Acoustic emission monitoring during laser shock cleaning of silicon wafers

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Optics and Lasers in Engineering 43 (2005) 1010–1020

Acoustic emission monitoring during laser shock cleaning of silicon wafers T. Kim, J.M. Lee, S.H. Cho, T.H. Kim Laser Engineering Group, IMT Co. Ltd., P.O. Box 25, Yongin, Kyunggi-Do 449-860, Republic of Korea Received 27 August 2003; accepted 29 July 2004 Available online 18 April 2005

Abstract A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N2 gas. r 2004 Published by Elsevier Ltd. Keywords: Laser shock cleaning; Acoustic emission monitoring; Power density; Ar; Microphone

Corresponding author. Department of Metallurgical system Engineering, Yonsei University, Seoul 120-749, Republic of Korea. Fax: +82 31 330 7377. E-mail address: [email protected] (T.H. Kim).

0143-8166/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.optlaseng.2004.07.004

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1. Introduction In semiconductor manufacturing, a wafer surface cleaning is one of the most important processes, since the contaminants on the surface of a wafer deteriorate device performance and drop the yield [1]. A well-known RCA wet cleaning chemistry has been applied in the semiconductor manufacturing conventionally. However, the strong chemicals could easily attack the sensitive semiconductor surfaces such as metal interconnect and low-k materials, and the application of 300 mm wafers causes the more consumption of chemicals inducing higher cost and environment impacts. In order to solve these problems, there have been many efforts to develop dry cleaning method [2–4]. Recently, a laser cleaning technique has attracted considerable interest as a new alternative cleaning technique to replace conventional wet chemical wafer cleaning, since it is a dry cleaning process [2–4]. This is seen to be non-obtrusive in the production process since no vacuum or special protective atmospheres are required and the beam of the laser can be highly localized allowing specific areas of contamination to be targeted. However, laser cleaning has its own many disadvantages. For example, the speed of laser cleaning is relatively slow due to small laser spot size, and direct interactions between the laser beam and the substrate surface cause severe surface damage easily on the delicate wafer surfaces. In addition, this conventional laser cleaning employing mostly ultraviolet radiation is expensive process and inorganic particles smaller than the laser wavelength is difficult to remove due to the limitation of cleaning force, which should be larger than adhesion force of particles [3]. Therefore, a more effective laser cleaning technique is required not only to obtain high throughput by high speed cleaning but also to remove nano-scale particles successfully without causing any surface damages. In this paper, a new dry cleaning methodology named ‘laser shock cleaning’ is applied to remove nano-scale silica particles from silicon wafer surfaces. The cleaning performance of the shock process was evaluated by scanning the wafer surfaces. In order to characterize the process, in-process acoustic monitoring is carried out in the change of process parameters such as laser power density and atmospheric gas.

2. Laser shock cleaning A laser shock cleaning technique is a new laser cleaning methodology for the effective removal of very small particles from solid surfaces, which uses an airborne plasma shock wave without any direct interactions between the laser beam and the wafer surface [5]. Fig. 1 shows the schematic diagram of laser shock cleaning. When the intense laser beam is focused in the air just above the wafer surface, the gaseous ambient constituents such as nitrogen and oxygen begin to breakdown and ionize, then produce intense airborne plasma. Subsequently, an intense shock wave is generated

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T. Kim et al. / Optics and Lasers in Engineering 43 (2005) 1010–1020 Microphone

Shock wave Pulsed Laser Beam Amplifier PC based signal analyser

Si substrate

Working Table

Fig. 1. The schematic diagram of the experimental system for acoustic monitoring in the laser shock cleaning process.

by the rapid expansion of the plasma and propagates spherically. If the force of shock wave is larger than the adhesion force between the particle and the substrate, the particles began to detach and remove from the surfaces [2,5,6]. During shock wave generation, an audible acoustic wave is produced like big snapping sound, which is detectable with an acoustic transducer. Accordingly acoustic emission can be monitored by microphone as shown in Fig. 1, and this is applied to characterize the laser shock cleaning process quantitatively [7,8].

