Electron beam dose control for scanning electron microscopy and critical dimension measurement instruments

A system and method for controlling electron exposure on image specimens by adjusting a raster scan area in-between scan frame cycles. A small, zoomed-in, scan area and the surrounding area are flooded with positive charge for a number of frame cycles between scan frames to reduce the voltage differential between the scan area and surrounding area, thereby reducing the positive charge buildup which tends to obscure small features in scanned images. The peak current into a pixel element on the specimen is reduced by scanning the beam with a line period that is very short compared to regular video. Frames of image data may further be acquired non-sequentially, in arbitrarily programmable patterns. Alternatively, an inert gas can be injected into the scanning electron microscope at the point where the electron beam impinges the specimen to neutralize a charge build-up on the specimen by the ionization of the inert gas by the electron beam.

FIELD OF THE INVENTION 
The present invention relates generally to enhanced feature measurement in 
scanning electron microscopy, and more specifically to a system and 
methods for controlling electron exposure on image specimens in scanning 
electron metrology, particularly in the inspection of features of 
micro-circuits. It may also apply to critical dimension measurement in 
similar instruments. 
BACKGROUND OF THE INVENTION 
In scanning electron microscopy, a beam of electrons is scanned over a 
specimen, and the resulting electrons that are returned from the specimen 
surface are used to create an image of the specimen surface. In some 
systems, the beam is arbitrarily controllable to make multiple scan passes 
over specific areas or portions of areas at different sample frequencies 
to magnify the image of the surface. 
On a specimen made of a substantially insulative material (e.g., a 
semiconductor material), performing multiple scans over the same small 
area may cause the specimen to accumulate an excess positive charge in 
that small area relative to the rest of the scanned area. That excess 
charge causes an image of that small area to appear dark, thus obscuring 
image features in that small area. 
SUMMARY OF THE PRESENT INVENTION 
One embodiment of the present invention is a system and method for imaging 
a specimen that acquires a charge when scanned with a scanning electron 
microscope comprising an electron source and apparatus for forming, 
accelerating, focusing, and scanning an electron beam across a portion of 
said specimen. That imaging being performed by raster scanning a selected 
small area of the specimen for a single frame cycle, and then raster 
scanning a substantially larger area of the specimen that includes the 
small area to brighten the image of the small area of said specimen by 
flooding the substantially larger area with electrons. 
A second embodiment of the present invention is a system and method of a 
scanning electron microscope to image a specimen that acquires a charge 
when scanned with a scanning electron microscope by injecting an inert gas 
at the point where the electron beam impinges on the surface of the 
specimen. That inert gas being ionized by the electron beam and thus 
neutralizing the charge as it builds up on the specimen.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Reference will now be made in detail to the preferred embodiments of the 
present invention, examples of which are illustrated in the accompanying 
drawings. 
FIG. 1 shows a block diagram of system 10 including an electron microscope 
subsystem 11 of the present invention. The electron microscope subsystem 
11 includes an electron beam source 12, a focusing column and lens 
assembly 14, and a scan controller 16 to scan an electron beam across 
selected regions of specimen 20. Also included in electron microscope 
system 11 is an electron detector 24 to detect secondary and backscattered 
electrons from specimen 20. In system 10 of the present invention, 
electron detector 24 is selected to have a bandwidth that is at least 
adequate to detect the secondary and backscattered electrons that form 
electron signal 28. For example, electron detector 24 may be a 
micro-channel plate, micro-sphere plate, semiconductor diode, or a 
scintillator/ photomultiplier (PMT) assembly, each well known in the art. 
Then the electrons of signal 28 received by detector 24 are processed and 
stored for display by image processor and display subsystem 26. 
In operation, electron beam 18 is scanned over specimen 20 and secondary 
and backscattered electron signal 28 is detected by electron detector 24. 
Further, electron beam 18 is focused on the surface of specimen 20 with 
the average current into specimen 20 determined by scan controller 16 that 
controls the raster scanning of beam 18. In the present invention electron 
beam 18, as discussed below, can be scanned for a single frame cycle, and 
then blanked for a period of one or more frame cycles. 
