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Formation of ultra-thin PtSi film by vacuum annealing
Vacuum 65 (2002) 133–136
Formation of ultra-thin PtSi film by vacuum annealing Liu Shuanga,*, Zhong Zhiyonga, Ning Yonggonga, Chen Aia, Zhang Huaiwua, Yang Jiadeb a
Institute of Information Material Engineering, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China b Chongqing Optoelectronics Research Institute, Chongqing 400060, People’s Republic of China Received 7 August 2001; accepted 26 September 2001
Abstract The quantum efficiency of PtSi infrared detector is restricted by PtSi film thickness. A new approach to the formation of ultra-thin PtSi film is introduced in this article. Based on solid phase reaction theory, an ultra-thin PtSi film of thickness 4 nm is obtained using vacuum annealing and wet etching. The analysis results of X-ray diffractometer, X-ray photoelectron spectroscopy and transmission electron microscopy are also given. It is shown that the approach has advantages in terms of low reaction temperature and short reaction time, and a uniform, continuous and smooth PtSi film of thickness 4 nm is formed by the new approach. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: PtSi thin film; Vacuum annealing; XPS; XRD; TEM
1. Introduction Thin silicide films are becoming more important with increasing packing density of integrated circuits [1,2]. Platinum silicide is a candidate for applications in making both ohmic and Schottkybarrier contacts to MOS [2] and bipolar devices. Now, the super conducting transition has been observed on the thin PtSi films down to d ¼ 4 nm [3]. Since Platinum silicide has a relatively high work function, the most attractive application is the formation of Schottky-barrier junction which is important for 3–5 mm infrared charge coupled device (IRCCD) imager arrays with P-type silicon [4]. *Corresponding author. Tel.: +86-28-3202563; fax: +86-283254131. E-mail address: [email protected] (L. Shuang).
Previous works show that the quantum efficiency of PtSi infrared detector is restricted by PtSi film thickness, one important technique for fabricating high performance Schottky-barrier detectors is the formation of very thin films [5]. Very thin PtSi films have a larger coefficient of absorption than thick films and also in the range of longer wavelength [6]. At a thickness of 9 nm, the quantum efficiency is about 10 times greater than at a thickness of 78 nm [5], The Schottkybarrier of a PtSi film of 10 nm or less shows the highest infrared reflectivity. Therefore tight control of the PtSi layer uniformity and thickness has become very important for the optimal detector behavior. Most work on the formation of ultra-thin PtSi film has been performed over the last few decades. The common techniques for the formation of the
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 1 9 - 5
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L. Shuang et al. / Vacuum 65 (2002) 133–136
thin PtSi films are electron beam deposition or sputter deposition of a Pt layer in clean vacuum conditions. Then annealing is performed to obtain the metal silicides under protective gases. Silicidation by means of conventional furnace annealing processes (RTP) has been reported [4–7]. This paper, reports a new technique for forming ultra-thin (4 nm) PtSi films using sputtering for metal Pt deposition under 105 Pa; vacuum annealing, wet etching rich Pt. X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) are used for characterizing the phase of PtSi films and transmission electron microscopy (TEM) is used for determining the continuity, smooth, uniformity and thickness of the PtSi films.
Fig. 1. XRD spectra of the sample.
2. Experimental procedures The samples are fabricated on P-type Si(1 0 0) wafers with a resistivity of 1271 O cm. The substrates are chemically cleaned and etched in diluted HF for 2–3 min to remove the native oxide. The wafers are immediately loaded into the vacuum chamber for deposition prior to sputter at a base pressure of 105 Pa, the substrate is heated to 2001C for 2 min and kept at this temperature during the Pt sputter deposition and the working pressure is 103 Pa. Sputter power value is 0.8–1 kW, the target voltage is 400–450 V, and sputter time is about 2 min. The sputter Pt layer is of about 20 nm thickness. On taking out the sample and removing the un-reacted Pt by wet etching in H2O+HCl+HNO3 at 751C, etching time is 3 min. After the samples are cleaned, they are loaded into a vacuum chamber for annealing. Thermal annealing is performed at temperatures between 2501C and 3501C and a pressure of about 104 Pa. The ultra-thin PtSi films are obtained at an annealing time of 5 min. XRD, XPS and high resolution TEM are used to characterize the formed layers.
