Silicon Solar Cell Laboratory

The Silicon solar cell laboratories at SERIS are spanning across levels 1 and 2 of the E3A building. The labs are capable of processing both wafer-based and thin-film based silicon solar cells of different types. Industrial tools with high silicon wafer processing rates (or 'throughput') ranging from 500 to 3600 wafers/hour are utilised to enable industry-relevant solar cell R&D.

Silicon Cleanroom 1A

  • The Quantum tube diffusion furnace from Tempress Systems is the latest-generation 5-stack high-throughput furnace featuring HD (high density) POCl3 (phosphorus oxychloride) diffusion, BBr3 (borontribromide) diffusion, thermal annealing, and dry and wet oxidations. The furnace has an integrated lift shuttle system for automated wafer handling. The furnace configuration allows all five process tubes to operate independently, with dedicated temperature and process controllers. With this furnace SERIS has the capability to perform industrial high-quality POCl3 and BBr3 diffusions for both multi- and monocrystalline wafers, dry and wet oxidation processes for dielectric passivation, oxide assisted drive-ins, and sacrificial oxide layers (for masking and other applications). The HD atmospheric pressure POCl3 process features high throughput, a small pitch, back-to-back wafer positioning, and a long flat zone. Using improved chemistry and hardware adaptions, an excellent sheet resistance uniformity is obtained for a wide range of targeted sheet resistances (up to 140 Ω/square). In the back-to-back arrangement, the POCl3 and BBr3 tubes have a process throughput of 1200 and 500 wafers per tube, respectively.

    5-stack Quantum tube diffusion furnace with integrated lift shuttle system.

  • The LINEA Pilot is an inline wet-chemistry tool custom-made for SERIS by Singulus-Stangl. The system’s versatile design makes it well suited for an industrial R&D pilot line for c-Si solar cells. It is used for several wet-chemical processes, including standard texturing of multi-Si wafers, single-side texturing of multi-Si wafers using SERIS-developed methods, and single-side etching for rear junction isolation for both multi- and mono-Si wafers. The system has two chemical baths: single-side etch (SSE) and acid texturing. The SSE can be used for rear junction isolation for both mono- and multi-Si wafers. The acid texturing bath is used for texturing multi-Si wafers. This setup is specifically designed to avoid change-over of chemicals and reduce the setup time for experiments. This increases the flexibility of the system to rapidly accommodate various experimental plans. A key feature of the tool is its unique wafer transport mechanism. Wafers are transported on a chain and supported on small pins at the rear surface. This leads to uniform wet-chemical processes without marks or stains on the front or the rear surface. This advanced transport system reduces process issues associated with traditional roller-type conveyor systems. The tool also includes an automatic chemical supply system and a waste collection system. An offline titration system allows to verify the concentrations of the chemicals used in the tool, which increases the stability and repeatability of the wet-chemical processes.

    (Left) LINEA Pilot tool. (Right) Silicon wafers on the plastic conveyor system.

  • The LINEA Lab is a lab-scale inline wet-chemistry tool designed for alkaline texturing and etching of mono-Si wafers. It is a manual dose system with excellent process flexibility. The system has an alkaline process bath and a hydrochloric acid (HCl) bath for subsequent neutralisation of the alkaline process. Several process parameters can be conveniently adjusted, including the chemical mixtures and concentrations, the bath temperature, and the wafer transport speed.

    (Left) LINEA Lab tool. (Right) Solar cells entering the tool.

Silicon Cleanroom 1B

  • SierraTherm (now SCHMID Thermal Systems) has been supplying in-line Atmospheric Pressure CVD (APCVD) tools into the photovoltaic industry since 2003. APCVD is the most cost-effective method for depositing SiO2, phosphorus-doped SiO2 (PSG), and boron-doped SiO2 (BSG) layers onto solar wafers. For this reason, leading manufacturers of high-efficiency n-type solar cells continue to expand production lines utilizing APCVD systems. The R&D scale 3-chamber APCVD system installed at SERIS is capable of depositing BSG, PSG, and SiO2 layers. The tool is highly flexible and enables precise control of the film thickness and dopant concentration. In a typical application, 2-12 wt% PSG films with a thickness of 35-65 nm are used as a phosphorus dopant source. Similarly, BSG films in the range of 2-6wt% and 35-65 nm thickness are used as a boron dopant source. SiO2 is an effective diffusion barrier and can be deposited onto the doped films in a single pass. Production-scale APCVD systems have been used in mass production for more than 12 years. Over 2 GW of high-efficiency silicon solar cells will be produced in 2016 using SCHMID APCVD tools. Production systems using belt transport have a capacity of 1800 wafers per hour, while the latest roller transport APCVD system (see photo) can process up to 4000 wafers per hour.