Typically, specimen 20 may be comprised of a variety of materials with the 
present invention particularly applicable to materials containing a 
substantial amount of insulative material (e.g., semiconductor material). 
Small area 22 of specimen 20 is shown to illustrate a particular area of 
interest to be scanned to determine features of the specimen in the image 
of small area 22 developed by image processor and display subsystem 26. 
For example, small area 22, may, in a degenerate case, be a single line or 
a single pixel element on specimen 20. In the present invention the peak 
current onto small area 22 is reduced by scanning electron beam 18 faster 
than the television rate commonly used in conventional SEM instruments. In 
system 10 of the present invention, electron beam 18 is typically scanned 
with a line period of 16 .mu.sec, or four times the rate normally used for 
TV raster scanning having a line period of 64 .mu.sec. 
FIG. 2 shows a partial cross-sectional view of electron microscope 
subsystem 11 of the present invention to reveal more detail. As shown 
here, subsystem 11 is shown with electron beam source 12 at the top which 
produces electron beam 34. One implementation that could be used includes 
an electron gun 36 that consists of a thermal field emitter (TFE) with the 
electrons accelerated by a surface field generated by power supply 32. 
Alternative electron gun embodiments could be employed. The electrons 
emitted by electron gun 36 are then, within beam source 12, directed 
through electrodes 38 and gun lens 39 (each also controlled by power 
supply 32) to form electron beam 34 that enters focusing column and lens 
assembly 14 to be directed to specimen 20. It should also be noted that 
electrodes 38 typically include both suppressor and extractor electrodes. 
In focusing column and lens assembly 14, electron beam 34 passes through an 
aperture 41, reducing the beam current from approximately 300 pA to a 
range of 5 to 100 pA forming what is labelled electron beam 34' in FIG. 2. 
A larger electron beam current (e.g., 100 pA) is particularly useful for 
pattern recognition. That larger beam current also reduces the integration 
time to achieve a given signal-to-noise ratio for the image or linescan 
which is well known in the art. Stated a little differently, there is a 
better signal-to-noise ratio for higher beam currents, however there is an 
improved image quality for lower beam currents. 
Electron beam 34' then passes through objective lens 42, including magnetic 
coils 43 and pole pieces 44, that generate a strong magnetic field. That 
magnetic field is used to focus beam 34' to form electron beam 18 with a 
spot size of approximately 5 nm when directed at specimen 20. 
Additionally, the location of electron beam 18 is controlled with scan 
plates 45, located within the magnetic field created by coils 43 and pole 
pieces 44, with scan plates 45 powered by raster generator 48 to direct 
beam 18 in both the x and y directions across specimen 20 by signals on 
lines 46 and 47, respectively. To tie FIGS. 1 and 2 together in this area, 
scan plates 45 and raster generator 48 correspond to scan control 16 in 
FIG. 1. 
Referring next to FIG. 3, there is shown a block diagram of one potential 
embodiment of raster generator 48. Included in this sample embodiment of 
raster generator 48 is a clock 60 to produce a timing signal that is 
applied to ramp generator 62 and counter 64. Ramp generator 62 in turn 
produces a ramp signal x', and counter 64 produces a digital signal which 
represents a preset count. The preset count from counter 64 being 
representative of the timing signal from clock 60. In turn, the preset 
count from counter 64 is applied to look-up table 65 wherein look-up table 
65 has been programmed to select individual y-axis lines on the surface of 
specimen 20 to be scanned that corresponds to the count from counter 64. 
It should be noted here that the y-axis lines to be scanned may be 
sequential; non-sequential; selected lines with one or more intermediate 
lines skipped; selected lines scanned repeatedly; or any combination or 
order desired for various regions on the surface of specimen 20. The 
output digital value of look-up table 65 is then applied to 
digital-to-analog converter (DAC) 66 to produce a stepped signal, y', that 
corresponds to the y-axis position on specimen 20 to be scanned. Next, 
signals x' and y' are directed to the rotation and scaling controllers 68 
(e.g., utilizing a multiplying D/A converter with a technique that is well 
known in the art) that produces signals x and y that are applied to scan 
plates 45 (see FIG. 2) via lines 46 and 47, respectively, to control the 
actual x and y positions electron beam 18 scans on specimen 20. 
