Focusing position detecting device in optical magnifying and observing apparatus

A focusing position detecting device in which a laser beam is directed through an objective lens toward the surface of an object to form a minute spot on the surface of the object, and the beam reflected from the surface of the object is led through the objective lens again toward a concentrating point, so as to detect the focusing position of the objective lens by measuring the position of the concentrating point. In the device, a photoelectric element is provided to detect the intensity of the reflected beam, and the intensity of the laser beam is so controlled that the output of the photoelectric element is maintained constant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a focusing position detecting device in an 
optical magnifying and observing apparatus which magnifies very small or 
fine patterns for observation with observing means such as a microscope. 
2. Description of the Prior Art 
LSI's of high integration density, bubble memories, photosensitive plates 
of image pickup tubes, and the like have very small or fine patterns of 2 
to 3 .mu.m, and microscopes of high magnification are used for the 
inspection of the external appearance of these parts. The depth of the 
focus of such a high-magnification microscope is not more than 1 .mu.m, 
and automatic focusing with high accuracy is therefore demanded. 
The basic principle of attaining focusing in an apparatus commonly used for 
optically magnifying and observing a fine pattern will be described with 
reference to FIG. 1, for a better understanding of the present invention. 
Generally, an optical magnifying and observing apparatus comprises two 
optical systems, i.e., an observing system and a focusing position 
detecting system. However, in view of the fact that the present invention 
is specifically concerned with a focusing position detecting device, the 
latter or focusing position detecting system is only shown in FIG. 1, and 
the former or observing system is not shown to avoid confusion. Briefly 
describing, this observing system is to be understood to be a system in 
which visible radiation is projected through an objective lens 4 on the 
surface of an object 5 and radiation reflected therefrom is received for 
observation. (In such a system, a half-mirror of predetermined design is 
essentially required.) 
In the example shown in FIG. 1, a laser beam 1 is employed for the purpose 
of focusing position detection. The laser beam 1 is diverged by a concave 
lens 2, reflected then by a half-mirror 3 and concentrated by an objective 
lens 4 on the surface of an object 5. In FIG. 1, the position of the 
object 5 is indicated by the solid line when the laser beam 1 is 
accurately focused by the objective lens 4, and a minute spot 6 is formed 
on the surface of the object 5 in that case. The beam 7 reflected from the 
surface of the object 5 passes through the objective lens 4 again and 
passes then through the half-mirror 3 to be concentrated on a point 8. 
The combination of a pin-hole plate 9 formed with a pin hole 10 and a 
photoelective element 11 is provided for detecting the position of the 
concentrating point 8. The pin-hole plate 9 is arranged to oscillate in a 
direction of the arrow X (that is, in a vertical direction) in FIG. 1. 
FIG. 2 shows the waveform of the output from the photoelectric element 11. 
In FIG. 2, the horizontal axis X represents the direction of oscillation 
of the pin-hole plate 9, and the vertical axis V represents the level of 
the output from the photoelectric element 11. 
When now the reflected beam 7 from the object 5 is concentrated on the 
point 8 in the pin-hole 10 of the pin-hole plate 9 as shown by the solid 
lines in FIG. 1, the output V from the photoelectric element 11 has a 
generally triangular waveform having a peak at the point 8 on the X-axis, 
as shown by a curve 12 in FIG. 2. The output V from the photoelectric 
element 11 has such a waveform since the pin-hole plate 9 is oscillating V 
in the direction of the X-axis around the concentrating point 8 of the 
reflected beam 7. It will be seen in FIG. 2 that the output V from the 
photoelectric element 11 becomes lower as the pin-hole plate 9 moves a 
greater distance away from the concentrating point 8. 
Suppose that the surface of the object 5 shown by the solid line is 
displaced away from the objective lens 4 by a distance Z and is now 
located at a position 5' as shown by the broken line. In such a case, the 
reflected beam 7' concentrates on another point 8'. Therefore, when the 
pin-hole plate 9 is moved to the concentrating point 8', the output V from 
the photoelectric element 11 has a generally triangular waveform having 
its peak at the point 8' on the X-axis, as shown by another curve 12' in 
FIG. 2. 
