MEMS optical interferometry-based pressure sensor using elastomer nanosheet developed by dry transfer technique

MEMS-based Fabry–Perot (FP) interferometers are promising for wavelength selective filters with high selectivity, which have been widely used for tunable optical devices and optomechanical sensors. With the actuation of a movable mirror of two parallel reflecting mirrors, tunable transmissive color filters^1,2) and tunable IR bandpass filters³⁾ have been proposed. A wide tunable range can be obtained owing to the high sensitivity of the mirror displacement. On the other hand, a spectral shift associated with membrane deflection caused by external force was utilized for physical^4,5) and chemical sensors.⁶⁾ In particular, a high capability for position detection of minimum 50 pm⁷⁾ provides highly sensitive physical and chemical sensors compared with a piezoresistive^8,9) or capacitive^10,11) detection method. Therefore, optical interferometry has been applied to pressure, tactile, and surface stress sensors that detect molecular adsorption caused by an antigen–antibody reaction.

To obtain the spectral shift applied with physical and chemical quantities, a suspended membrane with a narrow cavity above a substrate is required. Previously proposed optical interferometry-based sensors used a relatively complex fabrication process such as sacrificial etching for a suspended membrane followed by an encapsulation process for the nanocavity⁶⁾ or deep reactive ion etching to form a thin membrane and a high temperature wafer bonding process.¹²⁾ These fabrication processes for the sealed nanocavity structure impose structural and material restrictions such as chemical stability for etching gas and high-temperature tolerance.

We previously developed a series of freestanding polymeric ultrathin films (referred to as "nanosheets") for biomedical applications.^13,14) Recently, we have also achieved the preparation of an elastomer nanosheet using a polystyrene–polybutadiene–polystyrene (SBS) triblock copolymer whose thickness can be controlled to a minimum 100 nm by a microgravure roll technique.¹⁵⁾ The ultrathin elastomer sheet demonstrated unique physical properties such as flexibility, stretchability, low Young's modulus of 40 MPa, and high adhesiveness.^14–16) In particular, the adhesiveness of the elastomer nanosheet markedly increases with the thickness in the submicron range. In addition, the triblock copolymer provides controllability to the film thickness by the adjustment of concentrations during the roll-to-roll process, which allows for a self-supporting nanosheet by a simple method. The freestanding SBS nanosheet also demonstrated a large elastic deformation of 38% strain with cyclic straining.¹⁶⁾

In this paper, by using the nanosheet adhesiveness, we propose an elastomer-based FP interferometer by a dry transfer technique over a shallow trench in a substrate to form a nanocavity without vacuum and high-temperature processes. The newly proposed dry transfer technique using an SBS nanosheet can achieve a permanent bonding to an arbitrary substrate, which provides a simple and low-cost process by eliminating the conventional refill process of etching holes and high-temperature bonding process. With the pressure change, the freestanding SBS nanosheet was found to deform with good adhesion between the dry transferred SBS and the substrate.

For device fabrication, the high adhesive strength of elastomers is used for the pick-up and drop transfer of two-dimensional (2D) materials^17,18) and a microfluidic channel by adhesive bonding.¹⁹⁾ However, the molecular interaction force of elastomers is not extremely strong compared with that of plastics because the adhesion strength is derived from van der Waals force. While the typical elastic modulus of plastics is known to be several GPa, that of elastomers is 0.1–10 MPa, which is 2–4 orders of magnitude smaller. The contact area of materials with low elastic modulus to a substrate becomes larger with the same loading, resulting in high adhesive strength. Hence, it can be explained that the adhesion force in the elastomers is dominated by the large real contact area.²⁰⁾ It is also reported that adhesive strength is dependent on elastic modulus with a constant surface roughness.^21,22) In addition, the effect of adhesion force becomes larger with decreasing film thickness; therefore, a submicron-thick elastomer sheet can be used as an adhesive onto a solid substrate at room temperature and atmospheric pressure.

By means of this effect, an elastomer-based MEMS FP interferometer for physical and biological sensing is proposed, as shown in Fig. 1(a). A flexible sheet is suspended over a nanocavity whose length determines the free spectral range (FSR) of the FP interferometer, namely, a narrow gap provides a wide FSR. When an external force, such as pressure, and molecular adsorption using an antigen–antibody reaction is applied to the suspended membrane, interference peaks of reflected or transmitted light are shifted depending on the gap length. With the high adhesiveness of an elastomer nanosheet, a suspended membrane with a nanocavity is formed by a dry transfer process.

