Selective Laser Ablation Research Articles

Laser Ablation Mechanism Of Silicon Nitride Layers In A Nanosecond UV Regime

Selective laser ablation of silicon nitride (SiNx) layers is a crucial technological step to achieve alternative front side metallization like electrochemical contact. In this work, we discuss the mechanism of laser ablation with a nanosecond UV laser source. A model with two thresholds corresponding to the melting threshold of silicon and the ablation threshold of silicon nitride is proposed. A finite element method is used to solve the heat transfer equation and describe the ablated SiNx surface for a single laser pulse. Numerical results are compared to optical microscopy measurements of the ablated zone.

Selective Ablation of Thin Films by Ultrashort Laser Pulses

Laser ablation of bulk solid could obtain superior machining accuracy by utilizing ultrashort pulses due to the smaller laser-induced thermal diffusion length, which minimizes the heat affected zone. In selective laser ablation of thin films, the precise control of the heat affected zone in the vertical direction becomes critical in order to avoid the damage of the substrate. For this application an effective thermal penetration depth can be defined determined by not only the optical penetration depth, but also the lattice thermal diffusion length in short pulse ablation or the hot- electron penetration depth in case of ultrashort laser ablation. The ablation characteristics, such as the threshold fluence and the multi-pulse incubation effect, are strongly dependent on the film thickness when it is in the range of the effective thermal penetration depth.

Local aluminum–silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm 2 on a phosphorous diffused emitter with a peak doping density of 2×10 20 cm −3. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.

A new route to graphene layers by selective laser ablation

Selectively creating regions of spatially varying thickness may enable the utilization of the electronic properties of N-layer (N=1 or more) graphene and other similar layered materials (e.g., topological insulators or layered superconductors) for novel devices and functionalities on a single chip. The ablation threshold energy density increases dramatically for decreasing layer numbers of graphene originating from the dimensional crossover of the specific heat. For the 2D regime of graphite (up to N≈7) the dominant flexural mode specific heat (due to its N-1 dependence) gives rise to a strong layer number-dependence on the pulsed laser ablation threshold energy density, while for 3D regime (N>>7) the ablation threshold saturates due to dominant acoustic mode specific heat. As a result, several energy density windows exist between the minimum energy densities that are required for ablating single, bi, or more layers of graphene, allowing layer number selectivity.

Investigation of ablation mechanisms for selective laser ablation of silicon nitride layers

Abstract In this work SiNX deposited on silicon was locally ablated using laser irradiation. The focus was set on the investigation of the ablation mechanisms where a picosecond (ps) pulse laser is used with three wavelengths 1064, 532, 355 nm. The ablated areas were characterized by light microscopy and the threshold fluences were determined for various layer thicknesses. Furthermore, four-probe sheet-resistance and SunsVoc measurements were conducted. Light microscopy images were taken and compared to simulated color maps, which were calculated from spectral reflection coefficients. The results of sheet resistance and SunsVoc measurements show an influence on the underlying silicon for all three wavelengths used. However, light microscopy images reveal for the first time a change from indirect ablation (lift-off) to partial lift-off for a thin a-SiNx:H-layer (n ≈ 2.1, t ≈ 75 nm) by using a VIS picosecond laser. Thus, a first step towards selective laser ablation was made of dielectrics.

Layer selective laser ablation for local contacts to thin emitters

High efficiency solar cells require high generation and low recombination rates. High bulk lifetime, well passivated surfaces, and lowly doped thin emitters allow for low recombination rates. Thin passivated emitters should be contacted locally in order to avoid excessive contact recombination. This is common practice for front junction solar cells but is also advantageous for back junction cells. We analyze a novel layer selective laser ablation process. From a passivating stack composed of 70 nm silicon nitride that we deposit on top of 35 nm of amorphous silicon we selectively ablate the silicon nitride layer. Transmission electron microscopy investigations confirm the full ablation of the silicon nitride layer. After the ablation process, a 17 nm-thick amorphous silicon layer remains on the substrate. The crystalline silicon substrate shows no dislocations after the process. Evaporating aluminum on top of the locally ablated nitride layers forms local contacts of the aluminum to the silicon. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011.