3. Experiments A p-type bare silicon wafers in the size of 6 in were used. These wafers were contaminated with fumed silica particles, which have an average diameter of around 14 nm. The particles were deposited on the silicon wafer surfaces by an air spray method using purified N2 gas. The contaminated wafers are attempted to clean by laser shock cleaning technique. A Q-switched Nd:YAG laser with a fundamental wavelength of 1064 nm is used to generate the airborne plasma shock waves. The cleaning tests are carried out in class 10 cleanroom environment. In this experiment, the gap distance between laser beam focus and the wafer surface was 5 mm. In order to evaluate the cleaning performance, KLA-Tencor Surfscan 5500 was used to scan the wafer surface and count the particles. Fig. 1 shows the schematic diagram of the experimental system for acoustic monitoring during laser shock cleaning process. A wide-band microphone with a frequency range of 10 Hz–15 kHz was used to detect the acoustic emissions during the process. The detected acoustic signal was fed into a PC-based data acquisition system after passing through a pre-amplifier. The signal was digitized by the signal processor using an analogue-to-digital converter (ADC) with a sampling rate of

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300 kHz. The PC monitor showed the digitized acoustic waveform and fast Fourier transform (FFT) analysis was performed in the computer. In order to investigate the effect of process parameters, acoustic emission monitoring has been performed with a change of incident laser power density and atmospheric gas such as Ar, air and N2 at laser focus.

4. Results and discussion Fig. 2 shows the scanned image of the wafer surface after laser shock treatment in the change of incident laser power density at laser focus, i.e., (a) 8.9  1011 W/cm2, (b) 11.4  1011 W/cm2, (c) 14  1011 W/cm2 and (d) 16.6  1011 W/cm2. One laser pulse inducing one plasma shock wave irradiated at each 4 different positions on the wafer surface and Ar gas atmosphere was formed at laser focus. It is shown from Fig. 2 that the cleaned area by application of one shock wave becomes larger as the laser power density increases from (a) to (d). The calculated cleaned areas at (a), (b), (c), (d) are 13.2, 24.6, 37.4 and 51.5 cm2, respectively, as shown in Fig. 3. This implies that the strength of shock wave increase linearly as the incident laser power density at laser focus increases. This is probably because strongly dense electro-magnetic field induced by high laser power density at the laser focus can easily ionize the Ar molecules and generates the stronger airborne plasma at the laser focus. As a result, it is seen that higher laser power density at the focus position is more effective to remove the silica particles from silicon wafer surfaces. It is noted that the cleaning efficiency of the silica particles from silicon wafer is more than 95% when the whole wafer surface was scanned at the laser power density of 16.6  1011 W/cm2. In order to characterize the laser shock cleaning process, in-process acoustic emission monitoring was carried out in the change of the laser power density.

(d) 16.6x1011 W/cm2 (a) 8.9x1011 W/cm2

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Fig. 2. The scanned image of the wafer surface; (a), (b), (c) and (d) show the cleaned area after cleaning of the wafer surface at power density of 8.9  1011, 11.4  1011, 14  1011 and 16.6  1011 W/cm2, respectively.

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Fig. 3. The cleaned area by laser shock cleaning process as a function of the power density of laser beam.

Fig. 4 shows acoustic waves emitted from the laser focus in Ar gas atmosphere during the shock cleaning process. The laser power densities at the laser focus are (a) 5.7  1011 W/cm2, (b) 8.9  1011 W/cm2 (c) 12.7  1011 W/cm2 and (d) 16.6  1011 W/cm2. It is shown that the intensity of acoustic wave increased in proportional to the laser power density. In order to evaluate the acoustic intensity change quantitatively, a root mean square (RMS) value of the acoustic wave during the initial period till 0.03 s is measured and this is shown in Fig. 5. From the result, the acoustic intensity increased linearly as the laser power density at laser focus increased. This implies that the intensity of acoustic wave is strongly related to that of the shock wave. As a result, the acoustic emission monitoring gives a useful information on the laser-induced shock wave during the cleaning process. Accordingly the acoustic emission monitoring can be utilized not only to optimize the processing parameters but also to monitor the laser status. In order to investigate frequency characteristics of the acoustic wave during laser shock cleaning, a FFT analysis has been carried out in the computer. Fig. 6 shows the frequency spectra of the acoustic waves previously shown in Fig. 4(c) and (d), which are emitted from the laser focus in Ar atmosphere at the power density of 12.7  1011 and 16.6  1011 W/cm2, respectively. It is shown that the frequency spectrum has the largest peak around at 5.8 kHz and many large peaks are found from 10 to 12 kHz at the acoustic waves. It is also seen that the intensity of the largest peak become larger as the laser power density increases. From the results, the acoustic wave has a unique characteristic frequency at 5.8 kHz and its intensity is dependent on the incident laser power density at the laser focus. Fig. 7 shows the scanned image of the silicon wafer surface after laser shock treatment in the change of gas atmosphere at the laser focus. The laser-induced shock wave is generated at three different gas conditions such as: (a) Ar; (b) air; and (c) N2. The incident laser power density at the laser focus was 16.6  1011 W/cm2. It is shown from the figure that the cleaned area (a) at Ar gas is much larger than others of (b), (c). The measured cleaned areas at (a), (b) and (c) are 50.2, 36.3 and 35.8 cm2,

