Surface chemistry of thermal dry etching of cobalt thin films using hexafluoroacetylacetone (hfacH)

Abstract

Amechanism of thermal dry etching process of cobalt thin films by using 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoroacetylacetone, hfacH) was investigated. This process, relevant to atomic layer etching (ALE) technology directed towards oxidized cobalt films, requires adsorption of molecular organic precursor, such as hfacH, at moderate temperatures and is often thought of as releasing water and Co(hfac) at elevated temperatures. The reaction was analyzed in situ by temperature-programmed desorption (TPD) and the resulting surface was investigated ex situ by X-ray photoelectron spectroscopy (XPS). The changes in surface morphology during the process were monitored by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The removal of Co(hfac) from the surface was observed above 650 K, a temperature well above commercially desired etching conditions, suggesting that the thermal etching process is more complex than originally envisioned. In addition, the upper limit of thermal treatment is established at 800 K, as the microscopic techniques clearly indicated surface morphology changes above this temperature. In addition, the structure of the surface at the nanoscale is observed to be affected by the presence of surface bound organic ligands even at room temperature. Thus, further mechanistic studies should address the kinetic regime and surface morphology to make inroads into mechanistic understanding of the dry etching process.

Molecular mechanisms of atomic layer etching of cobalt with sequential exposure to molecular chlorine and diketones

“…For thermal desorption experiments, the chlorinated (and partially oxidized) cobalt film sample was loaded into the TPD chamber at UHV conditions, dosed with of hfacH (1 × 10 −6 Torr for 5 min) at several different surface temperatures, and the thermal desorption spectra were recorded at 2 K/s heating rate, stopping at approximately 425°C. As has been shown previously, 10 heating substantially above this temperature results in surface pitting of the film. Figure 7 compares the traces collected for selected representative Co-containing species desorbing from the surface.…”

“…In fact, although the surface roughness is not the focus of the present work, the observation that the ALE process does not increase the roughness of the cobalt film surface is consistent with the previous observations of surface "smoothing" in ALE processes. 10,[19][20][21] We also evaluated another beta-diketone precursor, acacH, for the Co ALE process. However, the Co film did not show a significant thickness change with sequential doses of Cl 2 and acacH at 140°C.…”

“…This study also implies that removing a metal adatom is substantially less energetically demanding than removing an atom from a smooth surface, which is fully consistent with the previous observations of surface smoothing during ALE. 10,[19][20][21] As shown above, the mechanism of ALE of metals can be very complex, so further experimental work is needed to identify and sort all the possible desorbing products containing metals and then to propose energetic diagrams for the identified processes. This last step could be correlated with FIG.…”

“…[5][6][7] A simple acetylacetone (2,4-pentanedione, acacH) and hexafluorinated version of this compound (1,1,1,5,5,5-hexafluoro-2,4,-pentanedione, hfacH) have been used successfully to etch oxidized cobalt surfaces; 8,9 however, it was also shown recently that the proposed volatile product, Co(hfac) 2 , does not form on a clean cobalt surface 10 and that the presence of surface oxygen is required for this product to form. Even when Co(hfac) 2 is formed on the oxidized cobalt surface in a self-limited reaction, it was not observed to desorb until approximately 350°C, 10 the temperature substantially higher than the industrial processing conditions and experimentally observed etching. Thus, the etching mechanism proposed previously for nickel, where surface metal diketonates are formed and desorbed as Ni(hfac) 2 in a self-limiting reaction, 11 is not appropriate for cobalt, and further understanding of the process is needed.…”

In silico design of a thermal atomic layer etch process of cobalt

“…This process is strongly temperature dependent with a variable etch rate of 0.2 nm/cycle at 140 C to 1.6 nm/cycle at 185 C. Moreover, the presence of surface F species was also confirmed with XPS in this study. In a similar fashion, chemical vapor etch processes that have utilized acacH and hfacH have also been reported 46,47 but similarly the resulting thin films exhibit non-negligible amounts of fluorine contamination. This is an undesirable effect which has been shown to be detrimental to the performance of CMOS devices and a key attribute of these processes to be avoided.…”