Even if you want to redesign the MEMS sensor of the drive motor, the goal is to make the job as quick and easy as possible. With the MEMS library in Tanner L-Edit, you can choose from a large number of different basic units of MEMS devices and then quickly assemble your MEMS layout. Using the Tanner design tool, you can use the design flow shown in Figure 1 to design a complete "design is right" MEMS process.
Figure 1: MEMS Creation Design Flow
There are many traditional design processes in the field of MEMS creation. Figure 1 shows a top-down design flow that connects the different design areas required for MEMS design. This process provides the design team with a seamless solution to help integrate digital design, analog design, and MEMS devices into an IC chip.
Initial steps: Create and verify the circuit diagram
First, create a new cell for the MEMS device in the S-Edit schematic editor, and then connect the basic cell symbols that make up the device in the test circuit. Next, use T-Spice to analyze the design and check the waveform of the working condition through the W-Edit waveform viewer to determine whether the device meets expectations.
Specify layer information
After being satisfied with the circuit diagram, use L-Edit to create the physical layout of the MEMS device. In L-Edit, you can use the MEMS basic cell device in the MEMS library.
You can control the layer information for each base unit and specify the layer information for two types of basic MEMS components:
• Surface micromachining. You can specify the number of structural layers and define the parameters for each layer. You can also specify design rules for each layer, such as spacing, bracketing, and minimum line width requirements.
• Fluid channels. You can specify the number of structural layers, as well as define the parameter sets that correspond to the fluid elementary units in the MEMS library.
This layer of information is automatically applied to the relevant basic units in the MEMS library.
Creating basic units using MEMS libraries
The MEMS library contains more than 40 parameterized basic units for layout layout of MEMS devices. The basic unit continues to increase with each new release of the tool. The basic units provided by the MEMS library are grouped by category, as shown in Table 1.
Table 1: Available Base Units
To create a MEMS device, simply use the Library Palette to select each basic cell from the MEMS library, as shown in Figure 2.
Figure 2: Assembling a MEMS device using the Library Palette for layout
For example, to create a Transverse comb drive resonator, call the desired base unit instance and make the connection in the layout. To illustrate the simplicity and directness of the process, we can create a MEMS transducer; using its resonant frequency's high sensitivity to physical parameters, it turns it into a sensor. Figure 3 describes the creation steps:
1. The instance invokes Plate. 2. The instance calls a comb drive and copies one. 3. Connect the two comb drives to the Plate. 4. The instance calls the folding spring, flips it over, and copies one. 5. Connect the two folding springs to the Plate. 6. The instance calls the Ground Plate, connecting it to the structure on the appropriate layer. 7. Instantiate the three pads and connect them to the appropriate layers and components. 8. For each instantiated base unit, adjust its parameters to meet your needs (see next section).
Figure 3: Completed Lateral Drive Resonator
Change parameters
You can change any of the defined parameters to affect the elements of the MEMS structure. For example, Figure 4 shows that the layout changes immediately after changing the key parameters of the basic unit of a harmonic-side drive motor.
Fig. 4: Changing the motor basic unit quickly by adjusting only one parameter
With the parametric basic unit, you can quickly build MEMS structures. The method is to make a specific base unit by calling the base unit, copying and pasting, changing direction, and changing parameters. Each basic unit contains a set of default parameter values ​​that you can use to quickly lay out basic units, then select each basic unit and change its parameters.
Design your own basic unit
The MEMS library contains more than 40 basic units that you can use in MEMS structures. However, the world of MEMS design is very broad, and sometimes you may need to define your own basic unit and put it on the Library Palette for your team to use. Figure 5 shows the flow of designing the basic unit.
Figure 5: Process Overview of Self-Designing Basic Units
The steps to create a basic unit yourself are as follows:
1. Make a circuit symbol. Use S-Edit to define the symbol entity, attribute, and port. 2. Create a SPICE model. In order to simulate this basic unit in T-Spice, you need to define the SPICE model. MEMS devices can be multi-domain systems: electrical and non-electrical. This requires defining a mapping between SPICE electrical variables and non-electrical domain variables. For example, the pressure on the Plate can be mapped to the current in the electrical domain. 3. Write a layout generator. Define a layout generator for a custom base unit using the C programming language and L-Edit UPI calls. The C program accepts the basic unit parameters and generates the required layout information. Ending the Process After building the MEMS device layout using the Library Palette in L-Edit, there are the last few steps to complete, as shown in Figure 6.
Figure 6: Steps to complete MEMS design
These steps include:
• Establish a connection relationship. An often overlooked step in ending the process is to establish a connection between the basic units in order to correctly extract the SPICE netlist. For each base unit instantiated with the Library Palette, the correct port is automatically created. L-Edit can establish a connection between two or more basic unit ports. In the resonator example, you should connect one of the plate's ports to one of the ports of the comb drive.
• Layout extraction. Using L-Edit to extract layouts results in a SPICE netlist consisting of MEMS devices and connection information. Use this extracted information to compare layout and circuit diagrams (so-called "layout vs. circuit diagram" or LVS).
• Circuit diagram extraction. For LVS, use S-Edit to extract schematic netlists. This netlist contains device descriptions, connections, and geometric parameters.
• Perform LVS. With the LVS tool, you can compare layouts and circuit diagrams to make sure that both describe the same device.
Each MEMS device you create can be adjusted to the manufacturing process to ensure that the structure meets the design rules. L-Edit allows you to choose from a set of predefined manufacturing processes. Process information includes layer definitions, design and extraction rules, model parameter values, macros, and process definitions.
Three-dimensional analysis and system-level simulation
With the completed layout data, you can automatically generate a three-dimensional view of the device. Interact with 3D views and add technical information before sending the design to a 3rd party 3D analysis tool to check the physical interactions between multiple domains (eg mechanical, thermal and electrostatic). You can then extract the behavioral model of the MEMS device to perform system-level simulations based on 3D analysis data.
Conclusion
With the Tanner MEMS parameterized basic cell library, you can quickly create complex surface micromachining or fluid MEMS devices. By instantiating each basic unit, setting parameters, and assembly layout in L-Edit, you can quickly define and analyze the device. Perhaps you are redesigning the sensor of the MEMS motor, but using the Tanner tool set, you can complete the task as quickly as possible.
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