阅读说明：本技术 用于同步辐射软X射线聚焦成像的Kinoform介质透镜及其制备方法 (Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging and preparation method thereof ) 是由 陈宜方 童徐杰 韦璐 于 2021-01-17 设计创作，主要内容包括：本发明属于电子束光刻技术领域,具体为用于同步辐射软X射线聚焦成像的Kinoform介质透镜及其制备方法。本发明的Kinoform介质透镜结构包括硅基底、薄膜窗口、具有斜面形貌的二维或者三维的介质材料Kinoform透镜；制备步骤包括：在薄膜衬底上旋涂光刻胶,利用电子束灰度光刻技术在光刻胶上曝光形成Kinoform透镜的设计图形,最后显影得到Kinoform透镜。本发明方法可用于制备一类具有类似于锯齿波带结构的X射线聚焦成像透镜,即圆形Kinoform平板透镜,实现对软X射线的高效率聚焦和成像；具有工艺稳定可靠、制备周期缩短和与现有的光刻工艺兼容等优点。制备的透镜适合于对生物细胞、有机材料和介质材料等的X射线高衬度三维成像。(The invention belongs to the technical field of electron beam lithography, and particularly relates to a Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging and a preparation method thereof. The Kinoform dielectric lens structure comprises a silicon substrate, a thin film window and a two-dimensional or three-dimensional dielectric material Kinoform lens with a slope appearance; the preparation method comprises the following steps: and spin-coating photoresist on the film substrate, exposing the photoresist by using an electron beam gray scale lithography technology to form a design pattern of the Kinoform lens, and finally developing to obtain the Kinoform lens. The method can be used for preparing an X-ray focusing imaging lens with a structure similar to a sawtooth wave band, namely a round Kinoform flat lens, and realizes high-efficiency focusing and imaging of soft X-rays; the method has the advantages of stable and reliable process, shortened preparation period, compatibility with the existing photoetching process and the like. The prepared lens is suitable for X-ray high-contrast three-dimensional imaging of biological cells, organic materials, dielectric materials and the like.)

1. A Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging is characterized in that the structure of the Kinoform medium lens is as follows from bottom to top in sequence: the optical lens comprises a silicon substrate, a thin film window and a two-dimensional or three-dimensional dielectric material Kinoform lens with a slope appearance; wherein, the silicon substrate and the film window respectively play a role in supporting and transmitting light, and the front surface of the Kinoform lens made of dielectric material is placed on the film; the converging direction of the lens is vertical to the film window; the soft X-ray is focused after penetrating through the film and the lens; the kinoform structure of the dielectric material with low absorption coefficient can realize high-efficiency focusing of soft X-rays.

2. The Kinoform dielectric lens of claim 1, wherein the lens material has a high transmittance for soft X-rays with energy of 200-2000 eV, and has an absorption coefficient of less than 10^-2Including but not limited to HSQ or PMMA.

3. The Kinoform dielectric lens of claim 1, wherein the lens material uses electron sensitive materials for electron beam gray scale lithography and development, including but not limited to HSQ or PMMA.

4. The Kinoform dielectric lens of claim 1, wherein the lens is in a shape of a long strip or a circle, and the variable period structure of the lens has an inclined appearance and has a converging effect on X-rays; the cross-sectional morphology of a single period is a right triangle, a right trapezoid or a crescent, or a mixture of shapes thereof.

5. The Kinoform dielectric lens of claim 1, wherein the thin film window material is selected from silicon nitride, silicon dioxide or silicon; the film window is square, the thickness of the film window is 100 nm-300 nm, and the side length of the film window is 1 mm-2 mm.

6. The Kinoform dielectric lens of claim 1, wherein the focusing efficiency is greater than 10% for the 280-530 eV water window energy range and greater than 15% for the 500-2000 eV energy range.

