A four-mirror reflective phase shifter for efficient generation of highly circularly polarized EUV light

Abstract

The application discloses a four-mirror reflection type phase shifter for efficiently generating highly circularly polarized EUV light, and circularly polarized light in an extreme ultraviolet (EUV) spectral range is a powerful tool for element-specific studies of chiral-sensitive light-matter interactions, and provides a possibility for realizing measurement of femtosecond dynamic magnetic dichroism. The application proposes that a Mo / B4C thin film sample is prepared by using a direct current magnetron sputtering technology under a high vacuum environment and by optimizing a sputtering parameter combination. The thin film sample can more effectively meet the requirement of multilayer films of a mirror of the four-mirror reflection type phase shifter for the extreme ultraviolet light. The thin film prepared by using the method not only has stable properties and clear boundary layers, but also has extremely low roughness and relatively high reflectivity.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of extreme ultraviolet (EUV) optics, and particularly provides a design method of a four-mirror reflective phase shifter that can efficiently generate highly circularly polarized EUV light. Specifically, a preparation technology of Mo / B4C thin film mirror suitable for EUV light reflection is provided, and a design scheme of a four-mirror reflective phase shifter with adjustable optical rotation and capable of generating highly circularly polarized EUV light is provided.

[0002] The present application uses a direct current magnetron sputtering technology to prepare Mo / B4C multilayer films. By precisely controlling the sputtering power and deposition time, the growth thickness of Mo and B4C layer films can be effectively controlled. By switching the top baffles of the Mo and B4C sources in the sample growth chamber, the alternating deposition of Mo and B4C can be realized. A Mo / B4C thin film mirror with precise thickness, low surface roughness, high reflectivity and clear layer interface is prepared. Based on the phase delay principle of metal mirror reflection, the four mirrors are installed on a support in an ultra-high vacuum chamber, and finally efficient generation of circularly polarized EUV light in a wide wavelength band is realized. BACKGROUND

[0003] Circularly polarized light in the deep ultraviolet (EUV) spectral region can be used as a powerful research tool to investigate chiral-sensitive light-matter interactions with element-specific depth. In particular, magneto-optical dichroism spectroscopy is suitable for transient absorption and reflection, time-resolved magnetic small-angle scattering, and coherent imaging experiments. This technology combines element-selective magnetic sensitivity and nanoscale spatial resolution, providing new possibilities for femtosecond-level dynamic magneto-optical dichroism measurements.

[0004] Although the polarization of electromagnetic radiation can be precisely controlled in the optical wavelength range using birefringent crystals or sheet polarizers, these methods are based on transmission principles. In particular, a significant problem with extreme ultraviolet light is that it is strongly absorbed by almost all known optical materials, which prompts us to explore different reflective solutions from traditional transmission methods. Unfortunately, reflective mirrors made of conventional optical materials still face significant loss problems when dealing with extreme ultraviolet light. Therefore, reducing the loss of materials to extreme ultraviolet light has become a problem to be solved. At the same time, in the spectral range of interest, the widely used material Au performs poorly, with low circular polarization degree.

[0005] Currently, researchers are deeply studying what kind of material and geometry can make the reflective phase-shifter polarizer play the optimal performance, so as to effectively generate high-quality circularly polarized extreme ultraviolet light. When light is reflected on the surface of an absorbing material with a complex refractive index, it will cause a characteristic phase shift. Specifically, when linearly polarized light with non-zero s and p components is incident on a metal surface and reflected, it is usually converted into elliptically polarized light. In fact, reflective devices have long been used in the study of polarization in the XUV spectral region and in the generation and analysis of circularly polarized light in synchrotron devices, which is embodied in the magnetic circular dichroism experiments of ferromagnetic materials such as Fe, Co and Ni. With the availability and performance improvement of laser-driven high-harmonic sources in the EUV spectral region, reflective polarizers have once again attracted attention and have been applied to ultrafast polychromatic magnetic absorption spectroscopy. Therefore, it is particularly important to develop a four-mirror reflective phase shifter that can efficiently generate high-circularly polarized EUV light in a wide spectral range. Such a phase shifter can quickly switch between left-handed and right-handed polarized light while maintaining the linearity of the optical path, which is crucial for performing femtosecond-level dynamic element resolution magnetic dichroism measurements. SUMMARY

