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The mobile phone you hold in your hand every day, the safety system in your car, and the watch on your wrist all contain microelectromechanical components, also known as MEMS, which are smaller than a human hair. They are like miniature machines: they can sense acceleration, pressure, sound, or angular velocity and convert these changes into electrical signals. How are these tiny things made? Starting with a flat silicon wafer, patterns are etched by coating with photoresist and exposure, thin functional films are grown on the surface, and then a three-dimensional structure is carved out using deep reactive ion etching. Finally, the chip and cover plate are bonded together to form a cavity and packaged for testing, so that it can work silently without you noticing. We will explain the entire process in a simple way so that you can understand the "birth certificate" of a MEMS sensor.
Manufacturing Overview
A MEMS (Micro-Mechanical System) is completed step-by-step from "design → front-end fabrication → mid-stage bonding and cavity formation → back-end packaging and testing". The front-end uses photolithography to define patterns on a silicon wafer, then uses thin-film deposition and etching to create structures such as cantilever beams, mass blocks, and movable electrodes. The mid-stage often uses glass or silicon cap wafer bonding to form a vacuum or controlled cavity, ensuring the structure has the designed damping. Finally, in the back-end packaging, an ASIC or readout circuit is added, and calibration, temperature drift compensation, and reliability verification are completed. While it resembles a semiconductor, MEMS has two additional elements: "mechanical structures in the thickness direction" and "cavities/moving gaps," thus requiring unique considerations in process selection and tolerance control.
Scientific principles and key design
The core of MEMS sensors is converting physical quantities into electrical signals. Taking a common capacitive accelerometer as an example, external acceleration causes displacement of a mass block, altering the distance between the movable and fixed electrodes, resulting in a change in capacitance. In a piezoresistive pressure sensor, the bending of the thin film under pressure causes strain in the silicon resistor, and the change in resistance can be read by a Wheatstone bridge. The design must simultaneously balance sensitivity, measurement range, signal-to-noise ratio, and bandwidth. Mechanically, the spring constant and resonant frequency are determined by geometric dimensions, material elastic modulus, and structural shape. Electrically, the capacitance is determined by the electrode area and gap. Signal processing further enhances usability through amplification, filtering, and modulation/demodulation. Due to the tiny size of MEMS, surface forces (such as adhesion and van der Waals forces) are more dominant than volume forces (inertia and gravity). Therefore, dry release, hydrophobic surface treatment, and preventing jamming are crucial for manufacturing yield. Furthermore, gas damping strongly affects dynamic response, so a vacuum chamber is often used to control the Q value. Temperature alters the elasticity and resistance of materials, requiring compensation mechanisms to be incorporated during the design and packaging stages. These cross-disciplinary couplings make MEMS design not just about layout and process, but a systems engineering that simultaneously optimizes physics, materials, and circuitry.
Lithophotos, deposition, DRIE and bonding
In practice, photoresist is first coated onto a silicon wafer using photolithography, followed by exposure and development, and then the pattern is transferred to a thin film or the silicon substrate. Thin film deposition commonly uses LPCVD/PECVD (such as SiN, SiO₂) or PVD (metal electrode); sacrificial layers (such as PSG or oxide layers) are used to define gaps, which are then removed by selective etching to release movable structures. When high aspect ratio trenches are required, DRIE's Bosch process is used, employing alternating etching/sidewall protection to achieve a sidewall uprightness of 10:1–30:1; pressure sensing films often achieve the target sensitivity through micromechanical back etching and thin film thickness control. Next comes bonding: anodic bonding (Si–glass) forms strong bonds at approximately 300–450 °C and several hundred volts, suitable for visual alignment of the transparent cover and the cavity; Si–Si bonding can be achieved using surface activation or hydrogen bonding, followed by annealing for strengthening. If a high Q value is required for the device, a vacuum is simultaneously evacuated during bonding, and a getter is introduced to maintain the cavity pressure.
From packaging and testing to mass production
System-in-Package (SiP) for MEMS and ASICs is becoming increasingly common, shortening traces and improving signal-to-noise ratio. Packaging materials must balance hermeticity and thermal expansion coefficient matching to prevent zero-point drift caused by temperature cycling. Automotive-grade components often use ceramic or metal packages to enhance hermeticity and reliability. Before mass production, electrical measurements, vibration scanning, and reliability tests such as temperature and humidity cycling and drop impact are required. Pressure and gas sensors also need to be traced back to standard measurement platforms for multi-point calibration and linearization. Factory calibration can be performed on a test machine using lookup tables or first/second-order polynomial coefficients written into non-volatile memory to ensure that each chip meets specifications with the same drive parameters at the system level.
Looking back on this journey, the birth of MEMS is essentially about carving "invisible micro-mechanisms" onto silicon, and then using hermetically sealed enclosures and circuits to reliably transmit signals. Every step is crucial: is the pattern accurate? Is there internal stress in the thin film? Are the etched sidewalls straight enough? Is the pressure in the cavity stable? All of these factors ultimately affect sensitivity, noise, and lifespan. The next time your phone automatically rotates its screen, your car's airbag reacts instantly, or your watch measures altitude changes, remember these structures that are only clearly visible under a microscope—they make everyday technology truly "feelable."
