Enhanced high-speed logarithmic photo-detector for spot scanning system

摘要

Disclosed are apparatus and methods for inspecting or measuring a specimen. An incident beam is directed across a plurality of consecutive scan portions of a specimen so that an output beam profile from each scan portion is consecutively collected by a photomultiplier tube (PMT), and the scan portions include at least one or more first scan portions and a next scan portion that is scanned after the one or more first scan portions. After or while the incident beam is directed to the one or more first scan portions of the specimen, an output signal for each first scan portion is obtained based on the output beam profile that is collected by the PMT for each first scan portion. An expected output beam profile for the next scan portion is determined based on the output signal that is obtained for each one or more first scan portions. As the incident beam is directed towards the next scan portion, a gain input to the PMT for the next scan portion is set based on the expected output beam profile so that the gain for such next scan portion results in a measured signal at the PMT that is within a predefined specification of the PMT or other hardware components that receive a measured signal from the PMT.

主权项

1. A method of inspecting or measuring a specimen, the method comprising:
directing an incident beam across a plurality of consecutive scan lines of a specimen so that an output beam profile from each scan line is consecutively collected by a photomultiplier tube (PMT), wherein the scan lines include one or more first scan lines and a next scan line that is scanned after the one or more first scan lines; after or while the incident beam is directed to the one or more first scan lines of the specimen and prior to the incident beam being directed to the next scan line, receiving data for the one or more output beam profiles for the one or more first scan lines from the PMT into a line buffer for holding data for the entire one or more first scan lines; prior to the incident beam being directed to the next scan line, determining an expected output beam profile for the next scan line based on data for the one or more output beam profiles that are received into the line buffer for the one or more first scan lines, wherein the expected output beam profile is determined to have a substantially same value or a same rate of gain increase or decrease as the one or more output beam profiles for the one or more first scan lines; and as the incident beam is directed towards the next scan line, setting a gain input to the PMT for the next scan line based on the expected output beam profile so that the gain for such next scan line results in a measured signal at the PMT that is within a predefined specification of the PMT or other hardware components that receive a measured signal from the PMT.