Lens evaluating device

A lens evaluating device for measuring the optical transfer function of the lens under evaluation for evaluating the lens. The lens evaluating device includes an exposure light source, a converging lens for converging a light beam radiated from the exposure light source at a point, a characteristics evaluating pattern onto which the light beam outputted from the converging lens is converged by the lens under evaluation to form an image on it, a detection unit for receiving a return light beam having scanned the characteristics evaluating pattern via the lens under evaluation for detecting changes in reflection intensity of the return light beam, and an analysis unit for executing frequency analysis of the detected results from the detection unit for measuring the optical transfer function. The results of detection by the detection unit are subjected to a frequency analysis using a spectrum analyzer for measuring the optical transfer function of the lens under evaluation for evaluating the lens. An exposure light source includes a first resonator and a second resonator. The first resonator has a laser medium illuminated by an excited light radiated by an exciting light source and a first non-linear optical crystal device for wavelength-converting the light beam radiated from the laser medium. The second resonator has a second non-linear optical crystal device for wavelength-converting a light beam radiated from the first resonator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a lens evaluation device and, more particularly, 
to a device for measuring the optical transfer function (OTF) of the lens 
for evaluating lens characteristics. 
2. Description of the Related Art 
A semiconductor exposure device has hitherto been employed for formation of 
a circuit pattern of a semiconductor device. With such semiconductor 
exposure device, a reticle on which a circuit pattern for an original 
picture corresponding to the circuit pattern to be formed on the 
semiconductor device is formed is illuminated by a light beam radiated 
from an exposure light source. An image of the circuit pattern of the 
original picture obtained by illuminating the reticle with the light beam 
is formed via a projecting lens on a semiconductor wafer (semiconductor 
device) for exposing the circuit pattern corresponding to the image of the 
circuit pattern of the original picture on a resist layer formed on the 
semiconductor wafer. 
FIG. 1 shows an arrangement of this type of a conventional semiconductor 
exposure device. 
Referring to FIG. 1, the semiconductor exposure device includes an exposure 
light source 131 for radiating a light beam for illuminating a reticle 133 
on which is formed a circuit pattern for an original picture corresponding 
to a circuit pattern to be formed on the semiconductor device. As the 
exposure light source 131, an excimer laser or an ultra-high pressure 
mercury arc lamp radiating the light of a shorter wavelength is employed. 
The light beam emanated from the exposure light source 131 is incident on 
an illuminating optical system 132 by means of which it is radiated to the 
reticle 133 on which there is formed the circuit pattern of the original 
picture corresponding to the circuit pattern formed on the semiconductor 
device. The image of the circuit pattern of the original picture is 
contracted in size to, for example, a one-fifth size, by a contracting 
projecting lens 113, so as to be projected on a resist layer formed on a 
semiconductor wafer 114, so that light exposure is made on the resist 
layer to a shape corresponding to the image of the circuit pattern of the 
original picture. 
The semiconductor wafer 114 is set on an XY stage 115. The XY stage 115 is 
moved along two mutually perpendicular axes for variably adjusting the 
position of the semiconductor wafer 114 set thereon for achieving relative 
position setting (registration) between the reticle 133 and the 
semiconductor wafer 114 in order to get the image of the circuit pattern 
of the original picture on the reticle 133 correctly formed at a pre-set 
position on the semiconductor wafer 114. Such relative position setting 
between the reticle 133 and the semiconductor wafer 114 is achieved by 
detecting position setting marks formed on the semiconductor wafer 114 and 
moving the XY table 115 in a controlled manner responsive to the detection 
output. 
For producing a fine circuit pattern of the semiconductor device to high 
accuracy, it is necessary for the image of the circuit pattern of the 
original picture formed on the reticle to be formed with high resolution 
on the semiconductor wafer. For forming the image of the circuit pattern 
of the original picture with high resolution and exposing the resist layer 
on the semiconductor wafer to light, it is necessary to employ not only a 
light source capable of radiating a light beam of an extremely short 
wavelength as a light source for exposure, but also a lens having 
extremely high resolution characteristics as a lens forming the image of 
the circuit pattern of the original picture on the semiconductor wafer. 
