Hitachi Creates MEMS Sensor Process
Hitachi researchers have developed a method to form cavities in the interconnect layers of CMOS ICs, allowing MEMS sensors to be created in the wiring layers. The technology results in ultrasmall, single-chip solutions, and will be applied to tire pressure sensors and other MEMS devices, Hitachi said.
Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 9/3/2009
Researchers from the Hitachi Central Research Laboratory (Tokyo) said they have developed a CMOS-compatible MEMS technology that builds MEMS sensors into a CMOS chip's interconnect layers. Speaking at the 20th Micro-Machine/MEMS Show in Tokyo, the Hitachi team said the approach delivers MEMS sensors with the smallest moving parts. The technology will be applied to sensors measuring the pressure of car tires, collision detection, and finger touch pressure.
Normally, there are compromises when using a CMOS process to monolithically integrate a MEMS sensor on a single chip. MEMS devices require deep etch steps compared with the shallow etching in CMOS processes, and MEMS often require hollow structures while CMOS calls for dense structures. To get around these differences, MEMS integration often involves a system-in-package (SiP) approach, putting a MEMS device and a CMOS chip in a single package that tends to be larger because the chips must be positioned side-by-side or stacked vertically.
The Hitachi researchers said they have developed a CMOS process that requires no compromises, forming the MEMS sensors in the multilevel interconnect after completion of the CMOS transistors. By fabricating the MEMS device with the interconnect process, the Hitachi team was able to form cavities in the thick insulation layer after formation of the metal layers using tungsten and tungsten silicide.
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The Hitachi process forms cavities in the insulation of wiring layers. |
The cavities are formed after formation of the lower electrode, when a thick SiO2 layer and the upper electrode are deposited. In the upper electrode, multiple holes are etched, a plasma or wet etchant for SiO2 is applied through the holes to attack the thick SiO2 layer, the SiO2 under the holes is removed and small cavities of SiO2 are formed and connected gradually. Finally the connected cavities produce a large cavity under the upper electrode. The final cavity size depends on the hole size and etching conditions, including time, gas flow rate and gas components.
Hitachi has used the process to create a prototype pressure sensor that integrates CMOS amplifiers and pressure sensors, detecting changes in the electrostatic capacitance. The pressure sensor chip measures just 0.9 × 0.8 mm, and consists of 50 cavities. Changing the diameter of the cavity results in a wide pressure range, and the sensitivity and precision can be adjusted by varying the combination of the cavities.
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Hitachi has used the process to create a single-chip pressure sensor. |
The project is being supported by the governmental New Energy and Industrial Technology Development Organization (NEDO).
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