Thermal Laser Separation Research Articles

A comparative experimental study on front and back laser cutting technology for mass separation of N-TOPCon crystal silicon solar cells

Generally, the Thermal Laser Separation (TLS) technology is used to pre-groove from the back of the cell, which is the first process of half-cell module preparation. However, this benchmark practice may result in forming new leakage points in the grooves at both ends of the cell. Hence, we investigate the mechanism behind the formation of leakage points after grooving and the impact on N-TOPCon's cell efficiency and module power. It is discovered that the diffusion of P element is the main reason for the increase in the proportion of leakage points. The front cutting is accordingly proposed to alleviate problem. A comprehensive experimental study on its feasibility has been conducted. It has been confirmed that the leakage current under the reverse bias voltage is smaller, the data convergence is better, and the breakdown performance is more stable for N-TOPCon. Furthermore, the module power is higher.

Thermal laser separation and high-throughput layer deposition for edge passivation for TOPCon shingle solar cells

This work demonstrates the reduction of cutting-induced losses on tunnel-oxide passivated contact (TOPCon) shingle solar cells via edge passivation using high-throughput layer deposition. TOPCon shingle solar cells with a size of 26.46 mm × 158.75 mm are separated from industrial full-square TOPCon host cells either by laser scribing and mechanical cleaving (LSMC) from the emitter-free rear side or by thermal laser separation (TLS) from the front side. TLS yields up to 0.2%abs more efficient shingle cells directly after singulation in comparison to shingle cells that have been separated by LSMC. Passivated edge technology (PET) is applied by depositing aluminum oxide (Al2O3) layers using two different tools: thermal atomic layer deposition (T-ALD) in a lab-scale tool with a throughput of tens of shingle cells per hour and a high-throughput plasma-enhanced ALD (PE-ALD) prototype tool with a throughput of about 60,000 shingle cells per hour. The energy conversion efficiency of the edge-passivated shingle cells after T-ALD Al2O3 PET is found to be 0.4%abs higher than directly after separation. This gain is the same regardless of whether LSMC or TLS is used for cell separation. For PE-ALD Al2O3 and TLS, a gain of 0.5%abs is measured after PET. Stringing tests with electrically conductive adhesive so far indicate that Al2O3 layers do not negatively affect the resistance of cell to cell interconnections. Thus, low-damage cell cutting in combination with high-throughput Al2O3 layer deposition for edge passivation is a very promising approach to maintain high efficiency for industrial TOPCon solar cells in shingled modules.

Stable passivation of cut edges in encapsulated n-type silicon solar cells using Nafion polymer

In this study, the edge passivation effectiveness and long-term stability of Nafion polymer in n-type interdigitated back contact (IBC) solar cells are investigated. For new module technologies such as half-cut, triple-cut, or shingled modules, cutting of the cells introduces unpassivated edges with a high recombination rate and this limits the module power. These cut edges can be “repassivated” after cutting and in this work Nafion polymer is used to achieve this. First, different edge types, namely emitter edges (n+/n/p+) and back surface field (BSF) edges (n+/n/n+), as well as different cutting techniques such as laser cut and cleave (L&C), thermal laser separation (TLS), and mechanical cleaving are evaluated. It is found that TLS and mechanical cleaving enable good repassivation on both BSF and emitter edges. Second, industrial-size IBC solar cells are made to assess the effect of the edge repassivation on performance. On 1/4-cut M2 size IBC cells with two emitter edges, efficiency is improved by over 0.3%abs. However, an efficiency improvement was not observed for similar cells with BSF edges, due to an insufficient passivation at the bulk edges. Last, the real-world stability of the Nafion repassivation is evaluated in industrially relevant module stacks by laminating the repassivated wafers with ethylvinylacetate (EVA) or polyolefin elastomer (POE) encapsulants and then exposing them to industry standard testing of 1000 h under damp heat conditions (85 °C, 85% relative humidity). The tests reveal that the repassivation is stable in EVA encapsulants but not in POE.

Investigation on the Processing Quality of Nanosecond Laser Stealth Dicing for 4H-SiC Wafer

Silicon carbide (SiC), due to its characteristic materials performance, gets more attention in Radio Frequecy (RC) and High-power device fabrication. However, SiC wafer dicing has been a tricky task because of the high hardness and brittleness. The blade dicing suffers from poor efficiency and debris contaminants. Furthermore, the laser ablation dicing and Thermal Laser Separation (TSL) can have thermal damage and irregular crack propagation. In this study, Stealth Dicing (SD) with nanosecond pulse laser method was applied to 4H-SiC wafer. A series of experiments were conducted to analyze the influences of different parameters on cross section and surface. An edge defect less than 3 μm and cross section with roughness of about 0.8 μm was achieved. And the three-point stress test was applied to obtain the die strength. Besides, a novel method of double pulse inducing cracks growth was proposed for the first time to optimize the surface edge. Finite Element Analysis (FEA) verifed the feasibility. Through experiments, the edge defect decreased to less than 2 μm. This work contributes to the wafer Stealth Dicing application for SiC and advance semiconductor materials.

