19 Leslie Court
Whippany, New Jersey 07981
EMULSITONE SOLUTION #163
Emulsitone Solution #163 is designed for use in the production of buried layers in
silicon ICs where sheet resistivity is in the range of 8 to 10 ohm cm and where freedom
from surface pitting is especially important. Emulsitone Solution #163 should be used in a
single step process where the deposition and drive-in is carried out without deglassing.
Prior to coating the wafers, the surfaces should be treated by soaking in a chromic acid-sulfuric acid solution, consisting of 3 grams of CrO3 in 100 ml. of concentrated sulfuric acid. The solution should be at 55oC, and soak time should be at least 10 minutes. After the chromic acid-sulfuric acid treatment, the wafers are rinsed in DI water, alcohol and blown dry.
The wafers are coated by spinning. With the wafers positioned on the spinner, several drops of solution are applied to the center of the wafer. In 2 to 3 seconds the solution will spread out nearly to the edge of the wafer. Spinning is started. A spin speed of 3000-4000 rpm is recommended. Five to ten seconds of spinning is sufficient to yield a thin glassy film.
POST-SPIN HEAT TREATMENT
The coated wafers should be heated at 150oC to 200oC for 15 minutes in air within one hour after coating.
For the diffusion process, the wafers are supported vertically in slotted boats. The orientation of the wafers, either parallel or perpendicular to the gas flow, yields similar results. The atmosphere for diffusion is nitrogen with 3 - 5% oxygen. Typical diffusion results are as follows with a temperature of 1200oC:
|Time (Hours)||Rs (ohms/square)||Xj (Microns)|
|1||15 - 16||1.5|
|4||12 - 13||3.1|
|8||9 - 10||4.2|
|16||9 - 10||6.1|
Diffusion in dilute oxygen atmospheres yields shallow depression of the window areas relative to the undoped wafer surface. After diffusion, the wafer windows may be deglassed in dilute HF solution or buffered HF solution, leaving the masking oxide. The wafers are then subjected to oxide growth which will further depress the window areas. Alternatively, the window areas may be further depressed by switching the diffusion atmosphere to strongly oxidizing during the last 15 minutes of the diffusion process.
To mask against penetration for 16 hours of diffusion at 1200oC, 10,000 Angstroms of thermal oxide is required. The thermal oxide should be free of pinholes and other defects, especially remnants of photo resist, to obtain pit-free surface which should result with Emulsitone Solution #163.
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