Effect of post-growth thermal annealing on structural and electrical properties of heavily phosphorus-doped epitaxial silicon

다국어 초록 (Multilingual Abstract)

Continued downscaling of metal-oxide semiconductor field-effect transistor (MOSFET) devices has significantly increased contact resistance in the source/drain (S/D) region. To reduce the contact resistance, extensive research on heavily doped Si films and dopant activation has been conducted. In particular, the use of in-situ phosphorus-doped epitaxial silicon (Si:P) film in the n-type MOSFETs (nMOSFETs) has attracted a lot of interest because its high P doping exceeds its solid solubility limit in Si and yields a low contact resistance. During the Si:P film growth, P atoms are located at substitutional sites in the Si lattice, which induces high tensile strain and prevents dopant diffusion. Furthermore, the advantages of heavily doped Si:P film (e.g., low contact resistance and high tensile strain) can be applied not only to the conventional planar devices but also to fin field-effect transistor (FinFET) devices.
Although the successful integration of the Si:P film into the S/D area of planar MOSFETs and FinFETs has been established for the applications of nMOS, there have been no fundamental studies on the dopant behaviors of Si:P films with extremely high doping levels during post-growth thermal annealing. Dopant redistributions, such as P pile-up at the SiO2/Si interface and dopant-vacancy clustering in Si:P films after post-growth thermal annealing, are crucial for the electrical properties of nMOS devices.
Among the reported various post-annealing methods, laser annealing combined with high P-doping technique is implemented to further increase the active carrier concentration. A high laser annealing temperature of the sub-melt region generates a high active dopant concentration above the solid solubility limit in Si. But, these active dopants are converted to electrically inactive states during the subsequent thermal processing, which deteriorates the electrical properties of nMOSFETs. A phosphorus-vacancy (V) cluster configuration, i.e. PnV (n = 1–4), is considered responsible for dopant deactivation. However, P4V clusters and their atomic rearrangement during the activation and deactivation of P have not been fully investigated.
This dissertation demonstrates a fundamental study on the physical and chemical properties of Si:P films, providing a pathway to elucidate the effects of post-growth thermal annealing on the microstructural, chemical, strain, and electrical properties of Si:P films for the applications of nMOS. The epitaxial Si:P films used in this study were grown on p-type Si(100) substrates and recessed S/D structures via reduced pressure chemical vapor deposition. The Si:P films with a high doping level and uniform box-shaped P profile facilitates precise characterizations of the dopant redistribution after post-growth thermal annealing. To investigate various material characteristics of Si:P films after post-growth thermal annealing, rapid thermal annealing, millisecond laser annealing, and nanosecond laser annealing were performed on the Si:P films. To observe the dopant redistribution in Si:P films directly, high-resolution transmission electron spectroscopy and energy-dispersive X-ray spectroscopy mapping were performed. The strain states and P concentrations during post-growth thermal annealing were characterized via high-resolution X-ray diffraction and secondary-ion mass spectroscopy. Chemical bonding states were investigated via high-resolution X-ray photoelectron spectroscopy (HR-XPS) to elucidate the chemical environment of P and atomic interactions in the Si–P binary system fully. Moreover, the activation- and deactivation-induced changes in the local atomic bonds around P were identified by analyzing the chemical bonding states in the laser-annealed Si:P films via HR-XPS. Chemical bonding states corresponding to active P atoms, which are distinguishable from chemical bonding states for inactive P atoms, were characterized when P activation and deactivation occurred. Density functional theory (DFT) calculations were performed to study the atomic rearrangement and consequent changes in the density of state of Si:P structures when P activation and deactivation occurred. The results of the DFT calculations indicate that the dissolution and formation of PnV clusters is attributed to the electric conduction of the Si:P film. In addition, the heat distribution in the Si:P films after nanosecond laser annealing was simulated based on the finite element method to determine the material and structural effects around the Si:P films grown on the recessed S/D structures. The experimental and theoretical approaches reported herein allowed us to comprehensively study the microstructural, chemical, strain, and electrical properties of the heavily doped Si:P films after post-growth thermal annealing, thus enabling us to modify their material properties and fulfill the relevant criteria for various semiconductor applications.

