Scanning electron microscope and method for measuring pattern

1. A scanning electron microscope comprising:

an electron-optical system including an electron source configured to emit primary electrons and an objective lens;

a stage on which a sample is placed;

a secondary electron detector disposed adjacent to the electron source relative to the objective lens and configured to detect secondary electrons emitted by interactions between the primary electrons and the sample;

a backscattered electron detector disposed between the objective lens and the stage and configured to detect backscattered electrons emitted by interactions between the primary electrons and the sample;

a backscattered electron detection system controller provided for the backscattered electron detector and configured to apply a voltage to the backscattered electron detector; and

a device-control computer configured to detect a state of an electrical charge carried by the backscattered electron detector based on signal intensity at the secondary electron detector when the primary electrons are applied to the sample with a predetermined voltage applied to the backscattered electron detector.

2. The scanning electron microscope according to claim 1 , wherein

the device-control computer acquires the signal intensity at the secondary electron detector when applying the primary electrons to the sample while changing the predetermined voltage applied to the backscattered electron detector, and calculates a voltage of the electrical charge carried by the backscattered electron detector based on a relationship between the signal intensity at the secondary electron detector acquired and the voltage applied to the backscattered electron detector.

3. The scanning electron microscope according to claim 2 , wherein

the device-control computer determines, based on the relationship between the signal intensity at the secondary electron detector acquired and the voltage applied to the backscattered electron detector, a voltage applied to the backscattered electron detector when the signal intensity at the secondary electron detector sharply changes, and calculates a voltage that results from positive/negative inversion of the voltage applied to the backscattered electron detector as the voltage of the electrical charge carried by the backscattered electron detector.

4. The scanning electron microscope according to claim 2 , further comprising a display device,

wherein the device-control computer causes the display device to display an error when the voltage of the electrical charge carried by the backscattered electron detector calculated falls out of an allowable range.

5. The scanning electron microscope according to claim 2 , wherein

the device-control computer applies, to the backscattered electron detector, an offset voltage corresponding to the voltage of the electrical charge carried by the backscattered electron detector calculated.

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

a sample table configured to hold the sample placed on the stage; and

a sample table voltage controller configured to apply a voltage to the sample table,

wherein the device-control computer applies, to the sample table, an offset voltage corresponding to the voltage of the electrical charge carried by the backscattered electron detector calculated.

7. The scanning electron microscope according to claim 2 , wherein

the device-control computer applies the primary electrons to the sample with the electron-optical system configured to satisfy a predetermined optical condition, and

the predetermined optical condition includes an accelerating voltage condition under which tertiary electrons are emitted in response to the backscattered electrons impinging on the backscattered electron detector, the backscattered electrons being emitted by interactions between the primary electrons applied to the sample under the predetermined optical condition and the sample, and the tertiary electrons emitted is larger in number than the backscattered electrons impinging on the backscattered electron detector.

8. The scanning electron microscope according to claim 7 , wherein

the device-control computer applies a negative voltage to the backscattered electron detector during a time period over which the primary electrons are applied to the sample with the electron-optical system configured to satisfy the predetermined optical condition.

9. The scanning electron microscope according to claim 7 , wherein

the backscattered electron detection system controller measures a current flowing between the backscattered electron detector and a ground during a time period over which the primary electrons are applied to the sample with the electron-optical system configured to satisfy the predetermined optical condition.

10. The scanning electron microscope according to claim 1 , wherein

the device-control computer acquires the signal intensity at the secondary electron detector when applying the primary electrons to the sample with a predetermined reference voltage applied to the backscattered electron detector, and compares the signal intensity at the secondary electron detector acquired to a threshold voltage to determine whether the backscattered electron detector takes on an electrical charge.

11. The scanning electron microscope according to claim 1 , wherein

the device-control computer applies the primary electrons to the sample with the predetermined voltage applied to the backscattered electron detector, acquires an image based on the secondary electrons detected by the secondary electron detector, and takes an average grayscale value of the image as the signal intensity at the secondary electron detector.

12. The scanning electron microscope according to claim 1 , wherein the backscattered electron detector includes an annular scintillator.