Wafer Bonding  


Direct Bonding - High T

Any two flat, highly polished, clean surfaces will stick together if they are brought into contact. The bond is of the Van der Waal’s, or hydrogen type and is of low strength, but can be significantly improved by thermal treatment. This process has been successfully exploited for MEMS fabrication, using silicon : silicon bonding with either plain or oxidised wafers. Because of the thermal treatment the technique has often been referred to as silicon fusion bonding.

The high temperature (typically 1000oC) that has been required to achieve acceptable bond strength has been a limitation with regard to applications using silicon direct bonding. However recent developments in surface preparation (e.g. using plasma activation) have demonstrated that high temperatures are not necessarily needed. This is of great interest in MEMS design as it enables stress-free bonds to be achieved at temperatures that are compatible with metallisation layers and opens up a much larger range of applications for silicon direct bonding.

The initial bonding is normally done at room temperature with some force applied. AML wafers bonders include a special pin to ensure that when the bond forms it does so starting at the centre and works towards the wafer edges, thus ensuring no trapped air at the interface. This reduces voids in the bonding and leads to better quality, higher yield bonds. Compared with anodic bonding, silicon direct bonding has the following benefits:

Exact thermal expansion match therefore minimal stress in bonded wafers
Fusion bonded wafers have higher temperature capability whereas anodic bonding limited by strain point of the glass
Fusion bonded wafers can be used for subsequent IC processing, whereas the anodic bonding process introduces alkali metal ions: not
allowed for CMOS processing

However there are also some drawbacks:

Surface roughness requirement for direct bonding is ~ few Ångströms compared with a few 10's nm for anodic bonding
Anodic bonding is more tolerant of surface particles and is generally a more robust process
Silicon direct bonding is widely used in the production of SOI wafers and has also been exploited for a wide range of MEMS devices.
MEMS fabrication is assisted in the AML wafer bonders via the ability to accurately align bonded wafers, in-situ, using infra-red split field
optics.

New AML In-Situ RADICAL Activation for Low Temperature Bonding

Aligner BonderINDUSTRY FIRST! ACTIVATE, ALIGN & BOND IN ONE MACHINE!

More stable, reliable, better bond & bond uniformity
Larger more useful process window after activation & subsequent bonding
No roughening of the surface! No change in AFM topographical roughness after activation
Reduced handling - higher yields
No exposure to energetic ions or electric fields reducing the risk of surface damage
In-situ promotes higher yields due to reduced presence of hydrocarbons

Only AML can: Align, Activate and Bond in one chamber, in one machine!