High-Performance MIM Capacitors Using a High-kappa TiZrO Dielectric

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High-Performance MIM Capacitors Using a High-

␬ TiZrO

Dielectric

C. H. Cheng,aH. C. Pan,cS. H. Lin,dH. H. Hsu,aC. N. Hsiao,cC. P. Chou,a F. S. Yeh,dand Albert Chinb,z

a

Department of Mechanical Engineering and bDepartment of Electronics Engineering, National Chiao-Tung University, Hsinchu, Taiwan

c

Instrument Technology Research Center, National Applied Research Labs, Hsinchu, Taiwan d

Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan

We have fabricated high-␬ Ni/TiZrO/TaN metal–insulator–metal 共MIM兲 capacitors. A low leakage current of 3.3 ⫻ 10−8_A_/cm2_{at −1 V was obtained for a 18 fF}_/␮m2_{capacitance density. For a 5.5 fF}_/␮m2_{capacitance density device, a small} voltage coefficient of capacitance␣ of 105 ppm/V2_{and temperature coefficient of capacitance of 156 ppm}_{/°C were measured.} © 2008 The Electrochemical Society. 关DOI: 10.1149/1.2993977兴 All rights reserved.

Manuscript submitted July 2, 2008; revised manuscript received September 9, 2008. Published October 14, 2008.

The continuously increasing capacitance density 共␧0␬/t␬兲 and

preserving low leakage current are the technology trends of the metal–insulator–metal 共MIM兲 capacitors.1-18To achieve this goal, the use of higher-␬ dielectrics for MIM capacitors is required. How-ever, the increasing␬ value usually decreases the conduction band offset共⌬E_C兲 to the metal electrode, where the ⌬E_Ceven becomes slightly negative共−0.1 eV兲 in SrTiO3 共STO兲.18Such a small⌬EC

increases the unwanted leakage current of MIM capacitors,16and therefore a trade-off between the ⌬EC and ␬ values is necessary.

Besides, the␬ value in STO is strongly dependent on the process temperature due to the formation of nanocrystals above 450°C.16 Unfortunately, such a high temperature is above the maximum al-lowable temperature of 400°C for very large-scale integration 共VLSI兲 back-end integration. In this paper, we report low leakage TiZrO MIM capacitors processed at 400°C. Using a low cost and high work function共5.1 eV兲 Ni electrode, low leakage currents of 3.3⫻ 10−8and 2.5⫻ 10−7A/cm2 at, respectively, −1 and −2 V, were measured for the Ni/TiZrO/TaN MIM capacitors at 18 fF/␮m2

density. These electrical characteristics are better than previously reported for Ir/TiTaO/TaN12,13 and Ni/TiHfO/TaN15 capacitors with a close leakage current but at a slightly lower capacitance den-sity of 14.3 fF/␮m2. Besides, a small quadratic voltage coefficient of capacitance共VCC ␣兲 of 105 ppm/V2_{and a temperature}

coeffi-cient of capacitance共TCC兲 of 156 ppm/°C were measured in the 5.5 fF/␮m2TiZrO MIM capacitor. Such excellent device character-istics are due to the higher ⌬E_C for ZrO₂ 共1.4 eV兲 than Ta₂O₅ 共0.3 eV兲 and better ␬ of ZrO2 than HfO2.19,20These good device

performances of Ni/TiZrO/TaN capacitors can be used for multiple functional system-on-chip共SoC兲 application.

Experimental

The high-␬ TiZrO MIM capacitors were fabricated on Si wafers. For VLSI back-end integration, a 2␮m thick SiO2isolation layer

was first deposited on the Si substrates. After that, the combined bottom electrode of 200 nm Ta and then 50 nm TaN were deposited by sputtering. The TaN surface was exposed to a NH₃+plasma treat-ment to increase the oxidation resistance during the following post-deposition annealing 共PDA兲.5,6 Then, a 16, 47, or 56 nm thick Ti_xZr_1−xO共x ⬃ 0.67兲 dielectric layer was deposited by physical va-por deposition共PVD兲. Because the as-deposited TiZrO by PVD at room temperature is highly defective, a 400°C PDA in oxygen am-bient was performed to reduce the defects in TiZrO and leakage current.3This 400°C PDA is also used in the VLSI back-end-of-line to fabricate the MIM capacitors. From the PDA

