Founded 1905 APPLICATION OF TITANIUM SILICIDE AS AN INTERCONNECT IN DEEP SUBMICRON INTEGRATED CHIP MANUFACTURING TAN CHENG CHEH, DENNIS (B.Eng. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF PH.D. DEPARTMENT OF MECHANICAL ENGINEERING THE NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to a number of individuals who have assisted in making the project possible and successful. Firstly, I would like to express my utmost appreciation to my supervisor Assoc. Prof. Lu Li, my mentors from Chartered Semiconductor Manufacturing Pte Ltd Dr. Alex See and Dr. Lap Chan for providing me with the opportunity for postgraduate study, but most crucially, for their invaluable supervision, advice and guidance throughout my candidature. It is both my privilege and honor to work with them. I would also like to acknowledge the National University of Singapore, Chartered Semiconductor Manufacturing Pte Ltd, and the National Science and Technology Board for providing me with the scholarship and financial assistance. Special thanks and appreciation to the staff and students of Materials Science Laboratory at the Department of Mechanical Engineering of the National University of Singapore for their technical expertise and enjoyable comradeship; namely, Mr. Thomas Tan Bah Chee, Mdm Zhong Xiang Li, Mr. Ng Hong Wei, Mr. Maung Aye Thein , Mr. Chua Beng Wah, Mr Gary Wong and Ms Sharon Nai. Special thanks and appreciation also to the staff from Chartered Semiconductor Manufacturing for their technical expertise; namely, Dr. Ho Chaw Sing, Dr. Randall Cha, Mr. Lim Eng Wah, Mr. Yap Kuan Pei, Mr. Tee Kheng Chok, Dr. Chen Shao Yin, Mr. Lai Chung Who, Dr. Soo Choi Pheng, Ms Ko Lian Hoon and Mr Pang Chong Hau. i Most importantly, I am eternally indebted to my family, especially my parents, for their unvarying support and encouragement. Lastly, I would like to express my gratitude to my most beloved wife, Ms. Teresa Soh, for her untiring patience and understanding. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS i TABLE OF CONTENTS iii SUMMARY vii LIST OF FIGURES ix LIST OF TABLES xiv LIST OF ABBREVIATION xv CHAPTER INTRODUCTION 1.1 Background 1.2 Objectives 1.3 Scope 1.4 Organization of Thesis CHAPTER LITERATURE SURVEY 2.1 Titanium Silicide in Integrated Chip manufacturing 2.2 Titanium Salicide Process 2.3 Area-Dependency of C49-to-C54 TiSi2 Phase Transformation 2.4 Pre-Amorphization Implant and Implant Through Metal 2.5 Kinetics of Thin Film TiSi2 Formation 2.6 Comparison with Other Metal Silicide 11 2.7 Rapid Thermal Annealing 12 iii 2.8 Thermal Budget 13 2.9 Differential Scanning Calorimeter 13 2.10 Kissinger’s Analysis 15 2.11 Raman Spectroscopy 16 2.12 X-Ray Diffraction 17 2.13 Scherrer’s Equation 19 CHAPTER EFFECTS OF RAMP UP RATES ON THE SALICIDE PROCESS 21 3.1 Introduction 21 3.2 Experimental Procedures 21 3.3 Results 23 3.4 Discussion 31 3.5 Summary 34 CHAPTER EFFECT OF SILICON SUBSTRATE AMORPHIZATION ON THE KINETICS OF THE REACTION BETWEEN THE TITANIUM THIN FILM AND SILICON 36 4.1 Introduction 36 4.2 Experimental Procedures 37 4.3 Results 40 4.3.1 Differential Scanning Calorimetry Data 40 iv 4.3.2 X-ray Diffraction Spectra 41 4.3.3 Crystallite Size Evaluation from XRD Spectra 44 4.3.4 Activation Energy Calculations for C49-TiSi2 Formation 46 4.3.5 Activation Energy Calculations for C49-TiSi2 Precursor Phase 4.4 Discussion 54 4.4.1 Deposition of Ti on Crystalline or Amorphous Si 54 4.4.2 Effect of Degree of Amorphization on TiSi2 Formation 56 4.4.3 Activation Energy 62 4.4.4 Effect of Degree of Amorphization on C49-TiSi2 Crystallite Size 4.5 52 Summary CHAPTER 67 67 STUDY OF TITANIUM SILICIDE FORMATION USING SPIKE ANNEALS 69 5.1 Introduction 69 5.2 Experimental Procedure 70 5.3 Results 71 5.3.1 Sheet Resistance 71 5.3.2 Gate-to-Source/Drain Leakage Current 77 5.3.3 Microscopic Characteristics 80 5.4 Discussion 81 5.5 Summary 88 v CHAPTER CONCLUSIONS 89 CHAPTER FUTURE RECOMMENDATIONS 91 7.1 Introduction 7.2 Spike Anneal with Pre-Amorphization-Implant/ 91 Implant-Through-Metal 91 7.3 Multi-Spike Anneal 92 7.4 Induction heating 94 CHAPTER REFERENCE 98 vi SUMMARY Titanium silicide (TiSi2) has been the choice interconnect