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Abstract
The proposed X-ray spatial light modulator (SLM) concept is based on the difference of X-ray scattering from amorphous and crystalline regions of phase change materials (PCMs) such as Ge2Sb2Te5 (GST). In our X-ray SLM design, the “on” and “off” states correspond to a patterned and homogeneous state of a GST thin film, respectively. The patterned state is obtained by exposing the homogeneous film to laser pulses. In this paper, we present patterning results in GST thin films characterized by microwave impedance microscopy and X-ray small-angle scattering at the Stanford Synchrotron Radiation Lightsource.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the visible light regime, spatial light modulators (SLMs) are powerful tools for the manipulation of amplitude, phase, or polarization of light by transmission or reflection from an SLM [1–3]. It is especially useful for perfecting a defective optical wavefront generated from a fluctuating light source. A typical SLM is composed of a 2-D array of pixels, addressable by light or electrical signals. The SLM is a programmable blazed grating with the ability to study diffraction properties based on wavelength, blazed angle, and pixel size. For X-ray optics, SLM converts the incident Gaussian X-ray transverse intensity profile to a flattop profile, resulting in a high-precision beam pattern with uniform effective undulator strength. [4] Although X-ray is widely used for diagnosis and analysis in clinical medicine [5], crystallography [6–7] and biological organisms [8–9], dynamic X-ray optics like SLM designed to enhance the performance of X-ray optics does not exist. [10]
In this work, we demonstrate the writing and erasing of a reflective X-ray grating on a laser addressable GST film on a silicon wafer. Although vanadium oxide, metal oxides, and GST have been used to demonstrate modulation of optical characteristics in integrated photonics, the control of optical properties is needed during the phase transition in these materials. [11–16] GST is a phase-change material (PCM) used in non-volatile electronic and optical memory devices. By controlling the input energy to GST, GST can be tuned the crystal structure of the materials. In the presence of a continuous X-ray power, both amorphous and crystalline GSTs are effectively stable without phase transition, which is critical for adopting SLM for X-ray optics. The scattering efficiency of X-rays is controlled by the charge density distribution in the material [17]. GST in its crystalline state has a higher density than the amorphous state [18]. The resulting contrast in charge density enables constructive interference of X-rays under specific angles, controlled by the pattern imprinted into the GST thin film using laser illumination. Reflective X-ray optics, such as gratings and zone plates, can be fabricated by patterning GST bands and zones in different phases on a GST film. A reflective X-ray SLM would be composed of a PCM film on silicon, patterned with gratings or off-axis zone plate switchable between on-state and off-state repeatedly.
We have studied the low-q X-ray diffraction pattern originating from the periodic density modulation of amorphous and crystalline regions of Ge2Sb2Te5 (GST). The effect of crystalline states of GST on X-ray reflectivity was investigated, using X-ray diffraction images at 8 keV. The X-ray radiation was provided by the Stanford Synchrotron Radiation Light Source.
2. Experimental method
The GST films with thicknesses ranging from 20 to 100 nm were deposited by magnetron sputtering using an alloy target in an Argon atmosphere on Si (100) wafers with a deposition rate of about 0.1 nm/second as in Fig. 1(a). The patterned gold membrane (shadow mask) was in contact with the GST film. The shadow mask was a gold membrane on a silicon frame. Patterns on the gold membrane were made by e-beam lithography. The GST film was then exposed to the light from a femtosecond laser through the nanometer apertures on the gold membrane as in Figs. 1(b) 1), 2) and 3). Amorphous GST turned into crystalline GST after multiple laser pulses exposure [19–20]. The patterns on the GST film were obtained upon excitation with a single 350 fs laser pulse at 800 nm wavelength for amorphization and 40 Hz repetition rate pulses for crystallization. Incident laser fluence per pulse was from 5.8 mJ/cm2 to 20.8 mJ/ cm2. The phase transformation of a GST film occurs by the melt-quenched process induced by the laser pulse. [21] Gratings composed of regularly spaced crystalline and amorphous GST strips were formed on the film after illumination. We were able to see the crystalline areas under a visible light microscope due to the optical reflectivity difference between the amorphous and crystalline states as in Figs. 1(b) 4), 5) and 6) [22–23]. To turn the crystalline GST back into the amorphous state, a single laser pulse was used. (Supplement 1 are provided with patterned 38 areas. The top image shows the overview of the array of areas with an increasing amount of energy deposited on each unit area.)