    The APCVD tool at SERIS

  • SINGULUS and SERIS jointly designed an inductively coupled plasma-enhanced chemical vapour deposition (IC-PECVD) tool suitable for bifacial heterojunction (abbreviated HET) solar cell architectures, the SINGULAR-HET. The machine has two stations for top-side deposition and two stations for bottom-side deposition. In this setup it is possible to deposit, for example, a-Si:H(i) and a-Si:H(p+) layers on the front side and a-Si:H(i) and a-Si:H(n+) layers on the rear side of the wafer, without breaking the vacuum. This enables deposition of all PECVD layers required for a HET solar cell within a single process cycle, with high throughput and high yield. The inductively coupled plasma (ICP) source of the SINGULAR-HET provides a high-density plasma with low kinetic ion energy, which is in the optimal range for high-quality silicon depositions without inducing plasma damage to the c-Si substrate. The high plasma density achieved in an ICP system, together with an automatic wafer handling unit, enables the high throughput required for PV applications. These properties make the SINGULAR-HET an ideal and versatile tool for HET solar cell fabrication.

    (Left) The SINGULAR-HET tool. (Right) The tool has an automated wafer handling unit which enables a high throughput

  • The ENERGi tool is a high-productivity ion implanter for doping crystalline silicon solar cells. Ion implantation is an alternative doping method to the typically used thermal diffusion processes. Doped regions are formed by implantation of p-type dopants like boron or n-type dopants like phosphorus into the n or p-type silicon wafers. The ENERGi has a fully automated platform and process capabilities for both phosphorus and boron implantation with a process throughput of 2400 wafers per hour per implant. Furthermore, the ENERGi has a unique ion source that produces a continuous flux ion beam and provides excellent doping quality at the lowest cost of ownership. High beam current is maintained even at low implant energies for phosphorus and boron doping, which provides the desired full amorphization of the surface layers. As c-Si solar cell process technology is driven beyond 20% cell efficiency, and the solar cell structures are getting more complex (for example rear-passivated p-type cells, n-type bifacial, or all-back-contact cells), there is a significant opportunity to simplify processing, and to reduce the number of process steps and thus costs, by using ion implantation. The process flow for solar cell fabrication is simplified due to single-sided doping without the use of sacrificial masking layers. The doped regions of the solar cells can be formed with superior lateral uniformity and with better repeatability compared to the standard thermal diffusion process.

    ENERGi tool

  • The R&D inline PECVD machine MAiA 2.1 from Meyer Burger (Germany) AG provides a range of functional thin-film coating options required for implementing new technological approaches to increase the efficiency of c-Si solar cells. It is a quasi-continuously operating high-throughput PECVD reactor (> 1000 wafers/hour for some processes). The c-Si wafers are transported through the machine in a flat carrier. The deposition process uses a ‘remote' plasma energised by 2.54-GHz microwaves, inducing lower damage to the Si wafers than the conventional parallel-plate approaches. The tool consists of three modules: load/buffer module (LM/BMI), process module (PM), and buffer out/unload module (BMO/UM). The loading module is equipped with an infrared lamp array for rapid substrate heating to the process temperature (350-550ºC). The buffer module is equipped with radiation heating plates. The process module with deposition zone includes an array of identical linear plasma sources.

