Deformation in Thin Films Deposited on Rigid Substrates

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DEFORMATION IN THIN FILMS DEPOSITED ON RIGID SUBSTRATES Masanori Murakami IBM Research Division, T. J. Watson Research Center,Yorktown Heights, New York 10598

ABSTRACT Stresses and strains are always present in thin films deposited on rigid substrates. They are introduced into the films during the deposition and/or subsequent heat and chemical treatments. Since the origins of the strains (stresses) in thin films and their shapes are different from those of bulk materials, the distribution of strains supported elastically by the films is different from that of the bulk materials. The present paper reviews deformation in thin films caused by elastic strains, using illustrative data of Pb and Nb films that were deposited on Si wafers. Also, x-ray diffraction techniques to detect such local deformation are briefly reviewed. INTRODUCTION The use of thin films has become very important in various components of electronic devices. Strains (or stresses) are always present in these thin film electrode materials and may deteriorate both the device performance and reliability. Two kinds of strains are present in thin films; "intrinsic" (or "grown-in") and "extrinsic" strains. The intrinsic strains are frequently observed in as-deposited "hard" films such as W, Fe, Mo, Cr, Mn, Nb, etc [1]. The origins of these strains have been discussed extensively by Doerner and Nix [21, Hoffman [31, and Chopra [41. The extrinsic strain is primarily caused by mismatch of thermal coefficients between the films and the substrates. The large strains are observed in "soft" films such as In, Au, Al, Pb, etc. which have thermal expansion coefficients much larger than those of commonly used substrates like Si, Ge, GaAs, etc. The universal problem caused by high strain is loss of adhesion between the electrode materials and the substrates. The problem is particulary serious for refractory materials used widely in VLSI devices as reviewed exhaustively by d'Heurle [5]. The problem has been frequently overcome by improving the substrate cleaning procedure, the deposition of thin adhesive layers, etc. Recently, to obtain the optimum device performance, both horizontal and vertical dimensions of the devices have been reduced significantly to almost the technological limit. The modern electron-beam lithography technique allows fabrication of devices with electrode width as small as 0. 1Am. Muliple-layer structures have been used in which extremely thin layers act as barriers between the two different electrode materials. For such devices, the microstructure changes in the electrode materials, as small as a few nanometers, caused shorts, leading to device failures. However, such small deformation is extremely difficult to observe directly even with the modern analytical techniques because of the complexity of the device structure. Therefore, it is very important to predict such deformation of thin film electrode materials induced by small strains (even within the elastic limit) either by theories or diagostic experiments in order to increase the reliabili