Image Sensing Using Micromachined Ultrasonic Sensor Arrays

Tu Xiang Zheng 

Ultrasonic sensors convert ultrasound waves to electrical signals or vice versa. They detect a wide range of materials, are not influenced by problematic surfaces, and are largely immune to environmental influences. They have many uses in medicine as well as in other various advanced technologies including electronics, chemicals and construction. As well known, an ultrasonic sensor applied to the abdomen of a pregnant woman sends ultrasonic waves into the body and receives the echoes back from the inside, which are used for making visual images. These real time images showing the appearance and movement of the fetus allow observation of the development of the fetus.

Silicon based capacitive ultrasonic sensor arrays bring revolutionary improvement in performance and represent a major advance in ultrasonic sensor technology.

These sensors benefit from the economies of scale found in semiconductor manufacturing and are well suited for high-volume applications that demand high-performance sensors at low costs. A similar sensor array can be found in the US Patent 6,359,276 B1, which is used for sensing infrared image.

As shown in the above figure, each sensor of the array comprises two electrodes facing each other, one of which is fixed and the other is movable. The two electrodes are separated by an insulating layer and an air gap. It can operate on transmit and receive mode, by converting electrical energy into acoustic energy or vice versa through the displacement of the movable electrode.

When a voltage is applied between both electrodes and the membrane is pulled down to the bottom electrode by electrostatic forces. The membrane moves until the electrostatic force has equilibrium with internal force of the membrane. AC signals cause vibrations of the thin diaphragm and generate ultrasonic waves. Furthermore, the receiver can detect an ultrasonic wave using the change of capacitance when displacement of the membrane is caused by the pressure of an arriving ultrasonic wave.

In according to this patent, the sensor array is disposed in a silicon substrate in which there already exists a CMOS circuit with readout electronics. Each sensor includes a silicon nitride membrane bridging a cavity recessed into the substrate. The membrane has four beams. The distal ends of the beams are anchored to the substrate, so that the membrane is supported by the substrate and the surface of the beams is aligned with the plane surface of the substrate. The surface of the membrane and beams is coated with silicon dioxide film. A metal layer is disposed on the surface of the silicon dioxide film. The end portions of the metal layer are disposed on the beams and keep in contact with the proximal end portions of electrical conductors which already exist on the surface of the substrate. The cavity is a narrow gap, so that the membrane can touch the bottom of the cavity without damage as it is forced to bend downward. The trenches between the membrane and the beams and between the beams and the edges of the substrate are also narrow, so that the membrane and beams can touch the edge of the substrate, as they are forced to bend in the lateral directions. 

It was reported by Andrew et al. that MEMS technology makes it possible to produce air and gas ultrasonic sensors that can operate at higher frequencies (200 kHz to 5 MHz). The ability to make microscopic structures with MEMS technology permits the fabrication of very small sensors that emit high-frequency ultrasound. The smaller the sensor implies the higher the frequency of the ultrasonic signal. This was realized by Lemmerhirt et al. They developed a CMOS based ultrasonic 32 X 32 sensor array. The array was built with CMOS process for 3D image acquisition. Each sensor has 100 µm diameter membrane with 60 µm diameter top electrode and 0.6 µm gap. The center frequency of each element is 1.8 MHz.