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TOF-SIMS study of adhesive residuals on device contact pads after wafer taping and backgrinding
Applied Surface Science 203±204 (2003) 445±448
TOF-SIMS study of adhesive residuals on device contact pads after wafer taping and backgrinding P. Lazzeria,*, G. Francob, M. Garozzob, C. Gerardib, E. Iacoba, A. Lo Farob, A. Priviterab, L. Vanzettia, M. Bersania a
ITC-irst Centro per la Ricerca Scienti®ca e Tecnologica, 38050 Povo, Trento, Italy b STMicroelectronics, Stradale Primosole 50, 95121 Catania, Italy
Abstract Wafer thinning prior to bonding and packaging of integrated circuits (IC) is accomplished by back-surface grinding using abrasive tools. At this stage of device fabrication, most of the IC surface but the contact pads is covered by the passivation layer. Being unprotected, the pads could thus be easily contaminated by particles and chemicals. An adhesive tape is commonly used to shield the IC front surface. However, adhesive residuals that can detrimentally affect the pad/wire bonding and seriously alter the sticking of the package plastic with the device surface can be likely present as a consequence. TOF-SIMS was used to investigate the adhesive residuals on the contact pads after using different tapes, allowing to recognise the compounds transferred from time to time on the IC surface. Semi-quantitative evaluations of the adhesive residual amount after exploiting several cleaning recipes were also obtained, allowing the tailoring of the cleaning process parameters after contamination identi®cation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; Microelectronic; Pad; Cleaning; Adhesive; Residuals
1. Introduction A key back-end process in microelectronic technology is wafer thinning prior to cutting, bonding and packaging of integrated circuits (IC). Wafer thinning is generally accomplished by back-surface grinding using abrasive tools. At this stage of device fabrication, most of the IC surface but the contact pads is covered by the passivation layer. Being exposed for the connection with the package I/ O pins and unprotected, the pads can be easily contaminated by particles and chemicals during the ®nal scraping and polishing steps of device production. To
avoid drawbacks on device performances due to impurities as well as to facilitate the handling of the wafer after thinning, an adhesive tape is commonly used to shield its front surface. Adhesive residuals can be likely present as a consequence, that can detrimentally affect the pad/wire bonding and seriously alter the sticking of the package plastic with the IC surface. TOF-SIMS was used to monitor the presence of adhesive residuals on device pads and to optimise the cleaning process parameters for adequate contaminants removal. 2. Experimental
*
Corresponding author. Tel.: 39-0461-314464; fax: 39-0461-810851. E-mail address: [email protected] (P. Lazzeri).
Two sets of 8 in wafers were employed to replicate the backside grinding operations requiring the use of
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 9 8 - 0
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P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
smaller than the pad dimensions (100 100 mm2). Samples prepared for this study are listed in Table 1.
Table 1 Sample IDs Cleaning process
Tape A
Tape B
No cleaning Wet, standard Wet, prolonged Oxygen plasma
A1 A2 A3 A4
B1 B2 B3 B4
the adhesive tape. Formerly, these samples identically undergone several steps of device production, including metal deposition and passivation. As a result, different materials (dielectrics, metals) were simultaneously available for the evaluation of the adhesive residuals on the different areas of the IC surface. Nevertheless, this work concerns with the TOF-SIMS characterisation of the device contact pads. Two types of commercially available tapes were tested. The adhesion of the tape on the wafers was ensured by using an automatic sticking tool. Four samples per batch were prepared that consequently undergone wafer thinning and polishing. After tape removal, three samples for each set were treated by wet or dry cleaning processes. In order to evaluate both the amount and type of surface contaminations, TOF-SIMS measurements were performed by a Cameca/ION-TOF instrument. A 25 keV Ga beam was used for these purposes. Spectra were acquired in S-SIMS conditions in both detection polarities. To unambiguously identify the contamination species, the utmost mass resolution was set operating the primary gun in ``bunched'' mode. The sampled area, supposedly to be 75 75 mm2, was nevertheless
3. Results In Fig. 1, the spectra (positive SI) obtained from (a) sample A1 (no cleaning) and (b) A4 (O2 plasma cleaning) are, respectively, shown. Fig. 1 reveals, besides the matrix elements Al and Ti, that several peaks selectively appear in the spectrum from the not cleaned sample. Intense signals are actually present in the range between mass 41 and 59 amu and other valuable peaks are also revealed up to mass 151 amu. The intensity of all these species markedly diminishes on sample A4. Conversely, in this latter case the peak of the DOP (di-octyl-phtalate) fragment at mass 149 amu is clearly detected. Surprisingly, the mass spectra obtained from the adhesive side of tape A had no common features with Fig. 1a. The presence of PDMS (poly-dimethylsiloxane) compounds on the tape surface was indeed clearly acknowledged by the occurrence of the siloxane typical ®ngerprint. However, the proof that tape A triggers residuals on the pad surface was provided by the data obtained from a silicon wafer after tape sticking. The identical ®ngerprint shown in Fig. 1a was in fact clearly detected on the contaminated wafer surface. Results obtained from samples B1 and B4 are shown in Fig. 2. Several contaminants were still selectively detected on sample B1. Again, despite different results were obtained from the direct analysis
Fig. 1. TOF-SIMS spectra (positive SI) from sample A1 (a) and A4 (b).
