ultrapure water for maximum efficiency of solar cells

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Ultrapure Waterfor Maximum Efficiency of Solar CellsIn the manufacture of solar wafers and solar cells the highest purity

requirements are imposed on the water used for cleaning and rinsing.

High-precision, modern conductivity and TOC instruments monitor in real

time the quality of ultrapure water (UPW) with regard to trace ionic and

organic contamination.

Ultrapure water in solar cell pro-ductionLarge quantities of ultra-high quality ultrapure water are required in solar cell production. After the raw wafers are cut from silicon ingots using wire saws, slurry is removed from the wafers in a cleaning process and they are then washed with UPW in a rinse step.

After texturing the surface using potassium hydroxide (KOH) or mixtures of hydroflu-oric acid and nitric acid (HF/HNO3), residues of the etching bath are removed with another rinse with ultrapure water. Then the p/n transition is created by phosphor diffusion. The resulting phos-phor glass layer on the surface must be removed by oxide etching, which requires another rinse, before a blue antireflection

coating is deposited for the purpose of reducing optical losses and for electrical surface passivation.

A clean wafer surface is indispensable prior to the phosphor diffusion and coat-ing process steps for achieving maximum efficiency of the subsequent solar cell. The clean surface of the special glass panes plays a critical role also at the time of processing the solar cells into modules in the matrix unit, before the cells are im-bedded in the glass foil sandwich in the laminator. A final rinse with high-quality ultrapure water ensures that the silicon and glass surfaces are not contaminated by deposits of salts or organic compounds and thus do not negatively affect effi-ciency.

THORNTON Leading Pure Water Analytics

MicroelectronicsPerspectives in Pure Water Analytics

2 METTLER TOLEDO Microelectronics News 1

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wire sawing

coating rinse etching P diffusion rinse

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UPWIngot Raw wafer Wafer

texturingKOHHF/HNO3

Solar cell String Solar modulemetallization

The quality of the ultrapure water after production and in the distribution system is monitored in-line using high-precision conductivity measurements and TOC de-terminations. In addition, the decision is made based on these measurements, whether low-contamination rinse water is recycled to the UPW treatment system or used otherwise (reclaimed), which can substantially reduce the not insignificant costs for water treatment.

High-precision conductivity mea-surementThe greatest demands on water purity are imposed in the semiconductor and photo-voltaic industry. Water is used having a conductivity equivalent to the theoretical value of 0.005501 μS / cm at 25 °C. In these industries the reciprocal value of the electrolytic conductivity, the specific resis-tance, is indicated for monitoring for ionic contaminants. Here the aim is a value of 18.18 MΩ 3 cm at 25 °C. With a coefficient of 4 to 7 % / K the temperature behavior of ultrapure water is clearly other than with other common electrolytes (e.g. approx. 2 % / K for KCl). Therefore special algorithms for ultrapure water

must be established for precise tempera-ture compensation. Along with this computation the exact determination of the cell constants of the conductivity sen-sors is of very great significance for the accuracy of the measurement.

Since 1964, Mettler-Toledo Thornton has operated very successfully in the field of ultrapure water analytics and is recog-nized as having the most precise measurement systems for determining specific resistance / electrolytic conductiv-ity in pure and ultrapure water. At the Thornton manufacturing plant the cell constants of the conductivity sensors are determined with a maximum unreliabil-ity of only 1 % on the special Autoloop test rig. Here ultrapure water is used as the calibration medium which corresponds also to the later operating range of the measurement cells in UPW applications. The cell constants are, moreover, trace-able to generally accepted standards such as those established by ASTM and NIST.

Fastest TOC determinationSince, as a rule, no statement on organic contamination can be made using spe-cific resistance, it must be monitored using a TOC analyzer. According to infor-mation from solar cell manufacturers the limit value for UPW is 1 or 0.5 ppb of car-bon.

