The material below is abstracted from our full publication:
J. Han, D. P. Neikirk, M. Clevenger, and J. T. McDevitt, "Fabrication and characterization of a Fabry-Perot based chemical sensor," SPIE Microelectronic Structures and MEMS for Optical Processing II, M. E. Motamedi and W. Bailey, editors, Proc. SPIE 2881, Austin, Texas, USA, 14-15 October, 1996, pp. 171-178.
Please refer to this full publication for complete details.
Also see the slides on this F-P chemical sensor used in our talk at the 96 SPIE MEMS meeting.
QuickTime movie of a chemical sensing bead.
For further information contact Professor Neikirk at:
neikirk@mail.utexas.edu .This file created: Feb. 7, 1997.
Jaeheon Han*, Dean P. Neikirk*, Mervyn Clevenger ^ and John T. McDevitt ^
* Microelectronics Research Center and Department of Electrical and Computer Engineering,
^ Department of Chemistry and Biochemistry
The University of Texas at Austin
ABSTRACT
A micromachined silicon Fabry-Perot interferometric sensor is demonstrated as an optical chemical sensor. This sensor is based on the combined nature of the amplifying and tuning characteristics of the Fabry-Perot microcavity structure and the doping effect of polymer films such as Poly(3-dodecylthiophene) (P3DDT) upon exposure to an oxidizer, in this case, iodine. The fabricated Fabry-Perot chemical sensors show reversible sensing behavior with a maximum change in transmitted optical intensity of 60 %. Significant improvement of the sensing performance is obtained from the Fabry-Perot microcavity structure compared to a simple planar single membrane structure, which indicates the resonant effect of the Fabry-Perot cavity on the chemical sensor. The measured sensing characteristics suggest that the change in absorptance of P3DDT polymer inside the microcavity plays a major role, while the deflection of a microcavity membrane by the P3DDT polymer-induced surface tension gives tunability of the sensor to maximize the amplification of output response by adjusting the Fabry-Perot microcavity gap spacing.
1. INTRODUCTION
The application of silicon micromachining techniques to the construction of Fabry-Perot cavity-based sensors have provide both a reduction in their size as well as improvements in performance and manufacturability. Here we discuss Fabry-Perot microcavity structures for use as chemical sensors fabricated using a single-wafer surface micromachining sacrificial layer process. This approach allows better device performance compared to hybrid fusion bonding through precise control of the cavity gap spacing, and higher manufacturing yield by enhancing the simplicity of fabrication and the compactness of the device.
For chemical or gas sensing applications, the identification of a method through which the chemical or physiochemical event can be coupled with a readout device is required; many prior chemical sensors have been based on catalytic reactions which occur at elevated temperatures, with the transduction event causing an electrical signal (usually a change in resistance) [1]. Prior work with micromachined chemical sensors has utilized the compact structure for thermal isolation and as an integrated heating source [2].
In this paper, the fabrication of a micromachined Fabry-Perot cavity filled with a chemically sensitive polymer, thus producing an optically interrogated chemical sensor system, will be discussed. Micromachined silicon Fabry-Perot chemical sensors are fabricated by combining the signal amplifying characteristics of a micromachined silicon Fabry-Perot microcavity structure with the optical changes induced by oxidation in the conducting polymer film Poly(3-dodecylthiophene) (P3DDT). The optical response of the fabricated chemical sensors will be presented to evaluate the role of the Fabry-Perot microcavity structure and the conducting polymer in this sensor. For possible applications in light emitting diodes (LEDs) and large area displays similar structures using other polymers have recently been reported [3,4].
2. DEVICE FABRICATION AND OPERATION
For these chemical sensors, a combination of bulk micromachining and surface micromachining processes have been adapted from those used to fabricate Fabry-Perot pressure sensors [5,6]. Figure 1 shows a schematic illustration of the fabrication procedure for a Fabry-Perot microcavity. Lightly doped 4-inch single crystal (100) silicon wafers are used as a substrate. The wafers are double-side polished and 300 um thick. The 0.7 um thick air gap is created after the sacrificial polysilicon embedded between two diaphragms is removed.
