First principles study on overall reaction mechanism for tungsten atomic layer deposition in the early stages

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공과대학 재료공학부
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서울대학교 대학원
학위논문 (박사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2019. 2. 윤의준.
Following Moores law for decades, thin film deposition techniques have been intensively advanced to meet the demand for miniaturized and highly integrated devices in the electronics industry. Recently, conformal film deposition techniques, which allows precise thickness control at atomic scale, are becoming very important. Nitride materials, such as titanium nitride and silicon nitride, have been deposited using conventional deposition methods such as low-pressure chemi-cal vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD). However, development of memory devices has required another deposition technique such as atomic layer deposition (ALD) to meet the demand for excellent step coverage and high conformality on extremely high aspect ratio structures. The ALD processes utilize well-controlled sequential surface reactions to obtain uniform and conformal films.

As one of the most essential materials in fabrication of fu-ture memory devices, tungsten (W) has been used for a metal gate with lower resistivity than other candidate materials, which results in enhancement of device performance. In the fabrication of recent memory devices, tungsten films have been deposited using ALD by alternatively exposing W precursors such as tungsten hexafluoride (WF6) and reducing agents such as diborane (B2H6) in an ABAB… sequence. In the ALD processes for W deposition, B2H6 dosing process can play an important role in deposition of W films with low resis-tivity and in removal of residual fluorine (F) atoms on the surface.

However, as the size of the memory device becomes smaller and smaller, it becomes difficult to deposit W films having excellent step coverage and conformality due to a severe problem that a seam or void is formed in the process of filling the W metal gate. This problem is a primary obstacle of the devel-opment for future memory devices. To treat this problem, theoretical comprehension of the ALD process for W deposition is required due to the experimentally limited observations on the sub-nanometer scale. Although a few experimental results on ALD W have been reported, there has been no theoretical report on the overall reaction mechanism for ALD W process.

Firstly, we have investigated the dissociation reaction of B2H6 on three different TiN surfaces, TiN (001), Ti-terminated TiN (111), and N-terminated TiN (111), using DFT calculations. N-terminated TiN (111) shows the lowest overall reaction energy for B2H6. These results imply that severe problems, such as a seam or void, in filling the W metal gate for memory devices could be attributed to the difference in the deposition rate of W films on TiN surfaces. From this study, it was found that the control of the texture of the TiN film is essential for improving the subsequent W nucleation.

Secondly, we have investigated the effects of H2 and N2 treatment on TiN surfaces for B2H6 dosing process. In our DFT calculated results, H2 treatment on the TiN surfaces is to make the surface to be H-covered TiN surfaces, which results in lowering the reactivity of B2H6 precursor since the overall reactions of the B2H6 on the H-covered TiN surfaces are energetically less favorable than the TiN surfaces. As a result, an effect of the H2 treatment is to decrease the reactivity of the B2H6 molecule on the TiN surface. However, N2 treatment on Ti-terminated TiN (111) surface is more likely to make the TiN surface to be N-terminated TiN (111) surface, which results in making a lot of N-terminated TiN (111) surfaces, having very reactive nature for B2H6 bond dissociation. As a result, the effect of N2 treatment serves as a catalyst to decompose B2H6. From the deep understanding of the effect of H2 and N2 during the B2H6 dosing process, the use of proper gas treatment is required for improvement of the W nucleation layers.

Lastly, we investigated overall ALD reaction mechanism for W deposition on the TiN surfaces based on DFT calculation as well as the detailed dissociative reactions of WF6. Our calculated results suggest that the overall reactions of the WF6 on the B-covered TiN surfaces are energetically much more favorable than the one on the TiN surfaces, which means that the high reactivity of WF6 with the B-covered TiN surface is attributed to the presence of B-covered surface made by B2H6 molecule. As a result, an effect of the B2H6 flow serves as a catalyst to decompose WF6 molecule. Two additional reaction processes right after WF6 bond dissociation, such as W substitution and BF3 desorption, were also explored to clearly understand the detailed reactions that can occur by WF6 flow. At the first additional reaction process, W atoms can be substituted into B site and covered on the TiN surfaces due to the strong bonding nature of W with the TiN surface than B atoms. At the sec-ond additional reaction process, remaining atoms, such as B and F, can be easily desorbed as by-product, that is, BF3 because BF3 desorption is energetically favorable reaction with low activation energy. Furthermore, we also investigated the effect of H2 post-treatment on W-covered TiN surface in order to remove residual F adatoms, which are known to cause severe problems that extremely degrade characteristics of memory devices. It was found that both H2 dissociative reaction and HF desorption can occur enough well under somewhat high temperature and H2 ambience, which is confirmed by the our DFT results and previously reported experimental results. These results imply that the understanding of the role of gas molecules used for W deposition gives us insight into improving the W ALD process for future memory devices.
우리는 DFT 기반으로 W 증착을 위해 사용되는 B2H6, H2 & N2 및 WF6와 같은 전구체의 분해반응을 통해 W ALD 증착 메커니즘을 연구하였다.

