All-silicon Fa-Per sensor fabricated by MEMS technology

Introduction: In recent years, the development of microelectromechanical system (MEMS) technology has injected new vitality into the field of optical fiber sensing, and combining it with optical fiber provides the possibility of high-sensitivity pressure measurement. Optical fiber MEMS Faber sensor has the characteristics of high consistency, mass production, excellent performance and easy stability.

Written by Tianjin University PhD student Dai Xiaoshuang (first author of the paper) & Wang Shuang (corresponding author)

01 Introduction

In recent years, the development of microelectromechanical system (MEMS) technology has injected new vitality into the field of optical fiber sensing, and combining it with optical fiber provides the possibility of high-sensitivity pressure measurement. Optical fiber MEMS Faber sensor has the characteristics of high consistency, mass production, excellent performance and easy stability. The traditional demodulation method can generally obtain accurate optical path differences (OPDs) under different pressures by demodulating the fast Fourier transform combined with the spectral peak tracing method, but this method has strict requirements on the length of the Fa-Per cavity, as follows: Short cavity length is beneficial to improve pressure sensitivity, but after fast Fourier transform, there is the problem of aliasing of spectral components, which is not conducive to the extraction and demodulation of subsequent signals. In this study, in order to achieve high-sensitivity pressure measurement in the case of short vacuum chamber length, from the perspective of Vernier effect, it is proposed to match the optical path difference between the silicon cavity and the silicon/vacuum hybrid cavity of the all-silicon Fabry chip produced by MEMS technology. , to achieve high-sensitivity pressure sensing by tracking the evolution of the spectral envelope. The research results were published in the optical journal OpTIcs Express under the title “High-sensiTIve MEMS Fabry-Perot pressure sensor employing an internal-external cavity Vernier effect”. The first author is Dai Xiaoshuang, a doctoral student at Tianjin University, and the corresponding author is Wang Shuang Associate Professor.

Cover image: Silicon/vacuum hybrid cavity reflectance spectra and reflectance envelope spectra based on the Vernier effect of the inner and outer cavities.

Source: OpTIcs Express (2022). https://doi.org/10.1364/OE.469369 (Fig. 3)

02 Research Background

In the Fa-Pert composite microcavity interference spectrum, cosine signals of different frequencies represent the interference information of different microcavities, and the interference spectrum of a single microcavity cannot be directly extracted from the composite spectrum for independent demodulation. In the previous research of the research group, the interference frequency of the composite microcavity was separated by Fourier transform, and a band-pass filter was constructed according to the frequency component characteristics of the frequency spectrum of the composite microcavity, and other frequencies were filtered out by inverse Fourier transform. component, only the independent interference spectrum corresponding to one of the microcavities can be obtained, and then the wavelength shift of the spectral peak can be traced by the single-peak tracing method. However, when the Faber cavity length is short, the above method is no longer applicable. The reason is that in the full spectrum range, the low-frequency signal obtained by the Fourier transform of the vacuum cavity with a shorter cavity length is very close to the DC fundamental frequency, which is not conducive to the construction of a band-pass filter, resulting in deviations in the experimental results. Based on this, the Vernier effect of the inner and outer cavities is considered, and an all-silicon Fa-Pop pressure sensor is designed, in which the silicon cavity is used as the inner cavity and the silicon/vacuum hybrid cavity is used as the outer cavity for optical path difference matching. The insufficiency of the frequency domain filtering method enables highly sensitive pressure measurement.

03 Innovative Research

3.1 Theoretical analysis of Vernier effect in inner and outer cavity

The vacuum chamber pressure sensitivity is:

The pressure sensitivity of the silicon/vacuum mixing chamber is:

Among them, m represents the interference order, L2 represents the cavity length of the vacuum cavity, n2 represents the refractive index of the vacuum cavity, L1 represents the cavity length of the silicon cavity, and n2 represents the refractive index of the silicon cavity.

