Gas-assisted etching with focused ion beam technology

The role of focused ion beam (FIB) systems in device failure analysis has been well documented 1 . FIB etching is used to cross-section features and to prepare TEM samples with better than 0.1 micron (μm) accuracy 2 , to open probe holes for mechanical and electron beam probing of circuitry, and to rewire circuits by cutting or adding connections. FIB is used to image cross sections with either secondary ions or secondary electrons, to measure metal grain size distributions through channeling contrast, and to measure process control parameters. System limitations include a relatively slow removal rate by beam sputtering of large volumes of material. Also, redeposition of sputtered material on the sidewalls of holes limits the achievable sidewall angle, and thus limits the aspect ratios of holes to approximately 6:1. With decreasing geometry sizes and multiple planarized metal layers, device modifications require cuts and interconnect holes of up to 10:1 aspect ratio, which up to now have been difficult with FIB. We will discuss recent progress in using gas-assisted etching (GAE) to enhance the FIB etching, to sharpen the etch profile, and to etch high aspect ratio holes. The hardware and GAE process will be briefly described. We will discuss the relative etch rates and selectivity of GAE with typical device materials, such as aluminum (Al), tungsten (W), silicon dioxide (SiO 2 ) and silicon (Si), using two halogen-based etchant gases, xenon difluoride (XeF 2 ) and chlorine (Cl 2 ). Images of sidewall profiles with and without GAE will be compared, and we will demonstrate several applications of GAE for e-beam and optical analysis.