REQUIREMENTS AND PERFORMANCE OF AN ELECTRON-BEAM COLUMN DESIGNED FOR X-RAY MASK INSPECTION

Production viable inspection of x-ray masks requires the resolution of a scanning electron microscope (SEM) at an imaging rate approximately 1000 times that of commercially available systems. This article analyzes the inspection task for x-ray masks and compares the requirements for beam-current density and imaging efficiency necessary to achieve reasonable throughput with technologies available on current SEM and electron-beam lithography systems. The resulting specifications have been translated into an electron-optical column with many novel features. The gun, which contains six thermal field-emission sources mounted on a turret, produces a Gaussian profile beam that is scanned over a continuously moving substrate. Dual electrostatic icosapole deflectors provide high speed telecentric deflection. Secondary electrons are separated from the primary beam by a Wien filter and accelerated into a semiconductor electron detector. An analog optical-fiber link is used to transmit the signal to the image computer for defect detection. Results are presented for the performance of the electron-optical column and imaging system and for the overall defect detection performance of SEMSpec, a dedicated production x-ray mask inspection system.