U.S. patent application number 17/039620 was filed with the patent office on 2022-03-31 for method and system for high speed signal processing.
This patent application is currently assigned to FEI Company. The applicant listed for this patent is FEI Company. Invention is credited to Rob BRAAN, Bart Jozef JANSSEN, Jeroen KEIZER, Henricus Gerardus ROEVEN.
Application Number | 20220103771 17/039620 |
Document ID | / |
Family ID | |
Filed Date | 2022-03-31 |
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United States Patent
Application |
20220103771 |
Kind Code |
A1 |
ROEVEN; Henricus Gerardus ;
et al. |
March 31, 2022 |
METHOD AND SYSTEM FOR HIGH SPEED SIGNAL PROCESSING
Abstract
A method and system for acquiring data from a pixelated image
sensor for detecting charged particles. The method includes reading
a pixel voltage of one or more of the multiple pixels multiple
times without resetting the image sensor and digitizing the pixel
into a first number of bits. The camera outputs a digitized
compressed pixel voltage in a second, less, number of bits. The
maximum range of the digitized compressed pixel voltage is less
than a maximum range of the pixel voltage.
Inventors: |
ROEVEN; Henricus Gerardus;
(Eindhoven, NL) ; BRAAN; Rob; (Eindhoven, NL)
; JANSSEN; Bart Jozef; (Eindhoven, NL) ; KEIZER;
Jeroen; (Eindhoven, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FEI Company |
Hillsboro |
OR |
US |
|
|
Assignee: |
FEI Company
Hillsboro
OR
|
Appl. No.: |
17/039620 |
Filed: |
September 30, 2020 |
International
Class: |
H04N 5/378 20060101
H04N005/378; H04N 5/363 20060101 H04N005/363; H04N 5/359 20060101
H04N005/359 |
Claims
1. A method for acquiring data from a camera including a pixelated
image sensor for detecting charged particles, comprising: obtaining
multiple pixel voltages of a pixel of the image sensor by reading
one or more pixels of the image sensor multiple times without
resetting the image sensor; digitizing each of the multiple pixel
voltages into a first number of bits; and compressing each of the
digitized multiple pixel voltages into a digitized compressed pixel
voltage in a second, lower, number of bits, wherein a maximum range
of the digitized compressed pixel voltage is less than a maximum
range of the pixel voltage, and wherein the digitized compressed
pixel voltage is generated by removing at least a most significant
bit (MSB) of the digitized pixel voltage.
2. The method of claim 1, wherein the digitized pixel voltage is
unsigned.
3. The method of claim 1, further comprising: for each pixel of the
one or more pixels of the image sensor, sequentially receiving a
first digitized compressed pixel voltage and a second digitized
compressed pixel voltage; determining a differential compressed
pixel voltage by calculating a difference between the first
digitized compressed pixel voltage and the second digitized
compressed pixel voltage; and generating a differential pixel
voltage by adjusting the differential compressed pixel voltage to a
valid range, wherein the valid range is determined based on a
predetermined noise offset and the maximum range of the digitized
compressed pixel voltage.
4. The method of claim 3, wherein the valid range is from the noise
offset to a sum of a threshold voltage and the noise offset, and
the threshold voltage is determined based on the maximum range of
the digitized compressed pixel voltage.
5. The method of claim 4, wherein adjusting the differential
compressed pixel voltage to the valid range includes adding the
threshold voltage to the differential compressed pixel voltage
responsive to the differential compressed pixel voltage lower than
the noise offset, and subtracting the threshold voltage from the
differential compressed pixel voltage responsive to the
differential compressed pixel voltage greater than the sum of the
threshold voltage and the noise offset.
6. The method of claim 1, wherein the second number of bits is
determined based on a maximum range of change in the pixel value
between sequential readouts.
7. The method of claim 1, wherein the digitized compressed pixel
voltage has a same signal precision as the digitized pixel
voltage.
8. The method of claim 1, wherein the digitized compressed pixel
voltage is generated by further removing one or more bits from a
least significant bit side of the digitized pixel voltage.
9. The method of claim 1, further comprising detecting image sensor
overexposure based on the digitized compressed pixel voltage of the
one or more pixels of the image sensor.
10. A method for acquiring data from a camera including a pixelated
image sensor for detecting charged particles, comprising:
repetitively reading the image sensor without resetting the image
sensor to obtain multiple pixel voltages of a pixel of the image
sensor; compressing each of the multiple pixel voltages into a
compressed pixel voltage, wherein the compressed pixel voltage is a
difference between the pixel voltage and a first threshold voltage
responsive to an amplitude of the pixel voltage not less than an
amplitude the first threshold voltage and less than an amplitude of
a second threshold voltage, and wherein a range of the compressed
pixel voltage is not greater than the amplitude of the first
threshold voltage, and the range of the compressed pixel voltage is
lower than a range of the pixel voltage; digitizing the compressed
pixel voltage; and outputting the digitized compressed pixel
voltage.
11. The method of claim 10, wherein compressing each of the
multiple pixel voltages further includes subtracting the second
threshold voltage from a particular pixel voltage of the multiple
pixel voltages responsive to the amplitude of the pixel voltage not
less than the amplitude of the second threshold voltage and less
than an amplitude of a third threshold voltage.
12. The method of claim 10, wherein the second threshold voltage is
two times of the first threshold voltage.
13. The method of claim 10, further comprising resetting the image
sensor after reading the image sensor a predetermined number of
times.
14. The method of claim 10, further comprising resetting the image
sensor in response to the amplitude of the pixel voltage greater
than a maximum pixel voltage amplitude.
15. The method of claim 10, wherein the pixel voltage between
adjacent sensor resets is a monotonic signal superimposed with a
noise signal.
16. The method of claim 10, further comprising: sequentially
receiving a first digitized compressed pixel voltage and a second
digitized compressed pixel voltage from the camera; dark correcting
the first digitized compressed pixel voltage and the second
digitized compressed pixel voltage; determining a differential
compressed pixel voltage by calculating a difference between the
dark-corrected first digitized compressed pixel voltage and the
dark-corrected second digitized compressed pixel voltage; and
generating a differential pixel voltage by adjusting the
differential compressed pixel voltage into a valid range, the valid
range determined based on a predetermined noise offset and the
first threshold voltage.
