Testable Design & Test of Microsystems
First, a global inventory is presented of the most commonly designed microsystems today, like microelectronics combined with relatively easily electrically compatible elements (e.g. electro-magnetic, optical MEMS) via medium electrically compatible devices (e.g. micro-mechanical, MEMS), to outright hostile elements (e.g. fluidic, BioMems). The relation between packaging, testing and repair is discussed.
Next, the currently used functional testing approach is discussed of several micro-system cases. The problems of only using this approach are discussed, such as the lack of testing-grade metrics, cumbersome test times and unsuitability in a mass-production environment, due to e.g. non-electrical (slow) stimuli and responses. As most front-end, embedded and back-end parts of microsystems include analogue/digital conditioning circuitry followed by data converters, it makes sense to use mixed-signal testing and Design-for-Test approaches for the combination of electronics and micro devices.
The way of how to test microsystems is very dependent on their application area with regard to cost. The differences in test budget range from high (space, safety-critical) to extremely low in consumer and disposable (e.g. pharmaceutical) micro-system products. This budget will determine which of the many technical solutions will be used.
Essential in the total microsystem design is to investigate the influence of faults in the total system, and provide the proper stimuli for detecting these faults. This involves the modeling of the fault-free as well as the faulty behavior of the non-electrical components. Crucial problem is to have a good knowledge of the (often not standardized) manufacturing processes, and possible errors that can occur. Often, processes are used which have a close link with microelectronic processes (e.g. dry etching), which helps in this respect. Some examples are given of possible defects in micro devices.
Several examples of fault modeling micro devices, a micro-mechanical and fluidic device, will be provided to show the analytical approach and FEM-based approach and illustrate current possibilities and limitations. Next, it is shown how to translate this information into a microelectronic CAD environment. In an example, the system consequences of local non-electrical behavior will be illustrated.
Equally important is to gain (electronically) access to the micro devices. Several methods will be described (e.g. IEEE 1149.4) to gain access, and the potential advantages and limitations of the different approaches discussed. Sometimes, indirect access is required, e.g. as result of extremely low voltages/currents. Hence, the correct (test) partitioning of micro-devices and their associated microelectronics is of the utmost importance. An example of a micro-flow sensor and associated electronics is provided.
One potential approach in microsystem testing is to reduce (cumbersome) functional tests by first carrying out relatively simple electrical tests. Micro-devices often contain electrical components (e.g. resistors, capacitors), or can be relatively easily enhanced with them. Sometimes they deliver or require voltages/currents which indirectly provides information with respect to their correct operation. One can consider this as Design-for-Test structures in or in close interaction with micro-devices. Several examples will be provided, including the link with self-calibration.
Another approach effectively eliminates the non-electrical stimulus or response in the micro devices by combining sensors and actuators, resulting in a completely electrical input/output combination. A derivative of this method is BIST (Built-In Self Test). An example will be given of an electro-magnetic microsystem, and a table of possibilities in different domains is presented.
In the last module, three microsystems (electro-magnetic, micro-mechanical and fluidics) will be evaluated in terms of Design-for-Test, test generation and testing. It will include digital tests, analogue / mixed-signal tests and (limited) functional tests. The different advantages and disadvantages of the test approaches are dealt with. The parallels with analogue signal generation in different domains (e.g. fluidics) will be given. Potential test systems are briefly discussed, and the use of using DSP techniques in evaluating microsystems is dealt with.