temperature-dependent X-ray diffraction analysis, the TiZrO maintained an amorphous phase even up to 450°C. Finally, a 40 nm Ni and/or 50 nm Al was deposited and patterned to form the top electrode. The metal thickness for both the top and bottom electrode should be as thin as possible for dynamic random access memory共DRAM兲 but relatively thick for radio-frequency共rf兲 application to decrease the series resistance. The bottom TaN was made thicker because of the larger resistivity. A large capacitor size of 180⫻ 180 ␮m was mea-sured. The devices were characterized by capacitance-voltage共C-V兲 and current-voltage共J-V兲 measurements.

Results and Discussion

Figures 1a and b show the C-V and J-V characteristics of Ni/TiZrO/TaN and Al/TiZrO/TaN capacitors. A high capacitance density of 18 fF/␮m2_{was measured at 500 kHz. At −2 V, the}

leak-age current of TiZrO MIM capacitors improves by 2 orders of mag-nitude using a high work function Ni共5.1 eV兲 as compared with Al 共⬃4.1 eV兲 electrode. At this 18 fF/␮m2 _{capacitance density, low}

leakage currents of 3.3⫻ 10−8 and 2.5⫻ 10−7A/cm2 at −1 and −2 V were measured in a Ni/TiZrO/TaN MIM capacitor, respec-tively. Table I summarizes the device performance of various MIM capacitors. The Ni/TiZrO/TaN device data are better than those of the Ir/TiTaO/TaN MIM capacitors with a lower 14.3 fF/␮m2 ca-pacitance density shown in Table I, even though a higher work func-tion Ir top electrode 共⬃5.27 eV兲 is used for the TiTaO capacitor than the Ni electrode共⬃5.1 eV兲 for TiZrO. This is mainly attributed to the larger conduction band offset of ZrO2共1.4 eV兲 than that of

Ta2O5 共0.3 eV兲.19The device performance of Ni/TiZrO MIM

ca-pacitors is also better than the Ni/TiHfO,15where a higher capaci-tance density is obtained in Ni/TiZrO with a comparable leakage current shown in Table I. This is due to the higher␬ for ZrO2than

HfO₂ with close ⌬E_C, which is the reason why ZrO₂ is used in DRAM to replace HfO2.20For analog integrated circuit共IC兲

appli-cation, a low VCC ␣ is required. Figure 1c shows the ⌬C/C-V characteristics of TiZrO MIM capacitors, where VCC␣ can be ex-tracted from the following equation: C共V兲 = C0共␣V2+␤V + 1兲; ␣

and␤ are the quadratic and linear VCC, respectively. The VCC ␣ is better using a Ni electrode than the Al. This may arise from the higher work function of Ni than Al, which exponentially decreases the free carrier injection from the electrode by Schottky emission and lowers the effect of charge relaxation.21

To further lower the VCC, we fabricated TiZrO dielectric capaci-tors at larger 47 and 56 nm thickness. Figures 2a-c show the C-V, J-V, and ⌬C/C-V characteristics of Ni/TiZrO/TaN capacitors at these TiZrO thicknesses. Low leakage currents of 6.7⫻ 10−8_and

4⫻ 10−8_{at −2 V were measured at a capacitance density of 6.5 and}

5.5 fF/␮m2, respectively. Both the VCC␣ and ␤ decrease with in-creasing TiZrO thickness or dein-creasing capacitance density. A small VCC␣ of 105 ppm/V2_{and a VCC}_{␤ of −757 ppm/V at 500 kHz}

z

E-mail: albertគachin@hotmail.com

Journal of The Electrochemical Society, 155共12兲 G295-G298 共2008兲

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were obtained at a 56 nm thickness of TiZrO with a capacitance density of 5.5 fF/␮m2_{. Besides, the small dissipation factor from}

