Thin-film SOI enables portable UV sensors

24 May 2007

OKI, the Japanese telecommunications manufacturer, claims to have developed the world's first UV sensor using thin-film SOI.

OKI says that its new ML8511 is the first UV sensor IC to use thin-film SOI technology. Samples of the device will be available next month for use in portable UV meters and volume production of 500 000 units per month is scheduled for December.

Conventional UV sensors tend to use compound semiconductors such as gallium nitride, making it difficult to mount peripheral circuits on a single chip. "Our approach means that there is no need to use a compound semiconductor or to attach a cut-filter, making it is easier to build peripheral circuits," Naomi Takeuchi from OKI, told optics.org. "The characteristic absorption wavelength depends on the thickness of the silicon thin-film. We leveraged this characteristic of thin-film SOI."

The ML8511 outputs an analog voltage that is proportional to the amount of UV light that falls on it. OKI says that the ML8511's maximum sensitivity wavelength is 365 nm and quotes an output voltage of 1.0 V in the dark up to 2.2 V at 10 mW/cm² at 365 nm. The company adds that because the ML8511 outputs a voltage it can be connected to an analog-digital converter directly and there is no need for an additional circuit to convert a photoelectric current.

"It can be used as a portable UV meter to calculate the UV light intensity from the amount of light it receives. The meter will be battery driven and directly connected to the A/D converter," said Takeuchi.

The company believes that because silicon-based UV sensors require an optical wavelength cut-filter, problems such as piece-to-piece variation in sensor characteristics and a decline in sensitivity can occur. "Advantages of this new UV sensor over existing products are the evenness of the photo sensitivity characteristics and tolerance to noise," commented Takeuchi.

The new ML8511 sensor uses thin-film SOI-CMOS technology to enable easy integration, allowing it to be mounted on digital and analog circuits. OKI believes that adding a built-in operational amplifier enables the analog voltage output function to exist on a single chip.

OKI intends to enhance its product line-up by including digital output circuits that connect to microcontrollers and combine ambient light sensors on a single chip. "Our next challenge is to find a low-cost method to achieve light sensitivity in the green for illumination sensors," concluded Takeuchi.