Carrier wafers made from borosilicate glass enable UV de-bonding

The high-performance material now also allows new forms of de-bonding. Laser de-bonding through a glass carrier wafer made of BOROFLOAT glass is possible thanks to deep UV light transmission at the relevant laser wavelength range. This allows for the production of larger and thinner silicon wafers in a stable production process.

The constant trend towards smaller, more lightweight devices with 3D design architecture calls for ultra-thin silicon wafers with very high flatness levels. In addition, this quickly growing market demands higher yield rates, making larger and thinner wafers necessary. To reliably produce such wafers, process stability becomes increasingly important. The temporary bonding of a silicon wafer to a carrier wafer is a necessary step during silicon wafer thinning. The quality of a carrier wafer is determined by its ability to allow for fast processing and de-bonding times and to achieve a default-free surface cleanliness of the silicon wafer.

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. SCHOTT’s BOROFLOAT glass offers the necessary optical properties to support laser de-bonding.

“The laser-activated release 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, SCHOTT in Jena.

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. “BOROFLOAT glass offers the unique thermal, mechanical and chemical properties needed to meet the requirements for gap-free and long-lasting material bonds,” commented Kloss.

The thermal expansion behaviour of BOROFLOAT 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 BOROFLOAT glass compared to other alternative substrates allows for the necessary mechanical strength and stability required for microstructuring.

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 planarisation processes. Even mask-based chemical etching technologies can be applied to SCHOTT’s borosilicate wafers in order to create high definition surface channels.