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Borosilicafilm 5 x 1019 is designed for applications where the surface concentration is required to be less than the solid solubility of boron in silicon. The spun-on film is very thin--of the order of 200 angstroms. Since the film is so thin, and the boron concentration is limited, the surface concentration obtained is sensitive to the ambient in which the heat soak is carried out. The solubility of boron in SiO2 is such that out-diffusion of boron from silicon into the SiO2 phase must be taken into account when using Borosilicafilm 5 x 1019 as a diffusion source. Lower sheet resistance is obtained in a nitrogen atmosphere; higher sheet resistance is obtained in an oxidizing atmosphere, the highest being obtained in oxygen or wet oxygen where the growth of SiO2 during diffusion is the most rapid.

In addition to offering a wide range of sheet resistance by choice of ambient, Borosilicafilm 5 x 1019 is formulated so that removal of the boron doped oxide after diffusion is easily carried out in mild HF solutions. In this way, the boron-doped oxide is removed without disturbing the original SiO2 film used for masking. Borosilicafilm 5 x 1019 contains no observable sodium ions or other impurities which are mobile in SiO2. The boron concentration introduced into the silicon is such that the large concentration of dislocations observed when silicon is saturated with boron will not be seen when the wafers are treated with etches designed to reveal dislocations.


Borosilicafilm 5 x 1019 may be applied to the wafer surface with a dropper. Sufficient material should be applied so that the wafer is completely covered with solution prior to the start of spinning. After about 15 seconds of spinning, a thin glassy film will form which consists of SiO2 containing dissolved boron. The wafers may then be subjected to the diffusion cycle. If it is desired to store the wafers for extended periods, the wafers should be heated to 70o-80oC for 10 to 15 minutes to remove residual solvent. The surface concentration achieved will not be sensitive to spin speed for shallow penetrations. However, film thickness will determine the time in the diffusion cycle when the source is depleted and the proceeds as a Gaussian rather than an erfc diffusion. For most applications, spin speeds between 2000 to 6000 rpm will suffice.


To obtain reproducible results, the wafer surface or the windows in the silicon dioxide must be clean and free of unremoved oxide. Any oxide remaining in the windows will dilute the Borosilicafilm deposited on the surface, resulting in lower Co. Typical diffusion results are obtained as follows:

Temperature--950oC Time--15 minutes
Ambient Spin Speed (rpm) Rs (Ohms/square) Co (atoms/cm3)
N2 3000 65 5X1019
N2:O2 4:1 3000 145 2X1019
O2 3000 7500 5X1018
O2 5000 15000 1X1017

Temperature--1200oC Time--15 minutes
Ambient Spin Speed (rpm) Rs (Ohms/square) Co (atoms/cm3)
N2 3000 25 5X1010
N2:O2 4:1 3000 45 2X1010
O2 3000 600 5X1018

Removal of Boron Doped Silicon Oxide Layer

For a film applied at 3000 rpm spin speed, the oxide layer is completely removed in two minutes of etching in a buffered HF solution consisting of 60 grams NH4F, 100 grams H2O, and 16 grams 48% HF. This film was produced in an N2 ambient. In a more oxidizing atmosphere, the films are removed more rapidly.

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