Improving soft X-ray imaging contrast uniformity of 50nm Au zone plate by optimizing pulse plating

The Fresnel zone plate plays a pivotal role in X-ray imaging systems, directly influencing the uniformity of imaging contrast. Achieving uniform imaging contrast imposes stringent requirements on the linewidth and height uniformity of the zone plate. The current mainstream method for zone plate fabrication involves utilizing electron beam lithography in conjunction with electroplating. However, in structures with large aspect ratios, precise deposition of gold to the bottom of the trench through electroplating becomes increasingly challenging to control. In this study, we employed a pulse plating process to enhance the uniformity of the plating height. Detailed investigations were conducted to analyze the effects of pulse current density, pulse duty cycle, and pulse frequency on plating height. Utilizing pulse plating, we successfully fabricated a zone plate with an outermost width of 50 nm and a thickness of approximately 245 nm. Structural analysis showed that the plating height uniformity is improved to better than 5%. The high uniformity imaging pattern was obtained by using the Siemens star test pattern at the soft x-ray imaging station at NSRL. The spatial resolution was down to 50 nm.


1.Introduction
With the rapid advances of science and technology toward the nanoscale [1], there is an increasing demand to observe the internal structure of materials at the nanoscale level.While the electron microscope can achieve sub-nanometer resolution and provide clear visualization of various fine structures, they are constrained by limited penetration capabilities and face challenges in observing large samples.High-resolution X-ray microscopes (such as full-filed transmission X-ray microscopes (TXM)) have gained widespread application in material science, life science, and environmental science due to their unique advantages of high penetration, non-destructive, and high-resolution imaging.The key optics in TXM is the Fresnel zone plate in noble metals [2], serving as the objective optics to magnitude the images on the detector.Its quality significantly influences the spatial resolution and diffraction efficiency of the imaging system.The optimization of processes and enhancement of the performance of Fresnel zone plates have become focal points of increasing attention among researchers, such as Qiucheng Chen's study on the simulation of photolithography [3]  and so on.
According to the structural design of the zone plate, the closer the processed structure is to the theoretical value, the better the corresponding contrast uniformity will be.Therefore, in actual processing, it is necessary to obtain a uniform plating as much as possible while ensuring the duty cycle.Various methods have been employed for manufacturing zone plates, including sputtering/dicing, soft X-ray lithography [4], zone-doubling technique [5], atomic layer deposition [6] for narrow zones, reactive ion etch combined with Au electroplating, and electron beam lithography (EBL) followed by electroplating.However, the combination of electron beam lithography and electroplating stands out as the most widely adopted, stable, and reliable method.The resolution performance of Fresnel zone plates is proportional to the width of the outermost ring [7].While the nanofabrication technique of zone plates by EBL has been developed to 12 nm resolution, several challenges persist in the field of electroplating.The mass transfer of solutions in large aspect ratio structures during electroplating can only be realized by diffusion.This limitation arises from the ease with which ions dissipate and are not promptly replenished, resulting in a suboptimal uniformity of the plated layer.Conventional DC plating proves ineffective in addressing this issue, as it continually depletes ions near the cathode, exacerbating concentration polarization.In contrast, pulse plating offers a viable solution.Not only does it foster grain refinement and reduce porosity, but it also enhances the dispersion ability of ion transfer and mitigates concentration polarization when compared to DC plating.Lindblom et al [8] studied the improvement of nickel height uniformity by pulse electroplating.On this basis, we further studied in detail the effects of parameters such as current density and duty cycle on the height uniformity of electroplating to obtain a more uniform plating.This work focuses on the study of plating height control in a pulse electroplating process for zone plates.The plating parameters are optimized in the planar to obtain finer grains.Subsequently, these parameters undergo further optimization within the grating structure, taking into account the modulation requirements of large aspect ratio structures.The objective of this approach is to establish a controllable pulse plating process, offering an optimization method specifically tailored for achieving highly uniform plating in large aspect ratio structures.