0.015 to 0.084 was measured with increasing frequency from 10 to 500 kHz. These results indicate that the Ni/TiZrO/TaN ca-pacitor is a good candidate for rf application. From the experimental

data presented in Fig. 1c and 2c, the VCC␣ improves with increas-ing the metal work function and dielectric thickness, where both cases give the lower charge injection into the capacitor. This was well explained by the charge injection model reported previously.21 These good device performances nearly meet the requirements of -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 14 16 18 20 22 24 100 kHz 500 kHz Ni/TiZrO/TaN Al/TiZrO/TaN

Voltage (V)

C

H

(

fF

/

m

2

)

(a)

-3 -2 -1 0 1 2 3 10-9 10-7 10-5 10-3 10-1 101 103 105 107 4.5eV Ni TiZrO TaN 5.1eV 4.1eV Al Ni/TiZrO/TaN Al/TiZrO/TaN

Current

Density

(

A/c

m

2

)

Voltage (V)

Gate injection Bottom injection

(b)

0.0 -0.5 -1.0 -1.5 -2.0 0.0 6.0x103 1.2x104 1.8x104 2.4x104 3.0x104 3.6x104 4.2x104

C~18 fF/

m

2 =7651 =3308 Ni/TiZrO/TaN Al/TiZrO/TaN

C/C

(ppm)

Voltage (V)

@500kHz

(c)

Figure 1.共a兲 C-V, 共b兲 J-V, and 共c兲 ⌬C/C-V characteristics of Al/TiZrO/TaN and Ni/TiZrO/TaN MIM capacitors.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2 4 6 8 10 C~6.5

fF/

m

2 C~5.5

fF/

m

2

Ni/TiZrO/TaN

100 kHz 500 kHz

Voltage (V)

C

H

(fF/

m

2

)

(a)

-3 -2 -1 0 1 2 3 10-9 10-7 10-5 10-3

Ni/TiZrO/TaN

C~6.5fF/m2 C~5.5fF/m2

Current

density

(

A/cm

2

)

Voltage (V)

(b)

0.0 -0.5 -1.0 -1.5 -2.0 0.0 2.0x103 4.0x103 6.0x103

Ni/TiZrO/TaN

C~6.5 fF/m2;=248,= -1840 C~5.5 fF/m2;=105,= -757

C/C

(ppm)

Voltage (V)

@500kHz

(c)

Figure 2.共a兲 C-V, 共b兲 J-V, and 共c兲 ⌬C/C-V characteristics of Ni/TiZrO/TaN MIM capacitors with 47 or 56 nm TiZrO dielectric thicknesses.

G296 Journal of The Electrochemical Society, 155共12兲 G295-G298 共2008兲

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bypass capacitors used for rf circuits listed in the International Tech-nology Roadmap for Semiconductors共ITRS兲 for the year 2012, with a capacitance density⬎5 fF/␮m2_{, a VCC}_{兩␣兩 ⬍ 100 ppm/V}2_{, and a}

VCC兩␤兩 ⬍ 1000 ppm/V.1

The TCC is an important factor, because modern ICs usually operate at elevated temperatures. Figure 3 shows the measured nor-malized capacitance as a function of temperature. Small TCC values of 179 and 156 ppm/°C were measured at 100 kHz for the 6.5 and 5.5 fF/␮m2 _{density TiZrO MIM capacitors, respectively. The}

de-creasing trend of TCC with dede-creasing capacitance density is similar to the VCC␣ case, which again may be related to the charge trap-ping and relaxation in MIM capacitors.21

Such an improving trend of VCC␣ with the decreasing capaci-tance density of MIM capacitors is summarized in Fig. 4. Here, the variation of␣ is plotted as a function of 1/C to show the dependence of capacitance equivalent thickness共=␧₀␬/C兲. The TiZrO shows a better chance to meet the ITRS requirement in 2012 than HfO2and

Ta2O5. Besides, for the same required VCC兩␣兩 ⬍ 100 ppm/V2, the

TiZrO can have a higher capacitance density than using HfO2and

Ta₂O₅.

