NCLA - Ultrafast Laser Materials Processing

Observations with the SEM initially recorded damage to the Silicon crystalline lattice at laser fluences of 0.17Jcm^-2. At this point a periodic ripple structure was observed on the surface in the irradiated region as shown in figure 1. The spatial frequency of these ripples was comparable to the wavelength of the laser beam.

Figure 1. SEM Image of the ripple structure observed with 120mm focal length lens at the ablation threshold

Analysis found that the direction of the ripples is independent of the orientation of the crystalline lattice but is dependent on the polarization axis of the laser beam as shown in figure 2. This is because the ripples always remain at right angles to the polarization axis of the laser beam even when the sample is rotated.

Figure 2. Scanning Electron Microscope (SEM) image of the ripple formation on a sample rotated at right angles.

This structure is a recorded phenomenon and was observed with numerous other materials. Experimentation found that the wavelength of the ripples varies as a function of the angle of incidence between the incident laser beam with wavelength and normal to the target surface

(Eq. 1)

EDX analysis of the ripple structure revealed that the amorphous Silicon that had solidified into the ripple structure was oxidized, refer figure 3. This indicated that this region of the irradiated Silicon was sufficiently heated by the laser so that it was able oxidize with the surrounding air.

Figure 3. EDX analysis result indicating oxidation of the silicon at the ripple structure.

Here, general EDX analysis was done because the resolution of the X-Ray signal was insufficient to analysis each crest independent of the surrounding area. No trace elements of oxygen were found in areas that were not exposed to the laser beam

This result in conjunction with the SEM images indicated that thermal processes were present at the ablation threshold for Silicon (0.17Jcm^-2)with pulses of 150fs.

Laser Fluences 0.17Jcm^-2 – 0.35Jcm^-2:

Further increases in the laser fluence above the ablation threshold level show that the ripple structure starts to breakdown, see figure 4. This is a result of higher amounts of energy being transferred to the Silicon atoms and then removing them from the bulk material.

Figure 4. Series of Scanning Electron Microscope (SEM) images showing the transformation from the periodic array of ripples to the formation of a crater by the removal of Silicon particles.

Compositional analysis of the Silicon structure at this stage indicates that the majority of the oxidized material occurs in the debris around the laser hole. In addition to this, trace amounts of carbon were also detected in the surrounding re-deposited material. One dimensional EDX spectral analysis across the length of the laser hole reveals the relative distribution of each element across the periphery of the laser hole as illustrated in figure 5.

Figure 5. EDX analysis showing the one dimensional distribution of carbon and oxygen around the periphery of the laser pulse.

This elemental distribution indicates that as the ablated material loses its energy and becomes deposited in and around the edge of the laser hole, it starts to oxidize in the atmosphere and also combine with other elements. A more detailed analysis on the origin of the carbon signal can be linked to the particle debris resting on the surface of the surrounding Silicon wafer, refer figure 6.

Figure 6. EDX analysis indicating that higher percentages of carbon is found in the Particle debris

Laser Fluences 0.35Jcm^-2 – 1.5Jcm^-2

At higher laser fluences the structure of the laser hole becomes better defined with most of the material irradiated with the laser having sufficient energy to become ablated. This ablated material can be observed at the upper edges of the laser hole and the surrounding area as shown in figure 7. Closer examination of the internal surface of the laser hole still finds evidence of thermal processes, as the molten Silicon can be seen to flow in accordance with the pressure gradients created by laser machining by each successive pulse. These thermal processes at the side of the laser hole can be attributed to the tail end of the Gaussian profile of the laser pulse, which does not have sufficient energy to ablate all the material it irradiates. At this point conventional vertical imaging of the laser micro-machined features yields insufficient information on the narrow depth profile of the holes. This problem is particularly noticeable with optical microscopy. However electron microscopy in conjunction with mechanical cross sectioning does have the resolution and depth of focus required to observe these structures.

Figure 7. Scanning Electron Microscope (SEM) images of a laser machined hole.

Laser Fluences 1.5Jcm^-2 – 5Jcm^-2

At higher fluences the laser holes have a greater depth and vertical profile. More ablated material is being deposited in the surrounding area and the lower pressure regions at the top of the laser hole. This is a result of the ablated material from the bottom of the laser hole approximately undergoing adiabatic expansion into the larger volume at the upper regions of the laser hole and cooling down. At the lower sections of the laser hole where no material has a chance to become deposited, all the material that was exposed to the laser has become ablated as evidenced in figure 8.

Figure 8. Scanning Electron Microscope (SEM) images of a mechanically prepared cross-section of a high fluence laser hole.

As an additional indication of the thermal effects occurring at different depths along the laser hole, point EDX analysis was carried out to determine the level of oxidation. This is because oxidation can be attributed to thermally heated Silicon becoming in immediate contact with air. Initially analysis at the top of the laser hole in the region where the ablated material was being deposition was carried out, and extensive oxidation and carbonization was observed. This observation can be directly related to the deposited material in that region in figure 9.

Figure 9. EDX Analysis showing the presence of carbon and oxygen in the recast material

Further down the laser hole the percentage of deposited material decreases and as a result so does the quantity of oxygen and carbon detected (figure 10.).

Figure 10. Further down the laser hole, the percentage of oxygen decreases and carbon is completely absent.

At the base of the laser hole, there was a complete absence of oxygen and carbon (figure 11.) in that region. SEM cross sectional imaging found limited evidence of any thermal processes in the form of Silicon in the molten phase. The absence of Oxygen at the base of the laser hole also indicates that the remaining Silicon material here was not sufficiently heated to bond with Oxygen once the laser machining ended. This observation can be explained by the fact that the laser pulse at higher fluences had sufficiently high energy to remove all the material exposed to the laser beam independent of the Gaussian cross sectional distribution of the laser pulse. The Oxygen and Carbon detected at the top of the laser hole is a result of the ablated material cooling down and combining with atmospheric elements. In the lower sections of the laser hole where ablation exceeds deposition, analysis found no evidence of any thermal oxidation in addition to SEM cross sectional imaging finding no characteristic structures associated with Silicon in the molten phase.

Figure 11. At the base of the laser hole only silicon could be detected

Conclusions on thermal damage to Silicon with 150fs laser pulses

At low laser fluences there is insufficient energy at all points along the Gaussian cross sectional distribution to remove all the material exposed to the laser and therefore the material that is not removed will dissipate its energy when cooling down to produce the observed internal thermal effects. EDX analysis of these molten thermal effects indicated the presence of Oxygen in that region and Carbon with the deposited particulate matter.

At higher fluences the reduction in molten material within the laser irradiated region and the more pronounced and cleaner nature of the laser hole can be explained by the fact that the entire laser beam independent of the Gaussian profile has sufficient energy to remove all material it irradiates. These observations made with SEM cross sectional imaging were also supported by EDX analysis. For higher laser fluences at the base of the laser hole there was no evidence of oxidation. Only at the top of the laser hole, where material has become deposited was Oxygen and Carbon found.