Scatterometry measurement of the exposure focal length of photolithography tools

Since the late 2000s with the advent of 45nm CMOS nodes, the control of critical dimensions (CD) of the structures in the photolithography stage has become critical to the reliability of printed circuit boards. Optical photolithography remains the most economical and widespread technique for high volume production in the semiconductor industry. For this type of equipment, manufacturers have focused on increasing the numerical aperture of the exposure lens, reducing sources of optical aberrations and on metrology to ensure efficient monitoring of their machines. These developments were possible at the expense of the depth of field of the exposure. To avoid altering the images transferred to the photosensitive resins, and ultimately have a device failure, it is essential to give a value as accurate and precise as possible of this size. To meet the growing needs of process control and lithography tools required by the most advanced technologies, metrology techniques based on analysis of reflected signals are massively used. Although this methodology is well suited to current CMOS technologies (CMOS14nm and earlier), it is unlikely to address more advanced technologies, so other techniques must emerge, such as techniques based on the analysis of the diffracted signal (scatterometry).