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Ion acoustic microscopy for imaging of buried structures based on a focused ion beam system
Microelectronic Engineering 57–58 (2001) 659–664 www.elsevier.com / locate / mee
Ion acoustic microscopy for imaging of buried structures based on a focused ion beam system a, a a b Ch. Akhmadaliev *, L. Bischoff , J. Teichert , K. Kazbekov a
Research Center Rossendorf Inc., Institute of Ion Beam Physics and Materials Research, P.O. Box 51 01 19, D-01314 Dresden, Germany b Kazakh National Technical University, Satpaev str. 22, 480000 Almaty, Kazakhstan
Abstract An intensity modulated focused ion beam (FIB) which is hitting the surface of a solid, leads to a temperature variation in the near subsurface region which is sufficient for thermal elastic wave generation. The measurement of these waves can be used for analysis and imaging of surface as well as subsurface structures in the material. The modified FIB equipment IMSA-100 working with 35 keV Ga 1 and Au 1 ions, and a current of about 3 nA was employed to obtain acoustic images from structures on silicon and glass targets. The acoustic signals were detected using a lead zirconium titanate (PZT) ceramic transducer delivering an output voltage of 50–100 nV. The modulation frequency was varied in the range of 60–170 kHz. The obtained lateral resolution of the ion acoustic images at these frequencies was about 15 mm on silicon and about 7 mm on glass, respectively. 2001 Elsevier Science B.V. All rights reserved. Keywords: Focused ion beam; Ion acoustic microscopy; Piezoelectric transducer
1. Introduction In recent decades scanning acoustic microscopy has become a very useful tool for non-destructive characterization and imaging of surface and sub-surface material properties. The excitation of thermal elastic waves in the material can be obtained by modulated beams like microwaves [1], pulsed lasers [2], or particles such as electrons [3] or ions [4,5]. For the detection of these waves several techniques were developed like the modulation of an optical beam caused by the time- and temperaturedependent change of the refractive index, or the optical interferometry which uses thermally induced surface deformation. A further technique employs a piezoelectric transducer to measure the thermoelastic response for imaging. A broad review is given elsewhere [6]. Each of these methods has its characteristic pros and contras, like a limited resolution due to the energy dissipation volume in the case of scanning electron acoustic microscopy (SEAM). Our approach is now to combine the * Corresponding author. E-mail address: [email protected] (Ch. Akhmadaliev). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00459-2
2001 Elsevier Science B.V. All rights reserved.
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
660
scanning acoustic microscopy (SAM) with a high resolution excitation tool, a focused ion beam (FIB), to a scanning ion acoustic microscope (SIAM). An advantage is that one has an analyzing and a modifying tool in one piece of equipment. Some problems arise from the small acoustic signal output due to the excitation power of only some hundred mW.
2. Theoretical estimations There are many processes giving rise to acoustic waves in solids during focused ion beam bombardment of the surface. The main reason for acoustic waves is periodical heating of a surface of the sample. Generation and propagation of acoustic waves in a homogeneous isotropic medium induced by heating can be described by the following equation [7]: ≠2 ¢ ¢ 2 3Bath =T ¢ 5r ] mDu¢ 1 ( l 1 m )=divu u¢ ≠t 2
(1)
where u¢ is the vector of displacement, r is the density of the material, a is the thermal expansion coefficient, m and l are the Lame´ coefficients, B 5 l 1 ]23 m is the bulk modulus of elasticity of the ¢ t) in the sample can be calculated using the thermal material. The temperature distribution T(r, diffusion equation ≠ ¢ t) 2 kDT 1 Cr ] T 5 Q(r, ≠t
(2)
¢ t) is the heating power density, k is the coefficient of thermal conductivity, r is the where Q(r, material density, C is the specific heat. The obtainable resolution d of the acoustic microscope consists of three main components — the excitation beam spot size d s , the diameter of the energy dissipation volume d v , and the thermal diffusion length d th : ]]]] d 5œd 2s 1 d v2 1 d 2th .
