JPH0156501B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for adjusting a single crystal lanthanum hexaboride (LaB ₆ ) electron gun according to optimum operating conditions.

Single-crystal LaB ₆ electron guns have recently come into use in electron beam equipment because of their high brightness and long lifetime.
However, the optimal conditions for using the LaB ₆ electron gun are not very clear. The operating temperature is 1400℃ or higher, which exposes a clean surface, and the evaporation rate is not very high.
It has been reported that a temperature below 1600°C is optimal.
Further, the present inventors conducted a comprehensive characteristic evaluation of the LaB ₆ electron gun and found that there is a unique optimal bias voltage for scanning electron microscopes and electron beam exposure systems that use crossover images.

An object of the present invention is to provide a method for adjusting a single-crystal LaB ₆ electron gun of an electron beam apparatus using a crossover image as a probe or critical illumination using optimal bias voltage conditions.

The details of the present invention will be explained below with reference to the drawings.

Figure 1 shows a Vogel type electron gun. 1
01 is LaB ₆ chip, 102 is heater, 103
is a holder for LaB ₆ chip 101 and heater 102,
104 is an insulator that mounts the LaB ₆ chip, heater, and holder. 105 is Wehnelt, 10
6 is an anode. Figure 2 shows a diagram of the LaB ₆ chip. The LaB ₆ chip is typically a four-sided pyramid with one end conical and the tip of the cone shaped into a spherical surface. The characteristics of the electron beam obtained from an electron gun with such a structure are as follows: chip temperature T, Wehnelt 105
Bias voltage V across chip 101, Wehnelt 1
05 Distance L between chips 101, Wehnelt hole diameter
Wd, distance S between Wehnelt 105 anodes 106,
It depends on the radius of curvature rt of the tip of the chip (see Figure 2), the half-apex angle θ formed by the cone at the tip of the chip, and the crystal orientation of the LaB ₆ chip axis <a, b, c>. Among these, S,
It is already known that changes in Wd affect the brightness and crossover system. The inventors are V,
The characteristics of the electron beam were investigated using T, rt, θ, L, and the crystal orientation of the chip as parameters. First S=10
mm, Wd=1.5mm, rt=15μm, θ=45°, L=0.2mm
The relationships between the brightness B, crossover shape, electron beam intensity angular distribution pattern (emission), and bias potential when the chip axis orientation is <100> are shown in Figure 3 A, B, C, and C under the following conditions, respectively. Shown below. Here, T ₁ = 1400℃, T ₂ = 1500℃, T ₃ = 1600
It is ℃. Maximum brightness Bmax as shown in Figure 3A
is given at a constant bias voltage V ₃ regardless of temperature (1400°C to 1600°C). At voltages higher than the bus voltage V ₄ , the brightness remains at a certain value regardless of the temperature T.
Converges to Bconst. FIGS. 3B and 3C show crossover images and emission patterns corresponding to bias voltages V ₁ , V ₂ , V ₃ , and V ₄ . When V ₁ V<V ₃ , the crossover image and the emission pattern are 4-fold symmetrical patterns reflecting the crystal plane anisotropy of the work function. When V ₃ V<V ₄ , the current intensity distribution in the crossover image is a Gaussian distribution, but the emission pattern is a 4-fold symmetrical pattern. At V ₄ V, both have a Gaussian distribution. The noteworthy point is V
At V ₃ , the current intensity of the crossover image becomes a Gaussian distribution and reaches its maximum brightness. According to experiments conducted by the inventors, this relationship is based on the radius of curvature of the tip of the chip.
rt, the half-apex angle θ formed by the cone at the chip tip, the crystal orientation of the LaB ₆ chip, and the distance L between the chip tip and Wehnelt. The crystal orientation of the LaB ₆ chip is related only to the shape of the crossover image and the emission pattern, and rt and θ are mainly related to the absolute value of the bias voltage and the strength of the anisotropy of the crossover image and the emission pattern. L was related only to the absolute value of the bias voltage. In conclusion, the relationship shown in Figure 3 was generally established for single-crystal LaB ₆ electron guns.

