To improve the energy resolution (Delta E) of Nb/Al superconducting tunnel junctions (STJs), an ozone (O-3) oxidation process has been developed to fabricate a thin defect-free tunnel barrier that simultaneously shows high critical current J(C) > 1000 A/cm(2) and high normalized dynamic resistance R(D)A > 100 M Omega.mu m(2), where A is the size of the STJ. The 50-mu m(2) STJs produced by O-3 exposure of 0.26 Pa.min with an indirect spray of O-3 gas, which is a much lower level of exposure than the O-2 exposure used in a conventional O-2 oxidation process, exhibit a maximum J(C) = 800 A/cm(2) and a high R(D)A = 372 M Omega.mu m(2). The 100-pixel array of the 100-mu m(2) STJs produced using the same O-3 oxidation conditions exhibits a constant leak current I-leak = 14.9 +/- 3.2 nA at a bias point around Delta/e (where e is half the energy gap of an STJ), and a high fabrication yield of 87%. Although the I-leak values are slightly larger than those of STJs produced using the conventional O-2 oxidation process, the STJ produced using O-3 oxidation shows a Delta E = 10 eV for the C-K alpha line, which is the best value of our Nb/Al STJ x-ray detectors.