An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, which sets up a transverse electric field in the channel in order to modulate the longitudinal conductance of the channel.
In typical IGFET processing, the source and drain are formed by introducing dopants of second conductivity type (P or N) into a semiconductor substrate of first conductivity type (N or P) using a patterned gate as a mask. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide a gate which provides a mask during formation of the source and drain by ion implantation. Thereafter, a drive-in step is applied to repair crystalline damage and to drive-in and activate the implanted dopant.
There is a desire to reduce the dimensions of the IGFET. The impetus for device reduction comes from two directions. One is the desire to increase the number of individual IGFETs that can be placed onto a single silicon chip or die. More IGFETs on a single chip leads to increased functionality. A second desire is to improve performance, and particularly the speed, of the IGFET transistors. Increased speed allows for a greater number of operations to be performed in less time. IGFETs are used in great quantity in computers where the push to obtain higher operation cycle speeds demands faster IGFET performance.
One method to increase the speed of an IGFET is to reduce the length of the conduction channel underneath the gate and gate oxide regions. However, as IGFET dimensions are reduced and the supply voltage remains constant (e.g., 3 V), the electric field in the channel near the drain tends to increase. If the electric field becomes strong enough, it can give rise to so-called hot-carrier effects. For instance, hot electrons can overcome the potential energy barrier between the substrate and the gate insulator thereby causing hot carriers to become injected into the gate insulator. Trapped charge in the gate insulator due to injected hot carriers accumulates over time and can lead to a permanent change in the threshold voltage of the device.
As shown above, a threshold point exist where heightened speed and reduced dimensions will lead to IGFET breakdown. Conventional approaches have encountered difficulty trying to reconcile the methods for decreasing the hot carrier effects and the methods for improving performance. Also, it is desirable to achieve these sought after results without adding costly processing steps. Thus, it is an objective to uncover newly configured IGFET structures and the methods to produce the same which will increase performance and not compromise the EGFET's longevity or fabrication costs.
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