Tunnel current detecting photo-acoustic spectrometer

The amount of deformations such as expansions or contractions of a substance caused in accordance with irradiation of cyclically interrupted monochromatic light is detected by either the changes in a tunnel current sensitive to the changing distance between a sample surface and a detection probe or the amount of changes in a probe fine feed mechanism keeping the tunnel current constant between a sample surface and a detection probe at all times, so that the absorption spectrum intrinsic to the substance relative to the irradiating optical energy according to its wavelength is measured to examine the optical properties of the substance.

BACKGROUND OF THE INVENTION 
The present invention relates to a spectrometer for examining the optical 
properties of a substance. 
If a substance is irradiated with light, it will absorb the optical energy. 
This energy will be converted into atomic vibrations, e.g., heat by a 
non-radiation transition, except that consumed either for another light 
emission (photoluminescence) or for photo-chemical reactions. The 
photo-acoustic spectrometry is defined to measure the thus generated 
calorie or the accompanying strain as a function of excitation optical 
energy. For this photo-acoustic spectrometry, there is known a device as 
shown in FIG. 7. This method is disclosed in the reference of the Bul. 
Electrotech Lab. Vol. 47 No. 2(1983). A sample 5 is placed in a sealed 
container 3 having an optical irradiation window 1 and an acoustic 
detector (or a highly sensitive microphone) 2 and is irradiated from the 
outside with interrupted monochromatic light 6. If the sample 5 absorbs 
the light and heats up, the temperature of gases surrounding the sample 5 
will also rise. As a result, the gas layer is expanded to act as a piston 
thereby to generate pressure waves 7 in the sealed container 3. If, 
moreover, the irradiation light is cyclically interrupted, the pressure 
waves change into sound waves. When the wavelength of the irradiation 
light is changed and the amount of heat resulting from absorption of light 
for each wavelength is measured by the microphone system, the absorption 
spectrum of the sample 5 is obtained. This is known as the gas microphone 
method for measurement of the photo-acoustic spectrum. There is also known 
a piezoelectric element method by which strain waves 8 induced in 
accordance with a local heat generation are directly detected by the use 
of a piezoelectric element 9 or the like, as shown in FIG. 8. 
The gas microphone method described above according to the prior art is 
accompanied by the following problems: The sealed container for placing 
the sample has to be filled up with some gas for propagating the 
deformation of the sample as the pressure waves to the microphone so that 
the sample being measured has its surface contaminated with the molecules 
of the gas, whereby the measurements cannot be accomplished with a clean 
sample surface or in a vacuum; and a substance having a small optical 
absorption constant is difficult to measure because of limited sensitivity 
since the pressure waves caused by the deformations of the sample are 
detected by the microphone. 
In order to improve the resolution in the depth direction, on the other 
hand, it is necessary to increase the interrupted (or modulated) frequency 
thereby to shorten the period of time for the heat propagation. In the gas 
microphone method, the signal is weakened with the frequency f of 
interruption in the form of f.sup.-1 or f.sup.-3/2 so that the 
interruption frequency f cannot be increased so much. In the piezoelectric 
element method, on the other hand, the light has to be interrupted (or 
modulated) with the resonance frequency of the element so as to improve 
the detection sensitivity so that the modulation frequency cannot be 
continuously changed to limit the depth resolution. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a tunnel current detecting 
photo-acoustic spectrum of a sample in a vacuum. 
Another object of the invention is to provide a tunnel current detecting 
photo-acoustic spectrometer which is possible to measure a photo-acoustic 
spectrum of a sample having a low photo-absorption character. 
Other and further objects, features and advantages of the invention will 
appear more fully from the following description.

DETAILED DESCRIPTION 
According to the present invention, the amount of deformations of a 
substance such as expansions or contractions accompanying the absorption 
of the irradiating optical energy is detected as either the change in the 
value of the tunnel current flowing between the sample and the detection 
probe or the amount of changes when the fine feed mechanism is deformed to 
keep the tunnel current value constant at all times, so that the 
absorption spectrum intrinsic to the substance relative to the irradiating 
optical energy may be measured to accomplish the optical analysis of the 
substance. 
