Method for simultaneously realizing single-point focusing and wavefront segmentation of extreme ultraviolet band light

Abstract

The application relates to a method for simultaneously realizing single-point focusing and wavefront segmentation of extreme ultraviolet light, and relates to the field of optics. The application aims to solve the problem that extreme ultraviolet light cannot simultaneously realize single-point focusing and wavefront segmentation. The method of the application is as follows: the angle between an optical axis and a central axis of an optical element is theta, the angle between a single incident light ray and a tangent direction of an incident point on an inner surface of the optical element is theta1, the angle between the single incident light ray and the optical axis is theta2, and the angle between a normal line of a single light incident point and the central axis of the optical element is theta3; the above angles need to satisfy the following formula: (90 DEG-theta1)+(theta2+theta)+(180 DEG-theta3) = 180 DEG; the relationship between the extreme ultraviolet light reflected by grazing incidence and the optical element is regulated according to the above formula, so that the method for realizing the single-point focusing and the wavefront segmentation of the extreme ultraviolet light reflected by grazing incidence is completed. The method of the application can obtain a very high reflectivity, greatly reduce optical distortion, has the wavefront segmentation function in addition to focusing, and realizes light field regulation to a certain extent.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optics, and particularly relates to a method for simultaneously achieving single-point focusing and wavefront segmentation of extreme ultraviolet band light. BACKGROUND

[0002] Extreme ultraviolet band laser generally refers to light waves with a wavelength of 10-100 nm. Due to high photon energy, theoretically, light of this band can ionize any atom or molecule. A big feature of this band laser is that the material has a very high absorption rate and a very short decay depth, which leads to the fact that such light cannot penetrate most materials.

[0003] Light field regulation of extreme ultraviolet band laser is a big technical difficulty in this field. Because the absorption rate of various materials to this band light source is very high, and the refractive index is close to 1, single-point focusing and simple light field regulation that are very easy to achieve in the optical band are very difficult in the extreme ultraviolet band. At present, the multilayer film reflection technology is generally used internationally in the extreme ultraviolet band. According to the Bragg formula, a multilayer film structure with appropriate parameters can have a high reflectivity to incident light at a small incident angle. But this so-called "high" reflectivity is generally only close to 50%. Moreover, the multilayer film production process is complex, expensive, easy to be damaged by incident light, and has a low service life. Moreover, the reflection surface of the multilayer film has selectivity to the wavelength and incident angle of the incident light, that is, for each wavelength and each incident angle of the incident condition, a corresponding multilayer film material needs to be produced, which has great limitations in use.

[0004] Using diffraction to focus and regulate light field is another commonly used method for extreme ultraviolet band. For example, wave band film and other optical elements. But the diffraction efficiency of the diffraction type optical element is low, and most of the incident light energy is lost. Moreover, the wave band film and other optical elements of the extreme ultraviolet band can only be made with a light aperture of hundreds of microns by the current process, which further reduces the utilization rate of the incident light. Therefore, although the diffraction type optical element has good focusing effect and flexibility of light field regulation for the extreme ultraviolet band, the utilization rate of the incident light is very low, and it cannot be well regulated for the light field of the extreme ultraviolet band laser.

[0005] Compared with the above methods, the grazing incidence reflection method is an effective way to ensure the reflectivity of the incident light. In the grazing incidence angle close to or less than the total reflection angle (the total reflection angle Wherein, delta is the real part of complex refractive index, which is a very small number in the extreme ultraviolet wave band, and a very high reflectivity can be achieved within the range. However, the traditional grazing incidence reflection type optical element is off-axis grazing incidence reflection, which causes the optical distortion of the focused spot to be extremely large, which brings great difficulty to subsequent experimental and theoretical analysis. Even if the effect of focusing and small optical distortion of the spot can be achieved, but limited by the structure of the optical element, it is impossible to simultaneously realize the wavefront segmentation of the extreme ultraviolet wave, and form interference fringes inside the focused spot, so as to achieve the purpose of light field regulation except single point focusing. SUMMARY

[0006] The purpose of the application is to solve the problem that the extreme ultraviolet wave cannot simultaneously realize single point focusing and wavefront segmentation, and to provide a method for simultaneously realizing single point focusing and wavefront segmentation of extreme ultraviolet wave.

[0007] The method for simultaneously realizing single point focusing and wavefront segmentation of extreme ultraviolet wave of the application is realized by adjusting the angle relationship between the extreme ultraviolet wave and the optical element by using the optical element, and the method is as follows:

[0008] The angle between the optical axis and the center axis of the optical element is theta, the angle between the single incident light and the tangent direction of the incident point on the inner surface of the optical element is theta1, the angle between the single incident light and the optical axis is theta2, and the angle between the normal line of the single light incident point and the center axis of the optical element is theta3; the above angles satisfy the following formula:

[0009] (90°-theta1)+(theta2+theta)+(180°-theta3)=180° (1)

[0010] According to the above formula, the relationship between the grazing incidence reflected extreme ultraviolet wave and the optical element is adjusted, that is, the method for realizing single point focusing and wavefront segmentation of the grazing incidence reflected extreme ultraviolet wave is completed.

