Electron microscope and calibration method
An electron microscope includes an electron optical system that irradiates a sample with an electron beam to form an image with electrons transmitted through the sample, a camera that includes an image sensor having sensor pixels and captures frame images that are based on output values output from each of the sensor pixels by incidence of the electrons on the image sensor, and a correction coefficient calculation unit that calculates correction coefficients for correcting sensitivities of the sensor pixels. The correction coefficient calculation unit determines, from the frame images captured under a condition under which the electrons incident on the image sensor follow a Poisson process, a mode value of the output values for each of the sensor pixels, and calculates the correction coefficients based on the mode value determined for each of the sensor pixels.
1 . An electron microscope comprising:
an electron optical system that irradiates a sample with an electron beam to form an image with electrons transmitted through the sample;
a camera that comprises an image sensor having sensor pixels and captures frame images that are based on output values output from each of the sensor pixels by incidence of the electrons on the image sensor; and
a correction coefficient calculation unit that calculates correction coefficients for correcting sensitivities of the sensor pixels,
the correction coefficient calculation unit configured to:
determine, from the frame images captured under a condition under which the electrons incident on the image sensor follow a Poisson process, a mode value of the output values for each of the sensor pixels; and
calculate the correction coefficients based on the mode value determined for each of the sensor pixels,
wherein the condition under which the frame images are captured is established by emitting the electron beam to a vacuum portion where the sample is not present to adjust the electron optical system such that an average of a number of the electrons incident on each of the sensor pixels per frame coincides with a dispersion thereof, thereby establishing the Poisson process.
2 . The electron microscope according to claim 1 , wherein the correction coefficient calculation unit is further configured to:
average the mode values determined for the sensor pixels to calculate an average mode value, and
divide the mode value by the average mode value for each of the sensor pixels to calculate the correction coefficients.
3 . The electron microscope according to claim 1 , wherein
the image sensor is a sensor in which the output values output from each of the sensor pixels by the incidence of the electrons on the image sensor follow a Landau distribution.
4 . The electron microscope according to claim 3 , wherein the camera is a direct detection camera that directly detects the electrons with the image sensor.
5 . The electron microscope according to claim 1 , further comprising:
an electron optical system control unit that controls the electron optical system; and
a camera control unit that controls the camera, wherein
the camera control unit is configured to acquire control information for the electron optical system from the electron optical system control unit, and
the camera control unit is configured to control the camera based on the acquired control information to acquire a dark-time image.
6 . The electron microscope according to claim 5 , wherein
the camera control unit is further configured to:
determine, based on the control information, whether the electrons are incident on the camera; and
control the camera to acquire the dark-time image when having determined that no electron is incident on the camera.
7 . A calibration method for an image sensor in an electron microscope comprising an electron optical system that irradiates a sample with an electron beam to form an image with electrons transmitted through the sample, and a camera that comprises the image sensor having sensor pixels and captures frame images that are based on output values output from each of the sensor pixels by incidence of the electrons on the image sensor, the calibration method comprising:
determining, from the frame images captured under a condition under which the electrons incident on the image sensor follow a Poisson process, a mode value of the output values for each of the sensor pixels; and
calculating correction coefficients for correcting sensitivities of the sensor pixels based on the mode value determined for the sensor pixels,
wherein the condition under which the frame images are captured is established by emitting the electron beam to a vacuum portion where the sample is not present to adjust the electron optical system such that an average of a number of the electrons incident on each of the sensor pixels per frame coincides with a dispersion thereof, thereby establishing the Poisson process.