Design and Test of a High-Sensitivity MEMS Capacitive Resonator for Photoacoustic Gas Detection

Trace gas detection is crucial in environmental monitoring, industrial safety, and medical diagnostics. Photoacoustic spectroscopy (PAS) has gained prominence due to its high precision and sensitivity. PAS converts absorbed laser energy into acoustic signals, contrasting traditional absorption spectroscopy. The PAS method produces no output without target gas input and remains unaffected by most background light. This inherent advantage allows for creating highly sensitive, compact, and cost-effective acoustic sensors that surpass photoelectric detectors in both affordability and size.

Cantilever-enhanced PAS (CEPAS) enhances sensitivity by detecting sound pressure via mechanical cantilevers. The advancements in Micro-electromechanical systems (MEMS) technology enable compact, cost-effective cantilevers, with displacement measurable through optical and non-optical methods. In capacitive transduction, capacitance increases as the electrode gap narrows, requiring micrometer-scale spacing for high detection sensitivity. However, the tiny gap causes significant viscous damping (squeeze film effect), decreasing sensor sensitivity. This study proposes a new MEMS capacitive resonator design for PAS, reducing the squeeze film effect.

This MEMS sensor uses a silicon resonator, sensing electrode, and glass substrate for gas detection. A laser generates acoustic waves, vibrating the resonator's cantilevered structure. The anchor secures it to the substrate while the receiving region interacts with the sound. The electrode with subelectrodes and metal leads balances potential.

Traditional sensors have varied electrode distances for capacitive detection that require narrow gaps (≤5μm). This increases gas damping, thus lowering the resonator's Q-factor and reducing sensitivity. The proposed design alters overlapping areas instead of varying electrode distances, maintaining a constant gap to minimize gas damping. Interlocking comb structures enhance sensitivity. The resonator's vertical vibration changes the capacitance, and raising the anchor reduces damping, improving sensitivity while keeping the electrode gap small.

The device was fabricated using silicon-on-glass (SOG) technology. A 200 nm gold layer was sputtered onto a 500 μm glass substrate to form lead wires and electrodes. Inductively coupled plasma (ICP) etching created bonding anchors and grooves in a 300 μm silicon wafer. Anodic bonding secured the silicon structure to the glass, followed by ICP etching to shape the 40 μm movable structure. Gold leads connected pads to the resonator and sensing electrode, and a transimpedance amplifier converted capacitance changes into voltage signals. Finally, the sensor was enclosed in a custom shell for dust proofing and electromagnetic shielding.

The resonator, tested under sound wave excitation, achieved a maximum sensitivity of 3749 mV/Pa at 1870 Hz with a 15 V bias. When integrated into a CO₂ detection system based on photoacoustic spectroscopy (PAS), it enabled precise gas concentration measurements, achieving a minimum detection limit of 0.58 ppb with a 49 s averaging time. The study utilized Normalized Noise Equivalent Absorption (NNEA) to standardize sensor performance, eliminating dependencies on laser power, gas absorption, and amplifier bandwidth, allowing for objective comparisons across PAS systems. The NNEA was 8.45 × 10−9 cm−1·W·Hz−1/2, highlighting its superior performance among capacitive PAS sensors.

This study aims to integrate a silicon resonator, semiconductor laser, and signal processing circuits onto a single chip, making photoacoustic gas sensors cheaper, compact, and more power-efficient, leading to broader adoption.