Книги+1 / 2013 [Chandan_Kumar_Sarkar]_Technology_CAD

3.11  Setting the Number of Carriers

ATLAS can solve both electron and hole continuity equations, or only for one or none. This choice can be made using the parameter CARRIERS. For example,

METHOD CARRIERS = 2

This specifies that a solution for both carriers is required. This is the default. With one carrier the parameter ELEC or HOLE is needed. For example, for hole solutions only,

METHOD CARRIERS = 1 HOLE

To select a solution for potential only, specify

METHOD CARRIERS = 0

3.12  Important Parameters of the METHOD Statement

It is possible to alter all of the parameters relevant to the numerical solution process. This is not recommended unless you have expert knowledge of the numerical algorithms [5]. All of these parameters have been assigned optimal values for most solution conditions. It is beyond the scope of this chapter to give more details.

Two parameters, however, are worth noting at this stage:

1.CLIMIT or CLIM.DD specifies minimal values of concentrations to be resolved by the solver. Sometimes it is necessary to reduce this value to aid solutions of breakdown characteristics. A value of CLIMIT = 1e-4 is recommended for all simulations of breakdown where the pre-breakdown current is small. CLIM.DD is equivalent to CLIMIT but uses the more convenient units of cm−3 for the critical concentration.

2. DVMAX controls the maximum update of potential per iteration of Newton’s method. The default corresponds to 1 V. For power devices requiring large voltages, an increased value of DVMAX might be needed. DVMAX = 1e8 can improve the speed of highvoltage bias ramps.

154Technology Computer Aided Design: Simulation for VLSI MOSFET

3.CLIM.EB controls the cut-off carrier concentration below which the program will not consider the error in the carrier temperature. This is applied in energy balance simulations to avoid excessive calculations of the carrier temperature at locations in the structure where the carrier concentration is low. However, if this parameter is set too high so that the carrier temperature errors for significant carrier concentrations are being ignored, unpredictable and mostly incorrect results will be seen.

3.12.1  Restrictions on the Choice of METHOD

The following cases all require METHOD NEWTON CARRIERS = 2 to be set for isothermal drift-diffusion simulations. Both BLOCK and NEWTON are permitted for lattice heat and energy balance:

•Current boundary conditions

•Distributed or lumped external elements

•AC analysis

•Impact ionization

3.12.2  Pisces-II Compatibility

Previous releases of ATLAS (2.0.0.R) and other PISCES-II based programs use the SYMBOLIC command to define the solution method and the number of carriers to be included in the solution. In the current version of ATLAS, the solution method is specified completely on the METHOD statement. The COMB parameter that was available in earlier ATLAS versions is no longer required, as it is replaced by either the BLOCK method or the combination of GUMM EL and NEWTON.

References

1.Sentaurus TCAD Manuals, Synopsys Inc., Mountain View, CA.

2.Taurus Medici Manuals, Synopsys Inc., Mountain View, CA.

3.ATLAS User’s Manual, Silvaco Int., Santa Clara, CA, May 26, 2006

4.M.K. Jain, S.R.K. Iyengar, and R.K. Jain, Numerical Methods for Scientific and Engineering Computations, New Age International, New Delhi, India, 2004.

5.M.K. Jain, Numerical Solution of Differential Equations, 2nd ed., New Age International, New Delhi, India, 2008.

Current major suppliers of TCAD tools include Synopsys, Silvaco, and Crosslight. Synopsys provides both process and device simulation tools. It supports a broad range of applications such as complementary metal- oxide-semiconductor (CMOS), power, memory, solar cells, etc. In addition, Synopsys provides tools for interconnect modeling and parasitic extraction procedure which are required for optimizing the performance of an integrated circuit chip.

In this chapter, we discuss techniques for device simulation using the Synopsys tool suite. The tool suite includes tools for constructing device structures, meshing, simulation, visualization of electrical characteristics, plotting of various curves, and extraction of performance parameters. The working of each tool is described with the help of examples. The principles of operations involved are also discussed. The details of each tool are not provided, but readers are referred to corresponding user guides.

4.2  Design Flow

A typical design flow consists of either the creation of a device structure by a process simulation (Sentaurus Process or Taurus TSUPREME-4) tool or through computer aided design (CAD) operations and process emulation steps (using tools like Sentaurus Structure Editor). The constructed device is meshed intelligently using tools like Sentaurus Mesh/Noffset 3D. Sentaurus Device is used to simulate the electrical characteristics of the device. It simulates numerically the electrical behavior of a single semiconductor device in isolation or several physical devices combined in a circuit. Terminal currents, voltages, and charges are computed based on a set of physical device equations that describes the carrier distribution and the conduction mechanisms. Finally, Tecplot SV is used to visualize the output from the simulation in 2D and 3D, and Inspect tool is used to plot the electrical characteristics. The design flow is shown in Figure 4.1.