7. The Kinoform dielectric lens of any one of claims 1-6, prepared by the steps of:

(1) spin-coating HSQ photoresist on a substrate with a thin film window;

(2) forming a photoresist pattern of the Kinoform lens on the photoresist by using an electron beam gray scale photoetching method;

(3) developing the sample obtained in the step (2);

(4) and (4) performing reactive ion beam etching on the sample obtained in the step (3).

8. The preparation method according to claim 7, wherein in the step (1), the thickness of the HSQ photoresist is 300 nm to 4000 nm; and baking to harden the material, wherein the baking temperature is 150 ℃ to 180 ℃ and the baking time is 2 min to 30 min.

9. The preparation method according to claim 7, wherein in the step (2), the electron beam gray scale lithography is used for exposure layout division and exposure dose distribution according to the three-dimensional appearance of the Kinoform lens, and the exposure basic dose is 500-3000 μ C/cm²The exposure relative dose is 0.05-2; the exposure area of the electron beam gray scale photoetching is circular or rectangular; the diameter of the circle is 20 mu m-1 mm; the length of the short side of the rectangle is 20 mu m-5 mm, and the length of the long side is 100 mu m-5 mm.

10. The method according to claim 7, wherein the ratio of the developing solution in step (3) is TMAH: h₂O =1:3, the developing temperature is 40-60 ℃, and the developing time is 1-10 min; the gas for ion beam etching in the step (4) is O₂，O₂The gas flow is 10 sccm-30 sccm, the power is 20W-100W, and the time is 1 min-3 min.

Technical Field

The invention belongs to the technical field of electron beam lithography, and particularly relates to a Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging and a preparation method thereof.

Background

The synchrotron radiation X-ray microscopic imaging technology is one of the important methods for the current research of basic subjects such as materials, physics, biology, medicine and the like. The X-ray focusing optics are a crucial component in X-ray imaging systems. In soft X-ray, especially in a water window waveband (280-530 eV) with natural contrast for biological imaging, a traditional diffraction focusing lens mainly based on a metal Fresnel Zone Plate (FZP) has a large absorption coefficient in the waveband, and the principle of diffraction focusing further limits the focusing efficiency, so that the actual focusing efficiency is lower than 5%. Whereas the theoretical efficiency of a refractive Kinoform lens can reach 100%. Even in the soft X-ray band, which is easily absorbed, the efficiency can still be much higher than the theoretical limit of the FZP. International Kinoform lens development mainly focuses on hard X-rays with energy of 5-10 keV, the lens material is mainly silicon or gold, and a deep silicon etching process is usually adopted to prepare a one-dimensional Kinoform lens with a focusing direction parallel to a substrate, but the etching depth has limitations. In addition, the two one-dimensional Kinoform lenses can be orthogonally superposed to realize point focusing, so that the efficiency is reduced, and the optical path is difficult to adjust. Due to the above problems, Kinoform lenses have not been applied to synchrotron radiation small-angle X-ray photon correlation spectroscopy, thin-layer material reflection spectroscopy, momentum analysis spectroscopy, and the like until the last two years.

In order to break the bottleneck that the soft X-ray wave band focusing lens is low in efficiency and difficult in process preparation, the Kinoform lens is prepared in one step by using a photoresist medium material with a low absorption coefficient in a soft X-ray wave band as a lens body and adopting an electron beam gray level exposure method. The design processing scheme is not only suitable for preparing a two-dimensional Kinoform lens, but also can realize the processing of a circular Kinoform lens, thereby fully exerting the high-efficiency focusing advantage of the Kinoform lens. The high-efficiency focusing of the soft X-ray wave band has wide application prospect in a plurality of X-ray optical fields of synchronous radiation micro-nano probes, full-field microscopic imaging, X-ray confocal microscopic imaging, novel X-ray fluorescence CT and the like, greatly improves the technical level of synchronous radiation detection in China, and has great significance for the research of basic subjects in China.

The process is not only suitable for the preparation of Kinoform lenses in the soft X-ray band, but also can be used for the preparation of other focusing and imaging elements at the energy.

Disclosure of Invention

The invention aims to provide a Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging and a preparation method thereof, which are simple, stable and efficient, so as to solve the problems mentioned in the background technology.