Fig. 1. (a) Schematics of Laser Stencil Lithography; 1) Placing the shadow Mask of gold membrane on the GST film, 2) Exposure with a fs laser, 3) Removal of the Shadow Mask with pattern left on the GST film, (b) optical images; 1) lower magnification of an optical image of patterns 2) higher magnification of an optical image, 3) an optical image with a laser spot on silicon wafer with a mask, and 4), 5), 6) after removing the Au mask, (c) Schematics of SAXS Experiment setup and X-ray detector, and (d) a schematic diagram of X-ray SLM.
The schematics of SAXS experiment setup and X-ray detector is shown in Fig. 1(c). The structure of the GST material was studied using X-ray diffraction with a stand-alone X-ray diffractometer (Bruker APEX II with Cu K-alpha source, distance from the sample to the detector: 70 mm, 2 Theta: −40°, Omega: −10°, Phi: −90°, and Chi: −90°). A schematic diagram of X-ray SLM concept shows in Fig. 1(d) that the “on” and “off” states correspond to a patterned and homogeneous state of a GST thin film. The pattern, which is imprinted into the GST thin film, allows constructive interference of X-rays at specified angles.
In this work, scanning Microwave Impedance Microscopy (MIM) is employed to resolve the local conductivity. MIM is a measurement of tip capacitance and dissipation, as affected by local sample conductivity [24]. The MIM-Imaginary signal (capacitive), is a monotonic function of the sample conductivity, while the MIM-Re signal (dissipative), has a peak at intermediate conductivities (around 100 ohm-cm).
The Small-Angle X-ray Scattering (SAXS) experiments were performed on the nano-patterned areas with a membrane mask (fabricated by E-beam lithography with a 100 nm thick Au film with a grating pattern of 300 nm width and spacings of 300 nm). The corresponding X-ray diffraction image was produced with an 8 keV X-ray beam of spot size 300 µm × 300 µm, from Beamline 1-5, SSRL, SLAC National Accelerator Laboratory (Supplement 1 are provided for the beamline setting). The position of the X-ray detector with respect to the incoming X-ray beam was calibrated based on the diffraction of silver behenate powder located at the sample position. The distance to the sample is determined as 2.92 m based on its pixel size of 80 × 80 µm and the X-ray photon energy of 8 keV. The scattered X-ray signal was recorded with a Mar165SX detector with a binned pixel size of 80 × 80 microns. The Nika package for Igor was used for data reduction [25].
3. Results and discussion
The phase change of GST films is confirmed by X-ray diffraction using a stand-alone X-ray diffractometer (Bruker APEX II) with a Cu K-alpha source. We see rings rather than dots because of the polycrystalline nature of the ‘crystallized’ GST as shown in Fig. 2(a). A comparison between the amorphous (off) and polycrystalline (on) states is accomplished by X-ray diffraction. Crystallization happens in nucleation-driven (rapid nucleation) PCMs through the stochastic creation of crucial nuclei and their subsequent expansion [26]. The recrystallized region is polycrystalline, with grains arranged in a variety of ways. The crystallized GST exhibit characteristic peaks at 27°, 29°, 42°, 53°, 62°and 71° in Fig. 2(b), corresponding to the diffraction of the (111), (200), (220), (222), (400) and (420) crystalline planes, respectively [27–28].
Fig. 2. X-ray diffraction patterns with different laser fluence conditions (a) 2D images and (b) line analysis.
The crystallization was further confirmed by using a scanning-tip MIM to plot the topography (see Fig. 3(a, b)) and measure the tip-sample capacitance and the conductivity across the film. Figure 3(c) shows the MIM-Im signal at the boundary between amorphous and crystalline GST. The MIM-Re signal also shows the conductivity change at the interface between amorphous and crystalline structures as shown in Fig. 3(d). Figure 3 shows a large increase in conductivity in the crystalline area after a lower laser fluence (5.8 mJ/cm2) illumination. Supplement 1 (see Visualization 1 and Visualization 2) are provided, demonstrating the ability to write and erase patterns on amorphous and crystalline GST films respectively.
Fig. 3. MIM images near the boundary between amorphous and laser treated crystalline area using an 800 nm femtosecond laser on amorphous Ge2Sb2Te5 40 nm/Si (100) (a) added a boundary line into the image as a guide (b) Topography, (c) MIM-Im, and (d) MIM-Re images (see Visualization 1 and Visualization 2).
To spatially modulate this crystallization process, we apply a patterned mask on top of the PCM prior to illumination. An e-beam patterned gold membrane mask was used to block out the unwanted light. The mask was a 100-nm thick free-standing gold film, and the pattern is fabricated by e-beam lithography with 300 nm width and 300 nm spacing between patterns. The data presented in Fig. 4 demonstrate that a crystalline nano-patterned area can be formed on a GST film by using an Au mask fabricated by E-beam lithography and a laser. The phase change is shown by the change in conductivity observed by MIM [27–28].
