fig2 illustrates that a provided metal piece 20 after being shaped can be cleaned with a degreasing agent solution any process such as machining or casting can form the metal piece 20 the metal piece 20 can be immersed in the degreasing agent solution having a temperature in an approximate range from 20 u00b0 c to 30 u00b0 c for 1 to 6 minutes the concentration of degreasing agent contained in the solution can be in an approximate range of 90 grams liter g l to 150 grams liter g l the metal piece 20 is washed with distilled water after being removed from the solution fig3 illustrates that a patterned photoresist layer 40 with a plurality of openings 41 can be formed on the surface 21 of the metal piece 20 in one exemplary embodiment each of the openings 41 can be substantially circular and arranged in an array in other exemplary embodiments the openings 41 can be substantially square or other shapes the openings 41 can be through holes or blind holes in the patterned photoresist layer 40 each of the openings 41 may have a diameter in a range from about 100 nm to about 300 nm fig4 illustrates that a plurality of first inclined holes 221 can be formed on the surface 21 of the metal piece 20 by plasma etching and the first inclined holes 221 can be arranged so as to be inclined toward the surface 21 in detail the first inclined holes 221 can be formed by a first plasma.