[0006] The present application proposes a four-mirror reflective phase shifter. The polarization phase shifter is based on the phase delay of the metal mirror reflection, and finally can efficiently and broadband generate circularly polarized EUV light in the extreme ultraviolet spectral range of 35eV to 90eV. The device allows fast switching (<10s) of the left and right-handedness of the EUV light beam, and maintains the collimation of the optical path through a vacuum precision driving system without changing the position of the light beam on the sample. After briefly describing the polarization in the form of Stokes and calculating the phase control through reflection, we describe the main design features of the four-mirror reflective phase shifter. The present application proposes to use a four-mirror reflective phase shifter configuration that can efficiently generate highly circularly polarized EUV light. The phase shifter is based on the phase delay of the metal mirror reflection. Finally, it can efficiently and broadband generate EUV light with a circular polarization degree of up to 90% in the extreme ultraviolet spectral range of 35eV to 90eV. The device allows fast switching (<10s) of the left and right-handedness of the EUV light beam, and can adjust the light beam position through a vacuum precision drive to accurately collimate the four mirrors.

[0007] A four-mirror reflective phase shifter for efficiently generating highly circularly polarized EUV light adopts a direct current magnetron sputtering method to grow a Mo / B4C thin film on a concave focusing mirror. The thin film is stable in properties and has high reflectivity, and is used as a mirror for the four-mirror reflective phase shifter. The four mirrors are installed on a support and placed in an ultra-high vacuum chamber, and a vacuum precision driving system is configured. The phase shifter can efficiently and broadband generate circularly polarized EUV light in the extreme ultraviolet spectral range while maintaining the collimation of the optical path, and can also achieve fast conversion of left and right-handed polarized light.

[0008] Mo / B4C thin film is grown on a concave focusing mirror by a direct current magnetron sputtering method, the background vacuum degree of the direct current magnetron sputtering chamber is 10 -7 Torr level, the target material is high-purity Mo target material and B4C target material with a purity higher than 99.95%, and Mo and B4C are sputtered by high-energy Ar + bombardment, and the alternating growth of Mo layers and B4C layers is realized by controlling the baffles at the top of Mo and B4C sources.

[0009] The specific steps of the direct current magnetron sputtering method are as follows:

[0010] (1) complete the preparation and setting of the equipment;

[0011] (2) perform preliminary treatment of the substrate;

[0012] (3) perform pre-operation before sputtering;

[0013] (4) perform the deposition process of the thin film.

[0014] The direct current magnetron sputtering chamber must have a background vacuum degree better than 10 -7 Torr level. In order to calibrate the deposition rate in situ, a quartz crystal oscillator is provided in the magnetron sputtering chamber, and a gas channel capable of introducing pure argon gas is also provided.

[0015] The concave focusing mirror has a size of (70*25*15mm 3 ), a radius >10km, and a surface roughness <0.5nm (rms), and is made of monocrystalline silicon. The concave focusing mirror is cleaned by sputtering treatment technology in the direct current magnetron sputtering growth chamber to remove gas adsorbed on the surface and reduce the roughness of the sample surface. This method helps to obtain a high-quality sample surface. When performing sputter cleaning, the appropriate power range is 5-10W, and the cleaning time is recommended to be controlled between 5-10 minutes.

[0016] In the direct current magnetron sputtering method, the power of the Mo target and the B4C target is set to 10W and 80W respectively, high-purity argon (99.999%) is introduced during sputtering growth, the pressure is stabilized at 7mTorr by adjusting the pumping rate, the distance between the substrate and the target material is 100mm, and the substrate rotates at a speed of 5rpm without public rotation.

[0017] The method for preparing Mo / B4C thin film samples by optimizing the sputtering parameter combination in a high-vacuum environment by using the direct current magnetron sputtering technology. This method can better meet the needs of the multilayer film of the extreme ultraviolet four-mirror reflective phase shifter mirror. The Mo / B4C thin film prepared by the present application is deposited on a specific concave focusing mirror using the magnetron sputtering equipment customized by Shanghai Real Road Vacuum Technology Engineering Co., Ltd.

[0018] The concave focusing mirror used in the experiment has a size of 70x25x15mm 3 , a radius greater than 10km, a surface roughness less than 0.5nm (rms), and is made of single crystal silicon material. The concave mirror needs to be pre-processed before being placed in the sputtering growth chamber. The surface cleaning process includes ultrasonic cleaning in acetone, isopropyl alcohol and deionized water, then rinsing with deionized water and drying. The vacuum degree of the sputtering growth chamber must reach the order of 10 -7 Torr. In order to avoid the adverse effects of the adsorbed gas on the surface of the concave mirror on the growth of the thin film, the concave mirror is annealed for 5 minutes before growth, with a temperature setting of up to 300℃.