3. Results and discussion Fig. 1 shows XRD spectra of the sample, peak 3 is wider, its 2y is 431–451. When I=I0 ¼ 100; 2y of
Fig. 2. XPS Pt4f spectra and the curve fitting of the spectra of the sample.
PtSi and Pt2Si are 43.581 and 44.681, respectively, peak 3 shows that the sample phase is a mixture of PtSi and Pt2Si, and when peak 3 is very small and weak, it shows that the film is very thin. Because the film is very thin, the phase of PtSi film was characterized using XPS for chemical analysis. Fig. 2 shows XPS spectra of Pt4f and the curve fitting of the spectra [branching ratio is 3:4, spin orbit spitting line width and line position are kept constant]. The spectra are taken using Mg Ka as the source. The pass energy of the hemisphere experiment is conducted in VG MICROLAB MK II.XPS analyzer, which is set at 20 eV. The binding energy of XPS core-level is calibrated by C1s ðEB ¼ 284:6 eVÞ: Because the Pt4f7/2 line positions of PtSi and Pt2Si are 72.5 and 71.9 eV, respectively [8], they prove the phase result of XRD analysis, and the fact that the phases of the film are PtSi and Pt2Si. Figs. 3 and 4 show TEM crystal lattice images of sample. The sample is first cut into two pieces,
L. Shuang et al. / Vacuum 65 (2002) 133–136
Fig. 3. TEM crystal lattice image of sample (larger area).
Fig. 4. TEM crystal lattice image of sample (smaller area).
which are then glued together with the grown layers face to face. After that, the cross-section is mechanically thinned to a thickness of about 80 mm, followed by dimpling to a thickness of
135
about 10 mm. The dimpled sample is finally ion milled to electron transparent, and then observed in a field emission 300 kV Philips CM 300 TEM. To know more about the continuity, smoothness and uniformity of the films, the larger area TEM crystal lattice images of sample were developed. As in Fig. 3, it is seen that the film is very continuous, smooth and uniform. Fig. 4 shows smaller area high resolution, TEM crystal lattice images of sample, which shows that films thickness is about 4 nm. When a thin layer of platinum on excess silicon is annealed (in vacuum or inert atmosphere) the sequential appearance of two phases is observed [9]. Initially, the reaction between the platinum metal and silicon produces the first phase, Pt2Si, the Pt2Si subsequently reacts further with the silicon to form the second phase PtSi, until all the Pt2Si has been consumed: Pt+Si-Pt2Si+SiPtSi. The growth kinetics of both phases exhibit t1=2 time dependence [10], indicating diffusionlimited reactions. It is observed that the two phases each grow in a well-defined laterally uniform manner and the temperature dependence of the reaction rate each follows an Arrbenius law, with activation energies of 1.370.2 and1.570.2 eV for Pt2Si and PtSi, respectively [10], over the temperature range 200–3251C. Because the activation energies for Pt2Si are lower than for PtSi, the annealing at shorter time (2 min) and lower temperature (B2001C) can lead to make the sputtered Pt film forming an ultra-thin continuous Pt2Si layer at first, and when the working pressure is not high, oxygen from the ambient diffuses into the Pt layer and reacts with the front of Pt2Si phase to form an oxide layer. The oxide layer thus formed hinders further growth of the Pt2Si layer [11], therefore, the Pt2Si layer cannot become thick. A thinner sputtered Pt layer (20 nm) can ensure film continuity, and can obtain a continuous Pt2Si film. On wet etching the Pt metal, only Pt2Si layer is left on the substrates. The Pt2Si forms PtSi by annealing. The annealing in vacuum is better than in gases such as N2, Ar; it can reduce the affect of oxygen and make Pt2Si+Si-PtSi reaction complete. The continuity of this film was improved and its thickness remained ultra-thin.
