Detailed Description
For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, and it is apparent that the described embodiments are some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
With the rapid development of information technology, the requirements of fields such as high-speed high-capacity data communication, high-performance computing, artificial intelligence and the like for advanced semiconductor chips with higher processing speed, lower power consumption and finer line width continue to increase. Extreme ultraviolet (extreme ultraviolet) lithography, particularly EUV lithography machines based on laser-produced plasma (LPP) light sources, have become an essential feature for advanced mass production of chips and for continued processing. Currently, commercial EUV lithography machines commonly employ EUV radiation having a wavelength of 13.5nm and a bandwidth of about 2% as an exposure light source, which radiation is generated by laser plasma radiation.
In order to meet the demands of large-scale wafer fabrication, laser plasma EUV light source systems must have the capability of outputting high power and high stability EUV light at a central focus (INTERMEDIATE FOCUS, IF), low contamination characteristics, and low operation and maintenance costs. Among them, high power, low fluctuation EUV light output at IF point is one of the core indicators that determine overall EUV lithography system performance.
The EUV light power of the laser plasma source at the IF point depends on several factors, such as the incident laser power, EUV light conversion efficiency (conversion efficiency, CE), overall optical system transmission efficiency, and source power stability. How to significantly improve the conversion efficiency of laser energy to EUV radiation has become an important research direction in the global EUV lithography field, and is a technical bottleneck that must be broken through to realize domestic EUV light sources.
In current commercial EUV lithography machines, laser plasma light sources commonly employ Sn droplets as the plasma target. The liquid tin is a preferable target form for realizing high CE and sustainable high-power output due to the characteristics of high-efficiency intrinsic radiation capacity, stable physicochemical property, moderate evaporation temperature (about 2602K) and the like in a 13.5nm wave band. The droplet target technology generates intense 13.5nm euv radiation by continuously generating tin droplets on the order of tens of microns in diameter in a high-speed jet manner, and then irradiating individual droplets with a high-energy pulsed laser to induce localized plasma formation.
However, although Sn droplet target technology has been widely used in current EUV lithography light sources, there are several key technical challenges and shortcomings:
First, the problem of splash debris contamination remains prominent. During the action of laser and tin liquid drops, a large amount of high-energy particles, tin clusters which are not ionized completely and tiny liquid drops can be generated, and the splashes can be deposited on the surfaces of optical elements such as a main reflector and the like, so that the reflectivity is reduced, the long-term stable operation of the system is influenced, and the maintenance frequency and the cost are increased.
Second, there are limitations to the controllability of droplet morphology and spatial positioning. Because of the small fluctuation of factors such as droplet diameter, speed, track jitter and the like, the targeting position and incidence angle of laser on the droplet are not easy to keep optimal, thereby influencing the plasma generation efficiency and the power stability of the EUV light source.
Third, energy utilization efficiency is limited. Only a part of laser pulse energy effectively participates in the excitation and radiation process of tin plasma, and lower CE becomes an important constraint factor for further improving the overall efficiency of the EUV light source and reducing the unit exposure energy consumption.
Furthermore, the complexity of the droplet preparation and delivery system is a non-negligible problem. The need for highly precisely controlled nozzle structures, stable liquid tin hydrodynamic conditions, and high frequency (above tens of kilohertz) droplet generation techniques places high demands on the overall reliability and engineering integration of the light source system.
In view of the above challenges, current research is focused on several directions including improving droplet pretreatment and plasma generation processes by optimizing pre-pulse and main pulse laser parameters, developing more advanced droplet detection and tracking techniques to improve targeting accuracy, studying low contamination target configurations (e.g., cluster targets, dual droplet targets, or aerosol targets) to mitigate specular contamination, and exploring alternative materials or composite target systems to further improve EUV radiation efficiency and system stability.
In order to solve the key technical challenges of splash pollution, insufficient stability, low energy utilization efficiency and the like existing in the application of a traditional Sn liquid drop target in a laser plasma extreme ultraviolet (LPP-EUV) light source, the invention provides a high-efficiency composite film target of an extreme ultraviolet lithography light source and a specific preparation method. The composite film target can be applied to an EUV lithography machine light source system, and is expected to remarkably improve the conversion efficiency and the system reliability of extreme ultraviolet light.
The preparation method of the high-efficiency composite film target material of the extreme ultraviolet lithography light source comprises the following steps (with reference to the accompanying figure 1):
And 101, depositing a Sn film on the substrate C target material by utilizing a laser film deposition technology to obtain the C-Sn film target material.
And depositing a Sn film on the substrate C target material by adopting a pulse laser deposition (Pulsed Laser Deposition, PLD) technology to form the C-Sn film target material. The basic principle of PLD is to irradiate a solid target material with high-power density laser pulse to induce the surface of the material to ablate and generate high-temperature high-density plasma plumes. Atoms, ions and molecules rich in the plume move at high speed and are deposited on the surface of the substrate, and finally a compact, uniform and component-controllable film is formed. Compared with other deposition technologies, PLD has the advantages of being capable of realizing accurate transfer of film components, being capable of preparing high-purity films, having excellent microstructure regulation and control capability and being applicable to various complex substrates.
PLD process is particularly suitable for preparing high quality Sn films. By precisely controlling the laser energy, the pulse frequency, the background atmosphere and the substrate temperature, the Sn film can be ensured to have excellent crystallinity, higher purity and good adhesion performance. In addition, PLD can prepare a multi-layer composite structure through multi-target switching or staged deposition, and provides a process basis for optimizing energy absorption and transmission in the subsequent laser ablation process.
During the film deposition process, the choice of the substrate material is critical to the performance of the final C-Sn film target. The substrate needs to meet the following conditions:
thermal stability, namely, the high temperature in the deposition and subsequent laser action process can be borne;
lattice matching, i.e. if epitaxial growth is needed, lattice constant matching is needed to be considered;
chemical compatibility, namely avoiding reaction with Sn or a deposition environment and preventing interface pollution or diffusion;
surface quality: lower surface roughness helps to improve film uniformity and adhesion;
application suitability, namely selecting according to the requirements of target application, such as optical transparency, conductivity and the like;
Economical efficiency, namely comprehensively considering the material cost and the processing feasibility.
