Patent Application
20240312665
Exemplary embodiments of the present disclosure provide a method comprising forming a plating base at a bottom of grooves for a uniform bottom-up electroplating.
Exemplary embodiments of the present disclosure provide a structure comprising a freestanding gold membrane mask with flat parallel gold bars.
The matters exemplified in this description are provided to assist in a comprehensive understanding of exemplary embodiments of the disclosure. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the claimed disclosure. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
Detailed descriptions of exemplary implementations of embodiments of the present disclosure set forth herein together with accompanying Figures describe non-limiting examples of implementations of certain exemplary embodiments of the present disclosure and aid in the description of technology associated therewith.
X-ray phase contrast imaging (XPCI) of biological samples offers improved image quality over conventional X-ray methods. Despite this significant advantage, the technique remains restricted to research laboratories and, in particular, to synchrotron radiation sources. The possibility to transfer XPCI into real medical applications in clinical laboratories offer the prospect of creating a significant change in medical X-ray imaging. Among many XPCI techniques, two approaches emerged recently that have allowed XPCI implementations with conventional X-ray sources. One is interferometric XPCI based on three diffraction gratings, and another one is intensity-modulation X-ray phase-based imaging (IM XPCI) [7]. In the latter method, the image resolution is determined by the aperture size of the mask, hence masks with sub-micrometer apertures provide sub-micrometer resolution.
Fabrication of high-aspect-ratio (the ratio between the absorber height and the aperture width) and high-resolution gratings is extremely challenging and considerable amount of effort was put in the process development. Fabrication based on gold (Au) electroplating into the mold made in the thick layer of photoresist supported by low X-ray absorbing substrates such as silicon (Si), graphite, silicon nitride or polyimide membranes using deep x-ray-lithography is the most often used approach. X-ray lithography requires dedicated synchrotron facilities and can be considered a bottleneck of the method. Resist's low structural rigidity requires an additional lateral support for high-aspect-ratio structures with a micron-to-submicron-footprint, which reduces gratings performance.
Another method of fabricating absorption gratings is based on etching deep grooves in silicon, then filling them with gold by electroplating. Recently, centrifugal gold and tungsten particle deposition was explored for filling the grooves. Molds made of Si are rigid, their fabrication does not require X-ray lithography, but Si absorbs X-rays, limiting the photon throughput potentially reducing image contrast when polychromatic sources are used. Attempts have been made to increase mask contrast by using low absorbing substrates such as SU-8 resist or polyimide films glued on a metal frame for mechanical support. The polymer materials tend to degrade under prolonged exposure to X-rays, which can compromise mask stability over time.
Fabrication of freestanding X-ray absorption masks is a long-standing key challenge in improving image quality, as well as in reducing radiation dose, which is essential for medical applications. There are a limited number of publications on this topic and most structures are made in a resist layer, limiting the absorber thickness. Freestanding metal grids of large dimensions obtained using X-ray lithography and a graphite substrate were reported. Despite many advances in x-ray mask fabrication, fabrication of freestanding absorption gratings requires further development and innovation, and, to our knowledge, has not yet been demonstrated.
Here, we present the development, fabrication details, and experimental evaluation of freestanding 10-μm-thick gold membrane masks with an array of 0.9-1.5 μm void slit apertures with a grating pitch of 7.5 μm to be used with a low energy (8 keV) x-ray source. The 6.0-6.6-μm-wide gold bars are supported by 3-μm-wide crosslinks at 400 μm intervals.
Fabrication of such gratings is not feasible by electroplating into the resist molds because of the insufficient mechanical strength of the polymer. The presented fabrication process involves creating grooves in Si using deep reactive ion etching (DRIE), depositing a voltage barrier (SiNx) and a sacrificial layer (SiO2), and forming a conductive plating base on the groove bottom for subsequent gold electroplating. A critical part of the fabrication process is the release step. In this step, the sacrificial Si is etched, and the space between the gold bars supported by the Si frame becomes void. The major challenge is to keep the long gold bars from collapsing onto each other during and after release. We have explored different release approaches utilizing a combination of DRIE with isotropic wet etch and xenon difluoride (XeF2) vapor etch.
