Technology News
08/01/2005
Wide-field pattern-inspection system cuts microscopy scan time
NanoGeometry Research Inc., Kawasaki, Japan, and Topcon Corp., Tokyo, have developed a wide-field pattern inspection system that cuts the time needed to check an entire 25×35mm exposure shot for defects down from several months to one day. The high-resolution scanning electron microscopy (SEM) system compares the pattern on the wafer to the design layout and flags and analyzes defects down to 5-10nm to provide quick feedback for fixing design problems and revising optical proximity correction (see figure).
Optical systems are fast but have limited resolution, while typical e-beam SEM systems have high resolution, but their narrow field of view makes inspecting large areas slow. The newly developed e-beam SEM inspection technology enlarges the field of view to a square 210μm on each side. The wide beam scans at high speed, and data is converted at 200Mpixels/sec. In most cases, one shot covers a whole die pattern. NanoGeometry’s software extracts the pattern outlines in real time for resist or etched patterns.
The operator specifies the types of circuit patterns to be measured and the defect criteria. The computer compares the outline it extracts from the actual pattern on the wafer to the corresponding layout data, measures how much the two differ, marks where the difference exceeds the specified defect criteria, and sorts the defects by severity and type. Repeating the analysis on the next die can show whether the defects are random or systematic. The system can measure average, maximum, and minimum gate critical dimensions across the whole die to determine exposure quality. And since it’s measuring actual pattern dimensions, not simulations, it can find defects not predicted by simulation, and does not flag areas that aren’t true defects.
The system can reduce the typical overwhelming number of defects, up to 50,000 or so, down to about 100 groups, sorted by type (line, end, corner, and gate) and cell library, so problem cell structures can be quickly found and corrected. The NGR 2100 inspection tool using this technology is now in beta evaluation at chipmakers. In repeat defect identification mode, the tool compares two shots, or two die, for systematic defects of 10nm or below. Currently it can inspect a 15×15mm area in 6 hr, but researchers think they can significantly improve that. The system runs on a cluster of a few to a dozen or so PCs.
- Masahiro Yamamoto, Tadashi Kitamura, NanoGeometry Research; M. Inoue, Topcon Corp., Pennwell partner Nikkei Microdevices
Applied goes bright on inspection
A just-introduced laser 3D [channel] brightfield inspection system, UVision, by Applied Materials uses a beam multiplier that sets up a column of beams derived from the same deep-ultraviolet (DUV) laser source to scan wafers in parallel. For signal collection, there is a proprietary optical device that is able to separate reflected light from scattered light and direct each of the signals to a different set of detectors.
According to Yogev Barak, chief marketing officer, Process Diagnostics and Control Group, the company was looking for an inflection point in requirements and technology in pursuing this new sector. Applied already has inspection tools in the remaining inspection areas: darkfield and e-beam.
Because the system uses a laser light source vs. traditional brightfield systems that use lamps, Barak likened the transition in this sector to what occurred in lithography when the industry moved to DUV technology. “When looking at defects at the 90nm, 65nm nodes, and beyond, the only way to see [with the required resolution] is by going from a lamp-based to a laser-based [brightfield] tool,” said Barak, referring to the decreasing intensity that accompanies the use of shorter lamp wavelengths that are necessary to see ever-smaller defects. The DUV laser used in the Applied tool has an 80nm pixel size, giving it a defect sensitivity of 30nm - the requirement at the 65nm node - but the company believes it is extendible beyond 65nm by changing the laser and optics.
Barak said customers could find critical layer defects such as a 40nm void in an STI layer, a 30nm particle in a poly layer, and a 30nm bridge in an etch layer using the new tool (see figure) that they couldn’t detect with conventional brightfield systems. - D.V.
New nanocrystal layer enhances image contrast in lithography
Pixelligent Technologies LLC has revealed that it is developing a new nanocrystal-based reversible contrast-enhancement layer (R-CEL) material that, according to company president Gregory Cooper, can help enable the extension of current 193nm lithography platforms to the 22nm design node or beyond.
Cooper said that the new nanocrystal material, which is a coating spun directly onto the photoresist, absorbs low-intensity light that diffracts around the photomask and onto the shadowed area of the resist. The R-CEL is designed to improve pattern resolution in multiple-exposure processes by preventing unwanted solubility changes in the resist that can otherwise result from the cumulative effects of successive low-intensity exposures. (Multiple exposures are essential for decreasing the size and pitch of features beyond the diffraction limit of information that can be patterned by a single mask image.)
What makes the new nanomaterial effective as a contrast enhancement layer, Cooper claimed, is that while it filters out low-intensity diffracted light, it also becomes transmissive where it is illuminated by high-intensity UV. Allowing the material to act essentially as an optical switch, this so-called “bleaching” property permits only the intended UV light to expose the resist, he says, thus creating a pattern with crisper, more uniform edges.
A second key characteristic of the nanocrystal is that after exposure, it quickly relaxes to its normal, opaque state and can be reused in situ, explained Ken Rygler, Pixelligent Technologies’ acting CEO. This reversibility feature is significant because it means that the wafer does not have to be removed from the stepper during multiple-exposure techniques. It therefore eliminates the added handling and process steps associated with traditional methods, he added, which translates to cycle-time, cost, and yield benefits.
“Other CEL materials have been used for higher-wavelength g-line (436nm) and i-line (365nm) UV light, but these have not proven extendible for wavelengths of 248nm and 193nm,” Rygler said. “This is a novel approach to expose a sharper image on the PR than has previously been possible at deep UV.”
“The R-CEL can be introduced into current semiconductor processes using conventional track lines without requiring new investment,” Cooper contended. “It is also compatible with other existing methods to improve resolution such as immersion lithography and phase shifting.”
Yet to be developed is a technique to strip the nanocrystal from the photoresist. Cooper estimated that the material, whose composition he declined to disclose, will be commercialized in 18-24 months. - P.L.