Common optional substrates include monocrystalline materials, oxide materials, metal materials, flexible materials and the like, but based on the requirements of the invention on laser plasma spectrum diagnosis (particularly on accurate analysis of the spectral line of Sn element), the single-element substrate is preferred to reduce spectral line interference, improve signal-to-noise ratio and ensure the repeatability and accuracy of experiments.
As compared with the system, the carbon (C) element is preferable as a substrate for the following reasons:
The high melting point and the thermal stability are that the melting point of the C element is up to about 3550 ℃, the C element shows excellent thermal stability in a high-energy laser ablation environment, can effectively bear high-temperature impact of deposition and subsequent laser action, and prevents the substrate from being damaged;
the excellent thermal conductivity is that the thermal conductivity of the C material is about 2000W/mK and is far higher than that of silicon (150W/mK) and germanium (60W/mK), so that the C material is favorable for rapidly radiating heat, maintaining the stability of plasma and improving the ablation efficiency;
the C material has good chemical inertia, almost has no chemical reaction with the Sn film, can ensure the chemical purity of the film, and avoids interface diffusion or reaction layer generation;
the high-quality C substrate can provide a surface with extremely low roughness, promote the uniformity and compactness of Sn film deposition, improve film adhesion and reduce defect density;
The thermal expansion coefficient of the C material is relatively low and is close to that of the Sn film, so that interface thermal stress caused by temperature change is reduced, and failure risks such as film cracking and peeling are reduced;
the C material has good thermal conductivity and electrical conductivity, and good optical transparency (in a visible-infrared band), so that the C material is convenient for subsequent application and expansion in multiple fields;
Under the action of high-energy laser, the laser ablates the Sn film to form high-density Sn plasma, when the laser continuously acts on the C substrate, the laser energy is mainly deposited on the surface of the substrate due to the extremely high melting point of the laser, the conversion of the energy to the thermal kinetic energy of the plasma is inhibited, the density and the ablation rate of the Sn plasma are further improved, and the radiation performance of the plasma is enhanced;
Adopting 10Hz pulse deposition frequency, allowing Sn atoms to be rearranged on the surface of the substrate after each deposition to form a nanoscale transition layer, improving the density of the film layer and the ablation efficiency of plasma, being beneficial to forming a good gradient interface structure and further optimizing the spectral characteristics and the conversion efficiency;
the spectral background is clean, namely the element C does not have strong characteristic spontaneous emission lines in an extreme ultraviolet band (especially 13.5nm plus or minus 2 percent), compared with elements such as Si, ge and the like, the self plasma excitation of the element C does not introduce extra line interference, the signal to noise ratio of Sn plasma lines is obviously improved, and the element C is very important for EUV radiation intensity and spectral purity measurement;
Compared with other main group elements, such as base materials of silicon (Si), germanium (Ge), lead (Pb) and the like, the carbon (C) has the optimal comprehensive performance in the aspects of thermal stability, spectrum background interference control, processing cost, reliability and the like. The Si substrate has a high melting point, but the abundant spontaneous emission spectrum lines easily introduce spectrum line interference in an EUV band, the Ge substrate has a low melting point and is easy to oxidize, the Ge substrate is not suitable for a high-temperature laser deposition environment, and the Pb substrate has a high density but has an excessively low melting point and toxicity, so that the requirements of high stability and safety cannot be met. Therefore, the C substrate is preferably used as a carrier of the composite film target material.
And 102, collecting a plasma extreme ultraviolet emission spectrum generated by the C-Sn film target material under laser irradiation by utilizing a plasma spectrum measurement technology, and calculating the extreme ultraviolet conversion efficiency according to the plasma spectrum to obtain a first efficiency calculation value.
After the preparation of the C-Sn film target is finished, a high-resolution extreme ultraviolet spectrometer is utilized to carry out real-time spectral measurement on plasma generated by irradiating the C-Sn film target with laser, and the extreme ultraviolet emission intensity in the 13.5nm wave band is monitored.
The euv light conversion efficiency (first efficiency calculation value) in the first stage is calculated by spectral data analysis.
And 103, optimizing parameters of pulse laser deposition according to the first efficiency calculated value, and adjusting the thickness of the Sn film until the extreme ultraviolet conversion efficiency corresponding to the optimized C-Sn film target material meets the preset requirement.
Based on the first efficiency calculation, a machine learning algorithm (e.g., bayesian optimization, genetic algorithm, or reinforcement learning) may be used to intelligently optimize pulsed laser deposition parameters including, but not limited to, laser power density, pulse frequency, background air pressure, deposition time, and substrate temperature.
The machine learning model establishes a training set through historical deposition data and real-time spectrum feedback data, and automatically adjusts parameter combinations to quickly converge to deposition conditions which optimize extreme ultraviolet light conversion efficiency.
Finally, the optimized C-Sn film target material is obtained, and the corresponding extreme ultraviolet light conversion efficiency meets or is superior to the design index.
The thickness of the Sn film after optimization in the embodiment is about 400nm, and the extreme ultraviolet light conversion efficiency can reach more than 2%.
And 104, continuing to deposit the C film on the C-Sn film target by utilizing a laser film deposition technology to obtain the sandwich-type C-Sn-C composite film target.
And (3) continuously adopting a pulse laser deposition technology to deposit a layer of C film on the surface of the Sn film on the basis of the C-Sn film target material which is optimized in the first stage, so as to form the C-Sn-C composite film target material.
The functions of the top layer C film include:
1. Splash inhibition. The top layer C film is used as a buffer barrier, can absorb and diffuse a part of energy pulse when laser is initially incident, avoids laser directly impacting the surface of the Sn layer, and further weakens the formation of a local super-heated area, can reduce material cracking and spraying caused by the rapid expansion of a local high-temperature-high-voltage area, and has high thermal stability and strong interface binding force, and can bear the initial pulse of laser-induced shock wave, thereby inhibiting the non-uniform expansion and particle release of the Sn layer. The effect of the method is to obviously reduce Sn particles and debris pollution and improve the service life and cleanliness of optical elements of the system.
