- Email: [email protected]
Back side SIMS analysis of hafnium silicate
Applied Surface Science 252 (2006) 7179–7181 www.elsevier.com/locate/apsusc
Back side SIMS analysis of hafnium silicate C. Gu a, F.A. Stevie a,*, J. Bennett b, R. Garcia a, D.P. Griffis a a
Analytical Instrumentation Facility, North Carolina State University, 2410 Campus Shore Drive, Raleigh, NC 27695, USA b ATDF, 2706 Montopolis Drive, Austin, TX 78741, USA Available online 27 April 2006
Abstract High-k dielectrics are under study as part of the effort to continually reduce semiconductor device dimensions and hafnium silicate (HfSixOy) is one of the most promising high-k materials. A requirement of the dielectric is that the constituent elements cannot diffuse into adjacent device regions during thermal processing. Analysis for inter-diffusion using front side SIMS of high-k dielectrics has been complicated by matrix and sputtering effects. Use of a back side analysis sample preparation procedure that was successful for copper diffusion and site specific studies produced a HfSiO specimen that has less than 250 nm silicon remaining and minimal slope over the analysis region. Magnetic Sector (CAMECA IMS-6F) SIMS analysis of this specimen with low energy O2+ bombardment does not show the matrix and sputtering effects noted in the front side data. Sufficient depth resolution was obtained to define the interface between the silicon substrate and the HfSiO layer and indicate what appears to be an interfacial layer. There is no indication of hafnium diffusion into the silicon substrate. # 2006 Elsevier B.V. All rights reserved. Keywords: SIMS; High-k dielectrics; Back side analysis; Hafnium silicate
1. Introduction High-k dielectrics are under study as part of the effort to continually reduce semiconductor device dimensions. Reduction in device size requires a thinner gate oxide and silicon oxide is approaching the process limit for thickness. Further reduction will cause increased gate leakage current and risk of oxide breakdown [1,2]. Use of a different dielectric constant material permits processing of a thicker layer and hafnium silicate (HfSixOy—subsequently referred to as HfSiO) is one of the most promising high-k materials. One requirement of the dielectric is that the constituent elements cannot diffuse into adjacent device regions during thermal processing. The depth resolution required to check for interdiffusion for the very thin (typically less than 10 nm thick) high-k dielectric films with the silicon substrate has not been achievable by sputtering through the film because of matrix and sputtering effects [3,4]. Depth profiles obtained using quadruple systems to achieve high depth resolution at low impact energy have shown an extended tail of hafnium into the silicon substrate
* Corresponding author. Tel.: +1 919 515 7037; fax: +1 919 515 6965. E-mail address: [email protected] (F.A. Stevie). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.099
[3,4]. Ion beam mixing or non-uniform removal of the hafnium may be the cause of this tail. Secondary ion yields appear to vary through the interface and within the HfSiO. The result of these analyses is that it is unclear whether hafnium penetrates into the underlying silicon. There is also interest in identifying what appears to be an intermediate layer between the silicon and HFSiO as noted in transmission electron microscopy (TEM) analysis [5]. Analysis from the back side after removal of most of the substrate should provide an improvement. Pioneering work in back side analysis [6,7] showed significantly improved depth resolution in certain cases where the substrate could be removed and the layers of interest analyzed by profiling from the back of the structure. The initial studies took advantage of the ability to remove the substrate using a chemical etch or polishing followed by chemical etch, and a layer in the specimen acted as an etch stop. Other back side analyses have used the chemical etch approach [8,9], but preparation using only polishing until recently had been achieved only for analysis of copper diffusion and for a site specific study [10–12]. Back side SIMS analyses of hafnium oxide or silicate have been performed after sample preparation using chemical etch or polishing plus chemical etch to reach the HfSiO layer, or by polishing to about 1 mm from the layer [13–15]. These
7180
C. Gu et al. / Applied Surface Science 252 (2006) 7179–7181
studies have not provided the desired depth resolution or have removed all of the silicon, so that the interfacial region between HfSiO and Si cannot be studied. The purpose of the current study is to optimize depth resolution by polishing as closely as possible to the HFSiO layer and by use of low impact energy analysis conditions. 2. Experimental method The mechanical polishing procedure to obtain sample flatness with respect to the layer of interest has been described in earlier work [10,11]. It is possible to remove on the order of 750 mm of silicon and achieve a planarity with respect to the sample surface of approximately 2.5 nm over a 60 mm diameter area of analysis as determined by a set of SIMS analyses at different points on a polished sample. An important aspect of the procedure is to prepare bevels on the sides of the specimen and to make frequent checks on planarity as the specimen is polished. Another significant factor for this study is the adhesion of the top layer of the sample to the epoxy used to hold the specimen for polishing. If the specimen is coated with a conductive layer to aid in analysis of an insulating layer, the coating must adhere sufficiently strongly to the sample surface to survive the polishing process. Coatings of Au and Al proved unsuccessful, but Ti has been found to provide sufficiently strong adhesion. The site specific study [11] also showed that a conductive path from the region of analysis to the sample holder is critical to prevent charging effects. SIMS analyses were obtained using a CAMECA IMS-6f with 30 nA O2+ primary beam rastered over 190 mm 190 mm and detection of positive secondary ions from a 60 mm diameter optically gated region at the center of the raster. The primary accelerating potential was at 3 kV and the sample potential at 1.75 kV with net impact energy of 1.25 keV and 498 angle of incidence with respect to normal. These analysis conditions provide a projected range of 2.7 nm for O2+ in silicon thus providing good depth resolution. The most abundant isotope 180 Hf was monitored and mass resolution of 2100m/Dm was used to reduce interference from SixOy species such as 28 30 Si Si4O2. The 25 nm thick HfSiO film is sufficiently thick to allow determination of the HfSiO sputtering rate. For the analysis conditions used, HfSiO sputtered at 0.78 nm/s and Si at 2.45 nm/s or a ratio of 0.32 for HfSiO/Si.
