[en] The discovery of high temperature superconductors (HTS) has stimulated much thin film research in the Y₁Ba₂Cu₃O_7-x system. Most of the films have been grown on substrates with the perovskite crystal structure such as strontium titanate and lanthanum aluminate. Magnesium oxide (MgO) which has a simple cubic crystal structure has also been used. The structural match between Y₁Ba₂Cu₃O_7-x and MgO is not as close as for the perovskite substrates, but very good quality films can still be grown. The low microwave loss of magnesium oxide has made it a common substrate choice for thin film microwave devices. The authors have grown in situ Y₁Ba₂Cu₃O_7-x films on MgO substrates using an inverted cylindrical magnetron sputter gun with a variety of deposition conditions to study the effects on the films' properties. The best films were grown at T_x=800 degrees C and a total pressure of 400 microns, half oxygen and half argon, and had inductive T_c's with onsets at 86 K and 1 K wide. Inductive J_c's were greater than 10⁶ A/cm² at 4 K and greater than 10⁴ A/cm² at 77 K. The authors studied the effect of varying the deposition temperature while keeping the pressure fixed at 300 microns, and found that the best overall film (sharpest T_c and smooth surface) occurred for T_s=750 degrees C. Other properties were optimized at other T_s. Initial measurements of the photoresponse of a 1500 angstrom film grown over 2000 angstrom step in the substrate showed the expected bolometric response near T_c and a possible non-bolometric response at low temperatures