Patent ID: 12191216

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG.1illustrates in perspective a laser processing apparatus10according to an embodiment of the present invention. As illustrated inFIG.1, the laser processing apparatus10includes a laser beam applying mechanism12for applying a laser beam and a holding table13for holding a workpiece W to be processed thereon. The laser processing apparatus10is arranged to move the laser beam applying mechanism12and the holding table13relatively to each other and apply the laser beam from the laser beam applying mechanism12to the workpiece W held on the holding table13, thereby processing the workpiece W with the laser beam.

The laser processing apparatus10will be described in relation to a three-dimensional coordinate system including X, Y, and Z axes extending respectively along X-axis, Y-axis, and Z-axis directions. The X-axis directions extend horizontally and include a +X-axis direction and a −X-axis direction, and the Y-axis directions extend horizontally perpendicularly to the X-axis directions and include a +Y-axis direction and a −Y-axis direction. The Z-axis directions extend vertically perpendicularly to the X-axis directions and the Y-axis directions and include a +Z-axis direction and a −Z-axis direction.

The workpiece W is a semiconductor wafer shaped as a circular plate, such as a silicon wafer. The workpiece W has a face side, illustrated as an upper side inFIG.1, including a plurality of rectangular areas demarcated by a grid of projected dicing lines and having a plurality of devices such as integrated circuits (ICs) and large scale integration (LSI) circuits formed in the respective rectangular areas. The workpiece W may be made of any of other materials including ceramic, glass, sapphire, or the like as well as semiconductor materials including silicon or the like.

The workpiece W is affixed to a tape T and fixedly supported on an annular frame F by the tape T. The workpiece W, the tape T, and the annular frame F jointly make up a workpiece unit U that will be handled as one piece on the laser processing apparatus10.

The laser processing apparatus10has a base11shaped as a rectangular parallelepiped. The base11supports on an upper surface thereof a moving mechanism14for processing-feeding the holding table13in the X-axis directions and indexing-feeding the holding table13in the Y-axis directions. An upstanding wall16is erected from the base11behind the moving mechanism14. The laser beam applying mechanism12is supported on an arm17that protrudes from a front face of the upstanding wall16in overhanging relation to the holding table13.

The moving mechanism14includes an indexing feed mechanism20for moving the holding table13and the laser beam applying mechanism12relatively to each other in indexing feed directions, i.e., the Y-axis directions, and a processing feed mechanism21for moving the holding table13and the laser beam applying mechanism12relatively to each other in processing feed directions, i.e., the X-axis directions.

The indexing feed mechanism20includes a pair of guide rails23disposed on the upper surface of the base11and extending parallel to the Y-axis directions, and a drive motor26for moving a Y-axis movable table24slidably mounted on the guide rails23in the indexing feed directions along the guide rails23. After the laser beam applying mechanism12has processed the workpiece W along one of the projected dicing lines thereon, the indexing feed mechanism20indexing-feeds the workpiece W on the holding table13by a predetermined distance in one of the indexing feed directions for the laser beam applying mechanism12to process the workpiece W along a next one of the projected dicing lines thereon.

A nut, not illustrated, is fixedly mounted on a lower surface of the Y-axis movable table24, and a ball screw25disposed between and extending parallel to the guide rails23is operatively threaded through the nut. When the drive motor26that is coupled to an end of the ball screw25is energized, it rotates the ball screw25about its central axis, causing the nut to move the Y-axis movable table24and the processing feed mechanism21and the holding table13that are mounted on the Y-axis movable table24in one of the Y-axis directions along the guide rails23. The indexing feed mechanism20may alternatively include a linear motor, not illustrated, as is the case with the processing feed mechanism21to be described below.

The processing feed mechanism21includes a pair of guide rails30disposed on an upper surface of the Y-axis movable table24and extending parallel to the X-axis directions, and an actuator33for moving an X-axis movable table32slidably mounted on the guide rails30in the processing feed directions along the guide rails30.

The actuator33includes a linear motor mounted on a lower portion of the X-axis movable table32and including electromagnetic coils, not illustrated, positioned in vertically facing relation to a magnet plate35disposed on the Y-axis movable plate24between the guide rails30and extending in along the X-axis directions. The electromagnetic coils are successively energized by three-phase alternating currents flowing therethrough in successive phases for generating a moving magnetic field that moves the actuator33itself and the X-axis movable table32in reciprocating strokes along the X-axis directions.

