2.1

Probe beam; spatially coherent soft x-ray laser at the wavelength of 13.9 nm

The SXRL in the transient collisional excitation (TCE) scheme at the wavelength of 13.9 nm is generated from the linearly formed nickel-like silver gain-medium plasmas pumped by the linearly focused CPA Nd:glass laser (TOPAZ). TOPAZ laser system has the zigzag slab-type amplifier chain, which enables us to operate this system with 0.1 Hz repetition-rate ^[16]. Schematic image of the spatially coherent soft x-ray laser is shown in figure 3. The pump laser pulses consist of the pre- and main pulses. Pre-pulse produces the pre-plasma for increasing the absorption of the main pulse, and the main pulse produces the population inversion between 4d - 4p levels in the nickel-like ions. The double target scheme is used for the generation of the spatially coherent x-ray laser. The second medium works as the amplifier and as the spatial filter for the SXRL from the first target that has about 10 mrad beam divergence. The output energy, beam divergence and the duration were 1 μJ, 1 mrad and 7 ps, respectively ^[14,15].

Figure 3.

Spatially coherent soft x-ray laser in JAEA.

2.2

Pump laser; Ti:Sapphire laser

Ti:Sapphire laser at the central wavelength of 795 nm with the duration of 80 fs was used for the pump source for the sample. The spatial profile and peak fluence of the pump beam were Gaussian (FWHM 100 μm) and 0.7 J/cm², respectively. The incident angle of the pump beam to the sample was almost normal and the polarization was linear. The oscillator of Ti:Sapphire laser was synchronized to that of TOPAZ laser system. The timing jitter (standard deviation) between Ti:Sapphire laser and TOPAZ was measured to be 5 ps (RMS) by using the streak camera (HAMAMATSU FESCA-200).

2.3

Double time fiducial system to compensate the timing jitter between the SXRL and Ti:Sapphire laser

In the previous experiment ^[13], temporal synchronization between the pump (Ti:Sapphire laser) and probe (SXRL) was obtained by the synchronization between the Ti:Sapphire laser and TOPAZ (pump laser for generation of SXRL). The causes of timing jitter between the pump and probe pulses are as follows; (i) timing jitter between the Ti:Sapphire laser and TOPAZ, (ii) the uncertainty of the rise-up time of the SXRL (timing jitter between TOPAZ and SXRL). In the case of (i), the timing jitter between the Ti:Sapphire laser and TOPAZ was measured to be 5 ps (see in section 2.2). In the case of (ii), there are several reports for the measurement of the temporal evolution of the small signal gain (SSG) of the SXRL by using the high-order harmonics seeding technique ^[17]. It have been measured to be 5 ~ 37 ps under the several conditions ^[18-20]. The rise-up time of the SXRL depends on the gain medium and irradiation condition, and the instability of the pump laser condition for generation of SXRL might be cause of the uncertainty of the rise-up time of the SXRL. In the present setup, we measured the timing jitter between the Ti:Sapphire laser and SXRL in every shots by adopting a portion of the Ti:Sapphire laser and SXRL as the time fiducials. The SXRL was spatially divided into the probe beam and the fiducial beam (fiducial (X) in figure 2) by the edge of the soft x-ray beam splitter. The soft x-ray beam splitter is a thin mirror which deposited Pt onto Si wafer. The Ti:Sapphire laser was divided into the pump beam and the fiducial beam (fiducial (T) in figure 2) by the plate beam splitter. The fiducial beams are measured by the x-ray streak camera in every shots. The Gold cathode of the x-ray streak camera (HAMAMATSU C4575-01) has the sensitivity for Ti:Sapphire pulse by the multiphoton excitation so it can measure both the SXRL and Ti:Sapphire laser pulse at the same time. The temporal resolution of the x-ray streak camera was 2 ps, therefore the timing jitter between the pump and probe pulses can be compensated to be 2 ps.

2.4

Soft x-ray imaging system with interferometer

The soft x-ray image induced the pump beam with the Gaussian spatial profile enable us to discuss the dependence for the pump laser local fluence of the ablation dynamics in single shot. It is suitable for the measurement around the ablation threshold in particular. However the ablation process that is quite sensitive for the pump fluence such as the spallative ablation ^[11,12] was not observed by the soft x-ray interferometer because the lateral resolution was not sufficient (~ 2 μm) in the previous experiment^[10,13]. In this study, the lateral resolution was improved by using the high precise imaging mirror with short focal length. The soft x-ray imaging system consists of the reflection optics (Mo/Si multi-layer mirrors or Pt mirror). The probe SXRL pulse illuminates the sample with the oblique incidence angle of θ = 20 deg. The image of the illuminated area on the sample is transferred to the CCD surface by the imaging mirror (f = 125 mm) with the magnification factor of about 40. A double Lloyd’s mirror consists of two flat Pt mirrors with a relative incline angle of 0.02 degree. The grazing incident angle of the soft x-ray to the mirrors was about 2 degree. One of the mirrors covers the reflection from the observation area on the sample, and the other covers that from another area used as the reference, and they are overlapped at the CCD position to make interference pattern. A fringe shift of one period corresponds to 20.3 nm under the present condition. The depth resolution depends on the separation of the interference fringes and CCD pixel size, and it was 1 nm in the present experiment. This system can be switched between interferometry and reflective imaging easily by the modification of the relative incline angle of the double Lloyd’s mirror.