for MOS (MetalOxide-Semiconductor) devices due to its low electrical resistivity, good thermal stability and low silicon consumption. Other than these advantages, most importantly titanium silicide can be integrated into a self-aligned silicide process (SAlicide). This is a process that does not require additional lithography while it selectively forms titanium silicide on areas where it is required: exposed silicon surfaces on the MOS device. This can be easily achieved because titanium when in contact with silicon forms titanium silicide when heated. Although titanium silicide is thermally stable at high temperatures in a face centred orthorhombic structure (C54-TiSi2), there exists a meta-stable phase at lower temperatures with a base centred orthorhombic structure (C49-TiSi2) which first forms. The final C54 phase is desired over the C49 phase titanium silicide not only because it is thermally stable, but also more importantly it has a low electrical resistivity which is essential to the performance of the MOS device. Demand for faster electronics led to higher circuit density in MOS devices and smaller feature sizes. It was then discovered that the phase transformation of titanium silicide from C49 phase to C54 phase is areadependent. The current challenge is to understand the obstacles inhibiting this phase change, to understand some of the common techniques used to overcome these obstacles while exploring new methods with existing manufacturing tools. The kinetics of the phase transformation and properties of the resulting titanium silicide from different processing techniques are hence studied in the present investigation. Both new and improved processing techniques based on current vii manufacturing tools are investigated and integrated into an existing manufacturing process, while newer novel techniques are explored and investigated. viii LIST OF FIGURES Figure 2-1. Schematic diagram of a typical Ti-salicide process. Figure 2-2. Schematic diagram of the differential scanning calorimeter 14 Figure 2-3. Schematic of a laser-Raman spectroscopy system. 16 Figure 2-4. Diffraction of x-rays by a crystal. 18 Figure 3-1(a). Sheet resistance of silicided undoped poly Si lines of varying line widths after RTA2. 23 Figure 3-1(b). Sheet resistance of silicided undoped poly Si lines of varying line widths after etchback. 24 Figure 3-2(a). Micro-Raman spectra for TiSi2 on undoped 0.35 µm poly Si lines after RTA2. 25 Figure 3-2(b). Micro-Raman spectra for TiSi2 on undoped 0.33 µm poly Si lines after RTA2. 26 Figure 3-2(c). Micro-Raman spectra for TiSi2 on undoped 0.30 µm poly Si lines after RTA2. 27 Figure 3-2(d). Micro-Raman spectra for TiSi2 on undoped 0.28 µm poly Si lines after RTA2. 27 Figure 3-2(e). Micro-Raman spectra for TiSi2 on undoped 0.25 µm poly Si lines after RTA2. 28 Figure 3-3(a). Gate-to-source/drain leakage current for specimens at 0.35 µm line width. 29 Figure 3-3(b). Gate-to-source/drain leakage current for specimens at 0.33 µm line width. 29 Figure 3-3(c). Gate-to-source/drain leakage current for specimens at 0.30 µm line width. 30 ix Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 0.00115 -9.2 (a) 1/Tm 0.0012 0.00125 0.00111 -9.2 (b) 0.00131 -9.6 Inφ/Tm2 Inφ/Tm2 -9.6 1/Tm 0.00121 -10 -10 -10.4 -10.4 -10.8 -10.8 0.00111 -9.2 (c) 1/Tm 0.00121 0.00131 Inφ/Tm2 -9.6 -10 -10.4 -10.8 Figure 4-8. Kissinger plots for the C49 TiSi2 pre-cursor phase formation on different specimens: (a) specimen D: 8x1014 cm-2 Si implant, (b) specimen E: 1x1015 cm-2 Si implant, (c) specimen F: 45nm amorphous Si deposition. Table 4-4. Activation energy for the C49-TiSi2 pre-cursor phase on Si substrates with different degrees of amorphization. Specimen Substrate description D Implant dose 8E14 cm-2 E Implant dose 1E15 cm-2 F 45 nm amorphous Si deposition Activation Energy C49-TiSi2 [eV] Correlation factor 2.49 ± 0.11 -1.00 2.45 ± 0.16 -0.99 2.35 ± 0.11 -1.00 Page 53 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 4.4 Discussion 4.4.1 Deposition of Ti on crystalline or amorphous Si In the as-deposited condition, specimen A whose Ti film was deposited on a crystalline Si substrate does not show Ti diffraction