Zoom In Zoom Out Reset image size

An SBS nanosheet was prepared on a poly(ethylene terephthalate) (PET) film with prelaminated poly(vinyl alcohol) (PVA) as a sacrificial layer with a microgravure coating technique. The SBS polymer dissolved in tetrahydrofuran (THF) was coated on the PVA/PET films with a thickness of approximately 600 nm by a microgravure roll technique (rotation speed: 30 rpm, line speed: 1.3 m/min, drying temperature: 80 °C). The SBS thickness can be controlled by adjusting the line speed from 100 to 1000 nm. The SBS nanosheet supported with a carrier tape (i.e., Kapton tape) was released from the PET film in hot water at 70 °C. To form a diaphragm structure above a nanocavity, shallow trenches on a silicon substrate were prepared. For the improvement of adhesion, a silane coupling agent was coated on the silicon wafer, followed by transferring a nanosheet at room temperature.

Figure 1(b) shows the freestanding SBS nanosheet released from the PET film supported with Kapton tape. The freestanding area was approximately 30 × 30 mm². With the state of freestanding form, the SBS nanosheet was transferred to a Si substrate with nanocavities as shown in Fig. 1(c). Owing to the physical adhesiveness of the elastomer nanosheets, strong adhesion with the substrate was obtained at atmospheric pressure and room temperature.

Figure 2(a) shows an optical microscopy image of the developed FP interferometer of 100 µm diameter. For the measurement of the reflection spectra, a vertical illumination-type optical microscope (Mitsutoyo VMU-LB), CCD camera (Olympus DP22), spectrometer (Ocean Optics USB2000TR), and a xenon light source (Asahi Spectra LAX-C100, 100 W) were used. The magnification of the objective lens was 20× and the measurement spot diameter was 10 µm. The reflection spectrum of the interferometer was measured by microscopic spectroscopy, as shown in Fig. 2(b). Four interference peaks of the reflection spectrum from the diaphragm in the visible range were observed. A fitting curve using rigorous coupled wave analysis (RCWA) method with a refractive index of 1.5 with a thickness of 625 nm for SBS and a gap of 510 nm was in good agreement with the experimental reflection spectrum. For the evaluation of aspect ratio dependence, the gap length was designed to be 100, 300, and 500 nm with the diameter range of 10–300 µm. A maximum aspect ratio of $1000:1$ with a 300 nm gap and a 300 µm diameter was obtained, as shown in Fig. 2(c). A minimum gap length of 120 nm with an 80 µm diameter was also achieved, as shown in Fig. 2(d).

Zoom In Zoom Out Reset image size

For pressure sensing, the developed optical interferometer was implemented in a vacuum chamber with an optical window. The interference property was evaluated under a vacuum condition by microscopic spectroscopy through the optical window. The vacuum chamber was evacuated at 0–50 kPa. The measurement spot size was set to be 5 µm by the combination of the 20× objective lens and optical fiber with the 100 µm fiber core mentioned above, which was small enough for the diameter of a deformable membrane. By decreasing the chamber pressure, the pressure response was characterized by the change in an interference property caused by membrane deformation with a pressure difference between the cavity and the vacuum chamber.

With an applied pressure of −20 kPa to the vacuum chamber, interference peaks were temporarily redshifted owing to the expansion of the FP cavity, which suggested that the dry-transferred SBS nanosheet effectively sealed the cavity, as shown in Fig. 3(a). The dry-transferred SBS nanosheet kept on attaching to the substrate under a vacuum condition. However, the interference peaks were found to blue-shift over time, and it returned to the initial position after 60 s. Note that the SBS nanosheet has a slight permeability to gas; hence, an additional thin film is required on the SBS interferometer for pressure sensing.

Zoom In Zoom Out Reset image size

To avoid gas leakage of the SBS nanosheet, a parylene-C thin film was coated on the SBS nanosheet. Parylene-C was deposited on the SBS nanosheet before being released from the PET film of 200 nm thickness, which was sufficiently impermeable to gas as discussed later. After that, the parylene-C/SBS film was released from the PET film by the sacrificial etching and transferred to the Si substrate by the same method mentioned above. The drift of the spectrum peak of the double-layered nanosheet under vacuum was evaluated in terms of gas leakage. Figure 3(b) shows the time–course reflection spectrum of the parylene-C/SBS interferometer at −20 kPa. No change in the optical property was observed for a minute, while that of the bare SBS was changed due to air leakage [Fig. 3(a)]. Under the vacuum condition, interference peaks of the parylene-C/SBS interferometer were found to redshift, which suggested that the parylene-C/SBS membrane deformed into an upper direction by a pressure difference and maintained the deformed convex shape. The gap length can be calculated by using the spectrum peak position and refractive indexes of parylene-C (1.66) and SBS (1.50) and was determined to be 1.38 µm. The result indicated that the 200-nm-thick parylene-C encapsulated the FP cavity, which performed pressure sensing corresponding to the membrane deformation. Therefore, with the additional thin layer deposited on the SBS nanosheet, optical pressure sensing using the elastomer nanosheet is possible. The SBS nanosheet works as the adhesion layer in the dry transfer process.

