High-throughput optimization of adhesion in multilayers by superlayer gradients

Abstract

For adhesion measurements, the superlayer method has recently been developed: an overlayer under high compressive stress (Mo film in our case) provides the loading and drives interface delamination and buckling. We combined the superlayer method with thickness gradients for high throughput adhesion measurements. In this way, the effect of small modifications of interface chemistry on adhesion in multilayers can be assessed in a single test through the identification of the threshold thickness. We used this methodology for an optimisation of the Ti interlayer thickness for adhesion at the SiO 2 /Ag interface (see figure 1a). An improvement of the adhesion could be observed in the range from 0 to ~ 3 A of Ti (figure 1b). At higher thicknesses of Ti the adhesion has become stronger than the range of the test. Similar tests have been performed on the Mo/SiO 2 interface with Ag interlayer. In this case, the increase of Ag thickness weakens the interface and causes the adhesion to decrease (figure 1c). The adhesion drops abruptly at an equivalent thickness of Ag of ~ 6 nm, corresponding to the Ag islands coalescence. Thus, the method has become a tool not only to measure the adhesion but to study film's growth as well.Another important film property is the residual stress. Using the wide range of buckle sizes generated by the thickness and adhesion gradients, we investigate the buckle morphology in details. The results indicate that the ratio between the height and an in-plane width of a telephone-cord like blister can be used to assess the value of the biaxial stress in the compressed layer. Finally we suggest that further understanding of the relation between buckle morphology and interfacial toughness is highly desirable.

How Does Adhesion Induce the Formation of Telephone Cord Buckles?

“…By varying the deposition parameters, various buckle morphologies can be obtained. For the present samples, the delamination occurred at the interface between silver and the native silicon dioxide of the substrate [26] and the kinematics of buckle formation was easily recorded by optical microscopy, a 29 m wide TC progressing over one wavelength in about 28 s. The buckle propagation was consistent with usual observations [27], with one notable addition, however: It was observed that the tip of the semicircular buckle sags when the TC completes one half-rotation and the front caves in (Fig. 1, see the Supplemental Material for a movie of the buckle propagation [28]).…”

Orientation-dependent chemistry and band-bending of Ti on polar ZnO surfaces

“…It is believed that the interfacial reaction leads to a reduced ZnO subsurface enriched in O-vacancies which act as donors and increase the carrier concentration 1,2,6 . Next to its use to create ohmic contacts 1,2,[6][7][8][9][10][11][12] , titanium is also known to promote adhesion for noble metals within optical coatings deposited on multilayeredglazings [13][14] , to enhance the gas sensor properties of ZnO 15 and to set up resistive random access memories 16 . Changes in Ti/ZnO stacking as a function of temperature are crucial regarding the optimization of contact resistivity 1,2,6-12 , gas detection limit 17 and mechanical properties of coatings 13 .…”

Reactive ZnO/Ti/ZnO interfaces studied by hard x-ray photoelectron spectroscopy

“…In this case, an ultrathin Ti layer serves as an adhesion enhancer between the oxide and the noble metal. 3 In both cases, the origin of the peculiar electronic behavior and of the bonding reinforcement is under discussion. [4][5][6] The reduction of ZnO by Ti results in the creation of O vacancies in the ZnO, giving rise to reduced interfacial resistivity and n-doping.…”