FIG. 4A shows an optical system of an electron beam exposure apparatus using a crossover reduced image used in the present invention.
701 is an electron gun, 702 is an anode, 703 is a first crossover, 704 is a first condenser lens, 705 is a first condenser lens aperture, 706 is a second crossover, 707 is a blanking plate, 708 is a second condenser lens,
709 is the second condenser lens perch, 7
10 is the third crossover, 711 is the deflection system 71
2 is an objective lens, 813 is an objective lens aperture, 714 is a fourth crossover, and 715 is a sample surface. In order to perform accurate drawing, the current intensity distribution of the crossover image 714 needs to have a Gaussian distribution, that is, a shape close to a circle, as shown in FIG. 3B. (That is, the current intensity distribution of the first crossover is a Gaussian distribution). The current intensity distribution of the emission pattern is also V=V ₄ in Figure 3C.
It is desirable that the emission pattern be _a Gaussian distribution as given by It must be of a shape that shows When V=V ₄ , as shown in FIG. 4B, even if the optical path of the electron beam changes from 721 to 722, the current flowing into the second crossover 706 does not change. On the other hand, if the emission pattern has anisotropy, as shown in FIG. 4C, when the electron beam optical path changes from 723 (solid line) to 724 (dotted line), the current flowing into the second crossover 706 changes. I end up. It is said that anisotropy in the emission pattern results in poor current stability, and it has conventionally been used in bias regions where anisotropy can be ignored. However, according to experiments conducted by the inventors, even when the emission pattern has anisotropy, the degree of vacuum in the space from the electron gun 702 to the objective lens 712 is approximately 1×10 ^-6 as shown in FIG. 4A. Torr
It turns out that the better it is, the more stable it will be. 1×
When the intensity was worse than 10 ^-6 Torr, the contamination inside the mirror became noticeable over time, and the stability of the beam current worsened. Detection of the crossover image and the shape of the emission pattern will be described later.

It has also been found that when the emission pattern has anisotropy, the optimal method for automatic axis alignment is a feedback circuit that maximizes the current passing through the aperture. FIG. 5 shows one example. Here, 801 is an m-th (m=1, 2, . . . ) crossover, 802 is an alignment coil, 803 is a lens, and 804 is an aperture of the lens 803, which also has a function of detecting the electron beam current. 805 is the (m+1)th crossover,
806 is an amplifier, 807 is an analog-to-digital converter, 808 is an amplifier, 809 is a digital-to-analog converter, and 810 is an interface that connects the computer 811 with these circuits. First, the alignment coil 802 is operated to two-dimensionally scan the electron beam over the aperture 804.
4. Find the alignment coil excitation condition that minimizes the current flowing through 4, and use this as the initial setting value. If necessary, store the minimum cut-off current flowing through the aperture 804 in the computer, and
The alignment coil 802 may be controlled so that the current flowing through 04 is always equal to this initial minimum current. Although the detection electrode and the lens aperture are used together, they may be used independently. Also, a feedback circuit that does not use a computer may be provided.

Such conditions for using single crystal LaB ₆ and alignment method are also effective for electron beam equipment using critical illumination. FIG. 6 shows an electron beam exposure apparatus using critical illumination. 901 is an electron gun, 902
is an anode, 903 is a first crossover, 904 is a crossover magnifying lens, 905 is an anti-scattering aperture, 906 is a blanking electrode, 907
is a condenser lens, 908 is an electron beam shaping aperture, 909 is a second crossover imaged on the electron beam shaping aperture, 91
0 is a deflection system, 911 is an objective lens, 912 is an anti-scattering aperture, 913 is a sample surface, and 914 is a projected image of an electron beam shaping aperture, that is, a shaping image. In such an electron beam device, the shape of the crossover must be circular and axially symmetrical. If the crossover has anisotropy, the electron beam shaping aperture 908 cannot be uniformly illuminated.
As a result, the intensity distribution of the electron beam 914 used for drawing becomes uneven, making accurate drawing impossible. The emission pattern is also preferably circular, but may be anisotropic under the same conditions as a crossover reduction type device.

