Standing wave effects

An additional complication associated with the standing wave effect occurs for the exposure of resist over topography. When a resist is spin-coated onto a substrate containing steps, the resist thickness varies from one area to another on the wafer. Since the standing wave effect is a strong function of resist thickness, exposure variations resulting from variation in resist thickness in the vicinity of the step result in changes in linewidth. 

Two component, positive photoresists (see Section 3.5.b) represent systems with unusual exposure characteristics caused by the standing wave effect (see Section 2.1.f) and "bleaching" or change in optical density during exposure (see Sections 3.5 and 3.9). Both of these phenomena result in nonlinear exposure throughout the thickness of the resist film, and result in uneven developing rates as a function of film thickness, making evaluation of these systems difficult. 

As discussed previously, an optional postexposure, predevelopment bake can reduce problems with the standing-wave effect in DNQ-novolac positive resists. However, such a postexposure bake step is indispensable in the image reversal of positive resists (37-41) and certain resists based on chemical amplification of a photogenerated catalyst (64-67, 77, 78). For both types of resists, the chemistry that differentiates between exposed and unexposed areas does not occur solely during irradiation. Instead, differentiation occurs predominantly during a subsequent bake. Therefore, to obtain acceptable CD control in these systems, the bake conditions must be carefully optimized and monitored. 

Figure 3.59). They also showed that standing wave effects, though not totally absent, were barely noticeable despite the quasimonochromaticity of the excimer laser radiation. 

Walker, Reduction of photoresist standing wave effects by post exposure bake, IEEE Trans. Electron Dev. ED-22(7), 464 466 (1975). 

M.A. O Reilly, Y. Huang, K. Hynynen, The impact of standing wave effects on transcranial focused ultrasound disruption of the blood-brain barrier in a rat model, Phys. Med. Biol. 55 (18) (2010) 5251-5267. 

Absolute power measurements are fairly difficult to perform in the (sub)mm part of the spectrum, because of uncertainties in the beam propagation, because of diffraction effects, and because of poor matching to detectors, which results in standing-wave effects and unterminated reflectances. Only a handful of researchers have addressed this problem, so their efforts can be summarized briefly. 

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