The mask design must meet challenging requirements such as being robust enough to perform the imaging experiments while being made of ten-micrometer-thick and six-micrometers-wide gold bars separated by micron-sized void slit apertures; the aperture length must be long enough to provide unblocked field of view, without collapse or deformation of the grating structure. We have chosen an individual aperture length of 400 μm with an overall 4×4 mm2 membrane grating area. Large membrane grating area was chosen to test the method's feasibility for various X-ray imaging experiments. The ideal mask should represent an array of flat, homogeneous, parallel rectangular gold bars with rigid supports at both ends.
100-mm-diameter, 500-μm-thick, double side polished (100) prime Si wafers with 5-20 Ohm-cm resistivity (University Wafers) were used in all experiments. The schematic of the fabrication processes is shown in
First approach. Fabrication starts with Cr (30 nm) deposition by electron beam evaporation (Temescal FC 2000) that acts as a mask for the following Si DRIE, and later as a sacrificial layer for lift-off removal of a plating base. After Cr deposition, the wafer was spin coated with a S1805 photoresist (Microchem) at 3000 rpm for 45 s, baked at 115° C. for 1 min. 1.5-μm-wide lines were lithographically patterned with a dose of 55 mJ/cm2 using the 405 nm laser of the Maskless Lithography Aligner (MLA, Heidelberg 150). After resist developing with ¼ diluted 351 developer, and pattern transfer into the Cr layer using a commercial Cr etch solution (Transene 1020), the wafer was cleaved into 25 mm×25 mm size pieces, and each piece was processed separately using various fabrication parameters. The grooves in Si were etched to a depth of about 12 μm by a Bosch-like DRIE process (Oxford PlasmaLab 100 DRIE system). In the optimized recipe, the passivation and etch steps were each 3 s long. The flow rate gradually decreases for C4F8 from 80 standard cubic centimeter per minute (sccm) to 20 sccm and increases for SF6 from 10 sccm to 70 sccm during seven cycles for etch step. For the passivation steps, 100 sccm of C4F8 gas and 1 sccm of SF6 gas were used. A source power of 700 W is used in both steps; a RF power of 10 W is used in the deposition step and of 30 W in the etching step, pressure is 30 mTorr, the etching temperature is 10° C. After DRIE was complete, a Ti/Au (5 nm/50 nm) plating base was evaporated at a rate of 1 Å/s (Temescal FC 2000) to ensure metal deposition under normal incidence to the bottom of the Si grooves. The plating base was lifted-off from the Si top surface using the Cr etch solution (Transene 1020). The sample was electroplated at 40° C. using a commercial plating solution Techni gold 25E (Technic).
In the second approach, a resist layer served as a mask for Si DRIE. Resist application, lithography and Si etching were done as described above. After DRIE, the samples were cleaned in oxygen plasma at 200 W power for 10 min and Nanostrip solution at 120° C. for 20 min followed 10 s dip in a 10:1 buffered oxide etch (BOE) solution (Sigma-Aldrich). Then, a 250-nm thick low-stress SiNx layer was deposited by PECVD at 400° C. (Applied Materials P 5000) followed by a 200-nm SiO2 layer deposited at 100° C. using SiH4 and N2O precursor gases (flow ratio SiH4:N2O=8.5:20) by PECVD (Oxford PlasmaLab 100 Inductively Coupled PECVD) and, finally, the Ti/Au (5 nm/50 nm) plating base was evaporated using Temescal FC 2000. SiO2 was etched in a 10:1 BOE solution for 90 sec to lift-off the plating base from the groove tops and lateral walls. Since the bottom of the grooves are wider than the top, the SiO2 under the Ti/Au layers remain mostly in place, with minor undercut, but firmly attached to the substrate. Gold is electroplated to fill the grooves using Techni gold 25E at a temperature of 40° C. and a direct current density of 1 mA/cm2 (Keithley 2400 Source Meter). After electroplating, the samples were annealed on the programmable hot plate (Torrey Pines) at 350° C. for 5 hr with heating and cooling ramps of 1°/min, for building up a small tensile stress in the gold layer, to prevent buckling and collapsing of the gold bars after release. To open a window, the back side of the sample was coated with an approximately 12-μm-thick AZ 4620 resist layer (Micro Chemicals GmbH) by spin coating at 1500 rpm for 45 s, followed by 10 s spinning at 4000 rpm to reduce the edge bed. The lithography step (exposure dose of 750 mJ/cm2) was performed on a Heidelberg Instruments MLA 150 aligner, which allowed back-to-front alignment of the final mask for DRIE through-etch. After developing with 351 developer diluted 1:3, the sample was flipped over and temporally bonded to an alumina