WO2010086865A1

WO2010086865A1 - System and method for high speed laser material processing

Info

Publication number: WO2010086865A1
Application number: PCT/IL2010/000090
Authority: WO
Inventors: Hagai Aran; Ron Kitay
Original assignee: Advanced Dicing Technologies Ltd.
Priority date: 2009-02-02
Filing date: 2010-02-02
Publication date: 2010-08-05

Abstract

A system and associated method for laser processing of material specimens comprising a laser mounted to scan onto a track wherein the specimens are forwarded along the track in a forwarding direction, the laser is scanned over said track in a second direction, and the substrates are angled to the track.

Description

SYSTEM AND METHOD FOR HIGH SPEED LASER MATERIAL PROCESSING FIELD OF INVENTION

The present invention relates to a novel system and method for high speed laser processing of materials, particularly but not exclusively for processing micro-components on silicon wafer and other substrates for use in the high tech micro-electronic industries.

BACKGROUND

Among materials processing techniques, the use of lasers is well established. Inter alia, lasers are used for a number of material processing techniques including: ablation, marking and scribing, cutting, welding, deposition and reactive processing, such as oxidation or sintering. In many cases, the processing throughput for mass produced component manufacturing is limited by the realignment times of the mechanical system that moves the material and / or the laser beam. This increases fabrication costs and may do so critically in markets where components are produced at high throughput and low cost, such as in the manufacture of solar cells used in the Photo Voltaic (PV) energy industry, for example. In solar cell production for example, components are fabricated on wafers and lasers are used for scribing to remove the silicon passivation material and for scribing the wafer edge to electrically isolate components and for selectively sintering electronic contacts on the reverse side of the component. Since the production rates in solar cell manufacturing may be 1200 wafers/hour or more, and in many applications the laser is required to traverse the substrate several times, the laser beam speed may move across the surface at speeds of lm/s or more, and in many cases the requirements are above 5m/s. It will be appreciated that a system which needs to accelerate to 5 m/s in a fraction of a second, to stop and to change direction requires a substantial mechanical substructure to withstand the heavy stresses induced. Since the time required for acceleration cannot be used for active material processing per se. this time is typically wasted and increases the overall wafer processing time.

Some systems use rotating prisms to create a circular or continuous beam movement thereby avoiding or at least minimizing the need for linear acceleration and the resultant dead time. However, such systems also have dead times whilst the substrate or beam is deflected from one scan position to another, as required to create the patterning over the full wafer surface. SUMMARY OF THE INVENTION

In accordance with a first embodiment, the present invention is directed to providing a system for laser processing of material specimens comprising a laser mounted to scan across substrates that advance along a track, the laser scanning being caused by a rotating optical element selected from the group consisting of a mirror arrangement and a prism arrangement.

In one embodiment, the track is a curved track and the specimens are forwarded along the curved track in a forwarding direction and a laser is scanned over said track in a second direction.

Optionally, the curved track is part of a loop of a helical track.

Optionally, the portion of the second direction resolved in direction of loop is in an opposite direction to the forwarding direction.

Optioanlly, the portion of second direction resolved in direction of loop is substantially in same direction to the forwarding direction.

Typically, the material specimens are substrates. For example, silicon wafers.

Typically, the laser processing is selected from the group consisting of cutting, scribing, marking, ablating, reactive processing, oxidation and sintering.

Optionally, the system further comprises an acoustic-optic deflector.

Optionally, the system further comprises a beam splitter for splitting incident laser beam into a plurality of beams for scribing a plurality of lines in one sweep.

Optionally, the system further comprises feedback from a power meter to control or stabilize output of the laser.

Optionally, the system further comprises comprising an optical lens upstream of the rotating optical element to process the laser beam by at least one process selected from the group of expanding, collimating and shaping.

Optionally, the system further comprises an F-theta lens downstream of the rotating optical element to compensate for bema smearing at different scan angles. Optionally, the system further comprises the laser beam traverses the substrate in one direction from a first side to a second side and is optically jumped back to the first side by a means selected from the group comprising an acoustic-optic deflector, a prism and a rotating mirror arrangement.

