Paper PS1-MoA1
Reduction of Plasma Induced Silicon-Recess During Gate Over-Etch Using Synchronous Pulsed Plasmas

Monday, October 18, 2010, 2:00 pm, Room Aztec

With the downscaling of CMOS devices in semiconductor industry, very thin layers (<1.5nm) are now introduced in transistor gate stacks. Integrating such thin layers presents tremendous challenges, particularly for the etch processes which have to be stopped selectively without inducing damage to the thin materials below. For instance, bulk silicon may be oxidized during the gate over-etch step through the thin gate oxide, which leads to silicon recess during the subsequent wet cleanings. In this contribution, we will precise the mechanisms of silicon oxidation through the thin gate oxide, and we will propose solutions to minimize this phenomenon by pulsing the plasma.

The experiments are performed on a state of the art 300mm AdvantEdge^TM etch reactor equipped with the Pulsync^TM system which provides full plasma pulsing capabilities at frequencies between 100 Hz and 20 kHz, with duty cycles between 10 and 90 %. In-situ spectroscopic ellipsometry is used to determine etch rates on thick silicon oxide and polysilicon layers, and to investigate plasma induced oxidation through a 2.5nm thin silicon oxide coated on bulk silicon. An angle resolved XPS system connected to the reactor allows quasi in-situ surface characterizations.

We show that an infinite selectivity of polysilicon over SiO₂ is obtained using an HBr/O₂/Ar gate over etch process on thick layers. However, when a thin layer of silicon oxide is exposed to the same process, the thin oxide layer thickness increases with the plasma exposure time. This thickness increase is related to plasma induced oxidation through the thin gate oxide. XPS analysis show that a Si-Br_x interface layer builds up between SiO₂ and Si, and that some bromine is incorporated in the oxide. This suggests that bromine implantation through the SiO₂ layer may generate a path in the oxide layer facilitating the oxygen and water diffusion (from the plasma or from the atmosphere) down to the SiO₂/Si interface

We show that plasma induced oxidation can be minimized by using synchronous pulsed plasmas. This way, we move from a highly dissociated plasma to a highly recombined plasma. As a consequence, radicals are larger and less prone to diffuse, and ions are molecular rather than atomic, which decreases the net energy of their components. Hence, bromine incorporation is highly limited and no Si-Br_x interface layer is created, which minimizes silicon oxidation through the thin gate oxide.

These experiments clarify the mechanisms of plasma induced oxidation through the thin gate oxide, and show the promises of synchronous pulsed plasmas to reduce silicon recess.