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 Integrated circuits Miniature electronic circuits produced within and upon a single semiconductor crystal, usually silicon. Integrated circuits range in complexity from simple logic circuits and amplifiers, about 1/20 in. (1. . Such films tend to develop sizable mechanical stresses as they are deposited. Though not well understood, these stresses can result from the nucleation nu·cle·a·tion n. 1. The beginning of chemical or physical changes at discrete points in a system, such as the formation of crystals in a liquid. 2. The formation of cell nuclei. and growth process (e.g., lattice-mismatched epitaxial growth) or, in the case of the widely used technique of electrodeposition e·lec·tro·de·pos·it tr.v. e·lec·tro·de·pos·it·ed, e·lec·tro·de·pos·it·ing, e·lec·tro·de·pos·its To deposit (a dissolved or suspended substance) on an electrode by electrolysis. n. The substance so deposited. , 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 (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. researchers have established a Class II (1 mW) HeNe optical bench dedicated to the in situ In place. When something is "in situ," it is in its original location. measurement of residual stress Residual stresses are stresses that remain after the original cause of the stresses (external forces, heat gradient) has been removed. They remain along a cross section of the component, even without the external cause. 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 mon·o·lay·er n. 1. A film or layer one molecule thick formed at the interface between water and either oil or air by a substance such as a partially esterified fatty acid that contains both hydrophobic and hydrophilic groups in the same , as well as the formation and coalescence coalescence /co·a·les·cence/ (ko?ah-les´ens) the fusion or blending of parts. co·a·les·cence n. See concrescence. coalescence a fusion or blending of parts. 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, (301) 975-6412; gery.stafford@nist.gov. |
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