Substrate Condition and Metrology Considerations in Poly Gate Doping Implants

The evolution from planar to 3D structures in advanced memory devices has resulted in semiconductor equipment manufacturers facing unprecedented challenges in delivering products that can demonstrate simultaneous compliance to the productivity, reliability and process requirements of their customers. In the field of ion implantation, these challenges are driven by: (i) the increasing prevalence of hard mask and removal of PR stripping process and (ii) the transition from the use of implants in dopant application to that of materials modification. These have resulted in large reductions in both the particle size and number density that can be tolerated from implant steps.One area where these issues have proven challenging is that of contact engineering. Low energy phosphorus implants are used to improve the contact resistivity of poly Si contact. This is critical for the read/write time of the storage node capacitor in DRAM operation. As devices shrink further, the thickness of the poly gate in the peripheral transistors become as low as a few hundred Å. This results in a phosphorus implant requirement of ~1keV. Depletion in the poly Si gate requires a few keV implant energy for poly doping for both NMOS and PMOS. In order to maintain proper gate operation, gate doping requires around E15 doses. This places a large amount of implanted phosphorus at or near the surface of the wafer.In this paper, a phenomenon is described where the magnitude of surface particles arising from phosphorus implants is a function of the reaction between implanted phosphorus and ambient atmosphere. Using SEM/EDX, spatial and morphological descriptors of the defect types arising from these reactions have been classified. The implications of these results will be discussed from both a process perspective, in terms of accumulated dose and implant energy, and a time-based effect whereby defects grow over time both in number and size. Mitigation paths for particle metrology are proposed, and guidelines for Fab operators in terms of material storage and particle monitoring protocols described, in particular the criticality of time to measurement and maximum implanted dose.