2.1.Plane plating optimization
In this study, the aspect ratio of the photoresist structure obtained by electron beam lithography reached 7. The large aspect ratio structure requires very small grain size and good uniformity of the plating layer during the plating process, which puts high demands on the plating process.Compared with DC plating, pulse plating not only promotes grain refinement and reduces porosity, but also improves the plating height uniformity.Therefore, under the premise of ensuring that ion dissipation is effectively supplemented, the current density should be increased as much as possible to form a plating layer with smaller grains and denser.
A seed layer of 5 nm Cr/10 nm Au for gold electroplating was coated on the silicon by thermal evaporation in a vacuum.Plating time is uniformly 5 minutes.The surface roughness of the plated layer was selected as a crucial parameter for evaluating densification and grain size.To optimize the pulse plating process parameters, we use orthogonal experiments to investigate the effects of three factors, pulse current density, pulse duty cycle, and pulse frequency on the roughness of gold plating.The surface roughness of the plated layer is obtained by using an atomic force microscope to measure several points at each of the corners, borders, and center of the plated layer, and then averaging the results.The optimization scheme of pulse parameters and experimental results are shown in Table 1.  1, it can be seen that the main order of the influence of pulse process parameters on gold plating roughness is: C→B→A.The average current density has the greatest influence and the pulse frequency and duty cycle have less influence.The preferred pulse parameters are a current density of 1.6 A/dm², a duty cycle of 20%, and a frequency of 10 kHz.

Large aspect ratio structure plating optimization
To further consider the impact of large aspect ratio structures, the optimized pulse process parameters were studied in detail based on the above experiment results.The effects of average current density, duty cycle, and pulse frequency on the uniformity of plating height were discussed respectively.When calculating the plating height uniformity, nine points were arbitrarily taken and the ratio of the standard deviation of the thickness to the average thickness was used to evaluate the uniformity of the coating thickness.The calculation method is as follows.
The structure used for electroplating is shown in Fig. 1(a).The grating has a period of 100 nm, a duty cycle of 1:1, and a length of 1.25 µm.The photoresist height is 350 nm.deposition rate and plating height uniformity.It is considered that there are fewer side reactions when the current efficiency is greater than 90%.In Fig. 1(b), when the pulse duty cycle is 20% and the frequency is 10 kHz, the pulse current density increases from 1.0 A/dm 2 to 2.2 A/dm 2 .The deposition rate showed a linear increasing trend, and the plating height uniformity became better first and then worse.When the current density is 1.6 A/dm 2 , its plating current efficiency is 92%.The ions were effectively replenished by diffusive mass transfer during the off time.When the current density is 2.2 A/dm 2 , the plating current efficiency drops to 75%.This indicates that there is some current consumption in the process, which exceeds the upper limit of the allowable current density, and a side reaction occurs.At a current density of 1.6 A/dm 2 , the plating height uniformity was good and the process conditions were relatively stable.In Fig. 1(c), when the pulse current density is 1.6 A/dm 2 and the pulse frequency is 10 kHz, the pulse duty cycle is increased from 10% to 50%.Increasing the duty cycle will cause the current efficiency to continue to decline.Consumption of ions through diffusion is difficult to effectively replenish and it is not conducive to the formation of a uniform plating layer.In Fig. 1(d), when the pulse current density is 1.6 A/dm 2 and the duty cycle is 10%, as the pulse frequency increases, the deposition rate rises slowly and oscillates.The difference in current efficiency is not large and the change in deposition rate is relatively small.

Fabrication process and results
In this work, the zone plate to be fabricated has the outmost zone-width of 50 nm and a radius of 60 µm on a 100-nm thick Si3N4 membrane.A seed layer of 5 nm Cr/10 nm Au for gold electroplating was coated by thermal evaporation in a vacuum.Then 350-nm-thick PMMA (MW: 950 k) was spin-coated and baked on a hot table for 5 minutes at 180 ℃.E-beam exposure for the zone plates was carried out by a state-of-the-art e-beam writer (JEOL 6300FS) with a Gaussian beam at 100 keV.The beam current used was 500 pA to ensure the beam-spot diameter of 7-10 nm.
After the E-beam exposure, the samples were developed in methyl isobutyl ketone (MIBK)/isopropyl alcohol (IPA) (1 : 3) for 60 s at 23 ℃, and rinsed in IPA for 30 s. Au electroplating was carried out in a K3Au(SO3)2 electrolyte (8.3 g/L concentration, PH 6.5, 50 ℃, supplied by TECHNIC), driven by a pulse current source delivered by Keithley Ltd.Finally, a lift-off process was done by soaking the plated zone plates in acetone for about 20 minutes to remove the resist area.The design parameters of the imaging zone plate are shown in the Table 2.In order to determine the uniformity of the overall structure height of the area plate, a portion of the structure was selected from the center along the radius to the edge to determine its uniformity.Lines as shown in Fig. 3(a)5 were selected in different regions and gratings were selected at regular intervals to calculate their height uniformity separately.The results are shown in Fig. 4. It can be seen that the uniformity of the plating height gets progressively worse with the increase in radius.This is because as the radius increases, the aspect ratio of the structure increases and the modulation becomes more pronounced.Multiple measurements show that the unevenness of the plating height is mainly caused by the structure and the process is stable.The height uniformity of the zone plate plating was good and remained within 5%.Based on the image, lines with a width of 50 nm in the center of the test pattern can be clearly identified.Even though the outermost ring of the zone plate has a width of 50 nm, lines below 50 nm line width are also observed with good imaging results.

