8.25 Sspd_chapter 6_part 7_introduction to athena 6_process simulator  (Page 6/7)

The minimum set of diffusion step parameters is as follows:

• Time (e.g., 60 minutes)

• Temperature (e.g., 1100° Celsius)

• Gas pressure (1 atmosphere is default)

The following input file statements will appear:

# DRIVE-IN

DIFFUSE TIME=60 TEMP=1100 NITRO PRESS=1.00

If you choose the Ramped box and End Temperature or Temperature rate, a ramped temperature thermal step is simulated. The temperature rate is a variable by default, but it can be set to a specific constant temperature rate by selecting Constant in the Rate box. If the End Temperature is set to 1000, the following lines appear:

# RAMPING DOWN

DIFFUSE TIME=60 TEMP=1100 T.FINAL=1000 NITRO PRESS=1.00

The same pull down menu used for inert diffusions is also used for oxidations described in the “Simulating Oxidation” Section on page 2-39. But, since there are special considerations for inert diffusions which come under the category of Rapid Thermal Anneals (RTA), the special notes pertaining to this specific set of conditions are described in the next section. These notes are very important for accurate simulation of high temperature, short duration anneals. We recommend that you read these notes before attempting to write the RTA section of the input file.

7.7.8: Simulating Rapid Thermal Anneals (RTA) Notes

The usual reason for employing a Rapid Thermal Anneal (RTA) in a process flow is to anneal out damage in the substrate that has been caused by a previous process step, usually an implant, while at the same time minimizing dopant diffusion. Dopant activation also occurs during this process. These anneals are usually high in temperature and low in duration for sound device physics reasons.

Once again, the key to accurate simulation of RTA lies in the accurate simulation of substrate damage behavior. The role of interstitials in enhanced dopant diffusion has already been explained in Section 2.4:“Choosing Models In SSUPREM4” to become familiar with the role of interstitials during process simulation.

The reason why an RTA usually employs high temperatures and short durations is because for a given high dose implant, if an anneal duration is selected so that a fixed percentage of the damage is annealed, the lower the anneal temperature, the more dopant diffusion occurs.

The above statement requires an explanation since intuitively, the opposite would seem more likely. A descriptive explanation of what is happening can be informative if the two extremes of anneal temperature are considered.

For the lowest anneal temperatures, the damage anneal rate is almost zero, so dopant diffusion rates are enhanced by a factor of 1000°C or more for the long time periods required to remove the damage. This results in high total dopant diffusion.

For the highest temperature anneals, a significant percentage of damage removal occurs in a fraction of a second. Almost zero damage enhanced diffusion or total diffusion occurs in this instance, and the anneal time to remove the damage is very short. Extrapolating between these extremes provides a qualitative explanation of what occurs for intermediate temperature anneals.