Laser process shapes microscopic parts.

By combining advances in lasers, chemistry and computer-aided design and manufacturing techniques, scientists have scaled machining down to microscopic proportions.

This new technology makes possible machined pieces one or two orders of magnitude tinier than the finest parts crafted by watchmakers, says Daniel J. Ehrlich, a physicist at the MIT Lincoln Laboratory in Lexington, Mass. Ehrlich and Theodore M. Bloomstein, a graduate student in electrical engineering at MIT, describe their three-dimensional micromachining process in the Aug. 10 Applied Physics Letters.

Using this process, "you can do almost anything and make any shape both along the [surface] and the depth," comments Howard R. Schlossberg, a physicist with the Air Force Office of Scientific Research in Washington, D.C.

To make their three-dimensional designs, Ehrlich and Bloomstein rely on a computer to guide a laser as it scans a piece of silicon. First they use computer-aided design and manufacturing (CAD/ CAM) software to draw and scale a part. A second program slices that computer blueprint into many parallel planes. The computer notes the coordinates of the part in a given plane and then directs the laser to etch plane by plane, leaving silicon only at those coordinates.

For shaping, Ehrlich and Bloomstein place a newly cleaned silicon crystal into a vacuum chamber and blow chlorine through the chamber. The instant the laser hits the silicon surface, it heats that spot to about silicon's melting point. The heated silicon combines with chlorine atoms hitting it and escapes as a silicon chloride gas. "Among reactions that occur on a surface, it's among the very fastest," Ehrlich says. The researchers then pump out this gas.

The computer can direct the laser to any 1-micron location on a square surface 256 microns wide. The laser then etches down in 1-micron steps. It can hit up to 20,000 spots per second and moves at a rate of 20 millimeters per second. The longer the laser dwells at a spot, the more deeply it etches the silicon, enabling the researchers to shape the part in the third dimension. Higher pressures of chlorine also speed etching, Ehrlich says. In addition, varying the chlorine pressure lets the researchers control the part's final texture.

The MIT group got the idea for this process in part from recent advances in stereolithography, in which a computer-directed laser causes liquid plastic to solidify into a particular shape. "We also have begun to build [up] three-dimensional things," says Ehrlich. For example, they have used the laser and a technique called chemical vapor deposition to lay down patterns of platinum or cobalt in the newly shaped silicon.

Ehrlich expects manufacturers to use the micromachining technique to make microscopic prototype parts, molds, and stamping tools. The MIT researchers have produced plastic parts from these micromolds, and they hope to use those parts to make valves and pumps for implantable medical devices. They are also refining the technique for micromachining metals and ceramics.