Single Event Upset in a 3D MOSFET

radex03.in : Single Event Upset in a 3D MOSFET

Requires: DevEdit 3D/Device 3D
Minimum Versions: Atlas 5.20.2.R

This example demonstrates simulation of the effects of different angles of incidence of an SEU in a MOSFET structure. The MOSFET structure is constructed using DevEdit 3D. The structure is then passed to Atlas for electrical testing. The input file consists of the following parts:

  • Construction of the device in DevEdit 3D
  • SEU simulation in Atlas: particle with normal incidence
  • SEU simulation in Atlas: particle with oblique incidence

Atlas contains many features to enable the simulation of Single Event Upset (SEU) phenomena. Models for charge generation exist both in 2D, 3D and quasi-3D cylindrical coordinates. It is also possible for the users to add their own position and time dependent charge generation using the C-Interpreter and incorporate these into Atlas simulations.

The first stage of the input file constructs the MOSFET geometry, material regions, doping profiles, electrodes, and subsequently generates the mesh in 3D. This is perfromed in DevEdit 3D by drawing the device regions in interactive mode, and specifying 3D doping distributions. The mesh was generated automatically by specifying basic mesh parameters and constraints with subsequent refinement based on the doping distribution.

DevEdit 3D generates 2 types of files: DevEdit 3D input file and the structure file. The first can be run in the DeckBuild to produce the corresponding structure file and is included here as the first portion of the input file. The second can be read in directly by Atlas in the MESH statement. Note that the DevEdit 3D input file can be edited as any other input file. It is straightforward to change type and value of the doping associated with each region or resize regions. More importantly DevEdit 3D input files can also be read directly into the graphical user interface of DevEdit 3D to provide all the menu options used to construct the structure.

The Atlas simulation begins by reading the structure from DevEdit 3D. DeckBuild provides an autointerface between DevEdit 3D and Atlas so that the structure produced by DevEdit 3D is transferred to Atlas without having to indicate the mesh statement (commented out in this example). Without the automatic DevEdit 3D/Atlas interface under DeckBuild the MESH statement is needed to load the structure and the mesh.

The first active statements in the Atlas portion of the input file are contact and model definitions. In the contact statement, the workfunction is specified for the gate contact to reflect the properties of polysilicon. The set of physical models used in the simulation includes Shockley-Read-Hall and Auger recombination as well as the CVT mobility model accounting for doping, parallel and transverse electric field mobility dependencies. The material statement and its parameters are not defined in the input file implying that the default values for silicon are used. A two-carrier solution of the problem is indicated through the parameter carrier=2 in the method statement. However, this parameter is omitted here since it represents the default setting.

In practical applications, while simulating the latched state of a transistor in a memory cell, a large lumped resistor must be attached to the drain since the latter is in the floating state. This should adequately represent the MOSFET in either a high or low state that may be used in the SRAM cell. However, this lies beyond the scope of the example considered here. The purpose of this example is to demonstrate general capabilities in simulating the SEU by analyzing the reaction of the device in the off state to the charge generation of the SEU.

The simulation is first performed to obtain the condition of the structure prior to the particle hit. This condition is with the drain voltage of 5V, and all other electrodes grounded. This condition of the structure is saved in a solution file for the use in the second atlas run.

The parameters of the charge track are specified in the singleeventupset statement. The general description of the SEU physical model employed in Atlas 3D mode as well as the description of all of the parameters for specifying position and time dependent generation is given in the Appendix to this example's description.

The first SEU simulation is performed for a normal particle incidence through the drain region (along Y direction) with a maximum carrier density generated of 1.e18 cm-3. The characteristic radius of the track was defined to be 0.05 micron, the characteristic time of the Gaussian time dependency - 2 ps, and the time instant of the peak of generation - 4 ps.

The transient simulation is performed to monitor the effects of SEU generation. The initial time steps in the solve statements are defined by the user (eg. 0.05 ps in the first solve statement), while the subsequent time steps are selected automatically. The simulation is performed up to 0.1 microsecond of physical time. To analyze the effect of carrier generation on internal physical distributions, an output structure file is saved for the instant of the peak of generation (t=4 ps).

The external terminals transient characteristics are saved in the .log file. The MOSFET transient response in the form of drain current transient characteristic is displayed using TonyPlot.

The simulation then proceeds to the second Atlas input file for simulating oblique particle incidence. This input file contains the same CONTACT and MODELS definitions as the previous one. The SEU definition of the particle hitting the device along its longer diagonal through the drain region (far left top corner) down to the near right bottom corner of the structure is specified in the SINGLE statement. All other parameters of the SEU are the same as in the previous Atlas simulation. The condition of the structure of Vds=5 V is loaded and the transient response of the SEU is simulated for the second SEU conditions.

The external terminals transient characteristics are saved in the .log file , and the structure information is saved in the solution file at the peak of generation.

Both drain transient current characteristics corresponding to the different SEU conditions are then displayed in TonyPlot. The peak of the current associated with the drift collection is achieved at approximately 6 - 7 ps. Note that the peak of generation is taking place at 4 ps. The drift collection continues for about ~100 ps when the charge in the depletion region is practically collected. The peak of the current for the diagonal incidence is calculated to be slightly higher than for the normal incidence. The decrease in current is however slower for the normal incidence making the total extracted charge greater.

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.