Surface Smoothing Effects of Materials Used in Underlayer of MTJ with Gas Cluster Ion Beams
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Surface Smoothing Effects of Materials Used in Underlayer of MTJ with Gas Cluster Ion Beams Noriaki Toyoda Graduate school of engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo, 671-2280, JAPAN. ABSTRACT Surface smoothing of Ru used as underlayer of magnetic tunneling junctions (MTJ) in magneto-resistive random access memory (MRAM) was carried out with gas cluster ion beam (GCIB) in order to improve device characteristics. For Ru films, surface smoothing with 5 kV N2-GCIB irradiation was effective, and CoFe films deposited on smoothed Ru surface also showed smooth surface. From the hysteresis loop measurements of MTJ formed on smoothed Ru with N2-GCIB, it showed improvement of inter-layer coupling magnetic field (Hin) with decreasing the surface roughness of underlayer Ru. It is expected that surface roughness of MgO in MTJ was also improved by smoothing of underlayer Ru with N2-GCIB. INTRODUCTION Recently, development of various type of non-volatile memory has been conducted enthusiastically. Among various non-volatile memories, spin-transfer torque magneto-resistive random access memory (STT-MRAM) attracts many attentions because of its characteristics of high-speed read/write and almost infinite rewrite cycles. Therefore, STT-MRAM is considered as an alternative of current cache memory or dynamic random access memory (DRAM). The structure of STT-MRAM is completely different from conventional Si-CMOS devices. STTMRAM uses magnetic tunneling junction (MTJ). MTJ is consisted of number of layer of ultrathin metal films, such as Ru, CoFe, Ta, PtMn, and MgO. Since these ultra-thin films are multilayered, surface roughness of underlayer film might affect the surface roughness of the film on upper layer. It has been reported that a tunnel magnetoresistance (TMR) ratio decreased when the surface roughness of MgO increased [1]. Therefore, surface smoothing of films in the bottom layer of MTJ is important to improve device characteristics of STT-MRAM. We have been developed gas cluster ion beam (GCIB) process for surface modification [2]. Gas cluster ions are aggregates of several thousands of gaseous atoms or molecules. Owing to their large size, energy per atoms can be easily reduced to several eV/atom, which realize lowdamage irradiations [3]. However, energy density of bombarded area by cluster ions becomes extremely high, which induces high-temperature (104 K) and high-pressure (10 GPa) at a short time (< psec) [4]. Therefore, enhancements of chemical reaction or sputtering of surface atoms are observed at low substrate temperature [5]. Besides these characteristics of GCIB, surface smoothing owing to lateral sputtering effects is one of the unique characteristics of GCIB [6]. When gas cluster ions bombard a target surface, most of atoms are sputtered in lateral directions
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even when the incident angle of GCIB is normal. These lateral motions of atoms induce surface smoothing [7]. In the previous study, we have demonstrated reactive etching of metals used in STTMRAM by GCIB irradiation with acetic acid
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