NanoEMS CMOS sensors: From single-chip to smart to super-smart

The meteoric growth of motion sensor deployment in mobile phones and tablets has led to eye watering price erosion.  Consequently, while creating a profitability headache for many MEMS manufacturers, the diminishing price-tag provides opportunity for these sensors to become ubiquitous - enabling electronic gadgets, devices and accessories with motion and position awareness, adding to the appeal of existing products and opening the door to some exciting new concepts.  However, for the trend to advance, the ever-decreasing price requires a fundamental change in the way sensors are traditionally made.  

One solution, NanoEMS technology, uniquely supports the fabrication of the entire set of motion sensors in standard CMOS fabs, resulting in much lower cost and enabling continued proliferation.  Even more significant, NanoEMS allows all of the sensors to be simultaneously fabricated on a single chip - together with the supporting analog (signal conditioning) and digital (DSP and intelligence) electronics, paving the way for smaller, smarter motion sensor modules at a much-reduced cost.

There are three key problems limiting the potential ubiquitous proliferation of motion sensors -- size, power consumption and complexity, all of which drive manufacturing cost.  NanoEMS shrinks all of the motion sensors and integrates them together with control electronics onto a single chip in the standard CMOS wafer process used to fabricate the majority of high-volume cost-sensitive chips today.  To understand the significance of this breakthrough, we will describe the problems posed by traditional MEMS approaches and show how NanoEMS systematically removes the issues, providing a clear pathway to smaller, cheaper, lower power intelligent solutions that will drive exciting new consumer product innovation.

Today’s problem: Discrete complexity

Four different motion sensors are required to provide sufficient position and motion information: compass (absolute 2D angular position); pressure sensor (altitude); accelerometer (linear acceleration), and gyroscope (angular acceleration).  Traditionally, each type of sensor is manufactured as a discrete device, using specific, non-standard, manufacturing processes or materials, and having its own ASIC for signal conditioning and conversion to a digital interface. Consolidating these devices is highly complex using traditional techniques.  Even though a single CMOS control ASIC could unify and optimize the electronics required for all four sensors, most manufacturers do not have access to all the specific sensor processes and typically form partnerships to offer all devices.  Furthermore, bringing MEMS devices together into the same package requires complex, specialist, multi-chip packaging that increases cost and size, and impacts reliability.

Incompatible processes limit integration

Traditional MEMS sensor devices (accelerometers, gyroscopes) are manufactured in specific MEMS fab lines, which enable mechanical structures to be formed on top of a silicon substrate by depositing additional materials and using micromachining to define the structures, or within cavities in the wafer that are created using costly, non-standard additional processes such as isotropic Deep Reactive Ion Etching (DRIE) to produce vertical-sided cavities.

Heading sensors (three-dimensional electronic compasses) are traditionally created using either the Hall effect, or a magneto-resistive material deposited onto the silicon to detect the Earth’s magnetic field.  To measure the field in three dimensions, separate sensing elements are required – one for each dimensional component of the field, and additional complexity is required in the device construction. In some cases, special devices called magnetic concentrators are bonded to the chip to rotate the X and Y (horizontal) components of the magnetic field into the z (vertical) dimension so they are perpendicular to the plane of the chip. Alternatively, two separate sensor chips are arranged in horizontal and vertical planes within the device package to measure the magnetic field in three dimensions. Such techniques result in mechanical alignment difficulties, which lead to cross-axis measurement inaccuracies.

Gyroscopes are typically created using resonant structures, requiring the MEMS to reside in a partial vacuum.  For traditional MEMS, this requires use of special manufacturing techniques, like wafer fusion bonding and wafer-level, eutectic-bonded, hermetic capping to create a vacuum within the package.  The use of chemical getters required to mitigate outgassing add further to the cost of hermetic packaging: these chemically absorb gases that may be released from the silicon and other package materials, during periods of elevated temperature experienced during package processing or solder reflow.

A mixed-signal ASIC is required to accept the small signals from the MEMS or magnetic sensor devices, condition them with low-noise amplifiers and gain stages, convert to digital then apply further digital signal processing conditioning and provide a digital interface.  The ASIC is a CMOS device and typically has to be manufactured separately from the MEMS devices and stacked with the MEMS in the final package using complex, multi-chip modules or specialist, wafer-bonding methods.