Silicon photonics technology to create thin-film LED arrays

Higher density single quartz film light emitting diode (LED) arrays have been investigated. Bonded epitaxial film (epi-film) LEDs about 2 μm thin have been fabricated on CMOS IC drivers and other dissimilar substrates via molecular forces ("epi-film bonding (EFB)" technology). This epitaxial film LED array provides characteristics good enough to supply LED print heads (small variable emitter power (< ±5%), and long lifetime (> 1000 h)). Fabrication tests were performed on 2-dimensional (2D) epitaxial film LED arrays; good performance was achieved with a 2D 1200 dpi epitaxial film LED array (a small emission area of ​​10 μm x 10 μm, and a good array intensity of 21.2 μm) to show its characteristics.

1200dpi epitaxial film LED arrays bonded to diamond-like carbon (DLC) films with high heat conduction were tested for the first time. Test results show that good connections of small epitaxial films (10μmx10μm) on DLC films can be constructed. LED arrays bonded to DLC film shapes on silicon substrates show high heat conduction characteristics; preliminary evaluation suggests LED temperatures of around 50°C even with a very high 20kA/cm2 LED current density.

In recent years, silicon photonics has a particular appeal as a "beyond the Moore" technology. Much research has been done to develop silicon photonics. One of the key points in silicon photonics is the integration of optical devices and silicon devices. The integration of optical devices and silicon devices has been studied by many methods, such as using silicon-based photonic devices, compound semiconductors grown on silicon, and wafer bonding. These technologies are all for device integration, and semiconductor thin film bonding seems to be particularly attractive for integrating dissimilar materials.

So far, many research laboratories have been studying the integration of semiconductor thin film connections into different material devices. However, no successful products have been applied to semiconductor thin film connections. It is difficult to control semiconductor thin films without any defects, especially at the wafer level, which seems to be a big reason why some laboratories try to use semiconductor thin film connections in device products. Building highly reliable thin film connection devices has become an urgent issue.

Semiconductor thin film interconnects have a major advantage of providing more flexible options for integrating device material combinations than compound semiconductors grown on silicon. Mismatched material properties and device processes limit the application of compound semiconductors grown on silicon to dissimilar material device integration. It will be possible to integrate devices that can be fabricated separately in the best manufacturing process when semiconductor thin film interconnects are applied. This will also lead to high performance and high reliability integrated devices. Another advantage is the use of semiconductor thin film interconnects, which are planar wire structures that are photolithographically shaped to connect to the integrated device. Metal thin film wires can be shaped to cover the edge areas of the connected thin film devices. Its wire structure will lead to more compact and integrated high density devices, compared to surface chip mounting structures using die links, wire links and flip chip links. The planar wire structure eliminates large connection pads. This will result in reduced device size and increased device integration density.

Light emitting diode print head (LED print head) is a key component, which is used in LED printers and used as a light source alone. LED printers are one type of photoelectric printers; the other type is for laser printers. Conventional LED print heads contain LED array chips and CMOS IC driver chips which can be mounted on a printed circuit board. The LED array chip and IC driver chip are connected with high-density wires. The light emitting area of ​​the LED array chip is small, for example, 20μmx20μm at 600dpi, but the size of the LED array chip is large because of the large wire connection pad. The installation of LED array chips and IC driver chips also limits the reduction of the size of LED print heads. To solve these problems, we have also studied the integration of thin film LED arrays and CMOS IC drivers, and have also successfully developed a technology for integrating 3D thin film LED arrays and IC drivers into LED print heads; we call this technology "epiphysally thin film bonding (EFB)" technology and call semiconductor thin films "epiphysally thin films".

EFB technology will be applied to integrate dissimilar materials and integrate epitaxial thin film LED arrays with IC driver LED heads. Application of EFB technology to ultra-high density integration of dissimilar material devices will be a valid future goal. Epitaxial thin film LED arrays fabricated in 2D have a good test EFB technology applied to ultra-high density integration. The density of 2D LED arrays is limited to an array intensity of about 1mm as long as the LEDs are arrayed with traditional mounting techniques. Much higher LED array densities are expected to be achieved by applying EFB technology to the fabrication of 2D LED arrays.

At higher density epitaxial thin film LED arrays, higher thermal conductivity is expected to the substrate to which the epitaxial thin film LED array is connected, especially when the LEDs are operated at higher LED current ranges. However, many studies have been studied and reported for bonding semiconductor thin films to high heat flux conductive materials.

