E+H Article
Precision Grinding of Semiconductor Wafers
Hubert Hinzen, Bert Ripper, George Müller, Nürnberg, Gemany


Figure 1. Schematic and photograph of a rotational grinding apparatus.
Wafer grinding is important in both the pre- and post-processing of silicon substrates. It occurs first following the slicing of wafers from the silicon ingot. Since slicing is not especially accurate, due to the limited axial stiffness of slicing tools, grinding is used to reduce variations-in wafer thickness. Polishing then provides the wafer with a very-low-damage, fine-finish surface.

After semiconductor chip formation is completed, grinding is employed again to reduce a processed wafer from a thickness of slightly less than 1 mm to a final thickness in the range of about 0.2 to 0.5 mm. This is a necessary step for the processes of die separation and packaging. To optimize manufacturing throughput, material removal rates during back side grinding should be as high as possible.

In advanced silicon device technology, the uniformity of silicon wafer thickness - a property tracked by the parameter Total Thickness Variation (TTV) [1] - has become increasingly important [2]. In this regard, rotational grinding technology has emerged as a significant method for achieving acceptable TTV values. As illustrated in Figure 1, rotational grinding is a straightforward concept. A wafer is placed on a vacuum chuck. The surface to be machined is located directly beneath the grinding "cup-wheel" tool with the center of the wafer situated below the fine-grain, diamond-coated cup-wheel rim. Both elements - the cup-wheel and the wafer-rotate. Downward displacement of the spindle unit removes material on the wafer surface and creates a flat plane. Wafer thickness is thus controlled directly through axial spindle displacement.

While the foregoing principles are simple, the tools of rotational grinding require sophisticated design. In general, two consecutive grinding operations are needed (Fig. 1): rough grinding with a relatively coarse-grain wheel removes most of the material, whereas the final surface is produced through fine-grain grinding. Various wafer surface qualities can be created with different tooling and process combinations.

The positional stability of the cup-wheel axis relative to the wafer axis directly determines the accuracy of the ground wafer's thickness distribution. In this regard, rotational grinding is especially advantageous because the number of degrees of freedom for the machine is minimized while optimum stiffness can be achieved. But, since machine stability has two components - mechanical and thermal - both require consideration.

During grinding, the machining forces react on the tool as well as the wafer, producing elastic deformation that results in improper inclinations of the rotational axes. The machine tool designer must use machine stiffness to counteract this, but, since Young's modulus is never infinite, high material stiffness alone is insufficient for achieving ultraprecision in machining. Furthermore, every grinding operation generates a certain amount of heat, which leads to thermal deformation. The result in rotational grinding can again be misalignment of the axes. To meet current and future semiconductor wafer accuracy requirements, an approach is needed that goes beyond classical mechanical methods. In this note we describe an application of auto control technology to achieve this goal.
Figure 2.The conventional approach to rotational grinding.
Figure 3. Rotational grinding with feedback employed
Rotational grinding tool with automatic control
Conventional Rotational grinding is illustrated schematically in Fig. 2. To improve this arrangement, one may first represent the sum of all grinding perturbations by a single parameter - the relative inclination of the two rotary axes. Knowledge of that angle then allows generation of a feedback signal to control grinding (Fig. 3). The level of control required is significant; current wafer manufacturing requirement demand an angular precision of ±10-4 degrees.

One may infer the relative angle between the rotary axes by measuring the wafer thickness distribution. Ideally, such measurements should be made in situ during grinding, but, since this is not practical, post-grinding measurements are employed. The approach shown in Fig. 4 (measurement of wafer thickness distribution) employs a noncontact capacitive probe and a measurement pattern aptly suited for the wafer's circular symmetry.
Figure 4. Wafer locations and apparatus for measuring wafer thickness distribution. The wafer thickness measurements provide data for the rotational grinding tool's control unit as well as for subsequent quality control evaluation.
In Figs. 3 and 4, the PC control unit directs positioning operations for the measurement sequence and then calculates the grinding axis angles from the measured thickness distribution data. This measurement and calculation generates precise angular correction values that eliminate any trial and error approach in adjusting the grinding tool.

Figure 5 illustrates the basic configuration for mechanically adjusting the grinding angles. Angular adjustments are made to the grinding spindle. To produce extremely delicate adjustments (in the range of 10-4 degrees) of the spindle angle and to avoid stiction problems, an elastically deformable element is used, as shown. Based on error correction values provided by the controller, the motorized element then automatically adjusts the spindle axis. In practice, the control loop implements spindle axis correction in two dimensions through two separate, orthogonally placed correcting elements.

Flatness accuracy

The effect of control-loop-operation is immediate. In Fig. 6, an initial TTV value of 3.01 µm is reduced first to 1.06 µm and then to 0.55 µm. While operation of the grinding apparatus over time does produce slowly varying angular disturbances of the spindle axis, these are amenable to rapid correction.

One interesting aspect emerging from the development of the rotational grinding process is that while one may compensate for global disturbances related to the relative angle between the rotary axes, angular adjustment will not compensate for local wafer disturbances such as dimples. In fact, dimples and other imperfections may actually cause improper response of the control loop. To prevent this, the measured data must be filtered to eliminate noncompensatable "background noise" inputs to the control unit. Background noise may be quite high if nonplanar irregularities are present on the wafer surface, but the problem can be reduced signifcantly through grinding the opposite side.

 
Figure 5.Angle correcting element
Still another application for high-precision rotational grinding relates to the preparation of silicon-on-insulator (SOI) wafers [3]. SOI technology presents substrates with a thin film structure - active device layer/insulating dielectric layer/supporting substrate - and places exceptionally stringent demands on wafer flatness and film thickness uniformity. With active device layers that are <=1 µm thick, SOI wafers require a TTV << 1 µm and local thickness variation (LTV) <0.5 µm The grinding spindle alignment strategy and control loop implementation described above have proven effective in meeting these requirements.

Figure 6. Results of the feedback- controlled grinding process: (a) wafer with initial TTV of 3.01 µm (b) first rotational grinding has reduced TTV to 1.06 µm, and (c) final finishing yields a TTV of 0.55 pm. Thickness distribution measurements were made with an E&H (Eichhorn und Hausmann) MX 102-8.
Conclusion

Automatic control applied to the rotational grinding process provides a means for meeting increasingly tight flatness specifications of modern semiconductor wafers. Rotational grinding for high-precision applications requires a closely coupled wafer thickness distribution measurement. By doing so, one systematically improves the TTV value without trial and error.

References

  1. ASTM F533 Standard Test Method for Thickness variation of Silicon Slices, American society for Testing and Materials, 1987

  2. T. Abe, "A Future Technology for Silicon Wafer Processing for ULSI," Precision Engrng., vol 13 (4), p. 251 (1991).

  3. L Peters, "SOI Takes Over Where Silicon Leaves Off," Semiconductor International, p. 48, March 1993.
Back