LASER MICROVIA DRILLING AND ABLATION OF SILICON USING 355 NM PICO AND NANOSECOND PULSES Paper M507

LASER MICROVIA DRILLING AND ABLATION OF SILICON USING 355 NM PICO AND NANOSECOND PULSES Paper M507 Henrikki Pantsar 1, Hans Herfurth 1, Stefan Heinemann 1, Petri Laakso 2, Raimo Penttila 2, Yi Liu 3, Golam Newaz 4 1 Fraunhofer USA, Inc. Center for Laser Technology, 46025 Port Street, Plymouth, MI 48375 2 VTT Technical Research Centre of Finland, Tuotantokatu 2, 53850 Lappeenranta, Finland 3 Wayne State University, 65 Chemistry, 5101 Cass Ave, Detroit, MI 48202 4 Wayne State University, 2135 Engineering, 5050 Anthony Wayne, Detroit, MI 48202 Abstract Laser ablation of silicon has become an intense research topic due to the rapidly growing interest in laser processing in the photovoltaics and electronics industries. Different types of lasers are being used for edge isolation, grooving, drilling among other applications, with the pulse width ranging from the ultrashort femtosecond regime up to long microsecond pulses. The results may vary significantly depending on the wavelength and pulse width delivered by the laser source. In this study, two frequency triplicated Nd:YVO4 lasers, delivering pulses of width 9 to 12 ps and 9 to 28 ns, were used to drill holes and form grooves in silicon wafers. The thickness of the wafers was 200 µm. Groove depth and geometry were measured using an optical 3D profiling system. Results revealed that the material removal rate was greatly influenced by the pulse energy and repetition rate when the nanosecond pulsed laser beam was used. With picosecond laser beam the volumetric material removal rate remained rather constant in the range of 100 to 500 khz, but the groove width and depth varied. Scanning and transmission electron microscopy were used to characterize the drilled holes. Microstructures were investigated by selected area electron diffraction patterns. According to the measurements, nanosecond pulses induce not only thermal, but also mechanical damage to the hole walls, while picosecond processing only results in a thin HAZ layer, which is partially covered with amorphous nanoparticles. Introduction Laser micromachining of silicon is of particular interest in applications such as photovoltaic applications and microelectronics. Laser ablation involves numerous concurrent processes including heating, melting, vaporization and ionization as the beam interacts with solid, liquid, vapor and plasma phases at or near the material surface [1]. The process characteristics are determined by the intensity, duration and wavelength of the laser pulse. Commercially available lasers for micromachining include lasers with pulse durations in the femto, pico and nanosecond timescale. Typical wavelengths include variations from uv to near ir. Femtosecond pulses are optimal for material processing in many aspects. In the case of sub-ps ultrashort pulses, the duration of the pulse is less than the characteristic thermalization time of the material and machining can be done with very few thermal effects. Especially in the low-fluence regime in which the average ablation rate is determined by the optical penetration depth, the thermal effects are negligible and close to zero heat affected zones are experienced.[2,3,4] Another advantage of ultrafast processing is that the fs pulses terminate before any material is expulsed from the surface. The complete energy of the pulse is thus deposited to the sample target without any laser-plasma interaction during the pulse.[1,5] Since the heat conduction losses within the material are minimal and no plasma shielding occurs, the ablation threshold of materials is the lowest at subps pulse widths. Material can be removed at extreme precision using low pulse energies. As the pulse energy, or the fluence, is increased, thermal ablation processes become more dominant even with femtosecond pulses. The complete energy of the pulse is still delivered into the material, but the ablation depth is determined by the effective heat penetration depth instead of the optical penetration depth. Ablation quality is decreased but the ablated depth per pulse increases strongly [2]. For applications in machining, laser systems have to be reliable, robust and affordable. Since technical effort increases with shortening the pulse duration, the latter should be as short as necessary, only, for achieving a Page 278 of 430

