- Email: [email protected]
Laser-generated surface acoustic waves in a ring-shaped waveguide resonator
Ultrasonics 49 (2009) 1–3
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short Communication
Laser-generated surface acoustic waves in a ring-shaped waveguide resonator A.A. Maznev 1 Philips Advanced Metrology Systems, 12 Michigan Drive, Natick MA 01760, United States
a r t i c l e
i n f o
Article history: Received 11 February 2008 Received in revised form 15 April 2008 Accepted 22 April 2008 Available online 29 April 2008 PACS: 43.35.+d 68.35.Iv
a b s t r a c t Surface acoustic wave (SAW) waveguide resonator is formed by a ring-shaped strip of copper 10 lm wide and 130 lm in diameter embedded into a 0.8 lm thick layer of silica on a silicon wafer. SAWs are excited at one side of the copper ring by a short laser pulse focused into a spatially periodic pattern and detected via diffraction of the probe laser beam overlapped with the excitation spot. SAW wavepackets with central frequency 460 MHz travel around the ring and are detected each time they make a full circle and pass trough the probe spot. Potential applications of ring resonators for SAWs are discussed. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Surface acoustic waves Laser ultrasonics Transient gratings SAW waveguide
Laser-generated surface acoustic waves (SAWs) have attracted considerable interest as a characterization tool for solid surfaces and thin films [1,2]. While a significant amount of work has been done on laser-generated SAWs on ‘‘blanket” thin films, studies involving more complex, ‘‘patterned” thin film microstructures are attracting increased attention [3–5]. SAW waveguides formed, for example, by a strip of a ‘‘slow” material deposited onto or embedded into a ‘‘fast” substrate are well known [6]. A ring-shaped SAW waveguide would naturally form a ring resonator. To the author’s knowledge, SAW resonators of this kind have not been studied previously, although spiralshaped waveguides have been proposed for delay line applications [7]. It should be mentioned that a resonator comprising a ringshaped interdigital transducer has been recently proposed [8] but not yet tested experimentally. This paper reports on laser generation and detection of SAWs in a circular waveguide formed by a microscopic copper ring embedded into a silica film on a silicon wafer. The sample manufactured by International SEMATECH contains various copper structures (mostly line patterns) embedded in a 0.8 lm SiO2 film on a (1 0 0) Si substrate. One structure found on the wafer is an array of 10 lm-wide copper lines with 20 lm spaces between the lines. Copper has significantly lower acoustic velocities compared to silica, therefore a copper strip embedded into a silica film should serve as a waveguide for SAWs. Another structure shown in Fig. 1a is a 10 lm wide ring-shaped strip, with the external diamE-mail address: [email protected] Present address: Laboratoire d’Acoustique, Université du Maine, 72085 Le Mans, France. 1
0041-624X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2008.04.007
eter of the ring 144 lm. This ring structure is part of a lettering, but it is fabricated in the same way as other structures on the wafer. Thus we have both linear and circular waveguides with the same cross-section (see Fig. 1b). Laser generation and detection of SAWs was performed using a technique referred to as transient gratings or Impulsive Stimulated Thermal Scattering (ISTS) [1]. Measurements were done with a commercially available thin film metrology system manufactured by Philips AMS. The optical apparatus is described in detail in Ref. [9]. In short, SAWs are generated by two crossed laser pulses (duration 0.5 ns, wavelength 532 nm, pulse energy 1 lJ) forming a spatially periodic interference pattern at the sample, with the period of the pattern defining the SAW wavelength. Surface ‘‘ripples” caused by SAWs are detected via diffraction of the probe beam focused at the center of the excitation pattern, with detection enhanced by optical heterodyning. The excitation spot size is 200x35 lm, and the probe spot size is 35 17 lm. Fig. 2 shows signal waveforms obtained from the linear and ring-shaped waveguides at the excitation pattern period 7.7 lm. The slowly decaying component of the signal is due to the ‘‘thermal grating”, i.e. surface displacement associated with the spatially periodic temperature profile. Fast oscillations are due to counterpropagating SAW wavepackets passing through the probe spot. Note that the signal from the linearwaveguide contains more oscillations, because the excitation spot overlaps with the linear copper strip over a greater length and, consequently, longer SAW wavepackets are generated. After the ‘‘thermal grating” decays via thermal diffusion and SAWs leave the probing area, the signal from the linear waveguide
2
A.A. Maznev / Ultrasonics 49 (2009) 1–3
a
a Probe spot
signal (a.u.)
