By Pieter Burggraaf
WaferNews Technical Editor
Among the 2001 ITRS grand challenges for yield enhancement, detecting defects associated with high aspect ratio trenches, contacts, and vias continues to be difficult and complicated by simultaneous needs for high sensitivity and high throughput with cost effective detection tools.
The ITRS specifically names high aspect ratio inspection (HARI, defined as defect detection into structures with an aspect ratio >3) as one of the five difficult inspection challenges for the 100nm node and beyond. The technical problems are poor transmission of energy into and back out of such structure, and the large number of contacts and via/wafer.
Conventional inspection technologies used in wafer fabs typically use dark field (e.g., laser scattering), bright field, or electron beam technologies that are capable of maximum aspect ratio of 1:1, 3:1 and in greater than 7:1 (under certain conditions), respectively. The general consensus is that evolutionary improvement for these technologies cannot continue to meet HARI requirements. Conventional optical technologies analyze the intensity of light reflected or scattered from surfaces and fail when insufficient light penetrates high aspect ratio structures. While electron beam inspection has the deepest aspect ratio capabilities, it is only capable of ~ 0.2 wafer/hr.
One revolutionary technology that addresses HARI needs is patented direct digital holography (DDH) capable of aspect ratios of 12:1 and higher (the strong signal returned from the bottom of contacts tested so far indicates that higher aspect ratios are possible). DDH is commercially available from nLine Corp., Austin, TX, in the form of its Fathom Tool for defect detection on patterned wafers. C.E. Thomas, while at Oak Ridge National Laboratory, did the underlying R&D.
Briefly described, DDH creates a hologram by combining reference and object beams of coherent light reflected off the surface under inspection. Combination of the two beams creates a complex pattern of interference fringes that contains information about the intensity and phase of the reflected light. The DDH technique digitally captures the hologram with a CCD then separates it into a clear, crisp intensity image (basically a bright field photograph) and a phase image. The latter is a graphic representation of the phase of the light falling at every pixel of the CCD. Since the phase of light at the CCD is a function of the path length between the inspected surface and the camera, the phase image can be converted to a topographic map of the inspected surface. Because phase measurement resolution is conservatively 1/100th the illumination wavelength, the topographic map has remarkably high vertical resolution as well.
Potential defect detection sensitivity for a DDH tool is about 1/13th the illumination wavelength (e.g., ~20nm for 266nm illumination) and is limited only by optical noise within the system. This is five times greater than the resolution of conventional, diffraction-limited intensity-based systems. DDH can detect defects that are 1/5th the size of those found by an intensity-based system operating at the same wavelength and camera pixel.
Among the advantages of DDH for wafer fabrication:
* It only detects defects caused by a change in surface topography. Buried defects that may cause a color change, but don’t create a surface bump or pit and color variations caused by non-uniform film thickness can removed from the inspection data.
* It is applicable to high aspect ratio structures because the holographic image is created using a heterodyne signal amplification. The object signal is strengthened by combination with the reference beam. A very weak signal returned from the bottom of a deep, narrow surface structure is enough to create a usable signal. Deep surface features that are dark to an intensity-based inspection system (e.g., thin etch residues) are light and detail rich with DDH.
* The heterodyne signal amplification technique also results in detecting surface defects in easily damaged films (e.g., low-k dielectrics) using very low illumination intensity.
Because phase detection is based on a difference in height measured by a camera pixel, the camera pixel corresponds to a certain projected area on the wafer determined by the magnification of the optical system (i.e., “pixel-area” multiplied by “phase-height” equals “phase-volume”). “This means that the minimum detectable signal for a given tool setup corresponds to a minimum detectable volume such as a bump or a pit,” says Bob Owen, nLine CEO. “The aspect ratio of the minimum detectable volume doesn’t matter in DDH for small defects; the defect can be short and broad, tall and thin, cubic, whatever. Equal volumes give equal signals independent of their aspect ratios.”