CPU, DRAM ride organic substrates at ISSCC

In a separate paper, Wei Zhang of the University of Minnesota described a 64-bit organic DRAM capable of a 12 millisecond read and a 20 millisecond write operation. The 8x8 array used a three-transistor DRAM cell and consumed 10nW per bit of refresh power.

Researchers have described organic versions of non-volatile and SRAM memories but have not previously worked on a general purpose DRAM, Zhang said. His next step will be to integrate the DRAM array with an organic flexible display, using the memory for image storage.

The device was made with an aerosol jet printer using an electrolyte with gate capacitance one or two order of magnitude higher than a traditional 65nm silicon DRAM. Like the organic processor it used only P-type transistors.

Coming to the rescue of both the CPU and DRAM designers, a researcher from the CEA-LITEN research lab in France described a new process for printing CMOS structures on a plastic substrate.

"This process has the capability to produce N and P transistors with matching performance for both digital and analog applications," said Anis Daami, author of the paper and a research engineer at CEA (Grenoble).

He showed a range of devices on foil sheets as large as 380x320mm created with laser ablation. Most of the designs were implemented with relatively wide 20 micron channels, however the process can support narrower lines.

"We are now we are working to lower channel values," he said.

Test devices produced to date include CMOS inverters that are fully functional even at very low voltage supplies. They exhibited relatively high swing, acceptable gains and noise margins at 30 percent of supply voltage.

Daami also described functioning ring oscillators in the 10-200 Hz range and a NAND gate. The process marks an advance over previous work that printed both N- and P-type transistors but could not fully print some elements or required use of a vacuum or extra lithography steps.

The CEA devices all passed functional testing without use of a vacuum at ambient temperature "on my desk," he said.

Finally, a researcher from the Institute for Microelectronics Stuttgart described a 6-bit digital-analog converter claimed to be 1,000 times faster and 30 times smaller than previously published organic DACs.

However, the DAC did consume 180-260 microwatts at 1-2V, significantly higher than previously published work at 0.7 microwatts. It used 129 transistors compared to 26 in an earlier DAC.

The design has a minimum channel length of four microns. It was fabricated on a new process using a glass substrate but without solvents and at a maximum processing temperature of 90 degrees C.

Due to process limitations, the design was not able to use resistors, eliminating some DAC architectures. Researcher Tarek Zaki implemented the DAC as a current-steering device for optimal speed and compactness.