Thus, whether or not the surface of the object 5 lies on the focusing 
position of the objective lens 4 can be detected by finding whether or not 
the output V from the photoelectric element 11 at the point 8 is maximum, 
provided that the pin-hole plate 9 oscillates around the concentrating 
point 8 in FIG. 1 (the point 8 on the X-axis in FIG. 2). Therefore, the 
laser beam 1 can be focused by the objective lens 4 on the surface of the 
object 5 by moving either the object 5 or the objective lens 4 until the 
photoelectric element 11 generates its maximum output V. 
FIG. 3 is a partly exploded perspective view of a prior art focusing 
position detecting device based on the principle above described. The 
structure and defects of the prior art device will now be described. In 
FIG. 3, the same reference numerals are used to designate the same parts 
appearing in FIG. 1. The observing system is disposed along the optical 
path indicated by the large arrow A, and its members are not especially 
shown to avoid confusion. A gas laser beam is used in FIG. 3, and the 
reference numeral 20 designates a gas laser beam emitter. The laser beam 1 
emitted from the gas laser beam emitter 20 is converged by a convex lens 
21 and is then diverged by a concave lens 2. After passing through a 
polarization beam splitter 22 and a quarter wavelength element 23, the 
laser beam 1 is reflected by a reflector 24 and is then concentrated by 
the condenser lens 4 to form a minute spot 6 on the surface of an object 
5. The reflected beam 7 from the object 5 is concentrated by the objective 
lens 4 again, and, after passing through the reflector 24 and quarter 
wavelength element 23, is deflected by the polarization beam splitter 22 
to be directed in a direction orthogonal with respect to the previous 
direction. Then, the reflected laser beam 7 passes through a concave lens 
25 and is concentrated by a condenser lens 26 to form a laser spot. The 
pin-hole plate 9 having the pin-hole 10 is oscillated, and the beam 
passing throug the pin-hole 10 is detected by the photoelectric element 
11. 
The waveform of the output V from the photoelectric element 11 is shown on 
the right-hand side of the photoelectric element 11 in FIG. 3. It will be 
seen that the displacement x of the pin hole 10 of the oscillating 
pin-hole plate 9 is plotted on the horizontal axis 27 and the output V 
from the photoelectric element 11 is plotted on the vertical axis 28. The 
curve 12 represents the signal waveform of the output V from the 
photoelectric element 11. Therefore, the object 5 is moved in the vertical 
direction for the purpose of focus adjustment until the maximum output V 
appears from the photoelectric element 11 at the center of oscillation of 
the pin-hole 10. 
However, a problem as pointed out presently is involved in the prior art 
device described above. 
It is generally acknowledged that, in such an observing apparatus, it is 
necessary to suitably adjust the focus of the optical system for 
observation of an object, and it is also necessary to observe various 
portions of the object while, for example, moving the object in a lateral 
direction. The direction of movement of the object 5 while attaining 
automatic focusing is indicated by the arrow B in FIG. 3 and similarly by 
the arrow B in FIG. 1. With the movement of the object 5 in the direction 
of the arrow B, a pattern 5a on the object 5 moves naturally in the same 
direction. Since the factor of reflection varies depending on the portion 
of the pattern 5a, the intensity of reflected beam 7 varies also with the 
movement of the object 5. 
Suppose now that a pattern 5a having a high reflection factor is present on 
the surface of the object 5, and such a pattern 5a is moving at a high 
speed in the direction of the arrow B in FIG. 1 and FIG. 3. The beam 7 
reflected from the pattern 5a having the high reflection factor passes in 
front of the pin-hole 10 while the pin-hole plate 9 is under oscillation. 
Consequently, the output V from the photoelectric element 11 within the 
oscillation range X of the pin-hole plate 9 will have a wafeform as shown 
in FIG. 4, and, it will be seen in FIG. 4 that a peak output V.sub.5a 
appears when the reflected beam 7 from the pattern 5a having the high 
reflection factor passes in front of the pin-hole 10. In this case, the 
level of the output V.sub.5a is higher than that of the output V.sub.5 
appearing at the exactly focused position 8. Consequently, when such a 
peak output V.sub.5a appears at the oscillating position x.sub.1 of the 
pin-hole plate 9, this position will be erroneously detected to be the 
exactly focused position of the object 5 relative to the objective lens 4. 
