高介電常數閘極介電質熱氧化氧化鋁在金氧半電晶體的電性及應用

國 立 交 通 大 學

電子工程學系電子研究所

博 士 論 文

高介電常數閘極介電質熱氧化氧化鋁在金氧半

電晶體的電性及應用

The Electrical Characteristics and Application in

MOSFETs of High k Gate Dielectric Al

2

O

3

Formed by

Aluminum Oxidation

研 究 生 ：廖金昌

指導教授 ：荊鳳德

高介電常數閘極介電質熱氧化氧化鋁在金氧半

電晶體的電性及應用

The Electrical Characteristics and Application in

MOSFETs of High-k Gate Dielectric Al

2

O

3

Formed by

Aluminum Oxidation

研 究 生：廖金昌 Student: C. C. Liao

指導教授：荊鳳德博士 Advisor: Dr. Albert Chin

國立交通大學

電子工程系電子研究所

博士論文

A Dissertation

Submitted to Institute of Electronics

College of Electrical Engineering and Computer Science

National Chiao Tung University

In Partial Fulfillment of the Requirements

For the Degree of Doctor of Philosophy

In

Electronic Engineering

January 2005

高介電常數閘極介電質熱氧化氧化鋁在金氧半

電晶體的電性及應用

研究生：廖金昌 指導教授：荊鳳德博士

國立交通大學

電子工程系電子研究所

摘要

由於矽積體電路元件尺寸的縮微，導致不斷的減少閘極氧化層的厚度。然 而，隨之而來的高閘極漏電流卻使電晶體的特性變差，並且使元件消耗的功率變 大。一個解決的辦法是以高介電常數介電質材料取代傳統熱氧化層，如此可以增 加元件的密度而不需要更進一步減少閘極氧化層厚度。雖然高介電常數介電質的 使用能有效的減少閘極漏電流，但仍然有一些相關新的問題必須解決。我們發展 出一種簡單的沈積介電質的方法，為了避免任何製程在氧化鋁上引起缺陷，先利 用物理氣相沈積的方式蒸鍍金屬薄膜，再將其氧化及退火。在我們研究中，我們 將氧化鋁沈積在矽基版上，並製作成金氧半電容和電晶體。為了瞭解其被應用為 閘極介電質的特性，我們量測了元件參數，閘極漏電流、遷移率和電晶體特性， 量測結果顯示，氧化鋁運用在金氧半電容上其漏電流遠低於相同等效厚度的二氧 化矽介電質，電子遷移率可比的上文獻發表的熱氧化二氧化矽數據。由電容電壓 特性曲線顯示氧化鋁的高介電常數。量測介電質崩潰電壓、定電流應力、定電壓 應力和應力產生的漏電流的可靠性測試，根據可靠性分析證實氧化鋁的高品質。

為了研究偏壓溫度不穩定性的影響，將10MV/cm 的電場和 85oC 溫度應力加在氧 化鋁閘極介電質電晶體上量測臨界電壓的改變，在高電場和高溫應力下，由臨界 電壓的改變可以推斷10 年壽命下所能承受的最大閘極電壓僅達到 1 伏特操作下 10% 的安全邊緣。此外，我們分別量測氧化鋁和二氧化矽介電質的閘極漏電流、 電荷對介電質的崩潰以及應力產生的漏電流在銅污染下的影響。結果顯示，氧化 鋁閘極介電質有強大的銅污染阻隔能力。因此，我們使用的這項製程方法，可以 將氧化鋁應用在金氧半電晶體所使用的閘極介電質，此外它同時具有簡單，並且 與現有超大型積體電路製程技術相容的優點。

The Electrical Characteristics and Application in MOSFETs

of High-k Gate Dielectric Al

2

O

3

Formed by Aluminum

Oxidation

Student: C. C. Liao Advisor: Dr. Albert Chin

Department of Electronics Engineering &

Institute of Electronics

National Chiao-Tung University

Abstract

The scaling-down of silicon integrated circuits has lead to a constant reduction of the thickness of gate oxide. However, the high gate leakage current degrades the performance of transistors and enlarges the power consumption of the devices. A solution is to replace SiO2 by high dielectric constant insulators, thus allowing an

increase of the packing density without a further reduction of gate oxide thickness. Although utilizing high-k dielectrics reduced the gate leakage current effectively, there are still some issues that we have to overcome. We have developed an approach to deposit high-k dielectric. We deposited the ultra thin Al film on Si substrate using PVD followed by oxidation and annealing. The MOS transistor and capacitor devices with Al2O3 dielectrics were fabricated. To investigate the characteristics of Al2O3 used

as gate dielectric, we measured the leakage current, mobility and transistor performance. The result indicates that the leakage current of Al2O3 as dielectric of

comparable to published mobility data from thermal SiO2. Capacitance-Voltage (C-V)

curve shows its higher dielectric constant (k). We also show the high quality of Al2O3

dielectric according to its reliability analysis. Reliability tests were carried out by measuring breakdown voltage, constant current stress, constant voltage stress and stress induced leakage current (SILC) effect. To investigate the Bias-Temperature Instability (BTI) effects on Al2O3 CMOSFETs, the ∆Vt changes have measured after

10MV/cm and 85o_{C stress. The 10 years lifetime V}

max-10years is from the extrapolation

of ∆Vt changes at high gate voltage and high temperature that can barely meet the

required 1 V operation with 10% safety margin. Furthermore, to characteristics the Cu contamination effect on Al2O3 gate dielectric, we measured the gate dielectric leakage

current, charge-to-breakdown and stress-induced leakage current on Al2O3 and SiO2,

respectively. The results show the Al2O3 gate dielectric has strong Cu contamination

resistance. Therefore, using this approach, we can fabricate Al2O3 high-k dielectric

that is suitable in MOSFETs application. More important, this approach is simple and fully compatible with current VLSI technology.

誌 謝

首先對我的指導教授荊鳳德教授致上最誠摯的謝意,對於他多年

來的指導和鼓勵，使我在研究的工作上獲益良多。還要感謝蔡中教授

的指導與教誨，使我在日常生活待人處世的原則獲益匪淺。

另外，感謝和我一起工作的實驗室夥伴們，感謝他們對於實驗上

的幫忙和討論使我的研究能得以順利進行，感謝林柏村博士、巫勇賢

博士、呂承翰先生、林全益博士、陳睎白博士和詹歸娣博士在實驗上

的幫忙。對國家豪微米實驗室(NDL)以及交大半導體中心所提供的良

好研究設備和支持在此由衷表示我的謝意。感謝屏東科技大學材料工

程研究所陳文照教授在

TEM 的協助。對於諸位口試委員的指正和建

議也在此僅申謝意。

最後，我要特別感謝我敬愛的父母，多年來的支持與照顧，以及

愛妻名鋼、妹妹淑慧、淑如、弟弟崇宏的關懷與包容，他們給于我最

大的自由發展空間，讓我無後顧之憂，得以順利完成學業，僅以此論

文獻給我最親愛的家人。

Contents

Abstract (in Chinese)

……….……….…...…... I

Abstract (in English)

