Scanning electron microscope

1. A scanning electron microscope comprising:

an electron source that produces an electron beam;

a trajectory dispersion unit that disperses a trajectory of the electron beam, which includes electrons with different energy values;

a selection slit plate having a selection slit that selects an energy range of the dispersed electron beam;

a deflection scanning unit that deflects and scans the electron beam over the selection slit plate; and

a transmittance monitoring unit that monitors a transmittance of the electron beam, the electron beam being transmitted through the selection slit;

wherein the trajectory dispersion unit is configured of a set of a magnetic deflector and an electrostatic deflector stacked on each other;

wherein, based on the electron beam, the transmittance monitoring unit measures one of a first electron signal transmitted through the selection slit, a transmitted current transmitted through the selection slit, a second electron signal reflected off the selection slit plate, and a slit current carried through the selection slit plate to obtain a measured value;

wherein, when a variation of the measured value exceeds a reference value, the transmittance monitoring unit measures the transmittance of the electron beam at the selection slit; and

wherein the transmittance monitoring unit determines a variation of the transmittance, and when the transmittance monitoring unit determines that the variation of the transmittance exceeds a reference value, the deflection scanning unit deflects and scans the electron beam over the selection slit.

2. The scanning electron microscope according to claim 1 , further comprising:

a second set of a second magnetic deflector and a second electrostatic deflector stacked on each other is provided,

wherein the set and the second set are disposed as sandwiching the selection slit.

3. The scanning electron microscope according to claim 1 ,

wherein the transmittance monitoring unit includes

a first measurement unit that measures the first electron signal or the transmitted current transmitted through the selection slit and outputs a first measured signal,

a second measurement unit that measures the second electron signal reflected off the selection slit plate or the slit current carried through the selection slit plate and outputs a second measured signal,

a first calculation circuit programmed to calculate a ratio between the first and second measured signals from the first and second measurement units, and

a second calculation circuit programmed to calculate a variation of the ratio.

4. The scanning electron microscope according to claim 1 ,

wherein the selection slit plate has a transmission opening near the selection slit, the transmission opening being larger than a width of the electron beam; and

the transmittance monitoring unit includes

a measurement unit that measures the measured value as a first measured value of the first electron signal or the transmitted current transmitted through the selection slit and a second measured value of the second electron signal or the slit current transmitted through the transmission opening,

a first calculation circuit programmed to calculate a ratio of the first measured value to the second measured value, and

a second calculation circuit programmed to calculate a variation of the ratio.

5. The scanning electron microscope according to claim 1 ,

wherein the deflection scanning unit is a deflector that deflects the electron beam or a slit moving mechanism that moves the selection slit plate.

6. The scanning electron microscope according to claim 1 ,

wherein the trajectory dispersion unit controls a width of the electron beam in a longitudinal direction of the selection slit so that the width of the electron beam is wider than when the electron beam is focused on the selection slit.

7. A scanning electron microscope comprising:

an electron source that produces an electron beam;

a trajectory dispersion unit that disperses a trajectory of the electron beam of electrons with a different energy value;

a selection slit plate having a selection slit that selects an energy range of the dispersed electron beam;

a transmittance monitoring unit that monitors a transmittance of the electron beam, the electron beam being transmitted through the selection slit; and

a deflection scanning unit that deflects and scans the electron beam over the selection slit plate;

wherein the transmittance monitoring unit determines a variation of the transmittance, and when the transmittance monitoring unit determines that the variation of the transmittance exceeds a reference value, the deflection scanning unit deflects and scans the electron beam over the selection slit;

wherein the deflection scanning unit is a deflector that deflects the electron beam or a slit moving mechanism that moves the selection slit plate;

wherein: in the deflection and scanning, the transmittance monitoring unit determines whether an accumulation value for correcting the electron beam using the deflector is a second reference value or more;

when the accumulation value is less than the second reference value, the deflector adjusts a position of the electron beam; and

when the accumulation value is the second reference value or more, the slit moving mechanism adjusts a position of the selection slit.

8. A scanning electron microscope comprising:

an electron source that produces an electron beam;

a trajectory dispersion unit that disperses a trajectory of the electron beam, which includes electrons with different energy values;

a selection slit plate having a selection slit that selects an energy range of the dispersed electron beam;

an objective lens that applies the electron beam to a sample;

a stage on which the sample is placed; and

a transmittance monitoring unit that monitors an electron signal or a transmitted current transmitted through the selection slit or an electron signal reflected off the selection slit plate or a slit current carried through the selection slit plate to obtain a measured value,

wherein, when a variation of the measured value is greater than a reference value, a focus on the sample is corrected by changing a voltage applied to the stage of the scanning electron microscope; and

wherein the trajectory dispersion unit is configured of a set of a magnetic deflector and an electrostatic deflector stacked on each other.