Excitation spot
-100
100
300
500
700
900
time (ns)
Ring waveguide
b
b
signal (a.u.)
100 μm
10 μm SiO2
Cu
0.8 μm
-10
10
30
50
70
180
200
time (ns)
Si substrate
c
Df ¼ ð2p=LÞdf =dk ¼ Vg =L;
ð1Þ
where L is the ring length, k is the wavenumber and Vg = dx/dk is the group velocity of the waveguide mode. From the spectrum presented in Fig. 3 we find that Df = 6.23 MHz. The median diameter of the ring is 134 lm which yields L = 421 lm, and, consequently, Vg = 2620 m/s. The phase velocity, on the other hand, can be estimated from the central frequency of the spectrum and the known
120
140
160
time (ns) Fig. 2. (a) Signal waveforms from the linear (top trace) and ring (bottom trace) waveguide structures. (b) and (c) show parts of the same waveforms on a finer time scale.
normalized FFT power (a.u.)
vanishes. However, in the ring waveguide SAW wavepackets return to the probe spot location each time they make a full circle about the ring. Note that the grating excitation yields two wavepackets traveling around the ring in the clockwise and counterclockwise directions. After each round trip, the wavepackets overlap at the probe spot location recreating the standing wave which is detected by the probe beam. Fig. 2c shows an enlarged view of the SAW signal after the first round trip. The decay time of the acoustic oscillations in the ring is 440 ns. To what degree this decay is caused by radiative losses is a question for further studies. Attenuation of SAWs in polycrystalline Cu films is known to be quite large and strongly dependent on the microstructure [11]. Selecting a different material for the waveguide may significantly decrease the losses in the resonator. Fig. 3 shows spectra of acoustic oscillations in the waveguides. The spectrum from the linear waveguide contains a single broad peak with the central frequency 460 MHz determined by the period of the excitation pattern and the SAW phase velocity. The ring waveguide spectrum consists of narrow peaks corresponding to the eigenmodes of the resonator. The central frequency is nearly the same as for the straight waveguide indicating that the waveguide curvature does not have a significant effect on the SAW velocity. The eigenmode frequency is determined by the requirement that the length of the ring be a multiple of the acoustic wavelength, and thus the frequency separation between the modes is given by
signal (a.u.)
Fig. 1. (a) Photograph of the ring waveguide structure with schematically shown excitation and probe laser spots. (b) Cross-section of the waveguide (not to scale).
1
0.5
0 400
420
440
460
480
500
520
frequency (MHz) Fig. 3. Power spectrum of acoustic oscillations in the ring waveguide (solid line) and the linear waveguide (dotted line).
acoustic wavelength of 7.7 lm which yields Vf = 3540 m/s. Theoretical estimates based on calculations for a ‘‘blanket” 0.8 lm Cu film on Si yield Vf = 3470 m/s and Vg = 2690 m/s. Thus the waveguide mode is still fairly similar to a SAW on the unpatterned Cu film.
A.A. Maznev / Ultrasonics 49 (2009) 1–3
The fact that the ring resonator yields narrow frequency peaks points to potential applications involving precise measurements of the SAW frequency. For example, one application of laser-generated SAWs is controlling thickness of Cu interconnects used in the integrated circuit manufacturing [1,10]. Using ring-shaped test structures could significantly increase the precision of the measurements compared, for example, to a square-shaped test box of the same size. Precise attenuation measurements may prove useful for diagnostics of copper microstructure [11]. Ring resonators may also find applications in electrically-driven SAW devices [8]. The author believes that more applications can be found and hopes that this study will attract interest of researches in various SAW-related fields to ring-shaped resonator structures. References [1] J.A. Rogers, A.A. Maznev, M.J. Banet, K.A. Nelson, Ann. Rev. Mat. Sci. 30 (2000) 117.