……….……….…... iii

Acknowledgment

……….………….……….…... v

Contents

……….………..…... vii

Figure Captions ………...……….………. xi

Table Captions

……… xvii

Chapter 1 Introduction

………..…...……….………… 1

1.1 Background and Motivation of Al2O3 High-k Dielectric ………..….…..…. 1

1.2 The Formation of Al2O3 High-k Dielectric ………...……… 5

1.3 The Measurement of the Devices ……….………… 6

1.4 Thesis Outline ………..……….……..…..…… 7

Chapter 2 Electrical characterization of Al

2

O

3

on Si from thermally

oxidized AlAs and Al

………….………..……….….. 14

2.1 Introduction ………..……….………..…….….. 14

2.3 Results and Discussion ………..……….……...…. 16

2.4 Conclusion ………..……….………... 19

Chapter 3 Device and Reliability of High-k Al

2

O

3

and La

2

O

3

Gate

Dielectric with Good Mobility and Low D

it …….….….. 24

3.1 Introduction ………..……….. 24

3.2 Device and Reliability of High-k Al2O3 Gate Dielectric ………..……….. 25

3.2.1 Experimental ………..………...… 26

3.2.2 Results and Discussion ………..……… 26

3.2.2.1 The physical property of Al2O3 …..………..…….. 26

3.2.2.2 Gate capacitor ………..…..………... 27

3.2.2.3 Mobility and transistor performance ………..…....…… 27

3.2.2.4 Reliability ………..………. 28

3.3 Device and Reliability of High-k La2O3 Gate Dielectric ……… 28

3.3.1 Experimental ………..…..………….…...……. 28

3.3.2 Results and Discussion ………..…………..……….…. 29

3.3.2.1 Gate capacitor ………..………….……….…... 29

3.4 Conclusion ……….……….……... 31

Chapter 4 Bias-Temperature Instability and Charge Trapping on

Fully-Silicided-Germanided Gates/High-k Al

2

O

3

CMOSFETs

.………..……….……… 55

4.1 Introduction ……….……….………..…………. 55

4.2 Experimental ………..……..………..…… 56

4.3 Results and Discussion ………..………..……….…….. 57

4.4 Conclusion ………..………..….…….……… 61

Chapter 5 The Copper Contamination Effect on Al

2

O

3

Gate

Dielectric on Si

……….…...……….. 72

5.1 Introduction ………..……….. 72

5.2 Experimental ……….……… 73

5.3 Results and Discussion ………..………...……….. 74

5.4 Conclusion ………..………..…….. 79

Chapter 6 Conclusion

………..……….……...… 85

Vita

………..……….….. 104

Figure Captions

Chapter 1

Fig. 1-1 ITRS projections for low power gate leakage.

Chapter 2

Fig. 2-1 SIMS depth profiles of oxidized AlAs at 500o_C.

Fig. 2-2 SIMS depth profiles of oxidized Al at 500o_C.

Fig. 2-3 J-V characteristic of MOS capacitors with deposited AlAs oxidized at a temperature of 500o_{C to form Al}

2O3 dielectric. The capacitor area is 800µm

×800µm.

Fig. 2-4 J-V characteristic of MOS capacitors with deposited Al oxidized at a temperature of 500o_{C to form Al}

2O3 dielectric. The capacitor area is 800µm

Chapter 3

Fig. 3-1 SIMS depth profile of oxidized Al at 500o_{C. The measured thickness is}

close to that measured by ellipsometer & cross-sectional TEM. Fig. 3-2 TEM photos of Al oxidized at (a) 400o_{C and (b) 500}o_C.

Fig. 3-3 J-V characteristics of the 48 Å Al2O3 MOS capacitor.

Fig. 3-4 C-V characteristics of 110 Å Al2O3 MOS capacitor.

Fig. 3-5 Interface trap density of the Al2O3-Si interface.

Fig. 3-6 Electron mobilities of MOSFET’s with 80 Å Al2O3 and conventional SiO2.

(M. S. Liang, J. Y. Choi, P. K. Ko, C. Hu, IEEE TED-33, no. 3, pp. 409-413, 1986.)

Fig. 3-7 Id-Vd characteristics of an 80 Å Al2O3 MOSFET. The high current drive in

Al2O3 gives a k-value of 9.8.

Fig. 3-8 Current density changes of the 48 Å Al2O3 MOS capacitor under a constant

voltage stress at 2.5V for 1000sec.

Fig. 3-9 SILC effect of the 48 Å Al2O3 MOS capacitor under a constant voltage

stress at 2.5V for 10,000sec.

Fig. 3-10 Voltage changes of 48 Å Al2O3 MOS capacitor under a 0.1mA/cm2 current

density stress for 1000s.

Fig. 3-11 SILC effect of 48 Å Al2O3 MOS capacitor under a 0.1mA/cm2 current

density stress for 1000sec.

Fig. 3-12 J-V characteristics for Al2O3 (9.6 & 16.5 Å EOT)and La2O3 (4.8 Å EOT)

capacitors. Stacked structure is adopted to reduce leakage current for Dit

measurement.

Fig. 3-13 Cross-section TEM of Al2O3 and La2O3. Very smooth interface is due to the

high thermal stability and native oxide free surface. Both dielectrics are amorphous.

Fig. 3-14 Cumulative distribution of leakage current and breakdown field for Al2O3

and La2O3 gate dielectrics.

Fig. 3-15 Hysteresis curves for Al2O3 and La2O3 gate dielectrics.

Fig. 3-16 Interface state density of Al2O3 and La2O3 on Si. Min Dit of 3x1010

eV-1_/cm2 _{is obtained for both dielectrics and close to SiO} 2/Si.

Fig. 3-17 Id-Vd characteristics of 30mmx1200mm NMOSFETs with 33 Å La2O3 gate

dielectric.

Fig. 3-18 Subthreshold characteristic and transconductance for 33 Å La2O3

NMOSFETs as a function of gate bias.

Fig. 3-19 Effective electron mobility versus electrical field for 33 Å La2O3

nNMOSFET.

C/cm2 _{for respective La}

2O3 and Al2O3.

Fig. 3-21 Stress induced leakage current and current variation for Al2O3 and La2O3

under -2.5V for 1hr.

Fig. 3-22 QBD distribution of La2O3 dielectricwith different Vg. For 50% MTTF and

Chapter 4

Fig. 4-1 The room temperature Ids-Vgs characteristics of NiSi/Al2O3 nMOSFETs

under double sweep measurements. The inserted figures are the enlarged view to show the hysteresis.

Fig. 4-2 The room temperature Ids-Vgs characteristics of NiGe/Al2O3 pMOSFETs

under double sweep measurements. The inserted figures are the enlarged view to show the hysteresis.

Fig. 4-3 The room temperature C-V characteristics of NiSi/Al2O3 nMOS capacitors

under double sweep measurements. The inserted figures are the enlarged view to show the hysteresis.

Fig. 4-4 The room temperature C-V characteristics of NiGe/Al2O3 pMOS capacitors

under double sweep measurements. The inserted figures are the enlarged view to show the hysteresis.

Fig. 4-5 The Ids-Vgs changes of NiGe/Al2O3 pMOSFETs stressed at 85oC and 10

MV/cm for 1 hour.

Fig. 4-6 The ∆Vt changes of NiGe/Al2O3 pMOSFETs stressed at 85oC and 10

MV/cm for 1 hour. For comparison, the data from poly-Si/SiON and TaN/HfAlO (from reference 6) are also added under the same stress condition.

Fig. 4-7 The Ids-Vgs change of NiSi/Al2O3 nMOSFETs stressed at 85oC and 10

MV/cm for 1 hour.

Fig. 4-8 The ∆Vt changes of NiSi/Al2O3 nMOSFETs stressed at 85oC and 10

MV/cm for 1 hour. For comparison, the data from poly-Si/SiON and TaN/HfAlO (from reference 6) are also added under the same stress condition.

Fig. 4-9 The extrapolated maximum operation voltage for 10 years lifetime of NiSi/Al2O3 nMOSFETs and NiGe/Al2O3 pMOSFETs under failure

condition of 50 mV change in Vt at 85oC. The data from oxynitride is also

Chapter 5

Fig. 5-1 The J-(VG-VFB) characteristics of MOS capacitors with 42 Å Al2O3 gate

dielectric (19 Å EOT) with or without Cu contamination. The MOS devices with 30 Å thermal SiO2 are also added for comparison. The devices were

contaminated by 10 ppb or 10 ppm Cu.

Fig. 5-2 The distribution of leakage current density for (a) 42 Å Al2O3 gate

dielectric (19 Å EOT), (b) 30 Å thermal SiO2 and (c) 36 Å SiON with

23%N content (30 Å EOT) gate dielectrics with or without Cu contamination.

