NSF Sponsored Workshop on Structured Design Methods for MEMS

IntelliCAD: The CAD-for-MEMS


Fariborz Maseeh
IntelliSense Corporation
16 Upton Drive
Wilmington, MA 01887
IntelliS@aol.com

Introduction

U.S. industry faces a large opportunity with microsensors and other microelectromechanical (MEM) devices. Because these devices are smaller in size and can be made at a very low cost, they are not only replacing traditional sensors but can find applications in new areas. The automotive industry, in particular, has invested heavily in MEM sensors because of the tremendous cost savings. Five years ago the annual production of MEM sensors for engine and transmission controls and diagnostics was more than 12 million units per year in the U.S. [High Technology Business, Sept.-Oct. 1989, p.29]. Indications are that this number has steadily increased since then with the drive for ever better mileage and lower emissions. Another large automotive application is silicon accelerometers for air bag release, anti-lock brakes and smart suspension. In 1994 the market for micromachined sensors was approximately $1.4 billion with applications in automotive, biomedical, process control, aerospace among others.

Silicon micromachining promises to dramatically reduce the cost of MEM devices because the process technology is very similar to that used in the high-volume manufacturing of semiconductor integrated-circuits (ICs). Rather than building electrical components like transistors and capacitors in silicon, manufacturers micromachine electromechanical elements for sensors and actuators. These miniature devices, with geometries measured in microns like ubiquitous ICs, have potential applications wherever traditional devices are found.

With the advantages of lower cost and widespread knowledge of semiconductor process technology, why is the MEM device market so small today? The reason is the lack of manufacturing knowledge in making the tremendous variety of miniature mechanical components. Semiconductor ICs have the advantage of commonality: the technology advances made for high-volume DRAM production spread quickly to other devices such as microprocessors and other types of memories.

The proper design and fabrication tools do not exist for MEM devices. Today's computer-aided design (CAD) systems are inappropriate because they do not have the necessary MEM design, material or fabrication information. As a result, manufacturers are forced to use time consuming and expensive design-fabrication trial and error to develop MEM devices. This enormous expense can be justified today only in applications such as in the automotive field which can amortize the costs over large volumes.

To address the need for design tools in MEMS, IntelliSense has developed an integrated software system, called IntelliCAD, in which a number of tools are integrated to assist developers of MEMS mimic the fabrication and modeling of MEMS in a workstation environment.

Users can design new micro fabrication processes with IntelliCAD by grouping logical sets of process steps into a process or by borrowing process steps from existing processes. Once a process exists, a user can change the physical makeup and material properties associated with that process by varying individual process components. Changes in the process components can then alter the corresponding look and performance of the device. The user can view a three dimensional representation of the resulting structure and can test the performance of the structure on-line by applying simulated loads and reviewing calculated effects. By using IntelliCAD, the entire design and manufacturing of a micro structure can be performed without entering a micro fabrication facility.

IntelliCAD Concept

A number of tools are integrated into a software system in which a user investigates a device from the conceptual stage to manufacturing. Each tool provides a specific functionality and is a transparent constituent of IntelliCAD. During the operation of IntelliCAD, the user interacts with the User Interface to perform various functions and receive information in a menu-driven or graphical format.

A process is first conceptualized through the Process Construction module with the Fabrication Database, and verified with the Process Check and Fabrication Simulation modules. By going back and forth among these modules the user will construct the process steps for the fabrication of a device. The process steps include a complete operational recipe with all the parameters required for an operator to perform each step. The user will periodically consult with the Fabrication Database and Material Database to select the parameter values for obtaining the material behavior of choice. The Material Database of the system has already been developed by IntelliSense to the point of commercialization. This tool, MEMaterial, consists of a material database, an estimation routine to simulate the material behavior of deposited films as a function of their manufacturing and a graphics user interface. Tool Interface passes material information to other constituents of IntelliCAD when needed.

The Solid Modeler, upon receiving the fabrication and material information from the aforementioned modules and the planar geometry information from the Layout Definition module, builds a three-dimensional model which can be used by the performance simulators for the analysis of mechanical, electrical, thermal and flow properties. The 3D models can also be displayed within the Graphics UI for visualization purposes at each step of the fabrication process. The Layout Definition module holds the mask information needed for definition of patterns in a process step. The layout information will be captured during the process construction.

Often users need to access standard (baseline) processes leading to the development of a certain MEM device. Standard process techniques are captured within the Design Database which can be accessed independently, modified when variations are needed at several steps. Simulations determine the effects of variations. The template processes stored within the Design Database may be tailored to capture an organization's standard or proprietary process. The Design Database contains information about sensors (products), both a sensor's performance and its design.

IntelliCAD Capabilities

Design Capabilities

A microstructure design process is made up of a group of fabrication steps (e.g., deposition, etching, diffusion). A user needs to create a complete list of fabrication steps to define a process. To do this, a user can either customize an existing process or define an entire set of process steps based on a concept. Often a design solution will incorporate both of these techniques. To support this process, IntelliCAD includes a database of complete development processes as well as detailed information of individual fabrication steps. This information gives a user process design freedom while also providing defined processes that can be used as design starting points to save time.

