Study of Ti/W/Cu, Ti/Co/Cu, and Ti/Mo/Cu Multilayer Structures
as Schottky Metals for GaAs Diodes
H.C. CHANG,1C.S. LEE,1,2S.H. CHEN,1E.Y. CHANG,1and J.Z. HE1
1.—Department of Materials Science and Engineering and Microelectronics and Information System Research Center, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China. 2.—E-mail: [email protected]
Schottky structures with copper and refractory metals as diffusion barrier for GaAs Schottky diodes were evaluated. These structures have lower series resistances than the conventionally used Ti/Pt/Au structure. Based on the electrical and material characteristics, the Ti/W/Cu and Ti/Mo/Cu Schottky structures are thermally stable up to 400°C; the Ti/Co/Cu Schottky structure is thermally stable up to 300°C. Overall, the copper-metallized Schottky struc-tures have excellent electrical characteristics and thermal stability, and can be used as the Schottky metals for GaAs devices.
Key words: Schottky, diffusion barrier, GaAs
Journal of ELECTRONIC MATERIALS, Vol. 33, No. 7, 2004 Letter
L15 (Received June 30, 2003; accepted January 30, 2004)
INTRODUCTION
The copper metallization process has become popular in silicon device manufacture and has been widely studied, but there are few reports about the copper metallization process for GaAs-based devices.1–3 In this work, copper-metallized GaAs-based Schottky diodes are studied. Traditionally, the Ti/Pt/Au Schottky contact is the most widely used structure for the fabrication of metal-semiconductor field-effect transistors, high-electron-mobility tran-sistors, and Schottky diodes. In this study, the top gold layer of the Schottky structure was replaced by copper. This has the advantages of reducing electri-cal resistivity and production cost when compared with the use of gold. The Pt layer was replaced by the transition metals, such as W, Co, and Mo, due to the better antidiffusion capability and the electrical conductivity of these transition metals.
From the phase diagrams, there are no inter-metallic compounds in the W/Cu, Mo/Cu, and Co/Cu binary systems. These binary systems have negligi-ble mutual solubility. However, in the Ti/W binary system, the maximum solubility of W in αTi is ap-proximately 0.2 at.% at 740%C and Ti has about 3 at.% solubility in (βTi-W) at 500°C. In the Ti/Mo system, there is one compound phase Mo9Ti4. The maximum solubility of Mo in (αTi) is approximately
0.4 at.% at 695°C. In general, both Ti/Mo and Ti/W systems have negligible mutual solubility,4whereas several TiCo intermetallic compounds such as Ti2Co, TiCo, and TiCo3are formed in the Ti/Co system. In this work, Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu Schottky structures were studied and compared with the traditionally used Ti/Pt/Au Schottky structure.
EXPERIMENTAL
Metal-organic chemical vapor deposition grown silicon-doped n-type (100) GaAs with thickness of 1 µm and concentration of 2.09 1017 cm3 was used for the fabrication of Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu Schottky diodes. The area of the diode is 3.14 mm2. The Ge/Au/Ni/Au ohmic metals were deposited by the electron beam evaporator and annealed by the rapid thermal annealing system at 400°C for 60 sec. The Schottky metals were deposited by sequentially DC sputtering the Ti (1000 Å), barrier layer (W, Co, and Mo) (400 Å), and Cu (5000 Å) targets through a metal mask. The conventional Ti/Pt/Au Schottky diode was also prepared for comparison.
MEASUREMENT AND CHARACTERIZATION
Figure 1 shows the I-V characteristics of the Schottky diodes with Ti/Mo/Cu, Ti/Co/Cu, and Ti/W/Cu structures as deposited and after anneal-ing at 200°C for 2 min. Applyanneal-ing thermionic emis-sion theory and considering series resistance, the
I-V characteristic of the Schottky diode can be expressed as
(1) where saturation current density J0 A*T2exp (qΦb/kT), q is the electron charge, V is the applied voltage, R is the series resistance, Aeffis the effective area of the Schottky diode, k is the Boltzman con-stant, T is the absolute temperature, A* is the effective Richardson constant of 8.0375 A cm2K2 for GaAs, Φb is the barrier height, and n is the ideality factor. All the ideality factors and barrier heights were calculated within the current range of 0.01–0.0001 mA/mm2.
Before the annealing treatment, the ideality fac-tors and barrier height were 1.11/0.76 eV, 1.15/0.78 eV, 1.11/0.7 eV, and 1.11/0.77 eV; after annealing at 200°C for 2 min, the values became 1.09/0.77 eV, 1.05/0.92eV, 1.12/0.73 eV, and 1.05/0.69 eV for Ti/Pt/ Au, Ti/Co/Cu, Ti/Mo/Cu, and Ti/W/Cu structures, re-spectively. The ideality factor and the Schottky bar-rier height remained fairly stable after annealing at 200°C for each structure. The barrier heights after annealing fall in the range of 0.7–0.9 eV, which is in the same range as the data reported by Sehgal et al.5
As shown in Fig. 1, for each diode structure at the same applied forward voltage, the diode current de-creased after annealing at 200°C for 2 min. The
Ti/Co/Cu structure shows the most serious degrada-tion (59.0% at 0.7 V), whereas the Ti/W/Cu structure has little change in current (17.2% at 0.7 V) after annealing treatment. The different characteristics of the I-V curves for each Schottky diode are due to the different material structures and the related series resistances of the diodes. In order to extract the series resistance of each diode from the I-V curve, a method proposed by Cheung and Cheung6is used. Rearranging and differentiating Eq. 1, we can obtain
(2) The slope of the line of d(V)/d(lnJ) versus J is equal to RAeff. Thus, we can easily determine R by Eq. 2. Figure 2 shows the plot of d(V)/d(lnJ) versus J of the Schottky diodes. The series resistances calculated are 39.92ohn, 43.72ohn, 57.41ohn, and 60.74ohn for Ti/ Co/Cu, Ti/Mo/Cu, Ti/W/Cu, and Ti/Pt/Au as-deposited structures, respectively. From the data of the resis-tance, all three of the copper-metallized Schottky structures have lower series resistance than the con-ventionally used Ti/Pt/Au structure.
