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Shave-off depth profiling: Depth profiling with an absolute depth scale
Applied Surface Science 252 (2006) 7293–7296 www.elsevier.com/locate/apsusc
Shave-off depth profiling: Depth profiling with an absolute depth scale M. Nojima a,c,*, A. Maekawa b,c, T. Yamamoto b,c, B. Tomiyasu c, T. Sakamoto d, M. Owari c, Y. Nihei a,b a
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan b Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan d Kogakuin University, 2665-1 Nakano-cho, Hachioji-city, Tokyo 192-0015, Japan Received 12 September 2005; accepted 15 February 2006 Available online 4 May 2006
Abstract Shave-off depth profiling provides profiling with an absolute depth scale. This method uses a focused ion beam (FIB) micro-machining process to provide the depth profile. We show that the shave-off depth profile of a particle reflected the spherical shape of the sample and signal intensities had no relationship to the depth. Through the introduction of FIB micro-sampling, the shave-off depth profiling of a dynamic random access memory (DRAM) tip was carried out. The shave-off profile agreed with a blue print from the manufacturing process. Finally, shave-off depth profiling is discussed with respect to resolutions and future directions. # 2006 Elsevier B.V. All rights reserved. Keywords: Shave-off; Depth profiling; FIB; SIMS
1. Introduction ‘‘What is the depth?’’ is an ultimate question for depth profiling. Depth profiling with an ion gun sample bombardment came into widespread use by beginning of the 1970s, together with the developments of powerful surface analysis tools [1]. The ‘‘depth’’ is described by a sputtering time or needs to be calibrated, assuming proportionality between depth and time, for example. Shave-off depth profiling is a process where a focused ion beam (FIB) is used for micro-machining to remove sample in a series of planes normal to the depth axis. This is completely different from depth profiling with a conventional ion bombardment process, even if both methods have their origin in same phenomenon; a sputtering event. Shave-off depth profiling can define the ‘‘depth’’ as a function of magnification projected on a display without a time function. In the shave-off process, the direction of the primary ion beam is parallel to the surface of the sample which is consequently shaved-off while obtaining the depth profile [2]. Schematic images in the process
* Corresponding author. Fax: +81 4 7123 9890. E-mail address: [email protected] (M. Nojima). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.144
of shave-off depth profiling are shown in Fig. 1. The features of shave-off depth profiling are described as follows: 1. The sputtering process is not a process of digging a crater from the surface but always one of micro-machining from the surface to the bulk: one can always keep the same surface conditions and constant qualities of ion detection efficiency and the depth resolution. 2. An absolute scale depth profiling: the scale of ‘‘depth’’ is defined by a voltage for beam scanning only a ‘‘depth’’ is not described by the elapsed time. 3. It is applicable to samples with arbitrary shapes or composed of different materials; the analyzing surface is always kept flat. Shave-off depth profiling is a powerful depth scaling method especially for samples with a rough surface or buried structures. By using shave-off depth profiling, we can obtain a depth profile with a 50 mm depth range and a 40 nm depth resolution at worst [3] without the influence of surface roughness or topographical effects. In this paper, some results from shave-off depth profiling will be shown and shave-off depth profile will be discussed with respect to resolution and a future perspectives.
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M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
Fig. 1. Schematic image sequence showing the process of shave-off depth profiling. (Each number represents a successive stage in the process of shave-off depth profiling.)
2. Shave-off depth profiling
2.2. Shave-off depth profiling with micro-sampling using an FIB
2.1. Results of shave-off depth profiling for a particle Fig. 2 shows results of conventional depth profiling and shave-off depth profiling for a particle a few micro-metres in diameter. The particle contains light elements: Si, Al, K and Na. On conventional depth profiling, all signals tend to decrease as the crater becomes deep. Shave-off depth profiling reflects the spherical shape of the particle and signal intensities have no relationship with depth. The horizontal scale indicates a scale of the absolute depth.
Shave-off depth profiling becomes widespread its applicability and flexibility by the introduction of the micro-sampling technique using a FIB. Nowadays micro-sampling is an indispensable technique for a TEM sample preparation. It enables shave-off depth profiling to be a pin point depth profiling. We can select points to be analyzed by shave-off depth profiling. Here, this is demonstrated by a dynamic random access memory (DRAM) tip selected and picked up during the manufacturing process.
Fig. 2. A comparison between conventional depth profiling and shave-off depth profiling.