satisfying result [6]. Nanosecond lasers fulfil the above criteria for the most part. The technology is well established and proven, rather simple in design and cost-effective. However, in some cases, the pulse is not short enough and the processing quality of these lasers does not meet the requirements. Picosecond laser sources have proven themselves as a compromize between the two aforementioned alternatives. Materials processing with laser pulses of width a few picosecond resembles much of that of high-fluence femtosecond processing. The ablation threshold is slightly higher than for fs-pulses, mainly due to heat conduction losses and plasma shielding [3]. At 1 ps pulses the plasma effects are negligible, raising up to a value of 20% at 10 ps during ablation of gold and similar findings have been obtained for silicon as well [1]. Overall, no drastic changes in terms of quality, thermal effects nor efficiency are observed when the pulse width remains less than 10 ps, even though the process can be considered to be purely thermal in nature [2,3,6,7]. In some cases the quality of ps processing can even exceed that of fs lasers. fs-laser induced pressure surges can cause mechanical damage to the material and lattice defects in silicon [8]. Nanosecond laser processing involves a complex mixture of concurrent physical processes. In contrast to femtosecond processing, the long pulse interacts with material in solid, liquid, vapor and plasma states. Considerable differences can be seen in the ablation process depending on the irradiance. For a given pulse energy, the maximum melt depth increases with longer pulses, i.e. lower irradiance (Al target)[7]. At the same time the recoil pressure, which is dependent on the irradiance [9], decreases causing incomplete melt ejection from the interaction area. In addition to these effects, the ablation threshold is higher than that observed using fs and ps pulses, mainly due to plasma shielding and greater heat conducton losses.[7] Studies comparing fs and ns pulses in drilling show even two times faster ablation rates for fs pulses in comparison to ns pulses (silicon, 266 nm radiation, 11 J/cm 2 ) [10,11]. However, at high fluence values, the rate of ablation with ns pulses increases strongly and exceeds that of fs and ps pulses [7]. During ns processing the mass ablation rate increases with laser power density following a power law dependence up to an irradiance of 0.3 GW/cm 2, almost independent of the target material (brass and glass, 248 nm KrF laser)[12]. At this point, plasma shielding starts to absorb the latter part of the pulse and the pulse becomes attenuated. Plasma will reflect and scatter the beam reducing ablation efficiency.[12] Experimental data shows that the ablation rate continues to increase in a linear fashion until an irradiance of 10 to 20 GW/cm 2 is reached [13,14,15,16]. At this point the ablation rate increases sharply. This behavior can be explained as homogeneous explosive boiling, which is responsible for ejection of large particles after a finite delay.[14,15,16] Overall, mass ejection during nanosecond ablation can be characterized by electron emission on a picosecond time scale, atomic/ionic mass ejection on a nanosecond timescale, and large particle ejection on a microsecond timescale, continuing up to tens of microseconds [16] When short nanosecond pulses or picosecond pulses are used, the irradiance is typically high enough to initiate plasma formation and result in plasma absorption. The plasma influence increases with the pulse duration, power density and wavelength. All of the energy absorbed by the plasma plume is not, however, lost from the process, but the plasma can in fact heat the target material [16]. If an ir laser is used, the beam mainly heats the peak of the expanding plume resulting in greater losses, whereas uv radiation mainly absorbs at the root of the plume delivering more energy to the material via plasma absorption [17]. Plasma absorption can also be exploited in some processes. When laser-induced plasma is formed in narrow bore drilling, hot plasma expands rapidly inside the channel and transports a large fraction of its energy by covection and radiation to the walls of the capillary, contributing to the radial expansion of the bore. This effect can stabilize ablation over a wide range of depths. [17] Drilling and ablation of silicon has been investigated in this study. The objective was to compare pico and nanosecond processing of silicon using 355 nm ultraviolet radiation. Based on previous referenced data pico and nanosecond laser sources would be in most cases the preferential choices for silicon processing and uv wavelength was selected to increase absorption, decrease the optical penetration depth into the underlying material, decrease losses due to plasma absorption and to reach a longer Rayleigh length together with a smaller focal spot diameter. Results have been evaluated based on optical measurements, SEM and TEM investigations. Experimental setup Experiments with nanosecond pulses were carried out using a q-switched Spectra-Physics HIPPO laser at 355 nm wavelength. The beam was delivered through a beam expander and a Scanlab Hurryscan 10 galvanometric scanner with 100 mm telecentric optics. The calcualted focal spot diameter with the setup was 10 µm. The pulse width of the laser varied with the Page 279 of 430