2. Energy redistribution and coupling optimization. The partial absorption and scattering effect of the top layer C film on the laser can form an energy slow-release layer, the laser energy delivery rate is delayed in the longitudinal direction, so that the Sn layer absorption is more uniform and mild, when the top layer C film is proper in thickness (such as 150 nm), the laser still maintains enough intensity to excite the Sn layer to form high-density plasma after penetrating, meanwhile, the laser energy peak value is prevented from being concentrated on a single surface point, the energy redistribution mechanism obviously improves the coupling efficiency of the laser energy and the Sn plasma, reduces the optical thickness of the Sn plasma, and further improves the EUV photon output efficiency. The effect is that the EUV emission intensity corresponding to single laser energy is increased, the spectral peak is concentrated near 13.5nm, and the spectral purity SP value is improved.
3. Plume stability and spatial uniformity enhancement. The top layer C film can promote Sn to be melted and gasified to form a stable evaporation interface on the surface, so that local plume instability is avoided from being formed in a high-speed gasification area, and the spatial constraint effect of the film structure on the plasma plume can reduce the expansion of the asymmetric plume, promote axisymmetric plasma to be formed and improve the focal stability of a light source. The effect of the device is that the output light intensity is more uniform, and the EUV focusing system can receive light and repeatedly operate for a long time.
4. And (3) the repeatability process control and the stability of the solid target are improved. Compared with the liquid instability and drop point drift of the liquid drop target, the composite film target is mechanically fixed and thermally stable, is favorable for process repeated control, has good mechanical strength and thermal shock resistance, and can avoid structural flaking or deformation in the laser repeated action process. The method has the advantages of prolonging the service life of the target material, reducing the replacement frequency and being beneficial to industrial mass production operation.
5. Spectral background advantage. The C element does not generate obvious spontaneous emission lines in the 13.5nm plus or minus 2% extreme ultraviolet band. This spectral characteristic allows the top C film to be partially broken down or excited even under laser ablation, without itself causing background interference or peak position superposition to the EUV main emission line of the Sn plasma (e.g., the band corresponding to Sn10+–Sn14+). Therefore, the introduction of the C film can not influence the Spectral Purity (SP) of an EUV emission measurement signal, but can improve the Spectral line signal-to-noise ratio and the focal quality of a light source by improving the spatial stability and the energy transport behavior of plasma. This feature also allows for higher reliability and application suitability of the C-Sn-C structure in EUV radiation spectroscopy diagnostics and radiation modeling.
And 105, collecting a plasma extreme ultraviolet emission spectrum generated by the C-Sn-C composite film target material under laser irradiation by utilizing a plasma spectrum measurement technology, and calculating the extreme ultraviolet conversion efficiency according to the plasma emission spectrum to obtain a second efficiency calculation value.
And analyzing plasma plumes generated by irradiating the C-Sn-C composite film target material with laser by utilizing a plasma spectrum measurement technology, collecting an extreme ultraviolet emission spectrum and calculating the extreme ultraviolet conversion efficiency (second efficiency calculation value) of the second stage.
And 106, further optimizing parameters of pulse laser deposition according to the second efficiency calculation value, and adjusting the thickness of the uppermost layer C film until the extreme ultraviolet light conversion efficiency corresponding to the optimized C-Sn-C composite film target material meets the preset requirement.
And the machine learning optimization strategy is adopted, and intelligent optimization is performed on the deposition thickness and the laser condition of the uppermost layer C film. And (3) continuously iterating through extreme ultraviolet spectrum feedback, and adjusting the thickness of the C layer to an optimal state, so that the overall extreme ultraviolet conversion efficiency and stability of the final C-Sn-C composite film target material are further improved.
The thickness of the C film after optimization in the embodiment is about 150nm, and the extreme ultraviolet light conversion efficiency can reach more than 6% under the condition.
In this embodiment, by monitoring the emission spectrum of the euv band of the laser plasma in real time, the pulse laser deposition parameters are dynamically adjusted to improve the quality of the deposited film and the euv conversion efficiency. The system can automatically complete data acquisition, CE calculation and closed-loop feedback optimization of deposition parameters, and effectively improves consistency and controllability of process stability and film performance.
The preparation of the composite target and the extreme ultraviolet light measurement process have a plurality of core points to be noted, and the preparation method is specifically as follows:
1. And controlling plasma generation and target breakdown, namely ensuring stable breakdown under the condition of single pulse by adjusting the power density of laser pulse in order to ensure that the C-Sn-C composite film target can be effectively broken down and excite clear characteristic spectral lines near 13.5nm of Sn plasma. The power density is estimated according to the following formula:
On the premise of determining the laser pulse width and the focal spot (the radius of the laser focal spot in the embodiment is kept to be 50 μm), the laser energy is gradually increased from zero until plasma flash (the white light continuous spectrum and the characteristic spectrum are simultaneously observed) is observed, and the occurrence of breakdown is confirmed. The power density at this point is set to a breakdown threshold above which the laser power density subsequently used to generate Sn plasma EUV radiation needs to be above to avoid energy starvation from breakdown.
In order to judge whether a top layer C film in a C-Sn-C composite film target material is effectively broken down by laser and realize excitation of an intermediate layer Sn, the invention establishes the following criterion system:
And (3) carrying out EUV spectral line characteristic recognition, namely after laser excitation, a strong and sharp emission peak appears for the first time in a 13.5nm +/-2% wave band, and the peak intensity is obviously higher than that of a continuous spectrum background, so that the laser penetrates through a top C film and excites an Sn layer to be effectively ionized. In this embodiment, as shown in fig. 3, only a weak continuum spectrum generated by the C plasma exists in the EUV spectrum in the non-breakdown state, no obvious spectrum exists near 13.5nm, and when the laser energy reaches the breakdown threshold, a clear Sn plasma emission peak appears in the same band, which indicates that the C film breakdown and Sn excitation are completed.
And (3) the excitation energy and thickness matching relation is that taking a C-Sn-C composite film target material with the top layer C film thickness of 40nm as an example, carrying out single-pulse excitation experiments under different laser energies, and analyzing the occurrence threshold and intensity response of Sn spectral lines by combining EUV spectrum measurement results (shown in figure 3). The results show that when the laser energy reaches about 60mJ, a significant emission peak is observed for the first time in the 13.5nm band. Under the thickness condition, stable laser penetration can be realized, and the excitation repeatability is good.
