1. Technical Field of the Invention
This invention relates to a wafer grinding method and a wafer grinding machine.
2. Description of the Related Art
With the ever-increasing trend toward higher integration and packaging of semiconductor devices in recent years, semiconductor chips (dies) have been correspondingly reduced in thickness. As a result, the back surface of the wafer is ground by a grinding means before dicing. The surface of the wafer back is ground with a protective tape attached to the front surface of the wafer to protect the circuit pattern formed on the front surface of the wafer.
Further, it common practice to polish the back surface of the water that has been ground to remove distortion.
With the reduction in wafer thickness, the finish thickness of the wafer requires high dimensional accuracy. Since the finish thickness of the wafer is normally considered to include the protective film; however the thickness variation of the wafer directly affects the finish thickness of the wafer.
In view of this, Japanese Unexamined Patent Publication No. 2007-335458 employs the means described below in order to secure the wafer having a target thickness, by accurately measuring the thickness of only the wafer, excluding the protective tape free of the effect of the nitride or the oxide on the back surface of the wafer.
Specifically, according to Japanese Unexamined Patent Publication No. 2007-335458, the back surface of the wafer is ground by measuring the total wafer thickness including the protective tape using a contact-type thickness gauge during the rough grinding process and by measuring the thickness of only the wafer using a noncontact-type thickness gauge during the finish grinding process. Once the target finish thickness is reached, the grinding process is ended thereby achieving an accurate target finish thickness.
A noncontact-type thickness gauge is described, for example, in Japanese Unexamined Patent Publication No. 2007-113980.
According to Japanese Unexamined Patent Publication No. 2007-113980, a hollow cylindrical probe for an eddy current displacement gauge is mounted on the bottom surface of the gauge body, and a coil is arranged at the forward end of the probe. On the bottom surface, a laser radiation unit and a laser light receiving unit of the laser displacement gauge are also arranged on the inside of the hollow cylindrical probe.
An AC magnetic field is generated by the coil and an eddy current is induced to a metal plate. The inductance change of the coil due to the eddy current thus induced is detected to calculate the distance L2 from the probe to the metal plate.
On the other hand, a laser beam is radiated from the laser radiation unit, and the laser beam reflected on the surface of the coating is detected by a laser receiving unit. The distance L1 between the laser displacement gauge and the coating surface is calculated according to trigonometry. The thickness of the coating is calculated based on distances L1 and L2.
During the grinding process; however the environment of the ground surface of the wafer is not stabilized due to the grinding liquid and the grinding sludge, and the use of the noncontact-type thickness gauge during the finish grinding process as described in Japanese Unexamined Patent Publication No. 2007-335458 would make a stable measurement difficult while at the same time fouling the measuring instrument. A measurement conducted while the work is not machined would be affected by the oxide film or the nitride film formed on the back surface of the water, thereby often making it impossible to measure the wafer thickness accurately.