News Column

Researchers Submit Patent Application, "Laser Processing Method for Workpiece", for Approval

May 7, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Okada, Shigefumi (Tokyo, JP); Ogoshi, Nobumori (Tokyo, JP), filed on October 8, 2013, was made available online on April 24, 2014.

The patent's assignee is Disco Corporation.

News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to a laser processing method of applying a laser beam to the inside of a thin platelike workpiece such as a semiconductor wafer from the back side thereof, thereby performing laser processing to the workpiece.

"In a semiconductor device fabrication process, a plurality of devices having electronic circuits are formed on the front side of a semiconductor wafer in a plurality of regions partitioned by a plurality of crossing division lines (process lines). The semiconductor wafer thus having the plural devices is divided along the division lines to thereby obtain a plurality of individual semiconductor chips respectively corresponding to the plural devices. As a method of dividing a workpiece such as a semiconductor wafer, there is a method including the steps of applying a laser beam having a transmission wavelength to the workpiece along each division line in the condition where the focal point of the laser beam is set inside the workpiece, thereby forming a modified layer inside the workpiece along each division line, and next applying an external force to the workpiece to divide the workpiece from the each modified layer as a break start point (see Japanese Patent No. 3408805, for example)."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "In performing this kind of laser processing by using laser beam applying means such as a laser head unit, the components of the laser beam applying means may be expanded or contracted by temperature changes, causing a possibility that a laser applied position may be deviated from a target position. When the laser applied position is deviated, there is a possibility that the modified layer may be formed at a device formed position. In dividing the workpiece along this modified layer by applying an external force in the above case, there arises a problem such that the workpiece is not properly divided to produce odd-shaped chips or damage the devices.

"It is therefore an object of the present invention to provide a laser processing method which can reduce the deviation of the laser applied position in applying a laser beam to the workpiece along each process line to perform laser processing to the workpiece.

"In accordance with an aspect of the present invention, there is provided a laser processing method of performing laser processing to a workpiece having a plurality of devices respectively formed in a plurality of regions partitioned by a plurality of crossing process lines formed on the front side of the workpiece, the laser processing method including a holding step of holding the front side of the workpiece by using holding means to expose the back side of the workpiece; an alignment step of detecting the process lines from the back side of the workpiece held by the holding means to align each process line with a laser beam having a transmission wavelength to the workpiece; a laser processing step of applying the laser beam to the workpiece from the back side thereof along each process line in the condition where the focal point of the laser beam is set inside the workpiece after performing the alignment step, thereby forming a plurality of modified layers inside the workpiece respectively along the plurality of process lines; and a positional deviation correcting step of imaging a predetermined one of the modified layers formed inside the workpiece from the back side of the workpiece with predetermined timing by using imaging means during the performance of the laser processing step, detecting a positional deviation of the predetermined modified layer from the corresponding process line to calculate a correction value, and adding the correction value to data on applied position of the laser beam to thereby make the applied position of the laser beam coincide with each process line.

"In the laser processing method of the present invention, the positional deviation correcting step is performed with predetermined timing during the performance of the laser processing step. That is, a positional deviation of a predetermined one of the modified layers already formed from the corresponding process line is detected to calculate a correction value according to the result of detection. Then, the correction value is added to data on applied position of the laser beam to make the applied position of the laser beam coincide with each process line after this correction. Accordingly, the deviation of the laser applied position can be reduced.

"Preferably, each modified layer is formed at a predetermined height from the front side of the workpiece in the laser processing step; and the positional deviation correcting step includes the steps of applying the laser beam along the predetermined modified layer to thereby form a correcting modified layer near the back side of the workpiece at a level higher than the predetermined height, imaging the correcting modified layer by using the imaging means, and detecting a positional deviation of the correcting modified layer from the corresponding process line to calculate the correction value.

"With this configuration, each correcting modified layer is formed as an index for detection of the positional deviation of each regular modified layer from the corresponding process line. Each correcting modified layer is formed at a vertical position near the back side of the workpiece above the corresponding regular modified layer. Accordingly, an image by electromagnetic waves reflected by each correcting modified layer can be obtained more clearly. As a result, the laser applied position can be aligned with each process line more accurately. Further, if each correcting modified layer is formed near the back side of the workpiece before forming the corresponding regular modified layer near the front side of the workpiece along the same process line, there is a possibility that the laser beam for forming the regular modified layer may be blocked by the correcting modified layer, so that the regular modified layer may not be formed. To the contrary, according to the present invention, the correcting modified layer is formed after forming the regular modified layer, so that the regular modified layer can be reliably formed.

"Preferably, the correcting modified layer is locally formed so as to have a predetermined length along the corresponding process line. With this configuration, the correcting modified layer is locally formed, so that a reduction in strength of the workpiece can be suppressed.

"Preferably, the predetermined height of each modified layer is greater than a finished thickness of the workpiece from the front side thereof; cracks are formed along each modified layer so as to extend from each modified layer to the front side of the workpiece; and the laser processing method further includes a grinding step of grinding the back side of the workpiece to reduce the thickness of the workpiece to the finished thickness after performing the laser processing step, thereby dividing the workpiece along each process line to obtain individual chips. By performing the grinding step, all of the correcting modified layers and all of the regular modified layers are removed, so that these modified layers are not left on each chip. As a result, the die strength of each chip can be improved as compared with the case where the modified layers are left on each chip.

"According to the present invention, it is possible to provide a laser processing method which can reduce the deviation of the laser applied position in applying a laser beam to the workpiece along each process line to perform laser processing to the workpiece.

"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 some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a perspective view of a wafer (workpiece) according to a preferred embodiment of the present invention;

"FIG. 2 is a perspective view showing a condition that the wafer is supported through an adhesive tape to an annular frame prior to performing a holding step in a laser processing method according to this preferred embodiment;

"FIG. 3 is a partially sectional side view showing the holding step;

"FIG. 4 is a plan view showing an alignment step in the laser processing method;

"FIG. 5 is a partially sectional side view showing a laser processing step in the laser processing method;

"FIG. 6 is an enlarged sectional view showing a modified layer formed inside the wafer by the laser processing step;

"FIG. 7 is a partially sectional side view showing a positional deviation correcting step in the laser processing method;

"FIG. 8 is a plan view for illustrating the positional deviation correcting step;

"FIG. 9 is a perspective view showing a grinding step in the laser processing method; and

"FIG. 10 is an enlarged sectional view for illustrating a modification of the positional deviation correcting step."

For additional information on this patent application, see: Okada, Shigefumi; Ogoshi, Nobumori. Laser Processing Method for Workpiece. Filed October 8, 2013 and posted April 24, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=2735&p=55&f=G&l=50&d=PG01&S1=20140417.PD.&OS=PD/20140417&RS=PD/20140417

Keywords for this news article include: Electronics, Semiconductor, Disco Corporation.

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Source: Electronics Newsweekly


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