Removal of Cu Impurities on a Si Substrate by Using (H 2 O 2 +HF) and (UV/O 3 +HF)

Journal of the Korean Physical Society, Vol. 33, No. 5, November 1998, pp. 579 583 Removal of Cu Impurities on a Si Substrate by Using (H 2 O 2 +HF) and (UV/O 3 +HF) Baikil Choi and Hyeongtag Jeon School of Materials Science and Engineering, Hanyang University, Seoul 133-791 (Received 9 March 1998, in final form 14 September 1998) Cleaning the Si surface has become one of the most critical steps of the ultra-large-scale integration process. One of the major concerns is the removal of metallic impurities on the Si surface. In this study, we concentrate on the metallic impurity Cu. The Si substrate was cleaned with piranha(h 2O 4 : H 2O 2 = 4 : 1) and HF (HF : H 2O = 1 : 100) solutions to eliminate the organic impurities and the native oxide. The initial Si substrate was then contaminated intentionally by dipping it into a 1-ppm standard solution of Cu followed by cleaning splits of a chemical HF solution combined with a UV/O 3 treatment and a chemical mixture of HF with H 2O 2. The initial substrate, which had been contaminated with the standard solutions of Cu, exhibited a contamination level of 10 14 atoms/cm 2. This substrate was cleaned and showed a Cu impurity concentration which had been reduced down to the levels of 10 9 10 10 atom/cm 2 as measured by total reflection X-ray fluorescence. Also, repeated treatments with these cleaning splits improved the surface microroughness of this initial substrate from 3.6 Å to 1.8 Å, as which by atomic force microscopy. The surface and the interface morphologies were examined by scanning electron microscopy and transmission electron microscopy. The results showed that the Cu impurities had been adsorbed on the Si surface not in a thin film but in a particular form with a hemispherical shape. Both the chemical compositions of the Cu impurities and the interface between the Cu and the Si substrate were investigated by Auger electron spectroscopy. I. INTRODUCTION To produce a high-performance, high-quality device, ULSI (ultra-large-scale integration) technology requires a more stringent and reliable means to control the surface smoothness and the organic, particulate, and metallic contaminants on the silicon substrate than does VLSI (very-large-scale integration) technology [1 5]. The most common process used today is the RCA wet-chemical cleaning process [6]. The RCA cleaning process was first developed by Kern and Poutinen and was published in 1970 [6]. In this cleaning method, the organic contaminants can be removed by the piranha (H 2 SO 4 - H 2 O 2 ) and SC1 (NH 4 OH - H 2 O 2 - H 2 O). The metallic contaminants are also eliminated by SC2 (HCl - H 2 O 2 - H 2 O) solution. However in the wet-chemical cleaning method, the metallic impurities, which exhibit higher electronegativity values than Si does, such as Cu and Ag, are very difficult to remove [7,8]. These days, new cleaning concepts, called dry-cleaning methods, are under development to improve the removal of such contaminants [9]. Dry-cleaning methods, such as HF/H 2 O vapor phase cleaning [10], UV/O 3 cleaning [11,12], UV/Cl 2 cleaning [13], and H 2 /Ar plasma cleaning [14] have been proposed and developed by some other scientists. The important advantages of these dry-cleaning methods are low chemical consumption, good process uniformity, and surface passivation to protect the Si surface from re-contamination with impurities. However, the wet-chemical cleaning method is still dominantly used in actual device manufacturing processes. The main purpose of this experiment is to compare the efficiency of removing Cu from a Si substrate for various cleaning splits. Among metals, Cu can contaminate a Si substrate during the process of ion implantation, reactive ion etching, or resist ashing [10]. However, metallic impurities, such as Cu, are difficult to remove by conventional wet-chemical cleaning because their