JP2001099753A - Method and device for evaluating optical element laser durability and exposing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and device for evaluating laser durability of an optical element capable of performing accurate laser durability evaluation even when using light with intensity distribution. SOLUTION: A laser beam 2 from a light source 1 passes through a beam forming optical system 3, an energy adjusting optical system 4, and an aperture 5, and it is separated into an irradiation beam 7 and a reference beam 12 by a beam splitter 6. Intensity of the irradiation beam 7 is monitored by a light quantity monitor sensor 13 and an oscilloscope 14 by using the reference beam 12. The irradiation beam 7 is irradiated on a sample 10. Irradiation beam intensity monitored by the light quantity monitor sensor 13 is read by a computer 15 via the oscilloscope 14. A uniformalizing optical system 18 is provided on a laser beam axis for enabling accurate LDT measurement even when light is used having spatially ununiform intensity distribution such as an excimer laser beam.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for evaluating laser durability of an optical element such as an optical material and an optical thin film, and a method for evaluating laser durability.

[0002]

2. Description of the Related Art In recent years, in order to increase the degree of integration of semiconductor devices, there is an increasing demand for a high-resolution reduction projection exposure apparatus (stepper) for semiconductor manufacturing. One method of increasing the resolution of photolithography using this stepper is to shorten the wavelength of the light source. Along with that,
Measurement and evaluation of optical elements in this short wavelength region are becoming indispensable.

[0003] The mechanism of destruction of an optical element by a laser has not been elucidated in detail. However, it is considered that the optical element is destroyed by melting of the substrate or the thin film due to absorption heat generation, brittle fracture by thermal stress, and dielectric breakdown by strong optical electric field.

In actual optical elements, the purity and homogeneity of the substrate material, the surface condition and roughness after processing, the residue of abrasives, the absorption of the coat, the distribution of the electric field intensity inside the coat, etc. These various factors also cause destruction of the optical element.

[0005] As a method of evaluating laser durability, a method often used is a laser damage threshold measurement method (laser damage threshold (LDT) measurement method).
This is a method for evaluating a change such as destruction of an optical element when the surface of an optical element sample is irradiated with a laser beam having a constant irradiation energy per unit area.

One example is shown in FIG. The laser light 2 from the light source 1 is shaped from a rectangle into a square by a beam shaping optical system 3, the irradiation light intensity (irradiation energy density) is adjusted by an energy adjustment optical system 4, and passes through an aperture 5.
The light beam has a predetermined sectional shape. Thereafter, the laser light 2 is irradiated with the irradiation light 7 and the reference light 1 by the beam splitter 6.
And 2. The intensity of the irradiation light 7 is monitored by the light amount monitor sensor 13 and the oscilloscope 14 using the reference light 12. The irradiation light 7 is shaped into a fixed shape by the condensing optical system 8 and is irradiated on the sample 10. The light passing through the sample 10 passes through a hole formed in the sample holder 9 and is reflected and absorbed by the beam stopper 11.

The intensity of the irradiation light is preferably controlled by the computer 15 so that the light intensity of the light source 1 is constant. In this example, the computer 15 has such a function. The computer 15 includes the light source 1
Instead of controlling the light intensity, the number of oscillation pulses of the light source 1 may be controlled. Computer 1
5 may have a function of integrating the light irradiated on the inspection object (sample) 10.

Further, the light intensity may be monitored by the light amount monitor sensor 13 and the light source 1 may be controlled so that the intensity of the irradiation light 7 is constant. Without providing such a feedback system, the light intensity of the light source 1 may be adjusted in advance so that the intensity of the irradiation light 7 becomes a predetermined value before irradiation to the sample 10. Alternatively, the light intensity may be controlled by using a light source having a function of keeping the light intensity constant in the light source 1 body.
After irradiating a certain amount of laser light, the sample 10 is removed from the sample holder 9 and observed with a microscope.

As another method for evaluating the laser durability of an optical element, attention is paid to the fact that the amount of light absorption changes as a precursor state of the destruction of the optical element, and it is caused by the light absorption when the optical element sample is irradiated with light. Laser durability characterized by measuring the amount of light absorbed by the optical element sample by measuring the acoustic signal generated by the change in the volume of the sample while keeping the irradiation light intensity constant, and observing the fluctuation. Evaluation method (for example, JP-A-10-232197)
Japanese Unexamined Patent Publication (KOKAI) No. 2000-222) is employed.

The outline is shown in FIG. 5, the same components as those shown in FIG. 4 are denoted by the same reference numerals. The laser beam irradiation mechanism and monitor mechanism in FIG. 5 are the same as those shown in FIG.

