For more details, please see our publications:

Youngmin Kim and D. P. Neikirk, "Micromachined Fabry-Perot Cavity Pressure Transducer," IEEE Photonics Technology Letters 7, Dec. 1995, pp. 1471-1473.

Y. Kim and D. P. Neikirk, "Micromachined Fabry-Perot Pressure Transducer with Optical Fiber Interconnects," in Micromachined Devices and Components, Ray Roop, Kevin Chau, Editors, Proc. SPIE 2642, pp. 242-249, Austin, Texas, USA, 23-24 October, 1995.

J. Han and D. P. Neikirk, "Deflection behavior of Fabry-Perot pressure sensors having planar and corrugated membrane," SPIE's Micromachining and Microfabrication '96 Symposium: Micromachined Devices and Components II, R. Roop and K. Chau, Proc. SPIE 2882, Austin, Texas, USA, 14-15 October, 1996, pp. 79-90.

 

Fabry-Perot Based Pressure Transducers

Youngmin Kim and Dean P. Neikirk

 

Microsensors embedded in mechanical systems should allow localized measurements, for example pressure and temperature, without disturbing measurand. For instance, local measurements will determine dynamic characteristics of the fluid film in a bearing, allowing bearing performance to be monitored. The microsensor for pressure measurement could be implemented using optical interferometry as shown in Figure 1. Pressure applied to the membrane is measured by detecting the deflection of the membrane. Advantages of optical measurement include: remote data acquisition can be achieved without loss of ratio of signal to noise (S/N ratio), pressure averaging effect reducing sensitivity of piezoresistive pressure sensor is avoided, and dimensions of device could be much smaller than capacitive pressure sensor.

The Fabry-Perot cavity and optical fiber are used as the sensing element and interconnect, respectively. The cavity is monolithically built by etching a sacrificial layer that lies between dielectric film stacks (Figure 2). The gap of the cavity can be precisely adjusted by controlling the thickness of a sacrificial layer grown using LPCVD. With LPCVD, multiple dielectric films (consisting of silicon dioxide and silicon nitride) can be deposited to form wavelength selective dielectric mirrors. The technique allows for batch fabrication of the pressure sensors with excellent alignment and parallelism of the two mirrors in the cavity. This alignment has been a problem in previous devices which use a hybrid assembly technique.

Fabry-Perot cavity-based sensors have been widely used for their versatility; for example they have been used to sense both pressure and temperature [1-4]. This kind of sensor detects changes in optical path length induced by either a change in the refractive index or a change in physical length of the cavity. Micromachining techniques make Fabry-Perot sensors more attractive by reducing the size and the cost of the sensing element. Another advantage of the miniature Fabry-Perot sensor is that low coherence light sources, such as light emitting diodes (LEDs), can be used to generate the interferometric signal, since the optical length of the miniature cavity is of the same order as the wavelength of the light, and shorter than the coherence length of a typical LED.

In these devices the cavity mirrors can be either dielectric layers or metal layers deposited or evaporated during the manufacturing process. The thickness of each layer must be tightly controlled to achieve the target performance of a sensor. However, there are unavoidable errors in thickness even though techniques of thickness control for thin films have rapidly improved [5]. For Fabry-Perot optical interference filters it has long been recognized that the performance of the filter is greatly influenced by random thickness variations in the films used [6,7]. For instance, the resonant wavelength is very sensitive to thickness variations. Unlike Fabry-Perot filters, in which the operating regime is usually near the resonance of a high finesse cavity, most Fabry-Perot sensors operate in a transition region between two resonances of a low finesse cavity. We have considered the impact of manufacturing errors on the performance of such sensors. In particular, we have considered how random errors in thickness of the cavity mirrors influence the accuracy with which gap can be measured. We have found that an optimum combination of initial gap and mechanical travel of the cavity exists for a given mirror design which gives the least variation in response curve. This should allow the high yield fabrication of sensors with a specified level of measurement accuracy [8].