    MAiA PECVD tool at SERIS

  • The InPassion LAB ALD tool from SoLayTec is a pioneering R&D system developed for the deposition of Al2O3 films using spatial ALD technology, where precursors are separated in space rather than in time. The deposition of Al2O3 films aims to improve surface passivation of c-Si solar cells to boost cell efficiencies as well as pave the way for new advanced solar cell concepts. Further, SERIS and SoLayTec jointly implemented an upgrade of this tool to include (for the first time) the deposition of intrinsic ZnO, Ga-doped ZnO and Al-doped ZnO thin films by spatial ALD for the development of transparent conductive oxides for application in heterojunction silicon solar cells. The spatial ALD system features two gas cabinets for the liquid precursors, a process deposition tool and an external abatement system. The deposition tool uses N2 for wafer transport by a double floating principle at atmospheric pressure and boasts a throughput of up to 100 wafers/hour (for film thicknesses of 10 nm). Deposition rates of up to 35 nm/min can be achieved with low precursor consumption. The system can process two substrate sizes (156 mm x 156 mm and 125 mm x 125 mm square wafers) with thicknesses in the range of 150 - 200 µm. Cassette to cassette loading is integrated into the system. For single-sided depositions, the film thickness can be adjusted for each wafer. This enables ultrafast ALD growth of functional thin films for industrial implementation in c-Si solar cells.

    Spatial ALD tool InPassion LAB from SoLayTec

Silicon Cleanroom 2A

  • The ILS LT is an R&D laser processing workstation designed for high-precision applications in the PV industry. The machine features three individual laser sources to provide highest flexibility: a 2-W UV continuous source, a 20-W green ns source, and a 30-W fs source operating at UV, green and IR wavelengths. This configuration enables a number of process applications for c-Si wafers: contact opening, IBC and PERC processing, junction isolation, drilling, cutting, and marking. An automatic loading system enables a high throughput of 500 wafers/hour. The laser system can also be used for providing the three scribes (P1, P2, P3) required for making thin-film (e.g., CIGS) PV modules.

    Innolas ILS LT laser system

Silicon Metallisation Lab 2B

  • The Meco Inline Plating tool at SERIS is a versatile system for pilot-scale (throughput > 100 wafers/hour) R&D on plated metallisation of solar cells. The plating tool is equipped with a vertical wafer transport mechanism and process channels for nickel, copper, tin and silver plating. Plating of nickel and copper may be carried out in either the light-induced plating (LIP) mode or the electroplating mode. A novel contact flipping mechanism enables an inline transition between LIP and electroplating. The tool is also equipped with process channels for inline resist removal and seed layer etch back to pattern plated metallisation on advanced cell structures like heterojunction cells and all-back-contact cells. Additionally, by exploiting the vertical transport mechanism, the Meco tool at SERIS can be used for simultaneously plating both surfaces of bifacial solar cells.

    The Meco Inline Plating Tool at SERIS

  • SERIS’ industrial screen printing line ‘Eclipse’ from DEK/ASM Assembly Systems has a throughput of 1200 wafers/hour per print. The fully automated line has a cassette loader to feed Si wafers into the Eclipse printing station. After printing, the wafers pass through a dryer (Heller) and are then collected in another cassette loader. The print line has three individual print stations. Station 1 is used for printing of silver pastes, station 2 is used for printing of aluminium pastes, and station 3 is used for printing of copper and dielectric pastes. Each print station includes an automated weighing station that measures the weight of the metal paste printed onto each wafer (without the need to handle the wafers manually for weight measurements). Stencil printing is allows printing of very fine metal lines (width approaching 30 µm) with high aspect ratio. The print stations have an alignment accuracy of ±10 µm and the prints can be aligned to the wafer edges or specific patterns on the wafer. This enables print-on-print applications and the metallisation of selective-emitter solar cell structures.

    (Left) Eclipse screen printing line. (Right) Weighing station integrated into the print stations.

  • The SinTerra is an automated fast firing furnace from BTU/AMTECH Systems. The furnace is equipped with automated cassette loading and unloading. The firing furnace has 6 zones with infrared lamps for heating c-Si wafers within a temperature range of 300-1000°C. The furnace uses a belt with edge support for the wafers. This minimizes belt marks on the wafers after firing. The furnace has the capability to independently control power to the top and bottom lamps of the high-temperature firing zones, which is beneficial for high-efficiency c-Si solar cell architectures like aluminium local back surface field (Al-LBSF) solar cells and n-type bifacial solar cells. Ramp and cooling rates can be precisely controlled in order to tailor the firing profiles.

    SinTerra fast firing furnace.

For further information, please contact:

Dr Thomas MUELLER
thomas.mueller@nus.edu.sg