R. J. Stoner

Phonon attenuation and velocity measurements in transparent materials by picosecond acoustic interferometry

A detailed analysis of the method of picosecond acoustic interferometry to study attenuation and velocity of longitudinal acoustic phonons in transparent materials in the Brillouin frequency range with picosecond laser pulses is presented. Experimental results for fused quartz from 90 to 300 K show good agreement with previous Brillouin scattering data. Measurements on a borosilicate glass (Corning 7059) and sapphire have also been made. This method makes these measurements possible under conditions where conventional approaches are not applicable.

Noninvasive picosecond ultrasonic detection of ultrathin interfacial layers: CFx at the Al/Si interface

A picosecond ultrasonics technique has been used to detect interfacial fluorocarbon (CFx) layers as thin as 0.5 nm between aluminum and silicon. The presence of the CFx material reduces acoustic damping and heat loss from the Al film into the Si substrate. This provides a means for noninvasive identification of organic/polymeric contaminants at the buried interface and potentially for characterizing interfacial mechanical properties.

Nondestructive detection of titanium disilicide phase transformation by picosecond ultrasonics

We demonstrate that picosecond ultrasonics is a sensitive nondestructive probe of the formation of TiSi2 from the reaction of titanium films on silicon annealed at temperatures of 300–800 °C. From the measured change in optical reflectivity, the responses due to electronic excitation, acoustic echoes, and thermal coupling to the underlying Si are resolved. The results show significant differences in the electronic response and the ultrasonic echo pattern before and after the structural phases C49 and C54 TiSi2 are formed. The longitudinal sound velocity is measured to be (8.3±0.2)×105 cm/s for C49 TiSi2, and about 5% lower for the C54 phase.

Nondestructive testing of microstructures by picosecond ultrasonics

We describe a new method we have developed which can be used to make ultrasonic measurements with picosecond time resolution, and give examples of its application to the study of the acoustic and mechanical properties of microstructures.

Detection of Titanium Silicide Formation and Phase Transformation by Picosecond Ultrasonics

Picosecond ultrasonics is employed to study the titanium silicide formation sequence for evaporated Ti films on silicon substrates annealed at temperatures between 300 and 800 °C. The measurements show significant differences in the ultrasonic echo pattern before and after the structural phases C49 and C54 are formed, thus indicating that picosecond ultrasonics is a sensitive non-destructive probe of silicide formation. The longitudinal sound velocity has been found to be (8.3 ± 0.2) × 10 5 cm/sec for C49 TiSi 2 , and about 5% lower for the C54 phase.

Ultrasonic experiments at ultra-high frequency with picosecond time-resolution

A method that can be used to make ultrasonic measurements with picosecond time resolution is described. In this method, short light pulses (duration <or=1 ps) are used to generate and detect ultrasonic waves. The excellent time resolution makes possible pulse-echo ultrasonic measurements in situations in which the acoustic path length is extremely short. It is possible to make ultrasonic pulse-echo measurements on thin-film microstructures of thicknesses down to a few hundred angstroms. As examples of the use of the method, measurements of sound propagation in glass films at frequencies up to 500 GHz, studies of the mechanical properties of microstructures, and investigations of high-frequency surface waves are described.<<ETX>>

Noncontact ultrasonic ULSI process metrology using ultrafast lasers

Subpicosecond laser ultrasonic techniques have been used for nearly a decade to study the physical and chemical properties of thin films and multilayers. Recent advances in computational methods, optical design and laser technology have made it possible to develop further related techniques for thin film process metrology. In this paper we describe the principles and applications of picosecond ultrasonics, with an emphasis on in-line characterization of metal and dielectric films used in ULSI chip manufacturing. Topics to be discussed include measurements of simple metal films ranging from 50 A to 2 microns thick, and opaque multilayers consisting of up to six sequentially deposited metal films.

Detection of cobalt silicide phase formations by ultrafast optical measurements

The evolution of the phases in the cobalt–silicon reaction is studied using ultrafast optical pump-and-probe measurements. Measurements have been made on 8.5-nm thick cobalt films sputter deposited onto Si(100) substrates, and then annealed at a series of temperatures up to 950°C in helium to form Co2Si, CoSi and CoSi2. From the optical measurements, the frequency of the fundamental acoustic thickness mode of the film was determined. This frequency was used to determine the film thickness of each silicide phase, and the result is in good agreement with Rutherford back-scattering measurements. From the evolution of the damping of the oscillations, the acoustic response can be used to study the roughness of the metallic films.

Picosecond ultrasonic study of the electrical and mechanical properties of CoSi/sub 2/ formed under Ti and TiN cap layers

We report noncontact measurements of CoSi/sub 2/ layers made using a commercial picosecond ultrasonic system. The layers were formed in a two step RTP-process beginning with samples with nominally 120 /spl Aring/ Co capped with either 150 /spl Aring/ PVD TiN, or 100 /spl Aring/ PVD Ti. The thickness, roughness and electrical resistivity of the final disilicide layers were investigated as functions of the first anneal temperature. The results indicate that the TiN-capped process yields a significantly smoother and more conductive disilicide film than the Ti-capped process over a wide range of first anneal temperatures.

Recent progress in picosecond ultrasonic process metrology

We discuss the use of the picosecond ultrasonics technique for characterizing several challenging processes which are common in sub-0.18 micron processes. Specific processes considered are reactive TaN, RTP-CoSi2, and damascene copper. We describe the transition from one- to two- dimensional acoustical excitations in damascene copper lines, and show how picosecond ultrasonics measurements can be used in to make high throughput, all-optical copper line CD measurements.