[0019] In order to improve the growth of Mo / B4C thin film, the Mo target and B4C target used are pre-sputtered for 15 minutes before the formal sample growth. After the pre-sputtering is completed, the distance between the substrate and the target material is set to 100mm, the high-purity Ar2 flux is controlled, the sputtering chamber pressure is maintained at 7mTorr, and the substrate is set to a self-rotation speed of 5rpm. First, open the Mo source shutter and close the B4C source shutter, set the Mo target sputtering power to DC 10W, and the deposition rate is 12min 38s, and a Mo layer with a thickness of 25nm is obtained. Then, we close the Mo source shutter and open the B4C source shutter, set the B4C target sputtering power to DC 80W under the same growth height and sputtering pressure, and the deposition rate is 3min 20s, and a B4C layer with a thickness of 2nm is obtained. In this way, the alternating plating of the two materials is realized. The deposition rate of the Mo target and the B4C target under the specified sputtering parameters is accurately calibrated by using a quartz crystal oscillator.

[0020] The present application proposes a four-mirror reflection type phase shifter configuration based on the phase delay of metal mirror reflection, which can efficiently generate highly circularly polarized EUV light and realize adjustable circular polarization degree.

[0021] To achieve the intended goal, the four-sided mirror is assembled on a support placed in a high-vacuum chamber, and the entire mirror assembly can be precisely controlled by vacuum driving, which can rotate around the beam axis with a rotation range of -60° to 60°, thereby realizing the rapid (less than 10 seconds) switching of the left and right rotation directions of the EUV light beam without changing the light beam positioning on the sample. During rotation, if there is a slight swing error of less than 30μrad causing a deviation in the light beam pointing, the mirror M4 is adjusted to compensate. Due to the wide-band characteristics of the Mo / B4C thin film, highly circularly polarized EUV light can be efficiently generated in the extremely deep ultraviolet spectral region of 35eV to 90eV.

[0022] The key technical index of the present application is that the circular polarization degree of EUV light is higher than 90%, the reflectivity of the mirror is higher than 75%, and the total transmittance is higher than 30%. The innovation of the present application is that the four-mirror reflection type design is conducive to maintaining the collimation of the light path, and high-efficiency generation of highly circularly polarized EUV light in a wide waveband range, while realizing rapid conversion of left and right circularly polarized light. The device is simple, solid and easy to operate. The scheme is a four-mirror reflection type phase shifter for high-efficiency generation of highly circularly polarized EUV light. The phase shifter can realize rapid conversion of left and right circularly polarized light on the basis of maintaining the collimation of the light path, and high-efficiency, wideband generation of circularly polarized EUV light in the extreme ultraviolet spectral range. The four mirrors are installed on a support and placed in an ultrahigh vacuum cavity to avoid mirror contamination accumulation. The entire mirror assembly can be driven by a vacuum precision drive motor to rotate around the beam axis (-60°<α<60°), thereby realizing rapid switching (<10s) of the left and right circular degrees of the EUV light beam without changing the light beam position on the sample. The light beam pointing deviation caused by the small swing error of <30μrad during rotation can be compensated by adjusting the mirror M4. The Mo / B4C thin film is used as the mirror of the four-mirror reflection type phase shifter, and cooperates with the vacuum precision drive system to accurately control the collimation of the four mirrors. The four-mirror reflection type phase shifter can realize high-efficiency generation of 90% circularly polarized EUV light in the wide waveband range of the extreme ultraviolet spectral range from 35eV to 90eV. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a schematic diagram of the mirror microstructure of the present application;

[0024] Figure 2 is a schematic diagram of the magnetron sputtering system used in the present application;

[0025] Figure 3 is a schematic diagram of the internal mirror arrangement of the four-mirror reflection type phase shifter involved in the present application;

[0026] Figure 4 is a schematic diagram of the overall configuration of the four-mirror reflection type phase shifter involved in the present application. DETAILED DESCRIPTION

[0027] In summary, the present application is to design a four-mirror reflection type phase shifter for high-efficiency generation of circularly polarized EUV light, to realize high-efficiency generation of highly circularly polarized EUV light in a wide waveband range of the extreme ultraviolet spectral range from 35eV to 90eV. And on the basis of maintaining the collimation of the light path, the left and right circularly polarized light can be rapidly converted. The technical index to be achieved by the present application is that the circular polarization degree of EUV light is higher than 90%, the reflectivity of the mirror is higher than 75%, and the total transmittance is higher than 30%.

