Development of spin funneling magnetic field sensor devices for high sensitive and high space-resolution magnetic imaging plates

为了实现高灵敏度和高空间分辨率的磁成像板，我们研究了自旋隧道磁场传感器器件的制备工艺。采用微波激励等离子体氧化或径向缝隙天线氮化的方法，在热氧化硅片上制备了bottom-electrode/Mn-Ir/Co-Fe/barrier/Co-Fe/Ni-Fe/upper-electrode叠层铁磁隧道结。该势垒是在沉积0.8-1.5 nm厚的金属铝膜后立即用X+O_2或X+N_2(X=He、Ar、Kr)混合气体等离子体氧化或氮化过程形成的。在1.5 nm厚的Al膜上氧化Al-O势垒的MTJ，最大TMR率达到58.8%，这是当时的世界纪录。在1.0 nm厚的Al膜上氮化Al-N势垒的MTJ中，最大TMR比超过了传统记录的33%，达到了49%。研究还发现，金属铝膜的等离子体氮化过程比金属铝膜的等离子体氧化过程缓慢。这意味着等离子体氮化适用于超薄势垒层的形成过程。研究了超薄金属薄膜的高浓度臭氧氧化工艺及其制备的MTJ的磁输运特性。实验结果如下：臭氧曝光法和等离子体曝光法对金属薄膜的氧化过程不同。臭氧过程的氧化速率(抛物线速率常数，k_p)由氧化物在氧化层中的扩散速度决定，比等离子体过程小两个数量级，而氧化初期立即在表面形成的氧化层厚度几乎相同。这意味着高浓度臭氧氧化法适用于超薄金属膜氧化的精确控制。

In order to realize a high sensitive and high space-resolution magnetic imaging plate, we investigated fabrication processes of spin tunneling magnetic field sensor devices. With the microwave excited plasma oxidizatioin or nitridation method using radial line slot antenna (RLSA) irradiator, ferromagnetic tunnel junctions (MTJs) in stack of bottom-electrode/Mn-Ir/Co-Fe/barrier/Co-Fe/Ni-Fe/upper-electrode were fabricated on a thermally oxidized Si wafer. The barrier was formed by the plasma oxidization or nitridation process with X+O_2 or X+N_2 (X=He,Ar,Kr) mixed gases, immediately after the deposition of 0.8-1.5 nm-thick metal Al films. In the case of MTJs with Al-O barrier oxidized from 1.5-nm-thick Al film, the maximum TMR ratio was achieved to 58.8%, which was the world record at the time. In the case of MTJs with Al-N barrier nitrided from 1.0-nm-thick Al film, the maximum TMR ratio overcame the conventional record of 33% and reached 49%. It was also found that plasma nitridation of metal Al films progresses more mildly than the plasma oxidization of them. This means that the plasma nitridation is suitable for the formation process of ultra-thin barrier layers. High concentration ozone oxidization process of ultra-thin metal films and magnetotransport properties of MTJs fabricated with this process were also investigated. As results followings were found. The oxidization process of metal films are different between the ozone exposure method and the plasma exposure method. The oxidizing rate (parabolic rate constant, k_p), dominated by the diffusion velocity of oxidizing species in the oxide layer, is smaller by two-orders in magnitude for the ozone process than that for the plasma process, while the thickness of oxide layer, immediately formed on the surface at the very early stage of oxidization, is almost same. This means that the high concentration ozone oxidization method is suitable for the precise control of oxidization of ultra-thin metal films.