The nature of silicon epitaxy on sapphire substrate (SOS) is a heterogeneous epitaxy process in which a thin layer of SI (typical thickness less than 0.6 µm) is grown on a sapphire wafer SOS is part of the CMOS technology silicon epitaxy on insulator substrate (SOI) because of its inherent radiation resistance, SOS is mainly used in aerospace and military applications.
Typically, high purity artificially grown sapphire crystals are used. The advantage of SOS is that its excellent electrical insulation prevents the diffusion of radiation caused by bulk currents into nearby components.
However, the commercialization of S0S is far from being possible, as it is difficult to apply to high-density applications in modern pins and tubes. This is a result of the formation of misalignment, twinning and stacking defects caused by the differences between the silicon and sapphire lattices during S0S. In addition, the silicon at the closest interface is contaminated with aluminum from the substrate.
|Parameters range for Silicon on Sapphire (SOS) Epi Wafers|
|Wafer diameter||76 mm, 100 mm, 150 mm|
|Orientation||(1012) ± 1º (R-plane)|
|Epi-layer thickness, µm||0,3 – 2,0|
|Epi-layer dopant||Phosphorous, Boron|
|Epi-layer resistivity, Ohm.cm|
|n-type||according to spec.|
|p-type||1,0 – 0,01|
5-6 Heterogeneous epitaxy of silicon
With the progress of large-scale and super large-scale integrated circuits, the application of epitaxial technology is becoming more and more widespread.
In addition to the homogeneous epitaxy of silicon on silicon substrates, the "SOS" epitaxial growth of silicon on sapphire and spinel substrates and the "SOI" epitaxy of silicon on insulating substrates have been developed. "on insulating substrates.
In this section, we briefly introduce these two techniques and SiGe/i
5-6-1 SOS Technology
SOS stands for "Silicon On Sapphire" and "Silicon On Spinel", which means the epitaxial growth of silicon on sapphire or spinel substrates.
Sapphire (a-Al2O3) and spinel (MgO-Al2O3) are good insulators, and the epitaxial growth of silicon on them can eliminate the interaction between integrated circuit components, which not only reduces leakage current and parasitic capacitance, enhances radiation resistance and reduces power consumption, but also improves integration and realizes double-layer wiring, which is the ideal material for large-scale and super large-scale integrated circuits. It is the ideal material for large scale and super large scale integrated circuits.
1. Selection of substrate material
In the selection of heterogeneous epitaxial substrate materials, the first thing to consider is the compatibility between the epitaxial layer and the substrate material. Among them, crystal structure, melting point, vapor pressure, thermal expansion coefficient, etc. have a great impact on the quality of the epitaxial layer, and the second must also consider the substrate to the epitaxial layer staining problem. Currently, the most suitable materials for heterogeneous substrates for silicon epitaxy are sapphire and spinel. Table 5-4 lists some of the main physical properties of these two materials and silicon for comparison.
In terms of crystal structure, sapphire is hexagonal and spinel is cubic, and three silicon cells and two spinel cells coincide, with a mismatch of 0.7% along the (100) direction. On the other hand, the thermal expansion coefficients of the substrate and epitaxial layer are similar, which is one of the important factors to get a good heterogeneous epitaxial layer. If the difference is large, a large stress will be generated near the interface when the temperature changes, which will increase the defects of the epitaxial layer and even warp, thus affecting the performance and thermal stability of the material and device.
Spinel is a better substrate material than sapphire in terms of lattice matching, thermal matching, self-doping reduction, and capacitive effects.
However, the properties of silicon epitaxial layers on spinel are strongly dependent on the substrate components, which vary depending on the preparation method and process conditions.
Therefore, although the silicon epitaxial layer on spinel substrate can be better than that on sapphire substrate, the reproducibility is poor, and the thermal conductivity of sapphire is high and the preparation process is more mature, so sapphire is widely used as silicon epitaxial substrate in current industrial production.
2.SOS epitaxial growth
The equipment and basic process of SOS epitaxial growth is the same as that of general silicon homogeneous epitaxy. The substrate cutting, grinding and polishing and cleaning are also largely the same, except that sapphire is harder than silicon, so the grinding and polishing time is longer.
In the epitaxial growth of SOS, it is worth noting that the self-doping effect is more serious, because under the epitaxial growth conditions, the following reactions will occur on the substrate surface:
Al2O3(s) + 2HCl(g) + 2H2(g) = 2A1Cl(g) ↑ + 3H20(g)
The chloride of low-valent aluminum is gaseous, and it causes the substrate to be corroded, resulting in defects in the epitaxial layer. In addition, H2 and precipitated silicon will also corrode the substrate, and the reaction is:
Before the substrate surface is completely covered by Si (at least the epitaxial layer grows to 10~20nm), the above corrosion reaction is going on. After the substrate surface is covered, these corrosion reactions also occur on the back side of the substrate.
This causes staining of A1O, etc. In addition, as the substrate surface is corroded, it will increase the defects in the epitaxial layer and even locally grow into polycrystals. Since the corrosion of SiCl4 on the substrate is greater than that of SiH4, it is more advantageous to use SiH4 thermal decomposition method for SOS epitaxial growth.
In order to solve the conflict between growth and corrosion, epitaxial growth methods such as dual-rate growth and two-step epitaxy can be used.
The dual-rate growth method is to first use a high growth rate (1~2μm/mn) to quickly cover the substrate surface (100~200nm growth). Then, a low growth rate (about 0.3m/min) is used to grow to the desired thickness.
The two-step epitaxy method is a combination of the advantages of both SiH4/H2 and SiCl4H2 systems.
The SiH4/H2 system is used in the first step to rapidly cover the substrate surface, and then the SiCl4/2 system is used in the second step to grow to the required thickness.
SOS epitaxial growth inevitably introduces a high density of dislocations, twins, intergranular boundaries and other lattice defects in the epitaxial layer due to mechanical damage on the substrate surface and corrosion between the growth component and the substrate lattice mismatch, improper valence bonding and strain effects.
These defects interact with Cu, Fe and other heavy metal impurities to form a series of deep energy levels in the forbidden zone.
In addition, there are crystal defects such as A1 local precipitation and its oxide in the epitaxial layer, which act as a compound, scattering and capture center to reduce the carrier concentration, mobility and minority carrier lifetime, so the quality of SOS epitaxial layer cannot catch up with the homogeneous epitaxial layer, and the thinner the epitaxial layer, the worse the performance.
Nevertheless, SOS materials can generally meet the requirements of MOS devices. In the future, it is an important issue for the development of SOS technology to improve the crystal integrity of SOS epitaxial layers, reduce self-doping, and make its performance close to that of homogeneous silicon epitaxial layers and have good thermal stability.