Silicon Solar Cell Laboratory

The silicon solar cell laboratories at SERIS are located on levels 1 (ground floor) and 2 of the E3A building. They are fully equipped to process both wafer-based (up to M4 size) and thin-film based silicon solar cells of different types. Industrial tools with high silicon wafer processing rates (or 'throughput') ranging from 200 to 3600 wafers/hour are utilised to enable industry-relevant solar cell R&D.

Silicon Cleanroom Lab 1A

  • Three batch wet chemistry tools (two in Lab 1A, one in Lab 1B) were custom-built for SERIS. They include process baths for standard silicon wafer cleaning sequences, various etching processes, and acidic and alkaline silicon texturing processes. The tools are compatible with both 5-inch and 6-inch silicon wafers and process 50 wafers per batch. They are used at SERIS for standard wet-chemical processing as well as to test new chemical mixtures and process additives designed for e.g. enhanced alkaline texturing of Cz-Si wafers and acid texturing of multi-Si wafers. The tools feature process tanks made of various materials such as PP (polypropylene), PFA (perfluoroalkoxy), PVDF/PTFE (polyvinylidene difluoride/polytetrafluoroethylene) to ensure compatibility with a wide range of chemicals. Most of the process baths are equipped with heaters and circulation pumps.

    (Left) Wet bench #1 for batch cleaning purposes. (Right) Close-up view of batch-based baths with a capacity of 50 silicon wafers per run.

  • The isopropyl alcohol (IPA) vapour dryer, also referred to as Marangoni dryer, is a competitive alternative to air-knifes or spin-rinse dryers. A single drying chamber provides deionised (DI) water rinsing and IPA vapour drying in one free-standing unit. IPA vapour is generated inside a standard 1-gallon bottle, so bottle change is very easy and quick. The IPA vapour is introduced through the top cover of the drying tank, ensuring even vapour distribution. This reduces IPA consumption and still provides surface tension drying across all wafers or substrates. Ozone could also be introduced to the drying process to eliminate trace organic impurities. Benefits of IPA vapour drying include very low IPA consumption, no watermarks, and no moving parts inside the drying chamber (this eliminates wafer breakage). Drying cycles are typically completed in 10 minutes.

    (Left) STG dryer #1 for surface tension gradient drying of a 50 wafer cassette of silicon wafers up to size M4. (Right) STG drier with open lid.

  • 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 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 of 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 is used 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) Silicon wafers entering the tool for processing.

  • The Quantum tube 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 mono-Si 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 /square). In the back-to-back arrangement, the POCl3 and BBr3 tubes have a process throughput of 1200 and 500 wafers per tube, respectively.

    (Left) 5-stack Quantum tube diffusion furnace with integrated lift shuttle system. (Right) View into the load station of the furnace.

  • The physical vapour deposition (PVD) sputtering platform at SERIS is designed for medium-throughput applications like solar cells, architectural glass, and flat panel displays. The machine can handle any flat substrate with a size of up to 300 mm × 400 mm, with a maximum thickness of 5 mm. Whilst the machine was designed for developing coatings on glass substrates, it is also capable of depositing transparent conductive oxides (TCOs) and metal films onto silicon wafers up to size M4. This state-of-the-art machine has dedicated chambers for the sputtering of metallic, dielectric and TCO layers. The processing chambers are equipped with planar magnetron sources for DC sputtering of metals, oxides and oxynitrides in the reactive mode, and with a cylindrical dual-magnetron source and planar sources for pulsed DC (DC+) sputtering of dielectrics and TCOs, with substrate heating up to 500°C. It is also possible to deposit graded layers, or multi-layer stacks of up to six different materials, without breaking the vacuum conditions. As the platform is comparable with large-scale production machines, the processes developed on this machine can easily be scaled up to industrial production lines. At SERIS this tool is used for depositing metal layers, TCOs and multi-layers for Si and CIGS thin-film solar cells, as well as heterojunction silicon wafer solar cells. The tool is able to deposit a variety of layers, including indium tin oxide, aluminium-doped zinc oxide, Ag, Al, Ti, Cu, In, ZnO and thin oxide and oxynitride tuned to specific requirements.

    (Left) In-line multi-chamber magnetron sputtering machine for the deposition of metals, dielectrics and TCOs. (Right) Close-up view of the vertical load carrier, here for 6 silicon wafers per run.

Silicon Cleanroom Lab 1B

  • Atmospheric pressure chemical vapour deposition (APCVD) is a cost-effective method for depositing SiO2, phosphorus-doped SiO2 (PSG), and boron-doped SiO2 (BSG) layers onto silicon wafers. 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 systems using belt transport (see photo) have a capacity of 1800 wafers per hour, while the latest roller transport APCVD system can process up to 4000 wafers per hour.

    (Left) Atmospheric pressure CVD tool in cleanroom lab 1B. (Right) Silicon wafers entering the tool for deposition.

  • The 8500 series elevator batch furnace from Sierratherm (now Schmid Thermal Systems) is designed for precision processing in manufacturing operations requiring performance of the highest quality and consistency. Rated up to 1050°C, this furnace features an ultra-clean low-mass refractory heating chamber. Multiple vertical heated zones, as well as power trimming to four element panels provide precise temperature stability and control throughout the process chamber. A sophisticated atmosphere inlet and exhaust system features four independently adjustable gas inlets and corresponding exhaust ports to efficiently extract burn-off effluents throughout the process chamber. Multiple tiers of graded, power-saving insulation minimise power consumption even at the highest processing temperatures. All process parameters, including temperatures, flows, and exhaust levels, are fully programmable for recipe design flexibility and precision. The applications at SERIS include diffusion of APCVD coated silicon wafers, co-annealing of B and P implanted silicon wafers, high-temperature long-cycle air atmosphere applications, and in-diffusion of other finite dopant sources.