The Thornton 5000TOC sensor is cur-rently the fastest analyzer on the market. Instead of batch oxidation that requires analysis times of typically 7 to 15 minutes, the 5000TOC uses the dynamic UV oxida-tion process. A sample stream of the UPW permanently flows through the analyzer in the open bypass. Initially, the entry conductivity is measured, then the sample flows through a silicon glass coil and is irradiated with high-energy UV light. The exposure to strong UV-light results in any organic molecules present in the solution breaking down into carbon dioxide and water. The carbon dioxide generated dur-ing UV exposure partly dissolves in water, which in turn leads to the formation of carbonic acid. The UPW now has a slightly higher conductivity, and using the difference in conductivity before and after UV oxidation the TOC content is calcu-lated.

Use of UPW in solar cell production

METTLER TOLEDO Microelectronics News 1 3

with impeller or vortex flow sensors. The Smart channels are generally populated with sensors for conductivity but sensors for pH, redox potential, dissolved oxygen and ozone can also be connected. A special feature of the Smart technology is the Plug and Measure functionality which auto-matically recognizes the sensors, loads their characteristic data and automati-cally configures the measurement system. In this fashion, the installation and start-up costs are reduced to a minimum and operating errors are eliminated. Moreover, it is equipped for connection to process control or a control system via eight analog 0/4 … 20 mA signals, 4 switch contacts, and a digital RS 232 interface that enables integration in PROFIBUS DP networks too.

This method is not only especially fast but also particularly maintenance-friendly, since moving parts such as pumps, and chemicals and membranes have been eliminated. Only the UV lamp must be replaced periodically. With a detection limit of 0.025 ppb C and a response time of < 1 minute the stringent requirements of the photovoltaic and semiconductor industry on the measurement system are completely satisfied.

High flexibility The Thornton 770MAX transmitter is a multi-channel measurement instrument that features two analog pulse inputs and four Smart channels. The analog pulse inputs are utilized for flow measurement

Product updateThe 5000TOC has now been superseded by the 5000TOC e. Physical and software improvements mean that the new model is even more reliable and robust than the previous version.

SummaryThe quality of ultrapure water is decisive for product quality of solar wafers, cells and modules. Modern ultrapure water analysis systems ensure the required water quality with regard to conductivity and TOC, and in virtue of their modular design are very versatile for utilization in system control and quality assurance.

Discover more at:www.mt.com/5000TOC

Quartz Coil

UV Oxidation

UV Lamp

Conductivity Sensor (1)

Conductivity Sensor (2)

770MAX transmitter and 5000TOC e sensor

Principle of dynamic UV oxidation


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wide range of measurement, low mainte-nance operation, and designed to operate on both the 770MAX and M300 multi-parameter instrument platforms.

Convenient multi-channel mea-surementIn most ultrapure water systems using dissolved ozone, multiple measurement points are needed to insure proper dosage and also to confirm the removal of resid-ual after ozone-destruct UV (ultraviolet light) exposure. Proper dosage equates to effective disinfection and allows a bacteria-free ultrapure water supply to process equipment. In contrast, detection of zero ozone is critical, as any residual becomes an unwanted contaminant at point of use and can contribute to increased oxygen levels, affecting performance of the downstream de-gassification process. Thornton’s multi-parameter, multi-channel measurement capability provides the benefit of lower cost per measurement point and the convenience of one instru-ment to operate.

Straightforward, rapid sensor maintenanceMettler-Toledo Thornton’s dissolved ozone sensor offers many advantages over other ozone sensors on the market. Sensor reli-ability is often a function of durability designed into a device. Our ozone sensor contains materials of construction calcu-lated to provide long-lasting performance.

The sensor body is made of stainless steel, offering high corrosion resistance. The reinforced silicone membrane maintains performance while offering the durability required for applications where mainte-nance downtime can be extremely costly. However, all dissolved ozone sensors re-quire periodic maintenance. This is where Mettler-Toledo Thornton’s ozone sensor design clearly separates itself from other ozone sensors.

The membrane cartridge design reduces time for periodic maintenance to a few minutes. The cartridge is removed by simply unscrewing the end of the sensor body, removing the existing cartridge, refilling a new cartridge with electrolyte, installing the new cartridge into the end cap and screwing it back onto the sensor body. This is a vast improvement over other dissolved ozone sensor designs.