The actuator33includes a temperature detector36for detecting a temperature of the electromagnetic coils of the linear motor. The temperature detector36detects a temperature of the actuator33. Since the actuator33moves the X-axis movable table32and hence the holding table13at a high speed in the reciprocating strokes when the processing feed mechanism21is in operation, the electromagnetic coils generate a large amount of heat, tending to thermally expand surrounding components.

The temperature detector36is incorporated in the actuator33for measuring the temperature of the electromagnetic coils. Alternatively, the temperature detector36may be disposed on the X-axis movable table32independently of the actuator33. Alternatively, the temperature detector36may detect a temperature of a holding mechanism37, which includes the X-axis movable table32and the holding table13, for holding the workpiece W, and may detect a temperature change caused by the heat of the actuator33from the detected temperature.

The holding table13is mounted on an upper surface of the X-axis movable table32. The holding table13is rotatable in a0direction about its central axis extending vertically in the Z-axis directions. A suction member made of porous ceramic is placed on an upper surface of the holding table13. Four clamps39are disposed around the holding table13at angularly spaced positions. The four clamps39are actuated by air actuators, not illustrated, to clamp the annular frame F of the workpiece unit U at the four positions around the holding table13to keep the workpiece W stably on the suction member.

The X-axis movable table32and the holding table13jointly make up the holding mechanism37for holding the workpiece W. The holding mechanism37is movable as a whole in the processing feed directions and the indexing feed directions.

The laser beam applying mechanism12has a processing head40mounted on a distal end of the arm17. The laser beam applying mechanism12includes an optical system, etc. housed in the arm17and the processing head40.

FIG.2illustrates in elevation the optical system, etc. of the laser beam applying mechanism12. The laser beam applying mechanism12includes a laser oscillator41for emitting a laser beam46, a mirror42for reflecting the laser beam46emitted from the laser oscillator41, a condensing lens43for converging the laser beam46reflected by the mirror42and applying the converged laser beam46to the workpiece W, and a converged spot position adjusting unit44for moving the condensing lens43in the Z-axis directions to adjust the position of the converged spot, i.e., focused spot, of the laser beam46.

The laser beam46emitted from the laser oscillator41is a YAG laser beam or a YVO laser beam, for example. The laser beam46applied to the workpiece W may process the workpiece W according to different processes. One of the processes is an ablation process in which a laser beam having a wavelength absorbable by the workpiece W is applied to form laser-processed grooves in the face side of the workpiece W. Another process is what is generally called a stealth dicing process in which a laser beam having a wavelength transmittable through the workpiece W is applied to form modified layers in the workpiece W.

Next, an arrangement for correcting the position of the converged spot of the laser beam will be described below.

FIG.3illustrates in block form a controller100of the laser processing apparatus10and components of the laser processing apparatus10that are controlled by the controller100. The controller100has a control unit102for controlling the moving mechanism14and the laser beam applying mechanism12, and a storage unit104for storing various kinds of data.

The control unit102controls the moving mechanism14to move the holding table13. Specifically, the control unit102controls operation of the indexing feed mechanism20, i.e., the drive motor26, to move the holding table13(seeFIG.1) in the indexing feed directions, i.e., the Y-axis directions. The control unit102also controls the processing feed mechanism21, i.e., the actuator33, to move the holding table13in the processing feed directions, i.e., the X-axis directions.

Furthermore, the control unit102controls operation of the converged spot position adjusting unit44to move the condensing lens43(seeFIG.2) in heightwise directions, i.e., the Z-axis directions, to change the position of the converged spot in the Z-axis directions. The temperature detector36is electrically connected to the controller100, so that the temperature detected by the temperature detector36can be input to the control unit102.

The storage unit104stores correlation maps M1and M2illustrated respectively inFIGS.4A and4B. The correlation map M1illustrated inFIG.4Ahas a horizontal axis representing temperature values (° C.) of the actuator33(seeFIG.1) and a vertical axis representing shifts (μm) in the −Y-axis direction of the converged spot in the workpiece W, and defines a correlation between the temperature values and the shifts. The correlation map M1is obtained on the basis of temperature values measured by the temperature detector36(seeFIG.1) and Y-axis coordinates of the converged spot detected at the time the temperature values are measured.