Referring next to FIGS. 4a and 4b, representative waveforms of signals x 
(46) and y (47), respectively, from raster generator 48 are shown. In FIG. 
4a, ramp segment 72 in the x signal (46) directs beam 18, via scan plates 
45, to scan a spot along a single line in the x-axis direction on specimen 
20. Since each segment of the signal in FIG. 4a is the same magnitude in 
voltage, alternatively the same duration in time, the length of each 
corresponding scan in the x direction is of the same length. Concurrently, 
in FIG. 4b each step segment 76 of the y signal (47) provides a y-address 
of a different signal value in the y-axis direction that is traced in the 
x direction of specimen 20 by the x signal. To illustrate what the x and y 
signals of FIGS. 4a and 4b are actually causing to happen relative to 
specimen 20, FIG. 4c is provided to show the paths scanned based on those 
signals, i.e., each line starts at x.sub.o and proceeds to x.sub.a at each 
of the corresponding y coordinates starting with y.sub.a and progressing 
through y.sub.e. 
However, scan signals x and y may be manipulated to vary the scan pattern 
in various ways. For example, in FIGS. 4d and 4e, each of lines y.sub.a, 
y.sub.b and y.sub.c on specimen 20 are each scanned twice along the x axis 
from x.sub.o to x.sub.a, and then back from x.sub.a to x.sub.o before 
progressing to the next y line. 
Another potential scan pattern is represented by FIGS. 4f and 4g where line 
y.sub.a is scanned three times in the x direction between x.sub.o and 
x.sub.a, always in the same x direction; then line Y.sub.b is scanned once 
between x.sub.o and x.sub.a ; next line Y.sub.c is scanned twice in the x 
direction between x.sub.o and x.sub.a, always in the same x direction; and 
finally each of lines Y.sub.d and Y.sub.e in that sequence, are scanned 
once between x.sub.o and x.sub.a. 
Yet another scan example is illustrated FIGS. 4h, 4i and 4j. Here two y 
lines, Y.sub.a and Y.sub.b, are scanned in sequence with each y line 
scanned once between x coordinates x.sub.a and x.sub.b, and then twice 
between the x coordinates x.sub.o and x.sub.c. 
An additional example is illustrated in FIGS. 4k and 41, assuming that the 
x axis scan is as illustrated in FIG. 4a. What is shown here is the 
non-sequential scanning of substrate 20 along a group of y lines, y.sub.a 
through y.sub.g, in the order of: y.sub.c, y.sub.f, y.sub.b, y.sub.d, 
y.sub.g, y.sub.a and y.sub.e. 
It should be kept in mind that the various scan patterns illustrated here 
simply illustrate the variations in scan patterns that can be used and 
they are not intended to be anything other than examples of the variations 
of scan patterns that may be used. 
Returning to FIG. 2, as beam 34' passes through the magnetic field of 
objective lens 42 and plates 45 it is focused into beam 18 and directed 
onto specimen 20. Given tolerances in today's applications, the spacing 
between column 14 (bottom of lens 42) and specimen 20 will typically be on 
the order of 2 mm, however that spacing is not critical to the operation 
of the present invention, it merely must be a known value. In addition, 
specimen 20 is biased to a selected potential by a second power supply 52 
(e.g., up to 5VDC) to create an extremely large decelerating field for the 
primary electrons of beam 18 as they approach specimen 20. The result is 
that the "landing energy" of those electrons as they reach specimen 20 is 
therefore much lower than the energy with which they are provided by 
electron gun 36 and with which they travel through column and lens 
assembly 14. The electron beam of the illustrated implementation starts 
out from electron gun 36 with an energy level of typically 5000 eV, and 
travels through column and lens assembly 14 with that energy level 
essentially unchanged. As electron beam 18 exits lens 42, the decelerating 
field radiating from specimen 20, created by the bias of second power 
supply 52, substantially decelerates the electrons within beam 18 to the 
desired landing energy. 
The effect of reducing the landing energy of the electrons by controlling 
the decelerating field allows for excellent optical performance by 
reducing the chromatic aberration coefficient of objective lens 42, and 
provides some immunity from stray magnetic fields in the environment 
(e.g., stray fields of 50 or 60 cycles from power lines). Thus, the beam 
landing energy can be adjusted by adjusting the bias applied to specimen 
20 from second power supply 52. 