carrier wafer (Kyocera) using Dow Corning vacuum grease (Dow Corning). The window back etching using DRIE Bosch-like process was stopped when a few micrometers of bulk Si remained. The sample was detached from the carrier wafer, cleaned with isopropanol, and the resist was removed in 1165 remover (Microchem). The SiNx layer on the mask front was removed by immersion into 48% HF for 3 min, which was controlled by aperture color change using the optical microscope. Then, etching was continued by XeF2 vapor isotropic etching (XETCH X4, Xactix Inc). Etching parameters were: XeF2−1 mTorr, N2−10 mTorr, the impulse duration 60 s. The sample was dehydrated at 115° C. for 5 min prior to etching. Examination and metrology during the fabrication process were performed using a Vega scanning electron microscope (Vega 3 SEM Tescan), a confocal microscope (VK-X1000 Keyence), and an optical microscope (Olympus). The stress of the electroplated 0.5-μm-thick gold film on a 4″ Si wafer was calculated from measuring the wafer curvature before and after gold deposition and annealing using KSA Multibeam Optical Sensor (k-Space Associates). Since the stress measurements were performed at room temperature, lower than the gold electrodeposition temperature, the measured values include a contribution from the stress arising from the differing coefficients of thermal expansion (CTE) of Si and Au.
The mask has been evaluated using a laboratory set-up designed for IM XPCI and consisting of a rotating anode (Cu) x-ray source, a confocal multilayer monochromator selecting the Cu K lines (8 keV) and focusing the beam to a 250 μm focal spot. The detector, featuring a 1.8×1.8 mm2 field of view and a 1.1 μm pixel pitch, was placed 2.2 m away from the source. The detector-to-mask distance varied between 1.5- and 25-mm. Samples were placed between mask and detector, with a mask to sample distance of 10 mm.
To assess the uniformity of the mask, visibility, which depends on mask parameters and that, in turn, affects the quality of the retrieved images, was used. It is defined as
with Imax and Imin corresponding to the maximum and minimum intensity of the beamlets created by the mask apertures, respectively.
A Siemens star, comprising twenty-four 500-nm thick Au spokes placed on a 200-nm thick SiN3 membrane, was used to assess the spatial resolution of an IM XPCI system employing the mask presented in this paper.
Additionally, a 1-mm thick foam layer was imaged and the transmission, refraction (proportional to the first derivative of the phase) and scattering contrast channels retrieved, using the method proposed in. The latter contrast channel is of particular interest in this context, as it is sensitive to length scales below the resolution of the system. Since in IM XPBI the resolution is determined by the size of the apertures, the production of micron-sized apertures makes the scattering channel sensitive to nanometric features.
The mold fabrication step determines the grooves profile and ultimately the quality of the mask. Vertical wall profile angle and low surface roughness are two important parameters. Anisotropic DRIE Bosch process consisting of alternating etch and deposition steps using SF6 and C4F8 gases is a reliable method of obtaining high-aspect-ratio microstructures in Si. Both steps ionize the gas in an ICP source, and only the etch step applies a bias voltage to the sample. During the etch step, free fluorine radicals etch Si isotropically, and the deposition step coats the sample with a fluorocarbon polymer. The subsequent etch step removes the polymer at the base of the grooves due to the high vertical flux and momentum of the ions; however, the sidewall polymer remains relatively intact due to minimal lateral ion bombardment. Since the etch step is isotropic, the sidewall has normally a “rippled” profile with shape modulations known as scallops. The sidewall slope and the depth of the scallops depend on the ratio and duration of the etching and passivation steps. By following the strategy proposed in of using C4F8 during the etch step and decreasing the duration of the etch and polymerization steps, we have obtained straight and smooth Si walls. Scanning electron microscopy (SEM) images of Si molds with 1-μm-wide and 12-μm-tall walls are shown in
(2a). The depth of the scallops is around 30 nm (2b); this is much smaller than the scallop sizes from the unmodified Bosch process, which are typically around a few hundred nanometers. The process is optimized to give a vertical profile on a certain critical feature size. Features larger than the critical feature size may have a concave profile, while those smaller than the critical feature size will have a positive profile angle.