Thus, the prior art device has been unable to accurately detect the true 
focusing position when a pattern having a high reflection factor is 
present on the surface of an object and has therefore been unable to 
attain the function of automatic focusing. While the above description has 
referred to the case in which a pattern having a high reflection factor is 
present on the surface of an object, the same applies also to the cases in 
which a pattern having a low reflection factor and a stepped pattern are 
present. In such cases too, the prior art device has been unable to attain 
the function of automatic focusing because a point at which the 
photoelectric element generates its maximum output does not necessarily 
coincide with the true focusing position. It has therefore been a common 
practice to detect the height of a pattern on the surface of an object by 
means of an air micrometer for the purpose of automatic focusing. However, 
blowing of a stream of air onto the surface of the object has resulted in 
blowing of fine dust particles at the same time. In the past, accumulation 
of fine dust particles on the surface of the object did not pose any 
substantial practical problem. However, with the recent trend of producing 
finer patterns in the fields of semiconductor and other industries, 
accumulation of such fine dust particles has frequently resulted in the 
production of defective patterns, and it has been the tendency that the 
air micrometer finds lesser applications in these industrial fields. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a novel and 
improved focusing position detecting device which obviates the prior art 
defects pointed out above and which ensures automatic focusing in spite of 
the presence of patterns such as a pattern having a high reflection 
factor, a pattern having a low reflection factor and a stepped pattern on 
the surface of an object. 
The present invention is featured by the fact that the reflected beam from 
the surface of an object is sensed by a second photoelectric element 
disposed independently of the aforementioned photoelectric element 
provided for the purpose of detecting the beam concentrating position, and 
the intensity of the laser beam is so controlled that the second 
photoelectric element generates always an output of constant level. 
The above and other relevant objects, features and advantages of the 
present invention will become apparent when the description based on the 
accompanying drawings and the novel matters pointed out in the appended 
claims are carefully read.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the focusing position detecting device according 
to the present invention will now be described in detail with reference to 
the drawings. 
FIG. 5 is a partly exploded prespective view showing in detail the 
structure of a first preferred embodiment of the present invention. In 
FIG. 5, the same reference numerals are used to designate the same parts 
appearing in FIG. 3. The device of the present invention shown in FIG. 5 
differs from the prior art device shown in FIG. 3 in the two points which 
will be described presently. In the first place, an additional beam 
splitter 30 is disposed between the concave lens 25 and the condenser lens 
26 in the focusing position detecting system so as to divert a portion of 
the beam 7 reflected from the object 5, and a control system is provided 
for controlling the intensity of the laser beam 1 depending the level of 
the thus diverted beam. This control system includes, in addition to the 
polarization beam splitter 30, a condenser lens 31, a second pin-hole 
plate 32, a second photoelectric element 33 and a control circuit 34. 
Secondly, a semiconductor laser oscillator 35 is employed as the laser 
beam source. This semiconductor laser oscillator 35 is featured by the 
fact that, in response to the control of the voltage applied thereto, the 
intensity of the laser beam emitted therefrom changes at a very high 
response speed. 
The operation of the device having the structure shown in FIG. 5 will now 
be described. The laser beam 1 emitted from the semiconductor laser 
oscillator 35 is converged by the convex lens 21, diverged by the concave 
lens 2, and, after passing through the polarization beam splitter 22 and 
quarter wavelength element 23, reflected by the reflector 24 and projected 
through the objective lens 4 to form a minute spot 6 on the object 5. The 
reflected beam 7 from the surface of the object 5 is concentrated by the 
objective lens 4 again, and, after being reflected by the reflector 24 and 
passing through the quarter wavelength element 23, reflected by the 
polarization beam splitter 22. The arrow A in FIG. 5 indicates the optical 
path of the observing system. 
The beam 7 reflected by the polarization beam splitter 22 is diverged by 
the concave lens 25 and is then split by the polarization beam splitter 30 
into two portions. One of the portions is directed toward the reflected 
beam intensity measuring optical system in the control system added 
according to the present invention, and the other portion is directed 
toward the focusing position detecting optical system described already 
with reference to FIG. 3. The latter or focusing position detecting 
optical system detects the focusing position of the laser beam 1 on the 
surface of the object 5 and includes the condenser lens 26, a slit plate 
9' having a slit 10', a bimorph oscillation element 9a' and the 
photoelectric element 11. The bimorph oscillation element 9a' for causing 
oscillation of the slit plate 9' is mounted in a cantilever mode. In the 
present invention, the conventional pin-hole 10 is replaced by the slit 
10' to deal with the displacement of the oscillating slit plate 9' in the 
direction shown by the arrow x. 
