WO2018011618A1 - Method and system for cleaving a substrate with a focused converging ring-shaped laser beam - Google Patents

Method and system for cleaving a substrate with a focused converging ring-shaped laser beam Download PDF

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Publication number
WO2018011618A1
WO2018011618A1 PCT/IB2016/054183 IB2016054183W WO2018011618A1 WO 2018011618 A1 WO2018011618 A1 WO 2018011618A1 IB 2016054183 W IB2016054183 W IB 2016054183W WO 2018011618 A1 WO2018011618 A1 WO 2018011618A1
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WIPO (PCT)
Prior art keywords
beam
laser
workpiece
ring
characterized
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PCT/IB2016/054183
Other languages
French (fr)
Inventor
Egidijus VANAGAS
Aivaras KAZAKEVICIUS
Dziugas KIMBARAS
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Evana Technologies, Uab
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Priority to PCT/IB2016/054183 priority Critical patent/WO2018011618A1/en
Publication of WO2018011618A1 publication Critical patent/WO2018011618A1/en

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/073Shaping the laser spot
    • B23K26/0734Shaping the laser spot into an annular shape
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/0604Shaping the laser beam, e.g. by masks or multi-focusing by a combination of beams
    • B23K26/0608Shaping the laser beam, e.g. by masks or multi-focusing by a combination of beams in the same heat affected zone [HAZ]
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/06Shaping the laser beam, e.g. by masks or multi-focusing
    • B23K26/067Dividing the beam into multiple beams, e.g. multifocusing
    • B23K26/0676Dividing the beam into multiple beams, e.g. multifocusing into dependently operating sub-beams, e.g. an array of spots with fixed spatial relationship or for performing simultaneously identical operations
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00Working by laser beam, e.g. welding, cutting or boring
    • B23K26/50Working by transmitting the laser beam through or within the workpiece
    • B23K26/53Working by transmitting the laser beam through or within the workpiece for modifying or reforming the material inside the workpiece, e.g. for producing break initiation cracks
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B23MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23KSOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00Materials to be soldered, welded or cut
    • B23K2103/50Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
    • B23K2103/56Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26 semiconducting

Abstract

A laser material processing method and a system is presented that is capable of processing a workpiece in the form of a wafer. The technique comprises a beam shaping optical system that manipulates a laser beam produced by a laser system to attain the shape of a ring and then tightly focuses it in way that its fluence distribution attains the shape of a narrow 'spike' that produces a narrow damaged area within the workpiece. The process is then repeated along a predetermined cutting axis to achieve a series of such damaged areas and then is further repeated until the workpiece is diced in pieces of selected size. The invention provides an effective laser processing method that allows to cut/scribe/cleave/dice or, generally speaking, separate, hard, brittle, and solid wafers that are either bare or have microelectronic devices formed on them.

Description

METHOD AND SYSTEM FOR CLEAVING A SUBSTRATE WITH A FOCUSED CONVERGING RING-SHAPED LASER BEAM

FIELD OF INVENTION

The present method relates to laser material processing. More particularly it relates to systems and methods for cleaving hard and brittle materials with specifically shaped laser beam. The invention is useful for separating semiconductor devices formed on a substrate.

BACKGROUND OF INVENTION

Wafer dicing plays a critical role in the fabrication of semiconductor devices, which are becoming ever smaller and more complex. The classical methods of dicing are based on the use of a diamond saw for silicon wafers thicker than 100 pm or by laser ablation in case the substrates are thinner.

Diamond disk saw technology is limited by its low processing speed (for hard materials). The diamond disk saw produces wide, chipped kerf and low quality edge in general, which in turn degrades device yield and lifetime. The technology is expensive, due to rapid diamond disk degradation, and unpractical owing to the need for water cooling and cleaning. Additionally, the performance is limited when the substrate that is being cut is thin.