If the electron gun is operated at the bias voltage _V3 shown in FIG. 3A, high brightness can be obtained at the same chip temperature, and the crossover image is circular and the emission pattern is anisotropic. In some cases, when using the device under conditions lower than the maximum brightness, if the bias voltage is set in a region higher than V ₃ and lower than V ₄ , the emission pattern will have anisotropy, but the crossover image will be circular. That is, V ₃ V
This means that the electron gun can be operated under _V4 conditions.

FIG. 7 shows a configuration diagram of an electron beam exposure apparatus in which the present invention is applied to the optical system shown in FIG. 4 so that the optimum bias voltage can be automatically set. This electron beam exposure apparatus is an apparatus using a cross-over reduced image similar to the one shown in FIG. 4A. 1001
is an electron gun, 1002 is an anode, 1003 is a first capacitor alignment coil, 1004 is a first capacitor, 1005 is an anti-scattering and electron beam detection aperture, 1006 is a second capacitor alignment coil, 1007 is a blanking electrode, 10
08 is a second condenser, 1009 is an anti-scattering and electron beam detection aperture, 1010 is an objective lens alignment coil, 1011 is a deflection electrode, 1
012 is an objective lens, 1013 is an objective lens aperture, 1014 is a Faraday cup, 101
5 is a knife edge, and 1016 is a backscattered electron detector. Next, the control system will be explained. 1017 and 1026 are amplifiers and D-A converters for operating the first capacitor alignment coil; 1018 and 1;
027 is an amplifier and an A-D converter for detecting the current flowing through the detection aperture 1005; 1019
and 1028 are amplifiers and D-A converters for operating the second condenser lens alignment coil, 10
20 and 1029 are amplifiers and A-D converters for detecting the current flowing through the detection aperture; 1021
and 1030 are amplifiers and D-A converters for operating the objective lens alignment coil, 1022, 102
3, 1031 are amplifiers 1022 and 1023 and a DA converter 102 for applying scanning waves to the deflection electrodes;
4 and 1032 are amplifiers and A-D converters for detecting reflected signals, and 1025 and 1033 are amplifiers and A-D converters for detecting the current flowing into the Faraday cup.
It is a D converter. 1035 is a bias resistance control mechanism for setting a predetermined bias resistance R, and a desired bias resistance can be set by a command from a computer 1041. 1037 is a high voltage circuit, 103
8 is a detection mechanism having a function of detecting the load current It flowing through the high voltage circuit and sending a digital signal to the computer. Bias voltage V is given by V=I _p R. The brightness B is the beam diameter d and the beam current
I _P is given by B=4I _p /(παd) ² (1), where α is the convergence half-vertex angle of the electron beam. I _P is detected by a Faraday cup, and d is observed from the rise of the reflected electron signal when the electron beam crosses the knife edge due to deflection. In addition, in order to measure I _P , each axis alignment coil 100 is
3,1006,1010 are operated. B is sampled as a function of V, and the values of V ₃ and V ₄ are determined.

Here, the shape of the crossover image can be detected using, for example, a Faraday cup 1014 and a knife edge 1015 shown in FIG. In other words, the electron beam is
By using a Faraday cup to detect the amount of current of the electron beam obtained when the electron beam is deflected and scanned so as to cut at 015, it can be determined whether the shape of the crossover image is circular. If the crossover image is not circular, an amplifier 10 is used to detect the current flowing through the Faraday cup.
25, the A/D converter 1033, and a computer to adjust the bias voltage so that the crossover image becomes circular.

