LOMONOSOV ET AL . VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX www.acsnano.org A C XXXX American Chemical Society Nanoscale Noncontact Subsurface Investigations of Mechanical and Optical Properties of Nanoporous Low-k Material Thin Film Alexey M. Lomonosov, †,^ Adil Ayouch, † Pascal Ruello, †, * Gwenaelle Vaudel, † Mikhail R. Baklanov, ‡ Patrick Verdonck, ‡ Larry Zhao, § and Vitalyi E. Gusev †, * † Laboratoire de Physique de l'Etat Condensé, UMR 6087 CNRSUniversité du Maine, Le Mans, France, ‡ IMEC, Kapeldreef 75, B-3001 Leuven, Belgium, and § GLOBALFOUNDRIES assignee at IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. ^ On leave from the General Physics Institute, Russian Academy of Sciences, Moscow, Russia. S patial in-depth nonuniformity of thin solid films is becoming a critical factor with the aggressive down-scaling of their thickness. The most crucial reason for the nonuniformity is related to the different mechanisms of chemical reactions occur- ring in the bulk and at interfaces and to the increasing surface/volume ratio. Many dif- ferent examples demonstrate this state- ment. For instance, the Young modulus of polycrystalline diamond films grown by microwave- 3 or e-beam-assisted 4 chemical vapor deposition continuously varies from the nucleation side to the growth side. The variation in the film texture with the increas- ing film thickness is considered a common phenomenon for film growth in general. 5 The variations of the material properties inside the film can take place at the scales from a few nanometers to hundreds of nanometers. 35 Inhomogeneity of thin films can also be caused by their postdeposition processing such as radiation-assisted curing used in the fabrication of low dielectric con- stant (low-k) films for modern microelectro- nic devices. 610 In addition, inhomogeneity can be introduced intentionally for the pro- duction of multilayered optical antireflec- tive and highly reflective coatings. 11 For these UV optics the individual layers, consti- tuting the coating, are of nanoscale thickness. Several techniques allow inspection of the physical and chemical properties of solids with a nanometric resolution. Scan- ning probe microscopy (SPM 12 ) is routinely employed, but it is limited to surface investi- gation. When subsurface information is re- quired, invasive treatments are usually needed. Typically, sub-nanometer resolution images of the microstructure are obtained after sample microtoming (cross section image with transmission electron microscopies 13 ). In-depth chemical and physical analysis can also be performed by slicing the material and analyzing either the removed matter (secondary ion mass spectroscopy 14 ) or the newly created surface (ion-sputtering- assisted X-ray and ultraviolet photoemission * Address correspondence to vitali.goussev@univ-lemans.fr, pascal.ruello@univ-lemans.fr. Received for review November 1, 2011 and accepted January 1, 2012. Published online 10.1021/nn204210u ABSTRACT Revealing defects and inhomogeneities of physical and chemical properties beneath a surface or an interface with in-depth nanometric resolution plays a pivotal role for a high degree of reliability in nanomanufacturing processes and in materials science more generally. 1,2 Nanoscale noncontact depth profiling of mechanical and optical properties of transparent sub-micrometric low-k material film exhibiting inhomogeneities is here achieved by picosecond acoustics interferometry. On the basis of the optical detection through the time-resolved Brillouin scattering of the propagation of a picosecond acoustic pulse, depth profiles of acoustical velocity and optical refractive index are measured simultaneously with spatial resolution of tens of nanometers. Furthermore, measuring the magnitude of this Brillouin signal provides an original method for depth profiling of photoelastic moduli. This development of a new opto-acoustical nanometrology paves the way for in-depth inspection and for subsurface nanoscale imaging of inorganic- and organic-based materials. KEYWORDS: picosecond laser ultrasonics . nanoacoustics . acousto-optics . depth profiling . transparent films . low-permittivity films ARTICLE