Crystal monochromator

A focusing X-ray crystal monochromator in which one or more crystal layers having different spacings of lattice plane are stacked on a crystal base. Due to different spacings of lattice plane, the angle of reflection and diffraction of a diverging incident X-ray beam can be so changed that the beam takes a parallel or focusing direction for monochromatization. Thus, the monochromator of the present invention can be applied to the X-ray lithography for transferring a pattern of high resolution or the X-ray analysis such as the fine X-ray diffraction.

BACKGROUND OF THE INVENTION 
The present invention relates to an applied technical field of X-rays 
having a wavelength of about 10 .ANG. or less and, more particularly, to a 
crystal monochromator suitable for an X-ray lithography, an X-ray analysis 
and an X-ray analyzer, in which an X-ray beam is monochromatized and 
focused to irradiate a specimen with a high intensity. 
In the prior art, in case an X-ray beam having a wavelength of about 10 
.ANG. is to be monochromatized and focused to have its intensity increased 
so that it may be used for the X-ray analysis or the like, a crystal 
monochromator made of a single-crystal material such as Si or Ge has to be 
bent (with a radius of curvature R of several tens cm to several cm) to 
focus its diffracted X-ray beam, as is disclosed in "Interpretation of 
Some Experimental Data Concerning Bent Monochromators for Synchrotron 
X-ray Radiation" by A. Boeuf et al., J. Appl. Crystal, pp. 265 to 267, 
Vol. 11, 1978. In order to bend the crystal material such as Si or Ge, 
however, it is usually necessary to slice the crystal to a thickness of 
several mm or less and to apply a stress 3 from the two ends, as shown in 
FIG. 1. This necessity makes it defectively difficult to hold a constant 
curvature at each portion of the crystal having lattice planes 2. 
SUMMARY OF THE INVENTION 
An object of the present invention is to eliminate the above-specified 
defect of the crystal monochromator according to the prior art and to 
provide a novel crystal monochromator which has a function to focus only a 
specific wavelength of an X-ray beam composed of a number of X-rays of 
different wavelengths without bending the crystal by the stress applying 
method or the like. 
In order to achieve the above-specified object, the crystal monochromator 
of the present invention uses a crystal assembly in which a number of 
crystal layers having different spacings of lattice plane are placed on a 
base crystal material. Here, the crystal layers partially may have the 
same spacing of lattice plane as that of the base crystal. In the case of 
a single crystal layer, on the other hand, the spacing of lattice plane of 
this crystal layer has to be different from that of the base crystal. 
The focusing action of the crystal monochromator of the present invention 
will be cursorily reviewed in the following with reference to FIG. 2. 
The present crystal monochromator 4 is constructed of a base 5 and a number 
of crystal layers 6 having different spacings of lattice plane. As a 
result, X-rays are so diffracted at the individual layers as to satisfy 
the Bragg condition so that the incident X-rays ranging from 10 (incident 
angle 12) to 11 (incident angle 13) are diffracted and focused into 
diffracted X-rays ranging from 16 to 17 (diffraction angle 14). 
In the case of the crystal monochromator using a single crystal according 
to the prior art, on the contrary, nothing but the single X-rays 
satisfying the Bragg condition is diffracted. Therefore, the intensity of 
the X-rays diffracted and focused by the crystal monochromator according 
to the present invention in which plural x-rays are diffracted is several 
to several tens times higher than the one according to the prior art. 
According to the present invention, moreover, the spacing of lattice plane 
of the crystal can be changed by adjusting the composition of the crystal 
and the concentration of a dopant. 
The present invention naturally accepts that the order of stacking the 
crystal layers may be different from that of the magnitudes of the 
spacings of lattice plane. 
The present invention can be practised if the ratio .DELTA.d/d of the 
displacement (.DELTA.d) of the spacing of crystal plane in the present 
monochromator to the spacing d of crystal plane of a reference crystal 
layer (including the base) is within a range of 4.times.10.sup.-5 to 
1.times.10.sup.-3 which is equivalent to a range of 4.times.10.sup.-3 % to 
1.times.10.sup.-1 %. The monochromatic function is not proper for the 
range less than 4.times.10.sup.-5, and the crystal becomes unstable for 
the range more than 1.times.10.sup.-3. 