electronegativity values are higher than that of Si so they, especially Cu, can contaminate the Si substrate easily by chemical adsorption. In this study, we applied the combined cleaning method of (UV/O 3 +HF) and (H 2 O 2 +HF) to remove the Cu impurities on a Si wafer [15]. A H 2 O 2 solution and UV/O 3 radiation were applied to oxidize the metallic contamination. The oxides formed by this process can be easily removed from the Si substrate by HF treatment. This cleaning effect was investigated by total reflection X-ray fluorescence(txrf). The surface of the Si substrate was rough after contamination with metallic impurities. We tried to improve this surface roughness and -579-

-580- Journal of the Korean Physical Society, Vol. 33, No. 5, November 1998 Fig. 1. Amount of Cu impurities after each cleaning split, as measured by TXRF. the removal efficiency by repeated treatments with the cleaning splits. An improvement in the surface roughness was observed by atomic force microscopy(afm). We also investigated the adsorption mechanism of the Cu impurity onto the Si substrate. The adsorption of Cu was thought to be due to chemical adsorption by the exchange of an electron between the Cu and the Si substrate [8,9]. To examine this mechanism, we used scanning electron microscopy(sem), transmission electron spectroscopy(tem) and Auger electron spectroscopy(aes). Using SEM and TEM, we examined the surface and the interface morphologies of the Cu impurities adsorbed on the Si substrate. Also, the chemical composition of the Cu impurity was investigated by AES. II. EXPERIMENTAL The wafer used in this experiment was a p-type, 4inch-diameter Si wafer with a resistivity of 10 23 Ωcm. The piranha and the HF cleanings were applied as a precleaning to remove initial organics and native oxides. The Si substrates were then intentionally contaminated Fig. 2. Surface roughness values after each cleaning split, as measured by AFM. Fig. 3. Three-dimensional images of the Si surfaces, as measured by AFM, (a) after intentional contamination and (b) after cleaning with Cu by dipping them into a 1-ppm standard CuCl2 solution for 3 minutes. The cleaning procedures were performed using 6 different cleaning splits. The cleaning splits 1, 2, and 3 are HF+H2 O2, HF+H2 O2 twice, Fig. 4. SEM image of the Cu-contaminated initial Si substrate.

Removal of Cu Impurities on a Si Substrate by Using Baikil CHOI and Hyeongtag JEON -581- Fig. 7. Auger line-scanning data for a Cu impurity contaminating a Si substrate. splits 4, 5, and 6 are UV/O3 +HF, UV/O3 +HF twice, and UV/O3 +HF three times, respectively. In the cases of the cleaning splits 1, 2, and 3, the chemical cleaning time was 1 minute, and the composition of the cleaning solution was (HF : H2 O2 : H2 O = 1 : 10 : 100). For cleaning splits 4, 5, and 6, the UV/O3 radiation time and the HF dipping time were 1 minute, respectively. After each cleaning split, the contamination level was measured by TXRF, and the surface roughness was measured by AFM. SEM and TEM were used to examine the surface and the interface morphologies of the metallic contaminants. AES analysis was carried out to analyze the chemical composition. III. RESULTS AND DISCUSSION Fig. 5. Cross-sectional HRTEM micrographs of Cu particles, which have a (a) particular shape and a (b) partially recessed particular shape, on Si substrates. and HF+H2 O2 three times, respectively. The cleaning Fig. 6. Scanning Auger microscope image of the Cu impurity on the Si substrate. After intentional contamination and cleaning, the amount of Cu impurities on the Si substrate was monitored by TXRF. Figure 1 shows the Cu contamination levels after each cleaning step. The amount of Cu impurities after intentional contamination was about 1014 atoms/cm2. The amount of Cu contamination was decreased down to the level of 1010 atoms/cm2 by cleaning Fig. 8. Auger depth-profile data for a Cu impurity contaminating a Si substrate.

