时钟树设计。时钟树，是个由许多buffer cell平衡搭建的网状结构，它有一个源点，一般是clock input port，也有可能是design内部某一个cell output pin，然后就是由一级一级的buffer cell搭建而成-Clock tree design. Clock tree, is balanced by the number of buffer cell to build the network structure, it has a source, usually