07/11/2017

Patterned Sapphire Substrate Process: Increase your yield with the use of intelligent FRT measurement technology

Whatever is produced, no matter what material and in which quantities – impeccable quality is achieved only by consistently high perfection. And this must be checked continuously. Sensors and measurement technology can help to increase productivity in process control, thereby reducing production costs. FRT measuring technology creates the ideal conditions for optimally coordinating the individual steps in a process chain.

In the production of LEDs, periodically structured sapphire substrates are used to increase the light output of the diodes. In the patterned sapphire substrate process (PSS process), structures such as cones or pyramids in the micro- to nanometer range are etched into smooth sapphire wafer structures. In the case of an unstructured wafer, a plurality of photons emitted from the active layer of an LED will never leave it due to total reflection. By structuring the PSS process, the amount of total reflection is reduced, so more photons leave the LED and the light output increases. In addition, the structuring of the substrate results in a higher quality of the subsequently deposited layers in the LED manufacturing process. As a result of this, the light output is also increased.

The structuring of the substrate surface is produced by nanoimprint or photolithography and subsequent etching. For photolithographic structuring, the structure of a photo mask is transferred by means of an exposure system to a photo resist layer and is thus predetermined for the etching process. The image of the mask can not cover the whole wafer, so the structure is successively transferred to the wafer in a special exposure device, called stepper. For the highest possible quality and yield of the exposure process in the stepper, the light must be equally well focused at all positions on the wafer. Deflections and thickness variations of the wafers lead to defocusing during the exposure and thus to fuzzy structuring, which then have to be reworked, sometimes with great effort, or even cause rejects. A prediction of the quality and the yield was so far not reliably possible.

At this point, the FRT measurement technology offers the possibility to classify the wafers before structuring. Depending on the quality of the wafer, corresponding steppers can be used accordingly, or the structuring process will not be performed at all if the wafer is too unsuitable. TTV measuring systems from FRT, such as the MicroProf® 200 TTV and MicroProf® 300 TTV, enable a highly precise measurement of the thickness variation of the wafers. In addition to global parameters, a local thickness variation (LTV) of the wafers can also be determined and displayed in a local wafer map. Our MicroProf® MHU can also be supplied with TTV option. It has a fully automatic wafer handling unit, with high throughput rates of over 200 wafers per hour. In addition, the samples can be sorted according to individual customer specifications.

Based on the measurements with the FRT devices, it is possible to predict the yield of the photolithographic process on the wafers for the different steppers and thus optimize the yield.

In this way, production costs as well as time losses can be reduced in the PSS process by post-processing. In addition, PSS structures can also be measured and evaluated after litography and etching by using a FRT multi-sensor system, e. g. equipped with a confocal microscope (CFM) and/or an atomic force microscope (AFM).

You see FRT measuring instruments create advantages – in many areas. We certainly have a solution for your specific application. Do not hesitate to contact us with any questions. Our experts will be glad to take care of your needs and work out individual solutions for you.