maxima (Figure 4-3) while the asdeposited specimen F (Figure 4-4) shows a strong Ti (101) diffraction peak. Since the C49-TiSi2 phase was formed subsequently at higher temperatures on all specimens, this proves the presence of Ti, only that there is no X-ray diffraction line for Ti (101) at θ = 2°. This observation suggests that perhaps the thin Ti film had a preferred orientation when it was deposited onto crystalline Si. To verify this postulate, a θ-2θ XRD scan was performed on specimen A after different thermal anneal treatments. Figure 4-9 shows the XRD spectra of specimen A annealed at different temperatures at a heating rate 60 Kminute-1. Strong Ti (002) and Ti (101) diffraction peaks were observed. This implies that the crystal orientation of the deposited Ti on crystalline (100) Si had a preferred (002) orientation whereas in the case of Ti on amorphous Si, the orientation of Ti was more random. It is also noted that the Ti(101) diffraction peak is the dominant peak for published values of randomly orientated Ti powders. Page 54 Ti (101) Ti (002) Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon * * 600°C * * 570°C * * 550°C * 520°C Intensity (a.u.) * * * 36 as dep. 41 46 51 θ (degree) Figure 4-9. θ-2θ x-ray diffraction scans of specimen A after different thermal annealing treatments. After thermal treatment of the specimen A, the two Ti diffraction peaks are shifted to lower diffraction angles, implying diffusion of Si into the Ti film. As a result of the change in the amount of silicon concentration in Ti, the lattice parameter was increased [36]. This is also seen in Figure 4-4 where the Ti (101) peak shifts to a lower diffraction angle with an increase in temperature. Page 55 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 4.4.2 Effect of Degree of Amorphization on TiSi2 formation Figure 4-2 show the DSC curves of specimens A to G where the heat flows, dH/dt, from various different specimens are plotted as a function of temperature. Distinct exothermic peaks are observed from all DSC measurements. No reaction was observed in the DSC curve for specimen G in Figure 4-2. This indicates that the DSC peaks observed from specimens A to F were not due to any re-crystallization from the amorphized Si film, nor were they due to reactions between the silicon substrate and the platinum pan. Hence, peaks X and Y in Figure 4-3 refer to reactions between the thin Ti film (45 nm) and the Si substrate. Heat flow from the reaction of the thin Ti film on the Si substrates can be measured because reduction in thermal mass by back grinding the specimen raised the sensitivity of the DSC to the heat from the reaction. Although the high heating rates resulted in high thermal lag, good contact between the flat sample and the metallic pan reduces this. As the heat due to the bulk heat capacity is high compared to the heat from the thin film reaction, it has to be removed by running the same sample twice and taking the difference between the two readings. This helps to isolate the heat release from the irreversible reaction. Almost all DSC curves in Figures 4-2 show one weak and one strong exothermic peak marked by X and Y. In general the DSC curves in Figure 4-2 can be divided into two categories. In the first category, the exothermic peak X with low formation enthalpy Page 56 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon can only be observed at a high heating rate (Figures 4-5(a) and (b)) while in the second category, peak X can clearly be seen even at a low heating rate (Figures 4-5(c) to (f)). After thermal annealing at 520 °C, a peak that matches with the Ti5Si3 phase is observed from specimen A (Figure 4-3). This suggests that the first DSC peak, X, shown in Figure 4-2(a) could be due to the formation of the Ti5Si3 phase. The same temperature was chosen for the annealing of specimen F. It is interesting to note that instead of the formation of the Ti5Si3 phase, the Ti5Si4 phase or Ti5Si4 with a trace of the Ti5Si3 phase was formed. This indicates that the resultant phase is controlled by the substrate structure. Thermodynamic analyses using Gibbs function were used to access the change of the