-582- Journal of the Korean Physical Society, Vol. 33, No. 5, November 1998 split 1 (HF+H 2 O 2 ). The contamination level was further reduced down to 10 9 atoms/cm 2 by split 2 and split 3. These results show that repeated treatments produce a better removal efficiency. Split 4 (UV/O 3 + HF) also exhibited a good Cu removal efficiency. The Cu contamination level was decreased down to 10 10 atoms/cm 2 after split 4 and reduced slightly farther after split 5 and split 6. The combination of the UV/O 3 and the HF cleaning steps also exhibited a good cleaning effect, and repeated treatments with this combination showed a trend similar to that of the previous cleaning splits (HF+H 2 O 2 ). Comparing these two different cleaning processes, HF + H 2 O 2 cleaning splits eliminated the Cu impurities more effectively than UV/O 3 +HF splits by one order of magnitude. Hence, we can conclude that the HF + H 2 O 2 cleaning split exhibited a slightly better cleaning effect than did the UV/O 3 + HF cleaning split. Surface roughness caused by these cleaning process was measured by AFM. Also, the effects of repeated treatments with these cleaning splits were also investigated and are shown in Fig. 2. Immediately after intentional contamination, the value of the RMS (root mean square) roughness exhibited a maximum value. This is due to adsorption and growth of the Cu impurity on the Si surface. After cleaning split 1, the surface roughness of the Si substrate was reduced significantly, but still exhibited a value which was a little high compared to that of the original bare Si substrate. It is thought that the metallic contamination degraded the surface smoothness. However, this roughness was improved by repeated treatments with split 1 and split 4. This improvement can be explained by repetition of oxide formation by the H 2 O 2 or the UV/O 3 in the cleaning splits and removal of oxide by the HF chemical solution. During these repeated treatments, the metal impurities were removed from the Si substrate and improved the Si surface roughness. Next, the adsorption mechanism of the Cu impurity was also studied. AFM, SEM, and TEM were used for morphological analysis, as shown in Figs. 3, 4, and 5. AES was used to investigate the chemical composition of the contaminating impurity and of the interface between Si and Cu. Firstly, the AFM (Fig. 3(a)) and the SEM (Fig. 4) images show that the Cu impurities were adsorbed on the Si surface not in a thin film but in a particle form with a hemispherical shape and a diameter of around 100-400 Å. The cross-sectional high-resolution TEM image shown in Fig. 5 exhibits a much clearer image of the particle and the interface between the Cu and the Si substrate. It is thought that the lattice images in Fig. 5 are due to crystallization of the Cu particle by the low-heating process during TEM specimen preparation. The height of the particle in Fig. 5(a) is about 30 Å, and its diameter is about 100 Å. In Fig. 5(b) we can see a slight recession of the Cu particle into the Si substrate. The recession of the Cu particle is thought to be due to the diffusion of Cu into the Si substrate. When this partially recessed the Cu impurity was removed by the cleaning process, small pits were left on the Si substrate, so these pits could result in an increase in the surface roughness of the Si substrate. These pits are called metal-induced pits (MIP). The electronegativity value of Cu is higher than that of Si, which results in the oxidation of Cu by the exchange of an electron between the Cu and the Si substrate. Using a HRTEM, we tried to observe this Si oxide. However, it was not observed because of the small amount of Si oxide and the overlap with the Si substrate. To analyze the chemical composition of the contaminated particle and of the interface between Cu and Si substrate, we utilized AES. The scanning Auger image is shown in Fig. 6. The line-scanning and the depth-profile data for the Cu particle, which was chosen by the scanning Auger image, are exhibited in Fig. 7 and Fig. 8, respectively. In Fig. 7, the particle, which was chosen in Fig. 6, was proved to be elemental Cu. The area analyzed for the AES depth-profiling result in Fig. 8 is several µm 2 and is centered around the Cu particle in Fig. 6. In Fig. 8, Cu, Si and oxygen were detected at the same time on the Si surface. The oxygen peak is thought to represent the native oxide, and this peak decreased as the depth profiling proceeded. However, at a depth of about 180 Å where the Cu peak was reduced to almost zero, the oxygen peak increased slightly even though the change was negligible. Novel metals, such as Au and Cu, whose electronegativity values are higher than that of Si, tend to be neutralized by taking an electron from the Si and precipitating it on the Si surface. That is to say, Cu 2+ in the contaminating solution could be reduced to Cu, and the Si surface could be oxidized to SiO x by means of a difference in the standard electrode potential [8,16]. Hence, Cu 2+ in the contaminating solution was precipitated on the Si surface with a change of electrons, and this reaction resulted in Si oxidation at the interface (2Cu 2+ + Si + 2H 2 O Cu + SiO x + 4H + ). Through the cleaning process, the Cu impurity adsorbed by chemical adsorption was oxidized by H 2 O 2 and UV/O 3 and etched away by the HF treatment. The removal of the Cu impurity caused the formation of metal-induced pits on the Si surface, which resulted in a rough Si surface. However, repeated treatment with (H 2 O 2 +HF) and (UV/O 3 + HF) could improve this surface roughness by repetition of the oxidation and the etching of the Si surface. In this study, we introduced the cleaning processes (HF + H 2 O 2 ) and (UV/O 3 + HF) and compared their efficiencies for removing Cu impurities. We also studied the mechanism for adsorption of Cu on the Si substrate. From the result, we concluded that both (HF + H 2 O 2 ) and (UV/O 3 + HF) cleaning exhibited good Cu removal. Also, repeated treatments with each cleaning split improved the surface roughness of Si and increased Cu-impurity removal efficiency. The AFM and the TEM images showed that Cu adsorbed on the Si surface as a hemispherical particle. We also investigated the mechanism of Cu contamination and its removal.