[0028] I. Preparation of EUV multilayer film mirror with low roughness and high reflectivity

[0029] For B4C / Si thin film structure applied to EUV four-mirror reflective phase shifter system mirror, low roughness and high reflectivity are important indicators to measure its performance. Obtaining high-quality, different layer thickness accurately controlled thin film structure becomes a key technology, because the surface roughness and the change of thin film thickness will significantly affect the reflection of multilayer film structure to extreme ultraviolet light.

[0030] In the present application, B4C / Mo thin film structure applied to EUV four-mirror reflective phase shifter system mirror is prepared by using direct current magnetron sputtering growth system. We need to stably control the deposition rate of different targets on the substrate at the best sputtering parameters. Thus, the accurate control of thin film thickness is realized, and the EUV multilayer film mirror with low roughness and high reflectivity is obtained.

[0031] In the present application, the deposition rate of Mo target and B4C target on the substrate at the best sputtering parameters is calibrated by crystal oscillator. Under the condition of a certain thin film growth rate, the thin film samples with different thicknesses grown on the substrate can be obtained by controlling different growth times. After calibration, we can obtain that when the height between the substrate and the target is 100 mm, the substrate keeps a self-rotation speed of 5 rpm without rotation, and the deposition rate is When the sputtering power of B4C target is 80 W, the deposition rate is The flux of high-purity Ar2 is controlled, and the sputtering pressure is kept at 7 mTorr. In the magnetron sputtering cavity, the baffles at the top of Mo and B4C sources can be switched, so that the growth time of B4C thin film layer and Mo thin film layer can be controlled and alternating film plating can be realized. Finally, the thickness of Mo layer is 25 nm, and the thickness of B4C layer is 2 nm.

[0032] In the present application, the roughness of the thin film surface grown by direct current magnetron sputtering can be accurately measured by atomic force microscope (AFM). Generally, the surface roughness of the thin film applied to EUV mirror is less than 0.7 nm.

[0033] II. Geometric configuration and overall configuration of four-mirror phase shifter assembly

[0034] The electromagnetic radiation in free space is described as a two-dimensional transverse wave, whose electric field vector E oscillates perpendicular to the propagation vector k. E can be decomposed into two orthogonal components E p and E s , parallel and perpendicular to the incident plane of the four-mirror reflective phase shifter, respectively:

[0035]

[0036] The oscillation frequency ω defines the time interval 2π / ω for the electric field vector to rotate one circle. The amplitude of the angular wave vector is given by k = 2π / λ, is the relative phase of oscillation along p and s directions. In the special case of linear polarization or π for perfect right / left circular polarization and E p = E s .

[0037] The polarization state is completely defined by the four-dimensional Stokes vector S:

[0038]

[0039] where the average brackets denote the time average. For perfect polarized light the Stokes parameters satisfy the identity:

[0040]

[0041] Assuming an angle α between the linearly polarized pulse and the circular polarizer, the normalized Stokes vector S in the sp coordinate system Pol can be calculated as:

[0042]

[0043] where I0is the intensity of the incident radiation and R(α) represents the rotation matrix around the propagation wave vector k:

[0044]

[0045] The linear p-(α = 90°) and s-(α = 0°) polarized light is S lin / I0= (1 ± 1 0 0)'. The right and left circularly polarized light is S circ / I0= (1 0 0 ± 1)'. The reflection of polarized light from a metallic mirror with complex reflection coefficient can be described by the Müller matrix M Pol :

[0046]

[0047] where and are the phase shifts due to the reflection from the absorbing metallic surface.

[0048] The Stokes vector after reflection from the polarizer is calculated as:

[0049] S = M Pol (Ψ Pol , Δ Pol ) S Pol

[0050] The linear degree of polarization P L and the circular degree of polarization PC may be simply defined as:

[0051]

[0052] To produce perfect circularly polarized light P C = ±1, the relation Δ Pol = ±π / 2 and a = Ψ Pol must be satisfied.

[0053] Now we use the above formalism to determine in which material and geometry a reflection-based phase-shifting polarizer can produce the best performance. To do so, we use the Darwin matrix formalism to calculate the complex reflection coefficient of different metallic mirror coatings This coefficient is a function of all the angles of incidence θ and of the photon energy range between 35 eV and 90 eV. For each metallic mirror coating and EUV photon energy, the parameters a and θ, which control the ratio between the s and p components of E, determine P C and the reflectivity R. While larger grazing angles of incidence lead to larger phase shifts Δ Pol and thus to an increased degree of circular polarization, the reflectivity decreases. Therefore, we define a figure of merit as and maximize it by varying the angles of incidence θ and the rotation angle a using an optimization algorithm. The results show that Mo(B4C(2 nm) / Mo(25 nm)) with a protective cap layer exhibits better performance in a wide wavelength range in the four-mirror geometry we applied. It efficiently and widely generates circularly polarized EUV light in the very deep ultraviolet spectral region between 35 eV and 90 eV.

