Method of formation of thin bonded ultra-thin wafers

DETAILED DESCRIPTION As the demand for high performance and diverse applications increases, semiconductor devices must be made with thinner layers that have more abrupt transitions between the layers.
Typical semiconductor devices are formed by material deposition of various kinds, including, for example, epitaxy.
Changing the content of the layers during such deposition, however, leads to a relatively gradual change in device characteristics.
Other semiconductor based devices require thin semiconductor films for use as membranes in microelectromechanical device systems or for use as a silicon on insulator (SOI) layer.
The substrate is generally either a virgin silicon wafer for SOS devices, a silicon wafer processed by growth or deposition of a secondary material for SOI devices, or an altered silicon wafer which has been etched or drilled for MEMS devices.
The substrate may also be a non-silicon material such as glass.
It is known to grow a thin crystalline silicon film on a substrate using an epitaxial growth process at elevated temperatures under a high vacuum.
However, this is a slow and expensive technique.
The thin film must be grown one atomic layer at a time in a costly reaction chamber.
Vapor phase epitaxy, for example, has a typical growth rate of 1 .
mu.
m per minute at 1200.
degree.
C.
Molecular beam epitaxy is even slower.
Moreover, epitaxy has the additional drawback that very complex recrystallization techniques are used.
This means that the epitaxy can only be performed using crystalline silicon substrates.
This eliminates the possibility of using some important materials, such as amorphous oxide.
As described above, the films which are formed in this way are relatively thick e.
g.
several .
mu.
m.
These materials also have uneven and relatively gradually-changing doping concentrations.
Many ideal SOS device applications are based on geometries which provide an abrupt transition from one uniform doping concentration to another.
The high temperatures and long times that are required for the thin film growth by epitaxy lead to dopant diffusion and junction broadening