The portion of the reflected beam 7 directed by the polarization beam 
splitter 30 toward the reflected beam intensity measuring optical system 
is concentrated by the condenser lens 31 and passes through the pin-hole 
of the pin-hole plate 32 to be incident upon the second photoelectric 
element 33. The photoelectric element 33 converts the intensity of the 
reflected beam 7 into an electrical quantity, and such an output from the 
photoelectric element 33 is fed back through the control circuit 34 to the 
semiconductor laser oscillator 35 to control the semiconductor laser 
oscillator 35 so that the output from the photoelectric element 33 can be 
maintained constant. Therefore, regardless of a variation of the 
reflection factor of the object 5 during the oscillation of the slit plate 
9', the intensity of the reflected beam 7 is controlled to be maintained 
constant, and the first photoelectric element 11 generates an output 
waveform as shown by a curve 12b in FIG. 5. Therefore, the desired 
automatic focusing can be attained by vertically moving the object 5 until 
the first photoelectric element 11 generates its maximum output at the 
center of oscillation of the slit 10'. 
The pin-hole plate 32 disposed in front of the second photoelectric element 
33 is provided so that the reflected beam 7 from the surface of the object 
5 can only be sensed. This pin-hole plate 32 acts to block incidence of 
that portion of the beam which is reflected from the back surface of the 
object 5 when the object 5 to be observed is, for example, a transparent 
one. 
FIG. 6 is a circuit diagram of one form of the control circuit 34 
preferably employed in the present invention. 
The purpose of the control circuit 34 shown in FIG. 6 is to control the 
voltage applied to the semiconductor laser oscillator 35 so that the 
output from the second photoelectric element 33 can be maintained at a 
predetermined voltage level V.sub.1. This predetermined voltage level 
V.sub.1 is determined by the combination of a regulated voltage source 36 
and a potentiometer 37. Referring to FIG. 6, the output from the 
photoelectric element 33 is amplified by an amplifier 38 to appear as an 
output voltage V.sub.2. The voltage V.sub.1 produced by the combination of 
the regulated voltage source 36 and the potentiometer 37 and the output 
voltage V.sub.2 from the amplifier 38 are applied to a differential 
amplifier 39, and its output is applied to the semiconductor laser 
oscillator 35. Thus, the semiconductor laser oscillator 35 generates the 
laser beam 1 in such a relation that the output voltage V.sub.2 from the 
amplifier 38 is always equal to the predetermined voltage V.sub.1. 
It will be seen fron the above detailed description that, by the use of the 
reflected beam intensity measuring optical system shown in FIG. 5 and the 
control system including the control circuit 34 shown in FIG. 6, the 
control circuit 34 acts to lower the level of the laser beam 1 emitted 
from the semiconductor laser oscillator 35 when a peak output V.sub.5a as 
shown in FIG. 4 is detected as a result of passing of the beam reflected 
from a pattern having a high reflection factor in front of the pin-hole 10 
(the slit 10') during oscillation thereof, as described with reference to 
FIGS. 3 and 4. FIG. 7 shows the waveform of the output V from the first 
photoelectric element 11 when the semiconductor laser oscillator 35 is so 
controlled. It will be seen in FIG. 7 that the peak output V.sub.5a shown 
in FIG. 4 is sufficiently suppressed as indicated by the arrow C. 
Therefore, the possibility of erroneous detection or mal-detection of the 
true focusing position is eliminated, and the true focusing position can 
be reliably detected. 
It will thus be understood that the present invention provides a highly 
universal automatic focusing device which dispenses with the use of the 
air micrometer and which reduces greatly the rate of production of rejects 
in the step of formation of very small patterns. 
