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Chemical and structural analysis of etching residue layers in semiconductor devices with energy filtering transmission electron microscopy
Materials Science in Semiconductor Processing 4 (2001) 109–111
Chemical and structural analysis of etching residue layers in semiconductor devices with energy filtering transmission electron microscopy S. Hensa,*, J. Van Landuyta, H. Benderb, W. Boullartb, S. Vanhaelemeerschb a
EMAT, RUCA, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium Interuniversity Microelectronics Center (IMEC), Kapeldreef 75, B-3001 Leuven, Belgium
b
Abstract The use of an energy-filtering field emission gun transmission electron microscope (CM30 FEG Ultratwin) allows, apart from imaging morphologies down to nanometer scale, the fast acquisition of high-resolution element distributions. Electrons that have lost energy corresponding to characteristic inner-shell loss edges are used to form the element maps. The production of Ultra Large-Scale Integration (ULSI) devices with dimensions below 0.25 mm requires among others the formation of a multilayer metallization scheme by means of repeatedly applying the deposition and etching of dielectrics and metals. In this work the evolution of the surface chemical species on etched Al lines in a post-etch cleaning process has been investigated by energy filtering transmission electron microscopy, with the aim to understand the role of each process step on the removal of the etching residues. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: EFTEM; Post-metal-etch strip; Aluminum
1. Introduction
2. Experimental
In this work elemental energy loss maps will be used to obtain detailed information about the formation of residue layers in a post-metal-etch cleaning process. The post-etch cleaning is important for preventing corrosion due to the remaining residue layers after reactive ion etching of the metallization structure. The main goal is to use energy filtering transmission electron microscopy (EFTEM) to study the composition and species of the dry-etch-related residues and their evolution during the cleaning process. This yields information about the cleaning step responsible for the removal of the different parts of the residue and to optimize the cleaning procedure.
Two different processes are studied. The first structure studied consists of regular lines of 0.4 mm wide and 1 mm spacing with a layer stack TiN/Ti/Al(0.2 at%Cu)/TiN/ Ti/Si which is patterned by i-line lithography. Dry etching is performed in a LAM TCP-9600 using Cl2/ BCl3 gases (L1). After etching, the wafers are transferred in-situ into a Downstream Quartz (DSQ) chamber, where either a H2O (L2) or H2O/CF4 (L3) plasma is used. After the DSQ treatment, the wafers are rinsed in the deionized (DI) water rinse station (L4). Also an ex-situ treatment in an aqueous organic mixture (EKC 265) is considered (L5). The second process is studied on lines with a width and spacing of 0.25, 0.4 and 1 mm with a layer stack TiN/ Ti/Al(0.2 at%Cu)/Ti/SiO2 patterned with DUV-lithography. The etching is now performed using Cl2/BCl3/ N2 gases (T1). After etching, these wafers received the same post-etch cleaning process as was used for the first
*Corresponding author. Tel.: +32-3-2180263; fax: +32-32180257. E-mail address: [email protected] (S. Hens).
1369-8001/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 1 4 7 - 5
110
S. Hens et al. / Materials Science in Semiconductor Processing 4 (2001) 109–111
process (T2–T5). After each step of the multi-step strip process, one wafer is taken for analysis. The surface chemistry is studied by means of energy filtering transmission electron microscopy. Energy-filtered images are taken with a Philips CM30 FEG microscope (300 keV) equipped with a Gatan Imaging Filter. For image calculation and processing Gatan’s software Digital MicrographTM is used. The TEM crosssection specimens are prepared with a FIB wedge milling technique for which the surface is protected with a sputtered glass- and a Pt-layer.
