Nanometer level autofocus air gauge.

Abstract A noninvasive, pneumatic, low-flow, differential, hot-wire, top-surface sensing device is described and demonstrated for use in the Etec ALTA® mask and reticle writer. This apparatus, a nanometer-level autofocus air gauge, is capable of sensing the distance from the final objective lens to the top surface of the photoresist with greater than 0.1 μm accuracy, 100-Hz bandwidth, and less than 10 nm of peak-to-peak noise. It can do this quite independent of the resist’s surface or bulk properties, such as the dielectric constant. The engineering requirements and efficacy of hot-wire sensing are developed and implemented. The theory, in general, for modeling hot wires in this device, as well as the fundamental time constants, transmission line effects, and constraints, are discussed. The necessary electronic circuitry for creating constant temperature bridges using hot wires and running them differentially is also developed and implemented. Results and data are included.