While the first embodiment of the present invention shown in FIG. 5 has 
been described specifically with reference to the case in which a pattern 
having a high reflection factor is present on an object, it will be 
readily apparent that the focusing position on an object having a pattern 
having a low reflection factor and a stepped pattern can be similarly 
reliably detected by the utilization of the technical idea of the present 
invention according to which the intensity of the laser beam is controlled 
on the basis of the result of measurement of the intensity of the 
reflected beam. 
In the course of repeated experiments using the device shown in FIG. 5, the 
inventors have found a phenomenon as described now. Such a phenomenon will 
be explained with reference to FIG. 5 again. In FIG. 5, the beam 7 
reflected from the surface of the object 5 is to be the only one to be 
incident upon the photoelectric elements 11 and 33. Actually, however, 
there is a beam 4a reflected from the objective lens 4 itself. That is, 
when the laser beam 1 emitted from the semiconductor laser oscillator 35 
is reflected by the reflector 24 and is then incident upon the objective 
lens 4, the beam 4a reflected from the objective lens 4 itself is directed 
toward the photoelectric elements 11 and 33. 
In the focusing position detection and the reflected beam intensity 
measurement for the purpose of the focusing position detection according 
to the present invention, the reflected beam 7 from the surface of the 
object 5 bears an important significance as will be apparent from the 
foregoing description. Therefore, an undesirable influence of the 
reflected beam 4a from the objective lens 4 itself, as described above, 
must be eliminated. 
The inventors have conducted an experiment for finding the ratio between 
the intensity of the reflected beam 4a due to the above phenomenon and 
that of the reflected beam 7 from the object 5 by measuring the output 
from the photoelectric element 33, that is, the ratio V.sub.b :V.sub.c 
between the component V.sub.b of the output from the photoelectric element 
33 due to the reflected beam 4a and the component V.sub.c of the output 
from the photoelectric element 33 due to the reflected beam 7. The 
inventors have obtained the following experimental results although the 
above ratio is variable depending on the kind of the object 5, that is, 
the reflection factor of the surface of the object 5: 
(a) When the object 5 has a high reflection factor, the ratio is given by 
EQU V.sub.b :V.sub.c =1:20. 
(b) When a thin film is formed on the surface of the object 5, and the 
object has a low reflection factor, the relation V.sub.b &gt;V.sub.c holds 
frequently, and the ratio is given by 
EQU V.sub.b :V.sub.c =5:1. 
In the experimental results above described, the relation V.sub.b &lt;&lt;V.sub.c 
holds in (a), and this means that the embodiment shown in FIG. 5 can 
sufficiently detect the focusing position. 
However, the inventors have invented a focusing position detecting device 
which solves also the problem pointed out in (b), in an effort to further 
enhance the universalization of the present invention. Embodiments of such 
a device will be described with reference to FIGS. 8 and 9. Such a 
focusing position detecting device has a structure based on the structure 
shown in FIG. 5 and can detect the focusing position even when the object 
has a low reflection factor or includes a plurality of patterns having 
different reflection factors. 
The two embodiments shown in FIGS. 8 and 9 are featured by the fact that, 
in addition to the provision of a reflected beam intensity measuring 
optical system including a second photoelectric element as shown in FIG. 5 
as its essential part, there are provided means for sensing the output 
from a laser beam emitter, converting means for multiplying the output 
from the laser beam output sensing means by a previously calculated 
coefficient to amplify or attenuate the sensor output thereby finding the 
converted output value indicative of the intensity of another beam 
incident upon the second photoelectric element together with the beam 
reflected from the surface of the object, and control means for 
subtracting the converted output from the output of the second 
photoelectric element to limit the output of the second photoelectric 
element to that corresponding to the component of the reflected beam from 
the surface of the object only and to apply the resultant output to 
control means controlling the laser beam emitter, whereby the intensity of 
the laser beam emitted from the laser emitter is so controlled as to 
maintain constant the output indicative of the result of substraction. 
The laser beam emitter output sensing means employed in the embodiments 
shown in FIGS. 8 and 9 embodies one of the following three forms: 
(1) A third photoelectric element using a portion of the laser beam 1 
diverted from the path of the laser beam 1 toward the objective lens 4. 
(2) A third photoelectric element sensing the laser beam emitted in a 
direction opposite to the direction of emission of the laser beam 1 from 
the semiconductor laser beam oscillator 35. 