On the other hand, the total thickness of the crystal is limited to 
5t.sub.h (wherein t.sub.h =(mc.sup.2 /e.sup.2).multidot.Vc/22d.sub.h 
.multidot.1/.vertline.FH.vertline.), at which the absorption of X-rays 
jumps. In the equation of t.sub.h : 
m: Mass of electron; 
c: Velocity of light; 
e: Elementary charge; 
Vc: Volume of unit cell; 
d.sub.h : Spacing of lattice plane; and 
Fh: Structure factor. 
According to the present invention, it is possible to provide a crystal 
monochromator which has a function to monochromatize and focus a diverging 
X-ray beam without bending the crystal by a method for stress application 
or the like. 
The crystal monochromator of the present invention is excellent in 
industrial effects because it can be applied to an X-ray lithography for 
transferring a fine pattern, an impurity analysis of a minute area, or an 
X-ray diffraction.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Example 1 
In this example, a crystal monochromator manufactured for focusing 
characteristic X-rays emitted from an X-ray tube will be described 
together with an X-ray lithography using the crystal monochromator and an 
apparatus therefor. 
In this example, the crystal monochromator has been manufactured by 
stacking twenty-five layers 6 (6-1, 6-2, . . . 6-n) of Ga.sub.x In.sub.1-x 
As, which has a constant spacing of crystal lattice plane of about 400 
.ANG. and the upper ones of which always have a larger spacing d of 
lattice plane than that of the lower ones by .DELTA.d/d=4.times.10.sup.-5 
(wherein .DELTA.d: the displacement of the spacing of lattice plane), on a 
base crystal of GaAs, which has its surface located on a (111) plane and a 
spacing of crystal lattice plane of 3.26 .ANG., by the molecular beam 
epitaxy (which will be abbreviated to the "MBE process") or the metal 
organic chemical vapor deposition (which will also be abbreviated to the 
"MOCVD process"). Thus, as shown in FIG. 2B with d representig the spacing 
of the lattice plane of the base crystal layer 5 and d.sub.1, d.sub.2, . . 
. d.sub.n representing the spacing of the lattice plane of the stacked 
crystal layers 6-1, 6-2, . . . 6-n, respectively, the spacing of the 
lattice plane of an upper crystal layer is greater than that of a lower 
crystal layer in accordance with the relationship d.sub.0 &lt;d.sub.1, 
&lt;d.sub.2, . . . &lt;d.sub.n-1 &lt;d.sub.n. The Ga.sub.x In.sub.1-x As can have 
its crystal lattice constant gradually changed by changing the composition 
of In little by little, as is disclosed in "Heterostructure Lasers" by H. 
C. Casey, Jr. et al., Academic Press, Inc. FIG. 3 presents the phase 
diagram of Ga.sub.x In.sub.1-x As. The GaAs crystal has a spacing of 
lattice plane of about 5.65 .ANG. on its (100) plane whereas the InAs 
crystal has a spacing of lattice plane of about 6.06 .ANG. on its (100) 
plane, as seen from FIG. 3. It is accordingly found from FIG. 3 that the 
spacing of crystal lattice plane of GaAs increases about 7.1% if the 
composition of Ga.sub.x In1.sub.-x. As changes from 19 to 20. 
As has been described hereinbefore, therefore, the change in the ratio 
.DELTA.d/d of about 4.times.10.sup.-5 between the adjoining layers can be 
effected by changing the In composition by about 0.06%. 
FIG. 4 shows an example in which the characteristic X-rays such as 
MoK.alpha..sub.1 rays emitted from a X-ray tube (not shown in the drawing) 
are focused by the crystal monochromator of the present example. An X-ray 
beam 8 emitted from a beam source 21 of about 1 mm width placed on a Mo 
target in the X-ray tube was applied to the crystal monochromator having a 
crystal surface width 23 of 10 mm for emitting a diffracted X-ray beam. In 
this example, the spacing 22 between the beam source 21 and the crystal 
monochromator 4 was set at about 100 mm. Moreover, this setting was 
supplemented by containing an angle of about 6.24 degrees between the 
optical axis of the X-ray beam 8 and the surface of the crystal 
monochromator 4. As a result, for the MoK.alpha..sub.1 rays having a 
wavelength .lambda. of about 0.709 .ANG., diffracted X-rays 26 satisfying 
the Bragg diffraction condition and having the wavelength .lambda. equal 
to that of the wavelength of MoK.alpha..sub.1 rays were emitted. 