[0054] The internal construction of the four-mirror reflection phase shifter is shown in Figure 3 Pulse light initially linearly polarized is reflected by the four mirrors and exits the device with unchanged beam position and pointing but with a high degree of circular polarization. The overall three-dimensional model is shown in Figure 4 The four mirrors are mounted on a holder, and the EUV light incidence angle is θ. The whole mirror assembly can be rotated around the beam axis (-60° < a < 60°) by an endless steel belt controlled by a vacuum precision drive. The device allows fast switching (<10 s) of the helicity of the EUV beam without changing the beam position on the sample. Approximately 400 microsteps correspond to a rotation of a = 1°. The vacuum precision drive system can also be used to compensate for small changes in the beam position and pointing. The beam pointing deviation due to a small wobble error of <30 μrad while rotating can be compensated for by fine adjustment of mirror M4. The maximum displacement of the beam in the horizontal and vertical directions is ±0.2 mrad and ±1 mrad, respectively (for a = 0). With this geometric configuration, the EUV four-mirror phase shifter will achieve a high degree of circularly polarized EUV light in a wide very deep ultraviolet spectral region between 35 eV and 90 eV.

[0055] The four-mirror phase shifter is placed in an ultra-high vacuum chamber built to vacuum standards, including ultra-high vacuum compatible materials, fabrication, assembly, hydrocarbon content, and particle-free. This is particularly important to avoid carbon contamination buildup in the mirrors.

[0056] It should be noted that the above examples are not intended to limit the scope of the present application, and any equivalent variations or substitutions made on the basis of the above technical solutions fall within the scope of the claims of the present application.

Claims

1. A four-mirror reflective phase shifter for generating highly circularly polarized EUV light, characterized in that: Mo / B4C thin films were grown on concave focusing mirrors using DC magnetron sputtering and used as mirrors in a four-mirror reflective phase shifter. The four mirrors were mounted on a support and placed in an ultra-high vacuum chamber, and a vacuum precision drive system was configured. Mo / B4C thin films were grown on a concave focusing mirror using DC magnetron sputtering, with a background vacuum of 10⁻⁶ Ω·cm in the DC magnetron sputtering chamber. -7 Torr-level targets are used, consisting of high-purity Mo and B4C targets with a purity exceeding 99.95%. Both Mo and B4C are derived from high-energy Ar. + Sputtering is performed by bombardment, and the alternating growth of Mo and B4C layers is achieved by controlling the switching of baffles at the top of the Mo and B4C sources, with a circular polarization degree of over 90%.

2. The four-mirror reflective phase shifter for generating highly circularly polarized EUV light according to claim 1, characterized in that: The specific steps of the DC magnetron sputtering method are as follows: (1) Complete the equipment preparation and setup; (2) Perform substrate preparation treatment; (3) Conduct pre-sputtering operations; (4) Perform the thin film deposition process.

3. The four-mirror reflective phase shifter for generating highly circularly polarized EUV light according to claim 2, characterized in that: To calibrate the deposition rate in situ, a quartz crystal oscillator is installed in the magnetron sputtering chamber, and a gas channel capable of introducing pure argon is required.

4. The four-mirror reflective phase shifter for generating highly circularly polarized EUV light according to claim 2, characterized in that: The concave focusing lens measures 70×25×15 mm. 3 With a radius >10 km and a surface roughness <0.5 nm, it is made of single-crystal silicon. The concave focusing lens cleans the gas attached to the surface and reduces the roughness of the sample surface through sputtering technology in the DC magnetron sputtering growth chamber.

5. The four-mirror reflective phase shifter for generating highly circularly polarized EUV light according to claim 2, characterized in that: In the DC magnetron sputtering method, the power supplies for the Mo target and the B4C target were set to 10 W and 80 W respectively. During sputtering growth, argon gas with a purity of 99.999% was introduced. By adjusting the pumping rate, the pressure during sputtering was stabilized at 7 mTorr. The distance between the substrate and the target was 100 mm. The substrate rotated at a speed of 5 rpm without revolution.

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