Etching of the DETA SAM due to Atomic Oxygen Exposure

Figure 5.2 displays the overall chemical composition for selected steps throughout the experiment. It can be seen that as the carbon and nitrogen signals decrease the silicon and oxygen signals increase. This is interpreted as evidence of the removal of the SAM which exposes more of the SiO2 substrate surface and in turn increases its signal. After the twenty-fourth exposure the carbon and nitrogen signals are just above half of their original intensity. The larger exposures at the end decrease the intensity of these peaks more rapidly. The SAM is never fully removed from the surface even though cumulatively the samples have been exposed to 7400 L of atomic oxygen. In the corresponding survey scans in Figure 5.3 these trends are seen.

Figure 5.2 Overall chemical composition from selected atomic oxygen experimental steps. The carbon and nitrogen signals sequentially decrease with each successive exposure.

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Figure 5.3 Survey spectra for selected experimental steps. With each exposure the C 1s and N 1s decrease in intensity.

It is worth noting that some iridium (Ir) from the capillary filament in the cracker is deposited on the surface during the exposures. XPS is very sensitive to Ir and chemical calculations show that the Ir 4d (~297 eV) represents < 1 % of the total surface. Therefore, it has not been included in the overall chemical composition. In order to understand the changes induced in the SAM following the atomic oxygen exposures, the C 1s and N 1s core level spectra were studied in detail. Figure 5.4 displays the peak fitted C 1s spectra from unnormalised data. Prior to atomic oxygen exposure, the primary component peak is the C-C / C-H peak at 284.8 eV30– 32_{. The C-H bonds from the DETA and the C-C bonds from adventitious carbon are}

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Figure 5.4 C 1s spectra showing the overall decrease in component peaks except for the O- C=O peak which grows with higher exposure to atomic oxygen.

The next peak at 286.6 eV is attributed to C-O / C-N bonds33,34_{. Due to the similar}

electronegativity values of oxygen and nitrogen it is very difficult to resolve the two different binding environments hence, they cannot be definitively distinguished. The C-O bonds are also indicative of adventitious carbon due to atmosphere exposure. While the C-N bonds are due to bonding in the DETA SAM. The third component peak visible in the spectrum at ~ 288.1 eV which shifts to 288.4 eV after the small exposures, has been ascribed to C-O-N bonds33,35_{. It appears that some}

oxygen has become incorporated into the DETA SAM and has bonded to the nitrogen and carbon in the terminal group. Following the twenty-fourth 100 L exposure the overall C 1s intensity is almost half of its original value. However, the

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growth of an additional peak at ~289.5 eV is observed and is indicative of O-C=O bonds33,36_{. This peak confirms that some oxygen is being incorporated into the}

DETA film. The next larger 1000 L exposure sees a decrease in all peaks except for the O-C=O component peak. This trend is seen for the subsequent exposures although, the growth of the O-C=O peak is very small. The final 2000 L exposure almost completely removes the SAM related signal from the surface.

The decrease in intensity of the C-C/C-H component peak, the increase in intensity of the C-O/C-N, C-N-O peaks and growth of the O-C=O peak indicates that two processes are occurring. Oxygen is being incorporated into the DETA SAM film which results in SAM decomposition and ultimately the removal of the SAM from the surface.

The deconvoluted N 1s spectra are shown in Figure 5.5. The initial N 1s contains two component peaks that have been attributed to N-H bonds at 399.3 eV 37 _{and a small}

component peak at 400.5 eV consistent with C-O-N bonds 35_{. Again, these C-O-N}

are due to oxygen bonding to the DETA terminal group. Similar, to the C 1s after the twenty-fourth 100 L exposure, there is a large decrease in the overall intensity of the N 1s leaving it at just above half of its original intensity. With the larger subsequent exposures, the overall intensity decreases. However, the C-O-N peak appears not to decrease at the same rate compared to the N-H peak.

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Figure 5.5 Peak fitted N 1s spectra for selected experimental steps. With each exposure the N 1s intensity decreases.

The loss of peak intensity for both the C 1s and N 1s alludes to two possible mechanisms for etching the DETA SAM. The atomic oxygen can be desorbing small moieties at a time or the intact DETA chains are being desorbed38_{. The carbon-}

nitrogen ratio plotted in Figure 5.6 as a function of oxygen treatments shows a small preferential removal of carbon over nitrogen. However, the overall reduction in intensity for the C and N 1s signals as a function of treatment shown in Figure 5.7 points to an etching process where the full chain is removed from the surface.

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Figure 5.6 The carbon nitrogen ratio displays a slight preference for the removal of C over N.

Figure 5.7 The C 1s and N 1s plotted as a function of atomic oxygen treatment showing the magnitude of the total reduction in these signals with atomic oxygen exposures.

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The O 1s and the Si 2p of the DETA SAM, illustrated in Figure 5.8(a) and (b) respectively, display very little change over the course of the experiment. Two component peaks are visible in the O 1s which are associated with SiO2 at 532 eV

and C-O at 530.3 eV39_{. The intensity of the SiO}

2 component peak is much larger

than the C-O peak making it difficult to observe changes in the carbon-oxygen bonding environment. There is a slight increase in the C-O peak with each exposure. A small change in the Si 2p can be associated with the formation of sub- oxides after exposure. Thickness calculations on the SiO2 peak reveal that there is

no change in the thickness of the SiO2 after any of the exposures. This signifies that

the head group of the DETA is unaffected by the atomic oxygen, whereas the rest of the SAM is progressively removed.

Figure 5.8 O 1s (a) and Si 2p (b) spectra show little to no change over the course of the study.

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Another DETA sample was investigated with large 1000 L exposures. This sample received exposures of 1000 L until the DETA SAM was almost completely removed.