Fig. 5-3 The (a) QBD and (b) tBD distribution of 42 Å Al2O3 gate dielectric (19 Å

EOT) MOS devices under different Cu contamination level. The distributions of 30 Å SiO2 MOS capacitors with or without Cu

contamination are also added for comparison.

Fig. 5-4 The stress effect on J-(VG-VFB) characteristics for MOS capacitors

contaminated by Cu with (a) 42 Å Al2O3 gate dielectric (19 Å EOT) and (b)

30 Å SiO2. The applied stress condition is at –3.3 V for 10,000 sec.

Fig. 5-5 The stress and Cu contamination effect on ∆J/J-V characteristics of (a) 42 Å Al2O3 gate dielectric (19 Å EOT) and (b) control 30 Å SiO2 MOS

Table Caption

Chapter 1

Table 1-1 The International Technology Roadmap of ITRS for Semiconductor 2002. Table 1-2 Physical properties for various high-k materials.

Chapter 1

Introduction

1.1

Background and Motivation of Al

2

O

3

High-k Dielectric

Continual the scaling-down of MOS devices has to a constant reduction of the thickness of the gate dielectrics. For sub-0.13 um gate length technologies, an equivalent SiO2 thickness lower than 15 Å is required. MOS transistor with gate oxide

thickness of 15 Å and 13 Å has been demonstrated, respectively [1.1], [1.2]. Nevertheless, silicon dioxide does not satisfy all criteria for an ideal technological insulator, such a low thickness would subjected to several problems for conventional dielectrics, such as silicon oxide, silicon nitride or oxynitride films, in terms of leakage current density and resistance to impurity diffusion. Thereby reducing the thickness with lower leakage current and high resistance to impurity diffusion plays an important role. A solution is to replace these materials by high dielectric constant (high-k) insulators, thus allowing an increase of the packing density without a further reduction of the insulator thickness. Fig. 1-1 shows ITRS projections for low power gate leakage. Actually, novel high-k gate dielectric has been identified in future R & D plan [1.3] as one of the most important factors for deca-nano CMOS technology.

thickness of gate oxide have be below 13 Å after 2005, and the use high constant dielectrics as insulator to replace SiO2 has become so widespread in the future gate

dielectric. It would seem almost unchangeable [1.4]. Since the leakage current is related to the physical thickness, the increasing thickness can reduce the gate leakage current of the devices. Although high-k dielectric films exhibit smaller band gap, weak bond, and charge trapping than SiO2. The high-k dielectric films with the same

effective oxide thickness (EOT) with SiO2 provide sufficient gate control with reduce

gate leakage. That is the reason why high-k dielectrics have drawn much attention for future gate dielectrics.

Recently, many high-k dielectric candidate materials have widely investigated and the characteristics and issues of those materials have also been reported. Table 1-2 shows the physical properties for various high-k materials. The high-k dielectrics show good performances are always accompanied by other drawbacks. Finding out the most suitable high-k dielectric for device and altering the device structure or process to meet the requirement of the high speed device are significant tasks to implant high-k dielectrics to the next VLSI generation. Many alternatives to SiO2

have been proposed. Silicon nitride (Si3N4) is an attractive candidate for this purpose

due to its higher dielectric constant (~7). However, Si3N4 has a poor interface with

trap density in the film. Therefore, it is not an attractive candidate for a future gate dielectric for ULSI [1.5].

Among the high-k simple metal oxide, such as Tantalum oxide (Ta2O5) and

Titanium oxide (TiO2) are seriously considered as candidates for advanced CMOS

gate dielectric applications. Because of they exhibit the merits including the better capacitance linearity than that SiO2, the lower process temperature and the larger band

gap (4.65eV) than that of the other high-k dielectrics and high reliability [1.6], [1.7]. However, Ta2O5 and TiO2 are not thermally stability when directly contacted to Si

substrate [1.8]. They react with Si and form an interfacial low-k dielectric layer. Additional barrier layer has been used to prevent the reaction, but it increases equivalent oxide thickness (EOT) and process complexity [1.9]. In addition, crystallization will cause from post implant RTA.

Recently, HfO2 and ZrO2 have attracted much attention due to their thermal

stability with poly or TiN and TaN [1.10], [1.11], [1.12]. In addition, they also exhibits suitable dielectric constant (25~30) and energy band gap, for the use of MOSFET in sub-micron generation. However, HfO2 is poor barrier to oxygen diffusion, oxygen

tends to diffuse though HfO2 and form oxide interlayer after high temperature process

and it becomes difficult to achieve the effective oxide thickness below 15 Å. Furthermore, it exists an important issue for the integration of HfO2 and ZrO2. The

boron penetration effect is severe when single HfO2 serves as gate dielectric of

transistors, while ZrO2 reacts with poly silicon to form Zr silicide after post implant

annealing. This Zr-silicide defects increase the leakage current and degrade the reliability. Besides, both HfO2 and ZrO2 will crystallize at the temperature higher than

500o_{C. After source/drain activation, this property increases the leakage current and}

power dissipation of the device.

The use of aluminum oxide (Al2O3) has attracted much attention because it has

some advantage over silicon dioxide (SiO2) which makes it promising; lower leakage

current, greater resistance to ionic motion, greater radiation hardness, the possibility of obtaining low threshold voltage MOSFETs, and possibility of use in nonvolatile memory devices. Balk [1.13] has proposed a set of criteria to be employed in the selection of insulators either instead of, or in conjunction with silicon dioxide (SiO2) :

(1) high dielectric constant, (2) high breakdown strength, (3) an effective barrier against contaminants such as alkali ions, copper and moisture, (4) etchable in liquid etchants at or near room temperature, (5) stable electrical properties when used as gate dielectric in MOSFET, (6) controllable electrical properties when used as nonvolatile memory elements. Al2O3 fulfills many of these requirements very well. Al2O3 is

believed to be more resistant to ion penetration than silicon dioxide (SiO2). Besides,

and memory application.

1.2 The Formation of Al

2

O

3

High-k Dielectric

Many methods to deposit ultra thin high-k dielectrics on substrates have been proposed in recent years and various methods exhibit the merits as well as some other issues that have to be solved.

In this study, we used the advance deposition method to fabricate Al2O3 high-k

dielectrics. We utilized the self-limit property of Al2O3 to prevent from the formation

of interface oxide and have the effect of aluminum doping at the same time. The pre-cleaned wafer was loaded in the chamber under high vacuum condition, and then aluminum layer was deposited in evaporation on Si Substrate. Because extremely high quality is required for gate dielectric, we have used thermally grown Al2O3 from

oxidized Al on Si-substrate. In comparison, sputtering and oxygen plasma have been used in the past to deposit aluminum oxide. However, high defects can be expected from Ar-ion bombardment and oxygen plasma respectively [1.14]. The MOS transistor and capacitor devices with Al2O3 dielectrics are fabricated and measured. It

is found in our study that the self-limiting oxidation mechanism is one of the important merits of Al2O3 for either reproducibility or uniformity control. We have

first characterized the electrical property of Al2O3 as dielectric of MOS-capacitor to

applied it to be gate dielectric of a MOSFET and characterized its electrical properties.

1.3 The Measurement of the Devices

To investigate the electrical characteristics of MOSFETs, we measured the leakage current, stress induced leakage current and Vt degradation using HP4156C

semiconductor parameter analyzer with high temperature chuck. Besides, HP4284A precision LCR meter was used to evaluate the capacitance at a high frequency 100KHz and a quasi-static frequency 10Hz. The thickness of the aluminum oxide film was measured by a single wavelength ellipsometer. We assumed the oxide layer was homogeneous with a refractive index n=1.76 (typical of bulk aluminum oxide). The structure and atomic composition of capacitor samples were investigated by micro analyses, such as secondary ion mass spectrometry (SIMS). For the SIMS analysis, we can determine the oxide and aluminum diffusion profile into silicon. The leakage current of 48 Å Al2O3 as dielectric of MOS-capacitor is already better than that of

SiO2 with the same equivalent thickness of 13 Å. Capacitance-Voltage (C-V) curve

shows its higher dielectric constant (k). Besides, the effective mobility of Al2O3 gate

MOSFETs is closed to the effective mobility of traditional SiO2 gate MOSFETs. The

show the high quality of Al2O3 dielectric according to its reliability analysis.

