1. Damaged abrasive discs inevitably produce damage layers of more than 10 microns; the damage caused by sandblasting may be greater! These damages can lead to the fragility of silicon wafers and the formation of diffusion channels. For larger mechanical damage, not only can it not be eliminated in the corrosion process, but it will be expanded and the surface pressure resistance will be greatly reduced. Cutting damage has a great impact on chip voltage. The damage layer on the surface of cut silicon wafer includes mosaic layer and stress layer. The surface of wafer is mosaic layer, and the lower layer is damage layer and stress layer with serious damage. Their thickness ranges from 15 to 25 microns, which is the average for the whole section.

2. Sources of stress: friction and extrusion between silicon wafers, clamping of metal tweezers, thermal mismatch between silicon wafers and quartz boats, rapid temperature rise and fall resulting in faster temperature change at the edge of silicon wafers than at the center, high temperature generated during corrosion, mismatch of atomic diameter when heavily doped, different thermal expansion coefficients of chips and plates... Stress can easily lead to breakage and warpage of silicon wafers.

3. Heavy metal impurities will greatly reduce minority carrier lifetime. They will cause large leakage current in PN junctions, and even make the voltage drop to zero. Heavy metals accumulate in the tail of single crystal and can be removed. In addition, after diffusion, phosphorus silica glass or borosilicate glass can be used to absorb at 950 - 1050 C for one hour, but the absorption has little effect on alkali metal (sodium, potassium) and alkaline earth metal (calcium, magnesium).

4. Because of the high concentration of phosphorus diffusion, large dislocations will occur in silicon at high temperature, and the dislocations of silicon single crystal itself will make phosphorus expand deeper along the places where dislocations are larger or more concentrated. Local penetration is easy to form when space charge region is widened. So the concentration of phosphorus diffusion should not be too high. It is necessary to prevent the diffusion of phosphorus and boron to produce alloy points, which will narrow the width of the base zone.

5. If the avalanche breakdown occurs in a small part of the PN junction, the rapidly increasing current will be concentrated in this area, which will be burned down due to the concentration of heat. This destructive breakdown is called thermal breakdown, which is irreversible. This is mostly due to the uneven surface or residual mechanical damage of PN junctions.

6. Surface ions contaminate negative charges and repel electrons to attract holes, forming P-type inverted layer channels, thus increasing leakage current; while positive charges attract electrons to repel holes, which is equivalent to reducing surface resistivity, narrowing surface space charge region and forming low breakdown on PN junction surface.

7. When the surface of PN junction only produces the effect of current, the waveform is still avalanche breakdown, and VB is basically unchanged. Generating current I = qGXmS, S is the surface area of PN junction. Therefore, inadequate corrosion, rough surface, a large number of composite centers, make the surface leakage current larger.

8. Cleaning treatment before and after diffusion and cleanliness of diffusion system itself have great influence on voltage. Soft properties are related to the entry of harmful impurities into the body, so all kinds of contamination should be avoided.

9. Control of junction depth: A breakdown voltage requires the base region to exceed the width of the maximum space charge region; deep junction is easy to manufacture high-voltage device B current capacity requirements, VF can not exceed the standard C switching speed requirements. The wider the base area, the slower the speed.

10. The decrease of minority carrier lifetime is mainly caused by heavy metal impurities (copper, iron, nickel, lead, zinc, etc.) forming strong recombination centers. In addition to the silicon wafer itself, process operation, chemical reagents and utensils will bring heavy metal impurities. In addition, diffusion furnace tube wall is also an important, sometimes even the most important source. There are also a lot of alkali metals, alkaline earth metals and some excessive rare earth elements in the tube wall. They can penetrate the quartz tube wall and enter the tube from the quartz crystal gap. They not only pollute the silicon wafer, but also disintegrate the quartz tube (from transparency to whitening, crispening, peeling!).

11. At room temperature, the maximum surface field intensity is located in the N region of low concentration, while at 80 C, the maximum surface field intensity shifts to the N+-N junction. Therefore, although this device exhibits a beautiful avalanche ionization breakdown at room temperature, its breakdown voltage begins to decrease at high temperature. This is a typical surface breakdown feature.