Fabrication of an inkjet-printed seed pattern with silver nanoparticulate ink on a textured silicon solar cell wafer

In this study, the possibility of using inkjet printing in the fabrication of a conductive seed pattern on a textured silicon solar wafer is investigated. Firstly, solar cell wafers were coated with a hydrophobic solution. It was found that the surface texture of a solar cell wafer causes a slight increase in the contact angle of silver nanoparticulate ink by 7.5° due to a weak Cassie–Baxter wetting state. After selective laser ablation of the hydrophobic coating and the SiN x layer in preparation for the surface energy-patterned finger electrode regions, silver nanoparticulate ink was deposited with a piezo drop-on-demand inkjet print head. Because the threshold laser fluence for the ablation of the hydrophobic coating is lower than that for the SiN x layer, the effective width of a surface energy-patterned finger electrode region with the Gaussian laser beam profile was found to be wider than the actual width of the SiN x -ablated region. Although this initially results in a widened deposition of silver nanoparticulate ink, the subsequent drying of silver nanoparticulate ink is found to cause a reduction of line width of around 6% to 14%. Therefore, the final line width of the seed pattern is the outcome of two opposing phenomena. The physical and electrical characteristics of the seed pattern are as narrow as 58.5 ± 1.2 µm, as thick as 1.81 µm on average and as conductive as 2.72 µΩ cm.

Selective ablation-assisted two-photon stereolithography for effective nano- and microfabrication

The two-photon stereolithography (TPS) process has strong merits for the direct fabrication of 2-D and 3-D microstructures with sub-100-nm resolution. In this paper, we report an effective fabrication process in which selective ablation-assisted TPS (SA-TPS) was used to ameliorate some of the limitations of the TPS process. In SA-TPS, two processes (namely, an additive process of two-photon induced photocuring and a subtractive process of selective laser ablation) were performed sequentially using a single femtosecond laser optical scanning system. The effectiveness of the proposed process was demonstrated in several applications, including precise high-resolution patterning at resolution levels higher than those achievable using the TPS process, and the fabrication of structures with high mechanical sensitivity.

Ablation of SiN Passivation Layers on Photovoltaic Cells with Femtosecond Laser Source

This paper presents results of high-speed selective laser ablation of Silicon Nitride (SiN) coatings, which are used as antireflection and passivation layer at monocrystalline and multicrystalline silicon wafers for solar cells. We discuss ablation mechanism of NIR femtosecond laser pulses and advantages for cold ablation with minimised lattice damage. We show practical results of singleshot ablation with a repetition rate of 100 kHz. Finally we compare layer ablation with NIR picosecond laser.

A novel technique using spectral-domain optical coherence tomography (Spectralis, SD-OCT+HRA) to image supine non-anaesthetized infants: utility demonstrated in aggressive posterior retinopathy of prematurity

To acquire high definition spectral domain optical coherence tomography (SD-OCT) images of supine, non-anesthetized infants in the office by a novel modification of a commercially available table-top, chin rest system. Spectralis, a combined HRA+OCT device (Heidelberg Engineering, Heidelberg, Germany) was modified to convert the table-top system into a hand-held device using a two-step modification of the existing system. This device was used to obtain high-resolution OCT images of four infants with aggressive posterior retinopathy of prematurity (APROP) who required no general anesthesia. Serial imaging at two time periods of the exact area of clinical interest were obtained and were comparable. The entire procedure was safely completed in the office. The obtained OCT images guided selective laser ablation of flat neovascularization which were missed on clinical examination. With this simple modification and technique, the ability of using the Spectralis, a popular combined angiography and OCT imaging device for imaging supine, non-anesthetized infants has been established. This possibility would allow the dual use of the table-top system to serve also as a hand-held device for paediatric cases that can be imaged in the office setting with limited operating room facility in a busy practice.