Capacitor reliability tests were carried out by measuring constant current stress, constant voltage stress and SILC (Stress Induced Leakage Current) effect. To investigate the Bias-Temperature Instability (BTI) effects on fully silicided-gate/Al2O3

CMOSFETs. The ∆Vt changes have measured after 10MV/cm and 85oC stress. The 10

years lifetime Vmax-10years is from the extrapolation of ∆Vt changes at high gate voltage

and high temperature. Furthermore, to characteristics the Cu contamination effect on Al2O3 gate dielectric, we measured the gate dielectric leakage current,

charge-to-breakdown and stress-induced leakage current on Al2O3 and SiO2,

respectively. These results suggest the possible application of aluminum oxide (Al2O3)

to the next generation of VLSI.

1.4 Thesis Outline

In this thesis, we performed a high-k dielectric Al2O3 for the MOSFETs device

by thermal oxidizing the thin Al and than studied the electrical properties and mechanism of Al2O3 dielectric. Moreover, we have studied the Bias-Temperature

Instability (NBTI) on fully Nickel silicide (NiSi) and germanided (NiGe) gate on high-k Al2O3 nMOSFETs and pMOSFETs, respectively. Finally, the Cu contamination

In Chapter 2, we have studied the Al2O3 to use as an alternative gate dielectric.

To ensure good quality, Al2O3 is thermally oxidized from MBE-grown AlAs or Al on

Si-substrates. Experimental results indicate that the leakage current from oxidized AlAs is larger than that from directly oxidized Al, which may be due to the weak As2O3 inside Al2O3. The leakage current of a 53 Å Al2O3 is already lower than that of

SiO2 with an equivalent oxide thickness of 21 Å.

In Chapter 3, firstly we have reported a very simple process to fabricate Al2O3

gate dielectric with k (9.0 to 9.8) greater than Si3N4. Al2O3 is formed by direct

oxidation from thermally evaporated Al. The 48 Å Al2O3 has ~6 orders lower leakage

current than equivalent 13 Å SiO2. Good Al2O3/Si interface was evidenced by the low

interface density of 1x1011 _eVcm-2 _{and compatible electron mobility with thermal}

SiO2. Good reliability is measured from the small SILC after 2.5 V stress for 10,000s.

Secondly high quality La2O3 was fabricated with EOT of 9.6 Å, leakage current of 0.4

A/cm2 _{and D}

it of both 3x1010 eV-1/cm2. The high-k is further evidenced from high

MOSFET’s Id and gm with low Ioff. Good SILC and QBD are obtained and comparable

with SiO2. The low EOT is due to the high thermodynamic stability in contact with Si

and stable after H2 annealing up to 550oC.

In Chapter 4, we have studied the Bias-Temperature Instability (BTI) on fully Nickel silicided (NiSi) and germanided (NiGe) gate on high-k Al2O3 nMOSFETs and

pMOSFETs, respectively. At an equivalent-oxide thickness (EOT) of 17 Å, the NiSi/Al2O3 pMOSFETs and NiGe/Al2O3 nMOSFETs have comparable threshold

voltage (Vt) change of -34 and 33 mV at 85oC and 10 MV/cm stress for 1 hour. This

result is quite different from the more severe native BTI (NBTI) degradation measured in oxynitride pMOSFET than positive BTI (PBTI) in nMOSFET. The extrapolated maximum voltage for 10 years lifetime is 1.16 and -1.12 V from NiSi-NiGe/Al2O3 CMOSFETs that can barely meet the required 1 V operation with

10% safety margin. However, further improvement is still required since the 18 Å oxynitride CMOSFETs have higher 10 years lifetime operation voltage of 2.48 and -1.52 V for PBTI and NBTI, respectively.

In Chapter 5, we have studied the Cu contamination effect on 42 Å thick Al2O3

MOS capacitors with an equivalent-oxide thickness (EOT) of 19 Å. In contrast to the large degradation of gate oxide integrity of control 30 Å SiO2 MOS capacitors

contaminated by Cu, the 19 Å EOT Al2O3 MOS devices have good Cu contamination

resistance with only small degradation of gate dielectric leakage current, charge-to-breakdown and stress-induced leakage current. This strong Cu contamination resistance is similar to oxynitride (with high nitrogen content) but the Al2O3 gate dielectric has the advantage of higher-k value and lower gate dielectric

Fig. 1-1 ITRS projections for low power gate leakage.

Material Dielectric constant (k) Band gap Eg(eV) ∆Ec (eV) to Si Contact stability with Si (KJ/mole) Si+Mox--> M+ SiO Crystal structure

SiO2 3.9 8.9 3.2 Stable Amorphous

Al2O3 9 8.7 2.8 63.4 Amorphous La2O3 30 4.3 2.3 98.5 Amorphous HfO2 25 5.7 1.5 47.6 Crystal T>700C ZrO2 25 7.8 1.4 42.3 Crystal T>400~800C TiO2 80 3.5 1.2 - Tetragonal

Chapter 2

Electrical characterization of Al

₂

O

₃

on Si from

thermally oxidized AlAs and Al

2.1 Introduction

It has been shown that the practical scaling limit for gate oxide is due to the leakage current by direct-tunneling process [2.1], although 11-15 Å direct-tunneling gate oxide has already been demonstrated [2.2], [2.3]. The thickness of thermally grown gate oxide is scaled down to 35-40 Å for the 0.18-µm VLSI generation, and further scaling down below 15 Å is required within a few years [2.4]. Unfortunately the large direct-tunneling current precludes the use of silicon dioxide (SiO2) below

15-20 Å thickness. However, continuously scaling down the gate oxide is necessary to increase the current drive capability of MOSFETs and the operation speed of ICs. The relationship of drive current and gate oxide thickness is shown in equation (1):

IDsat＝1 2

µ

n tox KA 0 L Wε (VG-VT) 2 (1)

where IDsat is the device saturation current, VG is the applied gate voltage and k is

the dielectric constant of gate capacitor. The only solution to overcome this difficulty and to continuously scale down the gate dielectric is to use a thicker dielectric with a higher k value. Therefore novel high-k gate dielectric has been identified as one of the

most important R & D plans [2.4] for deca-nano CMOS technology. Unfortunately, because of the stringent requirements for device quality gate dielectric, no satisfactory alternatives to SiO2 has so far been found. Recently, aluminum oxide (Al2O3) has

attracted much attention because of its very high dielectric constant (~10) that can be used for next generation DRAM and Flash memory application [2.5]. Moreover, Al2O3 has also been treated as a gate barrier in InAlAs/InGaAs MESFET using

plasma oxidation of deposited Al [2.6]. In this study, we have first characterized the electrical property of Al2O3 to use as an alternative gate dielectric of MOSFETs.

Because extremely high quality is required for gate dielectric, we have used thermally grown Al2O3 from oxidized AlAs or Al that were grown in an ultra-high vacuum

molecular beam epitaxy (MBE) system. In comparison, sputtering deposition and O2

plasma oxidation have been used in the past to deposit Al2O3; however high defects

can be expected from Ar+ _{bombardment and plasma damage, respectively. In this}

work, the leakage current of a 53 Å Al2O3 from direct oxidized Al is already better

than that of SiO2 with the equivalent thickness of 21 Å [2.2]. This result suggests the

possible application of Al2O3 to VLSI.