excess Gibbs free energies of the TiSi2, Ti5Si3 and Ti5Si4 phases via various reactions. These are then plotted onto an Ellingham plot across a temperature range similar to that of the DSC scans. Reactions r1 to r5 correspond to Equations (4-6) to (4-10) respectively where T is the temperature in Kelvin. All the thermodynamic data have been collected from Barin [37], Chase and co-workers (JANAF Tables) [38], Cox and co-workers [39] and Kubaschewski and Alcock [40]. Crystalline Si with a diamond structure is used for the calculations. ∆G (Ti5Si3) = -476.889 + 2.295T [kJ/mol] (r1: 5Ti + 3SiÆ Ti5Si3) (4-6) ∆G (TiSi2) = -746.594 - 0.122T [kJ/mol] (4-7) (r2: Ti5Si4 + 6SiÆ 5TiSi2) ∆G (TiSi2) = -113.792 + 0.463T [kJ/mol] (r3: Ti + 2SiÆ TiSi2) (4-8) ∆G (TiSi2) = -92.073 + 0.021T [kJ/mol] (4-9) (r4: Ti5Si3 + 7SiÆ 5TiSi2) ∆G (Ti5Si4) = 117.633 + 2.438T [kJ/mol] (r5: 5Ti + 4SiÆ Ti5Si4) (4-10) Page 57 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 50 r5. 5Ti + 4Si → Ti5Si4 r4. Ti5Si3 + 7Si → 5TiSi2 ∆G(kJ/mol) -50 r3. Ti + 2Si → TiSi2 -100 r2. Ti5Si4 + 6Si → 5TiSi2 -150 r1. 5Ti + 3Si → Ti5Si3 -200 -250 50 250 450 650 Temperature (°C) Figure 4-10. Ellingham plot of Ti-Si reactions. Change in Gibbs free energy for per mole of Si reactant. Crystalline Si in a tetrahedral structure is used for the calculations. Reaction r5 cannot take place since ∆G>0. As shown in Figure 4-10, the driving force for the formation of Ti5Si3 in the reaction r1 is always negative, indicating the possibility of formation. It is, however, noted from r5 that the reaction of 5Ti + 4SiÆ Ti5Si4 is not possible since the excess Gibbs free energy of the formation of Ti5Si4 phase is always positive. This analysis agrees well with the XRD results where there was no Ti5Si4 formed on specimen A where Ti was deposited on the crystalline Si wafer. Although the excess Gibbs energy for the Page 58 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon formation of TiSi2 is negative, it is more positive in comparison with that of the formation of Ti5Si3. Therefore the formation of TiSi2 is not favorable. With further heating to 570°C, the formation of C49-TiSi2 was discovered. It is noted from Figure 4-3 that the intensities of both C49-TiSi2 (131) and Ti5Si3 (112) peaks increased with heating temperature, indicating that the amount of the C49-TiSi2 and Ti5Si3 phases was raised at the same time. In general, the formation of C49-TiSi2 phase can take place through two routes, Ti5Si3 + 7Si Æ 5TiSi2 and Ti + 2Si Æ TiSi2. Although the driving force for the reaction between Ti and Si is more favorable than that between Ti5Si3 and Si, the reaction between Ti and Si would not take place since there exists a Ti5Si3 reaction layer between the unreacted Ti and Si substrate. Therefore, the second exothermic peak in the DSC plot is the formation of the TiSi2 from the precursor of Ti5Si3: ∆G (TiSi2) = -0.041 + 0.004T [kJ/mol] (Ti5Si3 + 7Si Æ 5TiSi2) (4-11) For the amorphized specimen F, after amorphization of the Si substrate, the Gibbs free energy of Si would be raised. The isobaric Gibbs free energy of formation of amorphous Si (a-Si) relative to the crystalline Si (c-Si) phase is given by [41]: T T ∆C p (T ' ) T0 T' ∆Gac (T ) = ∆H ac (T0 ) + ∫ ∆C p (T ' )dT ' − T∆S ac0 − T ∫ dT ' (4-12) where ∆Hac(T) is the enthalpy difference between a-Si and c-Si at a certain temperature T, ∆Cp(T) is the specific heat difference between a-Si and c-Si, and ∆Soac is the entropy difference at T = K between both phases. To find the Gibbs free energy of formation of Page 59 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon amorphous Si, we use figures obtained previously by Stolk et al. in Table 4-5 [42]. Stolk and co-workers prepared a-Si by implanting Si+ at energies ranging from 150 keV to MeV, with a dose of × 1015/cm2 onto Si clamped to a copper block cooled with liquid nitrogen. Relaxation of a-Si was done at 500 °C for hour. Using the figures presented by Stolk et al. [42], the excess Gibbs free energies for the reaction 5Ti + 4Si Æ Ti5Si4 is plotted in Figure 4-11. From Figure 4-11, it can be seen that although formation of Ti5Si4 is not favorable on crystalline Si (r3) or