Are there Tripods to film the bottom when writing on a paper?
There are a number of photo tripods that allow you to invert the center column.  When inverted, the camera sits between all the legs and can be pointed down.  Another tripod design has a center column that can also be shifted to a horizontal position.  This allows greater flexibility in terms of distance from the subject (and it also means you only have to shoot between the front two legs, rather than inside all three).