Selective fetoscopic laser ablation in 100 consecutive pregnancies with severe twin–twin transfusion syndrome

Selective fetoscopic laser ablation in 100 consecutive pregnancies with severe twin–twin transfusion syndrome

Fine Pitch 3D Dispensable Electrical Interconnects for System In Package Solutions

The use of dispensable conductive materials to create fine pitch 3D electrical interconnects has been proven by Vertical Circuits Inc. as a low cost, high reliability alternative to create System in Package (SIP) solutions. This paper discusses the benefits of dispensing the 3D electrical interconnects instead of using traditional wire bond or high cost TSV processes to create a System in Package. Pad pitches achieved are typically less than 65um. A conformal coating of a dielectric material with the subsequent selective laser ablation of the dielectric over the die pads allows all interconnect lines to be dispensed in a single application adjacent to each other. Compared to traditional wire bond applications, the overall interconnect inductance is lower with comparable capacitive and resistance values enabling higher frequencies to be achieved. Using this process, complex multi-step stacking and bonding applications that jeopardize known good die (KGD) and increase the XY footprint of the System in Package are eliminated. Furthermore, the entire die stacking process can be executed as a single step before the application of the electrical interconnect material. This method minimizes the overall material handling and repetitive die stack processes that increase the probability of defects leading to expensive yield losses. Methods for matching the Coefficient of Thermal Expansion (CTE) for the stacked components within a System in Package will be demonstrated as well as surface treatments to enable fine pitch dispense that result in increased product reliability. JEDEC L3 reliability data will be presented to demonstrate SIP robustness using this technology.

Selective fetoscopic laser ablation in 100 consecutive pregnancies with severe twin–twin transfusion syndrome

To report the perinatal outcomes of a large series of twin pregnancies with severe twin-twin transfusion syndrome (TTTS) managed with laser ablation surgery in an Australian tertiary perinatal centre and to compare the outcome with other large cohorts. The outcomes of 100 consecutive pregnancies with severe TTTS managed with selective fetoscopic laser ablation from March 2002 to June 2007 were examined. Survival and neonatal morbidity were analysed. Comparisons were made with the results from other studies of laser surgery with at least 100 pregnancies. There were 100 women with TTTS treated with laser ablation; 34 stage II, 44 stage III and 22 at stage IV. Median gestation at time of laser was 21 weeks (range 18-28) and median gestation at delivery was 31 weeks (range 20-39). Overall perinatal survival rate was 151 of 200 (75.5%). Eighty five per cent had one or more surviving twins. The survival rate for stage IV TTTS was 88.6%, significantly better than for stage II (69.1%) and stage III (73.9%) pregnancies. The perinatal mortality rate for donors (30%) was not significantly different from recipients (19%), but the fetal death rate for donors was significantly greater than that for recipients (P = 0.03). Severe cerebral abnormalities were present in only 2.8% of newborns. The overall survival rate was comparable to other large series. These results for the management of severe TTTS are comparable to the best reported international series. Long-term follow-up is required and more research needs to be undertaken to further improve these results.

Modification of atrioventricular conduction in dogs by laser irradiation of Koch’s triangle guided by balloon-tipped cardioscope

This study assessed the feasibility of modification of atrioventricular (AV) conduction under direct vision with a new ablation system using laser energy. In 23 anesthetized dogs, a combined laser and balloon-tipped endoscope was introduced into the right atrium after right thoracotomy. Koch's triangle was easily identified in all dogs endoscopically. Nd:YAG laser energy was delivered through an optical fiber positioned inside the saline-filled balloon to the middle portion of Koch's triangle in eight dogs (mid-Koch group) and to the posterior portion in 12 dogs (postero-Koch group). Complete AV block was achieved in five of eight dogs in the mid-Koch group. In the postero-Koch group, anterograde Wenckebach cycle length increased significantly from 178+/-23 ms to 202+/-37 ms (P = 0.008). Selective laser ablation of Koch's triangle is feasible with a combined laser and balloon-tipped endoscope.

