Images aerial

This is the intensity of the exposure radiation in the plane of the wafer. The extended source method, or Hopkins method,is often used to predict the aerial image of a partially coherent, diffraction-limited, low-numerical-aperture-aherrated projection system based on scalar diffraction theory. For very high NA, vector calculations involving the complete solution of Maxwell s equation are used. The illumination may be of a single wavelength or it may be broadband. The illumination source may be a conventional disk shape or other more complicated shapes as in off-axis illumination.  

---

Luft-bestandigkeit, /. stability in air, resistance to air. -bestandteil, m. constituent of air. -bild, n. aerial photograph (Optica) aerial image, -blaschen, n. (small) air bubble, -blase,/, air bubble air pocket air bladder, -bleiche, /. open-air bleaching, -bombe, /. aerial bomb, -brennstoffgemisch, n. fuel-air mixture. 

Contrast enhancement lithography is a clever procedure which uses a bleachable contrast enhancing layer to restore the distorted aerial image of the mask which has been blurred by diffraction effects into a sharp image at the underlying photoresist surface. The process is too complicated to explain in detail here and the interested reader is referred to the cited literature (60,61). Suffice it to say that the large extinction coefficients of most polysilane derivatives coupled with their ready bleachability make them ideally suited for such purposes and we have demonstrated this application at 313 nm (16,18). 

Figure 2.5. The intensity function or aerial image of a mask is ideally a square wave. However, projection optics operating near their diffraction limit degrade this square wave into a sinusoid with a small direct current dc) term. When this intensity function is imposed on the contrast-enhancement lithographic material, bleaching occurs most rapidly in the high-intensity areas such that the transmitted intensity function that exposes the resist is modified and thus leads to improved contrast.

The aerial image is the intensity distribution at the wafer plane and is the square of the magnitude of the electric field. ... 

The effects of aherrations on lithographic performance are determined more generally hy incorporating aherrations into the aerial image models. This is done hy modifying the pupil function of the lens to include phase error due to aherrations. ... 

Scalar models for calculating aerial image intensity... 

The scalar models for calculating aerial image intensity neglects the effects of polarization of light. This is a legitimate approximation for numerical apertures less than 0.7. Scalar models are the simplest of the aerial models and come in three main types the zero-order, first-order, and high-NA scalar models. 

This is referred to as the separability assumption in that the aerial image and the standing wave can be calculated independently and then multiplied together to yield the total intensity within the resist. ... 

Independently proposed hy Mack and Bernard, this model accounts for the effect of a changing aerial image focus (i.e., defocus effect) as it propagates... 

The logarithmic slope of the aerial image is typically called the log-slope, and it is a good metric of the quality of the latent image. It is customary to normalize the image log-slope hy multiplying it hy the nominal line width w of the feature to yield the normalized image log-slope (NILS) as... 

Defining a lumped parameter, the resist contrast y that relates the aerial image and the development rate r, one obtains the lithographic imaging equation... 

This equation shows how a gradient in aerial image intensity results in a solubility differential in the resist. The development rate contrast is maximized by a high resist contrast and a large log-slope of the aerial image. 

The radiation traversing the entire chain of illuminator and projection optics forms an aerial image of the mask that is projected onto the resist-coated wafers. A fast shutter within the illuminator assembly exposes the photoresist to the image for a very short period of time, on the order of a few tenths of a second. The integrated energy of each exposure must he repeatable to within 1%, such that the illumination of the reticle is fairly uniform (<+l%) over a large area, a feat it accomplishes with the aid of a number of methods, including the use the of fly s eyes and rods. " ... 

The exposure optics system comprises the set of optics (mirrors and lenses) that delivers the aerial image of the mask to the resist-coated wafer on the exposure stage. There are two categories of such systems. Contact and proximity optics systems are the simplest and the least expensive of the systems, but are not well suited for high-volume IC manufacture because of their high level of defectivity. The other system, the projection optics system, is predominantly used in modern semiconductor manufacturing. Both systems rely on the entire mask, or at least a portion of it, to be imaged simultaneously. 

The image intensity distribution, otherwise called the aerial image, is given by the square of the image amplitude ... 

Figure 13.34 Aerial image intensities for (a) large features [0.6(A/NA)] and (b) small features [0.4(A/NA)] with varying levels of partial coherence. (Reprinted with...

The NILS is usually the best single metric forjudging the lithographic usefulness of an aerial image. ... 

---