Surface grinding method of silicon wafers.

Dobrescu, Tiberiu ; Anghel, Florina

1. INTRODUCTION

The flatness value may be achieved by improvement of the traditional method, but it is not the final target of Si-wafers. For wafers over 200 mm in diameter, two technological disadvantages of the traditional method are exposed. One is the thermal and mechanical inhomogenity of a huge lapping exposed (Dobrescu, T., 1998).

The second is the difficulty of improving the stability of the etching rate, both wafer to wafer and within a wafer.

Because of the high normal forces induced when grinding brittle materials, unavoidable elastic deformation of the tool and workpiece occur. This is due to the kinematics of the cutting process, i.e. that the relative trajectories and speeds of the abrasive grains are varied. The elastic deformations caused by high normal forces bring about faults of geometry and surface and the sub-surface damage.

The conventional grinding methods for wafer have the above mentioned characteristics and therefore the wafer quality is limited. With the new rotation method, which is integrated in the ultra precision process, it is possible to suppress this disadvantage (Dorin, A. & Dobrescu, T., 2007).

2. THE PRINCIPLE OF THE ROTATION GRINDING METHOD

Figure 1 shows the principle of the rotation grinding method. A wafer is centered on a porous ceramic vacuum chuck. The work piece rotates relative to a cup wheel, which also rotates on its own axis. In this way, every point of the planar wafer surface comes into contact with the grinding wheel and the diminution of size is executed on for every point of the wafer. The cup wheel is moved in axial direction until the wafer has reached its final thickness (Inasaki, I., 1987).

Economical reasons require a splitting of the grinding process into a rough grinding and a finishing part. For this purpose, cup wheels with different grit sizes have to be used. In the rough grinding with a D46-cup wheel it is possible to reach high stock removal rates. Finishing with the grit size D7 permits a good surface quality (Dobrescu, T., 1996).

There are two construction principles for the rotation grinding method with stepped grit size:

* Concentric cup wheels. Two annular grinding wheels are mounted concentrically on one rotating support disk. The inner grinding-ring has the large grit size for rough grinding whereas the outer ring is provided for fine grinding. The two grinding rings are arranged in such a way that either the rough-grinding-wheel or the fine-grinding-wheel can be put in contact with the work piece whereby the other grinding wheel is not activated;

* Serially joined cup wheel. The grinding process is divided into two or more steps of serially joined cup wheels. The wafer runs through the workstations in order of declining grit size. In wafer rotation grinding, accurate straightness can be obtained theoretically. But in reality, the rotary axis of the grinding wheel and the work piece are not absolutely parallel because the plane parallelism of the ground wafer face cannot be achieved exactly.

[FIGURE 1 OMITTED]

The reasons for this fault are:

* machine deformation as a consequence of grinding forces;

* machine deformation due to thermal influence;

* insufficient adjustment;

* bearing clearance of tolerance.

Because of these reasons, a subsequent adjustment of the spindle is useful. The influence of the angular deviation on the shape of the ground work piece is quite simple.

The angular deviation can be divided up into a radial and tangential component of the angle between z--axis and the rotary axis of the grinding wheel.

If the spindle is inclined in positive [[alpha].sub.r] direction, the wafer has a convex profile on the wafer surface. In case of an inclination in negative [[alpha].sub.r] direction, the wafer would have a concave profile. Whether the inclination in a direction is positive or negative makes no difference. The wafer has a cone-like shape (Sun, W., Pei, Z. & Fisher, G., 2004).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The spindles deviation is not only limited on a purely [[alpha].sub.r] or [[alpha].sub.t] direction, but all combinations between these two components are possible, so that all shapes between convex and concave on one side and "cone-like" on the other side are possible (Matsui, S., 1988).

A certain relation between [[alpha].sub.r] and [[alpha].sub.t] inclination leads to a typical surface-profile which is shown in Figure 2.

It is characteristic for grinding in general that the processed surface is not an absolutely homogenous but shows a certain orientation of grinding marks. The aim for every high quality grinding process is to reduce the visibility of the grinding marks although they always exist. The grinding marks depend on the kinematics of the process.

Figure 3 shows an example of the surface of a wafer ground by the rotation method.

For the best possible surface roughness the grit size of the grinding wheel should be very fin. However, there are certain limits below which grinding is not longer possible because the self-dressing effect decreases and finally disappears.

If smaller grit size than D7 is utilized to produce smoother surface, the diamond in resin bond tends to be buried by the silicon dust created during the processing.

3. CONCLUSION

The following conclusions can be drowning from this experiment:

* Especially for large wafers (200 mm and more), rotation grinding with a cup-wheel represents future technology, because the geometrical contact zone is small (thermal induction) and the tool is defined in its geometry;

* To reach values of less than 0.5 [micro]m for the total thickness variation TTV, an automatic adjustment of the tool spindle will be necessary. The objective is to obtain a nearly perfect wafer quality after this step to reduce the polishing process expense;

* Lower wheel speed and higher feed rate produce rougher surface;

* In silicon wafer manufacturing, the removal amount of the subsequent polishing process to be large enough to eliminate all grinding marks generated in the simultaneous double side grinding or single side grinding operation. Further reduction of polishing amount necessitates optimization of the grinding process so that the grinding marks can be eliminated with minimum amount of polishing.

4. REFERENCES

Dobrescu, T. (1996), Rotation Grinding Method of Silicon Wafers, Research Reports, LAPT, University of Naples "FedericoII", pp. 31-34, Italy

Dobrescu, T. (1998), Cercetari privind optimizarea masinilor de superfinisat materiale fragile, PhD Thesess,Universitatea "Politehnica" din Bucuresti

Dorin, A & Dobrescu, T. (2004). A Study of Pad Dynamics in Wafer Polishing. Proceedings of the International Conference of Manufacturing Systems, Academy of Romania (Ed), pp. 513-516, Bucharest, October 2004, Romanian Journal of Technical Sciences

Inasaki, I. (1987). Grinding of Hard and Brittle Materials. Annals of the CIRP, no. 36, pp. 463-471

Matsui, S. (1988). An experimental study on the grinding of silicon wafers. Bulletin of the Japan Society of Precision Engineering, no. 22, pp. 295-300

Sun, W., Pei, Z. & Fisher, G. (2004). Fine grinding of silicon wafers: a mathematical model for the wafer shape. International Journal of Machine Tools & Manufacturing, no. 44, pp. 707-716