An introduction to MEMS

MEMS

This article will introduce you to the vast world of MEMS, which you use everyday without even knowing it! You will uncover the fascinating microscopic interfaces used in the industry between the realms of physics and electronics. Additionally, we will dive into an in-depth analysis of the widely used MPU6050 chip.

Please note that this article will not distinguish between MEMS (Micro-Electro-Mechanical Systems) and NEMS (Nano-Electro-Mechanical Systems). Specific details and subtle differences will also be omitted, as this article aims only to introduce the complex world of MEMS.

Historical introduction

MEMS, or Micro-Electro-Mechanical Systems, are miniaturized devices that combine mechanical and electrical elements on a single silicon chip. Their development began in the 1980s, alongside the rise of the semiconductor industry and microfabrication techniques, allowing for the production of devices that operate on a microscopic scale and produce effects on a macroscopic level.

MEMS technology, initially conceptualized in the 1950s, found its early applications in electronics and micromachining. Advances in materials science and photolithography paved the way for innovations in miniaturization. The first MEMS prototypes were produced in the 1960s and 1970s, though it was only in the 1980s and 1990s that MEMS found substantial commercial success, with accelerometers in automotive airbags as one of the earliest applications. These devices detected rapid deceleration and triggered airbag deployment, revolutionizing automotive safety and leading to widespread MEMS adoption in industries including medical, consumer electronics, and telecommunications

Today, MEMS technology is recognized as a key technology of the 21st century in numerous applications, ranging from medical biosensors to advanced motion detection in smartphones and drones, and continue to evolve with new fabrication techniques and materials.

The main types of MEMS

MEMS devices can be broadly classified into three primary types based on their functionality:

  • Sensors: Devices that detect changes in the environment by measuring mechanical, thermal, magnetic, or chemical information and converting it into electrical signals.

  • Actuators: Components that convert electrical energy into mechanical motion, used in applications requiring precise control at micro scales. Electrostatic and piezoelectric actuators are among the most common in MEMS.

  • Structural Elements: Purely structural MEMS parts form foundational devices within complex systems. These elements include gears, levers, and linkages, which can be manufactured in microscale but provide the same functionalities as their larger counterparts.

MEMS devices may also be hybrids, incorporating both sensing and actuation capabilities on a single chip. These hybrid devices enable applications that require feedback and control, such as in active suspension systems in vehicles.

Note

Certain MEMS operate on the same principles as traditional mechanical systems, such as gears, chains and motors.

Gears

Design and Operating Principle

The design of MEMS involves the integration of micro-scale mechanical components with microelectronic circuits on a silicon substrate, leveraging batch processing techniques from the integrated circuit (IC) industry. Three primary fabrication techniques are central to MEMS: bulk micromachining, surface micromachining, and high-aspect-ratio micromachining (HARM). These topics will not be covered in this article; however, you are encouraged to explore these processes further if interested.

Sensing

The function of MEMS relies on microsensors and microactuators that convert physical stimuli into electrical signals and vice versa. The main sensing principles include piezoresistive, piezoelectric, and capacitive sensing:

Info

Capacitance - measured in farads (F) - is based on the ability of two conductive plates, separated by an insulator, to store electric charge when a voltage is applied. The stored charge depends on plate area, distance between them, and the insulating material.

In MEMS sensors, changes in capacitance between plates detect motion or tilt with high precision.

  • Piezoresistive Sensing: Utilizes the piezoresistive effect, where mechanical stress alters a material’s resistance. Silicon’s strong piezoresistive properties make it ideal for pressure and force sensors.

  • Piezoelectric Sensing: Involves materials like quartz and ZnO, which generate an electric charge under mechanical stress. This property is particularly useful for pressure sensors and accelerometers.

  • Capacitive Sensing: Measures changes in capacitance between a set of plates as they move or change position relative to each other. MEMS accelerometers and gyroscopes commonly use capacitive sensing due to its high sensitivity to small displacements.

Actuation

Actuators, conversely, typically employ electrostatic, piezoelectric, or even thermal mechanisms to create movement or force. Electrostatic actuators are prevalent in MEMS due to their ease of integration and low power consumption.

  • Electrostatic Actuators: Leveraging the attraction between oppositely charged plates, these actuators are often seen in MEMS applications due to the ease of fabricating closely spaced components.

  • Piezoelectric Actuators: These actuators exploit the piezoelectric effect, where an electric field causes deformation in piezoelectric materials, allowing for precise control over mechanical movement.

Note

Wait, what is the difference between piezoresistive and piezoelectric materials ?