3. Results and discussion Fig. 1 shows bright-field TEM [0 1 1]Si cross-section images of the etched structures for both processes. During the specimen preparation, several problems were noticed. During mechanical thinning of the samples, the Al-lines deformed completely. To avoid this problem, the specimens are thinned using focused ion beam milling. Because EFTEM analysis requires very thin specimens (thickness smaller than the mean free path of the electrons traversing the specimen [1]), the specimens are thinned with the FIB-wedge technique. This means that the sample is rotated a few degrees during the end stage of the ion milling to obtain a wedge-shaped specimen [2]. Although SEM investigation detected the presence of a sidewall polymer, preliminary TEM analysis did not show such a sidewall polymer. Sputtering of a protective glass layer on top of the structures before the sawing and thinning of the samples prevented the removal of the residue layers during sample
preparation. However, the use of such a sputtered glass layer which is deposited on a small piece of the wafers, did not fully prevent the fall over of upstanding residue layers during the specimen preparation. Therefore on the second set of samples a protective oxide layer is spun on the whole wafers to prevent damage of the residue layers. Fig. 1a shows a cross-section BF TEM [0 1 1]Si image of sample L1. A large undercut (108–166 nm) is seen under the Ti/TiN antireflective coating. The undercut is reduced in the second process (sample T1; Fig. 1(b)–(d)). This difference in undercut is related to the use of N2 in the etching plasma. Comparing the sidewall layer in L1 and T1, a difference in contrast is visible indicating another composition in both cases. The thickness of the sidewall layer increases for the second process with increasing spacing between the lines (Fig. 1(b) 25 nm, (c) 37 nm, (d) 84 nm). The thickness of the residue layer reduces at the base of the structure. EFTEM analysis is used to investigate these observations (Fig. 2). For the first metal-etch process, elemental maps reveal the formation of an Al-oxide and the presence of carbon in the undercut. Elemental mapping also shows the formation of Al-oxide on the sidewalls. The carbon elemental map verifies the formation of a carbon-like polymer on the sidewall to enhance the anisotropic etching of the aluminum lines [3]. In the elemental maps shown, a carbon cluster is detected, whereas other lines from the same specimen show a more homogeneous C distribution on the sidewall. Earlier XPS-measurements [4] indicated also the presence of chlorine in the sidewalls, which is not confirmed by the EFTEM measurements. Probably the Cl quickly evaporates from
Fig. 1. Bright-field TEM [0 1 1]Si image of (a) L1 (W ¼ S ¼ 0:4 mm), (b) T1 (W ¼ S ¼ 0:25 mm), (c) T1 (W ¼ S ¼ 0:4 mm) and (d) T1 (W ¼ S ¼ 1 mm) sample.
S. Hens et al. / Materials Science in Semiconductor Processing 4 (2001) 109–111
Fig. 2. EFTEM images of Al, N, O, C and Ti for specimen L1.
the specimen during the EFTEM analysis or it might be lost during the specimen preparation. Further analysis of the other specimens (L2–L5), showed the removal of the carbon-like sidewall residue layers as a consequence of the applied plasma treatment. All investigated specimens (L1–L5) show the large undercut with carbon and aluminum-oxide trapped inside. The consequences of the second plasma treatment are also investigated with EFTEM. It is found that for all the investigated spacings and widths of the aluminum lines, the sidewall layer (visible on Fig. 1(b)–(d)) consists of aluminum oxide. The Al-oxide residue layer increases in thickness with increasing spacing and linewidth. Almost no carbon is detected at the sidewall in all investigated samples which received the Cl2/BCl3/N2 plasma etching. In Fig. 3, elemental maps from specimen T2 are shown. Upstanding walls remain after the first H2O cleaning plasma as can be clearly seen in the aluminum elemental map. These upstanding walls consist mainly of aluminum oxide. No carbon is detected in these walls, and also carbon is not present at the sidewalls of the aluminum. The use of a H2O/CF4 plasma removes more easily residue layers as is detected in sample L3 with EFTEM. In sample T3, the addition of CF4 to the plasma introduces Ti-fluoride material in the corner underneath the Ti/TiN ARC as can be seen in Fig. 4. This Tifluoride area is encapsulated by a Ti-oxide layer due to exposure to the air after the treatment and due to oxygen originating from the resist. Also a deposition of fluorine on the sidewall is detected. Further treatment (T4: water rinse and T5: EKC-rinse) does not remove the Ti-fluoride. The presence of a fluoride in the final specimen is a possible concern for the reliability of devices. Removal of this Ti-fluoride area has to be investigated in further work.
4. Conclusion EFTEM analysis is used to analyze etch residues in metallization structures. Contrary to the expectation
111
Fig. 3. EFTEM images of O, N, Ti and Al from specimen T2.
Fig. 4. EFTEM images of Ti, F, N and O from specimen T3.
that a carbon-based polymer layer is responsible for the anisotropic etching of the Al structures, such layer could only be observed for the samples etched with the Cl2/ BCl3 chemistry and turns out to be absent for the lines for the Cl2/BCl3/N2 plasma. In the latter case, a thick Al-oxide layer is detected. Its role in the anisotropic etching process needs further investigation. Obviously the presence of the Ti-F material under the ARC layer for the samples T3–T5 cannot be detected with any other analysis technique because of masking effects or their limited lateral resolution and hence clearly illustrates the power of the EFTEM technique.
Acknowledgements This work is supported by the Flemish Institute for the encouragement of scientific and technological research in the industry (IWT).
References [1] Grogger W, Hofer F, Kothleitner G. Micron 1998;29: 43–51. [2] Bender H. Inst Phys Conf Ser 1999;164:593–602. [3] Tonotani J, Saito S, Nishimura E. J Electrochem Soc 1997;144:2142–6. [4] Li H, Baklanov M, Boullart W, Conard T, Brijs B, Vandervorst W, Maex K, Froyen L. Solid State Phenomena 1999;65–66:177–80.
