MEMS pressure sensor teardown - part 2

The sensor I studied in my last post was delivered to me in a partially disassembled state. After returning to the e-waste dumpster we were able to find a fully intact unit.

SiliconPr0n wiki page: http://siliconpr0n.org/archive/doku.php?id=honeywell_awm2100v

The part number is clearly visible, it's a Honeywell AWM2100V airflow sensor. Some of my analysis from earlier was a bit off - it turns out that there's two ports on the device and some of the resistors on the membrane are heaters. One of the resistive elements is driven with a constant power and the resistance of the other one is measured to determine the membrane's temperature. Given the power input and the temperature increase above ambient (compared to unheated regions of the die and board) one can compute the airflow rate.

I tore this one down to the bare board but no further, die/board photos from the other unit are in part 1 of the post.

Sensor on the PCB

The original unit was a multi-board Honeywell process control module containing this sensor, a solenoid valve, and a large number of through-hole ICs including a Z88 family microprocessor (which may be covered in a future post - I want to get it decapped but haven't had time to do so yet).

I removed the sensor from the board using hot air. It's a six-pin SIL package with a plastic case snapped around the sensor board.

Packaging of sensor after removing from board

After removing the snapped-on casing we're left with the ceramic sensor board and a hose fitting on top. Under the host fitting is the actual sensor die, studied in detail in the previous post.

Fully disassembled

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.