Specific Process Knowledge/Thin film deposition/Deposition of Gold/Adhesion layers: Difference between revisions

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 image:Picture12.png|Fig. 9: (a) STEM-EELS linear scan of the 2nm Ti/2nm Au sample, showing the presence of oxygen in the Ti layer. (b) STEM-EELS linear scan of the 2nm Cr/2nm Au sample, which shows the presence of oxygen that is bounded to Cr and Cr diffusion into the Au layer. </gallery>
+The source of oxidation of the adhesion layer could have three origins: i) oxidation due to migration of oxygen from the SiO2 substrate, ii) oxidation during the e-beam deposition process, or iii) oxidation due to substrate contamination with water and oxygen molecules. To investigate the oxidation origin, XPS depth profile analysis were performed on 2nm Ti/20nm Au and 2nm Cr/20nm Au as-deposited samples.
-The source of oxidation of the adhesion layer could have three origins:
+The first analysis was performed to clarify if oxygen was originating from source i). To rule out possible oxygen migration from the oxygen-rich SiO2 substrate, the metal thin-films were deposited on amorphous Si3N4. For both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity decreased and Cr 2p and Ti 2p signals started to appear together with the O 1s signal (Fig. 10a for Ti and 10b for Cr, respectively).
-i) oxidation due to migration of oxygen from the SiO2 substrate, ii) oxidation
-during the e-beam deposition process, or iii) oxidation due to substrate
-contamination with water and oxygen molecules. To investigate the oxidation
-origin, XPS depth pro�le analysis were performed on 2-Ti/20-Au and
--Cr/20-Au as-deposited samples.
-The �rst analysis was performed to clarify if oxygen was originating from
-source i). To rule out possible oxygen migration from the oxygen-rich SiO2
-substrate, the metal thin-�lms were deposited on amorphous Si3N4. For
-both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity
-decreased and Cr 2p and Ti 2p signals started to appear together with the
-O 1s signal (Fig. 5.11a for Ti and 5.11b for Cr, respectively). For the Cr
-case, due to the lower spatial resolution of XPS with respect to STEM-EDX,
-it was not possible to verify the thickness of Cr-Au inter-di�usion in this
-case. The XPS electron escape depth at 1486.7 eV for Au is around 0.15
-nm [114], meaning that it is not possible to obtain information about interdi
-�usion for layers thinner than this value without collecting a signal from
-the underlying material.
-Peak �ts for the Ti and Cr signals were performed, and are reported in
-Fig. 5.12. The Ti 2p peak is a convolution of three components: a TiN
-doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and
-a TiOx signal, which forms the descending background tail [115]. The �t
-gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4
-substrate; 2) the adhesion layer was partially oxidized during deposition
-and not from source i). Metallic Ti could not be detected, but its presence
-cannot be excluded: metallic Ti is highly reactive with respect to oxygen
-and nitrogen, and the destructive sputtering process used for the depth
-pro�ling could have enhanced the mixing between Ti, O and N, catalyzing
-the reaction of the metallic Ti bound to Au to form an oxide or a nitride.
-The Cr 2p peak �t is formed by three components: a metallic Cr doublet
+[[File:Picture13.png|550px|center|thumb|Fig. 10: XPS depth pro�ling of 2nm Ti/20nm Au (a) and 2nm Cr/20nm Au (b) samples deposited on Si3N4 substrate. Both samples show the presence of a Au 4f signal from the surface to a depth of 20 nm. For a depth deeper than 20 nm, both Ti 2p and Cr 2p signals appear together with the O 1s signal.]]
-at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3
-doublet at 580 and 589.2 eV [115]. The result of the �t indicates a partial
-oxidation of Cr, as in the Ti case.
-The second analysis was performed to understand whether the oxygen
+Peak fits for the Ti and Cr signals were performed, and are reported in Fig. 11. The Ti 2p peak is a convolution of three components: a TiN doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and a TiOx signal, which forms the descending background tail. The fit gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4 substrate; 2) the adhesion layer was partially oxidized during deposition and not from source i). Metallic Ti could not be detected, but its presence cannot be excluded: metallic Ti is highly reactive with respect to oxygen and nitrogen, and the destructive sputtering process used for the depth profiling could have enhanced the mixing between Ti, O and N, catalyzing the reaction of the metallic Ti bound to Au to form an oxide or a nitride. The Cr 2p peak fit is formed by three components: a metallic Cr doublet at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3 doublet at 580 and 589.2 eV. The result of the fit indicates a partial oxidation of Cr, as in the Ti case.
-was originating from source ii) or iii). A 2-Ti/20-Au/2-Ti/20-Au sandwich
-structure was deposited and analyzed. Both layers of Ti were partially oxidized:
-the Ti 2p peak signals (Fig. 5.13a) are present at the same depth
-together with the O 1s signals (Fig. 5.13b). The O 1s signal in the Ti layer
-in contact with the Si3N4 substrate has higher intensity than the one of the
-Ti layer between the Au layers. Hence, the Ti layer in contact with the substrate
-is more oxidized, which suggests that Ti reacted with water adsorbed
-on the substrate surface. The conclusion is that the oxygen originated from
-sources ii) and iii).
-Water is present in most thin-�lm deposition systems and it is very
+[[File:Picture14.png|550px|center|thumb|Fig. 11: Left: Ti 2p XPS peak �t of the 2-Ti/20-Au sample. Right: Cr 2p XPS peak �t of the 2-Cr/20-Au sample.]]
-di�cult to remove because of its capacity to form hydrogen bonds with
-many materials. Ti reacts with water and oxygen and forms titanium oxides.
+The second analysis was performed to understand whether the oxygen was originating from source ii) or iii). A 2-Ti/20-Au/2-Ti/20-Au sandwich structure was deposited and analyzed. Both layers of Ti were partially oxidized: the Ti 2p peak signals (Fig. 5.13a) are present at the same depth together with the O 1s signals (Fig. 5.13b). The O 1s signal in the Ti layer in contact with the Si3N4 substrate has higher intensity than the one of the Ti layer between the Au layers. Hence, the Ti layer in contact with the substrate is more oxidized, which suggests that Ti reacted with water adsorbed on the substrate surface. The conclusion is that the oxygen originated from sources ii) and iii).
-Furthermore, the use of the same deposition conditions and of the same type
-of substrate with respect to Ti and a similar chemical reactivity of Ti and
+Water is present in most thin-�lm deposition systems and it is very di�cult to remove because of its capacity to form hydrogen bonds with many materials. Ti reacts with water and oxygen and forms titanium oxides. Furthermore, the use of the same deposition conditions and of the same type of substrate with respect to Ti and a similar chemical reactivity of Ti and Cr suggest that also the Cr layer gets further oxidized by the water and oxygen molecules present on the substrate surface.
-Cr suggest that also the Cr layer gets further oxidized by the water and
-oxygen molecules present on the substrate surface.
 == Adhesion layer effect on bilayer thin-film electrical resistivity ==