Time dependent preferential sputtering in the HfO 2 layer on Si(100)

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1 Available online at Thin Solid Films 516 (2008) Time dependent preferential sputtering in the HfO 2 layer on Si(100) S.J. Chang a, W.C. Lee a, J. Hwang a,, M. Hong a, J. Kwo b a Department of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu City, Taiwan b Department of Physics, National Tsing Hua University, Hsin-Chu City, Taiwan Received 11 October 2006; received in revised form 11 April 2007; accepted 1 June 2007 Available online 13 June 2007 Abstract The time dependent preferential sputtering in the HfO 2 layer on Si(100) has been investigated in-situ with X-ray photoelectron spectroscopy during Ar ion sputtering. Hf4f, O1s, and Si2p spectra show that three bonding environments (Hf 0+ from the Hf metal, Hf 2+ from HfO, and Hf 4+ from HfO 2 ) co-exist inside the HfO 2 layer during sputtering. The Hf 4+ doublet decreases with sputtering time in an exponential-like function. Both Hf 0+ and Hf 2+ doublets increase with sputtering time in opposite ways. Two concurrent sputtering mechanisms characterizing the formation of HfO and Hf due to preferential sputtering of oxygen within the HfO 2 layer can well explain the detailed bond breaking and re-formation process. The Hf metal is the final product and the HfO is an intermediate product during sputtering under vacuum. The HfO cannot be removed and acts as a residual component in the HfO 2 layer Elsevier B.V. All rights reserved. Keywords: Hafnium dioxide; Preferential sputtering; Time dependent; X-ray photoemission 1. Introduction In the development of complementary metal-oxide-semiconductor (CMOS) technologies, the SiO 2 oxide layer is required to be replaced by advanced high-k gate dielectrics due to scaling miniaturization. High-k gate dielectrics allow a thicker dielectric layer such that leakage current is reduced. Among high-k dielectrics, hafnium dioxide (HfO 2 ) is considered an important candidate due to its relatively high dielectric constant, large band gap, and high thermal stability [1,2]. Because of the excellent physical characteristics of HfO 2 for gate dielectrics, many research activities including the deposition methods of ultra thin HfO 2 films and the dielectric properties of HfO 2 have been reported in recent years [3 6]. Etching of HfO 2 is critical in the development of CMOS technologies. Both dry and wet etchings have been developed to remove patterned HfO 2 [7,8]; however, HfO 2 is hard to be removed due to its low etching selectivity over Si substrate [9]. The incomplete removal of HfO 2 after etching processes causes Corresponding author. Tel.: ; fax: address: jch@mx.nthu.edu.tw (J. Hwang). high contact resistance in the source and drain regions. Recently, Chen et al. reported a substantial increase of the etching rate after Ar ion bombardment on HfO 2 prior to the etching process [10].A mixture of Hf, HfO x (0bxb2), and HfO 2 within the HfO 2 film is formed by energetic Ar ions due to the preferential sputtering. Ar ions sputtering is also able to induce both physical and chemical effects to change the HfO 2 composition during the sputtering process [11,12]. However, HfO 2 may interact with Si (100) substrate. Lebedinskii and Zenkevich discovered the silicide formation after sputtering of Ar ions onto HfO 2 at several kev [13]. The mechanism of silicide formation is attributed to the depletion of oxygen at the interface, resulting from the preferential sputtering and Hf Si mixing at the HfO 2 /Si interface. Although the preferential sputtering of oxygen in HfO 2 films had been reported in the past two decades [10 14], the detailed interaction mechanisms for the Ar ion sputtering process were rarely reported. In this paper, we have studied the time dependent preferential sputtering in the HfO 2 layer on Si (100). X-ray photoelectron spectroscopy (XPS) is used to measure in-situ the chemical bonding information in order to understand the sputtering mechanisms in the HfO 2 /Si(100) structure during the Ar ion sputtering /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.tsf