A second aspect of the invention is directed to a method for laser processing of material specimens comprising forwarding specimens along a curved track in a first direction and scanning a laser in a second direction with respect to said track.

Optionally the second direction has a component parallel to the first direction such that the component of movement of the laser beam parallel to the first direction and the direction of movement of the specimens is the same direction.

Alternatively the second direction has a component parallel to the first direction such that the component of movement of the laser beam parallel to the first direction and the direction of movement of the specimens is the same direction.

Optionally the curved track comprises at least part of a loop of a helix.

Typically, the material specimens are substrates.

Optionally, the material specimens are silicon wafers.

Typically, the laser processing is selected from the list of cutting, scribing, marking, reactive processing, oxidation, ablating and sintering.

Optionally, the laser is rotated by at least one of the group comprising prisms and mirrors.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the invention and to show how it may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings: Fig. 1 is a plan view of an exemplary pattern on a substrate;

Figs. 2 and 3 are a simplified schematic isometric projection of a system in accordance with one embodiment of the invention, comprising a laser source, a beam rotation device, and a wafer transfer subsystem consisting of a looped track that is part of helix, arranged in such a manner that a laser beam can continuously transverse the substrates while maintaining continuous substrate movement along the track;

Fig. 4 is an enlargement showing a portion of the conveyor loop and substrates thereupon;

Fig. 5 shows a flattened view of the looped track of the conveyor, showing how the laser beam scribes a circular path; Fig. 6 is a view of an exemplary substrate with an array of components thereupon;

Fig. 7 is a flattened view of a multiple conveyor track;

Fig. 8 is a time graph accompanying the processing of substrates shown in Fig. 7; Fig. 9 is a flowchart showing the essential features of a method of the invention; Fig. 10 shows a sample on a conveyor and angled to the conveyor direction; Fig. 11 shows a laser system for unidirectional scanning of a substrate in accordance with another embodiment of the invention, and

Fig. 12 is a schematic illustration showing two possible implementations of the scan mode.

DESCRIPTION OF EMBODIMENTS The present invention is directed to a laser processing system wherein a laser moves in one direction only about a single axis. Samples to be processed, which are typically substrates such as silicon wafers and the like, are advanced along a track.

In one embodiment, the track is a looped section of a helix and the substrates are angled to the track. In this manner, despite the laser moving in one plane only, it can scan over the entire face of the substrate, and mark out complex patterns. In another embodiment, the swivels along an axis to move along a path that is perpendicular to the general direction of the substrate feeder. The substrates themselves are angled to the feeder. Again, it is the combination of laser movement, substrate movement, and substrate rotation angle that enables the whole surface of the substrate to be patterned by the laser, despite the fact that the laser beam only advances in a single direction in a single plane (absolute coordinates). It is a feature of embodiments of the invention, that there is no time wasted as the laser moves from the end of a scan line to the beginning of a scan line.

With reference to Fig. 1, by way of example only, a typical substrate 10 is shown. The substrate 10 features an array of components to be laser processed; the array being represented, for simplicity, as black squares 12 arranged in a grid of x rows and y columns. It will be appreciated that the laser has to jump from square to square over the array, and to somehow raster over the entire face of the substrate 10, typically row by row in x direction, column by column in y direction, back and forth (+x-x or +y-y, or in a spiral algorithm, typically from the center outwards or from the outermost components, inwards. With reference to Fig. 2, a system in accordance with an embodiment of the invention is shown. The system comprises a laser source 14, a beam rotation device 16, and a wafer transfer subsystem 18. Substrates 10 are arranged on the wafer transfer system 18 in such a manner that a laser beam 20 can continuously transverse the substrates 10 while maintaining continuous substrate movement along the wafer transfer subsystem 18. By moving the laser beam 20 in one direction and the material substrate 10 being processed in a different direction, or in the same direction at a different speed, using a novel approach that combines the laser scanning and the material handling, a novel system and method is disclosed that minimizes and may even substantially eliminate the dead time associated with shifting the laser beam 20 from one point on a substrate 10 to another, and with the physical movement of substrates 10. Time lost by acceleration and deceleration of the laser beam associated with scan direction change is also minimized.

