为了正常的体验网站,请在浏览器设置里面开启Javascript功能!

laser_microvia_drilling

2011-08-26 10页 pdf 2MB 18阅读

用户头像

is_699571

暂无简介

举报
laser_microvia_drilling LASER MICROVIA DRILLING AND ABLATION OF SILICON USING 355 NM PICO AND NANOSECOND PULSES Paper M507 Henrikki Pantsar1, Hans Herfurth1, Stefan Heinemann1, Petri Laakso2, Raimo Penttila2, Yi Liu3, Golam Newaz4 1 Fraunhofer USA, Inc. Center for Laser Tec...
laser_microvia_drilling
LASER MICROVIA DRILLING AND ABLATION OF SILICON USING 355 NM PICO AND NANOSECOND PULSES Paper M507 Henrikki Pantsar1, Hans Herfurth1, Stefan Heinemann1, Petri Laakso2, Raimo Penttila2, Yi Liu3, Golam Newaz4 1 Fraunhofer USA, Inc. Center for Laser Technology, 46025 Port Street, Plymouth, MI 48375 2VTT Technical Research Centre of Finland, Tuotantokatu 2, 53850 Lappeenranta, Finland 3 Wayne State University, 65 Chemistry, 5101 Cass Ave, Detroit, MI 48202 4Wayne 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 sub- ps 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 ICALEO® 2008 Congress Proceedings Laser Microprocessing Conference 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/cm2) [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/cm2, 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/cm2 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 ICALEO® 2008 Congress Proceedings Laser Microprocessing Conference 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 ion- beam milling machine (Gatan 691 Precision Ion Polishing System-PIPs) at 5kV with 6° tilting until the glue area not being fully removed. Results and discussion Grooves on silicon 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 ICALEO® 2008 Congress Proceedings Laser Microprocessing Conference 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 µm3, 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 µm2. At this fluence the amount of recast was small and the removed volume measured from the cross section of the groove was 24.2 µm3. 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 µm3 and taking the recast into consideration, the volume of the removed material was only 4.0 µm3. 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 ICALEO® 2008 Congress Proceedings Laser Microprocessing Conference 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/cm2. 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/cm2[12]. Since the average irradiance at 200 kHz was only 0.10 GW/cm2 and the peak irradiance at the center of the beam was 0.2 GW/cm2, 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. 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 µm3 compared to the 7.9 µm3 of picosecond pulses. At 200 kHz, the grooves became approximately equal in volume with the picosecond groove being 6.2 µm3 in volume and the nanosecond groove 5.8 µm3. However, a lesser amount of recast silicon was present at picosecond groove edges and the absolute material removal with picosecond pulses was 5.8 µm3 and 4.0 µm3
/
本文档为【laser_microvia_drilling】,请使用软件OFFICE或WPS软件打开。作品中的文字与图均可以修改和编辑, 图片更改请在作品中右键图片并更换,文字修改请直接点击文字进行修改,也可以新增和删除文档中的内容。
[版权声明] 本站所有资料为用户分享产生,若发现您的权利被侵害,请联系客服邮件isharekefu@iask.cn,我们尽快处理。 本作品所展示的图片、画像、字体、音乐的版权可能需版权方额外授权,请谨慎使用。 网站提供的党政主题相关内容(国旗、国徽、党徽..)目的在于配合国家政策宣传,仅限个人学习分享使用,禁止用于任何广告和商用目的。

历史搜索

    清空历史搜索