UCLA

In this work, a generalized methodology, combining thermodynamic assessment of various etching chemistries and kinetic verification of etching efficacy, is proposed. To screen various chemistries, reactions between the dominant vapor phase/condensed species at various partial pressures of reactants are first considered. The volatility of etch product is determined to aid the selection of viable etch chemistry. Magnetic tunnel junction (MTJ) based magnetic random access memory (MRAM) was chosen as a case study to address the challenge of patterning various metals. Ar ion beam milling was a traditional method in patterning MRAM devices; however, sidewall re-deposition became severe as the features size decreased while aspect ratio increased. Selected metals (Fe, Co, Pt) and their alloys within the MRAM were studied by the generalized approach. To validate the thermodynamic calculation, films were patterned using a modified reactive ion etch process of halogen discharge with subsequent H2 plasma exposure. The etch rate of Fe, Co, and Pt were enhanced 40%, 25%, and 20% respectively with secondary H2 chemistry. X-ray photoelectron spectroscopy (XPS) suggested chemical removal of non-volatile metal chlorides by H2 plasma. Moreover, characterization through superconducting quantum interference device (SQUID) proved that coercive field strength of magnetic alloy after Cl2 plasma can be recovered by additional H2 plasma exposure from 63.6 to 20.9 Oe.

To further improve selectivity to mask materials, the chemical organic etch of generating highly volatile etching product was investigated. The metal etch by organic chemical was first analyzed by thermodynamic calculation, and selectivities of Co to Fe etched by acac and hfac solutions were predicted. A series of etching experiment was verified the theoretical calculation through etch rate measurement and identification of etch products through inductively coupled plasma - mass spectroscopy (ICP-MS). Ar ion beam assisted chemical etch (IBACE) was then investigated to develop a vacuum-compatible and highly effective process in patterning magnetic metal stacks. The etching of Co by alternating Ar ion beam and acac vapor showed an enhancement of the etch rate by approximately 180% compared to that of Ar ion beam only. The etch rate of Co by alternating Ar ion beam and acac vapor was much greater than the sum of the two, suggesting a synergistic effect of chemical etching along with physical sputtering was developed. The final validation of this generalized approach was to assess the highly chemical etch in order to avoid the sputtering effect. A sequential etch with new surface modification of 500 W O2 plasma at 5 mtorr and organic vapor etch of formic acid was developed to pattern Fe, Cu, Co, Pd, and Pt, and the etch rate were 4.2, 3.7, 2.8, 1.2, 0.5 nm/cycle, respectively. A high selectivity of metal oxide to metal by formic acid vapor was achieved, suggesting one of the most important requirement in atomic layer etch, self-limiting reaction, was fulfilled. By controlling the thickness of oxides with O2 plasma oxidation, the etch rate of metals could be controlled in atomic scale level. The patterned samples of Co and MTJ stack were investigated in the optimized condition, and an isotropic etching profile, high selectivity of mask to metals, and retaining of magnetic characteristic were observed, suggesting a process with solely chemical etch was carried out. An anisotropic etching profile of Co was demonstrated by biasing sample with 200 V in O2 plasma, and an etch rate of 7.5 nm/cyc was achieved with sequential etch.