Through comprehensive verification of the criteria, whether the laser achieves complete breakdown of the C film and effective excitation of the Sn layer can be effectively judged, and therefore a foundation is laid for subsequent EUV radiation efficiency improvement and spectral line purity optimization.
2. And optimizing and guaranteeing the resolution ratio of the spectrum measuring system, namely selecting a grating with the line density of 1200g/mm to carry out extreme ultraviolet radiation spectroscopy according to the emission characteristic of the Sn target material in the EUV band to ensure the accurate identification of the element spectral line, and improving the spectral resolution ratio by reducing the slit width of the spectrometer to 50 mu m. Since high resolution configurations (narrow slits and high reticle gratings) may result in signal attenuation, compensation is performed by increasing the laser energy or accumulating the signal multiple times, while high pixel density CCD detectors are employed to ensure spectral lines span multiple pixels, improving measurement accuracy.
3. The invention constructs a set of time sequence control and space coupling strategy based on laser deposition-plasma excitation-spectroscopy measurement integration in order to realize the cooperative optimization of the film target preparation process and the extreme ultraviolet emission performance characterization thereof.
In the aspect of time sequence control, the whole system is uniformly managed by the first trigger in the figure 2, and mainly comprises three core modules, namely a pulse laser deposition module, a target material moving/converting module and an EUV spectrum measuring module. The system coordinates the operation time sequence of each module through a high-speed digital delay pulse controller (Stanford DG 535) so that a laser excitation program can be started automatically after each round of film deposition is completed, the triggering synchronization precision of laser pulse and spectrum exposure can be better than 10ns, and a clear time locking relation exists between laser deposition-laser excitation-spectroscopy acquisition, so that system jitter or measurement drift is avoided.
In the aspect of space coupling design, the thin film deposition area, the laser ablation area and the EUV signal acquisition path are uniformly designed through precise geometrical arrangement of light beams and targets. The laser deposition adopts a 45-degree incidence mode to control the thickness and uniformity of the deposited film, the laser excitation and spectral line acquisition path is set to be in a structure with an included angle of 90 degrees with the normal line of the target surface, and is assisted by an adjustable beam collimation module to ensure that a laser focusing spot stably covers a deposition area, and a high-precision 4-dimensional moving sample frame is used for realizing in-plane deposition uniformity correction and avoiding the overlapping of ablation areas.
In addition, in order to further improve the time resolution capability of spectral line data, the spectrum acquisition system supports linkage with the layer-by-layer film deposition step of the laser deposition system, forms a closed-loop control flow of deposition, excitation, measurement, feedback and redeposition, and is suitable for optimizing the thicknesses of the Sn film and the top C film buffer layer.
4. And (3) wavelength calibration and measurement precision improvement, namely absolute calibration is carried out by adopting a NIST database standard metal target to ensure the wavelength and intensity precision of a measurement system, spectrum acquisition parameters are optimized by adjusting focal length, energy and spot size, the wavelength error after calibration is controlled within 0.2nm, and the intensity relative error is less than 5%.
5. In the embodiment, laser pulse generated by a YAG laser with the frequency of 10Hz, the wavelength of 1064nm and the pulse width of 10ns is focused on the surface of the C-Sn film target in a vacuum cavity, so that high-temperature and high-density plasma is excited. The plasma emitted light enters a grazing incidence extreme ultraviolet spectrometer through a 50 μm slit, is collected by an absolute calibration back-illuminated CCD detector after being dispersed by a grating, and covers the wave band range of 11-16 nm.
And calculating extreme ultraviolet conversion efficiency according to the acquired plasma emission spectrum to obtain a first efficiency calculation value of the C-Sn film target. The euv light conversion efficiency CE can be calculated by the following formula:
,
Wherein, theAs laser absorption, electron density distribution dependent on the plasma, especially at near critical density, can produce strong absorption; for the radiation conversion ratio, the absorption of laser energy into radiation energy and the conversion of the radiation energy intoThe ratio of solid angle emission to the laser incident side; For spectral purity, the ratio of radiant energy in the total radiant energy over a 13.5nm bandwidth is expressed. The laser absorptivity and radiation conversion ratio are obtained by simulation of a two-dimensional radiation fluid dynamic program, and the spectral purity is obtained by observation spectral line analysis.
6. And the process optimization and feedback control of the C-Sn film target material, wherein the first efficiency calculated value of the C-Sn film target material provides key real-time feedback information for the deposition parameter optimization of the Sn film. In order to further improve the response speed and the response precision of film deposition and spectrum measurement, an intelligent parameter optimization module based on Machine Learning (ML) is introduced in the invention. Through training of a historical data set and dynamic input of real-time monitoring data, a Bayesian optimization, genetic algorithm or reinforcement learning method is adopted, and key parameter combinations such as optimal laser energy density, pulse frequency, target base distance and the like are rapidly deduced, so that deposition condition iteration is automatically guided, and optimization efficiency is greatly improved.
In the preparation and optimization process of the C-Sn film target, the influence of laser power density and the distance between the Sn target for coating and the C substrate on the film quality is also considered. In terms of laser power density, if the power density is too low, ablation is insufficient, resulting in a decrease in deposition rate and deterioration of film quality. If the power density is too high, sputtering tends to increase, and the conversion efficiency and film quality are lowered. The power density should therefore remain slightly above the ablation threshold. In this example, the laser power density was set to 6×1010W/cm2. In terms of the spacing between the Sn target for coating and the C substrate, if the spacing is too small, the particles are splashed and polluted, and if the spacing is too large, the deposition efficiency is reduced. The pitch is set to 5cm in this embodiment.