Thus the image-forming lens employed in the semiconductor exposure device 
has to be evaluated as to whether or not it is capable of forming an image 
of an extremely fine circuit pattern having a linewidth on the order of 
0.25 .mu.m with high accuracy on the resist layer of the semiconductor 
wafer. If the image-forming lens employed in the semiconductor exposure 
device is not capable of forming the image of the fine circuit pattern on 
the semiconductor wafer with high accuracy, or if lens characteristics are 
deteriorated through the use of the semiconductor exposure device, it 
becomes necessary to exchange the lens with a lens having satisfactory 
characteristics. 
The method for evaluating the characteristics, such as resolution, of a 
lens employed for forming the image of the circuit pattern of the original 
picture on the semiconductor wafer, employs a semiconductor exposure 
device constructed as shown in FIG. 1, in which a reticle having lines and 
spaces formed thereon as the reference of resolution is illuminated by a 
light beam outgoing from an exposure light source. An illuminated image of 
the line and spaces, thus formed on the reticle, is formed on the resist 
layer on the semiconductor wafer set on the XY stage via an image-forming 
lens which is to be evaluated. This resist layer is exposed and the 
exposed portion is developed. The cross-sectional shape of the pattern on 
the semiconductor wafer thus developed is measured using a scanning 
electron microscope. The exposure light volume distribution is calculated 
from the measured values of the cross-sectional shape and sensitivity 
characteristics of the resist layer on the semiconductor wafer. The 
optical transfer function of the image-forming lens is calculated form the 
calculated values of the exposure light volume distribution. 
The above-described method is described in "Technology of Semiconductor 
Lithography" by Koichiro Otori, published by SANGYO TOSHO Company, page 
93. 
The reason the above-described method is employed in evaluating optical 
characteristics of the image-forming lens employed in the semiconductor 
exposure device is that the light beam radiated from the exposure,light 
source employed in the semiconductor exposure device is low in coherence 
and it is extremely difficult for such light beam to be converged to form 
a point light source for evaluating lens characteristics. 
With the above-described method, consisting in developing an image of line 
and spaces formed on the reticle as the resolution reference on the resist 
layer on the semiconductor wafer, and evaluating lens characteristics from 
the developed picture, a series of measurement processes including a 
series of light exposure processes employing the semiconductor exposure 
device become complex. On the other hand, the operation of calculating the 
characteristics of the lens under measurement is a time-consuming 
operation, such that the lens characteristics cannot be known promptly. 
In addition, the process for producing the light exposure volume 
distribution using sensitivity characteristics of the photoresist formed 
on the semiconductor wafer is susceptible to errors. 
Furthermore, with the conventional semiconductor exposure device, since it 
is difficult to achieve highly accurate relative position setting between 
the reticle having lines and spaces as resolution reference formed thereon 
and the semiconductor wafer on which the image of these lens and spaces is 
to be formed, the image of the lines and spaces cannot be formed with high 
accuracy on the resist layer on the semiconductor wafer, as a result of 
which the characteristics of the lens under measurement cannot be obtained 
correctly. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a device for evaluating 
lens characteristics whereby the characteristics of the lens under 
evaluation can be evaluated promptly and in an error-free manner. 
It is another object of the present invention to provide a device for 
evaluating lens characteristics whereby the characteristics of the lens 
under evaluation can be directly evaluated without developing the lines 
and spaces as the resolution reference, such that evaluation of the lens 
characteristics may be made without being unaffected by measurement errors 
other than errors proper to the lens under evaluation. 