17. The method of claim 16, wherein the valid range is from the
noise offset to a sum of the first threshold voltage amplitude and
the noise offset.
18. A system for acquiring data from a sample, comprising: a
charged particle source for irradiating charged particles towards
the sample; a camera for detecting charged particles emitted from
the sample responsive to the irradiation, the camera includes an
image sensor with multiple pixels and one or more analog-to-digital
converters (ADCs), wherein the camera is configured to: convert
charged particles impinging a pixel of the multiple pixels into a
pixel voltage; compress the pixel voltage into a compressed pixel
voltage, wherein the compressed pixel voltage is a difference
between the pixel voltage and a first threshold voltage if an
amplitude of the pixel voltage is not less than an amplitude of the
first threshold voltage and less than an amplitude of a second
threshold voltage, and wherein a range of the compressed pixel
voltage is not greater than the amplitude of the first threshold
voltage, and the range of the compressed pixel voltage is lower
than a range of the pixel voltage; digitize the compressed pixel
voltage; and output the digitized compressed pixel voltage; an
image processor for receiving the digitized compressed pixel
voltage from the camera and generating a differential pixel voltage
based on the digitized compressed pixel voltage; and a controller
for forming an image of the sample based on the differential pixel
voltage.
19. The system of claim 18, wherein receiving the digitized
compressed pixel voltage from the camera and generating the
differential pixel voltage based on the compressed pixel voltage
includes: sequentially receiving a first digitized compressed pixel
voltage and a second digitized compressed pixel voltage;
determining a differential compressed pixel voltage by subtracting
the first digitized compressed pixel voltage from the second
digitized compressed pixel voltage; and generating the differential
pixel voltage by adjusting the differential compressed pixel
voltage into a valid range, the valid range determined based on a
predetermined noise offset and the first threshold voltage.
20. The system of claim 19, wherein the digitized compressed pixel
voltage and the differential pixel voltage have a same precision.
Description
INCORPORATION BY REFERENCE
[0001] This application relates to U.S. application Ser. No.
13/645,725 filed on Oct. 5, 2012, titled "Method for acquiring data
with an image sensor", by Janssen et al., which is incorporated
herein by reference in its entirety and for all purposes.
FIELD OF THE INVENTION
[0002] The present description relates generally to methods and
systems for data acquisition using a camera, and more particularly,
to high speed camera readout and real-time signal processing of the
readout data.
BACKGROUND OF THE INVENTION
[0003] A charged particle microscopy system may include a camera
for detecting charged particles emitted from a sample, digitizing
the detected raw signal, and outputting the digitized signal to an
image processor for real-time signal processing. In order to
increase the data acquisition speed, the microscopy system requires
a short sensor response time, high speed sensor data readout, and
real-time data processing and data storage. For cameras with
pixelated image sensor, the pixels may need to be reset when the
pixel voltage exceeding a predetermined level. One method to
increase the readout speed of such pixelated image sensor is
multi-frame correlative double sampling (mfCDS), disclosed in U.S.
application Ser. No. 13/645,725 by Janssen et al, filed on Oct. 5,
2012, titled "Method for acquiring data with an image sensor". In
mfCDS, multiple frames of raw data are readout from the image
sensor before resetting the image sensor or a particular pixel of
the image sensor. Particle counting can then be determined based on
the difference of sequentially acquired pixel voltages. However,
Applicant recognizes that the limited bandwidth within the camera
and/or between the camera and the image processor may become the
bottleneck for high speed signal processing.
SUMMARY
[0004] In one embodiment, a method for acquiring data from a camera
including a pixelated image sensor for detecting charged particles
comprises reading a pixel voltage of one or more pixels of the
image sensor multiple times without resetting the image sensor;
digitizing the pixel voltage into a first number of bits; and
outputting a digitized compressed pixel voltage in a second, lower,
number of bits, wherein a maximum range of the digitized compressed
pixel voltage is less than a maximum range of the pixel voltage,
and wherein the digitized compressed pixel voltage is generated by
removing at least a most significant bit (MSB) of the digitized
pixel voltage. In this way, pixel voltage at each pixel of the
image sensor may be readout and transferred from the camera to the
image processor in a lower number of bits without sacrificing data
quality or precision. High speed signal readout and processing may
be achieved with limited bandwidth within the camera and/or between
the camera and the image processor.
[0005] It should be understood that the summary above is provided
to introduce in simplified form a selection of concepts that are
further described in the detailed description. It is not meant to
identify key or essential features of the claimed subject matter,
the scope of which is defined uniquely by the claims that follow
the detailed description. Furthermore, the claimed subject matter
is not limited to implementations that solve any disadvantages
noted above or in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 shows a charged particle microscope.
[0007] FIG. 2 is a flow chart of a method for acquiring data from a
camera of the charged particle microscope of FIG. 1.
[0008] FIG. 3 illustrates the data flow of the method in FIG.
1.
[0009] FIG. 4A illustrates a method for compressing the pixel
voltage read from an image sensor.
[0010] FIG. 4B illustrates an example for implementing the pixel
voltage compression.
[0011] FIG. 4C illustrates another example for implementing the
pixel voltage compression.
[0012] FIG. 5 is a flow chart of a method for detecting sensor
overexposure.
[0013] FIG. 6A and FIB. 6B illustrate a method for adjusting a
pixel value of a differential compressed frame to a valid
range.
[0014] Like reference numerals refer to corresponding parts
throughout the several views of the drawings.
DETAILED DESCRIPTION OF EMBODIMENTS
[0015] The following description relates to systems and methods for
data acquisition and data processing in a microscopy system, such
as a charged particle microscope of FIG. 1. The charged particle
microscope may include a source for generating charged particles.
Responsive to irradiating a sample with the charged particles,
various types of charged particles emitted from the sample are
detected by different cameras or detectors.
[0016] The camera or detector may include a pixelated image sensor
for converting charged particles impinging a pixel of the image
sensor into the pixel voltage. The pixel voltage may be read out
from the image sensor using the mfCDS method. In particular, the
pixel voltage of a particular pixel is read out multiple times
before resetting the pixel voltage to a reset value. The number of
charged particles detected by the pixel may be determined based on
the difference between sequential readouts of the pixel voltage.
