Accuracy – Truncating a number in mathematics

I want to truncate a simple number to n decimal digits. For example, 2/3. used to

F[x_, n_] : = N[IntegerPart[x 10^n]/ 10 ^ n]

but I get it

F[2/3, 20] = 0.666667

There is a way to get

F[2/3, 20] = 0.6666666666666666666666 ?

I have searched a lot but I really have not found an answer. I'm looking for something like north or Round for truncation.