frequency being 10.2 ns at 50 khz, 18.6 ns at 100 khz and 28.4 ns at 200 khz. For the picosecond processing experiments a Lumera Rapid laser was used. The output wavelength of the beam was 355 nm. The optical setup comprised a beam expander and Scanlab Scangine 10 scanner with a 100 mm telecentric focusing lens. The calculated focal spot diameter for the optical setup was 10 µm. The pulse width of the laser was 9 to 12 ps. Laser power of 460 mw was used in all experiments. Material used for the experiments was 200 µm thick polished Ph-doped single crystalline silicon wafer. Samples were ultrasonically cleaned in acetone after processing. Loose particles and dust were swiped from the surface before optical measurement. Experiments for defining the ablation rate with ns and ps pulses were carried out by ablating grooves on silicon wafers with variable velocities and repetition rates. Groove profiles were measured using a Wyko NT3300 optical 3D profiling system. Holes were trepan drilled through the wafer using a specific beam path geometry to remove material more efficiently from the hole. The beam was programmed to move along a circle of 30 µm for 54 000 degrees, equal to 150 rotations. During this movement the beam was oscillated along a circular path at a frequency of 1500 Hz and an amplitude of 12 µm. The drilling time was 0.78 s. Focal position was set to the surface for the time of the drilling. Since the beam movement was created using scanner mirrors, it is unknown how precisely the beam follows the programmed path. The beam motion is presented in Figure 1. All experiments were carried out in ambient air. Figure 1. Beam movement during drilling. Yellow area shows the spot size, ablated area is shown in gray. The morphology of the holes was recorded by Hitachi S-2400 Scanning Electron Microscope (SEM) operating at 25kV. The microstructure on the edge of the holes was studied by JEOL FasTEM Transmission Electron Microscope (TEM) operating at 200kV. The TEM is equipped with an Electron X-Ray Dispersive Spectrometry (EDS). For TEM sample preparation, the holes were filled with M-Bond 610 epoxy to protect the wall of the holes not being removed by ion-beam milling as suggested in the literature[8]. The disks were then cured for two hours at 120 C. Both sides of the disks were ground by sand paper from 600 Grit down to 2400 Grit. The final thickness of the disks were about 40-70µm. Since the thinned disks are very fragile, they were glued to copper rings in order to obtain support. The disks were finally polished by ionbeam milling machine (Gatan 691 Precision Ion Polishing System-PIPs) at 5kV with 6 tilting until the glue area not being fully removed. Grooves on silicon Results and discussion Grooves were ablated on silicon surfaces at velocities of 20, 30, 45, 65, 100, 150, 225, 350 and 500 mm/s. Repetition rates for the nanosecond laser were varied between 20 and 200 khz, and for the picosecond laser from 100 to 500 khz. The nanosecond laser could not deliver the 460 mw power above 200 khz frequency and available power from the picosecond laser was limited below 100 khz. The ablation process was limited by the scanning velocity and frequency in two ways. First, the pulse to pulse overlapping had a minimum limit below which material expulsion from the groove was incomplete and significant amounts of silicon oxides started to form inside the groove. The upper limit for the scanning velocity was set by the maximum pulse to pulse distance, above which pulses form separate spots on the surface instead of a continuous groove. For nanosecond processing it was found that in the whole parameter range from 20 to 200 khz, clean consistent grooves with no oxide formation were achieved only when the pulse overlapping was less than 80 to 90%. The process tolerated greater overlapping when the pulse energy was low, i.e. the frequency was high. The feasible parameter area for picosecond processing was wider. The pulse overlapping at 100 and 200 khz frequencies could be up to 97% before oxide formation started to interfere with the process. Due to the parameter limits of the two lasers, a head to head comparison could be done only in the frequency Page 280 of 430