The lens evaluating device according to the present invention includes an 
exposure light source, a converging lens for converging a light beam 
radiated from the exposure light source at a point, a characteristics 
evaluating pattern onto which the light beam outputted from the converging 
lens is converged by the lens under evaluation to form an image on it, a 
detection unit for receiving a return light beam having scanned the 
characteristics evaluating pattern by the lens under evaluation for 
detecting changes in reflection intensity of the return light beam, and an 
analysis unit for performing frequency analysis of the detected results 
from the detection unit for measuring the optical transfer function. The 
results of detection by the detection unit are subjected to a frequency 
analysis using a spectrum analyzer for measuring the optical transfer 
function of the lens under evaluation for evaluating the lens. 
With the lens evaluating device of the present invention, a splitting 
optical device is arranged between the converging lens and the lens under 
evaluation for splitting the light beam outgoing from the converging lens 
from the return light beam through the lens under evaluation. 
An exposure light source includes a first resonator and a second resonator. 
The first resonator has a laser medium illuminated by an excited light 
radiated by an exciting light source and a first non-linear optical 
crystal device for wavelength-converting the light beam radiated from the 
laser medium. The second resonator has a second non-linear optical crystal 
device for wavelength-converting a light beam radiated from the first 
resonator. 
The evaluation pattern is formed on a photoresist surface of a 
semiconductor wafer. 
The lens evaluating device according to the present invention also includes 
a detection light source, a converging lens for converging a light beam 
radiated from the detection light source at a point, a characteristics 
evaluating pattern onto which the light beam outputted from the converging 
lens and converged at a point so as to be again diverged forms an image 
after being converged by a lens under evaluation arranged on the optical 
axis of the converging lens, movement means for moving the characteristics 
evaluating pattern relative to the converging lens within a plane 
perpendicular to the optical axis of the converging lens, a detection unit 
for receiving a return light beam having scanned the characteristics 
evaluating pattern moved by the movement means for detecting changes in 
reflection intensity of the return light beam, and an analysis unit 
supplied with a detection signal from the detection unit in order to 
perform frequency analysis on the detection signal and thereby to measure 
the optical transfer function. 
With the lens evaluating device, a splitting optical device is arranged 
between the converging lens and the lens under evaluation for splitting 
the light beam outgoing from the converging lens from the return light 
beam through the lens under evaluation and for deflecting the return beam 
from the optical axis of a light beam outgoing from the converging lens. 
The detection light source includes a first resonator and a second 
resonator. The first resonator has a laser medium illuminated by an 
excited light radiated by an exciting light source and a first non-linear 
optical crystal device for wavelength-converting the light beam radiated 
from the laser medium. The second resonator has a second non-linear 
optical crystal device for wavelength-converting a light beam radiated 
from the first resonator. 
The lens evaluating device also includes a light source unit for generating 
fourth harmonics of the basic wave of a solid state laser, a lens for 
collecting and converging a laser light from the light source means to a 
point light source, a characteristics evaluating pattern formed on a 
photoresist surface of a semiconductor wafer, movement means for moving 
the characteristics evaluating pattern on the semiconductor wafer relative 
to the converging lens, detection means for detecting changes in intensity 
of the light reflected via a lens under evaluation from the photoresist 
surface having the characteristics evaluating pattern formed thereon. The 
characteristics evaluating pattern is scanned by the operation of the 
movement means. Analysis means is also provided for performing frequency 
analysis on the detected results from the detection means for measuring 
the optical transfer function. The fourth harmonics outgoing from the 
light source means are transmitted through the lens under evaluation so as 
to form an image on the characteristics evaluating pattern formed on the 
photoresist surface of the semiconductor wafer for evaluating the lens 
under evaluation. 
Other objects and advantages of the present invention will become apparent 
from the following description of the preferred embodiments and the claims 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The device for evaluating lens characteristics according to the present 
invention evaluates the lens characteristics by transmitting a light beam 
from an exposure light source through the lens under evaluation to form an 
image on a pattern employed for evaluating lens characteristics, and is 
used for evaluating the characteristics of a lens employed for a 
semiconductor exposure device. 
