The semiconductor laser is one of the most widely used optoelectronic devices.
With the continuous progress of technology and the improvement of mass production ability of devices, it can now be applied to more fields.
The semiconductor laser is a kind of laser that mainly uses semiconductor materials as working materials. Because of the different material structure, the laser will be different.
Semiconductor lasers are characterized by small volume and long service life.
In addition to the communication field, it can also be used in radar, sound measurement and medical treatment.
Due to the large light output power of a single chip and the large heat generated per unit area, if the heat dissipation technology is not done well, the chip is easy to die and the performance will decline rapidly.
Classification of laser heat dissipation methods
At present, the main heat dissipation methods of laser are divided into traditional heat dissipation methods and new heat dissipation methods.
Traditional heat dissipation methods include:
- air cooling heat dissipation
- semiconductor cooling heat dissipation
- natural convection heat dissipation
The new heat dissipation methods include:
- flip heat dissipation
- microchannel heat dissipation
Heat dissipation structure and heat transfer process of semiconductor laser
Fig. 1 heat dissipation structure of the laser
The heat dissipation mechanism of semiconductor laser packaging is mainly composed of laser chip, welding layer, heat sink, metal layer, etc.
The welding layer in the heat dissipation structure of the semiconductor laser mainly connects the chip and heat sink by welding.
In order to reduce the thermal resistance when high-power semiconductor lasers are used, some materials with high thermal conductivity are often used during welding, such as gold tin solder.
There are many layers in the whole packaging process, including chip, solder layer, heat sink and the metal layer.
The heat transfer effect of the heat sink and the metal layer is used to transmit the heat energy of the laser chip, and finally make the semiconductor laser form good heat dissipation, so as to prolong the service life of the laser.
Matters needing attention in heat dissipation performance analysis
The heat dissipation performance of high-power semiconductor lasers is mainly evaluated by thermal resistance and thermal flux. In the evaluation, we should pay attention to the heat flux at limited temperature.
If it is found that the temperature difference between the two is relatively large during heat dissipation analysis, condensation will appear on the surface of the laser chip. After this problem occurs, it will not only affect the optical output power, but also affect the wavelength locking, and even damage the photoelectric performance of the circuit due to condensation, which will eventually affect the reliability.
At present, the common method to reduce thermal resistance is to use thermal conductivity materials. The emergence of thermal conductivity materials provides more optimization space for the laser to reduce the temperature.
Traditional heat dissipation method
1. Cooling and heat dissipation method of natural convection heat sink
Natural convection heat sink cooling is to use some materials with high thermal conductivity to take away the heat generated, and then dissipate heat through natural convection.
Scientists also found that fins can also help heat dissipation, and can maximize the heat transfer rate in the heat dissipation system.
When the temperature is the same, the fin spacing will decrease with the increase of fin height. When using the substrate to place the heat sink vertically, it is necessary to increase the height appropriately to improve the heat dissipation effect. This heat dissipation method will reduce a lot of costs when used.
In practice, copper or aluminum nitride is often used as the heat sink, but the heat sink can not fully meet the heat dissipation needs of high-power semiconductor lasers.
2. Semiconductor refrigeration and heat dissipation (electric refrigeration and heat dissipation) method
The main characteristics of semiconductor refrigeration and heat dissipation methods are small volume and strong reliability.
Semiconductor refrigeration and heat dissipation methods often appear in high-power semiconductor lasers. Because TEC refrigeration is added, the size of the package is increased accordingly, and the cost of the package is also increased accordingly. When in use, the cold end and heat sink of the semiconductor chip are connected together, and the hot end is dissipated through convection and TEC’s own heat,
Figure 2 is the working structure of TEC.
Figure 2 TEC structure
The cooling control effect of TEC can be improved by adjusting the internal parameters of TEC.
Researchers found that the best heat transfer area ratio can make the TEC characteristic coefficient reach the maximum.
It is also found that the ratio of heat transfer area has a great relationship with the characteristics of TEC material and exchange area.
3. Large channel water cooling and heat dissipation method
In order to reduce the temperature of the heat sink, a channel needs to be built in the heat sink. In order to achieve the cooling effect, a certain water source needs to be added to this channel, so that the work of the laser will not be delayed.
In view of this, researchers found that the heat dissipation effect of the spoiler structure will be better than that of the traditional cavity structure, but the pressure will also increase in the channel.
