Application of transmission electron detection to x-ray mask calibrations and inspection

Abstract

Masks used for the manufacture of integrated circuits by x-ray lithography can be calibrated and inspected in a scanning electron microscope by using the transmitted electron detection mode. By their nature, these masks present a measurement subject unique from most (if not all) other objects in semiconductor processing because the support membrane is, by design, x-ray transparent. This characteristic can be used as an advantage in electron beam- based mask metrology since, depending upon the incident electron beam voltages, substrate composition and substrate thickness, the membrane can also be essentially electron transparent. The areas of the mask where the absorber structures are located are essentially x-ray opaque as well as electron opaque. Viewing the sample from a perspective below an x-ray mask can provide excellent electron signal contrast (depending upon the instrument conditions) between the absorber structure and the membrane. Thus, the mask can be viewed in the transmitted electron detection mode of the scanning electron microscope, and precise and potentially accurate dimensional measurements can be made. One unique advantage to this is that in the transmitted electron detection mode, the modeling of the electron beam\specimen interaction becomes far less difficult than in the modeling of typical secondary electron images of opaque objects. The inelastically scattered beam electrons and the low- energy secondary electrons can be excluded from the detector and, therefore, need not be accurately modeled. Therefore, absorber structure width (linewidth) measurement standards can be potentially calibrated with less difficulty and higher accuracy than standards calibrated by more conventional means. The transmitted electron detection mode is also useful, because of the high contrast of the image, for the determination of mask defects and high- density particle detection as well as for registration measurements.