Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2000-09-21
2003-04-01
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C073S514210, C257S415000, C700S097000, C703S002000, C716S030000
Reexamination Certificate
active
06542829
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
A method for characterizing parameters describing microelectromechanical (MEMS) structures resulting from a device fabrication process or process variations is presented.
2. Description of Related Art
Monitoring the geometry of fabricated MEMS devices is as fundamentally important to predicting and evaluating device performance as tracking the material properties of the devices or variations in the device fabrication processes. Accurate knowledge and predictability regarding MEMS device geometries can improve device yields and reduce the need for post-fabrication processes such as trimming to achieve targeted device performance specifications. Although material properties generally affect critical performance parameters linearly, device geometry affects device performance characteristics to much higher powers, i.e., the critical performance characteristics of fabricated MEMS devices are generally much more sensitive to variations in device geometry than to variations in material properties.
FIG. 6
summarizes mathematically the extent to which material properties and device geometry affect common critical performance parameters for several different MEMS devices. As can be seen, geometric parameters such as length (L) and width (w) affect critical performance parameters such as mechanical displacement or mechanical force by as much as the third or fourth power, whereas material properties such as density or modulus of elasticity generally only affect such parameters to the first power, i.e., linearly.
Unknown and undesired variations in device geometry can also adversely affect the methods used to extract material properties for MEMS devices from MEMS test structures. Typically, values of selected material properties are extracted from such test structures by physically measuring intermediate parameters, such as pull-in voltages, resonant frequencies, and maximum deflections. The measured values of the intermediate parameters can then be mathematically reduced to derive the desired material property values. However, the measured values of the intermediate parameters depend on device geometry and are sensitive to variations in fabricated geometric dimensions. It is desirable to eliminate such variations as much as possible so that data reduction from the intermediate parameters results in an accurate determination of the values of the selected material properties. The uncertainty introduced by variations in the fabricated geometric dimensions in such test structures is significant and introduces at least first-order errors in the inferred values of the material properties. Variations in geometric dimensions as large as 20% compared to nominal values can easily propagate to errors in extracted material property data as high as 100%.
Conventionally, measurements of MEMS geometries are accomplished ex-situ, and include the use of SEM's, surface profilometers, ellipsometers, and interferometers. However, conventional ex-situ measurement techniques have various limitations and drawbacks. In some cases, measurements can be made to better than 5% accuracy, provided calibration standards for the measurement devices are carefully adhered to. This is particularly essential with respect to SEMs and surface profilometers. In addition, with a SEM, care is needed to align the viewing angle to be orthogonal to the surface of the device being measured, otherwise scaling offsets will be introduced in the dimensional measurements. Ellipsometers are specific to the substrates and layers used in device fabrication and are therefore of limited use. Further, ellipsometers require a laser spot size bigger than typical MEMS devices. Single-wavelength (&lgr;) interferometers may be useful for measuring certain geometric parameters. However, when depth measurements smaller than &lgr;/8 are required, errors can be introduced if the measurement techniques employed do not adjust for imperfect reflection off a device's surface or for thin-film effects resulting from internal reflection within a region of varying thickness or a gap region underneath the device. This may happen with polysilicon and other materials commonly used to fabricate MEMS when they are fabricated less than 3 &mgr;m thick and are partially transmissive at the optical wavelengths commonly used in interferometers.
In-situ electronic test structures for monitoring fabricated MEMS geometries and material properties are attractive because they offer ease of use, the repeatability and control of voltage application, compatibility with standard IC wafer-level probing techniques, limited device area requirements, and integration with real devices. In-situ measurement is especially useful for monitoring material properties which can be highly process-dependent. Mechanical property test structures using in-situ electrostatic actuation include beams and diaphragms actuated to pull-in, laterally resonant comb-drives, vertically resonant beams, and capacitance-voltage measurements of fixed-fixed beam bridges. Non-electrostatic methods for mechanical property measurement include load-deflection and bulge tests of membranes, measuring cantilever tip displacement with an externally applied force, and direct tensile measurement of strain. Non-electrostatic methods are typically carried out ex-situ, and require either specialized structures with modifications or additions to the fabrication process, or special apparatuses to make measurements and apply external forces.
Clearly, the ability to independently characterize the geometry of MEMS devices is essential to accurate, efficient, and successful device design, simulation, and material property extraction. Current methods of ex-situ and in-situ characterization have a variety of limitations and drawbacks. The present invention presents an approach for providing device geometry characterization which overcomes the limitations and drawbacks of the current methods and which advantageously relies only on an optical microscope and standard electronic test equipment used at the wafer-level for independent measurement of geometry.
Other work relevant to characterizing MEMS structures includes the following references, some of which are referred to by reference number in the following sections:
1. Raj K. Gupta, “Electrostatic Pull-in Test Structure Design for Mechanical Property Characterization of Microelectromechanical Systems (MEMS)”, Ph.D. Thesis, June 1997, M.I.T., Cambridge, Mass.
2. William C. Tang, “Electrostatic Comb Drive for Resonant Sensor and Actuator Applications”, Ph.D. Thesis, University of California at Berkeley, 1990.
3. R. I. Pratt, G. C. Johnson, R. T. Howe, and D. J. Nikkel, Jr., “Characterization of Thin Films Using Micromechanical Structures”, Materials Research Society Symposium Proceedings, 276 (1992) pp. 197-202.
4. H. Kahn, S. Stemmer, K. Nandakumar, A. H. Heuer, R. L. Mullen, R. Ballarini, and M. A. Huff, “Mechanical Properties of Thick, Surface Micromachined Polysilicon Films”, Proceedings IEEE MEMS 1996, San Diego, Calif., Feb. 11-15, 1996, pp. 343-348.
5. M. Biebl, G. Brandl, and R. T. Howe, “Young's Modulus of in-situ Phosphorus-doped Polysilicon”, Proceedings of Transducers' 1995, Volume II, Stockholm, SWEDEN, June 1995, pp. 80-83.
6. Jacob P. Den Hartog, “Mechanical Vibrations”, Fourth Edition reprint, Dover Publishing, Inc., Mineola, N.Y., USA, 1985, ISBN 0-486-64785-4.
7. M. A. Schmidt and R. T. Howe, “Silicon Resonant Microsensors”, Ceramic Engineering and Science Proceedings, 8, No. 9-10, September-October 1987, pp. 1019-1034.
8. G. K. Fedder, S. Iyer, and T. Mukherjee, “Automated Optimal Synthesis of Microresonators”, Proceedings of Transducers' 97, Volume II, Chicago, Ill., USA, Jun. 16-19, 1997, pp. 1109-1112.
9. K. Wang and C. T.-C. Nguyen, “High-Order Micromechanical Electronic Filters”, Tenth IEEE International Workshop on MEMS 1997, Nagoya, JAPAN, Jan. 26-30, 1997, pp. 25-30.
10. MEMCAD software is available from Microcosm Technologies, Inc., Cary, N.C.
11. W. H. Press, S. A. Teukolsky,
Coventor, Inc.
Jenkins & Wilson, P.A.
Le John
Shah Kamini
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