질화 산화 완충막을 이용한 에피텍셜 CoSi2 막의 형성

Abstract

With the continuous scaling-down of advanced semiconductor device sizes, a silicide process has become essential to improve device and circuit performance. Additionally, there is a stricter requirement to reduce the electrical resistance and propagation delay that are inherent properties of silicide. Among the many types of metal silicides, cobalt disilicide (CoSi₂) is a good choice for a nano-scale device contact materials because of its low resistivity (10 - 20 μΩm), good chemical stability, and high thermal stability. In particular, the similarity between the crystal structures and the relatively close lattice mismatch (~1.2 %) of CoSi₂ and Si makes epitaxial growth possible which in turn has the properties of high thermal stability against agglomeration, film uniformity, absence of grain boundary diffusion path, and compatibility with shallow junction formation. In a typical silicide process, Co films are deposited mainly by a sputtering method, and subsequent annealing forms the Co silicide on the active region. However, the high ion-induced substrate damage that occurs duringthe sputtering process has an undesirable effecton the active source and drain regions that are formed in the Si substrate. As an alternative to the sputtering method, the metal organic chemical vapor deposition (MOCVD) method, introduced to deposit Co films, can be used as it provides several advantages in termsof good conformal coverage and high growth rates without ion-induced substrate damage. During the cobalt silicidation process, the initial reaction between Co and Si resultsin the formation of polycrystalline Co-rich phases likeCo₂Si and CoSi, followed by the formation of a polycrystalline CoSi₂ phase at higher temperatures. The direct formation of CoSi₂ during the solid-state reaction between Co and Si without intermediate phases such as CoSi₂ and CoSi is related to the epitaxial growth of CoSi₂. Therefore, retardation of Co-diffusion into Si by a buffer layer at high temperatures could cause the formation of an epitaxial CoSi₂ layer on Si substrates. To retard Co diffusion into Si, various buffer layers have been introduced, including titanium, nitride, and oxide. In the past few years, it has been reported that epitaxial growth of CoSi₂ occurred on Si (100) using a Ti buffer layer. The Ti buffer layer method permits the epitaxial growth of CoSi₂ because the flux of Co atoms is controlled through a buffer layer, resulting in epitaxial CoSi₂ (100) on the Si (100) substrate. However, the Ti buffer layer method generateslateral encroachment and large voids under the edge of the oxide, these defects are known to lead to device failure due to deterioration of the gate and the shallow junction. The nitride buffer layer method is problematic in that nitridation requires a very high temperature and long heating time in order to form the nitride buffer layer. The oxide buffer layer method also exhibits several disadvantages. For instance, thick epitaxial CoSi₂ can be formed by repeated deposition of a thin Co layer (~2nm) and subsequent in-situ annealing this method, however, has been found to be sensitive to oxidation by oxygen contamination from oxygen sources such as annealing ambient and physisorbed water vapor, as well as other possible oxidants. In this paper we investigated the formation of an epitaxial CoSi₂ layer on Si (100) by forming a thin layer of SiO_(x)N_(y). Presently, The Co-silicide growth method using an oxide buffer layer was modified by the incorporation of nitrogen into chemical oxide (SiO_(x)), and three possibilities are expected. Firstly, this oxynitride layer may retard the growth of additional SiO_(x) by the oxidation of Co films that impede the growth of CoSi₂, indicating that the oxynitride buffer layer method permits the possibility of an epitaxial growth of CoSi₂ with almost no influence byoxygen contamination from the oxygen source, except the intentional air exposure. Next, the incorporation of nitrogen into chemical oxide is believed to reduce SiO diffusivity in oxide. This means that it takes a longer amount of time for Co atoms to pass through the dissolving layer into the Si substrate, indicating that the oxynitride layer plays an effective role as a buffer layer. Finally, oxynitride buffer layer method is expected to be able to grow thick epitaxial CoSi₂ layer without repetition of the process. Therefore we investigated the effects of an oxynitride buffer layer on the growth of Co-silicide on Si substrate.; 반도체 소자의 고집적화 될수록 기생저항의 증가로 인하여 누설전류를 증가시키는 문제점들이 대두되고 있다. 이러한 문제점들을 해결하기 위해 금속 박막을 증착한 후 급속 열처리하여 형성시킨 금속 실리사이드가 적용되어졌다. 금속 실리사이드는 비저항이 낮으며, 오믹 접합(ohmic contact)으로 인한 낮은 접촉저항의 특성을 보임으로써 ultra-large-scale integration(ULSI) 소자에 interconnects 와 contacts 재료로 광범위하게 적용되고 있는 물질로 연구가 진행되어 왔다. 이러한 금속 실리사이드 재료로써는 비저항이 낮은 TiSi₂와 CoSi₂가 일반적으로 많이 사용되어 왔다. 그러나 저저항의 실리사이드 공정을 위한 TiSi₂의 경우 소자의 고집적화에 따른 디자인 룰의 감소로 인해 면저항 값이 급격히 증가하는 한계를 가지고 있다. 반면에 CoSi₂의 경우는 배선선폭에 감소에 따른 면저항 값의 변화가 작으며, 주확산자가 Co 원자이기 때문에 실리사이드 공정 중 게이트 상부로의 측면 과도성장이 작은 장점을 가지고 있다. 현재 반도체 소자에는 PVD(Physical Vapor Deposition)을 이용하여 금속 박막을 증착시킨 후 금속 실리사이드를 형성하여 적용시키고 있다. 그러나 PVD는 단차도포성이 좋지 않은 단점이 있기 때문에 복잡한 구조의 contact에 균일한 두께로 증착이 불가능 하다. 반면에 CVD(Chemical Vapor Deposition)은 증착 물질과 기판의 표면 반응으로 인하여 종횡비가 큰 contact 구조에 균일하게 증착할 수 있는 장점을 나타내고 있다. 그러나 CVD는 증착 시 온도가 매우 높다는 점이 문제가 되어 상대적으로 낮은 온도에서 증착 할 수 있도록 증착 물질에 유기물을 배위시킨 MOCVD(Metal Organic Chemical Vapor Deposition)가 반도체 공정에 적용되어지고 있다. 본 연구에서는 oxynitride buffer layer와 Ti-capping layer를 이용하여, Co 박막의 oxygen contamination에도 불구하고 양질의 epitaxial CoSi₂를 형성 하였다. oxynitride buffer layer는 Co 박막의 oxygen contamination에도 Co와 Si 기판 사이에 additional SiO_(x)의 형성을 억제하여 oxynitride layer가 완충막으로써 역할을 잘 수행할 수 있게 하였고, Ti-capping layer는 oxygen contamination에 의해 생긴 Co 박막 내의 oxygen과 열처리 중 결합을 하여 실리사이드 반응 시 생기는 oxygen에 의한 악영향을 더욱 감소시키는 역할을 수행하였고, 더불어 형성되어진 epitaxial CoSi₂ layer의 roughness를 더욱 좋게 하는 긍정적인 영향을 미쳤다, 따라서, oxynitride buffer layer와 Ti-capping layer를 이용한 에피텍셜 CoSi₂ 박막의 성장은 공정 중 발생하는 oxygen contamination의 악영향에 상관없이 에피텍셜 코발트 실리사이드를 형성할 수 있는 방법으로 제안되어진다.