Further, regarding the detection of the emission pattern image, for example, from the beam current obtained when the electron beam is deflected to cross the beam detection aperture 1005, in almost the same manner as in the case of the crossover image described above, Its shape can be detected. The shape of the emission pattern image may be adjusted to exhibit anisotropy.

The quantities to be controlled in an electron beam exposure apparatus are the beam diameter d and the beam current I _P. By the way, in a region where equation (1) holds, B and I _P may be controlled instead of d and I _P. In an electron beam exposure apparatus, B=Bmax is not necessarily used. Depending on resist sensitivity conditions, B<Bmax may be used. In this case, it is sufficient to select V that provides the necessary hole within the range of V ₃ <V.

Bias voltage V is an increasing function of bias resistance R. Therefore, the electron gun may be controlled by determining the relationship between the brightness B and the bias resistor R as shown in FIG. 8, and manipulating the bias resistor R. In Figure 8, R ₁ , R ₂ , R ₃ ,
R ₄ corresponds to bias voltages V ₁ , V ₂ , V ₃ , and V ₄ , respectively. However, R ₁ , R ₂ , R ₃ , and R ₄ change depending on the chip temperature, so they must be recalculated each time the chip temperature is changed.

As mentioned above for single-crystal LaB ₆ , any electron gun that emits an electron beam from a chip with the structure shown in Figure 2 and made of single-crystal material will exhibit the electron beam characteristics as shown in Figure 3. . Therefore, tantalum carbide TaC, titanium carbide TiC, and zirconium carbide are effective for electron guns.
ZrC, hafnium carbide HfC, lanthanum 1
Single crystals such as La _0.3 Nb _0.7 B ₆ in which parts are replaced with neodymium are also effective in the present invention.

The above was an example of automating the optimal bias voltage in an electron beam exposure system using a crossover reduced image as shown in FIG. 4A, but an electron beam exposure system using critical illumination as shown in FIG. In other words, a cross-over shaped image projection type electron beam exposure apparatus may be used to automatically set the optimum bias voltage as shown in FIG.

In this case, the aperture image can be measured using the beam current from the Faraday cup as in FIG.
The emission pattern image is aperture 904 or 9
The beam current from 12 can be measured.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing an example of the configuration of a Vogel type LaB ₆ electron gun, Figure 2 is a perspective view showing the configuration of a LaB ₆ chip, and A in Figure 3 is obtained from the chip of the electron gun in Figure 1. B is an explanatory diagram showing the relationship between the crossover image and bias voltage; C is a characteristic diagram showing the relationship between the emission pattern and bias voltage; FIG. 4A is a configuration diagram showing the optical system of an electron beam exposure apparatus that uses a reduced image of a crossover image, B and C are explanatory diagrams showing electron beam shaping, and Figure 5 is a diagram showing the case where the emission pattern has anisotropy. A block diagram showing the alignment method, Figure 6 is a configuration diagram showing the optical system of an electron beam exposure apparatus using critical illumination,
FIG. 7 is a layout diagram showing an embodiment of an electron beam device according to the present invention, and FIG. 8 is a characteristic diagram showing the relationship between bias resistance and brightness.

Claims (1)

[Claims] 1. A cathode made of a single crystal chip with a tapered tip;
In an electron gun adjustment method in which an electron beam crossover image or a shaped image thereof formed by an electron gun equipped with a Wehnelt electrode and an anode is focused on a sample surface through an electron lens, the temperature of the cathode is adjusted.
The temperature is set at 1400 to 1600°C, and the shape of the crossover image or its shaped image and the emission pattern image of the electron beam is detected, and based on the detection results, the crossover of the electron beam is close to a circle, A method for adjusting an electron gun, comprising setting a bias voltage between the cathode and the Wehnelt electrode so that the emission pattern image exhibits an anisotropic shape. 2. The method for adjusting an electron gun according to claim 1, wherein the single crystal chip is composed of a lanthanum hexaboride single crystal.