A stress-concentrating spacer structure is a stack of an upper gate spacer
with a low Young's modulus and a lower gate spacer with a high Young's
modulus. The stacked spacer structure surrounds the gate electrode. The
stress-concentrating spacer structure may contact an inner gate spacer
that contacts the gate electrode or may directly contact the gate
electrode. The upper gate spacer deforms substantially more than the
lower gate spacer. The stress generated by the stress liner is thus
transmitted primarily through the lower gate spacer to the gate electrode
and subsequently to the channel of the MOSFET. The efficiency of the
transmission of the stress from the stress liner to the channel is thus
enhanced compared to conventional MOSFETs structure with a vertically
uniform composition within a spacer.