The lateral resolution was evaluated using the grooves fabricated on the Pt film (100 nm thickness) by a focused ion beam (FIB) tool. Figure 4 (a) is the image of the test pattern observed by a scanning electron microscope (SEM). The rectangular dark areas show the grooves. Each width and depth were 0.5 ~ 8 μm and about 6 nm, respectively. The intervals of pair of grooves from the edge of groove to the edge were same as the width of grooves. The single-shot soft x-ray image and the cross section at the area enclosed by a red dotted line are shown in figure 3 (b) and (c), respectively. The longitudinal scale was 0.35 μm / CCD pixel. The bright part inside of the grooves show that the surface roughness at the bottom of the grooves was better than a few nanometers. The pair of grooves of 1 μm width was clearly observed, and that of 0.5 μm width was not clear. Furthermore the soft x-ray intensity decreased within 2 pixels at the edge of most grooves. Therefore the lateral resolution was evaluated to be 0.7 μm. The lateral resolution was kept in the area of 400 μm x 400 μm, and it was sufficient to measure the ablation process of the spot size of 100 μm.

Figure 4.

Performance test of the soft x-ray imaging system. (a) The image of the test pattern observed by the scanning electron microscope (SEM). (b) Soft x-ray image of the test pattern. (c) Cross section of the soft x-ray image.

Figure 5.

Experimental results, (a), (c), (e): Temporal evolution of the reflective image of the laser induced R surface. (b), (d), (f): Temporal evolution of interferogram. (g) Spatial profile of the pump laser fluence. The peak laser fluence was 0.7 J/cm².

3.1

Experimental result

We observed the ablation dynamics of Pt (100 nm thickness) surface by using the soft x-ray laser probe system. The initial surface roughness of the sample was measured to be better than 1 nm by atomic force microscope. The peak fluence of the pump beam on the sample surface was about 0.7 J/cm². Figure 4 (a) ~ (f) show the temporal evolution of the reflective image ((a), (c), (e)) and the interferogram ((b), (d), (f)). The temporal evolution of the relative reflectivity of the soft x-ray compared with the outside of the ablating area shows that of the surface roughness (see in figure 1(b)). The temporal evolution of the shape of the surface (landscape) was observed from the interferogram. The fringe shift to left-ward of 1 period implies a positive dilation of 20.3 nm. The focal spot shape of the pump beam is shown in figure 4 (g). The local fluence at the edge of the crater was 0.2 J/cm², and it shows the ablation threshold. In figure 4 (a) and (b), at the time of around 60 ps after the pump laser irradiation, the reflectivity at the center of the ablating area was slightly decreased, and a thin dark-ring was appeared at the position of the edge of the crater. The interference fringes were continuous smoothly. The height at the center was measured to be 20 nm from the fringe shift. In figure 4 (c) and (d), at the time of around 150 ps, the reflectivity at the center of the ablating area was decreased, and a thin dark-ring became clear. The interference fringes shifted to right-ward at the position of the dark-ring. It shows the negative dilation. The height at the center was measured to be 50 nm. In figure 4 (e) and (f), at the time of over 10 s, the relative reflectivity in the ablating area was decreased to be 3 %, and the interference fringe shifted to right-ward slightly at center of the crater. They show that the bottom of the crater become almost flat (slope of the landscape ~ 20 nm/ 100 μm, surface roughness ~ 9 nm) and the apparent boundary (rim structure) was produced at the edge of the crater.

3.2

Discussion 1; High-density excitation area (center of the ablating area)

We discuss the experimental result in the area of high-density excitation (> 0.5 J/cm²). The relative reflectivity at the center was about 60 % and 7 % at t = 64 ps and 150 ps, respectively. They imply that the surface roughness increased from 3 nm (t = 64 ps) to 8 nm (t = 150 ps). The interference fringe were continuous smoothly, though the surface roughness grew up. The shape of the expansion surface was a dome shape, and the peak height were measured to be 20 nm (t = 61 ps) and 50 nm (t = 153 ps), respectively. The expansion of the surface cannot be explained by the thermal expansion of solid Pt (~ 2 nm) and the increasing of the volume of molten Pt (~ 8.5 nm). Therefore, the density under the surface decreased by the formation of the nano-bubbles structures ^[10,11,12] and the surface roughness grew up by the nano-bubbles reached to the surface. The surface expansion speed was estimated to be 330 m/s. It was constant in the present experiment (20 nm / 61 ps and 50 nm / 153 ps). It shows the tensile stress of the expansion surface at this area was small compared with the internal pressure of the sample.

3.3

Discussion 2; Low-density excitation area (around the ablation threshold area)

The thin dark-ring in figure 4 (a) ~ (d) were observed at the position of the edge of the crater. The relative reflectivity at dark-ring was measured to be about 10 % and less than 5 % at t = 64 ps and 150 ps, respectively. They imply that the surface roughness increased from 7.5 nm (t = 64 ps) to 8.5 nm (t = 150 ps). From the interference fringe shift, a few nm positive dilation (t = 61 ps) and 5 nm negative dilation (t = 153 ps) were observed. From these results, the ablation process around the ablation threshold was quite different from the case of high-density excitation. The surface roughness at the ablation threshold grew up rapidly compared with the high-density excitation area, and the surface expands with the vibration. They imply that the nano-bubbles structures produced in a shallow layer of a few nm in depth, and the tensile stress of the expansion surface was important. The rapid peeling of the solid surface layer occurred by the tensile strain have been predicted from the molecular dynamics simulations; it called the spallative ablation ^[11,12]. The tensile strain is caused by the compressive pressure wave between the internal pressure wave and reflected wave at the surface, therefore the spallative ablation is required the solid surface and observed only under the condition that the pump laser fluence close to the ablation threshold. It is expected that the vibration of the surface at this area produced by the compressive pressure at the surface.