Publications - Sensors and Actuators Lab

Miniature Fiber Optic Pressure Sensors Based on Polymer Process

Miniature Fabry-Perot (FP) pressure sensors have been of great interest because of their advantages of small sizes, high performance, and immunity to electromagnetic interference. Most of these sensors are built with silicon/silica materials that have good mechanical, chemical, and thermal stabilities. However, due to the large elastic modulus of silica/silicon, developing a high sensitivity miniature sensor becomes difficult. In addition, fabrication of these sensors often involves high temperature fusion bonding and harsh acid etching. On the other hand, to realize high sensitivity, miniature pressure sensors, a polymer material becomes an attractive choice since its Young’s modulus is significantly smaller than that of silicon/glass. Moreover, polymer processes can be performed under ambient pressure and temperature without hazardous chemicals. However, a significant drawback of a polymer-based sensor is the high temperature sensitivity, which must be compensated to obtain accurate pressure measurements.

Miniature temperature Compensated Fabry Perot Pressure Sensor Fabricated with Self-aligned Photolithography Process [1, 2]

Since there are many available processing techniques that can be used for polymer materials and performed in ambient pressure and temperature, sensor fabrication can be easy by using these processes. In this work, fiber optic Fabry-Perot sensor fabricated by using a self-aligned photolithography technique is presented. Instead of using the conventional mask and mask aligner for the photolithography process, an optical fiber waveguide is used to couple UV light for exposing and defining the optical cavity. This low-cost fabrication technique offers self-aligned property for sensor fabrication. Fiber Bragg gratings were embedded in the sensors to measure temperature and compensate temperature effect on the polymer cavities. Figure 1 illustrates the fabrication process of two types of sensors (i.e.: co-axial and cross-axial sensors) fabricated with the self-aligned photolithography. The cavities defined by the self-aligned photolithography were covered with another UV-curable polymer layer and thin metal layer. This polymer/metal composite layer was used as a pressure sensitivity layer for pressure measurements. The cross-axial sensor configuration has surface-mountable capability when a surface-flow should be avoided in pressure measurements.

Figure 1: Fabrication process of co-axial and cross-axial sensors

Scanning electron micrographs of the fabricated co-axial and cross-axial sensors are shown in Figure 2.

Figure 2: Scanning electron micrographs of fabricated sensors: (a) co-axial sensor and (b) cross-axial sensor.

Miniature Temperature Compensated Fabry-Perot Pressure Sensor Created by Using UV-molding process with an Optical Fiber Based Mold [3]

UV-molding process (or UV imprinting process) is one of fabrication processes that make use of a UV curable polymer. The process usually consists of several steps including master pattern fabrication, mold fabrication, dispensing/exposure, and demolding. If a master pattern is used as a mold, the first step can be omitted. This process has been applied to fabricate various optical elements with small features, which allows for enhanced thermo-mechanical stability, low birefringence, and low-cost. In this work, a Fabry-Perot (FP) pressure sensor with built-in temperature measurement and compensation capability is designed and fabricated based on a UV-molding process and the metal-polymer composite thin film fabrication method described previously. Figure 3 illustrate the fabrication process of the sensor.

Figure 3: Fabrication process of UV-molded fiber optic pressure sensor.

The sensor is composed of a UV-molded optical cavity and a polymer-metal composite diaphragm, which is fabricated at the end of a single mode fiber with a pre-written fiber Bragg grating, rendering the sensor the capability of temperature compensation and simultaneous measurement of pressure and temperature. The mold used for UV-molding process is built by assembling a polished ferrule and a cleaved optical fiber, which eliminates the need for a costly semiconductor or direct machining process and enables high accuracy optical alignment of the mold and the sensing fiber. The sensor fabrication follows simple, repeatable processes and safe procedures, and uses less expensive materials and equipment. This fabrication method can also be used for batch fabrication or wafer scale fabrication with minor modifications to the processes. Scanning electron micrographs of the fabricated co-axial and cross-axial sensors are shown in Figure 4.

Figure 4: SEM images of (a) fabricated sensor and (b) diaphragm close-up.

Publications on this work

[1] Bae, H., Zhang, X. M., Liu, H., & Yu, M., ”Miniature surface-mountable Fabry-Perot pressure sensor constructed with a 45 degree angled fiber,” Optics Letters, 35(10), 1701–1703 (2010).

[2] Bae, H., Dunlap, L., Wong, J., & Yu, M., “Miniature Temperature Compensated Fabry – Perot Pressure Sensors Created With Self-Aligned Polymer Photolithography Process,” IEEE Sensors Journal, 12(5) (2012).

[3] Bae, H., & Yu, M., “Miniature Fabry-Perot pressure sensor created by using UV-molding process with an optical fiber based mold,” Optics Express, 20(13), (2012).

Last updated on September 10th, 2010