Lanthanide (Tb)-doped HfO2 for high-density MIM capacitors

442 IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 7, JULY 2003

Lanthanide (Tb)-Doped HfO

₂

for

High-Density MIM Capacitors

Sun Jung Kim, Student Member, IEEE, Byung Jin Cho, Senior Member, IEEE, Ming-Fu Li, Senior Member, IEEE,

Chunxiang Zhu, Member, IEEE, Albert Chin, Senior Member, IEEE, and Dim-Lee Kwong, Senior Member, IEEE

Abstract—A high-density metal-insulator-metal (MIM) capac-itor with a lanthanide-doped HfO₂dielectric prepared by physical vapor deposition (PVD) is presented for the first time. A signifi-cant improvement was shown in both the voltage coefficient of ca-pacitance (VCC) and the leakage current density of MIM capac-itor, yet the high capacitance density of HfO₂dielectrics was main-tained by achieving the doping of Tb with an optimum concentra-tion in HfO₂. This technique allows utilizing thinner dielectric film in MIM capacitors and achieving a capacitance density as high as 13.3 fF/ m2with leakage current and VCC values that fully meet requirements from year 2005 for radio frequency (RF) bypass ca-pacitors applications.

Index Terms—Capacitance density, co-sputtering, HfO2, lanthanide, metal-insulator-metal (MIM) capacitor, voltage coefficient of capacitor (VCC).

I. INTRODUCTION

T

HE metal-insulator-metal (MIM) capacitor is a key pas-sive component in radio frequency (RF)/mixed signal ICs. Most foundries provide a MIM capacitor module with a ca-pacitance density ranging from 1 to 2 fF/ m using SiO - or Si N -based dielectrics [1]–[3]. Meanwhile, the industry will require capacitors with a capacitance density higher than 10 fF/ m for RF bypass capacitor applications from year 2005, according to the latest international technology roadmap for semiconductors (ITRS) [4]. This requirement can be achieved by using insulators with a dielectric constant higher than 57, and considering the present dielectric thickness of around 50 nm. Materials such as Ba, Sr, TiO , and TaO exhibit high di-electric constant values 60 or above if crystallized by a high tem-perature annealing [5], [6], which is however unrealistic in the backend of the line process. Instead, amorphous dielectrics such as Al O , Ta O , and HfO have recently been investigated for MIM capacitor application [7]–[10]. Considering their moder-ately high values of 9 to 25, the dielectric thicknesses of these materials should be reduced to thinner than 20 nm in order to meet the requirement for high-density bypass capacitor applica-tion. Use of thin dielectrics will however cause other problems

Manuscript received February 19, 2003; revised March 31, 2003. The review of this letter was arranged by Editor S. Kawamura.

S. J. Kim, B. J. Cho, M.-F. Li, and C. Zhu are with the Silicon Nano Device Laboratories, Department of Electrical and Computer Engineering, National University of Singapore (NUS), Singapore 119260 (e-mail: [email protected]).

A. Chin is with the Department of Electronics Engineering, National Chiao Tung University (NCTU), Hsinchu, Taiwan, R.O.C.

D.-L. Kwong is with the Department of Electrical and Computer Engineering, The University of Texas, Austin, TX 78752 USA.

Digital Object Identifier 10.1109/LED.2003.814024

such as high leakage currents and poor voltage coefficient of capacitance (VCC) [5], [10]. In this letter, a high-density MIM capacitor using 14-nm-thick Tb-doped HfO prepared by the cosputtering method is reported and the effects of Tb doping concentration on electrical properties of MIM capacitors are in-vestigated.

II. EXPERIMENTS

TaN/Hf Tb O/TaN multilayer MIM capacitor structures were fabricated on a 400-nm-thick SiO layer using a pulsed dc magnetron sputtering system. After 150-nm-thick TaN bottom electrode deposition, Hf Tb O films were reactively deposited at room temperature in a gas mixture of O (2 sccm) and Ar (23 sccm). The pressure was maintained at 3 mTorr, a dc power of 200 W was applied to the Hf target, and four different powers, 0, 40, 50, and 60 W were applied to the Tb target in order to obtain Hf Tb O films with different Tb doping concentrations. The corresponding Tb concentrations analyzed by X-ray photoelectron spectroscopy (XPS) were 0, 4, 10, and 14%, respectively. The film thickness measured by transmission electron microscopy (TEM) was 14 nm. The dielectric films were annealed at 420 C in a forming gas am-bient before forming 150-nm-thick TaN top electrodes, because our experimental experiences have shown that the forming gas annealing helps to reduce leakage current in sputter-deposited dielectric films on metal films. The top electrode was patterned by conventional optical lithography and dry etching. The area and the perimeter of the MIM capacitors used for electrical measurements are 2500 m and 2000 m, respectively.

