Femtosecond laser ultrasonic inspection of a moving object and its application to estimation of silicon wafer coating thickness

https://doi.org/10.1016/j.optlaseng.2021.106778Get rights and content

Highlights

  • Femtosecond laser enables a minimized contact time with a moving target.

  • Femtosecond pump-probe setup reduces signal distortion due to target movement.

  • Both pulse-echo and pitch-catch modes are investigated on a moving target.

  • Ultrathin coating thickness of a silicon wafer is successfully estimated.

Abstract

In this study, an ultrasound generation and sensing system using a femtosecond laser is developed specifically for noncontact inspection of a moving object. In the developed femtosecond laser ultrasonic system, a laser pulse source is divided into pump and probe laser pulses. Using a pump laser pulse with a subpicosecond duration, ultrasounds with ultrashort wavelengths (micrometer to tens of nanometers) are generated up to THz. Then, the resulting ultrasounds are measured using a probe laser pulse based on reflectometry at a sampling frequency of up to 1.5 THz. The developed system is used to generate and measure ultrasounds from a silicon wafer while the wafer is moving in a horizontal direction. Because of the ultrashort pulse duration of the probe and pump laser pulses, the contact time of these pluses with respect to a moving object is extremely short (subpicosecond), and the distortion of ultrasounds due to object motion is minimized. Ultrasounds are measured from the silicon wafer in both pulse-echo and pitch-catch modes, and it is validated that the ultrasounds acquired from a moving condition of the silicon wafer are in good agreement with those obtained from a stationary condition. Then, the thickness of a submicrometer coating layer deposited on the silicon wafer was successfully estimated while the silicon wafer was moving up to 20 mm/s.

Introduction

Ultrasonic techniques are widely used for the nondestructive evaluation and testing of various engineering components and systems [1], [2], [3]. In particular, laser ultrasonic techniques are attractive for situations where installation of contact-type transducers is difficult or inefficient [4]. One such situation can be online inspection of microdevices, such as silicon wafers used in semiconductors, during manufacturing [5]. However, the applicability of conventional nanosecond laser ultrasonic techniques to online inspection of these microdevices is limited mainly for the following two reasons.

First, the conventional nanosecond laser system cannot achieve the spatial and temporal resolutions necessary for microdevice inspection. For example, a 10 ns laser pulse can generate ultrasounds up to approximately 100 MHz, and the wavelength of the generated ultrasounds becomes approximately 84 µm in silicon materials commonly used for microelectronic devices (a longitudinal wave speed of 8436 m/s in silicon [6] is used). Additionally, the highest sampling frequency that can be achieved by the conventional interferometry system is generally less than gigahertz [7] (= 1 ns sampling time interval). These spatial (84 µm wavelength) and temporal (1 ns sampling time interval) resolutions are not high enough to inspect typical modern microdevices on the submicron and nanometer scales [5,6,[8], [9], [10]].

Second, ultrasound measurement is challenging with the conventional laser ultrasonic system when the target object is in continuous movement. For example, Ridgway et al. and Walter et al. inspected the velocity and waveform of ultrasounds measured from a moving paper [11,12] by using a nanosecond Nd:YAG pulse laser for ultrasound generation and an interferometer for ultrasound detection. Here, the continuous laser used for the interferometer was scanned synchronously with the paper movement using a rotating mirror to minimize the relative movement between the interferometer laser beam and the moving paper. Park et al. detected damage in a rotating steel fan using a nanosecond Nd:YAG laser for ultrasound generation and a piezoelectric sensor attached to the blade for ultrasound detection [13]. This study shows that ultrasonic generation using a nanosecond laser is feasible, but synchronously controlling the exact excitation location on the rotating fan is challenging. Gwashavanhu et al. analyzed the vibration of a rotating blade by using a laser Doppler vibrometer (LDV) [14]. In this study, the stead-state rotating speed of the rotation blade was measured, and the laser beam of the LDV was synchronously rotated using a galvanometer to minimize the relative movement between the LDV continuous laser beam and the rotating blade.

However, even when the laser beams used for ultrasound generation or detection are synchronized with the moving object, the relative movement between the laser beams and the moving object could not be fully eliminated. Consequently, the ultrasounds measured under moving conditions exhibited much higher levels of noise than those obtained under stationary conditions. Furthermore, as long as this relative movement between the laser beams and the moving object exists, the deviation of the measured ultrasounds from that measured from the stationary condition increases as the contact time of the laser beams with the moving object increases. The employment of a femtosecond laser instead of a nanosecond laser used for ultrasonic generation and a continuous laser used for ultrasonic measurement can substantially reduce this contact time with the moving object.

A femtosecond laser system in a pump-probe setup has been used for porosity inspection [15] and measurements of thermal conductivity [16] and electron-phonon coupling [17]. Additionally, ultrasonic measurements using a femtosecond laser system have been used for estimation of the elastic constant [18] and geometry [19] and imaging of surface wave propagation [20]. However, to the best of our knowledge, the development of a femtosecond laser ultrasonic technique for ultrasonic measurement under moving conditions has not yet been accomplished.

