Integration of Melting Excimer Laser Annealing in Power MOS Technology

The integration of excimer laser annealing (ELA) into the power MOS device technology has been studied and evaluated. The integration issues include patterning effect, extreme nonequilibrium kinetics of dopant and defects, material modification due to the melting-regrowth phenomena (in the melting regime), and residual implant damage. We demonstrated that ELA can be applied as a reliable, effective, and advantageous process in the context of semiconductor device fabrication. In particular, power MOS field-effect transistors were successfully fabricated with superior electrical characteristics than those fabricated according to the standard process. Optimization of the process was achieved through extensive characterization analyses, while an intense research effort was dedicated to the development of a technology computer-aided design tool for the simulation of the laser annealing process in Si-based devices. The electrical characterization of the transistor fabricated by ELA is presented, showing a device yield of 90% on wafer.

I. Introduction and State of the Art

Scaling the CMOS technology node below 90 nm will require source/drain (S/D) junction depths that are shallower than 50 nm [1]. Ultrashallow electrically active layers can be formed in Si through a combined process of ultralow energy ion implantation and high-ramp-rate (400°C/s) short-time ( 1 s) high-temperature “spike” annealing [2]. Although the ions are implanted at low energy and their range is very shallow, nonequilibrium diffusion can lead to increased diffusivity during postimplantation annealing (transient-enhanced diffusion) [3], seriously limiting the minimum junction depth. Excimer laser annealing (ELA) in the melting regime of ion-implanted Si [4], [5] has recently attracted renewed interest within the semiconductor community for its possible application to the formation of ultrashallow junctions in Si [6]–[9]. The technique also offers many advantages compared to other nonmelting-laser-based methods, such as exact control over the junction depth, higher dopant activation, and profile abruptness. In fact, when irradiating Si by laser light with sufficient energy density, a well-defined melted zone, with sharp transition from liquid to crystal phase, is formed. The diffusivity of dopants is raised in the liquid state (, [4]), and the dopants are able to redistribute uniformly within the melted layer, giving rise to boxlike profiles after regrowth. Due to the steep thermal gradient between the liquid and solid phases, immediately after irradiation, the liquid-crystal interface advances toward the surface at a rate of 3 m/s [6]. As a result of such rapid solidification (low-temperature solid-phase regrowth is typically at 550 °C), less dopant is segregated into the liquid phase at the liquid-crystal interface, and enhanced dopant trapping occurs. The fraction of the implant dose, which is retained within the semiconductor during ELA, is governed by segregation (during regrowth), evaporation (during the melted phase), and ablation (during energy deposition). The electrical activation of the retained dopant in the regrown layer following ELA is eventually limited by morphological instability at the liquid-crystal interface during regrowth, lattice strain, and thermodynamic limit [10].