Why Do Fabs Need (111) Wafers To Make BJT, While MOS Tubes Need (100) Wafers?

BJT and MOS select different directional silicon wafers (wafers) respectively. (111) and (100) orientation wafers have their specific application reasons in electronic component manufacturing, mainly related to the physical properties of the material and process requirements.

1. Basic understanding of crystal direction

Silicon wafers can have a variety of different crystal orientations, the most common of which are (100), (110), and (111). Crystal orientation refers to the orientation of crystal structure, and the physical and chemical properties of different crystal orientations are different.Figure: Lattice plane (hkl) represents a set of parallel planar silicon wafers usually with Flat or Notch marks, which are used to identify the crystalline orientation and doping type of the silicon Wafer. The following are common ways of marking: Orientation: (100) The orientation usually has a primary flat edge and a shorter secondary flat edge. (111) The crystal direction usually has only one main flat edge. Doping Type: N-type Doping (N-type Doping) usually has a long main flat edge. P-type Doping usually has a shorter main flat edge, or in some cases uses a missing corner marker.

2. BJT (bipolar transistor) and (111) crystal direction selection

Figure: BJT triode logo and its structure

a. Surface state density

The (111) crystalline orientation of the silicon surface has a high surface state density, which is not a big problem for BJT. The work of BJT mainly depends on carrier injection and diffusion, and the surface state has little influence on these processes.

b. Diffusion of embedded impurities

In the (111) crystal direction, the diffusion rate of impurities such as boron and phosphorus is high and uniform, which is conducive to the formation of high-quality PN junction. This is important for the performance of BJTS, which rely on good PN junction characteristics to achieve high gain and low noise.

c. Effect of crystal direction on current conduction

In silicon crystals with (111) orientation, the carrier mobility is slightly lower than that of (100) orientation, but this does not significantly affect the performance of BJT, because the working principle of BJT is mainly based on current density and carrier injection.

3. Selection of MOS device and (100) crystal direction

Figure: MOS tube and its logo

a. Surface state density

The (100) crystalline silicon surface has a lower surface state density, which is very important for MOS devices. Low surface state density can reduce the influence of interface states, thereby improving the threshold voltage stability and mobility of the MOSFET.

b. Mobility

In the (100) crystal direction, the electron mobility is higher than that in the (111) crystal direction, which is particularly important for NMOS devices. Higher electron mobility can improve the on-state current and switching speed of the MOSFET.

c. Interface quality

The (100) crystalline orientation of the silicon wafer makes it easier to form a high-quality SiO₂ insulation layer, which is essential for the insulation characteristics and reliability of MOS devices. A high-quality oxide layer helps to reduce leakage current and improve device stability and durability.

4. Process compatibility

a. Difficulty and cost of handling

The silicon wafer with (100) crystal orientation is relatively easy to handle in the processing process, and the surface is relatively flat, which is suitable for the manufacture of large-scale integrated circuits. In contrast, silicon wafers with (111) crystal orientation are more difficult to machine and cost more when cutting wafers.

b. Process standardization

Modern CMOS processes have become highly standardized, with (100) wafers becoming the industry standard. This standardization helps to improve production efficiency, reduce costs, and improve product consistency.