The system moves substrates 10, typically silicon wafers, along a looped conveyor path

25 of the wafer transfer system 18, which may be co-linear with the laser beam 20 emitted by the laser source 14. A 90° folding mirror 23 rotates around the center line 24 of the laser beam 20 path and creates a curved scanning line 26, as shown in Figure 3. Figure 4 shows a portion of the curved, and typically substantially circular conveyor path 25 of the wafer transfer system 18.

The scanning speed (i.e. the speed of the spot caused by the laser beam 20 on the substrate 10 relative to the substrate plane) and the substrate progression speed through the system is discussed herein below. It will be appreciated that the scanning speed is generally much higher than the substrate progress speed.

The scanning of the laser beam 20, over the substrate conveyor path 25 of the wafer transfer system 18, creates a diagonal path 26 of the laser. Figure 5 shows a flattened view of part of the conveyor path 25 and illustrates this diagonal path 26 inscribed by the laser.

Due to the geometry of the unique conveyor path 25 of the substrate 10 transfer system 18, i.e. the loop shape thereof, a circular movement of the laser beam 20 in a single direction in a single plane around its axis, enables all the substrates 10 to be traversed in turn. However, the laser beam 20 traverses each substrate 10 at a different location on its face in accordance with its relative position along the path 25. For example, with reference to Figure 4 above, it will be noted that the laser beam 20 traverses along the second line to the left of the substrate 1OA and on the 3rd line of substrate 1OB and so on.

The substrates 1OX are advanced along a curved path 25 through the transfer system 18 and progress both around the curved track, and through the system. Since the track is essentially part of a helix, the substrates worm forwards in relation to the scanning plane of the laser. In this way, despite the fact that the laser beam 18 rotates in a single plane and indeed in a single direction only, nevertheless the laser beam 18 effectively rasters over the entire width of the substrates 10X, since the substrate itself moves with respect to the plane of the laser as it moves along through the transfer system 18 along the loop 25. The schematic example shown in Figure 5 presented for illustrative purposes, demonstrates this, wherein it will be noted that the scanning line 26 is to the left of the substrate 1OD as it enters into the transfer system 18 at the left side of the picture, and to the right of the substrate 1OH as it exits the transfer system to the right.

The duty cycle of the beam 20 in one rotation of the folding mirror 16, is such that the beam 20 transverses on a particular substrate 1OX 1/n of the rotation time and on the rest of the substrates in the loop 25 (n-l)/n of the time, where n is the number of substrates 10 in the loop

25. For example, if there are twenty wafers or substrates 10 in the loop 25 at the same time and the rotation speed of the mirror 16 is 180 RPM, i.e. 1/3 sec for one rotation. Consequently, the laser beam 20 is on a particular substrate 10' for 1/20 x 1/3=1/60 second and off this substrate 19/20 x 1/3=19/60 seconds.

The 90° folding mirror 16 is rotated at an angular velocity ω that is compatible with the maximum linear scanning velocity on the substrate 10 surface required for the processing thereof. It will be noted that the perpendicular movement of each substrate 10 is a direct result of its progress along the conveyor 18 and around loop 25. The ratio between the scanning velocity, i.e. the rotation speed of the folding mirror 16, and the substrate 10 feeding velocity, i.e. the conveyor 18 speed, is selected so that the perpendicular movement during the time that the laser beam 20 is off the substrate 10 will be exactly one index, i.e. the interval between two adjacent scanning lines. For example, the substrate 610 shown in figure 6 has six scanning lines 612 and the index 611. In this case the laser beam 20 should transverse over each substrate 1OX six times. On each pass, the laser beam 20 scribes a new line 28, i.e. processes a new column 612 or row 614 of the array. This requires that the feeding velocity should be six times slower than the scanning speed, i.e. the laser beam 20 completes six passes by the time a substrate 10 completes one full loop 25 of the conveyor 18.

The substrates 10 are placed on the conveyor transfer system 18 at a predefined angle θ. An example, by way of illustration only is shown in Figures 5 and 7.

The predefined angle θ is a combination of two components. One component aligns the substrate 10 moving along the conveyor path 8 to the transverse line of the laser beam 20 while the second compensates the difference in feed velocity and scanning velocity.