range of 100 to 200 khz. Grooves ablated at these frequencies were measured in more detail to provide information about the groove depth and ablation rate. In addition to these, nanosecond experiments were run also at 50 khz repetition rate and picosecond experiments were continued up to 500 khz repetition rate. Scanning velocity was set to 225 mm/s. The profile of the groove was measured across the ablated line to reveal the depth and cross section area of the ablated and recast materials. The term groove volume here in after refers to the volume ablated below the original surface. The term removed material refers to the amount of silicon removed completely from the source; i.e. groove area minus recast area. Volume values here are presented in the units of µm 3, which is the area in question measured from the cross section multiplied by a length of 1 µm along the longitude of the groove. Since the profiles are derived from a line measurement across the groove and not from a measurement of the actual volume, the results are not precise. However, they represent a good estimation of the average cross section of the grooves. Results show that the ablation rate with nanosecond pulses was significantly impacted by the frequency or the pulse energy, whereas the ablation rate with picosecond pulses was independent of the frequency within the tested parameter area. With nanosecond pulses, the groove volume increased markedly with the pulse energy. 50 khz repetition rate, equaling to 9.2 µj pulse energy, created a groove with a cross sectional area of 26.3 µm 2. At this fluence the amount of recast was small and the removed volume measured from the cross section of the groove was 24.2 µm 3. Increasing the frequency resulted in a groove geometry, which was narrower and shallower than that created with higher pulse energies. Also the relative volume of recast compared to the groove volume increased significantly. At 200 khz repetition rate (2.3 µj) the groove volume was 5.8 µm 3 and taking the recast into consideration, the volume of the removed material was only 4.0 µm 3. In this case more than 30% of the material removed from the groove was being recast on the edges of the groove and not ablated away. The depth of the groove fluctuated significantly between 0 to 3.5 µm. Therefore, the profile for the 200 khz sample was derived from an average value of three individual measurements, in order to obtain a better estimation of the ablated volume. The cross sections of the grooves ablated with nanosecond pulses are presented in Figure 2. Grooves ablated at 225 mm/s scanning velocity using 50 and 200 khz repetition rates are presented in Figure 3 and Figure 4, respectively. Figure 2: Measured cross sections of grooves ablated with the nanosecond laser. Figure 3. Groove ablated by nanosecond pulses. Scanning velocity 225 mm/s, repetition rate 50 khz. Figure 4. Groove ablated by nanosecond pulses. Scanning velocity 225 mm/s, repetition rate 200 khz. As the line energy in each case was equal, a substantially greater part of the laser energy was being lost in the ablation process when the repetition rate was increased gradually from 50 to 200 khz. This increase Page 281 of 430

in the frequency caused the pulse width to change from 10.2 ns to 28.4 ns and the pulse energy to decrease from 9.2 to 2.3 µj. Both of these factors reduced the mean irradiance in the area of the beam, which changed from 1.15 to 0.10 GW/cm 2. At the same time, the process became more unstable and fluctuations in the groove depth and width were more evident. Longer pulses can be absorbed into or reflected from the laser induced plasma in a greater extent. The threshold for plasma formation for many materials is in the approximity of 0.3 GW/cm 2 [12]. Since the average irradiance at 200 khz was only 0.10 GW/cm 2 and the peak irradiance at the center of the beam was 0.2 GW/cm 2, plasma shielding should not play a role at higher repetition rates, but rather at low frequencies. Particles hovering above the interaction point can, however affect the ablation process, especially at higher repetition rates. The extent of such inter-pulse plasma/plume effects could not be estimated based on the conducted experiments. threshold, processing with nanosecond pulses becomes substantially more efficient. Figure 5: Measured cross sections of