The correlation map M1indicates that, as the temperature of the actuator33becomes progressively higher than a reference temperature T1, the converged spot is progressively shifted in the −Y-axis direction. Specifically, in the configuration illustrated inFIG.1, when the X-axis movable table32is thermally expanded due to a temperature rise of the actuator33, the holding table13is shifted in the +Y-axis direction from a reference position therefor. At this time, the relative position of the holding table13with respect to the laser beam applying mechanism12is shifted in the +Y-axis direction, causing the converged spot to shift in the −Y-axis direction within the workpiece W. At the reference temperature T1, the converged spot is not shifted in the Y-axis directions within the workpiece W, and is positioned at a designed position.

More specifically, as illustrated inFIG.5A, at the reference temperature T1, the holding table13has a center C whose Y-axis coordinate is represented by Ya, and the converged spot, denoted by P, is disposed at a predetermined position within the workpiece W. When the temperature rises to a temperature T2, as illustrated inFIG.5B, the center C of the holding table13moves Δy in the +Y-axis direction and has a Y-axis coordinate represented by Yb, and the converged spot P is shifted to a position Py that is spaced Δy in the −Y-axis direction.

When the holding table13is thus shifted in the +Y-axis direction, therefore, the converged spot P is shifted in the −Y-axis direction within the workpiece W. This is because the laser beam applying mechanism12is not moved in the Y-axis directions, and only the holding table13is moved in the Y-axis directions because of the thermal expansion.

Similarly, the correlation map M2illustrated inFIG.4Bhas a horizontal axis representing temperature values (° C.) of the actuator33(seeFIG.1) and a vertical axis representing shifts (μm) in the −Z-axis direction of the converged spot in the workpiece W, and defines a correlation between the temperature values and the shifts. The correlation map M2is obtained on the basis of temperature values measured by the temperature detector36(seeFIG.1) and Z-axis coordinates of the converged spot detected at the time the temperature values are measured.

The correlation map M2indicates that, as the temperature of the actuator33becomes progressively higher than the reference temperature T1, the converged spot is progressively shifted in the −Z-axis direction. Specifically, according to the present embodiment, in the configuration illustrated inFIG.1, when the X-axis movable table32is thermally expanded due to a temperature rise of the actuator33, the holding table13is shifted in the +Z-axis direction from a reference position therefor. At this time, the relative position of the holding table13with respect to the laser beam applying mechanism12is shifted in the +Z-axis direction, causing the converged spot to shift in the −Z-axis direction within the workpiece W. At the reference temperature T1, the converged spot is not shifted in the Z-axis directions within the workpiece W, and is positioned at a designed position.

More specifically, as illustrated inFIG.6A, at the reference temperature T1, the holding table13has a center C whose Z-axis coordinate is represented by Za, and the converged spot P is disposed at a predetermined position within the workpiece W. When the temperature rises to the temperature T2, as illustrated inFIG.6B, the center C of the holding table13moves Δz in the +Z-axis direction and has a Z-axis coordinate represented by Zb, and the converged spot P is shifted to a position Pz that is spaced Δz in the −Z-axis direction.

When the holding table13is thus shifted in the +Z-axis direction, therefore, the converged spot P is shifted in the −Z-axis direction within the workpiece W. This is because the laser beam applying mechanism12, i.e., the condensing lens43, is not moved in the Z-axis directions, and only the holding table13is moved in the Z-axis directions because of the thermal expansion.

Data for producing the correlation maps M1and M2vary from laser processing apparatus to laser processing apparatus and also from environment to environment in which to install the laser processing apparatus. The correlation maps M1and M2may be produced by performing a test process for data acquisition in a manufacturing process before the laser processing apparatus is shipped from a factory and then may be stored in the storage unit104, or alternatively may be produced by performing a test process for data acquisition after the laser processing apparatus has been installed and then may be stored in the storage unit104.

A control method of correcting the position of the converged spot on the laser processing apparatus10thus arranged will be described hereinbelow.FIG.7is a flowchart of the control method. The control method illustrated inFIG.7is carried out in a laser processing procedure such as an ablation process or a stealth dicing process referred to above.