P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
447
Fig. 2. TOF-SIMS spectra (positive SI) from sample B1 (a) and B4 (b).
Fig. 3. Tape markers (positive SI) on sample A1 (grey), A2 (light grey), A3 (black) and A4 (patterned).
of tape B, it was similarly proved that these species are certainly due to the use of the tape. The presence of distinctive peaks in spectra from samples A1 and B1 reveals that completely different chemical compounds are used for the manufacturing of the two tapes. Nevertheless, the features of data from sample A4 and B4 qualitatively resemble each other. Therefore, although different residuals were originally present on sample A1 and B1, the cleaning based on oxygen plasma is equally effective in removing them. In Fig. 3, the intensity of some selected tape A adhesive markers in samples A1±A4 are shown.
Values are normalised to the aluminium peak intensity. From this ®gure, it can be seen that the adhesive residuals are strongly reduced by both the wet and dry cleaning processes. Moreover, valuable differences in terms of contamination removal ef®ciency are observed for the adopted cleaning technologies. 4. Discussion More than 50 species that, by exact mass evaluation, are evenly assigned to different C, H and O
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P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
combinations were observed to yield the distinctive peaks of sample A1. Despite some of the organic compounds detected in its ®ngerprint are still unrecognised, it can be concluded that polypropylene glycol, or a PPG derivative, is certainly used as adhesive in tape A. PPG generates in fact some intense fragment peaks at mass 59, 73, 87, 99, 117 amu [1] that can be clearly recognised in Fig. 1a. Many of the markers of tape B adhesive still contain C, H and O atoms. Nevertheless, nitrogen was also detected to be present in some molecular ions. Peak intensities and number of residual species (about 15) due to tape B are much lower compared to the former case. Moreover, fairly small intensity variations were observed for tape B markers after plasma cleaning, meaning that the use of tape A yields indeed to an higher amount of adhesive contaminants on the IC front surface. Both the wet and dry processes are bene®cial for reducing the contamination species. Despite the standard wet process actually results in the poorest cleaning ef®ciency, a severe drop of the marker peaks intensity (more than an order of magnitude) can be accomplished even by using this treatment. Concerning cleaning effectiveness, the prolonged wet process, as expected, has a superior ef®ciency. The plasmabased treatment produces the highest reduction of adhesive residuals. On the other hand, this latter process is more expensive and characterised by a lower throughput. Moreover, the plasma cleaning appears to be ineffective in removing sodium, that is not present after the wet processes instead. The lack of PDMS peaks on sample A1 (and similarly concerning the species detected on the adhesive
side of tape B) clearly indicates that PPG and other compounds are actually selectively transferred from the tape to the IC surface. Dipolar forces are likely apt to explain the migration of these species from the tape, whereas PDMS barely adhere on the pad surface. Similar conclusions concerning PDMS behaviour were reported by [2,3]. 5. Conclusions Owing to its high sensitivity, TOF-SIMS allowed to successfully investigate the detrimental contaminations occurring after using the tapes. These results, although not quantitative yet, permit to evaluate the cleaning ef®ciency and to optimise the process parameters for achieving the required degree of cleanliness. The study of the contaminants behaviour undoubtedly leads to the choice of the most suitable tape for minimising unfavourable effects. Extremely valuable bene®ts in terms of cleaning process optimisation, fabrication cost reduction and device reliability can thus be obtained. References [1] D. Briggs, A. Brown, J.C. Wickermann, Handbook of Static Secondary Ion Mass Spectrometry, Wiley, 1989, p. 134. [2] F. Sugimoto, S. Okamura, J. Electrochem. Soc. 146 (1999) 2725. [3] M. Claes, S. De Gendt, C. Kenens, T. Conard, H. Bender, W. Storm, T. Bauer, P. Mertens, M.M. Heyns, J. Electrochem. Soc. 148 (3) (2001) G118.
