Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1998-07-26
2001-02-27
Anderson, Bruce C. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML, C250S310000
Reexamination Certificate
active
06194729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a particle beam apparatus in which an object is irradiated with a focused beam of electrically charged particles, such as electrons, positrons, or ions, and more particularly to such a particle beam apparatus in which the particle energy can be set at values below 1 keV. Such apparatuses are used in the form of scanning electron microscopes, particularly for inspection of semiconductor wafers and also for imaging and analysis of objects with low conductivity. In order to avoid charging of the specimen surface, or for analysis of regions near the surface, the energy when incident on the object should not exceed certain limiting energies which lie between 10 eV and 5keV.
2. Discussion of Relevant Art
Since the chromatic imaging errors of the electrostatic or magnetic lenses used for focusing the particle beam are inversely proportional to the particle energy, the resolution defined by the focal diameter of the particle beam is substantially worse at low particle energies than at higher particle energies. To solve this problem, it is known to first accelerate the particles to a substantially higher energy than the desired target energy when incident on the object and to reduce the particle energy by means of a decelerating device shortly before incidence on the object. Such a decelerating device usually consists of two or more electrodes to which an electrostatic decelerating field is applied. Braking of the particles can take place either in the rear portion of the objective or after leaving the objective, between the objective and the object.
Such heretofore known solutions can be divided into two groups.
In the first group of devices, the beam guiding tube is at ground potential from the anode at least as far as to the pole piece gap of the objective, and the object is at a the anode at least as far as to the pole piece gap of the objective, and the object is at a high voltage potential, which has the same sign as the cathode potential. Such arrangements are described in the article by KJ. Plasko et al., “Low energy electron beam lithography”,in Optical Engineering, March/April 1983, pages 195ff., in U.S. Pat. 5,389,787, and in European Patent EP-A2 0 769 799. At low target energies, particularly at target energies below 1 keV and an energy of the particles within the beam guiding tube of 10-20 keV, the specimen has to be placed at a correspondingly high potential of 9-19 kV. Besides insulation problems that arise at the specimen table, voltage flashovers between the object and the objective, or damage to the object due to internal currents arising from switching on and off the high voltage, can easily arise here. Detection of the secondary electrons emitted by the object is also very difficult, since these secondary electrons have to be extracted and accelerated again without affecting the primary beam.
In the second variant, the object and the electrode of the decelerating device on the object side are at ground potential, and the beam guiding tube from the anode as far as to the first electrode of the decelerating device is at a high anode potential, the sign of which is chosen such that a deceleration of the particles takes place when they leave the beam control tube. Such systems are described, for example, in German Patent DE 29 22 325 C2 and in European Patent EP 0 180 723 B1. With the arrangement according to German Patent DE 29 22 325 C2, the setting of the target energy takes place by variation of the potential of the beam guiding tube. A fixed but opposite potential between the cathode and the anode is superimposed on this variable anode potential. At target energies between 0.5 and 1 keV, the whole beam guiding tube from the anode to the objective is at a potential that is only 0.5-1 keV smaller in absolute value than the potential difference between cathode and anode. Since insulation from the surroundings has to be correspondingly set up, this arrangement can only be used at low maximum energies. For target energies below 0.2 keV, the emission becomes increasingly unstable with decreasing target energy, since the cathode potential nearly corresponds to the ground potential and therefore any stray field can affect the emission. This arrangement is therefore of little use for target energies below 0.2 keV.
In an arrangement according to European Patent EP 0 180 723 B 1, it is known from the apparatus LEO 1500 of LEO Elektronenmikroskopie GmbH, 73446 Oberkochen, Germany to keep the potential of the anode and of the beam guiding tube constant, independently of the target energy, as far as the braking device, and to vary the cathode potential in order to vary the target energy. This leads to the disadvantage that when there is a change of the target energy the energy of the particles within the beam guiding tube also changes. Consequently all the parameters within the imaging system, particularly the strength of the magnetic lenses, have to be correspondingly matched. This is especially critical at low energies, and thus low energies in the beam guiding tube, since small errors in settings already have large effects. Therefore the low energy region, particularly the region of target energies below 2 kV, has to be correspondingly finely equalized. Also, in this device, the cathode potential corresponds to the target energy, as against the ground potential, so that in this arrangement also, the emission is unstable at target energies below 0.2 keV.
SUMMARY OF THE INVENTION
The object of the invention is to provide an arrangement with which, even at the lowest target energies, which can be below 0.2 keV, a stable and constant operation can be achieved, without the object having to be placed at a high potential level compared to the ground potential. In addition, good detectability of secondary or back-scattered particles arising from the interaction of the particles with the object can be realized, even at small distances between the objective and the object.
This object is achieved by a particle beam apparatus with a particle beam generator including a cathode and an anode for acceleration to anode potential of the particles coming from the cathode, a beam guiding tube, an objective for focusing the particle beam onto an object, and a decelerating device. The anode and the beam guiding tube are at the same high voltage potential, which is high with respect to ground potential, and the decelerating device and the object are at the same potential, which deviates from ground potential at low target energies and is opposite to the anode potential.
The particle beam apparatus of the invention has a substantially known construction that consists of a particle generator or source with a cathode and an anode for emission and acceleration of the particles, a beam guiding tube, at least one objective for focusing the particle beam on an object, and a decelerating device. The anode and the beam guiding tube lie at the same high voltage potential, which is high with respect to the ground potential. At the same time, the last electrode of the decelerating device and the specimen lie at a common potential, which at least for the lowest target energies deviates from the ground potential and has a sign opposite that of the anode potential.
The invention thus departs from the prior art forms of construction in which at least either the beam guiding tube or the object is kept at ground potential. Instead, the known solutions are combined together such that the specific advantages of the known solutions are attained without the specific disadvantages of the individual solutions having to be taken into account. With application of a moderate high voltage potential below 5 kV to the object, no great problems now arise as regards insulation or object damage. Simultaneously even at the lowest target energies the cathode potential with respect to ground potential can be kept to a value of 0.2-5 kV, at which the emission is stable. And at high target energies, the energy setting is reached by appropriate raising of
Anderson Bruce C.
Leo Elektronenmikroskopie GmbH
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