With the semiconductor exposure device employing a lens evaluated by the 
device of the present invention, a light beam radiated from the exposure 
light source is radiated onto a reticle, on which is previously formed a 
circuit pattern of the original picture having a five-fold size of a 
corresponding circuit pattern to be formed on the semiconductor device, in 
accordance with the photolithographic technique in general. An image of 
the circuit pattern of the original picture, obtained by illuminating the 
light beam on the reticle, is projected to and formed on the semiconductor 
wafer constituting the semiconductor device to a size equal to one-fifth 
of that of the image on the reticle via a projecting lens for exposing the 
circuit pattern corresponding to the circuit pattern of the original 
picture on the resist layer provided on the semiconductor wafer. 
Referring to FIG. 2, the device for evaluating lens characteristics 
includes a converging lens 11 for converging a light beam outgoing from an 
exposure light source 10 to a point light source, and an XY stage 15 for 
causing relative movement of the above-mentioned evaluation pattern with 
respect to the lens. The evaluating device also includes a photodetector 
16 for detecting changes in intensity of the reflected light from a wafer 
14 via a small-sized projecting lens 13 under measurement. The wafer 14 
represents a surface on which is formed the above-mentioned evaluation 
pattern which is scanned as the XY stage 15 is moved. The evaluating 
device additionally includes a spectrum analyzer 17 for measuring the 
optical transfer function based upon frequency analyses of the detected 
results of the photodetector 16. 
The exposure light source 10 wavelength-converts the laser light emitted by 
diode laser excitation in a first resonator. The exposure light source 
also wavelength-converts the outgoing light from the first resonator in a 
second resonator of the external resonator type and radiates a light beam 
wavelength-converted to fourth harmonics with respect to the basic wave. A 
UV laser light source is employed as the exposure light source 10. The 
concrete construction of the exposure light source 10 will be explained 
subsequently. 
The outgoing light beam of the exposure light source 10, having the 
wavelength of the fourth harmonics, is of a narrow band of 266 nm in 
wavelength, and is of characteristics having high oscillation mode 
uniformity. 
The Nd:YAG laser light beam, for example, of the fourth harmonics, radiated 
from the UV laser light source constituting the exposure light source 10, 
is converged by the converging lens 11 to form a point light source FP at 
a focal point of the converging lens 11. The light beam of the fourth 
harmonics is highly coherent and has a coherence length of tens of kms. 
Besides, it is oscillated with a single transverse mode. Thus the light 
beam assures an ideal point light source. 
The light from the point light source FP is illuminated via a half mirror 
12 on the contracting, projector lens 13 under evaluation. An image of the 
light transmitted through the contracting projector lens 13 is formed on 
the semiconductor wafer 14 to form a spot. 
On the photoresist layer 14 on the semiconductor wafer 4 illuminated via 
the contracting projector lens 13, there is previously formed a test 
pattern comprised of lines and spaces. The surface of the photoresist 
layer 14 necessarily presents irregularities 14a, as shown in FIG. 2. The 
light flux scattered by these irregularities 14a, that is the reflected 
light from the photoresist layer 14, is transmitted through the projector 
lens 13 and thence supplied to the half mirror 12, whereby it is reflected 
at 90.degree. so as to be supplied to the photodetector 16. Since the 
photoresist layer 14 is moved relative to the projector lens 13, as the XY 
stage is moved, it is scanned by the light spot of the illuminating light. 
In the photodetector 16, there is produced an irregular noise in current 
changes brought about by light detection of the test pattern being moved. 
