Part Ⅰ. A study on Pt thin films as bottom electrode
The control of microstructure plays a key role in the properties of platinum (Pt) thin films. Pt thin films were deposited on $SiO_2$/Si substrates at 350℃ using dc magnetron sputtering system. As film thickness increased, both grain size and roughness increased. The increase of surface roughness might result from the groove deepening due to grain growth. The crystallographic texture changed from random to a strong (111) fiber texture during film thickening. The texture development was discussed in the view of the contribution of strain energy minimization and surface energy minimization to grain growth. The strain energy minimization was more contributed to the determination of crystallographic texture that the surface energy minimization in Pt thin film below 100 nm. In order to study the effect of annealing treatment, 30 nm-thick Pt thin films having 3.7Å rms roughness, they were fabricated on $SiO_x$/Si substrates at 350℃ using dc magnetron sputtering system, and annealed at 400℃ and 650℃. As annealing temperature increased, the surface roughness and the migration of Si atoms to Pt layer increased. The silicide formation was observed and analyzed by XRD, AFM, SEM and AES technique. We discuss about it in the view of the generation of Si cluster in $SiO_x$ layer with x＜2.0 near Pt layer. In order to evaluate the electrical properties, the resistivity of Pt thin films were measured. Below the critical film thickness, the resistivity increased rapidly.
Part Ⅱ. A study on PZT thin films as storage media
The dependence of piezoelectric properties of PZT thin films on film thickness was investigated using atomic force microscope (AFM) assisted domain imaging technique. PZT thin films were fabricated on Pt(111)/$TiO_x$/$SiO_2$/Si substrates at 375℃ by radio frequency (RF) magnetron sputtering. As the thickness of PZT thin film increased, the grain size increased and the preferred orientation of PZT thin film changed from (110) to (100). The voids were observed, and they increased with increasing film thickness. The volume density calculated by RBS decreased as film thickness increased. SAED patterns in plane view images changed from ring to spot patterns. Especially, 40 nm PZT thin films had a typical rosette structure, indicating the presence of two structures: a rosette structure and a poorly crystallized structure surrounding these rosettes. These were investigated using SEM, AFM, TEM and poling treatment. Because of the difference of work function between tip and bottom electrode, the shape of tip vibration amplitude was asymmetric, and the piezoresponse of PZT thin films increased with increasing film thickness. The dependence of piezo-response on film thickness may be related to grain size and grain boundary, crystallinity, surface decomposed layer and so on. We discuss about the relationship between microstructure and piezoelectric properties.
1. 얇고 평활한 Pt 전극의 제조와 분석
DC magnetron sputtering법을 이용하여 $SiO_2$/Si 기판 위에 350℃의 증착온도로 Pt 박막을 증착하였다. 박막의 두께가 증가함에 따라 grain 크기와 roughness가 증가하였다. 이러한 roughness 증가는 grain growth에 따른 groove deepening에 의해 비롯된 것 같다. Pt 박막은 (111) 우선 배향을 지녔다. (111) peak에 대한 rocking curve를 측정하였으며, 비대칭적인 모양을 non-uniform strain의 관점에서 해석하였다. 박막이 두꺼워짐에 따라 결정학적 구조는 random texture에서 강한 (111) fiber texture로 바뀌었다. 이러한 구조 변화를 surface energy minimization과 strain energy minimization의 관점에서 고찰하였다. Non-uniform strain은 strain energy minimization의 원인이 되고, 예측된 응력완화는 surface energy minimization에 영향을 줄 것으로 추론하였다.
2. 얇고 평활한 PZT 박막의 제조와 분석
계량된 AFM 장비를 이용하여 박막 두께 변화에 따른 압전 상수 변화를 관찰하고, 이러한 변화를 미세구조 변화의 관점에서 고찰하고자 하였다. PZT 박막은 다결정 구조를 지녔다. 박막 두께가 증가함에 따라 grain 크기가 증가하였다. 박막 두께가 증가함에 따라 RBS를 이용하여 계산된 부피밀도는 감소하였고, TEM으로 관찰된 void들은 증가하였다. 특히 40nm의 PZT 박막은 두 구조가 관찰되었다. Rosette 구조와 Poorly crystallized 구조인데, 이를 SEM,AFM, TEM, DC poling을 이용하여 관찰하였다. 외부 인가 전압 변화에 따른 tip vibration amplitude의 모양은 비대칭적이었으며, 박막 두께가 증가함에 따라 증가하였다. 비대칭적인 모양은 AFM tip과 하부전극 사이의 일함수 차이에 의한 것으로 생각된다. 두께 증가에 따른 압전 상수의 증가는 grain boundary, 결정성, surface decomposed layer, free surface affected region, volume density의 관점에서 고찰하였다.
