Semiconductor laser packaging technology
1. Technical introduction
Semiconductor laser packaging technology is mostly developed and evolved on the basis of discrete device packaging technology, but it has great particularity. In general, the die of discrete devices is sealed in the package. The main function of the package is to protect the die and complete the electrical interconnection. Semiconductor laser packaging is to complete the output of electrical signals, protect the normal operation of the die, output: visible light function, both electrical parameters and optical parameters of the design and technical requirements, it is impossible to simply use the packaging of discrete devices for semiconductor lasers.
2 light emitting part
The core light-emitting part of the semiconductor laser is a PN junction core composed of p-type and n-type semiconductors. When the minority carriers injected into the PN junction are combined with the majority carriers, it will emit visible light, ultraviolet light or near-infrared light. However, the photons emitted from the PN junction region are non directional, that is, there is the same probability to emit in all directions. Therefore, not all the light generated by the die can be released, which mainly depends on the quality of semiconductor materials, die structure and geometry, internal structure and packaging materials. The application requires to improve the internal and external quantum efficiency of semiconductor lasers. routine Φ 5mm semiconductor laser package is to bond or sinter a square tube core with a side length of 0.25mm on the lead frame. The positive pole of the tube core is bonded with the gold wire through the spherical contact point to connect the inner lead with one pin, and the negative pole is connected with the other pin of the lead frame through the reflection cup, and then its top is encapsulated with epoxy resin. The function of the reflecting cup is to collect the light emitted from the side and interface of the tube core and emit it to the desired direction angle. The epoxy resin encapsulated on the top is made into a certain shape, which has several functions: protecting the pipe core from external erosion; Adopt different shapes and material properties (with or without dispersant), function as a lens or diffuse lens, and control the divergence angle of light; The correlation between the refractive index of the tube core and the refractive index of the air is too large, so that the critical angle of total reflection inside the tube core is very small. Only a small part of the light generated by the active layer is taken out, and most of it is easy to be absorbed through multiple reflections inside the tube core, which is easy to cause excessive light loss. Epoxy resin with corresponding refractive index is selected as the transition to improve the light emission efficiency of the tube core. The epoxy resin used to form the pipe shell must have moisture resistance, insulation, mechanical strength, high refractive index and transmittance of light emitted to the pipe core. When packaging materials with different refractive index are selected, the influence of packaging geometry on photon escape efficiency is different. The angular distribution of luminous intensity is also related to the die structure, light output mode, material and shape of packaging lens. If the pointed resin lens is used, the light can be concentrated to the axis direction of the semiconductor laser, and the corresponding viewing angle is small; If the resin lens at the top is circular or planar, its corresponding viewing angle will increase.
3 drive current
Generally, the emission wavelength of semiconductor laser varies from 0.2-0.3nm / ℃ with temperature, and the spectral width increases, which affects the brightness of color. In addition, when the forward current flows through the PN junction, the heating loss causes the junction region to produce a temperature rise. Near the room temperature, the luminous intensity of the semiconductor laser will be reduced by about 1% for every 1 ℃ increase in temperature, so as to package and dissipate heat; It is very important to maintain the color purity and luminous intensity. In the past, the method of reducing the driving current is often used to reduce the junction temperature. The driving current of most semiconductor lasers is limited to about 20mA. However, the optical output of semiconductor lasers will increase with the increase of current. The driving current of many power semiconductor lasers can reach 70ma, 100mA or even 1a. It is necessary to improve the packaging structure, new semiconductor laser packaging design concept and low thermal resistance packaging structure and technology to improve the thermal characteristics. For example, the flip chip structure with large area is adopted, the silver glue with good thermal conductivity is selected, the surface area of the metal support is increased, and the silicon carrier of the solder bump is directly installed on the heat sink. In addition, in the application design, the thermal design and thermal conductivity of PCB are also very important.
After entering the 21st century, the efficiency, ultra-high brightness and panchromatic of semiconductor lasers have been continuously developed and innovated. The light efficiency of red and orange semiconductor lasers has reached 100im / W, that of green semiconductor lasers is 50lm / W, and the luminous flux of a single semiconductor laser has also reached tens of IM. Semiconductor laser chips and packages no longer follow Gong's traditional design concept and manufacturing mode. In terms of increasing the light output of the chip, R & D is not limited to changing the number of impurities, lattice defects and dislocations in the material to improve the internal efficiency. At the same time, how to improve the internal structure of the die and package, enhance the probability of photon emission in the semiconductor laser, improve the light efficiency, and solve the optimal design of heat dissipation, light extraction and heat sink, Improving the optical performance and accelerating the SMD process of surface mount is the mainstream direction of research and development in the industry.
