Smart Structures Technology As Part of Integrated Tool Management in Lithography Scanners

Moving standard deviation (MSD) values of under 10 nm for stage synchronization errors are beginning to be required by IC manufacturers around the world. This mandates multiple levels of management for the stage system in lithography tool. Furthermore, it is essential that these are incorporated into a single integrated tool management (ITM) system. In addition to the motion and trajectory control of the reticle and wafer stages, the stage support structures are actively stabilized in all six degrees of freedom (DOF). In order to minimize the internal disturbances in the tool often caused by large accelerations of the stages, several tool manufacturers employ actively controlled balancing masses. In addition, the main structural components of lithography tools are isolated from floor vibrations using increasingly sophisticated active isolation systems. While trajectory shaping, active reaction cancellation, and active isolation techniques greatly contribute to reducing the disturbances that reach the structural components of lithography tools, they do not eliminate them entirely. As a consequence, modal vibrations are excited in these structures in response to external and internal disturbances. Very low inherent damping in most structural components results in large dynamic amplification of that response. Often, resulting vibrations affect the stability of metrology system components supported by the structure, such as laser interferometer mirrors and beam splitters. This, in turn, limits the performance of the stage control system, resulting in higher MSD values. Interestingly, only low-tech, passive response attenuation solutions have found widespread application on lithography tools. These typically include tuned mass dampers (TMD) and visco-elastic components. Unfortunately, these solutions are not adequate for ultra-small feature lithography. Typical limitations of these methods include prohibitively high mass in the case of TMDs, and unacceptably high structural velocity values required by most visco-elastic components in order to dissipate enough energy. Additionally, these solutions offer no adaptive or tunable characteristics. ‘Smart structures’ is a term commonly applied to the active control of dynamic response of structural components using embedded actuators and sensors, as well as control electronics. Often the actuators used in smart structures applications are piezoelectric components used either in induced-strain or linear actuator configurations. These actuators offer low volume and mass, thus minimizing their effect on the controlled structure open-loop behavior. Further, these actuators can affect no rigid body motions, thus rendering them incapable of negatively impacting the stage motion control system. A properly designed smart structure control system can be effectively used to attenuate the modal response of selected structural components in lithography tools. This attenuation can be targeted at specific locations on the structure (such as supporting critical components of the metrology system), specific frequencies, and specific modes. Most importantly, perhaps, the application of smart structures technology for response attenuation allows for all the benefits of other active systems used in the tool to be brought to bear. Thus response attenuation systems can be made adaptive, tunable, and capable of communicating with the host and with other active systems used in the lithography tool. This paper presents a possible architecture for including smart structure technology in the context of advanced lithography tool management as the last missing link in the chain of active isolation, active reaction cancellation, and stage positioning and trajectory control.