The gap uniformity of the dry-transferred freestanding parylene-C/SBS nanosheet was evaluated, as shown in Figs. 4(a) and 4(b). A total of five points on the interferometer with a diameter of 200 µm were measured. The reflection spectrum of the fabricated parylene-C/SBS interferometer was in good agreement with a fitting curve of a gap length of 1.1 µm using 200-nm-thick parylene-C and 600-nm-thick SBS. The reflection spectra of the five different points were accurately matched; hence, the dry-transferred optical interferometer with a uniform gap was obtained.

Zoom In Zoom Out Reset image size

The SBS nanosheet has a large elastically deformed region. In a similar manner, parylene-C shows an elastic strain up to 2–3%. Hence, the fabricated optical interferometric pressure sensor is expected for repeated measurements. For the evaluation of repeatability, the membrane deformation was measured with respect to repeated pressure changes. The chamber vacuum was alternately turned on and off five times. Figures 5(a) and 5(b) show reflection spectra at atmospheric pressure and −20 kPa. Both spectra were quite identical to all the repeated trials, which indicated that the dry transferred parylene-C/SBS membrane can be used for pressure sensing with good repeatability.

Zoom In Zoom Out Reset image size

Applied pressure was controlled for the evaluation of the dry transferred nanosheet deformation dependent on pressure. Figures 6(a)–6(f) show the micrographs of the SBS nanosheet-based interferometer corresponding to a different applied pressure of 0 to −50 kPa. With increasing pressure difference in the chamber, the number of interference patterns was found to increase while uniform color was observed at the initial state. The interference stripes associated with applied pressure indicated that the parylene-C/SBS membrane was deformed to a convex shape. Note that the interference color change was observed only in the cavity area and no color change was observed in the field area. The result suggested that strong adhesion of dry transferred SBS nanosheet to the substrate was achieved.

Zoom In Zoom Out Reset image size

Figure 6(g) shows reflection spectra from the center of the interferometer according to the pressure difference. The interference order number increased with decreasing chamber pressure, which means that the air gap expanded. The interference property with applied pressure was used to calculate the membrane displacement by gap length fitting. Figure 6(h) shows the calculated membrane displacement as a function of pressure difference. With increasing pressure difference, the membrane displacement increased.

Conventional MEMS pressure sensors using silicon-based materials for a sensing diaphragm have been reported to exhibit a linear response with respect to applied pressure. However, the experimentally obtained membrane deflection as a function of applied pressure seems to be a quadratic curve. It is speculated that the elastomer holds a nonlinear characteristic in terms of Young's modulus associated with strain,²³⁾ resulting in softening the SBS nanosheet with increasing strain. Previously, the mechanical property of SBS nanosheets showed a decrease in the gradient of the stress–strain curve, which is defined as elastic modulus, with increasing strain.¹⁵⁾ In addition, the combined elastic property of the bilayer membrane was highly dependent on the SBS layer, which was three times thicker than the parylene-C layer, because the spring constant is proportional to the cube of film thickness. Hence, the obtained pressure response of the developed optical sensor is derived from the strain-dependent stiffness of the elastomer nanosheet. To improve the linearity of the pressure response, a material with strain-independent Young's modulus such as a metal thin film and a thick parylene-C film is used for a freestanding membrane. Furthermore, the SBS nanosheet is only used for the adhesion layer for film dry transfer and removed from the sensing area. For increasing pressure sensitivity, an approach of appropriately designing the structural parameters is reasonable because the sensitivity is inversely proportional to the cube of the film thickness and proportional to the fourth power of the radius. The SBS nanosheet with high supportability for a freestanding film has the design flexibility of structural parameters and we think that it is possible to design a highly sensitive pressure sensor.

In conclusion, we developed an elastomer-based Fabry–Perot interferometric pressure sensor by the dry transfer technique using an SBS nanosheet to make a nanocavity. The dry transferred SBS nanosheet can deliver high adhesiveness to a substrate and form a suspended structure without vacuum and high-temperature processes. A minimum gap length of 120 nm with an 80 µm diameter was achieved, as well as gap uniformity in the sensing area. For pressure sensing, a predeposited gas-impermeable layer on the SBS nanosheet was adapted to prevent the gas permeation of SBS. With the pressure change, the freestanding parylene-C/SBS membrane demonstrated deformation associated with pressure difference with good adhesion between the dry transferred SBS and the substrate. The proposed thin-film transfer technique using an elastomer nanosheet for a highly adhesive layer offers a low-cost and simple MEMS pressure sensor by lamination of the optimal sensing layer. In addition, the elastomer-based FP cavity structure can be applied to other optical components such as an optical fiber and a photodetector for a compact sensing system by postprocessing, and it enabled us to track the shift in a single optical interference by using a single-wavelength light source and measuring the change in transmittance at that wavelength.⁶⁾

Acknowledgments