In the present crystal monochromator, such a component 24 of the divergence 
9 of the incident X-ray beam as had an angle of divergence of about 
10.sup.-4 rad. , i.e. as had a divergence of about 90 .mu.m calculated in 
terms of the width 25 on the surface of the crystal surface was diffracted 
and focused on a point which was spaced by about 100 mm, as at 18, from 
the crystal monochromator 4. The angle between the optical axis of the 
diffracted X-rays 26 and the crystal surface was equal to the one between 
the optical axis of the incident X-ray beam 8 and the crystal 
monochromator 4 according to the Bragg diffraction condition. In this 
example, moreover, the beam source 21 had a width of about 1 mm so that 
the incident X-ray component 24 having the angle of divergence of 
10.sup.-4 rad. was applied to all over the area of the width 23 of the 
crystal monochromator 4. As a result, the diffracted X-ray beam 26 was 
reflected from the width 25 having a divergence of 90 .mu.m in an 
arbitrary position on the width 23 of the surface of the crystal 
monochromator 4 and was focused in a portion having a width 27 of 1 mm or 
less taken in its advancing direction until it reached the spacing 18. As 
a result, by placing a specimen 28 in a position of the spacing 18, the 
portion of the specimen 28 having a width of about 1 mm could be 
irradiated with a MoK.alpha..sub.1 monochromatic X-rays of several to 
several tens times stronger integrated intensity than that of the existing 
crystal monochromator. 
This example is accompanied by a problem that the intensity of the 
diffracted X-ray beam emitted from the lower crystal layers is decreased. 
However, this problem can be solved by making the lower crystal layers 
thicker than the higher layers to intensify the X-ray beam diffracted from 
the lower layers. 
On the other hand, the manufacture of the present crystal monochromator can 
be realized with similar effects not only by the method exemplified in the 
present example but also by a method of implanting an impurity in a 
changing concentration into the crystal of Si or the like to gradually 
change the spacings of crystal lattice plane or by a method of epitaxially 
growing layers of different impurity concentrations on the base crystal of 
Si or the like to stack layers of different spacings of crystal lattice 
plane. 
Moreover, the present crystal monochromator has a one-dimensional focusing 
effect for a diverging X-ray beam but is enabled to acquire a focusing 
effect for a two-dimensional divergence of X-rays by curving it with a 
radius of curvature R of several tens cm to several m. 
On the other hand, this example could be similarly practised by using Ge, 
emerald, quartz, garnet and InSb as its base crystal material. 
Example 2 
The X-ray lithography using the crystal monochromator of the present 
invention will be described with reference to FIGS. 5 and 6. 
The pattern to be transferred by the X-ray lithography was an Au pattern 30 
formed on the surface of a crystal 37. This crystal 37 was a Si crystal 
having a spacing of crystal lattice plane of about 3.14 .orgate. and a 
surface located on the (111) plane, and its Au pattern 30 was formed by 
the vacuum deposition. This Au pattern had a spacing of 2 .mu.m. 
An incident X-ray beam 32 emitted from the (not shown in the drawing) X-ray 
beam source generated a diffracted X-ray pattern at the portion 31 having 
no Au pattern. The X-rays 33 applied to the Au pattern 30 were not 
diffracted. 
The focusing crystal monochromator 4 of this example was manufactured by 
stacking thirty Ga.sub.x In.sub.1-x As layers, each having its In 
composition reducing by about 0.025% for each 600 .ANG., on a crystal base 
of GaAs which had a spacing of crystal lattice plane of about 3.26 .ANG. 
and a surface located on the (111) plane, by a method similar to that of 
the example 1. The change in this In composition brought a change of 
.DELTA.d/d=1/.7.times.10.sup.-5 in the spacing of lattice plane. 
In case both the spacing between the pattern crystal 37 and the focusing 
crystal monochromator 4 and the spacing between the crystal monochromator 
4 and a wafer 36 were set at 50 mm whereas the angle of incidence of 
X-rays into the crsytal monochromator 4 was set at 28.8 degrees, the 
aforementioned pattern of 2 .mu.m was projected as a pattern of about 1 
.mu.m on the wafer 36. 
