The downscaling of the DRAM devices is necessary to achieve higher speed with less power consumption. It is getting difficult to meet the new requirements with the existing $SiO_2$ or $Si_3N_4$ due to their low dielectric constants and tunneling leakage currents through the thin layers. For this reason, high-K materials enabling high-k and low leakage currents with physically thicker film have received considerable attention. $TiO_2$ film attracts particular attention as a promising alternative to $SiO_2$ or $Si_3N_4$ from high-k and low leakage current considerations. Along with the development of high-k dielectrics, there is a demand of new deposition technique in order to obtain the controllability of thickness in nanometer scale and superb step coverage. Among various deposition techniques, atomic layer deposition (ALD) is known as the most adequate technique because of its unique feature of the self-limited growth mechanism. This self-limited growth mechanism provides the exact control of the thickness even in ultra-thin film and the conformal deposition for 3-D capacitor structures.
In this study, the growth of $TiO_2$ films by plasma enhanced ALD (PEALD) using oxygen plasma as an oxygen reactant of Titanium tetra-Isopropoxide (TTIP) on Ir with and without $IrO_2$ seed layer was presented. $TiO_2$ crystallized into rutile due to $IrO_2$ interface layer, which was formed in $O_2$ plasma sequence by oxidation of Ir. The dielectric constant of rutile phased $TiO_2$ films deposited on Ir with and without $IrO_2$ seed layer was about 90 and 83, respectively. In spite of high dielectric constant, leakage current was too large ($0.002A/cm^2$ @ 1MV/cm ) to adopt DRAM capacitor.
To improve electrical properties, $TiO_2$/$Al_2O_3$/$TiO_2$ lamination films were investigated. Equivalent oxide thickness of $TiO_2$(4.6nm)/$Al_2O_3$(12.5nm)/$TiO_2$(4.6nm) laminated film was 0.9nm and leakage current characteristics is improved compare with rutile $TiO_2$ films. The dielectric constant of lower $TiO_2$ layer, $Al_2O_3$ layer, and Upper $TiO_2$ layer was about 80, 8, and 50, respectively. At 1V, For 40nm technology, EOT have to be reduces to 0.5nm and reducing the thickness of Al2O3 is most effective due to low dielectric constant of $Al_2O_3$ layer. However as the thickness of $Al_2O_3$ layer decreases, the leakage current of nano-laminated film increases rapidly. As a result, the electrical characteristic of nano-laminated films is improved compared with pure $TiO_2$ films. But it is necessary to improve that for 40nm technology.
The other method to reduce leakage current is the addition of $Al_2O_3$ into $TiO_2$ films. The compositions of TiAlO films are controlled by the unit-cycle number. As the concentration of Ti increase, the dielectric constants of TiAlO films increase slowly while Ti/(Al+Ti) atomic ratio is under 0.76. The dielectric constants of TiAlO films increase rapidly as Ti/(Al+Ti) atomic ratio exceeds 0.76, due to crystallization of TiAlO into rutile phase. The dielectric constants of TiAlO films deposited on $IrO_2$ seed layer are higher than those of TiAlO films deposited on Ir electrode. The small addition of $Al_2O_3$ to $TiO_2$ decreases the leakage current dramatically. When Ti/(Ti+Al) atomic ratio is 0.89, The leakage currents of TiAlO films are lowest. As Al/(Ti+Al) increase over 0.89, The leakage currents increase. Why the addition of $Al_2O_3$ decrease leakage current is that Al ion substitutes Ti ion and plays role as acceptor. The acceptor effect of Al is independent on the thickness of TiAlO films.
본 연구에서, Ir기판을 사용하여 열처리 없이 rutile구조의 $TiO_2$ 를 PEALD를 사용하여 증착하였다. 산소 플라즈마에 의해 증착 초기에 생성 된 Ir 산화층은 고유전율을 가진 루타일 $TiO_2$ 성장에 영향을 미친다. 이것은 FCC구조인 Ir의 격자상수가 anatase $TiO_2$ 의 a축 격자상수와 거의 같기 때문에, seed역할을 하기 때문이다. $TiO_2$ 의 결정성 향상을 위하여, Ir기판 위에 열처리를 통하여 rutile 구조의 $IrO_2$ seed layer를 생성시켰으며, $IrO_2$ seed layer 위에서 $TiO_2$ 를 증착하였다. $IrO_2$ seed layer위에서 $TiO_2$ 박막은 epitaxial 성장하는 것이 관찰 되었고, 그 결과 유전상수가 증가하였다.
Rutile 구조의 $TiO_2$ 의 누설전류 특성을 향상 시키기 위하여, TiAlO 박막을 증착하였다. Ir 기판에서 증착한 ATO 박막의 경우 $TiO_2$ 의 cycle이 10이하에서는 비정질 상을 보이며, 20이상의 cycle 수에서 rutile로 결정화 되기 시작하고, $TiO_2$ 10cycle이하에서는 Ti atomic ratio가 증가함에 따라, 느리게 유전상수가 증가하나, $TiO_2$ 20cycle이상의 박막에서는 rutile로 결정화 되면서, 빠르게 유전상수가 증가하게 된다. $IrO_2$ seed layer 위에서 증착 된 TiAlO 박막은 $IrO_2$ 의 영향으로 유전상수가 증가되는 것을 확인하였다.
TiAlO 박막은 Al이 첨가되면서 누설전류 특성이 급격히 좋아지다, $Ti_{0.89}Al_{0.11}O_x$ 의 조성에서 가장 좋은 누설 전류특성을 보였으며, 이 때 $TiO_2$ 와 $Al_2O_3$ 의 unit-cycle비는 40:1이다. Al양이 이것보다 더 증가하게 될 경우 누설 전류특성이 나빠지는 모습이 관찰되었다. $IrO_2$ seed layer가 삽입 된 경우 누설전류 특성이 더 좋아지는 것이 확인되었다. Ti0.89Al0.11Ox 박막은 두께가 줄어드는 경우에도 이러한 우수한 특성이 계속 유지 되었으며, $IrO_2$ seed layer위에서 증착 된 $Ti_{0.89}Al_{0.11}O_x$ 박막은 73의 유전상수를 가지며, 0.5nm의 EOT에서 1V 전압 인가시 $1*10^{-7}A/cm^2$ 우수한 누설 전류 특성을 보여, 40nm 와 그 이후 기술의 DRAM capacitor에 적용이 가능할 것으로 생각된다.