III. RESULTS ANDDISCUSSIONS

The capacitance densities of MIM capacitors using Tb-doped HfO dielectrics with four different Tb doping concentrations are shown in Fig. 1. From the capacitor with a 14-nm-thick pure HfO dielectric, denoted as 0% Tb concentration, a capacitance density as high as 13.7 fF/ m was achieved. A slight reduc-tion of capacitance density to 13.3 fF/ m was observed with 4% of Tb doping. Further doping of Tb has caused a substan-tial loss in the capacitance density, as observed in 10 and 14% Tb concentration samples. The degradation in the capacitance density for the Hf Tb O samples with a high concentration of Tb is attributed to the lower dielectric constant of Tb O . While the capacitance density varies with Tb concentration, individual capacitance values remain unchanged with measurement fre-quency up to 1 MHz regardless of the doping conditions,

KIM et al.: LANTHANIDE (Tb)-DOPED HfO FOR HIGH-DENSITY MIM CAPACITORS 443

Fig. 1. Capacitance densities of MIM capacitors using Hf Tb O with different Tb concentrations. High densities of 13.7 and 13.3 fF/m have been achieved for pure HfO and 4% Tb-doped samples, respectively.

Fig. 2. (a) Leakage current densities of MIM capacitors using HfO and Hf Tb O with different Tb concentrations. The lowest leakage current is found in 4% Tb-doped sample. (b) Leakage current densities of 0 and 4% Tb-doped HfO MIM capacitors measured at 125 C.

plying that the addition of Tb does not deteriorate the frequency response of the capacitance.

Fig. 2(a) shows the leakage current densities of Tb-doped HfO dielectrics with different Tb doping concentrations. The result shows that a small amount (4%) of Tb doping into HfO significantly increases the onset voltage for the dc-type conduc-tion, thereby the leakage current remains less than

A/cm at the bias of up to 3.3 V. The lower leakage current with lanthanide-doped HfO film is attributed to both less oxygen va-cancies and to the higher packing density of lanthanide-doped HfO that result from lower electro-negativity and larger atomic radii of lanthanide materials [11]. However, when the Tb doping concentration is increased to 10 and 14%, the leakage current property was severely deteriorated. The mechanism for the high leakage current in HfO film with an excessive amount of Tb doping is not clear at the moment. The leakage currents at high temperature for pure HfO and 4% Tb-doped HfO samples are shown in Fig. 2(b). The leakage current of the 4% Tb-doped HfO sample remains less than A/cm up to 2 V even at 125 C.

Dependence of Tb concentration in Hf Tb O film on both capacitance density and leakage current is summarized in Fig. 3. The leakage currents measured at 3.3 V and the capacitance densities obtained at 100 kHz are plotted together. From the result, we can see that a 4% Tb doping is the optimum condition

Fig. 3. Capacitance density at 100 kHz( ) and leakage current density at 3.3 V() against Tb doping concentration. The optimal Tb doping concentration is found at 4% in this work.

Fig. 4. Linear VCC, V , and quadratic VCC, V , against Tb doping concentration.V decreases with Tb concentration while the smallest V of0332 ppm/V was obtained at 4% Tb concentration. Virtually zero V is obtainable with Tb doping concentration at about 5%.

as it shows the lowest leakage current while maintaining a high capacitance density similar to that of a pure HfO dielectric.

The effect of Tb doping on voltage coefficients of a MIM ca-pacitor is shown in Fig. 4. Linear VCC on the left -axis in linear scale and quadratic VCC on the right -axis in log scale are plotted against Tb doping concentration in Hf Tb O film. The coefficients are extracted from the polynomial curve fitting on the capacitance–voltage (C–V) plot that is expressed as . According to the latest ITRS [4], less than 100 pm/V is required for analog circuit appli-cations and should be less than 1000 ppm/V for RF bypass capacitors. As seen in Fig. 4, an unacceptably high of 4843 ppm/V and of ppm/V are obtained from the pure HfO sample. The poor capacitance linearity of thin dielectric is expected, as VCC is known to be inversely proportional to the square of the dielectric thickness [12]. However, it was found that doping of Tb into HfO can significantly improve VCC as well. Fig. 4 and its inset table show both and can be dramatically changed by Tb doping. The decreases mono-tonically with Tb doping concentration, while the swings from negative to positive values by changing the Tb concentra-tion. The fact that curve against Tb concentration crosses the line in Fig. 4 indicates that it might be possible to obtain virtually zero by optimizing the Tb doping concentra-tion, which is about 5% of Tb doping into HfO in the figure. In

444 IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 7, JULY 2003

our experiment, the smallest of ppm/V was obtained at 4% Tb concentration. This value is less than half of the best one reported from the ALD HfO -based MIM capacitor [10], yet having even higher capacitance density and lower leakage current.

IV. CONCLUSION

We demonstrated that lanthanide doping in HfO can alle-viate the two undesired properties of thin dielectric MIM ca-pacitors, higher leakage current, and poor capacitance linearity. The 4% Tb-doped HfO dielectric MIM capacitor achieved a high capacitor density of 13.3 fF/ m leakage current less than A/cm at 3.3 V and the linear VCC as low as 332 ppm/V. Lanthanide-doped thin HfO film therefore has great potential for future RF bypass MIM capacitor applications.

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