In this paper, higher density epitaxial thin film LED arrays are integrated in 3D space of LED print head with CMOS IC driver. Higher density 2D epitaxial thin film LED arrays can also be applied to higher density integration of different raw materials devices by EFB testing. Thermal conductivity characteristics of epitaxial thin film LED array shapes by EFB are also described; testing of epitaxial thin film LEDs by EFB connected to diamond-like carbon (DLC) thin films is also reported.

Integration of LED array and CMOS IC driver

Figure 1 shows a microscope image of an LED array chip and an IC driver chip, which are mounted on a LED head printed circuit board with a customary 600 dpi. The LED array chip and the IC driver chip are electronically connected with high-density metal link wires, with the number of connected wires being approximately 3,000.

Figure 2 shows the manufacturing process of the new LED array, which is an epitaxial thin film LED array and a CMOS IC driver integrated into the EFB.

(a) The epitaxial thin film layer for the LED is grown on a GaAs base. A discarding layer is used to etch the GaAs base; and the epitaxial thin film layer is grown between the epitaxial thin film LED layer and the GaAs base. The epitaxial thin film LED layer contains AlGa, and there is a double heterostructure in the layer (the wavelength of the LED is about 750nm).

(b) The epitaxial thin film LED layer is mesa-etched in different insulating areas and the discard layer is exposed to the mesa etching. The insulating pattern for the epitaxial thin film LED layer is released from the other base (GaAs base) by chemical etching of the discard layer. The material is formed into a 20μmx20μm insulating area, and the area intensity of this LED array is 42.3μm to support the epitaxial thin film LED layer at 600dpi array intensity. The metal thin film line is properly released and the bonding process is formed at the edge area of ​​the epitaxial thin film LED without defects;

(c) The bonding of the epitaxial thin film LED layer to the metal thin film line with good step coverage and the bonding area is observed at the edge area of ​​the epitaxial thin film on the IC driver. The epitaxial thin film LED layer is tightly bonded to the IC driver surface by intramolecular forces at room temperature without any adhesion. In the bonding area, the IC driver surface is inactive before the epitaxial thin film bonding process.

(d) The support material is removed from the epitaxial thin film LED layer.

(e) The epitaxial thin film LED layer is etched into individual LEDs from the LED array mesa.

(f) Metal thin-film wires are formed by photolithography and connected to the epitaxial thin-film LEDs and IC drivers.

Figure 3 shows a SEM image of an epitaxial thin film LED connected to an IC driver. The illumination area (epithelial thin film) is 2μm. The epitaxial thin film LED is connected to a proper IC driver; no cracking and stagnation is observed even with very fine epitaxial thin film area of ​​about 150nm. Figure 4 shows a 600dpi SEM image of an epitaxial thin film LED array integrated with an IC driver. The epitaxial thin film LED array is properly connected to the IC driver on a CMOS IC wafer. The illumination area is 20μmx20μm. The intensity of the LED array is 42.3μm (600dpi array intensity). The metal thin film lines are properly formed with no defects at the edge of the epitaxial thin film LED.

Figure 5 shows a cross-sectional SEM image of an epitaxial thin film LED link to an IC driver. A very good link is constructed and no sag occurs at the link interface, clearly demonstrating that the atomic back-end range is not observed at the link interface. The different PLED and Vf distributions are small; the variation of PLED is about ±5% and the variation of Vf is about ±2%, and there are also traditional type LED array chips (LED array on GaAs substrate) with equivalent PLED and Vf. The different PLED and Vf distributions indicate different link interface characteristics. If sag/or cracking occurs in the epitaxial thin film LED array, different PLED and Vf distributions.

The results shown in Figure 7 are the life cycle test of a 600dpi LED print head. The LED print head contains 4992 LEDs. The test was conducted under higher normal operating conditions with LED current and duty cycle. The horizontal and vertical axes of Figure 7 represent the number of LEDs (#1~#4992) and the emitter power change (PLED(t)-PLED(0))/PLED(0), respectively, where PLED(0) is the starting emitter power and PLED(t) is the emitter power at time t. Figure 7 shows that no large emitter power drop occurs in the LED print head at t=1000h. The LED forward current at an operating time of 1000h is equal to 0.9mA, which is higher than 5 million sheets of paper printed; the machine also has a long life cycle to provide new LED print heads for use in LED printers.

Figure 8 shows a microscope image of the new LED print head (new LED array chips are placed on the printed circuit board) when all LEDs are turned on. Compared with the conventional LED print head (Figure 1), the connection wires connected to the LED array and the IC driver are completely limited, and the connection wires are connected to the input terminals of the printed circuit board and the IC driver. The number of connection wires is reduced to 1/5 and the number of chips placed on the LED print head is reduced to 1/2. As a result of reducing the number of connection wires and installed chips, the benefit of the print head product can be increased by about two times. That is, the quality of the print head product can be easily increased by more than two times without changing the structure of the print head product (wire connector and die connector).

