Near-Field Ultrasonic Imaging: A Novel Method for Nondestructive Mechanical Imaging of IC Interconnect Structures

The investigation of an alternate approach to nondestructive, nanoscale mechanical imaging for IC interconnect structures is reported. This approach utilizes a heterodyne interferometer based on a scanning probe microscope, also referred to as heterodyne force microscopy (HFM). This interferometer is sensitive to the relative phase difference of the two ultrasonic excitations due to spatial variations in the sample viscoelastic response and enables near-field, phase-sensitive imaging. Proof-of-feasibility demonstrations of this technique are presented for ultrasonic phase-imaging of Al/low-k interconnect structures. Spatial resolution < 10 nm is demonstrated. INTRODUCTION Spatial resolution limitations restrict the application of conventional ultrasonic or acoustic imaging to integrated circuit (IC) structures. Simply, the spatial resolution, w, of an acoustic microscope is given by [1]: . . 51 . 0 A fN v w o = where vo is the speed of sound in the coupling medium, f is the frequency of the acoustic/ultrasonic wave, and N.A. is the numerical aperature of the lens. For a frequency of 1 GHz the nominal spatial resolution attainable is approximately 1.5 μm. Higher resolution alternatives for nondestructive mechanical imaging include the atomic force microscope (AFM) or scanning probe microscope (SPM) platforms. Force modulation microscopy (FMM) [2], ultrasonic-AFM [3], ultrasonic force microscopy (UFM) [4] are a few examples. Each technique is traditionally sensitive to the static elastic properties of the sample surface. Recently, a high spatial resolution phase-sensitive technique has been demonstrated which employs an ultrasonic heterodyne methodology for imaging elastic as well as viscoelastic variations across a sample surface [5]. So-called heterodyne force microscopy (HFM) uses a near-field approach to measure time-resolved variations in ultrasonic oscillations at a sample surface. As such, it holds potential for overcoming the spatial resolution limitations of conventional phase-resolved acoustic microscopy (i.e. holography) by eliminating the need for far-field acoustic lenses. The work presented here investigates the application of HFM at low carrier frequencies (2.2 MHz) to IC interconnect test structures consisting of a 1-level Al/low-k polymer damascene wiring pattern to evaluate potential metrology applications for surface and subsurface nanomechanical imaging. Initial results reveal the high mechanical and viscoelastic image contrast capable with HFM on such structures. In addition, the variation of the viscoelastic phase with HFM operational parameters is also evaluated and indicates a strong, nonlinear relationship to sample and tip ultrasonic vibration amplitudes. HFM OPERATIONAL PRINCIPLES In conventional UFM the super-resonant vibration of the sample substrate results in a deflection, zc, of the SPM cantilever related directly to the elastic/adhesive properties of the Mat. Res. Soc. Symp. Proc. Vol. 716 © 2002 Materials Research Society