Part 2: How it works technically

The general idea for a motion sensor is as follows: acceleration or motion create forces, such forces causes displacements of or internal stress concentration in the mechanical structure, and the displacement and/or stress are turned into electrical signals.  There are various ways to accomplish the last-stage transduction. However, the need to have three-axis sensors places limitations on candidate transduction methods. (Some techniques that are good for making one axis sensor may not be able to make three axis sensors).

Today, the vast majority of MEMS sensor is based on capacitive sensing – a three dimensional microstructure, consisting of a moving mass suspended by springs, moves under applied acceleration or force.  The springs are made of three dimensional beams – precisely defined by photolithography, carved by plasma etching, and released by sacrificial etching.

The microstructure is conductive – it forms capacitors with its neighboring surfaces. Hence when the microstructure structures, the capacitance values change. The changes in capacitance can be measured by electronics circuitry, yielding either analog or digital output values.  

(click for high resolution image)

The following diagram illustrates how a MEMS gyroscope based on Coriolis force work. A mass, suspended using cantilevers (not shown), is parallel to the substrate of the chip. The mass has vertical side walls, and form four capacitors with neighboring anchored surfaces. The two driving electrodes are labeled d1, d2; the two sensing electrodes are labeled s1 and s2. Voltage can be applied to the mass and the anchored electrodes. Voltage can induce electric force to cause the mass to move. Movement of the mass can cause the capacitance values to change – thereby providing electrical read out of the positions.

A mass moving with a linear velocity V in a rotational frame with a rotation rate W experiences an induced Coriolis force.

Let’s explore how a one axis (yaw rate) gyroscope can be designed. Since the mass can only oscillate back and force, we view the mass as going through infinite series of round-trip movements. During each segment of the one-way movement, if the chip is rotated (say, in the yaw axis), a Coriolis force will develop, causing the mass to move sideways. The sideway movement is measured using capacitance measurement.

Here we use a parallel electrode capacitor. The springs that connect the mass to the chip substrate are purposefully not shown. Note that in reality, the capacitive electrode design are much more complicated, and the spring designs affect every aspect of the sensor performance.

The leading companies – Analog Devices, InvenSense, and STMicroelectronics, use different designs and materials for the moving pass and springs.  Their methods result in different thickness, geometries, material stress levels, and uniformity.

Below is a comparison of gyro principles and designs for three major companies in this space: STMicroelectronics, InvenSense, and Analog Devices.

 

Table: Comparison of chip technologies of major MEMS motion sensor companies, as of 2010.

 

Analog Devices

STMicroelectronics

InvenSense

Transduction principle

Capacitive sensing

Capacitive sensing

Capacitive sensing

Circuit-MEMS integration

Monolithic integration

Circuit and MEMS on separate chips; Side-by-side packaging

Circuit and MEMS on separate chips; Side-by-side packaging

Proof mass layer material

LPCVD grown polysilicon film

Single crystal silicon

Epitaxial grown silicon

Thickness of mass layer

1-5 micrometer

15-50 micrometer

10 to 100 micrometer

The above description pertains to how the device work.  The following discussion is about how the device is made and manufactured. There are three general steps to turn a piece of silicon into a chip that gets soldered on an iPhone board:

-         first: on chip processing must be done to make the device on silicon;

-         second: the device must be separated, packaged, and encapsulated to form a chip;

-         third: the chip must be sent to iPhone manufacturers and gets attached to circuit boards.

MEMS moving mass and springs are made on a silicon wafer. After processing, the wafer must be diced and mounted on an electrical wire frame. Wire leads are formed by bonding – between the chip and the electric wire frame. The prevailing method of packaging involves casting the entire device in a polymer matrix and then cutting the slab of cured polymer into individual chips. Such chips are directly mounted on circuit boards using surface mount techniques.

The moving parts must be protected and encapsulated in a stable environment. The MEMS moving parts must be protected from particles and environmental elements, during dicing, handling, and packaging. The environment typically involves low level vacuum.

The encapsulation is a process that Integrated Circuit manufacturers don’t need to be concerned about, since circuits do not contain moving parts and won’t be affected by particles, polymer cast, or packaging materials. Circuits are also not vulnerable to pressure and humidity, whereas MEMS devices are.  As a result, MEMS packaging and testing often constitute between 30-70% of the overall cost.  

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