The invention provides a high-efficiency Kinoform medium lens for soft X-ray focusing imaging, which sequentially comprises the following structures from bottom to top: the optical lens comprises a silicon substrate, a thin film window and a two-dimensional or three-dimensional dielectric material Kinoform lens with a slope appearance; wherein, the silicon substrate and the film window respectively play a role in supporting and transmitting light, and the front surface of the Kinoform lens made of dielectric material is placed on the film; the converging direction of the lens is vertical to the film window; the soft X-ray is focused after penetrating through the film and the lens; the kinoform structure of the dielectric material with low absorption coefficient can realize high-efficiency focusing of soft X-rays.

In the invention, the lens material is a material with high transmittance to soft X-rays with energy of 200-2000 eV, and the absorption coefficient is less than 10^-2Including but not limited to HSQ or PMMA.

In the present invention, the lens material uses electron sensitive material for electron beam gray scale lithography and development, including but not limited to HSQ or PMMA.

In the invention, the lens is a strip-shaped or round Kinoform lens, and the variable-period structure of the lens has an inclined appearance and has a convergence effect on X rays; the cross-sectional topography of a single period includes, but is not limited to, right triangles, right trapezoids, crescents, and mixtures of shapes.

In the present invention, the thin film window material includes, but is not limited to, silicon nitride, silicon dioxide, or silicon.

In the invention, the thin film window is square, the thickness is 100 nm-300 nm, and the side length of the window is 1 mm-2 mm.

The Kinoform dielectric lens for synchrotron radiation soft X-ray focusing imaging has the focusing efficiency of more than 10% in a 280-530 eV water window energy range and more than 15% in a 500-2000 eV energy range.

The invention provides a preparation method of the Kinoform medium lens for synchrotron radiation soft X-ray focusing imaging, which comprises the following specific steps:

(1) spin-coating HSQ photoresist on a substrate with a thin film window;

(2) forming a photoresist pattern of the Kinoform lens on the photoresist by using an electron beam gray scale photoetching method;

(3) developing the sample obtained in the step (2);

(4) and (4) performing reactive ion beam etching on the sample obtained in the step (3).

In the step (1), the silicon nitride film window is square, is grown by PECVD, and has the thickness of 100 nm-300 nm and the side length of 1 mm-2 mm.

In the step (1), the thickness of the HSQ photoresist is 300 nm-4000 nm; and baking to harden the material, wherein the baking temperature is 150 ℃ to 180 ℃ and the baking time is 2 min to 30 min.

In the step (2), the electron beam gray scale lithography is to divide the exposure layout and distribute the exposure dose according to the three-dimensional appearance of the Kinoform lens, and the exposure basic dose is 500-3000 mu C/cm²The exposure relative dose is 0.05-2.

In step (2) of the present invention, the exposure area of the electron beam gray scale lithography is circular or rectangular. The diameter of the circle is 20 μm to 1 mm. The length of the short side of the rectangle is 20 mu m-5 mm, and the length of the long side is 100 mu m-5 mm.

In step (3), the ratio of the developing solution is TMAH: h₂O =1:3, the developing temperature is 40-60 ℃, and the developing time is 1-10 min.

In the step (4), the ion beam etching is used for removing part of rough residual photoresist to improve the hardness of the lens, and the etching gas is O₂，O₂The gas flow is 10 sccm-30 sccm, the power is 20W-100W, and the time is 1 min-3 min.

In the invention, the Kinoform dielectric lens for synchrotron radiation soft X-ray focusing imaging has the focusing efficiency of more than 10% in the energy range of a 280-530 eV water window and more than 15% in the energy range of 500-2000 eV.

