referring to fig1 , an embodiment of a method for fabricating led units is described as follows : in step s 1 , a sapphire substrate 10 , which has a round flat surface , is provided . in step s 2 , referring to fig2 , a gan epitaxial layer 20 is formed on the flat surface of the sapphire substrate 10 . in one embodiment , the gan epitaxial layer 20 is formed using metal organic chemical vapor deposition ( mocvd ). the gan epitaxial layer 20 includes a lamination of an n - gan layer 21 , an active layer 23 , and a p - gan layer 25 . in alternative embodiments , the gan epitaxial layer 20 may also be formed with other appropriate epitaxial growth methods , such as molecular beam epitaxy ( mbe ) or hydride vapor phase epitaxy ( hype ). in step s 3 , referring to fig3 , a plurality of exhaust trenches 27 is formed in the gan epitaxial layer 20 using reactive ion etching , preferably inductively coupled plasma ( icp ) reactive ion etching . the exhaust trenches 27 pass through the n - gan layer 21 , the active layer 23 and the p - gan layer 25 of the gan epitaxial layer 20 . a width of each exhaust trench 27 is preferably in a range from about 10 microns ( μm ) to 500 μm . referring also to fig8 , the plurality of exhaust trenches 27 is arranged substantially perpendicular to each other , and defines a preserve region 28 in a middle portion of the gan epitaxial layer 20 ( bounded by a broken line ) and a sacrifice region 29 in a marginal portion of the gan epitaxial layer 20 surrounding the preserve region 28 . in the preserve region 28 , the exhaust trenches 27 further define a plurality of individual led units 281 having the same shape . each of the individual led units 281 is beneficially a square having a width in a range from about 100 μm to about 2000 μm . the individual led units 281 are arranged in a matrix defined by the exhaust trenches 27 . formation of the exhaust trenches 27 includes forming scribe lines and etching . scribe lines are formed on the gan epitaxial layer 20 using a photo - resist coating , which is beneficially fabricated from a relatively hard photo - resist material that withstands intense plasma . the photo - resist coating is then patterned to form the scribe lines . the gan epitaxial layer 20 is then etched along the scribe lines using an icp etcher to form the exhaust trenches 27 . in alternative embodiments , the exhaust trenches 27 may be formed using other etching methods such as chemical etching . in step s 4 , referring to fig4 , a reflective layer 30 is formed on the gan epitaxial layer 20 using plasma - enhanced chemical vapor deposition ( pecvd ). the reflective layer 30 may be a prague reflective layer , or a metal reflective layer comprising ag , ni , al , cu , or au . the reflective layer 30 is configured to reflect light generated in the active layer 33 to direct the light towards the n - gan layer 21 . in alternative embodiments , the reflective layer 30 may be formed using physical vapor deposition ( pvd ), sputtering , electroplating , or other suitable means . it should be noted that , during the formation , a photo - resist or a mask can be used to prevent the reflective layer 30 from forming in the exhaust trenches 27 . in step s 5 , referring to fig5 , the reflective layer 30 is attached to a conductive substrate 40 using wafer boding or electroplating . in this embodiment , a nickel layer is electroplated on the reflective layer 30 to form the conductive substrate 40 . in step s 6 , referring to fig6 , the sapphire substrate 10 is removed form the gan epitaxial layer 20 using an llo process . in this embodiment , a laser beam 50 emitted by an excimer laser irradiates the sapphire substrate 10 from a side thereof without forming the gan epitaxial layer 20 . the laser beam 50 having a wavelength of about 300 nm almost completely passes through the sapphire substrate 10 , and is then absorbed almost completely in the gan epitaxial layer 20 , whereby a temperature of an interface of the sapphire substrate 10 and the gan epitaxial layer 20 rapidly rises . this result in decomposition of gan into gallium and nitrogen and formation of a gap 11 between the sapphire substrate 10 and the gan epitaxial layer 20 . nitrogen generated at this time can exhaust from the exhaust trenches 27 , such that cracks on the gan epitaxial layer 20 can be effectively reduced with no pressure applied to the individual led units 281 . after the removal of the sapphire substrate 10 , excessive gallium is drained by acid etching or the like . referring also to fig9 , in order to increase the efficiency of the laser , the laser beam 50 projects a light spot 51 on the gan epitaxial layer 20 . the light spot 51 is substantially square , and of the same size as that of the individual led unit 281 . the llo process further includes : projecting an initial light spot 51 on a marginal portion of the sacrifice region 29 aligned with a first row of the individual led units 281 . the initial light spot 51 intercrosses a rim of the gan epitaxial layer 20 , in other words , part of the initial light spot 51 extends beyond the gan epitaxial layer 20 . accordingly , the gap 11 formed by the initial light sport 51 directly communicates with the environment , and nitrogen generated at this time can also exhaust from the gap 11 . moving the initial light spot 51 along the exhaust trench 27 downward into the preserve region 28 , thus scanning one individual led unit 281 of the first row . because the initial light spot 51 is aligned with the first row , and the initial light spot 51 has the same shape and size of the individual led unit 281 , the initial light spot 51 can entirely cover the individual led unit 281 , such that the whole of the individual led unit 281 separates the sapphire substrate 10 immediately . continuing moving the initial light spot 51 along the exhaust trench 27 and scanning each individual led unit 281 in the first row . when the initial light spot 51 passes through the preserve region 28 and reaches another sacrifice region 29 , stopping emission of the laser beam 50 . adjusting the excimer laser such that a new light spot 51 is projected on the original sacrifice region 29 aligned with a second row of the individual led units 281 . the second row is adjacent to the first row . repeating movement of the initial light spot 51 until each row of the individual led units 281 is scanned . in step s 7 , referring to fig7 , the conductive substrate 40 is cut along the exhaust trenches 27 to be separated into chip - shaped pieces by dicing or the like . the gan epitaxial layer 20 in the sacrifice region 29 is discarded , and a plurality of individual vertical topology gan led units 60 are then obtained by the individual led units 281 in the preserve region 28 . the present method for fabricating led units 60 includes forming a plurality of exhaust trenches 27 on the gan epitaxial layer 20 , such that nitrogen generated during the llo process can be exhausted from the exhaust trenches 27 . thus , cracking caused by high - energy laser beam irradiation on the gan epitaxial layer 20 is reduced , and fabrication yield of the led units 60 can be increased . it is believed that the present embodiments and their advantages will be understood from the foregoing description , and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages .