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Hydrophilically activated direct wafer bonding is a technique for gluelessly attaching oxide-coated wafers together. This ability is a vital step in the construction of many microelectronic and microelectromechanical (MEMS) devices. In particular this technique is widely used in the production of 3d interconnected devices due to the lack of interlayer. © 2014 The Electrochemical Society. [DOI: 10.1149/2.007403jss] All rights reserved.Manuscript submitted June 5, 2013; revised manuscript received December 9, 2013. Published February 3, 2014 This review covers the key papers relating to the theory, techniques and quantification of hydrophilically activated silicon wafer bonding. This begins with a review of the history and development of the art. Bond strength characterization is then reviewed followed first by models of physical deformation and then by plasma and radical activation techniques.In the interests of clarity and succinctness this review is not an attempt to exhaustively catalog all direct bonding wafer literature. Excluded are hydrophobic and UHV bonding, and the many papers applying bonding techniques to differing combinations of wafer materials. This paper concludes with a summery of the state of the art of direct wafer bonding and a summation of the current wafer-bonding model derived from the papers reviewed. History and Establishment of the Art

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Infra-red video sequences were taken of directly bonded silicon wafer pairs undergoing the razor blade crack length test for bond strength in a specially designed jig. A series of tests were carried out under controlled atmospheres of nitrogen at various relative humidities. Analysis of the video images showed that the crack continues to propagate rapidly for several minutes after the blade has stopped moving, and that the presence of moisture has a strong positive influence on the rate of crack propagation under static loading. A new Maszara protcol is suggested based on modelling crack growth using our experimentally derived constants.

A new method of exposing silicon/semiconductor wafers to a mixture of radicals is described, in which these species are generated in an oxygen-rich gas discharge confined between a concentric pair of annular mesh electrodes surrounding the wafers. This approach allows the wafer surfaces to be treated without damage from the energetic ions, strong electric fields, and high UV fluxes associated with direct treatment by exposure to gas discharge plasmas. The process is compared with direct oxygen plasma activation for its latitude with respect to treatment duration, effect on wafer surface roughness and bond strength. Wider process latitude and reduced surface roughening are obtained for treatment by radicals compared with direct plasma exposure. Comparative analysis of treated and untreated silicon surfaces by X-ray photoelectron spectroscopy indicate that traces of fluorine present on the wafer surface before treatment are removed with great efficiency by the process.