According to the method described above, the sample need not be placed in a 
gas environment for its measurements but can be measured in the vacuum 
keeping a clean surface. The tunnel current value exponentially changes 
with the changes in the distance and hence is sensitive to the distance 
between sample and probe. As a result, it is possible to measure the 
changes of atomic order (or several .ANG.) in the substance relative to 
the irradiating optical energy and accordingly even the fine changes in a 
substance having a small optical absorption coefficient. Moreover, the 
tunnel current value sufficiently responds to even a high-speed change in 
the distance so that the measurements can be accomplished in a 
high-modulation frequency. 
First Embodiment 
The present invention relates to a measuring system for the absorption 
spectrum intrinsic to a substance by detecting (measuring) the amount of 
deformations of the substance such as expansions or contractions 
accompanying the irradiating optical energy. These will be done either by 
measuring the changes in the value of a tunnel current flowing between a 
sample and a detection probe or by measuring the mount of changes 
resulting from the deformations of a fine feed mechanism to keep the 
tunnel current value constant at all times. The present invention will be 
described in the following in connection with the embodiments thereof with 
reference to the accompanying drawings. 
FIG. 1 is a schematic diagram showing a tunnel current detecting 
photo-acoustic spectrometer according to the present invention. White 
light 101 emitted from a light source 100 is made monochromatic (according 
to a wavelength selection) by a monochrometer 102 and is flickered into 
interrupted monochromatic light 105 in synchronism with a signal (having a 
frequency f.sub.0) 104 coming from an oscillator 103. A portion of this 
interrupted monochromatic light 105 is divided by a beam splitter 106, and 
this beam of light is guided into a pyroelectric detector amplifier 107 
and is tuned and amplified with an interrupted frequency f.sub.0 by a 
lock-in amplifier 108 until it is processed by a microprocessor 109 into a 
signal (A(.lambda.)e.sup.-i.phi..sbsp.A.sup.(.lambda.), where A(.lambda.): 
intensity; and .phi..sub.A (.lambda.): phase) for correcting the 
fluctuations of the optical intensity and phase accompanying the 
wavelength scanning. Most of the interrupted monochromatic light 
irradiates a sample 1 and is absorbed and turned into the amount of 
deformations such as expansions or contractions of the sample 1. The 
amount of deformations is detected as the change in the tunnel current 
flowing between the sample 1 and a probe 3 disposed at a spacing of about 
1 nm from the sample 1 and is amplified by an I/V (i.e., current/voltage) 
amplifier 110 and a logarithmic amplifier 111. This signal is introduced 
partly through a comparator 112, an integrator 113 and a high-voltage 
amplifier 114 into a fine feed mechanism 4 so that it is changed into a 
distance control signal over a relatively long period of time due to the 
thermal drift between the sample and the probe and partly through the 
terminal A of a switch 115 into a lock-in amplifier 116 so that it is 
changed into a signal 
(B(.lambda.).sub.e.sup.-i.phi..sbsp.B.sup.(.lambda.), wherein B(.lambda.): 
intensity; and .phi..sub.B (.lambda.): phase). Moreover, the flickering 
components (with each wavelength and time) of the light source are 
corrected by the calculations (to have an intensity of 
B(.lambda.)/A(.lambda.) and a phase of .phi..sub.A (.lambda.)-.phi..sub.B 
(.lambda.)) of the microprocessor 109. The results are outputted as the 
intensity and phase to an X-Y recorder 117, a CRT monitor 118 and so on, 
as shown in FIG. 9. 
FIG. 9 plots and example of the signal changes in the vicinity of the 
energy gap of a semiconductor. The sample measured is identified from the 
value of the wavelength .sub.l at which the intensity and phase components 
change in accordance with the energy gap intrinsic to the sample material. 
This is suitable for measurements within frequency bands of a feed back 
loop. When change in the tunnel current value is detected without 
responding the fine feed mechanism 4 relative to the deformations of the 
sample 1. The switch 115 can change these two measuring modes. 
Next, the unit for detecting the tunnel current will be described in the 
following. 
FIG. 2 shows the unit for detecting the tunnel current flowing between the 
sample and the detection probe. A thin sample 5 is mounted on a sample 
stage 10 having a light hole, and a detection probe 11 is placed to face 
the sample. The detection probe 11 is fixed on a fine feed mechanism 12 
and is fixed through a joint 13 to a coarse feed mechanism 14 for 
positioning of the distance between the sample 5 and the detection probe 
11. In the present embodiment, a precise micrometer is used as the coarse 
feed mechanism 14. This coarse feed mechanism 14 in turn is fixed in a box 
15, and the joint 13 is held without any chatter by a pole-spring-screw 
mechanism 16 attached to the box 15. This box 15 is equipped with a peep 
window or a light source window 17 which is used for monitoring coarse 
positioning of the probe relative to the sample. In order to prevent the 
light from leaking to the fine feed mechanism, it is also possible to use 
as a light-guided pipe for irradiating light. 