Fig. 1. Front side depth profile of 25 nm HfSiO/Si.
resulting from the reduced sputter rate through HfSiO which results in a build up of oxygen just past the HfSiO/Si interface. The SIMS analysis method used in this work takes advantage of the contrast improvement in viewing the sample obtained by use of a red filter. Fig. 2 is a grey scale representation of the red filter image of the back side polished sample showing height contours and several SIMS craters. One of the craters (crater 2) can be seen to be in a region which has minimal slope variation, i.e. it is contained within a single approximately circular contour line (93 nm). Fig. 3 shows SIMS depth profiles taken at craters 1 and 2 in Fig. 2. There is some slope for the profile at crater 1, but crater 2 shows better depth resolution in the region noted as mostly flat based on the red filter image. The difference between these two profiles shows that the planarity of the sample after polishing is crucial for obtaining the best depth resolution. Fig. 4 shows the SIMS depth profile obtained from the flat region back side profile shown in Fig. 3B (crater 2 of Fig. 2) after sputter rate correction. The first 0.2 mm of silicon is not shown. Depth resolution is significantly improved compared
3. Results and discussion Both front and back side analyses were obtained. Fig. 1 shows a SIMS depth profile taken from the front side. The results differ somewhat from a previously reported 500 eV O2+ quadruple SIMS analysis of HfO2 [3] in that matrix and sputtering variations are reduced. However, the front side profile in Fig. 1 still shows what appears to be hafnium penetration into silicon. The depth resolution of this analysis is insufficient to accurately determine the HfSiO/Si interface. Fig. 1 also shows a difference in silicon matrix intensity between the silicon substrate and the silicon just below the HfSiO. This increase may be due to oxygen yield enhancement
Fig. 2. Image obtained using a red filter to enhance contrast (converted to grey scale for publication). Grayscale image of the HfSiO sample polished from the back side; several SIMS craters are visible.
C. Gu et al. / Applied Surface Science 252 (2006) 7179–7181
7181
Fig. 3. As acquired back side SIMS depth profiles of Si/HfSiO. Less than 0.4 mm Si remains after polishing from sample back side. Profile A is from crater 1, and profile B from crater 2 as indicated in Fig. 2.
diffusion of hafnium into the silicon substrate. Additional study is planned of thinner HfSiO layers and of similar samples which have had nitrogen added to the HfSiO layer to reduce dopant penetration and leakage current. Further work on the preparation method will be directed at improving the size of the uniformly flat region after polishing.
References
Fig. 4. Expanded view of HfSiO region after sputter rate correction.