M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
The surface of the tip consists of thousand of via holes and this perforated structure makes it difficult to maintain a flat surface during a conventional bombardment process. Shave-off depth profiling can eliminate topographical effects and surface roughing. A piece of the tip was previously micro-machined by a FIB, picked up and planted on another substrate in a scanning ion microprobe chamber (SMI3050 SIINT). The piece on the substrate was transferred into the nano-beam SIMS apparatus, which consists of the Ga-FIB, the Mattauch-Herzog type mass filter and the 120-channel parallel ion detection system [4]. The top of the piece revealed sections through via holes and faced to the ion detector. On the bottom of via holes, via plugs stand in rows. The via plugs consist of aluminum and are covered with a titanium nitride barrier. Tungsten wiring connects via plugs to the base of the tip and is also covered with the barrier metal. Knowledge of the local coordinates of the titanium is important in estimating the barrier effect, and in detecting the diffusion of metals from plugs or electrodes into the dielectric region. The experimental conditions were as follows. The acceleration voltage and current of Ga-FIB were 24 kV and 35 pA, respectively. Total time for the depth profiling was 60 min for a 3 mm depth. Fig. 3 shows a result of shave-off depth profiling for the DRAM tip. The SEM image suggests depth information for this profile. When the FIB has reached the top of the sample, the edge of the FIB begins to shave the silicon oxide insulation region, including via holes. After that, the shaving reveals the face of the barrier metal. The two split peaks of Ti+ come from the package of via plugs covered with barrier. On the top of Al+ profile, signals rise gently. This reflects the tapered shape of the via plugs. From the first sharp peak of Ti+, the thickness of barrier metal was estimated to be 50 nm in FWHM. The value matches with the blue print of the manufacturing process.
7295
Fig. 4. A typical shave-off depth profile of a thin layer.
3. For ultimate depth resolution The shave-off depth profile is essentially a convolution of the beam profile with the solid chemical distribution. Sharpness of the FIB controls a depth resolution. The beam profile of the FIB consists of two distributions [5]. The main one is assumed to be a sharp Gaussian. The other is a low intensity tail outside the FIB. Fig. 4 shows typical shave-off depth profile of a titanium thin layer (30 nm). The profile rises gently as the long tail of the FIB reaches the metal. Soon after, the signals rise to a maximum and then drop suddenly. This indicates that on approaching the feature, one side of the tail strongly affects the profile, whereas the other side of the tail has no effect, because once the sample has been analyzed, the feature has been completely sputtered and has disappeared. Therefore, we can say that the depth resolution of shave-off depth profile is limited by the rising tail of the profile. Another point, which determines the depth resolution, is the thickness of the damaged amorphous layer during the FIB micro-matching process. It is reported that the surface damage amorphous layer by the FIB micro-machining is some nano-meters thick on a TEM image [6]. The minimization of the effect of the rising tail is the most important factor in improving the depth resolution of the technique. One approach to correct the rising tail is to deconvolute the shave-off profile. However, the rising tail involves several uncertain factors and this approach cannot solve the problem of the rising tail fundamentally or completely. Another approach is to produce an ultra sharp FIB. There are three factors which need to be taken into account: (1) Aberrations on each lens, mainly the spherical and chromatic aberrations of the objective lens. (2) Columbic interactions on FIB. (3) The intrinsic energy spread of a liquid metal ion source, insignificant for a normal Ga tip.
Fig. 3. Shave-off depth profile for a dynamic random access memory (DRAM) tip. (The SEM image suggests depth information for this profile.)
To achieve nano-metre depth resolution, an epoch-making FIB for shave-off depth profiling will be developed.
7296
M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
4. Concluding remarks We demonstrate that shave-off depth profiling provides an absolute scale determined by the geometry of the beam scan and is free from surface roughing and sample topography in the examples studied so far. From shave-off depth profiling of a DRAM tip, the thickness of barrier metal was estimated to be 50 nm FWHM. The value matches with a blue print of the manufacturing process. A shave-off depth profile is essentially a convolution of the beam profile with the solid chemical distribution; the depth resolution is strongly controlled by the long tail of the FIB. Acknowledgements This research was partially supported by MEXT, grant-inaid for Young Scientists (B), 16750066, 2004. We wish to
express our gratitude to Ms. Tomoko Arimitsu and Ms. Ikuko Nakatani of SIINT for technical support and helpful suggestions.
References [1] S. Hofmann, Philos. Trans. R. Soc. Lond. A 362 (2004) 55. [2] H. Satoh, M. Owari, Y. Nihei, J. Vac. Sci. Technol. B6 (1988) 915. [3] M. Toi, A. Maekawa, T. Yamamoto, T. Sakamoto, M. Owari, M. Nojima, Y. Nihei, J. Surf. Anal. 12 (2005) 170. [4] M. Nojima, M. Toi, A. Maekawa, B. Tomiyasu, T. Sakamoto, M. Owari, Y. Nihei, Appl. Surf. Sci. 231–232 (2004) 930. [5] Y. Kawanami, T. Ishitani, K. Uemura, S. Shukuri, J. Vac. Sci. Technol. B5 (1987) 1364. [6] S. Sadayama, Y. Sugiyama, K. Iwasaki, A. Yasaka, J. Electr. Microsc. 53 (2004) 493.