국문 초록 (Abstract)

지속적인 소자 사이즈의 미세화는 소스/드레인 영역에서 발생하는
직렬 저항 중 접촉 저항의 기여를 증가시켰다. 컨택 저항을 감소시키기
위해서는 고농도 도핑 된 소스/드레인 물질의 사용이 필요하다. nMOSFET
소자의 소스/드레인 물질로는 화학 기상 증착법을 이용하여 증착 된 고농도 P
도핑 된 Si (Si:P) 박막이 각광받고 있다. Si:P 박막은 Si 의 고용한계 이상 P 를
포함하고 있어 접촉 저항을 감소시키고, P 와 Si 의 격자 상수 차이에 의해
박막에 스트레인이 인가되어 채널 영역에서 전자의 이동도를 향상시킨다.
이러한 Si:P 박막은 여러 nMOSFET 소자에 적용되고 있음에도
불구하고, 후속 열처리에 따른 고농도 도핑 하에서 P 의 거동에 대한 근본적인
연구는 여전히 부족한 상황이다. 열처리 후 Si/SiO2 계면에 P 의 pile-up
현상이나 도펀트-공극 클러스터링과 같은 현상은 nMOS 소자의 전기적 특성에
중요한 영향을 주기 때문에 이에 대한 근본적인 연구가 필요하다.
여러 후속 열처리 방법 중, 레이저 열처리는 활성 캐리어 농도를
더욱 증가시키는데 사용된다. 높은 레이저 열처리 온도는 Si 의 고용한계
이상의 활성 캐리어를 생성한다. 하지만, 이렇게 높은 활성 캐리어는 레이저
열처리 이후 수반되는 열처리에 의해 비활성 캐리어 상태로 전환되며,
nMOSFET 의 전기적 특성을 열화 시킨다. PnV (n=1?4)로 나타내어지는 도펀트공극 클러스터는 P 의 전기적 비활성 현상의 원인으로 대두되고 있다. 그러나
PnV cluster 의 형성 및 원자의 재배열에 의한 전기적 활성 상태 변화에 대한
연구는 미흡한 상황이다.
본 연구에서는 종래의 열처리 기술인 퍼니스(furnace) 열처리와 급속
열처리(RTA) 이 후 Si:P 박막에서 P 의 재배열과 이에 따른 구조적, 응력,
전기적 변화에 대해 연구하였다. 화학 기상 증착법을 이용해 만들어진 Si:P
박막은 박스 모양의 일정한 도펀트 개형을 가지고 있기 때문에 열처리 이후
재배열 되는 P 의 모습을 효과적으로 관찰할 수 있었다. TEM, EDS, HR-XPS,
XRD, SIMS 분석을 이용하여 후속 열처리 이후 P 의 거동을 분석하였다. 더
나아가, 도펀트 활성화에 필수적인 레이저 열처리 후 진행되는 공정의 열에
의해 도펀트가 비활성화되는 메커니즘에 대해 연구하였다. XPS 를 이용해
레이저 열처리 이후 형성되는 활성화된 P 로부터 기원한 화학 결합 상태를
관찰하였다. 밀도 함수 이론 (density functional theory)를 이용한 상태 밀도
(density of state) 계산을 통해 P4V 의 형성과 분해가 Si:P 의 전기적 비활성화 및
활성화에 기여하는 것을 밝혀내었다. 이러한 결과로부터 본 연구는 열처리에
따른 P 의 거동에 대한 근본적인 정보를 제공하여 Si:P 를 차세대 nMOS
소자에 적용하는데 기여할 수 있을 것이다.

