– How to Improve the Reliability of Silicon Carbide Materials?
The silicon carbide industry chain includes silicon carbide powder, silicon carbide ingots, silicon carbide substrates, silicon carbide epitaxy, silicon carbide wafers, silicon carbide chips and silicon carbide device packaging. Among them, substrate, epitaxial wafer, wafer, and device packaging and testing are the four most critical parts in the silicon carbide value chain. The cost of the substrate accounts for 50% of the total cost of silicon carbide devices. The costs of epitaxy, wafer, and packaging and testing are respectively 25%, 20% and 5%.
The reliability of silicon carbide materials is of great significance to the performance of the final device. We explore characteristics of the material and the causes of defects from all parts in the industry chain, and improve the reliability of silicon carbide power devices through cooperating with upstream and downstream companies.
1. Growth and Preparation Method for Silicon Carbide Crystal Ingot
There are more than 250 kinds of isomers of silicon carbide, while 4H-SiC single crystal structure is mainly used to make power semiconductors. During the growth of silicon carbide single crystals, the 4H crystal form has a small growth window and has strict standards for temperature and pressure design. Inaccurate control during the growth process will result in silicon carbide crystals with other structures such as 2H, 3C, 6H, and 15R.
In the industry, there are three methods for the preparation of silicon carbide single crystal ingots, namely sublimation PVT, HT-CVD, and LPE (solution growth method). Among them, sublimation PVT is the most mainstream preparation method, and about 95% of silicon carbide ingots for commercial use are grown by PVT. Heat the silicon carbide powder in special equipment during the process, and the powder begins to sublime after the temperature rises to 2200-2500°C.
Since silicon carbide has only gaseous and solid, but no liquid state, ingots will crystallize on the top after sublimation. The growth rate of silicon single crystal is about 300mm/h, and the growth rate of silicon carbide single crystal is about 400um/h. It is clearly see that the difference between the two is nearly 800 times. For example, the formation of five or six centimeter ingots requires continuous and stable growth for 200-300 hours. This shows that the preparation rate of silicon carbide ingots is very slow, which makes the ingots expensive.
2. Defects of Silicon Carbide Single Crystal Ingot and Substrate
Both silicon carbide ingots and substrates contain many crystal defects, like stacking faults, microtubules, penetrating screw dislocations, penetrating edge dislocations, base plane dislocations, etc.
Silicon carbide ingot defects will greatly affect the yield of the final device, which is a very important topic in the industry chain, and various substrate manufacturers are sparing no effort to reduce the defect density of silicon carbide ingots.
3. Reliability of Silicon Carbide Substrate
The substrate is the product obtained by cutting the crystal ingot into thin slices, smoothing and polishing. The substrate obtains a good surface quality after the polishing process, which can suppress the generation of defects in the epitaxial growth, thereby obtaining a high-quality epitaxial wafer. Its surface quality includes flatness, near-surface dislocations and residual stress.
In order to suppress the occurrence of defects in the initial stage of epitaxial growth, the surface of the substrate must be stress-free and no near-surface dislocations. If the residual damage near the surface is not sufficiently removed, the epitaxial growth on the substrate will cause macroscopic defects. Therefore, the quality level of the substrate will seriously affect the quality level of the subsequent epitaxial growth.
4. Silicon Carbide Epitaxial Growth and Reliability
Epitaxy refers to the growth of a single crystal material 4H-SiC on the upper surface of the substrate, which is homogeneous with the substrate. Silicon carbide has many isomers. In order to ensure the preparation of high-quality epitaxial materials, special techniques are required to avoid introducing other crystal types. The current standardized process is to use 4° beveled 4H-SiC single crystal substrates with steps control growth technology.
The current commonly used process is the CVD method. The commonly used equipment is a hot-wall horizontal epitaxial furnace, and the often used reaction precursor gases are silane (SiH4), methane (CH4), ethylene (C2H4), etc., and nitrogen (N2) and trimethylaluminum (TMA) are used as a source of impurities. The typical growth temperature range is 1500~1650 ℃, and the growth rate is 5~30 μm/h.
The growth of the epitaxial layer can eliminate many surface or near-surface defects introduced in crystal growth and wafer processing, making the crystal lattice neat and improving surface morphology. Thick epitaxial layer, good surface morphology and low doping concentration are of great significance to increase the breakdown voltage. Such epitaxial wafers are used to manufacture power devices, which can greatly improve parameter stability and yield.
5. The Relationship between Silicon Carbide Epitaxial Wafers and Substrate Defects
As mentioned above, the defects of the silicon carbide epitaxial layer are related to the substrate and the growth process. The epitaxial layer defects include surface topography defects, microtubule defects, dislocations and other types. The surface morphology defects include carrot defects (comet-shaped in some cases), shallow pits, triangular defects, and dropouts; the microtubule defects in the substrate will be copied to the epitaxial layer. At present, the density of microtubes in the substrate has been far lower than 0.1/cm2, which is almost eliminated.
Most of the dislocations in the silicon carbide epitaxial layer originate from the substrate dislocations. The substrate dislocations mainly have TSD, TED, and BPD. Common dislocations and carrot defects introduced by epitaxy are important issues that affect the quality of silicon carbide epitaxy.
6. The Impact of Silicon Carbide Epitaxial Wafer Defects on the Final Device
During the epitaxial growth process, about 98% of the TSD in the substrate is converted to TSD, and the rest is converted to Frank SFs; 100% of TED is changed into TED; about 95% of BPD is turned into TED, and a small amount of BPD is maintained.
TSD and TED do very little affect on the performance of the final silicon carbide device, but BPD will cause the degradation of device performance. So people pay more attention to BPD. Stacking faults, carrot defects, triangle defects, falling objects and other defects are killer defects. Once these defects appear on the device, the device will fail the test, resulting in a lower yield, e.g. bipolar devices. Triodes and IGBTs are more sensitive to BPD.
|Defects/devices||SBD||MOSFET, JFET||pin, BJT, Thyristor, IGBT|
|TSD(No pit)||None||None||None, but it will decrease the lifetime of partial carrier|
|TED(No pit)||None||None||None, but it will decrease the lifetime of partial carrier|
|BPD(Including interface dislocations, half-ring arrays)||None, but it will cause MPS diode degradation||None, but will cause body diode degradation||Bipolar degradation (on-resistance and leakage current increase)|
|Endogenous stacking fault||VB reduction (20%-50%)||VB reduction (20%-50%)||VB reduction (20%-50%)|
|Carrot defect, triangle defect||VB reduction (30%-70%)||VB reduction (30%-70%)||VB reduction (30%-70%)|
|Dropping defects||VB reduction (50%-90%)||VB reduction (50%-90%)||VB reduction (50%-90%)|
7. Two Challenges Faced by Silicon Carbide Materials
One of the important challenges faced by silicon carbide materials is that the price for promotion is too high, and the price of the substrate is much higher than that of silicon and sapphire substrates. At present, the mainstream diameter of silicon carbide substrates is 4 to 6 inches, and more mature growth techniques are needed to expand the size to reduce the price.
On the other hand, the dislocation density of silicon carbide is in the order of 102-104, which is much higher than that of silicon and gallium arsenide. In addition, silicon carbide still has a large stress, which can cause problems with surface parameters. Improving the quality of the silicon carbide substrate is an important way to improve the quality of the epitaxial material, the yield of the device preparation, the reliability and the lifetime of the device.