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We present a miniature Fabry-Perot pressure sensor fabricated at the tip of an optical fiber with a pre-written Bragg grating by using UV-molding polymer process. The mold is constructed by integrating an optical fiber of 80 μm diameter with a zirconia ferrule. The optical fiber based mold makes it possible to use optical aligning method to monitor the coupled intensity between the mold-side and replica-side fibers, rendering a maskless alignment process with a submicrometer accuracy. A polymer-metal composite thin diaphragm is employed as the pressure transducer. The overall sensor size is around 200 μm in diameter. Experimental study shows that the sensor exhibits a good linearity over a pressure range of 1.9-7.9 psi, with a sensitivity of 0.0106 μm/psi. The fiber Bragg grating is exploited for simultaneous temperature measurements or compensation for temperature effects in pressure readings. The sensor is expected to benefit many fronts that require miniature and inexpensive sensors for reliable pressure measurement, especially biomedical applications.
©2012 Optical Society of America
1. Introduction
Miniature fiber-optic pressure sensors have attracted much interest for pressure monitoring due to their advantages of small size, high sensitivity, immunity to electromagnetic interference, and convenience of light guiding/detection through optical fibers [1]. An extrinsic Fabry-Perot (FP) interferometer created between a fiber endface and a reflective diaphragm that is suspended over a housing structure is one of the most common sensor configurations. Many types of miniature FP pressure sensors have been reported in literature. In terms of the sensor material, miniature FP pressure sensors can be categorized into the following categories: i) all silicon/silica sensors with housing and diaphragm both made of silicon/silica materials [2–7], ii) sensors with polymer housing and polymer/silica diaphragm with metal reflective layer [8–10], iii) sensors with silicon/silica housing and polymer (or polymer/metal composite) diaphragm [11–14], iv) sensors with zirconia housing and metal diaphragm [15,16].
Among various sensor materials, silicon/silica and polymer materials are the most popular choices. Silicon/silica based sensors have superior chemical/thermal stability and mechanical strength due to their material characteristics. However, the fabrication processes of silicon/silica sensors often involve high temperature and hazardous chemical. These processes are costly and require extra caution due to the special equipment needed and harsh chemicals used. Furthermore, the locally elevated temperature during the process can cause thermal shock or stress, which result in unexpected deformation of the structure. For sensors made by using chemical etching processes, the signal-to-noise ratio of the sensors is usually low due to the relatively high surface roughness of the processed surface. In addition, silicon/silica based sensors usually have limited sensitivities due to the high elastic moduli of the materials.
On the other hand, polymer materials have become excellent choices for making optical components including optical sensor structures due to their advantages of easy fabrication, low-cost processing, and versatility. A low modulus of elasticity renders the polymer based pressure sensors adequate sensitivity even for sensors with a miniature size. However, sensors made of polymer materials are not as thermally and chemically stable as those made of silicon/silica materials. The coefficient of thermal expansion (CTE) of polymer is about 5 to 15 times greater than that of glass or silicon. Due to the large CTE, polymer based pressure sensors inevitably suffer from the drift due to temperature variations. In order to obtain accurate pressure readings, the temperature effect should be addressed properly. For example, temperature compensation can be performed by measuring the temperature at the vicinity of the sensor and calibrating the temperature effect [17,18], or adopting a self-temperature compensated mesa structure or inner structure in the optical cavity [19,20].
Currently, most polymer-based pressure sensors that make use of polymers as FP housing structure materials are fabricated by using conventional semiconductor processes such as photolithography, chemical vapor deposition (CVD), and/or reactive-ion etching (RIE) in wafer scale [8,9]. Batch fabrication of multiple sensor heads can be obtained with these fabrication methods. However, these processes involve several high precision masks for selective removal processes and are performed by using expensive equipment in a cleanroom. Moreover, assembling the fabricated sensors heads to optical fiber ends can be challenging and tedious because of small size of the components.