(3)
In the case of electron beam excitation the penetration depth is several microns. The energy dissipation diameter for 35 keV electrons in silicon amounts to about 12 mm. Therefore, in spite of the very small spot size the SEAM technique has a limited resolution. In the case of applying pulsed laser beams the limiting factor is the spot size of about 1 mm. To overcome these problems this work deals with the fundamentals of an SIAM based on a FIB system. The penetration depth of 35 keV heavy ions like gallium or gold in silicon is about 30 to 50 nm only. The straggling of the ions in the cascade is in the same order of magnitude, i.e. the energy dissipation volume can be neglected. The spot size of an FIB is usually about 100 nm operating with a sufficient current for SIAM application. So the thermal diffusion length is the resolution determining parameter which depends on material properties as well as, very sensitively, on the beam modulation frequency and is given as: ]] k d th 5 ]] p Cr f
œ
(4)
where f is the beam pulse frequency. The thermal diffusion length decreases by increasing the beam
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
661
Fig. 1. Thermal diffusion length as a function of ion beam pulse frequency for different materials.
pulse frequency. In the case of a silicon sample a submicron range of the thermal diffusion length can be reached at beam modulating frequencies of about 20 MHz which is shown for three different materials in Fig. 1.
3. Experimental To prove that the FIB is suited for acoustic wave generation, first experiments were carried out some time ago [5]. The main part of the equipment is the focused ion beam system IMSA-100 [8]. For investigations of the ion acoustic effect Ga 1 and Au 1 ions with an energy of 35 keV and a current of about 3 nA were used. Due to the high current needed and the frequency limited electronics a sufficient spot size of about 1 mm was used. The FIB could be pulsed only up to 170 kHz with a duty factor of 1:1. The acoustic waves were detected using a piezoelectric transducer with an integrated preamplifier. This transducer is a plate of lead zirconium titanate (PZT) ceramic (19 mm30.5 mm) glued on a 0.3-mm-thick brass plate. The amplification factor of the preamplifier is about 55,000. In order to obtain the best condition the pulse frequency of the ion beam was chosen equal to the resonance frequency of the two investigated PZT plates at 80 kHz, 85 kHz and 61 kHz, 169 kHz, respectively. For the experiments a silicon chip glued on the top of the brass plate was investigated. A silver powder containing glue was used to obtain a good electrical contact between the sample and the base in order to minimize charging of the sample. This system showed a resonance frequency of about 80 kHz. Due to the low signal it was necessary to decrease the scanning velocity of the ion beam. This leads to a compression of the signal spectrum. In this way the signal to noise ratio can be improved
662
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
using a lock-in technique with a large time constant of the output low-pass filter. The signal to noise ratio also depends on the total scanning time T as S / N~T 0.5 . The amplitude of the signal also depends on the acoustic properties of the glue used for sample fixing.
4. Results and discussion Fig. 2 represents an image of a corner of a chip obtained using the secondary electron imaging system (a) and the ion acoustic effect (b, c). An Au 1 ion beam of 35 keV with a current of 2.6 nA and a beam blanker frequency of 80 kHz was used. The lateral resolution of these acoustic images (1003100 pixel) is about 20 mm on silicon which corresponds to the calculated thermal diffusion length of about 15–20 mm (see Fig. 1). The total scanning time was about 40 min for each acoustic image. Due to the thickness of the silicon chip of about 0.2 mm the difference between the acoustic signal from the brass plate and from the sample was quite large. The weak structure at the top of the picture is caused by the glue. The dissipation of acoustic waves in the soft material of the glue is high and leads to a large difference of the signals. A better quality of the phase image can be seen here. Fig. 3 shows a transistor structure obtained using secondary electron (a) and ion acoustic amplitude (b) and phase (c) signals. The SE images reveal that there is no visible damage caused by the acoustic measurement. The beam parameters were the same as in the previous imaging. The total scan time was about 1 h in the case of the amplitude image and 2 h 40 min in the case of the phase image. There are some differences between phase and amplitude pictures. In the latter the transistor is darker. This means that in the material the sound velocity is nearly the same as in the silicon base but the energy dissipation of the acoustic waves is higher. The thickness of the implanted structure is less than a micrometre but the thermal diffusion length in silicon at a beam pulse frequency of about 80 kHz is nearly 15–20 mm and so in the same range as the structure size. Fig. 4 shows an acoustic image of two vertical scratches on the top of a glass sample. A horizontal scratch on the opposite surface with
Fig. 2. The secondary electron image (a), the amplitude (b) and phase (c) ion acoustic images of a silicon chip corner at a resonance frequency of the detector of 86 kHz. The size of the images is 1003100 pixels corresponding to 3003300 mm 2 . The thickness of the chip is about 200 mm. The scanning time was 250 ms / pixel (b, c).
