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Device Templates

Device templates model the geometry, materials, heating and cooling characteristics of a circuit element such as a resistor or field effect transistor (FET), or of a subsystem composed of many such devices. A device template describes the thermal analysis problem to SYMMIC with sufficient detail to allow the software to compute a finite element temperature solution to the heat transfer partial differential equations. For a concrete example of a device template, see the section describing a Generic FET Template.

Why Templates?

Real device physics is complicated. Successful modeling and simulation is largely about what to leave out of a model of the real device. This is especially true at the design stage of the project when reasonably accurate but approximate simulations must be performed quickly to support the design iterations that are required to optimize performance. We believe this is best accomplished through the template framework.

The device template allows the designer to work with a simplified representation with just enough detail to obtain an accurate temperature distribution at the locations that most affect performance and reliability. As demonstrated later in the Top-Down Analysis chapter, sufficient detail can be included in a template to get accurate junction temperatures for high-electron mobility field-effect transistors in power amplifiers, but a template is not forced to have the same level of detail for other parts of the layout. Working directly from the CAD layout files, however, forces thermal simulations to process a lot more information, most of which is irrelevant to thermal analysis, slowing down the design process or forcing thermal performance to be ignored during the circuit design phase. The level of detail in a template can be tailored to the particular type of thermal analysis needed, such as including the individual gates in power amplifier FETs while simplifying other types of devices to rectangular heat sources.

The sophisticated, open-source, and fully-documented template format allows each engineering team to develop its own design kit for thermal analysis. Team members familiar with the thermal aspects of the manufacturing process (3D device structure, chip packaging, material properties, etc.) can easily create a family of device templates that will provide the circuit designers with high-fidelity finite element models that can be incorporated into their design process. Important features of the model can be parameterized inside the template. These design parameters are then exposed to the circuit designers through the SYMMIC user interface to allow temperature-aware design. For example, a FET could be parameterized by the number and width of the gates, gate-to-gate spacing, gate-to-drain distance, thickness of secondary metal layers, length of the field plate, heating profile as a function of field plate and gate voltage, and so on. Just about anything that is modifiable in the foundary process can be made adjustable in device templates. Circuit designers are then free to make the necessary tradeoffs to obtain optimal performance without needing new models or simulations from other team members expert in using general purpose finite-element software.

The following list summarizes the advantages of SYMMIC's template-based thermal analysis:

• Fast thermal analysis with the desired level of detail

• Temperature-aware circuit design iterations

• Electro-thermal co-simulation support

• Design parameters in the template are automatically made available to designers

• Rapid design space exploration for optimizing thermal performance

• Hierarchical layout and subsystem construction

• High-resolution modeling over a wide range of spatial and temporal scales

• Support for determining accurate thermal boundary conditions

• Automated meshing and finite-element problem setup

• Easier to learn and use than general purpose finite-element software

• User-modifiable, orthotropic, temperature-dependent material properties

• Continuous and pulsed-power for steady-state and transient analysis

• Complete representation of thermal analysis in compact, open file format

• More productive interactions between thermal engineers and circuit designers

What's in a Template?

Every device template may contain the following information:

• 3D device geometry grouped into named components

• Geometrical dimensions as user-adjustable parameters

• Meshing resolution for each geometric feature

• A list of materials and their thermal properties

• Boundary conditions describing joule heating and paths of cooling

• Square-wave timing of device on/off power

• Initial conditions for the thermal problem when not steady-state

• Choice of solution times and the size of integration time steps between them

• Choice of method for solving heat equation and related parameters

• A history of user-defined and other events in the life of the template

The device geometry is described as a set of rectangular parallelepipeds (hexahedrons or boxes) that are aligned with the Cartesian (x,y,z) axes. While this does not permit the description of non-rectilinear device geometry, it provides an efficient and reliably meshable description that is adequate for performing a detailed thermal analysis of most integrated circuit devices.