Continuing the discussion of the operation of the system illustrated in 
FIG. 2, secondary and backscatter electrons 28 are released as a result of 
the interaction of electron beam 18 with specimen 20 and are directed back 
toward lens 42. As electrons 28 are released, they spiral through lens 42 
as a result of the magnetic field, and then travel toward detector 55 as 
they leave the field within lens 42. The electron signal received by 
detector 55 is then collected by collector plate 56 which in-turn 
generates a signal that is amplified by transimpedance amplifier 58 before 
being applied to image generator 59. Other input signals to image 
generator 59 are signals x and y from raster generator 48 on lines 46 and 
47, respectively, to form a video signal representing an image of specimen 
20, or selected portions thereof. Again correlating the relationship 
between FIGS. 1 and 2, electron detector 24 includes detector 55 and 
collector plate 56, while image subsystem 26 includes amplifier 58 and 
image generator 59. Additionally, electron beam source 12, focusing column 
and lens assembly 14, and specimen 20 are all contained within a vacuum 
chamber 23. 
Note also that when a high electron beam current 18 is used, the 
integration time for detector 55 to achieve a given signal-to-noise ratio 
for an image or linescan is reduced. This shorter acquisition time allows 
faster pattern recognition in automated systems, and reduces sensitivity 
to low frequency vibration and electronic and electromagnetic noise in the 
system. 
In a system as described herein it is useful to look at the ratio of the 
detected electron beam current 28 from specimen 20 to the incoming 
electron beam current 18 to specimen 20, with that ratio referred to as 
the "emission coefficient". There are several variables that affect the 
value of the emission coefficient, some of which are the specimen 
material, the topography of the sample area, the bias voltage on the 
specimen and the landing energy of the primary electron beam. In cases 
where the emission coefficient is greater than one (e.g., for silicon 
specimens) -- that is, more electrons are being generated at the scanned 
area than are arriving at it -- the specimen tends to build up a positive 
charge in the scanned area. For other materials the emission coefficient 
will have differing values, greater than, less than, or equal to, one when 
a positive charge builds on a specimen of that material. The field which 
decelerates the primary beam (i.e., resulting from the bias of second 
power supply 52) further tends to accelerate the electrons of beam 28 as 
they leave the specimen surface, which accentuates the depletion of 
electrons from specimen 20. 
As mentioned previously, the electron microscope of the present invention 
is able to select small areas, including a single line, for raster 
scanning. Incoming electron beam 18 is further controllable so that any 
particular line or area on specimen 20 may be scanned several times. This 
creates a problem, however, in scanning situations where the emission 
coefficient is greater than one (e.g., for silicon specimens), or for 
whatever value for other materials that might constitute specimen 20. 
Attempting to zoom in on an image and measure very small areas results in 
the accumulation of a large positive charge in that area, and electrons 
are prevented from escaping from specimen 20 by the resulting 
electrostatic field. In the present invention, this problem is solved by 
flooding the surrounding area with electrons during a number of frame 
cycles as discussed below. 
As shown in FIG. 5, a small area 22 (see FIG. 1) may be scanned line by 
line. In the present invention that scan could begin with electron beam 18 
at a top left pixel 102, proceed to the right across that y line in the 
increasing x direction to pixel 104, then proceed downward to a pixel 105 
in another y line with the same x coordinate, from there proceed to the 
left in the negative x direction across that new y line to pixel 106, and 
continue scanning in that back and forth fashion in various y lines across 
small area 22. Then, when that scan reaches pixel 112, the beam is 
"blanked" (i.e., temporarily turned off) while electron beam 18 is 
returned to pixel 102. Alternatively, the scan may be controlled in one of 
the alternative patterns discussed in relation to FIGS. 4d-4l-- what ever 
is appropriate for specimen 20. 
