UCLA

This work has focused on the study and development of atomic layer etching for metallic and intermetallic thin films commonly used in front and back end of line applications. A thermodynamic equilibrium based framework consisting of minimization of Gibbs free energy and volatility assessment was used in screening and selecting promising plasma modification and chemical etchants. Challenges, such as the nonvolatility of metal etch products in front end of line processing of Cu for use in interconnects as well as back end of line patterning of magnetic and noble metals such as Co, Fe, CoFeB and Pt used in MRAM devices were addressed using the aforementioned approach to identify and develop reactive ion and atomic layer etch processes in parallel.

The efficacy of modification chemistries, including inductively coupled O2 and Cl2 plasmas, was assessed first for Cu. Alternating Cl2/H2 plasma exposure has been predicted and subsequently validated in literature to be capable of etching Cu at sufficiently high rates. Assessment of the Cl2 and subsequent H2 plasma exposure were predicted to enhance the vapor pressure of volatile etch products for patterning CoFeB as well as Pt. Experimental studies resulted in up to a 36% enhancement in etch rate while surface characterization indicated the removal of the non-volatile metallic chloride surface upon exposure to H2 discharge. For CoFeB specifically, damage to the static magnetic properties was shown to also have been recovered to nearly 70% of its original value, suggesting that this process could also be viable with additional optimization.

In order to achieve a self-limiting etch process for use in atomic scale patterning, O2 plasma modification in conjunction with the use of organic solution and vapor chemistries was studied as well. In concentrated solutions of acetylacetone, hexafluoroacetylacetone, acetic acid, and formic acid, CuO was observed to etch selectively over metallic Cu at values up to 285, confirming thermodynamic calculations. Formic acid vapor was subsequently found to etch CuO at rates up to 18 nm/min while no etch of metallic Cu was observed, indicating near infinite selectivity. Incorporating this formic acid chemistry with inductively coupled plasma oxidation resulted in etch rates up to 3.5 nm/cycle at 80 �C and 148 Torr. Similar self-limited chemical vapor etch of modified oxide layers were observed for Fe, Pt, Pd, and Co with etch rates of up to 4.8, 0.5, 1.1, and 2.8 nm/cycle and were shown to depend on the thickness of the modified metal oxide layer, giving rise to a route for controlling the etch rate. Reduction in etch per cycle through controlled oxidation was subsequently demonstrated for highly selective etch of CoFeB, resulting in a self-limited etch rate of 1.8 nm/cycle and subsequent chemical confirmation of selective oxide surface removal. Anisotropic etch profiles for patterned 70 nm � 10 �m lines of Cu and Co and 1 �m � 1 �m Cu square pads were achieved through application of a -100 and -200 V bias during the modification step and demonstrated that directional atomic layer etching can be achieved for patterning metallic thin films through precise control of ion energy, modification time, and subsequent chemical vapor etch.