Cinecom.net: How to Chroma/Green Screen Effectively in Premiere Pro

Third party chroma keyers certainly offer more, but the Ultra Key in Premiere Pro is more than sufficient for most green screen shots. Of course, as Jordy Vandeput mentions in the beginning of this Cinecom.net tutorial, the basis of a good key is the quality of your footage. Assuming you have a good shot, Jordy will who you how to get a high quality key in Premiere Pro. At 2:06, Jordy demonstrates the value of garbage matte (a mask that eliminates problem areas in your shot) and at 3:08, he shows how the alpha channel output view can help you clean up your key. 

Learn how to pull a clean chroma/green key in Adobe Premiere Pro. See the options of the Ultra Key effect to achieve a professional result. Remember the last film you saw? Or watched the weather forecast? Chances are it was full of green screens. —Cinecom.net

Source: https://youtu.be/SNNZ-ZG_j4M