent Cr tensile stress; these possibilities will be explored in our further studies. CONCLUSIONS Room temperature deposition of Cr on glass results in high inherent tensile stresses that reach to about 1400 MPa in 0.5 urn thick films. E-beam deposition leads to higher stresses, most likely because it results in a smaller grain size and larger defect density, which means larger potential for structural relaxation during continued deposition or subsequent anneal. A thermal anneal after deposition, as during processing steps for TFT, provides a possibility for more relaxation and results in an increase of the inherent stresses, unless grain boundary and defect structure have already been stabilized. Both E-beam and sputter deposition appear to result in high, and roughly similar, inter-facial fracture energies, Gf > 5-8 J/m*. Therefore, the difference in the cracking behavior between the E- beam and sputter deposited Cr on glass appears to derive from the different inherent stress levels. The inherent tensile stresses in the Cr on glass system are the cause for the apparent loss of adhesive strength at larger Cr-layer thicknesses. ACKNOWLEDGEMENT This work was supported by National Science Foundation through the Materials Science Center at Cornell University. CopyightO JCPDS-International Cenlre for Difkaction Data 1997

8 REFERENCES A.G. Evans and J.W. Hutchinson, Acta Metall. Mater. 43,2507 (1995) A.G. Evans and M. Ruhle, MRS Research Bull. 15,46 (1990) D. Stone, W.R. LaFontaine, P. Alexopoulos, T.-W. Wu, and C.-Y. Li, J. Mater. Res 3, 141 (1988) K. Umbach, Private Communication, February C.W. Kim, C.O. Jeong,H.S. Song, Y.S. Kim J.H. Kim, J.H. Choi, M.K. Hur, H.G. Yang, and J.H. So&, SID 96 Digest, 337 (1996) M.A. Korhonen, V.K. Lindroos, and L.S. Suominen, Adv. X-ray Anal. 32,407 (1988) M.A. Korhonen, L.S. Suominen, and C.-Y. Li, in Nondestructive Characterization of Materials IV, edited by C.O. Ruud et al. (Plenum Press, New York, 1991) p. 15 H.K. Pulker, Thin Solid Films 89, 191 (1982) V. Guilbaud-Massereau, A. Celerier, and J. Machet, Thin Solid Films 258, 185 (1995) Copyright 0 JCPDS-International Cede for Diffraction Data 1997