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
663
Fig. 3. Secondary electron (a) and ion acoustic amplitude (b) and phase (c) images of a transistor on a chip. The images are similar in size to Fig. 2.
lower intensity is also visible which demonstrates the evidence of buried structures. The thickness of the glass sample was 100 mm. The resolution was determined to be 7 mm.
5. Conclusions The results show that a FIB system combined with a piezo-electric sensor allows to generate and detect acoustic signals from surface and sub-surface structures, like buried paths or material defects with a lateral resolution in the mm-range up to now. It can be used for finding as well as for analyzing of buried features applying the well-known advantages of an FIB. A further improvement of the
Fig. 4. Acoustic image of two vertical scratches on the top surface of a glass sample. A horizontal scratch on the opposite surface is also visible here. The thickness of the glass sample was 100 mm. The resolution was determined to be 7 mm.
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Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
resolution of the ion-acoustic microscope requires to optimize the sensor geometry, to increase the beam pulse frequency as well as the application of a related high frequency and low noise registration electronics. Acknowledgements The authors would like to thank the Deutsche Forschungsgemeinschaft for financial support of the ¨ work (contract no. BI 503 / 7-1), and Dr. B. Kohler from the Fraunhofer-Institute for Nondestructive Testing, Dresden for helpful discussions and suggestions. References [1] [2] [3] [4] [5] [6]
A.L. Tronconi, M.A. Amato, P.C. Morais, K.S. Neto, J. Appl. Phys. 56 (5) (1984) 1462. K.L. Muratikov, A.L. Glazov, D.N. Rose, J.E. Dumar, J. Appl. Phys. 88 (3) (2000) 2948. L.J. Bulk, Adv. Electron. Electron Phys. 71 (1988) 1. D.N. Rose, H. Turner, K.O. Legg, Can. J. Phys. 64 (1986) 1284. ¨ J. Teichert, L. Bischoff, B. Kohler, Appl. Phys. Lett. 69 (11) (1996) 1544. J.C. Murphy, J.W. Maclachlan, L.C. Aamodt, IEEE Trans. Ultrason. Ferroelectrics Freq. Cont. UFFC-33 (5) (1986) 529. [7] R.M. White, J. Appl. Phys. 34 (12) (1963) 3559. [8] L. Bischoff, J. Teichert, E. Hesse, D. Panknin, W. Skorupa, J. Vac. Sci. Technol. B 12 (6) (1994) 3523.
Ion acoustic microscopy for imaging of buried structures based on a focused ion beam system a, a a b Ch. Akhmadaliev *, L. Bischoff , J. Teichert , K. Kazbekov a
Research Center Rossendorf Inc., Institute of Ion Beam Physics and Materials Research, P.O. Box 51 01 19, D-01314 Dresden, Germany b Kazakh National Technical University, Satpaev str. 22, 480000 Almaty, Kazakhstan
Abstract An intensity modulated focused ion beam (FIB) which is hitting the surface of a solid, leads to a temperature variation in the near subsurface region which is sufficient for thermal elastic wave generation. The measurement of these waves can be used for analysis and imaging of surface as well as subsurface structures in the material. The modified FIB equipment IMSA-100 working with 35 keV Ga 1 and Au 1 ions, and a current of about 3 nA was employed to obtain acoustic images from structures on silicon and glass targets. The acoustic signals were detected using a lead zirconium titanate (PZT) ceramic transducer delivering an output voltage of 50–100 nV. The modulation frequency was varied in the range of 60–170 kHz. The obtained lateral resolution of the ion acoustic images at these frequencies was about 15 mm on silicon and about 7 mm on glass, respectively. 2001 Elsevier Science B.V. All rights reserved. Keywords: Focused ion beam; Ion acoustic microscopy; Piezoelectric transducer
1. Introduction In recent decades scanning acoustic microscopy has become a very useful tool for non-destructive characterization and imaging of surface and sub-surface material properties. The excitation of thermal elastic waves in the material can be obtained by modulated beams like microwaves [1], pulsed lasers [2], or particles such as electrons [3] or ions [4,5]. For the detection of these waves several techniques were developed like the modulation of an optical beam caused by the time- and temperaturedependent change of the refractive index, or the optical interferometry which uses thermally induced surface deformation. A further technique employs a piezoelectric transducer to measure the thermoelastic response for imaging. A broad review is given elsewhere [6]. Each of these methods has its characteristic pros and contras, like a limited resolution due to the energy dissipation volume in the case of scanning electron acoustic microscopy (SEAM). Our approach is now to combine the * Corresponding author. E-mail address: [email protected] (Ch. Akhmadaliev). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00459-2
2001 Elsevier Science B.V. All rights reserved.
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
660
scanning acoustic microscopy (SAM) with a high resolution excitation tool, a focused ion beam (FIB), to a scanning ion acoustic microscope (SIAM). An advantage is that one has an analyzing and a modifying tool in one piece of equipment. Some problems arise from the small acoustic signal output due to the excitation power of only some hundred mW.