2.2 Experimental

this study. After modified RCA cleaning, HF dipping, rinsing in DI water and spun dry, the wafer was treated by HF-vapor passivation and immediately loading into the MBE chamber. The HF-vapor passivation has been used to reduce thermal budget to desorb the native oxide [2.7]. Then 40-80 Å AlAs or 40-55 Å Al were grown by MBE at 500o_{C. The oxidation process was performed in a standard furnace at 500}o_{C for 90}

min., and the dielectric thickness was measured by an ellipsometer. After oxidation, 3000 Å poly-Si was deposited followed by phosphorus doping using POCl3 at 850oC.

The wafers were then received a 30-min. nitrogen anneal at 800o_{C. This anneal was}

found to be very effective to reduce dielectric leakage current. Subsequently, 3000 Å Al was deposited and gate electrode was defined by patterning and wet etching. The gate dielectrics were electrically characterized by I-V leakage current using a semiconductor parameter analyzer, and the composition profiles were measured using secondary ion mass spectroscopy (SIMS).

2.3 Results and Discussion

Fig. 2-1 presents the SIMS profiles of oxidized AlAs at 500oC, where O, Al and As are detected within the oxidized layer. The possible AlAs oxidation process is in the following equation:

Therefore both Al2O3 and As2O3 are formed during AlAs oxidation. As shown in Fig.

2-1, significant Al and As diffusion into Si is observed. The diffused As into Si will behave as a n-type dopant and change the threshold voltage, while the amount of diffused Al is dependent on the solid solubility of Si. However, none of these effects are desirable for MOS devices. The reduced surface concentration of As is due to the out-diffusion into ambient during oxidation.

Fig. 2-2 shows the SIMS profile of oxidized Al. The O2 concentration decreases

to its background value as increasing depth beyond the oxidized layer. This is because the Si oxidation rate is negligibly slow at this temperature. In sharp contrast to AlAs oxidation, the oxidized Al shows a much sharper SIMS profiles. Possible reason may be due to the absence of As-enhanced diffusion during oxidation. Another advantage of direct Al oxidation may be due to the strong bonding energy of Al2O3 instead of

As2O3 from oxidized AlAs. Therefore, the oxidized Al films may provide not only

better material quality but also lower diffusion into Si during oxidation.

We have further characterized the electrical behavior of these oxides. Fig. 2-3 shows the typical J-V characteristics of MOS capacitors from oxidized AlAs. The capacitor leakage current reduces with increasing dielectric thickness from 80 to 130 Å. This is a typical behavior because a thicker insulating barrier has lower electric field that can block the electron transport more easily. However, this thicker insulating

barrier did not successfully reduce the leakage current at low voltages less than ~0.7 V. Possible reasons may be due to trap-assisted tunneling at low electric field [2.8]. The traps may be generated by the weak bonding strength of As2O3 inside Al2O3 matrix or

vacancies by As out-diffusion.

To investigate the effect of As-related weak As2O3 or vacancies, we have studied

the electric characteristic of capacitors with directly oxidized Al. In comparison the J-V characteristics of ~ 80 Å oxides in Figs. 2-3 and 2-4, a much lower leakage current is observed at gate voltage below 1 V from directly oxidized Al. This leakage current at low gate electric field is related to the intrinsic defects [2.8] that is similar to stress-induced leakage current (SILC) [2.7], [2.9]. Therefore the As-related weak As2O3 or vacancies are responsible to the increased oxide leakage current. The almost

same current at high voltages is due to the dominated tunneling mechanism. We have further compared the leakage current of Al2O3 with SiO2 from the published I-V

characteristic [2.2]. It is important to note that the leakage current of 53 Å Al2O3 is

already better than the leakage current of MOS capacitors with the same equivalent thickness of 21 Å thick SiO2 [2.2].

2.4 Conclusion

We have studied the thermally oxidized AlAs and Al to use as an alternative gate dielectric. The leakage current from AlAs oxidation is larger than that from Al

oxidation at low voltages. The leakage current of a 53 Å Al2O3 is already lower than

SiO2 with an equivalent oxide thickness of 21 Å. These results suggest that scaling

Fig. 2-1 SIMS depth profiles of oxidized AlAs at 500oC. 0 100 200 300 400 500 101 102 103 104 105 106 Al₂O₃/Si interface As Al O Secondary Ion C o unts Depth (Angstroms)

Fig. 2-2 SIMS depth profiles of oxidized Al at 500oC. 0 100 200 300 400 500 101 102 103 104 105 106 Al₂O₃/Si interface Al O Secondary Ion C o unts Depth (Angstroms)

Fig. 2-3 J-V characteristic of MOS capacitors with deposited AlAs oxidized at a temperature of 500oC to form Al2O3 dielectric. The capacitor area is 800µm×

800µm. 0.0 0.5 1.0 1.5 2.0 2.5 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 130 Angstroms 80 Angstroms C urrent D ensi ty ( A /c m 2 ) Voltage (V)

Fig. 2-4 J-V characteristic of MOS capacitors with deposited Al oxidized at a temperature of 500o_{C to form Al}

2O3 dielectric. The capacitor area is 800µm×

800µm. The J-V characteristic of 21 Å SiO2 is from reference 2.

0.0 0.5 1.0 1.5 2.0 2.5 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 21 Angstroms SiO₂ 53 Angstroms Al₂O₃ 73 Angstroms Al₂O₃ C urrent D ensi ty ( A /c m 2 ) Voltage (V)

Chapter 3

Device and Reliability of High-k Al

₂

O

₃

and La

₂

O

₃

Gate Dielectric with Good Mobility and Low D

_it

3.1 Introduction

By continuously scaling down the CMOS technology, ultra-thin high-k gate dielectrics with low leakage current are required to replace the direct-tunneling current dominated thermal SiO2. Recently Si3N4 gate dielectric has been studied

extensively to replace thermal SiO2 [3.1]; however, the marginal improvement beyond

SiO2 is due to the relatively lower k and slightly higher leakage current. Although

stacked high-k materials have been studied to achieve higher k value than that of Si3N4 [3.2], [3.3], it may require complicated process steps and the leakage current is

still higher than expected. In this study, we report a very simple process to fabricate Al2O3 as an alternative gate dielectric with a k value (9.0 to 9.8) greater than Si3N4. To

avoid any process related damage, Al2O3 gate dielectric is formed by direct oxidation

from thermally evaporated Al. In addition to high-k, the 48 Å Al2O3 has ~6 orders

lower leakage current than equivalent 13 Å SiO2. Good mobility, interface trap density,

and reliability are also achieved using this process.

reduction of EOT may be limited by the interface reaction region between high-k material and Si [3.4], [3.5]. Therefore, the search for thermodynamically stable high-k dielectric directly on Si is important to meet future sub-10 Å requirement. Besides the required good electrical properties such as low interface trap density (Dit), low

leakage current, high breakdown field (EBD) and good reliability, high-k material must

also be compatible with existing VLSI process. Thus, good stability with H2 and high

transition temperature from amorphous to crystal [3.5] are necessary to prevent dielectric degradation by H2 and crystalline structure created defects or dislocations

during strain relaxation in process. Previously, we have worked that amorphous Al2O3

directly on Si can meet near all the requirements and stable up to 1000o_{C [3.6], except}

that EOT (21 Å) and Dit (1x1011 eV-1/cm2) are still high. The high Dit is unacceptable

for IC because of the increased noise [3.7]. In this study, we have used amorphous La2O3 (k~27) to achieve 4.8 Å EOT and reduced Al2O3 EOT to 9.6 Å, where La2O3

has similar property as Al2O3 but with even better thermal stability on Si (Table 1-2).

In addition to respective low leakage current of 0.06 and 0.4A/cm2 _{for La}

2O3 and

Al2O3,both dielectrics now have good Dit (3x1010 eV-1/cm2), EBD, SILC, and QBD as

compared with SiO2.