Compared with the prior art, the method has the beneficial effects that:

firstly, the Kinoform lens morphology is obtained by adopting a preparation method mainly based on electron beam gray scale lithography, and the Kinoform lens morphology is suitable for preparing two-dimensional rectangular and three-dimensional circular Kinoform lenses. Compared with the traditional deep silicon etching process for preparing a two-dimensional Kinoform lens and the traditional multi-step alignment process for preparing a discrete step type Kinoform lens, the method has the process advantages of simplicity, rapidness, stability and wide applicability;

secondly, the invention uses photoresist medium (HSQ, PMMA, etc.) with low absorption coefficient to soft X-ray as Kinoform lens material, which saves the process step of metallization and improves the focusing efficiency under the energy section to 2-3 times of the original one;

thirdly, the invention can control the slope morphology of the lens in any period by adjusting the dose of electron beam gray level exposure; kinoform lenses of single periodic cross-section such as right triangle, right trapezoid, crescent and mixture of multiple shapes were prepared to optimize their optical performance. In addition, the method can also be used for preparing any photoresist three-dimensional structure, and has high process freedom and wide application prospect.

The method can be used for preparing a novel X-ray focusing imaging lens with a structure similar to a sawtooth wave band, namely a circular Kinoform flat lens, and realizes high-efficiency focusing and imaging of soft X-rays. It is theoretically possible to increase the focusing efficiency from the theoretical limit (10%) of the conventional zone plate to 15%. The method has the advantages of stable and reliable process, shortened preparation period, compatibility with the existing photoetching process and the like. The prepared X-ray focusing imaging lens is particularly suitable for X-ray high-contrast three-dimensional imaging of materials with low contrast, such as biological cells, organic materials, dielectric materials and the like.

Drawings

FIG. 1 is a silicon wafer with a thin film window.

FIG. 2 corresponds to step 1: and spin-coating HSQ photoresist on the substrate.

FIG. 3 corresponds to step 2: the HSQKinoform lens pattern is formed by electron beam gray scale lithography.

FIG. 4 corresponds to step 3: the photoresist was developed to obtain a HSQKinoform lens pattern.

Fig. 5 corresponds to step 4: and removing residual glue after etching to obtain the HSQKinoform lens graph with high focusing efficiency.

FIG. 6 is a schematic diagram of a circular HSQ Kinoform lens for high efficiency focusing corresponding to the water window band (280-530 eV).

FIG. 7 corresponds to step 1: and spin-coating PMMA photoresist on the substrate.

FIG. 8 corresponds to step 2: and E-beam gray scale lithography is carried out to form a PMMA Kinoform lens pattern.

FIG. 9 corresponds to step 3: the photoresist was developed to obtain a PMMA Kinoform lens pattern.

Fig. 10 corresponds to step 4: and removing residual glue after etching to obtain the PMMAKinoform lens pattern with high focusing efficiency.

FIG. 11 is a schematic diagram of a two-dimensional HSQ Kinoform lens for high-efficiency focusing in the water window band (280-530 eV) prepared correspondingly.

FIG. 12 is a schematic view of a 1-1.5 KeV high efficiency focusing round PMMA Kinoform lens.

FIG. 13 is a schematic representation of a lens structure of the present invention.

Detailed Description

The invention will be further described with reference to the drawings and examples, but the invention is not limited to the examples. All the simple changes of the process parameters in the embodiments are within the protection scope of the present invention.

Example 1: the preparation method of the round HSQ Kinoform lens with the water window wave band (280-530 eV) capable of efficiently focusing comprises the following specific steps:

(1) selecting a silicon wafer with a silicon nitride window, wherein the thickness of the silicon nitride window is 100nm, and the size of the silicon nitride window is 2mm multiplied by 2mm, as shown in figure 1;

(2) after an HMDS adhesion layer is coated on a substrate in a spin mode, HSQ electron beam photoresist with the thickness of 600 nm is coated in a spin mode, and the substrate is baked in an oven at the temperature of 180 ℃ for 2 min to be hardened, and the result is shown in figure 2;

(3) exposing the sample under an electron beam lithography machine,the exposure pattern is circular, the diameter is 100 μm, and the exposure basic dose is 2000 μ C/cm^2,Carrying out gray scale division on the graph according to the appearance, wherein the relative dose is 0.05-1, the gradient change is 0.01, and the graph after exposure is shown in figure 3;

(4) TMAH/H at 50 ℃ in a volume ratio of 1:3₂Development in O Mixed solution for 2 min followed by H₂O for 30s, the result is shown in fig. 4;

(5) removing residual glue by reactive ion beam etching with etching gas of O₂，O₂The gas flow is 20sccm, the power is 50W, the time is 1 min, the final obtained lens is shown in FIG. 5, and the overall appearance is shown in FIG. 6.

