Figure shows the SEM images of the AZO/Ag/AZO structure irradiated with a single laser pulse of 1.7 J/cm2. An irradiated region can be clearly observed in Figure a with no damage in the surroundings or cracking in the glass substrate. Figure b illustrates the well-defined cutting edges that leave the bare substrate uncovered with a flat and clean surface. It should be noted that both edges present modulated profiles such as the ones obtained if a laceration occurred. This quite large rip (approximately 200 μm wide) ensures an excellent isolation between the not irradiated DMD structure and the central area of the laser spot (see Figure c). Such an isolation is further guaranteed by the trilayer lift off from the substrate at the line border, as evident from the cross-sectional SEM image reported in Figure d.
Figure 2 SEM micrographs of the irradiated AZO/Ag/AZO electrode. The laser irradiation is a single pulse, at a wavelength of 1,064 nm, duration of 12 ns and energy fluence of 1.7 J/cm2. The corresponding laser-irradiated spot size is 9.1 mm2. (a) Overview of the (more ...)
The structural modification of the central area of the laser spot was confirmed by means of the RBS technique. Figure compares the energy spectra of He+ backscattered by AZO/Ag/AZO samples outside and inside the irradiated region of Figure a. Three peaks are well distinguished in the as-deposited DMD. The one centred at 1.7 MeV is relative to He+ backscattered from Ag atoms, while the two peaks at 1.56 and 1.51 MeV are due to backscattering from the Zn atoms in the top and bottom AZO layers, respectively. Such a well-defined multilayer structure, present in the as-deposited DMD, disappears after laser irradiation, showing that both Ag and Zn atoms are now located at the surface (Ag signal shifted towards higher energy). The smaller area of Ag and Zn peaks after laser irradiation also indicates that a partial removal of these materials has occurred, while the broader shape of the signals is related to the loss of the sharp multilayer structure. This will have a noticeable effect on the electrical properties, as discussed in the following.
Energy spectra of He+ backscattered by AZO/Ag/AZO samples outside and inside the irradiated area. A scheme of the RBS experimental setup is reported in the inset.
Figure shows the separation resistance measured between two points, at a distance of 1.2 mm from each other, inside and across the laser spot, on our thin AZO/Ag/AZO sample irradiated with various laser fluences. With the increase of the pulse energy, the resistance measured inside the laser spot continuously increases up to saturation above 10 MΩ reached for fluence values higher than 3 J/cm2 ensuring a complete electrical insulation. This is due to the more efficient ablation and damage of the film with the laser power, as also indicated by the spot area reported in the top x-axis scale. The increase of the laser fluence implies a steeper temperature gradient across the multilayers resulting in a damage of the DMD structure, thus, in an electrical insulation, more and more pronounced. Most interestingly, the measured resistance values across the edge of the laser spot show an excellent insulation even at the lowest used beam fluence with an increase, with respect to the as-deposited multilayers, of more than 8 orders of magnitude. Such high separation resistance is maintained also for higher laser fluences and can be attributed to the occurrence of the DMD laceration, as showed in Figure b. Similar separation resistance was not observed in the case of a reference thick AZO layer, irradiated under the same condition and included in Figure for comparison. To understand how the separation resistance can be related to the laceration, a further description of the DMD irradiation process is needed.
Figure 4 Dependence of the separation resistance on laser fluences. The irradiated spot size enlargement, evaluated through SEM imaging, is reported on the top x-axis. The cyan dashed area corresponds to the situation of excellent separation resistances (≥ (more ...)
The DMD removal process with nanosecond pulse irradiation occurs in three consecutive steps: absorption of the laser energy at the transparent electrode/glass interface, steep temperature increase of the irradiated area, and fracture and damage of the continuous conductive multilayers. To accurately describe this process, a thermal model was applied [20
]. The time-dependent temperature distribution in the irradiated samples is calculated according to the heat conduction equation:
are the mass density, the thermal capacity and the thermal conductivity of the material, respectively. The recession velocity, vrec
, is neglected in view of relatively low laser fluences which are insufficient for heating of the considered materials above the melting threshold and, thus, to initiate thermal vaporization [17
]. The laser source term is given by
where α and R are the absorption and reflection coefficients of the material, respectively. Q(x,y) is the incident laser pulse intensity with a Gaussian spacial profile, and f(t) is the square-shaped pulse in the time domain:
Equation 1 is calculated for each layer of the structure using the material properties summarized in Table .
Figure shows the simulations of the thermal process (in XZ-plane) on two samples irradiated with a single pulse, at a wavelength of 1,064 nm, duration of 12 ns and the lowest used fluence of 1.15 J/cm2. The samples (both 90 nm thick on glass substrates) differ only for the presence of a 10-nm Ag mid-layer and are initially at room temperature. Interestingly, immediately after the laser pulse, the maximum temperature reached in the multilayer structure is 150 K higher than that in the single AZO film, probably due to the higher absorption coefficient of the noble metal material at this wavelength. This is also indicated by the temperature distribution centred at the Ag depth in Figure a with respect to Figure b where the highest value is located at the surface of the AZO film. The same can be claimed by observing the spatio-temporal curves, reported in Figure c,d. Here, the green lines indicate the temperature values after 10 ns from the beginning of the laser pulse, and it is clear as the temperature is higher for the DMD sample and how the maximum value coincides with the Ag location, whereas this is not the case for the single AZO film. Also, the evolution of temperatures with time is quite different for the two samples, with a faster cooling after the laser process for the multilayer sample. Such a behaviour can be related to the higher thermal conductivity of Ag with respect to AZO. In addition, the simulations performed on a 10 times thicker AZO film (not reported here) show that the maximum temperature reached after the laser pulse is similar to the ultra-thin DMD structure, but the cool down process is even slower. These observations indicate that a 10-nm-thin Ag mid-layer greatly affects the heat flow during and after the laser irradiation, with noticeable effects on film removal thresholds. In fact, we experimentally observed that for DMD thin film, a much lower laser energy fluence is required to induce the film cracking.
Figure 5 Simulations of the thermal process. Temperature distribution on 40-nm AZO/10-nm Ag/40-nm AZO on glass (a, c) and on 90-nm AZO on glass (b, d). The laser irradiation is a single pulse, at a wavelength of 1,064 nm, duration of 12 ns and energy fluence of (more ...)