by St.J. Dixon-Warren
Engineering and Process Analysis Manager, Chipworks
Apple's iPhone 4 is a market leading consumer electronics device. The previous article in this series discussed Chipworks' teardown of the iPhone 4, followed by a discussion of the ST LIS331DLH three-axis accelerometer technology. This second article in the series discusses our findings on the three-axis gyroscope device used in the iPhone 4. The gyroscope, in combination with the accelerometer and an electronic compass device, discussed in third part of this series, provides full nine degree-of-freedom (9DoF) motion sensing.
iPhone 4 Three-Axis Gyroscope: STMicroelectronics L3G4200D
STMicroelectronics entered the MEMS gyroscope business in June 2008, when it launched its first single-axis gyroscope. A year later, in June 2009, ST launched a family of two-axis gyro products, and in February 2010, they announced the L3G4200D three-axis gyroscope. ST has been primarily targeting the consumer electronics market with these new gyroscope offerings.
The L3G4200D comes packaged in a 4 mm x 4 mm x 1 mm thick LGA package, which incorporates separate MEMS and ASIC die, as can be seen in the side-view X-ray in Figure 1.
The L3G4200D is essentially a tuning fork gyroscope. The operation is illustrated in Figure 2. The tuning fork is driven into resonant in-plane vibration (a). Rotation around the input axis results in vibrational energy transfer to the orthogonal mode (b), due to the effect of Coriolis forces. Panasonic actually makes piezoelectric gyroscopes, such as the EWTS64N, that operate in exactly this manner. The ST gyroscopes, however, use capacitive sensing, and are fabricated with the same THELMA process that is used to build the LIS331DHL.
Figure 3 presents a photograph of the gyroscope sensor die found inside the L3G4200D, after removal of the hermitic cap. The vibrating proof mass is comprised of four loosely coupled elements. Interdigitated polysilicon drive capacitor plates, located on the left and right sides of the die, induce in-plane horizontal vibration of the left and right proof mass elements. This vibrational energy is transferred through springs located near the corners to proof mass elements located above and below the central area. The result shows that the four proof mass elements exhibit a "breathing" type vibrational motion.
The four proof mass elements are shown in more detail by the tilt-view SEM in Figure 4. Rotation of the device around the horizontal (pitch) axis will result in differential out-of-plane deflections for the proof mass elements located above and below the central area, due to the effect of the Coriolis forces. Similarly, rotation around the vertical (roll) axis results in out-of-plane deflections of the left and right proof mass elements. These deflections will be detected by polysilicon capacitor plates located beneath the vibrating proof mass elements; this can be seen in Figure 5, which is a photograph of the gyroscope die with the top MEMS polysilicon layer removed to reveal the bottom interconnect polysilicon layer.
The L3G4200D is also sensitive to rotation around the yaw axis, perpendicular to the surface of the die. Coriolis forces induce lateral in-plane displacements, which are sensed by banks of interdigitated capacitor plates, located near the top and bottom edges of the device. The Coriolis force induced displacements are sensed by the ASIC that provides the signal to the A4 processor via digital I2C/SPI serial interface standard output. The final part of this series will discuss the electronic compass technology used in the iPhone 4 and earlier iPhone 3GS.
St. J. (Sinjin) Dixon-Warren manages the Process Analysis group in the Technical Intelligence business unit at Chipworks. His group provides technical competitive analysis services to the semiconductor industry, currently with a special focus on the analysis of MEMS, CMOS images sensor, advanced CMOS and advanced power devices. He is the Sector Analyst for MEMS analysis at Chipworks. Dr. Dixon-Warren holds a PhD in physical chemistry from the University of Toronto and a BSc in chemistry from Simon Fraser University. Dixon-Warren joined Chipworks, in 2004, as a member of the process analysis group. He is author of about 50 publications and of about 100 Chipworks reports. Dr. Dixon-Warren can be reached at [email protected].
Copyright 2011 MEMS Investor Journal, Inc.