Using mfCDS, multiple frames can be readout before resetting the
image sensor. The resetting noise is reduced by calculating the
difference between the sequential readouts. Further, the maximum
frame rate of image sensor readout may be increased by reducing the
frequency of resetting the pixel voltage.
[0017] The pixel voltage read from the image sensor may be
digitized by an analog to digital converter (ADC) and transferred
from the camera to an image processor for further processing.
Though high frame rate readout from the image sensor can be
achieved using the mfCDS method, the data transfer rate (i.e.
bandwidth) between the ADC and the readout electronics within the
camera and/or the data transfer rate between the camera and the
image processor may limit the overall data acquisition rate and
data quality of microscopy system. In order to increase the frame
rate of data received at the image processor, the bit depth (i.e.
the number of bits) of the digitized pixel voltage for each pixel
has to be reduced. However, reducing the bit depth may reduce data
precision. Herein, precision of a digital signal is the value
represented by the least significant bit (LSB) of the digitized
signal.
[0018] In order to address the above issue, a method for high speed
camera readout and real-time processing of the data received from
the camera is presented in FIG. 2. The dataflow is illustrated in
FIG. 3. Specifically, the pixel voltage of the image sensor is
readout multiple times before sensor reset. The pixel voltage
readout is compressed before being transferred from the camera to
the image processor. The image processor receives the compressed
pixel voltage from the camera, and generates a differential voltage
between sequentially received compressed pixel voltages. A sample
image may be formed based on the differential voltage. Between
adjacent image sensor resets, the pixel voltage is approximately
monotonic. That is, between sensor resets, the pixel voltage of
each pixel is a monotonic signal superimposed with a noise. The
noise amplitude is within 1% of the maximum amplitude of the pixel
voltage. The sensor is reset before the pixel voltage amplitude
exceeding the maximum pixel voltage amplitude. Because of the
approximately monotonical change of pixel voltage between sensor
resets and the change of the pixel voltage between sequential (or
adjacent) image sensor readouts is within a threshold level, no
information is lost even though the compressed pixel voltage is
transferred between the camera and the image processor.
[0019] The maximum range of the compressed pixel voltage is less
than the maximum range of the pixel voltage. The pixel voltage
amplitude may be zero to the maximum pixel voltage amplitude. The
pixel voltage may be compressed by subtracting a first threshold
voltage from the pixel voltage responsive to the amplitude of the
pixel voltage not less than the amplitude of the first threshold
voltage and less than the amplitude of a second threshold voltage.
The first threshold voltage may be determined based on the amount
of change in the pixel voltage between adjacent pixel readouts. For
example, the amplitude of the first threshold voltage is greater
than the amount of change of the pixel value between adjacent pixel
readouts. To further reduce the maximum range of the compressed
pixel voltage, the pixel voltage may be further compressed by
subtracting the second threshold voltage from the pixel voltage
responsive to the amplitude of pixel voltage not less than the
amplitude of the second threshold voltage and less than the
amplitude of a third threshold voltage, and subtracting the third
threshold voltage from the pixel voltage responsive to the pixel
voltage not less than the amplitude of the third threshold voltage
and less than the amplitude of a fourth threshold voltage. The
first to fourth threshold voltages may be an analog voltage with
the unit of volt. In one example, the pixel voltage of the image
sensor increases responsive to charged particle impinging the
pixel, and the threshold voltages are positive. In another example,
the pixel voltage of the image sensor decreases responsive to
charged particle impinging the pixel, and the pixel voltages are
converted to approximately monotonically increased positive
voltages before subtracting the positive threshold voltages. In yet
another example, the pixel voltage of the image sensor decreases
responsive to charged particle impinging the pixel, and the
threshold voltages are negative. The compressed pixel voltage may
be digitized into a lower number of bits than the pixel voltage
readout from the image sensor. In this way, the pixel voltage is
compressed or wrapped to a reduced range comparing to the range of
the uncompressed pixel voltage. The range of the compressed voltage
is not greater than the amplitude of the first threshold
voltage.
[0020] In another example, the pixel voltage readout from the image
sensor is digitized into a first number of bits. The digitized
pixel voltage is compressed to a digitized compressed pixel voltage
having a second, lower, number of bits. The digitized pixel voltage
and the digitized compressed pixel voltage have the same precision.
The digitized pixel voltage may be unsigned. In one example, the
digitized pixel voltage may be converted to be unsigned if the
pixel voltage decreases approximately monotonically and is
negative. The compression processes of subtracting threshold
voltages from the pixel voltage may be implemented by removing one
or more bits from the digitized pixel voltage. In one example, the
digitized pixel voltage may be compressed by removing at least the
MSB of the digitized pixel voltage. In another example, the
digitized pixel voltage may be compressed by preserving a first bit
to a second bit, and removing the rest bits, of the digitized first
pixel voltage, wherein neither the first bit nor the second bit is
the MSB. In one example, neither the first bit nor the second bit
is the least significant bit (LSB). FIGS. 4A-4C illustrate example
methods for compressing the pixel value.
[0021] The camera continuously and repetitively reads out pixel
voltages from each pixel in a region of the image sensor (i.e., a
frame of pixel voltages) and sends the compressed pixel voltages or
digitized compressed pixel voltages (i.e., compressed frame) to the
image processor. Differential frames are reconstructed based on the
difference of sequentially received compressed frames. Sample image
may then be generated based on the differential frame. In one
example, for each pixel, a differential compressed pixel voltage is
the difference between a first compressed pixel voltage and a
second compressed pixel voltage. The first compressed pixel voltage
corresponds to the pixel voltage readout at a first time point, and
the second compressed pixel voltage corresponds to the pixel
voltage readout at a second time point, immediately after the first
time point. There is no reset of the pixel or the image sensor
between the first and second time points. As shown in FIGS. 6A-6B,
the differential pixel voltage is reconstructed by adjusting the
differential compressed pixel voltage to a valid range. The valid
range is determined based on the first threshold voltage for
compressing the pixel voltage and a predetermined noise amplitude.
For example, the valid range is from a noise offset to a sum of the
first threshold voltage amplitude and the noise offset. The noise
offset is determined based on the noise amplitude, and may be
negative or zero. Adjusting the differential compressed pixel
voltage into the valid range includes adding the first threshold
voltage amplitude to the differential compressed pixel voltage
responsive to the differential compressed pixel voltage lower than
the noise offset, and subtracting the first threshold voltage
amplitude from the differential compressed pixel voltage responsive
to the differential compressed pixel voltage greater than the sum
of the first threshold voltage amplitude and the noise offset. The
precision of the differential pixel voltage is the same as the
digitized compressed pixel voltage.
