  We developed an electron beam size monitor for extremely small beam sizes. It
uses a laser interference fringe for a scattering target with the electron
beam. Our target performance is < 2 nm systematic error for 37 nm beam size and
< 10% statistical error in a measurement using 90 electron bunches for 25 -
6000 nm beam size. A precise laser interference fringe control system using an
active feedback function is incorporated to the monitor to achieve the target
performance. We describe an overall design, implementations, and performance
estimations of the monitor.
