CMOS MEMS

Creating microstructures and MEMS in and out of CMOS materials (CMOS MEMS) is a unique technology that originated and continues to be developed at the MEMS Laboratory of Carnegie Mellon University in Pittsburgh, Pennsylvania. Devices being made with the CMOS-MEMS process include: x-y-z microstages for a probe-based mass-data-storage system on a chip, biologically inspired microsensors, ultrasonic sensor arrays, accelerometers and gyroscopes for an on-chip inertial measurement unit, and acoustic MEMS earphones.

An acoustic CMOS MEMS earphone chip mounted and bonded onto a standard electronic package and packaged into a plastic earphone housing.

Acoustic CMOS MEMS

Acoustic CMOS MEMS structures and devices are an extension of CMOS-MEMS that enables the fabrication of an integrated membrane or membrane arrays to be used either as sensing elements (microphones to measure sound) or as actuating elements (microspeakers to produce sound). To construct a membrane using CMOS-MEMS for either microphone or speaker applications, we make an airtight membrane on the surface of the chip by defining a mesh or screen-like structure and coating the mesh with a polymer to make it air-tight. The fabrication steps that create acoustic CMOS MEMS devices are described and illustrated below:

1. The chip comes from a standard CMOS fabrication facility covered with a layer of protective glass (silicon dioxide). Regions meant for mechanical structures are patterned in one of the metal layers, usually the topmost layer.
   
   
2. The oxide is etched anisotropically (directionally) down to the silicon substrate, the metal layers acting as a mask to define the mesh structure.
   
   
3. The underlying silicon substrate is etched with an anisotropic deep etch followed by an isotropic (all directions) etch to release the metal-dielectric layers form the silicon substrate underneath. At this point, the membrane mesh structure is released from the underlying silicon and the desired cavity is formed. In the figure we see a CMOS-MEMS beam (looking from the end), and the metal layers inside which can be used as electrodes for sensing and actuation, or wires for connecting to the on-chip circuitry.
   
   
4. In the final step, the released CMOS-MEMS structure is coated with polymer in a chemical vapor deposition process. The polymer conforms to all sides of the beams, until all the gaps are sealed, creating an airtight membrane suspended over the gap. The metal layers inside the beams allow the membrane to be treated as an electrode for either capacitive sensing or electrostatic actuation.
   
   

A scanning electron micrograph (SEM) of the mesh-polymer membrane. Two holes have purposely been etched into the membrane to illustrate the mesh-membrane structure and to reveal the cavity underneath the membrane.

In addition to the simple fabrication sequence, the acoustic CMOS-MEMS devices have the advantage of integrated, on-chip electronics. For microphone applications, sensing circuitry (capacitive or piezoresistive) can be placed close to the mechanical structure, minimizing electromagnetic pickup and parasitic capacitance. For speaker applications, driver circuitry can be placed nearby the mechanical structures. In both microphone and speaker applications, signal processing electronics for signal conditioning (for example, noise cancellation and equalization) are also on-chip, increasing Akustica product performance, reliability and affordability.

Demonstrated Devices

Acoustic CMOS-MEMS enables Akustica to fabricate single and multiple membranes on standard and state-of-the-art computer and signal processing chips for revolutionary microphonic, speaker or combined microphonic-speaker applications addressing a variety of different markets.

To date, acoustic CMOS MEMS have been used to fabricate single-membrane earphone speakers that are 2mm x 2mm square, a microphone less than 1 mm x 1mm and speaker arrays with up to eight (8) individually controllable speakers in an area about twice the area of the eagle head on a quarter.

Below is a picture of a 2 mm x 2 mm microphone chip with a single membrane and integrated electronics. The 0.7 mm x 0.7 mm membrane is the dark blue area in the center with the associated and integrated electronics located beneath the green rectangles distributed about the perimeter of the chip.

The multi-membrane chip below is an array of eight membranes, each individually controllable and electrically isolated from the other membranes.

Array of eight octagonal speaker membranes.