IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 13, NO. 10, OCTOBER 2003 431
Very High Density RF MIM Capacitors (17
fF=m
2
)
Using High-
Al
2
O
3
Doped
Ta
2
O
5
Dielectrics
M. Y. Yang, C. H. Huang, Albert Chin, Senior Member, IEEE, Chunxiang Zhu, B. J. Cho, Senior Member, IEEE,
M. F. Li, Senior Member, IEEE, and Dim-Lee Kwong
Abstract—Using high- Al2O3 dopedTa2O5 dielectric, we have obtained record high MIM capacitance density of 17fF m2 at 100 KHz, small 5% capacitance reduction to RF frequency range, and low leakage current density of8 9 10 7A cm2. In combination of both high capacitor density and low leakage current density, a very low leakage current of5 2 10 12 A is calculated for a typical large 10 pF capacitor used in RF IC that is even smaller than that of a deep sub- m MOSFET. This very high capacitance density with good MIM capacitor characteristics can significantly reduce the chip size of RF ICs.
Index Terms—Capacitor, dielectric constant, frequency
depen-dence, high , MIM, RF.
I. INTRODUCTION
T
HE MIM capacitors [1]–[6] are widely used in RF circuit for impedance matching and DC filtering which usually consume a large portion of whole RF IC area. To reduce the chip size and cost, the capacitance density must be increased. To achieve higher capacitance density of in MIM capacitor, the application of high dielectric constant material [7]–[13] is the ideal choice because the reducing dielectric thickness will increase the undesired leakage current. In addition, the proper high- dielectric requires not only higher- but also process compatible to existing VLSI backend technology. Thus, high quality dielectric must be formed at a very low temper-ature of 400 limited by backend process [11]. Previously, we have reported that high performance RF MIM capacitor can be achieved using high- dielectric with relatively high capacitance density of 5 [13]. In this letter, we have further increased the capacitance density to 17 using high- doped . Low leakage current density of is also obtained. This excellent result sug-gests the developed high- MIM capacitor can be used for RF circuit with largely reduced device size.Manuscript received February 10, 2003; revised June 17, 2003. This work was supported by the NSTB/EMT/TP/00/001.2. The review of this letter was arranged by Associate Editor Dr. Arvind Sharma.
M. Y. Yang, C. H. Huang, and A. Chin are with the Department of Electronics Engineering, National Chiao Tung Univ., Hsinchu 300, Taiwan, R.O.C. (e-mail: achin@cc.nctu.edu.tw).
C. Zhu, B. J. Cho, and M. F. Li are with the Silicon Nano Device Labora-tory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore, 119260.
D.-L. Kwong is with the Department of Electrical & Computer Engineering, The University of Texas, Austin, TX 78752 USA.
Digital Object Identifier 10.1109/LMWC.2003.818532
II. EXPERIMENTAL
For VLSI integration, we have first deposited 500 nm PECVD on Si substrates. Then bottom electrode and RF transmission line are formed using Pt/Ti bi-layer metal. A second PECVD oxide was deposited for isolation and followed by patterning the active capacitor area. Thin Al:Ta (1:8) layer was deposited on bottom electrode, and oxidized at 400 for 45 min to form high- doped . The formed high- dielectric was further annealed at 400 for 15 min to improve the quality. Two dielectric thicknesses of 11.5 and 14 nm are formed and confirmed by ellipsometer measurement. The reason for doping into is to preserve the merit of good MIM capacitor integrity by adding dielec-tric [10]–[13]. Finally, Al metal was used for both top capacitor electrode and transmission line and the formed MIM capacitor area is . The MIM capacitors were characterized by HP4284A precision LCR meter from 10 KHz to 1 MHz and S-parameters measurement using HP8510C network analyzer from 200 MHz to 20 GHz. The parasitic pad, series inductor and resistor in transmission line are de-embedded from a same line length through transmission line [14], [16].