with the front side profile. The rise of the hafnium profile is 1.3 nm/decade. There is no evidence of hafnium diffusion into the silicon substrate. Note that Fig. 4 profile appears to show a layer approximately 2 nm thick between the silicon and HfSiO. A layer of 0.5–2 nm has been identified in similar specimens using TEM [5]. The composition of this layer was suspected to be SiO2 but from this analysis it appears that hafnium may also be present because the hafnium signal begins to be detected and then shows a monotonic increase starting at the point where the silicon signal begins to decrease. 4. Summary Back side analysis requires significant sample preparation but provides results that have been previously unobtainable by any other method. The matrix and sputtering effects that have been noted in front side depth profiles are not present in the back side results. The HfSiO layer studied shows no
[1] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [2] R.M. Wallace, Appl. Surf. Sci. 231/232 (2004) 543. [3] J. Bennett, M. Beebe, C. Sparks, C. Gondran, W. Vandervorst, Appl. Surf. Sci. 231/232 (2004) 565. [4] W. Vandervorst, J. Bennett, C. Huyghebaert, T. Conard, C. Gondran, H. De Witte, Appl. Surf. Sci. 231/232 (2004) 569. [5] G.D. Wilk, D.A. Muller, Appl. Phys. Lett. 83 (2002) 3984. [6] C.J. Palmstrom, S.A. Schwarz, E.D. Marshall, E. Yablonovich, J.P. Harbison, C.L. Schwartz, L. Florez, T.J. Gmitter, L.C. Wang, S.S. Lau, Mater. Res. Symp. Proc. 126 (1988) 283. [7] R.T. Lareau, in: A. Benninghoven, A.M. Huber, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS VI,, Wiley, New York, 1988, p. 437. [8] P. Ronsheim, D. Chidambarrao, B. Jagannathan, D. Hunt, J. Vac. Sci. Technol. B 20 (2002) 448. [9] K.L. Yeo, A.T.S. Wee, R. Liu, F.F. Zhou, A. See, J. Vac. Sci. Technol. B 21 (2003) 193. [10] C. Gu, A. Pivovarov, R. Garcia, F. Stevie, D. Griffis, J. Moran, L. Kulig, J.F. Richards, Vac. Sci. Technol. B 22 (2004) 350. [11] C. Gu, R. Garcia, A. Pivovarov, F. Stevie, D. Griffis, Appl. Surf. Sci. 231/ 232 (2004) 663. [12] C. Hongo, M. Tomita, M. Takenaka, M. Suzuki, A. Murakoshi, J. Vac. Sci. Technol. B 21 (2003) 1422. [13] W. Nieveen, B.W. Schueler, G. Goodman, P. Schnabel, J. Moskito, I. Mowat, G. Chao, Appl. Surf. Sci. 231/232 (2004) 556. [14] C. Hongo, M. Takenaka, Y. Kamimuta, M. Suzuki, M. Koyama, Appl. Surf. Sci. 231/232 (2004) 594. [15] J. Sameshima, R. Maeda, K. Yamada, A. Karen, S. Yamada, Appl. Surf. Sci. 231/232 (2004) 614.
Back side SIMS analysis of hafnium silicate C. Gu a, F.A. Stevie a,*, J. Bennett b, R. Garcia a, D.P. Griffis a a
Analytical Instrumentation Facility, North Carolina State University, 2410 Campus Shore Drive, Raleigh, NC 27695, USA b ATDF, 2706 Montopolis Drive, Austin, TX 78741, USA Available online 27 April 2006
Abstract High-k dielectrics are under study as part of the effort to continually reduce semiconductor device dimensions and hafnium silicate (HfSixOy) is one of the most promising high-k materials. A requirement of the dielectric is that the constituent elements cannot diffuse into adjacent device regions during thermal processing. Analysis for inter-diffusion using front side SIMS of high-k dielectrics has been complicated by matrix and sputtering effects. Use of a back side analysis sample preparation procedure that was successful for copper diffusion and site specific studies produced a HfSiO specimen that has less than 250 nm silicon remaining and minimal slope over the analysis region. Magnetic Sector (CAMECA IMS-6F) SIMS analysis of this specimen with low energy O2+ bombardment does not show the matrix and sputtering effects noted in the front side data. Sufficient depth resolution was obtained to define the interface between the silicon substrate and the HfSiO layer and indicate what appears to be an interfacial layer. There is no indication of hafnium diffusion into the silicon substrate. # 2006 Elsevier B.V. All rights reserved. Keywords: SIMS; High-k dielectrics; Back side analysis; Hafnium silicate
1. Introduction High-k dielectrics are under study as part of the effort to continually reduce semiconductor device dimensions. Reduction in device size requires a thinner gate oxide and silicon oxide is approaching the process limit for thickness. Further reduction will cause increased gate leakage current and risk of oxide breakdown [1,2]. Use of a different dielectric constant material permits processing of a thicker layer and hafnium silicate (HfSixOy—subsequently referred to as HfSiO) is one of the most promising high-k materials. One requirement of the dielectric is that the constituent elements cannot diffuse into adjacent device regions during thermal processing. The depth resolution required to check for interdiffusion for the very thin (typically less than 10 nm thick) high-k dielectric films with the silicon substrate has not been achievable by sputtering through the film because of matrix and sputtering effects [3,4]. Depth profiles obtained using quadruple systems to achieve high depth resolution at low impact energy have shown an extended tail of hafnium into the silicon substrate
* Corresponding author. Tel.: +1 919 515 7037; fax: +1 919 515 6965. E-mail address: [email protected] (F.A. Stevie). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.099