In FIG. 5, a sample 10 is provided with an acoustic detection element 16 (such as a piezoelectric element). The sample holder 9 is made of metal (or bakelite or the like), and the acoustic detection element 16 is
Is firmly fixed with an adhesive or the like, and stable acoustic matching is achieved. The electric signal from the sound detection element 16 is amplified by the sound detection element amplifier 17 and
4 is monitored.

The sample 10 set in this way is
The laser durability is evaluated by continuously irradiating laser light at a constant irradiation light intensity and measuring a change in output of the acoustic detection element with respect to the number of irradiations.

[0013]

In the laser durability evaluation method as described above, it is necessary that the intensity distribution of the light applied to the optical element as the subject is constant. On the other hand, as described above, the wavelength of the laser beam used for the stepper tends to be shorter,
An excimer laser, which is a high-power laser with a wavelength of less than m, has been used as a light source.

In an excimer laser, a laser light source is a substantially rectangular surface light source determined by an electrode interval and an electrode width. The intensity distribution of the laser light on the light source surface largely depends on the excited state of the internal laser medium viewed from the optical axis. Normally, when the medium is excited uniformly, an almost uniform intensity distribution is obtained in the range of electrode spacing × discharge width viewed from the optical axis. However, when the electrode shape changes due to long-term use and the discharge is offset, the intensity distribution is also offset. In addition, impurities are generated during use in the chamber in which the laser medium of the excimer laser is contained, so that the inside of the window from which the laser light is emitted becomes dirty, which causes unevenness in the intensity distribution of the laser light.

Even if the beam is diverged or converged by the lens, the intensity distribution is propagated as it is. Originally, it should be evaluated by irradiating a beam with a uniform irradiation energy density on the sample surface.However, if the intensity of the beam irradiating the sample is uneven, the high energy density part in the beam will be destroyed. It becomes easy and accurate evaluation cannot be performed.

For the reasons described above, it has been difficult to perform accurate measurement, especially when measuring the laser durability of an optical element using an excimer laser. The present invention has been made in view of these problems,
It is an object of the present invention to provide an optical element laser durability evaluation apparatus and an optical element laser durability evaluation method capable of performing accurate laser durability evaluation even when using light having an intensity distribution such as an excimer laser. I do.

[0017]

A first means for solving the above problems is an evaluation apparatus for observing a change in an optical element after irradiating the optical element with a laser beam. In the path of light to the optical element
An apparatus for evaluating laser durability of an optical element, further comprising a light amount equalizing mechanism for equalizing light amount unevenness of a light source laser.

In this means, the spatial intensity distribution of the laser light from the laser light source is made uniform by the light quantity equalizing mechanism until it reaches the optical element which is the subject. Therefore, it is possible to irradiate the optical element, which is the subject, with a spatially uniform laser beam, and to perform accurate laser durability evaluation.

A second means for solving the above-mentioned problem is:
The first means, wherein the light quantity equalizing mechanism comprises an optical system having a fly-eye lens (Claim 2).

By using the fly-eye lens, the laser light is a collection of light emitted from a number of point-like light sources, and even if the light from the laser light source has spatial intensity unevenness, the laser light is uniform. Light having a spatial intensity distribution is obtained.

A third means for solving the above problem is as follows.
A method for evaluating laser durability of an optical element by irradiating a laser light emitted from a laser light source to an optical element, and irradiating the optical element after uniforming light amount unevenness of the laser light. This is a method for evaluating the laser durability of an optical element (claim 3).

[0022] This means is to provide a light quantity equalizing mechanism between the laser light source and the object to be inspected even if the laser light from the laser light source has spatial intensity unevenness. Since a uniform laser beam can be applied to the optical element, it is possible to measure each part of the optical element which is the subject under the same conditions. Therefore, accurate measurement can be performed.

A fourth means for solving the above-mentioned problem is:
An optical element whose durability has been evaluated by the laser durability evaluation apparatus as the first means or the second means, and an optical element whose durability has been evaluated by the laser durability evaluation method as the third means. An exposure apparatus according to a fourth aspect of the present invention.

The exposure apparatus is used to convert an image of a pattern formed on a transfer member, such as an exposure apparatus for manufacturing a semiconductor, into an image.
A material having a function of forming an image on a transfer target. In addition, "the optical element whose durability was evaluated by the laser durability evaluation device, the optical element whose durability was evaluated by the laser durability evaluation method" necessarily means only the optical element whose durability was actually evaluated. Optical elements belonging to a lot for which the durability of a representative sample has been evaluated by sample inspection, and optical elements manufactured under the same conditions as those for which the durability of a representative sample has been evaluated by sample inspection in the design stage. Is also included.

