One shortcoming of large area, thinned scientific CCDs has been their lack of planarity. Areas of the silicon membrane that lie outside the depth of focus of the optical system lose resolution and cause a distorted image. Reticon has developed a thinning procedure that eliminates the ripples and bow often seen in thinned CCDs. This paper discusses the thinning procedure used to produce a flat, thinned, 2048 x 2048 pixel scientific CCD. The pixels are 13.5 micrometers square, making the image area of the CCD larger than 1' on a side. The final silicon membrane is between 16 and 18 micrometers thick and is flat to less than 5 micrometers peak-to- peak across the imaging area. Frontside devices have demonstrated a low dark current of 50 pa/cm2 at 20 C and full well of 120k electrons when the CCD is run in MPP mode. The charge transfer efficiency exceeds 0.99999 and the readout noise is less than 2.5 electrons when the CCD is operated at 20 kHz.