Introduction to MEMS Motion Sensors
A MEMS (Micro-Electro-Mechanical Systems) motion sensor is a tiny microchip that contains microscopic mechanical structures that can detect motion and acceleration. MEMS motion sensors have revolutionized the field of motion sensing and are used in countless applications today.
Some key facts about MEMS motion sensors:
- They are extremely small, often just a few millimeters in size. This allows them to be integrated into tiny electronic devices.
- They are inexpensive to manufacture in high volumes using semiconductor fabrication techniques. This makes them cost-effective options for motion sensing.
- They can accurately measure motion, acceleration, vibration, tilt, and rotation along one, two, or three axes.
- They have extremely low power consumption, making them suitable for battery-powered devices.
- They can withstand high G-forces and harsh environments.
In this article, we will explore the underlying working principles and architecture of MEMS motion sensors. We will focus on two main types:
- Accelerometers – measure linear acceleration.
- Gyroscopes – measure angular velocity and orientation.
How Does an Accelerometer MEMS Motion Sensor Work?
An accelerometer is a MEMS sensor that measures proper acceleration or g-force. This is the acceleration experienced relative to freefall, which is caused by inertial forces. Accelerometers are essential components in devices where motion detection and tilt sensing are needed, such as smartphones, fitness trackers, game controllers, and drones.
The basic working principle of a MEMS accelerometer is that a small proof mass is suspended by springs inside a cavity. When the sensor experiences an acceleration, the proof mass deflects due to Newton’s second law of motion (F=ma). This deflection is then measured electrically to determine the acceleration.
Key Components
A MEMS accelerometer contains the following key components:
- Proof mass – A tiny block of silicon attached to the substrate by suspension springs. It moves when accelerated.
- Suspension springs – Hold the proof mass in place and allow it to deflect when accelerated. Made of silicon.
- Capacitor plates – Stationary plates that measure capacitance changes as the proof mass moves between the plates.
- Sensor substrate – A silicon wafer that forms the base. Etched using MEMS fabrication techniques.
Capacitive Sensing
The most common way accelerometers detect proof mass movement is by capacitive sensing. The principle is that the capacitance between two plates changes when the gap between them changes.
The proof mass forms one plate of a capacitor, while the substrate has fixed plates attached to it. As the proof mass moves due to acceleration, the gap between the plates changes, causing a change in capacitance. This change is converted to an electrical signal that is proportional to the acceleration.
By detecting capacitance changes along two or three axes, the accelerometer can measure acceleration in multiple dimensions.
Types of MEMS Accelerometers
There are several types of MEMS accelerometers based on their internal structure and sensing mechanism:
- Piezoresistive – Uses piezoresistors on the spring arms to detect stresses.
- Piezoelectric – Uses piezoelectric materials that produce a voltage when stressed.
- Thermal – Contains heated gas in the cavity that changes temperature distribution when accelerated.
- Tunneling – Uses quantum tunneling effects between tip and surface electrodes.
However, capacitive sensing is by far the most common approach due to its simplicity, low-power, and robustness.
How Does a Gyroscope MEMS Motion Sensor Work?
A gyroscope is a sensor that can measure angular velocity and orientation. MEMS gyroscopes are essential for inertial guidance systems, stabilizing cameras, VR headsets, drones and other applications where precise rotational motion sensing is needed.
While early gyroscopes contained spinning masses, MEMS gyroscopes work completely differently by using microscopic structures that leverage the Coriolis effect.
The Coriolis Effect
The Coriolis effect causes a deflection of moving objects when viewed in a rotating reference frame. On Earth, this effect causes moving air masses to be deflected, creating large-scale weather patterns and ocean currents.
In a MEMS gyroscope, the proof mass is driven to vibrate at a constant frequency by electrostatic forcing. When the sensor rotates, the vibrating proof mass gets deflected orthogonal to its original vibration axis due to the Coriolis effect. This deflection is measured capacitively to determine angular velocity.
By detecting deflections along multiple axes, the 3D rotational motion can be measured.
Types of MEMS Gyroscopes
There are two main types of MEMS gyroscopes:
- Vibratory – Uses a vibrating proof mass as described above. Most common type.
- Optical – Uses a resonating cavity and detects changes in light patterns when rotated. Complex but highly accurate.
Both types are able to measure angular velocity with a high degree of precision.
