PVD HfO2 for high-precision MIM capacitor applications

IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 6, JUNE 2003 387

PVD HfO

₂

for High-Precision MIM

Capacitor Applications

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

Xiongfei Yu, Chunxiang Zhu, Member, IEEE, Albert Chin, Senior Member, IEEE, and

Dim-Lee Kwong, Senior Member, IEEE

Abstract—Metal–insulator–metal (MIM) capacitors are

fabri-cated using sputtered HfO₂with Ta and TaN for top and bottom electrodes, respectively. High-capacitance densities from 4.7 to 8.1 fF m2have been achieved while maintaining the leakage current densities around1 10 8A cm2within the normal circuit bias conditions. A guideline for the insulator thickness and its dielectric constant has been obtained by analyzing the tradeoff between the linearity coefficient and the capacitance density.

Index Terms—Capacitance density, HfO₂, metal–insu-lator–metal (MIM) capacitor, sputter, voltage coefficient of capacitor (VCC).

I. INTRODUCTION

M

OST of the high-precision metal–insulator–metal (MIM) capacitors, one of the key passive components in RF/mixed signal circuit devices [1]–[7], currently use PECVD SiO or Si N dielectrics with capacitance density of around 1 fF m [3], [7]. Recently, there has been great interest in replacing those conventional dielectrics with higher dielectric constant materials such as Al O , Ta O , TiO , HfO , etc. [9]–[12] to increase capacitance density while maintaining low-leakage current. It has been reported that HfO is one of the promising materials for the MIM capacitor, showing good performances such as high capacitance density, low-leakage current, and small VCC and TCC [12]. However, the deposition method, pulsed laser deposition (PLD), used in the literature is not a practical technique for mass production. More recently, it has been demonstrated that the HfO Al O MIM capacitor prepared by atomic layer deposition (ALD) provides a high capacitance density of 3.5 fF m and low VCC value ppm V [13]. However, since the dielectric thickness range for an analog MIM capacitor is much thicker than the gate dielectric and DRAM capacitor applications, the ALD process has a low throughput problem when it is used for analog MIM capacitor application. In this paper, the feasibility of sputtered HfO MIM capacitors for future RF/mixed signal circuit applications is investigated. Results on capacitance

Manuscript received March 3, 2003. The review of this letter was arranged by Editor C.-P. Chang.

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

A. Chin is with the Department of Electronics Engineering, National Chiao-Tung University, 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.2003.813381

Fig. 1. Capacitance densities of MIM capacitors using three different HfO thicknesses. Densities from 4.7 up to 8.1 fF=m have been achieved. Inset shows percentile changes in capacitances with respect to the values at 1 kHz. Dispersions range from 1.2% to 3.2%.

density and leakage are compared with recent publications and the ITRS requirement. In addition, the relationship between the linearity coefficient, VCC, and capacitance density has also been studied.

II. EXPERIMENTS

Ta HfO TaN multilayer MIM capacitor structures were fabricated on a thick field oxide layer. After TaN bottom electrode deposition, HfO was reactively deposited at room temperature in a magnetron sputtering system flowing a gas mixture of O (2 sccm) and Ar (23 sccm). The pressure was maintained at 3 mtorr and a dc power of 200 W was employed. Samples with three different HfO thicknesses, 16.4, 21.8, and 27.1 nm measured by TEM, were prepared. From the TEM analysis and capacitance value, the extracted effective dielectric constant of the sputtered HfO is 15. The films were annealed at 420 C in a forming gas ambient H N before Ta depo-sition for the top electrode. The area and the perimeter of the MIM capacitors are 168 100 m and 2576 m, respectively.

III. RESULTS ANDDISCUSSIONS

A. Capacitance Density and Leakage Currents

Capacitance densities of the sputtered HfO MIM capacitors are shown in Fig. 1 as a function of measured frequency. High densities, ranging from 4.7 to 8.1 fF m , have been obtained

388 IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 6, JUNE 2003

Fig. 2. Leakage currents for 16.4- and 27.1-nm-thick HfO MIM capacitors, which are low enough for most RF and analog applications. Requirements for specific applications are indicated with dashed lines.

