the first-order means linear approx. literally. But it is a little bit misleading in our case. My understanding is that first-order means major effect. e.g. gm is a first-order parameter while ro is not.
implantation is a IC process, you can find in lots of textbook. It can implant donnor or acceptor atoms into selected area in the bulk, with better density and vertical distribution control than diffusion.
This term goes back to the days of hand calculations where functions were of polynomial form. If you used the x term it was first order, if you kept the x squared term it was second order. Thus you had a first order or a second order approximation of the function.