|Thermal Oxide or SiO2 is one of the "building block" films used in making both simple
and complex semiconductor devices. If grown correctly using a high purity low defect Silicon
substrate, it can be an excellent dielectric (Insulating) thin film. It is normally found on a device
as "field oxide" electrically isolating Polysilicon, metal or other conductive thin film(s) from the
Silicon substrate or "bulk" of the device. It is also found on the device as a "gate oxide," this
is a very thin thermal oxide located over the gate or active region of the individual transistors.|
Silicon Dioxide is a "grown" oxide not CVD (chemical vapor deposition) and as such it has a higher integrity than most CVD oxide films and so far has demonstrated higher uniformities, less defects and higher dielectric strength than deposited Oxide thin films. For reference, the target index of refraction number for thermal oxide is 1.462.
|Thermal Oxide is normally grown in a diffusion furnace (either vertical or horizontal) or it can also be grown in a Rapid Thermal Processor. RTP systems are normally used for thin dry Thermal Oxides or Implant annealing on devices where tight thermal budgets are an issue. Large Horizontal or Vertical diffusion furnaces are usually the tool of choice when growing most Thermal Oxides. The picture at the right is of 300mm thermal oxides being unloaded in one of Process Specialties' diffusion areas. At PSI we only grow Thermal Oxide on 300mm wafers at a maximum temperature of 900°C. This results in almost zero flatness changes and diffusion related "slip" in the wafers. These flatness changes and slip are cumulative, and over time with successive strips and re-grows can induce serious damage into the wafers.|
|Growth methods: Thermal oxide is grown at high temperatures from 800°C - 1100°C
using either a "Wet" or "Dry" growth method. Today's Wet Oxides are grown pyrogenically
using Hydrogen and Oxygen gases in a internal or external torch. Most large IC manufactures
use external torch technology, this is where H2 & O2 gases are combined in a quartz torch unit
outside of the quartz furnace tube. This torch unit is housed in it's own small high temperature furnace element (separate from the
main furnace) that is heated to a
temperature >800°C. At this
temperature the gases will ignite
and burn on their own without an
outside ignition source. The flame
produced is blue in color and
usually contained in a secondary
quartz vessel called a steam
chamber, the result of this blue
flame is pure steam.
This high purity steam is then piped into the what is commonly called the "Source end" of the diffusion furnace, once the steam is inside furnace tube it rapidly expands and if the gas flows are correct the steam will distribute uniformly within the furnace tube. This produces a high quality, very uniform, low MCL and defect Thermal Oxide. Process Specialties developed one of the first high flow torch systems for its 300mm oxidation furnaces back in 1996 and because of this, we have been able to achieve less than 1% non-uniformities in all of our 300mm oxide furnaces on high quality prime and test grade wafers.
Now for a little History, in the 60's and 70's wet Thermal Oxides were grown using a very different method of producing steam. In those days steam was produced using an ancient system that consisted of a "quartz bubbler" filled with deionized (DI) water that was heated to boiling by a device called a mantle. Into the quartz bubbler Oxygen was piped below the surface of the DI water and is bubbled up through the water. From here the Oxygen laden with steam from the boiling DI water was piped into the Source end of the diffusion tube. Although much cheaper to produce (no High Purity Hydrogen gas is used) this steam source produced a poor quality Oxide that was prone to a lot of metallic contamination and oxide defects. We wanted to mention this ancient method not only as a history lesson, but also to point out that some companies still use this method to produce thermal oxide today!
Wet Thermal Oxide: Growing Thermal Oxide using Hydrogen/Oxygen steam accelerates oxide growth and is used to produce a thicker oxide 2,000-25,000Å, this thick oxide is usually called "field oxide" and it is found in areas on the device where high dielectric strength is needed. Remember, thermal oxide growth occurs at the Silicon/Oxide interface where it actually grows from the bottom-up and each new layer is in essence is being "pushed-up" instead being deposited or "piled on" like CVD or PECVD oxides. For this reason thermal oxide growth is not linear, as the oxide grows the rate of growth slows down, because as more oxide is present on the surface of the Silicon it takes longer for the Oxygen atoms to penetrate and reach the Silicon interface to then combine with Silicon atoms to form more SiO2 (Silicon dioxide). As shown in the drawing at the RIGHT the Oxygen atoms diffuse through the present oxide at high temperature to reach the Silicon, and the higher the temperature the faster this reaction takes place.
When oxide growth takes place and oxide is present on the Silicon surface, it takes longer and longer for the Oxygen atoms to diffuse through the Thermal Oxide to form more oxide. As an example, at a temperature of 1000°C it takes about 5 hours of growth time in wet steam to yield an oxide layer 10,000Å thick. To show how diffusion affects oxide growth rate, it takes approximately 24 hours to grow a Thermal Oxide 25,000Å thick. That is almost 5 times the length of growth time, to achieve an oxide a little over twice as thick. That doesn't include temperature ramp up, ramp down times, push-pull and load-unload times. So a 25,000Å oxide thickness in reality is normally closer to a 28 hour process!
We should point out that all silicon wafers exposed to air at room temperature will grow between 12-19Å of what is called "native oxide". When this oxide reaches about 19Å it stops growing, any additional oxide must be grown at high temperature of >800°C. The picture below at the LEFT is an actual view of Oxygen atoms (light blue) combined with Silicon atoms to form SiO2 at the Silicon interface (picture taken with an atomic force microscope).
Dry Thermal Oxide is grown in much the same way as wet thermal oxide except instead of pyrogenic steam as the oxidation gas, only pure Oxygen is used. A dry oxidation produces a more uniform and denser thermal oxide with even higher dielectric strength than wet oxide. The major difference in the growth of wet and dry oxide is the growth rate, dry oxide grows much slower than wet oxide. For this reason dry oxides normally do not exceed 1000Å in thickness and are primarily used for thin gate and capacitor oxides where high uniformity and high dielectric strength are needed. Dry thermal oxides are also very sensitive to defects and resistivity issues within the Silicon wafers and for this reason Process Specialties recommends using only high quality Prime material when attempting to grow very uniform high quality dry thermal oxides. Remember, all thermal oxide growth both wet and dry is very dependent on the Quality of the Silicon start material, defects, resistivity problems, metallic or other types of contamination can all effect the uniformity and overall quality of thermal oxide. So if you are looking for a very uniform oxide <1 - 2% across the wafer, we strongly recommend using only factory sealed Prime or Test, non-Epi, non-Reclaim Silicon start material.
Process Specialties offers precision low metal Dry and Wet Thermal Oxide processing on 50mm to 300mm wafers, in a thickness range from 35Å to 25,000Å. See the Processing section of this web site for more information.
© 1998 Process Specialties, Inc.