In this means, the state of the destruction occurring in the minute portion is inspected, and only the optical element having excellent durability without being destroyed is provided in the exposure apparatus, so that the durability of the exposure apparatus itself is also improved. . Further, since the optical element does not have a defect due to destruction or the like, if this is incorporated in an exposure apparatus, highly efficient and accurate exposure can be performed, and the production efficiency of a semiconductor element and the like is improved.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a laser durability evaluation device that is a first example of an embodiment according to the present invention. In FIG. 1, 1 is a light source, 2 is a laser beam, 3 is a beam shaping optical system, 4 is an energy adjusting optical system, 5 is an aperture, 6 is a beam splitter, 7 is irradiation light, and 8 is a condensing optical system (objective lens). ), 9 are sample holders, 10
Is a sample as an object (optical element), 11 is a beam stopper, 13 is a light amount monitor, 14 is an oscilloscope,
Reference numeral 15 denotes a computer, and reference numeral 18 denotes a homogenizing optical system.

In a chamber (not shown) of the laser durability evaluation device, a beam shaping optical system 3 for shaping the excimer laser beam 2 from the light source 1 and a uniforming optical system for reducing the intensity distribution unevenness of the light source. 18, an energy adjusting optical system 4 for adjusting the light amount, a beam splitter 6 for separating the laser light whose amount has been adjusted into irradiation light (measuring light) 7 and reference light 12, and condensing the laser light on the sample 10 A condensing optical system 8, a light amount monitor sensor 13 for receiving a reference light 12, a sample holder 9 on which a sample 10 as an object optical element is mounted, and a beam stopper 11 for absorbing measurement light transmitted through the sample. is set up.

A laser beam 2 from a light source 1 is shaped into a square from a rectangle by a beam shaping optical system 3, the intensity of irradiation light (irradiation energy density) is adjusted by an energy adjustment optical system 4, and a predetermined sectional shape is formed by an aperture 5. Is adjusted to have The laser beam that has passed through aperture 5 is
The beam is split into the irradiation light 7 and the reference light 12 by the beam splitter 6. The intensity of the irradiation light 7 is monitored by the light amount monitor sensor 13 and the oscilloscope 14 using the reference light 12. The irradiation light 7 is formed into a fixed shape by the condensing optical system 8 and is irradiated on the sample 10.

The irradiation light intensity monitored by the light amount monitor sensor 13 is read into a computer 15 via an oscilloscope 14, and the computer 15
The light source 1 is controlled so that the intensity of the light is constant, and the light amount of the irradiation light 7 irradiating the sample 10 is integrated. After the irradiation of the prescribed amount of light, the sample 10 is removed from the sample holder 9 and the change of the sample 9 is observed with a microscope.

The above operation is the same as that of the conventional example shown in FIG. The difference from the conventional example is that the homogenizing optical system 18 is provided on the laser optical axis. The homogenizing optical system 18 is an optical system for homogenizing spatial light intensity unevenness, and an example shown in FIG. 2 is conceivable.
FIG. 2 is a cross-sectional view of the homogenizing optical system 18.
18A is a fly-eye lens and 18B is an auxiliary lens. The fly-eye lens 18A is a combination of a large number of quadrangular prism convex lenses.
Is converted into an aggregate of point light sources emitted from the center of the convex lens by the action. Auxiliary lens 18B
Is for imaging light from these point light sources on an appropriate surface. With such a structure, even when light having a spatially non-uniform intensity distribution, such as excimer laser light, enters the homogenizing optical system 18, the light emitted from the homogenizing optical system 18 is spatially uniform. It has a strong intensity distribution.

Since the apparatus shown in FIG. 1 has such a homogenizing optical system 18, the sample 10 can be irradiated with light having a spatially uniform intensity distribution. Therefore, accurate LDT measurement can be performed even when light having a spatially non-uniform intensity distribution such as excimer laser light is used.

FIG. 3 is a schematic configuration diagram of a laser durability evaluation apparatus according to a second embodiment of the present invention.
3, the same components as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 3, reference numeral 16 denotes an acoustic detection element, and 17 denotes an amplifier for the acoustic detection element.

The configuration of the embodiment shown in FIG. 3 is almost the same as that shown in FIG. 1, and is different only in that the acoustic detection element 16 and the acoustic detection element amplifier 17 are provided. The description of the same parts will be omitted, and only different parts will be described.

The acoustic detection element 16 is composed of a piezoelectric element or the like, and is firmly bonded to the sample 10 with an adhesive or the like, and stable acoustic matching is achieved. The acoustic detection element 16 has a cylindrical shape, and is inserted from the back of a hole (not shown) provided in the sample holder 9, and is in contact with the back surface of the sample 10 (the side opposite to the laser light incident direction). Fixed. The wiring is drawn out from one end of the acoustic detection element 16.

