Potential analyzer

A potential analyzer for analyzing a potential at a specimen such as an LSI device or the like. This analyzer comprises: a detector to detect secondary electrons to be emitted from a specimen by radiating a primary electron beam thereto; a retarding grid, provided between the specimen and the detector, for controlling a detection amount of the secondary electrons; a circuit for applying a voltage to the retarding grid to keep the output of the secondary electron detector at a constant value; and a setting circuit for automatically setting the gain of the detector so as to allow an operating range of the voltage to the retarding grid to be within a specified value.

BACKGROUND OF THE INVENTION 
The present invention relates to a potential analyzer for analyzing a 
potential at a surface of an object to be analyzed (for example, a 
potential in an LSI device) using an electron beam as a probe. 
It is known that a potential at a portion where the electron beam is 
radiated can be measured with a scanning electron microscope by equipping 
it with an apparatus for analyzing energy of secondary electrons (refer to 
Japanese Patent Publication No. 51024/72). 
FIG. 1A shows this principle. A retarding grid G is arranged between a 
specimen and a secondary electron detector 4 disposed to face the specimen 
1. The retarding grid G forms a potential barrier to discriminate energy 
of secondary electrons 3 emitted from the specimen 1 due to its 
irradiation with an electron beam 2. FIG. 1B is a diagram showing the 
operation of this potential barrier. In the case where the retarding grid 
G is not used, every secondary electron 3 is detected by the secondary 
electron detector 4. An energy distribution of the secondary electrons 
emitted from the specimen 1 at zero potential is as shown by A in FIG. 1B. 
When the potential at the specimen 1 is at -5 V, the resulting secondary 
electron energy distribution is as indicated by B. When the retarding grid 
G is provided and a voltage of -5 V is applied thereto, secondary 
electrons to be detected are limited to those having 5 eV or more, so that 
a change occurs in the detection quantity of secondary electrons to be 
detected in dependence upon the potential at the specimen 1. In this way, 
since the secondary electron detection quantity relates to the potential 
at the specimen, the potential at the specimen 1 can be known from the 
detection quantity of secondary electrons. 
However, in this method of analyzing the potential on the basis of only the 
arrangement of the retarding grid G, it is difficult to quantitatively 
analyze the potential since there is not a linear relation between the 
potential at the specimen 1 and the detection quantity of secondary 
electrons. Therefore, for linearization of the above-mentioned relation, a 
feedback loop operation is known in which the potential at the retarding 
grid is adjusted by a specific circuit so as to always maintain the 
detection quantity of secondary electrons constant (H. P. Feurbaum et al, 
IEEE Journal of solid state circuits, Vol. SC-13, No. 3, 1978). 
FIG. 2 is a block diagram to describe this feedback loop operation. An 
output of the secondary electron detector 4 is compared with a reference 
voltage 6 and the difference is amplified by an amplifier 5 and its output 
is given to the retarding grid G. Since the potential at the retarding 
grid G decreases in association with an increase in the detection quantity 
of secondary electrons, even if the potential at the specimen 1 changes 
arbitrarily, the secondary electron detection quantity will be kept 
constant. Since an amount of change in the potential at the specimen is 
made to have a one-to-one correspondence to an amount of change of the 
retarding grid G, a change in the potential at the specimen 1 can be 
quantitatively known by measuring the potential at the retarding grid G. 
However, to accurately perform this measurement, a variable range of the 
potential at the retarding grid G has to be set within a predetermined 
range. The gain of the secondary electron detector 4 is usually adjusted 
to make the detector 4 operative within this range. However, if the 
intensity of the primary electron beam changes or if the material is 
different from a specimen to another, this operating point will be 
changed, which will cause a large error in measurement. For example, in 
the case of measuring voltage waveforms on circuit lines in an LSI, there 
are a number of points to be measured therein and the resulting voltages 
are compared with each other. However, the materials of the circuit lines 
in the LSI are not always identical (for instance, Al, poly-Si, gold, 
etc.). Due to this, an emission efficiency of secondary electrons differs, 
so that the gain of the secondary electron detector 4 has to be adjusted 
to match with a specified operating point (potential at the retarding grid 
G). This adjustment is needed almost whenever the measurement point is 
changed. Namely, in the foregoing method, the adjustment has to be 
performed while always paying attention to the operating point, which 
makes this method complicated and brings about measurement errors therein. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of such points and an 
object of the invention is to provide a potential analyzer which can 
automatically adjust its operating point. 
To accomplish the above object, a potential analyzer according to one 
aspect of the present invention intends to automatically set an operating 
point of a retarding grid G and comprises: a detector for detecting 
secondary electrons to be emitted from a specimen by radiating a primary 
electron beam thereto; a retarding grid, disposed between the specimen and 
the detector, for controlling a detection amount of the secondary 
electrons; a circuit for applying a voltage to the retarding grid to 
maintain the output of the secondary electron detector at a constant 
value; and a setting circuit for setting the gain of the detector so that 
the operating range for the voltage to be applied to the retarding grid is 
within a specified value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described in detail hereinbelow with 
reference to an embodiment. 
