Measuring instrument and laser beam machine for wafer

A measuring instrument for a wafer for measuring the thickness of a wafer held on a chuck table, comprising:

• a laser beam oscillator for irradiating a laser beam having wavelengths which pass through and are reflected by a wafer;
• a condenser for condensing the laser beam oscillated from said laser beam oscillator and irradiating the laser beam on a wafer held on said chuck table;
• light reception means for receiving reflected light of the laser beam irradiated upon the wafer held on said chuck table;
• convergence light point changing means for changing the convergence light point of the laser beam condensed by said condenser;
• and
• control means for measuring the thickness of the wafer based on a change signal from said convergence light point changing means and a light reception signal from said light reception means;
• said convergence light point changing means including optical path length changing mirror means which includes a pair of mirrors disposed such that reflecting faces are opposed in a spaced relationship by a predetermined distance and extend in parallel to each other, an angle adjustment actuator for adjusting the installation angle of said mirrors, and an installation angle detection sensor for detecting the installation angle of said mirrors and outputting a detection signal to said control means;
• said control means including a memory for storing a thickness control map which defines a relationship between the difference of the two installation angles of said mirrors and the thickness of the wafer, said control means being operable to control said angle adjustment actuator to change the installation angle of said mirrors and detect two peaks of a light amount based on the reception signal from said light reception means, determine the difference between the two installation angles of said mirrors based on the detection signal from said installation angle detection sensor when the two peaks of the light amount are inputted and compare the installation angle difference with the thickness control map to determine the thickness of the wafer.