Another classical laser processing technology, namely laser ablation, is also limited by its low processing speed and a kerf width which reaches 10-20 m and is too wide for most applications. Furthermore, laser ablation induces cracks, leaves melted residuals and contaminates the cutting area with debris. A wide area heat affected zone can reduce the lifetime and effectiveness of a semiconductor device. Together with ablation the diamond disk saw technique cannot be used for specialty wafers where there may be other surface features, such as dye-attached films for adhesive stacking. Such additions make the traditional sawing or ablation processes more difficult and vulnerable to debris. In order to improve the quality of separated devices other laser processing based methods and apparatus have been developed.

One of such methods is a laser process and laser processing apparatus described in a US patent No. US6992026, published on 31 -01 -2006. The said method and apparatus allows cutting a work-piece without producing traces of fusion and cracking perpendicularly extending out of a predetermined cutting line on the surface of the workpiece. The surface of the work-piece is irradiated with a pulsed laser beam according to the predetermined cutting line under conditions sufficient to cause multi-photon absorption, where the beam is aligned to produce a focal spot (or condensed point: a high energy/photon density zone) inside the bulk of the work-piece, consequently forming modified area along the predetermined cleaving line by moving the focal spot in the cleaving plain. After creating the modified area, the work-piece can be mechanically separated with a relatively small amount of force.

The said processing method and its variations are currently known in the art as 'stealth dicing'. All its variations are based on production of internal perforations by a focused pulsed laser beam at a wavelength for which the wafer is transparent, but which is absorbed by nonlinear processes at the focus, e.g. as in the internally etched decorative blocks of glass. The internal perforation leaves the surface top and bottom pristine. The wafers are usually placed on a plastic adhesive tape that is mechanically stretched causing the perforations to crack. It is claimed that no debris, surface cracking or thermal damage, occurs unlike with prior processes. In addition to specialty and multilayer wafers, microelectromechanical (MEM) system devices can also be separated this way.

The disadvantages of stealth dicing become apparent as, typically, in order to perform Stealth Dicing a high numerical aperture (NA) lens must be applied, which results in a small depth of focus (DOF) and provides tight focusing conditions. This results in multiple cracks extending to random directions on the surface of cleaving and affects the lifetime of devices produced from of said cleaved wafers. Also, stealth dicing has it's draw backs when processing sapphire. These specific disadvantages are not apparent when wafers and substrates are of thicknesses of up to ~ 120 - 140 pm and only require one pass per separation line to be diced. However, for thicker wafers (usually 4'; 6' sapphire wafers are >140 pm to 250 pm or more), a number of passes per separation line are required. As a consequence, the material is exposed to laser radiation for prolonged periods of time which has unfavorable influence to final device performance and yield. In addition, multi-pass processing slows down the total processing speed and throughput.

Another method for material processing is disclosed in a US patent application No. US2013126573, published on 23-05-2013. This method is provided for the internal processing of a transparent substrate in preparation for a cleaving step. The substrate is irradiated with a focused laser beam that is comprised of pulses having a pulse energy and pulse duration selected to produce a filament within the substrate. The substrate is translated relative to the laser beam to irradiate the substrate and produce an additional filament at one or more additional locations. The resulting filaments form an array defining an internally scribed path for cleaving said substrate. Laser beam parameters may be varied to adjust the filament length and position, and to optionally introduce V-channels or grooves, rendering bevels to the laser-cleaved edges. Preferably, the laser pulses are delivered in a burst train for lowering the energy threshold for filament formation, increasing the filament length, thermally annealing of the filament modification zone to minimize collateral damage, improving process reproducibility, and increasing the processing speed compared with the use of low repetition rate lasers. The application of this method results in rough processing applicable only to bare materials and is inconvenient for dicing owing to higher pulse energies required, which leads to unfavorable impact on final semiconductor device performance. In particular, if wafers are diced using this method, resulting light-emitting diodes (LED) are characterized by an increased leakage current, which in case of high brightness (HB) and ultrahigh brightness (UHB) LEDs negatively impacts performance.

