The chip structure of GaN-based blue/white LEDs strongly depends on the substrate material used. Most manufacturers currently use sapphire as the substrate material, and the chip structures are divided into four main categories, as shown in Figure 1.
Front mount chip structure
This type of chip is widely used in low and medium power packaging products, the advantage is the low price; the disadvantage is that due to the poor thermal conductivity of sapphire, so the chip heat dissipation is poor, P-type material conductivity is also poor, so the injection current is limited. In addition, the chip is five-sided light-emitting, usually need to place the chip in the holder, as shown in Figure 2, the opening area of the reflector is much larger than the chip light-emitting area, resulting in low luminous flux per unit area, the chip surface color uniformity is very poor, the chip is blue above, while the outer ring is yellow. This is in use, such as spotlights, flat panel lights, etc., the reason for the yellow circle.
In order to overcome the defects of traditional front-mount structure such as poor heat dissipation and limited injection current, some scientists have proposed the flip-mount structure. Heat can be transferred directly from the chip to the substrate material, such as ceramic, instead of passing through the sapphire substrate with poor thermal conductivity, so the injection current can be significantly increased; the luminous flux per unit area can also be significantly increased. The disadvantage is that the chip is five-sided light, to the need for accurate secondary optical design of the occasion, such as small angle spotlights, cell phone flash, car lights, ultra-thin backlight and flat panel lights, etc., brought a lot of inconvenience, in addition, there is also the problem of uneven color faced by the front mounted five-sided light out of the chip.
Thin-film flip-chip structure
In order to solve the problems of flip chip structure, a thin film flip chip structure has been proposed, based on the flip chip, by removing the sapphire substrate with laser peeling technology to obtain a single-sided light-emitting thin film chip. The thin film chip has a high efficiency, the light is concentrated in the upper part of the chip, which is conducive to secondary optical design, in addition to the specific very good surface color uniformity. However, to use the complex process of laser stripping technology, high cost, low yield, the chip itself is also prone to defects, in addition, because of the flip-chip structure, there is a gap between the chip and the (ceramic) substrate, to a certain extent reduces the chip's thermal conductivity; at the same time, because the chip is very thin, the chip is prone to cracks, leakage and other problems in use.
With the thin-film flip-chip structure, there are vertical structure chip, similar to flip-chip, sapphire substrate is stripped, the bottom of the chip is plated with a high inverse material and then add the conductive thermal conductivity of the substrate material, so that the chip has the advantages of a single-sided light-emitting chip at the same time, but also has a very good thermal conductivity, conductivity, reliability is also very high. Vertical structure is currently the most widely used thin-film chip structure, but due to the need to use laser stripping of sapphire substrates, low yield, high cost, limiting the vertical structure of the chip more widely used.
The Silicon Substrate LED Technology Project Team claims that, compared to traditional sapphire substrates, silicon substrate LEDs have the following advantages.
(1) Not only is the substrate cost much lower than sapphire material when grown, but the substrate can be stripped by chemical etching, which is much higher in efficiency and yield than sapphire substrates that must be stripped by laser methods, resulting in high-quality, low-cost vertically structured chips.
(2) Combined with the white light chip process, on the one hand, to reduce the light-emitting area, on the other hand, to achieve higher performance, including the unit area luminous flux and chip surface color uniformity.
(3) Vertical structure of the chip, not only can be used ceramic packaging process, but also can be used more inexpensive bracket packaging method, further reducing the customer's cost of use, as shown in Figure 3, which provides unlimited possibilities for high-quality lighting. The use of silicon substrate LED chips can achieve better lighting quality, lower system costs, more flexible design solutions, and more innovative LED lighting solutions.
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