I doubt on the leakage of chamber. how to make sure that the pressure of the chamber is good enought?
Making sure you have a leak free system is important. You don't want water vapor in your chamber. Each process parameter plays a role in how the graphene grows so you need to determine what type of graphene you want. Reducing adlayers can be accomplished via long annealing times at high temperature (i.e. a smooth catalyst surface). Also, hydrogen partial pressure plays a role in growth dynamics so shape and adlayer formation can be controlled through that route as well.
A good way to check the system is to evacuate to a low pressure. Then, close off each end of the tube. If the system is leak free, the vacuum should stay close to the same. It may increase at a slow rate (tens of mtorr/min) but I have found that this is OK. If you have a large leak the pressure will return to atmosphere very quickly.
If I bring the pressure down to 10mtorr, it should not go above 500mtorr. Now, keep in mind, a perfect seal would mean no leakage. Also, each system is different so I would setup a few design of experiment studies and see how your system performs. Obviously, the end goal is high quality graphene so the exact values for your system parameters are not critical. Each group has their own recipe. The key is optimizing your unique system.
I can't design your experiment for you; however, I have added a citation below that will give you a good place to start.
Li, X., Cai, W., An, J., Kim, S., Nah, J., Yang, D., ... & Ruoff, R. S. (2009). Large-area synthesis of high-quality and uniform graphene films on copper foils. Science, 324(5932), 1312-1314.
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