FIG. 3 is a block diagram showing one embodiment of the present invention. 
Generally, the potential at the specimen 1 (e.g., LSI device) to be 
analyzed is periodically changing. As already described in the 
conventional example of FIG. 2, the potential at the retarding grid G 
corresponds to the potential at the portion where the electron beam was 
radiated, owing to the use of the feedback loop circuit. A waveform B in 
FIG. 4A shows a retarding grid potential change when an operating 
potential of the retarding grid G is within a specified range C by 
adjusting the gain of the secondary electron detector 4. When such 
adjustment is wrong, an amplitude of the retarding grid potential change 
becomes small as shown by a waveform A, so that the measurement is not 
accurately performed. 
In this embodiment, a low-pass filter 8 is connected to the retarding grid 
G and the output of the low pass filter 8 and a reference voltage source 9 
for an operating point are compared using a differential amplifier or 
comparator 10 for setting an operating point, and its difference signal is 
fed back to an amplifier 7 for the secondary electron detector 4 so as to 
set the gain of the detector. The cut off frequency of the low pass filter 
8 is set for a frequency which is sufficiently lower than the frequency to 
be measured in the specimen 1. 
This operation will be explained using FIG. 4B. For example, it is now 
assumed that the operating point is inappropriate as indicated by the 
waveform A shown in FIG. 4A. In such a case, the output of the low pass 
filter 8 will be in such a waveform as indicated by A' in FIG. 4B, in 
which the high frequency components of the change in the retarding grid G 
have been removed. Although it is illustrated here that some variations 
still remain, if the frequency of the potential change in the specimen 1 
is, for example, 100 Hz or higher and the cutoff frequency is 1 Hz, the 
waveform A' will become almost a straight line. The waveform A' is 
compared with a set potential E for the operating point. Since there is a 
large differential voltage D, the differential amplifier 10 generates a 
large output and acts to reduce the amplification degree of the amplifier 
7. This operation is continued until the differential voltage becomes zero 
and finally stops with the waveform B' attained. In this way, the use of 
this method allows the mean value of the potentials at the retarding grid 
with respect to time to coincide with the reference voltage set for the 
operating point. 
In this embodiment, although the amplification degree of the amplifier 7 
has been adjusted for adjusting the gain of the detector, the gain of the 
secondary electron detector 4, e.g., photomultiplier voltage may be 
adjusted. FIG. 5 shows an example of such a method of adjusting the 
photomultiplier voltage. The voltage to be applied to the photomultiplier 
is generally a DC voltage of 0 to 1000 V. This DC voltage is formed by 
generating a high frequency voltage from a high-frequency generator 51 and 
rectifying this by, for example, a so-called Cockcroft-Walton type 
rectifier 53. By varying the output of the high-frequency generator 51, 
the photomultiplier voltage is preset for a value near the operating 
point. This operation is in most cases performed when the secondary 
electron image is observed to determine the analyzing portions. The 
automatic adjustment of the photomultiplier voltage is performed by 
providing a modulator 52 in the post stage of the high-frequency generator 
51. This modulator 52 will pass the output of the generator 51, without 
its modulation, to the rectifier 53 when an input voltage 54 is zero, will 
attenuate the output of the generator 51 when the input voltage 54 is 
positive, and will amplify the output of the generator 51 when the input 
voltage 54 is negative. This operation equivalently corresponds to that of 
the amplifier 7 and provides the effect of the invention. 
The method whereby the mean value of the potential change of the retarding 
grid G is compared with the reference value has been described in 
conjunction with FIGS. 4A and 4B. Instead of using the mean value obtained 
from the low pass filter to compare with reference value, the highest 
value or lowest value of the changes may be detected and used for the 
automatic adjustment of operating point. This method is also useful. 
FIG. 6 shows an example of a circuit to detect the highest value in the 
case of detecting the highest value in another embodiment of the 
invention. The circuit example of FIG. 6 may be substituted for the low 
pass filter 8 in FIG. 3. A terminal 66 is in electrical connection to the 
grid G and a terminal 67 is in electrical connection to the differential 
amplifier 10 for setting the operating point. Since the voltage at the 
terminal 66 changes within a range from a positive to negative value, a DC 
component is added by means of resistors 68 and 62 and a positive DC power 
source 61 so that the voltage at a terminal 69 is always positive. Only 
when the potential at the terminal 69 is higher than that across a 
capacitor 64 having a capacitance C, a current flows through a diode 63 to 
the capacitor 64. Thus, the highest potential value is stored in the 
capacitor 64. A discharge resistor 65 having a resistance R is provided 
for this capacitor 64. This discharge time constant is determined by R and 
C and is practically set to be substantially equal to the time constant of 
the low pass filter 8 which may be one second or so. In this example 
although the case where the highest value is used has been described, it 
is also possible to perform such automatic adjustment similarly using the 
lowest value. 