3.2.1 Experimental

To avoid any possible process-related damage in Al2O3 gate dielectric [3.8], we

have used direct thermal oxidation from thermally evaporated Al on Si. Native oxide is suppressed by a HF-vapor passivation and desorbed in-situ inside an ultra-high vacuum MBE environment [3.9], followed by an immediate Al evaporation. Deposited Al was oxidized at a temperature of 400o_{C to form Al}

2O3. Then the Al2O3

was annealed in nitrogen ambient to reduce defects. Poly-Si gate MOS capacitor and transistor were fabricated to evaluate the electrical characteristics. The suppression of native oxide is important to achieve higher k in Al2O3, and we have used similar idea

to achieve atomically smooth ultra-thin oxide with good electrical characteristics and reliability [3.10]. For comparison, MOS capacitor and transistor were also fabricated by thermal SiO2.

3.2.2 Results and Discussion

3.2.2.1 The physical property of Al

2

O

3

We have first measured the SIMS profile in Fig. 3-1 to confirm the formation of Al2O3. Although Al diffusion into Si can be observed, the concentration reduces

observed in Al2O3 bulk oxidized at 500oC. We have also used C-V measurement to

determine the Al diffusion and the concentration is lower than 1x1016 _cm-3_.

Furthermore, the field-dependent mobility also suggests the low Al diffusion into Si.

3.2.2.2 Gate capacitor

Fig. 3-3 presents the J-V characteristic of a 48 Å Al2O3 capacitor. The leakage

current is ~6 orders of magnitude lower than that of the equivalent 13 Å thermal SiO2.

In order to obtain an accurate k value, we have also measured the C-V of a thick 110 Å Al2O3 capacitor. As shown in Fig. 3-4, a k value of 9.0 is measured that is higher

than Si3N4. In Fig. 3-5 presents the measured interface trap density with a mid-gap

value of 1x1011 _eVcm-2 _{from this capacitor. This low interface trap density is suitable}

for MOSFET application.

3.2.2.3 Mobility and transistor performance

To further characterize the Al2O3/Si interface, we have fabricated a wide gate

MOSFET to measure the electron mobility. Fig. 3-6 shows the effective mobility of an 80 Å Al2O3 MOSFET. The electron mobility is comparable to published mobility data

than that of thermal SiO2 also proves the high-k value in Al2O3 MOSFET and a k of

9.8 is obtained.

3.2.2.4 Reliability

Fig. 3-8 and Fig. 3-9 show the 48 Å Al2O3 gate dielectric under a 2.5 V constant

stress for 10,000 sec, and good reliability is evidenced from the very small current change and stress-induced leakage current (SILC) respectively. Fig. 3-10 and Fig. 3-11 present the same Al2O3 gate dielectric under a much higher stress of ~5.4 V with

a 0.1 mA/cm2 _{constant current density for 1000 sec. The small voltage changes less}

than 0.04 V and the small SILC also suggest the excellent reliability. Therefore, the Al2O3 gate dielectric is suitable for continuously operation at VLSI generations of 2.5

V and beyond.

3.3 Device and Reliability of High-k La

2

O

3

Gate Dielectric

3.3.1 Experimental

To avoid any k value reduction, interfacial native oxide is suppressed by HF-vapor passivation and in-situ desorption [3.6] followed by an immediate La or Al evaporation. Because La or Al is highly reactive with O2, low oxidation temperatures

≤ 400o_{C is used to reduce metal diffusion into Si. The formed oxides were further}

annealed in N2 at 900oC. To reduce gate depletion, Al gate is used for MOS capacitor

and transistor to evaluate the electrical characteristics. H2 annealing at 450-550oC is

performed to study the stability withH2. Besides achieved higher k, suppression of

native oxide is important to obtain a smooth interface, low Dit, and high reliability in

our previously achieved atomically smooth ultra-thin oxides.

3.3.2 Results and Discussion

3.3.2.1 Gate capacitor

Fig. 3-12 presents the J-V characteristics of La2O3 and Al2O3 capacitors.

Comparable leakage current for La2O3 on Si or Si0.3Ge0.7 I s obtained that is important

for high mobility PMOS [3.11]. The stacked Al2O3/La2O3 is used to reduce leakage

current for C-V to obtain Dit. In order to get accurate k and EOT, the oxide thickness

is carefully examined by both ellipsometer and TEM in Fig. 3-13. The very uniform oxide and smooth interface are due to native oxide free surface and high thermal stability in Table 1-2 as contact with Si. Therefore, low EOT can be expected. Fig. 3-14 shows the cumulative values for high-k oxides, and leakage current of 0.06 A/cm2 _{for La}

4.8 Å and 9.6 Å EOT (without QM correction). Small hysterisis of 11 and 22 mV are measured for respective dielectrics that indicates good quality because of applied high annealing temperature without transition to crystal structure [3.5]. Fig. 3-16 shows the measured Dit of 3x1010 eV-1/cm2 from both capacitors. This low Dit close to thermal

SiO2 is extremely important for circuit to lower 1/f noise [3.7].

3.3.2.2 Transistor performance with 4.8 Å EOT

We have further fabricated wide gate MOSFETs with 4.8 Å EOT. Figs. 3-17 shows the device Id-Vd, and important Id-Vg and gm are plotted in Fig. 3-18. The very

high current drive and gm are due to high-k that gives a k of ~27 consistent with C-V

measurement. Good device pinch-off Ioff < 10-10A/µm and small sub-threshold swing

of 75 mV/decade are observed, and the small swing also suggests the low Dit. The

effective mobility is further plotted in Fig. 3-19. The electron mobility is comparable with published universal mobility data for thermal SiO2 because of low Dit.

3.3.2.3 Reliability

Fig. 3-20 shows the gate dielectrics under a -2.5 V constant stress for 1 hour with total Qinj of 1.3x103 and 1.2x105C/cm2 for La2O3 and Al2O3, respectively. No

significant charge trapping is occurred during stress, and small SILC for both dielectrics is observed in Fig. 3-21. Good reliability for 4.8 Å EOT La2O3 is

evidenced from the high QBD in Fig. 3-22 and comparable with current SiO2 [3.12].

The good SILC and QBD may be due to the high lattice energy in Table 1-2. From the

50% failure time, an extrapolated max voltage of 2.3 V is obtained for 10 years lifetime that suggests good reliability for VLSI application with 4.8 Å EOT and small leakage of 0.06 A/cm2 _{at 1 V.}

3.4 Conclusion

We report a very simple process to fabricate Al2O3 gate dielectric with k (9.0 to

9.8) greater than Si3N4. Al2O3 is formed by direct oxidation from thermally

evaporated Al. The 48 Å Al2O3 has ~6 orders lower leakage current than equivalent 13

Å SiO2. Good Al2O3/Si interface was evidenced by the low interface density of 1x1011

eVcm-2 _{and compatible electron mobility with thermal SiO}

2. Good reliability is

measured from the small SILC after 2.5 V stress for 10,000 sec.

High quality La2O3 and Al2O3 are fabricated with EOT of 4.8 and 9.6 Å, leakage

current of 0.06 and 0.4 A/cm2 _{and D}

it of both 3x1010 eV-1/cm2, respectively. The

Fig. 4-1

SIMS depth profile of oxidized Al at 500oC. The measured thickness is close to that measured by ellipsometer & cross-sectional TEM.

0

100

200

300

400

500

10

1

10

2

10

3

10

4

10

5

10

6 Al₂O₃/Si interface

Al

O

Se co nda ry I o n Co unt s Depth (Angstroms)

(a)

(b)

Fig. 3-2 TEM photos of Al oxidized at (a) 400oC and (b) 500oC.