TOF-SIMS study of adhesive residuals on device contact pads after wafer taping and backgrinding P. Lazzeria,*, G. Francob, M. Garozzob, C. Gerardib, E. Iacoba, A. Lo Farob, A. Priviterab, L. Vanzettia, M. Bersania a
ITC-irst Centro per la Ricerca Scienti®ca e Tecnologica, 38050 Povo, Trento, Italy b STMicroelectronics, Stradale Primosole 50, 95121 Catania, Italy
Abstract Wafer thinning prior to bonding and packaging of integrated circuits (IC) is accomplished by back-surface grinding using abrasive tools. At this stage of device fabrication, most of the IC surface but the contact pads is covered by the passivation layer. Being unprotected, the pads could thus be easily contaminated by particles and chemicals. An adhesive tape is commonly used to shield the IC front surface. However, adhesive residuals that can detrimentally affect the pad/wire bonding and seriously alter the sticking of the package plastic with the device surface can be likely present as a consequence. TOF-SIMS was used to investigate the adhesive residuals on the contact pads after using different tapes, allowing to recognise the compounds transferred from time to time on the IC surface. Semi-quantitative evaluations of the adhesive residual amount after exploiting several cleaning recipes were also obtained, allowing the tailoring of the cleaning process parameters after contamination identi®cation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TOF-SIMS; Microelectronic; Pad; Cleaning; Adhesive; Residuals
1. Introduction A key back-end process in microelectronic technology is wafer thinning prior to cutting, bonding and packaging of integrated circuits (IC). Wafer thinning is generally accomplished by back-surface grinding using abrasive tools. At this stage of device fabrication, most of the IC surface but the contact pads is covered by the passivation layer. Being exposed for the connection with the package I/ O pins and unprotected, the pads can be easily contaminated by particles and chemicals during the ®nal scraping and polishing steps of device production. To
avoid drawbacks on device performances due to impurities as well as to facilitate the handling of the wafer after thinning, an adhesive tape is commonly used to shield its front surface. Adhesive residuals can be likely present as a consequence, that can detrimentally affect the pad/wire bonding and seriously alter the sticking of the package plastic with the IC surface. TOF-SIMS was used to monitor the presence of adhesive residuals on device pads and to optimise the cleaning process parameters for adequate contaminants removal. 2. Experimental
*
Corresponding author. Tel.: 39-0461-314464; fax: 39-0461-810851. E-mail address: [email protected] (P. Lazzeri).
Two sets of 8 in wafers were employed to replicate the backside grinding operations requiring the use of
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 6 9 8 - 0
446
P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
smaller than the pad dimensions (100 100 mm2). Samples prepared for this study are listed in Table 1.
Table 1 Sample IDs Cleaning process
Tape A
Tape B
No cleaning Wet, standard Wet, prolonged Oxygen plasma
A1 A2 A3 A4
B1 B2 B3 B4
the adhesive tape. Formerly, these samples identically undergone several steps of device production, including metal deposition and passivation. As a result, different materials (dielectrics, metals) were simultaneously available for the evaluation of the adhesive residuals on the different areas of the IC surface. Nevertheless, this work concerns with the TOF-SIMS characterisation of the device contact pads. Two types of commercially available tapes were tested. The adhesion of the tape on the wafers was ensured by using an automatic sticking tool. Four samples per batch were prepared that consequently undergone wafer thinning and polishing. After tape removal, three samples for each set were treated by wet or dry cleaning processes. In order to evaluate both the amount and type of surface contaminations, TOF-SIMS measurements were performed by a Cameca/ION-TOF instrument. A 25 keV Ga beam was used for these purposes. Spectra were acquired in S-SIMS conditions in both detection polarities. To unambiguously identify the contamination species, the utmost mass resolution was set operating the primary gun in ``bunched'' mode. The sampled area, supposedly to be 75 75 mm2, was nevertheless
3. Results In Fig. 1, the spectra (positive SI) obtained from (a) sample A1 (no cleaning) and (b) A4 (O2 plasma cleaning) are, respectively, shown. Fig. 1 reveals, besides the matrix elements Al and Ti, that several peaks selectively appear in the spectrum from the not cleaned sample. Intense signals are actually present in the range between mass 41 and 59 amu and other valuable peaks are also revealed up to mass 151 amu. The intensity of all these species markedly diminishes on sample A4. Conversely, in this latter case the peak of the DOP (di-octyl-phtalate) fragment at mass 149 amu is clearly detected. Surprisingly, the mass spectra obtained from the adhesive side of tape A had no common features with Fig. 1a. The presence of PDMS (poly-dimethylsiloxane) compounds on the tape surface was indeed clearly acknowledged by the occurrence of the siloxane typical ®ngerprint. However, the proof that tape A triggers residuals on the pad surface was provided by the data obtained from a silicon wafer after tape sticking. The identical ®ngerprint shown in Fig. 1a was in fact clearly detected on the contaminated wafer surface. Results obtained from samples B1 and B4 are shown in Fig. 2. Several contaminants were still selectively detected on sample B1. Again, despite different results were obtained from the direct analysis
Fig. 1. TOF-SIMS spectra (positive SI) from sample A1 (a) and A4 (b).
