MML researchers have established a Class II (1 mW) HeNe laser optical bench (see figure) dedicated to the in situ measurement of stress evolution during electrochemical processing of thin films using the wafer curvature method. Forces on the order of 0.008 N/m (23 km radius of curvature) can be resolved, sufficient to study the deposition or adsorption of single layers of molecules onto the electrode surface.
Metal thin films are used by the microelectronics community to produce, for example, solderable surface finishes, magnetic recording media, and copper "wiring" in printed circuit boards and integrated circuits. Such films tend to develop sizable mechanical stresses as they are deposited. Though not well understood, these stresses can result from the nucleation and growth process (e.g., lattice-mismatched epitaxial growth) or, in the case of widely used electrodeposition techniques, from the use of solution additives and alloying elements needed to achieve desired deposition characteristics and materials properties. These stresses often approach or exceed the yield stress of the bulk material and can lead to loss of adhesion and the generation of bulk and surface defects. As feature sizes in microelectronic components in particular continue to shrink, the stresses associated with the earliest stages of film growth raise serious concerns in the industry about device performance and reliability.The measurements system has been used to measure the surface stress associated with reversible monolayer adsorption of metals and nonmetallic adsorbates. With the systems studied, we have demonstrated measurement sensitivity to both substrate-adsorbate and adsorbate-adsorbate lateral interactions. This could lead to a general in situ metrology for control of surfactants and adsorbate based surface modification for a variety of molecular electronics and sensor applications.