How Does Sand Make Silicon Wafers Step by Step: Silicon Wafer Preparation Process

How do silicon rods turn into silicon wafers?

The following is the silicon wafer preparation process:

1) Truncation

As shown in Figure 1, the two ends of the silicon rod are removed, one end is the seed end (where the seed crystal is located), and the other end is the non-seed end (opposite to the seed crystal), that is, the head and tail of the monocrystalline silicon are cut off.

2) Diameter grinding

Because the control of diameter and roundness in crystal growth can not be very accurate, so the silicon rods have to grow slightly larger, the size of the diameter is not uniform, so it is usually necessary to diameter grinding, so that the diameter of monocrystalline silicon to achieve consistency and meet the requirements of different product diameters, as shown in the figure.

3) Grinding the positioning surface

Single crystal has the characteristics of anisotropy, must be cut according to the specific crystal direction, in order to meet the needs of production. The principle is to use a beam of visible light or X-ray to shine at the end face of a single crystal rod, because of the difference in the direction of the crystal on the end face, its reflected pattern is also different. Based on the reflected image, the crystal orientation of the single crystal rod can be corrected. Once the crystal is oriented on the cutting block, a reference plane is ground along the axis, often referred to as the main reference plane of the wafer. In many wafers, the edge will also have a second, smaller reference surface, called a sub-reference surface, to distinguish the type of conduction. The Angle of the primary and secondary positioning edges identifies the type of silicon chip, as shown in the figure.

 4) Slice

After the shape of the rod is processed, the silicon rod is then cut into silicon wafers. There are two ways to slice the wafer, one is to cut the wafer from the crystal with an internal blade coated with diamond. These blades are thin, round pieces of steel with round holes in the center. The second is line slicing, which is achieved by the movement of the metal wire with diamond particles. When slicing monocrystalline silicon, the thickness, crystal direction, warpage and parallelism of the silicon wafer are the key parameters, which need to be strictly controlled. The requirements for chip slicing are: thickness meets the requirements, flatness and curvature should be small, no defects, no cracks, and shallow knife marks. The diagram of the two methods is shown in the figure.  
 

5) Grinding

After the slicing is completed, the surface of the silicon wafer should be grinded and machined, as shown in Figure 5. The purpose of the grinding process includes the following two points: first, to remove the scar on the surface of the silicon wafer, so that the surface processing damage of the silicon wafer is uniform. The second is to adjust the thickness of the silicon wafer, so that the thickness difference between the wafer and the wafer is gradually reduced, and the surface flatness and parallelism are improved.
 

The most commonly used is the planetary grinding method, as shown in Figure 6. The double-sided film machine has two grinding plates, the planet sheet is placed in the middle, and the silicon sheet is placed in the hole of the planet sheet. When grinding, the disc does not rotate, the inner gear and the center gear rotate, so that the planetary motion between the planetary disc and the grinding disc is done to drive the planetary motion of the silicon wafer, and the purpose of grinding is achieved under the action of the abrasive.   
 

6) Chamfer

The chamfering process, as shown in the figure, is to use a wheel with a specific shape to grind away sharp edges, edges and cracks on the edge of the silicon chip.

There are three main purposes of chamfering.

(1) Prevent wafer edge breakage. Wafer in the process of manufacturing and use, the edge of the wafer is often hit by the manipulator and so on, resulting in the formation of stress concentration area. These areas of stress concentration will cause the wafer to constantly release pollution particles during use, which will affect the yield of the product.

(2) Prevent the concentration of thermal stress. When wafer is in use, it will experience numerous high-temperature processes, such as oxidation, diffusion, etc. When the thermal stress generated in these processes exceeds the strength of the silicon lattice, dislocation and dislocation defects will be generated, and wafer edging can avoid such defects in the crystal edge.

(3) Increase the flatness of the epitaxial layer and the photoresist layer at the wafer edge. In the epitaxial process, the growth rate of the acute Angle area will be higher than that of the plane, so it is easy to create bumps at the edge with the wafer without grinding. Similarly, when the photoresist is applied by the rotary homogenizer, the photoresist solution will accumulate on the edge of the wafer, and these uneven edges will affect the focusing accuracy of the mask.
 

7) Polishing

Polishing is the last important processing process of the silicon surface, and it is also the most delicate surface processing. The purpose of polishing is to remove the fine damage layer on the surface and obtain a smooth surface with high flatness, as shown in Figure 8. Finally, you get the final product: silicon wafers.