Turning to FIG. 6 presenting the concept of an X-ray lithographic system 
using the crystal monochromator described above, the X-ray beam 32 emitted 
from its source 40 was diffracted and focused by the pattern-bearing 
crystal 37 and the focusing crystal monochromator 4 and was projected on 
the surface of the wafer 36 having a resist. This wafer 36 was supported 
by a jig 41. Incidentally, the X-ray beam source 40 used was a known X-ray 
tube or synchrotron radiation source. 
Example 3 
In this example, the crystal monochromator was manufactured by stacking 
twenty five crystal layers, which had a constant spacing of crystal 
lattice plane d at every 450 .ANG. of the layer thickness and in which the 
spacing of lattice plane of the upper layers was always larger by about 
.DELTA.d/d=4.times.10.sup.-5 than that of the lower layers, by the CVD 
(i.e. chemical vapor deposition) method, on a Si base, which had a surface 
located on the (111) plane and a spacing of crystal lattice plane of 3.13 
.ANG.. 
In order to change the spacings of lattice plane of the individual crystal 
layers in this example, the dosage of an impurity was uniform in each 
layer but changed in the adjacent layers. If phosphor P was adopted as the 
impurity to dope the Si base in a concentration of 10.sup.21 
atoms/cm.sup.3 the crystal had a lattice strain of about 10.sup.-5. In 
order to change the ratio .DELTA.d/d between the adjoining layers by about 
.DELTA.d/d=4.times.10.sup.-5, it was sufficient to increase the P 
concentration in the upper layers by 4.times.10.sup.19 atoms/cm.sup.3. 
Reverting to FIG. 4, the monochromator 4 was set such that the spacing 22 
between the beam source 21 having a width 1 mm for a MoK.alpha..sub.1 
X-ray beam 8 having a wavelength .lambda. of about 0.709 .ANG. and the 
surface of the crystal monochromator 4 having the width 23 of 10 mm was 
about 100 mm and that the angle between the optical axis of the X-ray beam 
8 and the surface of the crystal monochromator 4 was 6.49 degrees. 
As a result, the component 24 having an angle of divergence of about 
10.sup.-4 rad of the divergence 9 of the incident X-ray beam 8, i.e., the 
incident X-ray component 24 having a divergence of about 100 .mu.m 
calculated in terms of the width 25 on the crystal surface could be 
diffracted and focused at a point of the spacing 18 of about 100 mm from 
the crystal monochromator 4. 
Since, moreover, the beam source 21 had the width of 1 mm in this 
embodiment, the diffracted X-rays 26 could be generated from all the area 
of the width of the crystal monochromator 4 to irradiate the area, which 
had the width of 1 mm of the specimen 28 placed at the spacing 18, with 
the MoK.alpha..sub.1 monochromatic X-rays of several to several tens times 
stronger integrated intensity than that of the ordinary crystal 
monochromator. 
Incidentally, although the present example used P as the impurity, similar 
results were obtained even if B, Al, Ga, P, As and Sb were used. 
Example 4 
An X-ray diffraction of a minute area using the crystal monochromator 
described in the example 3 will be described in the following. 
Turning to FIG. 7, an X-ray beam 55 was focused and monochromatized by the 
focusing crystal monochromator 4 used in the example 3. The surface of a 
specimen 56 was irradiated with the X-ray beam 55 having a diameter of 0.5 
.mu.m. The resultant diffracted image 58 was photographed on a silver salt 
film 57. By this method, a diffracted X-ray image of the minute area of 
the crystal could be obtained to clarify the crystal structure. 
Example 5 
The example to be described is directed to the analysis of a minute area of 
a specimen surface by the X-ray photoemission spectroscopy using the 
focusing crystal monochromator according to the present invention. 
Turning to FIG. 8, the surface of a specimen (e.g., a Si crystal containing 
Ge or Ga as an impurity 52) 50 was irradiated with an X-ray beam (which 
had a diameter of 0.5 .mu.m on the specimen surface) 51 , which was 
focused and monochromatized by the (not shown in the drawing) focusing 
crystal monochromator. The photoelectrons 53 from the impurity 52 were 
detected by a detector 54 to measure the concentration of the impurity in 
the minute area. 
Throughout the examples of the present invention, the base crystal material 
could be similarly practised even using Ge, emerald, quartz, garnet and 
InSb, and the individual crsytal layers could be practised within a 
thickness range of 30 to 2,000 .ANG. and within a layer number of 1 to 
500.