  We present the results of our project for the design and construction and
on-sky test of silicon grisms. The fabrication of such devices is a complex and
critical process involving litho-masking, anisotropic etching and direct
bonding techniques. After the successful fabrication of the silicon grating, we
have optimized the bonding of the grating onto the hypotenuse of a silicon
prism to get the final prototype. After some critical phases during the
experimentation a silicon grism with 363 grooves/mm and a blaze angle of 14
degrees has been eventually fabricated. The application of an A/R coating on
both the surfaces has been the last step: this procedure is critical because of
the groove geometry of the diffraction grating, whose performace might be
compromised by the coating. Then, the grism was inserted in the filter wheel of
the Near Infrared camera NICS, at the focal plane of the National Galileo
Telescope (TNG), the 3.5 m Italian facility in the Canary Islands (E). The
result of the on-sky tests are given in detail.
