Sapphire substrate thinning and polishing technology research

Preparation in blue LEDchip, usually adopt the method of growth gallium nitride materials. In the commercialization of LED , most sapphire as epitaxial substrate materials. Because sapphire material thermal conductivity is bad, so as to prevent the LED a source area of high temperature affect the light output characteristics and life, in the completion of electrode preparation process, must carry on the back thinning of sapphire substrate, in order to improve the cooling performance of the device. In addition, due to the sapphire mohs hardness up to 9.0, to meet the requirements of scribing, lobes and subsequent process, the substrate thickness thinning is also need to a certain extent. After thinning on the back of the substrate surface damage layer exists, the residual stress will lead to the epitaxial wafer after thinning bending deformation and easy broken out in subsequent process, which affect the yield. After thinning, therefore, must to polishing on substrate back, to remove the surface damage layer and eliminate the residual stress. In general, need to the thickness of epitaxial wafer thin from more than 400 mm to 100 mm or thinner nearby. Due to the high hardness sapphire, the back thinning and polishing work usually takes a long time, so, back thinning and polishing machining efficiency and quality has become a key problem. Through comparative experiments, this paper studied the abrasive particles in different degrees during back thinning, removal rate and surface roughness after grinding and grinding speed and grinding pressure, the relationship between surface roughness and polishing and polishing time, the relationship between for back thinning and polishing process provide a basis for the further optimization.

2 .the sapphire substrate grinding mechanism

Material removal of grinding way usually has three [1] : the mechanical abrasion function of abrasive grain and grinding surface melting and plastic flow, slightly abrasive surface active material in chemistry. The melting point of sapphire is as high as 2045 ℃, according to the conclusions of Bowden and Hughes, sapphire substrate is given priority to with mechanical abrasion function in the grinding. Under the action of abrasive, sapphire substrate surface will appear a lot of micro cracks, beneath the surface, and extended to the formation of a surface layer damage. In the grinding process, the micro cracks extend continuously, if cross each other and its surrounding areas of the sapphire material will fall off, form tiny pits and grooves, so as to achieve the grinding effect. Due to the high hardness sapphire, when polishing the removal volume is small, so before polishing, the damage on the reverse of the epitaxial wafer thickness must be down to the right degree, ability to go to net when polishing. The formation and extension of micro cracks, and surface damage layer thickness, are closely associated with the grinding process parameters. Known from the analysis of the above, the surface damage of the sapphire substrate layer will directly affect the condition of the grinding surface condition, which affects the surface roughness. Therefore, through studying the surface roughness, along with the change of the process parameters can be indirectly analysis layer surface damage.

                           3.Experiment

Use the Logitech company's precision polishing machine for LED epitaxial wafer back thinning and polishing. Waxes for the epitaxial wafer stick on the flat glass, and then fixed on the grinding fixture. Will clamp on apply good abrasives abrasive disk and start the machine, the fixture is driven rotating abrasive friction force on the plate, the abrasive cutting movement relative to the epitaxial wafer, which is thinning. This way of lapping movement belongs to single eccentric type, the same situation, the condition of abrasive particle size and so on the way to obtain a good surface quality and uniformity. Through the automatic feeding device on the machine to quantitatively offers fresh abrasives grinding disc. Grinding time through the timer to monitor on the machine, and grinding removal amount can be attached on the fixture thickness gage real-time monitoring. After grinding, thoroughly clean the glass and the epitaxial wafer, and nitrogen blow dry, then use thickness gauge to measure the thickness of epitaxial wafer, the thickness of the reference before grinding, with its thickness difference divided by the corresponding milling time, can remove rate are obtained. Grinding and polishing of the surface roughness are through the measured steps. Respectively prepared with 240 mesh and 600 mesh boron carbide abrasive abrasives grinding experiments. Mainly studied using 240 mesh boron carbide abrasive removal rate and surface roughness and grinding speed and grinding pressure, the relationship between fixed with the grinding pressure is 2300 g, respectively to obtain the speed of 35, and 55 RPM removal rate and surface roughness; Fix for 45 RPM speed, respectively for the grinding pressure for 1600230 0 and 3000 g of removal rate and surface roughness. Using a 600 mesh boron carbide abrasive, will take 45 RPM speed, the grinding pressure is set to 3000 g, got the removal rate and surface roughness after grinding. Use a alkaline SiO2 colloidal suspension to polishing on the epitaxial wafer after thinning back. In order to obtain the surface roughness with the changing rule of the polishing time, in the process condition unchanged, measured the polishing time is 20,40,60 respectively and 80 min of surface roughness, and on the basis of the comparison.

