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PHOSPHOROSILICAFILM 7969

Phosphorosilicafilm is a dopant formulation designed to produce phosphorous diffused layers in semiconductor wafers. Phosphorous diffused layers may be produced routinely with sheet resistance profiles over the wafer surface flatter than 1-2%. Similar reproducibility from wafer to wafer and from run to run is easily achieved. When applied by spinning or spraying, a film forma which consists of silica with phosphorous dissolved in it. This phosphorous doped silica layer provides an erfc source, and the sheet resistance decreases linearly with the square root of the diffusion time.

Phosphorous concentrations required for emitters with high emitter efficiency in high frequency transistors and integrated circuits are obtained with films about 2000 angstroms thick. For high voltage devices requiring deep penetration and low sheet resistances less than 1 ohm/square, sprayed-on films are used.

Phosphorosilicafilm contains ethyl alcohol as solvent. In addition to ethyl alcohol, methanol or isopropyl alcohol may be used as diluents if it is desired to dilute the "as-received" formulation.

Phosphorosilicafilm is hygroscopic and for extended storage, the container should be securely fastened to prevent access of water vapor. The bottle may be left open for reasonable periods of time with no deleterious effects. After films have been applied by spinning or spraying, the wafers may be processed directly. However, if it is desired to store the wafers for extended periods of time after film application and prior to diffusion, the films should be hardened so that the layer is insoluble in water, and the wafers may be stored with no loss in doping characteristics. If it is desired to photo-etch Phosphorosilicafilm, higher temperature or longer heat soak is necessary for good adhesion of the photo-resist and to prevent leaching of phosphorous from the glass film.

For special application where higher sheet resistance and lower Co's are required, formulations yielding Co's as low as 1016 are available from stock.

Application
The viscosity of Phosphorosilicafilm is quite low, and the material wets readily to silicon surface. One or two drops are sufficient to cover a 1 1/2" wafer. For shallow penetrations where the source is not depleted, the sheet resistance obtained is independent of spin speed. A spin speed should be selected which minimizes the lip which builds up at the edge of the wafer. In 10 to 15 seconds of spinning, the film will be formed. For spraying, the material should be diluted to yield a film less than 10,000 angstroms thick. A thicker film will develop cracks on drying.

Diffusion
Typical diffusion results are obtained as shown in the table following.

Spin Speed (rpm) Diffusion Temperature
(oC)
Time (min.) Ambient Rs (ohms/square) Co (Atoms/cm.)
3000 1050 20 N2:O2 4:1 5 1021
3000 1200 60 N2:O2 4:1 2 1021
Sprayed-on 6000 angs. 1200 480 N2:O2 1:1 0.5 1020



Surface Condition
Phosphorosilicafilm will yield no surface damage or deposits. The phosphorous doped silica film is easily removed in dilute HF solution.

"LOW Co" PHOSPHOROSILICAFILM
"LOW Co" Phosphorosilicafilm formulations, which yield surface concentrations less than solid solubility, are similar to the standard formulations with the exception that the phosphorous concentration in the SiO2 layer is reduced. The Low Co materials are applied in the same way as the standard materials.

Diffusion Process
The surface concentration and sheet resistance realized with these materials will depend upon the atmosphere in which the diffusions are carried out. In an oxidizing atmosphere, the growth of SiO2 at the silicon surface will eventually prevent the access of dopant atoms from the doped oxide to the silicon. As a standard for reference, the Low Co Phosphorosilicafilm materials, except for the 1017 to 1016 range are evaluated by diffusion in N2 at 1200oC. Films are applied at 3000 rpm to silicon wafers of 10 ohm cm. resistivity. The diffusion time is fifteen minutes. Surface concentration is estimated from the sheet resistance and junction depth, determined by angle lapping and staining. For the materials in the lowest Co range, 1017 to 1016, the ambient is N2:O2 5:1, with the same temperature, 1200oC, for 15 minutes.