Figure 1. Schematic view of fabrication procedure for a micromachined Fabry-Perot microcavity: (a) formation of bottom diaphragm and etch windows; (b) patterning and deposition of polysilicon and top diaphragm; (c) blanket backside and sacrificial polysilicon etching.
After depositing the top diaphragm layers, all layers stacked on the back side of the wafer, including the sacrificial dielectric silicon dioxide and nitride layers, are removed by multiple blanket wet and plasma etching steps. Then, the sacrificial polysilicon is exposed through the etch windows through the backside of the bottom diaphragm. The polysilicon is etched using a KOH solution. Both the top and the bottom diaphragms are then released to form the Fabry-Perot microcavity structure. Successful fabrication of the chemical sensor requires a stiffer top diaphragm than previously fabricated pressure sensors [5] due to the extra asymmetrical stress caused by the sensing polymer film that coats the inner surface of the top diaphragm of the Fabry-Perot microcavity. The extra residual stress deflects the top diaphragm toward the bottom diaphragm and eventually causes sticking between the two diaphragms. This is not a problem for the coated bottom diaphragm since the polymer film is coated on both front and back sides of the bottom diaphragm, so the stress is compensated.
Three approaches were pursued to improve the stiffness of the top diaphragm. The first approach is to increase the total thickness of the top diaphragm while maintaining appropriate mechanical stability and optical characteristics. This has the limitation that it is hard to achieve the desirable thickness along with proper tuning of the optical and mechanical effects. The second approach is to reduce the size of the top diaphragm. This reduces the area of the fabricated device and restricts the amount of the sensing polymer film contained within the microcavity structure. The last approach is to reinforce the top diaphragm by placing the "structural columns" inside the microcavity structure, such as adding a polysilicon sidewall.
The polysilicon sidewall can be placed without adding extra process steps to the existing planar top diaphragm pressure sensor process flow. This is accomplished by deliberately leaving some of the polysilicon around the edge of the microcavity structure during the final sacrificial polysilicon etching step by intentionally under-etching. The remaining polysilicon, called the polysilicon sidewall, will behave like a structural column holding a ceiling, effectively reducing the area of the top membrane, and hence reducing its compliance. The size and shape of the polysilicon sidewall inside the microcavity can be adjusted by the sacrificial polysilicon etch time and the etch windows geometry, respectively. All three approaches are used to optimize the device structure for the Fabry-Perot chemical sensor.
The conducting polymer poly(3-dodecylthiophene) (P3DDT) was utilized to create the chemically sensitive element for the demonstration sensor. This polymer displays characteristic changes in optical properties (absorbance, transmittance, dielectric effects) when its oxidation state is changes . For demonstration purposes, molecular iodine vapor was used here as the analyte material. Prior studies have shown that iodine serves to oxidize readily polymers such as P3DDT [9].
Figure 2. (a) Molecular structure of poly (3-dodecylthiophene) (P3DDT) and (b) its doping reaction with iodine (from [7]).
To coat the membranes of the micromachined Fabry-Perot cavity with the P3DDT, the polymer is first dissolved in a solvent such as chloroform or xylene. Here a solution of 0.01 g P3DDT/ml was then applied to the device, and rapidly enters the cavity through the etched windows in the bottom diaphragm, aided in part by capillary action. To increase the thickness of the polymer layer, the solution can be re-applied after the solvent has evaporated from the prior treatment. As shown by the data presented below, each polymer coating cycle leads to a gradual, but systematic increase in the amount of cavity-localized polymer material. This is thought to occur due to diffusion restricted re-dissolution of the polymer during each re-application cycle. The micromachined Fabry-Perot chemical sensor after application of the P3DDT polymer is illustrated in Figure 3.
Figure 3. Schematic illustration depicting the micromachined silicon Fabry-Perot structure used to hold the chemically sensitive polymer.