The first component is as shown in Figure 5 and is equal to the width of the conveyor 18 divided by the length of the loop 25, i.e. the perimeter thereof.

As mentioned hereinabove, the substrates 10 may progress continually along the transfer system 18. Thus whilst the beam 14 is aimed on a substrate 10, the substrate 10 may progress through the system. This configuration would typically result in a diagonal scanning line but this may be avoided by orienting the substrates 1OX at a predetermined angle θ to compensate for this. The required angle θ is equal to the ratio between the perpendicular velocity and the effective scanning velocity; where the perpendicular velocity is the velocity component in the perpendicular direction and the effective scanning velocity is the difference between the laser linear scanning velocity and the feed velocity. The substrate placement angle θ is the sum of both angles. The distance between adjacent substrates along the transfer system 18 is selected to enable smooth line transit between the substrates.

An alternative embodiment is described in Figure 7. In this system 700, the laser beam 20 scans a portion of a circle instead of the full loop 25 (fig. 3) and the substrate transfer system

718 is built from multiple parallel conveyors 731, 732, 733 assembled perpendicularly to the plane of the laser beam plane 725, i.e. the scanning progress of the laser beam 720 is perpendicular to the substrate progress direction 750, and in such a way that the normals to the conveyors 731, 732, 733 meet at the axis of the laser 714 where the laser beam mirror arrangement 716 is placed. In the implementation of Fig. 7, three conveyors are shown. It will be appreciated that this is by way of example and for convenience of viewing only. In other embodiments of the invention, other numbers of conveyors may be used. Instead of inscribing a circle, the beam 720 traverses a segment 725 of a circle that incorporates the width of the conveyors, and repeats itself continuously, again scanning in one direction only, but either jumping back to the beginning or performing a full circular rotation.

The beam 720 traverses over the substrate 710 placed on a first conveyor 731 and may continue to a substrate 712 on a second conveyor 732 and so on until it traverses a substrate 711 on the last conveyor (in the example shown, the third conveyor 733), then the beam 720 starts again from the first conveyor 731. Fig. 8 shows the beam location with respect to time. One way of achieving this optical behavior is by means of three curved (i.e. hyperbolic) mirrors mounted in a triangular arrangement that rotate around the center of the laser track.

The substrates are continuously placed into and progress through the system. The substrates are placed at an angle θ to the wafer transfer system in such a way as to compensate for the progress of the substrate during the time that the beam traverses over the substrate. The angle θ is calculated by the conveyor 18 velocity divided by linear scanning velocity of the laser beam 720.

The beam manipulation system (mirror of prism) is preferably rotated at an angular velocity ω that corresponds to the maximum effective linear scanning velocity over the substrate surface, which enables the desired material processing to occur.

The ratio between the beam manipulation rotation speed and the substrate feed velocity are defined such that a substrate 10 progresses between consecutive scan lines (or patterned lines) during the time that the beam 20 is aimed at other substrates 10X. It will be appreciated that the basic system described hereinabove is capable of sophisticated adaption and variation for specific implementations. For example, specific embodiments, not shown, of the system may incorporate multiple image sensors and a vision computer to support substrate alignment and laser beam alignment across the path as well as laser power measurement along its trajectory. Such sensors can also be used to synchronize between laser pulses and the substrate location.

Preferred embodiments may include further refinements, based on these configurations, such as by splitting the laser beam emitted by the laser 14 into two or more beams by an appropriate beam splitter means. The two or more beams are then scanned in parallel thereby enabling a faster processing of the substrates, for example.