We have further studied the thermal stability of a Ni/TiZrO/TaN capacitor. Figures 5a and b display the C-V and J-V characteristics of a Ni/TiZrO/TaN device before and after thermal annealing at 350°C for 20 min under N2ambient. Only a small degradation of

the capacitance density and leakage current was found, which indi-cates the good thermal stability of both the top Ni electrode and the TiZrO dielectric.

Conclusions

We have investigated the device characteristics of Ni/TiZrO/TaN capacitors. A low leakage current and high capaci-tance density were obtained and better than previously reported MIM capacitors using a TiTaO or TiHfO dielectric. A low leakage current, a small VCC␣ of 105 ppm/V2, and a TCC of 156 ppm/°C have been achieved in Ni/TiZrO/TaN MIM devices at 5.5 fF/␮m2 capacitance density. This high-performance device is capable of be-ing integrated into a VLSI back-end and bebe-ing used in multiple functions associated with SoC.

25

50

75

100

125

0.0 2.0x10

4

4.0x10

4

6.0x10

4

8.0x10

4

Ni/TiZrO/TaN

C~6.5 fF/

m

2

; 179 ppm/

o

C

C~5.5 fF/

m

2

; 156 ppm/

o

C

Temperature (

o

C)

Norm

alized

capacitance

(ppm

)

@100kHz

Figure 3. TCC characteristics of Ni/TiZrO/TaN MIM capacitors with 47 or 56 nm TiZrO dielectric thicknesses.

0.00 0.04 0.08 0.12 0.16 0.20 0.24 101 102 103 104 1050 2 4 6 8 Al2O3/Ta2O5/Al2O3 Tb doped HfO₂ HfO₂ TiZrO

CET (nm)

VCC-

(ppm

/V

2

)

1/C

(

m

2

/fF)

2012 ITRS

Figure 4.⌬C/C-1/C plot for various MIM capacitors.

Table I. Comparison of MIM capacitors with various dielectrics and metal electrodes.

HfO2a

Tb-HfO2b TiTaOc TiHfOd ITRS@2012e TiZrOf

Top electrode Ta Ta Ir Ni — Ni Work-function 共eV兲 4.2 4.2 5.27 5.1 — 5.1 C Density 共fF/␮m2_兲 13 13.3 14.3 14.3 5 18 6.5 5.5 J共A/cm2_兲 @25°C 6⫻ 10−7 共2 V兲 1⫻ 10 −7 共2 V兲 2⫻ 10 −7 共2 V兲 8.4⫻ 10 −8 共1 V兲 — 3.3⫻ 10 −8 共1 V兲 2.5⫻ 10−7 共2 V兲 6.7⫻ 10−8 共2 V兲 4⫻ 10 −8 共2 V兲 ␣ 共ppm/V2_兲 ₆₀₇ ₂₆₆₇ ₆₃₄ ₃₃₉₂ _{␣ ⬍ 100} ₃₃₀₈ ₂₄₈ ₁₀₅ TCC共ppm/°C兲 — 123 — 123 — — 179 156 a_Ref.8 b_Ref.10 c_Ref.12,13 d_Ref.15 e_Ref.1 f_{This work.} G297

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Acknowledgment

This work was supported in part by NSC 95-2221-E-009-275 of Taiwan.

National Chiao-Tung University assisted in meeting the publication costs of this article.

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-2

-1

0

1

2

16

18

20

22

24 Fresh

After N

₂

PDA

Ni/TiZrO/TaN @500 kHz

Voltage (V)

C

H

(fF/

m

2

)

350oC N₂PDA 20min

(a)

-3 -2 -1 0 1 2 3 10-9 10-7 10-5 10-3 10-1 101 103

Fresh

After N

₂

PDA

350oC N2PDA 20min Ni/TiZrO/TaN

Current

density

(

A/c

m

2

)

Voltage (V)

(b)

Figure 5. Thermal stability behavior of共a兲 C-V and 共b兲 J-V characteristics for Ni/TiZrO/TaN capacitors after a 350°C N2anneal for 20 min.

G298 Journal of The Electrochemical Society, 155共12兲 G295-G298 共2008兲

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