Parts of the device geometry are grouped together into named components so that they can be easily identified by the designer. One example of a component could be the Gates of a FET, which all have the same geometric parameters can be treated as group in terms of the design parameters describing them. The user modifies the components to configure the thermal analysis to a particular device design. Sometimes components can be added and removed from the device. One example of this is increasing the Number of Gates which adds all the components associated with each additional finger of the transistor.

Most components in a device template are permanent and cannot be removed from the model, but their dimensions are usually adjustable in a fixed parameter range. Parameter ranges in a device template are chosen by the template designer to allow flexibility while maintaining meshing efficiency and accuracy of thermal analysis. It is not possible to eliminate components just by setting their dimensions to zero because mesh elements with zero dimensions would be generated as a result. However, permanent components can often be reconfigured to remove them from the thermal analysis by changing the material used. For example, by changing the material to match the material of neighboring components. For more on reconfiguring device templates, see the documentation for particular devices and the walk-throughs in the Top-Down Analysis chapter.

Template Units and Resolution

Device templates use micrometer (micron) units of length. Consequently, thermal conductivity and other physical properties involving units of length are specified with respect to micrometers. The smallest feature length representable in a device template is 1 nm. Mesh resolution can be finer, up to the limit of 0.0005 nm beyond which locations are considered identical. Positional resolution for placement of devices in a layout is truncated at 0.5 nm. The effective range of all real values is limited by the ~16 decimal digits able to be represented by 64-bit floating-point numbers. For indexed objects, such as vertices, the number of items is limited by the range of positive 32-bit integers. The allowable ranges that can be employed in a single model are summarized in the following table.

The number of temperatures per time point is the same as the number of (x,y,z) vertices in the mesh.

Template Errors

When a device template is opened, (or included as part of a layout,) the template will be interpreted as a meshable model for thermal analysis. One component of this process is resolving all the symbolic parameters in the template file into a consistent set of numerical values. If the parameters cannot be resolved, one or more error message will be displayed in the console, and no device model will be displayed in the main window. A dialog box will indicate that the program was “unable to build model from template”. Parameter lists can be examined to help determine which parameter(s) might be causing the error. Look at parameters highlighted in red, since a red parameter may have an invalid [min,max] range. In such cases, save the bad template to a new file. Then return to the last known good template and selectively modify or import parameters to avoid re-creating the problem. If the template was created by CapeSym, attach the bad template to a bug report emailed to SYMMIC_support@capesym.com.

XML Syntax and Validation

Device template files are written in XML format (see appendix for details). When the XML syntax of a template file is not valid, the device usually cannot be opened and viewed in SYMMIC. To display the contents of such corrupted files, use the Validate XML file... command from the File menu. This command will attempt to open and parse the device template file using the Microsoft XML libraries. When an XML error is encountered, the partially-parsed XML text will be displayed with the correct part written in black text, and the part following the first error written in red text. In some cases, the XML validator may not be able to display any text, but typically an error message is displayed indicating what problem was encountered. If no errors are found, the message “The XML is valid” will be displayed.

The XML syntax of a template is defined by a Document Type Definition (DTD) file (template_format.dtd). The XML validator will need to read the DTD file to validate the XML syntax, so the file must be present in the same folder as the device template being validated. If the DTD file is not present, no text will be displayed since validation is not possible. The template_format.dtd file is now installed into the same folder as the device templates, and may also be obtained from the installation CD or the download area on www.symmic.net/SYMMIC/downloads.

Successful XML validation of a device templatefile.

After successful validation, the device template XML will be displayed in a context-sensitive format in an browser-based window. The displayed XML may be edited, if desired, and then revalidated using one of the context menu commands that appear when the right mouse button is clicked within the window. Any new errors will be reported and displayed after the attempted validation. Black and red text resulting from XML syntax errors may also be edited and revalidated to arrive a correct XML document. The final, corrected document may be saved into the original file using the Save command.

The XML validator is designed to validate device templates using the template_format.dtd, but a layout template may also be validated as long as the DTD is changed to mmic_format.dtd in the DOCTYPE statement prior to validation.