Very high-density (23 fF/mu m(2)) RF MIM capacitors using high-k TaTiO as the dielectric

728 IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 10, OCTOBER 2005

Very High-Density (23 fF

=m

2

) RF MIM Capacitors

Using high-

 TaTiO as the Dielectric

K. C. Chiang, C. H. Lai, Albert Chin, Senior Member, IEEE, T. J. Wang, H. F. Chiu, Jiann-Ruey Chen,

S. P. McAlister, Senior Member, IEEE, and C. C. Chi

Abstract—A very high density of 23 fF

m

has been measured

in RF metal–insulator–metal (MIM) capacitors which use

high-TaTiO as the dielectric. In addition, the devices show a small

reduc-tion of 1.8% in the capacitance, from 100 kHz to 10 GHz. Together

with these characteristics the MIM capacitors show low leakage

currents and a small voltage-dependence of capacitance at 1 GHz.

These TaTiO MIM capacitors should be useful for precision RF

circuits.

Index Terms—Capacitor, RF metal–insulator–metal (MIM),

TaTiO.

I. I

A

CCORDING to International Technology Roadmap for

Semiconductors (ITRS), continuous down-scaling of the

size of metal–insulator–metal (MIM) capacitors is required to

reduce chip size and the cost of analog and RF ICs [1]. The use

of a high-- dielectric [2]–[15] is the only way to achieve this

goal, since decreasing the dielectric thickness

to achieve

high capacitance density

degrades the leakage

cur-rent, loss tangent and voltage-dependence of the capacitance

. Hence, the high- dielectric in MIM capacitors has

evolved from using SiON

[3]–[5] and Al O

[13] to

[7]–[11] or Ta O

[12], [14]. To increase the

value beyond 25, the dielectric

TiO is a potential candidate, since it can display very

high-( 80). However, the large leakage current from crystallization

of the TiO is a major limitation for device applications. Here,

we report the use of TiTaO as the dielectric, and show capacitors

with low leakage current and without crystallization, even after

backend processing. We report devices with a high density of 23

fF

m , a high- value of 39–45 (beyond the previous

Manuscript received May 25, 2005; revised July 19, 2005. This work was supported in part by the National Science Council of Taiwan, R.O.C. under Grant 92-2215-E-009-031. The review of this letter was arranged by Editor C.-P. Chang.

K. C. Chiang, C. H. Lai, and T. J. Wang are with the Department of Elec-tronics Engineering, Nano-Science Technology Center, National Chiao-Tung University, Hsinchu 300, Taiwan, R.O.C.

A. Chin is with SNDL, Department of Electrical and Computer Engi-neering, National University of Singapore, Singapore, on leave from the Department of Electronics Engineering, Nano Science Technology Center, National Chiao-Tung University, Hsinchu 300, Taiwan, R.O.C., (e-mail: albert_achin@hotmail.com).

H. F. Chiu and J.-R. Chen are with the Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C.

S. P. McAlister is with the National Research Council of Canada, Ottawa, ON, Canada.

C. C. Chi is with the Department of Physics, National Tsing-Hua University, Hsinchu 300, Taiwan, R.O.C.

Digital Object Identifier 10.1109/LED.2005.856708

Fig. 1. XRD patterns of TiO and TiTaO dielectric layers,28 nm thick, after 400 C O oxidation and N annealing.

barrier), and low leakage current of

A/cm . This

performance of the TiTaO MIM capacitors is accompanied by a

small

of 550 ppm at 1 GHz. Compared with current

tech-nology these high performance capacitors can drastically reduce

the RF capacitor area [1], yet can be fabricated with full

com-patibility with current VLSI process lines.

II. E

P

High-

TiTaO MIM capacitors were fabricated on 4-in Si

wafers. First, a 2- m-thick isolation SiO was deposited on

the Si substrates. The bottom capacitor electrodes were formed

by depositing 0.05- m TaN on a 1- m Ta layer, followed by

patterning. Then, a 17-nm-thick Ti Ta

O

was

deposited on the TaN/Ta electrode, followed by 400 C

oxi-dation and annealing. Finally, Al was deposited and patterned

to form the top capacitor electrode and RF transmission lines.

For comparison purposes devices with TiO as the dielectric

was also fabricated using the same process. The fabricated RF

MIM capacitors were characterized using an HP4284A

preci-sion LCR meter from 10 KHz to 1 MHz, and an HP8510C

network analyzer for the S-parameter measurements from 200

MHz to 20 GHz [13]–[15]. The series inductance and RF pads

were deembedded from a “through” and “open” transmission

lines [16], [17], respectively. The RF frequency capacitance was

extracted from the measured S-parameters using an equivalent

circuit model [15].