Chemical and structural analysis of etching residue layers in semiconductor devices with energy filtering transmission electron microscopy S. Hensa,*, J. Van Landuyta, H. Benderb, W. Boullartb, S. Vanhaelemeerschb a
EMAT, RUCA, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium Interuniversity Microelectronics Center (IMEC), Kapeldreef 75, B-3001 Leuven, Belgium
b
Abstract The use of an energy-filtering field emission gun transmission electron microscope (CM30 FEG Ultratwin) allows, apart from imaging morphologies down to nanometer scale, the fast acquisition of high-resolution element distributions. Electrons that have lost energy corresponding to characteristic inner-shell loss edges are used to form the element maps. The production of Ultra Large-Scale Integration (ULSI) devices with dimensions below 0.25 mm requires among others the formation of a multilayer metallization scheme by means of repeatedly applying the deposition and etching of dielectrics and metals. In this work the evolution of the surface chemical species on etched Al lines in a post-etch cleaning process has been investigated by energy filtering transmission electron microscopy, with the aim to understand the role of each process step on the removal of the etching residues. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: EFTEM; Post-metal-etch strip; Aluminum
1. Introduction
2. Experimental
In this work elemental energy loss maps will be used to obtain detailed information about the formation of residue layers in a post-metal-etch cleaning process. The post-etch cleaning is important for preventing corrosion due to the remaining residue layers after reactive ion etching of the metallization structure. The main goal is to use energy filtering transmission electron microscopy (EFTEM) to study the composition and species of the dry-etch-related residues and their evolution during the cleaning process. This yields information about the cleaning step responsible for the removal of the different parts of the residue and to optimize the cleaning procedure.
Two different processes are studied. The first structure studied consists of regular lines of 0.4 mm wide and 1 mm spacing with a layer stack TiN/Ti/Al(0.2 at%Cu)/TiN/ Ti/Si which is patterned by i-line lithography. Dry etching is performed in a LAM TCP-9600 using Cl2/ BCl3 gases (L1). After etching, the wafers are transferred in-situ into a Downstream Quartz (DSQ) chamber, where either a H2O (L2) or H2O/CF4 (L3) plasma is used. After the DSQ treatment, the wafers are rinsed in the deionized (DI) water rinse station (L4). Also an ex-situ treatment in an aqueous organic mixture (EKC 265) is considered (L5). The second process is studied on lines with a width and spacing of 0.25, 0.4 and 1 mm with a layer stack TiN/ Ti/Al(0.2 at%Cu)/Ti/SiO2 patterned with DUV-lithography. The etching is now performed using Cl2/BCl3/ N2 gases (T1). After etching, these wafers received the same post-etch cleaning process as was used for the first
*Corresponding author. Tel.: +32-3-2180263; fax: +32-32180257. E-mail address: [email protected] (S. Hens).
1369-8001/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 0 ) 0 0 1 4 7 - 5
110
S. Hens et al. / Materials Science in Semiconductor Processing 4 (2001) 109–111
process (T2–T5). After each step of the multi-step strip process, one wafer is taken for analysis. The surface chemistry is studied by means of energy filtering transmission electron microscopy. Energy-filtered images are taken with a Philips CM30 FEG microscope (300 keV) equipped with a Gatan Imaging Filter. For image calculation and processing Gatan’s software Digital MicrographTM is used. The TEM crosssection specimens are prepared with a FIB wedge milling technique for which the surface is protected with a sputtered glass- and a Pt-layer.