3
[2] A. Lomonosov, A.P. Mayer, P. Hess, in: M. Levy, H. Bass, R. Stern (Eds.), Modern Acoustical Techniques for the Measurement of Mechanical Properties, Academic Press, San Diego, 2001, p. 65. [3] L. Dhar, J.A. Rogers, Appl. Phys. Lett. 77 (2000) 1402. [4] Y. Sugawara, O.B. Wright, O. Matsuda, Rev. Sci. Instrum. 74 (2003) 519. [5] M. Profunser, O.B. Wright, O. Matsuda, Phys. Rev. Lett. 97 (2006) 055502. [6] A.A. Oliner, in: A.A. Oliner (Ed.), Acoustic Surface Waves, Springer, Berlin, 1978, p. 187. [7] L.R. Adkins, A.J. Hughes, J. Appl. Phys. 42 (1971) 1819. [8] S.V. Biryukov, G. Martin, M. Weihnacht, Appl. Phys. Lett. 90 (2007) 173503. [9] A.A. Maznev, A. Mazurenko, L. Zhuoyun, M. Gostein, Rev. Sci. Instrum. 74 (2003) 667. [10] M. Gostein, M. Banet, M. Joffe, A.A. Maznev, R. Sacco, J.R. Rogers, K.A. Nelson, in: A.C. Diebold (Ed.), Handbook of Silicon Semiconductor Metrology, Marcel Dekker, New York, 2001, p. 167. [11] A.A. Maznev, A. Mazurenko, M. Gostein, G. Alper, J. Tower, R. Carpio, in: Proceedings of 2004 IEEE/SEMI Advanced Semiconductor Manufacturing Conference, IEEE, Boston, 2004, p. 477.
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Short Communication
Laser-generated surface acoustic waves in a ring-shaped waveguide resonator A.A. Maznev 1 Philips Advanced Metrology Systems, 12 Michigan Drive, Natick MA 01760, United States
a r t i c l e
i n f o
Article history: Received 11 February 2008 Received in revised form 15 April 2008 Accepted 22 April 2008 Available online 29 April 2008 PACS: 43.35.+d 68.35.Iv
a b s t r a c t Surface acoustic wave (SAW) waveguide resonator is formed by a ring-shaped strip of copper 10 lm wide and 130 lm in diameter embedded into a 0.8 lm thick layer of silica on a silicon wafer. SAWs are excited at one side of the copper ring by a short laser pulse focused into a spatially periodic pattern and detected via diffraction of the probe laser beam overlapped with the excitation spot. SAW wavepackets with central frequency 460 MHz travel around the ring and are detected each time they make a full circle and pass trough the probe spot. Potential applications of ring resonators for SAWs are discussed. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Surface acoustic waves Laser ultrasonics Transient gratings SAW waveguide
Laser-generated surface acoustic waves (SAWs) have attracted considerable interest as a characterization tool for solid surfaces and thin films [1,2]. While a significant amount of work has been done on laser-generated SAWs on ‘‘blanket” thin films, studies involving more complex, ‘‘patterned” thin film microstructures are attracting increased attention [3–5]. SAW waveguides formed, for example, by a strip of a ‘‘slow” material deposited onto or embedded into a ‘‘fast” substrate are well known [6]. A ring-shaped SAW waveguide would naturally form a ring resonator. To the author’s knowledge, SAW resonators of this kind have not been studied previously, although spiralshaped waveguides have been proposed for delay line applications [7]. It should be mentioned that a resonator comprising a ringshaped interdigital transducer has been recently proposed [8] but not yet tested experimentally. This paper reports on laser generation and detection of SAWs in a circular waveguide formed by a microscopic copper ring embedded into a silica film on a silicon wafer. The