System measures miniature electromechanical structures

In the development of pressure transducers and accelerometers, designers are now using microelectromechanical systems (MEMSs) etched in silicon. A MEMS pressure transducer can consist of one or more micromachined diaphragms measuring from 25 mm to 5 mm in diameter and 10 mm deep. In operation, applied pressure produces a deflection in the silicon diaphragm that can be amplified and calibrated in pounds per square inch.

Jan 1st, 1999

System measures miniature electromechanical structures

In the development of pressure transducers and accelerometers, designers are now using microelectromechanical systems (MEMSs) etched in silicon. A MEMS pressure transducer can consist of one or more micromachined diaphragms measuring from 25 mm to 5 mm in diameter and 10 mm deep. In operation, applied pressure produces a deflection in the silicon diaphragm that can be amplified and calibrated in pounds per square inch.

To inspect the thickness of these MEMSs, Virginia Semiconductor (Fredricksburg, VA) and Virginia Technologies (Charlottesville, VA) have developed the OMMS-1, an automatic wafer-inspection system capable of calculating and displaying a thickness map over the entire wafer region. In operation, the system uses laser light that is attenuated by the silicon wafer. By calibrating the attenuation to known silicon thickness standards, the system can measure micromachined features as thin as 5 mm and as small as 100 mm wide.

During the development of this system, PC-based off-the-shelf hardware/software tools were used exclusively. The OMMS-1 incorporates a 200-MHz Pentium Pro running Microsoft Windows NT. The system also uses data-acquisition, image-capture, and motion-control hardware from National Instruments (Austin, TX ). System software was developed using the National Instruments LabView G-graphical programming language.

The OMMS-1 system interfaces a TM-1010 monochrome digital CCD camera from Pulnix (Sunnyvale, CA) and a National Instruments PCI-1424 digital image-acquisition board. For positioning its x-y stage, the system uses the National Instruments PC-Step-4CX motion-control board and related ValueMotion software. Digital and analog I/O for laser control and safety interlock switch monitoring is accomplished using the National Instruments AT-MIO-16XE-50 data-acquisition board.

During system operation, laser light is passed through the wafer under evaluation and imaged onto a CCD array. The OMMS-1 then calculates the wafer`s thickness as a function of its optical absorption. After image acquisition, National Instruments Imaq Vision VI libraries process the image and derive the wafer thickness. A LabView intensity graph then displays the wafer region under evaluation as a gray-scale thickness map. The technician can then select vertical-, horizontal-, or diagonal-line profiles to show the thickness values across the evaluation region.

"By using LabView, an estimated 50% reduction in development time was achieved over an alternative solution based on custom hardware and Visual C++ programming," says Robert A. Ross, president of Virginia Technologies. The OMMS-1 is currently being beta-tested by a manufacturer of MEMS pressure transducers.

More in Boards & Software