It is found that although large channels are widely used, due to the continuous improvement of laser output power, large channel water cooling and heat dissipation can not meet the heat dissipation requirements of high-power semiconductor lasers.
New heat dissipation method
With the increasing requirements for lasers in various fields, the traditional heat dissipation methods can not meet the current requirements, and more new heat dissipation methods need to be studied.
At present, there are several new ways of heat dissipation.
4. Flip chip mounting method
Figure 3 is a flip chip diagram.
The flip chip packaging still adopts TEC mode. The traditional method of pasting laser chip and heat sink chip adopts the front of the chip facing upward, and the back cooling surface and heat sink are connected through solder.
However, the heat generated in the active area of the chip is mainly concentrated in the area of several microns on the upper surface, and the distance between the upper surface and the lower surface is generally hundreds of microns.
The heat is transmitted to the heat sink through such a long distance, and then to TEC refrigeration. The heat dissipation effect is limited.
Figure 3 flip chip
By improving the internal structure of the chip, adjusting the surface structure of the chip and the heating layer in the active area, and using the chip flip chip technology, the main heating surface of the chip is directly connected with the heat sink after passing through the welding layer, and the heat dissipation efficiency of the laser can be improved by 20% or higher;
Because the performance of the optical chip is strongly related to the temperature, the higher the temperature, the more severe the wavelength drift, and the optical output power will be reduced or saturated.
The flip chip mounting method can greatly improve the heat dissipation effect, make the photoelectric output of the chip more stable, and greatly improve the performance of the whole laser. The final performance needs to meet the performance requirements of the national military standard gr-468-core. Some indicators are shown in Table 1.
Table 1 requirements for environmental stress test without actuation
|Test||Reference section||Additional information||Environment||Scope of application|
|High temperature storage||126.96.36.199||85℃，2000h||R||R||All photoelectric modules and integrated modules|
|Low temperature storage||188.8.131.52||-40℃，72h||O||O||All photoelectric modules and integrated modules|
|Temperature cycle||3.3.222||50 cycles – 40 ℃ / + 85 ℃||O||R||Laser diodes, LEDs, photodiodes and EA modulators|
|40 ℃ / + 85 ℃, 100 cycles||R||－||All photoelectric modules and integrated modules for CO environment|
|40 ℃ + 85C, 500 cycles||－||R||All photoelectric modules and integrated modules for CO environment|
|Humidity temperature||184.108.40.206||85 ℃ /% 85rh, 500 cycles||R||R||Specify all photodiodes for unsealed modules, as well as all photoelectric modules and integrated modules|
5. Microchannel heat dissipation method
There are two main ways of microchannel heat dissipation:
- Microchannels defined according to channel size;
- Microchannels defined according to the effect of surface tension.
Fig. 4 is a typical cooling structure diagram of microchannel heat sink.
Fig. 4 Cooling structure of microchannel heat sink
During the research, the researchers did an experiment with microchannels as cooling devices.
The heat dissipation characteristics of microchannel are found through experiments. The reason why microchannel heat sink can dissipate heat is that it has a certain high heat flux.
At the same time, it is also found that microchannels have better heat dissipation effect.
In addition, some people found that different groove shapes of microchannel heat sink will also affect the heat dissipation effect.
After numerous studies, it is found that the heat dissipation characteristics of cosine channel are the best of all shapes.
In addition, researchers also found that the cooling device combined with microchannel and glass microchannel can meet the heat dissipation requirements of high-power semiconductor lasers.
When the laser is used, it will be applied to the microchannel because the microchannel will have better heat dissipation effect than the traditional heat dissipation method, and can meet the heat dissipation requirements of high-power lasers.
However, there is a disadvantage in the use of microchannels, that is, the microchannels are often blocked by thermal deformation cooling medium particles, which affects the heat dissipation effect. Therefore, nano fluids need to be used to improve the heat transfer performance of the whole process.
6. Spray cooling heat dissipation method
Spray cooling is through the help of pressure, spray the cooling liquid to the surface of heat transfer by atomization, and achieve the purpose of cooling.
The main characteristics of spray cooling are large heat transfer coefficient and low flow rate of coolant.
Researchers found that when using water as medium and solid conical nozzle for experiments, the surface of microstructure can increase the effect of heat exchange.
During the study, it was found that the cooling of spray cooling was related to the spray velocity.
In addition, researchers have also found a spray phase change cooler, which has a close relationship with the nozzle height and heat dissipation in the spray cooling device.