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Fig. 4. Acoustic waves emitted from the shock wave during laser shock cleaning process using Ar gas. A power density of laser is (a) 5.7  1011 W/cm2, (b) 8.9  1011 W/cm2, (c) 12.7  1011 W/cm2 and (d) 16.6  1011 W/cm2.

respectively, as shown in Fig. 8. The cleaned area at Ar gas is about 39% larger than others. This implies that the strength of shock wave at Ar gas atmosphere is larger than that at air and N2 gas atmosphere. In order to verify this result, acoustic emission monitoring has been carried out with the different gas conditions at the laser focus. Fig. 9 shows acoustic waves emitted from the laser focus at different gas conditions, i.e. (a) Ar, (b) air, and (c) N2. The laser power density was the same as 16.6  1011 W/cm2 used in Fig. 8. From the figure it could be seen that the intensity of the acoustic wave at Ar is stronger than air and N2. In order to understand the difference of gases clearly, FFT analysis has been carried out. Fig. 10 shows the

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frequency spectra of the acoustic waves previously shown in Fig. 9(a)–(c). It is shown that the frequency spectrum at Ar atmosphere has the largest peak at 5.8 kHz, which has been known as a characteristic frequency in Fig. 6. Meanwhile, much smaller peaks at 5.8 kHz are observed at other gases. From the spectra, the acoustic intensity at Ar atmosphere is much stronger than air and N2 at same laser condition. As a result, frequency spectra provide a clear indication of the acoustic waves in the change of gas atmosphere during laser shock cleaning process.

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Fig. 8. The cleaned areas by laser shock cleaning process using Ar gas, air and N2 gas.

A RMS analysis of the acoustic wave during the initial period till 0.03 s is performed to evaluate the acoustic intensity quantitatively, which is shown in Fig. 11. It is clearly shown that the acoustic intensity at Ar atmosphere is much stronger than air and N2. At the power density of 16.6  1011 W/cm2, the acoustic RMS value at Ar is larger than other gases by 33%. This is well in accordance with the cleaned area in Fig. 8, where Ar is 39% larger than others. From Fig. 11, it is seen that the threshold power densities to generate the airborne plasma shock wave are obviously different with gas species, i.e. 5.7  1011 W/cm2 at Ar, 9.5  1011 W/cm2 at air and 10.2  1011 W/cm2 at N2. This implies that laser-induced ionization and plasma generation is strongly dependent on gas species at the laser focus. As a result,

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it is known that Ar is the easiest gas to generate a stronger laser-induced plasma shock wave for laser shock cleaning. It was also known that gas breakdown threshold of N2 gas is approximately 2 times higher than Ar gas at atmospheric pressure [9].

5. Conclusions The local removal of nano-scale silica particles from silicon wafer surfaces was carried out by laser shock cleaning technique. In order to characterize the process,

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Fig. 10. Frequency spectra of the previous acoustic waves (Fig. 9) emitted from the laser focus in (a) Ar, (b) air and (c) N2 atmosphere during the laser shock cleaning process at the power density of 16.6  1011 W/cm2.

acoustic emission monitoring was performed in the change of important process parameters such as laser power density and atmospheric gas at the laser focus. The cleaned area became larger with an increase of laser power density since the shock wave intensity is proportional to the laser power density. It was found from acoustic analysis that the acoustic intensity is closely related to the shock wave intensity. From FFT analysis, the acoustic wave emitted during airborne plasma generation has an unique characteristic frequency around at 5.8 kHz and its frequency intensity is dependent on the laser power density. It was also found that Ar gas has stronger acoustic wave intensity and lower threshold power density than air and N2 gas. This means that Ar is most effective gas to obtain stronger shock wave at same power density for laser shock cleaning process. As a result, acoustic waves provide useful indication not only to optimize the cleaning process but also to monitor the status of laser conditions.

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