Exposure stage

The exposure of an entire wafer necessarily involves the movement of the beam over the entire area. The deflection of the beam by the electromagnetic or electrostatic scan coils is not without errors because of the inherent limits to how far these coils can accurately and precisely deflect the beam. Good pattern quality requires the edge gradient of the electron-beam profile, the distortion of the exposed pattern, and the positional stability of the beam to be held below a small fraction of the minimum feature size. These considerations mandate that the size of the scan field be limited to a few millimeters at most and necessitate mechanical exposure stages to move the substrate through the deflection field of the electron-beam column.  

Bom and E. Wolf, Principles of Optics, 4th ed., p. 559, Pergamon Press, New York (1970) P. Givet, Electron Optics, Pergamon Press, Oxford, (1965) H.C. Pfeiffer, Variable spot shaping for electron beam lithography, J. Vac. Sci. Technol. 15(3), 887 (1978). 

Bowden, The hthographic process the physics, in Introduction to Microlithography, L.F. Thompson, C.G. Willson, and M.J. Bowden, Eds., p. 113, American Chemical Society, Washington, DC (1994). 

An alternative method of operating the exposure stage in a continuous mode is one in which the pattern is written on the substrate while the stage is moving. This strategy was developed at Bell Laboratories in the 1970s and is employed today on all MEBES mask-making machines. 

The maximum dose delivered by the exposure tool per unit dwell time defines the resist sensitivity required to maximize the throughput of the tool.  

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UV light treatment (circles) and without treatment (triangles). For the hybrid film with UV-light treatment, a sharp growth of diffraction efficiency was observed at an early exposure stage (circles). The diffraction efficiency reached a maximum of 1.1% in 250 mj cm", corresponding to ca. 5.0 s exposure at 50 mW cm". Further exposure reduced the efficiency to give a constant value around 0.5%. 

Lymphocytopenia Symptoms are anorexia, nausea and vomiting. Onset occurs 1 h to 2 days after exposure. Stage lasts for minutes to days... 

Nausea, vomiting, watery diarrhea, burning sensation of skin, disorientation, fever, extreme nervousness, confusion, impairment of cognitive function prostration, hypotension, ataxia and convulsions onset within minutes of exposure stage lasts for minutes to hours... 

Today, interferometers are used widely in the alignment system of lithographic exposure tools for coordinating the movement of the exposure stage. In particular, they have found application in high-precision measurements of extremely small distances, between different objects in the exposure tool. Furthermore, interferometric lithography owes its very existence to the Michelson-Morley experiment. 

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. 

Figure 15.1 shows a schematic diagram of an EBL exposure system, which consists of three main subsystems, namely, the electron source (gun), electron optical column (heam-forming system), and exposure stage. A computer is used to control the various machine subsystems and transfer pattern information to the beam deflection coils. 

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