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Frequently Asked Questions (FAQ) about MaxMile EpiEL Technologies/systems

  1. What is EpiEL?
  2. Why is EpiEL useful?
  3. Can EpiEL system characterize EL of wafers with insulated substrate, such as GaN on sapphire substrate?
  4. Is there any difference between your EpiEL measurement and device test?
  5. The EpiEL mapping and device data show some similar patterns and some quite different results. Am I misinterpreting your maps?
  6. EL results will be different for different device structures, especially for different light extraction approach. Can EpiEL system reflect these differences?
  7. We have the similar system already, but it is not accuracy & right. Many parameters effect epi-wafer, such kind technology can not tell the truth.
  8. We can use device process to control quality. If the processed wafer is OK, the rest wafers of same batch would pass the quality control. This procedure is pretty reliable. Why do we need EpiEL system?
  9. How can you demonstrate the capability of your system? Do you provide on-site system demonstration?
  10. Do you provide wafer scanning service?
  11. If I send my samples, how is the turn-around time of your wafer characterization service?
  12. How about throughput/speed of your EpiEL system? Can it meet high throughput requirement?
  13. Is there any consumer goods need to prepare?
  14. What is the probe size?
  15. Can I use EpiEL probes alone and develop own system?
  16. Is there any competitive products?  What is superiority of EpiEL?
  17. I have a PL mapper already, why do I need an EpiEL system?
  18. What is EpiEL/PL mapping wavelength range?
  19. Is there any contamination you expect during probing? I need to process the wafer after probing; do I need to do some surface pre-treatment after EpiEL probing?
  20. Will your tool damage the surface of wafer after EpiEL mapping measurement?
  21. I was wondering what the distance between two probes is and what kind of metal you use for the probe. Is it Pt or Cu?
  22. Please tell me any restrictions on materials for your EL mapping. As you know, it is hard to make a contact on p-GaN itself. How do you make p-contacts and n-contacts without fabricating device?
  23. Where can I find more information about EpiEL?
  24. What is lead/delivery time if place the order?

  • What is EpiEL?

EpiEL is a technology developed by MaxMile, which can be used for electroluminescence (EL) measurement directly on epitaxial materials, such as LED epiwafers.

EL measurement is usually performed on the finished devices (such as LEDs) since it needs a device structure to inject current. Conventional electroluminescence evaluation could not provide fast response for material development since the fabrication of devices is usually time-consuming and costly. MaxMile EpiEL mapping technology overcomes this limitation by temporarily forming a well defined light emitting diode (LED) inside the material. It characterizes the electroluminescence behavior of light emitting material as the finished device functions through electroluminescence.

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  • Why is EpiEL useful?

EpiEL is an unprecedented characterization/simulation solution for optoelectronic industry. It provides virtual LED device fabrication & characterization system which can be used to measure LED device parameters directly on epiwafers. Without any costly and time-consuming device fabrication, yet as through a finished device, EpiEL reveals not only the electro-luminescence (EL) but also the electrical properties of the material. With such unique capability, EpiEL can enable device-level quality control at early stage and on every wafer, provide instant feedback for material development, enable "fabless" LED material and CVD system development. EpiEL is also a powerful tool which can be used to simulate the different light extraction approaches in different device structure. It can assemble any device-level optical and electrical properties of the material in a fabless manner which could significantly reduce the R&D time and cost.

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  • Can EpiEL system characterize EL of wafers with insulated substrate, such as GaN on sapphire substrate?

Yes. EpiEL system uses proprietary electrode contact technology which can be used to obtain EL from any type of LED epi-wafers with insulated or conductive substrates.

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  • Is there any difference between your EpiEL measurement and device test?

The correlation between EpiEL and device depends on device structures and test conditions.

Generally speaking, the wavelength related information, such as WLP, WLD, WLC, FWHM, provided by EpiEL will be the same as those from device test if both test conditions are similar (with same equivalent current). Since the spectrum of LED will change as the driving current changes, the wavelength-related information might be slightly different between EpiEL and device results due to different device size and driving current. This difference can be correlated through the procedure called equivalent driving current.

Due to a variety of device structures, the correlation of EL intensity-related information, such as emissive intensity, peak intensity, external quantum efficiency, between EpiEL measurement and device results will be strongly related to their corresponding light extraction approaches. The intensity-related information revealed by EpiEL and device test will be similar if both light extraction methods are similar. EpiEL system can be configured to best simulate the specific device structure.