III. RESULTS ANDDISCUSSION
We have first measured the low frequency characteristics of high- doped MIM capacitors. As shown in Fig. 1, record high capacitance density of 17 at 100 KHz is measured for 11.5 nm doped device and a value of 22 is obtained. The equivalent-oxide thickness (EOT) values for these high density capacitors are calculated to be 2.0 and 2.4 nm for respective 11.5 and 14 nm physical dielectric thickness using , where is the dielectric constant. Such small EOT of 2.0 nm without quantum correction is even suitable for high- gate dielectric usage.
Fig. 2 shows the J–V characteristics of doped MIM capacitors. The asymmetrical J–V and breakdown volt-ages under positive and negative bias are due to the different work function of top Al and bottom Pt electrodes. The leakage current is increased by trading off the increasing capacitance density, and values of and are measured at 2 V for respective EOT of 2.4 and 2.0 nm. For a typical large capacitor of 10 pF used in RF ICs, a leakage current of only A is calculated that is even smaller than the off-state current of a deep sub- MOSFET [17]. Therefore, the leakage current density is low enough for RF IC applica-tion. We have also added the stress effect in the inserted figure.
432 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 13, NO. 10, OCTOBER 2003
Fig. 1. C–V characteristics of (a) 2.0 and (b) 2.4 nm EOTAl O doped Ta O MIM capacitors at different frequencies. The measured area is 50 m 2 50 m.
Fig. 2. J–V characteristics of 2.0 and 2.4 nm EOT doped Ta O MIM
capacitors. The insert figure is the stress induced leakage current under62.5 V stress for 1000 s. The asymmetric J–V and breakdown voltages are due to the different bottom Pt and top Al electrodes.
The good dielectric reliability can be evidenced from the small increase of leakage current under a stress condition of 2.5 V for 1000 s.
To further study the RF frequency characteristics, we have measured the S-parameters up to 20 GHz. Fig. 3(a) shows the measured (de-embedded) and modeled S-parameters for doped MIM capacitors, where the modeled data is from the equivalent circuit model shown in Fig. 3(b). The , , , and C in the model are the parasitic series resistor, series in-ductor, parallel resistor, and capacitor, respectively. Good agree-ment between measured and modeled data is obtained for both capacitors with different thickness, which can be used for ca-pacitance extraction.
Fig. 4 shows the frequency-dependent capacitance reduction and quality (Q) factor for high- doped MIM ca-pacitors. The Q-factors are directly from the measured
for frequency below 1 MHz and calculated from S-parameters at RF frequencies. The capacitance values at RF frequencies are extracted from the circuit model where well matched data
Fig. 3. (a) Measured and simulated scattering parameters of 2.0 and 2.4 nm EOTAl O doped Ta O MIM capacitors and (b) equivalent circuit model and numerical values of elements for capacitor simulation at RF regime.
Fig. 4. Frequency-dependent capacitance and Q-factor of 2.0 and 2.4 nm EOT
Al O doped Ta O MIM capacitors.
of modeled and measured S-parameters are shown in Fig. 3. Only a small amount of capacitance reduction (5%) is obtained from 10 KHz to 20 GHz that indicates the excellent perfor-mance for RF application. The Q-factor decreases as increasing capacitance density that is due to the higher resistor loss and consistent with the larger leakage current trend. However, an average Q-factor of 40 is obtained for 17 capacitor that also suggests the good RF performance. A resonant fre-quency of 8 GHz is observed due to the very large capacitor (42.5 pF) and residual inductance even after de-embedding. The high capacitor density, low leakage current, small frequency de-pendence and good Q-factor of doped MIM ca-pacitors are useful for circuit application at RF frequencies.
YANG et al.: VERY HIGH DENSITY RF MIM CAPACITORS 433
IV. CONCLUSION
We have achieved record high 17 capacitance den-sity, small 5% capacitance reduction to RF frequency range, and low leakage current of using
high-doped MIM capacitors and processed at 400 . This high capacitance density with good device integrity can greatly reduce the chip size of RF ICs.
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