Example 2: the method for preparing the two-dimensional HSQ Kinoform lens with the water window wave band (280-530 eV) capable of being efficiently focused comprises the following specific steps:

the main differences between this embodiment and the above example are: the lens of this example is a two-dimensional topography. Besides, the parameters used in some steps are slightly different, and other steps are the same as the previous example:

(1) selecting a silicon wafer with a silicon nitride window, wherein the thickness of the silicon nitride window is 100nm, and the size of the silicon nitride window is 2mm multiplied by 2mm, as shown in figure 1;

(2) after an HMDS adhesion layer is coated on a substrate in a spin mode, HSQ electron beam photoresist with the thickness of 600 nm is coated in a spin mode, and the substrate is baked in an oven at the temperature of 180 ℃ for 2 min to be hardened, and the result is shown in figure 2;

(3) exposing the sample under an electron beam lithography machine, wherein the exposure figure is a rectangular Kinoform lens of 100 mu m multiplied by 1000 mu m, and the exposure basic dose is 2000 mu C/cm^2,Carrying out gray scale division on the graph according to the appearance, wherein the relative dose is 0.05-1, the gradient change is 0.01, and the graph after exposure is shown in figure 3;

(4) TMAH/H at 50 ℃ in a volume ratio of 1:3₂Development in O Mixed solution for 2 min followed by H₂O for 30s, the result is shown in fig. 4;

(5) removing residual glue by reactive ion beam etching with etching gas of O₂，O₂The gas flow rate was 20sccm, the power was 50W, and the time was 1 min, and the final lens was as shown in FIG. 5, which is a wholeIntended as shown in fig. 11.

Example 3: the preparation method of the 1-1.5 KeV high-efficiency focused circular PMMA Kinoform lens comprises the following specific steps:

the main differences between this embodiment and the above example are: the photoresist in this example is PMMA, which is dissolved after the exposed portions are developed. Meanwhile, since PMMA is greatly affected by the proximity effect compared to HSQ, the embodiment optimizes the slope morphology of the region with a small period at the peripheral portion of the lens, changes the cross section of the right triangle into a right trapezoid, and also has a high focusing efficiency. Besides, the parameters used in some steps are slightly different, and other steps are the same as the previous example:

(1) selecting a silicon wafer with a silicon nitride window, wherein the thickness of the silicon nitride window is 100nm, and the size of the silicon nitride window is 2mm multiplied by 2mm, as shown in figure 1;

(2) after the HMDS adhesion layer is coated on the substrate in a spin mode, PMMA electron beam photoresist with the thickness of 3000 nm is coated in a spin mode, the PMMA electron beam photoresist is baked in an oven at the temperature of 180 ℃ for 10min to be hardened, and the result is shown in FIG. 7;

(3) exposing the sample under an electron beam lithography machine, wherein the exposure graph is circular, the diameter is 100 mu m, and the exposure basic dose is 1500 mu C/cm^2,Carrying out gray scale division on the graph according to the appearance, wherein the relative dose is 0.05-1, the gradient change is 0.01, and the graph after exposure is shown in figure 8;

(4) IPA/H at 23 deg.C in a volume ratio of 7:3₂Development in O Mixed solution for 4 min followed by H₂O for 30s, the result is shown in fig. 9;

(5) removing residual glue by reactive ion beam etching with etching gas of O₂，O₂The gas flow is 20sccm, the power is 50W, and the time is 1 min, and the final lens is shown in FIG. 10, and the overall schematic diagram is shown in FIG. 12.