2. Theoretical estimations There are many processes giving rise to acoustic waves in solids during focused ion beam bombardment of the surface. The main reason for acoustic waves is periodical heating of a surface of the sample. Generation and propagation of acoustic waves in a homogeneous isotropic medium induced by heating can be described by the following equation [7]: ≠2 ¢ ¢ 2 3Bath =T ¢ 5r ] mDu¢ 1 ( l 1 m )=divu u¢ ≠t 2
(1)
where u¢ is the vector of displacement, r is the density of the material, a is the thermal expansion coefficient, m and l are the Lame´ coefficients, B 5 l 1 ]23 m is the bulk modulus of elasticity of the ¢ t) in the sample can be calculated using the thermal material. The temperature distribution T(r, diffusion equation ≠ ¢ t) 2 kDT 1 Cr ] T 5 Q(r, ≠t
(2)
¢ t) is the heating power density, k is the coefficient of thermal conductivity, r is the where Q(r, material density, C is the specific heat. The obtainable resolution d of the acoustic microscope consists of three main components — the excitation beam spot size d s , the diameter of the energy dissipation volume d v , and the thermal diffusion length d th : ]]]] d 5œd 2s 1 d v2 1 d 2th .
(3)
In the case of electron beam excitation the penetration depth is several microns. The energy dissipation diameter for 35 keV electrons in silicon amounts to about 12 mm. Therefore, in spite of the very small spot size the SEAM technique has a limited resolution. In the case of applying pulsed laser beams the limiting factor is the spot size of about 1 mm. To overcome these problems this work deals with the fundamentals of an SIAM based on a FIB system. The penetration depth of 35 keV heavy ions like gallium or gold in silicon is about 30 to 50 nm only. The straggling of the ions in the cascade is in the same order of magnitude, i.e. the energy dissipation volume can be neglected. The spot size of an FIB is usually about 100 nm operating with a sufficient current for SIAM application. So the thermal diffusion length is the resolution determining parameter which depends on material properties as well as, very sensitively, on the beam modulation frequency and is given as: ]] k d th 5 ]] p Cr f
œ
(4)
where f is the beam pulse frequency. The thermal diffusion length decreases by increasing the beam
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
661
Fig. 1. Thermal diffusion length as a function of ion beam pulse frequency for different materials.
pulse frequency. In the case of a silicon sample a submicron range of the thermal diffusion length can be reached at beam modulating frequencies of about 20 MHz which is shown for three different materials in Fig. 1.
3. Experimental To prove that the FIB is suited for acoustic wave generation, first experiments were carried out some time ago [5]. The main part of the equipment is the focused ion beam system IMSA-100 [8]. For investigations of the ion acoustic effect Ga 1 and Au 1 ions with an energy of 35 keV and a current of about 3 nA were used. Due to the high current needed and the frequency limited electronics a sufficient spot size of about 1 mm was used. The FIB could be pulsed only up to 170 kHz with a duty factor of 1:1. The acoustic waves were detected using a piezoelectric transducer with an integrated preamplifier. This transducer is a plate of lead zirconium titanate (PZT) ceramic (19 mm30.5 mm) glued on a 0.3-mm-thick brass plate. The amplification factor of the preamplifier is about 55,000. In order to obtain the best condition the pulse frequency of the ion beam was chosen equal to the resonance frequency of the two investigated PZT plates at 80 kHz, 85 kHz and 61 kHz, 169 kHz, respectively. For the experiments a silicon chip glued on the top of the brass plate was investigated. A silver powder containing glue was used to obtain a good electrical contact between the sample and the base in order to minimize charging of the sample. This system showed a resonance frequency of about 80 kHz. Due to the low signal it was necessary to decrease the scanning velocity of the ion beam. This leads to a compression of the signal spectrum. In this way the signal to noise ratio can be improved
662
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
using a lock-in technique with a large time constant of the output low-pass filter. The signal to noise ratio also depends on the total scanning time T as S / N~T 0.5 . The amplitude of the signal also depends on the acoustic properties of the glue used for sample fixing.
