Standard Open Tool Packages for MEMS-enabled Products
MEMS DESIGN SOFTWARE SPEEDS PRODUCTION
BY MARK DA SILVA
Similar to IC development, packaging costs for microelectromechanical system (MEMS) devices may reach as high as 80% of the total product cost. A well-recognized commercialization barrier, this is a significant technological issue for the MEMS industry and has resulted in custom packaging for each application. Today, there are options for reducing MEMS packaging costs to bring MEMS devices to market faster.
Packaging MEMS components differs significantly from the packaging of microelectronics, which is well established. The primarily difference is because, unlike microelectronics, the functional specification of the MEMS chip is critical to package design. Industry experts also recognize that a lack of attention may be a contributing factor to the overall problem within organizations, as well as externally with industry vendors and suppliers.
MEMS Packaging Complexity
Microsystem packaging involves multiple layers of interfaces enabling connectivity from the chip to the outside world. Any unforeseen effects of a MEMS device on a package can ultimately inflate product development cycles, time-to-market and production costs. To move standard packaging procedures to the MEMS market, multiple effects must be considered at all phases. Beginning with the chip (for example, the MEMS device), to module, card, board and finally, frame gate. MEMS typically contain moving parts that are sensitive to package effects, and depending on the application, often need to interact with the outside world in some way. In the working environment, packages often need to provide a controlled environment that is hermetically sealed to protect against environmental stresses and corrosion, as well as mechanical and electrical isolation to improve device robustness. For all these reasons, the complexity of MEMS packaging becomes dependent on the specific application, forcing designers to either consider these effects seriously and come up with unique solutions or spend time going back to the drawing board.
MEMS devices are fragile and must be protected from damage during wafer-level processes such as dicing and cleaning. Wafer-level encapsulation or protection of the devices during back-end fabrication processes is part of the manufacturing flow for MEMS mirrors and inertial sensors, and is an inexpensive technique for increasing yield. In certain applications, wafer-level packaging (WLP) is sufficient for final packaging of the device.
While MEMS' manufacturing processes are somewhat similar to ICs, the functionality of these systems is different and presents unique challenges for MEMS packaging. In microelectronics, open tool packages may be used for several chip designs as long as they meet size and connectivity requirements, which is quite unlike MEMS packaging. Open tool packages, however, serve as useful starting concepts for MEMS packaging. Combined with wafer-level, die-level packaging or encapsulation solutions, this offers an inexpensive path to MEMS product commercialization.
Packaging During the Design Phase
The complex, 3-D, mechanical (suspended) nature of MEMS makes them sensitive to their package environment, significantly more so than conventional IC products. Currently, MEMS design is focused on the component level using specific MEMS CAD tools that address the inherently complex device design. Although some techniques exist that allow for package device co-design, they assume decisions about the kind of package, size, materials etc. have already been made, which is typically not a straight-forward task. Product design groups rely heavily on the expertise of package designers (with experience typically from micro- or opto-electronics industries) to choose a package based on a variety of factors such as cost, availability, customization, etc., and then design the package based on specifications of the MEMS device. This typically has led product developers to postpone package selection until after the initial device design has been completed, because design modifications are time-consuming, complex, and costly to rectify. This also has led to inefficiencies in the design process because of the time involved per design iteration. Although the industry is starting to recognize the specific differences between MEMS and IC packaging, and the need to protect the MEMS during fabrication, addressing these challenges often requires access to industry-specific tools and know-how that often resides with package suppliers. It is important for suppliers to share data with designers to overcome these barriers.
Reducing MEMS Packaging Co-design Complexity
Until now, there was no simple way to share detailed package design data in the early stages of the MEMS design process. Recent availability of a set of standard open tool IC packages within an existing MEMS CAD tool environment provides detailed package design data in an easy-to-use, accessible environment. New ground has been broken by the collaboration of a package supplier* and a MEMS design tool provider**. These two groups have standard packaging libraries that may be used to select and analyze the effects of a package on a MEMS device in a single environment (Figure 1). The goal is to preempt some of the issues discussed earlier, arising from MEMS packaging, and ultimately speed their commercialization.