In this paper, a miniature pressure sensor is presented which exploits a unique UV-molding process for fabrication and has built-in temperature measurement and compensation capability. Figure 1 illustrates the UV-molded pressure sensor, which consists of a cleaved or polished optical fiber with a pre-written fiber Bragg grating (FBG), a polymer housing structure directly molded at the fiber end, and a metal-polymer composite sensing diaphragm. Although UV-molding process has been used to fabricate micro/meso optical components such as micro lens array [21,22], imaging/illumination lens [23–26], and optical waveguide/optoelectronic elements [27], to the best of the authors’ knowledge, this is the first attempt of using UV-molding process for fabrication of miniature FP pressure sensors. There are several advantages of UV molding process, making it an excellent process for sensor fabrication. The process can be operated at room temperature under relatively low applied pressure or atmospheric pressure. Furthermore, the components fabricated by the UV-molding process have low birefringence and high repeatability with several tens of nanometers feature size [22,28]. The UV-molding process can be performed in batch fabrication by adopting a wafer scale fabrication technique [27,29]. Another advantage of UV-molding process is its flexibility in choosing materials due to the vast variety of polymer materials. Based on application needs, polymer materials with appropriate properties, such as biocompatibility, optical properties, and thermal/mechanical properties can be chosen. In addition to the aforementioned advantages of conventional UV molding process, the sensor fabrication process employs a unique optical fiber based mold, which enables a high accuracy, maskless, optical aligning of the sensor cavity during fabrication.
2. Sensor fabrication
The sensor fabrication is composed of two steps. The first step is to fabricate a mold that can be used for optical cavity molding. The second step is to perform cavity molding and cover a metal-polymer composite diaphragm over the molded cavity. The detailed fabrication process of the mold is shown in Fig. 2 . A zirconia ferrule (OD: 1 mm, ID: 80 μm) is polished at its end face to obtain flat and smooth surface, which will be replicated to form the top flat surface of the housing structure. The outer surface of the ferrule adjacent to the polished end face is then polished with an arbitrary angle (≈45°) to reduce outer diameter to be around 200μm, which will define the outer diameter of the fabricated sensors. A single mode optical fiber with a diameter of 80 μm (3M, FS-SC-3611) is cleaved and inserted into the ferrule by using a 5-axis translation stage. The aligning process is monitored by using an optical profilometer (TMS 1200, Polytec) that has a high vertical measurement resolution (0.3 nm). When desired height difference between the fiber and ferrule end face is obtained, UV adhesive (Dymax, OP-54) is used to fill the gap between the optical fiber and the ferrule. When the gap is completely filled, the adhesive is cured by using a UV light source (LC5, Hamamatsu). The fabricated mold is then cleaned following a general optical fiber cleaning process and treated with an organosilane to decrease the surface energy of zirconia and silica surface for easiness of releasing during the following molding process. By using commercially available optical elements and assembling technique, a high accuracy, inexpensive mold can be fabricated. The extrusion height (i.e., cavity length) can be precisely controlled with a resolution of less than 1 nm by using the optical profiler. The fabricated mold has a high durability due to its material characteristics and good surface releasing property with the organosilanes surface treatment, rendering extended lifetime of the mold under harsh molding conditions. A microscopic image of the fabricated mold and a scanning electron micrograph (SEM) of the molded optical cavity are shown in Fig. 3 .
Fig. 3 (a) Microscopic image of the fabricated mold. (b) Scanning electron micrograph (SEM) of a molded optical cavity.
