Backside Mounting Procedures for Semiconductor Wafer Processing

Craig P. Orlando, Joel L. Goodrich and Ernest L. Gosselin
M/A-COM: Tyco Electronics

Throughout the semiconductor industry there has been a procedure for backside wafer processing where a product wafer is mounted to a backing host. The standard procedure of backside mount and thin down is essential to produce a consistent and uniform overall wafer thickness. The final product, a semiconductor chip, is placed into a manufacturing package where thickness variations must be kept to a minimum, otherwise devices will not set properly into a package causing performance variations that would lead to poor electrical yields.

With this in mind, our focus has been to tighten the limits of the procedure during the backside wafer process. The backside process in the Semiconductor Technology Center in Burlington Massachusetts has two distinct mounting adhesive procedures for performing bulk removal of material. For the past two years, we have used an excellent temporary thermoplastic-mounting adhesive for backside wafer processing. This procedure combines a liquid adhesive and a unique backing host that makes the backside process visually superior and gives us the ability to automate various aspects of the remaining processes.

Additionally, there has been a need to use a second mounting adhesive for a number of products that have very specific process requirements. The end results for these products are that they need to be etched through to chip separation and not dismounted as wafers. When using the standard adhesive, the products that would be targeted to go for etch-through would always separate at every stage of the process after the thin down procedure had been performed. The separation of product wafer from the backing host creates massive device loss and lowers the overall yield for each wafer. These product types are very unique in that, they are being thinned to approximately two- (2) mils total wafer thickness. The product wafer is subjected to additional heated backside processing, while staying mounted to the backing host. These heated process steps would be the catalysts that would separate the product wafer from the backing host.

To avoid this separation problem, we are using the second mounting adhesive. It gives us the ability to structurally strengthen the bond between the product wafer and the backing host wafer during any heated operations. To remove the product wafer from the backing host, it is necessary to place the wafer into an environmentally friendly solution for dismounting. The evolution of these two procedures, using two mounting adhesives, will be discussed in detail below.