TiW Film Research Articles

Thermal and oxidation stability of TixW1−x diffusion barriers investigated by soft and hard x-ray photoelectron spectroscopy

The binary alloy of titanium-tungsten (TiW) is an established diffusion barrier in high-power semiconductor devices, owing to its ability to suppress the diffusion of copper from the metallization scheme into the surrounding silicon substructure. However, little is known about the response of TiW to high-temperature events or its behavior when exposed to air. Here, a combined soft and hard x-ray photoelectron spectroscopy (XPS) characterization approach is used to study the influence of post-deposition annealing and titanium concentration on the oxidation behavior of a 300 nm-thick TiW film. The combination of both XPS techniques allows for the assessment of the chemical state and elemental composition across the surface and bulk of the TiW layer. The findings show that in response to high-temperature annealing, titanium segregates out of the mixed metal system and upwardly migrates, accumulating at the TiW/air interface. Titanium shows remarkably rapid diffusion under relatively short annealing timescales, and the extent of titanium surface enrichment is increased through longer annealing periods or by increasing the bulk titanium concentration. Surface titanium enrichment enhances the extent of oxidation both at the surface and in the bulk of the alloy due to the strong gettering ability of titanium. Quantification of the soft x-ray photoelectron spectra highlights the formation of three tungsten oxidation environments, attributed to WO2, WO3, and a WO3 oxide coordinated with a titanium environment. This combinatorial characterization approach provides valuable insights into the thermal and oxidation stability of TiW alloys from two depth perspectives, aiding the development of future device technologies.

Microstructure and stress gradients in TiW thin films characterized by 40 nm X-ray diffraction and transmission electron microscopy

The functionality of TiW diffusion barrier is often correlated with the presence of impurity atoms, diffusion of neighbor layer atoms, and depletion of titanium from the film and seldom attributed to the microstructure of the film. Here in this work, morphological aspects of sputter deposited TiW thin films as well as the influence of process parameters on residual stress of the film were investigated to understand their role in the functionality of TiW as a barrier. Two films, one tensile (+2 GPa) and another with compressive stress (−2 GPa), deposited by varying process conditions show distinctive differences in their microstructure. Both films were composed of α–W crystal structure but film with compressive stress has densely packed microstructure with no inter-granular porosity unlike to the tensile stressed film, where substantial porosity was present. Furthermore, TiW films were studied using a cross-sectional synchrotron 40 nm X-ray nano-diffraction technique which revealed that a gradual stress relaxation takes place in tensile stressed TiW film during its growth while in the compressive film no such relaxation was observed. The stress relaxation is mainly attributed to the increase in grain size during film growth of the tensile film. Based on these investigations, a growth model was put forth to correlate the deposition conditions and morphology of the TiW films grown.

Molybdenum Capping Layer Effect on Electromigration Failure of Plasma Etched Copper Lines

