New Temporary Bonding Solution Based on a Vacuum Wafer Carrier

Tuesday, 7 October 2014: 10:20
Expo Center, 1st Floor, Universal 9 (Moon Palace Resort)
T. Rogers and R. Santilli (Applied Microengineering Ltd)
There are many applications that require wafers to be thinned to high thickness uniformity. In addition there are requirements for the thinned wafer to go through high temperature and vacuum based process steps. The present solution to these requirements involves the temporary bonding of the device wafer to a carrier wafer. This temporary bonding solution has been very difficult to solve, and until now it has been down to the crude technique of gluing the wafers together using an adhesive. This technology process has many flaws:  complex procedures, low yields, and very expensive equipment to name but three. These drawbacks have been the main reason for holding back the implementation of volume production in the important application area of 3D IC’s.
This paper presents a novel solution and uses the vacuum chuck concept, instead of adhesives, for holding the device wafer in place. This transportable vacuum chuck is known as a vacuum wafer carrier. 

The vacuum wafer carrier is used in conjunction with a wafer bonding / debonding tool and the principle of operation is as follows:
a)    Load device wafer and carrier into vacuum chamber of the bonding / debonding tool
b)    Pump chamber to bonding pressure (typically 10-3 mBar)
c)    Bring device wafer into contact with carrier (this traps the vacuum in the cavities of the carrier)
d)    Vent the vacuum chamber

The device wafer can then be put through the various required process steps (eg grinding , DRIE PECVD, electroplating). This makes it compatible with all 3D IC processing variants  (via first, via middle, and via last). Once these processes are completed, the carrier is replaced in the vacuum chamber for the debonding step.
The carrier design ensures that the wafers are in rigid contact with the surface of the carrier during the grinding step, and the dimensions of the cavities are designed to ensure the deflection of the device wafer during grinding is minimised to produce thickness tolerances of <1µm. In addition the recessed cavities of the carrier can be designed to accommodate any surface topography, such as solder balls, on the contact surface of the device wafer.
The vacuum seal between the carrier and the device wafer has been optimised to include sufficient stiction to ensure that the device wafer remains adhered to the carrier even when the carrier is being transported in a vacuum environment. The debonding step has been designed to overcome this stiction and release the wafer. During the release step a receiving platen is positioned just below (typically ~0.5mm) the device wafer in the vacuum chamber and the thinned wafer can then be picked up from this platen by the robotic handler.
This new temporary bonding solution will enable fab lines to substitute the current adhesives based solutions and gain from cost reductions, simplified procedures, fewer processing steps, increased yields and lower manufacturing cost.