TOPCon shingle solar cells: Thermal laser separation and passivated edge technology

AbstractThis work shows the first demonstration of thermal laser separation (TLS) and post‐metallization passivated edge technology (PET) applied to tunnel‐oxide passivated contact (TOPCon) shingle solar cells. The shingle solar cells with 26.46 mm × 158.75 mm size are separated from industrial full‐square TOPCon host solar cells. The singulation is performed either by TLS from the front side (emitter side) or by conventional laser scribe and mechanical cleaving (LSMC) from the rear side (emitter‐free side). The TLS optimized in this work yields up to 0.2%abs more efficient shingle cells after separation in comparison with LSMC‐separated shingle cells. The most promising PET sequence identified for the singulated TOPCon cells consists of depositing an 8‐nm‐thin aluminum oxide layer by thermal atomic layer deposition at a temperature below 200°C in conjunction with subsequent hotplate annealing at 250°C. Application of the PET yields a boost of up to 0.5%abs in energy conversion efficiency for edge‐passivated TOPCon shingle cells in comparison with their performance directly after separation. This efficiency‐increasing impact of the PET sequence is found not to be strongly dependent on the separation process applied. The most efficient TOPCon shingle cell after PET achieves an efficiency of 22.0% and has been singulated by TLS.

Thermal Laser Separation of PERC and SHJ Solar Cells

This article investigates thermal laser separation (TLS) on p-type Czochralski-grown silicon (Cz-Si) passivated emitter and rear cells (PERC) and n-type Cz-Si heterojunction (SHJ) solar cells. The TLS comprises of two laser-based processes: the crack initiation by a scribe laser and the crack propagation by a cleave process leading to separated cells with smooth edge surfaces. The sole impact of the cleave process on the cells' passivation layers is examined by performing it without the initial scribe, not separating the samples. By means of photoluminescence imaging, different cleave laser powers PC on passivated PERC and SHJ cell precursors are investigated, and the optimum PC values are then chosen for the processing of metallized and fired host cells with half cell and shingle cell layouts. Both, the cleave process as well as the complete TLS process are then separately performed in order to investigate the effect of each individual process by SunsVOC measurements taken before and after processing. For monofacial PERC cells, a drop in pseudofill factor ΔpFF = - 0.3%abs is recorded after TLS of the host cell into five shingle cells with 31.35 mm cell width. The effect on bifacial SHJ cells is stronger with ΔpFF = - 2.1%abs. Thereby, the cleave process itself does not induce significant losses in the cell performance for both cell types. The main losses are attributed to the recombination at the newly created unpassivated edges after complete TLS.

Postmetallization “Passivated Edge Technology” for Separated Silicon Solar Cells

This article introduces a postmetallization “passivated edge technology” (PET) treatment for separated silicon solar cells consisting of aluminum oxide deposition with subsequent annealing. We present our work on bifacial shingle solar cells that are based on the passivated emitter and rear cell concept. To separate the shingle devices after metallization and firing, we use either a conventional laser scribing mechanical cleaving (LSMC) process or a thermal laser separation (TLS) process. Both separation processes show similar pseudo fill factor (pFF) drops of − 1.2%abs from the host wafer to the separated state. The pFF of the TLS-separated cells increases by up to +0.7%abs from the as-separated state after PET treatment due to edge passivation, while the pFF of LSMC-separated cells increases by up to +0.3%abs. On cell level, the combination of TLS and PET allows for a designated area output power density of p out = 23.5 mW/cm², taking into account an additional 10% rear side irradiance.

TLS-Dicing for SiC - Latest Assessment Results

With the gaining demand for SiC semiconductor devices it is more and more challenging to meet the requirements for SiC volume production with the state of the art wafer dicing technology. In order to overcome this challenge the laser based dicing technology Thermal Laser Separation (TLS-DicingTM) was assessed for SiC volume production within the European project SEA4KET. This paper presents the key results of this project. It could be demonstrated that the demand of SiC volume production regarding throughput and cost as well as edge quality and electrical performance of diced chips can be met with TLS-DicingTM.

Advances in thermal laser separation: process monitoring in a kerf-free laser-based cutting technology to ensure high yield

Thermal Laser Separation (TLS) is an ablation free and therefore kerf free laser based cutting technology. This novel dicing technology is utilized to separate microelectronic devices on semiconductor wafers. In order to broaden the use of TLS method towards smaller chips and new materials on the one side and towards an advanced process monitoring on the other side, a novel contact-free measuring method for monitoring the TLS process has been developed. This new method is based on heating the semiconductor wafer with a near-infrared (NIR) laser and analyzing the subsequent temperature gradients induced by heat diffusion with contact-less temperature measurement. Thus, the TLS-crack can be detected due to its isolating effect to heat flux. The feasibility of the method is presented and the influence of parameters like sample size, distance to the TLS-crack and the laser setup in the measurements is reported.

