BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a wafer producing method for producing a wafer from an ingot after forming a separation layer inside the ingot by setting a focal point of a laser beam having a transmission wavelength to the ingot inside the ingot at a predetermined depth from one end surface of the ingot, the predetermined depth corresponding to the thickness of the wafer to be produced, and next applying the laser beam to the ingot.

Description of the Related Art

Various devices such as integrated circuits (ICs), large scale integrations (LSIs), and light emitting diodes (LEDs) are formed by forming a functional layer on the front side of a wafer formed of silicon (Si), sapphire (AL2O3), or the like and partitioning this functional layer into a plurality of separate regions along a plurality of crossing division lines.

The division lines of the wafer having these devices are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus to thereby divide the wafer into a plurality of individual device chips respectively corresponding to the devices. The device chips thus obtained are used in various electrical equipment such as mobile phones and personal computers. Further, power devices or optical devices such as LEDs are formed by forming a functional layer on the front side of a wafer formed of hexagonal single-crystal silicon carbide (SiC) and partitioning this functional layer into a plurality of separate regions along a plurality of crossing division lines.

In general, the wafer on which the devices are to be formed is produced by slicing an ingot with a wire saw. Both sides of the wafer sliced from the ingot are polished to a mirror finish (see Japanese Patent Laid-Open No. 2000-094221, for example). However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, 70% to 80% of the ingot is discarded to cause a problem of poor economy. In particular, a hexagonal single-crystal SiC ingot has high hardness and it is therefore difficult to cut this ingot with the wire saw. Accordingly, considerable time is required for cutting of the ingot, causing a reduction in productivity. Furthermore, since this ingot is high in unit price, there is a problem in efficiently producing a wafer in this prior art.

A technique for solving this problem has been proposed by the present applicant. This technique includes the steps of setting the focal point of a laser beam having a transmission wavelength to hexagonal single-crystal SiC inside an SiC ingot, next applying the laser beam to the ingot as scanning the laser beam on the ingot to thereby form a separation layer in a predetermined separation plane inside the ingot, and next separating a wafer from the ingot along the separation layer (see Japanese Patent Laid-Open No. 2016-111143, for example). Further, a similar technique has also been proposed. This technique includes the steps of setting the focal point of a laser beam having a transmission wavelength to hexagonal single-crystal Si inside an Si ingot at a predetermined depth from an end surface of the ingot, the predetermined depth corresponding to the thickness of a wafer to be produced, next applying the laser beam to the ingot as scanning the laser beam on the ingot to thereby form a modified portion at this predetermined depth inside the ingot, and next separating the wafer from the ingot along this modified portion (see Japanese Patent Laid-Open No. 2011-060862, for example).

SUMMARY OF THE INVENTION

According to the techniques described in Japanese Patent Laid-Open No. 2016-111143 and Japanese Patent Laid-Open No. 2011-060862 mentioned above, the amount of the ingot to be discarded in producing the wafer from the ingot is reduced as compared with the case of using a wire saw to slice the ingot, so that some effect can be exhibited against the problem of poor economy. However, it is not easy to separate the wafer from the ingot along the separation layer or the modified portion formed inside the ingot by applying a laser beam, so that there still remains a problem in production efficiency.

It is therefore an object of the present invention to provide a wafer producing method which can efficiently produce a wafer from a hexagonal single-crystal SiC ingot.

In accordance with an aspect of the present invention, there is provided a wafer producing method for producing a wafer from an ingot after forming a separation layer inside the ingot by setting a focal point of a laser beam having a transmission wavelength to the ingot inside the ingot at a predetermined depth from one end surface of the ingot, the predetermined depth corresponding to a thickness of the wafer to be produced, and next applying the laser beam to the ingot, the wafer producing method including: a first ultrasonic vibration applying step of applying ultrasonic vibration to a given area of the ingot at a first density to thereby form a partially broken portion where a part of the separation layer is broken; a second ultrasonic vibration applying step of applying the ultrasonic vibration to a whole area of the ingot larger than the given area at a second density lower than the first density, after performing the first ultrasonic vibration applying step, thereby forming a fully broken portion where the separation layer is fully broken in such a manner that breaking starts from the partially broken portion; and a separating step of separating the wafer from the ingot along the fully broken portion.

Preferably, the ultrasonic vibration is applied through a layer of water to the ingot in the first ultrasonic vibration applying step and the second ultrasonic vibration applying step.

Preferably, the ingot includes a hexagonal single-crystal SiC ingot having a c-axis and a c-plane perpendicular to the c-axis, and the separation layer is composed of a modified portion and cracks extending isotropically from the modified portion along the c-plane, the modified portion being formed by setting a focal point of a laser beam having a transmission wavelength to hexagonal single-crystal SiC inside the SiC ingot at a predetermined depth from the one end surface of the SiC ingot, the predetermined depth corresponding to the thickness of the wafer to be produced, and next applying the laser beam to the SiC ingot to thereby decompose SiC into Si and C.

Preferably, the c-axis is inclined at an off angle with respect to a normal to the one end surface of the SiC ingot, the off angle being formed between the c-plane and the one end surface, the focal point of the laser beam is moved in a direction perpendicular to a direction of formation of the off angle to thereby continuously form the modified portion in the direction perpendicular to the direction of formation of the off angle, and the focal point of the laser beam is indexed in the direction of formation of the off angle in a range not greater than a range of formation of the cracks, whereby a plurality of linear modified portions are arranged side by side in the direction of formation of the off angle in such a manner that the cracks extending from any adjacent ones of the plurality of linear modified portions are overlapped.

According to the wafer producing method of the present invention, the amount of the ingot to be discarded in producing the wafer from the ingot can be greatly reduced as compared with the case of using a wire saw to slice the ingot. Further, the first ultrasonic vibration applying step is performed to apply ultrasonic vibration to a part of the ingot at a high density, thereby forming the partially broken portion. Thereafter, the second ultrasonic vibration applying step is performed to apply ultrasonic vibration to the whole of the ingot at a low density, thereby forming the fully broken portion as spread from the partially broken portion. Thus, the fully broken portion is formed from the separation layer previously formed inside the ingot. Accordingly, the wafer to be produced can be separated from the ingot along the fully broken portion easily and efficiently.

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.