Figure 1. This MEMS layout editor can include changes in design.
Ceramic technology provides superior performance because of its mechanical strength, thermal conductivity and heat durability, and is durable enough for harsh environments and cost effective for the consumer market. Through the availability of open-tooled packaging solutions, MEMS designers have more choices for initial package selection in terms of chip size, number of electrical connection, etc., and to consider the effects of packaging early in the design phase. With access to package geometry and materials data, designers can choose specific package concepts from a variety of package types, initiate performance-based design (such as thermomechanical effects), and modify package data to specific microsystem requirements, to shorten the design cycle, reduce risk and decrease time-to-market (Figure 2). The supplier's packaging libraries are now bundled with a MEMS design software suite. This software is used to develop MEMS-based products. The availability of these package libraries to such a wide group of designers, at no additional cost, will help accelerate the design process.
Figure 2. After simulated modeling, die deformations can be transferred to new anchor models using this software.
Packaging Library Content
For MEMS design, packaging data is best introduced in the form of 3-D models. The library allows designers to create the 3-D geometry from supplied data, modify the existing provided geometry or import it in standard format. A complete description of the materials used in each package type is available. Since a variety of views exist for the package, it is possible for a designer to modify the package geometry and material properties completely — allowing for custom package design. These custom designs can be sent back to the supplier for feedback or fabrication.
The package library contains models of a variety of open tool packages types including surface mount devices (SMD), side brazes (S/B), pin grid arrays (PGA), cerdips (C-DIP), leadless chip carriers (LCC), flat packs (F/P), and chip scale packages (CSP). Each package model includes material and geometry data (including mesh), as well as the ability to modify these parameters. The library is embedded within this existing software interface, for ease of modeling and analysis, and contains detailed documentation.
During the design phase, the availability of package information enables creation of physically precise packages in terms of geometry and material properties. A package model or several models can be brought up and co-simulated with the MEMS device to predict a variety of typical package effects, such as coupled thermoelectromechanical or thermofluidic effects or coupled RLC analysis. In the working environment, packages experience a variety of loads such as high-G loads, shock impact, electrical currents or fields, operational temperature ranges, ESD, etc., and it is important to understand the effects of these loads on the operation of the MEMS device. The designer also needs to understand the effects of other external stimuli such as noise and vibration. The MEMS designer must be able to observe the coupling between the various subsystems — MEMS, IC, and packaging. System-level modeling is a powerful technique to allow the MEMS product group to couple the various subsystems in a single environment. Elements of the package library can be converted directly into macro-models that are available to system-level descriptions of the product, thereby allowing for a higher level of optimization of the product.
The ability to bring in standard open tool packages for consideration early in the MEMS design phase, offers product development groups a unique way for the package and device designers to communicate within the same CAD environment to solve problems that are unique to the technology (Figure 3). The ability to import or build, analyze and troubleshoot packaging effects on MEMS devices early in the design cycle enables designers to limit product risk and reduce cost. Further, it offers package designers a path to start with an available package and make changes with the microsystem in mind, and to feed those changes directly back to the supplier for custom development if necessary. Ideally, these packaging capabilities are coupled with a complete MEMS design methodology that begins with schematic-based, system-level design and simulation. It will then enable physical design from 2-D layout and fab process descriptions to 3-D geometry..The next step is to incorporate 3-D field solvers for detailed finite element method and binary element method analysis of electrostatics and coupled electromechanics and other properties. Finally, the methodology will produce extractions of reduced-order behavior models for use in standard EDA or other analysis tools. Ultimately, combining this methodology with packaging analysis capabilities will bring MEMS-enabled products to market faster and at a lower cost.
Figure 3. Example of a simulated MEMS package.
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**CoventorWare from Coventor.
MARK DA SILVA, Ph.D., manager applications engineering, may be contacted at Coventor Inc., 625 Mount Auburn Street, Cambridge, MA 02138; (617) 497-6880 X249; e-mail:.firstname.lastname@example.org.