                4 .The experimental results and analysis

4.1 Use 240 mesh and analysis about boron carbide abrasive of the results

4.4.1 surface roughness, the relationship between removal rate and the speed

Substrate surface roughness trend along with the change of abrasive disk rotational speed, it can be seen that with the increase of rotational speed, the surface roughness decrease. Overall, the roughness value in the range of 485 + 80 nm change, keep in the same order of magnitude. Found in practical operation, due to the large granularity, 240 mesh boron carbide in the dispersion of abrasives is poor. Uneven distribution of abrasive on the disk, easy to produce deeper scratches, the surface roughness is bigger at low speed. The uniformity and the roughness decreases if improve speed. Substrate removal rate trend along with the change of abrasive disk rotational speed, with the increase of rotational speed, the removal rate is obviously rising trend. Grinding removal rate and unit time lining bottom as opposed to a tray of abrasive for cutting the number of the movement. After rising speed, abrasive particles move faster, grinding removal rate is correspondingly rise. Can shorten the grinding time, the increase of removal rate under the condition of the allowed, shall be appropriately increasing the grinding disc rotation speed.

4.1.2.  The surface roughness, the relationship between removal rate and 

grinding pressure

Substrate surface roughness trend along with the change of ground stress, it can be seen that with the increase of pressure, surface roughness of approximate linear downward trend. Overall, the roughness value in the range of 455 + 40 nm change, keep in the same order of magnitude. Since 240 for larger purpose boron carbide particles and distribution to a certain extent, the pressure is small, the sapphire substrate main contact with larger particles, so big pressure on the particles of the scratches deep accordingly. And has certain spacing between the larger particles, causing higher roughness. After the increase pressure, abrasive layer is extruded, the smaller particles can also contact with the substrate face, though within the larger particles into the sapphire substrate depth increase, but the smaller particles can scratch leveling effect of large particles, particles and spacing decreases, and thus overall, roughness can be reduced. Substrate removal rate trend along with grinding pressure.The removal rate of the approximate linear upward trend. After the increase pressure, larger particles into the depth inside the sapphire substrate, cutting ability to rise, and particle spacing decreases, and the actual participation in grinding particle number increase, so the removal rate will increase. If the pressure is too large, abrasive particles may be crowded broken, grinding rate will decline. Therefore, we should increase the grinding pressure at the appropriate scope.

4.2 The use of 600 mesh and analysis of the results of boron carbide abrasive 

With 600 mesh after boron carbide abrasive grinding, has found its removal rate measured 0.346 mu m/min, the surface roughness is 60.45 nm, and the same conditions, grinding disc of 45 RPM, the grinding pressure of 3000 g) using a 240 mesh abrasive, the removal rate of 3.4 mm/min, the surface roughness is 416.21 nm, visible removal rate and surface roughness has a change of order of magnitude. The cutting depth can be explained by the abrasive particles have a change of order of magnitude. Therefore, abrasive granularity is the decisive factors influencing the removal rate and surface roughness, and the influence of the rotational speed and pressure, can only make the removal rate and surface roughness in the same order of magnitude. In the back thinning, in order to shorten the time, shall, first of all, the larger the particle size, abrasive. In the epitaxial wafer thickness near the target thickness, and then switch to the smaller the particle size, abrasive, will damage layer thickness on the back to the extent polishing operation allows.

4.3 The surface roughness with the polishing time change

Polished surface roughness with the change of the polishing time, at the start of the polishing, thinned with the surface of the damage layer of the larger fluctuation under combined polishing liquid and polishing mat quickly removed, thus roughness fell rapidly, as the polishing, rolling on the surface of the epitaxial wafer gradually joint and soft polishing pads, roughness decreased speed slow, finally, the surface roughness reaches a stable value. Found in actual operation, the value depends on the choice of technological conditions such as pressure, for the polishing conditions can obtain the minimum surface roughness. According to the analysis above, should be to control the polishing time just can make the epitaxial wafer meet minimum roughness on the back.

5 Conclusion

Using 240 mesh boron carbide abrasive, through contrast experiment, found that the surface roughness increases with the rotational speed of grinding disc and decline, along with the increase of the grinding pressure decreases, range within the same order of magnitude, respectively for 485 + 80 + 40 nm and 455 nm; Grinding removal rate along with the increase of grinding speed and grinding pressure were obvious rising trend, which changes the approximate linear relationship. With 600 mesh abrasive removal rate and surface roughness after comparison, concluded that the abrasive granularity is the decisive factors affecting both, but speed and pressure can make the change in the same order of magnitude. After different polishing time by measuring the surface roughness value, found that when polishing the surface roughness decreased the rate of slow down gradually, finally reached a steady value, its size depends on the choice of technological condition. The conclusion of sapphire base LED epitaxial wafer back thinning and polishing technology provide the basis for the further optimization.