Electron beam irradiating apparatus and electric signal detecting apparatus

To prevent electric charge up from being accumulated on the plane scanned by an electron beam and further to improve the S/N ratio, an electron beam irradiating apparatus comprising: position information signal outputting section for outputting position information signals, in sequence to designate positions at which an electron beam is irradiated on a plane scanned by the electron beam, so as to designate the irradiation positions at random; and irradiation controller for controlling the electron beam to irradiate the electron beam at the irradiation positions in response to the outputted position information signals. Further, to integrate an photoelectric signal over a sufficient time interval within the period of the pixel clock signal, the electric signal detecting circuit comprises a plurality of sample hold circuits and a selecting circuit for selecting and activating the sample hold circuits in sequence.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electron beam irradiating apparatus, 
and specifically to an electron beam irradiating apparatus which can 
irradiate an electron beam upon a plane scanned by an electron beam, by 
designating electron beam irradiation positions. Further, the present 
invention relates to an electric signal detecting apparatus incorporated 
in the electron beam irradiating apparatus, and more specifically to the 
detecting apparatus for generating a video signal in response to a 
photoelectric signal generated by the electron beam irradiating apparatus. 
2. Description of the Prior Art 
In the semiconductor manufacturing process for instance, the microstructure 
of a semiconductor sample is observed with the use of an electron beam 
irradiating apparatus such as an electron microscope, for instance. 
In the prior art electron beam irradiating apparatus, a saw-tooth waveform 
signal referred to as a horizontal scanning signal and a vertical scanning 
signal is applied to a scanning coil to scan irradiation positions on a 
scanning plane in sequence continuously. Further, a horizontal blanking 
signal is applied to the scanning coil to return the electron beam to the 
start position for each scanning line. Additionally, whenever all the 
scanning operation ends once on one scanning plane, a vertical blanking 
signal is applied to the scanning coil, so that the same area can be 
irradiated with the electron beam repeatedly. 
In the case where the semiconductor sample is observed in the semiconductor 
manufacturing process, the surface of the semiconductor sample is usually 
formed of an insulating substance such as photoresist. Therefore, when the 
sample as described above is continuously irradiated with the electron 
beam of the prior art electron microscope, there arises a problem in that 
the sample is charged up electrically because the conductive path of the 
irradiated electrons is small and narrow. 
Once this charge-up phenomenon exists, since the irradiation direction of 
the electron beam applied upon the sample is distorted by the electric 
charge, an object to be observed by the electron microscope cannot be 
accurately photographed, or noise is superposed upon the object detection 
signals. 
To reduce the electric charge accumulated on the sample, various 
countermeasures have been so far proposed, for instance such that the 
voltage for accelerating the electron beam is lowered (low voltage 
method); the electron density is reduced (low doze method); the scanning 
speed is increased (high speed scanning method), etc. In the 
above-mentioned prior art methods, however, it is still extremely 
difficult to hold all over the scanned sample surface in an equipotential 
level. 
One of the major reasons is that in the prior art electron beam irradiating 
apparatus, the line scanning is repeated, except a special case as in an 
automatic focusing operation. In this repeated line scanning operation, as 
far as the conductive sample is being scanned, there exists no specific 
problem. However, when the insulating sample is being scanned repeatedly, 
the electric charge up increases in proportion to the number of repeated 
scanning operation. When the scanning width is as narrow as several pixels 
in particular, the charged electrons can be well discharged, so that the 
electric charge is not accumulated, in the case of where the scanning 
width extends to about 50 pixels, with the result that it becomes 
difficult to observe the sample of insulating material. 
Further, in the prior art electric signal detecting apparatus incorporated 
in the electron beam irradiating apparatus, when the electric signal such 
as photoelectric signals are integrated by the sample hold circuit in 
response to the pixel clock signal of the video signal within the period 
of the clock signal, since the number of the sample hold circuit is only 
one, there exists a problem in that it is impossible to integrate the 
photoelectric signal over a sufficient time interval within the period of 
the clock signal, because the same sample hold circuit must respond to the 
succeeding clock signal. 
SUMMARY OF THE INVENTION 
With these problems in mind, therefore, it is the object of the present 
invention to provide an electron beam irradiating apparatus and the method 
therefor, by which an image of a high S/N ratio can be obtained without 
producing the electric charge up accumulated on a sample to be observed. 
Further, it is another object of the present invention to provide an 
electric signal detecting apparatus which can integrate the electric 
signals over a sufficient time interval within the period of the clock 
signal, to obtain a reliable video signal on the basis of the 
photoelectric signal generated by the electron beam irradiating apparatus. 
