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iPhone 4 takes consumer handhelds to 9 degrees of freedom

Posted: 01 Sep 2010 ?? ?Print Version ?Bookmark and Share

Keywords:iPhone? inertial? sensor?

Chipworks looks under the hood of the iPhone 4 to reveal the first portable consumer device featuring full nine degrees-of-freedom (9-DoF) inertial sensing. Apple has accomplished this by integrating a three-axis accelerometer and a three-axis gyroscope from STMicroelectronics (ST), together with an electronic compass from AKM.

The original 2G iPhone,released in 2007, featured an ST 3mm x 5mm LIS302DL three-axis accelerometer. This provided the screen orientation function, by using the acceleration of gravity to determine the orientation, and it was also available to app developers for simple games. The iPhone 3G also only contained an accelerometer but in this case Apple used the 3mm x 3mm LIS331DL MEMS device, also from ST. Next up, the iPhone3GS added digital compass capability by adding an AKM AK8973 electronic compass. This provided improved GPS location functionality, even though it required regular recalibration by waving the phone in a figure-eight motion.

Figure 1ST Microelectronics LIS331DLH MEMS Die (click to enlarge)

The iPhone 4 features an ST LIS331DLH three-axis accelerometer and an AKM AK8975 three-axis electronic compass. To get the 9-DoF, Apple added an ST L3G4200D three-axis gyroscope sensor, packaged in a 4mm x 4mm LGA package.

According to ST the LIS331DLH is an ultra-low power, digital three-axis accelerometer. The package contains two stacked die, a V583A ASIC with 2007 die marks and a MEMS die with C5L12B 2008die marks. Figure 1 shows the MEMS die, with its hermetic cap removed, illustrating the integrated XY sensor structure and a Z-axis sensor structure.

The XY sensor uses interdigitatedfinger capacitors to sense the motion of a proof mass that is constrained to move in the XY plane, while the Z sensor uses an underlying capacitor plate to sense vertical deflection of a proof mass mounted on a torsion spring. STfabricated the part using their well-established THELMA process (ThickEpi-Poly Layer for Micro-actuators and Accelerometers). The THELMA process uses a thick top polysilicon layer (poly 2) to form the micro-machined electro-mechanical structures plus a thin buried polysilicon layer (poly 1) for connection to the bond pads, as well as for the bottom capacitor plate of the Z-sensor.

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