(3) A circuit by which the voltage itself applied to the semiconductor 
laser oscillator 35 is introduced to an amplifier or attenuator. 
As described hereinbefore with reference to FIG. 5, a portion 42 (FIG. 8) 
of the reflected beam 4a from the objective lens 4 and a portion 43 (FIG. 
8) of the reflected beam 7 from the object 5 are incident upon the second 
photoelectric element 33. Therefore, for the purpose of the control of the 
laser beam 1 emitted from the semiconductor laser oscillator 35, the 
output component V.sub.b resulting from the incidence of the portion 42 of 
the reflected beam 4a from the objective lens 4 may be substracted from 
the output (V.sub.b +V.sub.c) of the photoelectric element 33, and the 
remaining output component, that is, the output V.sub.c of the 
photoelectric element 33 corresponding to the portion 43 (FIG. 8) of the 
reflected beam 7 from the surface of the object 5 may be maintained 
constant. 
The inventors will now explain how to find the component V.sub.b of the 
output of the photoelectric element 33 corresponding to the incident 
portion 42 of the reflected beam 4a from the objective lens 4. The 
intensity of the reflected beam 4a from the objective lens 4 is 
proportional to that of the laser beam 1 emitted from the semiconductor 
laser oscillator 35. Therefore, by detecting the intensity of the laser 
beam 1, the intensity of the portion 42 of the reflected beam 4a from the 
objective lens 4 can be found, and, consequently, the intensity of the 
portion 43 of the reflected beam 7 from the object 5 can be found. 
The structure of the second embodiment of the present invention will now be 
described with reference to FIG. 8. As described hereinbefore, the 
important feature of the device according to the present invention resides 
in the fact that the output from the semiconductor laser oscillator 35 is 
so controlled as to maintain constant the intensity of the beam 7 
reflected from the surface of the object 5. Therefore, the structure of 
the reflected beam intensity measuring optical system in the second 
embodiment shown in FIG. 8 is entirely the same as that in the first 
embodiment of the present invention shown in FIG. 5. The second embodiment 
shown in FIG. 8 includes additional means added to the structure shown in 
FIG. 5. Therefore, explanation of the same parts as those shown in FIG. 5 
is omitted to avoid repetition. 
The laser beam 1 emitted from the semiconductor laser oscillator 35 is 
partly reflected by the polarization beam splitter 22, and the reflected 
beam portion 40 is directed toward a third photoelectric element 41 from 
which an output V.sub.a appears. 
On the other hand, the second photoelectric element 33 senses the portion 
42 of the reflected beam 4a from the objective lens 4 and the portion 43 
of the reflected beam 7 from the object 5, and the corresponding output 
components V.sub.b and V.sub.c appear from the second photoelectric 
element 33. Thus, the actual output from the second photoelectric element 
33 is given by (V.sub.b +V.sub.c). 
The output V.sub.a from the third photoelectric element 41 is applied to an 
amplifier 44 having an amplification factor V.sub.b /V.sub.a, and an 
output V.sub.d which is equal to V.sub.b appears from the amplifier 44, as 
follows: 
##EQU1## 
This output V.sub.d (=V.sub.b) from the amplifier 44 is applied to a 
differential amplifier 45 together with the output (V.sub.b +V.sub.c) from 
the second photoelectric element 33, and an output equal to V.sub.c 
appears from the differential amplifier 45. This output V.sub.c is applied 
to the differential amplifier 39 together with the voltage V.sub.1 
determined by the combination of the regulated voltage source 36 and the 
potentiometer 37, and the resultant output from the differential amplifier 
39 is used to control the intensity of the laser beam 1 emitted from the 
semiconductor laser oscillator 35. Thus, the voltage applied to the 
semiconductor laser oscillator 35 is controlled so that V.sub.c can be 
maintained constant at the level of V.sub.1, hence, the intensity of the 
beam 7 reflected from the surface of the object 5 can be maintained 
constant. In this manner, the intensity of the reflected beam 7 from the 
object 5 can be maintained constant without being effected by the 
reflected beam 4a from the objective lens 4. 