2 S.J. Chang et al. / Thin Solid Films 516 (2008) Experimental An HfO 2 film was deposited on a 2 in. n-type Si (100) wafer by electron-beam evaporation at room temperature. An HfO 2 ceramic target was used as the evaporation source for HfO 2 deposition, where the electron beam evaporator was operated at 1 kw under ultrahigh vacuum (UHV). Before the HfO 2 deposition, a Si (100) wafer was cleaned by the Radio Corporation of America (RCA) method. A buffered hydrofluoric (HF) acid solution was used to remove the native SiO 2 layer. The chamber pressure was maintained at Pa during deposition in order to prevent the possible formation of SiO 2. Other experimental details for the HfO 2 deposition process were described elsewhere [15,16]. The as-deposited HfO 2 film is amorphous, determined by reflection high-energy electron diffraction. The film thickness is about 10 nm, determined using ellipsometry and high-resolution transmission electron microscopy. The UHV-deposited HfO 2 on Si has achieved an atomically sharp oxide/si interface, an important aspect not obtained using atomic layer deposition (ALD). While the ALD grown dielectrics have exhibited good electrical properties, the inevitable formation of an interfacial layer (SiO 2 ) due to its inherent growth mechanism has lowered the dielectric constant of the overall gate stack, and has hampered further progress of reductions of the equivalent oxide thickness (EOT), defined as κ SiO2 *(thickness of high κ dielectrics)/(κ high κ ), to be substantially below 1.0 nm. The UHV-deposited HfO 2 on Si has employed them as templates to initiate subsequent ALD growth of high κ dielectrics, and to suppress effectively the unwanted interfacial layer formation near Si. Excellent electrical properties in terms of low electrical leakage current densities at a small EOT have been reported [17]. The chemical bonding and electronic states information were taken by PHI 1600 XPS using Al Kα ( ev). The Ar ion energy was Fig. 2. O1s spectra emitted from the HfO 2 /Si(100) structure at different Ar ion fixed at 3 kev, and the raster size was 2 2 mm 2 during Ar ion sputtering. 3. Results and discussion Fig. 1 shows a series of Hf4f spectra emitted from the HfO 2 / Si(100) structure at different Ar ion sputtering times. All the spectra are calibrated by setting the C1s peak at ev in order to remove the charging-induced energy shift due to X-ray exposure. Before sputtering, the Hf4f doublet appears at 17.1 ev (4f 7/2 ) and 18.7 ev (4f 5/2 ) in the binding energy scale which have been assigned to be emitted from the Hf 4+ ions in the HfO 2 structural configuration [18]. The previously reported energy position of the Hf4f doublet in HfO 2 varies from 16.6 ev to 18.2 ev [2,18 20]. The difference in the reported values Fig. 1. Hf4f spectra emitted from the HfO 2 /Si(100) structure at different Ar ion Fig. 3. Si2p spectra emitted from the HfO 2 /Si(100) structure at different Ar ion

3 950 S.J. Chang et al. / Thin Solid Films 516 (2008) Fig. 5. Concentrations of Hf and O in the HfO 2 thin layer at different Ar ion sputtering times. The Hf/O ratios are calculated by the peak area ratio of the Hf4f and O1s spectra. The relative sensitivities of Hf and O are and 0.711, respectively, used in the calculation. Fig. 4. Deconvolutions of Hf4f spectra taken from the HfO 2 /Si(100) structure sputtered by Ar ions for 120 s. The Hf4f spectrum taken from the as-cleaned Hf metal foil is used for comparison. probably results from the calibration process of the charginginduced energy shift. Two new features appear in the Hf4f spectra after sputtering. One is the reduction of the Hf4f intensity due to removal of Hf atoms during sputtering. The other is the appearance of two broad peaks next to the original Hf4f doublets on the lower-binding energy side. This implies that the formation