Specifically, as illustrated inFIG.1, the workpiece W to be processed has a grid of projected dicing lines in two groups perpendicular to each other. The holding table13that is holding the workpiece W thereon is moved in a processing feed direction, i.e., one of the X-axis directions, during which time the workpiece W is processed by the laser beam46along one of the projected dicing lines extending in a first direction. Thereafter, the holding table13is moved in an indexing feed direction, i.e., one of the Y-axis directions, and then the workpiece W is processed by the laser beam46along a next adjacent one of the projected dicing lines extending in the first direction. After the workpiece W has been processed along all the projected dicing lines extending in the first direction, the holding table13is turned 90 degrees about its central axis. Then, the workpiece W is processed by the laser beam46along the projected dicing lines extending in a second direction perpendicular to the first direction. The control method is carried out in the above laser processing procedure, as follows:

<Temperature Detecting Step>

A temperature detecting step S1is a step in which the temperature detector36detects the temperature of the holding mechanism37or the temperature of the actuator33. The temperature detector36may detect the temperature in real time at all times during the laser processing procedure or may detect the temperature at certain timings, and the storage unit104may store the temperature thus detected.

<Corrective Value Calculating Step>

A corrective value calculating step S2is a step in which the correlation map defining the correlation between the temperature values and the shifts in the converged spot position is referred to, a shift in the converged spot position that corresponds to the temperature value detected by the temperature detector36is determined from the correlation map, and a corrective value for canceling out the shift is calculated.

Specifically, at the temperature T2, for example, the correlation map M1illustrated inFIG.4Ais referred to, and the shift Δy is determined from the correlation map M1. In this case, as illustrated inFIG.5B, the converged spot is moved by the shift Δy in the −Y-axis direction within the workpiece W.

Then, a corrective value is defined as a numerical value for cancelling out the shift Δy. The corrective value may be another numerical value calculated on the basis of the shift Δy, instead of being identical to the shift Δy. Similarly, with respect to the shift in the Z-axis direction illustrated inFIGS.4B and5B, a corrective value is defined on the basis of the shift Δz.

<Converged Spot Position Correcting Step>

A converged spot position correcting step S3is a step in which the position of the converged spot in the workpiece W is corrected using the calculated corrective value. The position of the converged spot in the workpiece W may be corrected in real time at all times during the laser processing procedure or may be corrected at certain timings. For example, the position of the converged spot in the workpiece W may not be corrected while the holding table13is being moved in the processing feed directions, i.e., the X-axis directions, but may be corrected when the holding table13is moved, i.e., indexing-fed, in the indexing feed directions, i.e., the Y-axis directions, after the workpiece W has been processed by the laser beam along a certain projected dicing line.

Specifically, as illustrated inFIG.5C, the holding table13is moved by the corrective value Δy, i.e., the shift Δy, in the −Y-axis direction on the basis of the corrective value calculated in the corrective value calculating step S2, bringing the position in the Y-axis directions of the converged spot in the workpiece W into alignment with the position at the reference temperature T1.

Similarly, as illustrated inFIG.6C, the converged spot position adjusting unit44is actuated to change the position in the Z-axis directions of the condensing lens43(seeFIG.2), moving the position of the converged spot by the corrective value Δz in the +Z-axis direction, to thereby bring the position in the Z-axis directions of the converged spot in the workpiece W into alignment with the position at the reference temperature T1.

By thus carrying out the converged spot position correcting step S3, the position of the converged spot in the workpiece W can be aligned with the position at the reference temperature T1, so that the converged spot in the workpiece W is prevented from being shifted due to the thermal expansion of the holding mechanism37.

In the corrective value calculating step S2, instead of determining a corrective value by referring to the correlation map, a corrective value per unit temperature change may be determined. The position of the converged spot in the workpiece W may be corrected using the corrective value thus determined.

Specifically, providing it is confirmed that, when the temperature of the actuator33(seeFIG.1) rises by 5° C., the position in the Z-axis directions of the converged spot in the workpiece W is shifted by −20 μm and the position in the Y-axis directions of the converged spot in the workpiece W is shifted by +3 μm, the position of the converged spot in the workpiece W is corrected by +4 μm in the Z-axis directions and by −0.6 μm in the Y-axis directions per unit temperature change, i.e., per temperature rise of 1° C.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.