Ion effects in hydrogen-induced blistering of Mo/Si multilayers

Abstract

The role that energetic (>800 eV) hydrogen ions play in inducing and modifying the formation of blisters in nanoscale Mo/Si multilayer samples is investigated. Such samples are confirmed to be susceptible to blistering by two separate mechanisms. The first is attributed to the segregation of H atoms to voids and vacancies associated with the outermost Mo layer, driving blister formation in the form of H 2 filled bubbles. This process can occur in the absence of ions. A second blister distribution emerges when energetic ions are present in the irradiating flux. This is attributed to an ion-induced vacancy clustering mechanism that produces void blisters. The defects and strained states associated with the Moon -Si interfaces provide the preferred nucleation points for blistering in both cases. The effects of ions are ascribed to promotion of hydrogen uptake and mobility, in particular through the Si layers; to the generation of additional mobile species in the Si and Mo layers; and to the creation of new blister nucleation points. In addition to directly stimulating blistering via vacancy clustering, ions modify the development of H 2-filled blisters. This is most evident in the formation of multi-component structures due to overlapping delaminations at different layer interfaces. This affect is attributed to the introduction of active transport of hydrogen from the H 2 filled blisters across the outermost Moon -Si interface to the underlying layers. Ion-induced variations in hydrogen uptake and distribution and in the rates of blister nucleation and growth produce lateral differences in blister size and areal number density that create a macroscopic concentric pattern across the surface.

Temperature dependencies of hydrogen-induced blistering of thin film multilayers

“…The Mo/Si ML samples investigated in this work are similar to those studied previously [23][24][25][26] . They were deposited on a super-polished Si wafer by magnetron sputtering with additional ion polishing of the deposited Si layers.…”

“…In addition to the neutral atomic and molecular hydrogen flux, the source also produced an ion current on the order of ~75 nA. The majority of ions produced have energies >800 eV, due to the positive bias (+1000 V) used to e-beam heat the capillary 24 .…”

“…This is attributed the strained states and defects that are introduced near these interfaces by the transition between materials and the local atomic structures 24 . In the case of exposure to a neutral-only hydrogen flux, blistering was the result of layer delamination occurring exclusively at the outermost Mo-on-Si interface.…”

“…These samples are composed of alternating layers of nanometer thick amorphous Si (a-Si) and polycrystalline Mo with mixed Mo-Si interfacial regions 22 . They are susceptible to two distinct hydrogen-induced blistering processes [23][24][25][26] , which are attributed to the formation of H2-filled blisters and to the formation and clustering of hydrogen-vacancy complexes producing void blister 24 . The former occurs under the influence of thermal H-atom irradiation; the latter process is observed when energetic (100's of eV) ions are present in the irradiating flux.…”

“…In particular, the inclusion of energetic ions in the irradiating flux modifies the development and growth of H2-filled blisters 24 .…”