relaxed a-Si (r2), its formation is possible on a highly strained amorphized Si substrate as seen in r1. This explains the absence of Ti5Si4 on specimen A and the presence of it on specimen F. Table 4-5. Parameters used in the free-energy calculations of Figure 4-11. Structural state as-implanted (77K) a-Si as-implanted (RT) a-Si 500 °C relaxed a-Si ∆θb ∆Hexc ∆S0ac (deg) (KJ/mol) (R) 0.0135 13.3 12.6 0.26 2.52 1012 ± 150 0.0045 11.3 5.2 0.22 1.63 1261 ± 50 0.0003 9.0 0.0 0.20 1.00 1412 ± 20 χ a η Tm (K) χ is the defect fraction, ∆θ is the bond-angle distortion, ∆Hexc is the calculated excess enthalpy relative to the 500 °C relaxed a-Si state, ∆Soac is the 0K enthalpy, η is the scaling parameter for the specific heat, and Tm is the calculated melting temperature. Page 60 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 25 r3. 5Ti + 4c-Si → Ti5Si4 20 ∆G(kJ/mol) 15 r2. 5Ti + 4a-Si(2) → Ti5Si4 10 r1. 5Ti + 4a-Si(1) → Ti5Si4 -5 -10 50 250 450 650 Temperature (°C) Figure. 4-11. Ellingham plot of Ti5Si4 formation from Ti reacting with Si of different structures: r1: with as-implanted (77K) a-Si(1); r2: with relaxed (at 500 °C) a-Si(2); r3: Ti with crystalline (tetrahedral) Si. Excess Gibbs free energy normalized to per mole of Si reactant. It is noted from Figure 4-4 that the formation of C49-TiSi2 in the amorphized specimen starts from 550 °C, which is lower than that in specimen A at about 600 °C. The amount of the C49-TiSi2 phase increases with an increase in the annealing temperature. In addition to the formation of TiSi2 at 550 °C in specimen F, a transition from Ti5Si4 to Ti5Si3 was also detected after annealing at 600 °C with almost complete disappearance of Ti5Si4 (Figure 4-4). Finally Ti5Si3 phase disappears at 620°C, which is lower than in that of the specimen A. Therefore, the second exothermic peak Y was associated with the formation of C49-TiSi2. Page 61 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon XRD results shown in Figures 4.3 and 4.4 also indicate the existence of a critical level of amorphization below which the formation of Ti5Si3 dominates and above which the formation of Ti5Si4 is pronounced. 4.4.3 Activation energy In Table 4-3, the activation energy for the formation of C49-TiSi2 shows an increase with an initial increase in degree of substrate amorphization followed by a decrease. The activation energies range from 1.53~2.04 eV which agree with previous research. Various authors have reported that the activation energies for thin film C49TiSi2 formation are 1.26~3.1 eV [12-17]. The discrepancies are mainly due to different sample preparation and testing methodologies. Murarka and Fraser reported an activation energy of 1.8 eV by measuring the change in electrical resistivity to track the amount of C49-TiSi2 formed from 1000 Å Ti deposited onto 4300 Å poly Si [12]. Using backscattering spectroscopy, Hung and coworkers reported an activation energy of 1.8 eV from 900 Å Ti deposited on 2500 Å of amorphous Si [13]. Thompson and co-workers employed electrical resistivity, they reported an activation energy of 1.47 eV for 800 Å of co-evaporated TiSi2 on Si substrate and 1.26 eV on a SiO2 substrate [14]. Clevenger and co-workers used the maximum rate of change of resistivity (dR/dt = 0) as an indication of the maximum reaction rate, and reported an activation energy of 2.6 eV for 575 Å of Ti deposited on poly Si [15]. Ma and co-workers [16] employed DSC to track 10 multilayers of Ti/a-Si (19.2:10.8 nm) films deposited onto photoresist-coated glass slides, and reported an activation energy of 3.1 Page 62 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon eV. More recently, Stark and co-workers [17] used backscattering spectroscopy measurements on 80 nm of Ti sputtered onto Si wafers annealed to different temperatures and obtained an activation energy of 2.3 eV. As an interconnect used in integrated circuit manufacturing, TiSi2 is usually formed via the salicide process which involves annealing of single Ti film which were deposited on Si wafers. In the present study, preparation of samples was done as close to the actual IC manufacturing scheme as possible. As seen in Tables 