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4. Conclusion Taking advantage of the fact that activation energy for Pt2Si is lower than for PtSi, and oxygen from the ambient can hinder further growth of the Pt2Si layer, the ultra-thin PtSi film can be obtained by two-step vacuum annealing and wet etching unreacted Pt. The ultra-thin PtSi film thickness is about 4 nm. The approach has advantages of low reaction temperature and short reaction time, and the formed PtSi ultra-thin films have good uniformity, continuity and smooth.
References [1] Jin S, Bender H, Donaton RA, Maex K. J Mater Res 1999;14(6):2577.
[2] Donaton RA, Jin S, Bender H, Conard T, Wolf ID, Maex K, Vantomme A, Langouche G. Electrochem Solid-State Lett 1999;2(4):195. [3] Oto K, Ishida S, Takaoka S, Murase K. J Appl Phys 1994;76(9):5339. [4] Wang LP, Yang JR, Wang JH. J Appl Phys 1993;74(10):6251. [5] Kimata M, Denda M, Daido M, Tsunoda R, Kanno T. Opt Eng 1987;26(3):209. [6] Uematsu S. Vacuum 1992;43:1039. [7] Tanabe A, Konuma K, Teranishi N, Tohyama S, Masubuchi K. J Appl Phys 1991;69(2):850. [8] Morgen SJ, Williams RH. Appl Surf Sci 1992;56–58:493. [9] Mcleod JE, Wandr MAE, Pretorius R, Comrie CM. J Appl Phys 1992;72(6):2232. [10] Crider CA, Poate JM, Rowe JE, Sheng TT. J Appl Phys 1982;52:2860. [11] Chin-An Chang. J Appl Phys 1985;58(3):1412.
Formation of ultra-thin PtSi film by vacuum annealing Liu Shuanga,*, Zhong Zhiyonga, Ning Yonggonga, Chen Aia, Zhang Huaiwua, Yang Jiadeb a
Institute of Information Material Engineering, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China b Chongqing Optoelectronics Research Institute, Chongqing 400060, People’s Republic of China Received 7 August 2001; accepted 26 September 2001
Abstract The quantum efficiency of PtSi infrared detector is restricted by PtSi film thickness. A new approach to the formation of ultra-thin PtSi film is introduced in this article. Based on solid phase reaction theory, an ultra-thin PtSi film of thickness 4 nm is obtained using vacuum annealing and wet etching. The analysis results of X-ray diffractometer, X-ray photoelectron spectroscopy and transmission electron microscopy are also given. It is shown that the approach has advantages in terms of low reaction temperature and short reaction time, and a uniform, continuous and smooth PtSi film of thickness 4 nm is formed by the new approach. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: PtSi thin film; Vacuum annealing; XPS; XRD; TEM
1. Introduction Thin silicide films are becoming more important with increasing packing density of integrated circuits [1,2]. Platinum silicide is a candidate for applications in making both ohmic and Schottkybarrier contacts to MOS [2] and bipolar devices. Now, the super conducting transition has been observed on the thin PtSi films down to d ¼ 4 nm [3]. Since Platinum silicide has a relatively high work function, the most attractive application is the formation of Schottky-barrier junction which is important for 3–5 mm infrared charge coupled device (IRCCD) imager arrays with P-type silicon [4]. *Corresponding author. Tel.: +86-28-3202563; fax: +86-283254131. E-mail address: [email protected] (L. Shuang).