[0022] In some example, before reconstructing the differential
frames, a dark frame may be subtracted from the compressed frame to
dark-correct the compressed frame. The dark correction process may
be used to remove fixed patterns present in the image from sensor
that is not exposed to radiation. Further, sensor overexposure may
be detected based on the digitized compressed pixel voltage
received by the image processor, as shown in FIG. 5.
[0023] In this way, the pixel voltage may be transferred between
the camera and the image processor with a reduced number of bits.
Because the characteristics of the pixel voltage, that are,
approximate monotonicity and limited change over time, the
difference between sequential pixel voltage readouts can be
losslessly reconstructed at the image processor despite reduced
dynamic range of signal transferred between the camera and the
image processor.
[0024] Turning to FIG. 1, a transmission-type charged particle
microscope 100, such as a transmission electron microscopy (TEM)
system or scanning transmission electron microscopy (STEM) system,
is shown. The microscope includes a vacuum enclosure 2 and a
charged particle source 4 for producing a charged particle beam 111
that propagates along a primary axis 110 and traverses an
electron-optical illuminator 6. The electron-optical illuminator 6
serves to direct/focus the charged particles onto a chosen part of
sample 60 (which may, for example, be (locally)
thinned/planarized). Also depicted is a deflector 8, which can be
used to effect scanning motion of the beam 111.
[0025] The sample 60 is held on a specimen holder 61 that can be
positioned in multiple degrees of freedom by a positioning
device/stage 62, which moves a cradle 63 into which holder 61 is
(removably) affixed; for example, the specimen holder 61 may
comprise a finger that can be moved (inter alia) in the XY plane
(see the depicted Cartesian coordinate system; typically, motion
parallel to Z and tilt about X/Y will also be possible). Such
movement allows different parts of sample 60 to be
illuminated/imaged/inspected by the electron beam 111 traveling
along primary axis 110 (in the Z direction) (and/or allows scanning
motion to be performed, as an alternative to beam scanning). If
desired, an optional cooling device (not depicted) can be brought
into intimate thermal contact with the specimen holder 61, so as to
maintain it (and the sample 60 thereupon) at cryogenic
temperatures, for example.
[0026] The electron beam 111 will interact with the sample 60 in
such a manner as to cause various types of "stimulated" radiation
to emanate from the sample 60, including (for example) secondary
electrons, backscattered electrons, X-rays and optical radiation
(cathodoluminescence). If desired, one or more of these radiation
types can be detected with detector 22, which might be a combined
scintillator/photomultiplier or EDX (Energy-Dispersive X-Ray
Spectroscopy) module, for instance; in such a case, an image could
be constructed using basically the same principle as in scanning
electron microscopy (SEM). However, alternatively or
supplementally, one can study electrons that traverse (pass
through) the sample 60, exit/emanate from it and continue to
propagate (substantially, though generally with some
deflection/scattering) along axis 110. Such a transmitted electron
flux enters projection lens 24, which will generally comprise a
variety of electrostatic/magnetic lenses, deflectors, correctors
(such as stigmators), etc. In normal (non-scanning) TEM mode,
projection lens 24 can focus the transmitted electron flux onto
detector 26, which, if desired, can be retracted/withdrawn (as
schematically indicated by arrows 27) so as to get it out of the
way of axis 110. An image (or diffractogram) of (part of) the
sample 60 will be formed by projection lens 24 on detector (such as
screen) 26, and this may be viewed through a viewing port located
in a suitable part of a wall of enclosure 2. The retraction
mechanism for detector 26 may, for example, be mechanical and/or
electrical in nature, and is not depicted here.
[0027] As an alternative to viewing an image on detector 26, one
can instead make use of the fact that the depth of focus of the
electron flux leaving projection lens 24 is generally quite large
(e.g. of the order of 1 meter). Consequently, various other types
of analysis apparatus can be used downstream of detector 26, such
as TEM camera 30, STEM camera 32, and spectroscopic apparatus
34.
[0028] At TEM camera 30, the electron flux can form a static image
(or diffractogram) that can be processed by image processor 220 and
controller 50. When not required, camera 30 can be
retracted/withdrawn (as schematically indicated by arrows 31) so as
to get it out of the way of axis 110.
[0029] An output from STEM camera 32 can be recorded as a function
of (X,Y) scanning position of the beam 111 on the sample 60, and an
image can be constructed that is a "map" of output from camera 32
as a function of X,Y. Camera 32 may comprise a matrix of pixels.
When not required, camera 32 can be retracted/withdrawn (as
schematically indicated by arrows 33) so as to get it out of the
way of axis 110 (although such retraction would not be a necessity
in the case that camera 32 is a donut-shaped annular dark field
camera, for example; in such a camera, a central hole would allow
flux passage when the camera was not in use).
[0030] In addition to imaging using cameras 30 and/or 32, one can
also invoke spectroscopic apparatus 34, which could be an EELS
module, for example. The EELS module includes a spectrometer 35 for
dispersing the charged particles based on the particle energy and a
detector/camera 36 for capturing the spectrum.
[0031] It should be noted that the order/location of detectors 26,
30, 32, 34 and 36 is not strict, and many possible variations are
conceivable. For example, spectroscopic apparatus 34 can also be
integrated into the projection lens 24.
[0032] The controller 50 is connected to various illustrated
components via control lines. The controller comprises a processor
54 and non-transitory memory 55. Instructions may be stored in the
non-transitory memory 55, when executed, causes the controller 50
to provide a variety of functions, such as synchronizing actions,
providing setpoints, processing signals, performing calculations,
receiving operator input from user input device 53 and displaying
messages/information on display device 51. The controller 50 may be
(partially) inside or outside the enclosure 2, and may have a
unitary or composite structure, as desired.
[0033] One or more detectors 22 and 26, cameras 30 and 32, and
spectroscopic apparatus 34 may be electrically connected with image
processor 220. The image processor may include a processor, a
memory, and one or more Field-programmable gate arrays (FPGAs).
Embedded software may be run in the image processor to process
image data received from the cameras and/or detectors at high frame
rate. Processed data from the image processor may be transferred
from the image processor to the controller for further processing.
