by St.J. Dixon-Warren
Engineering and Process Analysis Manager, Chipworks
The iPhone 4 represents the next step in evolution of Apple's market leading smartphone technology. The previous two parts of this series reviewed the results of our teardown of the iPhone 4, plus included a discussion of the 3-axis accelerometer and gyroscope technology. This final part reviews the electronic compass technology, and then provides some concluding remarks. The integration of these three devices provides full nine degree-of-freedom (9DoF) motion sensing.
iPhone 4 Electronic Compass: AKM AK8975/3
The third motion sensor in the iPhone 4 is the AKM AK8975 electronic compass. The AK8975 device is very similar to the AK8973, which was found in the iPhone 3GS. This discussion will focus on the AK8973.
The AKM compass devices are based on the Hall Effect, which is one of a number of methods for detecting magnetic fields. A benefit of Hall sensors is that they can be fabricated as planar devices using a CMOS process flow. Such devices, however, are only sensitive to a magnetic field oriented perpendicular to the die surface. The incorporation of a magnetic concentrator enables the detection of field lines parallel to the die surface.
The magnetic concentrator is formed using a disk of high permeability material that is deposited over the Hall sensor structures on the die. Figure 1 below shows a side-view package X-ray of the AK8973. The die is wire bonded to a metal lead frame within the plastic mold package. The concentrator is visible as a dark gray strip just above the die.
Cross-sectional analysis of the package reveals the magnetic concentrator to be formed as a circular disc of FeNi alloy deposited over a die coat layer. The magnetic concentrator serves to bend magnetic field lines that are oriented parallel to the die surface, to give a component oriented perpendicular to the die surface, hence allowing the detection of parallel components of the field by the Hall sensors that are embedded in the die surface.
Figure 3 shows a photograph of the Hall sensor structure found on the AK8973. The location of the magnetic concentrator, which was lost during the sample decapsulation, is annotated as a blue circle. The Hall sensor structure is comprised of eight discrete Hall sensor devices, located in pairs beneath the four quadrants of the magnetic concentrator. Each of the Hall sensor devices is formed using an N-well in the P-type substrate. The devices operate by passing a current diagonally between two contacts to the N-well. The application of a magnetic field, oriented perpendicular to the die surface, results in a small potential between the two contacts oriented on the other diagonal. Deconvolution by the A4 processor of the signal from the Hall sensors, beneath each quadrant of the concentrator, allows the direction vector for the magnetic field to be calculated.
The integration of 9DoF sensing in the iPhone 4 represents a milestone in the evolution of portable consumer electronics devices. We can expect the other manufacturers to follow Apple's lead. In addition, we expect component suppliers, such as STMicroelectronics and InvenSense, to incorporate these sensing functions into integrated solutions, thus allowing the component supplier to grab a larger part of the value.
For example, InvenSense has recently launched the MPU6000, which integrates a 3-axis accelerometer and a 3-axis gyroscope with a digital motion processor that is designed to interface with a third-party compass device, to provide full 9-axis sensing.
Software integration is critical to the effective use of the data provided by these sensors. The user is typically not interested in the raw signal provided by the sensors, but rather in the human-machine interactions that they enable. In addition, in combination with wireless technologies, these low cost devices will enable the creation of dispersed sensor networks.