"Light stamps" could simplify optical lithography

Zurich, Switzerland, July 15, 1998—Scientists at IBM Research - Zurich have demonstrated that a surprisingly simple optical technique can create exceedingly fine patterns onto silicon wafers without using the expensive lenses required today. The key to the new method is a transparent "light stamp" made of a rubber-like plastic that is placed directly on a silicon wafer during processing. Molded from a master design, the reusable light stamps have a pattern of raised areas that conform snugly to the silicon surface. Light passing through these raised areas can define on the wafer features with dimensions less than half the wavelength of the illuminating light source&$151;smaller than is possible by traditional optical lithography techniques.

If this method proves to be reliable in a manufacturing environment, it could be an economical way of making a variety of patterned optical devices&$151;such as polarizers, filters and gratings&$151;that require only one lithography step. Using light stamps to make microelectronic circuits, which require dozens of steps precisely aligned with each other, is currently difficult, but the scientists are also working on thin stamps fused with highly accurate quartz plates that will make the stamps suitable for multiple step lithography.

"We have employed light stamps to form structures down to 100 nm (0.1 micrometer) using the 248-nm light source now commonly applied in the semiconductor industry," says Bruno Michel, project leader of the microcontact processing team at IBM Research - Zurich.

"The exposure field size is also less constrained because aberrations that can occur in a lens system are no longer an issue," adds Heinz Schmid, who is responsible for this research effort. "Moreover, stamps can be reproduced easily and cheaply from the master. We expect each stamp will be good for about 1,000 impressions."

Technical details

In conventional optical "projection" lithography, precision lenses focus laser light through a large stencil-like mask containing a circuit pattern and then down onto a thin layer of light-activated photoresist material that coats a silicon wafer. The pattern of light and shadow defined by the mask causes chemical reactions in the photoresist that, together with additional processing and material deposition steps, creates an identical pattern of greatly reduced features on the wafer. Today's complex microelectronic circuits and interconnecting wires are made of many layers, each formed by repeating this photolithography process several times. The minimum feature size that can be created by projection lithography is determined by the wavelength of the light used as well as the quality of the optics, processing equipment and mask. In contrast, contact lithography, uses a mask placed directly on the silicon surface, eliminating the need for complex and expensive focusing optics. The mask, however, must have the same feature size as the finished product. Contact lithography has not yet played a significant role in high-resolution lithography, however, because the traditional chrome-on-glass masks made it difficult to achieve uniform contact over relatively large areas without mechanical damage or wear.

Zurich's soft light stamp overcomes this limitation. The pliable stamp material makes uniform contact over large areas, despite any small height variations in the wafer's surface.

The light stamp is inexpensively molded from a precision silicon master, which is made by a combination of direct electron-beam writing and reactive ion etching. Protruding features on the light stamp form the pattern to be transferred onto the wafer. These features also concentrate and intensify the light, which is coupled to the photoresist coating the wafer only within the areas of direct contact. When it passes through the stamp, the light is converted to a shorter wavelength, thus enabling features smaller than the wavelength of the original light source. The high-intensity light within the well-defined channels in the stamp leads to the high contrast required to make well-defined pattern boundaries.

The "soft lithography" contact processing technique was pioneered in the early 1990s by Professor George M. Whitesides, now at Harvard University. The IBM Zurich team is also developing similar methods to create patterns of nanoparticles, liquids and fragile biological molecules that could permit structures to be made with materials and on substrates not currently possible with conventional photolithography techniques.

The members of the team that developed the light stamp are Heinz Schmid, Hans Biebuyck, and Bruno Michel of IBM Research - Zurich, together with Olivier J.F. Martin of the Swiss Federal Institute of Technology in Zurich, Switzerland. The scientific report of their findings was published in a recent issue of the technical journal Applied Physics Letters (Vol. 72, No. 19, pp. 2379-81, May 11, 1998).