IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007 235
High-Temperature Leakage Improvement in
Metal–Insulator–Metal Capacitors by
Work–Function Tuning
K. C. Chiang, C. H. Cheng, H. C. Pan, C. N. Hsiao, C. P. Chou,
Albert Chin, Senior Member, IEEE, and H. L. Hwang, Fellow, IEEE
Abstract—Using low-cost and high work–function Ni, a low
leakage current of 5× 10−6 A/cm2 at 125 ◦C is obtained in a high 25-fF/µm2-density SrTiO3 metal–insulator–metal (MIM) capacitor processed at 400◦C. This is approximately two orders of magnitude better than the same device using a TaN electrode, with added advantages of improved voltage and temperature co-efficients of capacitance. This work–function tuning method also has merit for achieving both low thermal leakage and high overall
κ value beyond previous laminate structure.
Index Terms—Capacitor, high temperature, metal–insulator–
metal (MIM), Ni, thermal leakage.
I. INTRODUCTION
A
S THE VERY large scale integration (VLSI) scaling con-tinues, the thermal effect on IC becomes a serious issue due to the increasing integration density, large power density, and high chip temperature. This becomes even worse by the large dc leakage and power consumption from ultrathin gate oxides. Such leakage current at elevated temperature is especially important for advanced charge-storage MIM capacitors [1]–[14], where the smaller conduction band offset (∆EC) in higher κ dielectric [15] further degrades the thermal leakage but is needed for the required higher capacitance density. To address this issue, an Al2O3-HfO2laminate structure [8] is used, and a high∆EC Al2O3 is used to decrease the thermal leakage. However, the improved high-temperature leakage current is traded off with the capacitance density andκ value. To overcome the above problems, in this letter, we have used work–function (φm) tuning to improve the thermal leakage without scarifying overall κ in MIM
Manuscript received December 6, 2006. This work was supported in part by Technology Development Program of Academia (TDPA) Department of Industrial Technology (DOIT) Ministry of Economic Affairs (MOEA) under Grant 94-EC-17-A-01-S1-047 and National Science Council (NSC) under Grant 95-2221-E-009-275 of Taiwan. The review of this letter was arranged by Editor A. Wang.
K. C. Chiang and A. Chin are with the Department of Electronics Engineer-ing, National Chiao-Tung University, Hsinchu 30010, Taiwan, R.O.C. (e-mail: [email protected]).
C. H. Cheng and C. P. Chou are with the Department of Mechanical Engineering, National Chiao-Tung University, Hsinchu 30056, Taiwan, R.O.C. H. C. Pan and C. N. Hsiao are with the Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan, R.O.C. H. L. Hwang is with the Department of Electrical Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan, R.O.C.
Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2007.891265
devices. The near two orders lower leakage current at 125◦C are obtained using low-cost and high-φmNi (5.1 eV) on high-κ SrTiO3 than control TaN (4.5 eV). Using the Ni electrode also has the merit of much lower cost than the expensive and rare Noble metals [11]. The measured 125-◦C leakage of 5×10−6A/cm2 at 25-fF/µm2 capacitance density is also one of the lowest reported data without trading off the high-κ value [1]–[14], with added merits of improved voltage and temperature coefficients of capacitance (VCC and TCC) needed for the analog/RF application.
II. EXPERIMENTALPROCEDURE
The TaN/Ta bilayer metal was deposited on SiO2/Si sub-strate by sputtering and patterned to form the bottom capac-itor electrode. The TaN surface was further treated by NH3 plasma nitridation [4], [10]–[14] to enhance the diffusion bar-rier property, which in turn improves oxygen deficiency and capacitance density by reducing the interfacial TaON formation during postdeposition anneal (PDA) [13], [14]. Then, 23- or 46-nm-thick SrTiO3 dielectric layer was deposited using an RF magnetron sputter system with ceramic target, followed by the subsequent 400-◦C furnace annealing for 1 h under oxy-gen ambient for dielectric quality improvement. This process temperature is lower than previous 450-◦C-formed nanocrystal SrTiO3[13], [14] for better backend integration and improved device uniformity, although the overallκ value is lowered due to the mixed amorphous and crystalline phase. Finally, Ni, control TaN, or Al was deposited and patterned to form the top capacitor electrode, and the fabricated devices were character-ized by capacitance–voltage (C–V ) and density–voltage (J–V ) measurements.
III. RESULTS ANDDISCUSSION
Fig. 1(a) and (b) shows the C–V and J–V characteristics of SrTiO3MIM capacitors with different Ni, TaN, or Al metal electrode, respectively. At ∼12-fF/µm2 capacitance density, the device with a Ni electrode shows the desired less voltage and frequency dependences, which are related to the mea-sured significant lower leakage current. The leakage current improvement increases greatly with increasing temperature to 125◦C due to the exponential temperature dependence, where more than two orders of magnitude lower leakage current are
236 IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007
Fig. 1. (a)C–V and (b) J–V characteristics of ∼12-fF/µm2SrTiO3MIM capacitors with different Ni, TaN, or Al top electrode, respectively. Both the 25-◦C and 125-◦CJ–V data are measured.
measured using Ni electrode than TaN. Furthermore, a high breakdown voltage and a field of 10 V and 2.2 MV/cm were measured for Ni electrode, which shows the good potential for real application.