In this all-silicon Fabry sensing structure, the optical path difference between the silicon cavity and the silicon/vacuum hybrid cavity is matched. The amplification factor is:

The sensitivity of the reflection spectral envelope is then:

The designed sensor is simulated and tested in the pressure range of 10~300 kPa and the wavelength range of 1500~1600 nm, and the pressure sensitivity and temperature sensitivity results of the vacuum chamber length of 30 μm are obtained (Figure 2).

Figure 2 (a) Pressure sensitivity in the pressure range of 10~300 kPa and wavelength range of 1500~1600 nm; (b) temperature sensitivity at the central wavelength of 1550 nm.

Source: OpTIcs Express (2022). https://doi.org/10.1364/OE.469369 (Fig. 4) 3.2 Fabrication and verification of all-silicon pressure sensor

The all-silicon sensor chip consists of two layers of silicon wafers. A SOI wafer with a thickness of 70 μm monocrystalline silicon and a single crystal silicon wafer with a thickness of 500 μm double-sided polishing are used as raw materials. The size of the wafer is Both are 4 inches and the crystal orientation is <100>. In order to reduce the influence of the interfacial bonding strength on the deformation repeatability of the diaphragm, etching was performed on the surface of the SOI wafer with an etching depth of 30 μm, and the thickness of the top silicon diaphragm was 40 μm. As shown in Figure 3, the prepared sensor and its reflection spectrum information. The low-frequency signal of the vacuum cavity is too close to the fundamental frequency of the direct component, which is not conducive to its efficient extraction and filtering. The envelope peak tracking method based on the Vernier effect of the inner and outer cavity is suitable for the short vacuum cavity to improve the pressure sensitivity.

Figure 3 (a) Sensor chip; (b) MEMS pressure sensor; (c) reflection spectrum of the sensor; (d) Fourier transform amplitude-frequency curve.

Source: Optics Express (2022). https://doi.org/10.1364/OE.469369 (Fig. 5) 3.3 High Sensitivity Pressure Measurement

By tracing the envelope peak of the reflection spectrum, a good linear relationship between the pressure value and the peak wavelength and a pressure sensitivity of -1.028 nm/kPa can be obtained, as shown in Fig. 4(a). Tracking the envelope of the reflection spectrum under the condition of short Faper cavity length solves the problem that the low frequency signal of the vacuum cavity length is very close to the DC component after fast Fourier transform and cannot be accurately demodulated.

Verify the temperature sensitivity of the sensor at 110 kPa. In the range of 0–80 °C, the envelope peak evolution of the reflection spectrum is shown in Fig. 4(b), with a temperature sensitivity of 0.041 nm/°C. In a sense, this is normal. Considering the thermal expansion effect of the residual gas pressure in the vacuum chamber leads to an unexpected change in the length of the vacuum chamber, which is ignored in the theoretical analysis. Therefore, the fabrication process of the MEMS sensor can be improved, the influence of residual gas pressure can be reduced, and lower temperature sensitivity can be achieved.

Figure 4 (a) Variation of the wavelength of the envelope peak of the reflection spectrum with pressure; (b) Variation of the wavelength of the envelope peak of the reflection spectrum with temperature. Source: Optics Express (2022). https://doi.org/10.1364/OE.469369 (Figs. 8 and 9)

04 Application and Outlook

To sum up, the all-silicon Fa-Per sensor fabricated by MEMS technology in this work innovatively realizes the Vernier effect of the inner and outer cavity, and achieves highly sensitive pressure measurement by tracking the envelope evolution of the reflection spectrum. The shortcomings of the frequency-domain filtering method in the case of short Fa-Per cavity length are eliminated. The sensors realized by MEMS technology have good consistency and can be mass-produced, which lays the foundation for the realization of product industrialization. It can be seen that the sensor has broad application prospects and potential in high-sensitivity pressure measurement.

(Source: China Power Grid, author: Dai Xiaoshuang Wang Shuang)