Another US patent application No. US2012234807, published on 20-09-2012, describes a laser scribing method with extended depth affectation into a work-piece. The method is based on focusing of a laser beam in such a way that intentional aberrations are introduced. The longitudinal spherical aberration range is adjusted to be sufficient to extend depth of focus into a work-piece with a limited transverse spherical aberration range. The method also results in rough processing by high energy pulses to obtain vertical damage traces inside the work piece. High pulse energy is necessary due to the fact that a low numerical aperture lens (having a focal length of tens of millimeters) must be used, which results in loose focusing conditions - the focal spot has a very smooth spatial intensity profile, therefore resulting in operational conditions where the damage threshold must be exceeded in a large area. Due to the increased requirements for pulse intensity (which is needed for optical breakdown) an increase in pulse energy is required and makes the process not attractive for HB and UHB LED production where the LED leakage current and high roughness of the chip wall are critical as mentioned above.

Another method is disclosed within patent CN204504505 that was published on 29-07-2015. The patent discloses an optical system that is to be used for laser beam shaping. The first part of the system is comprised of collimating optics. The collimated laser beam is then directed into an axicon lens that shapes the laser beam into a circular distribution that is allowed to propagate for a certain distance. Therefore, the intensity distribution of the laser radiation has its maximum values on the periphery of the beam and minimum values in the center of the beam. Furthermore, it is noted that the resulting beam is focused with a low numerical aperture spherical lens resulting in an extended depth of focus. Combined with the spherical aberrations of the optical system the method creates a possibility to cut relatively thick workpieces like for example a steel sheet having a 50 mm thickness. Of course, this kind of optical system might not be suitable for cutting semiconductor wafers or other materials that require a high cutting precision. Because the highest intensity is concentrated on the periphery of the beam before the low numerical aperture objective lens, the focused beam peak should have a relatively wide FWHM and therefore fabrication process might result in a wide kerf on the workpiece, which is undesirable while processing high precision requiring semiconductor wafers.

WO 2014/079570 A1 discloses the use of Bessel-like beams for layered material processing, more particularly, laser processing of tempered glasses by means of Bessel- like beams. The presented device includes an axicon lens that is used for Bessel beam creation, and a two-step optical system that is used to transfer and reconstruct the Bessel beam within the material. This results in elongate laser damage regions within the workpiece. However, the described method avoids tight focusing conditions which may further reduce the size of the lengthened laser damage regions. In fact, by using tightly focused circular laser beam, the diameter of the damaged volume might be brought very close to the diffraction limit of the beam. Yet another PCT patent application WO2016059449 describes a method of laser processing for separating semiconductor devices formed on a single substrate or separating high thickness, hard and solid substrates, which is rapid. During preparation of the device or substrate for the cleaving/breaking/dicing procedure an area of damage is achieved by obtaining deep and narrow damage area along the intended line of cleaving. The laser processing method comprises a step of modifying a pulsed laser beam by a focusing unit, such as that an 'spike'-shaped beam convergence zone, more particularly an above workpiece material optical damage threshold fluence (power distribution) in the bulk of the workpiece is produced. During the aforementioned step a modified area (having a 'spike'-type shape) is created. The laser processing method further comprises a step of creating a number of such damage structures in a predetermined breaking line by relative translation of the workpiece relative the laser beam focal point.