In the present embodiment, a fine feed mechanism shown in FIG. 3(a) and 
3(b) or a fine feed mechanism shown in FIG. 4 is used to realize the 
high-speed response of the aforementioned fine feed mechanism 12. In the 
fine feed mechanism shown in FIG. 3, hollow cylindrical piezoelectric 
elements 41 are assembled in a cross shape and in a perpendicular position 
and each has its one end fixed to an insulated box 42 and its other end 
fixed to an insulated receptacle 43. This insulated receptacle 43 is 
internally threaded to secure a metallic detection probe bed 44 which is 
also internally threaded. Moreover, the detection probe 11 is attached to 
the probe bed 44 through a detection probe holder 45 which is also 
internally threaded. The aforementioned insulated box 42 is covered with a 
shielding plate 46. The operations will be described in the following. The 
fine feed in the Z-axis direction is accomplished by expanding and 
contracting the cylindrical piezoelectric element positioned upright 
perpendicularly to the cross, and the fine feeds in the X and Y-axis 
directions are accomplished by expanding one of the crossing piezoelectric 
elements while contracting the other. 
In another fine feed mechanism shown in FIG. 4, an insulating member 52 and 
an internally threaded metallic detection probe bed 53 are fixed in a 
hollow cylindrical piezoelectric element 51, and the detection probe 11 is 
attached to the detection probe bed 53 through an internally threaded 
detection probe holder 54. Moreover, a common electrode is arranged inside 
of the cylindrical piezoelectric element 51, and Z-axis electrodes and 
staggered X- and Y-axis electrodes are arranged outside of the cylindrical 
piezoelectric element 51. The operations of this fine feed mechanism will 
be described in the following. The Z-axis feed is accomplished through 
expansions and contractions by applying a plus or minus voltage to the 
inside common electrode, and the X- and Y-axis feeds are accomplished 
through bending motions resulting from expansions and contractions by 
applying a plus voltage to one of the crossing electrodes relative to the 
inside common electrode and a minus voltage to the other. 
Next, the operations for detecting the tunnel current will be described in 
the following. As shown in FIG. 5 (in case the sample is nonconductive), a 
bias voltage is applied between the detection probe 11 and a conductive 
metal 18 formed by either sputtering or evaporation. Thus, the tunnel 
current is detected by operating the Z-axis of the fine feed mechanism 12 
to further bring the sample 5 and the detection probe 11 close to several 
nm. Then, the sample 5 is irradiated with the interrupted monochromatic 
light 6 through the hole of the sample bed 10 so that the resultant 
deformations (in the direction of arrow 19) of the sample 5 are measured 
by either the changes in the tunnel current value or the amount of 
deformations of the fine feed mechanism. Moreover, the aforementioned 
controls of the irradiation light, the tunnel current value detection and 
the fine feed mechanism are accomplished by the optical control system and 
the tunnel current detecting and fine feed mechanism control systems shown 
in FIG. 1. 
With the construction thus far described, the photo-acoustic spectrometer 
can measure the fine change of a substance accompanied by the irradiating 
optical energy. 
Second Embodiment 
FIG. 6 shows the leading end of a tunnel current detection unit according 
to a second embodiment of the present invention. The leading end is 
different from the foregoing first embodiment: an optical pipe 20 for 
irradiating the sample 5 sideways with light is attached. It has been 
confirmed that this unit can also attain the effects similar to those of 
the first embodiment. 
Thus, the amount of deformations of a substance such as expansions or 
contractions relative to the irradiating optical energy is detected as 
either the changes in the value of a tunnel current flowing between the 
sample and the detection probe or the amount of changes resulting from 
deformations of the fine feed mechanism to keep the tunnel current value 
constant at all times. It is possible to measure the sample in a vacuum in 
which the surface is clean and the fine changes of a substance having a 
small optical absorption coefficient in terms of the changes in the tunnel 
current value which is very sensitive to the distance. Thus, the range of 
samples to be measured can be widened. Since, moreover, the tunnel current 
responsible to the high-speed changes is detected, the heat propagation 
time can be shortened by increasing the interruption (or 
modulation)frequency, so that the resolution in the depth direction can be 
improved.