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Galliumsilicafilm is a formulation designed to form a glassy layer
consisting of gallium oxide and silica. The solution should be applied by spinning; a
typical photo-resist spinner is adequate for this purpose.
The best results will be obtained if the Galliumsilicafilm is applied to a wafer where the silicon surface is hyrophilic. To obtain a suitable surface, the wafer should be soaked in a solution of chromic acid-sulfuric acid. This solution, consisting of about 3-4 grams of chromic oxide dissolved in 100 ml of concentrated sulfuric acid, should be heated to 55oC or higher, and the wafers should be soaked for at least 15 minutes in this solution. Other solutions such as H2O2 + H2SO4 may also be used. After soaking the wafers are rinsed in DI water, alcohol, and blown dry. The wafers should not be dipped in HF solutions after the previous treatment.
After the surface treatment, the wafers positioned on the spinner chuck and a few drops of solution are applied to the center of the wafer. With a suitably treated surface, the solution will wet out over the wafer surface in one or two seconds. The spinner is started, and in 5-10 seconds a uniform film will form on the surface. At 3000 rpm, the film thickness will be 1000-1500 angstroms thick. At the completion of the spinning process, the wafers should be air dried for at least one hour prior to diffusion. It is not necessary to heat the wafers for film densification prior to diffusion.
The surface concentration obtained with Galliumsilicafilm will depend upon the position of the wafers during the diffusion process. For example, if the wafers are positioned vertically, and are not in contact, one will obtain a sheet resistivity of 350 ohms/square after one hour of diffusion at 1200oC in N2. If the wafers are coin stacked with a coated surface in contact with the uncoated surface of the wafer above, one obtains a sheet resistivity of 35 ohms/square after a one hour diffusion at 1200oC in N2 on both the coated and the uncoated surface. These differences in sheet resistivity occur because the gallium atom diffuses rapidly through SiO2 and escapes into the atmosphere.
The surface concentration is sensitive to the ambient atmosphere during the diffusion process. The presence of oxygen will reduce the surface concentration, and if present in appreciable concentration, no gallium will diffuse from the film into the silicon. The diffusion ambient, therefore, should be pure N2 or N2 + a few percent H2 (forming gas). Most of the data quoted here was obtained using N2 as the ambient. The following results were obtained for wafers diffused both vertically and coin-stacked at 1200oC and 1250oC.
|Temperature||Wafer Position||Time||Rs (Ohms/Sq.)||Xj (Microns)|
|1200oC||Coin Stack||1 Hr.||30||5|
|1250oC||Coin Stack||1 Hr.||15||7.5|
For longer diffusion times, the penetration will increase with the square root of the time of diffusion. However, since the gallium leaves the film by evaporation after the first hour of diffusion, the sheet resistivity will not change indicating the diffusion process is a limited source diffusion, and the surface concentration decreases with the square root of the time of diffusion. After the diffusion process, the doped SiO2 layer of Galliumsilicafilm may be removed in HF solution. In a few minutes in HF, a clean surface will be obtained with no pitting or remnants of SiO2 or gallium oxide. However, when the diffusion process is carried out in N2 at these high temperatures for long times, some formation of silicon nitride may occur. To prevent this, it is advisable after the first hour or two of diffusion, to change the ambient from pure N2 to N2 + O2 or air. This will prevent any wafer stains from forming.
The diffusion results one obtains will exhibit excellent uniformity in sheet resistivity both from wafer to wafer and across the wafer surface. One would obtain a sheet resistivity variation less then 5% throughout the run. P-N junctions produced with Galliumsilicafilm will exhibit low leakage current and sharp reverse breakdowns.
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