The thickness of the Sn film in the preparation and optimization process of the C-Sn film target material needs to take the characteristic spectral line intensity of Sn in the wavelength range of 13.5nm (+ -2% bandwidth) as an evaluation basis, and meanwhile, the C element spectral line is synchronously monitored in the experimental process, so that the phenomenon that the Sn signal is covered due to the fact that the C spectral line signal is too strong is avoided. Determining the optimal experimental condition according to the Sn spectral line intensity peak value change, and determining the total deposition time according to the following formula by combining the pulse frequency and the single pulse deposition rate:
,
in this embodiment, the optimal Sn film thickness of the C-Sn thin film target is about 400nm, and the corresponding Sn film deposition time is 30 minutes.
7. The design of the C-Sn-C composite film target material and the optimization of the sandwich structure are that, as a preferred implementation mode, after the C-Sn film target material is prepared, a layer of C film is further deposited on the surface of the C-Sn film target material by utilizing a laser film deposition technology, so that the sandwich type C-Sn-C composite structure is formed. And then irradiating the C-Sn-C composite film target material by laser, and collecting the plasma emission spectrum of the target material to obtain a second efficiency calculation value. And continuously optimizing the deposition parameters by using a machine learning method according to the value until the optimized composite film target material meeting the extreme ultraviolet light source efficiency of more than 6% is obtained. The influence of laser energy density and target spacing on film quality still needs to be paid attention to in the preparation and optimization processes of the C-Sn-C composite film target material. In this example, the laser power density for producing the top C film was set to 1011W/cm2, and the distance between the C target for plating and the C-Sn thin film target was set to 6.5cm. In this embodiment, the optimal C film thickness of the C-Sn-C composite film target is about 150nm, and the corresponding C film deposition time is 10 minutes. The sandwich type target structure realizes the efficient absorption and utilization of laser energy through the functional collaborative design of different material layers, remarkably improves the uniformity of plasma and the stability of a film, and improves the spectral purity and the conversion efficiency.
The preparation of the C-Sn-C composite film target material for the extreme ultraviolet lithography light source and the corresponding extreme ultraviolet spectrum measurement are carried out by the pulse laser deposition-plasma spectrum measurement integrated device shown in the figure 2. The integration device comprises a pulse laser deposition module and a plasma spectrum measurement module, and realizes synchronous operation by controlling laser triggering, target movement and spectrum acquisition through time sequence. YAG laser is shared by the pulse laser deposition module and the spectrum measurement module, so that a closed-loop efficient integrated operation flow is formed. Referring to fig. 2, fig. 2 shows a schematic structural diagram of a pulse laser deposition-plasma spectrum measurement integrated device provided by the invention. Through the integrated design, the problems that the traditional spectrum measuring device cannot be diagnosed in real time, the repeated measuring conditions are inconsistent, the data processing is complicated and the like are effectively solved, and the experimental efficiency and the result reliability in the deposition preparation process are further improved.
The pulse laser deposition module mainly comprises a first trigger 1, a first laser 2, a first laser reflecting lens 3, a second laser reflecting lens 4, a first laser focusing lens 5, a vacuum transparent window 6, a vacuum chamber 7, a first target frame 8, a first moving platform 9, a first controller 10, a second target frame 11, a vacuum gate valve 12, a second moving platform 13 and a second controller 14. The first trigger 1 is used to trigger the first laser 2 and the first controller 10 simultaneously. The first laser 2 outputs an ablative laser beam having a wavelength of 1064nm and a pulse width of 10 ns. The ablated laser is vertically reflected by the first laser reflecting lens 3 (the included angle between the incident beam and the reflected beam is 90 degrees) to the second laser reflecting lens 4, and is further reflected by the second laser reflecting lens 4, and then is focused by the first laser focusing lens 5 and the vacuum transparent window 6 to be incident on the first target frame 8 in the vacuum chamber 7. The first laser focusing lens 5 employs a quartz lens having a focal length of 300 mm. Solid Sn or C targets are placed on the first target frame 8, and high-energy density ablation is initiated after laser beam focusing, so that high-temperature Sn or C plasma plumes are formed. The first target frame 8 is arranged on the first moving platform 9 and is controlled by the first controller 10 to realize accurate displacement and collimation in the two-dimensional plane direction. The second target frame 11 is connected with the second moving platform 13, and is used for bearing a C substrate target or a C-Sn film target and receiving Sn or C plasma sputtering matters to form the C-Sn film target or the C-Sn-C composite film target. The second mobile platform 13 is controlled by a second controller 14 to achieve accurate two-dimensional position adjustment. The vacuum chamber 7 provides the required vacuum environment for the entire deposition and measurement process. The vacuum gate valve 12 is arranged in the vacuum system and is used for controlling the chamber to be blocked or deflated during target changing, so as to ensure independent adjustment and stable maintenance of the vacuum degree of the system.
The plasma spectrum measuring module comprises a vacuum chamber 7, a second target frame 11, a vacuum gate valve 12, a second movable platform 13, a second controller 14, a second laser 15, a third laser reflecting mirror 16, a fourth laser reflecting mirror 17, a second laser focusing lens 18, a spectrometer entrance slit 19, an extreme ultraviolet spectrometer 20, a CCD camera 21 and a second trigger 22. The second trigger 22 is used for coordinated control of timing triggers of the second laser 15, the CCD camera 21 and the second controller 14. The second laser 15 likewise outputs a laser beam with a wavelength of 1064nm and a pulse width of 10 ns. The laser beam is first reflected by the third laser mirror 16 by 90 degrees, then deflected by the fourth laser mirror 17, and finally focused by the second laser focusing lens 18 onto the deposition target surface on the second target frame 11. The second laser focusing lens 18 is a quartz lens with a focal length of 150mm, and the focusing angle is designed to be 45 degrees.
The plasma extreme ultraviolet emission light formed under the action of the laser passes through the spectrometer entrance slit 19 and enters the extreme ultraviolet spectrometer 20 for light splitting. The euv spectrometer 20 is configured to receive and perform a spectroscopic treatment of the plasma euv light. The CCD camera 21 is connected to the rear end of the extreme ultraviolet spectrometer 20, and is used for enhancing and detecting the split optical signals.
The first trigger 1 and the second trigger 22 are connected through a synchronous control circuit, so that time sequence coupling of laser ablation and spectrum measurement is realized, and plasma emission signals can be accurately collected and recorded in an optimal time window after each pulse laser action, so that the repeatability and analysis reliability of data are remarkably improved.
