Red LED Thin Film Waferganwafer
LED epitaxial wafers are located in the upstream link of the LED industry chain, which is the link with the highest technical content in the semiconductor lighting industry and the greatest impact on the quality and cost control of the final product. Ganwafer can supply red LED epitaxial wafers grown on GaAs substrate. Just offer one epi structure of LED thin film wafer for reference as following:
1. Epitaxial Structure of Red LED Wafer
|Buffer layer n-GaAs||–|
2. Technical Support for GaAs LED Thin Film Epitaxy on Etching
One of our customers buys this structure to make light emitting devices. During the LED thin film device fabrication processes, they have met following problems at GaAs etching step, as figure 1 shows:
1) The LED layer is damaged. LED layers below the buffer layer are etched;
2) Non uniform etching of the LED chips in the same LED wafer
– Within the one wafer, LED chips have different height;
– Group A LEDs are etched to the p-GaP, Group B LEDs to the DBR layer, and Group C LEDs to the GaAs substrate, as figure shown below;
3) LED layer is too thick; therefore, they want the LED layer thickness to be 3 um.
Fig.1 Etching of Group A, B, C LEDs
Our technicians explain that it looks like flip-chip. Structurally, it is positive subepitaxy etching. But when it comes to chip height inconsistencies, it seems that it is because of your uneven etching. For damage of the epitaxy layer, we are not sure it is caused by coarsening or other process operation after the chip is cut. According to our knowledge, it should be cut into chips to make a similar roughening process. Thickening the buffer layer to protect the epitaxy layer, but we don’t think the effect is necessarily good.
We speculate about specific reasons because specific chip technology is not known:
1) The high inconsistency should not be the problem below or the uneven surface etching.
2) If the buffer layer is corroded, it should be side corrosion. If the solution corrodes GaAs, the thickness of side corrosion will not necessarily alleviate.
If it is not intentional to keep GaAs buffer, the dry etching is no problem, and the problem would be in wet etching, so as to solve the problem of non-uniform etching, why not add a corrosion stop layer (etch stop layer), like GaInP? GaInP is usually used as etch stop layer. It can be added between the n-GaAs buffer layer and the DBR layer of the ulta thin red led thin film wafer. Normally, it’s no problem that stop layer will protects the LED layers from the GaAs echant (mixture of citric acid and hydrogen peroxide) during GaAs wet etching process. Please note the remaining GaInP would affect forward voltage, but not too much, and optical power would be decreased of LED chip after GaAs wet etching process.
3. Measures to Improve the Uniformity of Wet Etching of Thin Film LED Epi Wafer
Etching uniformity is an etching parameter that measures the level of wet etching process. Etching uniformity has a great impact on the quality of semiconductor wafers, including LED thin film materials. Incomplete etching or over etching will reduce the quality of GaAs red LED thin film semiconductor, and even lead to the scrapping of etched devices. Therefore, it is necessary to strictly control the etching uniformity in the wet etching work to ensure the quality of the etched wafers. The measures are proposed as following:
1) Proper mixing: By installing a stirring device in the etching tank, continuous stirring is carried out during the GaAs LED wafer etching process, so as to ensure that the temperature and concentration of chemical agents can be evenly distributed, thereby improving the uniformity of wet etching.
2) Overflow circulation of chemical etchant: The temperature, concentration and flow field of chemical solution can be uniformly distributed through overflow circulation, thus improving the uniformity of wet etching.
3) Rotating wafer during etching: In the chemical solution treatment process of the LED epitaxy wafer, by rotating the wafer, a part of the wafer is prevented from always being in the slot of the wafer box, thus improving the uniformity of chemical solution corrosion on the wafer.