The EpiEL voltage will be related to probe device size and contact resistance.  For nitride-based application, electrical information provided EpiEL is strongly related to p-layer activation condition; and some wafers might indicate larger values due to improper p-activation. If the size of device is similar, usually the driving voltage at small driving current (including turn-on voltage) provided by EpiEL will be similar to that from real device, while driving voltage at high driving current will be slightly higher than those in real device.

MaxMile latest EpiEL technology gives out device Vf estimation which is close to real device value.

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  • The EpiEL mapping and device data show some similar patterns and some quite different results. Am I misinterpreting your maps?

In order to correlate EpiEL results with device data, please pay attention to the following factors:

  • Wafer orientation.

  • Color encoding system: different color encoding in EpiEL and device test could cause misunderstanding. MaxMile EpiEL system has the capability to customize its color encoding system. MaxMile also offers the service to generate the color-encoded mapping from customers' device data.

  • Test condition: different test conditions could lead to different results. For more information, please check the question "Is there any difference between your EpiEL measurement and device test?" in this FAQ.

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  • EL results will be different for different device structures, especially for different light extraction approach. Can EpiEL system reflect these differences?

There is a way for EpiEL technology to correlate its test results with specific device structure. EpiEL system (EpiEL-700) can be configured to best simulate the corresponding device structure.

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  • We have the similar system already, but it is not accuracy & right. Many parameters effect epi-wafer, such kind technology can not tell the truth.

EL measurement provided by MaxMile EpiEL system is pretty accurate and reliable. To our knowledge, currently there is no other similar system which has same performance level (device-level EL measurement directly on epiwafer).

There are a couple of quick tests which have been used by the LED industry. One of them uses indium dots temporarily soldered on wafer surface, the other one uses simple metal probes. The EL measurement provided by these techniques may not be accurate and reliable, and corresponding formation of contacts could damage wafers.

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  • We can use device process to control quality. If the processed wafer is OK, the rest wafers of same batch would pass the quality control. This procedure is pretty reliable. Why do we need EpiEL system?

It is a matter of efficiency and cost effectiveness. The device process "destroys" control wafer, and it is costly and takes much longer time. MaxMile EpiEL system provides the device level results just as device process does. The rapid and nondestructive nature of EpiEL system enable the manufacturer to perform "device test" on each wafer (100% quality control), and provide instantly feedback for material development. Furthermore, EpiEL can use a near identical device test condition, it will provide an improved wafer evaluation for quality control.

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  • How can you demonstrate the capability of your system? Do you provide on-site system demonstration?

Currently MaxMile offers free sample characterization service to demonstrate the capability of EpiEL system. Samples will be returned right after EpiEL scanning. Customer can evaluate MaxMile EpiEL system through correlating EpiEL mapping results with their device test results.

MaxMile can arrange for on-site system demonstration if the customer is serious in purchasing EpiEL system. MaxMile suggests customer to evaluate EpiEL systems through free sample characterization first.

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  • Do you provide wafer scanning service?

Yes. MaxMile does provide wafer scanning service. First couple samples will be free for each customer to evaluate the effectiveness of EpiEL system. There will be a service charge for characterizing more wafers. MaxMile offers favorable rate service if customer signs a contract for routine wafer characterization service.

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  • If I send my samples, how is the turn-around time of your wafer characterization service?

For small amount samples (<10), we can finish the EpiEL test and send you the report within a day or two. For the first time of characterizing your sample, we usually fully characterize your sample and might take a little more time than regular EpiEL test.

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  • How about throughput/speed of your EpiEL system? Can it meet high throughput requirement?

Yes. EpiEL is a rather flexible system which can meet requirements spanning from comprehensive research and development to rapid quality control.

Usually EpiEL measure 32 points full LIV for 5-10 minute in a 2-inch wafer, depending on test configuration. For rapid quality control, EpiEL could take only a couple minutes to determine the quality of the wafer.

MaxMile also provides a high throughput version of EpiEL to meet a demanding application need.

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  • Is there any sample preparation needed before test?

No. EpiEL is rather simple to use. Customer just needs to load epiwafers (right after growth) into system and select the corresponding characterization purpose, the system will finish the rest.

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  • What is the probe size?

It depends on specific application and test purpose. EpiEL uses different probe sizes to meet different application requirements.

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  • Can I use EpiEL probes alone and develop own system?

EpiEL probes are specially developed for MaxMile EpiEL mapping systems which uses proprietary instant-device-formation technique to obtain device-level EL measurement directly on LED epiwafers without any device fabrication. EpiEL probes alone could not work.