In this embodiment, the first laser 2 and the second laser 15 may each be a Nd-YAG laser (neodymium-doped yttrium aluminum garnet laser). YAG laser has the characteristics of high energy output and multiple wavelength selection, and is particularly suitable for generating high-altitude and high-density plasmas through interaction with solid targets. By reasonably regulating and controlling parameters such as laser wavelength, single pulse energy and the like, the dynamic behavior of plasma plumes can be effectively controlled, so that a film with uniform deposition and pure components is deposited on the surface of a substrate target material. In addition, the Nd-YAG laser has good stability and adaptability, can continuously and efficiently generate high-temperature and high-density plasmas under different laser parameter configurations, and is an ideal laser source for laser plasma generation and film deposition.
In the process of carrying out plasma measurement for multiple times, in order to ensure the stability of laser plasma and avoid the local excessive ablation of a target, the first target frame 8 is controlled to move in a two-dimensional plane through the first moving platform 9 in the design of the invention, so that uniform laser irradiation of different areas on the surface of the target is realized, and the damage of the target caused by the continuous action of laser pulses on the same point is prevented. Meanwhile, the second moving platform 13 controls the second target frame 11 to move in a two-dimensional plane, so that laser ablation measurement can be carried out on the deposited C-Sn film target or C-Sn-C composite film target at different positions, and plasma extreme ultraviolet emission spectrum data corresponding to the C-Sn film target or the C-Sn-C composite film target under different experimental conditions can be obtained. The euv spectrometer 20 receives the plasma euv light signal incident through the spectrometer entrance slit 19 and outputs a corresponding plasma euv band emission spectrum.
In each set of spectral measurements, six separate plasma extreme ultraviolet spectra are typically acquired. In order to improve the accuracy of the data, preprocessing is required to be performed on each group of acquired spectrum data, namely, firstly screening out abnormal spectrum curves, and then summing and averaging the screened spectrum signals so as to obtain a high-quality plasma spectrogram with abnormal values removed. Through the data processing steps, systematic fluctuation and random errors in the measuring process are effectively eliminated, the representativeness and the repeatability of plasma spectrum data are improved, and a reliable basis is provided for the accurate calculation of the subsequent extreme ultraviolet Conversion Efficiency (CE) and the film performance evaluation.
By the pulse laser deposition-plasma spectrum measurement integration device, preparation and real-time spectrum measurement of the composite film target material can be efficiently completed on the same platform, closed-loop process control from deposition to diagnosis is realized, process development efficiency and target material preparation consistency are greatly improved, and important technical support is provided for efficient development of an extreme ultraviolet lithography machine light source system.
In this embodiment, the preparation process of the composite film target needs to comprehensively consider key details such as material characteristics, process parameters, environmental control and the like, and the optimization of film quality and performance is realized by finely adjusting and controlling environmental variables, laser parameters, substrate states and post-treatment processes. The specific process steps are as follows:
First, a chamber is prepared. Since Sn is very easily oxidized to form SnO2, once exposed to air or oxygen-containing atmosphere, the resistivity of the film is significantly improved and the performance is reduced, so that the operation is required to be performed in a high vacuum or inert gas (such as argon and nitrogen) filled environment, and the oxidation reaction is avoided to the greatest extent. In this embodiment the chamber is evacuated to a vacuum of less than 10-5 Pa.
In the pretreatment link of the substrate, the Sn target is firstly subjected to ultrasonic cleaning and drying treatment, and then is subjected to mechanical polishing or ion sputtering cleaning so as to remove a surface oxide layer and adsorb pollutants, thereby ensuring the surface to be clean. The substrate temperature also needs to be precisely controlled to regulate and control the crystallinity and compactness of the deposited film. The substrate temperature in this example was maintained at 25 ℃.
The target mounting step is divided into two stages. The first stage is the preparation of the C-Sn film target material and extreme ultraviolet spectrum measurement. In this stage, a Sn target for high purity plating is fixed to the first target frame 8. A C substrate is placed on the second target frame 11 for producing a C-Sn thin film target. The second stage is the preparation of the C-Sn-C composite film target material and the extreme ultraviolet spectrum measurement. In this stage, the high purity plating film is fixed on the first target frame 8 with the C target. The prepared C-Sn substrate is placed on a second target frame 11 for producing a C-Sn-C composite thin film target.
And in the laser parameter setting step, nd-YAG laser is selected to output laser with the wavelength of 1064nm, and proper laser power density is set, so that the Sn target material can be effectively ablated, and meanwhile, large-size particles are avoided. As a preferred embodiment, when preparing the C-Sn film target, the laser power density is set to 6×1010W/cm2, the frequency is set to 10Hz, and the substrate temperature is 25 ℃. When preparing the C-Sn-C composite film target, the laser power density is increased to 1011 W/cm2 in the embodiment, the frequency is 10Hz, and the substrate temperature is kept at 25 ℃.
In the deposition process, the film thickness, the deposition rate and the film uniformity are balanced by optimizing the target base distance and the deposition time. If the distance of the target base is too small, local heat accumulation is caused on the substrate area by the plasma plume, so that film non-uniformity or microstructure defects are easily caused, and if the distance is too large, the plume density is obviously attenuated, so that the target utilization rate is reduced and the film thickness is insufficient. At the same time, the choice of deposition time plays a key role in the final film thickness and film quality. Under the condition of fixed laser energy and frequency, the thickness of the film layer is basically linearly increased, so that the deposition time is comprehensively determined according to the matching relation of the target film thickness, the deposition rate and the adhesive force. In addition, the deposition time is too short, which may cause discontinuous film or insufficient nucleation, and too long may cause adverse effects such as thermal stress accumulation and loose structure, and needs to be reasonably set in cooperation with plasma plume monitoring and pre-experiment data. In the embodiment, when preparing the C-Sn thin film target, the distance between the substrate C target and the Sn target is set to be 5cm, the pulse laser deposition time is set to be 30 minutes, and meanwhile, plasma plumes are monitored in real time, while when preparing the C-Sn-C composite thin film target, the distance between the C-Sn substrate and the C target is set to be 6.5cm, the pulse laser deposition time is set to be 10 minutes, and the plasma plumes are monitored in real time.