Previous works show that the quantum efficiency of PtSi infrared detector is restricted by PtSi film thickness, one important technique for fabricating high performance Schottky-barrier detectors is the formation of very thin films [5]. Very thin PtSi films have a larger coefficient of absorption than thick films and also in the range of longer wavelength [6]. At a thickness of 9 nm, the quantum efficiency is about 10 times greater than at a thickness of 78 nm [5], The Schottkybarrier of a PtSi film of 10 nm or less shows the highest infrared reflectivity. Therefore tight control of the PtSi layer uniformity and thickness has become very important for the optimal detector behavior. Most work on the formation of ultra-thin PtSi film has been performed over the last few decades. The common techniques for the formation of the
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 4 1 9 - 5
134
L. Shuang et al. / Vacuum 65 (2002) 133–136
thin PtSi films are electron beam deposition or sputter deposition of a Pt layer in clean vacuum conditions. Then annealing is performed to obtain the metal silicides under protective gases. Silicidation by means of conventional furnace annealing processes (RTP) has been reported [4–7]. This paper, reports a new technique for forming ultra-thin (4 nm) PtSi films using sputtering for metal Pt deposition under 105 Pa; vacuum annealing, wet etching rich Pt. X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) are used for characterizing the phase of PtSi films and transmission electron microscopy (TEM) is used for determining the continuity, smooth, uniformity and thickness of the PtSi films.
Fig. 1. XRD spectra of the sample.
2. Experimental procedures The samples are fabricated on P-type Si(1 0 0) wafers with a resistivity of 1271 O cm. The substrates are chemically cleaned and etched in diluted HF for 2–3 min to remove the native oxide. The wafers are immediately loaded into the vacuum chamber for deposition prior to sputter at a base pressure of 105 Pa, the substrate is heated to 2001C for 2 min and kept at this temperature during the Pt sputter deposition and the working pressure is 103 Pa. Sputter power value is 0.8–1 kW, the target voltage is 400–450 V, and sputter time is about 2 min. The sputter Pt layer is of about 20 nm thickness. On taking out the sample and removing the un-reacted Pt by wet etching in H2O+HCl+HNO3 at 751C, etching time is 3 min. After the samples are cleaned, they are loaded into a vacuum chamber for annealing. Thermal annealing is performed at temperatures between 2501C and 3501C and a pressure of about 104 Pa. The ultra-thin PtSi films are obtained at an annealing time of 5 min. XRD, XPS and high resolution TEM are used to characterize the formed layers.
3. Results and discussion Fig. 1 shows XRD spectra of the sample, peak 3 is wider, its 2y is 431–451. When I=I0 ¼ 100; 2y of
Fig. 2. XPS Pt4f spectra and the curve fitting of the spectra of the sample.
PtSi and Pt2Si are 43.581 and 44.681, respectively, peak 3 shows that the sample phase is a mixture of PtSi and Pt2Si, and when peak 3 is very small and weak, it shows that the film is very thin. Because the film is very thin, the phase of PtSi film was characterized using XPS for chemical analysis. Fig. 2 shows XPS spectra of Pt4f and the curve fitting of the spectra [branching ratio is 3:4, spin orbit spitting line width and line position are kept constant]. The spectra are taken using Mg Ka as the source. The pass energy of the hemisphere experiment is conducted in VG MICROLAB MK II.XPS analyzer, which is set at 20 eV. The binding energy of XPS core-level is calibrated by C1s ðEB ¼ 284:6 eVÞ: Because the Pt4f7/2 line positions of PtSi and Pt2Si are 72.5 and 71.9 eV, respectively [8], they prove the phase result of XRD analysis, and the fact that the phases of the film are PtSi and Pt2Si. Figs. 3 and 4 show TEM crystal lattice images of sample. The sample is first cut into two pieces,
L. Shuang et al. / Vacuum 65 (2002) 133–136
Fig. 3. TEM crystal lattice image of sample (larger area).