12. Leakage current is another important parameter to characterize high voltage devices. The high temperature characteristics of devices with room temperature soft characteristics are certainly not good. Only devices with avalanche breakdown characteristics at room temperature can have good high temperature characteristics, but not necessarily. Some devices are still hard at room temperature or even up to 80 degrees Celsius, but at high temperature (diode 150 degrees Celsius), they become soft, resulting in low stability and reliability. This can be divided into two cases: A. Room temperature is a hard characteristic, but the starting current is large, that is, the diffusion current is large, which is commonly referred to as the "back chair". One of the common reasons for this is that

The protective material is impervious to solidification, and the surface channel is induced by the high density negative surface charge, which forms a large initial current. This phenomenon can be eliminated by improving the curing process and eliminating the channel. B. Room temperature is also a hard characteristic, but it produces a large current, that is, leakage current increases rapidly with the increase of voltage. The reason is that the surface damage layer caused by cutting or grinding is contaminated by heavy metals, and there are at least two impurity levels, which improves the recombination probability. Deep corrosion process can reduce the large leakage current after removing the damaged layer and contamination and eliminating surface contamination.

Surface ions contaminate negative charges and repel electrons to attract holes, forming P-type inverted layer channels, thus increasing leakage current; while positive charges attract electrons to repel holes, which is equivalent to reducing surface resistivity, narrowing surface space charge region and forming low breakdown on PN junction surface.

When the surface of PN junction only produces the effect of current, the waveform is still avalanche breakdown, and VB is basically unchanged. The generated current is proportional to the surface area of PN junction. Therefore, inadequate corrosion, rough surface, a large number of composite centers, make the surface leakage current larger.

The first purpose of corrosion (pickling) is to remove all kinds of contamination on the grain table, such as flux, dust particles, metal atoms or ions... The method is to dissolve it with strong oxidizing acid to form soluble substance.

The second is to remove the damage caused by cutting and splitting on the grain surface (the damage layer may be 50-100 microns thick), and to form a smooth surface. Flat and smooth surfaces help to reduce leakage current and prevent the formation of local strong electric fields. Local strong electric field easily causes leakage current to rise with voltage before breakdown, forming rounded corners or backrest chairs (channel breakdown).

Thirdly, the edges and corners of polygons (square, rectangle, hexagon) are passivated or even rounded, because the chemical reaction at the edges and corners is faster. The passivation edge can reduce the angular electric field of the p-n junction in reverse operation and increase the breakdown voltage of the diode. For circular grains, there is no such requirement.

Determined resistivity, diffusion concentration and junction depth are equivalent to an ideal breakdown voltage BVpp (assumed to be a planar junction). In order to improve the voltage withstand of devices, it is necessary to make the surface of p-n junction reach or approach the breakdown voltage of p-n junction. Two problems need to be solved: first, the contamination on the surface of p-n junction and the structural defects of single crystal must be eliminated, and the surface of p-n junction must be effectively insulated; second, the surface of p-n junction must have a reasonable shape to make p-n junction table. The surface electric field is not higher or close to the internal electric field intensity.

Because of the principle of electrical neutrality, the space charge region must bend at the surface of p-n junction for bevel modeling of mesa. Positive oblique angle (for ordinary P+N type diodes, the area of N plane is small, the area of P plane is large, and the negative oblique angle is opposite) is advantageous to the broadening of depletion zone, while the negative oblique angle can achieve satisfactory results only at a very small angle (3 6). The maximum field intensity of negative oblique angle is on the side of high concentration, while the maximum field intensity of positive oblique angle is on the side of low concentration.

If avalanche breakdown occurs in a very small part of the PN junction, the rapidly increasing current will be concentrated in this area, and it will be burned down due to the concentration of heat. This destructive breakdown is called thermal breakdown, which is irreversible. This is mostly due to the uneven surface or residual mechanical damage of PN junctions.