JP4353193B2 - Electron source and electron beam irradiation apparatus equipped with the electron source - Google Patents

Description

  The present invention relates to an electron source employed in an application apparatus using an electron beam such as an electron microscope or an electron beam drawing apparatus.

In recent years, as a new electron source, a surface diffusion type in which, for example, Zr, Ti, Hf, etc. and oxygen atoms are adsorbed to a single atom on the surface of a single crystal chip of a high temperature resistant metal material such as tungsten (W) or molybdenum (Mo). An electron source has been put into practical use. As a general configuration, a W single crystal line having a certain crystal orientation is joined to the apex of a W hairpin filament, and the tip of the single crystal line is sharpened by electrolytic polishing. Then, a hydrogen compound powder such as hydrogenated Zr is attached between the W hairpin filament and the single crystal line, and heat treatment is performed in a vacuum atmosphere having a partial pressure of oxygen gas, thereby promoting the diffusion of Zr and the like. An adsorbed layer of Zr and O is formed on a specific crystal plane (US Pat. No. 4,324,999). The case where such an electron source is used in a weak electric field region where field emission does not occur is particularly called a Schottky emission. Zr / O / W has been put to practical use as this electron source (Journal, Vacuum, Science, Technology, B3 (1), 1985, P220 (J. Vac. Sci. Technol. B3 (1), 1985, p220). )). A control electrode (generally called a suppressor electrode) and an extraction electrode are added to the electron source to constitute an electron gun. The basic structure is shown in FIG. 1 is a single crystal chip of W (100), 2 is a hairpin-shaped filament made of a polycrystalline wire of W, 4 is a terminal such as stainless steel on which the filament 2 is spot welded, and 5 is a ceramic insulator. A Zr oxide replenishment source 3 having a work function lower than that of the W single crystal chip 1 is attached to the center, root or filament 2 of the single crystal chip 1. The oxide replenishment source 3 is heated from about 1500 K to about 1900 K, and is thermally diffused along the single crystal chip 1 to the tip of the chip. The metal (Zr) oxide diffused to the tip of the single crystal tip 1 forms a Zr monoatomic layer with oxygen at the tip of the single crystal tip 1. At this time, it is selectively adsorbed and formed on a specific crystal plane (100) having high surface diffusion and activation energy. When a single crystal line whose crystal plane is the tip of the single crystal chip 1 is used, only the shaft tip of the single crystal chip 1 can be kept in a low work function state. Thereby, a high emission electron current density can be obtained from that portion. 6 is called a suppressor electrode, and functions to suppress thermionic electrons from the W hairpin filament heated at 1500K to 1900K. Reference numeral 7 denotes an extraction electrode, which functions to apply an electric field to the tip of the single crystal and extract Schottky emission (hereinafter abbreviated as SE). Unlike the field emission, this SE has a field intensity applied to the tip that is extremely lower than that in the field emission state. Therefore, the emitted electrons are thermal electrons, and tunnel electrons are not included. This means that there is no fluctuation of the emission electron flow peculiar to field emission electrons, and an extremely stable emission electron current can be obtained. Furthermore, since the operating temperature is lower than that of a normal thermionic source, for example, LaB ₆ or W hairpin, the energy width of the emitted electron stream can be reduced.

  The amount of electron beam emitted from the SE electron source depends on the electric field strength at the tip of the chip as described in the above document. For this reason, the electron dose is generally adjusted by the extraction voltage applied to the extraction electrode.

  However, when the extraction electrode is changed, there is a problem that the optical axis is shifted every time the change is made.

  Further, as can be seen from FIG. 1, the gap between the control electrode and the tip and the gap between the control electrode and the extraction electrode are extremely small. For this reason, the voltage value applied to these electrodes is desired to be small. However, in the structure of the conventional electron gun, the adjustment range of the electron dose is narrow, and the voltage applied to the electrode is large and easy to discharge.