Fig. 4. Crystalline nano-patterned area with a membrane mask using a 800 nm femtosecond laser on amorphous Ge2Sb2Te5 40 nm/Si (100). (a) Topography, (b) MIM-Im, and (c) MIM-Re images. While the stripes of the mask are clearly seen in the GST conductivity, they are not homogeneous and show partial crystallization which is most likely caused by incomplete nucleation, which could be overcome by further laser irradiation.
SAXS can be performed on the patterned two-phase area to observe the constructive interference of X-rays scattered from the laser-induced pattern [29]. The scattering images of SAXS demonstrated the periodicity between amorphous and crystalline patterns in the substrate. To discriminate between surface roughness induced scattering and bulk crystallization change, SAXS performed with different incident angles from 0.3° to 1.0° for a 480-second data collecting time. Figure 5 shows the integrated SAXS patterns for different angles of incidence (a) and for different azimuthal angles (b). In both cases, the scattering efficiency of the pattern decreases. The pattern periodicity can be extracted by SAXS images. If the modulation of the scattering efficiency measured by SAXS is higher than what we expect from the bulk density change, then the surface must play a role. The density of amorphous GST (Ge2Sb2Te5) is 5.87 g/cm3 and changes to 6.27 g/cm3 upon crystallization [30]. The SAXS intensity was evaluated with various rotation angles from −0.5° to + 0.5° after alignment of the sample to confirm the high-quality data. Figure 6 shows a relatively good representation of the SAXS pattern at 0° rotation angle in our SAXS setting. The SAXS pattern obtained from the GST sample after laser exposure is shown as the inset in Fig. 6, which shows the integrated X-ray intensity along qx. The periodicity of the maxima corresponds to a scattering contrast in real space with 570 ± 20 nm periodicity, which matches the periodicity of the crystallized pattern of 600 nm quite well. The discrepancy is likely due to a minor misorientation of the pattern against the nominal phi = 0° alignment, because the effective spacing is seen to increase with decreasing phi, approaching the nominal value of 600 nm.
Fig. 5. Far-field diffraction patterns (Fourier transform) of the patterned GST (a) with different incident angles (Omega) from 0.3 degree to 1.0 degree, (b) with different rotation angles (Phi) from - 0.5 degree to + 0.5 degree at 0.3-degree incident angle for 480-second data collecting time, (c) distance between patterns, which shows 0.56 µm ± 0.02 µm.
Fig. 6. SAXS pattern obtained from the scattering contrast between amorphous and crystalline regions of the PCM GST. The inset shows the raw detector image (with intensity in log-scale), whereas the integrated pattern reveals a scattering contrast greater than a factor of six.
These data show that the X-ray scattering contrast between amorphous and crystalline PCMs can be used to design beam-shaping objects. In the present case, the intensity ratio of maxima over minima is 6 ± 1, which can be further enhanced in case a two-dimensional (e.g. zone plate) pattern is employed and in case full nucleation of the patterned area is achieved.
4. Conclusion
We demonstrated the feasibility of building an X-ray SLM with a PCM by the phase transformation of a GST film by the melt-quenched process using the laser pulse. Crystallization happens in nucleation-driven (rapid nucleation) GSTs and the recrystallized region is polycrystalline. By patterning GST bands and zones in distinct phases on a GST film, reflective X-ray SLM can be made. It is noted that other PCMs can also be considered in the future for this application. The main challenges to overcome will be 1) the confinement of crystal growth within well-defined patterns, and 2) ensuring nucleation over the irradiated area.
Funding
Gordon and Betty Moore Foundation (EPiOs grant GBMF4536); National Science Foundation (DMR1305731); Basic Energy Sciences (DE-AC02-76SF00515); Defense Advanced Research Projects Agency (QORS).
Acknowledgments
We thank Richard Tiberio, Clifford Knollenberg, Feng Xiong, and Vijay Parameshwaran. Part of this work was performed at the Stanford Nano Shared Facilities and at the Stanford Nanofabrication Facility. We also thank Kurt Rubin for his helpful discussions.
Disclosures
Z.X.S. is a cofounder of PrimeNano Inc., which licensed MIM technology from Stanford for commercial instrumentation. SWF and HSPW are supported in part by member companies of the Stanford Non-Volatile Memory Technology Research Initiative (NMTRI), which, however, did not support this work.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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