sample manufactured by International SEMATECH contains various copper structures (mostly line patterns) embedded in a 0.8 lm SiO2 film on a (1 0 0) Si substrate. One structure found on the wafer is an array of 10 lm-wide copper lines with 20 lm spaces between the lines. Copper has significantly lower acoustic velocities compared to silica, therefore a copper strip embedded into a silica film should serve as a waveguide for SAWs. Another structure shown in Fig. 1a is a 10 lm wide ring-shaped strip, with the external diamE-mail address: [email protected] Present address: Laboratoire d’Acoustique, Université du Maine, 72085 Le Mans, France. 1
0041-624X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2008.04.007
eter of the ring 144 lm. This ring structure is part of a lettering, but it is fabricated in the same way as other structures on the wafer. Thus we have both linear and circular waveguides with the same cross-section (see Fig. 1b). Laser generation and detection of SAWs was performed using a technique referred to as transient gratings or Impulsive Stimulated Thermal Scattering (ISTS) [1]. Measurements were done with a commercially available thin film metrology system manufactured by Philips AMS. The optical apparatus is described in detail in Ref. [9]. In short, SAWs are generated by two crossed laser pulses (duration 0.5 ns, wavelength 532 nm, pulse energy 1 lJ) forming a spatially periodic interference pattern at the sample, with the period of the pattern defining the SAW wavelength. Surface ‘‘ripples” caused by SAWs are detected via diffraction of the probe beam focused at the center of the excitation pattern, with detection enhanced by optical heterodyning. The excitation spot size is 200x35 lm, and the probe spot size is 35 17 lm. Fig. 2 shows signal waveforms obtained from the linear and ring-shaped waveguides at the excitation pattern period 7.7 lm. The slowly decaying component of the signal is due to the ‘‘thermal grating”, i.e. surface displacement associated with the spatially periodic temperature profile. Fast oscillations are due to counterpropagating SAW wavepackets passing through the probe spot. Note that the signal from the linearwaveguide contains more oscillations, because the excitation spot overlaps with the linear copper strip over a greater length and, consequently, longer SAW wavepackets are generated. After the ‘‘thermal grating” decays via thermal diffusion and SAWs leave the probing area, the signal from the linear waveguide
2
A.A. Maznev / Ultrasonics 49 (2009) 1–3
a
a Probe spot
signal (a.u.)
Excitation spot
-100
100
300
500
700
900
time (ns)
Ring waveguide
b
b
signal (a.u.)
100 μm
10 μm SiO2
Cu
0.8 μm
-10
10
30
50
70
180
200
time (ns)
Si substrate
c
Df ¼ ð2p=LÞdf =dk ¼ Vg =L;
ð1Þ
where L is the ring length, k is the wavenumber and Vg = dx/dk is the group velocity of the waveguide mode. From the spectrum presented in Fig. 3 we find that Df = 6.23 MHz. The median diameter of the ring is 134 lm which yields L = 421 lm, and, consequently, Vg = 2620 m/s. The phase velocity, on the other hand, can be estimated from the central frequency of the spectrum and the known
120
140
160
time (ns) Fig. 2. (a) Signal waveforms from the linear (top trace) and ring (bottom trace) waveguide structures. (b) and (c) show parts of the same waveforms on a finer time scale.
normalized FFT power (a.u.)
