Wafer Thinning
Wafer Thinning - The Concept
Back lapping is the thinning of semiconductor wafers by removing material from
the backside which is the unpolished or unprocessed face. Wafers are often
fabricated thicker than necessary in order to allow the user to trim the
material to the desired thickness and also to give the manufacturer easier
tolerances to deal with at the fabricating stage. Through wafer thinning, the
user will be looking to change the electrical, optical or mechanical properties
of the material to suit their end application. Normally whole wafers are lapped
simultaneously, although it is not uncommon for individual devices or parts of a
wafer to be prepared individually. For example a pre-sawn part being used in a
fibre-optic application may require additional thinning and polishing to suit
it's attachment to the fibre-optic.
Silicon is the most common semiconductor material wafer thinning is performed on
although the process can also be applied to other compound semiconductors for
example gallium arsenide and indium phosphide. Common applications that require
backgrinding are often flip chip related and often are in used in applications
such as card systems, mobile phones and other RF devices. Lapping may also be
needed to guarantee an additional uniformity to the back surface of the wafer by
removing saw marks caused during sawing of the compound ingot and naturally
occurring defects during the compound growth.
WWafer thinning is a delicate process due to the fragile nature of the materials
in questions. Without expertise and experience it is possible to overstress the
wafer which can lead to cracking and widespread splintering of the wafer caused
by uneven thinning on the back side. To ensure overall quality and a better
yield at saw, the wafer must be de-stressed by a secondary polishing process.
The polishing process removes the distortions which cause the instability across
the wafer thereby de-stressing the wafer. It is important to have the technical
knowledge and experience to account for the many variables such as wafer size,
wafer material, passivation, desired thickness and reduction ratio. Final
thickness requirements can vary between 0.050 and 0.200 mm, certain processes
can also require thinning to 0.010 mm (10 microns). Another important point is
to ensure protection of the active side of the wafer during the removal process.
Wafer thinning - The Process
The active wafer surface is protected by mounting it face first onto a
specialised ultra clean adhesive tape. Using a high-precision polishing machine,
the wafer is then mounted onto a scrolled cast iron plate and the backside is
brought into contact with an abrasive slurry to remove material from the
backside. The slurry is a combination of lapping oil and silicon carbide or
aluminium oxide. For fine finishes aluminium oxide is the recommended lapping
material to apply the finishing polish and remove wafer stresses. Careful
control of wafer loading towards the end of the polishing process ensures the
highest quality finish. The wafers are then washed in a controlled solution and
the protective tape removed from the front of the wafer.
Wafer thinning - Capabilities
- Up to 8" wafers
- Low stress finishes #350-#2000
- Bumped and non-bumped wafers
- Fast turn-around for volume orders