III. R

D

Fig. 1 shows the X-Ray diffraction (XRD) patterns of

28-nm-thick TiO and TiTaO layers, which were used to examine the

CHIANG et al.: VERY HIGH-DENSITY (23 fF m ) RF MIM CAPACITORS 729

Fig. 2. (a) C–V and (b) J–V characteristics of TiO and TaTiO capacitors. The leakage current is lower in the TiTaO capacitors. The leakage currents at both 25 C and 125 C from top and bottom electrodes are shown for comparison. The capacitor size is20 2 20 m .

Fig. 3. Scattering parameters of a TiTaO MIM capacitor, from 200 MHZ to 20 GHz, after deembedding the through transmission line. Insert: the equivalent circuit model used for capacitance extraction. The capacitor size is20220m .

thermal stability on their amorphous structure. Significant

crys-tallization of the TiO was measured after a 400 C O

treat-ment for 10 min which became worse after subsequent 30 min

N annealing. In contrast the TiTaO was amorphous after the

same thermal cycle. This good stability after backend thermal

treatment is important in reducing the leakage current in RF

MIM capacitors. Further trading off the Ti composition

0.6 in

Ti Ta

O with thermal stability is necessary if higher thermal

cycle is used such as 450 C and above.

Fig. 4. (a)1C=C–V characteristics of a TiTaO MIM capacitor. The data for frequencies>1 MHz were obtained from the S-parameters. (b) Frequency dependent capacitance density,1C=C,  and  for a TiTaO MIM capacitor biased at 2 V. (c) Temperature-dependence of capacitance (TCC). The capacitor size is20 2 20 m .

Fig. 2(a) and (b) shows the C–V and J–V characteristics of

TaTiO and TiO MIM capacitors. For the TiTaO device a very

high capacitance density of 23 fF

m was measured, giving

a high- value of

39, although more detailed study in a

dif-ferent experiment with Transmission Electron Microscopy for

thickness calibration gives a

value of

45 [18]. This

value

is greater than the

value for HfO and Ta O which

are used in DRAM. However the TiO MIM capacitor showed

an unusual capacitance variation at voltages above

0.75 V. The

730 IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 10, OCTOBER 2005

poor C–V for the TiO MIM capacitor is thought to be related

to its large leakage current, Fig. 2(b), which may be due to the

current conduction through grain boundaries of the

poly-crys-talline TiO . In contrast, constant capacitance values, with little

voltage and frequency dependence, were found for the TiTaO

MIM capacitor. The TiTaO MIM capacitors have

5–7 orders

of magnitude lower leakage current than that for the TiO

de-vices. Note that although the ITRS only specifies the

capaci-tance density and

-factor [1], the leakage current was as low

as 5 pA

A/cm

at 1 V for a large 9.2 pF

capac-itor (

m in size) and lower than the leakage current

of sub-100 nm transistors [16]. The leakage current, injecting

electrons from the Al contact, is lower than that from using the

lower TaN electrode. This is due to the better interface for the

Al, which also gives better voltage and frequency dispersion in

the C–V curves. The leakage current is even worse at 125 C

and the using high work-function metal electrode to reduce the

leakage current will be needed [19]–[21].

Fig. 3 shows the measured S-parameters for a TiTaO MIM

capacitor, where the capacitance at RF frequencies can be

extracted from S-parameters using the equivalent circuit model

shown in the insert. Fig. 4(a) displays the

char-acteristics, where the data

1 MHz were calculated from the

measured S-parameters using a circuit-theory derived equation

[10]. The frequency dependent capacitance value is shown in

Fig. 4(b). The capacitance reduction of 1.8% from 100 kHz to

10 GHz indicates good device performance over the IF to RF

range. However, the rapid

reduction with increasing

frequency above megahertz regime may be due to the trapped

carriers being unable to follow the high frequency signal [10],

[15], [17]. Here, the typical carrier lifetime of trap-related

Shockley–Read–Hall recombination is in the range ms to

s.

The first order voltage linearity

and quadratic voltage

lin-earity

[7] are also shown in Fig. 4(b). The small

of

550 ppm, low

of 81

and

of 98 ppm/V at 1 GHz are

important for high-speed analog/RF IC applications [7]–[11].

Fig. 4(c) shows the temperature-dependence of capacitance

(TCC). Again, similar reduction of both capacitance and TCC

are found with increasing frequency.

IV. C

Very high 23 fF

m capacitance density, with a capacitance

reduction of 1.8% from 100 kHz to 10 GHz, and a small 550

ppm

at 1 GHz were simultaneously achieved in novel

high- TiTaO MIM capacitors processed at 400 C. These MIM

capacitors should be suitable for precision RF circuits.

R

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