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Inorganic Solution

Making a chip as powerful as a Pentium by traditional means is not an easy feat. While semiconductor makers like Intel have learned to make transistors smaller and smaller over the past few decades, squeezing vastly more performance into the microprocessors, the basic mechanics of chip making haven’t changed much. The base material remains silicon, sliced into thin wafers. An insulating layer of silicon dioxide goes on top of the wafer; a thin layer of “photoresist” (a light-sensitive material) is deposited on the silicon dioxide. Light beams project the pattern of the circuit onto the photoresist through a stencil; the pattern is then etched out by acids or reactive gases. Additional layers of silicon are added, “dopants” such as boron or arsenic are put into the mix, and finally the transistors are linked by means of tiny aluminum wires.

The resulting microchips are a marvel of engineering and are largely responsible for fueling the Information Revolution. Using multibillion-dollar manufacturing plants, Intel and others can now make transistors as small as a few hundred nanometers across (a nanometer is a billionth of a meter), packing tens of millions of them on a single chip. The downside is that the several hundred manufacturing steps take upward of two weeks and require clean rooms hundreds or thousands of times more pristine than your average laboratory.

Last fall, Jacobson and his student Brent Ridley described in the journal Science the first printed inorganic transistors. Several other research groups, most notably at Lucent’s Bell Labs and Cambridge University in Britain, have also printed transistors. These groups, however, are using organic polymers; such materials could have great promise in the electronics required to make cheap, flexible displays. But organic transistors appear to be inherently limited in computing speed. Jacobson’s big breakthrough is that he and his colleagues at the Media Lab have created liquid suspensions of inorganic semiconductors-the same class of materials used in your Pentium chip-so that they can be used in a printing process. In other words, rather than carving logic into a solid piece of silicon, Jacobson is simply printing it onto a substrate.

Jacobson’s optimism is justified by his group’s rapid advances in synthesizing “semiconductor ink.” Under normal conditions, semiconducting materials such as silicon, cadmium selenide and gallium arsenide form bulk crystals with melting points well over 1000 C. Jacobson and his team, however, have found a way to synthesize a solution of tiny “nanocrystals” of 100 atoms or less. This semiconductor ink can be patterned or printed onto a variety of substrates, including thin sheets of plastic, at temperatures under 300 C. The particles, Jacobson notes, are small enough to form 200-nanometer structures-about the scale of complex integrated circuits like Intel’s Pentium chip.

The suspension of nanoparticles is so similar to conventional inks that Jacobson and his co-workers are able to use an inkjet printer manufactured by Hitachi to fabricate tiny machines called MEMS, or microelectromechanical systems. MEMS, which are one of the fastest-growing new areas in materials technology (see “May the Micro Force Be With You,” TR September/October 1999), are typically made using many of the same arduous techniques used to fabricate conventional silicon microchips. Using the inkjet printer, Jacobson and his students have managed to fashion both a working thermal actuator and a linear-drive motor with features on the order of 100 micrometers by simply depositing hundreds of layers of ink. And they are able to form the tiny machines without a clean room and at temperatures well under 300 C.

The group has also used the inkjet printer to produce much more intelligent radio-frequency identification tags. Others are also working on such tags but are relying on logic using organic transistors. Jacobson thinks that the faster logic possible with inorganics can make his version of the tags far more intelligent, allowing companies to track everything from expensive goods to the packages in a supermarket. A radio signal detector could read the devices, update them and integrate them into inventory systems. A person could walk into a supermarket, pick up some items and walk out, and the money would be automatically tallied up and deducted from his or her bank account-and from the supermarket’s inventory system.

Using printed circuitry like that is just the beginning. Because the computer logic is printed, it can be put on the surface of almost anything: soup can labels, textiles, soda cans. “You could add intelligence to almost anything you want,” claims Colin Bulthaup, one of Jacobson’s students. “One thing we want to do is build a digital camera in a business card: everything embedded into the card itself. There’s no reason to have all these clunky silicon chips. You can pattern your semiconductor, your photodetector-all the materials together-and integrate them into a single device, one that is incredibly small, incredibly cheap and incredibly quick to produce.”

Making such devices using an inkjet printer, however, is still a far cry from printing high-quality logic circuits. That requires fabricating transistors and other electronic components at the scale of a few hundred nanometers-the level of precision in a Pentium chip. For that, Jacobson has made use of polymer stamps that don’t look all that different from the stamps used to certify documents. In one version, the stamp has the architecture of the circuit in positive relief and is dipped in the nanoparticle ink; the circuitry is then transferred by hand onto a substrate. Also promising is a negative stamp that “embosses” a thin layer of ink previously deposited onto a plastic surface. The stamp’s features push aside the ink at certain points, forming whatever feature is engraved on the stamp at resolutions of 200 nanometers.

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