Mask-wafer contact, defects

Contact or proximity printing is no longer widely used because of defects caused by mask/wafer contact. It is still used in non-critical situations when such defects can be tolerated, or when the resolution required is low enough to allow an adequate gap to be left between mask and wafer. 

Unfortunately, contact printing was plagued by a number of practical and technical difficulties. Bringing the mask and the wafer into direct contact causes damage to the mask and creates defects, which are reproduced in subsequent exposures. In addition, nonflatness of the wafer and mask results in alignment... 

In proximity printing spacing the mask away from the substrate minimizes defects that result from contact, but increases the diffraction of the transmitted light which reduces the resolution. The degree of reduction in resolution and image distortion depends on the mask to substrate distance which may vary across the wafer. For proximity printing, equation (3.1) can be rewritten as... 

Particle Ramoval (Semiconductor Process Fluids). The fluids that come in contact with photo masks and wafers during integrated circuit (IC) production are often contaminated with particles which, if not eliminated, can cause defects and reduce production yields. 

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. 

However, due to the defectivity problem associated with contact printing, as discussed above, the first noncontact aligners, called proximity aligners, were introduced in 1973 by Kasper Instruments. In proximity printing mode, the mask does not come into direct physical contact with the wafer [see Fig. 13.24(a)]. Rather, it is held 10-25 p.m above the upper surface of the resist-coated wafer. This gap, however, limited resolution on account of diffraction. The theoretical resolution limit of proximity printing is given by the expression... 

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