Electroluminescence Mapping Tool Accelerates LED Development
LED manufacturing usually involves optical measurements of epiwafers at various stages of production. Photoluminescence is frequently used after epitaxial growth to obtain a quick assessment of the optical properties of the material, while electroluminescence characterization is usually only carried out after full device fabrication.
This approach has several downsides, though, because these two optical measurements involve different physical mechanisms and produce different spectra. Photoluminescence is governed by the material s optical properties, but electroluminescence also depends on the physical structure of the layers, the electrical properties of their doped regions, and the properties of the electrical contacts.
The difference between the two spectra has several consequences. For example, it means that a measurement of the photoluminescence spectrum cannot directly predict the electroluminescence peak - in fact, for green LEDs the top contact can produce a significant wavelength shift in the photoluminescence. The difference also suggests that photoluminescence data should not be used to optimize device performance, and explains why epiwafers with high photoluminescence efficiency may not actually lead to devices with good electroluminescence characteristics.
Addressing the need for a tool that can provide epiwafer measurements is MaxMile Technologies, a characterization start-up headquartered in Lexington, SC, that has just released the first instrument that can provide electroluminescence mapping of as-grown LED epiwafers. This feature differentiates the tool from other LED wafer testing instruments such as the "BlueRay" system being developed by Suss MicroTec, which can only map the characteristics of LED die.
MaxMile s chief research scientist Max Ma revealed that it is the company s probe technology that has enabled it to make electroluminescence measurements on as-grown epiwafers. The instrument operates by using a metallic probe with a relatively large tip to form an LED in the epiwafer, which avoids any physical damage to the material as the tip is not sharp enough to scratch the surface. The probe s low current of 100 μA also prevents damage due to device heating, says Ma, although it does result in a small offset compared
with the packaged LED s emission peak wavelength, which is due to the differences in current density and operating temperature.
"We came to this idea from the research perspective, where we felt there was a need for a tool that can quickly determine device performance," explained Ma. The tool enables fast, non-destructive measurements of the epiwafer s electroluminescence, which can speed device development and lower the associated costs.
Volume LED manufacturers can also benefit from the rapid acquisition of electroluminescence data because they can use this information to optimize growth. This is not the only benefit the instrument can bring to the fab, though, says Ma, because it can also be used to streamline LED production by screening material immediately after it comes out of the reactor. The electroluminescence efficiency of a new LED design can even be predicted once an epiwafer featuring that design has been fully processed, as the data can provide a form of calibration. This reference data is required because the detected emission intensity can be influenced by various factors, including the type of substrate and whether it is polished.
The instrument can map various characteristics of 2, 3 and 4 inch epiwafers, and can complete four electrical or optical measurements on 100 sampling points in 10 minutes. It can determine the external quantum efficiency of the LED formed in the epiwafer, and its peak wavelength and total emission intensity. Other forms of LED characterization are also possible, such as the electroluminescence spectra at specific currents or voltages, and the device s current-voltage behavior and peak emission wavelength at various drive currents.
MaxMile s first instrument in the range, the EL-100, is available with either an ultraviolet-visible or visible- infrared detector, making it suitable for all forms of LED. In the coming months the company will also launch two other versions of the instrument, an EL-300, which can provide photoluminescence spectra, and an EL-500, which is capable of detecting diode currents as low as 1 pA, six orders of magnitude below the detection capability of the EL-100.