Find out more at:

Ozone Measurement TechnologyIncreased Reliability, Reduced MaintenanceFor facilities using ozone as a means to disinfect or sanitize ultrapure

water, METTLER TOLEDO offers a proven dissolved ozone sensor designed

for high performance and low cost of ownership.

Instrumentation reliability is criticalThe use of dissolved ozone in the produc-tion of ultrapure water is commonplace in semiconductor facilities around the world. It is seen as an economical and practical approach to controlling bacteria levels in final treatment processes of ultrapure water. Microorganisms in ultrapure water can lead to increased levels of organic and particulate contamination, ultimately contributing to decreased product yields. As many plants operate around the clock, all year, it is obvious that the instrumen-tation used to monitor ozone be reliable, robust and low maintenance.

Optimized ozone sensorIn-line dissolved ozone measurement has typically been accomplished using instru-mentation that ranges from sophisticated high cost, maintenance-intensive equip-ment giving good performance, to low cost, less reliable equipment with flow sensitive readings. Mettler-Toledo Thornton offers a proven polarographic dissolved ozone sensor, optimized for a

www.mt.com/THORNTONOzone measurement system


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including microelectronics, power and pharmaceutical, as well as the pure water treatment industry itself. ASTM Standards take the form of: Methods for measure-ment, calibration and accuracy determination; Specifications for the pu-rity or other characteristics of water or treatment materials; Practices that outline accepted procedures; and Guides that describe a variety of approaches to measurements.

Sharing expertiseThese standards not only provide widely accepted means for industry standardiza-tion, they also serve as important learning tools to those new to particular measure-ment or testing technologies. For over 15 years, Mettler-Toledo Thornton has spon-sored its personnel in participating in the ASTM Water Committee with industry colleagues, contributing in their areas of expertise. These areas have included continuous conductivity measurement and temperature compensation in high purity waters, total organic carbon deter-mination, pH measurement (especially in low conductivity waters), oxidation-reduc-tion (redox) potential measurement, dissolved oxygen measurement in ppb ranges, ozone measurement, plus related pure water treatment methods and mate-rials. At the same time, their participation keeps Thornton personnel up to date with new treatment methods, user-developed measurement techniques and industry trends.

ASTM International CommitteeMettler-Toledo Thornton ParticipationWater Committee standards from ASTM are widely used in the industries

utilizing pure waters such as microelectronics, as well as the pure water

treatment industry itself. Mettler-Toledo Thornton contributes to this

important work, enabling customers to benefit from optimized

measurements.

Globally respected bodyASTM International, originally known as the American Society for Testing and Materials (ASTM), was formed in 1898. Today it continues to develop voluntary consensus standards with input from ex-pert producers, users, consumers, government and academia. The concept of consensus is effectively built into the standards approval process. All committee members have a voting interest and all objections must be addressed fairly and objectively by the group. As a result, the ASTM plays a respected leadership role in addressing the standardization needs of the global marketplace.

Water CommitteeOf special interest to Mettler-Toledo Thornton and its customers is the ASTM Committee D19 on Water. It was formed in 1932 and currently has approximately 340 members in nearly 30 countries, with jurisdiction of over 300 standards docu-ments. Key areas of committee interest are the development of standard methods for sampling, identification, and analysis of water and materials dissolved or sus-pended in water. In addition, it develops standards on the performance of water treatment materials and the determina-tion of the corrosivity or deposit-forming properties of water.

Water Committee standards are widely used in the industries utilizing pure waters

Thornton conductivity / resistivity sensors for pure water measurements are all manufactured, factory calibrated and certified in accordance with ASTM stan-dards D1125 and D5391 which define standard conductivity solutions and the techniques for handling pure water mea-surements. Thornton high purity pH and dissolved oxygen sensors are designed and operate in accordance with ASTM stan-dards D5128 and D5462 respectively.

Participants on the Committee have found it to be a rewarding, on-going challenge to articulate technology into clear and concise standards and to keep them up to date. In addition, the work has helped enable Thornton and Thornton customers to benefit from optimized measurement techniques over the years.

www.mt.com/THORNTON


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specifications for all contaminant levels in UPW systems.

