25-nm-Device Topology Description:

This is a 25 nm Leff NMOSFET in bulk silicon with a super-halo channel and source/drain doping profile.

The device is based upon the super-halo doping given by Taur et al. [1], but the profiles are analytic functions. Reasonable device geometries can be designed for simulations in the deep sub-100-nm regime. The n+ polysilicon gate has a doping of 5e20 cm-3 and a height of 60 nm. The poly doping was chosen by scaling up from the 90 nm device so that the ratio of poly depletion to tox is more or less constant. This doping scheme degrades current relative to metal gate by less than 10% in the devices. The physical tox is 15 Angstroms. Leff (as defined between where the source/drain dopings fall to 2e19 cm-3) was extracted and found to be 25 nm. Lpoly was arbitrarily set at 50 nm. Because the device is symmetric, the origin for the lateral grid is at the middle of the channel (x = 0); the interface between the gate oxide and bulk is chosen as the depthwise origin (y = 0). In the simulations, only the portion of the MOSFET up to the source/drain regions for a lateral distance, Lsd = 22.5 nm, from the edge of the gate to model boundary, is used. The source and drain contacts are one-node-thick electrodes on the left and right boundaries of the silicon extending down a depth of 10 nm from the depthwise origin. The particular electrode to silicon contact resistances for this model were assumed to be zero.

A full 2-D doping profile (placed in the doping directory) was developed. The file sh25.doping contains a 2-D profile represented as three columns: the first is lateral position, x; the second is depth position, y; the third is the cumulative doping of the source/drain region and super-halo at those points, Nd(x,y) - Na(x,y). There is a separate background uniform p-type doping of 1e15 cm-3. The profile is simply generated from an analytic formula, as in sh25.analytic, that describes two 2-D gaussian functions: one for the n+ source/drain, one for the p+ halo. The device profile is mirror symmetric about x = 0.

A set of simulated Id vs. Vgs curves (PREVIEW) are stored in the idvg directory. The characteristics have been generated using the Drift-Diffusion (DD) model as described in MEDICI [2] and taking into consideration poly- depletion (PD) and quantum mechanical (QM) effects. A sample MEDICI file inputfile25 is included for reference. The I-V characteristics are text files labeled according to their drain bias (where Vbs = 0 V); for example, ddd0.1 has a Vds of 0.1 V. Each file has two columns of data: the first is Vgs in 0.1 V steps and the second is normalized Id (A/µm).

This device was scaled down to a 20 nm and up to a 30 nm Leff by shifting the gaussian center in the x-direction of both the source/drain and halo dopings simultaneously. Simulated values of threshold voltage (defined where Id = 1e-6 A/µm and Vd = 1V) and DIBL are plotted as a function of Leff in the following GRAPH.

References

[1] Y. Taur et al., IEDM proceedings, p. 789, 1998
[2] MEDICI manual, Technology Modeling Associates