The current proportional to the detected light enters the spectrum 
analyzer 17. The spectrum analyzer 17 is a frequency analysis device for 
executing the frequency analysis based upon the input current. The results 
of the frequency analysis by the spectrum 
analyzer 17 are shown in FIG. 3 in which the frequency and the frequency 
spectrum of the light current are plotted on the ordinate and on the 
abscissa, respectively. In the analysis pattern of FIG. 2, a spectrum 
peak. N.sub.p in FIG. 3 is represented in terms of the spatial frequency 
of the lines and spaces of the test pattern and the frequency determined 
by the speed of movement of the wafer 14. The spectrum of broad 
distribution, which is decayed with increase in frequency, that is at 
higher frequencies, is the irregular noise N attributable to surface 
roughnesses of the photoresist layer 14 on which the test pattern is 
formed. This irregular noise N, which inherently is close to the white 
noise, presents the spectrum of broad distribution decayed at higher 
frequencies because the light beam transmitted through the contracting 
projector lens 13 under evaluation is acted upon by the optical transfer 
function (OTF) as later described. Such spectrum affords the correct OTF 
for the lens. 
On the other hand, the relative noise intensity, that is the power spectral 
intensity of the above-described irregular noise divided by the mean 
photodetector current, integrated over the entire frequency range, is 
equivalent to the mean square value of the irregular noise attributable to 
surface roughness of the photoresist 14. By utilizing such relation, the 
irregular noise may be frequency analyzed for quantitatively evaluating 
the surface roughnesses of the photoresist 14. 
The reason such analysis becomes possible is that, while the surface 
irregularities of the photoresist layer are jagged and present uniform 
frequency components, the irregularities having high frequency components 
scatter the light beam at a larger angle, and the light beam scattered at 
larger angles cannot pass through the converging lens, and hence the 
scattered light transmitted through the converging lens are of frequency 
characteristics identified with the transfer characteristics of the lens. 
The OTF of the lens under inspection may be measured by evaluating the 
noise by taking advantage of surface irregularities of the photoresist 
surface. 
On the other hand, the OTF of the projector lens 13 on its optical axis may 
be measured by placing the point light source FP on an extension of the 
optical axis of the projector lens 14 under inspection. The off-axis OTF 
of the projector lens 14 may be measured by placing the point light source 
FP at an offset position from the optical axis of the projector lens 14. 
Thus the converging lens 11 is adapted for being moved in a direction 
perpendicular to the optical axis for coping with the necessity for 
measuring the off-axis OTF. 
Meanwhile, the green leakage light of the second harmonics, radiated from 
the first resonator having the optical axis common to the optical axis of 
generation of the fourth harmonics, may be utilized for setting the wafer 
position as described above. 
The device for evaluating lens characteristics according to the present 
invention may be easily implemented by substituting the detection optical 
system made up of the converging lens, half-mirror 12 and the 
photodetector 16 for the illuminating optical system 132 in the 
semiconductor exposure device shown in FIG. 1. On the other hand, the 
movement of the photoresist layer 114 may be produced without raising the 
cost by utilizing the movement mechanism of the XY stage 115 of the 
exposure system. 
The basic principle and construction and the concrete construction of the 
exposure light source 10 of the evaluating device of the present invention 
will be explained with reference to FIG. 4 and 5 and with reference to 
FIG. 6, respectively. 
A laser light source 111 shown in FIG. 4 is constituted by a second 
harmonics generating laser light source (SHG laser light source) as later 
explained, and radiates the basic wave laser light. The basic wave laser 
light is phase-modulated by a phase modulator 112 employing an 
electro-optical (EO) device or an audio-optical (AO) device so as to be 
incident on an external resonator 115 via a reflecting surface 113 for 
detecting the light reflected by the resonator and a light-collecting lens 
114. The external resonator 115 is comprised of a non-linear optical 
crystal device 118 arranged between a concave reflective surface 116 of a 
concave mirror 116 and a planar reflective surface of a plane mirror 117. 