[0011] Further, the total reflection angle of grazing incidence light is theta c , then The calculation formula of the real part of complex refractive index delta is:

[0012]

[0013] Wherein, r e is the classical electron radius, N is the atomic density, lambda is the wavelength of incident light, and f1(omega) is the real part of atomic scattering factor.

[0014] Further, the total reflection angle of grazing incidence light is theta c ; when theta1<=theta c , the single light is totally reflected on the inner surface of the optical element, and when theta1>theta cWhen the grazing incidence angle of the single light ray is greater than θ c , the single light ray is not totally reflected at the inner surface of the optical element.

[0015] Further, the port radius of the light source entry end of the optical element is r1, the port radius of the light source exit end is r2, the incident spot radius of the extreme ultraviolet waveband light is less than or equal to r1, and the spot radius of the extreme ultraviolet waveband light on the focal plane is less than or equal to r2.

[0016] Further, the surface shape of the inner surface of the optical element is a surface shape that totally reflects or nearly totally reflects the incident light ray of the grazing incidence reflected extreme ultraviolet waveband light.

[0017] The grazing incidence reflected extreme ultraviolet waveband light of the optical element adopted by the present application needs to be controlled so that the incident spot radius of the extreme ultraviolet waveband light is less than or equal to r1, the spot radius of the extreme ultraviolet waveband light on the focal plane is less than or equal to r2, and the included angle θ between the incident light optical axis and the central axis of the element is related to the required reflectivity, which is discussed as follows:

[0018] Total reflection angle of grazing incidence light The real part δ of the complex refractive index is calculated according to the following formula:

[0019]

[0020] To obtain a higher reflectivity, the grazing incidence angle of most of the light rays in the light beam needs to be less than or close to θ c . When the grazing incidence angle of the single light ray is greater than θ c , the single light ray is not totally reflected at the inner surface of the optical element.

[0021]

[0022] Both of the above two methods can estimate the reflectivity of the element. Considering that the reflective surface of the element has a certain roughness, the value calculated by the theory is generally slightly greater than the value measured by the experiment.

[0023] Figure 2The light ray is incident on the inner surface of the element. Wherein θ1 is the included angle between the incident light ray and the tangent direction of the incident point of the inner surface of the element (if the inner surface is conical, the inner surface of the incident light ray), θ2 is the included angle between the incident light ray of a single light ray and the optical axis, θ3 is the included angle between the normal of the incident point of a single light ray and the central axis of the element (related to the surface type), and θ is the included angle between the optical axis of a single light ray and the central axis of the element. Then Figure 2 The angles in the above formula satisfy the following relationship:

[0024] (90°-θ1)+(θ2+θ)+(180°-θ3)=180° (1)

[0025] Assume Figure 2 The light ray is the most marginal light ray in the light beam, and θ2 is the divergence angle of the light beam. Therefore, the angles satisfy the above constraint relationship. When the other angles are unchanged, the greater θ is, the greater θ1 is. When θ1>θ c , the greater θ1 is, the lower the reflectivity is. The specific range of θ that can be adjusted is determined comprehensively according to the acceptable reflectivity of the user and the wavefront interference effect to be achieved. The reflectivity can also be measured by experiment.

[0026] The present application comprises the following beneficial effects:

[0027] The method of the present application can obtain a very high reflectivity while greatly reducing optical distortion, and has a wavefront splitting function in addition to focusing, thereby realizing light field regulation to a certain extent. Since the absorption of extreme ultraviolet light by materials is extremely serious, there are very few optical elements that can regulate the light field of extreme ultraviolet band laser. The present application can simultaneously realize single-point focusing and wavefront splitting, thereby increasing the diversity of light field regulation means of extreme ultraviolet band laser. From the application point of view, the energy density peak value of the interference fringes generated by wavefront splitting is exponentially increased by focusing, so that the single exposure of the interference fringes formed by a single element on the material surface can realize the machining of micro-nano structures and be used for interference plasma diagnosis, etc. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 The figure is a schematic diagram of the optical path of the optical element of the present application;

[0029] Figure 2 The figure is a schematic diagram of the included angle between the optical axis of the incident light and the element of the present application;

[0030] Figure 3 The figure is a schematic diagram of the optical element of the present application;

[0031] Figure 4 The figure is a contrast diagram of the light spot in defocus (left) and focus (right) (same scale);

[0032] Figure 5The image shows the changes in the spot size before, at, and after the focal point of the 46.9nm laser light field detected under an optical microscope after being modulated by this invention.