Currently, chemical mechanical polishing (CMP) rather plasma etching is the most popular production process for Cu lines [1,2]. This is due to the low volatility of the copper halide, e.g., CuClx or CuBrx, at room temperature [3,4]. A plasma-based Cu etch process was invented by the authors’ group [5-7]. This process has been successfully used in fabricating ICs and TFT LCDs [8,9]. The Cu layer was converted into a CuClx or CuBrx layer in a parallel-plate plasma reaction, which was then stripped off with a HCl solution. The Cu conversion rate can be very high, e.g., 140 nm/min at room temperature. The reliability of the Cu line has been studied using the electromigration (EM) stress method [10,11]. The TiW capping layer effect on the Cu lifetime was reported [11]. In this study, the molybdenum (Mo) capping layer effect on the Cu failure process is studied. Mo/Cu and Mo/Cu/Mo stacks were deposited on the Corning glass and etched into a 4-point pattern for the EM test. All films were sputter deposited. The Mo capping and barrier layers were reactive ion etched in a PlasmaTherm 700C system at CF4/O2 10/10 sccm, 60 mTorr, 600W for 2 minutes. The Cu to CuClx conversion reaction was done in the same RIE reaction at HCl/CF4 20/5 sccm, 70mTorr, 600 W for 2 minutes. The CuClx layer was dissolved in a H2O:HCl (8:1 v/v) solution for 1 min. The Cu samples were stressed at room temperature at fixed current densities (J). Changes of line color and shape were recorded. Figure 1 shows the top view of a 30 µm wide Mo/Cu/Mo line after J = 0.6 × 106 A/cm2 stress for 10 cycles. The line broken spot occurred at the cathode or anode pad which was not observed with the TiW capped Cu line. This is because the mechanical property of the Mo film is weaker than the TiW film is [12,13]. When the probe needle was applied to the pad, a high stress was generated locally, which initiated the failure process. Figure 2(a) shows the resistance-time (R-t) curve and temperature-time (T-t) curve of a 30 μm Mo/Cu/Mo line stressed at J = 0.4 × 106 A/cm2. The resistance gradually increased with the increase of the stress time, and the resistance increased dramatically near the line broken time. The temperature of the Cu line was calculated using the JEDEC standard guidelines of JESD33-B [14]. Since the temperature increase is caused from the Joule heating effect, the T-t curve follows the same trend as that of the R-t curve. Figure 2(b) shows the top view of the Mo/Cu/Mo line at different stress time at J = 0.4 × 106 A/cm2. The original line had the sage green color. It changed to the aquamarine color with the increase of the stress time. A white region appeared before the line was broken, which became dark upon the line breakage. The color change is related to the change of the local temperature. The EM failure phenomena of the Mo/Cu and Mo/Cu/Mo lines were compared. The result will be discussed from their R-t and T-t curves as well as the color changes. The capping layer material and properties are important to the EM lifetime of the Cu line. Authors acknowledge the financial support of this work through NSF CMMI project 1633580. 1. A. Gladkikh and Y. Lereah, Appl. Phys. Lett., 66(10), 1214 (1995). 2. C. S. Hau-Riege, S. P. Hau-Riege and A. P. Marathe, J. Appl. Phys., 96(10), 5792 (2004).3. Y. Kuo and J.R. Crowe, JVST A 8(3), 1529 (1990). 4. H. Miyazaki, K. Takeda, N. Sakuma, S. Kondo, Y. Homma and K. Hinode., JVST B 15(2), 237 (1997).5. Y. Kuo and S. Lee, Jpn. J. Appl. Phys., 2(39), L188 (2000). 6. S. Lee and Y. Kuo, J. Electrochem. Soc., 148, G254 (2001). 7. Y. Kuo and S. Lee, Appl. Phys. Lett., 78, 1002 (2001). 8. Y. Kuo and S. Lee, Vacuum, 74(3-4), 473 (2004).9. J. Yang, Y. Ahn, J. Bang, W. Ryu, J. Kim, J. Kang, M. S. Yang, I. Kang, I. Cung, ECS Trans., 16(9), 13 (2008).10. C. S. Hau-Riege, Microelectronics Reliability, 44, 195 (2004).11. M. Li, J. Q. Su and Y. Kuo, 235th ECS Meeting, Abst. #120271, May 26-31 (2019).12. R. Farraro, R.B. McClellan, Metal. Trans. A, 8(10), 1563 (1977).13. A. Roshanghias, G. Khatibi, R. Pelzer and J. Steinbrenner, Surf. and Coatings Tech., 259, 386 (2014).14. EIA/JESD33-B, Standard Method of Measuring and Using the Temperature Coefficient of Resistance to Determine the Temperature of a Metallization Line (2004). Figure 1