[3,4]. Ion beam mixing or non-uniform removal of the hafnium may be the cause of this tail. Secondary ion yields appear to vary through the interface and within the HfSiO. The result of these analyses is that it is unclear whether hafnium penetrates into the underlying silicon. There is also interest in identifying what appears to be an intermediate layer between the silicon and HFSiO as noted in transmission electron microscopy (TEM) analysis [5]. Analysis from the back side after removal of most of the substrate should provide an improvement. Pioneering work in back side analysis [6,7] showed significantly improved depth resolution in certain cases where the substrate could be removed and the layers of interest analyzed by profiling from the back of the structure. The initial studies took advantage of the ability to remove the substrate using a chemical etch or polishing followed by chemical etch, and a layer in the specimen acted as an etch stop. Other back side analyses have used the chemical etch approach [8,9], but preparation using only polishing until recently had been achieved only for analysis of copper diffusion and for a site specific study [10–12]. Back side SIMS analyses of hafnium oxide or silicate have been performed after sample preparation using chemical etch or polishing plus chemical etch to reach the HfSiO layer, or by polishing to about 1 mm from the layer [13–15]. These
7180
C. Gu et al. / Applied Surface Science 252 (2006) 7179–7181
studies have not provided the desired depth resolution or have removed all of the silicon, so that the interfacial region between HfSiO and Si cannot be studied. The purpose of the current study is to optimize depth resolution by polishing as closely as possible to the HFSiO layer and by use of low impact energy analysis conditions. 2. Experimental method The mechanical polishing procedure to obtain sample flatness with respect to the layer of interest has been described in earlier work [10,11]. It is possible to remove on the order of 750 mm of silicon and achieve a planarity with respect to the sample surface of approximately 2.5 nm over a 60 mm diameter area of analysis as determined by a set of SIMS analyses at different points on a polished sample. An important aspect of the procedure is to prepare bevels on the sides of the specimen and to make frequent checks on planarity as the specimen is polished. Another significant factor for this study is the adhesion of the top layer of the sample to the epoxy used to hold the specimen for polishing. If the specimen is coated with a conductive layer to aid in analysis of an insulating layer, the coating must adhere sufficiently strongly to the sample surface to survive the polishing process. Coatings of Au and Al proved unsuccessful, but Ti has been found to provide sufficiently strong adhesion. The site specific study [11] also showed that a conductive path from the region of analysis to the sample holder is critical to prevent charging effects. SIMS analyses were obtained using a CAMECA IMS-6f with 30 nA O2+ primary beam rastered over 190 mm 190 mm and detection of positive secondary ions from a 60 mm diameter optically gated region at the center of the raster. The primary accelerating potential was at 3 kV and the sample potential at 1.75 kV with net impact energy of 1.25 keV and 498 angle of incidence with respect to normal. These analysis conditions provide a projected range of 2.7 nm for O2+ in silicon thus providing good depth resolution. The most abundant isotope 180 Hf was monitored and mass resolution of 2100m/Dm was used to reduce interference from SixOy species such as 28 30 Si Si4O2. The 25 nm thick HfSiO film is sufficiently thick to allow determination of the HfSiO sputtering rate. For the analysis conditions used, HfSiO sputtered at 0.78 nm/s and Si at 2.45 nm/s or a ratio of 0.32 for HfSiO/Si.
Fig. 1. Front side depth profile of 25 nm HfSiO/Si.
resulting from the reduced sputter rate through HfSiO which results in a build up of oxygen just past the HfSiO/Si interface. The SIMS analysis method used in this work takes advantage of the contrast improvement in viewing the sample obtained by use of a red filter. Fig. 2 is a grey scale representation of the red filter image of the back side polished sample showing height contours and several SIMS craters. One of the craters (crater 2) can be seen to be in a region which has minimal slope variation, i.e. it is contained within a single approximately circular contour line (93 nm). Fig. 3 shows SIMS depth profiles taken at craters 1 and 2 in Fig. 2. There is some slope for the profile at crater 1, but crater 2 shows better depth resolution in the region noted as mostly flat based on the red filter image. The difference between these two profiles shows that the planarity of the sample after polishing is crucial for obtaining the best depth resolution. Fig. 4 shows the SIMS depth profile obtained from the flat region back side profile shown in Fig. 3B (crater 2 of Fig. 2) after sputter rate correction. The first 0.2 mm of silicon is not shown. Depth resolution is significantly improved compared
3. Results and discussion Both front and back side analyses were obtained. Fig. 1 shows a SIMS depth profile taken from the front side. The results differ somewhat from a previously reported 500 eV O2+ quadruple SIMS analysis of HfO2 [3] in that matrix and sputtering variations are reduced. However, the front side profile in Fig. 1 still shows what appears to be hafnium penetration into silicon. The depth resolution of this analysis is insufficient to accurately determine the HfSiO/Si interface. Fig. 1 also shows a difference in silicon matrix intensity between the silicon substrate and the silicon just below the HfSiO. This increase may be due to oxygen yield enhancement
Fig. 2. Image obtained using a red filter to enhance contrast (converted to grey scale for publication). Grayscale image of the HfSiO sample polished from the back side; several SIMS craters are visible.