Vital measurementIn-line, real-time analysis of total or-ganic carbon (TOC) is a powerful and critical technique for monitoring organic contaminants in ultrapure water and for maintaining the ’health’ of the UPW sys-tem. To accurately and quickly measure contamination in the single ppb and ppt range requires instruments that are not only accurate at that level but also cali-brated to perform in the required range. The Mettler-Toledo Thornton 5000TOC e is designed and calibrated for this specific critical application.

Monitoring the TOC in the UPW system after the deionization (electro-deioniza-tion) and polishing stage is an important checkpoint for organics to prevent them from entering the polishing and distribu-

tion loop. Additional measurements upstream and downstream can help diag-nose where the organics are breaking through, e.g. whether it is reverse osmosis membrane failure or ion exchange resin deterioration or a pump failure. Further measurement in the distribution loop re-duces the risk of organic contamination in the production process.

Value and limitationsThe key to understanding the value and limitations of TOC data lies in under-standing the great potential variety of organic impurities in water and in the relationship between the TOC level and the various organic compounds poten-tially present in ultrapure water. The percentage of carbon in organic com-pounds found in water can vary from less than 5 % to more than 80 %. Therefore, water with a reported TOC level could contain any combination of natural and

TOC Measurement in UPW Systemsfor the Semiconductor IndustryAs integration on silicon chips becomes greater, so does the necessity

for ever purer water. With fast, accurate measurement down to 0.025 ppbC,

the 5000TOC e sensor is the perfect measurement instrument.

Ever-increasing integrationOver the past four decades advances in chip architecture, the reduction in line width and the increase in the number of transistors on semiconductor wafers has exponentially increased on silicon chips. The increase in the number of circuits has significantly decreased the line-widths on the wafers and the advances in design continue with the recent announcement of the 22 nm SRAM chip and work begin-ning on 16 – 15 nm and even 8 nm chips. The 22 nm SRAM chip puts 364 million bits of SRAM memory and more than 2.9 billion transistors packed into an area the size of a fingernail. This advancement increases by magnitudes the requirements for sensitive, accurate and continuous measurement and control of the UPW system.

The semiconductor manufacturing pro-cess has the most stringent specifications and requirements for ionic, particulate and organic contamination in their pure and ultrapure water systems. Managing and measuring these contaminants in their UPW systems has assisted and en-abled semiconductor manufacturers to improve product quality and maximize yields. Advances in water purification technology and innovations in instru-mentation for monitoring these contaminants have played a significant role in improving and measuring the quality of ultrapure water. These ad-vances, in turn, have driven lower 5000TOC e sensor


bacteria that may exist in the distribution system, storage tanks, and the process technology or treatment modules. Accurate, fast and convenient measure-ment of the organic level in a UPW system can save millions of dollars in lost prod-uct.

The 5000TOC e sensor provides the perfor-mance needed to meet these requirements, while offering the added benefits of continuous in-line measurement in a low-maintenance, industrial package with proven multi-parameter technology. The 5000TOC e sensor interfaces directly with the 770MAX instrument, which pro-vides additional measurement parameter capability. The 770MAX is the ’gold mea-surement standard’ for the semiconductor industry and the 5000TOC e sensor increases the significance of this multi-parameter platform.

Standard parameterTOC testing has been a standard for the Semiconductor industry for many years but because of the cost and inconvenience of some analyzers it was employed in only a limited number of points on the UPW skid and distribution loop. The 5000TOC e sensor and 770MAX multi-parameter in-strument package make multiple testing points not only economical but critical investments for risk reduction and im-proved yields.

manmade organic compounds and the concentration of those compounds can vary widely in the UPW.

TOC measurements do not provide ana-lytical detail of the organics in the water or the concentration of a specific impurity. Instead, the TOC measurement provides a baseline for the total organic contamina-tion in the UPW stream. Improving the accuracy of the TOC measurement will help insure that the organic level is well within the required specifications for to-day’s critical wafer production.

The 5000TOC e and 770MAX are specifi-cally designed and calibrated for use in the measurement of UPW in a semicon-ductor facility. The 5000TOC e undergoes a stringent special calibration in a UPW system that is high resistivity (18.178 MΩ 3 cm) and ultra low TOC < 2 ppb. This special system calibration of the 5000TOC e and 770MAX assures the fast-est and most accurate measurement of TOC in UPW systems.