Removal of Cu Impurities on a Si Substrate by Using Baikil CHOI and Hyeongtag JEON -583- IV. SUMMARY In this study, the (HF + H 2 O 2 ) and the (UV/O 3 + HF) cleaning processes were applied to remove Cu impurities from intentionally contaminated Si substrates. Also, the removal efficiencies of the two different cleaning methods and of repeated treatments with each cleaning split were compared. The Cu concentration on the intentionally contaminated Si wafer was about 10 14 atoms/cm 2, and the cleaning splits of (HF + H 2 O 2 ) and (UV/O 3 + HF) effectively removed the Cu impurities to a level of 10 10 atoms/cm 2. Thus the Cu contaminant was effectively removed by the (HF + H 2 O 2 ) and the (UV/O 3 + HF) the cleaning splits. Also, repeated cleaning treatments resulted in improved metallic-impurity removal. The surface roughness of the Cu-contaminated wafer was about 3.6 Å (RMS value). The surface roughness of the Si substrate was also improved by repeated treatments with (HF + H 2 O 2 ) and (UV/O 3 + HF). The Cu impurities contaminated the Si surface not in thin films but as particles with a hemispherical shape. It seemed that Cu adsorbed on the Si surface by exchange of an electron between the Cu and the Si substrate and caused metal-induced pits after the cleaning process. ACKNOWLEDGMENTS This study was supported by the Ministry of Education (Semiconductor Field, ISRC-96-E-1047) and by the 97 intramural research fund in Hangyang University. REFERENCES [1] K. Saga and T. Hattori, J. Electrochem. Soc. 143, 3279 (1996). [2] G. Zoth and W. Bergholz, J. Appl. Phys. 67, 6764 (1990). [3] T. Ohmi, T. Imaoka, I. Sugiyama and T. Kezuka, J. Electrochem. Soc. 139, 3317 (1992). [4] J. Alay, S. Verhaverbeke, W. Vandervorst and M. Heyns, Jpn. J. Appl. Phys. 32, 358 (1993). [5] J. Yoshinobu, S. Tanaka and M. Nishijima, Jpn. J. Appl. Phys. 32, 1191 (1993). [6] W. Kern and D. A. Puotinen, RCA, Rev 31, 187 (1970). [7] H. Park, C. R. Helms, D. Ko, M. Tran and B. B. Triplett, Mat. Res. Soc. Symp. Proc. 315, 353 (1993). [8] H. Morinaga, M. Suyama and T. Ohmi, J. Electrochem. Soc. 141, 2834 (1994). [9] Chongmu Lee, M. G. Park, Hyeongtag Jeon and T. H. Ahn, J. Korean Phys. Soc. 30, 781 (1997). [10] S. M. Sze, ULSI Technology (Mc Graw Hill, New York, 1996), p. 66. [11] H. Jeon, H. Choi and T. Ahn, J. Korean Phys. Soc. 29, 781 (1996). [12] H. Jeon, H. Choi, Y. Cho, K. Ryoo and S. Jung, Mat. Res. Soc. Proc. 386, 297 (1995). [13] J. Ruzyllo, A. M. Hoff, D. C. Frystak and S. D. Hossain, J. Electrochem. Soc. 136, 1602 (1989). [14] J. Cho, T. P. Schneider, J. Vanderweide, H. Jeon and R. J. Nemanich, Appl. Phys. Lett. 59, 1995 (1992). [15] Wonjun Lee and Hyeongtag Jeon, J. Korean Phys. Soc. 30, 307 (1997). [16] T. Ahn, M. Park, C. Lee, J. Park and H. Jeon, Jpn. J. Appl. Phys. 36, 5779 (1997).