Electromigration of Plasma Etched Copper Lines of Various Widths and Lengths

Copper (Cu) interconnect lines are widely used in advanced, high-density ICs. Compared with aluminum (Al), Cu has many advantages, such as the higher conductivity and longer lifetime. However, Cu is difficult to etch into fine lines using the plasma etching method because the reaction product is nonvolatile. Another problem of Cu interconnect is that it has poor adhesion to the dielectric film unless an adhesion layer is used. Recently, Kuo’s group solved the etching problem with a novel room-temperature process that consumes the patterned Cu thin film with a plasma reaction and then removes the reaction product with a liquid solution [1,2]. This method has been used in the fabrication of ICs and TFT LCDs [3]. One of the most critical issues in applying Cu lines in products is the reliability - lifetime prediction, which is usually done by the electromigration (EM) measurement [4]. Previously, the Kuo’s group has studied temperature and mechanical bending effects on the Cu fine line’s lifetime [5,6]. However, there are few studies on the geometry effects of the Cu line on the lifetime. In the paper, authors investigated the relationship between the width or length of the Cu line and the EM lifetime. The TiW (10 nm)/Cu (200 nm) thin film stack was sputter deposited on a Corning glass. The TiW film was deposition at 5 mTorr in Ar at 75 W for 15 minutes. The Cu film was deposited at 10 mTorr in Ar at 80W for 60 minutes. The Cu film was patterned with a lithography process using a line-and-space mask. Then, the sample was exposed to a Cl2 /CF4 plasma in the PlasmaTherm 700C system operated in the RIE mode at room temperature. The condition was Cl2/CF4 10/5 sccm at 50 mTorr, 600 W for 3 minutes, corresponding to -VDC = 237 V [7]. After the RIE reaction, the sample was dipped in the 10:1 diluted HCl solution to dissolve the CuClx reaction product. Then, the underneath TiW was RIE etched with CF4 5 sccm at 40 mTorr, 600W for 2 minutes corresponding to -VDC = 273V. The failure time was determined by stressing a sample with the constant current density at room temperature until the current suddenly jumped by several orders of magnitude. Relationships between the line width or length and the EM failure time at different current densities were measured, as shown in Figures 1 and 2, where “L” means the line length and “W” means the line width. Several conclusions can be summarized from these figures. First, the line failure time is shortened with the increase of the current density. Second, under the same current density, the narrow line has a longer lifetime than the wide line. Third, the influence of the line length to the failure time is highly dependent on the line width. In Fig. 1, for 30 µ wide lines, the 800 µ long line has a slightly longer lifetime than that of the 400 µ long line. However, in Fig. 2, for 10 µ wide lines, the 20 µ long line has a lifetime slightly longer than that of the 200 µ long line at the current density of 2.5x106 A/cm2. Separately, for the same 20 µ line length, the lifetime of the 3 µ wide line is longer than that of the 10 µ wide line. For all 10 µ wide lines, the lifetime appears to be longer with the shortening of the line length. The further study on the Cu line width and length effect for narrow Cu lines, i.e., less than 3 µ, is in progress of which the result will be reported. In addition, the breakdown of the Cu line may be related to the edge roughness or the TiW/Cu interface quality, which will also be clarified and reported. Authors acknowledge to support of this work through the NSF CMMI 1633580 project. 1. Y. Kuo and S. Lee, Jpn. J. Appl. Phys. 39(3AB), L188-L190 (2000). 2. Y. Kuo and S. Lee, Appl. Phys. Lett., 78(7), 1002-1004 (2001). 3. Y. Kuo, invited, Proc. 16th Intl. Workshop on Active-Matrix Flat Panel Displays and Devices, 211-214 (2009). 4. J. R. Black, IEEE Reliability Physics Symposium, 142-149 (1974). 5. G. Liu and Y. Kuo, Electrochem. Soc., 156(7), H579-H584. (2009). 6. C. C. Lin and Y. Kuo., Journal of Applied Physics 111(6), 265-10130 (2012). 7. S. Lee and Y. Kuo, Electrochem. Soc., 148(9), G524-G529 (2001). Figure 1

Wafer-level Au–Au bonding in the 350–450 °C temperature range

Metal thermocompression bonding is a hermetic wafer-level packaging technology that facilitates vertical integration and shrinks the area used for device sealing. In this paper, Au–Au bonding at 350, 400 and 450 °C has been investigated, bonding wafers with 1 µm Au on top of 200 nm TiW. Test Si laminates with device sealing frames of 100, 200, and 400 µm in width were realized. Bond strengths measured by pull tests ranged from 8 to 102 MPa and showed that the bond strength increased with higher bonding temperatures and decreased with increasing frame width. Effects of eutectic reactions, grain growth in the Au film and stress relaxation causing buckles in the TiW film were most pronounced at 450 °C and negligible at 350 °C. Bond temperature below the Au–Si eutectic temperature 363 °C is recommended.

X-ray Stress Evaluation in Phase Change GeSbTe Material and TiW Electrodes

Stress generation and relaxation upon Ge2Sb2Te5 (GST) crystallization were studied by X-ray diffraction technique, which allows the stress in either GST or Ti3W7 (TiW) to be evaluated independently within TiW/GST/TiW multilayer film. The GST crystallization results in tensile stress in the GST film and additional compressive stress in the TiW film, due to volume shrinkage of the GST film. Moreover, the tensile stress in the GST film is significantly increased when it is sandwiched between TiW films. Interfacial effect is proposed to attribute the dependence of stress on the capping layer thickness.

Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications

This paper describes the ohmic contacts to single-crystalline 3C-SiC thin films heteroepitaxially grown on Si (001) wafers. In this work, a TiW (titanium–tungsten) film was deposited as a contact material by RF magnetron sputter and annealed through the vacuum and rapid thermal anneal (RTA) process. Contact resistivity between the TiW film and the n-type 3C-SiC substrate was measured by the circular transmission line model (C-TLM) method. The contact phases and interface of the TiW/3C-SiC were evaluated with X-ray diffraction (XRD), scanning electron microscope (SEM) and Auger electron spectroscopy (AES) depth-profiles. The TiW film annealed at 1000°C for 45s with the RTA plays an important role in the formation of ohmic contact with the 3C-SiC film and the contact resistance is less than 4.62×10−4Ωcm2. Moreover, the inter-diffusion at the TiW/3C-SiC interface was not generated during, before and after annealing, and was kept in a stable state. Therefore, the ohmic contact formation technology of single-crystalline 3C-SiC films by using the TiW film is very suitable for high-temperature micro-electro-mechanical system (MEMS) applications.

초고온 MEMS용 다결정 3C-SiC의 Ohmic Contact 특성

This paper describes the ohmic contact formation of polycrystalline 3C-SiC films deposited on thermally grown Si wafers. In this work, a TiW (titanium tungsten) film as a contact material was deposited by RF magnetron sputter and annealed with the vacuum process. The specific contact resistance (<TEX>${\rho}_{c}$</TEX>) of the TiW contact was measured by using the C-TLM (circular transmission line method). The contact phase and interfacial reaction between TiW and 3C-SiC at high-temperature as also analyzed by XRD (X-ray diffraction) and SEM (scanning electron microscope). All of the samples didn't show cracks of the TiW film and any interfacial reaction after annealing. Especially, when the sample was annealed at <TEX>$800^{\circ}$</TEX> for 30 min., the lowest contact resistivity of <TEX>$2.90{\times}10{\Omega}cm^{2}$</TEX> was obtained due to the improved interfacial adhesion. Therefore, the good ohmic contact of polycrystalline 3C-SiC films using the TiW film is very suitable for high-temperature MEMS applications.

GaN MSM UV photodetectors with titanium tungsten transparent electrodes

GaN metal-semiconductor-metal (MSM) ultraviolet photodetectors with titanium tungsten (TiW) transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW film deposited with a 300-W RF power can still provide a reasonably high transmittance of 75.1% at 300 nm, a low resistivity of 1.7/spl times/10/sup -3/ /spl Omega//spl middot/cm and an effective Schottky barrier height of 0.773 eV on u-GaN. We also achieved a peak responsivity of 0.192 A/W and a quantum efficiency of 66.4% from the GaN ultraviolet MSM photodetector with TiW electrodes. With a 3-V applied bias, it was found that minimum noise equivalent power and maximum D/sup */ of our detector were 1.987/spl times/10/sup -10/ W and 6.365/spl times/10/sup 9/ cmHz/sup 0.5/W/sup -1/, respectively.

초고온 MEMS용 단결정 3C-SiC의 Ohmic Contact 형성

This paper describes the ohmic contact formation of single crystalline 3C-SiC thin films heteroepitaxially grown on Si(001) wafers. In this work, a TiW (Titanium-tungsten) film as a contact matieral was deposited by RF magnetron sputter and annealed with the vacuum and RTA (rapid thermal anneal) process respectively. Contact resistivities between the TiW film and the n-type 3C-SiC substrate were measured by the C-TLM (circular transmission line model) method. The contact phases and interface the TiW/3C-SiC were evaulated with XRD (X-ray diffraction), SEM (scanning electron microscope) and AES (Auger electron spectroscopy) depth-profiles, respectively. The TiW film annealed at <TEX>$1000^{\circ}C$</TEX> for 45 sec with the RTA play am important role in formation of ohmic contact with the 3C-SiC substrate and the contact resistance is less than <TEX>$4.62{\times}10^{-4}{\Omega}{\cdot}cm^{2}$</TEX>. Moreover, the inter-diffusion at TiW/3C-SiC interface was not generated during before and after annealing, and kept stable state. Therefore, the ohmic contact formation technology of single crystalline 3C-SiC using the TiW film is very suitable for high temperature MEMS applications.

GaN MSM photodetectors with TiW transparent electrodes

TiW films were deposited onto glass substrates and GaN epitaxial layers by RF magnetron sputtering. It was found that TiW film deposited with a 300 W RF power could provide us a high transmittance and a low resistivity. GaN-based ultraviolet metal–semiconductor–metal (MSM) photodetectors were also fabricated. It was found that photocurrent to dark current contrast ratios were 4.35, 4.6 × 10 2 and 5.7 × 10 4 for the photodetectors with ITO, TiN, and TiW electrodes, respectively. The large photocurrent to dark current contrast ratio could be attributed mainly to the fact that TiW can form a high Schottky barrier height on the surface of u-GaN epitaxial layers.