4-3 and 4-4, activation energies for the formation of the C49TiSi2 precursor phase are higher than that of the C49-TiSi2 phase indicating that the ease of forming the precursor phase is slower than that of the C49-TiSi2 phase. However, the scarcity of Si in the Ti film in the early stages of the Ti-Si reaction prevented the formation of TiSi2 thus permitting the formation of Ti5Si4. Si is established in previous studies as being the main diffusing species in the Ti-Si reaction [43]. The activation energies for the formation of the C49-TiSi2 precursor shown in Table 4-4 reduces with an increasing degree of Si substrate amorphization. This reduction in activation energy is actually due to the increase in the Gibbs free energy of the initial state of the Si substrate, as schematically represented in Figure 4-12. The reason is that the Gibbs free energy increases with increasing degrees of amorphization due to a higher strained structure. Hence, the combined Gibbs free energy of say Ti and Si, in specimen F, G2, would be higher than that of specimen D, G1. This would result in specimen F having a lower activation energy than D. Page 63 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon G Ea2 Ea1 Activated state G2 G1 Initial state Final state T Figure 4-12. Schematic diagram of the formation of precursor phases for specimens A and F, where the initial Gibbs free energy of specimens A and F are G1 and G2 respectively. The absence of DSC peak X for specimens A, B and C in Figure 4-5 can also be explained by the reduction in activation energy with increasing degrees of amorphization observed in Table 4-4. With the reduction in Si implant dose, the degree of Si substrate amorphization and crystal lattice strain would be lower in specimens A, B and C. This would mean that the activation energies for the formation of the C49-TiSi2 precursor phase in specimens A, B and C would be higher. Hence, a slower rate of formation could be expected, and coupled with the fact that the formation enthalpy of formation of Ti5Si3 (-73.8 ± 2.0 kJ/mol) is lower compared to that of TiSi2 (-170.9 ± 8.3 kJ/mol) [44, 45], the DSC peak would be too subtle to be observable. In fact, we can observe this in Figures 45 (a), (b) and (c) where the DSC peak X was absent at the lower heating rates but was later observable at the higher heating rates. This observation indicates that the formation Page 64 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon of the TiSi2 precursor phase was present but its slow rate of reaction and formation enthalpy obscures it in DSC scans with low heating rates. Table 4-3 shows the activation energies for the formation of C49-TiSi2 in specimens A to F. The activation energies for specimens A and B are about the same. The highest activation energy can be seen from the specimen C followed by a continuous decrease in the activation energy with increase in Si substrate amorphization. From XRD measurements, it is known that there are two kinds of precursors formed at low temperature during annealing, namely, Ti5Si3 and Ti5Si4, which have different activation energies for the transition to TiSi2. Measurement shows that the activation energy for the transition from Ti5Si3 to C49-TiSi2 is higher than that from Ti5Si4 to C49-TiSi2. We believe that without sufficient Si substrate amorphization, Ti5Si4 could not be formed as indicated by the Ellingham plots in Figure 4-10. Although the lower degrees of Si substrate amorphization could not lead to the formation of Ti5Si4, it could however increase the rate of Ti5Si3 formation. Hence, a larger amount of Ti5Si3 would form prior to C49-TiSi2. This reduces the overall Gibbs free energy of the system, and causes the initial increase in the activation energy for C49-TiSi2 formation from specimens A and B to C as in Table 4-3 which is represented in Figure 4-13. From Figure 4-13, it is clear that the formation of the precursor leads to an increase in activation energy for C49-TiSi2 formation. Page 65 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon G Ea1 G1 Ea2 G2 Initial state Formation of pre-cursors Final State – C49 T Figure 4-13. Schematic diagram on the formation