  We report a self-aligned, monolithic electron interferometer, consisting of
two 45 nm thick silicon layers separated by 20 $\mu$m. This interferometer was
fabricated from a single crystal silicon cantilever on a transmission electron
microscope grid by gallium focused ion-beam milling. Using this interferometer,
we demonstrate beam path-separation, and obtain interference fringes in a
Mach-Zehnder geometry, in an unmodified 200 kV transmission electron
microscope. The fringes have a period of 0.32 nm, which corresponds to the
$\left[\bar{1}\bar{1}1\right]$ lattice planes of silicon, and a maximum
contrast of 15 %. This design can potentially be scaled to millimeter-scale,
and used in electron holography. It can also be applied to perform fundamental
physics experiments, such as interaction-free measurement with electrons.
