Humidity

Humidity can cause degradation of MEMS as discussed previously. Mitigation tech­niques for humidity and salt environments include use of hermetic sealing, moisture-resistant materials, dehumidifiers or desiccants, protective coatings/covers, and reduced use of dissimilar metals.

Deleterious effects are often exacerbated with high humidity. For example, pack­age crack growth has a dependence on moisture that is well documented [41]. Also, electrical performance may change as moisture condenses in gaps causing surface tension that may induce a piezoresistive stress effect [42]. Perhaps best known is the relationship of adhesion and friction of polycrystalline silicon MEMS [43]. This dependence is reduced, but not eliminated, when molecular coatings are applied to the surfaces. Such anti-stiction coatings have the ability to penetrate into the intri­cate side wall and under-surface spaces in three dimensions. Thus, these coatings extend the operating life of MEMS devices by reducing stiction [44].

12.10.2 MEMS Packaging and Quality Assurance

We return to our first table of the chapter discerning differences in the microcircuit and MEMS arenas in respect to packaging. The hermetic packaging segment of the United States grew out of the US military's use of hermetic packaging. The two documents that serve as a starting point for developing qualification and assurance tests for MEMS packaging are MIL-PRF-38535 Integrated Circuits (Microcircuits) Manufacturing, General Specification and MIL-STD-883 Test Method Standard.

MIL-PRF-38535 specification establishes general reliability requirements for integrated circuits or microcircuits. The quality and reliability assurance require­ments must be met for their acquisition. The intent of this specification is to allow the device manufacturer the flexibility to implement best commercial practices to the maximum extent possible while still providing a product that meets military performance needs. MIL-STD-883 Microcircuit Test Methods establishes uniform methods, controls, and procedures for testing microelectronic devices suitable for use within military and aerospace electronic systems including basic environmen­tal tests. These tests determine robustness to deleterious effects of natural elements and conditions surrounding military and space operations; mechanical and electri­cal tests; and workmanship and training procedures. This standard applies only to microelectronic devices. However, MEMS devices in microcircuit packages may be tested in accordance with MIL-STD-883.

Package tests should be defined and tailored in respect to the final applica­tion, usage, life expectancy, shelf life time, and criticality. Table 12.13 offers some suggested tests to be considered for screening MEMS packages.

Whether following the above prescriptive assurance specifications or deriving new specifications, it is important to consider the application and to then apply the appropriate test. As an example, Analog Device have developed stress tests called "Random Drop" and "Mechanical Drop". "Random Drop" is the random-orientation batch drop of packaged devices from a height of 1.2 m onto a marble surface. The drop is repeated for 10 times, with a basic functionality check done between each drop. In a "Mechanical Drop" test, devices are dropped one by one from a height of 0.3 m onto a marble surface, first in the X-axis, then the Y-axis, and finally the Z-axis directions. An electrical screen is performed, and the drop test procedure repeated from a height of 1.2 m [45]. This work by Analog Devices™ is an excellent example of the need to tailor test plans to achieve a reliable program.

12.11 Case Studies

It is rather difficult to show packaging cases as examples, since most MEMS devices require their own individual package design, matching, for example, the application, the interface, or the environment it will be used. There can be hundreds of examples found in the literature for MEMS packages, each one unique. In this section, we will discuss some of the commercial success stories: the Analog Devices accelerometer,

the TI micro-mirror array, MEMS RF devices, and MEMS microphones, as well as more special applications such as biomedical and space applications.

Date: 2015-02-28; view: 1319