In situ monitoring and control of material growth for high resolution electron beam induced deposition

During electron beam induced deposition on electron transparent membranes, the transmitted annular dark field (ADF) signal can be monitored. A method was developed to use the ADF signal to obtain insight into the growth process and to control the mass of individual nanometer-sized deposits. Arbitrary two-dimensional patterns can be defined. The smallest sampling time of the ADF signal monitoring is presently about 40ms. For arrays of dots that were deposited, the growth of each individual dot was monitored. It is observed that the growth is different for each dot, although the average deposit growth rate is linear with the dwell time. Apart from monitoring the ADF signal during the growth, the amount of deposited mass can be controlled for individual deposits by terminating the growth process when the ADF signal exceeds a threshold value. The dynamic ADF feedback control was applied to reduce variations in deposit mass. This attempt did not succeed, but the method was successfully applied to prevent the occurrence of a proximity effect. When the electron beam irradiates the side of an already existing structure, the amount of deposited material is higher than if the electron beam irradiates an area that is under normal incidence. With the dynamic ADF feedback control, this effect can be compensated in situ and the amount of deposited material that is probed by the beam is constant regardless of the local growth rate. The mass deposition resolution of the feedback system is estimated by assuming a volume and a density of the deposits. It is estimated that the ultimate mass resolution is a single molecule.