of precursor phases in specimen F leading to a drop in Gibbs free energy from G1 to G2. When the degree of amorphization of Si substrate is further increased, Ti5Si4 forms. The Gibbs free energy of Ti5Si4 is higher than that of its reactants Ti and Si as shown in Figure 4-10. This causes the sudden decrease in the activation energy for formation of the C49-TiSi2 phase for specimens C and D. With increasing Si substrate amorphization, the grain size of newly-formed Ti5Si4 may become smaller and the energetic state of Ti5Si4 becomes higher. Due to the increase in the initial energy state of Ti5Si4 with increasing amorphization, the activation energy to the C49-TiSi2 transition is, therefore, reduced. This is clearly indicated by the measurements of the activation energy, which continuously decrease from specimens D to F. Page 66 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 4.4.4 Effect of Degree of Amorphization on the C49-TiSi2 Crystallite Size Peak broadening of the XRD spectra can be induced either by reduction in crystallite size or the presence of microstrains within the diffracting domains. However, the Scherrer equation assumes broadening of diffraction peaks due purely to crystallite size. Although strain in TiSi2 films have been reported [46-48], a uniform residual strain does not cause broadening of XRD peaks, only a non-uniform residual strain will result in broadening of XRD peaks. The mean crystallite size in specimen F is smaller than that in specimen A, indicating a higher C49-TiSi2 nucleation rate. Previous TEM studies by Kittl and Hong shows smaller C49-TiSi2 grains as a result of pre-amorphized implants [49]. Combined with results from sections 4.4.2 and 4.4.3, a substantial amount of Ti5Si4 and the unknown phase present in specimen F resulted in a higher nucleation rate and thus smaller C49-TiSi2 grains. The smaller grains would in turn enhance the C49-to-C54 TiSi2 phase transformation at the higher temperatures as C54-TiSi2 nucleates on the triple-grain boundary junctions of C49-TiSi2. 4.5 Summary 1. The formation of TiSi2 by thermal annealing of a single titanium film deposited on silicon substrates with varying degrees of amorphization has been detected using a differential scanning calorimeter. An exothermic peak corresponding to the formation of C49-TiSi2 has been observed in all cases. Page 67 Effect of Silicon Substrate Amorphization on the Kinetics of Reaction between Titanium Thin Film and Silicon 2. The activation energy for the formation of the C49-TiSi2 was found to be in the range of 1.53~2.04 eV. 3. With increasing Si substrate amorphization, the activation energy first increases before it decreases. 4. A second exothermic peak was recorded in the DSC traces when there was a sufficient degree of Si substrate amorphization. This new peak occurs before the formation of C49-TiSi2 and was not due to recrystallization of the Si substrate. 5. XRD spectra suggest that the new exothermic peak could be due to the formation of Ti5Si3 and Ti5Si4. 6. It is believed that the formation of Ti5Si3 reduces the Gibbs free energy of the system prior to C49-TiSi2 formation. This resulted in an increase in activation energy for the formation of C49-TiSi2. 7. A higher degree of Si substrate amorphization raises the Gibbs free energy to allow formation of Ti5Si4. Although the formation of Ti5Si4 also reduces the Gibbs free energy of the system, the free energy activation barrier for the formation of C49-TiSi2 reduces. 8. This suggests that the Gibbs free energy of Ti5Si4 is higher than its reactants in crystalline form: Ti and Si, which is also in agreement with the calculated Gibbs free energies. Page 68 [...]