Oki Integrates AlGaAs LEDs on Silicon Mounting Chips

Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 4/24/2008 7:02:00 AM

Oki Digital Imaging Corp. (Tokyo) is in mass production with LED print heads that mount ~5000 AlGaAs light-emitting diodes (LEDs) on CMOS driver ICs.

The print heads are being mass produced at a rate of 100,000-200,000 units monthly with high reliability, Oki engineers said at the Fourth International Nanotechnology Conference on Communication and Cooperation (INC4) held in Tokyo earlier this month.

LED AlGaAs layers 2 µm thick are initially grown on a normal GaAs substrate. The company’s Epi Film Bonding (EFB) technology integrates AlGaAs epitaxial layers on CMOS silicon ICs. The approach delivers a higher number of GaAs LEDs from a single GaAs wafer because the LED layers do not waste real estate on bonding pad areas. Also, the LED print head has a smaller footprint.

With no bonding pad areas, the width of the LED array is just 70 µm, resulting in 5× more LED chips from a single GaAs wafer compared with the conventional approach. Connections between the LED layer chip and CMOS driver chip are made by a conventional metal interconnect process.

Oki’s Epi Film Bonding (EFB) technology integrates AlGaAs epitaxial layers on CMOS silicon ICs. (Source: Oki Digital Imaging)

Conventional LED print heads mount silicon CMOS driver chips and AlGaAs LEDs on a wider substrate, connecting the CMOS and LED chips with multiple bonding wires. For conventional LED print heads, the length of the light-emitting portions was 20 µm, and the bonding pad length was 280 µm, leading to an LED array width of 350-400 µm.

To remove the LED epitaxial layer from the GaAs substrate, a sacrificial layer is etched between the epitaxial layer and the GaAs substrate. The most important technique is how to smooth the rear surface of the epitaxial film, said Mitsuhiko Ogihara, general manager of the company’s R&D department. Attachment of the epitaxial film to a silicon CMOS driver chip is completed through intermolecular force, similar to attaching one silicon wafer to another. A buffer layer or smoothing layer is introduced between the silicon wafer and the epitaxial film. In actual production, multiple LED epitaxial films are bonded together to boost productivity.

For LED print heads, the Oki approach mounts more LEDs than conventional print heads, leading to higher printing resolution. An Oki LED print head for A4-sized paper with 600 dpi includes 26 one-dimensional LED array chips, each chip with 192 LED films, resulting in a 192 × 26 array for 4992 total dots. A single-array chip is 8 mm long.

The Oki approach results in much higher LED density because real estate is not wasted on bonding pads. The approach may be applied to flexible displays in the future. (Source: Oki Digital Imaging)

Oki worked closely with researchers at Hiroshima University at the initial stage of the technology, Ogihara said. The Ministry of Education, Culture, Sports, Science & Technology provided funds to the university for the project.

Looking to future applications, Ogihara said the technology may be applied to 2-D arrays for high-resolution LED displays, and could be used with flexible substrates.