Methods disclosed in prior art impose limitations on substrate thickness, material type and processing quality used for wafer separation. In order to process thicker materials the above mentioned technologies require an increase in laser power or number of laser beam passes per separation line. As a consequence, this has advert effects both to the semiconductor device performance and the yield of production. SUMMARY

This invention is made to improve the results of previously described systems or at least overcome the drawbacks, especially in semiconductor wafer dicing applications. During preparation of the device or substrate for the cleaving/breaking (dicing) procedure an area of damage is achieved that is characterized by the obtained deep and narrow damaged areas along the intended line of cleaving. The present method does not require multiple laser beam passes per cutting line therefore increasing the yield of production. Henceforth the term 'workpiece' will be defined to include the terms substrate, wafer, wafer sheet, device or similar item that is prepared for processing and subsequent mechanical separation and will be used interchangeably. The laser processing method comprises a step of modifying a pulsed laser beam by an optical system (2), in which the beam is affected by a group of axicon lenses (8, 9) to attain a ring-shaped intensity distribution and then focused into a workpiece in a way that a 'spike-shaped' beam convergence zone or, more particularly, a 'spike-shaped' fluence distribution is formed within the workpiece at the intersection of the adversative parts of the ring shaped beam. Moreover, the 'spike-shaped' fluence distribution (10) is created exceeding the optical damage threshold in the bulk material of the workpiece (4). The material is partially or completely transparent to the wavelength of the said laser radiation and during the aforementioned step a modified area, which can also be referred to as a damaged structure, of the workpiece is created due to multiphoton absorption, preferably under sufficient conditions to produce localized melting or Coulomb explosions. Prior to the beam shaping with axicon lenses (8, 9) an additional optical system (6, 7) is used in order to change the divergence of a beam, which enters the group of axicon lenses (8, 9). Induction of negative or positive divergence of the ring shaped beam results in change of the focal plane distance after the focusing lens (3). In case of positive divergence, the focal plane (12) is formed in a larger distance than the intersection area (14) and in case of negative divergence, the focal plane (1 1 ) is formed prior to the intersection area (13). The divergence of the ring shaped beam can be changed in order to lengthen or shorten the laser modified area within the workpiece, based on the thickness of the workpiece (4) and preferred processing conditions.

The laser processing method further comprises a step of creating a number of such damaged structures along a predetermined breaking line by translation of the object relatively to the laser beam focal point. It should be apparent to a person skilled in the art that after forming such cutting line and by employing relatively small mechanical force the object can be separated or cut in to two or more smaller pieces having a defined separation boundary, defined by the sequence of laser-damaged areas. BRIEF DESCRIPTION OF DRAWINGS

Drawings are provided as a reference only and in no way should limit the scope of the invention. None of the nodes depicted in the drawing shall be deemed as limiting, rather just as an example of many possible embodiments.

Fig. 1 depicts a schematic of a laser processing system that is in accordance with the present method and optical system.

Fig. 2 depicts a detailed principle scheme of the optical system that is in accordance with the present invention.

Fig. 3 depicts a conventional numerical ray tracing representation of laser radiation focusing within a workpiece achieved by simulating laser beam propagation through the disclosed optical system and the focal plane of the propagating beam is set to be above the workpiece (1 st configuration).

Fig. 4 depicts a numerically calculated irradiance distribution within the workpiece along the optical axis of the optical system that is in accordance with the present invention (1 st configuration).

Fig. 5 depicts a numerically calculated peak irradiance distribution within the workpiece along the optical axis of the optical system that is in accordance with the present invention (1 st configuration).

Fig. 6 depicts a conventional numerical ray tracing representation of laser radiation focusing within a workpiece achieved by simulating laser beam propagation through the disclosed optical system and the focal plane of the propagating beam is set to be below the workpiece (2nd configuration).

Fig. 7 depicts a numerically calculated irradiance distribution within the workpiece along the optical axis of the optical system that is in accordance with the present invention (2nd configuration).

Fig. 8 depicts a numerically calculated peak irradiance distribution within the workpiece along the optical axis of the optical system that is in accordance with the present invention (2nd configuration). Fig. 9 depicts resulting damage of LiNb03 wafer that was processed using method that is in accordance with the present invention. The left hand picture presents the surface of the processed wafer before separation of pieces. The right hand picture presents profile of the cutting line.