FIG. 7.
Relationship of imaging contrast uniformity along the radius direction.The imaging contrast uniformity was calculated by drawing circles at 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, and 100nm linewidths in Fig. 6.The results are shown in Fig. 7.As the line width gradually increases, the uniformity of the contrast becomes better.This is because as line width increases, the contrast becomes correspondingly better.In addition, the uniformity of the plating height becomes better, and the contrast uniformity is improved accordingly.

4.Conclusions
In this work, by optimizing the parameters of the pulse plating process, a more uniform plating was obtained.Using the optimized process parameters for plating, the zone plate with 50 nm as the outermost ring line width was successfully prepared, which improved the line width and sidewall height uniformity and further enhanced the performance of the zone plates.Optical imaging of a 50 nm Siemens star by the fabricated zone plates has

FIG. 1 .
FIG. 1.(a) Plating structure diagram.(b)(c)(d) Effect of current density, duty cycle and pulse frequency ondeposition rate and plating height uniformity.It is considered that there are fewer side reactions when the current efficiency is greater than 90%.In Fig.1(b), when the pulse duty cycle is 20% and the frequency is 10 kHz, the pulse current density increases from 1.0 A/dm 2 to 2.2 A/dm 2 .The deposition rate showed a linear increasing trend, and the plating height uniformity became better first and then worse.When the current density is 1.6 A/dm 2 , its plating current efficiency is 92%.The ions were effectively replenished by diffusive mass transfer during the off time.When the current density is 2.2 A/dm 2 , the plating current efficiency drops to 75%.This indicates that there is some current consumption in the process, which exceeds the upper limit of the allowable current density, and a side reaction occurs.At a current density of 1.6 A/dm 2 , the plating height uniformity was good and the process conditions were relatively stable.In Fig.1(c), when the pulse current density is 1.6 A/dm 2 and the pulse frequency is 10 kHz, the pulse duty cycle is increased from 10% to 50%.Increasing the duty cycle will cause the current efficiency to continue to decline.Consumption of ions through diffusion is difficult to effectively replenish and it is not conducive to the formation of a uniform plating layer.In Fig.1(d), when the pulse current density is 1.6 A/dm 2 and the duty cycle is 10%, as the pulse frequency increases, the deposition rate rises slowly and oscillates.The difference in current efficiency is not large and the change in deposition rate is relatively small.

TABLE 2 .FIG. 2 .
FIG. 2. Comparisons of the resultant zone plates after electroplating of Au and lift off process, plated by three different plating parameters.(a) DC plating, the current density is 1 A/dm 2 .(b)The plating parameters are 1.6A/dm 2 , 10%, 6 kHz.(c) The plating parameters are 1 A/dm 2 , 30%, 10 kHz.Figure2(a) shows that DC plating is not suitable for application to nanometer fine structures.The results in Fig.2(b) and Fig.2(c) show that the height of the zone plate plate with the optimized parameters is more uniform.Its duty cycle is closer to 1:1 and no spindle-shaped lines appear.

Figure 2 (
FIG. 2. Comparisons of the resultant zone plates after electroplating of Au and lift off process, plated by three different plating parameters.(a) DC plating, the current density is 1 A/dm 2 .(b)The plating parameters are 1.6A/dm 2 , 10%, 6 kHz.(c) The plating parameters are 1 A/dm 2 , 30%, 10 kHz.Figure2(a) shows that DC plating is not suitable for application to nanometer fine structures.The results in Fig.2(b) and Fig.2(c) show that the height of the zone plate plate with the optimized parameters is more uniform.Its duty cycle is closer to 1:1 and no spindle-shaped lines appear.

3. 2 .FIG. 4 .
FIG. 3(a)Electron microscope image with a tilt angle of 30°.(b) Enlarged view of selected 3 regions.The outermost ring of the Fresnel zone plate uses photoresist as a reinforcement.The length of each grating segment is 1.25 µm as shown in Fig.3(b).Electron microscopy results show good overall height uniformity.

3. 3 .FIG. 5 .FIG. 6 .
FIG. 5.The layout of imaging experiment station at Hefei Light Source.The test optical path diagram for the imaging test is shown in Fig.5.Full-field soft X-ray imaging by the fabricated 50 nm FZPs was taken at NSRL, using the Siemens star as the object.The test imaging is shown in Fig.6.

TABLE 1
Pulse parameter optimization scheme and experimental results.