In this paper, a femtosecond laser system for ultrasonic inspection of a moving object is developed, achieving a submicrometer spatial resolution and a subpicosecond temporal resolution. Using the proposed femtosecond laser ultrasonic system, ultrasounds were successfully generated and measured from a silicon wafer while the laser incident was fixed and the wafer was moving in a horizontal direction. Then, the coating thickness of the silicon wafer was estimated using the measured ultrasounds. The advantages of the developed system include the following: (1) The ultrashort pulse duration of the femtosecond laser minimizes the contact time of the laser pulses with the moving object and reduces signal distortion due to target object movement; (2) because the proposed femtosecond laser system can drastically shorten the duration of the measured ultrasound signal, the shifting of the probe laser beam during this time duration due to the object moving can also be minimized; and (3) Owing to its ultrahigh spatial and temporal resolutions, the developed system is attractive for ultrasonic inspection of microsystems in a moving condition, such as ultrathin coating thickness estimation of a silicon wafer presented in this study.

Section snippets

Configuration of the developed system

The overall configuration of the developed femtosecond laser ultrasonic system is shown in Fig. 1. First, the femtosecond pulse laser source is divided into pump and probe pulse lasers. The pump laser is modulated by an acousto-optic modulator (AOM), and its wavelength is shifted to half of its original wavelength using a beta barium borate (BBO) crystal so that the pump and probe laser pulses can be easily separated by an optical filter and a dichroic mirror. When the pump laser pulse arrives

Pulse-echo mode

The effectiveness of the developed femtosecond laser ultrasonic system for ultrasound generation and sensing on a moving object is examined using a silicon wafer with Au/Cr coating layers. The 675 µm thick silicon wafers with a 15.24 cm diameter were coated with a 100 nm thick Au layer and a 10 nm thick Cr layer. The ultrasounds were generated in both pitch-catch and pulse-echo modes, and the wafer was diced for test convenience, as shown in Fig. 3.

In the pulse-echo mode, a 350 fs laser pulse

Application to silicon wafer coating thickness estimation

The developed femtosecond laser ultrasonic system was used to estimate the coating (deposited Au layer) thickness of a silicon wafer while the silicon wafer was in a moving condition. Silicon wafers with three different Au thicknesses (approximately 50, 100, and 150 nm) were prepared and classified into Cases 1 to 3. The layout of these silicon wafers is identical to the previous silicon wafer described in Section 3 (Fig. 3) except for the thickness of the Au layer. The ground-truth thickness

Discussion

In Sections 3 and 4, it is shown that the distortion of the ultrasound signals measured under a moving condition was small when ultrasounds were generated and measured using the developed femtosecond laser system. The advantages of the developed femtosecond laser ultrasonic system for ultrasound generation and measurement under moving conditions are further discussed here.

Fig. 9 illustrates the shifting of the pump and probe laser pulses when the target object is moving in a horizontal

Conclusion

In this study, femtosecond laser ultrasonic inspection of a moving object was accomplished in pulse-echo and pitch-catch modes. Owing to the subpicosecond contact time of a laser pulse with the moving object and ultrahigh spatial (micrometer to tens of nanometers) and temporal (subpicosecond) resolutions, the developed system is suitable for inspection of a moving object with submicron dimensions. It is shown that ultrasound signals can be reliably measured under both stationary and up to

CRediT authorship contribution statement

Kiyoon Yi: Conceptualization, Investigation, Methodology, Writing – original draft. Peipei Liu: Validation, Data curation, Writing – review & editing. Seong-Hyun Park: Data curation. Hoon Sohn: Supervision, Writing – review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019R1A3B3067987).

References (42)

  • F. Hadj-Larbi et al.

    Sezawa SAW devices: review of numerical-experimental studies and recent applications

    Sens Actuators A Phys

    (2019)
  • Y. Gan et al.

    Thermomechanical wave propagation in gold films induced by ultrashort laser pulses

    Mech Mater

    (2010)
  • J. Jang et al.

    Silicon wafer crack detection using nonlinear ultrasonic modulation induced by high repetition rate pulse laser

    Opt Lasers Eng

    (2020)
  • C.B. Scruby et al.

    Laser ultrasonics: techniques and applications

    (1990)
  • P. Aryan et al.

    An overview of non-destructive testing methods for integrated circuit packaging inspection

    Sensors

    (2018)
  • M.K. Song et al.

    Crack detection in single-crystalline silicon wafer using laser generated lamb wave

    Adv Mater Sci Eng

    (2013)
  • P. Liu et al.

    Estimation of silicon wafer coating thickness using ultrasound generated by femtosecond laser

    J Nondestruct Eval Diagn. Progn Eng Syst

    (2020)
  • J. Yang et al.

    Laser ultrasonic technique for evaluating solder bump defects in flip chip packages using modal and signal analysis methods

    IEEE Trans Ultrason Ferroelectr Freq Control

    (2010)
  • Q. Shan et al.

    Surface-breaking fatigue crack detection using laser ultrasound

    Appl Phys Lett

    (1993)
  • F Lefevre et al.

    Laser generated guided waves and finite element modeling for the thickness gauging of thin layers

    Rev Sci Instrum

    (2010)
  • JB Walter et al.

    Fabry-perot laser ultrasonic elastic anisotropy measurements on a moving paper web

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