Fabry-Perot Transducer Experimental Results

We have recently completed fabrication of a surface micromachined Fabry-Perot pressure transducer. An air-gap cavity has been formed by etching a sacrificial layer selectively using windows in the "top" or "front" side mirror, as shown in Fig. 2 and Fig. 3. Polysilicon is used as a sacrificial layer and KOH as the etching solution. Depending on the geometry of the polysilicon sacrificial layer and the geometry of the etch windows, it was observed that membranes formed by undercut etching tend to be fractured when they become larger than 50 microns. This phenomenon has been reported elsewhere [9]. The fracture of the membrane is believed to occur at stress concentration points in the membrane that are determined by the shape of the windows. Stress induced bending of the top mirror must also be avoided in the optically active areas; this can be accomplished even in devices larger than 50 microns by carefully selecting the shape and placement of the windows, as shown in Fig. 3.

Optical measurements have been made on the device shown in Fig. 3 to verify the accuracy of our plane wave models for the Fabry-Perot cavity. Future devices will use sacrificial etch windows placed in the lower membrane, allowing the poly etch to be performed through the back access hole. This will produce a top membrane that is completely sealed, as required for our bearing applications. Initial measurements have been made in the configuration shown in Fig. 3. This configuration does not have optimum sensitivity since the membrane that bends (the lower membrane) is pinned at its edges, thus producing no change in interference when pressure is applied. Even so, Fig. 4 clearly shows the impact of applied pressure on the optical reflectivity of the cavity. Comparison between our model and the measurements is quite good, indicating the validity of the simple plane wave approach used to calculate the characteristics of the Fabry-Perot cavity.

References

1. B. Halg, "A silicon pressure sensor with a low-cost contactless interferometric optical readout," Sensors and Actuators A, vol. 30, pp. 225- 229, 1992.

2. J. P. Dakin, C. A. Wade, and P. B. Withers, "An optical fiber pressure sensor," SPIE Fiber optics '87:Fifth International Conference on Fiber optics and Opto-electronics, vol. 734, pp. 194 - 201, 1987.

3. C. E. Lee and H. F. Talyor, "Fiber-optic Fabry-Perot Temperature Sensor Using a Low-Coherence Light Source," Journal of Lightwave Technology, vol. 9, pp. 129 - 134, 1991.

4. R. A. Wolthuis, G. L. Mitchell, E. Saaski, J. C. Hartl, and M. A. Afromowitz, "Development of medical pressure and temperature sensors employing optical spectrum modulation," IEEE Trans. on Biomedical Engin., vol. 38, pp. 974 - 980, 1991.

5. H. A. Macleod, Thin-film optical filters. New York: McGraw-Hill, 1986.

6. P. Bousquet, A. Fornier, R. Kowalczyk, E. Pelletier, and P. Roche, "Optical filters: monitoring process allowing the auto-correction of thickness errors," Thin Solid Films, vol. 13, pp. 285 - 290, 1972.

7. H. A. Macleod, "Thin film narrow band optical filters," Thin Solid Films, vol. 34, pp. 335 - 342, 1976.

8. Y. Kim and D. P. Neikirk, "Design for Manufacture of Micro Fabry-Perot Cavity-based Sensors," submitted to Sensors and Actuators A, 1994.

9. O. Tabata, K. Shimaoka, and S. Sugiyama, "In Situ Observation and Analysis of wet etching process for Microelectromechanical Systems," Proc. IEEE Workshop on Microelectromechanical Systems, 1991, pp. 99-102.

Figures

Figure 1: Schematic diagram illustrating an optical-interrogated Fabry-Perot pressure sensor interconnectd via an optical fiber.

Figure 2: Cross-sectional SEM of a air-gap Fabry-Perot cavity fabricated using surface micromachining.

Figure 3: Photomicrograph (top view) and cross-sectional diagram of an air-gap Fabry-Perot cavity formed by surface micromachining. The top and bottom mirrors for the cavity are silicon dioxide/silicon nitride dielectric stacks and the sacrificial layer is polysilicon. Optical access is provided by bulk-micromachining through the back of the wafer using standard anisotropic etch procedures. An optical fiber is then inserted into the hole, and the reflected light intensity monitored.

Figure 4: Reflectance of a FP cavity pressure sensor as a function of pressure. A multimode optical fiber was used as the interconnect and a He-Ne laser operating at a wavelength of 633 nm was used as a light source.