The external oscillator 115 is excited into resonant oscillation when the 
optical path length L.sub.R between the reflective surfaces 116, 117 of 
the resonator 115 is of a pre-set length and the phase difference of the 
optical path .DELTA. become an integer number multiple of 2.pi., with the 
reflectance and the phase of reflection being acutely changed in the 
vicinity of the phase of resonant oscillation. At least one Of the 
reflective surfaces 116, 117 of the resonator 115, for example, the 
reflective surface 117, is driven along the optical axis by an 
electromagnetic actuator 119. 
If the SHG laser light source is employed as the laser light source 111, 
and the single mode laser beam is generated and introduced to the external 
resonator 115, barium borate (BBO), for example, is used for a non-liner 
optical crystal device 118 within the resonator 115, and the laser beam 
which represents second harmonics with respect to the incident light is 
generated under the non-linear optical effects of BBO. The laser beam 
represents fourth harmonics if the incident light is the SHG laser. The 
reflective surface 116 of the concave mirror 115 of the external resonator 
115 reflects substantially all of the incident laser light, while the 
reflective surface 117 of the plane mirror is a dichroic mirror reflecting 
substantially all of the incident light and transmitting substantially all 
of the outgoing light reduced in wavelength to one-half of the wavelength 
of the incident light. 
An oscillator 121 outputs a modulated signal for driving the optical phase 
modulator 112. The modulated signal is routed via a driver (driving 
circuit) 122a to the phase modulator 112. The reflected light (return 
light) of the laser beam supplied to the resonator 115 is detected via the 
reflective surface 113 by a photodetector 123, such as a photodiode, a 
detection signal of which is supplied to a synchronous detector 122b, to 
which the modulated signal from the oscillator 121 is also supplied after 
wave-shaping and phase-delay as the occasions may require. The modulated 
signal is multiplied by the detection signal of the reflected light for 
performing synchronous detection. An output detection signal from the 
synchronous detecting circuit 122b is passed through a low-pass filer 
(LPF) 122c so as to become an error signal for the optical path length of 
the resonator. This error signal is routed to a driver 122d, a driving 
signal which drives the actuator 119 for shifting the reflective surface 
117 along the optical path by way of performing servo control of reducing 
the error signal to zero, thereby controlling the optical path length 
L.sub.R of the external resonator 115 to a length corresponding to the 
locally maximum point of reflectance (resonant point). 
The electro-magnetic actuator 119 may be designed as a voice coil driven 
type actuator, with which the duplex resonant frequency may be adjusted to 
tens of kHz to one hundred kHz or higher. By such increase in the resonant 
frequency and decrease in phase differences, the cut-off frequency of the 
servo range may be enlarged and the driving current may be reduced to 
simplify the circuit construction. Thus a system in which changes in the 
resonator length L.sub.R of the external resonator 115 may be stably 
suppressed to 1/1000 to 1/10000 of the wavelength, that is to less than 1 
.ANG., may be constructed inexpensively. 
The external resonator 115, which is a so-called Fabry-Perot resonator, is 
in resonant oscillation when the optical path phase difference .DELTA. is 
equal to an integer number multiple of 2.pi., and is changed significantly 
in reflective phase in the vicinity of the phase of resonance. As 
disclosed in "Laser Phase and Frequency Stabilization Using an Optical 
Resonator", by R. W. Drever et al, Applied Physics B 31.97-105 (1983), 
frequency control of the resonator may be performed by taking advantage of 
such phase change. It is this technique that is utilized in the external 
oscillator 115. 
Since an ultraviolet light of a shorter wavelength is required in the lens 
evaluating device of the present invention, an arrangement shown in FIG. 5 
is employed as a laser light source. Such laser light source 21 is a SHG 
laser oscillator. 