Piezoresistive materials change their electrical resistance when strained, while piezoelectric materials generate an electric charge when mechanically deformed, but also deforms under an electric field.

A wide range of applications

Due to their relatively low cost, usefulness and reliability, MEMS have found their way into a wide range of applications, including:

Automotive

As discussed earlier, MEMS sensors are widely used in critical automotive safety and control systems. Accelerometers for airbag deployment, gyroscopes for stability control, and pressure sensors for advanced tire pressure monitoring have significantly improved vehicle safety. MEMS technology is set to become the driving force behind advanced driver-assistance systems (ADAS).

Aerospace and Defense

As you probably expect, MEMS accelerometers and gyroscopes provide accurate orientation and stabilization data for navigation and guidance in drones and missiles. MEMS are essential in miniaturizing instruments for reconnaissance and other critical defense applications.

Medical Devices

In healthcare, MEMS will be used in near future in blood pressure sensors and micro-pumps for implantable drug delivery systems. These devices are compact, allowing for integration into minimally invasive medical devices and enabling precise measurements within the human body.

A section view of a micro-pump :

Micropump

Microfluidic devices for lab-on-a-chip applications illustrate MEMS’ capability to deliver precise, reliable, and miniaturized medical solutions.

Note

Lab-on-a-chip devices are MEMS that integrates laboratory functions on a single integrated circuit, allowing for rapid chemical analysis, diagnostics, or biological testing with tiny fluid samples.

Telecommunications

Optical MEMS, or MOEMS, are now usefull in fiber-optic networks, enabling high-speed data transmission through compact, low-power optical switches. RF MEMS have optimized mobile devices, creating smaller, more efficient components like filters, switches, and oscillators that support faster, more reliable wireless communication.

Consumer Electronics

MEMS technology is integral to modern smartphones, tablets, and gaming consoles. Accelerometers and gyroscopes enable features such as screen orientation adjustments and motion control in gaming enhancing user experience, and are mandatory to VR immersion.

MEMS are also present in your printer, as inkjet printer heads utilize this technology.

In projectors, where millions of mirrors (each a few microns) define each pixel of the image (DMD: Digital Micromirror Device or DLP:Digital Light Processor).

Additionally, due to limited space, fancy wireless earphones use MEMS microphones for compact and relatively efficient sound capture.

In-depth analysis of a MEMS: MPU6050

As stated earlier in this article, accelerometers and gyroscopes are among the most impactful MEMS devices.

The MPU6050 is a popular Motion Tracking device that combines a three-axis gyroscope and a three-axis accelerometer on a single integrated circuit. Let’s take that chip, and take a look on its insides.

Accelerometer

An accelerometer detects linear acceleration through a proof mass suspended by polysilicon springs within the microfabricated structure. As the structure moves, the capacitance between stationary plates and those connected to the suspended mass changes, with the capacitance shift being directly related to the acceleration along the corresponding axis.

Accelerometer

The same principle applies when acceleration occurs along the X, Y, or Z axes, except that the structure designed for the Z axis requires a more vertical configuration.

Note

In practice, the three-axis accelerometer looks more like this.

Gyroscope

Gyroscope

Gyroscopes measure angular rotation through vibrating structures and the Coriolis effect. When a vibrating mass within the gyroscope experiences rotational motion, the Coriolis force causes perpendicular displacement, which is detected by capacitive (or sometimes piezoelectric) sensors, and converted into a voltage signal.

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  1. Under no angular motion, the 4 masses are rapidly vibrating along the horizontal plane.
  2. Pitch, 3. Yaw, 4. Roll motions.

Note

In real life, the three-axis gyroscope appears more like this.

Gyroscope

A Conclusion

In this article, we have merely begun to explore the diversity and complexity of MEMS. Nevertheless, you are now aware of the widespread nature of this field, which remains largely unknown to the general public.

The field of MEMS, with its extensive integration of mechanical, electrical, and structural components at a microscale, represents a significant milestone in technological innovation. Moreover, MEMS technology encompasses not only sensors and actuators but also purely electronic and structural elements, making it a versatile tool in modern engineering.

While the versatility of MEMS has enabled its application across diverse sectors, the technology still faces many challenges. Those challenges include manufacturing complexity, integration with electronics, ensuring reliability and longevity, cost reduction, market competition, standardization and regulation, ongoing R&D investment, and addressing environmental impacts.

So, will MEMS face the same obstacles and limits as conventional electronics? Or will the course of innovation stop by itself, exhausted of ideas?

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