To fill the mold with the metal, two electroplating techniques are used: (i) bottom-up electroplating from the conductive bottom, (ii) a full-surface electroplating from the plating base created by atomic layer deposition (ALD) on all surfaces. The second method is used for the electroplating of gratings with 0.5 duty cycle [6,16]. This approach is hard to implement in the fabrication of the freestanding masks because of challenges in achieving uniform filling over the entire groove depth and avoiding void formation due to prematurely groove sealing. Gold deposition exhibiting mostly bottom-up evolution has been recently demonstrated using fully metallized gratings by reducing gold deposition on the groove top using bismuth. However, the undesirable gold deposition on the top of the structure still occurs and this will degrade the quality of the low-duty-cycle mask. Uniform void-free filling, essential to mask quality, can be obtained for the low-duty-cycle masks only when the plating proceeds from the bottom of the grooves. To achieve this, in the first approach (
A new approach (figurelb) has been developed to form a plating base at the bottom of the grooves for the uniform bottom-up electroplating. We took advantage of the well-known fact that SiO2 formed from SiH4 and N2O at low temperature can be easily removed by wet etch. We used SiO2 as a sacrificial layer to lift-off the subsequently evaporated plating base from the unwanted areas. The isotropic nature of wet etching provided a means to remove SiO2 along with the evaporated metals from the groove tops and sidewalls, while SiO2 under the Ti/Au layers remained at the bottom of the grooves. To avoid gold nucleation on Si, an insulation layer of PECVD SiNx has been deposited prior to SiO2 deposition. This approach allowed us to achieve uniform bottom-up electroplating (
To ensure good adhesion of the silicon nitride film to the sidewalls, the deposition conditions of the silicon nitride were optimized. Initial films were near stoichiometric conditions with an index of approximately 1.9.
One of the major issues in obtaining a high-quality gold mask is gold residual stress, which consists of thermal stress due to mismatch between Au and Si CTE and intrinsic stress that comes from gold growth factors during electroplating. A low-stress (ca.-35 MPa) fine grain gold deposit to provide mask dimensional accuracy was obtained using parameters optimized for X-ray mask fabrication. The stress value is in a good agreement with that reported in for this plating solution.
A critical part of the fabrication process is the release step, in which the sacrificial Si is etched away and the spaces between the gold bars become voids. The major challenge is to keep the long gold bars parallel during and after release. Meeting these challenges requires reducing the bar buckling by changing the slightly compressive stress in the electroplated gold film to some tensile value by annealing.
Electroplated gold is a polycrystalline material. The microstructural characteristics of polycrystalline films define their properties and performance and can be altered (a) by electroplating parameters and (b) by annealing after electrodeposition using slow thermal ramps. Following electroplating, we have achieved a low-stress deposition essential for to the mask dimensional accuracy; subsequently, to avoid gold bars buckling, the stress must be increased to some tensile value by annealing. Through annealing, gold atoms migrate from the grain boundaries, where they are in energy-unfavorable states, towards the grain top surfaces, where they fill lower energy states, such as those favoring crystal growth with higher rates. Since atoms depart from grain boundaries, tensile stress builds up when cooling, and reaches about 200 MPa tensile in 0.5-μm-thick gold films after annealing at 200° C. for 10 min. Considering that the thickness of the to-be-released gold layer is 20 times higher, and that release of the gold membrane with a stress value close to the maximum achievable tensile stress is more stable, we have annealed the samples for 5 hr. at 350° C. Systematic evaluation of the optimal annealing parameters is outside the scope of this paper.
