Measuring the through-plane elastic modulus of thin polymer films in situ

Abstract

Due to the ongoing increase of the transistor density on a chip, industry has replaced the silicon oxide dielectric layers, traditionally used in the back-end interconnect stack, by low- K polymer films with a thickness down to several hundred nanometers. The use of these polymer dielectric films has introduced new failure modes. To have a better understanding of these failures, knowledge of the mechanical properties is necessary. Due to surface effects, the material properties of thin films may differ in the in-plane and trough-plane direction. Most techniques available for measuring these properties are only capable of obtaining the in-plane modulus. To have an in situ measurement of the through-plane modulus, a parallel plate capacitor (PPC) under hydrostatic pressure is used in combination with an interdigitated electrode (IDE) to capture the change in dielectric constant. Since it is believed to be mechanical isotropic, a benzocyclobutene (BCB) film is used to provide a reference measurement. The through-plane elastic modulus and change in permittivity for a 1 μm thick film sandwiched by two aluminum electrodes on a silicon wafer are reported. Two circular PPCs and four IDEs were tested at a pressure of 0, 5, 7.5 and 10 MPa. An initial relative dielectric constant of the film of 2.66 ± 0.05 was obtained. This yields a change in constant equal to 1.241 × 10 −4 ± 2.1 × 10 −5 per MPa pressure at room temperature. The through-plane modulus showed a linear elastic behavior equal to 4.73 ± 0.46, 4.11 ± 0.39 and 3.64 ± 0.31 GPa for 20°, 50° and 75 °C, respectively. The modulus at room temperature is in good agreement with the values found in literature.