Microstructural Analysis of Copper Interconnections Using Picosecond Ultrasonics
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Microstructural Analysis of Copper Interconnections Using Picosecond Ultrasonics James M.E. Harper, Sandra G. Malhotra, Cyril Cabral Jr., Christian Lavoie, Hsin-Yi Hao*, Wadih Homsi*, Humphrey J. Maris* IBM T.J. Watson Research Center, Yorktown Heights, NY 10598 *Department of Physics, Brown University, Providence, RI 02912 ABSTRACT We demonstrate that picosecond ultrasonics provides detailed information on the structure and properties of patterned arrays of copper fine lines used in silicon chip interconnections. In this method, the sample surface is momentarily heated several oC using a pump laser beam, and the transient change in the optical reflectivity is measured by a probe laser beam. Measurements of the optical reflectivity are made on time scales ranging from picoseconds to nanoseconds, revealing information on electronic, acoustic and thermal properties. We have applied this method to samples consisting of copper line arrays of 0.4 µm linewidth, 0.65 µm pitch and 0.35 µm depth in SiO2 on silicon wafers. For comparison, we examined the picosecond ultrasonic response of 200 nm-thick blanket copper thin films. The patterned Cu lines are found to have long-term oscillations at frequencies of 4.39 and 8.29 GHz with lifetimes at least 10 times longer than the oscillations in the blanket Cu film. A twodimensional mechanical analysis was developed which uses as input parameters the dimensions and sound velocities of the materials in the sample, and finds the normal mode frequencies and displacements. The main vibrational modes are identified and described for the patterned lines, and the simulations confirm that the lowest frequency modes have very small damping coefficients. Also, the time-dependent signal is shown to reveal details of interface layers and integrity of the copper/liner interface. INTRODUCTION Picosecond ultrasonics was developed to probe material properties and interfaces at length scales that are inaccessible to lower frequency acoustic techniques [1,2,3]. Since the sound velocity in copper is about 4.5 nm/ps, a 45-nm thick copper film is traversed by an acoustic wave in 10 ps. To examine film thicknesses below the tens of nm range, it is therefore necessary to produce and detect acoustic signals with picosecond time resolution. The development of stable, pulsed, mode-locked lasers has made possible the application of quantitative picosecond ultrasonics as a practical thin film measurement method [4]. In its simplest application, the method provides a film thickness measurement on multilayer samples that need not be transparent. However, as has been previously demonstrated, the sample response covers a wide range of time scales from ps to ns, providing information on electronic [5], mechanical [6] and thermal properties [7,8] of the sample. In this paper, we demonstrate that this method can provide detailed information on the structure and properties of patterned copper fine line arrays in addition to blanket copper thin films. With patterned fine line arrays, we show that the normal modes of osc
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