The operation condition is less than the breakdown voltage and is investigated by the time-to-breakdown (tBD) study.
Fig. 2(a) shows thetBDlifetime and stability of SrTiO3devices after a thermal treatment of 350◦C for 1 h. A still high field of 1.4 MV/cm for the extrapolated ten-year lifetime is obtained after thermal treatment, indicating the good reliability and process integration capability. In addition, we further studied the φm effect on higher density capacitor of 25 fF/µm2. Fig. 2(b) shows the J–V characteristics of SrTiO3 MIM ca-pacitors with various metal electrodes. Similar remarkably large improvement of thermal leakage is obtained at 125 ◦C, where low current of only 5× 10−6 A/cm2 is measured at a 25-fF/µm2capacitor. This is one of the lowest reported thermal leakage currents at 25-fF/µm2density [1]–[14].
Fig. 3 shows the∆C/C−V characteristics of SrTiO3MIM capacitors with 12- or 25-fF/µm2 density. Similar to the previous works [9]–[14], the quadratic VCC (α) improves with decreasing leakage current and capacitance density. At
∼12-fF/µm2 density, low 125-◦C leakage current of 2×
10−7 A/cm2 at 2 V, and smallα of 392 ppm/V2 are
simulta-Fig. 2. (a) Time-to-breakdown lifetime and thermal stability test for Ni/SrTiO3/TaN capacitor. (b) 25-◦C and 125-◦C measuredJ–V characteristics of∼25-fF/µm2 SrTiO3 MIM capacitors with different Ni, TaN, or Al top electrode.
Fig. 3. ∆C/C–V characteristics of ∼12- or ∼25-fF/µm2 SrTiO3 MIM capacitors with different Ni, TaN, or Al top electrode.
neously measured, which are comparable with or better than the best reported data in literature [1]–[14]. Similar improved TCC is also measured for SrTiO3MIM capacitors, where TCC of 804, 423, and 334 ppm/◦C are measured for 12 fF/µm2 -density devices with Al, TaN, and Ni electrodes, respectively.
CHIANG et al.: HIGH-TEMPERATURE LEAKAGE IMPROVEMENT IN MIM CAPACITORS 237
Fig. 4. Measured and calculated 125-◦C J−E1/2 characteristics of ∼25-fF/µm2 SrTiO3 MIM capacitors with different Ni, TaN, or Al top
electrode. The inserted figure is the band diagrams under thermal equilibrium.
To investigate the large leakage current difference for SrTiO3 MIM capacitors with various Ni, TaN, and Al electrodes, we have plottedln(J) versus E1/2in Fig. 4. The measured data fit well with the calculation by either Schottky emission (SE) or Frenkel–Poole (FP) conduction as follows:
J ∝ exp γE1/2− V b kT (1) γ = e3 ηπε0K∞ 1/2 . (2)
TheK∞is the high-frequency dielectric constant (= n2, where
n is the refractive index). The η is a constant with its value
equal to 1 or 4 for FP or SE, respectively, which gives the slope
γ to be 1.6 × 10−5 or 3.18× 10−5 eV(m/V)1/2 by applying
n = 2.4 of SrTiO3[14], [16] into the previous equations. From
the good agreements between measured and (1) calculated data, the 125-◦C leakage current of SrTiO3 devices with Al or TaN electrode is dominated by SE over the whole electric field due to small metal/insulator barrier height(φb). In sharp contrast,
the leakage from Ni electrode is dominated by SE only at a low field but governed by a trap-conducted FP mechanism at a high field, which is due to a largeφb, shown in the inserted thermal equilibrium-band diagram.
IV. CONCLUSION
Much improved thermal leakage current at 125◦C is obtained by using low-cost and high-φm Ni electrode in SrTiO3 MIM capacitors fabricated at 400◦C for VLSI backend integration. The lower leakage also shows improved VCC and TCC and important for analog/RF ICs.
ACKNOWLEDGMENT
The authors from National Chiao-Tung University would like to thank Prof. H. Iwai’s valuable discussion.
REFERENCES
[1] C.-M. Hung, Y.-C. Ho, I.-C. Wu, and K. O, “High-Q capacitors imple-mented in a CMOS process for low-power wireless applications,” in Proc. IEEE MTT-S Int. Microw. Symp., 1998, pp. 505–511.
[2] J. A. Babcock, S. G. Balster, A. Pinto, C. Dirnecker, P. Steinmann, R. Jumpertz, and B. El-Kareh, “Analog characteristics of metal–insulator– metal capacitors using PECVD nitride dielectrics,” IEEE Electron Device Lett., vol. 22, no. 5, pp. 230–232, May 2001.