The detailed optical cavity molding and diaphragm covering process of the sensor is illustrated in Fig. 4 . First, an optical fiber (SMF28, Corning) is prepared and aligned to the fabricated mold [Fig. 4(a)]. The optical fiber is cleaved (or polished) at one end prior to going through a general fiber cleaning process. The fiber is then chemically treated with an organosilane to obtain a better adhesion between the optical fiber endface and the UV-curable polymer. Then, the mold and the fiber are preliminarily aligned in line on two 5-axis stages by using images captured from two optical microscopes with CCD cameras positioned with a 90-degree angle separation. Next, the cleaved fiber end is deposited with a UV-curing polymer with a dipping method [Fig. 4(b)]. The fiber is retracted to ensure enough spacing between the fiber and the mold for the deposition process while being mounted on the stage. Another optical fiber, which has a small polymer drop at the tip, is approached to the chemically treated fiber on the stage until the polymer drop is transferred. After the deposition process, the fiber with the polymer drop is carefully moved toward the mold until it touches the mold surface to minimize the residual polymer layer between the optical fiber and the mold. The final alignment is performed to minimize the misalignment between the mold and the fiber, in which the fiber cores of the mold and the fiber on which the sensor is to be fabricated (i.e., sensing fiber) are aligned by monitoring the coupling intensity. To measure the coupling intensity, light from a superluminescent diode (SLD) (Exalos, EXS13G1–2111) is coupled to the sensing fiber and the coupled light intensity is measured from the fiber in the mold by using a photodetector (New Focus, Model 2011) together with a DAQ board and a computer. The relationship between the coupling intensity and the misalignment between two fiber cores is shown in Fig. 5 . It is noted that the accuracy of the alignment is determined by the resolution of the stage (0.5 μm for the stage used in experiment). It is possible to obtain even better alignment accuracy by using a motorized stage with a higher resolution. After a good alignment is achieved, UV-light is exposed to the UV-curing polymer. The mold is then released from the cured polymer to complete the optical cavity molding. This process renders simple sensor fabrication since the optical cavity is fabricated and securely attached to an optical fiber end face with a single process. Further, post-baking is performed to finalize the housing structure fabrication, in which the sample is baked at 150 °C for 5 hours for obtaining better thermal and mechanical stability of the UV-cured polymer housing. The 3D surface topographies of the mold and molded cavity are shown in Fig. 6 . The shrinkage of molding process in diameter and depth was measured to be 0.87% and 0.65%, respectively (see Fig. 7 ). In order to get better accuracy in terms of cavity dimension, the shrinkage factors can be considered in the mold design stage to compensate for the actual shrinkage. Finally, a polymer-metal composite layer is deposited on the housing structure to form a suspended diaphragm [Fig. 4(e)]. A SEM image of the fabricated sensor and a close-up of the diaphragm are shown in Fig. 8 . The diaphragm is bulged in the figures because of the pressure difference of the SEM machine vacuum chamber and the sensor air cavity. The detailed diaphragm deposition process can be found in reference [14].
Fig. 5 Coupled intensity as a function of the misalignment between the sensing fiber and the fiber in the mold during the alignment process. The results were obtained experimentally with a movement step of 0.5 μm from the stage.
As can be seen in Fig. 7, the diameter of the air cavity is well defined by that of the mold (i.e., the optical fiber diameter (80 μm)). The molding process can ensure a good repeatability of sensor fabrication. To verify the repeatability of the cavity molding process, 10 cavity samples were molded and their cavity diameters and depths were measured by using an optical profilometer (TMS 1200, Polytec). Maximum deviation of the cavity depth was found to be 0.12% with respect to the mean cavity depth of the 10 samples. The deviation of the cavity diameter was not able to be detected due to the limitation of the lateral resolution of the profilometer (~0.1 μm), which is much larger than the vertical resolution of the equipment (~0.3 nm). This indicates that the lateral deviation of the diameter is smaller than 0.1 μm for a molded cavity with an 80 μm diameter. The repeatability of the cavity molding process can be further enhanced by tightly controlling the process parameters such as UV light source intensity and UV curable polymer volume.
At the given diameter of the cavity, the sensitivity of the sensors can be easily tuned by changing the thickness of the metal layer in the diaphragm according to the pressure range requirements in different applications. The extrusion height of the mold is chosen to be 64.0 μm so that the wavelength spectrum of the fabricated FP cavity will have enough number of peaks for easiness of optical signal processing. The measured extrusion height (63.6 μm) is slightly smaller than the designed value due to shrinkage of the adhesive during the curing process. The diaphragm of the sensor is made of a urethane acrylate-based film coated with a silver layer. The parameters of the sensor are summarized in Table 1 . Since the diaphragm can be modeled as an edge-clamped circular plate, a finite element method (FEM) based model was used to predict the sensitivity of the sensor. The model predicated sensitivities is 0.0157 μm/psi.
Table 1. Parameters of two representative sensors fabricated by UV molding process
In order to apply the abovementioned fabrication method for batch fabrication, a specially designed fiber holder should be fabricated to ensure the device-to-device uniformity of the fabrication processes. A silicon wafer with etched through holes can be exploited to align and fix fibers that are positioned in an array and leveled to one of the surfaces of the wafer. The guiding holes can be fabricated by using photolithography and deep reactive ion etching (DRIE) processes with a positive clearance of 3~5 μm between the hole and the fiber outer diameter. The fibers can be fixed in the holes by using mounting wax during the rest of the processes. Covering of the UV curable polymer diaphragm can be done in a wafer-scale with a good device-to-device uniformity since a uniform polymer layer with a diameter much larger than the wafer can be easily obtained by choosing a proper polymer material and controlling the volume of the polymer dispensed on water surface. Uniformity of metal layer deposition on the polymer diaphragm can be attained by using proper process parameters of a sputtering or evaporation process.
