Accelerometers: Shaswata Bose 17me167 Dept. Of Mechanical Engineering Nitk Surathkal

ACCELEROMETERS

Shaswata Bose 17ME167 Dept. of Mechanical Engineering NITK Surathkal

WHAT IS AN ACCELEROMETER? • An accelerometer is a device used to sense the absolute or relative acceleration of a device.

• The fundamental concept behind the working of any MEMS accelerometer is based on the deflection of a certain body known as proof mass attached to a rigid body via springs under acceleration

TYPES OF ACCELEROMETERS •

All accelerometers work on the same principle, but the process by which deflection of the proof mass is converted into electric signal varies.

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The different types of accelerometers I have found so far:1. Piezoresistive 2. Capacitive 3. Tunneling 4. Resonance

BRIEF DESCRIPTION OF THE WORKING OF SOME TYPICAL MEMS ACCELEROMETERS

PIEZORESISTIVE MEMS ACCELEROMETERS •

In Piezoresistive accelerometers, a piezoresistive piece is embedded in the proof mass.

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The motion of the proof mass under acceleration results in the motion of the embedded piezoresistor which causes a change in its resistance.

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This change in resistance can be used to determine the acceleration acting on the setup

A schematic representation of a piezoresistive MEMS accelerometer

CAPACITIVE MEMS ACCELEROMETERS • In a capacitive MEMS accelerometer, the proof mass has tiny fingerlike potrusions that constitute the metal plates of capacitors attached to a fixed base

• As the proof mass moves under acceleration, the gap between the capacitor plates changes, which results in a change of capacitance of the setup.

• Using suitable electrical connections, the variation in gap between the plates will represent the acceleration acting on the setup

• Plates like this can be arranged so that the accelerometer can read accelerations in all the 3 directions

Schematic representation of a 3-Axis capacitive MEMS accelerometer

Perspective view of the Z proof mass to show the Z acceleration sensing capacitors

TUNNELING MEMS ACCELEROMETERS •

These accelerometers are used to detect minor acceleration acting on a body

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The setup consists of a proof mass and a tunneling tip close to the proof mass so as to allow electrons to tunnel through the air gap between the tip and the proof mass

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When some acceleration acts on the proof mass, the tunneling gap increases. In order to maintain the tunneling current, some actuation mechanism can be used

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The actuation can then be converted into acceleration readings

Schematic representation of a tunneling accelerometer used for micro and sub-micro g measurements

SEM picture of the tunneling tip

RESONANCE MEMS ACCELEROMETERS •

In these type of accelerometers, the proof mass is anchored to a rigid base via tuning fork-like resonators. The resonators are electrically actuated to vibrate at their resonant frequencies.

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The acceleration acts axially on the setup. This results in a shift of the resonant frequency of the setup under axial accelerations

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This frequency variation can be used to detect the acceleration acting on the body

Schematic representation of a resonance MEMS accelerometer

MANUFACTURING PROCESSES FOR MEMS • Various micromachining methods are available which can be used to fabricate these devices, namely:-

1. 2. 3. 4. 5. 6.

Bulk micromachining Surface micromachining Wafer bonding Deep Reactive Ion Etching(DRIE) LIGA Hot Embossing

BULK MICROMACHINING •

In this process, material is removed by chemical etchants.

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The region where the material is not to be removed is covered by a masking agent like silicon nitride

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It is of two types:- isotropic and anisotropic etching

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Difficult to control etch depth

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Etch stops are thus used to control the depth of those etches

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In the substrate

SURFACE MICROMACHINING •

In this process, a sacrificial layer is applied on the substrate and the pattern is etched on the layer using photolithography

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The gaps are filled then filled using Si to form the structural layer

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The rest of the sacrificial layer is removed to get the complete product

DEEP REACTIVE ION ETCHING (DRIE) •

In this process, the unmasked region is alternately exposed to SF6 and C4F8 which alternately etches and covers the etch with a protective layer.

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The anisotropy of the etch removes the base faster than the sidewall.

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Alternate etching and protective coating results in deep vertical cuts.

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The sidewalls are not smoothed and are scalloped

LIGA • Used to form deep, smooth and nearly vertical etches in PMMA molds, which can then be used to form the part

• A special X-ray mask is needed for protecting the areas not to be etched • The process is very expensive as it requires a special X-ray mask and an X-ray synchrotron

• Sometimes, the final product is used as as a tool insert to imprint the shape of the tool into a polymer layer.

• Electroplating of metal in the mold is then done and the polymer mold is removed to obtain the final product

HOT EMBOSSING •

In this method, a tool insert is fabricated using LIGA, which is then heated along with the polymer substrate at a temperature higher than its glass transition temperature and the tool insert is embossed in the polymer substrate

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The system is then allowed to cool down and the polymer substrate is then removed from the insert, which can then be used as the product

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The process needs to be performed in complete vacuum in the absence of any impurities in order to ensure high dimensional accuracy

THANK YOU!