X-RAY-INTERFEROMETRIC DETERMINATION OF ANGSTROM-SCALE LATTICE SHIFTS AT THE SURFACE OF SILICON-CRYSTALS - THE ANALOG TO LIGHT-OPTICAL INTERFERENCE MICROSCOPY

The paper presents a new imaging method based on a refined and better stabilised X-ray Michelson interferometer [APPEL, BONSE]. Interference fringes are detected which mark local Angstrom-scale shifts of the la ttice at the surface of the interferometer's phase-shifter crystal. Co nventional double-crystal topography (DCT), lacking phase information of the beam reflected by the specimen crystal, does not detect mere la ttice shifts. The new method appears to be also more sensitive to any combined lattice strain delta = Delta d/d + cot theta(B) Delta theta ( theta(B): Bragg angle, d: lattice parameter) present in the crystal la yer contributing to the surface Bragg reflection. delta is known to be the dominant part of the DCT image [BONSE, 1958; BONSE, HARTMANN]. Th e technique presented here is the X-ray analogue to the well known lig ht-optical interference microscope. A possible application is the test ing of silicon crystals for minute lattice imperfections causing latti ce-incoherent positions of atoms in a surface layer of typical 1 to 10 mu m thickness. For instance striations, which under certain conditio ns arise during crystal growth, cause such incoherence.