Measurement of TOC in UPW systemsMicrobial contamination can result in the loss of millions of dollars in defective wafers and chips. While a direct correla-tion between organics in the water system and microbial contamination is difficult, organic levels are a significant indicator of the ’health’ of the water system. Organics provide a food source for any

Using the 5000TOC e sensor throughout the UPW skid to monitor the integrity of reverse osmosis membranes, the effective-ness of TOC destruct UV lamps, resin bed and electro-deionization performance and organics shedding is a cost-effective way to improve the performance of the UPW pad.

Pure and ultrapure water production re-quires the monitoring of organic contamination throughout the treatment process. The 5000TOC e sensor provides continuous, fast, and reliable monitoring of TOC levels from post RO waters to point-of-use. With continuous in-line measurements, the 5000TOC e sensor en-sures TOC excursions will not be missed.

Flexible solutionThe Thornton 5000TOC e sensor with 770MAX multi-parameter instrument provides the ideal combination of accu-racy, speed and capability in one analytical package. The 770MAX can ac-cept three other analytical sensors in addition to a TOC sensor, including con-ductivity / resistivity, pH, ORP or dissolved oxygen, plus pressure, tank level or two flow sensors. The 770MAX can also inter-face with two 5000TOC e sensors, leaving two additional channels for conductivity, pH or DO measurement and two pulsed flow channels are also available.

Find out more at:www.mt.com/THORNTON


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THORNTON solution Mettler-Toledo Thornton is the mar-ket leader in critical ultrapure and pure water analytics.

Our 770MAX instrument is a multi-parameter meter with a broad range of measurement sensor options.

The in-line 5000TOC rapidly detects organic contamination in real time.

As well as TOC, THORNTON produc-es sensors for monitoring pH / ORP, DO2 / DO3, conductivity, flow and pressure.

■ 770MAX multi-parameter transmitter

■ 5000TOC sensor for continuous Total Organic Carbon measure-ments

■ pH for ultrapure water applica-tions

■ 2-electrode, 4-electrode and inductive conductivity sensors

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n One Transmitter for Multi-Parametersfor TOC, pH, Conductivity, DO …

Wide range of instruments for your processes …

Smart

Flow

DO2 / DO3

pH / ORP


One Transmitter for Multi-Parametersfor TOC, pH, Conductivity, DO …

Wide range of instruments for your processes …

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Conductivity

Pressure / Tank levelTransmitter 770MAX

THORNTON benefits

■ All-in-one supplier with sensors for all your pure water analytics needs

■ Multi-parameter transmitter capabilities saves panel space and lowers costs per parameter

■ 5000TOC allows a real-time TOC measurement, in-line, all the time

■ No gases or reagents to handle, store or replace for TOC sensor; absence of moving parts mini-mizes routine maintenance

■ Smart Sensor technology offers “Plug and Measure” communica-tion from sensor to transmitter

■ Sterilizable sensors designed for exceptionally long lifetime

Visit for more informationwww.mt.com/pro

Take Conductivity to a Digital Level With measurement technology built into each sensor, Intelligent Sensor Management (ISM) conductivity sensors from Mettler-Toledo Thornton have the ability to store unique sensor identity, measurement, configu-ration and calibration data, all within the sensor!

Installation and startup are effortless when utilizing pre-calibrated ISM technology. Each sensor automatically communicates via an elec-tronic “handshake” to the transmitter providing quick and consistent installation for simple Plug and Measure performance.

Sensors send digital signals to the transmitter with less ambient noise and interference, extending cable lengths and improving measure-ment performance. When simplicity and installation speed are impor-tant, think ISM conductivity sensors from Mettler-Toledo Thornton.

www.mt.com/cond

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Mettler-Toledo Thornton, Inc. 36 Middlesex TurnpikeBedford, MA 01730, USATel: +1 781 301 8600Fax: +1 781 271 0214Toll Free (USA): 1 800 510 PUREEmail: [email protected]

ultrapure water for maximum efficiency of solar cells

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