Saturday, July 14, 2012

MEMS pressure sensor teardown - part 1


While dumpster diving the e-waste bins on campus, one of my roommates found a control board of some sort that had an unusual sensor on it. We decided to take a closer look.

Sensor board
The board substrate is ceramic, most likely alumina. There are three electrically conductive layers visible on the board - gray (first level metalization), black (thick-film resistors), and gold (second level metalization and bond pads). A bluish dielectric separates M1 and M2 at crossing points but is not present over the remainder of the board.

The brownish reside on the top of the board is adhesive residue from the encapsulation over the sensor die.
Sensor die

The sensor die is approximately 1600μm along each side and appears to be made from a <100> oriented silicon wafer. Two metal layers are visible - one of a resistive material (most likely polysilicon) and one of gold (used for bond pads). For the sake of discussion I will define the upper center pin to be pin 1.

The die consists of six resistors and is entirely passive, with no transistors whatsoever.

The active sensing element consists of two membranes made out of what appears to be silicon nitride. The membranes are suspended over a cavity defined by an anisotropic wet etch using a KOH or related chemistry.

There are a total of three resistors between the membranes, whose values presumably change as the membrane is stressed. Pins 1 and 2, as well as 3 and 4, are connected to thin zigzag resistors on the left and right membranes respectively. Pins 7 and 8 connect to another resistor which starts on the lower left of the upper membrane, loops around at the far side, and goes back to the lower left on the lower membrane.

In addition, there are three resistors on the silicon substrate - betweens pins 8 and 9, 7 and 6, and 6 and 5. While they are all bonded out to pads and would be easy to measure, I have not yet attempted to get resistance readings.

The resistor between pins 5 and 6 has very unusual geometry and is not the typical zigzag I would expect. There is no obvious reason for this pattern.

3 comments:

  1. Very interesting reading.. I also recently looked under the skirt of a pressure sensor too.. See http://www.varesano.net/blog/fabio/inside-ms5611-high-resolution-barometer

    Unfortunately, I destroyed it by mistake while handlying so I couldn't make 400x magnified pictures..

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  2. This is flow sensor. Membrane is actualy thermal insulation geometry. There is wire that is heat generator and resistor that change resistans based on temperature which is different based on air flow (direction and intensity).

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  3. Yeah, I figured that out when I found a datasheet for the part (see my next post in the series).

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