3. Results and discussion Fig. 1 shows bright-field TEM [0 1 1]Si cross-section images of the etched structures for both processes. During the specimen preparation, several problems were noticed. During mechanical thinning of the samples, the Al-lines deformed completely. To avoid this problem, the specimens are thinned using focused ion beam milling. Because EFTEM analysis requires very thin specimens (thickness smaller than the mean free path of the electrons traversing the specimen [1]), the specimens are thinned with the FIB-wedge technique. This means that the sample is rotated a few degrees during the end stage of the ion milling to obtain a wedge-shaped specimen [2]. Although SEM investigation detected the presence of a sidewall polymer, preliminary TEM analysis did not show such a sidewall polymer. Sputtering of a protective glass layer on top of the structures before the sawing and thinning of the samples prevented the removal of the residue layers during sample
preparation. However, the use of such a sputtered glass layer which is deposited on a small piece of the wafers, did not fully prevent the fall over of upstanding residue layers during the specimen preparation. Therefore on the second set of samples a protective oxide layer is spun on the whole wafers to prevent damage of the residue layers. Fig. 1a shows a cross-section BF TEM [0 1 1]Si image of sample L1. A large undercut (108–166 nm) is seen under the Ti/TiN antireflective coating. The undercut is reduced in the second process (sample T1; Fig. 1(b)–(d)). This difference in undercut is related to the use of N2 in the etching plasma. Comparing the sidewall layer in L1 and T1, a difference in contrast is visible indicating another composition in both cases. The thickness of the sidewall layer increases for the second process with increasing spacing between the lines (Fig. 1(b) 25 nm, (c) 37 nm, (d) 84 nm). The thickness of the residue layer reduces at the base of the structure. EFTEM analysis is used to investigate these observations (Fig. 2). For the first metal-etch process, elemental maps reveal the formation of an Al-oxide and the presence of carbon in the undercut. Elemental mapping also shows the formation of Al-oxide on the sidewalls. The carbon elemental map verifies the formation of a carbon-like polymer on the sidewall to enhance the anisotropic etching of the aluminum lines [3]. In the elemental maps shown, a carbon cluster is detected, whereas other lines from the same specimen show a more homogeneous C distribution on the sidewall. Earlier XPS-measurements [4] indicated also the presence of chlorine in the sidewalls, which is not confirmed by the EFTEM measurements. Probably the Cl quickly evaporates from
Fig. 1. Bright-field TEM [0 1 1]Si image of (a) L1 (W ¼ S ¼ 0:4 mm), (b) T1 (W ¼ S ¼ 0:25 mm), (c) T1 (W ¼ S ¼ 0:4 mm) and (d) T1 (W ¼ S ¼ 1 mm) sample.
S. Hens et al. / Materials Science in Semiconductor Processing 4 (2001) 109–111
Fig. 2. EFTEM images of Al, N, O, C and Ti for specimen L1.
the specimen during the EFTEM analysis or it might be lost during the specimen preparation. Further analysis of the other specimens (L2–L5), showed the removal of the carbon-like sidewall residue layers as a consequence of the applied plasma treatment. All investigated specimens (L1–L5) show the large undercut with carbon and aluminum-oxide trapped inside. The consequences of the second plasma treatment are also investigated with EFTEM. It is found that for all the investigated spacings and widths of the aluminum lines, the sidewall layer (visible on Fig. 1(b)–(d)) consists of aluminum oxide. The Al-oxide residue layer increases in thickness with increasing spacing and linewidth. Almost no carbon is detected at the sidewall in all investigated samples which received the Cl2/BCl3/N2 plasma etching. In Fig. 3, elemental maps from specimen T2 are shown. Upstanding walls remain after the first H2O cleaning plasma as can be clearly seen in the aluminum elemental map. These upstanding walls consist mainly of aluminum oxide. No carbon is detected in these walls, and also carbon is not present at the sidewalls of the aluminum. The use of a H2O/CF4 plasma removes more easily residue layers as is detected in sample L3 with EFTEM. In sample T3, the addition of CF4 to the plasma introduces Ti-fluoride material in the corner underneath the Ti/TiN ARC as can be seen in Fig. 4. This Tifluoride area is encapsulated by a Ti-oxide layer due to exposure to the air after the treatment and due to oxygen originating from the resist. Also a deposition of fluorine on the sidewall is detected. Further treatment (T4: water rinse and T5: EKC-rinse) does not remove the Ti-fluoride. The presence of a fluoride in the final specimen is a possible concern for the reliability of devices. Removal of this Ti-fluoride area has to be investigated in further work.
4. Conclusion EFTEM analysis is used to analyze etch residues in metallization structures. Contrary to the expectation
111
Fig. 3. EFTEM images of O, N, Ti and Al from specimen T2.
Fig. 4. EFTEM images of Ti, F, N and O from specimen T3.
that a carbon-based polymer layer is responsible for the anisotropic etching of the Al structures, such layer could only be observed for the samples etched with the Cl2/ BCl3 chemistry and turns out to be absent for the lines for the Cl2/BCl3/N2 plasma. In the latter case, a thick Al-oxide layer is detected. Its role in the anisotropic etching process needs further investigation. Obviously the presence of the Ti-F material under the ARC layer for the samples T3–T5 cannot be detected with any other analysis technique because of masking effects or their limited lateral resolution and hence clearly illustrates the power of the EFTEM technique.
Acknowledgements This work is supported by the Flemish Institute for the encouragement of scientific and technological research in the industry (IWT).
References [1] Grogger W, Hofer F, Kothleitner G. Micron 1998;29: 43–51. [2] Bender H. Inst Phys Conf Ser 1999;164:593–602. [3] Tonotani J, Saito S, Nishimura E. J Electrochem Soc 1997;144:2142–6. [4] Li H, Baklanov M, Boullart W, Conard T, Brijs B, Vandervorst W, Maex K, Froyen L. Solid State Phenomena 1999;65–66:177–80.