To achieve the above-mentioned object, the present invention provides an 
electron beam irradiating apparatus comprising: position information 
signal outputting means for outputting position information signals for 
designating positions at which an electron beam is irradiated on a plane 
scanned by the electron beam in sequence, so as to designate the 
irradiation positions at random; and irradiation control means for 
controlling the electron beam to irradiate the electron beam at the 
irradiation positions in response to the outputted position information 
signals. 
Further, to achieve the above-mentioned object, the present invention 
provides an electron beam irradiating apparatus comprising: position 
information signal outputting means for outputting position information 
signals for designating positions at which an electron beam is irradiated 
on a plane scanned by the electron beam in sequence, so as not to 
designate the adjacent irradiation positions continuously; and irradiation 
control means for controlling the electron beam to irradiate the electron 
beam at the irradiation positions in response to the outputted position 
information signals. 
Further, to achieve the above-mentioned object, the present invention 
provides an electron beam irradiating method comprising the steps of: 
outputting position information signals for designating positions at which 
an electron beam is irradiated on a plane scanned by the electron beam in 
sequence, so as to designate the irradiation positions at random; and 
controlling the electron beam to irradiate the electron beam at the 
irradiation positions in response to the outputted position information 
signals. 
Further, to achieve the above-mentioned object, the present invention 
provides an electric signal detecting apparatus comprising: a plurality of 
sample hold circuits for integrating electric signals in response to a 
clock signal over within a period of the clock; and a selecting circuit 
for selecting and activating said sampling hold circuits for integrating 
the electric signals, in sequence. 
In the electron beam irradiating apparatus according to the present 
invention, since the scanning plane is irradiated with the electron beam 
at random, without irradiating the same irradiation position continuously; 
or alternatively in accordance with a predetermined order, without 
irradiating the adjacent irradiation positions continuously, it is 
possible to prevent electric charge up from being accumulated on the 
scanning surface. 
Further, in the electric signal detecting apparatus according to the 
present invention, since a plurality of sample hold circuits are provided, 
and the sample hold circuits are activated in sequence by the select 
circuit, the respective sample hold circuits can integrate the electric 
signals over a sufficient time interval within the period of the clock 
signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the electron beam irradiating apparatus according to 
the present invention will be described hereinbelow with reference to FIG. 
1. 
In FIG. 1, an x deflection memory 1 and a Y deflection memory 2 are of 
EPROM (erasable programmable read-only-memory) for storing random numbers 
X.sub.P and Y.sub.P, respectively generated by random number generating 
means such as a host computer, for instance. These random numbers are any 
numbers from 1 to 512 and outputted in a random order. The X deflection 
memory 1 and the Y deflection memory 2 are provided with addresses from 1 
to 512, to which random numbers X.sub.P1, X.sub.P2, . . . , Y.sub.P1, 
Y.sub.P2, . . . , outputted from the random number generating means at 
random are stored, respectively. In the X or Y deflection memory 1 or 2, 
the abscissa denotes the address number, and the ordinate denotes the 
random number. 
A clock 3 generated from a clock generating means (not shown) is inputted 
both an X deflection counter 4 and a counter 6. The X deflection counter 4 
counts the number of the inputted clock 3. On the basis of this counted 
value, the address is designated. A random number value X.sub.P of a data 
stored in the designated address is outputted to an X deflection register 
7, so that the electron beam is scanned to the X axis position in response 
to the outputted random number, without scanning the Y axis position. 
Further, when the number of clock 3 reaches 512 beginning from 1, the 
counter 6 is cleared up and simultaneously the Y deflection counter 5 is 
counted up by one clock. On the basis of the clock number of the Y 
deflection counter 5, the address of the Y deflection memory 2 is 
designated, so that the random number value Y.sub.P of a data stored at 
the designated address is outputted to a Y deflection register 8. 
Accordingly, the electron beam is irradiated at the Y axis position 
shifted by the position corresponding to the next random numbers Y.sub.p. 
As described above, when the value of the Y deflection counter 5 reaches 
512, the irradiation of electron beam is completed by one frame. 
The random numbers X.sub.P and Y.sub.P outputted to the X deflection 
register 7 and the Y deflection register 8 are D/A converted by an X 
deflection D/A convertor 9 and a Y deflection D/A convertor 10. 