High Quality Half-cell Processing Using Thermal Laser Separation

Half-cell modules are promising candidates for a new generation of PV-modules as the electrical losses can be reduced while the optical gains are increased. This technological approach requires a cell separation process which does not induce any significant electrical or mechanical damages on cell level. Thermal laser separation (TLS) is a damage free and kerfless dicing technology for brittle materials such as silicon. We investigate the applicability of TLS for the solar cell splitting and compare it to a reference cell-splitting process based on laser scribing and subsequent cleavage. It is found that the electrical properties of the TLS-half-cells are slightly better compared to the reference process and that compared to a laser separation there is no mechanical damage due to the TLS process.

Thermal Laser Separation – A Novel Dicing Technology Fulfilling the Demands of Volume Manufacturing of 4H-SiC Devices

One challenge for volume manufacturing of 4H-SiC devices is the state-of-the-art wafer dicing technology – the mechanical blade dicing which suffers from high tool wear and low feed rates. In this paper we discuss Thermal Laser Separation (TLS) as a novel dicing technology for large scale production of SiC devices. We compare the latest TLS experimental data resulting from fully processed 4H-SiC wafers with results obtained by mechanical dicing technology. Especially typical product relevant features like process control monitoring (PCM) structures and backside metallization, quality of diced SiC-devices as well as productivity are considered. It could be shown that with feed rates up to two orders of magnitude higher than state-of-the-art, no tool wear and high quality of diced chips, TLS has a very promising potential to fulfill the demands of volume manufacturing of 4H-SiC devices.

Comparison of Different Novel Chip Separation Methods for 4H-SiC

Mechanical blade dicing is a state-of-the-art technique for the chip separation of SiC devices. Due to the hardness of SiC this technique suffers from low feed rate and high wear of the diamond coated dicing blade, resulting in the risk of uncontrolled tool breakage during the dicing process. With the upcoming transition to 150 mm diameter of SiC wafers this technique will most probably reach its limit. For dicing SiC wafers of those diameters on a productive scale three alternative dicing technologies are considered in this paper: ablation laser dicing, Stealth Dicing and Thermal Laser Separation. All these methods are based on laser processing. The benefits of these technologies are discussed in detail and compared to the classical mechanical diamond blade dicing, including a brief summary of first experimental results on each of the three laser dicing technologies.

High quality and high speed cutting of 4H-SiC JFET wafers including PCM structures by using Thermal Laser Separation

ABSTRACTThe silicon carbide (SiC) market is gaining momentum hence productivity in device manufacturing has to be improved. The current transition from 100 mm SiC-wafers to 150 mm SiC-wafers requires novel processes in the front-end as well as the back-end of SiC-chip production. Dicing of fully processed SiC-wafers is becoming a bottleneck process since current state-of-the-art mechanical blade dicing faces heavy tool wear and achieves low throughput due to low feed rates in the range of only a few mm/s. This paper presents latest results of the novel dicing technology Thermal Laser Separation (TLS) applied for separating SiC-JFETs. We demonstrate for the first time that TLS is capable of dicing fully processed 4H-SiC wafers, including back side metal layer stacks, process control monitoring (PCM), and metal structures inside the dicing streets with feed rates up to 200 mm/s. TLS thus paves the way to efficient dicing of 150 mm SiC-wafers.

Advanced Vacuum Wafer Drying for Thermal Laser Separation Dicing Assessment Results from European Collaborative “SEAL” Project

Based on recent progress on laser-based wafer dicing equipment and process, the partners adixen, Fraunhofer IISB and JENOPTIK investigated the use of a vacuum based decontamination process to dry and to decontaminate the substrate surface of the diced wafers from water residuals, which are a side-effect of the TLS (thermal laser separation) approach. The decontamination process was achieved by using an adixen vacuum drying module prototype further to the JENOPTIK TLS dicing process. Within the frame of the European collaborative project SEAL, supported by the European Commission, experimental assessment was conducted by Fraunhofer IISB (research institution) together with JENOPTIK and Adixen.

Ablation Free Dicing of 4H-SiC Wafers with Feed Rates up to 200 mm/s by Using Thermal Laser Separation

ABSTRACTThis paper presents Thermal Laser Separation (TLS) as a novel dicing technology for silicon carbide (SiC) wafers. Results of this work will play an important role in improving the SiC dicing process regarding throughput and edge quality. TLS process parameters were developed for separating 4H-SiC wafers. Separated SiC dies were analyzed and compared with results produced with current state of the art blade dicing technology. For the first time, fully processed 100 mm 4H-SiC wafers with a thickness of 450 μm, including epi-layer and back side metal layers, could be separated with feed rates up to 200 mm/s. Besides the vastly improved dicing speed, the TLS separation process results in two important features of the separated SiC devices: First, edges are free of chipping and therefore of higher quality than the edges produced by blade dicing. Second, the TLS process is kerf free, which allows for reducing the necessary dicing street width and hence increasing the number of devices per wafer.