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Improve metrology for sapphire substrates used in LED production

Matt Novak, Bruker Nano- October 16, 2012

Strategies United predicts that by 2015 the global market for high brightness LEDs (HB-LEDs) will generate revenues of $18.9 billion, representing a compound growth rate of 11.8%. A key factor in the growth of HB-LEDs is the use of patterned sapphire substrates (PSS) to enhance light extraction efficiency. PSS’s demanding metrology requirements can have a major impact on wafer yield. Specialized 3D optical microscopes can quantify the height, width and pitch of PSS features on production wafers and readily spot defective features with greater speed and, hence, higher sampling ratios than other techniques. The end result in many cases is a substantial increase in yield that can pay back the cost of the instrument in a short period of time.

PSS background
Over the last several years, HB-LEDs have penetrated a number of new market applications such as display chips and solid-state lighting. A key factor in growth of HB-LEDs is the use of PSS to increase light extraction efficiency. The PSS technique consists of etching a repetitive grid structure into the sapphire to scatter light confined in the Gallium Nitride (GaN) layer. Besides increasing light extraction efficiency, PSS structures reduce dislocation defect density in the GaN layer, resulting in improved efficiency. As a general rule, the smaller the feature size of the grid structure, the greater the improvement in light extraction efficiency. Obtaining maximum efficiency for the reflector requires achieving consistent PSS grid geometry.

Dry etching is the most common method for producing PSS wafers. A layer of photoresist is deposited onto the sapphire substrate. Lithography exposes a step pattern onto the photoresist. Exposure to fluoride-based plasma gas and microwave energy anisotropically etches the crystalline structure to form a dense, uniform patterned of cones or pyramids. Dry etching rates typically run in the order of 50 to 200 nanometers (nm) per minute, and a standard 2-inch wafer typically takes between 30 and 60 minutes to etch.

Role of metrology
Metrology tools play a critical role by ensuring PSS structures are created with the right height, width and pitch. The gold standard for PSS shape measurement is the atomic force microscope (AFM), which is widely used by both wafer suppliers and LED manufacturers because it can directly measure the entire PSS surface, even where it is steeply sloped. In atomic force microscopy, a sharp probe is brought into close proximity to a sample. Probe and sample are subsequently moved relative to each other in a raster pattern, and a quantity is measured in a serial fashion at discreet locations. Interaction between the tip and sample is measured by monitoring the displacement of the free end of the attached cantilever.

Substrate wafer suppliers typically utilize the AFM to measure the smoothness of the wafer after the polishing process and to spot film defects in epitaxial layers. They also use the AFM to provide the dimensional characterization of the PSS at atomic resolution. Dedicated instruments of this type play a critical role in both research and development and production applications by performing automated measurement, data collection, data analysis and report generation on PSS wafers.

However, wafer suppliers involved in production applications usually require greater measurement speed than can be achieved by an AFM. It typically takes these instruments several minutes to process a single site on the wafer, and wafer suppliers typically need to process 5 to 10 locations per wafer depending on wafer size and feature specifications. AFM measurement speed typically limits the proportion of wafers that can be sampled. Thus, a process deviation that causes out of specification wafers to be produced may not be detected until a considerable number of bad wafers have been etched. Wafers that have not been correctly patterned need to undergo re-polish, which reduces the thickness of the wafer to the point that it cannot go through the patterning process again. The wafer normally must be sold as an unpatterned sapphire substrate at a considerably lower selling price.