In situ stress measurements during electrodeposition of thin films studied.

Thin films of various metals 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 the widely used technique of electrodeposition, from the use of solution additives and alloying elements needed to achieve desired deposition rates and mechanical properties. Often these stresses can 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 continue to shrink, the stresses associated with the earliest stages of film growth raise serious concerns in the industry about device performance and reliability.

To address these concerns, NIST researchers have established a Class II (1 mW) HeNe optical bench dedicated to the in situ measurement of residual stress during electrodeposition using the wafer curvature method. The substrate is a 60 mm X3 mm X0.1 mm wafer of borosilicate glass onto which 250 nm of gold is evaporated. The curvature of the substrate is monitored during electrodeposition by reflecting the laser off of the glass/metal interface, through a series of mirrors and onto a position-sensitive detector. The average in-plane stress of a metal film electrodeposited onto the Au can be calculated from the deflection of the beam as a function of time. The apparatus can resolve surface stresses on the order of 0.3 N/m while the beam is in solution, thus allowing researchers to observe the stresses associated with the entire deposition process.

As a demonstration, NIST researchers have followed stress development in the first 50 nm of Cu deposited onto Au. This system is known to follow classical Stranski-Krastanov growth, where three-dimensional islands grow on top of one or more Cu monolayers. Researchers have quantified the surface stresses associated with the formation of the first Cu monolayer, as well as the formation and coalescence of discrete Cu nuclei. It is expected that measurements such as these will allow the researchers to determine the root cause of stress in electrodeposited thin films and to propose mitigation strategies.

CONTACT: Gery Stafford, [phone]; [e-mail].