4. Results and discussion Fig. 2 represents an image of a corner of a chip obtained using the secondary electron imaging system (a) and the ion acoustic effect (b, c). An Au 1 ion beam of 35 keV with a current of 2.6 nA and a beam blanker frequency of 80 kHz was used. The lateral resolution of these acoustic images (1003100 pixel) is about 20 mm on silicon which corresponds to the calculated thermal diffusion length of about 15–20 mm (see Fig. 1). The total scanning time was about 40 min for each acoustic image. Due to the thickness of the silicon chip of about 0.2 mm the difference between the acoustic signal from the brass plate and from the sample was quite large. The weak structure at the top of the picture is caused by the glue. The dissipation of acoustic waves in the soft material of the glue is high and leads to a large difference of the signals. A better quality of the phase image can be seen here. Fig. 3 shows a transistor structure obtained using secondary electron (a) and ion acoustic amplitude (b) and phase (c) signals. The SE images reveal that there is no visible damage caused by the acoustic measurement. The beam parameters were the same as in the previous imaging. The total scan time was about 1 h in the case of the amplitude image and 2 h 40 min in the case of the phase image. There are some differences between phase and amplitude pictures. In the latter the transistor is darker. This means that in the material the sound velocity is nearly the same as in the silicon base but the energy dissipation of the acoustic waves is higher. The thickness of the implanted structure is less than a micrometre but the thermal diffusion length in silicon at a beam pulse frequency of about 80 kHz is nearly 15–20 mm and so in the same range as the structure size. Fig. 4 shows an acoustic image of two vertical scratches on the top of a glass sample. A horizontal scratch on the opposite surface with
Fig. 2. The secondary electron image (a), the amplitude (b) and phase (c) ion acoustic images of a silicon chip corner at a resonance frequency of the detector of 86 kHz. The size of the images is 1003100 pixels corresponding to 3003300 mm 2 . The thickness of the chip is about 200 mm. The scanning time was 250 ms / pixel (b, c).
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
663
Fig. 3. Secondary electron (a) and ion acoustic amplitude (b) and phase (c) images of a transistor on a chip. The images are similar in size to Fig. 2.
lower intensity is also visible which demonstrates the evidence of buried structures. The thickness of the glass sample was 100 mm. The resolution was determined to be 7 mm.
5. Conclusions The results show that a FIB system combined with a piezo-electric sensor allows to generate and detect acoustic signals from surface and sub-surface structures, like buried paths or material defects with a lateral resolution in the mm-range up to now. It can be used for finding as well as for analyzing of buried features applying the well-known advantages of an FIB. A further improvement of the
Fig. 4. Acoustic image of two vertical scratches on the top surface of a glass sample. A horizontal scratch on the opposite surface is also visible here. The thickness of the glass sample was 100 mm. The resolution was determined to be 7 mm.
664
Ch. Akhmadaliev et al. / Microelectronic Engineering 57 – 58 (2001) 659 – 664
resolution of the ion-acoustic microscope requires to optimize the sensor geometry, to increase the beam pulse frequency as well as the application of a related high frequency and low noise registration electronics. Acknowledgements The authors would like to thank the Deutsche Forschungsgemeinschaft for financial support of the ¨ work (contract no. BI 503 / 7-1), and Dr. B. Kohler from the Fraunhofer-Institute for Nondestructive Testing, Dresden for helpful discussions and suggestions. References [1] [2] [3] [4] [5] [6]
A.L. Tronconi, M.A. Amato, P.C. Morais, K.S. Neto, J. Appl. Phys. 56 (5) (1984) 1462. K.L. Muratikov, A.L. Glazov, D.N. Rose, J.E. Dumar, J. Appl. Phys. 88 (3) (2000) 2948. L.J. Bulk, Adv. Electron. Electron Phys. 71 (1988) 1. D.N. Rose, H. Turner, K.O. Legg, Can. J. Phys. 64 (1986) 1284. ¨ J. Teichert, L. Bischoff, B. Kohler, Appl. Phys. Lett. 69 (11) (1996) 1544. J.C. Murphy, J.W. Maclachlan, L.C. Aamodt, IEEE Trans. Ultrason. Ferroelectrics Freq. Cont. UFFC-33 (5) (1986) 529. [7] R.M. White, J. Appl. Phys. 34 (12) (1963) 3559. [8] L. Bischoff, J. Teichert, E. Hesse, D. Panknin, W. Skorupa, J. Vac. Sci. Technol. B 12 (6) (1994) 3523.