grooves ablated with the picosecond laser. More likely causes for low material removal rates at high frequencies are related to the pulse irradiance. Working closer to the ablation threshold with longer pulses leads to a situation where a greater part of the pulse energy is being used to heat the material in the solid and liquid phases than to evaporate and remove material. At the same time the recoil pressure, which is proportional to the irradiation [9,18], is decreased reducing melt expulsion from the groove. Material removal with ns pulses was approximately twice as efficient than with picosecond pulses when the repetition rate was 100 khz (4.6 µj pulse energy). Nanosecond pulses created a groove volume of 16.7 µm 3 compared to the 7.9 µm 3 of picosecond pulses. At 200 khz, the grooves became approximately equal in volume with the picosecond groove being 6.2 µm 3 in volume and the nanosecond groove 5.8 µm 3. However, a lesser amount of recast silicon was present at picosecond groove edges and the absolute material removal with picosecond pulses was 5.8 µm 3 and 4.0 µm 3 with nanosecond pulses. The cross sections of the grooves for picosecond experiments are presented in and Figure 5. The removed volumes and and groove volumes are presented as a function of the repetition rate and pulse energy in Figure 6. Similar findings about the relationship between pulse duration and removal rates have been obtained using a twotemperature model for aluminum ablation [19]. Picosecond laser ablation is more efficient compared to nanosecond ablation when operating slightly above the ablation threshold of nanosecond pulses. When the laser fluence notably exceeds the nanosecond ablation Figure 6: Cross section areas for grooves and removed material. The repetition rate had only a slight effect on the material removal rate with picosecond pulses and these changes can be approximated to be within measurement errors. The removed volume was in all cases between 5.8 and 6.7 µm 3 and the recast volume was in each case less than 10% of the removed material volume. As the irradiance at 100 to 500 khz frequencies far exceeds the ablation threshold of silicon, the ablation rate is related to the line energy rather the than pulse energy, as experienced during nanosecond processing. The main difference between grooves machined at low or high repetition rates was the width of the groove, making grooves ablated at high repetition rates deeper. The groove ablated at 500 khz showed an surface area Page 282 of 430

of width 15 µm, where laser treatment is visible. At 300 and 200 khz, the width of this area was 16 and 18 µm, respectively. When the frequency was reduced to 100 khz, the width increased to 25 µm, with traces of laser ablation up to 20 µm from the centerline of the track. Similar effects were seen in tracks ablated at lower scanning velocities of 100 and 150 mm/s as well. Widening of the ablated track with increasing pulse energy can be partially explained by the increase of effective spot diameter, i.e. the portion of the gaussian profile laser beam, in which the irradiation exceeds the ablation threshold. According to calculations, the effect of the effective beam diameter should be only in the range of few microns. A more likely cause for this effect would be plasma absorption and beam scattering. Tracks ablated at 500 and 100 khz frequency are presented in Figure 7and Figure 8, respectively. Holes in silicon Holes were drilled through 200 µm silicon wafer using a scanning path shown in Figure 1. The linear velocity of the beam was 20 mm/s and the circumferential velocity along the oscillated path was approximately 115 mm/s. Initially holes were drilled with both lasers at a 100 khz repetition rate resulting in a pulse energy of 4.6 µj. Incomplete expulsion of melt and ablated material limited the use of these parameters in nanosecond laser drilling. At the used circumferential velocity, the pulse to pulse overlap was close to 90% and as seen from the groove experiments, the nanosecond laser required less than 80% overlapping to ablate material efficiently. At 100 khz the hole became filled with silicon dioxide blocking and scattering the incoming laser beam and through penetration could not be achived. Frequency was reduced to 30 khz in order to create clean through holes in the sample. This resulted in a 333% increase in the the pulse energy and reduction in the pulse width from 18.6 to approximately 9 ns. Overall, the average intensity across the area of the beam was increased by a factor of 7 to a value of 2.2 MW/cm 2. Peak intensity