Increased LED density reduces the size of the epitaxial thin film LEDs. The result of the reduced size of the epitaxial thin film LEDs is a reduction in the strength of the epitaxial thin film LEDs bonded to the bonding area. This is one of the main arguments for manufacturing higher density LED arrays, where a strong epitaxial thin film LED array is bonded to the epitaxial thin film LED array and IC driver via EFB. Bonding tests on smaller epitaxial thin film LEDs to IC driver were shown to bond to smaller epitaxial thin film LEDs with greater strength. FIG9 shows a 1200 dpi epitaxial thin film LED array bonded to an IC driver, which is completed with the 1200 dpi epitaxial thin film LED array and IC driver fabricated in FIG2. During the fabrication of the 1200 dpi epitaxial thin film LED array, no epitaxial thin film LEDs were separated from the bonding area of ​​the IC driver. The size of the light emitting area is 10μmx10μm, and the intensity of the LED array is 21.2μm (1200 dpi array intensity).

High-density 2D epitaxial thin film LED array

In the traditional 2D LED array usage model, the LED is mounted on a circuit board, and the thickness of the LED array is greater than 1mm; even when traditional LED chips are used, the connection wires connected to the LED and mounted on the circuit board will cause a larger LED array thickness. When epitaxial thin film LEDs are used, all wires can be formed on a fine pattern of metal thin film. When integrating epitaxial thin film LED arrays and IC drivers, the key to making higher density 2D LED arrays is to construct epitaxial thin film LED arrays with high connection strength on the base.

Because the epitaxial thin film LED is very thin, 2μm, it is also flexible. This flexible epitaxial thin film LED feature allows us to use a higher epitaxial thin film LED array on a flexible substrate. Higher density 2D epitaxial thin film LED arrays can be formed on glass and plastic substrates respectively. The manufacturing process of this 2D epitaxial thin film LED array is similar to the process of integrating the epitaxial thin film LED array and IC driver.

Shown in FIG10 is a 2D epitaxial thin film LED array (24 dots x 24 dots) which is attached to a plastic base. The thickness of the plastic base is 0.2 mm, the area of ​​the epitaxial thin film LED is 300 μm x 300 μm and the array thickness is 600 μm. The epitaxial thin film includes an AlGaAs layer (wavelength is about 750 nm) or an AlInGaP layer (wavelength is about 650 nm). The thickness of the epitaxial thin film is about 2 μm. The 2D epitaxial thin film LED array is properly attached to the base and no idling and no cracking are found. As shown in FIG10 , the base is flexible. No idling and no cracking are found when the base is bent. The 2D epitaxial thin film LED array shown in FIG10 is a characteristic display under a bent base.

Figure 11 shows a 600dpi 2D epitaxial thin film LED array attached to the glass base of the EFB. The size of the epitaxial thin film LED is 20μm x 20μm, and the LED array intensity is 42.3μm (600dpi), providing high-density display characteristics; the 2D LED array area is about 1mmx1mm (24 dots x 24 dots). The manufactured 1200dpi 2D epitaxial thin film LED array was also tested. Figure 12 shows the characteristics of the 1200dpi 2D epitaxial thin film LED array. The size of the light emitting area is 10μmx10μm, and the LED array intensity is 21.2μm (1200dpi array intensity). The 1200dpi 2D LED array contains 24 dots x 96 dots, and the 2D LED array area is as small as about 0.5mmx1mm. The test results show that the 1200dpi epitaxial thin film LED array has good performance.

Thin film LED array attached to DLC thin film

Since the epitaxial thin film LED structure is directly connected to a base, the thermal conductivity of the base is the main characteristic that determines the heat flow of the epitaxial thin film LED. When another layer is formed on the base and the epitaxial thin film LED is bonded to this layer, the heat flow conduction and the thin layer on the base also affect the heat flow characteristics of the epitaxial thin film LED. DLC is one of the materials with high heat flow conductivity. DLC thin film can be formed with a smooth surface of nanometer scale; the smooth surface of nanometer scale is necessary for the bonding layer to construct a high bond directly on the epitaxial thin film. The advantage of the chemical impedance characteristics of DLC thin film will also be used for DLC thin film as a bonding layer. However, there is no report on bonding epitaxial thin film to DLC thin film.

The bonding test of epitaxial thin film LED to DLC thin film is first described. The efficiency characteristics of DLC thin film to epitaxial thin film LED are also demonstrated. DLC thin film is formed on Si substrate by chemical vapor deposition (CVD). The epitaxial thin film layer is bonded to DLC thin film through EFB. The bonded epitaxial thin film layer is processed by 1200dpi epitaxial thin film LED array. Metal thin film electrodes and lines are formed by photolithography.

