  We present a study of direct laser driven electron acceleration and scaling
of attosecond bunch compression in unbound vacuum. Simple analytical
expressions and detailed three-dimensional numerical calculations including
space charge reveal the conditions for compression to attosecond electron
sheets. Intermediate emittance minima suitable for brilliant x-ray generation
via coherent inverse Compton scattering (ICS) are predicted. We verify the
coherent emission properties of the resulting x-ray fields and demonstrate
feasability for realistic laser parameters.