... explain the success of this technique The use of a spike anneal on the formation of titanium silicide on electronic structures down to 0.275 micrometers is studied The integration of this technique into the existing manufacturing process of integrated chips was investigated Last but not least, the feasibility of other new novel techniques will be investigated Page 2 Introduction 1.4 Organization of Thesis... remaining parts of the thesis are organized as follows: Chapter Two provides background information on titanium silicide and some of the experimental techniques used in this research including the formation of titanium silicide on designated areas of the chip, comparison with other metal silicides and current techniques to overcome the area-dependency of the C49-to-C54 titanium silicide phase transformation... Differential Scanning Calorimetry ITM Implant Through Metal PAI Pre-Amorphizing Implant RTA Rapid Thermal Annealing TEM Transmission Electron Microscopy XRD X-Ray Diffraction xv Introduction Chapter 1 Introduction 1.1 Background Since the invention of the integrated circuit (IC) by Kilby & Noyce in 1959, investigation of the materials and their integration into the manufacturing processes has been an integral... many of these metal silicides contain secondary and metastable phases, hence in- depth studies of their properties are needed before any metal silicide can successfully be integrated into the IC chip Titanium silicide has been widely used as an interconnect in the Metal-OxideSemiconductor (MOS) device for many technological generations due to its low resistivity and good thermal stability However, the application. .. phase conversion, but be low enough to prevent agglomeration of the C54-TiSi2 film 2.3 Area-Dependency of C49-to-C54 TiSi2 Phase Transformation The difficulty of using titanium silicide as a metal silicide begins to show at deep sub-micron device features The sheet resistance of titanium silicide after RTA2 increases as the linewidth decreases below 0.35 µm, and at 0.25 µm the sheet resistance was... resistivity, but also on the ease with which the silicide can be formed Other criteria also include the amount of Si substrate consumed and the main diffusing species during formation Titanium silicide is widely used as an interconnect to the gate and source/drain regions due to its low resistivity and good thermal stability [1] Titanium silicide first forms as the metastable C49 phase, a body centred orthorhombic... Process Titanium salicide or the self-aligned silicide process manufactures a film of titanium silicide selectively on the gate and source/drain regions of the MOSFET Since only areas where the silicon surface is exposed can react with the titanium film, the process is achieved without the use of an additional masking step, thus, the phrase: Self-Aligned Figure 2-1 shows the 4 basic steps in the titanium. .. characterized using microRaman spectroscopy The effect of different ramp-up rates on the formation of the titanium salicide is also studied The process employed in 0.25-micron technology for the formation of C54 phase titanium silicide is studied To understand the kinetics of the phase transformation of titanium silicide, differential scanning calorimetry and X-ray diffraction are used Based on these... 2 is titanium deposition, which is usually done using Physical Vapour Deposition A timelag between steps 1 and 2 of not more than 8 hours prevents any excessive SiO2 from reforming before titanium is deposited In order to avoid contamination of the titanium film, a layer of titanium nitride can sometimes be deposited onto the titanium film as seen in Figure 2-1 Step 3 is the first Rapid Thermal Anneal... part of the development for faster and more powerful integrated circuit chips This is because performance of the integrated circuit is very much affected by material selection and its successful integration into existing manufacturing processes Study on metal silicides forms a large part of this investigation because they can make low resistance and reliable contacts to shallow p-n junctions However many . Founded 1905 APPLICATION OF TITANIUM SILICIDE AS AN INTERCONNECT IN DEEP SUBMICRON INTEGRATED CHIP MANUFACTURING TAN CHENG CHEH, DENNIS (B.Eng. (Hons.),. studied in the present investigation. Both new and improved processing techniques based on current vii manufacturing tools are investigated and integrated into an existing manufacturing process,. Background Since the invention of the integrated circuit (IC) by Kilby & Noyce in 1959, investigation of the materials and their integration into the manufacturing processes has been an integral