of new Hf-related chemical bonds occurs during sputtering. The two broad peaks abruptly increase in intensity and gradually reach a saturated magnitude at sputtering time around 40 s. Fig. 2 shows the corresponding O1s spectra emitted from the HfO 2 /Si(100) structure at different Ar ion sputtering times. The O1s signal appears at ev, which is emitted from the O 2 ions in the HfO 2 structural configuration [1,21]. Similar to Hf4f doublets, the O1s signal also reduces with sputtering time. This implies that a removal process of Hf and O atoms occurs simultaneously during sputtering. However, no new O1s signals were generated and Table 1 The de-convoluted data extracted from the Hf and O spectra taken at different sputtering times Sputtering time (s) Concentrations (at.%/s.) Hf Hf 4+ Hf 2+ Hf 0+ O Three Hf bonding environments inside the HfO 2 layer are Hf 0+ from the Hf metal, Hf 2+ from HfO, and Hf 4+ from HfO 2. clearly observed as the Hf4f spectra in Fig. 1. The O1s signal slightly shifts toward lower binding energy by 0.2 ev after sputtering. This probably results from formation of new chemical bond of Hf and O. In order to characterize the possible chemical bond reformation leading to the generation of two new Hf4f broad peaks and the energy shift of O1s, two steps of analysis are used to clarify the mechanisms resulting in the changes in the Hf4f and O1s spectra. First, Si2p spectra are taken to make sure if the changes in the Hf4f and O1s spectra resulted from the interaction between HfO 2 and Si(100) during sputtering. Second, the Hf4f spectra are de-convoluted to identify the possible Hf-related bonds formed during sputtering. Fig. 3 shows a series of Si2p spectra emitted from the HfO 2 / Si(100) structure at different Ar ion sputtering times. Before sputtering, the Si2p signal appears at about 99.0 ev which has been assigned to be emitted from the Si Si (Si 0+ ) bonds in Si (100) [22]. The Si2p increases in intensity with sputtering time, which is an indication of the removal of the HfO 2 layer during sputtering. Note that no shift of the Si2p peak was observed as shown in Fig. 3. This supports that there is no interaction Fig. 6. The relative amount of three Hf components (Hf 0,Hf 2+,Hf 4+ ) at different Ar ion sputtering times. The three Hf components are derived from the deconvolutions of Hf4f spectra in Fig. 4.

4 S.J. Chang et al. / Thin Solid Films 516 (2008) between HfO 2 and Si(100) at the interface within the sputtering time period of 250 s in our study. Silicon thus plays no role in the changes in the Hf4f and O1s spectra in Figs. 1 and 2. The possible chemical bond formations during sputtering can be extracted from the de-convolution of the Hf4f spectra in Fig. 1. Fig. 4 shows the de-convoluted data of a typical Hf4f spectrum taken from the HfO 2 /Si(100) structure after Ar ion sputtering for 120 s. Three Hf4f doublets are required to obtain a consistent fit for all the Hf4f spectra in Fig. 1, and are assigned as Hf 0+ from the Hf metal, Hf 2+ from HfO, and Hf 4+ from HfO 2. The binding energies of the three doublets are summarized in Table 1. The Hf4f doublet taken from a clean Hf metal is inserted in Fig. 4 in order to confirm that the energy position of Hf4f 7/2 of the Hf 0+ doublet is at 14.0 ev in the binding energy scale. The clean Hf metal was prepared by sputtering an Hf metallic foil with Ar ions for 6 min in order to reveal the Hf4f 7/2 of the Hf 0+ doublet. The Hf4f 7/2 at 16.2 ev has been suggested as the Hf4f 7/2 of the Hf 2+ doublet from HfO during sputtering [19]. The Hf4f 7/2 at 17.1 ev is assigned as the Hf4f 7/2 of the Hf 4+ doublet from HfO 2, which exhibits the same energy position of that from the as-grown HfO 2 shown in Fig. 1. As mentioned earlier, the Si(100) substrate does not interact with the HfO 2 layer during sputtering in our study. The concentrations of Hf and O inside the HfO 2 layer are thus calculated based on Hf4f and O1s intensities and their relative sensitivities and plotted