우선, ALD W 증착에서 B2H6 주입 공정에서 하부막 TiN 층의 반응 메커니즘을 탐구하기 위해 DFT 계산을 기반으로 하여 B2H6 분자에 대해 세 가지 TiN 표면에 대한 반응성을 조사하였다. TiN 증착 시 주로 (001) 및 (111) texture를 갖는 poly-crystalline TiN 박막이 관측되기 때문에 본 연구에서는 TiN (001), Ti-terminated TiN (111), and N-terminated TiN (111) 세 가지 표면에 대해 연구를 진행하였다. 결과적으로, 세 가지 표면 중 N-terminated TiN (111) 표면에서 B2H6 분해 반응이 가장 잘 일어나며 표면 방향에 따른 W 증착속도 차이로 인해 void 형성 문제가 발생할 수 있음을 해석하였다.

다음으로, B2H6 주입 과정에서 H2 및 N2 처리가 TiN 표면에 미치는 영향을 조사하였다. 우리의 DFT 계산 결과에서, TiN 표면에 대한 H2 처리는 TiN 표면을 H 커버된 TiN 표면으로 만드는 것으로 B2H6 precursor의 반응성을 낮추는 결과를 낳았다. 그 이유는 H 커버된 TiN 표면에서의 B2H6 분해반응은 bare TiN 표면에 비해 에너지적으로 불안정해지는 반응이기 때문이다. 결과적으로, H2 처리의 효과는 TiN 표면의 B2H6 분자의 반응성을 감소시켜서 표면을 passivation 시키는 역할을 한다. 그러나, N2 처리는 TiN 표면을 N-termiated TiN (111) 표면을 많이 만들어서 B2H6 분해반응이 잘 일어나도록 해준다. 결과적으로, N2 처리의 효과는 B2H6를 분해시키는 촉매 표면을 만드는 역할을 한다. B2H6 주입 공정 동안 H2 및 N2의 적절한 gas 사용은 W 박막 특성에 큰 영향을 미칠 수 있을 것으로 기대된다.

마지막으로, 우리는 WF6의 분해반응뿐만 아니라 DFT 계산에 기초한 TiN 표면위에서의 W 증착에 대한 전반적인 ALD 반응 메커니즘을 조사했다. 결과적으로 bare TiN 표면 대비 B-covered TiN 표면에서 WF6 분자의 분해반응이 에너지 적으로 훨씬 유리한 것으로 나타났으며, 이를 통해 B2H6 주입의 효과는 WF6 분자를 분해시키는 촉매 표면을 만드는 역할을 한다. W 치환과 BF3 탈착과 같은 WF6 분해반응이 일어난 직후의 두 개의 추가 반응 과정도 정확한 반응 메커니즘을 이해하기 위해 수행되었다. 첫 번째 추가 반응 과정에서 W 원자는 B 원자에 비해 TiN 표면과의 binding energy가 훨씬 높아서 B 원자와 치환이 가능하다. 두 번째 추가 반응 과정에서, BF3 탈착은 낮은 에너지배리어와 에너지 적으로 안정해지는 반응이기 때문에 B 및 F와 같은 잔류 원자들은 BF3로 쉽게 탈착 될 수있다. 또한, 우리는 메모리 소자의 특성을 극도로 저하시키는 심각한 문제를 일으키는 것으로 알려진 잔류 F 원자를 제거하기 위해 W-covered TiN 표면에 대한 H2 후 처리의 영향을 조사하였다. H2 분해반응과 HF 탈착은 우리의 DFT 결과 및 이전에 보고된 실험 결과에 의해 확인되는 다소 높은 온도 및 H2 분위기 하에서 충분히 잘 일어날 수 있는 것으로 밝혀졌다. 이러한 결과는 W 증착에 사용된 가스 분자의 역할에 대한 이해가 향후 메모리 소자에 대한 W ALD 공정 개선에 대한 통찰력을 제공할 것으로 기대된다.
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Materials Science and Engineering (재료공학부)Theses (Ph.D. / Sc.D._재료공학부)
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