Advances in high precision sample positioning stages for electron-beam microfabrication and metrology

Abstract

An improved computer controlled X–Y stage has been built for electron-beam microfabrication and metrology. The stage servo drives and the laser stage position sensors are mounted directly to the stage vacuum chamber to provide maximum rigidity. An ’’L’’ shaped chromium carbide mirror provides a reference with 1/4 arc-s accuracy for the stage position sensing laser, while allowing the sample to be in the plane of the laser beams to eliminate position sensing errors due to pitch and yaw of the stage. A reference chip is mounted on the stage in the plane of the sample to aid in initial focus and registration. For precision manual control of the stage a digital joystick is used. Nonlinear shaping networks on a standard linear taper joystick enable the operator to easily position the stage to 0.01 μm and still be able to move at full slew speed (1 cm/s). For each axis, V–F converters are used to convert the analog output of the shaping network to a digital pulse train which feeds a 24 bit closed loop servosystem. A digital tach is used for damping. Rate feedback is derived from the laser interferometer system rather than the conventional generator coupled to the servomotor. High torque motors are used for improved acceleration and dynamic braking. Servomotor-to-stage-drive coupling incorporates a friction lead screw to eliminate backlash. Under computer control, repeatability of positioning has been determined to be 0.01 μm. Accuracy is 0.01 μm/cm plus 0.005 μm/cm of stage travel (laser interferometer error).