For example, the controller generates sample images based on the
data received from the image processor. The cameras and/or
detectors may have separate image processors or a shared image
processor. In one embodiment, the image processor and the
controller may be integrated together as one component. In another
embodiment, the image processor may be integrated with the
camera.
[0034] Though a transmission type electron microscopy is described
by way of example, it should be understood that the imaging system
may be other types of charged particle microscopy system, such as a
SEM or a focused ion beam combined with scanning electron
microscopy (FIB-SEM). The charged particle may be electron, ion, or
x-ray. One or more of the detectors or cameras, such as detectors
22 and 36, cameras 30 and 32, may include one or more image sensors
with multiple pixels. The pixelated image sensor may be operated
according to the methods disclosed below.
[0035] FIG. 2 shows method 200 for reading and processing data
acquired by a camera including at least a pixelated image sensor.
The camera may detect charged particles emitted from a sample in a
microscope, such as the charged particle microscope 100 of FIG. 1.
The dataflow among components of the microscope while executing
method 200 is shown in FIG. 3. The camera data is read out
utilizing the mfCDS method combined with data compression to
increase the transfer rate of frames within the camera and from the
camera to the image processor.
[0036] At 201, the data acquisition parameters of the microscope
are set. The data acquisition parameters may include one or more of
the dose of the charged particle beam at the sample plane, the
imaging/scan area, the data readout rate at the image sensor, and
the number of frames N readout between adjacent image sensor reset.
The number of frames N between adjacent sensor resets may be
determined based on an estimated pixel voltage change between
adjacent pixel voltage readouts and the full-well capacity of the
pixel. For example, the pixel of the image sensor is reset before
reaching a predetermined maximum pixel voltage amplitude. The
maximum pixel voltage amplitude is lower than the full-well
capacity of the pixel. The pixel voltage change between adjacent
pixel voltage readouts may be estimated based on the dose of the
charged particle beam and the sample type.
[0037] At 204, the charged particle beam is directed to the sample.
Responsive to the irradiation of the charged particles, various
types of charged particles, such as the secondary electrons and the
x-ray, are emitted from the sample. The multiple cameras (or
detectors) in the microscope sense the emitted charged particles.
For example, the cameras may include one or more of the TEM camera,
the STEM camera or detector, the EDX detector, and the detector in
the spectroscopic apparatus for sensing the EELS spectra. The
camera includes a pixelated image sensor. The pixel voltage of a
particular pixel changes approximately monotonically responsive to
one or more charged particles impinging the pixel.
[0038] At 206, the pixel voltages are read out at the frequency
determined at step 201 from the image sensor and digitized into a
first number of bits. In one example, the pixel voltages of
multiple pixels of the image sensor are read out according to a
predetermined pattern to form a frame of pixel voltage. During
image sensor readout, the image sensor is read out repetitively at
the frame rate determined at step 201. After consecutively
acquiring N frames, the image sensor is reset by resetting the
pixel voltage of each pixel to a reset voltage. The reset voltage
may be different for each reset. For each pixel of the multiple
pixels, the pixel voltage is readout once during each frame
readout. The pixel voltage of each pixel of the multiple pixels is
repetitively readout N times before resetting the image sensor.
[0039] At 208, the pixel voltage is compressed, and the compressed
pixel voltage is output to the image processor. In one example, the
compressed pixel voltage may be digitized and transferred to the
image processor. In another example, the pixel voltage is digitized
before being compressed. The digitized compressed pixel voltage has
a second number of bits, lower than the first number of bits of the
digitized pixel voltage. The pixel voltage is compressed to a range
less than the maximum range of the pixel voltage. The maximum range
of the compressed pixel voltage is not greater than a first
threshold voltage amplitude. In one example, the first threshold
voltage is subtracted from the pixel voltage responsive to the
amplitude of the pixel voltage not less than the amplitude of the
first threshold voltage and less than the amplitude of a second
threshold voltage. In another example, the digitized pixel voltage
is compressed by removing at least the MSB. The first threshold
voltage can be presented by a number of bits lower than a number of
bits used for representing the maximum of the pixel voltage.
[0040] As shown in FIG. 3, in one example configuration, camera 301
includes image sensor 302, ADC 303, and readout electronics 304.
The pixel voltage read from the image sensor 302 is digitized into
the first number of bits by the ADC, and then compressed to the
second number of bits. The readout electronics 304 may control the
timing of data readout and outputs the compressed digitized pixel
voltage to image processor 320.
[0041] FIGS. 4A-4C illustrate the process of compressing the pixel
voltage when the pixel voltage increases responsive to charged
particles impinging the pixel. The y-axis of FIG. 4A is the pixel
voltage or the corresponding digitized pixel voltage of a
particular pixel of the image sensor. The x-axis represents time.
Time increases as indicated by the arrow. The solid plot 403 is the
uncompressed pixel voltage readout from the image sensor. The
uncompressed pixel voltage may be an analog signal or a digital
signal. The dashed plot 404 is the compressed pixel voltage. At T0,
the image sensor is reset. As a result, the pixel voltage is reset
to a reset voltage. Herein, the reset voltage is zero. In other
examples, the reset voltage may be a non-zero value. The reset
voltage may vary upon each reset, therefore introduce a reset
noise. From T0, as more charged particles impinging the pixel, the
pixel voltage 403 increases from T0 to T4. At T4, the image sensor
is reset again. Arrows 401 and 402 indicate the image sensor reset
event. The pixel voltage is readout at a frequency of 1/.DELTA.T.