In the second embodiment above described, a portion of the laser beam 1 
emitted from the semiconductor laser oscillator 35 is reflected by the 
beam splitter 22 to be sensed by the third photoelectric element 41 
functioning as the laser beam output sensing means. However, in the case 
of the semiconductor laser emitter 35, a laser beam is also emitted in the 
rearward direction. Therefore, a photoelectric element for sensing such a 
laser beam may be provided to attain the same effect. 
The third photoelectric element 41 provided for sensing the portion of the 
laser beam 1 for the purpose of laser beam output detection may be 
eliminated, and the voltage applied to the semiconductor laser oscillator 
35 may be detected in lieu of the detection of the laser beam output, 
since, in the case of the semiconductor laser oscillator 35, the intensity 
of the laser beam emitted therefrom is approximately proportional to the 
level of the voltage applied thereto. Suppose that V.sub.f is the voltage 
applied to the semiconductor laser oscillator 35 in the third embodiment 
shown in FIG. 9. Then, the laser beam 1 emitted from the semiconductor 
laser oscillator 35 has an intensity substantially proportional to the 
level of the applied voltage V.sub.f, and the beam 4a of proportional 
intensity is reflected from the surface of the objective lens 4. 
Consequently, the portion 42 of the reflected beam 4a of proportional 
intensity is incident upon the second photoelectric element 33. At the 
same time, the portion 43 of the reflected beam 7 from the surface of the 
object 5, having an intensity proportional to the laser beam intensity, is 
also incident upon the second photoelectric element 33. Consequently, the 
output appearing from the photoelectric element 33 is given by (V.sub.b 
+V.sub.c) corresponding to the intensities of the beam portions 42 and 43. 
Since the output component V.sub.b resulting from the incidence of the 
portion 42 of the reflected beam 4a is proportional to the applied voltage 
V.sub.f, V.sub.b is expressed as: 
EQU V.sub.b =kV.sub.f 
where k is the proportional constant. 
Therefore, when the voltage V.sub.f applied to the semiconductor laser 
oscillator 35 is applied to an amplifier 46 having an amplification factor 
k, the output V.sub.g from the amplifier 46 is given by V.sub.g =kV.sub.f 
=V.sub.b. This voltage V.sub.g =V.sub.b is applied to the differential 
amplifier 45 together with the voltage (V.sub.b +V.sub.c), as described 
with reference to FIG. 8, and the output V.sub.c appears from the 
differential amplifier 45. Then, this voltage V.sub.c and the regulated 
voltage V.sub.1 are applied to the differential amplifier 39, and the 
output from the differential amplifier 39 is used to control the intensity 
of the laser beam 1 emitted from the semiconductor laser oscillator 35. In 
this manner, the voltage V.sub.c is controlled to be constant so that the 
intensity of the reflected beam 7 from the object 5 can be maintained 
constant. 
In the aforementioned embodiments of the present invention, the 
semiconductor laser oscillator 35 has been referred to as a preferred 
laser beam source. However, a laser beam source, for example, a He-Ne 
laser oscillator 47 capable of emitting a constant output 48 as shown in 
FIG. 10 may be employed in lieu of the semiconductor laser oscillator 35. 
In such a case, a modulator 49, for example, an AO modulator (an acoustic 
type beam modulator) or an EO modulator (an electronic type beam 
modulator) capable of changing the intensity of the laser beam at a high 
speed is preferably inserted in the optical path of the laser beam output, 
so that, by application of a control signal 51 to the modulator 49, a 
laser beam 50 having a changeable intensity entirely similar to that 
described in the embodiments employing the semiconductor laser oscillator 
35 can be obtained. 
The prior art device has been unable to optically detect the focusing 
position on an object 5 having a very low reflection factor, because the 
intensity of the beam 4a reflected from the objective lens 4 is higher 
than that of the beam 7 reflected from the object 5. In sharp contrast, it 
has become possible, according to the device of the present invention, to 
optically detect the focusing position on an object even when the 
reflection factor of the object is not more than 0.1%. 
In future, the object and the objective lens will more frequently be 
immersed in oil for the purpose of observation of very small patterns with 
a high resolution, and, in such a case, the intensity of the beam 
reflected from the object will be extremely attenuated. In such a special 
case, the focusing position detection will be nearly impossible unless the 
device according to the present invention is relied upon. Thus, the 
industrial importance of the device of the present invention will increase 
more and more.