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  • Is there any competitive products? What is superiority of EpiEL?

To our knowledge, currently there is no similar system provided by competitors. PL mapping, device test and some quick tests (such as indium dot) have been used for quality control in LED industry. EpiEL is developed to provide a better solution:

  • Compared to device process, EpiEL provides improved quality control with identical test condition, and enables 100% quality control (“device test" on every wafer)

  • Compared to PL mapping, EpiEL enables true EL test directly on epi-wafer.

  • Compared to quick tests, EpiEL provides reliable and wafer-level mapping results.

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  • I have a PL mapper already, why do I need an EpiEL system?

The photoluminescence (PL) is mainly determined by the optical properties of the material, while the electroluminescence is determined by a number of factors such as the optical properties and physical structures of the optically active layers, the electrical properties of two conductive regions which are used for cathode and anode contacts, and the properties of the electrical contacts through which the electrical current injected.

It is well known that photoluminescence is not equivalent to electroluminescence. High photo-luminescence efficiency is necessary but not sufficient for good light-emitting materials or wafers. A wafer with high photoluminescence efficiency may or may not exhibit high electroluminescence efficiency and hence good light emitting diodes (LEDs). As indicated in green LED application, the different emission mechanism between PL and EL could also result in different emission wavelength/intensity. From quality control of view, PL mapping could not provide sufficient information for epiwafer quality control.

Compared PL, EpiEL system enables device-level EL measurement directly on epiwafers. As a finished device, EpiEL reveals not only the electroluminescence, but also the electrical properties of the material. The characterization performed by EpiEL is rapid, nondestructive. The powerfulness and low-cost nature of EpiEL provides an excellent balance between quality control and cost-effectiveness for LED industry. EpiEL makes it possible to perform "device test" on every wafer (100% quality control).

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  • What is EpiEL/PL mapping wavelength range?

Standard EpiEL/PL wavelength will be one of the following options. We can also customize the system to meet specific needs.

  • UV: 200-400nm / 200-450nm / 200-600nm

  • VIS: 350-700nm / 380-760nm / 400-800nm

  • VIS/NIR: 650-1050nm / 550-1050nm / 350-1050nm

  • UV/VIS: 200-700nm / 300-800nm / 200-850nm

  • UV/VIS/NIR: 200-950nm / 300-1050nm / 200-1050nm

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  • Is there any contamination you expect during probing? I need to process the wafer after probing; do I need to do some surface pre-treatment after EpiEL probing?

There is no contamination after EpiEL measurement. It has been proven at least for nitride-based applications that wafers with EpiEL test can be used for device fabrication as those without EpiEL test.

If your wafers were tested through our sample service, you might need to clean the wafer using regular semiconductor cleaning process to remove the dust, etc., since our EL mapping service is not performed under cleaning room environment.

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  • Will your tool damage the surface of wafer after EpiEL mapping measurement?

EpiEL must use some mechanism to touch wafer in order to inject the current into the wader to light up EL emission. For most applications, this contact is instant and nondestructive under recommended test condition. If your samples are very sensitive in any contact, we suggest you to validate the EpiEL measurement first through our free sample service.

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  • I was wondering what the distance between two probes is and what kind of metal you use for the probe. Is it Pt or Cu?

MaxMile EpiEL system uses a special and proprietary LED probe technology to measure the electroluminescent and electrical properties of unprocessed epiwafer.

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  • Please tell me any restrictions on materials for your EL mapping. As you know, it is hard to make a contact on p-GaN itself. How do you make p-contacts and n-contacts without fabricating device?

EpiEL works best on nitride LED material. There is no restriction on materials for EpiEL measurement. Any kind of LED epiwafers can be evaluated by EpiEL, though the level of information provided by EpiEL might be different for different materials. EpiEL uses a special and proprietary LED probe technology which can instantly and nondestructively form both anode and cathode contacts directly on epiwafer.

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  • Where can I find more information about EpiEL?

MaxMile home page (http://www.maxmiletech.com/) is the best place to find the information about EpiEL. In order to stimulate and foster relevant researches and technology innovations, MaxMile Technologies shares most of its findings and technological advances with industrial peers through journals and technical conferences. It also develops these findings and advances into practical application notes via its own website. MaxMile Technologies believes that these publications will help customers to use its service and technologies more effectively.

If you could not find information for which you are looking, please contact MaxMile at Info@MaxMileTech.com.

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  • What is lead/delivery time if place the order?

Normally two months.

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