In other words, the image sensor is readout at a frame rate of
1/.DELTA.T. From T0 to T1, the pixel voltage 403 is between the
reset voltage and the first threshold voltage V1, and the
compressed pixel voltage 404 equals the pixel voltage 403. From T1
to T3, responsive to the pixel voltage 403 not less than the first
threshold voltage V1 and less than the second threshold voltage V2,
the compressed pixel voltage 404 equals the pixel voltage 403
subtracting the first threshold voltage V1. The second threshold
voltage V2 is twice of the first threshold voltage V1. From T2 to
T3, responsive to the pixel voltage 403 not less than the second
threshold voltage V2 and less than the third threshold voltage V3,
the compressed pixel voltage 404 equals the pixel voltage 403
subtracting the second threshold voltage V2. The third threshold
voltage V3 is three times of the first threshold voltage V1. From
T3 to T4, responsive to the pixel voltage 403 not less than the
third threshold voltage V3 and less than the fourth threshold
voltage V4, the compressed pixel voltage 404 equals the pixel
voltage 403 subtracting the third threshold voltage V3. The fourth
threshold voltage V4 is four times of the first threshold voltage
V1. At T4, since N frames have been acquired from previous reset at
T0, the pixel voltage is reset again to the reset voltage. From T4
to T5, since the pixel voltage 403 is lower than the first
threshold voltage, the pixel voltage 403 is the same as the
compressed pixel voltage 404. After T5, as the pixel voltage 403
increases to be above V1 and lower than V2, the compressed pixel
voltage 404 equals the pixel voltage 403 subtracting V1. As such,
the compressed pixel voltage 404 is between zero and V1. For
digitized signals, the compression process illustrated in FIG. 4A
may reduce the bit depth of the digitized compressed pixel voltage
by 2 bits from the bit depth of the digitized pixel voltage. For
example, the digitized pixel voltage has 12 bits, and the digitized
compressed pixel voltage has 10 bits. The first to fourth threshold
voltages are 1024, 2048, 3072, and 4096, respectively. Value
aliasing is introduced to the compressed pixel voltage through the
compression. For example, pixel voltages between T1-T2 are aliased
with (therefore not distinguishable from) pixel voltages between
T0-T1. The value aliasing can be corrected or resolved in the image
processor, by adjusting the pixel value of the differential
compressed frame to the valid range.
[0042] If the pixel voltage is digitized, the compressed digitized
pixel voltage may be generated by preserving a first bit to a
second bit, and removing the rest bits, of the digitized first
pixel voltage. The neither the first bit nor the second bit is the
MSB. In one example, the subtraction of threshold voltages from the
pixel voltage may be achieved by removing one or more bits from the
side of the MSB, as shown in FIG. 4B. As an example, the digitized
pixel voltage 410 has 12 bits. The compression illustrated in FIG.
4A may be implemented by removing 2 bits from the MSB side. The
digitized compressed pixel voltage is the 10 bits from the LSB side
as shown by 412. As such, in FIG. 4A, D1 is 1024, D2 is 2048, D3 is
3072, and D4 is 4096.
[0043] In another example, the subtraction of threshold voltages
from the pixel voltage may be achieved by removing one or more bits
from both the MSB side and the LSB side, as shown in FIG. 4C. As an
example, the digitized pixel voltage 410 has 12 bits. The digitized
compressed pixel voltage is bit 1 to bit 10 as shown by 421. In
this example, the signal precision of the digitized compressed
pixel voltage is reduced comparing to the digitized pixel voltage
in order to increase the data transfer rate.
[0044] FIG. 4A shows uncompressed pixel voltage increasing
monotonically between consecutive sensor resets. In another
embodiment, the uncompressed pixel voltage read out form the image
sensor decreases monotonically between consecutive resets. In one
example, the pixel voltage may be compressed by subtracting
negative threshold voltages from the uncompressed pixel voltage. In
another example, the uncompressed pixel voltage may be converted to
monotonically increased pixel voltage, such as by subtracted from a
threshold pixel voltage, before being compressed as shown in FIGS.
4A-4C.
[0045] Turning back to FIG. 2, at 210, the image processor receives
the compressed pixel voltage or digitized compressed pixel voltage
from the camera and forms compressed frames with the compressed
pixel voltage. In one example, as shown in FIG. 3, the image
processor 320 may include one or more FPGAs 322 and memory 323. The
FPGAs 322 have direct memory access to memory 323. Image processor
320 may optionally include a processor 321 for controlling the
data/image processing within the FPGAs 322.
[0046] At 212, sensor overexposure is determined based on the
compressed frame. The sensor overexposure may be determined based
on the pixel value and the variance of the pixel values of a
compressed frame. Dose protection block 324 of FIG. 3 represents
the process of determining the sensor overexposure. Details of
overexposure detection are presented in FIG. 5. If sensor
overexposure is detected, at 214, method 200 may prevent the
charged particles from reaching the image sensor, for example, by
closing a shutter. The method 200 may send out notification to the
operator indicating sensor overexposure. The method 200 may further
adjust the data acquisition parameters of the current image session
or stop the current image session. If the sensor overexposure is
not detected, method 200 moves to 218.
[0047] At 218, a differential compressed frame is generated by
subtracting the compressed frame from the previously acquired
compressed frame. The pixel values of the differential compressed
frame are then adjusted to a valid range. For example, a
differential compressed frame is obtained by subtracting a first
compressed frame acquired at a first time point t1 from a second
compressed frame acquired at a second time point t2, immediately
after acquiring the first compressed frame, that is
E.sub.t2-E.sub.t1.
[0048] Step 218 may optionally include dark correcting the
differential compressed frame before the subtraction. That is, the
differential compressed frame is generated by subtracting
sequentially acquired dark-corrected compressed frames. For
example, as shown in FIG. 3, a dark frame 331 stored in memory 323
may be optionally subtracted from the compressed frame at 325 to
generate a dark-corrected compressed frame 326. The dark-corrected
compressed frame 326 is temporarily stored in memory 323. The
dark-corrected compressed frame 326 is also sent to 328 to subtract
the previously saved compressed frame. After a delay 327, upon
receiving the next dark-corrected compressed frame, the
dark-corrected compressed frame 326 is subtracted from the next
dark-corrected compressed frame at 328 to generate a differential
compressed frame.
[0049] The pixel value of the differential compressed frame is
adjusted to the valid range at block 329 of FIG. 3. The valid range
is determined based on a predetermined noise amplitude and the
first threshold voltage used for compressing the pixel voltage at
208 of FIG. 2. The noise may include one or more sensor dark noise,
sensor thermal noise, sensor readout noise, and sensor quantization
noise. The noise amplitude may be determined a priori from
inspection of image frames acquired without irradiation. For
example, the noise amplitude is determined based on the standard
deviation of the pixel values in the image frame acquired without
irradiating the sample with the charged particle beam. A noise
offset is determined based on the noise amplitude. The noise offset
may be the negative of the noise amplitude. In one example, the
valid range is from the noise offset to the sum of the first
threshold voltage and the noise offset, wherein the noise offset is
non-positive. If the pixel value is less than the noise offset, the
first threshold voltage is added to the pixel value. If the pixel
value is greater than the sum of the first threshold voltage and
the noise offset, the first threshold voltage is subtracted from
the pixel value. The first threshold voltage is the pixel voltage
V.sub.1 or the digitized pixel voltage D.sub.1 used for compressing
the pixel voltage at 208.