In the above-described embodiment, the circuit for setting the operating 
point is always operating. In still another embodiment of the invention 
shown in FIG. 7, the circuit for setting the operating point is not always 
made operative. That is, in this embodiment, the output of the 
differential amplifier 10 for setting the operating point is connected to 
the amplifier 7 through a switch 12 and a memory circuit 11. The switch 12 
is turned on immediately before the measurement is started. The operating 
point is set in one to ten seconds for a point determined by the reference 
voltage source 9 for giving the operating point, owing to the circuit 
structure including the filter 8, source 9, amplifier 10 and memory 
circuit 11. It may be apparently set for the mean value, highest value or 
lowest value. After the switch 12 is turned off, the voltage of the output 
of the differential amplifier 10 when the switch 12 is on is recorded in 
the memory circuit 11 and the amplification degree of the amplifier 7 is 
determined in response to this voltage and the operating point is not 
changed. The measurement is performed (measurement data is obtained) after 
the switch 12 is turned off. The simplest example of the memory circuit 11 
is constituted by a combination of a motor and a potentiometer. Since use 
is often made of a microcomputer to control the memory circuit, the memory 
circuit may be implemented by a semiconductor device or the like. In such 
a case, the ON-OFF operation of the switch 12, collection of data and the 
like are automatically performed in response to an execution instruction 
of measurement. With such a constitution, it is possible to eliminate an 
error of measurement which sometimes occurs when the frequency to be 
analyzed becomes a low frequency near the cutoff frequency of the low pass 
filter 8 in the foregoing embodiment. 
Although the number of measuring portions, i.e., the number of portions 
where the primary electron beam is radiated is one in the above 
embodiment, in many cases in the actual measurement, a plurality of 
portions are measured and the data obtained from those portions are 
compared. In such a case, the present invention can be effectively 
utilized. FIG. 8 shows an embodiment for this purpose. In this embodiment, 
an X deflector 18 and a Y deflector 19 which are used to move the portion 
to be irradiated by the primary electron beam 2 are interconnected to the 
operating point setting circuit. The X deflector 18 and Y deflector 19 are 
driven by an X deflection amplifier 16 and a Y deflection amplifier 17, 
respectively. The output of the X deflection amplifier 16 is varied 
through a change-over switch 13a by position setting voltage circuits 14a, 
14b and 14c. The output of the Y deflection amplifier 17 is varied through 
a change-over switch 13b by position setting voltage circuits 15a, 15b and 
15c. The position setting voltage circuits 14a, 14b, 14c, 15a, 15b and 15c 
are arbitrarily changed by an operator. For example, the change-over 
switches 13a and 13b are set at position .circle. and the measuring 
portion (A) is determined by the position setting voltage circuits 14a and 
15a. This operation may be alternatively performed at positions .circle. 
and .circle. . Then, the measuring portions (A), (B) and (C) are 
determined. Thereafter, the measurement is performed. 
First, the interlocked change-over switches 13a, 13b and 13c are set at 
positions .circle. . In this case, the switch 13c is turned on and the 
corresponding operating point setting circuit is made operative, so that 
the operating point is automatically set. Then, the switches are set at 
position .circle. and the measurement is done, thereby allowing the 
potential at the retarding grid G to be recorded or to be memorized by a 
circuit. Subsequently, the switches are set at position .circle. and the 
similar measurement is done. By subtracting the value of the reference 
voltage source 9 at the operating point from the resulting measurement 
values at (B) and (C), the voltages at the points (B) and (C) are analyzed 
using the voltage at the point (A) as a reference. If the point (A) is 
used as the earth electrode, the potentials at the points (B) and (C) 
could be analyzed by this measurement. In addition, by using the method 
whereby the switching operations of (A), (B) and (C) are repeatedly 
performed a number of times, it is also possible to correct the 
measurement error (variation in the primary electron beam or the like) 
regarding the time. The operation in this embodiment can be easily 
performed by increasing the measuring points or the like due to the 
control by a computer. 
The method of the present invention can be applied using the substantially 
similar circuit even in case of the potential measurement by a so-called 
stroboscopic method in which the electron beam is pulse modulated in 
synchronism with a change in the specimen (for example, the driving 
frequency for the LSI) and thereby to also enable the high frequency 
potential of the order of 1 to 100 MHz to be analyzed. 
As described above, according to the present invention, since the operating 
point of the feedback loop circuit for analyzing the potential can be 
automatically set, it is possible to remove the complicated factors which 
causes measurement errors as in the conventional analyzer and the 
operability is remarkably improved. The invention will be effective if it 
is used to examine the functions and defects and the like of LSI devices 
or the like.