X-Ray Diffraction Analysis
Glancing angle x-ray diffraction (XRD) with Cu Kα radiation was used to identify the material phases. The annealing treatment was done up to 500°C for 30 min. The Ti/Co/Cu Schottky structure
L16 Chang, Lee, Chen, Chang, and He
Fig. 1. Forward I-V characteristics of Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu structures as deposited and after annealing at 200°C for 2 min: ——●●—— Ti/Co/Cu (as deposited), ——■■—— Ti/Mo/Cu (as deposited), ——∆—— Ti/W/Cu (as deposited), ——●—— Ti/Co/Cu (annealing at 200°C, 2 min), ——■—— Ti/Mo/Cu (annealing at 200°C, 2 min), and ——▲—— Ti/W/Cu (annealing at 200°C, 2 min).
Fig. 2. Plot of d(V)/d(lnJ) versus J. The series resistances of the Ti/W/Cu, Ti/Mo/Cu, Ti/Co/Cu, and Ti/Pt/Au Schottky structures is proportional to the slope of the fitted line: ——◆—— Ti/Pt/Au (as deposited), ——∆—— Ti/W/Cu (as deposited), ——■■—— Ti/Mo/Cu (as deposited), ——●●—— Ti/Co/Cu (as deposited), and ———— linear fitting. J J q V JRA nkT eff = − − 0 exp 1 ( ) d V d InJ RA J nkT q eff ( ) ( ) = + JEM_1095-L1 6/21/04 8:01 PM Page 16
formed CoTi phase after annealing at 400°C. All these Schottky structures showed atomic interdiffu-sion after annealing at 500°C for 30 min. Figure 3 shows the XRD results of the Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu Schottky structures after annealing at 500°C. For the Ti/W/Cu Schottky structure, the CuTi phase was formed after 500°C annealing. Ti4Mo9 phase was formed in the Ti/Mo/Cu structure and CoTi phase was formed in the Ti/Co/Cu structure after annealing at 500°C. From the XRD results, the Ti/W/Cu and Ti/Mo/Cu Schottky structures were thermally stable after annealing up to 400°C for 30 min; Ti/Co/Cu Schottky structure was thermally stable only up to 300°C after annealing for 30 min. Table I is a summary of XRD results for the struc-tures after annealing at different temperastruc-tures from 200°C to 500°C for 30 min.
CONCLUSIONS
GaAs Schottky structures with copper and refrac-tory metal were evaluated. The Ti/Co/Cu Schottky structure shows the lowest series resistance, but the change of ideality factor is larger than the other structures after annealing treatment at 200°C. From the XRD results, the Ti/W/Cu and Ti/Mo/Cu Schottky structures are thermally stable after an-nealing up to 400°C, and Ti/Co/Cu Schottky struc-ture is thermally stable after annealing up to 300°C. Experimental results show that the copper-metal-lized Schottky structures have comparable electrical characteristics and thermal stability as compared to the traditional Ti/Pt/Au structure. The Ti/Co/Cu, Ti/Mo/Cu, and Ti/W/Cu metal stacks can be used as Cu metallization of the Schottky structures for GaAs devices.
ACKNOWLEDGEMENTS
The work was sponsored jointly by the Ministry of Education and the National Science Council, Republic of China, under Contract No. 89-E-FA06-2-4.
REFERENCES
1. C.Y. Chen, L. Chang, E.Y. Chang, S.H. Chen, and D.F. Chang, Appl. Phys. Lett. 77, 3367 (2000).
2. C.Y. Chen, E.Y. Chang, L. Chang, and S.H. Chen, Electron.
Lett. 36, 1318(2000).
3. C.Y. Chen, E.Y. Chang, Li Chang, and S.H. Chen, IEEE
Trans. Electron Dev. 48, 1033 (2001).
4. T.B. Massalski and B. Thaddeus, Binary Phase Diagrams (Metals Park, OH: ASM, 1986).
5. B.K. Sehgal, R. Gulati, A.A. Naik, S. Vinayak, D.S. Rawal, and H.S. Sharma, Mater. Sci. Eng. B48, 229 (1997). 6. S.K. Cheung and N.W. Cheung, Appl. Phys. Lett. 49, 85 (1986).
Study of Ti/W/Cu, Ti/Co/Cu, and Ti/Mo/Cu Multilayer Structures
as Schottky Metals for GaAs Diodes L17
Fig. 3. XRD patterns of Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu structures after annealing at 500°C for 30 min.
Table I. Summary of XRD Results for Ti/W/Cu, Ti/Mo/Cu, and Ti/Co/Cu Schottky Structures after Annealing at 200–500°C for 30 min
Annealing
Temperature (°C) Ti/W/Cu Ti/Mo/Cu Ti/Co/Cu
200 Stable Stable Stable
300 Stable Stable Stable
400 Stable Stable Formed CoTi phase
500 Formed CuTi phase Formed Ti4Mo9phase Formed CoTi phase Schottky
Structure JEM_1095-L1 6/21/04 8:01 PM Page 17