3. Optical interrogation system
In the experiment, the fabricated sensor was connected to a broadband optical interrogation system (SM130, Micron Optics) [Fig. 9(b) ], which provides a spectrum range of 1510 to 1590 nm. Representative interference spectra obtained from the sensor at two different temperatures (25 and 45 °C) are shown in Fig. 9(a). The sinusoidal pattern in the spectrum is generated by the interference in the Fabry-Perot cavity and the sharp peak at wavelength of 1522.26 nm is from the FBG integrated in the pressure sensor for temperature measurement and compensation. Both the shift in peaks for the optical pressure sensor and the shift in Bragg wavelength for the FBG can be seen with respect to the temperature change.
Fig. 9 (a) A representative interference spectrum obtained by using the fabricated sensor with a built-in Bragg grating. (b) Schematic of the experimental arrangement for pressure measurement.
The absolute cavity length L of the sensor, retrieved from the reflection spectrum of the spectrometer, can be written as
where λ1 and λ2 are any two center wavelengths of adjacent peaks of the reflection spectrum of the FP cavity and FSR = | λ2 - λ1| is the free spectral range. The cavity length L was calculated by averaging several cavity lengths retrieved from a pair of two adjacent peaks with good visibility to reduce the random errors. After retrieving the absolute cavity length, the one peak tracing method was used to further reduce the measurement error and data processing time [30]. The initial cavity length measurement, one peak tracing process, and pressure measurements were all carried out in real-time, which were controlled by a LabVIEW program.4. Sensor calibration
Calibration of the sensor was performed in a pressure chamber with a reference pressure sensor to evaluate performance of the optical sensor, as shown in Fig. 9(b). A conventional pressure sensor (LL-080-35A, Kulite Semiconductor) was used as the reference. The internal chamber pressure was controlled by using a pressure regulator (R-68825-08, Marsh Bellofram), which has a pressure range of 1.7 to 60 psi. In the experiments, the pressure in the chamber was first increased and then decreased with a step of 0.4 psi within a range of 1.9 to 7.9 psi at the room temperature (24 °C). The calibration result of the UV-molded pressure sensor is shown in Fig. 10(a) . It can be seen that the sensor exhibits good linearity (R2 = 0.9996) over the entire tested pressure range. Based on linear regression analysis of the measured data, the pressure sensitivities were calculated to be 0.0106 μm/psi. This result is slightly smaller than the value obtained from the numerical simulations (0.0157 μm/psi). Residual stresses generated during the polymerization and metallization process of the diaphragm are the possible reasons for the lower measured sensitivity. The hysteresis error of the sensor is obtained as 1.5%. In the calibrations, the small zero shift and non-linearity of the calibration curves are believed to be due to the room temperature variations and random errors from the interrogation system.
Fig. 10 (a) Calibration curve of the sensor at room temperature (24 °C), (b) calibration curves of the sensor at three difference temperatures (30, 40, and 50 °C).
The pressure sensitivity calibration was also performed at different temperatures (e.g., 30, 40, and 50 °C) to see the effect of temperature on pressure sensitivity using the experimental setup shown in Fig. 9(b). The result of calibration is shown in Fig. 10(b). In all three cases the measurements show good linearity (R2 ≈0.999) over the measured pressure range. It is noted that there is about 5.4% of pressure sensitivity variation over the temperature range of 24 to 50 °C. This deviation of the sensitivity value is believed to be due to the error of the temperature controller, random error of the measurement system, and change of the sensor material properties due to the temperature change.