Study of TiW/Au Thin Films Metallization Stack for High Temperature and Harsh Environment Devices on 6H Silicon Carbide

Study of TiW/Au Thin Films Metallization Stack for High Temperature and Harsh Environment Devices on 6H Silicon Carbide

Influence on the Long‐Term Stability of Polysilicon Resistors from Traces of Titanium and Tungsten

Trace amounts of a residue, formed during patterning of the TiW film by etching in , cause a significant improvement in the long‐term stability of polysilicon integrated circuit resistors. The stabilization is the result of a reduction in the amount of hydrogen reaching the grain boundary dangling bonds during processing. This appears to be due to a retardation in the diffusion of hydrogen caused by an ability of the TiW residue on the borophosphosilicate glass surface to change the relative amounts of atomic and molecular hydrogen. The amounts of hydrogen involved in the resistivity changes have been evaluated. Typically, for 500 nm thick boron‐implanted chemical vapor deposited polysilicon films with a grain size of 150 nm, the concentration of grain boundary traps was . The amount of hydrogen passivating dangling bonds after hydrogen annealing and plasma‐enhanced chemical vapor deposition nitride passivation was . After a 1000 h electrical and thermal stress at 150°C saturated the resistance drift, the amount of hydrogen that left the dangling bonds was . The presence of TiW residue reduced the amount of hydrogen reaching the dangling bond by and that leaving after stressing by . © 2000 The Electrochemical Society. All rights reserved.

Ti concentration effect on adhesive energy at Cu/TiW interface

Changes in adhesive energy at Cu/TiW interfaces caused by varying the Ti concentration were evaluated by means of contact angle measurement. The adhesive energy was evaluated by applying the Young–Dupré equation to the contact angle. Copper particles were fabricated by annealing thin Cu film deposited on the TiW film. The adhesive energies at the Cu/TiW interface were evaluated as 1.5, 2.1, and 2.6 N/m for a Ti concentration of 0, 10, and 20 wt. %, respectively. The adhesive energies were found to increase almost linearly as the Ti concentration was increased. These results were applied to prevent TiW/Cu/TiW interconnects fabricated by using infrared-assisted reactive ion etching from peeling at the Cu/TiW interface. In annealing Cu films on TiW substrates at 600 °C in a vacuum, it was found that the Cu peeled from the TiW when the Ti concentration was 10 wt. %, but it stuck to that at 20 wt. %. The effect of Ti on the adhesion strength was also studied from the results of molecular calculation by using the method.

Evaluation of Cu adhesive energy on barrier metals by means of contact-angle measurement

The adhesive energies at Cu/SiO2, Cu/TiN, and Cu/TiW interfaces were investigated by measuring the contact angles of Cu particles to the substrate. To form the Cu particles, thin Cu films deposited on each substrate were annealed at 500 °C for 50 h. The adhesive energies are determined to be 0.8, 1.8, and 2.2 N/m for the SiO2, TiN, and TiW, respectively. The Cu particle on TiW film shows the highest adhesion. When TiW is used as a barrier metal, fine Cu lines are fabricated by reactive ions etching without peeling. On the other hand, Cu lines on the TiN film are peeled during the etching. This is consistent to the evaluation result that TiW has higher adhesive energy than TiN.

Electrical degradation of Al/TiW/CoSi2 shallow junctions

TiW barrier properties have been investigated by electrical testing of shallow CoSi2 junctions. Rutherford backscattering and Auger analyses have been used to study Al(0.5% Cu)/TiW/CoSi2/Si structure. Based on the electrical data, it was demonstrated that 900 Å of TiW will serve as a good diffusion barrier up to 475 °C/30 min. The junctions failed at 450 °C when TiW thickness was reduced to 450 Å. The electrical data show the stress of TiW film does not show any significant effects on its properties. Furthermore, shallower junctions (i.e., p+n ) are more vulnerable to barrier failure.

Interaction of TiW film with GaAs

The interactions of co-evaporated Ti0.3W0.7 films with GaAs have been studied after annealing at temperatures in the range of 650 to 900° C for 15 min employing Rutherford backscattering, transmission and scanning electron microscopy. X-ray diffraction and energy-dispersive X-ray analysis techniques. Reaction has been found to take place at 650° C as evidenced by the presence of an AsTi compound in the region near the interface of TiW film and GaAs. Gallium diffuses out to the surface at temperatures above 750° C and causes surface morphological degradation, which can be related to the instability of the TiW Schottky barrier height at higher temperatures ≳750° C as reported in the literature.