On a specimen made up of a substantially insulative material (e.g., a 
semiconductor die), each scan may result in the release of secondary 
electrons, increasing the positive charge of the area of interest with 
each scan. As a result of repeated scans, small area 22 acquires a higher 
positive charge than the surrounding area of specimen 20. Such a positive 
charge will be displayed as a darkened area by image processor and display 
subsystem 26 in the resulting image. Depending on the level of positive 
charge on small area 22 relative to the surrounding area, features of 
small area 22 may be difficult to discern in that image. 
As shown in FIG. 6, the technique of one embodiment of the present 
invention alleviates that darkened image problem by performing a sequence 
of scans which includes flooding an image area 120 (includes small area 22 
and the area surrounding small area 22) during an integer number of raster 
scans. For example, in a first scan frame cycle, n.sub.1, each line of 
only small area 22 is scanned. In each of a selected number of subsequent 
frames n.sub.1 +1 , n.sub.1 +2, . . . , n.sub.1 +m, each line in all of 
image area 120 is scanned in each frame sequentially, each time scanning 
the significantly increased image area of image area 120 as compared to 
small area 22, thus essentially flooding the image area 120. The next 
small area 124 on specimen 20 (may be the same, overlapping, adjoining, or 
separated from, small area 22) and the surrounding larger image area is 
similarly scanned in frame n.sub.2. This process is thus repeated until 
all of the small areas 22, 124, . . . , of interest are scanned. It must 
also be kept in mind that each subsequent small area to be imaged may be 
the same as the previously scanned small area, or offset from that 
previously scanned small area. Also during scanning of subsequent small 
areas (e.g., small area 124), the image area (e.g., 120') to be flooded, 
may include a substantial portion of the image area (e.g., 120) of the 
previously imaged small area (e.g., 22) since at least the image areas, if 
not the small areas as well, may overlap each other. 
Flooding the scanned small area and surrounding image area with positive 
charge effectively reduces the voltage differential between the small area 
(e.g., 22) to be imaged and the surrounding image area (e.g., 120 less 
22), thus allowing electrons to continue to escape from the imaged small 
area. The overall charge that builds up on specimen 20 while imaging each 
small area can be adjusted by changing the ratio between the number of 
frames in which only the small image area is scanned (zoomed-in-frame) 
versus the number of frames during which the larger image area is scanned 
(zoomed-out-frame). 
FIG. 7 provides a sample implementation of an electron detector and image 
processor subsystem 128 that performs the combined function of electron 
detector 24 and image processor and display subsystem 26 of FIG. 1. 
Specifically, subsystem 128 includes a detector 130 that detects the 
reflected and backscattered electrons from specimen 20 with the output 
signal from detector 130 applied to amplifier 132. Amplifier 132 
subsequently supplies an amplified signal to digitizer 134 where the 
signal is digitized for application to image processor 136. In the lower 
path of subsystem 128 there is an oscillator 138 that applies a signal to 
frequency divider 140 to generate signals to control both digitizer 134 
and image processor 136 with the operation of subsystem 128 discussed more 
completely below. Additionally, comparing the components illustrated in 
FIG. 7 to those shown in FIG. 2: detector 130 relates to detector 55 and 
collector plate 56; amplifier 132 relates to amplifier 58; and the 
remainder of the circuit in FIG. 7 relates to image generator 59. 
In the embodiment illustrated in FIG. 7, each y line scan signal in an area 
of interest on specimen 20 is strobed at four times the conventional video 
rate (i.e., 160 MHz, the frequency of oscillator 138, corresponds to a 
four times interleaving using a standard video rate). In the lower path of 
subsystem 128 a 160 MHz signal is generated by oscillator 138 and applied 
to frequency divider 140 that performs two functions. 
One function of frequency divider 140 is to divide the 160 MHz signal by 
four to present a 40 MHz signal to image processor 136. The second 
function of frequency divider 140 is to phase split a 160 MHz signal from 
oscillator 138 into four 160 MHz strobing signals, each with a different 
phase relative to each other (i.e., .phi..sub.1 =0.degree., .phi..sub.2 = 
90.degree., .phi..sub.3 = 180.degree. and .phi..sub.4 = 270.degree.). Each 
of those four different phase strobing signals, .phi..sub.1, .phi..sub.2, 
.phi..sub.3 and .phi..sub.4, are applied to a different terminal of 
digitizer 134 to cause digitizer 134 to divide each y line scan signal 
from amplifier 132 into four different y line scan signals. 