[0050] FIG. 6A illustrates adjusting the pixel value of the
differential compressed frame to the valid range when there is no
noise or zero noise. Because the compressed pixel voltage is from
zero to the first threshold voltage, the pixel value of the
differential compressed frame (that is, the difference between two
compressed pixel voltage) is from the negative first threshold
voltage -V.sub.1 to the first threshold voltage V.sub.1. The valid
range 610 is from zero to V.sub.1. If the pixel value is within the
invalid range (that is, outside of the valid range 610) as
indicated by the shaded area, the pixel value is adjusted into the
valid range by adding the first threshold voltage to the pixel
value. For example, pixel value 611 is adjusted to pixel value 612.
As such, pixel values in the invalid range are moved to the valid
range as indicated by arrow 613.
[0051] FIG. 6B illustrates adjusting the pixel value of the
differential compressed frame to the valid range when noise is
present in the pixel voltage. The noise offset 601 is negative. The
valid range 620 is from the noise offset 601 to the sum 602 of the
first threshold voltage V.sub.1 and the noise offset 601. The
shaded areas indicate invalid range. If the pixel value is from
-V.sub.1 to noise offset 601, the first threshold voltage V.sub.1
is added to the pixel value, so that the pixel value is moved to
the range from 0 to sum 602, as indicated by arrow 621. If the
pixel value is from sum 602 to V.sub.1, the first threshold voltage
V.sub.1 is subtracted from the pixel value. As the result, pixel
value in range 623 is moved to range 624, as indicated by arrow
622.
[0052] At 220, sample image is formed based on the differential
image. As shown in FIG. 3, the differential image is transferred
from the image processor 320 to controller 50 for generating the
sample image. Step 220 may include pre-processing the differential
image before forming the sample image.
[0053] In this way, by reading out data from the camera at a bit
depth lower than the bit depth for digitizing the image sensor
readout. The camera may be operated at a maximum frame rate for
reading out the sensor data, and the overall frame rate for data
acquisition can be increased. The compression of pixel voltage can
be executed at high speed by removing one or more bits from the MSB
of the digitized pixel voltage. The change in pixel voltage during
sequential image sensor readout can be losslessly reconstructed by
adjusting the pixel value of the compressed differential frame to
the valid range. Note that operations described sequentially herein
may in some cases be rearranged or performed concurrently.
[0054] FIG. 5 shows method 500 for detecting sensor overexposure
based on the compressed frames from camera output. The sensor
overexposure may be detected based on the amount and variance of
the pixel values of the compressed frame. In one example, the
overexposure is determined based on one or more compressed frames
acquired immediately after a sensor reset.
[0055] At 502, each compressed frame is divided into multiple
subframes, each subframe including one or more pixels. The
subframes may overlap with each other.
[0056] At 504, the sum and variance of all pixel values in the
subframes are calculated and compared with a threshold sum at a
threshold variance, respectively, at 506. In one example, the
variance may the mathematical variance of the pixel values in the
subframe. In another example, the variance may be calculated
through other simplified approximate method. The threshold sum may
be determined based on the number of pixels for each subframe, the
full-well capacity of each pixel, and the number of frames after
the most immediate sensor reset. The threshold variance may be
determined by measuring the pixel values with actual deliberate
(non-damaging) overexposure. If the sum of all pixel values of any
subframe is greater than the threshold sum and the variance of the
subframe is lower than the threshold variance, overexposure is
detected at 508. Otherwise, no overexposure is detected at 510.
[0057] The technical effect of compressing the pixel voltage read
out from the image sensor is to achieve high frame rate of data
transfer even when the bandwidth between the ADC and the readout
electronics and/or the bandwidth between the camera and the image
sensor is limited. The technical effect of compressing the pixel
voltage by removing one or more bits from the MSB of the digitized
pixel voltage is that the compression can be implemented at a high
speed. The technical effect of generating the differential frame
based on sequentially acquired compressed frame is that the change
in pixel voltage responsive to charged particles impinging the
sensor is determined. The technical effect of correcting the range
of the differential compressed frame to obtain the differential
frame is that the aliasing due to compression is corrected. The
precision of the digitized compressed pixel voltage and the pixel
value in the differential frame are the same.
[0058] In one presentation, a method for acquiring data from a
camera including a pixelated image sensor for detecting charged
particles comprises receiving a first and a second digitized
compressed pixel voltages from the camera; determining a
differential compressed pixel voltage by calculating a difference
between the first digitized compressed pixel voltage and the second
digitized compressed pixel voltage; generating a differential pixel
voltage by adjusting the differential compressed pixel voltage to a
valid range determined by a predetermined noise offset and a first
threshold voltage; and forming an image of the sample based on the
differential pixel voltage.
[0059] In another presentation, a camera for detecting charged
particles comprises an image sensor and one or more ADCs, wherein
the camera is configured to: read a pixel voltage of one or more
pixels of the image sensor multiple times without resetting the
image sensor; digitize the pixel voltage into a first number of
bits; and output a digitized compressed pixel voltage in a second,
lower, number of bits, wherein a maximum range of the digitized
compressed pixel voltage is less than a maximum range of the pixel
voltage, and wherein the digitized compressed pixel voltage is
generated by removing at least a most significant bit (MSB) of the
digitized pixel voltage.