A key aspect of preferred embodiments is the optimization of the laser tool and its synchronization with the scan mechanism. It will be appreciated that for most applications, pulsed lasers are preferable due to their reduced heat-affected zone and faster material removal rates. The pulse repetition rate, pulse temporal width, and beam shape have to be mutually optimized to obtain a low power, high throughput system. For example for continuous line scribing, the beam width in the scanning direction is defined so as to provide a target overlap (typically 30-40%) between consecutive pulses, the time between pulses (the laser frequency mode) is determined by calculating the distance traveled between the firing of consecutive pulses divided by the system angular velocity in the earlier embodiments above, and by feed rate in the later described embodiments. To ensure repeatability and accuracy of the pattern scribing, the pulses are required to be synchronized with the beam and substrate location. In other applications, where the system is used to define non-continuous elements, a Q switched solution is preferred where the repetition rate is defined according to the number of processed elements per second and the pulse energy is defined to enable a single pulse processing. Typically the distance between the processed elements will determine the linear scanning velocity driven by the repetition rate - X(mm) x F(Hz) = V(mm/sec), where X is the distance between two elements on the same scanning line, and F is the laser frequency. With reference to Fig. 9, a flowchart illustrating a method for laser processing of material specimens is shown. The method consists of (a) forwarding specimens along a curved track in a first direction and (b) scanning a laser in a second direction with respect to said track.

The novel method combines manipulation of the laser source 14, the substrates 10 and an angle shift between the direction of the scan laser beam 20 and the direction of the moving sample to scan the laser beam 20 over the substrate 10 to process the substrate surface; the characteristics of the substrate material, the laser type and spot size defining the scribing geometry on the substrate.

Firstly, the laser type suitable for the specific task in question is defined, in terms of pulse duration for pulsed lasers, the rise time, energy, and wavelength. Then the dimension of the prism or folding mirror and the rotation velocity of the scanner V₈^_1x are fixed in accordance with the size of the sample, the required laser pattern, or dots geometry along the scribe line, and, assuming that there are no other limitations, n terms of line size / time to scribe one line, where the time to scribe one line is equivalent to the time to scribe a sample divided by the numbers of lines scribed.

Referring to Fig. 10, the progress velocity V_conveyer of the specimen 1010 (substrate) on the conveyer 1018 is defined by L (distance between lines)/ (time to scribe one line). The ratio y — ≡n^be_ d_et_ermmes t_ne angle α between the beam direction d and the substrate direction s. conveyer

It will be appreciated that the angle α between the laser beam 1020 and the moving substrate 1010 can be achieved either by positioning the substrate 1010 at an angle to the axes of the conveyer 1018 as defined by the laser/substrate velocities ratio, or by rotating the laser scanner device in a horizontal plan parallel to the conveyer plan to create this angle. The vertical rotation device (arrangement of mirrors or prisms) shifts the laser beam

1020 at the required scribe line velocity, at the end of each line (corresponding to an end of one of the rotating prism arrangements or rotating mirror arrangements) the beam will jump back to θ =0 (perpendicular to the scribed plan) at zero timing at the location on the next scribe line and then the next rotating prism starts to shift the beam along the next scribe line. The combination of angle of substrate s to conveyor axis c and the progress velocity V_comeyer of the specimen or substrate 1010 are set to ensure straight and parallel scribe lines on the substrate 1010.

With reference to Figure 11, a laser system 1100 in accordance with an embodiment of the invention is schematically shown. Laser system 1100 includes a scanner 1116, which is typically a rotation prism or a mirror arrangement, a laser source 1114, preferably an acoustic optical modulator 1106 to split small off the incoming beam for power control purposes, a power meter 1108, optical components 1112 such as lenses to expand, collimate and / or shape the beam 1120 prior to its entering the scanner 1116, and an F-theta lens 1118 to compensate for the so-called beam smearing effect at different angles. For example, a circular shaped beam will become an ellipse as the scan angle changes without use of an F-theta lens to compensate for and thus cancel this effect. Notably, the beam 1120 returns to the start position at "zero time" as it scans the substrate 1110, i.e. the scanning is performed in one direction only.

This embodiment is based on the combination of zero time for beam return and the substrate shift angle. The substrate shift angle negates the time normally required to travel from one scribe line to the next one and the scanner's "jump back" at zero time to the origin point, saves the time spent by a standard galvo-scanner for acceleration/deceleration when there is a need to change scribe direction. With reference to Figure 12, two possible embodiments for implementing the required scanning are shown. Embodiment A shows a laser beam 1220 incident on a bending mirror 1216 and then on an acoustic-optic deflector AOD 1218. The voltage and current provided to the AOD change the density and therefore the deflection angle provided to the laser beam 1225. At zero voltage/current the laser beam 1225 "jumps" back and passes the AOD 1218 without deflection.

