Study of ion beam damage in magnetic tunnel junction on FIB prepared samples

Abstract

Magnetic Random Access Memory (MRAM) has emerged as the leading candidate for future universal memory due to its non-volatility, excellent endurance and read/write performance. The magnetic tunnel junction (MTJ) is a data storage element in MRAM and is basically composed of two ferromagnetic layers separated by the magnesium oxide (MgO) tunnel barrier. MgO between two ferromagnetic layers was adopted to enlarge the resistance difference between two kinds of magnetic arrangements by tunneling current through MgO. Like this, it is important to understand characterization of MgO for developing Mram. Due to thin thickness of MgO, FIB milling should be used for the preparation of TEM specimens in Mram. The major problem in MgO sampling by FIB milling is the transform of MgO between two ferromagnetic due to FIB induced damage, which leads to high tunnel current through MgO and high resistance difference between two kinds of magnetic arrangements. An understanding of FIB generated artifact on MgO is important to analysis Mram and to optimize the sample preparation process. The normal ion beam damage are compared with low-keV FIB ion beam damage on blanket MgO wafer. Experiments were performed using Helios 450 FIB(FEI) and XV-200TBs(SII) with gallium ion sources operated at 30 keV to 2 keV, respectively. As a preliminary, the thicknesses of all specimens were fixed at 100nm for the final ion beam milling currents of 210 pA(30 keV) by Helios 450 FIB(FEI). Specimens of 100nm were transferred to low-keV FIB (Helios 450/XV-200TBs) to do the low-keV ion milling. Then each specimen had a 2 keV cleaned surface and a 30 keV FIB prepared surface. In this paper, we understand the normal ion beam damage on blanket MgO through changing beam current and beam voltage. Then we present the optimized recipe and which equipment is better to analysis.