Dielectric for laser trimming

DETAILED DESCRIPTION The present invention relates generally to laser-trimming and more specifically to laser-trimming above a semiconductor material layer which is on a dielectric material layer.
Laser-trimming of thin-film resistors is used extensively to produce improved accuracy in analog integrated circuit technology.
In integrating laser trimming into dielectrically isolated circuit technology, a difficulty peculiar to dielectric isolation has been identified.
Trimming is generally accomplished by use of an infrared laser for improved control.
Silicon is nearly transparent at the wavelengths used; this results in laser energy penetrating to the bottom of the dielectrically isolated island, reflecting back and transferring some of the reflected energy back to the resistor.
The result is poor control due to interference effects.
These effects are variable due to changes in dielectrically isolated island-depth and proper control over trim-energy hence becomes very difficult.
An existing technique addresses the problem by simply placing the thin-film resistor over the polycrystalline silicon used to support the dielectrically isolation regions.
The polysilicon is much thicker than the single-crystal islands (typically 10 mils vs.
1 mil) and energy scattering off the grain boundaries soon dissipates the laser beam.
The resulting lack of reflection and interference produces enhanced controllability.
The resistor is deposited over a polycrystalline surface which is not perfectly flat but in fact possesses considerable relief at the grain boundaries.
In addition, large polycrystalline areas tend to "dish out" during the grind and polish operation which complicates laser-focusing and reduces photoresist definition, resulting in poor control over resistor geometries.
One solution is suggested in U.
S.
Pats.
Nos.
4,468,414 and 4,510,518 to N.
W.
Van Vonno.
An opening is provided in the dielectric isolation at the bottom of the single crystal island exposing the polycrystalline support.
Thus, infrared radiation passes into the support and does not reflect back to the top surface of the single crystal island