Improving 300mm wafer yield using x-ray diffraction inspection
04/01/2006
Petra Feichtinger, Bede X-ray Metrology, Englewood, Colorado
Digital x-ray diffraction inspection of bare and patterned 300mm wafers is challenging conventional semiconductor wafer inspection systems. Digital x-ray imaging for crystallographic defects has the potential to deliver substantial savings in a semiconductor manufacturing environment by identifying defects that may cause device failure or wafer breakage during ultra-fast annealing.
X-ray metrology using x-ray diffraction (XRD) is now becoming an established tool for monitoring layer thickness and composition, and it can also be used to inspect semiconductor wafers for grown-in or process-induced structural defects. XRD inspection (XRDI), also called x-ray topography, is recognized as a powerful tool for directly imaging defects in single crystals, such as semiconductor substrates and epitaxial thin films [1].
For wafers without defects, the crystalline lattice is essentially perfect, leading to a constant x-ray intensity diffracted from the wafer. However, defects that disturb the perfection of the lattice (i.e., those that introduce strain and/or tilt) alter the diffraction conditions, giving rise to variations in the diffracted intensity near the defects.
Tools that took advantage of this phenomenon were widely used in the 1960s and 1970s for process development to improve silicon wafer quality. The complexity of these historic instruments, however, restricted their implementation in manufacturing for larger diameter wafers until recently, when fast computers and large-area CCDs with small pixel size became available. XRDI is now capable of providing automated quantitative data in digital format, making it suitable for routine defect recognition in automated factory environments.
Both reflection and transmission geometries can be employed in digital XRDI at the same resolution. In reflection geometry, both the x-ray source and the detector are at the same side of the sample. Strain fields of defects close to the wafer surface are imaged. Transmission geometry, on the other hand, is capable of identifying defects through the whole depth of the wafer, from the back side to the front surface, including the area under the bevel.
Orientation contrast is controllable in digital XRDI, down to sensitivity of only a few arc seconds, without re-exposure, providing not only sensitivity to crystalline defects but also gross lattice misorientation from wafer bowing or crystal twinning. The spatial resolution of the technique is limited to around 3µm by the pixel size of current CCD cameras, which is adequate for the majority of structural defects, including misfit or thermal dislocations that produce image widths of several microns.
XRDI ensures rapid identification of the following critical lattice defects in bare and patterned device wafers:
- crystalline lattice defects close to the wafer surface and in epitaxial layers,
- crystalline lattice defects in the wafer bulk,
- mechanical damage around the wafer edge (surface and buried), and
- cross-sectional defect depth.
Edge-exclusion zone monitoring
With the proposed adoption in SEMI standards of an edge exclusion zone 1mm wide, wafer edges will require closer monitoring for defects. Thermal slip dislocations or, in the worst case, wafer breakages preferentially nucleate at mechanical sub-surface damage sites, which may be left over from mechanical edge-shaping or may have been introduced during wafer handling. With XRDI, such surface cracks and buried tilted regions can be detected independently of the polished wafer edge shape or patterning state, even if there is no indication of defects at the wafer surface level.
The defect images produced by the BedeScan [2] XRDI instrument can show mechanical-edge damage and induced slip lines in a doped wafer after thermal processing. Knowing that any possible damage will be located near the wafer edge, the instrument can be programmed to survey only the 10mm wafer periphery, without spending time measuring the usually perfect wafer center.
XRDI reveals mechanical edge damage and thermal slip dislocations in a doped wafer after thermal processing. An XRDI overview scan in low-resolution transmission mode quickly shows the location of crystalline defects. A following high-resolution zoom-in provides sufficient detail to allow for defect analysis including determination of likely generation mechanisms.
Thermal slip monitoring
XRDI determines thermal slip dislocation content regardless of whether these defects have caused a modification of the surface topology. The technique will also identify slip independently of wafer doping level, backside treatment, patterning level of the wafer up to a few metal layers, or the electrical activity state of the dislocations. Transmission mode scans reveal dislocations at any depth within the wafer. In reflection scan mode, threading dislocations ending at the wafer surface are visible, while the majority of dislocations buried in the wafer bulk are not detected in this measurement mode.
Figure 1 shows defect images around a pin-mark generated during thermal annealing of a bare wafer. The fast transmission-mode scan in low resolution allows for the identification of the defective region. Zooming in with high resolution reveals the mechanical edge damage of a pin mark, and shows that dislocations originally nucleated at this backside damage site induced a thermal slip during subsequent annealing. XRDI can image the threading ends of this thermal dislocation terminating at both the front- and backside of the wafer.
Relaxation onset monitoring
Growth of a strained epitaxial layer beyond the critical thickness may result in misfit dislocation nucleation and thus layer relaxation. X-ray diffraction defect imaging technology is capable of measuring the onset of relaxation, i.e. the existence of initial, sparse dislocations in epitaxial wafers and patterned wafer pads, which would not cause a detectable lattice parameter change.
 Figure 2. XRDI defect image of a silicon wafer (left), with respective defect recognition map indicating potentially defective dice (right). |
Digital XRDI is also capable of producing automatically generated defect recognition maps (Fig. 2, left). The number of “good” versus “bad” dice on a wafer can thus be predicted (Fig. 2, right).
Acknowledgment
BedeScan is a trademark of Bede X-Ray Metrology.
References
- D.K. Bowen, B.K. Tanner, X-ray Metrology for Semiconductor Manufacturing, CRC Press, Taylor and Francis, Boca Raton, 2006
- D.K. Bowen, M. Wormington, P. Feichtinger, L. Pina, “X-ray Topography Apparatus,” US Patent 6,782,076, 2004.
Petra Feichtinger is a senior technologist and product manager for the BedeScan x-ray diffraction inspection tool at Bede X-Ray Metrology, Belmont Business Park, Durham, DH1 1TW, UK; ph 44/191-332-4813, [email protected].