thus reached a value of 4.3 MW/cm 2 at the center of the gaussian profile beam. Holes drilled with nanosecond and picosecond pulses are presented in Figure 9 and Figure 10, respectively. Drilling time was 0.78 s in both cases. The differences in the hole entrance diameters result from differences in the scanner performances. Figure 7. Profile of groove ablated with ps pulses at 500 khz repetition rate and 225 mm/s scanning velocity. Figure 9. Entrance (left) and exit (right) of a hole drilled using nanosecond pulses. Pulse energy 15.3 µj. Figure 8. Profile of a groove ablated with ps pulses at 100 khz repetition rate and 225 mm/s scanning velocity. Preliminary investigation of the entrance side shows that both holes were rather similar in quality. The main difference was that the resolidification formations in the nanosecond processed samples were axially deposited, whereas the picosecond processed sample showed radial rings around the hole walls. The exit sides revealed larger differences depending on the Page 283 of 430

pulse width. The nanosecond hole walls were covered with what appears to be a recast layer. But in the case of the picosecond laser, the hole walls near the exit of the hole are very smooth and show no signs of any resolidified material. Longer drilling time would have resulted in a more circular/elliptic exit hole geometry with picosecond pulses. In both cases the beam was shut off after 150 revolutions with basically no refinishing. dislocation direction was always perpendicular to the surface of the hole. The dislocations are located in the single crystal silicon and may arise from thermally induced stresses experienced during drilling. As shown in Figure 12, the area marked A contained some small grains which are crystallines as indicated by selected area electron diffraction (SAED) patterns, Figure 12 b). EDS analysis from A area showed that this area contained only Si. The reason of the formation of these small grains is unknown. However there are two possibilities; one is that they recrystallized from the recast material first melted by the nanosecond pulses, the other is that the area A was broken down into small grains directly from the Si wafer. Figure 10. Entrance (left) and exit (right) of a hole drilled using picosecond pulses. Pulse energy 4.6 µj. TEM observations from the center of the 200 µm wafer indicated that the microstructure at the edges of the holes fabricated by picosecond and nanosecond pulses were totally different. Figure 11 shows that defects (dislocations) were introduced by nanosecond drilling, while the main feature in the picosecond pulse drilled hole was a layer of nanoparticles adjacent to the hole wall. Figure 12. a) Dislocations on the edge of the holes introduced by nanosecond pulsed laser beam. b) selected area electron diffraction pattern from area A. Observation on another area in the sample drilled by nanosecond pulses is shown in Figure 13. SAED pattern obtained from area B shows that the nanoparticles in this area were mainly Si nanoparticles although EDS spectrum also showed a small amount of O in this area. Oxygen might have been contributed by the glue, or a small amount of SiO 2. In Figure 14, the area marked as D shows amorphous features containing Si and a small amount of O, which might be also contributed from glue area. Figure 11. The microstructure of edge areas of holes fabricated by nanosecond pulses (left) and picosecond pulses (right). Figure 12 shows the dislocations introduced by nanosecond pulsed laser beam. It was found that the Page 284 of 430

Figure 13. a) Another area at the edge of a hole drilled by nanosecond laser pulses, b) SAED patterns from area B. was outlined by a 50 to 100 nm thick layer. This layer appeared similar to the melt film described in previous publications [8]. It can thus be assumed that the film was molten silicon which has resolidified into an amorphous state. The film is shown in Figure 15 with arrows. Nanoparticles with a diameter around 100 nm were found in the glue close to the resolidification layer, Figure 15. The selected area electron diffraction pattern (SAED) from the area containing nanoparticles shows amorphous feature, indicating that the nanoparticles were noncrystallines, Figure 15 b). As indicated by EDS analysis, Figure 16, the glue area contained C, O and small amount of Cl, while Si detected from the glue area should have come from the Si wafer. Cu (peak not visible in Figure 16) should have come from the copper ring glued onto the sample. In the nanoparticle area, as indicated by Figure 16 b), EDS analysis