at different Ar ion sputtering times, as shown in Fig. 5. The O concentration decays with sputtering time in an exponential function. And the Hf concentration increases with sputtering time in an opposite way. Preferential sputtering of oxygen occurs in the HfO 2 layer since Hf concentration increases with sputtering time. Moreover, the sputtering rate of Hf and O are time dependent during sputtering. The detailed information of the preferential sputtering of oxygen can be extracted from the de-convoluted data of Hf spectra tabulated in Table 1. Fig. 6 shows relative concentrations of three Hf doublets (Hf 0+,Hf 2+,Hf 4+ ), plotted together with the total Hf concentration, at different sputtering times. Two interesting features are described as follows. First, the Hf 4+ doublet decreases with sputtering time in an exponential-like function. Second, both Hf 0+ and Hf 2+ doublets increase with sputtering time in opposite ways. The sputtering rates of Hf 0+ and Hf 2+ are about the same at sputtering time less than 40 s. As the sputtering time exceeds 40 s, Hf 2+ ceases to increase and Hf 0+ continues to increase gradually and to reach a saturated value. Two concurrent sputtering mechanisms are proposed to well explain the sputtering data of Hf, Hf 4+,Hf 0+,andHf 2+ shown in Fig. 6. The two sputtering mechanisms are stated in Eqs. (1) and (2) shown below. HfO 2ðsÞ Y Ar sputtering HfO ðsþ þ 1=2O 2ðgÞ HfO ðsþ Y Ar sputtering Hf ðsþ þ 1=2O 2ðgÞ : First, both Eqs. (1) and (2) can explain the preferential sputtering of oxygen since oxygen is in a form of gas and pumped out of the chamber. This is also in good agreement with ð1þ ð2þ the reduction of the Hf 4+ signal in the HfO 2 structural configuration as shown in Fig. 6. Second, the appearance of Hf 0+ and Hf 2+ signals simultaneously indicates that both equations occur at the same time during sputtering. Both Hf 0+ and Hf 2+ have about the same intensity at sputtering time less than 40 s, indicating that only part of the HfO bonds are sputtered to form Hf metallic bonds. At sputtering time longer than 40 s, Hf 2+ stays at a nearly constant magnitude. This suggests that the production rate of Hf 2+ (Eq. (1)) is about the same as the sputter rate of Hf 2+ (Eq. (2)). The nearly constant Hf 2+ intensity is also supported by the fact that the reduction rate of Hf 4+ equals to the production rate of Hf 0+ shown in Fig. 6. The time dependence of oxygen preferential sputtering is actually governed by Eqs. (1) and (2) to produce HfO and Hf, respectively. HfO is an intermediate product during sputtering, which serves as an intermediate state to move oxygen out of HfO 2. The existence of HfO makes the sputtering process more complex. The HfO cannot be removed and acts as a residual component in the HfO 2 layer until the HfO 2 layer are completely sputtered away. 4. Conclusions The sputtering rates of Hf and O in the HfO 2 layer are time dependent during Ar ion sputtering. Three types of Hf bonding environments (Hf 0+ from the Hf metal, Hf 2+ from HfO, and Hf 4+ from HfO 2 ) co-exist inside the HfO 2 layer, being resulted from the preferential sputtering of oxygen during sputtering. Two concurrent sputtering mechanisms characterizing the formation of HfO and Hf within the HfO 2 layer can well explain the detailed bond breaking and re-formation process. The Hf metal is the final product and the HfO is an intermediate product during sputtering under vacuum. The HfO cannot be removed and acts as a residual component in the HfO 2 layer. Acknowledgement The authors would like to thank the support from the National Science Council through NSC M References [1] M. Oshima, S. Toyoda, T. Okumura, J. Okabayashi, H. Kumigashira, K. Ono, M. Niwa, K. Usuda, N. Hirashita, Appl. Phys. Lett. 83 (2003) [2] G. He, M. Liu, L.Q. Zhu, M. Chang, Q. Fang, L.D. Zhang, Surf. Sci. 576 (2005) 67. [3] D. Triyoso, R. Liu, D. Roan, M. Ramon, N.V. Edwards, R. Gregory, D. Werho, J. 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