Fabrication of MEMS Motion Sensors
MEMS motion sensors leverage the same fabrication techniques used to manufacture integrated circuits and microprocessors. This enables mass production and integration with CMOS electronics. Here is an overview of the manufacturing process:
Silicon Wafer Preparation
Fabrication begins with blank silicon wafers about 1 mm thick. These wafers are highly polished on one side. An epitaxial layer is then grown on the polished side under vacuum and high heat. This layer defines the thickness and properties of the MEMS structures.
Depositing and Patterning Thin Films
Next, layers of conductive, insulating and piezoelectric materials are deposited on the wafer using techniques like chemical vapor deposition (CVD), sputtering, oxidation, and molecular beam epitaxy. Each material is patterned by photolithography and selective etching to form the electrodes, capacitor plates, and piezoelectric elements needed.
Bulk Micromachining
The silicon wafer is then etched from the backside to form cavities and tunnels where the MEMS structures will move. This bulk micromachining utilizes deep reactive-ion etching to carve extremely precise shapes out of the silicon.
Surface Micromachining
Additional layers are then deposited and patterned on the top surface to construct the delicate springs, weights, and gaps that make up the MEMS mechanical structures. The gaps are often sealed under vacuum or inert gas.
Capping and Dicing
In the final steps, the MEMS elements are sealed using wafer bonding under a protective cap. The wafer is then diced into hundreds of tiny MEMS motion sensor dies using a diamond saw. The dies are assembled into packages with electronics.
This entire process leverages the same equipment used to fabricate computer chips, allowing MEMS motion sensors to be manufactured in large volumes.
Applications of MEMS Motion Sensors
MEMS motion sensors are enabling many disruptive applications and technologies due to their microscopic size, low cost and excellent performance:
Consumer Electronics
- Smartphones – Accelerometers for screen orientation and gyroscopes for pitch/roll/yaw sensing.
- Game controllers – Accurate tilt and motion tracking for immersive gameplay.
- Digital cameras – Image stabilization with gyroscope and accelerometer data.
Automotive Systems
- Electronic stability control – Gyroscopes detect yaw and prevent skidding.
- Airbag deployment – Accelerometers sense rapid deceleration for airbag release.
- Navigation systems – Accelerometers augment GPS data for accurate position tracking.
Industrial
- Condition monitoring – Vibration monitoring with accelerometers helps predict machine failures.
- Platform stabilization – Gyroscopes and accelerometers enable stable positioning.
- Robotics – Motion sensing for navigation, mapping and coordination.
Fitness and Sports
- Wearable fitness trackers – Built-in accelerometers count steps and analyze workouts.
- Smart footwear – Inserted sensors provide gait analysis and improve athlete performance.
- Motion tracking – Accurately tracks complex motions like swings, pitches and more.
As MEMS fabrication continues to evolve, motion sensors will find new applications across industries. Their small size and low cost makes them ideal for integrating motion sensing capabilities into mass-market consumer products and niche industrial solutions alike.
Conclusion
MEMS motion sensors have completely transformed the fields of inertial guidance, motion tracking, and platform stabilization due to their microscopic size and excellent performance. By leveraging capacitive and Coriolis sensing principles, accelerometers and gyroscopes respectively provide precise measurements of linear and angular motion. With robust, cost-effective fabrication techniques adapted from the semiconductor industry, MEMS motion sensors will continue to enable innovative motion-aware devices and technologies.
Frequently Asked Questions
What are the key components inside a MEMS accelerometer?
MEMS accelerometers contain a movable proof mass suspended by springs, fixed capacitor plates to detect proof mass motion capacitively, and a substrate that houses these components. Acceleration deflects the proof mass and causes capacitance changes proportional to acceleration.
How does a MEMS gyroscope measure angular velocity?
It uses a vibrating proof mass. When the sensor rotates, the Coriolis effect causes the vibrating proof mass to deflect orthogonal to its vibration axis. This deflection is measured capacitively and is proportional to angular velocity.
What fabrication process is used to manufacture MEMS motion sensors?
Semiconductor micromachining techniques adapted from integrated circuit fabrication are used. This includes deposition, photolithography, bulk and surface micromachining, and wafer bonding/dicing to mass produce tiny MEMS structures on silicon wafers.
What are some applications of MEMS motion sensors?
Key applications are in consumer electronics like smartphones and game controllers, automotive systems like airbag deployment, industrial platforms and robotics, fitness wearables, and motion tracking in sports. Their small size and low cost make them widely usable.
What are the advantages of MEMS motion sensors over conventional motion sensors?
Compared to larger conventional sensors, MEMS motion sensors are microscopic in size, extremely low cost at high volumes, and have very low power consumption. Their performance is often superior despite the tiny footprint.