The reactant and detecting gas iodine (I2) is supplied by room temperature sublimation from granules of solid iodine placed close to the device (on the order of 1 cm from the device), in an otherwise open ambient atmosphere. The sublimed iodine vapor penetrates through the etch windows (that are approximately 10 um square) in the bottom diaphragm, diffuses into the Fabry-Perot cavity, and oxidizes the P3DDT. The oxidation serves to create delocalized charge carriers along the backbone of the polymer which lead to large changes in the spectral properties of the material. This also changes the molecular structure of P3DDT so that it becomes a conducting polymer. Figure 2 schematically illustrates the molecular structure of the P3DDT polymer and its reaction with iodine [7]. These changes are quite dramatic as the initial neutral polymer layer appears red in color and the oxidized, iodine-exposed material displays a dark brown coloration [8,9].
3. MEASUREMENT RESULTS
The fabricated Fabry-Perot chemical sensors show reversible on-off sensing behavior with 90% reaction time of 3 seconds and 90% recovery time of 2 seconds, as shown on Figure 4, for an illumination wavelength of 633 nm. The sensing characteristics of the Fabry-Perot microcavity structure and a simple single membrane structure (equivalent to the bottom diaphragm of the Fabry-Perot microcavity) as a function of the number of polymer coating cycles are compared in Figure 5 (again at the illumination wavelength = 633 nm).
Figure 4. Change in transmitted intensity ([lambda] = 633 nm) through the polymer-filled Fabry-Perot cavity; at t = 10 seconds, the device was exposed to iodine vapor produced by sublimation of solid iodine at room temperature placed within about 1 cm of the sensor in an otherwise open atmosphere; at t = 30 sec. the solid iodine was removed from the vicinity of the sensor.
Figure 5. Transmitted intensity change as a function of the number of polymer coating cycles for different micromachined support structures; square: Fabry-Perot cavity; cross : single membrane coated on one side with polymer; circle: single membrane coated on both sides.
In contrast, there is a significant change in the characteristics of the polymer-filled Fabry-Perot device as a function of the number of coating cycles. The maximum change in transmitted intensity is about 50% after three polymer coating steps. Importantly, the largest signal is obtained at a polymer loading thickness which is less than that required to completely fill the microcavity. Thus, the behavior clearly suggests the occurrence of a signal "amplification" due to interference effects in the Fabry-Perot microcavity.
The sensing characteristics of a Fabry-Perot microcavity structure having a 200 um diameter circular top diaphragm and a 140 um length square bottom diaphragm with four symmetrical polysilicon sidewalls is shown in Figure 6. Figure 7 shows optical micrographs of the above device after (a) zero, (b) five and (c) ten coating cycles of P3DDT, respectively. It is clear that the more polymer there is in the microcavity, the more change there is in absorptance and subsequently in the change in transmitted intensity. On the other hand, the presence of additional polymer increases the stress between the diaphragm and polymer film, causing deflection of the diaphragm.
Figure 6. Variation of transmitted intensity as a function of the number of polymer coating cycles for a device having a 200 um diameter circular top diaphragm and a 140 um length square bottom diaphragm. In this case, optimum performance was obtained after three coating cycles, and evidence of sticking appeared after five coating cycles.
(a)
(b)
(c)
Figure 7. Optical micrographs of the device: (a) no P3DDT polymer film; (b) after five coating cycles of the polymer (the small dot in the center of the diaphragm indicates sticking); and (c) after ten coating cycles.
5. CONCLUSIONS
Micromachined silicon Fabry-Perot chemical sensors, which combine the amplifying and tuning characteristics of Fabry-Perot microcavity structure, and the optical effects of the conducting polymer film such as Poly(3-dodecylthiophene) (P3DDT), have been fabricated and characterized. The polymer-filled microcavity devices show reversible sensing behavior in response to exposure to iodine with about 50% maximum change in the transmitted intensity.
The measured sensing characteristics suggest that the change in absorptance of the P3DDT polymer film after application of iodine plays a major role in sensing efficiency. The deflection of the diaphragm by polymer induced stress gives tunability to maximize the amplification of output responses by adjusting the microcavity gap spacing.
6. ACKNOWLEDGMENTS
This work was supported in part by the National Science Foundation, the Welch Foundation, and by the Defense Advanced Research Projects Agency (DARPA) Embedded Microsystems Program.
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