Figure 13 shows the epitaxial film array bonded to the DLC film. The size of the epitaxial film is 10μmx10μm. The strength of the epitaxial film is 21.2μm (1200dpi array strength). The tape test showed that the epitaxial film was bonded to the DLC film without release. As shown in Figure 13, the epitaxial film was properly bonded including the edge area of ​​the epitaxial film. This indicates that the epitaxial film is properly bonded mechanically from a high density epitaxial film LED array to the DLC film.

In Figure 14, a 1200 dpi epitaxial film LED array is shown, which is connected to a DLC film. The size of the light emitting area is 10μmx10μm. The LED array intensity is 21.2μm (1200 dpi array intensity). The scattered LED characteristics on a chip (emitter power LED, and forward bias Vf) will indicate different connection characteristics. Waste and cracking occur in the epitaxial film LEDs resulting in greatly different LED characteristics. In Figure 15, a 600 dpi epitaxial film LED array is connected to a DLC film with PLED and Vf distributed on a chip. The results shown in Figure 15 can be compared to the results shown in Figure 6 (600 dpi epitaxial film LED array connected to an IC driver). Different PLED, and Vf distributed in the epitaxial film LED array on the DLC film is almost equal to that on the IC driver. The PLED data and Vf distribution in FIG. 15 indicate that the epitaxial thin film LED array constructed on the DLC thin film has good connection and small difference in connection characteristics.

The increase of LED temperature has an impact on LED characteristics. The temperature increase will change the efficiency of the emitter light power. The LED current (If)-emitter light power (PLED) characteristics of the epitaxial thin film LED array were measured on the DLC thin film bonded to the silicon base. The If-PLED characteristics of the epitaxial thin film LED array were also compared when measured on the silicon base bonded to various technology layers. The heat flow conduction of the various technology layers of the DLC thin film will be very small.

The study of the heat flow characteristics of DLC thin films is a new process. The large difference in heat conduction will lead to large differences in the characteristics of epitaxial thin film LEDs with DLC thin films and various technology layers. Figure 16 shows the If-PLED characteristics of a 1200dpi epitaxial thin film LED array connected to a DLC thin film. Figure 16 also shows the If-PLED characteristics of epitaxial thin film LED arrays connected to various technology layers.

The emission power of epitaxial thin film LED array is equal to that of various technology layers in DLC, but is higher than that of various technology layers when the LED current range is considered high. The PLED of epitaxial thin film LED array reaches the maximum PLED when the DLC thin film is about 12mA (current density is about 20kA/cm2), and reduces If by about 12mA. The PLED of epitaxial thin film LED array reaches the maximum PLED of various technology layers at very low If, and decreases about 3mA faster than If.

The temperature of the epitaxial thin film LED (Ts) was preliminarily evaluated, measuring the If dependent wavelength distribution of the light emitted from the epitaxial thin film LED. Preliminary simulations showed that Ts was about 50°C even with a very high current density of about 20kA/cm2 (If was about 12mA) for the epitaxial thin film LED bonded to the DLC thin film on the silicon substrate; Ts also increased to about 100°C at a current density of 5kA/cm2 (If was about 3mA) for the epitaxial thin film LED on various technology layers. It was demonstrated that the bonded epitaxial thin film LED had a higher heat flux on the DLC thin film, ensuring higher thermal conductivity characteristics, as well as better LED characteristics; even at a higher LED current density range.

Conclusion

Manufacturing higher density epitaxial thin film LED arrays, which feature 3D integration with CMOS IC drivers through EFB technology. 1200dpi epitaxial thin film LED arrays can also be integrated into IC drivers through EFB. Epitaxial thin film LED arrays integrated into IC drivers show good LED characteristics and high reliability. This EFB technology provides integrated light and CMOS devices and is guaranteed to be completed in silicon photonics technology.

Higher density 2D integration of epitaxial thin film LEDs can be accomplished on glass and flexible plastic substrates. Array thickness of 2D LED arrays can be as thin as 21.2μm and can be fabricated on heterogeneous material substrates through EFB. Test results also show that EFB technology can be applied to ultra-high density integration of heterogeneous material devices. This study is the first to propose that epitaxial thin film LEDs can be directly bonded to DLC thin films formed on silicon substrates. Tests of epitaxial thin film LED arrays bonded to DLC thin films show that different bonding characteristics with smaller and better LED characteristics can be constructed. EFB technology will provide new high-density integrated device production reformation and contain heterogeneous materials in the system, and is expected to have newer electronic components in the future.