Id/Vgs and Threshold Voltage Extraction

mos1ex01.in : Id/Vgs and Threshold Voltage Extraction

Requires: SSuprem 4/S-Pisces
Minimum Versions: Athena 5.22.1.R, Atlas 5.22.1.R

Basic MOS Athena to Atlas interface example simulating an Id/Vgs curve and extracting threshold voltage and other SPICE parameters. No advanced features are used in this example so as to demonstrate simple functionality. This example demonstrates:

  • Process simulation of a MOS transistor in Athena
  • Process parameter extraction (eg. oxide thicknesses)
  • Autointerface between Athena and Atlas
  • Aimple Id/Vgs curve generation with Vds=0.1V
  • Parameter extraction for Vt, linear gain (beta) and mobility rolloff (theta)

The process simulation in SSuprem 4 follows a standard LDD MOS process. The process steps are simplified and default models are used to give a fast runtime. The polysilicon gate is formed by a simple geometrical etch. Before this point the simulation is essentially one dimensional and hence is run in Athena's 1D mode. After the poly etch, the structure converts to 2D.

The grid used in this example is defined quite tightly. However the statement init ... spac.mult=3 relaxes the mesh in X and Y directions by a factor of three. A more typical mesh for MOS simulation can be obtained by setting spac.mult=1 .

Using DeckBuild's auto-interface, the process simulation structure will be passed into Atlas automatically. This auto-interface therefore allows global optimization from process simulation to device simulation to SPICE model parameter extraction.

The extract statement at the end of the file is used to calculate the oxide thickness at that point. The value returned here may be used as an optimization target for calibration. Refer to the Interactive Tools of the Virtual Wafer Fab manual for instruction in the use of the Optimizer . This value will be appended to a file in the current working directory called results.final. When using the VWF automation tools, extract values are logged to the worksheet for RSM modeling. Significant use of extract statements is strongly recommended, especially at points where in-line Fab measurements are taken.

Electrodes are defined at the end of the process simulation. Metal is deposited and patterned. Then electrode statements are used to define the metal regions plus the polysilicon as electrodes for use in Atlas.

In Atlas the first task is to define the models and material parameters for the simulation. The contact statement is used to define the workfunction of the gate electrodes, while the interface statement defines the fixed charge at the silicon/oxide interface. For simple MOS simulation the parameters CVT and SRH define the recommended models. CVT sets a general purpose mobility model including concentration, temperature, parallel field and transverse field dependence. For more complete MOS simulations of short channel lengths, energy balance simulation might be used.

The statement solve init is used to solve the thermal equilibrium case. After this the voltages can be ramped. It is recommended to use small steps at first when ramping voltages. Once two non-zero biases have been obtained, the program uses a projection as initial guesses to further bias points. The projection method allows larger voltage steps to be taken. The syntax method trap enables Atlas to cut user-defined voltage steps in half if convergence is not obtained. This is a highly recommended option and is turned on by default.

The unique feature of this example is the IV data simulated and the extraction syntax used. The model, interface and contact statements in Atlas are also as in the previous example.

The sequence of solve statements is set to ramp the gate bias with the drain voltage at 0.1V. Solutions are obtained at 0.25V intervals up to 3.0V. All terminal characteristics are saved to the file mos1ex02_1.log as specified in the log statement.

The extract statements at the end of the file are used to measure the threshold voltage and other SPICE parameters. The results from the extract statements are printed in the run-time output, saved to a file called results.final and optionally used in the optimizer or VWF automation tools. The syntax used in these statements is freely composed of operators such as maximum value (max) and simulation results such as drain current (i."drain"). The name parameter specifies only a user-defined label. The routines are not hard-coded to these names. Thus the first extraction statement reads: extract the value called nvt found by taking the x intercept of the maximum slope to the curve of drain voltage vs. drain current and subtracting half the drain voltage. This is just one possible definition of threshold voltage. Current search methods are also possible and are described under the DIBL example later in this section. The VWF interactive tools manual has details of the extract syntax.

The second extract statement measures the gain (or Beta). This is defined as the value of the steepest slope to the Id/Vgs curve divided by the drain voltage. The final extraction is for the SPICE level 3 mobility roll-off parameter (or Theta). This syntax shows the use of the syntax: $"nvt" and $"nbeta" . This tells DeckBuild to substitute the previously extracted values of threshold and beta into this places in the equation.

To load and run this example, select the Load example button in DeckBuild. This will copy the input file and any support files to your current working directory. Select the run button to execute the example.