[3] Y. L. Tu, H. L. Lin, L. L. Chao, D. Wu, C. S. Tsai, C. Wang, C. F. Huang, C. H. Lin, and J. Sun, “Characterization and comparison of high-κ metal–insulator–metal (MIM) capacitors in 0.13µm Cu BEOL for mixed-mode and RF applications,” in VLSI Symp. Tech. Dig., 2003, pp. 79–80. [4] Y. K. Jeong, S. J. Won, D. K. Jwon, M. W. Song, W. H. Kim, O. H. Park,
J. H. Jeong, H. S. Oh, H. K. Kang, and K. P. Suh, “High quality high-k MIM capacitors by Ta2O5/HfO2/Ta2O5 multilayered dielectric and NH3plasma interface treatments for mixed-signal/RF applications,” in VLSI Symp. Tech. Dig., 2004, pp. 222–223.
[5] X. Yu, C. Zhu, H. Hu, A. Chin, M. F. Li, B. J. Cho, D.-L. Kwong, P. D. Foo, and M. B. Yu, “A high-density MIM capacitor (13 fF/µm2) using ALD HfO2dielectrics,” IEEE Electron Device Lett., vol. 24, no. 2, pp. 63–65, Feb. 2003.
[6] S. J. Kim, B. J. Cho, M.-F. Li, C. Zhu, A. Chin, and D. L. Kwong, “HfO2 and lanthanide-doped HfO2 MIM capacitors for RF/mixed IC applications,” in VLSI Symp. Tech. Dig., 2003, pp. 77–78.
[7] C. Zhu, H. Hu, X. Yu, S. J. Kim, A. Chin, M. F. Li, B. J. Cho, and D. L. Kwong, “Voltage and temperature dependence of capacitance of high-κ HfO2MIM capacitors: A unified understanding and prediction,” in IEDM Tech. Dig., 2003, pp. 879–882.
[8] H. Hu, S. J. Ding, H. F. Lim, C. Zhu, M. F. Li, S. J. Kim, X. F. Yu, J. H. Chen, Y. F. Yong, B. J. Cho, D. S. H. Chan, S. C. Rustagi, M. B. Yu, C. H. Tung, A. Du, D. My, P. D. Fu, A. Chin, and D. L. Kwong, “High performance HfO2-Al2O3laminate MIM capacitors by ALD for RF and mixed signal IC applications,” in IEDM Tech. Dig., 2003, pp. 379–382. [9] S. J. Kim, B. J. Cho, M. B. Yu, M.-F. Li, Y.-Z. Xiong, C. Zhu, A. Chin, and
D. L. Kwong, “High capacitance density(>17 fF/µm2) Nb2O5-based MIM capacitors for future RF IC applications,” in VLSI Symp. Tech. Dig., 2005, pp. 56–57.
[10] K. C. Chiang, A. Chin, C. H. Lai, W. J. Chen, C. F. Cheng, B. F. Hung, and C. C. Liao, “Very high-κ and high density TiTaO MIM capacitors for analog and RF applications,” in VLSI Symp. Tech. Dig., 2005, pp. 62–63. [11] K. C. Chiang, C. C. Huang, A. Chin, W. J. Chen, S. P. McAlister,
H. F. Chiu, J. R. Chen, and C. C. Chi, “High-κ Ir/TiTaO/TaN capacitors suitable for analog IC applications,” IEEE Electron Device Lett., vol. 26, no. 7, pp. 504–506, Jul. 2005.
[12] K. C. Chiang, C. H. Lai, A. Chin, T. J. Wang, H. F. Chiu, J. R. Chen, S. P. McAlister, and C. C. Chi, “Very high density (23 fF/µm2) RF MIM capacitors using high-κ TiTaO as the dielectric,” IEEE Electron Device Lett., vol. 26, no. 10, pp. 728–730, Oct. 2005.
[13] K. C. Chiang, C. C. Huang, A. Chin, W. J. Chen, H. L. Kao, M. Hong, and J. Kwo, “High performance micro-crystallized TaN/SrTiO3/TaN ca-pacitors for analog and RF applications,” in VLSI Symp. Tech. Dig., 2006, pp. 126–127.
[14] K. C. Chiang, C. C. Huang, A. Chin, G. L. Chen, W. J. Chen, Y. H. Wu, A. Chin, and S. P. McAlister, “High performance SrTiO3 metal– insulator–metal capacitors for analog applications,” IEEE Trans. Electron Devices, vol. 53, no. 9, pp. 2312–2319, Sep. 2006.
[15] J. Robertson, “Band offsets of wide-band-gap oxides and implications for future electronic devices,” J. Vac. Sci. Technol. B, Microelectron., vol. 18, no. 3, pp. 1785–1791, May 2000.
[16] F. Gervais, Handbook of Optical Constants of Solids II, E. D. Palik, Ed. New York: Academic, 1991, p. 1035.
