![]() ![]() A residual stress can alter the crystal lattice of the semiconductor material thereby changing the electronic energy bands and affecting the electron and hole mobilities. Residual stresses also impact the behavior of electronic devices. A tensile residual stress results in the element exhibiting a higher mechanical stiffness than otherwise, while a compressive residual stress will lower the stiffness and can also result in Euler buckling of the element. For example, a mechanically compliant element of a MNS device behaves differently if a significant residual stress is present in the material layer(s) composing the device as shown in a later section. The impact of residual stresses on MNS device behavior, performance, workability and reliability can be very large. ![]() There a several reasons for this, including: many MNS devices employ one or more thin-film material layers as mechanically or electro-mechanically functional layers in the design the performance of MNS devices having mechanical or electro-mechanical functionality can be significantly impacted by the values of the residual stresses in these layers the behavior of purely electronic devices are also impacted by residual stresses, increasingly so as the device critical dimensions are scaled downwards the residual stresses in thin-film layers can vary over very large ranges of values depending on the specific processing conditions used during deposition and many MNS devices employ stacks of thin-film layers thereby involving complex materials systems wherein each of the materials has a different residual stress value. The material property of deposited thin-film layers used in the manufacturing of micro- and nano-systems (MNS) (The abbreviation “MNS” is used in this text for “micro- and nano-systems.”) often having the considerable interest is the residual stress. ![]() Methods which can be used to control the stresses and mitigate the impact of residual stresses in micro- and nano-systems device design and fabrication are then covered, followed by some recent development of interest. A review of some of the literature to illustrate the level of variations in residual stresses depending on processing conditions is then provided. Then, a review of the reported methods used to measure residual stresses in thin-films are described. This is followed by a review of thin-film deposition methods outlining the process parameters known to affect the resultant residual stress in the deposited layers. The outline of this paper is as follows: a discussion of the origins of residual stresses in deposited thin-film layers is given, followed by an example demonstrating the impact on device behavior. The impact of this is micro- and nano-systems device development takes longer, is considerably more expensive, and presents higher risk levels. Obviously, this is a far less than ideal situation. Hence, device designers usually do not have sufficient information about the residual stresses values when they perform the device design. As a consequence, there is still no generally applicable theory to predict residual stresses in thin-films. The origins of residual stress can involve a number of complex and interrelated factors. Residual stresses in deposited layers are known to be highly dependent on a number of factors including: processing conditions used during the deposition type of material system (thin-films and substrate materials) and other processing steps performed after the thin-film layer has been deposited, particularly those involving exposure to elevated temperatures. It is not uncommon for residual stresses in deposited thin-film layers to vary over extremely large ranges of values (100% percent or more) and even exhibit changes in the sign of the stress state. While many material properties of deposited thin-film layers are dependent on the specific processing conditions, the residual stress often exhibits the most variability. A residual stress is defined as the presence of a state of stress in a thin-film material layer without any externally applied forces wherein the residual stress can be compressive or tensile. This review paper covers a topic of significant importance in micro- and nano-systems development and manufacturing, specifically the residual stresses in deposited thin-film material layers and methods to control or mitigate their impact on device behavior. ![]()
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