Recently, the research on chip and epitaxy has found that the performance of chips made by the same epitaxy process and chip process is very different, which has changed the “world view” of experimental design. The quality of substrate is very important.
Today, I’m going to talk about GaAs epitaxial substrate and GaAs epitaxy. At present, gallium arsenide has the largest volume and is mainly used in the communication field (5g mobile phone PA communication RF chip), with a global market of nearly 10 billion US dollars. Indium phosphide is mainly used for optoelectronic devices in the communication field (such as the transmitting chip in the optical module).
Gallium arsenide also accounts for a large proportion in optoelectronic lasers and led fields. As a mature second-generation compound semiconductor, GaAs Power chips and optoelectronic chips grow different material film structures by epitaxial growth on GaAs substrates. MOCVD technology is commonly used in industry to deposit on the upper surface of GaAs substrate through compound thermal decomposition reaction.
MOCVD (metal oxide chemical vapor deposition) is a new type of vapor phase epitaxial growth technology developed on the basis of vapor phase epitaxial growth (VPE). It uses the organic compounds of group III and II elements and the hydrides of group V and VI elements as the crystal growth source materials, carries out vapor phase epitaxy on the substrate by means of thermal decomposition reaction, and grows a variety of thin layer single crystal materials of group III-V, II – VI compound semiconductors and their multi-component solid solutions.
We can see that epitaxy is growing from nothing to layer on the substrate, so the selection and process treatment of the substrate are very important. Because the effective working layer of a real device may be only a few microns to tens of microns thick.
Substrate is not only the support of devices, but also the growth seed of epitaxial layer. It has a great impact on the performance of optoelectronic devices. We need to have several basic requirements for selecting substrate:
1) Substrate crystal plane orientation;
2) Surface corrosion pit density;
3) Impurity type and density‘
4) Substrate thickness and size.
For example, a silicon doped n-type gallium arsenide substrate:
Si-Dopant GaAs Wafer
Diameter ： 100．5±0．5mm（4”）
LaserMark:Back side major flat．
GaAs and InP are two commonly used substrates. Generally, (100) plane is selected as the epitaxial growth plane, sometimes ± 0.1 ° or ± 0.5 ° away from the crystal plane. In the laser chip manufacturing process, such (011) plane is the dissociation plane, which can be used as the cavity plane.
How to check the defects on the crystal surface of the substrate?
Common methods: GaAs materials use hf:cro3+h2o=2:1 as corrosion solution. CrO3 is first dissolved in water (33% by weight). For InP, hcl:h2o=4:1. the corrosion pits in the above figure appear, and then observe and count them under a high-power microscope. For GaAs less than 2000/cm2, InP less than 50000/cm2 has good epitaxial quality.
At the same time, corrosion or “remelting” is often carried out before epitaxy, so the substrate thickness will also change. Generally, the thickness of epitaxial substrate is 350 ~ 400um.
The main process step before LPE growth is to weigh the materials used according to the composition of solid solution, clean and corrode them, and then put them into each molten pool for growth.
For example, gallium arsenide system, growth of GaAlAs epitaxial layer on GaAs substrate:
Theoretically, the equilibrium weight X of GaAs and the weight y of Al in each gram of GA solution can be determined according to the following equations.
Taking gaalas/gaas DH Laser as an example, the parameters of each layer produced by liquid phase epitaxy and the cleaning and corrosion process parameters of each material are given below:
Learn more about the process of depositing GaAs epitaxial layer by MOCVD.
The earliest source materials used for MOCVD growth of GaAs are tmga and ash3. many other combinations are also used later. Its typical growth conditions are as follows:
AsH3 flow (7 ~ 9) *10*-4mol/min
Tmga flow 10*-5mol/min
The growth temperature is 600 ~ 700 ℃
V/iii ratio is 30 ~ 45
H2 total flow 21 ~ 31
The basic process of growth is:
1; After the processed substrate is loaded into the base, adjust the settings related to the tmga source, such as flow, temperature, etc.
2: Then the system vacuumizes, passes H2 and adjusts the pressure in the reaction chamber.
3. when the temperature rises to 300 ℃, AsH3 is applied to form as atmosphere in the reaction chamber to prevent GaAs decomposition.
4. after the temperature rises to the epitaxial growth temperature, the tmga crystal growth is introduced.
5. after the growth, stop feeding tmga, cool down to 300 ℃, and then stop feeding ash3
6 after the temperature drops to room temperature, open the furnace and take out the film.
Gallium arsenide industry chain distribution
Compound semiconductor industry is similar to traditional silicon-based semiconductor, but it has its own unique features. Gallium arsenide also includes chip design, OEM, packaging and testing. Most of them are epitaxial stages of epitaxial wafer growth.
Upstream of the industry are GaAs substrates and EPI wafers. The substrate is the most basic material of GaAs wafer (GaAs substrate), and the manufacturers mainly include Sumitomo of Japan; Freiberger, Germany AXT, USA
After the production is completed, it will be sent to EPI Wafer Factory (GaAs epitaxy), and different material layers will be added on the surface of GaAs wafers by leading manufacturers such as Iqe in the UK, vpec in Taiwan, Sumitomo in Japan, etc. At present, most of the midstream volume is occupied by the three major IDM manufacturers in the United States, skyworks, qorvo and Broadcom (Avago). The global volume of GaAs in 2019 is $8.544 billion; Another route is to separate design from OEM.
In the past, because the volume of the second and third generation semiconductors was small and the products were diversified, the IDM route could closely connect design and manufacturing. However, as the second and third generation semiconductors become more large-scale and large-scale, it has become a trend to outsource more production tasks to foundries. In foundries, win, GCS and AWSC are the main ones.
In 2019, the scale of GaAs device OEM is USD 881million. In addition to the chip design business, Avago and skyworks also have their own factories. When their own capacity is insufficient, they will hand over some orders to the Taiwan, China OEM. Avago’s OEM is Wenmao, and skyworks’s OEM is Acer technology. Qorvo has sufficient capacity, mainly self-produced, and will also provide OEM services.
Because Hongjie’s technology is authorized by skyworks, when skyworks takes back the orders for internal digestion, Hongjie’s revenue drops sharply and needs transformation urgently. However, the transformation is successful at present. However, Huanyu, the third in the millennium, is an independent technology with steady growth. In 2016, San’an optoelectronics wanted to acquire Huanyu, but failed to pass the approval of the United States. Finally, it retreated to the second place to establish a joint venture (San’an Huanyu) to license the HBT technology of 4G Mobile phone PA to San’an for production (excluding 5g Technology). However, the joint venture has been cancelled at the end of 2019. San’an optoelectronics now cooperates with Huawei to produce 5g RF PA chips developed by Huawei and 5g base station RF chips of gallium nitride.