P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
447
Fig. 2. TOF-SIMS spectra (positive SI) from sample B1 (a) and B4 (b).
Fig. 3. Tape markers (positive SI) on sample A1 (grey), A2 (light grey), A3 (black) and A4 (patterned).
of tape B, it was similarly proved that these species are certainly due to the use of the tape. The presence of distinctive peaks in spectra from samples A1 and B1 reveals that completely different chemical compounds are used for the manufacturing of the two tapes. Nevertheless, the features of data from sample A4 and B4 qualitatively resemble each other. Therefore, although different residuals were originally present on sample A1 and B1, the cleaning based on oxygen plasma is equally effective in removing them. In Fig. 3, the intensity of some selected tape A adhesive markers in samples A1±A4 are shown.
Values are normalised to the aluminium peak intensity. From this ®gure, it can be seen that the adhesive residuals are strongly reduced by both the wet and dry cleaning processes. Moreover, valuable differences in terms of contamination removal ef®ciency are observed for the adopted cleaning technologies. 4. Discussion More than 50 species that, by exact mass evaluation, are evenly assigned to different C, H and O
448
P. Lazzeri et al. / Applied Surface Science 203±204 (2003) 445±448
combinations were observed to yield the distinctive peaks of sample A1. Despite some of the organic compounds detected in its ®ngerprint are still unrecognised, it can be concluded that polypropylene glycol, or a PPG derivative, is certainly used as adhesive in tape A. PPG generates in fact some intense fragment peaks at mass 59, 73, 87, 99, 117 amu [1] that can be clearly recognised in Fig. 1a. Many of the markers of tape B adhesive still contain C, H and O atoms. Nevertheless, nitrogen was also detected to be present in some molecular ions. Peak intensities and number of residual species (about 15) due to tape B are much lower compared to the former case. Moreover, fairly small intensity variations were observed for tape B markers after plasma cleaning, meaning that the use of tape A yields indeed to an higher amount of adhesive contaminants on the IC front surface. Both the wet and dry processes are bene®cial for reducing the contamination species. Despite the standard wet process actually results in the poorest cleaning ef®ciency, a severe drop of the marker peaks intensity (more than an order of magnitude) can be accomplished even by using this treatment. Concerning cleaning effectiveness, the prolonged wet process, as expected, has a superior ef®ciency. The plasmabased treatment produces the highest reduction of adhesive residuals. On the other hand, this latter process is more expensive and characterised by a lower throughput. Moreover, the plasma cleaning appears to be ineffective in removing sodium, that is not present after the wet processes instead. The lack of PDMS peaks on sample A1 (and similarly concerning the species detected on the adhesive
side of tape B) clearly indicates that PPG and other compounds are actually selectively transferred from the tape to the IC surface. Dipolar forces are likely apt to explain the migration of these species from the tape, whereas PDMS barely adhere on the pad surface. Similar conclusions concerning PDMS behaviour were reported by [2,3]. 5. Conclusions Owing to its high sensitivity, TOF-SIMS allowed to successfully investigate the detrimental contaminations occurring after using the tapes. These results, although not quantitative yet, permit to evaluate the cleaning ef®ciency and to optimise the process parameters for achieving the required degree of cleanliness. The study of the contaminants behaviour undoubtedly leads to the choice of the most suitable tape for minimising unfavourable effects. Extremely valuable bene®ts in terms of cleaning process optimisation, fabrication cost reduction and device reliability can thus be obtained. References [1] D. Briggs, A. Brown, J.C. Wickermann, Handbook of Static Secondary Ion Mass Spectrometry, Wiley, 1989, p. 134. [2] F. Sugimoto, S. Okamura, J. Electrochem. Soc. 146 (1999) 2725. [3] M. Claes, S. De Gendt, C. Kenens, T. Conard, H. Bender, W. Storm, T. Bauer, P. Mertens, M.M. Heyns, J. Electrochem. Soc. 148 (3) (2001) G118.