Fig. 4. TEM crystal lattice image of sample (smaller area).
which are then glued together with the grown layers face to face. After that, the cross-section is mechanically thinned to a thickness of about 80 mm, followed by dimpling to a thickness of
135
about 10 mm. The dimpled sample is finally ion milled to electron transparent, and then observed in a field emission 300 kV Philips CM 300 TEM. To know more about the continuity, smoothness and uniformity of the films, the larger area TEM crystal lattice images of sample were developed. As in Fig. 3, it is seen that the film is very continuous, smooth and uniform. Fig. 4 shows smaller area high resolution, TEM crystal lattice images of sample, which shows that films thickness is about 4 nm. When a thin layer of platinum on excess silicon is annealed (in vacuum or inert atmosphere) the sequential appearance of two phases is observed [9]. Initially, the reaction between the platinum metal and silicon produces the first phase, Pt2Si, the Pt2Si subsequently reacts further with the silicon to form the second phase PtSi, until all the Pt2Si has been consumed: Pt+Si-Pt2Si+SiPtSi. The growth kinetics of both phases exhibit t1=2 time dependence [10], indicating diffusionlimited reactions. It is observed that the two phases each grow in a well-defined laterally uniform manner and the temperature dependence of the reaction rate each follows an Arrbenius law, with activation energies of 1.370.2 and1.570.2 eV for Pt2Si and PtSi, respectively [10], over the temperature range 200–3251C. Because the activation energies for Pt2Si are lower than for PtSi, the annealing at shorter time (2 min) and lower temperature (B2001C) can lead to make the sputtered Pt film forming an ultra-thin continuous Pt2Si layer at first, and when the working pressure is not high, oxygen from the ambient diffuses into the Pt layer and reacts with the front of Pt2Si phase to form an oxide layer. The oxide layer thus formed hinders further growth of the Pt2Si layer [11], therefore, the Pt2Si layer cannot become thick. A thinner sputtered Pt layer (20 nm) can ensure film continuity, and can obtain a continuous Pt2Si film. On wet etching the Pt metal, only Pt2Si layer is left on the substrates. The Pt2Si forms PtSi by annealing. The annealing in vacuum is better than in gases such as N2, Ar; it can reduce the affect of oxygen and make Pt2Si+Si-PtSi reaction complete. The continuity of this film was improved and its thickness remained ultra-thin.
136
L. Shuang et al. / Vacuum 65 (2002) 133–136
4. Conclusion Taking advantage of the fact that activation energy for Pt2Si is lower than for PtSi, and oxygen from the ambient can hinder further growth of the Pt2Si layer, the ultra-thin PtSi film can be obtained by two-step vacuum annealing and wet etching unreacted Pt. The ultra-thin PtSi film thickness is about 4 nm. The approach has advantages of low reaction temperature and short reaction time, and the formed PtSi ultra-thin films have good uniformity, continuity and smooth.
References [1] Jin S, Bender H, Donaton RA, Maex K. J Mater Res 1999;14(6):2577.
[2] Donaton RA, Jin S, Bender H, Conard T, Wolf ID, Maex K, Vantomme A, Langouche G. Electrochem Solid-State Lett 1999;2(4):195. [3] Oto K, Ishida S, Takaoka S, Murase K. J Appl Phys 1994;76(9):5339. [4] Wang LP, Yang JR, Wang JH. J Appl Phys 1993;74(10):6251. [5] Kimata M, Denda M, Daido M, Tsunoda R, Kanno T. Opt Eng 1987;26(3):209. [6] Uematsu S. Vacuum 1992;43:1039. [7] Tanabe A, Konuma K, Teranishi N, Tohyama S, Masubuchi K. J Appl Phys 1991;69(2):850. [8] Morgen SJ, Williams RH. Appl Surf Sci 1992;56–58:493. [9] Mcleod JE, Wandr MAE, Pretorius R, Comrie CM. J Appl Phys 1992;72(6):2232. [10] Crider CA, Poate JM, Rowe JE, Sheng TT. J Appl Phys 1982;52:2860. [11] Chin-An Chang. J Appl Phys 1985;58(3):1412.