After the preparation of the C-Sn thin film target and the C-Sn-C composite thin film target is completed, thin film characterization is needed. The invention comprehensively adopts structural characterization and emission spectrum diagnosis means to evaluate the structural quality and radiation performance of the C-Sn thin film target and the C-Sn-C composite thin film target.
First, the crystalline state of the thin film sample was analyzed using X-ray diffraction (XRD). In this example, XRD patterns of the C-Sn thin film target and the C-Sn-C composite thin film target are shown by FIGS. 4 and 5.
XRD patterns (FIG. 4) of the C-Sn thin film targets show that the samples were based on highly crystalline Graphite (Graphite-2H, PDF # 41-1487) on the surface of which a metallic Sn film was successfully deposited. The strong (002) diffraction peak at 2 theta ≡ 26.5 deg. in the figure comes from the graphite substrate, indicating that it has a high preferential orientation. The diffraction peaks of beta-Sn (corresponding to PDF#04-0673) in the film layer appear in the range of 30-70 degrees, which shows that the deposited tin film is in a polycrystalline structure but has weaker crystallinity. No hetero-phase peaks such as SnO2 or SnC are seen in the spectrum, which indicates that no obvious chemical reaction occurs in the laser deposition process, and the obtained C/Sn layered structure is purer.
XRD patterns (figure 5) of the C-Sn-C composite film target material show that after Sn and C films are sequentially deposited on a graphite substrate through a laser deposition process, the bottom layer graphite shows a typical (002) diffraction peak (2 theta approximately 26.5 degrees), and the peak is strong and sharp, thus indicating that the graphite substrate has excellent crystal orientation. The polycrystalline structure of the intermediate layer Sn film is preserved, and the characteristic peak is matched with PDF#04-0673 beta-Sn, and is still clearly distinguished despite the relatively weak strength. No hetero-phase peaks such as SnO2 or SnC are seen in the spectrum, which shows that no obvious reaction exists in the deposition process, and the obtained pure C/Sn/C composite structure. The main reason for the absence of significant diffraction peaks in the top C film in the composite film target is that the penetration depth of XRD (typically on the order of μm) is much greater than the film thickness (top C film thickness in this example is about 150 nm). For X-rays, the diffraction signal produced by such films is extremely weak, often buried in the base signal or masked by the instrument noise floor. 2. The (002) peak of the bottom C substrate is extremely strong, and the middle Sn film also has diffraction contribution. The top C film, even with weak diffraction, is easily masked by the base main peak or indistinguishable.
In addition, the extreme ultraviolet emission characteristics of the target under the excited condition are subjected to diagnostic analysis through laser plasma excitation spectrum measurement. Under the excitation condition of standard single pulse laser, EUV emission spectrum is collected, and the emission line intensity, spectral line purity SP and background continuous spectrum are analyzed. The spectral response characteristic can be used as a feedback basis for target structural design and film formation optimization. FIG. 6 shows EUV emission spectra of a C-Sn thin film target after excitation under different deposition conditions, and FIG. 7 shows spectral response comparison of the corresponding C-Sn-C composite thin film target and the corresponding C-Sn thin film target. The results in fig. 6 show that under the condition of laser energy of 250mJ, the emission intensity of the C-Sn film target material with the Sn film deposition time of 30 minutes in the 13.5nm wave band is obviously enhanced, the spectral line purity is improved, the continuous spectrum interference is reduced, and the C-Sn film target material has better energy coupling efficiency and radiation stability. The results in fig. 7 show that in the case of laser energy of 350mJ, a significant emission peak from Sn appears in the EUV spectrum of the C-Sn-C composite thin film target, indicating that the top C film is completely broken down, sufficient laser energy has reached the middle Sn layer and ablated to create a high temperature, high density Sn plasma.
In the process of analyzing and inverting EUV spectrum of laser plasma, spectrum wavelength calibration is needed first. And (3) accurately calibrating the high-Z element Sn plasma spectrum data by measuring the characteristic spectral line of low-Z element (such as Al and Si) plasmas, and determining the corresponding relation between the Pixel point and the wavelength of the abscissa of the spectral line. In the spectral line identification step, the measured spectrum is compared with standard spectral line data in an NIST database, the transition energy level and ionization state corresponding to each spectrum peak are defined, and spectral line interference caused by a C substrate target material is identified and eliminated.
The diagnosis of the plasma temperature and density parameters adopts the Boltzmann diagram method and the Stark broadening method to jointly deduce, and simultaneously combines the Cowan program to calculate and analyze the Sn ion energy level structure and transition information, so that key physical quantities such as the temperature, the density and the charge state distribution of the plasma are accurately extracted. Configurations considered in the Cowan program calculation include the ground state, the dual excited state, the tri-excited state, and the higher multiple excited states of Sn6+- Sn14+ ions.
The invention further prepares the C-Sn film target material under different Sn film deposition time (20/30/40 minutes), and systematically researches the influence of the deposition time on the extreme ultraviolet emission performance of Sn plasma through measurement and analysis of laser plasma spectrum. FIG. 6 shows EUV spectra for different C-Sn thin film targets and pure Sn targets at a laser energy of 250 mJ. The result of the attached drawing shows that the C-Sn film target material prepared by adopting the deposition time of the Sn film for 30 minutes has the advantages of improving the SP value of the spectral purity and reducing the background noise while maintaining good spectral intensity.
In this example, the laser absorptivity fL of the C-Sn thin film target at a deposition time of 30 minutes of the Sn film calculated by the radiation hydrodynamic program is 0.4, the radiation conversion ratio CR is 0.5, the spectral purity SP is 0.12, and the overall conversion efficiency CE is 2.4%. Indicating that the C-Sn/30 min thin film target achieves the intended CE target.