    Coin Stack Diffusion (CSD) furnace for several thousand silicon wafers per process run.

  • 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 HET solar cells 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) SINGULAR-HET ICPECVD tool, loading station in the front, plasma chambers in the background. (Right) Automated wafer handling unit which enables a high throughput of up to 2400 wafers/hour.

  • 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 well 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.

    (Left) ENERGi Ion Implant tool in cleanroom lab 1B with automatic cassette loading and unloading. (Right) Close-up view of the loading station with three parallel belts, which enables a high throughput of up to 2400 wafers/hour.

  • 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 (350-550⁰C). The buffer module is equipped with radiation heating plates. The process module with deposition zone includes an array of identical linear plasma sources.

    (Left) MAiA PECVD tool in cleanroom lab 1B for the deposition of AlOx, SiOx and SiNx films. (Right) Twenty 6-inch silicon wafers can be deposited per run.

  • The InPassion LAB ALD tool from SoLayTec is a pioneering R&D system developed for the deposition of Al2O3 films using spatial atomic layer deposition (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 and 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.

    (Left) Spatial ALD tool InPassion LAB. (Right) 70 nm of Al2O3 deposited onto a mono-Si wafer.

  • The tube furnace TS81254 from Tempress Systems is an R&D 4-stack high-throughput furnace featuring various R&D options: low pressure chemical vapour deposition (LPCVD) of doped and intrinsic poly-Si for passivated-contact solar cell applications, doped and intrinsic silicon nitride deposition, oxidation for surface passivation applications, as well as co-annealing of implanted samples. The furnace has an integrated lift shuttle system for automated wafer handling. The furnace configuration allows all four process tubes to operate independently, with dedicated temperature and process controllers.

    Tube furnace #2 in cleanroom lab 1B for low-pressure CVD deposition of intrinsic and doped poly-Si and SiNx films.

Silicon Cleanroom Lab 2A

  • The ILS LT is an R&D laser processing workstation designed for high-precision applications in the PV sector. The machine features three individual laser sources to provide excellent flexibility: a 2-W UV continuous source, a 20-W green ns source, and a 30-W fs source that can be tuned to operate at either UV, green or IR wavelengths. This configuration enables a number of process applications for c-Si wafers, including contact opening, IBC and PERC processing, junction isolation, wafer drilling, wafer cutting, and laser 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.

    (Left) Innolas ILS LT laser system with cassette loading and unloading station. (Right) View inside the laser process station, with either fixed optic and x-y table movement or scanner optics for 6-inch silicon wafers.

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.

    (Left) The Meco Inline Plating Tool at SERIS. (Right) Wafer loading from a horizontal cassette loading belt to the vertical wafer transport system used during plating.

  • 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 masking 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 enables 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) Screen printing process.

  • 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 PERC solar cells and n-type bifacial solar cells. Ramp-up and ramp-down rates can be precisely controlled in order to tailor the firing profiles.

    SinTerra fast firing furnace.

  • The IP410 at SERIS is a versatile pilot-scale inkjet printer for solar cell metallisation, masking and patterning applications. The printer supports a wide range of functional inks including solvent-based metal inks, hotmelt inks and UV curable inks. Multiple printing modules are available at SERIS to quickly switch between different ink types while avoiding cross contamination. An ink evaluation module with disposable cartridges is also available to test small volumes of experimental inks. Printing patterns are digitally defined and therefore can be quickly updated, which makes the inkjet printer ideal for process development and rapid prototyping. The tool is fully automated and includes cassette loading and unloading. Cassette loading and unloading is combined in a compact module with an innovatively designed robotic arm. This compact module allows the tool to maintain a relatively small footprint.

    Pilot-scale inkjet printer for masking with hotmelt wax or direct metal printing on silicon wafers.

  • The SCT Thermal Evaporator is a lab-scale tool for depositing metal layers (Al, Ag, etc) onto various substrates (silicon wafers, glass sheets, etc). The deposition chamber uses a cryo pump and can achieve very low vacuum conditions (< 10-6 Torr) to ensure high-purity metal films. The evaporator features dual resistively heated sources and a rotating substrate table to ensure uniform depositions across large-area substrates.

    Thermal evaporator for metal films.

  • The inline sputtering tool from FHR Anlagenbau, Germany, is dedicated for the deposition of Cu-Ga (CG), Cu-In-Ga (CIG), Cu-Zn-Sn and other precursors as well as ZnOS buffer layers onto glass and metal substrates with sizes of up to 300 mm x 400 mm. The system has three planar magnetron sources, whereby two sources are connected to a DC power supply for DC sputtering of metal layers, while the third source is connected to an RF power supply for the deposition of ZnOS buffer layers. The substrates can be heated up to 200ºC. The tool is also used for deposition of Cu and other metal seed layers for subsequent plating on silicon wafers.

    Inline multi-chamber sputter machine #2 in lab 2B for sputtering of various thin films.

For further information, please contact:

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