Fundamental concepts about Lithography

https://dunham.ece.uw.edu/ee528/notes/Chapter5.pdf Text: Silicon VLSI technology fundamentals, practice, and modeling

Aerial image is the pattern of optical radiation striking the top of the resist

Latent image is the 3D replica produced by chemical processes in the resist

在这里插入图片描述

Ray tracing (assuming light travels in straight lines) works well as long as the dimensions are large compared to lamada At smaller dimensions, diffraction effects dominate

在这里插入图片描述

If we want to image the aperture on an image plane (resist), we can collect the light using a lens and focus it on the image plane 如果我们希望将 aperture 的pattern 在 image plane(reisit) 上成像，我们可以用 Collimating Lens 光照 pattern, 再用 Focusing Lens 收集光成像

在这里插入图片描述

Diffraction is usually described in terms of two limiting cases Fresnel diffraction - near field. 菲涅耳衍射 Fraunhofer diffraction - far field. 弗朗霍菲衍射

Light travels as an electromagnetic wave through space. when one is intergested in the behavior of an optical system in which all of the dimensions are very large compared to the wavelength of light, the light can usually be treated as particales traveling in straight lines between the optical components. This simplifies the problem to one of " ray tracing".

In projection lithography systems, for example, this approach can often be used in describing the optical source and condenser lens, but it fails when the light passes through the mask because the feature dimensions on the mask are comparable to the wavelength of the light. To understand the behavior of the light as it passes through the mask, the objective lens, and on to the wafer, we must include the wave nature of light. The single most important effect that we must account for is diffraction.

Diffraction effects occur because light does not in fact travel in straight lines. when the aperture is small, the light pattern which strikes the screen(image plane） covers a much larger area than can be accounted for by simply drawing straight lines(ray tracing) to describe the light propagation. In fact the smaller the aperture becomes, the more spread out the screen image becomes. This is very much like the situation we find in modern lighography where light passes through a mask with apertures(clear regions) with dimensions on the order of the light wavelength.

Huygens-Fresnel principle states that every unobstructed point of a wavefront at a given instant in time acts as a source of a spherical secondary wavelet of the same frequency as the primary wave. The amplitude of the optical field can be found by superimpoing all these wavelets considering their relative magnitdues and phases.

Projection Systems (Fraunhofer Diffraction)

在这里插入图片描述

NA represents the ability of the lens to collect diffracted light

在这里插入图片描述

modulation transfer function or MTF, defined as shown below

在这里插入图片描述

spatial coherence of the light source

在这里插入图片描述

Contact and Proximity Systems (Fresnel Diffraction) • Contact printing systems operate in the near field or Fresnel diffraction regime. • There is always some gap g between the mask and resist

在这里插入图片描述

Summary of wafer printing systems

在这里插入图片描述

在这里插入图片描述

在这里插入图片描述