  An object of this invention is to provide the electron source which can solve the said problem, and the electron beam irradiation apparatus using the electron source.

  In the present invention, in order to achieve the above object, a needle-like cathode whose tip is formed at a cone angle of 15 degrees or less, a control electrode for controlling an electron dose from the cathode, and an electron from the needle-like cathode Discloses an electron source having an extraction electrode for extracting the electron.

  Furthermore, according to the configuration of the present invention, there is also disclosed an electron beam irradiation apparatus that changes the electron dose by changing the negative voltage value applied to the control voltage while keeping the positive voltage applied to the extraction electrode constant.

  The tip of the cathode was formed to have a tip radius of curvature of less than 0.5 μm.

  The above configuration is based on the following reason.

  First, adjusting the control voltage applied to the control electrode is more advantageous than adjusting the electron dose with the extraction voltage, such as less change in the optical axis. In other words, it is ideal if all the used electron doses can be adjusted by changing the control voltage under a constant extraction voltage.

  Further, the electric field strength at the tip of the SE chip depends on the structure of the electron source itself.

  The structural factors that influence the electric field strength are described in the literature journal, Apply, Physics, Vol. 44, No. 5, 1973 (J. Appl. Phys., Vol. 44, No. 51973). Two parameters, cone angle (2β) and radius of curvature (r).

  FIG. 2 shows SEM photographs of the tips of two types of chips used in this example. The magnification is 10,000 times in both figures. FIG. 2A shows a conventionally used chip having a cone angle (2β) = 26 degrees and a radius of curvature (r) = 0.55 μm. FIG. 2B is an SEM image of the tip of the present invention, where the cone angle (2β) = 8 degrees and the radius of curvature (r) = 0.30 μm. According to the above document, both parameters should be made as small as possible to increase the electric field strength. Thereby, a large electric field strength can be produced even with a small extraction voltage. However, the above document does not describe anything about the influence of the control voltage. From our experiments, it was found that the electric field generated by the control voltage applied to the control electrode also becomes stronger by reducing the two parameters.

  For this reason, according to the said structure, compared with the conventional thing, the electric field strength change of a chip | tip tip with respect to a fixed voltage change can also be enlarged.

  In the present invention, it is possible to provide an electron source that can cover the range of all the electron doses used by adjusting one control voltage by adjusting the control voltage, and an electron beam irradiation apparatus equipped with the electron source.

  The radiation current density J of the SE electron source is given by the following equation. T is the chip temperature, φ is the work function, a is a constant, F is the electric field strength, and k is a constant.

J = 120T ^ 2exp (-(φ-a√F) / kT) (A / cm ^ 2) (1)
As is clear from the above equation, the radiation current density depends on the temperature T of the chip, the work function φ of the radiation surface, and the electric field strength F at the tip of the chip. The electric field strength F depends on the cone angle and the radius of curvature of the tip itself, and the electric field strength F also depends on the distance between the SE tip and the extraction electrode. As shown in FIG. 1, the distance between the SE tip and the extraction electrode is extremely small, and it is difficult to adjust further. For this reason, the cone angle and the curvature radius can be adjusted, and the change range of the electric field strength at the tip of the tip in an arbitrary voltage range of the control voltage can be increased.

  Although it has been known that electron beams are easily emitted simply by sharpening, in the present invention, a Schottky emission electron source sharpened to 15 degrees or less and an electron dose adjustment are applied to the control electrode. The combination of unprecedented requirements to control by controlling the control voltage increases the range of probe currents that can be obtained with respect to changes in the control voltage. Become.

  If the desired voltage adjustment range cannot be obtained by the control electrode adjustment range, it is necessary to secure the desired probe current adjustment range by adjusting the extraction voltage, but other parameter adjustments by changing the extraction voltage are necessary. Considering troubles and adverse effects such as optical axis misalignment, it is ideal to secure a desired probe current range with one extraction voltage. This is based on the premise that the optical axis and the position of the virtual light source do not change depending on the change of the control electrode.