The laser light source 111 of the ultraviolet laser light radiating unit 10 
is comprised of a laser medium 24, such as Nd:YAG, and a non-linear 
optical crystal device 25, such as KTP (KTiOP.sub.4), between a pair of 
reflective surfaces 22 and 23 of the resonator 21, as shown in FIG. 6. The 
basic wave laser of a wavelength from the laser medium 24 is passed 
through the non-linear optical crystal device 25 and driven into resonant 
oscillations for generating the SHG laser light which is routed to an 
external resonator 26. One of the reflective surfaces 27, 28 of the 
external resonator 26, for example, the reflective surface 27, is driven 
along the optical axis by the electro-magnetic actuator 29. From a 
non-linear optical crystal device 30, such as BBO, enclosed in the 
external resonator 26, a laser light beam which represents second 
harmonics of the incident laser light, that is fourth harmonics of the 
original basic laser light beam, is produced and taken out at the external 
resonator 26. 
By way of a concrete example, the ultraviolet laser light radiating unit 10 
is constructed as shown for example in FIG. 6, in which parts or 
components common to those shown in FIGS. 4 and 5 are correspondingly 
numbered and not specifically explained and specific numerical values are 
given for the laser wavelength and so forth. 
A laser diode 110, a semiconductor laser, transmits a laser beam of a 
wavelength of 810 nm, to the laser light source 111 which employs a laser 
medium Nd:YAG and causes the basic wave laser light with the wavelength of 
1064 nm to be oscillated in resonance at the KTP 25 to output second 
harmonics having a wavelength of 532 nm equal to one-half the 
above-mentioned wavelength to an electro-optical modulator (EOM) 112. The 
laser beam of a frequency f.sub.c of the laser light source 111, such as 
on the order of 500 to 600 THz, is phase-modulated by a phase modulator 
112 with a frequency f.sub.m, such as 10 MHz, and a side band f.sub.c 
.+-.f.sub.m is set. The phase-modulated carrier frequency signal is 
supplied from the signal source (oscillator) 121. 
The return light from the external resonator 26, having the resonant 
frequency f.sub.0, is routed to the photodetector 123 where changes in the 
optical path length of the resonator are detected by the photodetector 123 
by the FM sideband method detecting the beat between the frequency f.sub.c 
and f.sub.c .+-.f.sub.m in order to detect a position error detection 
signal having a polarity corresponding to the minimum reflectance position 
of the resonator. The reflective surface 27 is moved along the optical 
axis by a voice coil motor 29 of the of the electro-magnetic actuator 
until the position error difference signal is reduced to zero. For 
achieving high-precision position error detection, the error signal is 
taken out by a lock circuit 122 made up of the synchronous detector 122b, 
LPF 122c and the drivers 122a, 122d, as shown in FIG. 2. 
By controlling the optical path length to L.sub.R so that the error signal 
is reduced to zero, and outputting the result via the BBO 30 in the 
external resonator 26, the second harmonics are converted into fourth 
harmonics, that is a laser light of an ultraviolet wavelength, which is 
outputted at a dichroic mirror 28. 
The laser medium is not limited to Nd:YAG and Nd:YVO.sub.4, Nd:BEL or LNP 
may also be employed. The non-linear optical crystal device may be 
exemplified by LN, QPN, LN, LBO or KN, in addition to KTP and BBO. 
With the above-described arrangement, the outgoing light employed as a 
light source is extremely high in coherence, and is of the single 
transverse mode, so that an ideal point light source may be produced. 
Since the OTF of the lens under inspection may be may be inspected at the 
time of evaluating lens characteristics without employing the projection 
exposure process for printing the photoresist, the measurement process may 
be simplified and accurate results of evaluation may be obtained in a 
short time. Since calculations of the exposure distribution based on 
sensitivity characteristics of the resist, as required with the 
conventional practice, are not made, the results may be error-free. Since 
the OTF of the exposure lens may be inspected without the light exposure 
process, the difficulty of the correct pattern not being formed due to 
problems other than those related with the lens may be eliminated from the 
measurement process, thus making it possible to comprehend the performance 
of the lens itself. 
The device for evaluating lens characteristics according to the present 
invention is not limited to the projection exposure device employing the 
reflective optical system described above, but may also be applied to a 
reflective optical system or to a proximity exposure device.