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Zettacore Re-inventing Memory in Molecules
Pradeep Shankar
Monday, November 1, 2004
A score of chemists at ZettaCore are racing to build—hold your breath—the next-generation memory chips. Chemists and chips? Yes.

In this Denver, CO-based company, which resembles a modest chemistry lab; a new technology that could shape the future of semiconductor industry is evolving. ZettaCore is developing molecular memory, that reads and writes data by adding and removing electrons off nanometer-sized molecules. Molecular electronics is predicted to be the future of the chip memory. If today’s integrated circuits are made of millions of silicon transistors, tomorrow molecules can replace the silicon transistors.

The semiconductor industry continues to operate on distinct geometries on a regular scale. Keeping pace with Moore’s Law, whenever you shrink you get benefits from it—power, performance and cost. However, the traditional semiconductor chip is approaching some fundamental physical limits. Increasingly, especially for memory chips, the benefits obtained by shrinking the chip are not at the same rate as before.

Most of the memory chips in use today are DRAMs. In a DRAM memory chip there is a single transistor and a capacitor for each bit of information. Building a capacitor at 90nm and below is more complex and adding to the cost. ZettaCore gets around this problem by replacing capacitors with molecules.

The Porphyin Magic
ZettaCore uses a class of molecules known as porphyrins to develop the memory chips. Porphyrins are organic molecules composed of mostly carbon, nitrogen and hydrogen atoms—and like any other atom, there are protons, neutrons and electrons. Chemists at ZettaCore engineer porphyrin molecules by altering the chemistry of the molecules to store charge (information). When linked in a chain, the molecules can carry either electrical or optical signals, making them useful for everything from optical networks to computing.

“When you work at the molecular level you are working at as small a geometry as is humanly feasible,” says Subodh Toprani CEO of ZettaCore. “Each molecule we use is about one nanometer in size. That’s 100,000 times smaller than the width of one strand of your hair.”

The Advantage
In a DRAM fabrication process there are two ways to build the capacitor that holds the charge—trench or stack. A trench involves building millions of three-dimensional tall tunnels on the surface of the semiconductor wafer. This is built in order to increase the surface area of capacitance to hold charge. The aspect ratio (height versus width) of such a tunnel is approaching 100:1. At a 90 nm level and possibly below, the aspect ratio increases and etching circuit elements becomes complicated. A stack process involves similar complexities.

The entire process of building a trench or a stack takes about 10 to 15 days. ZettaCore’s technology not only reduces the complexity but also the time, and this is where chip manufacturers can save significant cost. Instead of building the three-dimensional structures, molecules can be introduced in a planar fashion, and the layering of molecules takes about just a day.

ZettaCore’s molecules are designed to assemble automatically in the right place in an electronic circuit. This allows the molecules to attach only to a particular type of surface (silicon or metals or oxides), to pack tightly on that surface, and to align properly on the surface for electronic operation. Because of this chemical self-assembly, molecular memory chips can be manufactured using equipment and processes common in the semiconductor industry.

A typical DRAM will have thousands of capacitors arranged in an array. These capacitors hold charge (information) for a fraction of second. Hence they have to be refreshed every few milliseconds. In molecular memory chips, the molecules hold charge for longer time and the refresh cycle is in the order of seconds. This difference in refresh cycle has significant advantage in the power consumed.

A typical molecular memory chip will have molecules—instead of capacitors—arranged in an array (see diagram). An individual molecule can have four states, so it stores two bits of information. Using chemical self-assembly techniques, the molecules are applied to all of the memory elements of an array. At each location in the array there may be a few thousand to a million molecules. This provides excellent signal to noise characteristics and defect tolerance through redundancy. The failure of any single molecule will not affect the operation of a memory element. The array is connected to custom-designed electronic circuits, and fabricated using conventional logic that read and write the array.

Playing to Rule
Building a new technology is challenging and can make you a genius—but an equally challenging and daunting task is to make sure that the technology impacts our daily life in one way or another. Adopting anything different, especially within the semiconductor industry, has always had significant resistance. The founders of ZettaCore knew this early on. They have developed a technology that implements memory functions in the same way they are conventionally implemented now: storing data as a charge.

ZettaCore has also made sure that the manufacturing of molecular memory chips is compatible with existing infrastructure of the semiconductor industry.

Founded in 1999, ZettaCore is backed by leading venture capitalists like Kleiner Perkins Caufield & Byers and Draper Fisher Jurvetson. The company has raised $23 million till date. For the last one year, ZettaCore has been building prototypes. The company claims that it has built prototypes for a few prospective customers.

“There appears to be no end to the demand for memory,” quips Toprani. ZettaCore’s technology is promising as it is “least disruptive”. And what could be bigger than a technology that might keep Moore’s law humming decades after silicon reaches its limit?

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