Influence of O₂Gas on Etch Profile of Magnetic Tunnel Junction Stacks Etched in a CH₄/O₂/Ar Plasma

Abstract

The etch characteristics of magnetic tunnel junction (MTJ) stacks patterned with W/TiN films were examined using an inductively coupled plasma of a CH 4 /O 2 /Ar gas mix. The effect of the O 2 concentration on the etch rate, etch selectivity and etch profile of the MTJ stacks was examined. The etch profile of the MTJ stacks in 60% CH 4 /Ar gas appeared to be the best. The addition of 10% O 2 gas in CH 4 /Ar gas led to an improved etch profile with less redeposition on the sidewall of the MTJ stacks. This was attributed to the increase in [H]/[Ar] and [O]/[Ar] intensity ratios with increasing O 2 concentration to 20%. Transmission electron microscopy of the etched MTJ stacks revealed redeposited materials on the sidewall of the MTJ stacks etched in CH 4 /Ar gas that were indentified to be mainly Pt, Mn, Co, Ru and Ti. On the other hand, the amount of redeposited materials decreased significantly with the addition of 10% O 2 in CH 4 /Ar gas. The formation of metal oxides and protection layer in CH 4 /O 2 /Ar gas mix resulted in a high degree of anisotropy without redeposited materials in the etch profile of MTJ stacks.The rapid growth of smart devices such as smart phone, smart tab and smart TV requires new emerging semiconductor memory with the features of high density, fast speed, high endurance and nonvolativity. Among a variety of memory devices containing phase-change random access memory (PCRAM), resistive random access memory (ReRAM) and magnetic random access memory (MRAM), MRAM devices have attracted considerable attention as a replacement for current dynamic random access memory (DRAM) and flash memory. MRAM devices can provide nonvolatility, fast access time, unlimited read/write endurance, low operating voltage, and high storage density. 1,2 MRAM devices are composed of a magnetic tunnel junction (MTJ) stack and CMOS. The MTJ stack is a key part of MRAM devices and the realization of high-density MRAMs as a nonvolatile emerging memory requires new etching processes of MTJ stacks that can delineate the nanometer-sized patterns on the MTJ stacks. 3,4 The development of an etch process for patterning MTJ stacks, which consist of magnetic materials, metals and metal oxides, is important for the fabrication of high density MRAM with nanometersized patterns. Generally, dry etching of magnetic thin films is extremely difficult because the metals and magnetic materials barely react with the chemically active species in plasma. Initially, ion beam etching of magnetic thin films was used to etch the magnetic materials and metals but often, several undesirable attributes were produced, including slow etch rate, sidewall redeposition, and etching damages because of the etch mechanism by physical sputtering. Therefore, ion beam etching was of limited use in etching MTJ stacks as well as magnetic films. Chemically assisted ion etching and reactive ion etching were applied to etch the magnetic films but similar etch results to those obtained by ion beam etching were achieved. 5-7 Recently, high density plasma etching ...

Inductively Coupled Plasma Reactive Ion Etching of Magnetic Tunnel Junction Stacks in a CH3COOH/Ar Gas

“…4,13 Recent studies have focused on developing etching processes that employ non corrosive gas mixtures containing C, H, O and N, such as CO/NH 3 , CH 3 OH/Ar, CH 3 OH/H 2 O, CH 4 /Ar and CH 4 /O 2 /Ar. [5][6][7][8][9][10][11][12] MTJ stacks etching in such non corrosive gas mixtures mainly proceeds via oxidation of metals in the MTJ stack that compose each layer, formation of a protective layer and ion bombardment, which enables moderate etch rates and good etch profiles without any post etch degradation of the MTJ stacks. However, the lack of chemically reactive species in the plasma chemistry that can produce volatile etch byproducts results in heavy sidewall redeposition under certain conditions.…”

“…However, previous studies reported the formation of a protective layer and oxidation of the etched material. [9][10][11] The improved etch profile is attributed to partial oxidation of the film, which allows easy sputtering of the oxide layer and formation of a protective layer on the sidewall. Owing to fast etch rates and good etch profiles, 25% CH 3 COOH was selected as the standard concentration for the investigation of the etch profile evolution as a function of time.…”