Fig. 10 depicts a variation of the invention, wherein the optical path is split into two arms by using a beam splitter and beam shaping is performed separately in both arms. Later the two optical paths are made coincident again.

Fig. 1 1 depicts a variation of the invention, wherein the optical path is split into two arms by using a beam splitter and change of beam radius and divergence is performed separately in both arms. Later the two optical paths are made coincident again and conversion of the Gaussian beam to a ring shaped beam is performed jointly by a group of axicon lenses (8, 9).

Fig. 12 depicts a variation of the invention, wherein the change of laser beam radius and divergence is performed by telescope (6, 7) before splitting, then the optical path is split into two arms by using a beam splitter and laser beams are initially shaped into a ring shape beams by a group of axicon lenses (8, 9) separately in both arms. Later the two optical paths are made coincident again.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention provides a laser processing method for separating semiconductor devices formed on a single substrate or in general separating hard and solid substrates. During preparation of a sample for the cleaving/breaking procedure an area of damage is formed which is characterized by the obtained deep and narrow (high aspect ratio) damaged areas along the intended line of cleaving.

In the most preferred embodiment, the processing method comprises a step of irradiating a workpiece (4) with a focused ring-shaped pulsed laser beam through an optical system in such a way, that the beam convergence zone (focal plane) is formed above the top surface of the workpiece (4) or below the bottom surface of the workpiece and therefore as the beam propagates within the workpiece an interferential 'spike- shaped' pulse intensity pattern is created inside the material, at the intersection of the opposite parts of the focused beam. Furthermore, a laser induced damaged areas are created within the workpiece that are similar or resemble the beam intensity pattern, when beam fluence is above the optical damage threshold of the material. The term 'damage' may be referred to as any kind of local modification of the material, by which the mechanical properties are altered enough to produce a controlled crack (extending along the separation boundary) formation during later cleaving steps. The modifications, or damage structures (locally damaged zones, areas), are introduced by the mechanism of multiphoton absorption, which is possible if the workpiece material is partially or completely transparent (more particularly, the material is not affected by single photon absorption of the incident laser radiation) to the central wavelength of laser radiation used, i.e. the material bandgap exceeds the energy of a single photon energy, preferably multiple times. In order to induce a material breakdown, sufficient photon density must be achieved by using short and ultrashort pulses while employing beam focusing. This method is particularly relevant to materials, which have an energy bandgap exceeding 0.9 eV.

The processing method further comprises repeated irradiation of the sample at spaced positions where a series of damaged structures form a breaking/separation line. This is preferably achieved by mounting the workpiece on a motorized assembly of linear translation stages (5) and moving the workpiece (4) in a desired direction along the intended cleaving line, thus forming the cleaving plane in the bulk of the material. It should be apparent to a person skilled in the art that the different configurations of translation stages can be employed, including rotational stages or mobilizing the focusing unit, as long as the relative movement between the focusing unit and the workpiece is ensured. Sapphire, silicon carbide, diamond, lithium niobate, gallium nitride, aluminum nitride substrates or other high hardness materials that are difficult to process mechanically can be used as workpieces.

In the most preferred embodiment, the most appropriate way of realizing the disclosed steps is by using a pulsed laser beam source (1 ), which preferably emits a laser beam of a circular or elliptical Gaussian intensity distribution, a beam shaping unit (2), which comprises a beam expander (6, 7) and a group of axicon lenses (otherwise - an axicon telescope) (8, 9), a high numerical aperture aspherical focusing lens (3), and a workpiece (4), which can be translated by means of a motorized translation stage assembly (5), as shown in Fig. 1 and Fig. 2.