Given this embodiment, 512 subpixel samples are obtained during each phase 
shifted strobe, and four consecutive strobes of the same y line scan 
signal are each strobed with an incremental offset of phase (i.e., a 
quarter pixel width). These phase shifted y line scan signals are 
interlaced, resulting in a total of 512 times 4, or 2048 samples per line 
(e.g., when there are a total of 2048 sample pixels for a y line scan 
signal, the first strobe with a 0.degree. phase shift strobes pixels 0, 4, 
8, etc.; on the next strobe with a 90.degree. phase shift pixels 1, 5, 9 
etc. are strobed; on the next scan with a 180.degree.phase shift pixels 2, 
6, 10 etc. are strobed; and on the fourth scan with a 270.degree. phase 
shift pixels 2, 7, 11 etc. are strobed). 
The four phase implementation discussed with respect to FIG. 7 represents 
an economical way of extracting the data from the y line scan signals 
using less expensive 10 MHz equipment rather than 40 MHz equipment that 
would be needed without the strobing routine. 
FIG. 8 illustrates one of the aspects of the system of the present 
invention in which arbitrary programming of the direction of a line scan, 
and non-sequential line scans, can be used to obtain several critical 
dimension measurements on a substrate without having to reposition the 
substrate between each measurement. 
Before illustrating that, it would be helpful to introduce the concept of 
"charge induced asymmetry". Basically when a feature is scanned, such as a 
line on a wafer, the video signal from the leading edge of that scan 
provides a different image than the trailing edge of that scan as a result 
of the scanning process depositing a charge on the wafer during the 
scanning process thus affecting the resultant video image. That difference 
in image is referred to as "charge induced asymmetry". 
During the development of the present invention it was observed that line 
scan direction reversal during scanning reduces charge induced asymmetry 
in the line scan profiles. Therefore, multiple arrays of line scan data 
may be acquired, wherein the position, length, and orientation of each 
line scan over the specimen is arbitrarily programmable. 
Specifically FIG. 8 shows a portion of a conductive trace 150 dog-legging 
around a conductive pad 152 on an insulative material. Scan lines 154 and 
156 have been added to illustrate two potential locations where multiple, 
independent measurements may be made sequentially without having to 
reposition the substrate. Stated in another way, the scanning control 
system of the present invention can be programmed to deflect the electron 
beam to separated regions of the portion of the specimen surface beneath 
the beam deflection window of the electron beam column without moving the 
specimen. This ability to average over very small areas, and over 
different orientations, allows for accurate rapid metrology directly off 
the segment, without repositioning the sample. 
Another embodiment of the present invention to neutralize the charge 
build-up on a substrate during scanning, and the resultant darkened region 
is shown in FIG. 9. Enclosed within vacuum chamber 200 are electron beam 
source 12, focusing column and lens assembly 14 and specimen 20 as in FIG. 
2. Additionally, a capillary tube (or capillary array) 202 (such as made 
by Galileo of Sturbridge, Ma.), made of an electrically conductive 
material, is inserted into chamber 200 between lens assembly 14 and 
specimen 20 with an orifice of tube 202 directed at the point where the 
electron beam impinges on specimen 20. External to vacuum chamber 200 is 
supply tank 206 to contain an inert gas (e.g., argon) for delivery to 
capillary tube 202 via the serial connection of leak value 204 (e.g., 
Varian Model 951-5106) and a gas supply tube 208. The purpose of leak 
valve 204 is to control the rate at which the inert gas is injected into 
chamber 200 to maintain the vacuum at the desired level (e.g., 10-.sup.4 
Torr). Thus, by injecting the inert gas into chamber 200 at the point at 
which the electron beam scans specimen 20 the gas ionizes and in so doing 
neutralizes the charge build-up on specimen 20. 
Although the invention has been described in relation to various 
implementations, together with modifications, variations, and extensions 
thereof, other implementations, modifications, variations and extensions 
are within the scope of the invention. Other embodiments may be apparent 
to those skilled in the art from consideration of the specification and 
invention disclosed herein. The invention is therefore not limited by the 
description contained herein or by the drawings, only by the scope of the 
claims.