shows Si, C and O. Although the C and O may come from the glue, comparison between the ratio of C and O in the glue area and the ratio of C and O in the nanoparticle area suggests that, at least part of the amorphous nanoparticles have been deoxidized. The SAED pattern from the edge area of the hole show diffraction pattern of single crystal, Figure 15 c). Figure 14. Dislocations and amorphous Si at the edge of a hole drilled by nanosecond pulses. SAED patterns of areas C and D are shown. Even though nanosecond pulses produce thermal and mechanical damage to the hole walls, the thickness of the damaged layer between the outermost layer of the modified material and single crystalline silicon was in all investigated locations less than 1 µm. This suggests that the high recoil pressure generated by low repetition rate uv laser pulses removes melt efficiently from the hole and no significant recast layer is formed on the wall of the hole. It is also possible that due to the 355 nm wavelength, only a small amount of heat convection to the hole walls is generated through plasma absorption, and the heat affected zone remains thin. Figure 15. Microstructure analyses on the edge of the hole drilled by picosecond pulses. a) Nanoparticles at the edge of the hole, and selected area electron diffraction patterns from b) nanoparticle area and c) Si wafer. Figure 15 shows a close inspection on the edge of hole fabricated by the picosecond laser beam. The silicon wafer was undamaged and no mechanical defects were found in TEM investigations. The single crystal silicon Page 285 of 430

Picosecond ablation did not exhibit a similar relationship between the ablation rate and the pulse energy. The ablation rate remained essentially similar between repetition rates of 100 and 500 khz, which correlate to 4.6 and 0.9 µj pulse energies, respectively. The main effect of the pulse energy was the width of the ablated line, which increased with increasing energy. Figure 16. EDS analyses on a) glue area, b) nanoparticles and c) Si wafer area. Based on TEM investigations it can be concluded that in comparison to nanosecond pulses, picosecond processing causes negligible thermal effects to the parent material with no signs of mechanical damage. Nanosecond processing generates both thermal and mechanical damage to the hole walls in the form of dislocations, recast and recrystallized material, while picosecond drilling causes only a thin, <100 nm, resolidification layer to the wall of the hole. The surface became partially covered by amorphous nanoparticles, which presumably consist of at least partially oxidized silicon. All these observations indicate that more processes, which originate from greater heat input into the material, occur during nanosecond pulsed drilling than during picosecond pulsed drilling. Conclusions Grooves and holes have been fabricated in 200 µm single crystalline silicon wafers using 355 nm nanosecond and picosecond pulsed lasers. Processing results have been measured and characterized using optical measurements, TEM microscopy and SEM microscopy. Results show that the ablation rate is substantially affected by the pulse energy during nanosecond ablation. The increase in material removal rate was more than 600% when the pulse energy was increased from 2.3 to 9.2 µj by decreasing the frequency from 200 to 50 khz. Thermal losses have a major effect on the removal rate at irradiances close to the ablation threshold, as greater fraction of the pulse is heating the material in the solid and liquid phases instead of evaporating and removing material. Therefore, the dependence between pulse energy and material removal rate can be expected. The nanosecond ablation efficiency exceeded that of picosecond ablation at 100 khz frequency, but at 200 khz frequency the material removal rate of the ps laser was faster. In both processes, drilling and groove ablation, the optimal parameter area for nanosecond ablation was at less than 100 khz repetition rate, where as the picosecond laser delivered good results at 100 khz and above. Evaluating by SEM pictures, the quality of holes drilled with nanosecond and picosecond pulses was rather similar. When the nanosecond laser was operated at 30 khz and the picosecond laser at 100 khz frequency, the drilling times were equal. Nanosecond laser drilling became slower and eventually impossible when the repetition rate was increased. The pulse overlapping exceeded the defined feasible value of 80% and also the resulting low pulse energy and irradiance were inadequate in removing material from the