by high drain voltages. It has been shown that these short-channel effects can be avoided by scaling down the vertical dimensions (e.g., gate insulator thickness, junction depth, etc.) along with the horizontal dimensions, while also proportionately decreasing the applied voltages and increasing the substrate doping concentration [7], [8]. Applying this scaling approach to a properly designed conventional-size MOSFET shows that a 200-Å gate insulator is required if the channel length is to be reduced to 1 µ.

A major consideration of this paper is to show how the use of ion implantation leads to an improved design for very small scaled-down MOSFET’s. First, the ability of ion implantation to accurately introduce a low concentration of doping atoms allows the substrate doping profile in the channel region under the gate to be increased in a controlled manner. When combined with a relatively lightly doped starting substrate, this channel implant reduces the sensitivity of the threshold voltage to changes in the source-to-substrate (“backgate”) bias. This reduced “substrate sensitivity” can then be traded off for a thicker gate insulator of 350-Å thickness which tends to be easier to fabricate reproducibly and reliably. Second, ion implantation allows the formation of very shallow source and drain regions which are more favorable with respect to start-channel effects, while maintaining an acceptable sheet resistance. The combination of these features in an all-implanted design gives a switching device which can be fabricated with a thicker gate insulator if desired, which has well-controlled threshold characteristics, and which has significantly reduced interelectrode capacitances (e.g., drain-to-gate or drain-to-substrate capacitances).

This paper begins by describing the scaling principles which are applied to a conventional MOSFET to obtain a very small device structure capable of improved performance. Experimental verification of the scaling approach is then presented. Next, the fabrication process for an improved scaled-down device structure using ion implantation is described. Design considerations for this all-implanted structure are based on two analytical tools: a simple one-dimensional model that predicts the substrate sensitivity for long channel-length devices, and a two-dimensional current-transport model that predicts the device turn-on characteristics as a function of channel length. The predicted results from both analyses are compared with experimental data. Using the two-dimensional simulation, the sensitivity of the design to various parameters is shown. Then, detailed attention is given to an alternate design, intended for zero substrate bias, which often some advantages with respect to threshold control. Finally, the paper concludes with a discussion of the performance improvements to be expected from integrated circuits that use these very small FET’s.

III. DEVICE SCALING

The principles of device scaling [7], [8] show in a concise manner the general design trends to be followed in decreasing the size and increasing the performance of MOSFET switching devices. Fig. 1 compares a state-of-the-art n-channel MOSFET [9] with a scaled-down device designed following the device scaling principles to be described later. The larger structure shown in Fig. 1(a) is reasonably typical of commercially available devices fabricated by using conventional diffusion techniques. It uses a 1000-Å gate insulator thickness with a substrate doping and substrate bias chosen to give a gate threshold voltage Vt of approximately 2 V relative to the source potential. A substrate doping of 5×1015 cm–3 is low enough to give an acceptable value of substrate sensitivity. The substrate sensitivity is an important criterion in digital switching circuits employing source followers because the design becomes difficult if the threshold voltage increases by more than a factor of two over the full range of variation of the source voltage. For the device illustrated in Fig. 1(a), the design parameters limit the channel length L to about 5 µ. This restriction arises primarily from the penetration of the depletion region surrounding the drain into the area normally controlled by the gate electrode. For a maximum drain voltage of approximately 12–15 V this penetration will modify the surface potential and significantly lower the threshold voltage.

image

Fig. 1. Illustration of device staling principle with κ = 5. (a) Conventional commercially available device structure, (b) Scaled-down device structure.

In order to design a new device suitable for smaller values of L, the device is scaled by a transformation in three variables: dimension, voltage, and doping. First, all linear dimensions are reduced by a unit less scaling factor κ, e.g., image where the primed parameters refer to the new scaled-down device. This reduction includes vertical dimensions such as gate insulator thickness, junction depth, etc., as well as the horizontal dimensions of channel length and width. Second, the voltages applied to the device are reduced by the same factor (e.g., image). Third, the substrate doping concentration is increased, again using the same scaling factor (i.e., image). The design shown in Fig. 1(b) was obtained using κ = 5 which corresponds to the desired reduction in channel length to 1 µ.

The scaling relationships were developed by observing that the depletion layer widths in the scaled-down device are reduced in proportion to the device dimensions due to the reduced potentials and the increased doping. For example

image

The threshold voltage at turn-on [9] is also decreased in direct proportion to the reduced device voltages so that



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