UCM ultrasonic fine-cleaning system for research tasks
In optimizing the diamond wire saw process for slicing silicon wafers or cutting other hard materials, cleaning is but a secondary matter. Or is it? For evaluation and further processing, the surfaces obtained should be free of contaminants – so on the one hand the cleaning media ought to be perfectly matched to other process media, while on the other a suitable cleaning process is required. With a view to developing both, a public call for tenders was held for the supply of a cleaning system on behalf of the Fraunhofer center of semiconductor materials technology (THM). The bid was won by a custom-built UCM ultrasonic fine-cleaning system providing maximum flexibility.
Fraunhofer THM is a joint department of the Fraunhofer Institute for Solar Energy Systems (ISE) and the Frauhofer Institute for Integrated Systems and Device Technology (IISB). Based in Freiberg/Saxony, THM is dedicated to the production and separation of crystalline materials. It assists companies in handling industrial contracts and publicly funded projects by contributing its expertise in material preparation, machining and characterization – including issues such as crystal growth and epitaxy, wafer formation, and development of suitable consumables for upstream and downstream processes such as, e.g., cleaning. The two last-mentioned tasks, along with material characterization, fall into the remit of the working group on wafering. The latter's activities comprise the optimization of diamond wire saw processes, the main applications of which still lie in making wafers for the photovoltaics and semiconductor industries today. "Compared to the conventional sawing process relying on abrasive grit suspended in a carrier fluid (slurry), diamond wire sawing is much faster, and with a decline in the prices of diamond wire the technology can yield not just time gains but also cost advantages", comments Dr. Rajko Buchwald, Project Manager in the Wafering Department. Other application fields for this sawing technology lie in the cutting of hard materials such as ceramics, glass, sapphire and porous or sintered materials.
Another difference to conventional sawing lies in the residues present on the surfaces of the sawn wafers. Apart from deposits of the sawing coolant, these are particles which, in 99% of cases, lie in the under 1 µm size range. Hence, they are markedly more difficult to remove than particles left from the slurry sawing process which measure 4 µm on average, and the use of water as the main coolant constituant further compounds this challenge. As such contamination cannot be adequately removed with a standard system, THM needed a highly flexible cleaning solution adapted to its special research and development needs. Key specification criteria called for a 6-tank system configuration, a dryer, an ultrasound system supporting different frequencies, plus the ability to work with different cleaning media and water qualities. The main focus lay on an absolutely free programmability, versatility in using the tanks, and an exceptionally compact design in view of limited space availability. With this requirement profile, the project was put out to public tender. Among the bidders, UCM AG of Switzerland, a SBS Ecoclean Group company, scooped up the contract.
The ultrasound cleaning system comprises three cleaning and three rinsing stations, with each tank being assignable to both cleaning and rinsing. The tanks are designed to support the use of municipal, demineralized or ultra-pure water; they are connected to their own filter circuit and comprise conductivity measuring probes. In addition, they are equipped with heating and cooling so that the cleaning process can be performed within a defined temperature range. Another particularity of this system is that each of the three water qualities mentioned above can be heated separately using a high-grade steel continuous-flow water heater.
In each tank the medium is supplied from below, moves to the top and then overflows on all sides. This technology, developed by UCM for ultra-fine cleaning applications, ensures a very good and uniform treatment of the wafers. At the same time, the overflow feature guarantees that all removed particles and residue will get reliably conveyed out of the tank at once. Re-contamination of the substrates during withdrawal, as well as the formation of dirt pockets in the tanks will thus be prevented.
Three of the tanks are fitted with integrated multi-frequency ultrasound devices in the bottom and at the sides. This way, different frequencies can be applied to the wafers successively in the cleaning process. For the cleaning treatment the product is placed in special carriers holding 100 wafers each.
Thus, in the first cleaning step where major amounts of particles and dirt need to be removed, it is possible to use 25, 40 and 50 kHz, for instance. In the second tank, ultrasound frequencies of 40, 80 and 120 kHz might be applied, whereas 80, 120 and 250 kHz can then be adopted for the third tank. The wafer cleaning sequence is freely programmable. In the same way, the user can set the temperature, the ultrasound power and the product dwell time in each tank, which will be met with precision. Apart from the operating time of each ultrasonic transducer, an operating mode – continuous or pulsed – can likewise be defined for each. The controller provides a total of 16 program channels to this end. "Thanks to the system's free programmability and the use of separate ultrasound generators in each transducer bank, we can supply the ultrasound selectively in each tank, e.g., in a left – bottom – right sequence. This has the advantage of improving the media exposure of the wafers in the product carrier", Project Manager Dr. Rajko Buchwald explains.
The conductivity and pH level of all media are monitored continuously. For the vertical product movement, the number of strokes per defined interval can be preset via the frequency converter. Drying is performed on the capillary drying principle (lift-out) as well as by means of a downstream hot-air dryer. The process and tank parameters for each cleaning process are recorded in a CSV file. This ensures a highly accurate traceability and documentation of process sequences.
The cleaning medium, too, can be optimally adapted to the process. To this end, the UCM system in question supports the use of alkaline, neutral and acidic cleaning media. These are automatically supplied by metering units in each tank. In this field, THM cooperates with producers of specific coolants for the sawing process as well as with cleaning product manufacturers. The cleaning medium can thus be adapted perfectly to the coolant and vice versa.
„UCM has implemented our specifications in a really outstanding manner. We now possess a cleaning system which gives us full freedom in developing cleaning processes for diamond wire cut wafers and other material substrates", Dr. Rajko Buchwald concludes.
Author: Doris Schulz