목차 (Table of Contents)

• TABLE OF CONTENTS i
• List of Figure Captions v
• List of Table Captions x
• Abstract xi
• Chapter 1. Introduction 1
• 1.1. Heavily phosphorus-doped epitaxial Si film 4
• 1.1.1 In-situ doping technique for high phosphorus incorporation 4
• 1.1.2 Electrical properties of heavily phosphorus-doped Si film 6
• 1.2. Dopant behaviors in phosphorus-doped Si materials 18
• 1.2.1 Phosphorus pile-up at the interface between SiO2 and Si 18
• 1.2.2 Phosphorus activation and deactivation 19
• 1.3. Overview of the dissertation 25
• 1.4. References 27
• Chapter 2. Experimental 31
• 2.1. Sample preparation 31
• 2.1.1 In-situ P-doped epitaxial Si films growth by reduced pressure chemical vapor deposition (RPCVD) 31
• 2.2. Post-growth thermal annealing 31
• 2.2.1 Rapid thermal annealing (RTA) 31
• 2.2.2 Millisecond laser annealing 32
• 2.2.1 Nanosecond laser annealing 33
• 2.3. Analytical techniques 33
• 2.3.1 High-resolution transmission electron microscopy (HR-TEM) 33
• 2.3.2 Energy dispersive X-ray spectroscopy (EDS) 34
• 2.3.3 High-resolution X-ray diffraction (HR-XRD) 35
• 2.3.4 High resolution X-ray photoelectron spectroscopy (HR-XPS) 37
• 2.3.5 Secondary-ion mass spectrometry (SIMS) 38
• 2.3.6 Hall effect measurement 39
• 2.3.7 Density functional theory (DFT) 40
• 2.4. References 47
• Chapter 3. Dopant redistribution of Si:P films grown on Si (001) substrate after millisecond laser annealing 49
• 3.1. Introduction 49
• 3.2. Experimental 53
• 3.3. Effects of millisecond laser annealing on electrical and structural properties of Si:P films 57
• 3.3.1 Electrical properties of Si:P films 57
• 3.3.2 Strain states and P profiles of Si:P film 59
• 3.4. Effects of millisecond laser annealing on chemical bonding states of near-surface region of Si:P films 62
• 3.4.1 Chemical bonding states with different phosphorus doping concentration 62
• 3.5. Effects of millisecond laser annealing on dopant redistribution of near-surface region of Si:P films 76
• 3.5.1 Dopant pile-up at the near-surface region of Si:P films 76
• 3.6. Summary 84
• 3.7. References 85
• Chapter 4. Phosphorus activation and deactivation mechanism of Si:P films grown on Si (001) substrate 90
• 4.1. Introduction 90
• 4.2. Experimental 93
• 4.3. Electrical property of as-grown and millisecond laser-annealed Si:P films after RTA 95
• 4.3.1 Active carrier concentration 95
• 4.4. Strain states and dopant profiles of as-grown and millisecond laser-annealed Si:P films after RTA 103
• 4.4.1 Induced tensile strain states 103
• 4.4.2 Dopant profiles 104
• 4.5. Chemical bonding states of as-grown and millisecond laser-annealed Si:P films after RTA 107
• 4.5.1 Electrical activation-induced changes in the local atomic bond of P atoms 107
• 4.5.2 Electrical deactivation-induced changes in the local atomic bond of P atoms 111
• 4.6. Theoretical investigation of P activation and deactivation mechanism 120
• 4.6.1 Thermodynamic favorability of Si:P structure 120
• 4.6.2 Structure modeling 121
• 4.6.3 Density of states 124
• 4.6.4 Electron localization function 124
• 4.7. Summary 131
• 4.8. References 133
• Chapter 5. Effect of nanosecond laser annealing on Si:P films selectively grown at recessed source/drain structures for NMOS applications 139
• 5.1. Introduction 139
• 5.2. Experimental 142
• 5.3. Melting and diffusion of Si:P films grown at recessed source/drain structures during nanosecond laser annealing 144
• 5.3.1 Melting depth depending on the laser power densities 144
• 5.3.2 Redistribution of P after nanosecond laser annealing 150
• 5.4. Strain properties of Si:P films grown at recessed source/drain structures after nanosecond laser annealing 154
• 5.5. Theoretical investigation of heat distribution in the Si:P films grown at recessed source/drain structures during nanosecond laser annealing 157
• 5.6. Summary 161
• 5.7. References 162
• Chapter 6. Conclusion 164
• Abstract in Korean (국문요약) 166

참고문헌 (Reference)

1. The valence-shell electron-pair repulsion (VSEPR) theory of directed valency, R.J. Gillespie, 295–301. https://doi.org/10.1021/ed040p295, , 1963