1. Field of the Invention
The present invention relates to a charged particle beam application apparatus, or more particularly, to a charged particle beam application apparatus that is used to observe, inspect, and analyze a wafer sample, which has a minute circuit pattern, with a high resolution using a low-acceleration electron beam.
2. Description of the Related Art
Various techniques have been employed in detecting a defect which occurs in fabrication of a microscopic circuit such as an LSI, measuring the length of the defect, or assessing the shape of the defect. For example, an optical inspection apparatus produces an optical image of the microscopic circuit and inspects the image to detect an abnormality. However, the resolution of the optical image is not high enough to identify a very small shape-related feature, and is not high enough to discriminate a harmful defect from a harmless defect in terms of fabrication of a circuit. A sample to be handled by such a measurement/inspection apparatus has become more and more microscopic along with advancement of technologies. For example, in a recent DRAM manufacturing process, the width of a metal wire is 90 nm or less. For a logic IC, a gate dimension has reached 45 nm.
A defect inspection technique using an electron beam provides a resolution that is high enough to image a microscopic shape-related feature of a contact hole, a gate, or a wire and a shape-related feature of a microscopic defect, and can therefore be used to classify or detect a grave defect on the basis of a contrast of a shaded image of a detective shape. Therefore, for measurement/inspection of a microscopic circuit, a measurement/inspection technique employing a charged particle beam has an advantage over the optical inspection technique.
A scanning electron microscope (SEM) that is a type of charged particle beam apparatus focuses a charged particle beam emitted from an electron source of a heating type or a field emission type so as to form a thin beam (probe-like beam), and sweeps the probe-like beam over a sample. Secondary charged particles (secondary electrons or reflected electrons) are emitted from the sample due to the sweep. Synchronously with the sweep of the primary charged particle beam, a scan image is acquired by using the secondary charged particles as a luminance signal of image data. A typical scanning electron microscope accelerates electrons emitted from the electron source using an extracting electrode interposed between the electron source, at which a negative potential is developed, and a ground at which a ground potential is developed, and irradiates the resultant electrons to the sample.
The resolution offered by a scanning charged particle microscope such as an SEM and the energy of a charged particle beam have a close relationship. When a primary charged particle beam of high energy reaches a sample (that is, when the landing energy of a primary charged particle beam is large), since primary charged particles deeply invade into the sample, an emissive range on the sample from which secondary electrons and reflected electrons are emitted expands. As a result, the emissive range becomes wider than the probe diameter of the charged particle beam, and an observational resolution is markedly degraded.
When the energy of a primary charged particle beam is excessively reduced in order to lower the landing energy, the probe diameter of the charged particle beam greatly increases due to aberrations. Eventually, the observational resolution is degraded.
Further, a contrast of an SEM image is affected by the value of a current carried by a primary charged particle beam to be irradiated to a sample. When the beam current decreases, the ratio of a secondary signal to a noise (signal-to-noise ratio) is greatly lowered and a contrast of a scan image is degraded. Preferably, the beam current value should be controlled to be as large as possible. When the energy of the primary charged particle beam is reduced, formation of a thinner probe-like beam becomes hard to do due to the Coulomb's law. When the energy of the primary charged particle beam is excessively controlled to become small, a beam current required for producing the scan image becomes insufficient. This makes it hard to acquire the scan image with a high magnification and a high resolution.
For observation with a high resolution, the energy of a primary charged particle beam, or especially, landing energy has to be appropriately controlled according to an object of observation.
As a control technology for landing energy, a retarding method is widely adopted. In the retarding method, a potential causing a primary charged particle beam to decelerate is developed at a sample in order to decrease the energy of the charged particle beam to a desired level of energy immediately before the charged particle beam reaches the sample.
For example, in JP-A-6-139985, an invention that controls the timing, at which a negative potential for retarding is developed at a sample, responsively to mounting or replacement of a sample has been disclosed.
An invention disclosed in JP-A-2001-185066 is such that: when the slope of a sample is observed according to the retarding method, the magnetic poles of an objective lens are separated into upper and lower ones, and a potential identical to the one at the sample is developed at the lower magnetic pole in efforts to minimize the adverse effect of an asymmetric retarding electric field derived from the slope of the sample (to minimize occurrence of astigmatism or reduction in efficiency in detecting secondary electrons).
In JP-A-6-260127, an invention of a potential measurement apparatus employing an electron beam has been disclosed. In the potential measurement apparatus described in JP-A-6-260127, an objective lens is divided into a yoke part for excitation and a magnetic-pole part, and is formed with two magnetic circuits. An electric field for pulling up secondary electrons is applied to the magnetic-pole part. According to JP-A-6-260127, since the objective lens is divided into two parts, the magnetic circuits can be readily designed according to the working distance between a sample and the objective lens. The diameter of the spot of an electron beam can be appropriately controlled irrespective of the working distance.