5. Temperature sensitivity and temperature compensation
As mentioned previously, due to the large CTE of the polymer material, the polymer processed optical pressure sensors are expected to have much larger temperature sensitivity than that of pressure sensors made of silicon/silica. Therefore, this effect should be evaluated and compensated for in order to obtain accurate pressure measurements. First, the temperature effect on the cavity length change is investigated. In the experiment, the temperature sensitivity of the UV-molded pressure sensor was obtained by monitoring the optical cavity length change with respect to controlled temperature variations. Temperature control was achieved by using a thermo controller (Omega Engineering Inc., CN77333), a thermocouple (Omega Engineering Inc., CO1-K), and two polyimide-insulated flexible heaters (Omega Engineering Inc., KH 103/10). The heaters were attached at the vicinity of the pressure sensor to control the temperature locally, while minimizing the heating time and the temperature effect on the reference pressure sensor. To measure its temperature sensitivity, the sensor was first heated from 26 °C to 50 °C with an increment of 3 °C under a constant pressure of 1.9 psi. The cavity length was measured at each temperature step. The obtained cavity length change as a function of temperature is shown in Fig. 11(a) . According to the result, a linear relationship can be observed with R2=0.9983 and the temperature sensitivity of the FP pressure sensor can be obtained as 0.0158 µm/°C. To compensate for the temperature effects, the temperature at the vicinity of the pressure sensing area can be measured with the FBG embedded in the sensor to relate the temperature to cavity length variations. Figure 11(b) shows the measured FBG peak wavelengths at a constant pressure level by using the same optical interrogation system and the LabVIEW code. The measurement was performed simultaneously with the temperature sensitivity measurement of the pressure sensor. The temperature sensitivity and the resolution of the FBG sensor was obtained as 0.0120 nm/°C and 0.12 °C, respectively. By using the pressure sensitivity and temperature sensitivity of the FP pressure sensor and temperature sensitivity of the FBG, temperature compensation as well as simultaneous temperature and pressure measurements can be achieved.
Fig. 11 (a) Temperature sensitivity of the pressure sensor. (b) Temperature sensitivity of the FBG embedded in the pressure sensor.
Even though there is a small variation in the pressure sensitivity of the sensor at different temperatures (<5.4%), it was shown that the cavity length of the pressure sensor changes linearly with respect to both pressure and temperature, and the Bragg wavelength of the FBG also shifts linearly with respect to temperature. Therefore, in the working temperature range (24~50 °C), these linear relationships can be expressed in a matrix form as the following [18,31]:
where ΔλFBG is Bragg wavelength shift of the FBG, ΔL is cavity length change of the pressure sensor, ΔP and ΔT is the pressure change and temperature change, respectively. Pressure sensitivity of the FBG can be neglected since the nominal pressure sensitivity value of the FBG (2.02×10−6 MPa−1) is three orders of magnitude smaller than that of the optical pressure sensor. By taking inverse of the sensitivity matrix S, Eq. (2) can be rewritten as:Based on Eq. (3), by using an optical system (e.g., the system shown in Fig. 9(b)) that can measure both the Bragg wavelength shift and the cavity length change of the FP pressure sensor, the temperature and pressure values can be retrieved simultaneously. Note that in an application that requires high accuracy pressure measurements, the measurement error due to the pressure sensitivity variation with respect to temperature changes can be reduced by using an formula based on Taylor expansion with nonlinear terms [32]. Another way of reducing this error is to use the support vector regression (SVR) method to obtain an approximate non-linear function in the whole region through a statistical machine learning process [33]. Therefore, by adding an inherently embedded FBG that can be used to monitor the temperature change during pressure measurements, the temperature effect of the pressure sensor can be compensated.
6. Concluding remarks
In this article, a miniature Fabry-Perot pressure sensor fabricated with the UV-molding process is presented. The sensor is composed of a UV-molded optical cavity and a polymer-metal composite diaphragm, which is made 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. Due to the relatively low stiffness of the polymer-metal composite diaphragm, sensors with extra miniature sizes but high sensitivity can be obtained. The experimental study of this sensor has shown that the sensor has good linearity in the designed pressure range. Effective temperature compensation by using the built-in FBG has also been demonstrated. Since the materials used for the sensor housing and diaphragm have good biocompatibility, this sensor will become a favorable choice for biomedical applications such as arterial, intracranial, urethral, or intradiscal pressure measurements.
Acknowledgments
This work was partially supported by the National Science Foundation (NSF) (CMMI0644914), Maryland Proof of Concept Alliances funds from Army Research Lab (ARL) under a Cooperative Agreement (W911NF0920028), the United States Army Research Office (USARO) Defense University Research Instrumentation Program (DURIP) (W911NF0710215), and the Maryland NanoCenter and its NispLab. The NispLab is supported in part by the NSF as a MRSEC Shared Experimental Facility.
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