[0060] In one embodiment, a method for acquiring data from a camera
including a pixelated image sensor for detecting charged particles,
comprises reading a pixel voltage of one or more pixels of the
image sensor multiple times without resetting the image sensor;
digitizing the pixel voltage into a first number of bits; and
outputting a digitized compressed pixel voltage in a second, lower,
number of bits, wherein a maximum range of the digitized compressed
pixel voltage is less than a maximum range of the pixel voltage,
and wherein the digitized compressed pixel voltage is generated by
removing at least a most significant bit (MSB) of the digitized
pixel voltage. In a first example of the method, the digitized
pixel voltage is unsigned. A second example of the method
optionally includes the first example and further includes for each
pixel of the one or more pixels of the image sensor, sequentially
receiving a first digitized compressed pixel voltage and a second
digitized compressed pixel voltage; determining a differential
compressed pixel voltage by calculating a difference between the
first digitized compressed pixel voltage and the second digitized
compressed pixel voltage; and generating a differential pixel
voltage by adjusting the differential compressed pixel voltage to a
valid range, wherein the valid range is determined based on a
predetermined noise offset and the maximum range of the digitized
compressed pixel voltage. A third example of the method optionally
includes one or more of the first to the second examples, and
further includes, wherein the valid range is from the noise offset
to a sum of a threshold voltage and the noise offset, and the
threshold voltage is determined based on the maximum range of the
digitized compressed pixel voltage. A fourth example of the method
optionally includes one or more of the first to the third examples,
and further includes, wherein adjusting the differential compressed
pixel voltage to the valid range includes adding the threshold
voltage to the differential compressed pixel voltage responsive to
the differential compressed pixel voltage lower than the noise
offset, and subtracting the threshold voltage from the differential
compressed pixel voltage responsive to the differential compressed
pixel voltage greater than the sum of the threshold voltage and the
noise offset. A fifth example of the method optionally includes one
or more of the first to the fourth examples, and further includes,
wherein the second number of bits is determined based on a maximum
range of change in the pixel value between sequential readouts. A
sixth example of the method optionally includes one or more of the
first to the fifth examples, and further includes, wherein the
digitized compressed pixel voltage has the same signal precision as
the digitized pixel voltage. A seventh example of the method
optionally includes one or more of the first to the sixth examples,
and further includes, wherein the digitized compressed pixel
voltage is generated by further removing one or more bits from a
least significant bit side of the digitized pixel voltage. An
eighth example of the method optionally includes one or more of the
first to the seventh examples, and further includes detecting image
sensor overexposure based on the digitized compressed pixel voltage
of the one or more pixels of the image sensor.
[0061] In one embodiment, a method for acquiring data from a camera
including a pixelated image sensor for detecting charged particles,
comprises repetitively reading a pixel voltage of a pixel of the
image sensor without resetting the image sensor; compressing the
pixel voltage into a compressed pixel voltage, wherein the
compressed pixel voltage is a difference between the pixel voltage
and a first threshold voltage responsive to an amplitude of the
pixel voltage not less than an amplitude the first threshold
voltage and less than an amplitude of a second threshold voltage,
and wherein a maximum range of the compressed pixel voltage is not
greater than the amplitude of the first threshold voltage, and the
maximum range of the compressed pixel voltage is lower than a
maximum range of the pixel voltage; digitizing the compressed pixel
voltage; and outputting the digitized compressed pixel voltage. In
a first example of the method, the method further includes, wherein
compressing the pixel voltage further includes subtracting the
second threshold voltage from the pixel voltage responsive to the
amplitude of the pixel voltage not less than the amplitude of the
second threshold voltage and less than an amplitude of a third
threshold voltage. A second example of the method optionally
includes the first example and further includes wherein the second
threshold voltage is two times of the first threshold voltage. A
third example of the method optionally includes one or more of the
first to the second examples, and further includes resetting the
image sensor after reading the pixel voltage of the pixel a
predetermined number of times. A fourth example of the method
optionally includes one or more of the first to the third examples,
and further includes resetting the image sensor in response to the
amplitude of the pixel voltage greater than a maximum amplitude of
the pixel voltage. A fifth example of the method optionally
includes one or more of the first to the fourth examples, and
further includes, wherein the pixel voltage between adjacent sensor
resets is a monotonic signal superimposed with a noise signal. A
sixth example of the method optionally includes one or more of the
first to the fifth examples, and further includes sequentially
receiving a first digitized compressed pixel voltage and a second
digitized compressed pixel voltage from the camera; dark correcting
the first digitized compressed pixel voltage and the second
digitized compressed pixel voltage; determining a differential
compressed pixel voltage by calculating a difference between the
dark-corrected first digitized compressed pixel voltage and the
dark-corrected second digitized compressed pixel voltage; and
generating a differential pixel voltage by adjusting the
differential compressed pixel voltage into a valid range, the valid
range determined based on a predetermined noise offset and the
first threshold voltage. A seventh example of the method optionally
includes one or more of the first to the sixth examples, and
further includes wherein the valid range is from the noise offset
to a sum of the first threshold voltage amplitude and the noise
offset.
[0062] In one embodiment, a system for acquiring data from a sample
comprises a charged particle source for irradiating charged
particles towards the sample; a camera for detecting charged
particles emitted from the sample responsive to the irradiation,
the camera includes an image sensor with multiple pixels and one or
more analog-to-digital converters (ADCs), wherein the camera is
configured to: convert charged particles impinging a pixel of the
multiple pixels into a pixel voltage; compress the pixel voltage
into a compressed pixel voltage, wherein the compressed pixel
voltage is a difference between the pixel voltage and a first
threshold voltage if an amplitude of the pixel voltage is not less
than an amplitude of the first threshold voltage and less than an
amplitude of a second threshold voltage, and wherein a maximum
range of the compressed pixel voltage is not greater than the
amplitude of the first threshold voltage, and the maximum range of
the compressed pixel voltage is lower than a maximum range of the
pixel voltage; digitize the compressed pixel voltage; and output
the digitized compressed pixel voltage; an image processor for
receiving the digitized compressed pixel voltage from the camera
and generating a differential pixel voltage based on the digitized
compressed pixel voltage; and a controller for forming an image of
the sample based on the differential pixel voltage. In a first
example of the system, the system further includes wherein
receiving the digitized compressed pixel voltage from the camera
and generating the differential pixel voltage based on the
compressed pixel voltage includes: sequentially receiving a first
digitized compressed pixel voltage and a second digitized
compressed pixel voltage; determining a differential compressed
pixel voltage by subtracting the first digitized compressed pixel
voltage from the second digitized compressed pixel voltage; and
generating the differential pixel voltage by adjusting the
differential compressed pixel voltage into a valid range, the valid
range determined based on a predetermined noise offset and the
first threshold voltage. A second example of the system optionally
includes the first example and further includes, wherein the
digitized compressed pixel voltage and the differential pixel
voltage have the same precision.
* * * * *