Al

₂

O

₃

Glue

100Å

Al

₂

O

₃

Glue

100Å

Crystal

100Å

Crystal

100Å

0 1 2 3 4 5 6 7 8 10-11 10-9 10-7 10-5 10-3 10-1 101 103 105 48 A Al 2O3 13 A SiO 2

Jg (A/cm

2

)

Vg (Voltage)

Fig. 3-4 C-V characteristics of 110 Å Al2O3 MOS capacitor. -2 -1 0 1 2 0 100 200 300 400 500 600 Capacitance (pF) V (Voltage)

Fig. 3-5 Interface trap density of the Al2O3/Si interface. -0.6 -0.4 -0.2 0.0 0.2 0.4 1010 1011 1012 1013 1014 Di t (ev/cm 2 ) Energy (ev)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 100 200 300 400 500 600 700 Al₂O₃ gate 80A SiO₂ gate u eff (cm 2 /V-s e c ) E s,eff (MV/cm)

Fig. 3-6 Electron mobilities of MOSFET’s with 80 Å Al2O3 and conventional SiO2.

(M. S. Liang, J. Y. Choi, P. K. Ko, C. Hu, IEEE TED-33, no. 3, pp. 409-413, 1986.).

Fig. 3-7 Id-Vd characteristics of an 80 Å Al2O3 MOSFET. The high current drive in Al2O3 gives a k value of 9.8. 0 1 2 3 4 5 0.000 0.001 0.002 0.003 0.004 0.005 Al₂O₃ VG from 0V~5V SiO₂ VG from 0V~5V I D (A ) V_D (Voltage)

Fig. 3-8 Current density changes of the 48 Å Al2O3 MOS capacitor under a constant

voltage stress at 2.5 V for 10,000 sec. 0.0 _2.0x103 4.0x103 6.0x103 8.0x103 1.0x104 10-9 10-8 10-7 10-6 10-5 J (A/cm 2 ) Time (sec)

Fig. 3-9 SILC effect of the 48 Å Al2O3 MOS capacitor under a constant voltage stress at 2.5 V for 10,000 sec. 0 1 2 3 4 5 10-11 10-9 10-7 10-5 10-3 Before 2.5V 10000s stress After 2.5V 10000s stress J (A/cm 2 ) V (Voltage)

Fig. 3-10 Voltage changes of 48 Å Al2O3 MOS capacitor under a 0.1 mA/cm2 current

density stress for 1000 sec.

0 200 400 600 800 1000 0 1 2 3 4 5 6 V g (V oltag e) Time (Sec) -0.10 -0.05 0.00 0.05 0.10 ∆V Voltage Vo ltage Chan ge ∆ V ( V ol ta ge)

Fig. 3-11 SILC effect of 48 Å Al2O3 MOS capacitor under a 0.1 mA/cm2 current

density stress for 1000 sec.

0 1 2 3 4 5 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3

Before 0.1mA/cm2 stress 1000s After 0.1mA/cm2 stress 1000s

J (A/cm

2 )

Fig. 3-12 J-V characteristics for Al2O3 (9.6 & 16.5 Å EOT)and La2O3 (4.8 Å EOT)

capacitors. Stacked structure is adopted to reduce leakage current for Dit

measurement. 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 10-9 10-7 10-5 10-3 10-1 101 103 EOT=4.8A 33A La₂O₃ on Si_0.3Ge_0.7 EOT=16.5A 36A Al₂O₃ on Si EOT=9.6A 21A Al₂O₃ on Si EOT=21.8A 36A Al₂O₃ / 33A La₂O₃ / Si EOT=4.8A 33A La₂O₃ on Si Gate Voltage (V) C u rre n t D e n s ity (A /c m 2 )

Fig. 3-13 Cross-section TEM of Al2O3 and La2O3. Very smooth interface is due to

the high thermal stability and native oxide free surface. Both dielectrics are amorphous. La2O3 Glue 10nm Glue Si Al2O3 Si

Fig. 3-14 Cumulative distribution of leakage current and breakdown field for Al2O3

and La2O3 gate dielectrics.

11 12 13 14 15 0 20 40 60 80 100 C u m u la ti v e F a ilu re (% ) Breakdown Field (MV/cm) 0.01 0.1 1 Al₂O₃ 21A La₂O₃ 33A

Fig. 3-15 Hysteresis curves for Al2O3 and La2O3 gate dielectrics. -3 -2 -1 0 1 2 0 100 200 300 400 500 600 700 800 Area=1x10-4 cm2 21A Al₂O₃ V △ _FB=22mV 33A La₂O₃ V △ _FB=11mV Gate Voltage (V) Capaci tance ( pF)

Fig. 3-16 Interface state density of Al2O3 and La2O3 on Si. Min Dit of 3x1010

eV-1/cm2 is obtained for both dielectrics and close to SiO2/Si.

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

1010

1011

1012

EOT=21.8A

EOT=16.5A 36A Al2O3 / 33A La2O3 / Si

36A Al₂O₃/Si D it (e v -1 /c m 2 ) Energy (ev)

Fig. 3-17 Id-Vd characteristics of 30 mm x 1200 mm nMOSFETs with 33 Å La2O3 gate dielectric. 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 0 5 10 15 20 25 Vg=0.75V Vg=0.6V Vg=0.45V Vg=0.3V Vg=0.15V Dr ai n Cur re n t ( m A) Drain Voltage (V)

Fig. 3-18 Subthreshold characteristic and transconductance for 33 Å La2O3

nMOSFETs as a function of gate bias.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 10-6 10-5 10-4 10-3 10-2 10-1 SW=75 mV/decade Vd=0.4V Dr ai n Cu rr e nt ( A ) Gate Voltage (V) 0 5 10 15 20 25 30 35 40 g m ( m S /mm)

Fig. 3-19 Effective electron mobility versus electrical field for 33 Å La2O3 nMOSFET. 0.1 1 102 103 E_eff (MV/cm) u ef f (cm 2 /V -sec )

Fig. 3-20 Time evolution of Ig under -2.5 V for 1 hr with Qinj of 1.3x103 and 1.5x105

C/cm2 for respective La2O3 and Al2O3.

0 400 800 1200 1600 2000 2400 2800 3200 3600

10-1

100

101

102

33A La₂O₃ CVS under -2.5V for 3600sec

21A Al₂O₃ CVS under -2.5V for 3600sec

C u rre n t D e n s ity (A /c m 2 ) Time (second)

Fig. 3-21 Stress induced leakage current and current variation for Al2O3 and La2O3 under -2.5 V for 1hr. 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 Cu rre n t D e ns ity (A /cm 2 ) 33A La₂O₃ 21A Al₂O₃ Gate Voltage (V)

Solid Symbol: Fresh device Hollow Symbol: Stressed device

0 5 10 15 20 25 30 (I stres s ed - I F resh ) / I fre s h (% )

Fig. 3-22 QBD distribution of La2O3 dielectricwith different Vg. For 50% MTTF and

10 years lifetime, a max operation voltage of 2.3 V is obtained.

