Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-12-27
2003-05-20
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754120, C324S765010, C250S310000, C250S492200
Reexamination Certificate
active
06566897
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the detection of defects in patterned substrates, such as semiconductor wafers, by inspection using a charged particle beam. More particularly, the present invention relates to improving the uniformity and voltage contrast of an image produced by a charged particle beam inspection tool.
2. Description of the Related Art
Defect detection is an important aspect in the manufacture of semiconductor devices. Early detection, preferably at multiple stages of fabrication, enables a source of defects to be identified and eliminated before large numbers of wafers are affected. Currently, the majority of in-line inspection is performed using optical inspection tools, such as the 21XX-series wafer inspection tools from KLA-Tencor. These optical tools, however, are limited in their capabilities by their small depth of focus and blurring due to diffraction. The small depth of focus of these optical tools is an inherent limitation of the large numerical aperture objective lenses required to image sub-micron features. Any defect that is not at the surface of the device will be substantially out of focus and therefore undetectable. Examples of such sub-surface defects include polysilicon gate shorts, open vias and contacts, and metal stringers. In addition, the diffraction-limited resolution of optical tools blurs small surface defects rendering them undetectable as minimum critical dimensions (CDs) shrink below 0.25 &mgr;m. These include defects such as ~0.1 &mgr;m particles and regions of missing or extra pattern which are at or below the minimum CD.
Charged particle beam inspection will likely become one of the critical technologies in advanced semiconductor manufacture. Charge particle beam inspection tools, which include conventional scanning electron microscopes (SEMs), focused ion beam microscopes (FIBs) and electron-beam (E-beam) defect detection systems, have a much higher resolution than optical tools and are able to detect smaller size defects. E-beam defect detection systems can also detect sub-surface defects by measuring the voltage contrast change resulting from the electrical effect of killer defects, i.e., “open” and “short” type defects. See, for example: T. ATON et al.,
Testing integrated circuit microstructures using charging
-
induced voltage contrast
, J. VAC. SCI. TECHNOL. B 8 (6), November/December 1990, pp. 2041-2044; K. JENKINS et al.,
Analysis of silicide process defects by non
-
contact electron
-
beam charging,
30
TH
ANNUAL PROCEEDING RELIABILITY PHYSICS 1992, IEEE, March/April 1992, pp. 304-308; J. THONG, ED., ELECTRON BEAM TESTING TECHNOLOGY, Pelnum Press 1993, p. 41; and T. CASS,
Use of the Voltage Contrast Effect for the Automatic Detection of Electrical Defects on In
-
Process Wafers
, KLA Yield Management Seminar 1996, pp. 506-2 through 506-11.
Schlumberger's E-beam defect detection technology operates in either a positive or negative voltage contrast mode. In either mode, floating electrical conductors on the wafer under inspection are raised to a potential by pre-charging the surface of the wafer with charged particles (e.g. electrons). Because they appear in different contrasts, the floating and grounded connectors can therefore be distinguished. In a positive voltage contrast mode the floating conductors are charged to a more positive voltage than the grounded conductors, while in a negative voltage contrast mode the floating conductors are charged to a more negative voltage. A focused, low voltage particle (electron) beam interrogates the charge states of the wafer's conductors. By comparing the voltage contrast image (or partial image) of a die with that of a reference (e.g. a neighboring die), one can locate defects in the die. Because this technique relies on voltage contrast variation to identify defects, it is important to have: (1) a uniform voltage contrast image in which the background contrast is uniform; (2) a consistent contrast for a circuit when that circuit is located in different areas of the field of view; and (3) a distinctive contrast (e.g., a large difference) between circuit elements with different underlying connections.
One problem with charged particle beam inspection systems is that the resulting images are often non-uniform in quality. Unwanted variations in the topographic contrast or voltage contrast of an image often exist. Non-uniformity in the voltage contrast can result from uneven charging of a patterned substrate (wafer or die). Surface charging can affect secondary electron collection efficiency and the on-going charging process during primary beam irradiation. E-beam defect detection systems operate between two crossover voltages, at which a primary electron induces more secondary electron emission current than primary current. This means that floating conductors within a field of view (FOV) will charge positively. Uncaptured secondary electrons which are returned to the wafer can negatively charge the area surrounding the FOV, thereby creating a “micro” retarding field (MRF). The MRF affects the surface charging process and can cause several problems with the voltage contrast of an image. First, the MRF can cause some secondary electrons to be rejected back into the FOV area of the wafer, thereby reducing the positive voltage contrast. Second, if the magnification of the system is increased for detailed inspection, the MRF can cause a positive voltage contrast mode to switch to negative. At a high magnification, a strong MRF will retard back enough secondary electrons back to the FOV area, thereby negatively charging the FOV area. Third, the MRF can create unpredictable “ghost features” and site dependent contrast variations in an image. The MRF is non-isotropic at the edge of the FOV, and the intensity of returned secondary electrons at the edge of the FOV can differ significantly from those at the center of the FOV. This results in uneven charging of floating structures. In addition, the efficiency of detecting secondary electrons emitted from the center and the edge of the FOV can differ greatly. These problems produce false contrast differences which greatly degrade the reliability of E-beam defect detection systems.
Since 1995, Schlumberger has used E-beam probers such as the commercially available IDS 10000 system on passivated integrated circuits (ICs) to measure waveforms at a high beam current. The E-beam prober scans a large area and then images a smaller area. A high current vectored beam is pulsed to measure capacitive AC waveforms on the passivated IC. Imaging the small area prior to scanning the large area reduces the unstable surface charging in the small area, thereby producing a more stable and accurate voltage waveform (as a function of time). The uniformity or contrast of an image is not a concern because the measurement is taken at the area of a conductor on an individual die. This method is only applicable to functioning integrated circuits connected to an electrical stimulus, rather than to unfinished patterned substrates.
It is also known to try to improve measurements produced by charged particle beam tools. International Application No. PCT/US98/00782 published on Jul. 23, 1998 as International Publication No. WO 98.32153 is directed to measuring critical dimensions of microcircuits using SEMs. Multiple scans of a SEM over a small scan area result in dark images, obscuring the features of the area. Scanning a larger area brightens the image. This method, however, merely brightens an image rather than enhances the image contrast differences between features with different underlying connections. In addition, simply brightening an area will not improve the uniformity of an image.
Accordingly, there is a need to improve the uniformity and contrast quality of an image produced by a charged particle beam inspection tool, in order to enhance the detection of defects on a patterned substrate. In particular, it would be desirable to enhance the voltage contrast of the image.
SUMMARY
In accordance wit
Kanai Ken-ichi
Lo Chiwoei Wayne
Applied Materials Inc.
Cuneo Kamand
Einschlag Michael B.
Tang Minh N.
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