4.3  Sentaurus Structure Editor

Sentaurus Structure Editor is a device editor and process emulator based on CAD technology, used for constructing a semiconductor device. As shown in Figure 4.2, it has three distinct operational nodes: 2D structure editing, 3D structure editing, and 3D process emulation. Using Sentaurus Structure Editor, device structures are constructed or edited interactively using a graphical user interface (GUI) facility. Alternatively, devices can be generated

whenever Sentaurus Device is run)

takes *_msh.dat and *_msg.grd files as input

2D/3D Scientific Visualization and Plotting of Simulated Data by Tecplot_SV

Curve Display and analysis by Inspect

FIGURE 4.1

Design flow of device simulation process using ISE-TCAD, Synopsys.

in batch mode using Scheme scripting language. When a GUI action is performed, Sentaurus Structure Editor prints the corresponding Scheme command in the command-line window. From the GUI, 2D and 3D device models are created geometrically using 2D or 3D primitives such as rectangles, polygons, cuboids, etc. In the process emulation mode (Procem), Sentaurus Structure Editor translates processing steps such as etching and deposition,

Sentaurus

Structure

Editor

FIGURE 4.2

Operational modes of Structure Editor.

patterning, fill, and polish into geometric operations. Procem supports various options such as isotropic or anisotropic etching and deposition, rounding, and blending. The working of the Structure Editor is explained with the help of a design example, which is presented in the next sub-section.

4.3.1  Design of a 2D Bulk Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) of Channel Length 100 nm

The 2D metal-oxide-semiconductor (MOS) structure that we attempt to create in Sentaurus Device Editor (SDE) is shown in Figure 4.3. The step-by-step design process using commands is illustrated pictorially in sequence in Figures 4.4 to 4.12.

Here we illustrate the steps required to create a rectangle that will be used as bulk silicon material. This involves the creation of a rectangle whose diagonally opposite corner coordinates are required to be specified, and commands are as follows. SDE will consider rectangular material as “Silicon” and this will be named “region_1”. As this is a 2D MOS Device, the Z coordinate is considered to be 0.

y

FIGURE 4.3

Schematic of 2D MOS structure. Positive X and Y axes are right-hand side and downward direction, respectively. For 2D MOS device, Z coordinate is always considered to be 0.

FIGURE 4.4

Schematic of bulk silicon creation in SCE by drawing a rectangle using a command whose corner coordinates are (–0.1 0.0 0.0) and (0.1 0.2 0.0).

To start Sentaurus Structure Editor, on the command line, enter:

sde (sdegeo:create-rectangle

(position -0.1 0.0 0.0) (position 0.1 0.2 0.0) “Silicon” “region_1”)

(sdegeo:create-rectangle

(position -0.05 -0.002 0.0) (position 0.05 0 0.0) “Oxide” “region_2”)

FIGURE 4.5

The creation of oxide on the bulk silicon by using sequential command.

FIGURE 4.6

Molybdenum gate on oxide material. This is the actual MOS structure. Rectangular molybdenum material corner coordinates are (–0.05, –0.002, 0.0) and (0.05, –0.08, 0.0).

One needs to run the sequential command to get the entire structure [1–4]. Sequential command will now create another rectangle on the bulk silicon material which will be used as oxide or gate oxide. The material has been selected by command “Oxide” whose diagonally opposite corner coordinates are (–0.05 –0.002 0.0) and (0.05 0 0.0), and this oxide region is named “region_2”.

(sdegeo:create-rectangle

(position -0.05 -0.002 0.0) (position 0.05 -0.08 0.0) “Molybdenum” “region_3”)

(sdegeo:define-contact-set “gate” 4 (color:rgb 1 0 0) “##”) (sdegeo:set-current-contact-set “gate”)

FIGURE 4.7

MOS structure with gate contact on top.

FIGURE 4.8

Body contact in which contact edge point coordinate (0.0 0.2 0) has been selected by the program.

(sdegeo:define-2d-contact (list (car (find-edge-id (position 0.0 -0.08 0)))) “gate”);gate contact position and contact name

For the external connection to the 2D MOS device, a 2D gate contact has to be defined. Here contact is defined at the gate edge with proper coordinate point. Though the coordinate can be anywhere at the gate edge, for this device the middle point of the gate edge has been selected with coordinates (0.0 -0.08 0) and contact name “gate”.

(sdegeo:define-contact-set “body” 4 (color:rgb 1 0 0) “##”) (sdegeo:set-current-contact-set “body”)

FIGURE 4.9

Source/drain 2D contact at position (–0.075 0 0) “source”.

FIGURE 4.10

SDE shows the final MOS structure with all the contacts. Here another source/drain contact point is created here whose coordinate is (0.075 0 0) and name of “drain.”

(sdegeo:define-2d-contact (list (car (find-edge-id (position 0.0 0.2 0))))”body”)

(sdegeo:define-contact-set “source” 4 (color:rgb 1 0 0) “##”) (sdegeo:set-current-contact-set “source”) (sdegeo:define-2d-contact (list (car (find-edge-id (position -0.075 0 0)))) “source”)

(sdegeo:define-contact-set “drain” 4 (color:rgb 1 0 0) “##”) (sdegeo:set-current-contact-set “drain”)

FIGURE 4.11

The SDE diagram after refinement window selection, source/drain and bulk material doping.