0 20 40 60 80 100 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 Vg=-3.3V Vg=-3.4V Vg=-3.5V Cu m u la tiv e Fa il u re (% ) Q_BD (C/cm2)

Chapter 4

Bias-Temperature Instability and Charge Trapping

on Fully-Silicided-Germanided Gates/High-k Al

₂

O

₃

CMOSFETs

4.1 Introduction

As continuous scaling down the MOSFET devices into sub-100 nm scale, oxynitride or metal-oxide high-k gate dielectric, with ultra-thin equivalent oxide thickness (EOT), is required to replace the conventional SiO2 to reduce gate dielectric

leakage current. To further increase the drive current in MOSFET, metal gate will be integrated with these high-k gate dielectrics [4.1]-[4.5]. However, one of the main concerns of such metal-gate/high-k MOSFETs is the poor Bias-Temperature Instability (BTI) [4.1], [4.6]-[4.8]. In particular, the negative BTI (NBTI) of pMOSFETs is becoming an increasingly serious problem of the CMOS reliability. It is known that the NBTI of oxynitride gate dielectric pMOSFETs is mainly related to nitrogen traps [4.9]-[4.12], hydrogen [4.13]-[4.14], moisture (H2O) [4.14], and

impurity diffusion [4.6] etc. Both the NBTI and positive BTI (PBTI) are even worse in metal-oxide high-k CMOSFETs than oxynitride devices [4.1], [4.6]-[4.8]. In this work, we have studied the BTI effect on fully silicided-germanided (NiSi-NiGe) dual

gates on high-k Al2O3 CMOSFETs [4.5], [4.15]-[4.19] and compared with a

benchmark oxynitride devices. The fully silicided gate has advantage of fully process compatible to current VLSI fabrication technology. The Al2O3 gate dielectric has

reasonable high-k and good thermal stability with amorphous type up to 1000o_{C [4.5],}

[4.12], [4.15]-[4.21]. In contrast to the worse NBTI than PBTI in oxynitride devices, close NBTI and PBTI is found in fully NiSi-NiGe gates/Al2O3 CMOSFETs. At 17 Å

EOT, the extrapolated maximum operation voltage (Vmax-10years) for 10 years lifetime,

with 50 mV threshold voltage (Vt) change at 85oC, is 1.16 V and -1.12 V from PBTI

and NBTI, respectively. These results are comparable or better than the reported HfAlON [4.6] and HfSiON data [4.7]. The high Vmax-10years and close value for both

NBTI and PBTI may be due to the process without hydrogen and H2O that were used

during high-k HfAlON and HfSiON deposition by Atomic-Layer Deposition (ALD) using NH3 and H2O sources [4.6]. However, the inferior PBTI of NiSi-NiGe/Al2O3

CMOSFETs to oxynitride devices suggests that further improving the high-k dielectric quality is required.

4.2 Experimental

Standard n- and p-type (100) Si wafers with a typical resistivity of ~10 Ω-cm were used in this study. After standard cleaning, the device active region was formed

by thick field oxide and patterning. The source and drain region were implanted by 35 KeV Phosphorus or 25 KeV Boron for nMOSFETs or pMOSFETs respectively, followed by RTA activation. Then the ~39 Å Al2O3 was formed by physical-vapor

deposition, 400o_{C oxidation for 20 min. and 400}o_{C annealing for 20 min. From the}

C-V measurement, a k value of 8.9 and EOT of 17 Å were obtained. The slightly low-k value than bulk Al2O3 (k=10) is due to the oxidation of Si during post

deposition annealing. The fully NiSi and NiGe gates were formed by depositing 150 Å amorphous Si or Ge on Al2O3, 150 Å Ni for both n- and p-devices, and followed by

silicidation and germanidation at 400o_{C RTA for 30 sec [4.5], [4.15]-[4-19]. For}

comparison, control oxynitride was formed by decoupled plasma nitridation [4.22] on a 18 Å SiO2 that was grown by oxygen at 700oC for 12 min. The formed oxynitride

has peak N concentration of 9% near poly gate interface, followed by post deposition annealing at 1000o_{C RTA. The fabricated CMOSFETs were further characterized by}

C-V I-V, and Bias-Temperature (BT) measurements.

4.3 Results and Discussion

We have first measured the Ids-Vgs characteristics to examine the hysteresis

effects in Al2O3. Fig. 4-1 and Fig. 4-2 show the measured hysteresis on NiSi/Al2O3

nMOSFET, the double sweep of gate voltage from 0 to 1.5 V results in Vt shifted to

positive by +10 mV. This result suggests generating negative charge traps in nMOSFET. For pMOSFET, the double sweep of gate voltage from 0.5 to -1.5 V results in VT shifted to negative by -10 mV, which also indicates the generating

positive charge traps in bulk Al2O3 and Al2O3/Si interface. These amounts of charge

trappings may cause NBTI in pMOSFETs and PBTI in nMOSFETs. The poor Ioff in

nMOSFETs is due to the insufficient high temperature annealing (only 950o_{C RTA)}

for ion-implanted damage. The limited RTA temperature also gives a relative poor sub-threshold swing because of the insufficient annealing of high-k dielectric defects. Further improving the leakage current and sub-threshold swing can be done by increasing RTA temperature to typical 1000-1050o_{C or using LaAlO}

3 high-k dielectric

[4.24], [4.25].

We have also used the C-V to measure the hysteresis effects. Fig. 4-3 and Fig. 4-4 show the CV hysteresis characteristics, measured at 500 KHz, on NiSi/Al2O3

nMOSFETs and NiGe/Al2O3 pMOSFETs respectively, where conventional

bi-directional sweeps from inversion to accumulation were applied to these transistors. The double sweeping hysteresis of +10 mV and ~0 mV are measured for nMOS and pMOS capacitors, respectively. The smaller flat band voltage shift of pMOS capacitor than the Vt shifts of Ids-Vgs curves in NiGe/Al2O3 pMOSFETs may be due to the

relative slower and deeper traps that were unable to follow the AC signal [4.23]. Similar effect is also observed in other high-k dielectrics from quasi-static to RF frequency range [4.24]-[4.27].

Figs. 4-5 and Fig. 4-6 shows the NBTI characteristics of Ids-Vgs and Vt change

(∆Vt) for NiSi/Al2O3 pMOSFETs stressed at 10 MV/cm electric field and 85oC

ambient to 1 hour, respectively. This stress condition was chosen to compare with published data in the literature [4.6], [4.7]. For reference and comparison, the ∆Vt of

SiON pMOSFETs and the data of TaN/HfAlO [4.6] are also plotted in Fig. 4-6. After the 1 hour stress at 10 MV/cm and 85oC, a ∆Vt of -33 mV is measured suggesting that

the NBTI is one of the most serious reliability issues for fully NiSi gate/high-k Al2O3

pMOSFETs. The measured ∆Vt changes are comparable or better than the published

HfAlON [4.6], HfSiON [4.7] and HfTaO [4.1] where additional Al, N, Si or Ta must be added to HfO2 for BTI reliability improvement. However, the measured ∆Vt is still

larger than that of oxynitride devices with plasma generated N at top poly-Si/oxynitride interface. The possible reason of the inferior NBTI is due to the degraded interface of Al2O3/Si and bulk oxide change in Al2O3, where oxygen-riched

SiN interface on Al2O3 will be the possible solution for this important reliability issue

[4.10]-[4.12].

NiGe/Al2O3 nMOSFETs stressed at 10 MV/cm electric field and 85oC to 1 hour,

respectively. The ∆Vt of SiON nMOSFETs are also added in Fig. 4-8 for comparison.

After the 1 hour BT stress, a ∆Vt of 34 mV is measured for PBTI that is close to the

-33 mV value in NBTI. In sharp contrast, the NBTI of only 0.76 mV is measured in poly-Si/oxynitride (9% peak at poly-Si interface) nMOSFETs after the same BT stress that is lower than the 6.6 mV change in oxynitride pMOSFETs. The better PBTI than NBTI is normal for oxynitride MOSFETs where the mechanism is attributed to hole injection to break the H-Si bonds and created interface traps [4.9]-[4.14]. However, the measured NBTI and PBTI value are less than that of HfO2 [4.7], [4.8],

which may be due the strong A-O bond and consistent with the results in HfAlO [4.6]. The close PBTI and NBTI absolute value after BT stress in Al2O3 MOSFETs

suggesting the higher number of interface and bulk traps, which are normal for high-k gate dielectric, are the main causes and for larger ∆Vtchange than oxynitride

devices.

We have also measured BTI at other gate electric field for 10 years lifetime projection, where the lifetime at each gate voltage was defined by a ∆Vt=50 mV

change during stress. Such high gate voltage or electric field is especially required for poly-Si/SiON CMOS due to the excellent reliability. Figs. 4-9 show the time as a function of Vgs of nMOSFETs and pMOSFETs, respectively. The 10 years lifetime