BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a grinding method for grinding a wafer.

Description of the Related Art

When a wafer is ground to be reduced in thickness, the rigidity of the wafer is reduced to cause a problem such that the wafer may become hard to handle in subsequent steps. To cope with this problem, there has been proposed a grinding method using a grinding wheel having a plurality of abrasive members annularly arranged so as to form a circular ring having an outer diameter smaller than the diameter of the wafer, in which the back side of the wafer is ground at a central portion thereof by using the abrasive members to thereby form a circular recess at the central portion and simultaneously form an annular projection (reinforcing portion) around the circular recess (see Japanese Patent Laid-open No. 2007-173487 and Japanese Patent Laid-open No. 2015-74042, for example).

In removing the annular projection of the wafer ground by the above grinding method, the bottom surface of the circular recess of the wafer is held on a holding surface of a holding table in a cutting apparatus, for example, and the front side of the wafer opposite to the bottom surface of the circular recess is cut by a cutting blade along the inner circumference of the annular projection to thereby remove the annular projection (see Japanese Patent Laid-open No. 2009-141276, for example). The annular projection must be supported on a holding table during cutting of the wafer in order to prevent falling of the annular projection. Known as such a holding table is a stepped holding table having a central holding portion holding the bottom surface of the circular recess and an annular holding portion holding the end surface of the annular projection.

If the annular projection is not supported by the annular holding portion of the holding table, a difference in inclination may arise between the annular projection and the circular recess in cutting away the annular projection. As a result, cracks may be generated in the central portion of the wafer where the circular recess is formed or abnormal wearing may occur in the cutting blade. To solve such a problem, the following invention has been proposed. Before cutting away the annular projection, a measuring gauge is brought into contact with the upper surface of the annular projection to monitor the height of the upper surface of the annular projection. Then, the upper surface of the annular projection is ground as monitoring the height of the upper surface of the annular projection by using the measuring gauge such that the height of the upper surface of the annular projection becomes a preset height. The height of the upper surface of the annular projection is changed according to the depth of the circular recess, or according to the thickness of the central portion where the circular recess is formed, thereby making constant the amount of projection of the annular projection (see Japanese Patent Laid-open No. 2012-146889, for example).

SUMMARY OF THE INVENTION

In the invention described in Japanese Patent Laid-open No. 2012-146889, a measuring gauge for measuring the height of the upper surface of the annular projection must be provided separately from a measuring gauge for measuring the height of the bottom surface of the circular recess. Alternatively, it is necessary to provide gauge moving means horizontally moving the measuring gauge for measuring the height of the bottom surface of the circular recess. In any case, the configuration of the grinding apparatus becomes complicated and the height of the annular projection cannot be accurately adjusted.

It is therefore an object of the present invention to provide a wafer grinding method which can eliminate the need for measurement of the height of the upper surface of the annular projection and can grind the upper surface of the annular projection such that the height of the annular projection becomes a preset height.

In accordance with an aspect of the present invention, there is provided a wafer grinding method using a grinding apparatus including a holding table having a holding surface for holding a wafer, grinding means including a rotatable grinding wheel having a plurality of abrasive members annularly arranged so as to form a circular ring having an outer diameter less than a diameter of the wafer, the abrasive members being adapted to grind a central portion of the wafer held on the holding table to thereby form a circular recess at the central portion of the wafer and also form an annular projection around the circular recess, vertical moving means vertically moving the grinding means in a direction perpendicular to the holding surface, height recognizing means recognizing a height of the grinding means moved by the vertical moving means, horizontal moving means relatively moving the holding table and the grinding means in a horizontal direction parallel to the holding surface, and a setting section setting a predetermined value for a height of the annular projection of the wafer. After the circular recess is formed, an upper surface of the annular projection is ground by the abrasive members of the grinding means to thereby adjust the height of the annular projection to the predetermined value set by the setting section. The wafer grinding method includes a center grinding step of grinding the central portion of the wafer by using the abrasive members to thereby form the circular recess and simultaneously form the annular projection around the circular recess, a height storing step of recognizing the height of the grinding means after performing the center grinding step by using the height recognizing means and next storing the height recognized above, and an annular projection grinding step of grinding the upper surface of the annular projection by using the abrasive members in a condition that a height raised from the height of the grinding means stored in the height storing step by the predetermined value previously set by the setting section is calculated as a grinding end height where the grinding of the annular projection by the grinding means is ended.

Preferably, the grinding apparatus further includes wafer height measuring means measuring a height of an upper surface of the wafer held on the holding table. Preferably, the wafer grinding method further includes a depth calculating step of calculating a depth of the circular recess from a difference between the height of the upper surface of the wafer measured by the wafer height measuring means before performing the center grinding step and a height of a bottom surface of the circular recess measured by the wafer height measuring means after finishing the center grinding step, and an annular projection grinding start height calculating step of calculating a height raised from the height of the grinding means stored in the height storing step by the depth of the circular recess calculated in the depth calculating step as a grinding start height where the grinding of the annular projection by the grinding means is started, both the depth calculating step and the annular projection grinding start height calculating step being performed before starting the annular projection grinding step.

According to the present invention, it is unnecessary to measure the height of the annular projection by bringing a measuring gauge into contact with the upper surface of the annular projection. That is, the annular projection can be ground by the abrasive members to thereby adjust the height of the annular projection to the predetermined value set by the setting section without using a measuring gauge to monitor the height of the annular projection. Accordingly, any mechanism is not required to be added to the conventional grinding apparatus in performing the present invention, so that the height of the annular projection can be easily adjusted according to the present invention.

Further, in the case that the depth calculating step of calculating the depth of the circular recess is performed after finishing the center grinding step and that the annular projection grinding start height calculating step of calculating the grinding start height of the grinding means where the grinding of the annular projection by the grinding means is started is performed before starting the annular projection grinding step, the height of the annular projection can be adjusted with high accuracy to the predetermined value set by the setting section.

The above and other objects, features, and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention.

