Geometric Compensation of Focused Ion Beam Machining Using Image Processing

Abstract

During the process of focused ion beam (FIB) machining, the redeposition of the sputtered material during machining decreases the geometric accuracy of the process. In this paper, a new approach to reducing the geometric error in FIB machining is introduced. The new algorithm measures the amount of redeposited material after each production cycle and modifies the next process plan. Information on the amount and shape of the geometric error is determined using an image processing method. The geometric error compensation method is applied to a real FIB, and the experimental results demonstrate considerable improvement over uncompensated milling. S ince microcomponents are widely used in a variety of applications, their accuracy after production is important. As a result, manufacturing processes capable of producing microfeatures in patterns and threedimensional (3D) structures have been studied by many researchers. Focused ion beam (FIB) machining is a micromachining process that has nanometer-scale accuracies and can be used for microfabrication applications. The FIB, which was originally developed for microscopy and utilizes accelerated metal ions, is also useful for microfabrication applications because of its milling capability. 1,2) FIB milling is a material removal process that sputters using accelerated metal ions (mostly Ga þ ). During sputtering, atoms from a solid surface are removed by the bombardment of energetic particles. The momentum of the incident atom is transferred to the atoms on the target material surface. This momentum transfer leads to the ejection of surface atoms, 3) thus generating the desired topography or surface pattern.In the FIB milling process, an ion beam with a diameter of less than a few hundred nanometers is precisely controlled and follows a predesigned path. The energy of the incident ion beam is also controlled to remove a specific amount of material. However, FIB milling methods have limited shape accuracy. 4) The shape of a machined surface is determined not only by the ion beam energy, but also by other aspects of the machining process. Three dominant factors that affect the shape accuracy of machined surfaces are: (1) the Gaussian profile of the beam energy density, (2) the variation in sputtering yield with the ion beam incident angle, and (3) the redeposition of sputtered particles onto the machined substrate. 5) As the milling process progresses, the factors listed above cause distortions in the intermediate shapes of an FIB-milled surface. Because the energy distribution of the ion beam follows a Gaussian distribution, the cross-section of the machined surface becomes Gaussian shaped. The Gaussianshaped texture and the 3D features of the machined surface result in an uneven sputter yield due to the variation in the ion beam's incident angle. Furthermore, the sputtered particles reattach to the machined surface, which is called redeposition.When the machined feature size is large relative to the ion beam spot size, redeposition is the predominant cause of geometric err...