New ultrafast technique for laser-based ultrasonics

The characterization of multilayered thin films via laser-based ultrasound has been a well-established technique since the late 1980s.¹ Laser-based ultrasound employs pulses from a picosecond or femtosecond laser, which are absorbed in a thin metallic or multilayer film. The heat generated close to the surface produces a strain pulse, which propagates through the material at the speed of sound. At interfaces, part of the strain pulse reflects back toward the surface. After returning to the sample surface, it can be detected with a time-delayed probe pulse. For example, in crystalline silicon a transit time of 118ps corresponds to a 1μm layer thickness. By using femtosecond lasers we can determine the thickness of layers noninvasively with subnanometer resolution. The spectrum of acoustic phonons in multilayer films is strongly modified in comparison with bulk material, which suggests that we could also probe the structural properties of multilayer films using picosecond ultrasound technology.

In the semiconductor industry, laser-based ultrasound has already been established as a standard tool for metrology during the production process.² In conventional laser-based ultrasound devices, the time delay between the pump and probe pulses is accomplished via a mechanical delay line. However, this approach has three inherent drawbacks: it moves slowly; it is always a source of mechanical vibration; and scans with a time delay longer than a nanosecond require several tens of centimeters of mechanical translation, which introduces changes in the spot size as well as introducing pointing variations of the laser spot on the sample.

We recently introduced a high-speed asynchronous optical sampling (ASOPS) technique for performing laser-based ultrasound at very high scan rates without any mechanically moving parts.^3,4 High-speed ASOPS is based on two Ti:sapphire lasers that emit femtosecond pulses at a repetition rate close to 1GHz.⁵ The repetition rate of the lasers is stabilized such that one repetition rate has an offset frequency of 10kHz with respect to the other. With one laser serving as the pump laser to create the strain pulse and the other laser operating as the probe laser, a time delay of 1ns (the inverse repetition rate) is scanned within 100μs (the inverse of 10kHz) with 10fs resolution. The basic principle of ASOPS is shown in Figure 1. Within a sampling time of 1s a signal-to-noise ratio on the order of some 10⁷ is achieved, which allows highly sensitive detection of the picosecond ultrasound pulses. The whole setup consists of a dual femtosecond laser system housed in a single 30 × 30cm box (pumped by a single green solid-state laser), the synchronization electronics, optics to focus the beams onto the sample, a detector, and a PC including an A/D converter board.

We applied ASOPS laser-based ultrasonic methods to several material systems relevant to microelectronics.⁴ One application demonstration investigated Si/Mo multilayers that were fabricated as Bragg mirrors in the extreme UV.⁶ Such multi-layers have previously been investigated with pump-probe spectroscopy based on a conventional system employing a mechanical delay line.⁷ The Bragg mirror consists of 50 Si/Mo layers with a period of 6.84nm. The ratio of the thickness of the Mo layer to the total period is 0.38. The reflectivity change of this structure after impulsive optical excitation is shown in Figure 2 for the first 150ps. The signal shows a dominant exponential contribution due to electronic relaxation that is modulated by coherent acoustic phonons within the multilayer in the first 20ps. After 110ps an acoustic echo is observed that corresponds to the round-trip time of an acoustic pulse propagating from the surface to the substrate and back. In Figure 2(b) the extracted phonon signature in the first 20ps time delay is shown. The Fourier transform—see Figure 2(c)—reveals a pronounced maximum at 1.12THz and a minimum at 1.02 THz. A second smaller feature occurs at 0.4THz. All features can be modeled and are well known to be very sensitive to the exact layer thickness and composition.⁷ The specific details of this spectrum allow the noninvasive characterization of the composition of the EUV mirrors such as the ratio of the Mo/Si thickness and the interface quality. Because we can tune the wavelength of the two lasers independently, the system also provides flexibility for spectroscopy, e.g., when investigating strongly scattering samples where stray light from the pump pulse can be suppressed with a bandpass filter in front of the detector.

Finally, we note that ASOPS can be applied to all time-resolved optical techniques. The performance of an ASOPS-based system is that of an all-optical oscilloscope with 1GHz frequency resolution and 5000GHz bandwidth. For example, we used the same laser system described here to perform high-resolution and high-sensitivity time-domain terahertz spectroscopy.⁸ The full advantage of ASOPS-based techniques will be exploited in 2D mapping applications and in situ process monitoring.

The authors acknowledge financial support through the Ministry of Science, Research, and the Arts of Baden-Wrttemberg. We thank S. Baun (Fraunhofer Institute, Dresden, Germany) for providing the extreme UV Bragg mirror.