Contributed by St.J. Dixon-Warren, Manager, Process Analysis, Chipworks Inc.
In a good example of Apple’s superior media hype, when the latest 3GS iPhone was launched in June, some note was made of the addition of an electronic compass to improve the accuracy of the GPS map applications. The mobile phone media appears to have completely missed commenting on the fact that the Nokia N97 smart phone also features an electronic compass, and was released some six months before the iPhone 3GS. According to iSuppli’s web teardown the iPhone contains an AKM AK8973 compass chip.
Chipworks’ teardown of the Nokia N97 found the STMicroelectronics LIS302DL three-axis accelerometer and an AKM AK8974 mounted on the main board of the phone, as shown in Figure 1 (the same parts are also used in Nokia’s N86 and 6720 Classic phones). We have seen the STMicroelectronics accelerometer in many recent devices; however, this was the first time we had seen an AKM magnetic compass in a consumer device. The fabrication of an electronic compass capable of measuring the direction of the fairly weak ~20 µT magnetic field of the earth is non-trivial, and thus we felt some further digging was warranted.
Figure 1 Nokia N97 Main Board
There are a number of ways to make a magnetic compass sensor. Chipworks recently analyzed the MEMSIC MMC312xM 3-Axis Magnetic Sensor, which uses anisotropic magnetoresistance. This device required three discrete magnetic sensor die, one for each axis, plus a separate ASIC. By contrast, the AKM compass chip is based on silicon monolithic Hall sensor technology, as disclosed in an AKM press release dated March 14, 2007. Which raises the question – “How do they do that?” – since a conventional planar Hall sensor is only sensitive to field perpendicular to the die surface, and hence some clever engineering will be required to achieve the three magnetic sense axes, needed for compass applications.
Figure 2 shows the 4 mm x 4 mm AK8974 package photograph, after removal from the N97 board. Top-view and side-view X-ray photography, presented in Figure 3 and Figure 4, reveals the internal structure of the device. The package substrate, wire bonds, and die are visible in the X-ray photographs. Of particular interest is a small ~0.3 mm diameter disk of denser material located over the die. This disk serves as a magnetic concentrator and is critical to the operation of the device.
Figure 2 AKM AK8974 Package Photograph
Figure 3 AKM AK8974 Package X-ray – Top View
Figure 4 AKM AK8974 Package X-ray – Side View
The magnetic concentrator is more clearly seen in the package cross section, presented in Figure 5. The cross-section also shows the package substrate and die.
Figure 5 AKM AK8974 Package Cross Section
The operation of a CMOS based single chip electronic compass has been described by Schott and Huber. In essence, the magnetic concentrator, which should be made of a ferromagnetic material, serves to bend magnetic field lines lying in the XY plane to give a component in the Z direction. Four discrete Hall Sensors, located at the four quadrants of the disk, as shown in Figure 6, can then sense the magnetic field in the X and Y directions. The Z field lines are unaffected by the concentrator. Extraction of the magnetic field vector direction can be achieved by appropriate addition and subtraction of four measured Hall signals.
Figure 6 Three-Axis Hall Sensor Structure
Decapsulation of the AK8974 die, seen in Figure 7, confirms the four-fold symmetry of the Hall sensor device. The die also shows a large block of CMOS logic, required for extraction of the direction of the earth’s magnetic field vector from the Hall sensor signals.
Figure 7 AKM AK8974 Die
Chipworks is presently looking at the AK8973, used in the iPhone 3GS, and we are completing detailed analyses of both of these two innovative electronic compass devices. We’ll see what differences there are!
- MEMSIC MMC312xM Tri-axis Magnetic Sensor with I2C Interface (MEMS) (MPR-0907-801, Chipworks, 2009).
- AKM AK8974 Magnetic Compass (MEMS) (EXR-0908-802, Chipworks, 2009).
- C. Schott and S. Huber, Modern CMOS Hall Sensors with Integrated Magnetic Concentrators (Lecture Notes in Electrical Engineering, Springer Berlin Heidelberg, 2008).
- AKM AK8973/4 Electronic Compass Process Review (MPR-0910-801, Chipworks, in preparation).