[0033] Figure 6 The interference fringes formed by wavefront segmentation are simultaneously focused to form a hundred-nanometer periodic micro-nano fringe pattern on the sample surface;

[0034] Figure 7 This is a reflectivity diagram of multiple 46.9nm laser pulses measured in the experiment. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the spirit of the contents disclosed in the present invention will be described in detail below. After understanding the embodiments of the present invention, any person skilled in the art can make changes and modifications based on the technology taught in the present invention without departing from the spirit and scope of the present invention.

[0036] The illustrative embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0037] Example 1

[0038] This embodiment discloses an optical element for extreme ultraviolet light field modulation based on grazing incidence reflection. The optical element has a frustum-shaped structure, and its inner surface is parabolic. The radius of the bottom circle is r1 (4.4 mm), and the radius of the top circle is r2 (2.5 mm). The angle between the optical axis and the central axis of the optical element is θ. The angle between a single incident ray and the tangent at the incident point on the inner surface of the optical element is θ1. The angle between a single incident ray and the optical axis is θ2. The angle between the normal at the incident point and the central axis of the optical element is θ3. These angles must satisfy the following formula:

[0039] (90°-θ1)+(θ2+θ)+(180°-θ3)=180° (1)

[0040] The stated θ is 0.2°, and the maximum value of θ2 (i.e., the beam divergence angle) is 3 mrad. The wavelength of the extreme ultraviolet band of the grazing incidence reflection is 46.9 nm.

[0041] The comparison diagram of the light spot in this embodiment, showing the defocused (left) and focused (right) light spots at the same scale, is shown below. Figure 4 As shown, by Figure 4 It can be concluded that this element has a significant focusing effect in the extreme ultraviolet band, with the spot size at the focal point reaching the scale of hundreds of micrometers. The changes in the spot size before, at, and after the focal point, after the 46.9 nm laser field detected under an optical microscope was modulated by this invention are shown in the following diagram. Figure 5 As shown, by Figure 5It can be seen that the focused extreme ultraviolet laser spot has the smallest spot at the focal point, and expands to different degrees before and after the focal point, and the wavefront segmentation phenomenon can be clearly seen. The interference fringes formed by the wavefront segmentation form the hundred-nanometer periodic micro-nano fringes on the surface of the sample as shown in Figure 6 , Figure 6 The feasibility of micro-nano machining using the method and the extreme ultraviolet laser of the embodiment is verified. Figure 7 The reflectivity map of a plurality of 46.9nm laser pulses measured in the experiment is shown in Figure 7 It can be concluded that the reflectivity of the element to the extreme ultraviolet band light source can reach more than 90%.

Claims

1. A method for simultaneously achieving single-point focusing and wavefront segmentation of extreme ultraviolet light, characterized in that, The method utilizes optical elements to adjust the angular relationship between extreme ultraviolet light and the optical elements, and the method includes: The inner surface of the optical element is configured such that the incident light rays of the extreme ultraviolet band light subjected to grazing incidence reflection are totally or nearly totally reflected. Let the port radius of the light source entry end of the optical element be r1, and the port radius of the light source exit end be r2. Let the incident light spot radius of the extreme ultraviolet band light be less than or equal to r1, and the light spot radius of the extreme ultraviolet band light on the focal plane be less than or equal to r2. Let θ be the angle between the optical axis and the central axis of the optical element; θ1 be the angle between a single incident ray and the tangent direction at the point of incidence on the inner surface of the optical element; θ2 be the angle between a single incident ray and the optical axis; and θ3 be the angle between the normal to the point of incidence and the central axis of the optical element. These angles must satisfy the following formula: (90°-θ1)+(θ2+θ)+(180°-θ3)=180° (1) The relationship between the grazing-incidence reflected extreme ultraviolet light and the optical element is adjusted according to the formula, thus completing the method for achieving single-point focusing and wavefront segmentation of the grazing-incidence reflected extreme ultraviolet light; wherein, the adjustment includes: Let the angle of total internal reflection of the grazing incident light be θ. c By adjusting θ and then θ1, the desired reflectivity and wavefront interference effect can be obtained, so that the energy density peak of the interference fringes generated by wavefront segmentation can be increased exponentially while focusing. Specifically, when adjusting θ, θ1 is made less than or equal to θ. c or close to θ c When θ1≤θ c When θ1 > θ, a single ray undergoes total internal reflection at the inner surface of the optical element; when θ1 > θ c In this case, a single ray of light undergoes non-total internal reflection on the inner surface of the optical element, and the reflectivity is obtained through measurement or calculation.

2. The method for simultaneously achieving single-point focusing and wavefront segmentation of extreme ultraviolet light according to claim 1, characterized in that, The formula for calculating the real part δ of the complex refractive index is as follows: Where, r e Let f1(ω) be the classical electron radius, N be the atomic density, λ be the incident light wavelength, and f1(ω) be the real part of the atomic scattering factor.

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