Deep p+ junctions formed by drive-in from pure boron depositions

This paper presents a new method of supplying the high doses of boron needed for creating several micron deep p⁺n junctions. Chemical vapor deposition (CVD), in a Si/SiGe epitaxial reactor, of nanometer-thick pure boron layers is used to fabricate 5 μm deep p⁺n junctions. The 10 min B deposition is combined with a 195 min drive-in at 1100°C to give a resulting sheet resistance of 3.1 Ω/sq. For as-deposited B-layers in windows through an silicon dioxide isolation to the Si substrate, reactions of the Si with oxide at the perimeter of the deposited windows will be enhanced by the presence of the B-layer during the high-temperature drive-in. Detrimental effects such as lateral contact window widening, small surface defects and/or large spikes formation, are avoided by capping the surface of the windows with either thermal oxide in a selective process or a low-pressure CVD (LPCVD) oxide during the drive-in. A good electrical quality of the oxide capping layer was achieved. The surface morphology was investigated by atomic force and scanning electron microscopy (AFM/SEM) analysis and found to depend on the overall method of fabrication.