Diffusion of Solid Source Dopants
Solid source dopant materials(Sb2O3, Zn3P2, and others) are sometimes required for many semiconductor process applications. For example, antimony diffusion into silicon is a desirable process for introducing slow diffusing n-type impurities into silicon. This process can be used for the formation of buried n+-layers for bipolar transistors or any other device structure, requiring a slow-diffusing n+-layer to minimize out-diffusion in subsequent heat treatment of the silicon wafer.
The most widely used antimony source material for Sb diffusions is antimony trioxide (Sb2O3). The Sb2O3 source material is placed into a separate source furnace with operating temperatures from 620 - 660ºC. At those temperatures the vapor pressure of Sb2O3 is sufficient for transfer of Sb2O3 vapor with a suitable carrier gas into the actual diffusion zone where the silicon wafers are located. The Sb concentration in silicon, or the sheet resistivity of the diffused Sb layer, which can be attained depends on:
- Sb2O3 vapor pressure, or Sb2O3 source temperature
- Silicon wafer or diffusion temperature
- Diffusion time
Tystar Thermal CVD System
Antimony diffused layers with sheet resistivities of 10 to 60 Ohms/square or surface concentrations from 5 x 1018 to 5 x 1019/cm3 are typically attained.
Sb2O3 diffusions can be readily performed in the Tystar TYTAN diffusion furnace.
The diffusion temperatures are typically from 1230ºC to 1280ºC and require quartzware for high temperature operation. A thick-wall quartz tube is recommended. SiC wafer carriers and SiC cantilever rods are recommended to minimize quartz deformation at the higher temperatures. The gas inlet part of the process tube is extended beyond the actual diffusion zone and extends through a source furnace, which is attached to the diffusion zone heater. The source furnace consists of a small 3-zone heater, which extends into the skirt of the diffusion tube. It is essential that there is a smooth temperature transition from the source furnace to the diffusion zone. There can be no dip in the temperature or the Sb2O3 flux cannot be controlled. The temperature in the diffusion zone is controlled to < +/- 1ºC in the diffusion zone over a distance of 34”/860 mm, and in the source heater over a distance of 6”/150 mm.
The Sb2O3 material is introduced into the source furnace in a quartz boat, which is loaded from the gas inlet port of the extended diffusion tube. A tapered quartz flange or a ball joint is used to connect the gas inlet to the process tube. Process gases used for the Sb2O3 diffusion are either a combination of N2 with O2 or pure, dry argon. The Argon process minimizes the oxidation of Sb2O3 into Sb2O4, which has a much lower vapor pressure than Sb2O3 and results in a reduced consumption of Sb2O3. Several wafer loads can be processed with one charge of Sb2O3 in the source furnace. At the exhaust end of the process tube the Sb2O3 and Sb2O4 condenses and can present a particulate problem when the wafers are loaded and unloaded. A condensing cup is normally attached to the source end to capture the condensed Sb2O3 and Sb2O4. The Tystar TYTAN furnace offers a significant advantage for this problem. The patented heat plug design permits process operation at much reduced gas flows without back streaming from the scavenger. With proper provisions, minimal condensation is observed inside the tube resulting in much lower particulate generation.
- Sheet Resistivity: +/- 5%
- Oxide Thickness: +/- 3%
- Junction Depth: +/- 5%