Wafer technology has evolved rapidly in recent years to respond to PV market requirements. Reduced use of silicon (Si) has pushed the cost levels of wafer production to new lows. Most manufacturers have adopted diamond wire for sawing. Extending the cost savings into the future will be the next challenge. In this article, we will review the history of sawing technology, present the challenges faced by the wafer industry today and introduce Meyer Burger’s groundbreaking DW291, a new product innovation in diamond wire technology.
Wire saw technology has made major progress in recent years. The expansion of Chinese manufacturers between 2009–2011 relied on the proven slurry process, based on loose abrasive grit, and Meyer Burger’s DS271 machines. This allowed wafers to be cut with about 150 µm of Si material loss, corresponding to about 9 g of Si/wafer. Attempts to economically recycle kerfs loss remained largely unsuccessful. Thus, for a typical 180 µm thick wafer a mere 55% of the Si was actually used. Further yield losses included out-of-spec wafers and non-usable brick end pieces. Depending on the process, cutting times were long, taking around 8 hours to slice one load of wafers.
Meyer Burger’s diamond wire equipment revolutionized cutting technology. It uses a fixed abrasive tool, which boosts the relative speed of the cutting grit against the Si surface by a factor of 2. This advantage was first captured in upgraded slurry tools, reducing cutting times to around 4 hours.
Since technology introduction, diamond wire equipment underwent ongoing improvements. Initially, relatively thick wires (120 µm) were used, with kerf loss similar to that of slurry technology. In short time the equipment became more compact and operated at higher speeds, while wire management was optimized to reduce wire consumption. As a true innovation, Meyer Burger has introduced the Diamond Wire Management System (DWMS).
DWMS eliminates self-induced wire wear and reduces wire tension fluctuations.
The system enables lower wire consumption and usage of thinner wires. The machine DW288 Series 3, launched in 2016, achieved cutting times of below two hours, corresponding to a capacity of 35 MW. It used 60 µm diamond wires and produced a mere 80 µm kerf loss, corresponding to 4 g of Si/wafer. This resulted from major improvements in equipment design, diamond wire manufacturing, the slicing process and cutting fluid management.
Present challenges in wafering
Many of the improvements described above have been due to the increased usage of mono materials, which are amenable to diamond wire. Also a significant number of large multi wafering companies are firmly convinced of the benefits of diamond wire and invested accordingly.
Diamond wire manufacturers still face difficulty producing high quality thin wire at low cost and high volume. Wiresaw manufacturers face the problem that, with increasing thinness, wires are more prone to breakage, and aim to develop systems that help to control wire tension. An alternative is to produce thinner wafers. However, adoption has been slow and has only been under- taken by a few niche players with the relevant expertise.
A further challenge stems from the need to make wafering more profitable, and in particular, cut operating expenditure. With wafer prices tumbling, it is essential to have access to inexpensive bricks and convert them into wafers at minimum cost. Manufacturers are therefore aiming to improve yields, increase outputs and reduce working capital requirements by further shortening the cycle time.
New DW291 – mastering diamond wire technology
In view of these challenges, Meyer Burger developed further its wafer product portfolio and identified the key technology drivers for boosting manufacturers’ performance. Meyer Burger developed the new DW291 diamond wiresaw, delivering a comprehensive set of innovations that push the boundaries of PV wafering.
The purpose of slicing solutions is to produce a maximum number of wafers from a given input material (Si brick) at lowest conversion cost, with a minimum of resources employed (capital, labor, land, etc.).
The DW291 has been designed for the utmost efficacy in this respect, focusing on the following levers:
The DW291 combines multiple innovations to achieve a superior performance. Meyer Burger has therefore improved its wire management system and radically redesigned the ma- chine structure to shorten the unguided wire length by more than 50%.
This reduces wire twisting with subsequent risk of wire break- age. As a result, the DW291 features wire tension fluctuations of below +/- 0.3N, 40% better than the previous machine generation DW288 Series 3. This enables cutting with 50 µm wires, and could potentially go down to 40 µm in the future.
Because handling wires half as thin as a human hair is a challenge to operators, Meyer Burger has eliminated more than 50% of operator–machine interactions. Improved access allows operators to reach all relevant areas of the DW291 to set-up the wire web from a single position while the redesigned cutting room gives them a better view of the ongoing cut. Finally, many process steps have been automated.
These innovations optimize operators’ work conditions, ultimately resulting in longer uptime and higher production yields even when using the thinnest wires.
A new drive train concept has increased the load length of the DW291 by 28% compared to the previous machine. The resulting extra output per tool of 10 MW/year makes the DW291 attractive also for the multi players.
In addition, improvements to the motor and bearing design enable higher cutting speed (+10% / +2 MW/saw) and acceleration (+25% / +3 MW/saw). Overall, DW291 boasts over 50 MW per tool, 43% more than the DW288 Series 3.
The higher cutting speed and acceleration required a new machine frame to guarantee mechanical stability and avoid detrimental vibrations. Meyer Burger combined four structural parts to make a single unified machine frame. A special system ensures high precision and flexibility.
Featuring these innovations, DW291 is set to have a lasting economic impact on the wafer industry, because every extra 10 µm helps to save 0.6 g of Si/wafer, the same amount of material yields more than 4% additional wafers (180 µm thick).
Meyer Burger’s proven DWMS technology has been enhanced to include a resharpening process (patent pending) that enables optimal usage conditions for the diamond wire. In turn, less wire is used for each cut, saving costs.
Also with its 50 MW throughput, the DW291 outperforms the DW288 Series 3 by 15 MW (+43%). Thus, customers require fewer tools, operators and smaller factories for the same output. This directly reduces the cost of wafer production.
The future of the wafer industry remains extremely exciting. The impressive set of innovations of Meyer Burger’s DW291 are a big step forward for the PV wafering industry and have pushed cost levels to new lows.
The award winner DW291 is contributing to superior sustainability of photovoltaics and wafering. The new DW291 will play a significant role in making solar energy more affordable and accelerating the adoption of renewable energy.
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