vanishes. However, in the ring waveguide SAW wavepackets return to the probe spot location each time they make a full circle about the ring. Note that the grating excitation yields two wavepackets traveling around the ring in the clockwise and counterclockwise directions. After each round trip, the wavepackets overlap at the probe spot location recreating the standing wave which is detected by the probe beam. Fig. 2c shows an enlarged view of the SAW signal after the first round trip. The decay time of the acoustic oscillations in the ring is 440 ns. To what degree this decay is caused by radiative losses is a question for further studies. Attenuation of SAWs in polycrystalline Cu films is known to be quite large and strongly dependent on the microstructure [11]. Selecting a different material for the waveguide may significantly decrease the losses in the resonator. Fig. 3 shows spectra of acoustic oscillations in the waveguides. The spectrum from the linear waveguide contains a single broad peak with the central frequency 460 MHz determined by the period of the excitation pattern and the SAW phase velocity. The ring waveguide spectrum consists of narrow peaks corresponding to the eigenmodes of the resonator. The central frequency is nearly the same as for the straight waveguide indicating that the waveguide curvature does not have a significant effect on the SAW velocity. The eigenmode frequency is determined by the requirement that the length of the ring be a multiple of the acoustic wavelength, and thus the frequency separation between the modes is given by
signal (a.u.)
Fig. 1. (a) Photograph of the ring waveguide structure with schematically shown excitation and probe laser spots. (b) Cross-section of the waveguide (not to scale).
1
0.5
0 400
420
440
460
480
500
520
frequency (MHz) Fig. 3. Power spectrum of acoustic oscillations in the ring waveguide (solid line) and the linear waveguide (dotted line).
acoustic wavelength of 7.7 lm which yields Vf = 3540 m/s. Theoretical estimates based on calculations for a ‘‘blanket” 0.8 lm Cu film on Si yield Vf = 3470 m/s and Vg = 2690 m/s. Thus the waveguide mode is still fairly similar to a SAW on the unpatterned Cu film.
A.A. Maznev / Ultrasonics 49 (2009) 1–3
The fact that the ring resonator yields narrow frequency peaks points to potential applications involving precise measurements of the SAW frequency. For example, one application of laser-generated SAWs is controlling thickness of Cu interconnects used in the integrated circuit manufacturing [1,10]. Using ring-shaped test structures could significantly increase the precision of the measurements compared, for example, to a square-shaped test box of the same size. Precise attenuation measurements may prove useful for diagnostics of copper microstructure [11]. Ring resonators may also find applications in electrically-driven SAW devices [8]. The author believes that more applications can be found and hopes that this study will attract interest of researches in various SAW-related fields to ring-shaped resonator structures. References [1] J.A. Rogers, A.A. Maznev, M.J. Banet, K.A. Nelson, Ann. Rev. Mat. Sci. 30 (2000) 117.
3
[2] A. Lomonosov, A.P. Mayer, P. Hess, in: M. Levy, H. Bass, R. Stern (Eds.), Modern Acoustical Techniques for the Measurement of Mechanical Properties, Academic Press, San Diego, 2001, p. 65. [3] L. Dhar, J.A. Rogers, Appl. Phys. Lett. 77 (2000) 1402. [4] Y. Sugawara, O.B. Wright, O. Matsuda, Rev. Sci. Instrum. 74 (2003) 519. [5] M. Profunser, O.B. Wright, O. Matsuda, Phys. Rev. Lett. 97 (2006) 055502. [6] A.A. Oliner, in: A.A. Oliner (Ed.), Acoustic Surface Waves, Springer, Berlin, 1978, p. 187. [7] L.R. Adkins, A.J. Hughes, J. Appl. Phys. 42 (1971) 1819. [8] S.V. Biryukov, G. Martin, M. Weihnacht, Appl. Phys. Lett. 90 (2007) 173503. [9] A.A. Maznev, A. Mazurenko, L. Zhuoyun, M. Gostein, Rev. Sci. Instrum. 74 (2003) 667. [10] M. Gostein, M. Banet, M. Joffe, A.A. Maznev, R. Sacco, J.R. Rogers, K.A. Nelson, in: A.C. Diebold (Ed.), Handbook of Silicon Semiconductor Metrology, Marcel Dekker, New York, 2001, p. 167. [11] A.A. Maznev, A. Mazurenko, M. Gostein, G. Alper, J. Tower, R. Carpio, in: Proceedings of 2004 IEEE/SEMI Advanced Semiconductor Manufacturing Conference, IEEE, Boston, 2004, p. 477.