Lecture 25 (CHE 323) CVD, part 2

[@ChrisMack]

PDF copies of all the slides in this course are available at: www.lithoguru.com/scientist/CHE323/course.html

[@animeshmondal1208]

At lower pressure, there is a lesser number of collision. So diffusivity increases a lot while the distance the gas needs to travel does not change a lot. The result is much higher mass transfer at low pressure.

[@possible-realities]

In the diffusion controlled regime, as you increase the total pressure, it takes longer time for the reactant to reach the surface, right? Does this mean that you actually have to lower the partial pressure of the reactant to keep down the ratio of homogeneous reaction to heterogeneous reaction as the total pressure goes up?

[@xiangyuliu4761]

At 12:42, should the boundary layer thickness increases with increasing pressure? Here with higher pressure, gas velocity decreases, which reduces the Renold number, hence leads to thicker boundary layer?

[@tinyluo123]

Another great thanks to Chris from my friend who watch the video using my account , haha

[@tinyluo123]

to I would like to answer all the question 1. rxn limited and diffusion limited regime are distinguished by temp since rxn rate is more sensitive to temp than diffusion coefficient. At high temp, it is diffusion limited and at low temp , it is rxn limited. 2. in cvn design, we want to achieve high dp rate while making the process easier to control as well as better quality of thin film (uniform, step coverage). Hence, inherently, diffusion limited regime at high temp is chosen first due to its inherent high dp rate nature. however, gas flow is harder to control than temp. then, we switch to rxn limited. Moreover, we find that lower pressure by adding inert purge gas helps to increase the diffusion coefficient and extend the rxn limited regime to high temp and thus overcome its problem of slow throughput. 3. decreasing total pressure increase diffusion coefficient and increase the dp rate largely for diffusion limited regime and rxn limited regime at high temp. partial pressure of reactant gas is maintained to prevent homo nucleation. 4. Ads are obvious , apcvd is bad because gas flow is hard to control and high pressure --shadowing effect,poor step coverage. lpcvd is bad because sometimes high temp is not prefered and deposition on the hot wall need to regularly cleaned. pecvd still has a limited output and may induce contaminants from plasma. A great thanks to Chris. You are my enlightened tutor in thin film deposition of wafer fabrication

[@Hndm27]

Hot wall system must be Diffusion controlled regime bcz you have shown in graph that at high temperature equivalent Kt is hg.please help