The output signals of the X deflection D/A convertor 9 and the Y deflection 
D/A convertor 10 are amplified by deflection amplifiers, respectively, and 
then inputted to a scanning coil (not shown) as a horizontal scanning 
signal (H. SCAN) and a vertical scanning signal (V. SCAN). The scanning 
coil generates deflection voltages corresponding to the random numbers 
X.sub.P and Y.sub.P, to irradiate the electron beam at irradiation 
positions corresponding to the random numbers X.sub.P and Y.sub.P on the 
scanning plane. 
Further, since the electron beam is irradiated upon the scanning plane at 
random, a blanking time during which the electron beam is not irradiated, 
as shown in FIG. 4, is provided for each time required to shift the 
electron beam. 
With reference to FIG. 2, the shift motion of the irradiation position of 
the electron beam on the scanning plane 11 will be described. Here, the 
numeral 12 denotes an irradiation position at which the X axis position is 
X.sub.P1 and the Y axis position is Y.sub.P1. The electron beam is shifted 
from the irradiation position 12 to the irradiation position 13 with the Y 
axis position Y.sub.P1 kept as it is, and then shifted finally to the 
irradiation position 15 via the irradiation position 14. Thereafter, the Y 
axis position is shifted from Y.sub.P1 to Y.sub.P2, and the electron beam 
is shifted from the irradiation position 16 to the irradiation position 19 
finally via the irradiation positions 17 and 18 with the Y axis position 
Y.sub.P2 kept as it is. Successively, the Y axis position is shifted from 
Y.sub.P2 to Y.sub.P3, so that the irradiation position is shifted to 20. 
As described above, the electron beam is irradiated at all the irradiation 
positions corresponding to all the pixels of 512.times.512 pixels on the 
scanning plane 11, so that the irradiation for one frame can be completed. 
The above-mentioned operation is repeated in the same way. 
In the construction of this embodiment, since the scanning plane is 
irradiated at random with the electron beam, it is possible to suppress 
the electric charge up accumulated on the scanning plane. 
In the above-mentioned embodiment, the scanning is first made along the X 
axis direction with the Y axis position kept at a fixed position. Without 
being limited thereto, however, it is of course possible to scan both the 
X and Y axis positions simultaneously. 
Further, the frame construction in which one frame is composed of 
512.times.512 pixels has been explained, by way of example. Without being 
limited thereto, it is also possible to easily apply the present invention 
to such a frame construction that one frame is composed of 512.times.480 
or 1024.times.1024 pixels, by simply modifying the design values to the X 
deflection counter 4 and the Y deflection counter 5, and modifying the 
memory size of the X deflection memory 1 and the Y deflection memory 2. 
A second embodiment of the present invention will be described hereinbelow. 
In this second embodiment, the random number generating means is not used, 
but position information signals for designating 1 to 512 irradiation 
positions are stored previously in the respective addresses of the x 
deflection memory 1 and the Y deflection memory 2. 
The above-mentioned position information signals are so determined as not 
to designate two adjacent irradiation positions continuously. Further, the 
position information signals are arranged appropriately according to the 
distribution of the electrically conductive region and the electrically 
non-conductive regions which constitute the scanning plane, in order to 
prevent electric charge up from being accumulated at the electron beam 
irradiation positions on the scanning plane 11. 
In more detail, when the electrically conductive region on the scanning 
plane is irradiated with the electron beam, the relatively adjacent 
regions are irradiated collectively to reduce the load applied to the 
scanning coil. On the other hand, when the electrically non-conductive 
region is irradiated with the electron beam, the irradiation positions 
continuously irradiated with the electron beam is scattered. The 
above-mentioned irradiating method can reduce the electric charge up 
accumulated on the scanning plane. 
Further, with respect to the arrangement of the electron beam on the basis 
of the position information signals, it is also preferable to previously 
prepare a plurality of patterns corresponding to the objects to be 
observed and to exchange the patterns according to the object to be 
observed. 
In the construction of the above-mentioned embodiment, it is possible to 
determine an appropriate irradiation conditions according to the pattern 
of an object to be observed, for preventing electric charge up from being 
accumulated on the scanning plane. 
A third embodiment of the present invention described hereinbelow with 
reference to FIGS. 3 to 6, which relates to an electric signal detecting 
apparatus incorporated in the electron beam irradiating apparatus 
according to the present invention. The electric signal detecting 
apparatus is used to obtain a video signal on the basis of the 
photoelectric signal obtained by the electron beam irradiating apparatus. 
FIG. 5 shows an ordinary electric signal detecting circuit for generating a 
video signal on the basis of secondary electrons emitted by irradiating 
the scanning plane with an electron beam generated by an electron beam 
irradiating apparatus. 