capillary, presumably due to a decreased recoil force. TEM investigations showed that nanosecond laser drilling resulted in thermal and mechanical damage to the silicon wafer. The affected layer on the hole wall was up to 1 µm in thickness and contained amorphous features, polycrystalline silicon as well as monocrystalline areas with dislocations. Picosecond pulsed drilling was not found to cause mechanical damage to the material. The hole was outlined by a thin layer, which supposedly consists of amorphous resolidified silicon. The thickness of the layer was 50 to 100 nm. No further damage to the material was found. References [1] Pronko, P.P; Dutta, S.K; Du, D. (1995) Thermophysical effects in laser processing of materials with picosecond and femtosecond pulses, J. Appl. Phys 78 (10) 6233-6240. [2] Le Harzic, R; Breitling, D; Weikert, M; Sommer, S; Föhl, C; Valette, S; Donnet, C; Audouard, E; Dausinger, F. (2005) Pulse width and energy influence on laser micromachining of metals in a range of 100 fs to 5 ps, Appl. Surf. Sci 249, 322-331. Page 286 of 430

[3] Nolte, S; Momma, H; Jacobs, H; Tünnermann, A, Chichkov, B.N; Wellegehausen, B; Welling, H. (1997) Ablation of metals by ultrashort laser pulses, J. Opt. Soc. Am. B14, 2716-2722. [4] Nolte, S. (2003) Micromachining, in Fermann, M.E; Galvanauskas, A; Sucha, G (eds) Ultrafast lasers, Marcel Dekker Inc, 359-394. [5] Le Drogoff, B; Margot, J; Chaker, M; Sabsabi, M; Barthélemy, O; Johnston, T. W; Laville, S; Vidal, F; von Kaenel, Y. (2001) Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys, Spectrochim. Acta B, 56, 987-1002. [6] Dausinger, F; (2003) Femtosecond technology for precision manufacturing: Fundamental and technical aspects, RIKEN Review No. 50, 77-82. [7] Breitling, D; Ruf, A; Dausinger, F. (2004) Fundamental aspects in machining of metals with short and ultrashort laser pulses, Proc. SPIE 5339, 49-61. [8] Kaspar, J; Luft, A; Nolte, S; Will, M; Beyer, E. (2006) Laser helical drilling of silicon wafers with ns to fs pulses: Scanning electron microscopy and transmission electron microscopy characterization of drilled through holes, J. Laser. Appl. 18, 85-92. [15] Yoo, J.H; Jeong, S.H; Greif, R; Russo, R.E. (2000) Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon, Appl. Phys. Lett. 76, 783-785. [16] Russo, R.E; Mao, X.L; Liu, H.C; Yoo, J.H; Mao, S.S. (1999) Time-resolved plasma diagnostics and mass removal during single pulse laser ablation, Appl. Phys. A 69[Suppl.], S887-S894. [17] Breitling, D; Schittenhelm, H; Berger, P; Dausinger, F; Hügel, H. (2001) Shadowgraphic and interferometric investigations on Nd:YAG laserinduced vapor/plasma plumes for different processing wavelengths, Proc. SPIE 4184, 534-538. [18] Miyamoto, I; Asada, S; Sano, T; Ohmura, E. High speed drilling of thin silicon wafer by uv laser, in Proceedings of the 20 th International Congress on Applications of Lasers and Electro-Optics, Jacksonville, Fl, USA, 1612-1619. [19] Ruf, A. (2004) Modellierung des Perkussionsbohrens von metallen mit kurz- und ultrakurzgepolsten lasern, Ph.D. Thesis, University of Stuttgart, Germany. [9] Lee, D.J; Jeong, S.H. (2004) Analysis of recoil force during Nd:YAG laser ablation of silicon, Appl. Phys. A 69, 1341-1344. [10] Zeng, X; Mao, X.I; Greif, R; Russo, R. (2005) Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon, Appl. Phys. A 80, 237-241. [11] Zeng, X; Mao, X; Greif, R; Russo, R.E. (2004) Ultraviolet femtosecond and nanosecond laser ablation of silicon: Ablation efficiency and laser-induced plasma expansion, Proc. SPIE 5448, 1150-1158. [12] Mao, X; Russo, R.E. (1997) Observation of plasma shielding by measuring transmitted and reflected laser pulse temporal profiles. Appl. Phys. A 64. [13] Karnakis, D.M. (2006) High power single-shot laser ablation of silicon with nanosecond 355 nm, Appl. Surf. Sci 252, 7823-7825. Meet the author Dr. Henrikki Pantsar is a Senior Engineer at the Fraunhofer Center for Laser Technology in Plymouth, MI. He holds Master and Doctor of Science degrees in Mechanical Engineering from the Lappeenranta University of Technology in Finland. He has previously worked in the laser materials processing research groups at the Lappeenranta University of Technology and VTT Technical Research Centre of Finland. He joined Fraunhofer CLT in 2007 where he specializes in laser micromachining and laser processing for alternative energy device manufacturing. [14] Yoo, J.H; Jeong, S.H; Greif, R; Russo, R.E. (2000) Explosive change in crater properties during high power nanosecond laser ablation of silicon, J. Appl. Phys, 88, 1638-1649. Page 287 of 430