C. Gu et al. / Applied Surface Science 252 (2006) 7179–7181
7181
Fig. 3. As acquired back side SIMS depth profiles of Si/HfSiO. Less than 0.4 mm Si remains after polishing from sample back side. Profile A is from crater 1, and profile B from crater 2 as indicated in Fig. 2.
diffusion of hafnium into the silicon substrate. Additional study is planned of thinner HfSiO layers and of similar samples which have had nitrogen added to the HfSiO layer to reduce dopant penetration and leakage current. Further work on the preparation method will be directed at improving the size of the uniformly flat region after polishing.
References
Fig. 4. Expanded view of HfSiO region after sputter rate correction.
with the front side profile. The rise of the hafnium profile is 1.3 nm/decade. There is no evidence of hafnium diffusion into the silicon substrate. Note that Fig. 4 profile appears to show a layer approximately 2 nm thick between the silicon and HfSiO. A layer of 0.5–2 nm has been identified in similar specimens using TEM [5]. The composition of this layer was suspected to be SiO2 but from this analysis it appears that hafnium may also be present because the hafnium signal begins to be detected and then shows a monotonic increase starting at the point where the silicon signal begins to decrease. 4. Summary Back side analysis requires significant sample preparation but provides results that have been previously unobtainable by any other method. The matrix and sputtering effects that have been noted in front side depth profiles are not present in the back side results. The HfSiO layer studied shows no
[1] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [2] R.M. Wallace, Appl. Surf. Sci. 231/232 (2004) 543. [3] J. Bennett, M. Beebe, C. Sparks, C. Gondran, W. Vandervorst, Appl. Surf. Sci. 231/232 (2004) 565. [4] W. Vandervorst, J. Bennett, C. Huyghebaert, T. Conard, C. Gondran, H. De Witte, Appl. Surf. Sci. 231/232 (2004) 569. [5] G.D. Wilk, D.A. Muller, Appl. Phys. Lett. 83 (2002) 3984. [6] C.J. Palmstrom, S.A. Schwarz, E.D. Marshall, E. Yablonovich, J.P. Harbison, C.L. Schwartz, L. Florez, T.J. Gmitter, L.C. Wang, S.S. Lau, Mater. Res. Symp. Proc. 126 (1988) 283. [7] R.T. Lareau, in: A. Benninghoven, A.M. Huber, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS VI,, Wiley, New York, 1988, p. 437. [8] P. Ronsheim, D. Chidambarrao, B. Jagannathan, D. Hunt, J. Vac. Sci. Technol. B 20 (2002) 448. [9] K.L. Yeo, A.T.S. Wee, R. Liu, F.F. Zhou, A. See, J. Vac. Sci. Technol. B 21 (2003) 193. [10] C. Gu, A. Pivovarov, R. Garcia, F. Stevie, D. Griffis, J. Moran, L. Kulig, J.F. Richards, Vac. Sci. Technol. B 22 (2004) 350. [11] C. Gu, R. Garcia, A. Pivovarov, F. Stevie, D. Griffis, Appl. Surf. Sci. 231/ 232 (2004) 663. [12] C. Hongo, M. Tomita, M. Takenaka, M. Suzuki, A. Murakoshi, J. Vac. Sci. Technol. B 21 (2003) 1422. [13] W. Nieveen, B.W. Schueler, G. Goodman, P. Schnabel, J. Moskito, I. Mowat, G. Chao, Appl. Surf. Sci. 231/232 (2004) 556. [14] C. Hongo, M. Takenaka, Y. Kamimuta, M. Suzuki, M. Koyama, Appl. Surf. Sci. 231/232 (2004) 594. [15] J. Sameshima, R. Maeda, K. Yamada, A. Karen, S. Yamada, Appl. Surf. Sci. 231/232 (2004) 614.