  As described above, by adjusting the cone angle and the radius of curvature, and adjusting the electron dose with the control electrode, the change range of the electron dose emitted from the electron source with respect to any voltage change of the control voltage is increased. It becomes possible to do.

  In the electron gun using the SE electron source adopting the present invention, the variable range of the obtained probe current becomes larger with respect to the change of the control voltage, so that the electron dose in a wide range with respect to one extraction voltage. Setting is possible. Furthermore, as a result, a necessary variable range of the probe current can be obtained without moving the extraction voltage, and the extraction voltage can be set to a fixed value, which can contribute to prevention of misalignment due to a change in the extraction voltage. In addition, since the value of the extraction voltage is small, the chip is not damaged by the discharge, and the stability of the electron gun is increased.

  Examples of the present invention are described below. FIG. 3 shows a test apparatus evacuated to an ultra-high vacuum for evaluating an SE electron gun. It is divided into an electron gun section and an exhaust mount section. This is an apparatus for evaluating the characteristics of many electron guns by exchanging the electron gun part. First, the electron gun with the SE chip mounted is placed on an exhaust stand and ultra-high vacuum exhaust is performed. When the vacuum is exhausted to ultra high vacuum, the hairpin filament is heated by energization from the constant current power source Vf. The temperature of the SE chip is set to an arbitrary constant temperature from 1600K to 1800K.

  Then, SE electrons are emitted from the tip of the SE chip by supplying a positive voltage from the DC high voltage power source Ve to the extraction electrode. Further, a negative voltage is applied to the control electrode (suppressor electrode) by a DC voltage power source Vs. This eliminates the emission of electrons outside the electron line.

As shown in the figure, most of the emitted SE electrons reach the aperture plate, and a part thereof reaches the Faraday cup. The energy at this time is determined by the DC high-voltage power supply Va. In the length measuring SEM, a low acceleration voltage is used and it is about 1 kV or less. The SE electron incident on the Faraday cup is used as a probe current and used in an actual electron microscope or electron beam application apparatus. The opening angle is on the order of several mrad, and the current is several pA to several hundred pA. The extraction voltage Ve applied to the extraction electrode is kept constant, the control voltage is changed within a certain range, and the probe current obtained at that time is measured.

  By controlling the electron dose based on the voltage value of the negative voltage applied to the control voltage in this way, there is no need to control the electron dose by the extraction voltage, and the extraction voltage can be fixed, resulting in axis misalignment. It is possible to easily set the electron dose.

  Generally, when the extraction voltage is changed, the ratio between the extraction voltage and the acceleration voltage for accelerating the beam is changed, so that the optical axis position is changed by the lens action of the electrostatic lens, and as a result, the axis is shifted.

  In the present embodiment, in view of the above, the control voltage is changed with the extraction voltage fixed.

  And in the Example of this invention, in view of the above fact, the above measurement was implemented using two chips | tips from which the cone angle and curvature radius which are shown in FIG. 2 differ.

  The result is shown in FIG. The change range of the control voltage is 100V to 900V. From FIG. 4, the change range of the probe current is 22 pA to 57 pA in the conventional tip (old tip), and can be changed in a very large range from 2.3 pA to 67 pA in the present invention (new tip). Also, the extraction voltage Ve can be reduced to 1.5 kV as compared with the conventional chip.

  As described above, when the electron gun of the present invention is employed, the extraction voltage can be reduced and the change range of the probe current with respect to the change of the control voltage is increased. As described above, the change range of the probe current depends on the cone angle and the radius of curvature with respect to the constant change of the control electrode.