The release process started with backside Si DRIE process to open a window; the process was stopped when a few micrometers of bulk Si remained. We then continued with a gentle XeF2 vapor phase etching to remove the remaining sacrificial Si. Although XeF2 etching is an exothermic process, the mask can be released at almost ambient temperature by slowing the etch rate, lowering the process pressure, increasing impulse duration, and diluting XeF2 with nitrogen. Prior to XeF2 etching, SiNx insulation layer on the mask front must be removed by immersion into 48% HF. This is an important step to avoid defects caused by the broken silicon nitride layer and is discussed further in Section 3.4. Images of the flat gold membrane with homogeneous uniform gold bars and parallel apertures along their length are shown in
SEM and optical microscopy show an array of flat, parallel, and fully open apertures. Optical microscope image (
It is important to note that etching of SiNx by XeF2 is not efficient and requires a separate processing step to remove it. Otherwise, the remaining SiNx causes aperture widening defects: if a silicon nitride membrane gets broken in the one of the apertures, the tensile stress of the intact SiNx membranes in the neighboring apertures pulls the gold bars apart as shown in
Optical flatness measurements of the gold freestanding mask were performed with a VK-X1000 Keyence laser confocal microscope and show the freestanding membrane buckling, with deviations from the ideal planar shape below 5 μm along the gold bars and below 37 μm across the gold bars over the entire gratings area. While requirements for out-of-plane grid deformations are permissive for the XPCI, portions of 2×2 mm2, as necessary for the experiments, can be selected from the middle of the membrane, with deviations from flatness bellow 5 μm in both directions, as shown in
Membrane buckling suggests that there is not enough tensile stress in electroplated gold to keep the gold bars straight. With annealing time increase, it should be possible to reduce the gold bars buckling, and this is a subject for further work. Flatness of the membrane can also be improved by reducing the size of the window in Si. If larger mask area is needed, a thin layer (˜5 μm) of polymer such as polyimide or polymethyl methacrylate (PMMA) spun over the front side of the mask prior to the Si etching using XeF2 improves membrane flatness substantially. The downside of this approach is that polymers under X-rays are degrading with time. Reducing the length of the aperture would also improve membrane rigidity though is not desirable because of reducing field of view.
A mask with 1.5-μm-wide apertures has been tested with 8 keV X-rays.
The retrieved refraction image (differential phase) of the Siemens star is shown in
Exemplar images of retrieved transmission, refraction and scattering for a thin foam layer, imaged with an 8 keV x-ray beam, indicating the complex refraction index of the imaged sample as n=1−δ+iβ, the transmission image (
A microfabrication process was developed to fabricate the mask prototypes for a novel low-energy, phase-based X-ray microscope. The masks are 10-11-μm-thick freestanding gold membranes 4×4 mm2 in size with an array of 0.9-μm-1.5-μm-wide and 400-μm-long void slit apertures with 7.5-μm periodicity. An aspect ratio of 12 has been demonstrated. The method developed herein allows for further increase in the aspect ratio as we target higher resolution (i.e. narrower apertures) or higher x-ray energies, which are among our plans for future development. We overcame several challenges in the fabrication process and have (a) optimized the DRIE Bosch process to obtain 12-μm-tall and 0.9-1.5 μm-wide structures with smooth vertical walls, (b) developed a technique to form a plating base at the bottom of the Si grooves for bottom-up electroplating. A SiNx layer was used to insulate the structure's lateral and top surfaces, and a SiO2 sacrificial layer was used to remove the evaporated plating base from the unwanted areas by lift-off, (c) achieved uniform bottom-up gold electrodeposition, (d) tailored residual stress in electroplated gold to keep gold membrane flat upon release from Si, (e) obtained the freestanding gold membrane mask with flat parallel gold bars. From the presented data, aperture length and the overall mask size, homogeneity of the electrodeposited gold layer, SiNx and gold residual stresses, and SiNx layer removal from the mask front are crucial factors to control the uniformity of the released mask. We further demonstrated the possibility to obtain high-resolution high-contrast 2D images of biological samples using an incoherent, rotating anode X-ray source; while in this case, the beam was monochromatized; use of polychromnatic spectra with low average energy is equally possible. Future work will focus on further reducing aperture width down to 500 nm and on the optimization of the fabrication parameters for reliable manufacturing of the freestanding masks.
While the present disclosure has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure.
This application claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Application Ser. No. 63/413,898, filed on Oct. 6, 2022, the entirety of which, including the contents of all the references cited therein, are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63413898 | Oct 2022 | US |
