Oligonucleotide microarrays, also called "DNA chips," are currently made by a light-directed chemistry that requires a large number of photolithographic masks for each chip. Here we describe a maskless array synthesizer (MAS) that replaces the chrome masks with virtual masks generated on a computer, which are relayed to a digital micromirror array. A 1:1 reflective imaging system forms an ultraviolet image of the virtual mask on the active surface of the glass substrate, which is mounted in a flow cell reaction chamber connected to a DNA synthesizer. Programmed chemical coupling cycles follow light exposure, and these steps are repeated with different virtual masks to grow desired oligonucleotides in a selected pattern. This instrument has been used to synthesize oligonucleotide microarrays containing more than 76,000 features measuring 16 microm 2. The oligonucleotides were synthesized at high repetitive yield and, after hybridization, could readily discriminate single-base pair mismatches. The MAS is adaptable to the fabrication of DNA chips containing probes for thousands of genes, as well as any other solid-phase combinatorial chemistry to be performed in high-density microarrays.

Extreme ultraviolet ͑EUV, ϭ13 nm͒ lithography is considered to be the most likely technology to follow ultraviolet ͑optical͒ lithography. One of the challenging aspects is the development of suitable resist materials and processes. This development requires the ability to produce high-resolution patterns. Until now, this ability has been severely limited by the lack of sources and imaging systems. We report printing of 38 nm period grating patterns by interferometric lithography technique with EUV light. A Lloyd's Mirror interferometer was used, reflecting part of an incident beam with a mirror at grazing incidence and letting it interfere with the direct beam at the wafer plane. High-density fringes ͑38 nm pitch͒ were easily produced. Monochromatized light of 13 nm wavelength from an undulator in an electron storage ring provided the necessary temporal and spatial coherence along with sufficient intensity flux. This simple technique can be extended to sub-10 nm resolution. © 1999 American Institute of Physics. ͓S0003-6951͑99͒02841-7͔Extreme ultraviolet lithography ͑EUVL, ϭ13.4 nm͒ is one of the candidate technologies pursued for printing semiconductor devices with critical dimensions of 70 nm and below.1 The approaches currently developed are based on the use of 4ϫ reduction optics and reflective masks. The reflectivity of near-normal incidence optics is enhanced to ϳ60%-70% by multilayer interference coatings ͑/4 stacks͒. In this spectral region, all materials have high absorption coefficients bringing some special requirements on EUVL optical systems and materials. For example, photoresist thickness has to be limited to less than ϳ0.1 m in order to obtain a sufficiently uniform exposure along the film thickness.Evaluation of resist materials require the ability to expose patterns in order to obtain measurements of exposure dose sensitivity, contrast and resolution using a given material. A prototype EUV projection camera has been built at the Sandia National Laboratory that is capable of printing sub-0.1 m lines and spaces but access is limited. 2 We have developed a system using EUV-interferometric lithography ͑EUV-IL͒ for studying imaging at EUV wavelengths, and for testing photoresist materials in which we have demonstrated unprecedented printing of features with periods as small as 38 nm. IL was not used before to print such small features in the EUV regime, and the features are the smallest ever printed using a photon-based lithography. Fabrication of quantum confinement devices is a clear application, and another is the study of the properties of materials confined in ultrasmall structures ͑polymeric resists in particular͒.

No abstract

We studied the surface properties of patterned Al͑Cu͒ lines related to the electromigration phenomena using photoemission spectromicroscopy techniques. We stressed the lines for electromigration in situ in the ultrahigh vacuum microscope chamber and observed the changes on the line surface. Our results show surface precipitation of Cu beneath the Al 2 O 3 layer on the line surface as well as on side walls. Enrichment of grain boundaries in Cu due to electromigration flux was observed in areas downstream of voids with respect to the electron flow. © 1999 American Institute of Physics. ͓S0003-6951͑99͒01801-X͔ Electromigration in Al͑Cu͒ interconnect lines has been studied extensively as an important reliability problem in microelectronic circuits. In particular the role of Cu in improving resistance to electromigration damage has drawn much attention. 1,2 Cu is usually swept away from an area by electromigration before fast Al diffusion leads to appreciable damage in the line. 3 Dominant paths for electromigration flux of atoms are believed to be along grain boundaries and interfaces. Cu has very low solubility in Al at operation temperatures and has been shown to segregate into phase Al 2 Cu precipitates 4 as well as to grain boundaries and interfaces. 3,5 In electron microprobe measurements grain boundaries were shown to become rich in Cu depending on the prior thermal treatment of the film. 2,3 Cu must be effective in reducing the electromigration flux along these dominant paths to produce the observed improvement. Because of its critical role in the electromigration process, it is essential to obtain information on the distribution and chemical state of Cu in the grain boundaries, interfaces, precipitates, and grains during operation and test conditions. Photoemission spectroscopy is a powerful method used in studying physical and chemical properties of solid surfaces. However, the sampling area is ordinarily too large to obtain spatially resolved information on the scale that is necessary for analysis of microscopic changes associated with the electromigration process in interconnect lines. This limitation is overcome in MAXIMUM ͑installed at the Advanced Light Source of Lawrence Berkeley National Laboratory͒, a scanning photoemission spectromicroscope with a spatial resolution better than 0.1 m and an energy resolution of 300 meV. 6 In this study we used a number of contrast mechanisms afforded by this microscope including elemental and chemical sensitivity, and beam induced charging to investigate this problem.For this experiment a 600 nm thick Al 4 wt % Cu alloy was dc magnetron sputter deposited on Si wafers with 200 nm thermally grown SiO 2 . The wafers were not heated during the deposition and the base pressure of the deposition system was better than 1.3ϫ10 Ϫ5 Pa. Single and parallel lines were patterned using conventional photolithography techniques and wet etching. Samples were later annealed at 450°C for 30 min in forming gas and quenched. No passivation was deposited on the patterned wafers. Pattern...

X-ray point sources are an interesting alternative to synchrotrons for small to medium scale production. These sources are by nature highly divergent, and thus require the use of collimation for delivering an acceptable lithographic illumination. We present the design of an aspheric collimator for a point source such as a dense plasma focus system. Designing a collimating mirror for a point source presents different challenges than designing one for a synchrotron source, although both cases require that the radiation be condensed and collimated to deliver the radiation with 1%-2% uniformity and acceptable runout over a large field area. The collimator is 3 cm wide and 31 cm long, accepts 20 mrad in the vertical and 70 mrad in the horizontal, and delivers the x-ray flux to a 3 cmϫ3 cm field on a mask located 3 m from the source with a runout of 6 mrad. The figure is an asphere, not dissimilar to those developed previously. The scanning mechanism, however, is radically different, since the virtual rotation point is located at the source, rather than at the mirror pole. This design provides excellent uniformity since the mirror is always scanned inside the radiation cone while maintaining a constant nominal incidence angle. The design principles and predicted performance are discussed in detail, together with a consideration of the manufacturing challenge for such a mirror. The collimator design presented has the capability of delivering the required power density and collimation from an x-ray point source to satisfy the requirements of x-ray lithography.