IntelliCAD includes simulation tools that define new fabrication steps and verify the integrity of design processes. The fabrication simulation tool is used for creating variations of existing fabrication steps and can model the majority of known fabrication process steps. To perform this simulation, a known algorithm that defines physical design characteristics is invoked on a given process step using the characteristics of the process as input parameters. For fabrication steps that do not have known algorithms, a user defined algorithm can be user entered to override the fabrication simulation routine. The other simulator is the process simulation tool which is rule-based and contains fabrication process sequence rules developed using the collection and organization of micro fabrication expertise. It is used to verify process sequence continuity and completeness. If a process is not presented logically, the process simulation tool will report logic problems for user investigation. Before three dimensional visualization and performance simulation can occur the process simulation tool must verify process (or partial process) integrity.

Layout Definition

One step in designing a process sequence is defining a mask or sequence of masks that will create an anticipated geometry. In IntelliCAD a user can choose a two dimensional mask and can view the geometric impact that the application of that mask (with given etch parameters) would have on a structure. A user can experiment with different masks and masking sequences in an effort to create a desired geometry. The layout definition feature replaces the mask definition, preparation, and application procedures that occur in the micro fabrication facility.

Material Properties Definition

For deposition process steps, the designer has the ability to manipulate deposition thickness using the fabrication simulator and has the added capability to manipulate the material properties associated with thin film depositions. IntelliSense has commercialized a material analysis tool called MEMaterial that examines the material properties of thin films as a function of process parameters. The user can change parameters and MEMaterial will predict resulting material property changes. IntelliSense has integrated the MEMaterial product into the IntelliCAD package. When a deposition step is defined in IntelliCAD, the user can access MEMaterial and model the material properties of the deposited thin film. Thus the user can control the physical properties and material characteristics of thin film depositions.

MEMaterial contains a database of microelectronic materials that can be analyzed using estimation and optimization routines developed at IntelliSense. These routines compute material properties (based on measured data) for any given set of parameters. MEMaterial also allows users to analyze data using two and three dimensional graphs and generates plots that give users insight into material parameter dependencies.

On-line Visualization

IntelliCAD contains a Solid Modeler Tool to provide on-line three dimensional visualization of user defined processes. As input, the solid modeler uses out-of-plane geometry from the fabrication and process building routines and in-plane geometry from the mask simulator. Based on the geometries inherent in the structure the Solid Modeler creates an on-line representation that appears as if it were fabricated in the lab. As an additional feature, the user can change the viewing perspective of the three dimensional structure to simulate a lab technician rotating the device to enhance viewing. The user does not need to create a complete process to use the solid modeler. Any group of coherent process steps can be evaluated.

Process Simulation

The final step of the micro structure development process is performance simulation. The geometry that results from the Solid Modeler combined with the associated material properties from MEMaterial are used as input into the process simulation module. Performance simulation is achieved through finite element method (FEM) analysis. Multiple performance simulators including thermal, mechanical, electrostatic and electromagnetic types are interwoven within the IntelliCAD performance simulation module to create compete analysis and testing capabilities. From the GUI the user can apply various loads to a structure and view the effects that the loads have on the structure.

Graphical User Interface

The graphical user interface is completely menu driven and all functions are clearly defined. A new user should be able to use the IntelliCAD software without referencing the user manual.

Data Availability

The scope and magnitude of micro fabrication data associated with micro fabrication techniques and processes is continually expanding and it too fragmented to be captured in a single data repository. Therefore, IntelliCAD databases will provide a wealth of verified micro fabrication data and will allow users to incorporate their own data into a user defined data area. The user has the option of performing analysis based on IntelliCAD data only, user defined data only, or a combination of user defined data and IntelliCAD data. In addition to creating facilities for users to enter their own process data, IntelliSense is committed to providing updates of all IntelliCAD databases on a regular basis.

Conclusion

Microelectromechanical system (MEMS) technology emerged as an off-shoot of the semiconductor industry and much of MEMS technology is borrowed from the semiconductor industry. However, MEMS technology has unique manufacturing problems that require solutions that can not be borrowed from the semiconductor industry. A large demand exists for MEMS technology but we have failed to adequately manufacture the supply. Fast access to high quality manufacturing know how is essential for the MEMS market to reach its potential. The aim of this presentation is to target the obstacles responsible for the slow growth of this high potential technology and to suggest ways of overcoming them.

IntelliCAD is aimed to circumvent some of the difficulties present in the MEMS manufacturing. It promises to reduce the number of design-fabricate-test iterations required to manufacture a MEMS device. The reduced number of iterations will translate to reductions in time to market and manufacturing cost and will act as a catalyst for new and creative micro fabrication efforts. Currently, there are no fully integrated micro fabrication tools that support the manufacture of MEMS structures.

IntelliCAD will provide the following benefits to the micro fabrication community:


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