Embodiment B shows a different configuration. Laser beam 1020 incident on a rotating polygon 1022 constructed from light reflecting mirrors 1024 arranged to rotate around an axis 1026. During the rotation of the polygon 1022 of mirrors 1024, the laser beam 1022 reflected off the mirrors changes its position and scans the substrate 1010 below. The incident beam 1020 moving to the next mirror causes the reflected beam 1022 to "jump" back to the original location on the substrate 1010. If the substrate is also advanced, the beam 1022 will thus scan across the substrate.

The applicants have found that a CO₂ laser is well suited for ablation of SiN or SiOx materials as typically used as passivation material on silicon based substrates. For such application, peak powers of 1.1 J/mm² are sufficient to ablate the material.

Another laser apparently suitable for this application is a pico-second (half- nanosecond) laser. This laser, with a UV/green wavelength, can remove typical passivation coating layers Such as SiN or SiO₂ with good depth controlled and no heat effects due to the short pulse range.

Example

In proof of concept experiments, it has been found that a CO₂ laser is well suited for ablation of SiN or SiOx materials as typically used as passivation material on silicon based substrates. For such applications, peak powers of 1.1 J/mm² are sufficient to ablate the material. Features shown with some specific embodiments may be incorporated with other embodiments. Thus the scope of the present invention is defined by the appended claims and includes both combinations and sub combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description. In the claims, the word "comprise", and variations thereof such as "comprises",

"comprising" and the like indicate that the components listed are included, but not generally to the exclusion of other components.

Claims

1. A system for laser processing of material specimens comprising a laser mounted to scan across substrates that advance along a track, the laser scanning being caused by a rotating optical element selected from the group consisting of a mirror arrangement and a prism arrangement.

2. The system of claim 1 wherein the track is a curved track and the specimens are forwarded along the curved track in a forwarding direction and a laser is scanned over said track in a second direction.

3. The system of claim 2 wherein the curved track is part of a loop of a helical track.

4. The system of claim 2, wherein portion of second direction resolved in direction of loop is in an opposite direction to the forwarding direction.

5. The system of claim 2, wherein portion of second direction resolved in direction of loop is substantially in same direction to the forwarding direction.

6. The system of claim 1 wherein the material specimens are substrates.

7. The system of claim 1 wherein the material specimens are silicon wafers.

8. The system of claim 1 wherein the laser processing is selected from the group consisting of cutting, scribing, marking, ablating, reactive processing, oxidation and sintering.

9. The system of claim 1 further comprising an acoustic-optic deflector.

10. The system of claim 1 further comprising a beam splitter for splitting incident laser beam into a plurality of beams for scribing a plurality of lines in one sweep.

11. The system of claim 1, further comprising feedback from a power meter to control or stabilize output of the laser.

12. The system of claim 1 further comprising an optical lens upstream of the rotating optical element to process the laser beam by at least one process selected from the group of expanding, collimating and shaping.

13. The system of claim 1 further comprising an F-theta lens downstream of the rotating optical element to compensate for bema smearing at different scan angles.

14. The system of claim 1 wherein the laser beam traverses the substrate in one direction from a first side to a second side and is optically jumped back to the first side by a means selected from the group comprising an acoustic-optic deflector, a prism and a rotating mirror arrangement.

15. A method for laser processing of material specimens comprising forwarding specimens along a curved track in a first direction and scanning a laser in a second direction with respect to said track.

16. The method of claim 15 wherein the second direction has a component parallel to the first direction such that the component of movement of the laser beam parallel to the first direction and the direction of movement of the specimens is the same direction.

17. The method of claim 15 wherein the second direction has a component parallel to the first direction such that the component of movement of the laser beam parallel to the first direction and the direction of movement of the specimens is the same direction.

18. The method of claim 15 wherein the curved track comprises at least part of a loop of a helix.

19. The method of claim 15 wherein the material specimens are substrates.

20. The method of claim 15 wherein the material specimens are silicon wafers.

21. The method of claim 15 wherein the laser processing is selected from the list of cutting, scribing, marking, reactive processing, oxidation, ablating and sintering.

22. The method of claim 15, wherein the laser is rotated by at least one of the group comprising prisms and mirrors.