Shave-off depth profiling: Depth profiling with an absolute depth scale M. Nojima a,c,*, A. Maekawa b,c, T. Yamamoto b,c, B. Tomiyasu c, T. Sakamoto d, M. Owari c, Y. Nihei a,b a
Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan b Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan d Kogakuin University, 2665-1 Nakano-cho, Hachioji-city, Tokyo 192-0015, Japan Received 12 September 2005; accepted 15 February 2006 Available online 4 May 2006
Abstract Shave-off depth profiling provides profiling with an absolute depth scale. This method uses a focused ion beam (FIB) micro-machining process to provide the depth profile. We show that the shave-off depth profile of a particle reflected the spherical shape of the sample and signal intensities had no relationship to the depth. Through the introduction of FIB micro-sampling, the shave-off depth profiling of a dynamic random access memory (DRAM) tip was carried out. The shave-off profile agreed with a blue print from the manufacturing process. Finally, shave-off depth profiling is discussed with respect to resolutions and future directions. # 2006 Elsevier B.V. All rights reserved. Keywords: Shave-off; Depth profiling; FIB; SIMS
1. Introduction ‘‘What is the depth?’’ is an ultimate question for depth profiling. Depth profiling with an ion gun sample bombardment came into widespread use by beginning of the 1970s, together with the developments of powerful surface analysis tools [1]. The ‘‘depth’’ is described by a sputtering time or needs to be calibrated, assuming proportionality between depth and time, for example. Shave-off depth profiling is a process where a focused ion beam (FIB) is used for micro-machining to remove sample in a series of planes normal to the depth axis. This is completely different from depth profiling with a conventional ion bombardment process, even if both methods have their origin in same phenomenon; a sputtering event. Shave-off depth profiling can define the ‘‘depth’’ as a function of magnification projected on a display without a time function. In the shave-off process, the direction of the primary ion beam is parallel to the surface of the sample which is consequently shaved-off while obtaining the depth profile [2]. Schematic images in the process
* Corresponding author. Fax: +81 4 7123 9890. E-mail address: [email protected] (M. Nojima). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.144
of shave-off depth profiling are shown in Fig. 1. The features of shave-off depth profiling are described as follows: 1. The sputtering process is not a process of digging a crater from the surface but always one of micro-machining from the surface to the bulk: one can always keep the same surface conditions and constant qualities of ion detection efficiency and the depth resolution. 2. An absolute scale depth profiling: the scale of ‘‘depth’’ is defined by a voltage for beam scanning only a ‘‘depth’’ is not described by the elapsed time. 3. It is applicable to samples with arbitrary shapes or composed of different materials; the analyzing surface is always kept flat. Shave-off depth profiling is a powerful depth scaling method especially for samples with a rough surface or buried structures. By using shave-off depth profiling, we can obtain a depth profile with a 50 mm depth range and a 40 nm depth resolution at worst [3] without the influence of surface roughness or topographical effects. In this paper, some results from shave-off depth profiling will be shown and shave-off depth profile will be discussed with respect to resolution and a future perspectives.
7294
M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
Fig. 1. Schematic image sequence showing the process of shave-off depth profiling. (Each number represents a successive stage in the process of shave-off depth profiling.)
2. Shave-off depth profiling
2.2. Shave-off depth profiling with micro-sampling using an FIB
2.1. Results of shave-off depth profiling for a particle Fig. 2 shows results of conventional depth profiling and shave-off depth profiling for a particle a few micro-metres in diameter. The particle contains light elements: Si, Al, K and Na. On conventional depth profiling, all signals tend to decrease as the crater becomes deep. Shave-off depth profiling reflects the spherical shape of the particle and signal intensities have no relationship with depth. The horizontal scale indicates a scale of the absolute depth.
Shave-off depth profiling becomes widespread its applicability and flexibility by the introduction of the micro-sampling technique using a FIB. Nowadays micro-sampling is an indispensable technique for a TEM sample preparation. It enables shave-off depth profiling to be a pin point depth profiling. We can select points to be analyzed by shave-off depth profiling. Here, this is demonstrated by a dynamic random access memory (DRAM) tip selected and picked up during the manufacturing process.
Fig. 2. A comparison between conventional depth profiling and shave-off depth profiling.
