(A) study of pulsed ICP, CCP etch process and pulse plasma diagnostics for highly selective etching

다국어 초록 (Multilingual Abstract)

As the device dimension continues to shrink, precise etching technology is required. Especially, due to the decrease of critical dimensions (CD) below the resolution of photolithography, to fabricate the extremely narrow patterns for dynamic random-access memory (DRAM), 3D NAND flash memory devices, logic devices, etc., multiple patterning technologies such as double patterning technology (DPT) and quadruple patterning technology (QPT) are being applied. In addition, for highly selective etching processes over mask materials and underlayers are required in many other applications for the nanometer scale etching with a high aspect ratio.
In recent years, various pulsed plasmas have been introduced for the etching of fine patterns in semiconductor industry. In fact, in pulsed plasmas, there are numerous modes of operation by controlling pulse parameters such as duty ratio, pulse frequency, pulse phase between source power pulse and bias power pulsing for multiple pulsing, multiple frequencies, etc. Depending on the combination of the pulse parameters, the plasma parameter distribution and plasma chemistry are repeatedly changed instantaneously as a function of time and they react with material surfaces for etching. Therefore, even though it is generally known that the pulsed plasma etching is beneficial for nanoscale etching due to high etch selectivity and highly anisotropic etch profiles, the detailed change in plasma chemistry and the fundamental plasma-surface interactions that lead to etching and/or deposition for highly selective etching in the pulse plasmas are poorly understood.
In this thesis, the effect of various pulsed plasma characteristics has been investigated using pulsed inductively coupled plasma (ICP) and capacitively coupled plasma (CCP) etcher. In the etching of spin-transfer torque magnetic random-access memory (STT-MRAM): We applied the pulsed-bias power to increase the volatility of etch residue during pulse-off time while the 13.56 MHz rf source power is continuously applied in the ICP system. In this work, the effect of pulse-biased inductively coupled plasma (ICP) etching process operated at 400 kHz-13.56 MHz, which can potentially have a significant effect on the control of the ion energy distribution with which ions bombard surfaces.
In the etching of DRAM: The capacitively coupled plasma (CCP) system was operated with synchronous pulsed plasma and embedded pulse plasma was studied. Especially, for nano-scale high aspect ratio contact (HARC) etching, we investigated SiO2 etching masked with amorphous carbon layer (ACL).
In the etching of Patterning: In order to better understand the behavior taking place in the pulsed fluorocarbon plasma etch process and for better control of the critical pulse plasma parameters, we investigated the relation of fluorocarbon-based pulsed plasmas composed of CF4(CHF3)/O2/Ar gas mixtures to the etching of SiO2, Si3N4, SiON, SOH, and ACL by investigating process parameters and by analyzing the pulsed plasma parameters in variously pulsed ICPs using 27.12 MHz ICP source power and 13.56 MHz bias power. Through the mass spectrometry and optical emission spectroscopy, the dissociated species such as CF3, F, etc. related to materials etching and the species such as CF2, CHF2, etc. related to the surface polymerization preventing etching could be identified
Therefore, in this thesis, it was confirmed that to satisfy the requirements of etch process optimization for many devices, controlling pulsed plasma parameters with complex gas mixtures is essential for controlling the plasma chemistry and plasma-surface interactions.

국문 초록 (Abstract)

DRAM, 3D NAND, Logic 등으로 대표되는 반도체 식각 공정의 경우 소자 미세화에 따른 재료적 한계 및 기술적 난이도의 증가로 식각 균일도 및 임계치수(critical dimension) 제어, 식각 선택도(etch selectivity) 및 식각 프로파일 확보, 전하축적, 식각 손상(etch damage: structural and electrical), 패턴 profile 왜곡 등의 문제가 지속적으로 대두 되고 있다. 또한 STT-MRAM 과 같은 new memory 의 개발로 나노미터급에서 새로운 물질의 식각 공정을 진행함에 있어 식각 공정의 난이도 및 공정 step 수도 지속적으로 증가하고 있다. 따라서 나노미터급의 미세공정에서 보다 향상된 식각 공정을 위해 다양한 식각 파라미터를 조절하여 이를 해결하는 문제는 메모리 분야의 축소화 요구에 대한 기술적 난관을 극복하고 경쟁력 향상을 위해 지속적으로 연구 개발되고 있다. 본 학위논문에서는 이러한 식각 공정의 높은 기술적 난이도를 극복하기 위한 advance etch 방법으로 pulse plasma 를 사용하여 공정 parameter 를 극대화 하여 STT-MRAM, DRAM, Pattering 공정에서 etch profile, etch selectivity 등의 식각 문제를 해결하였다. 또한 SEM, XPS, TEM을 통한 식각 특성 분석 뿐만 아니라 OES, Langmuir probe, PSM 등을 사용하여 식각 공정시 pulse parameter 조절에 따른 pulse plasma 에 대한 연구를 진행하여 pulsed etch process 에 대한 이해도 높이고자 하였다.
따라서 본 학위논문에서 연구된 pulsed low rf bias frequency 를 적용한 MTJ pattern 식각, Synchronous, Embedded pulse plasma 를 적용한 SiO2 hole pattern 식각, 낮은 duty ratio 를 적용한 Synchronous, Asynchronous pulse plasma 의 patterning 물질 식각 및 플라즈마 분석을 통해 각 물질에 따른 pulse plasma 의 공정 parameter 조절이 식각에 매우 중요한 영향을 미치는 것을 확인할 수 있었다. 따라서 DRAM, patterning, STT-MRAM 등의 다양한 반도체 식각 공정에 이용한 펄스 식각 기술은 현 식각 공정의 높은 기술적 난이도를 극복하기 위해 효과적으로 적용될 수 있는 매우 중요한 기술임을 확인할 수 있다.

목차 (Table of Contents)

• [Chapter 1] Introduction 1
• 1.1. Basic Mechanism of the Dry Etch Process 1
• 1.2. Plasma Etching Process for Nanoscale Memory Device Fabrication 3
• 1.3. Undesirable Phenomena 8
• 1.4. Motivation and Objective 11
• [Chapter 2] Advanced Dry Etch Process: Pulsed Plasma Etching 14
• 2.1. Inductively Coupled Plasma Process 14
• 2.2. Capacitively Coupled Plasma Process 16
• 2.3. Fundamentals of Pulsed Plasma 18
• [Chapter 3] Analytical Tools and Plasma Diagnostics 23
• 3.1. Field Emission Scanning Electron Microscopes (FESEM) 23
• 3.2. Transmission Electron Microscopy (TEM) 23
• 3.3. XRay Photoelectron Spectroscopy (XPS) 25
• 3.4. Optical Emission Spectroscopy (OES) 25
• 3.5. Langmuir Probe 28
• 3.6. Plasma Sampling Mass Spectrometer (PSM) 30
• [Chapter 4] Bias Pulsing 32
• 4.1. STT-MRAM 32
• 4.2. Experimental Methods 34
• 4.3. Pulsed RF and DC Bias Etching of MTJrelated Materials 35
• 4.4. Pulsed Low Frequency RF Bias ICP Etching 37
• [Chapter 5] Synchronous/Asynchronous/Embedded Pulsing 42
• 5.1. DRAM 42
• 5.2. Experimental Methods 43
• 5.3. Pulsed Dual Frequency CCP Etching 44
• [Chapter 6] Synchronous/Asynchronous/Embedded Pulsing and Diagnostics 50
• 6.1. Patterning 50
• 6.2. Experimental Methods 52
• 6.3. Various Pulse Plasma in ICP Etching 56
• [Chapter 7] Conclusions 75
• [References] 77