Evaluation of backside particle contamination and electrostatic chuck design on the cleanliness of EUV reticle mask blanks in a multilayer Mo/Si ion beam deposition system

A key requirement for the success of EUV lithography is a high volume supply of defect-free Mo/Si multilayer (ML)- coated mask blanks. The process of fabricating mask blanks is particularly sensitive to particle contamination because decoration by the deposition of the reflective stack on sub-lithographic (< 22 nm) particles can create larger, printable defects. One possible source of added defects is the mask substrate fixturing method, which, in the Veeco ion beam deposition (IBD) system used to deposit our ML coatings, must allow tilt and rotation of a vertically oriented substrate. As commonly practiced, an electrostatic chuck (ESC) is used instead of a mechanical clamping fixture to avoid transferring particles to the front surface of the mask by mechanical clamping and declamping operations. However, a large number of particles can be introduced to the backside of the mask by electrostatic clamping. Up to now, there has been little concern about such backside particles, except for relatively large particles (> 1 micron) that may affect out-of-plane distortion of the mask in an EUV lithography tool. As the cleanliness of the EUV masks and mask blank fabrication approaches perfection, however, there is more concern that particles transferred from the backside to the frontside of the mask may be a significant issue. Such transfer may occur in the deposition chamber, in the substrate cassette, or in the transfer module and may be indirect. In this paper, we present data from characterizing the amount, size, shape, composition, and location of the backside particle defects generated by electrostatic clamping, using a particle counter and scanning electron microscope (SEM), and compare results for a pin-type e-chuck, which has a small contact area, with the standard flat e-chuck. The key result is a 10X to 30X reduction in the total number of backside particles for the pin chuck. Also, preliminary data indicates that the pin chuck stays cleaner under service conditions than the flat chuck. The exact elemental composition of the defects is sensitive to the clamping method and type of backside Cr coating. In general, for the flat chuck, Al defects, attributed to particles from the alumina chuck surface, are dominant. For the pin chuck, Si,Cr,N,O defects from the mask surface are mainly observed.