Selective laser ablation of SiNx layers on textured surfaces for low temperature front side metallizations

AbstractFor an alternative front side metallization process without screen printing of metal paste the selective opening of the front surface anti‐reflection coating could be realized by laser ablation. A successful implementation of this scheme requires direct absorption of the laser light within the anti‐reflection coating, since the emitter underneath must not be damaged severely. Additionally, the ablation must be feasible on textured surfaces. In this paper, we show that laser light with a wavelength of 355 nm and a pulse length of approximately 30 ns is absorbed directly by a typical silicon nitride anti‐reflection coating. Based on lifetime measurements on ablated samples it is shown that a damage free laser ablation of SiNx layers on planar surfaces is possible. The characteristic ablation structure on textured surfaces is explained and quantified by rigorous coupled wave analysis (RCWA) simulations. Finally, high efficiency solar cells with a standard emitter (Rsh approx. 50 Ω/sq) have been processed using laser ablation of the silicon nitride anti‐reflection coating. These cells show efficiencies of up to 19·1%, comparable to the reference solar cells using photolithographically opened contact areas. Copyright © 2008 John Wiley & Sons, Ltd.

Lithography-free high-resolution organic transistor arrays on polymer substrate by low energy selective laser ablation of inkjet-printed nanoparticle film

Inkjet direct writing of functional materials provides a promising pathway towards realization of ultra-low-cost, large-area printed electronics, albeit at the expense of lowered resolution (∼20–50 μm). We demonstrate that selective laser sintering and ablation of inkjet-printed metal nanoparticles enables low-temperature metal deposition as well as high-resolution patterning. Combined with an air-stable carboxylate-functionalized polythiophene, all-inkjet-printed and laser-processed organic field effect transistors with micron to submicron critical feature resolution were fabricated in a fully maskless sequence, eliminating the need for any lithographic processes. All processing and characterization steps were carried out at plastic-compatible low temperatures and in air under ambient pressure.

Fluorescence spectroscopy in selective percutaneous nucleotomy using the excimer laser--experimental studies

Our intention was to test fluorescence spectroscopy as a possibility for selective laser ablation. In an experimental setup a XeCl excimer laser (308 nm) with pulse energies between 5 and 50 mJ was used for fluorescence excitation in 20 human and 100 pig vertebral segments. Tissue fluorescence was detected via a quartz fiber bundle and analyzed by a polychromator and optical multichannel analyzer. Both low and high energy levels led to a broad band fluorescence of nucleus pulposus, intervertebral muscle, spinal cord and vertebral end plate and a typical fluorescence of anulus fibrosus with peak maxima at 385 and 435 nm. Tissue fluorescence was independent of surrounding medium (with air and normal saline used) and excitation energy. Borderlines between different tissues could be clearly indicated in all cases. Using intensity rations at 350, 385 and 435 nm (I1 = I350/I385, I2 = I385/I435) tissue discrimination was possible in all cases. This results demonstrate, that discrimination between intervertebral disc and surrounding tissue is possible and could be used for selective percutaneous laser nucleotomy in the near future.

Site-Selective Doping of Dyes into Polystyrene-block-Poly(4-vinyl pyridine) Diblock Copolymer Films and Selective Laser Ablation of the Dye-Doped Films

The site-selective doping of dyes into spin-cast thin films with different phase separation structures of polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) diblock copolymers and the dopant-induced laser ablation of the dye-doped films fabricated were investigated. The selective doping of dyes into P4VP parts was carried out by immersing the films in a methanol solution containing the dyes, in which the driving force was induced by hydrogen bonds between the nitrogen atoms of pyridyl groups and the carboxylic groups of the dyes. Schematic models for the adsorption process of the dyes in the methanol solution were discussed. Dopant-induced laser ablation was carried out on the dye-doped block copolymer films, resulting in the ablation of dye-doped P4VP parts only. Factors affecting selective ablation, such as dyes, the morphologies of phase separation structures, the number-average molecular weights of the diblock copolymers, and irradiation environments, were discussed.

OP10.10: The CHOP cardiovascular score for twin–twin transfusion: the effect of treatment with selective laser ablation

OP10.10: The CHOP cardiovascular score for twin–twin transfusion: the effect of treatment with selective laser ablation

OP10.13: Placental discordance before selective laser ablation as a predictor of perinatal outcome and birth weight discordance in twin–twin transfusion syndrome (TTTS)

OP10.13: Placental discordance before selective laser ablation as a predictor of perinatal outcome and birth weight discordance in twin–twin transfusion syndrome (TTTS)