In FIG. 5, the secondary electrons are transformed into a light signal by a 
scintillator 30. The transformed light signal is introduced via a light 
guide 31 to a photomultiplier 32 by which the light signal is transduced 
into an electric signal and further the transduced signal is amplified. 
The current output signal of the photomultiplier 32 is further amplified 
by a charge amplifier including a resistor 33, and then converted into a 
voltage signal through a current-voltage converting circuit 34. The output 
signal of the current-voltage converting circuit 34 is passed through a 
low-pass filter 35 to remove high frequency noise, further passed through 
an amplifier 36 to regulate the gain level, and then inputted to a sample 
hold circuit 37. 
As depicted in FIG. 6, the sample hold circuit 37 integrates the output 
signal of the amplifier 36 over the time interval within the period of the 
clock signal in response to a pixel clock 38 for prescribing the pixels of 
a video signal 39, to output the video signal 39 of 8 bits. 
In this embodiment, the sample hold circuit 37 is composed of a plurality 
of sample hold circuits 37a, 37b . . . as shown in FIG. 3. 
In FIG. 3, a selecting circuit 40 activates the sample hold circuits 37a, 
37b, . . . in sequence, respectively via respective switching means 41a, 
41b, . . . , whenever one pixel clock 38 is inputted to the selecting 
circuit 40. The sample hold circuits 37a, 37b, . . . integrate the output 
signal of the amplifier 36 during an integration time T determined on the 
basis of the electron beam scanning time. The integrated signal is A/D 
converted into a video signal of 8 bits representative of 256 gray levels, 
for instance. The video signal outputted from the respective sample hold 
circuits 37a, 37b, . . . are added through an adder 42 and then outputted 
finally as a video signal 39. 
The operation of the present embodiment will be described hereinbelow. In 
response to the pixel clock, the selecting circuit 40 closes any one of 
the switch means 41, for instance only the switch 41a to activate only the 
sample hold circuit 37a connected to the closed switch 41a. 
As depicted in FIG. 4, the sample hold circuit 37a integrates the output of 
the gain level amplifier 36 over the integration time T.sub.I within the 
period T of the pixel clock 38. It is preferable to determine this 
integration time T.sub.I as long as possible within the period T of the 
pixel clock 38, in order to average the statistic factor of the emitted 
secondary electrons and further to improve the S/N ratio. Therefore, the 
integration time T.sub.I is determined so as to satisfy the condition as 
T.sub.I -T-T.sub.B, where T.sub.B denotes a blanking time required to 
shift the electron beam to another irradiation position. 
After having been integrated within the integration time T.sub.I, the input 
signal of the sample hold circuit 37a is outputted through the adder 42. 
After the integration time T.sub.I has elapsed, when a holding circuit of 
the sample hold circuit 37a is turned off, a discharge circuit is turned 
on, so that the sample held signal is discharged. In the present 
invention, it is not required that the sample hold circuit 37a is 
activated in response to the succeeding pixel clock 38; in other words, it 
is unnecessary to complete the discharge within the period T of the pixel 
clock 38. Therefore, it is possible to determine the integration time 
T.sub.I freely, as far as T.sub.I -T-T.sub.B is satisfied, thus enabling 
the integration time T.sub.I to be determined longer than is conventional. 
Successively, when the succeeding pixel clock 38 is inputted, the selecting 
circuit 40 activates only the sample hold circuit 37b. In the same way as 
described above, the sample hold circuits are activated in sequence, 
repeatedly. 
In the construction of the present invention, since a plurality of sample 
hold circuits are activated in sequence, each of the sample hold circuits 
can integrate the electric signal over a sufficient time interval within 
the period of the clock signal. 
As a result, it is possible to remove noise difficult to prevent in the 
case of instantaneous sampling operation or short time integrations, so 
that the S/N ratio can be improved. In addition, since no low-pass filter 
is required, images of higher S/N ratio can be obtained. 
Further, since the statistic factor of the secondary electrons emitted when 
the electron beam is irradiated can be averaged sufficiently, the signals 
of higher reliability can be generated. 
As described above, in the electron beam irradiating apparatus according to 
the present invention, since the irradiation positions on the scanning 
plane are irradiated with the electron beam at random, it is possible to 
prevent the scanning plane from being charged up. 
Further, since a plurality of sample hold circuits are provided so as to be 
activated in sequence by the selecting circuit, it is possible to 
integrate the electric signal over a sufficient time interval within the 
period of the clock signal.