Patent Number: 
Section: description

The principle of the emission noise reduction technique is shown in FIG. 1. The primary electrons are extracted from the Schottky emitter 10, focused by the source lens 30, accelerated to a final beam voltage of 1 keV and refocused with the final lens 70 onto the sample 90. As is known in the art, the electron-optical lenses may be either electrostatic lenses, magnetic lenses, or combination of the two. When a periodic voltage is applied to the deflection plates 60, the focused beam is swept across the sample 90 and generates secondary electrons. (As is known in the art, deflection coils could be used in place of the deflection plates.) Secondary electrons which escape from the sample surface strike the detector 80 and contribute to the signal Id which is used to create a secondary electron image. However, only a small fraction of the emitted electrons hit the sample. The majority of the emitted electron current Ie, typically 50-200 xcexcA, is collected by the extraction electrode 20 (which has an extraction electrode aperture extending through it). A small portion of the electron current, typically 100-300 nA, passes through the first lens 30. In a conventional set-up, the majority of this current is collected by the beam-limiting element 50 (having a beam-limiting aperture extending through it), and only a small fraction Ib, typically 1-50 nA, is utilized for imaging. The novel approach of the present invention utilizes a screening element 40 (having a screening aperture extending through it) located between the emitter 10 and the beam-limiting aperture 50, which screening aperture 40 collects most of the current transmitted by the first lens 30. Only a small fraction Iba of the electron current, approximately 1-10%, is collected by the beam-limiting aperture 50. (As used herein, references to the xe2x80x9cbeam-limiting aperturexe2x80x9d and xe2x80x9cscreening aperturexe2x80x9d should be understood to encompass the blocking or truncating structure that defines the aperture.) In order to achieve good noise suppression, the screening aperture 40 should let through only a portion of the beam in which the electrons are correlated. For electron emission along the axis of a Schottky emitter, the electrons are correlated within an emission half cone angle a given approximately by   α  =            2              π              ⁢                  kT        Φ             where T is the tip temperature, k is Boltzmann""s constant and "PHgr" is the electron energy. At 1800K, xcex1 is 14 mrad for 1 kV electrons, which is more than typically used in the microcolumn operation (5-10 mrad). The current Iba collected by the beam-limiting aperture 50 is then used as a reference signal in the image processing. Specifically, current measuring circuitry coupled to the beam-limiting aperture 50 measures the portion of the electron beam that is blocked. An implementation of the noise suppressing scheme is illustrated also in FIG. 2. The current Iba (top graph) collected by the screened beam-limiting aperture 50 shows an emission noise peak 100 made while the electron beam Ib is scanned (for example along the x-axis) across the sample 90. The secondary electron signal Id (middle graph) includes an emission noise peak 110, superimposed on the imaging signal representing useful substrate information. This additional peak, due to the fluctuation in the emission current, could be interpreted as a substrate defect. The spurious emission noise peak 110 can then be suppressed or eliminated from consideration by processing the secondary electron signal Id data using the current Iba data collected by the screened beam-limiting aperture 50 (bottom graph). For example, the secondary electron signal Id may be divided by the current Iba collected by the beam-limiting aperture 50 or, alternatively, the current Iba collected by the beam-limiting aperture 50 may be subtracted from the secondary electron signal Id. (If needed, prior to such subtraction or division, either or both of the electron signal Id data or the current Iba data may linearly transformed with a shift of the origin or multiplication by a scaling factor.) The correction of the secondary electron signal Id data to account for emission noise by using the current Iba data collected by the screened beam-limiting aperture 50 can be suitably carried out by a processor. The elimination of the effect of the emission noise increases the detection sensitivity of an inspection tool, in particular to defects smaller than the beam spot size. This allows the use of a larger spot size and the imaging of the substrate on a more coarse pixel grid. Such imaging in turn reduces the total number of required pixels and therefore increases the throughput of the tool. The role of the screening aperture is crucial. If, for example, the current from the extractor electrode aperture or an un-screened beam-limiting aperture were used as a reference signal, the probability of noise suppression would be significantly reduced. This is due to the fact that the electron emission from the tip is strongly localized, and varies on a microscopic scale. Consequently, the electron beam varies spatially such that the noise in one part of the beam may be quite independent from the noise in a different part of the beam. The majority of the emitted electron current Ie, collected by the extraction electrode, includes thermal emission from the shank of the emitting tip, and is therefore not a sensitive measure of emission noise near the tip apex. Similarly, the current collected by an un-screened beam-limiting aperture contains emission from emitting regions that do not contribute to the beam current Ib. The electron current collected by the extraction electrode or an unscreened beam-limiting aperture has been used before as a means of trying to stabilize the emitted electron current using a direct feedback loop. This earlier approach did not prove practical, for the reasons described above. The use of a feedback loop to control the electrostatic field applied to the emitter has the further disadvantage of disturbing the dynamic equilibrium between electrostatic forces, surface migration and electron emission at the tip, which results in varying electron emission conditions and electron-optical properties. The scope of the present invention is meant to be that set forth in the claims that follow and equivalents thereof, and is not limited to any of the specific embodiments described above.