The sample holder 9 is made of metal (or bakelite or the like). The electric signal from the sound detection element 16 is amplified by the sound detection element amplifier 18 and monitored by the oscilloscope 14.

The acoustic detection element 16 is used for the sample 10
This is to measure an acoustic signal generated due to a volume change of the optical element caused by light absorption when a laser beam is irradiated to the (optical element as an object). Then, by observing this behavior, the laser durability is measured.

The measurement sample 10 set as described above is continuously irradiated with the irradiation light intensity set by the energy adjusting optical system 4, and the output change of the acoustic detection element with respect to the number of irradiations is measured, thereby obtaining the laser durability. To evaluate.

The above operation is the same as that of the conventional example shown in FIG. 5, but in the present embodiment, the point that the homogenizing optical system 18 is provided on the laser optical axis is different from the conventional example shown in FIG. It is different from the example. The operation of the homogenizing optical system 18 is the same as that described in the description of the embodiment shown in FIG. 1. By providing the homogenizing optical system 18, the spatially non-uniform intensity such as excimer laser light is obtained. Even when light having a distribution is used, each portion of the sample 10 can be irradiated with laser light of the same intensity. Therefore, the effect of the laser beam on each part of the sample 10 is the same, so that the laser durability can be accurately measured.

The optical element evaluated as described above can be incorporated in an exposure apparatus having a conventionally known configuration. In this case, since only an optical element having excellent durability can be used, the exposure apparatus itself can have excellent durability. Further, since the optical element has no defect due to destruction or the like, more efficient and accurate exposure can be performed, and the manufacturing efficiency of the semiconductor element and the like is improved.

[0040]

As described above, according to the first to third aspects of the present invention, it is possible to irradiate an optical element which is an object with a spatially uniform laser beam. By using this, accurate laser durability evaluation can be performed.

If a fly-eye lens is used as a uniforming mechanism, the laser light is a collection of light emitted from a number of point light sources, and even if the light from the laser light source has spatial intensity unevenness. As a result, light having a uniform spatial intensity distribution can be obtained as an output.

Furthermore, even when the laser light from the laser light source has spatial intensity unevenness, the optical element as the object can be uniformly irradiated with the laser light. Can be measured under the same conditions. Therefore, accurate measurement can be performed.

According to the fourth aspect of the present invention, since the exposure apparatus is provided with the optical element having excellent durability, the durability of the exposure apparatus itself can be improved. Further, since the optical element does not have a defect due to destruction or the like, if this is incorporated in an exposure apparatus, highly efficient and accurate exposure can be performed, and the production efficiency of a semiconductor element and the like is improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a laser durability evaluation device that is a first example of an embodiment according to the present invention.

FIG. 2 is a schematic diagram illustrating an example of a uniforming optical system using a fly-eye lens.

FIG. 3 is a schematic configuration diagram of a laser durability evaluation device which is a second example of the embodiment according to the present invention.

FIG. 4 is a schematic configuration diagram showing an example of a conventional laser durability evaluation device.

FIG. 5 is a schematic configuration diagram showing another example of a conventional laser durability evaluation device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Laser light, 3 ... Beam shaping optical system, 4
... Energy adjustment optical system, 5 ... Aperture, 6 ... Beam splitter, 7 ... Irradiation light, 8 ... Condensing optical system (objective lens), 9 ... Sample holder, 10 ... Sample as an object (optical element), 11 ... Beam stopper, 13 ...
Light amount monitor, 14 oscilloscope, 15 computer, 18 uniform optical system, 18A fly-eye lens, 18B auxiliary lens

Claims (4)

[Claims] 1. An evaluation device for observing a change in an optical element after irradiating the optical element with a laser beam, wherein a light amount unevenness of a light source laser is observed in a light path from a laser light source to the optical element as an object. An apparatus for evaluating laser durability of an optical element, comprising a light quantity equalizing mechanism for equalizing light. 2. The apparatus according to claim 1, wherein said light amount equalizing mechanism comprises an optical system having a fly-eye lens.
A laser durability evaluation apparatus for an optical element according to item 1. 3. A method for irradiating a laser beam emitted from a laser light source to an optical element to evaluate the laser durability of the optical element, wherein the unevenness in the amount of the laser beam is made uniform before irradiating the optical element. A method for evaluating the laser durability of an optical element. 4. An optical element whose durability has been evaluated by the laser durability evaluation device according to claim 1 or 2, or an optical element whose durability has been evaluated by the laser durability evaluation method according to claim 3. An exposure apparatus comprising an element.