During Display Week, in Room 3017 at the Sheraton Seattle Hotel, Peter Brody of Advantech showed a 4-inch, 100 dpi, QVGA OLED with a cadmium selenide (CdSe) TFT backplane fabricated by evaporating TFT films through a shadow masks rather than using. photolithography.

Ken Werner
Senior Analyst and Editor

Brody, who is often credited with building the world’s first active-matrix LCD in 1972, says this approach can produce uniform power transistors for driving AMOLED color sub-pixels over large areas. "But," says Brody, "CdSe is only half the answer. The other half is that it lends itself to evaporative deposition, so we can use a very simple and inexpensive manufacturing technology."

In a presentation prepared by Advantech Global (Brody’s company) for a well-known Chinese TV manufacturer, Brody compared the costs of producing a 17-inch display panel with current a-Si LCD processes on a Gen 5 fab, LTPS OLED processes, and with three iterations of Advantech’s AMAX miniLine in-line manufacturing system. He concludes that the simple evaporative CdSe process, which requires a relatively small capital investment, is capable of delivering OLED’s well-appreciated image-quality advantages at a cost that is actually less than LCD.

The 4-inch demonstrator Advantech showed in Seattle was made in a single deposition system, but the company is currently installing a multi-million-dollar in-line fabrication system in Pittsburgh, in which each circuit level is deposited in a dedicated chamber, with all chambers operating simultaneously. The system will be capable of a finished-circuit TACT time of 5 minutes, Brody says, with 2 minutes possible in later generations.

Advantech made the demonstrator’s CdSe backplane. OLED processing, including the tri-color emitters, was performed by BOE in Beijing. Each pixel switch in the demonstrator consists of two TFTs, a storage capacitor and an output terminal, with each sub-pixel measuring 83×250 microns.

Back in the ’70s, CdSe’s potential was appreciated, but many people were concerned with the toxicity of Cadmium. Brody says CdSe is safe, and makes the following points:

• Cd is ionically bound to Se and can only be separated from the Se with strong acids.
• If display products containing CdSe backplanes are disposed of in the ground, Cd does not leach out and therefore does not pose an environmental hazard.
• The weight of the cadmium in a typical backplane will be on the order of 1.6 x 10^-7 gm/cm², or less than 1 part in 10⁶ of total weight, which is well below the European requirement of 1 part in 10⁴.
• The average daily human intake of unbound Cd in food is 10 to 25 micrograms, equivalent, says Brody, "to eating about 100 cm² of our backplanes!"

After years of struggle, it turned out that 3- to 4-inch-class WVGA AMOLEDs were the killer app that pushed PenTile Matrix technology over the top. In a similar way, could it be that the well-documented problems with AMOLED backplane complexity, stability, uniformity, and scalability will provide CdSe with its long-awaited opportunity? We will be tracking the ongoing developments at Advantech with great interest.