And a C layer is further deposited on the basis of the C-Sn film target material to form a C-Sn-C composite film target material, so that the plasma emission intensity, the spectral purity and the conversion efficiency can be further improved. FIG. 7 shows EUV spectra of a C-Sn/30 min-C/10 min composite thin film target, a C-Sn/30 min thin film target, and a pure Sn target at a laser energy of 350 mJ. The graph results show that the EUV spectral line intensity, the spectral purity and the CE value of the C-Sn-C composite film target are further improved compared with those of the C-Sn film target. Wherein the peak spectral line intensity is enhanced by about 2 times, the spectral purity SP is enhanced by about 2.3 times, and the conversion efficiency CE is enhanced by about 3 times to 6%. Indicating that the composite film target material of C-Sn/30 min-C/10 min achieves the preset CE target.
The invention provides a preparation method of a high-efficiency composite film target material of an extreme ultraviolet lithography light source and a matched laser deposition-plasma spectrum integrated system, which are specially applied to a light source system of a laser plasma extreme ultraviolet (LPP-EUV) lithography machine, and have the following remarkable technical advantages:
The invention adopts the design of device integration and automatic control to construct a double-laser system cooperative working mode, wherein a first laser is used for preparing a C-based Sn film by Pulse Laser Deposition (PLD), and a second laser is special for plasma spectrum excitation and real-time diagnosis. The two sets of light paths are independently and parallelly designed, so that optical cross interference is effectively avoided, and system stability is improved. The triggering of the double lasers, the accurate synchronization of target movement and spectrum data acquisition are realized through a high-precision time sequence controller (such as Stanford RESEARCH SYSTEMS DG) so as to ensure that the experimental process has good repeatability and controllability.
Real-time spectral feedback and process closed-loop optimization are a big bright point of the system. The glancing Emitter Ultraviolet (EUV) spectrometer and the high-time-resolution CCD camera are integrated, the plasma emission evolution process can be captured in a nanosecond time scale, the transition characteristic spectral line (near 13.5 nm) of Sn8+–Sn14+ high-charge-state ions is monitored in real time, and the extreme ultraviolet Conversion Efficiency (CE) is calculated in real time based on the spectral line intensity change. By combining a built-in algorithm, the system can dynamically adjust key parameters such as laser energy, target base distance and the like, and intelligent closed-loop control integrating measurement, optimization and deposition is formed.
In order to avoid local damage caused by laser single-point ablation, the target is arranged on a high-precision four-dimensional moving platform (the stepping precision is less than or equal to 1 mu m), programming path planning (such as spiral scanning track) is supported, the service life of the target is effectively prolonged, and the uniformity and compactness of a deposited film are improved.
In the aspect of film design, the invention creatively provides a multilayer film structure strategy, and a C-Sn-C composite film target is formed by alternately depositing a Sn film and a C buffer layer. And the splash inhibition of Sn particles and the great improvement of the spectral purity and the conversion efficiency are realized by using the top layer C film. In the embodiment, the radiation intensity of the C-Sn/30 min-C/10 min composite film target material in the 13.5nm wave band is improved by 2 times compared with that of a single-layer Sn film, the improvement of the spectral purity SP is about 2.3 times, and the conversion efficiency CE is about 3 times. The important application prospect of the C-Sn-C composite film target material prepared by the invention in extreme ultraviolet lithography is proved.
Aiming at CE calculation, the invention introduces a CE inversion frame based on two-dimensional radiation fluid dynamics simulation, and performs parameter inversion and correction by combining experimental actual measurement spectrum. By combining the laser absorptivity fL, the radiation conversion ratio CR and the spectral purity SP, the CE value of the C-Sn-C composite film target material can reach 6%, which is obviously better than about 5% of the liquid drop Sn target, and is much higher than 2% of the solid Sn target.
In terms of technical compatibility and expandability, the invention carries out comprehensive optimization design. The target frame adopts a modularized structure, supports quick replacement of different targets (such as Sn, mo, si and the like), can realize preparation of a Sn-Mo/Si multilayer film structure by combining a PLD process, and supports development requirements of novel EUV reflector materials. The system integrates a spectrum database (based on NIST standard data) internally, and is assisted with a machine learning algorithm to perform spectral line identification, parameter inversion and automatic generation of process optimization suggestions, so that Sn ion transition characteristics can be matched quickly. In addition, the system design is compatible with a CO2 laser (10.6 mu m) and a 2 mu m solid laser, and is suitable for experimental requirements of different wave bands, so that a wide space is provided for future short-wave laser efficient CE process research.
The invention has obvious advantages in the aspects of economic benefit and industrialization potential. By adopting Pulsed Laser Deposition (PLD) technology to replace the traditional droplet spraying target system, the complex droplet generator is omitted, and the overall cost can be greatly reduced. Meanwhile, the utilization rate of the solid Sn target material is increased to more than 90%, so that the material consumption and the operation cost are greatly reduced. The system adopts a modularized vacuum chamber design (such as an integrated gate valve structure) to support rapid target changing and cavity cleaning operation, so that the maintenance period is shortened from 48 hours to 4 hours of the traditional system, and the equipment availability and the industrial mass production adaptation capability are greatly improved.
In conclusion, the invention remarkably improves the conversion efficiency, stability and reliability of the target in the laser plasma extreme ultraviolet lithography light source system through target structure innovation, real-time spectrum diagnosis, data driving process optimization and system integration design, has the comprehensive advantages of high efficiency, low cost and easy expansion, and provides solid support for autonomous breakthrough of the domestic high-end EUV lithography technology.
The invention also provides a high-efficiency composite film target of the extreme ultraviolet lithography light source, which is prepared by adopting the preparation method of the high-efficiency composite film target of the extreme ultraviolet lithography light source. The high-efficiency composite film target material of the extreme ultraviolet lithography light source has the characteristics of stable structure, uniform components, excellent adhesive force, high purity, high conversion efficiency and good processability, can be effectively applied to a laser plasma extreme ultraviolet (LPP-EUV) lithography light source system, and remarkably improves the extreme ultraviolet radiation output efficiency and the system operation stability.
It should be noted that the above-mentioned embodiments are merely for illustrating the technical solution of the present invention, and not for limiting the same, and although the present invention has been described in detail with reference to the above-mentioned embodiments, it should be understood by those skilled in the art that the technical solution described in the above-mentioned embodiments may be modified or some technical features may be equivalently replaced, and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solution of the embodiments of the present invention.