M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
The surface of the tip consists of thousand of via holes and this perforated structure makes it difficult to maintain a flat surface during a conventional bombardment process. Shave-off depth profiling can eliminate topographical effects and surface roughing. A piece of the tip was previously micro-machined by a FIB, picked up and planted on another substrate in a scanning ion microprobe chamber (SMI3050 SIINT). The piece on the substrate was transferred into the nano-beam SIMS apparatus, which consists of the Ga-FIB, the Mattauch-Herzog type mass filter and the 120-channel parallel ion detection system [4]. The top of the piece revealed sections through via holes and faced to the ion detector. On the bottom of via holes, via plugs stand in rows. The via plugs consist of aluminum and are covered with a titanium nitride barrier. Tungsten wiring connects via plugs to the base of the tip and is also covered with the barrier metal. Knowledge of the local coordinates of the titanium is important in estimating the barrier effect, and in detecting the diffusion of metals from plugs or electrodes into the dielectric region. The experimental conditions were as follows. The acceleration voltage and current of Ga-FIB were 24 kV and 35 pA, respectively. Total time for the depth profiling was 60 min for a 3 mm depth. Fig. 3 shows a result of shave-off depth profiling for the DRAM tip. The SEM image suggests depth information for this profile. When the FIB has reached the top of the sample, the edge of the FIB begins to shave the silicon oxide insulation region, including via holes. After that, the shaving reveals the face of the barrier metal. The two split peaks of Ti+ come from the package of via plugs covered with barrier. On the top of Al+ profile, signals rise gently. This reflects the tapered shape of the via plugs. From the first sharp peak of Ti+, the thickness of barrier metal was estimated to be 50 nm in FWHM. The value matches with the blue print of the manufacturing process.
7295
Fig. 4. A typical shave-off depth profile of a thin layer.
3. For ultimate depth resolution The shave-off depth profile is essentially a convolution of the beam profile with the solid chemical distribution. Sharpness of the FIB controls a depth resolution. The beam profile of the FIB consists of two distributions [5]. The main one is assumed to be a sharp Gaussian. The other is a low intensity tail outside the FIB. Fig. 4 shows typical shave-off depth profile of a titanium thin layer (30 nm). The profile rises gently as the long tail of the FIB reaches the metal. Soon after, the signals rise to a maximum and then drop suddenly. This indicates that on approaching the feature, one side of the tail strongly affects the profile, whereas the other side of the tail has no effect, because once the sample has been analyzed, the feature has been completely sputtered and has disappeared. Therefore, we can say that the depth resolution of shave-off depth profile is limited by the rising tail of the profile. Another point, which determines the depth resolution, is the thickness of the damaged amorphous layer during the FIB micro-matching process. It is reported that the surface damage amorphous layer by the FIB micro-machining is some nano-meters thick on a TEM image [6]. The minimization of the effect of the rising tail is the most important factor in improving the depth resolution of the technique. One approach to correct the rising tail is to deconvolute the shave-off profile. However, the rising tail involves several uncertain factors and this approach cannot solve the problem of the rising tail fundamentally or completely. Another approach is to produce an ultra sharp FIB. There are three factors which need to be taken into account: (1) Aberrations on each lens, mainly the spherical and chromatic aberrations of the objective lens. (2) Columbic interactions on FIB. (3) The intrinsic energy spread of a liquid metal ion source, insignificant for a normal Ga tip.
Fig. 3. Shave-off depth profile for a dynamic random access memory (DRAM) tip. (The SEM image suggests depth information for this profile.)
To achieve nano-metre depth resolution, an epoch-making FIB for shave-off depth profiling will be developed.
7296
M. Nojima et al. / Applied Surface Science 252 (2006) 7293–7296
4. Concluding remarks We demonstrate that shave-off depth profiling provides an absolute scale determined by the geometry of the beam scan and is free from surface roughing and sample topography in the examples studied so far. From shave-off depth profiling of a DRAM tip, the thickness of barrier metal was estimated to be 50 nm FWHM. The value matches with a blue print of the manufacturing process. A shave-off depth profile is essentially a convolution of the beam profile with the solid chemical distribution; the depth resolution is strongly controlled by the long tail of the FIB. Acknowledgements This research was partially supported by MEXT, grant-inaid for Young Scientists (B), 16750066, 2004. We wish to
express our gratitude to Ms. Tomoko Arimitsu and Ms. Ikuko Nakatani of SIINT for technical support and helpful suggestions.
References [1] S. Hofmann, Philos. Trans. R. Soc. Lond. A 362 (2004) 55. [2] H. Satoh, M. Owari, Y. Nihei, J. Vac. Sci. Technol. B6 (1988) 915. [3] M. Toi, A. Maekawa, T. Yamamoto, T. Sakamoto, M. Owari, M. Nojima, Y. Nihei, J. Surf. Anal. 12 (2005) 170. [4] M. Nojima, M. Toi, A. Maekawa, B. Tomiyasu, T. Sakamoto, M. Owari, Y. Nihei, Appl. Surf. Sci. 231–232 (2004) 930. [5] Y. Kawanami, T. Ishitani, K. Uemura, S. Shukuri, J. Vac. Sci. Technol. B5 (1987) 1364. [6] S. Sadayama, Y. Sugiyama, K. Iwasaki, A. Yasaka, J. Electr. Microsc. 53 (2004) 493.