from the capacitors using three different HfO thicknesses. The capacitance obtained is around 5 8 times higher than current SiO or Si N -based MIM capacitors [3], [7]. The capacitance values remained nearly unchanged throughout the measured frequency up to 1 MHz. The percentile variations with respect to the value at 1 kHz are shown in the inset of Fig. 1. The variations are 3.2%, 2.1%, and 1.2% for 21.8-, 16.4-, and 27.1-nm-thick HfO samples, respectively. These values are comparable or smaller than those of ALD HfO and Al O MIM capacitors [13]. Fig. 2 shows the leakage current densities of 16.4- and 27.1-nm-thick HfO MIM capacitors. Desired device requirements for on-chip capacitors for RF and mixed-signal applications are indicated with dashed lines in the figure for easy assessment. The leakage current densities are fairly low within the normal bias range from 3 to 3 V meeting ITRS requirements.

The performances of the 16.4- and 27.1-nm-thick HfO MIM capacitors are compared with published results and ITRS requirements on both capacitance density and leakage current density in Fig. 3. The dashed line indicates the capacitance density requirement. According to the latest ITRS, a capacitor density of 4 fF m or higher is required for precision analog capacitors from year 2005 to 2007 [14]. As for the leakage current, 7 fA pF V or lower is required, which corresponds to the area below the solid line in Fig. 3. The hatched area in the figure is where both the capacitance and the leakage current densities meet the ITRS requirements. From this figure, one can see that our PVD HfO MIM capacitors data lie within that boundary, meeting ITRS requirements for analog capacitor applications up to year 2007. Although Al O MIM capacitor [11] also exhibits sufficiently high capacitance density, it suffers from severe leakage current.

B. Linearity Coefficients and Capacitance Density

The relationship between the linearity coefficient of a capac-itor and its density was investigated and shown in Fig. 4(a) and (b). The quadratic VCC, , of the capacitors with three different HfO thicknesses are plotted in Fig. 4(a). The and linear VCC, , are usually extracted from the polynomial curve fitting on the CV plots which is expressed as C Co V V.

Fig. 3. Performance comparison with results in recent publication. Hatched area is where both the capacitance and the leakage current densities meet the ITRS requirement.

Fig. 4. (a) Quadratic VCC,, versus HfO thickness. HfO thicker than 42 nm is required to meet the requirement of 100 ppm=V . (b) Capacitance density versus the reciprocal of dielectric thickness. HfO film thinner than 32 nm is required ifk = 15 is to meet the requirement of 4 fF=m , while the film thinner than 42 nm is required ifk = 19.

The quadratic term indicates the variance of the capacitance on the applied bias, while the linear term shows the bal-ance of the capacitbal-ance. While the capacitors with capacitbal-ance densities higher than 4 fF m are of interest, an additional capacitor using thick HfO was also fabricated in order to provide more reliable projections in Fig. 4(a) and (b). The values as a function of HfO thickness exhibit a gradient of 1.93 in log–log scale. This result is consistent with that of the Ta O MIM capacitor, which was once reported to be inversely proportional to the square of the dielectric thickness [9]. From the plot in Fig. 4(a), we can see that, in order to meet ITRS 2007 requirement of VCC ppm V for the precision analog capacitor [14], the HfO film must be thicker than 42 nm. Meanwhile, when capacitance densities are plotted against the reciprocal of dielectric thickness as shown in Fig. 4(b), we can see that the extrapolation of the line crosses 4 fF m at 32 nm on the axis. This indicates that HfO must remain thinner than 32 nm in order to meet density requirement for precision capacitor. The dashed line in Fig. 4(b) is from calculations assuming that the value is 19. The result of crosses 4 fF m at 42 nm. From the TEM analysis and capacitance value, the effective dielectric

KIM et al.: PVD HfO FOR HIGH-PRECISION MIM CAPACITOR APPLICATIONS 389

constant of our PVD HfO is found to be 15. A slight im-provement on PVD HfO film quality to increase the effective dielectric constant or the leakage current density will satisfy both linearity and capacitance density requirements. Further improvement of our PVD HfO film quality by optimizing the process conditions is underway.

IV. CONCLUSION

High-performance MIM capacitors have been fabricated using sputtered HfO . The capacitance densities and the leakage current characteristics satisfy the ITRS requirements for analog capacitor up to year 2007, indicating that the sput-tered HfO is a very promising candidate for precision MIM capacitors. In addition, a preliminary guideline for the insulator thickness and its dielectric constant has been obtained from the tradeoff between the linearity coefficients and the capacitance density.

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