MEMS (Micro Electro-Mechanical Systems) Technology
In less than 20 years, MEMS (micro electro-mechanical systems) technology has gone from an interesting academic exercise to an integral part of many common products. But as with most new technologies, the practical implementation of MEMS technology has taken a while to happen. The design challenges involved in designing a successful MEMS product (the ADXL2O2E) are described in this article by Harvey Weinberg from Analog Devices.
In early MEMS systems a multi-chip approach with the sensing element (MEMS structure) on one chip, and the signal conditioning electronics on another chip was used. While this approach is simpler from a process standpoint, it has many disadvantages:
* The overall silicon area is generally larger.
* Multi chip modules require additional assembly steps.
* Yield is generally lower for multi chip modules.
* Larger signals from the sensor are required to overcome the stray capacitance of the chip to chip interconnections, and stray fields necessitating a larger sensor structure.
* Larger packages are generally required to house the two-chip structure.
Of course, history teaches us that integration is the most cost effective and high performance solution. So Analog Devices pursued an integrated approach to MEMS where the sensor and signal conditioning electronics are on one chip.
Figure 1
The latest generation ADXL2O2E is the result of almost a decades worth of experience building integrated MEMS accelerometers. It is the world's smallest mass-produced, low g, low cost, integrated MEMS dual axis accelerometer.
The mechanical structure of the ADXL2O2E is shown in Figure 1 along with some key dimensions in Figure 2.
Figure 2
Polysilicon springs suspend the MEMS structure above the substrate such that the body of the sensor (also known as the proof mass) can move in the X and Y axes. Acceleration causes deflection of the proof mass from its centre position. Around the four sides of the square proof mass are 32 sets of radial fingers.
These fingers are positioned between plates that are fixed to the substrate. Each finger and pair of fixed plates make up a differential capacitor, and the deflection of the proof mass is determined by measuring the differential capacitance.
This sensing method has the ability of sensing both dynamic acceleration (i.e. shock or vibration) and static acceleration (i.e. inclination or gravity).
The differential capacitance is measured using synchronous modulation/demodulation techniques. After amplification, the X and Y axis acceleration signals each go through a 32KOhm resistor to an output pin (Cx and Cy) and a duty cycle modulator (the overall architecture can be seen in the block diagram in Figure 3). The user may limit the bandwidth, and thereby lower the noise floor, by adding a capacitor at the Cx and Cy pin.
The output signals are voltage proportional to acceleration and pulse-width-modulation (PWM) proportional to acceleration.
Using the PWM outputs, the user can interface the ADXL2O2 directly to the digital inputs of a microcontroller using a counter to decode the PWM.
Figure 3
Challenges in MEMS Design
The mechanical design of microscopic mechanical systems, even simple systems, first requires an understanding of the mechanical behaviour of the various elements used. While the basic rules of mechanical dynamics are still followed in the miniaturised world, many of the materials used in these structures are not well mechanically characterised. For example, most MEMS systems use polysilicon to build mechanical structures. Polysilicon is a familiar material in the IC world, and is compatible with IC manufacturing processes.
Until recently, little work has been done to fully understand polysilicon's mechanical properties. In addition, many materials mechanical properties change in the microscopic world. Again, polysilicon is a good example. In the macro world it is rarely used as a mechanical element. It is too brittle and fragile to withstand all but small mechanical deflections. But in the
extremely small movements of MEMS structures (less than a few pm), it turns out to be an almost ideal material.The electronic design of MEMS sensors is very challenging. Most MEMS sensors (the ADXL2O2E included) mechanical systems are designed to realise a variable capacitor. Electronics are used to convert the variable capacitance to a variable voltage or current, amplify, linearise, and in some cases, temperature compensate the signal. This is a challenging task as the signals involved are very minute.
In the case of the ADXL2O2E for example, the smallest resolvable signal is approximately 2OzF and this is on top of a common mode signal several orders of magnitude greater than that! Of course, for cost reasons the
electronics must be made as compact as possible at the same time.
The integrated approach presented further challenges.
Many standard production steps that improve the mechanical structure degrade the electronics and vice versa. For example, the usual method for flattening out the Polysilicon mechanical structure is annealing (where the structure is exposed to controlled high temperatures). While the annealing process is beneficial to the mechanical structure, it can degrade or destroy the BiMOS transistors used in the signal conditioning electronics. So compatible mechanical and electronic process methods had to be devised.Another roadblock for the MEMS designer has been the unavailability of standard design software. Modern integrated circuits are rarely designed by hand. Complex CAD and simulation software is used to help design and optimise the designers concepts.
MEMS design software is still in its infancy, and most MEMS manufacturers develop part or all of their CAD and simulation software to suit their particular needs.
The fabrication process design challenge is perhaps the greatest one. Techniques for building three-dimensional MEMS structures had to be devised. Chemical and trench etching can be used to "cut out" structures from solid polysilicon, but additional process steps must be used to remove the material underneath the patterned polysilicon to allow it to move freely.
Standard plastic injection molded IC packaging cannot be used because of the moving parts of the MEMS structure. A cavity of some type must be maintained around the mobile MEMS structure. So alternative low-cost cavity packaging was developed.
In addition, this package must also be mechanically stable as external mechanical stress could result in output changes.Even mundane tasks, such as cutting the wafer up into single die, becomes complicated. In a standard IC the particle residue created by the sawing process does not effect the IC. In a moving MEMS structure these particles can ruin a device.
The Users Challenge
MEMS sensors, like almost all electronic devices, do not exhibit ideal behaviour. While most designers have learned how to handle the non-ideal behaviour of op-amps and transistors, few have learned the design techniques used to compensate for non-ideal MEMS behaviour. In most cases, this type of information is not available in textbooks or courses, as the technology is quite new. So generally designers must get this type of information from the MEMS manufacturer.Analog Devices, for example, maintains a web site with design tools, reference designs, and dozens of application notes specific to its MEMS accelerometers to ease the users work.
Conclusion
As with all new technologies both designers and users of MEMS devices have a learning curve to overcome. The effort is worthwhile, as the latest generation MEMS devices high performance and low cost have enabled innovative new products in dozens of markets.
____________________________________________________Harvey Weinberg is an Applications Engineer for Analog Devices Inc.
Micromachined Products Division in Cambridge, Massachusetts.For more information, please contact :-
Analog Devices - Tel: +44(0) 1932 266013
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