FEATURE WAFER PROCESSING
Borosilicate glass carrier wafers enable faster laser de-bonding
Thomas Kloss, Senior Innovation & Product Manager at Schott explores the latest improvements in glass UV de-bonding for advanced semiconductor wafer production
will be achieved through irradiation using a high power excimer laser at a fairly low wavelength of 248nm or 308nm. Extra-low iron Borofloat glass of desirable 0.5mm carrier thickness shows over 90% transmission at 308nm and still over 35% for 248nm, thus significantly outperforming other thin flat glasses,” said Thomas Kloss, Senior Innovation & Product Manager for Borofloat at Schott. Anodic bonding is widely used to combine
silicon wafers with borosilicate glass to cap MEMS, other electronic and optical parts or to seal microfluidic devices. A perfect match between the two substrates is essential for good bonding behaviour. “This glass offers the unique thermal, mechanical and chemical properties needed to meet the requirements for gap-free and long-lasting material bonds”, stated Kloss. The thermal expansion behaviour of this
he performance requirements for glass wafers used for anodic bonding or as carrier wafers in wafer thinning processes are mainly determined by their ability to perfectly match those of the silicon wafer to which they shall be permanently or temporarily bonded. Well- adapted thermal expansion behaviour is as important as excellent flatness and process robustness. Glass wafers made from Schott’s Borofloat borosilicate glass provide such remarkable material properties along with exceptionally high UV transmission at the relevant laser wavelength range – a special requirement for high speed laser de-bonding processes. This allows for the production of larger and thinner silicon wafers in a stable production process. The constant trend towards smaller,
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more lightweight devices with 3D design architecture calls for ultra-thin silicon wafers with very high flatness levels. At present glass wafers with total thickness deviations smaller than a micron are available. To achieve the steadily increasing functionality demanded by the market, higher yield rates are required and this will ultimately necessitate larger wafers at lower cost in the near future. However with larger and thinner wafers, process stability often becomes an issue. One solution is to temporarily bond the
20 WINTER 2015 | MICROMATTERS
silicon wafer to a tooling part called the “carrier wafer” which serves as flat support during silicon wafer thinning. There are many different temporary wafer-bonding technologies and as processing time is an essential cost factor in semiconductor wafer production, de-bonding time and achieving a surface cleanliness without imperfections play an important role.
Laser de-bonding through glass carrier
wafers currently offers the fastest de- bonding time as well as a good price- performance -ratio. Deep UV light transmission at the relevant laser wavelength range is crucial for the principal feasibility and efficiency of this type of wafer de-bonding. This glass offers the necessary optical properties to support laser de-bonding. “The laser-activated release
Figure 1: Thermal expansion behaviour of Borofloat glass over a wide temperature range compares to that of silicon thus making it ideal for anodic bonding processes
Figure 2: Mechanical strength and stability during a process where thousands of accurate features have to be machined are essential in order to produce high precision textured wafers with a consistently perfect surface pattern and accurate size. Images: Curtesy of Schott
glass over a wide temperature range compares to that of silicon and thus makes it ideal for anodic bonding processes. Furthermore, many wafers require microstructures created via ultrasonic drilling, powder blasting or a combination of photolithography and dry etching. The high abrasion resistance of this glass compared to other alternative substrates allows for the necessary mechanical strength and stability required for micro-structuring. In addition, Borofloat glass also offers
high chemical resistance which is relevant when the wafers are exposed to chemicals throughout highly sophisticated etching and chemical mechanical planarization (CMP) processes. Even mask-based chemical etching technologies can be applied to the company’s borosilicate wafers in order to create high definition surface channels. Borofloat is one of the leading, most well- established glass wafer materials used in the semiconductor industry today. Over the years, it has acquired an outstanding reputation worldwide as a result of its broad adoption across many existing and emerging applications. Indeed the material is often now used as the high quality benchmark against which all other glass types are measured.
SCHOTT
www.schott.com/uk T: +49 (0)6131/66-0
/ MICROMATTERS
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