A Predictive Length-Dependent Saturation Current Model Based on Accurate Threshold Voltage Modeling

Khee Yong Lim*, Xing Zhou*, and David Lim

*School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798
Fax: (65) 791-2687.  Email: exzhou@ntu.edu.sg
Chartered Semiconductor Manufacturing Ltd., 60 Woodlands Industrial Park D, Street 2, Singapore 738406

Proc. of the 2nd International Conference on Modeling and Simulation of Microsystems (MSM99)

San Juan, Puerto Rico, U.S.A., April 19-21, 1999, pp. 423-426.

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This paper presents a compact length-dependent saturation current (Idsat) model for deep-submicron MOSFET’s based on accurate modeling of the threshold voltage (Vth). A predictive bias-dependent Vth model that relates to process conditions has been developed [1], [2]. An extension of the Vth model [1] is used in this work, which consists of only five important process-dependent parameters and two auxiliary DIBL parameters, namely, Ns, l, a, b, k and i, j, respectively. Each parameter has its own physical representation that characterizes the individual short-channel effects. The first parameter Ns represents an effective vertical channel nonuniform doping profile, whereas the parameters k and b characterize the reverse short channel effect (Vth roll-up). The normal short channel effect (Vth roll-off) is modeled by l and a. The auxiliary parameters i and j are introduced in this work to fine tune the DIBL dependence, which model the asymmetric nature of the source and drain depletion region at high Vds condition. The Vth model requires a five-steps parameter extraction based on only five sets of measurement data: Vth(Vbs) at long channel, Vth(Lg, low Vds, high Vbs), Vth(Lg, low Vds, low Vbs),  Vth(Lg, high Vds, low Vbs),  and Vth(Lg, high Vds, high Vbs). With additional long-channel Vth(Vbs) data for different wafers, the Vth model can correlate to process variables such as Vth adjust implant dose and punchthrough implant energy.
Once a good Vth model is available, developing an Idsat model is mainly a matter of mobility and series resistance modeling. The Idsat model in [3] is employed, combined with our Vth model to predict experimental data from the same wafer. The Idsat model considers all the important short-channel effects, which include mobility degradation, velocity saturation, as well as source/drain series resistance. The low-field mobility (m0) and series resistance (Rs) are extracted by fitting to one set of Idsat versus Lg data at Vds=Vgs=Vdd. Once these two parameters are determined, a complete compact Idsat model with drawn gate length (Lg), bias voltages (Vgs, Vds, Vbs) and process variables (implant dose F and energy E) as input parameters is available.

The excellent prediction of the Idsat model (lines) to the experimental data (symbols) has been demonstrated in Figs.1-4. The extracted empirical correlation between the channel doping parameter (Ns) and the implant dose and energy (shown in the insets of Figs. 3 and 4) applies equally well to the saturation current. This work demonstrates an efficient and accurate approach to modeling deep-submicron MOSFET’s, which is very useful for reducing experimental wafer spilt-lot and for process control and optimization.



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  2. [5] X. Zhou and K. Y. Lim, "A compact MOSFET Ids model for channel-length modulation including velocity overshoot," Proc. 1999 International Semiconductor Device Research Symposium (ISDRS-99), Charlottesville, VA, Dec. 1999, pp. 423-426.
  3. [11] X. Zhou, K. Y. Lim, and D. Lim, "A general approach to compact threshold voltage formulation based on 2-D numerical simulation and experimental correlation for deep-submicron ULSI technology development," IEEE Trans. Electron Devices, Vol. 47, No. 1, pp. 214-221, Jan. 2000.
  4. [3] X. Zhou and K. Y. Lim, "A novel approach to compact I-V modeling for deep-submicron MOSFET's technology development with process correlation," Proc. 3rd International Conference on Modeling and Simulation of Microsystems (MSM2000), San Diego, CA, Mar. 2000, pp. 333-336.
  5. [13] K. Y. Lim, X. Zhou, and Y. Wang, "Modeling of threshold voltage with reverse short channel effect," Proc. 3rd International Conference on Modeling and Simulation of Microsystems (MSM2000), San Diego, CA, Mar. 2000, pp. 317-320.
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  8. [19] X. Zhou, S. B. Chiah, K. Y. Lim, Y. Wang, X. Yu, S. Chwa, A. See, and L. Chan, "Technology-dependent modeling of deep-submicron MOSFET's and ULSI circuits," (Invited Paper), Proc. 6th International Conference on Solid-State and Integrated-Circuit Technology (ICSICT-2001), Shanghai, Oct. 2001, Vol. 2, pp. 855-860.
  9. [13] K. Y. Lim, X. Zhou, and Y. Wang, "Physics-Based Threshold Voltage Modeling with Reverse Short Channel Effect," J. Modeling Simulation Microsystems (JMSM), Vol. 2, No. 1, pp. 51-55, 2001.