  Furthermore, in a scanning electron microscope, a device handling a semiconductor is required to reduce the probe current in order to reduce the influence of charge-up and contamination. This is because the magnitude of the probe current is proportional to the occurrence frequency of charge-up and causes a sample image failure, and also in the contamination, the probe current becomes heavily contaminated with the sample. For this reason, the probe current requires a minimum current amount of 10 pA or less. As a result of experiments to obtain this minimum current amount, it was possible to obtain an electron dose that met the above-mentioned conditions for the first time when a chip having a cone angle of 15 degrees and a curvature radius of 0.5 μm was used. (Line A in FIG. 4) It was found that at a cone angle larger than 15 degrees, the minimum current amount exceeds 10 pA and does not meet the desired conditions as described above.

  Therefore, if the electron source is formed with a cone angle smaller than this, the probe current range can be adapted to the above-mentioned conditions. However, advanced technology is required to make the tip shape an acute angle, and 5 to 6 degrees is the limit in the current formation technology.

Further, when the tip radius of curvature is set to 0.5 μm or more, Schottky emission is difficult to occur, and the above-described range of the beam current amount cannot be obtained. Also 0.2
If it is less than μm, the beam becomes unstable due to field emission.

  Although the above embodiment is implemented for the SE electron source, it is obvious that the same effect can be obtained for an electron gun that varies the probe current with a similar control voltage. For example, a TFE electron source, a CFE electron source, and the like can be implemented using the control electrode.

  Furthermore, the present invention can be applied to general apparatuses equipped with these electron sources, and the same effect can be obtained even when applied to a scanning electron microscope, a transmission electron microscope, an electron beam drawing apparatus, and the like.

  When the electron source shown in the embodiment of the present invention is applied to an electron beam irradiation apparatus, in order to obtain a variable range of probe current required for one extraction voltage setting, which is an effect of the present invention, at least the tip of the cathode is As a result of the experiment, it was found that it was necessary to make it 15 degrees or less.

The block diagram of SE electron source. The electron micrograph of the cathode of this invention and a prior art. 1 shows a test apparatus for a chip of the present invention and a prior art chip. The comparison figure of the chip of the present invention and the chip by the prior art.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Single crystal wire, 2 ... Hairpin filament, 3 ... Oxide supply source, 4 ... Terminal, 5 ... Ceramic insulator, 6 ... Control electrode, 7 ... Extraction electrode, 8 ... SE electron, 9 ... Diaphragm plate, 10 ... Faraday cup.

Claims (3)

A needle-like cathode having a tip angle of 8 degrees and a radius of curvature of the tip of 0.30 μm ;
An extraction electrode for forming an electric field for extracting electrons from the cathode;
A control electrode disposed between the cathode and the extraction electrode to control the amount of electron beam;
A negative voltage applied so as to control an electron beam current emitted from the cathode in a range of at least 67 pA to 10 pA or less, connected to the control electrode and applying a positive voltage to the extraction electrode. An electron source comprising a negative voltage application power source for adjustment. A needle-like cathode having a tip angle of 8 degrees and a radius of curvature of the tip of 0.30 μm, an extraction electrode for forming an electric field for extracting electrons from the cathode, and an electron beam disposed between the cathode and the extraction electrode; An electron source having a control electrode for controlling the amount of
A negative voltage applied so as to control an electron beam current emitted from the cathode in a range of at least 67 pA to 10 pA or less, connected to the control electrode and applying a positive voltage to the extraction electrode. An electron beam irradiation apparatus comprising: a negative voltage application power source for adjustment . A needle-like cathode;
An extraction electrode for forming an electric field for extracting electrons from the cathode;
An electron source comprising a control electrode disposed between the cathode and the extraction electrode and controlling the amount of electron beam;
The needle-shaped cathode has a tip angle of 8 degrees and a tip radius of curvature of 0.30 μm, and is connected to the control electrode and emits electrons from the cathode in a state where a positive voltage is applied to the extraction electrode. An electron source comprising a negative voltage application power source for adjusting a negative voltage applied so as to control a line current in a range of at least 67 pA to 10 pA or less .

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