30-09-2015 | | By Paul Whytock
The constant engineering drive towards ever flatter and thinner silicon wafers has received a significant boost following the development of new laser de-bonding techniques involving Borofloat, a borosilicate glass.
There is nothing new about borosilicate glass. It was first developed by German glassmaker Otto Schott in the late 1800s and sold under the brand name Duran. Then in 1915 Corning Glass Works introduced Pyrex kitchenware and the name became a synonym for borosilicate glass. These days Borofloat borosilicate glass from the Schott technology group is well recognised as a standard material for Micro-Electro-Mechanical Systems (MEMS) processes such as anodic bonding, mainly because of its characteristics of very accurate flatness and predictable thermal performance.
Anodic bonding is widely used to combine silicon wafers with borosilicate glass to cap MEMS, other electronic and optical parts or to seal micro fluidic devices. A perfect match between the two substrates is critical to ensure good bonding behavior.
The breakthrough from Schott has created new forms of de-bonding. Laser de-bonding through a glass carrier wafer made of BOROFLOAT glass is possible because of deep UV light transmission at the relevant laser wavelength range. It is this operational factor that facilitates the production of larger and thinner silicon wafers in what is a perdurable production process.
As mentioned previously there is a constant drive towards smaller, lighter electronic devices with 3D design architecture and this is attainable with ultra-thin silicon wafers with extreme flatness levels.
In addition to today's make-them-smaller-and-lighter design pressures there is also the demand for higher yield rates which makes larger and thinner wafers necessary.
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 permit fast processing and de-bonding times and to achieve flawless surface cleanliness of the silicon wafer. Deep UV light transmission at correct laser wavelength ranges is an imperative for this type of wafer de-bonding.
The Borofloat glass laser-activated release is achieved through irradiation using a high power excimer laser at a low wavelength of 248 or 308 nm.
The thermal expansion characteristics of Borofloat glass over a wide temperature range is similar to silicon which is another important indication that makes it suitable for anodic bonding processes.
And because 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 provides adequate mechanical strength and stability. It also has the necessary chemical resistance to cope with etching and chemical mechanical planarisation processes.