The previously mentioned laser beam source (1 ) is, preferably, a laser that is capable of stably emitting successive laser pulses of a defined polarization state and having a well-defined temporal envelope, preferably Gaussian, having a pulse duration set in the range of 100 to 15000 fs, a central wavelength set in the range of 500 to 2000 nm, a pulse repetition rate set in the range of 10 kHz to 2 MHz and having sufficient average output power to produce 1 to 100 μϋ energy pulses after the focusing unit (3). The previously mentioned beam expander (6, 7) might be, for example, a Keplerian or Galilean telescope, or any other optical element assembly, which can be used for the preferred beam width and divergence adjustment. The beam focusing element (3) comprises a high numerical aperture aspherical lens or, simply, an objective lens (3) and means to stabilize the distance between the focusing element and the workpiece with an error of few micrometers for workpiece translation speed equal, or below, or above 300 mm/s. The numerical aperture of the focusing lens (3) should be in the range of 0.5 - 0.9 and the working distance stabilization unit could be for example a piezoelectric nanopositioner or any other device capable of accurately maintaining the described distance. The nanopositioner is not shown in the drawings, but ideally it should be combined with the focussing unit (3) and is used essentially to translate the focussing lens in the Z (vertical) direction.

The overall optical system should be arranged in a way that the focal plane of the ring-shaped beam should be positioned closely above the top surface of the workpiece or closely below the bottom surface of the workpiece (4). This can be accomplished, for example, by setting positive or negative divergence of the beam by means of the telescope (6, 7). In case of convergence (negative divergence), the formed ring-shaped beam will focus (1 1 ) before the opposite sides of the beam intersect (13). And vice versa - in case of positive divergence, the focus (12) of the ring shaped beam will be formed after the intersection area (14). In all cases, the intersection area (13, 14) have the same axis of symmetry as the focusing lens and the ring-shaped portion of the beam. Furthermore, the beam interference pattern should create a 'spike-shaped' fluence distribution (13, 14), which is positioned with respect to the workpiece (4) such that the part (10), which is situated within the thickness of the workpiece (4) exceeds the optical damage threshold of the material. Preferably, the damage threshold is exceeded in a distance, extending from the top to the bottom surfaces of the workpiece (4) and produces a narrow damaged area that resembles the geometry of the fluence distribution within the material. In some other embodiments, the damage threshold might be exceeded just partially, if only this induces a crack formation, which extends throught the complete thickness upon application of mechanical force, thermal gradient or using other conventional separation methods.

In the most preferred embodiment, the ring shaped portion of the beam is formed by a set of concave (negative) and convex (positive) axicons (8, 9). In order to form a collimated ring-shaped beam, the axicons (8, 9) are selected to have the same cone angle. Different cone angles can be used for creating convergent or divergent ring-shaped beams, which results in a changed distance of the intersection area (13, 14) with respect to the focusing unit (3).

An exemplary geometrical optics representation of rays propagating through the workpiece is depicted in Fig. 3 and Fig. 6. Fig. 3 represents system configuration wherein the focal plane (1 1 ) of the beam is positioned above the top surface of the workpiece (4). The corresponding fluence distribution, which is created by the intersection of adversative parts of the focused ring-shaped beam is depicted in Fig. 4. More particularly, the 'spike shaped' fluence distribution is created at the intersection (13, 14) of the opposite parts of the beam. Also, peak fluence distribution is depicted in Fig. 5. Likewise, Fig. 6 represents a system configuration wherein the focal plane (12) of the incident ring-shaped beam is positioned below the bottom surface of the workpiece (4). The corresponding fluence and peak fluence distributions are depicted in Fig. 7 and Fig. 8 respectively. Furthermore, in case the focusing is done below the bottom surface, the distance of the focal plane from the bottom surface of the workpiece (4) should be increased and the position of the peak fluence portion is selected such that the laser beam does not damage the dicing tape.

The lateral distance between adjacent laser pulses, which are delivered onto the surface of the workpiece (4) can be in range from 0.1 m to 10 μιη and can be adjusted by changing the movement velocity of the motorized translation stage assembly (5), in case the pulse repetition rate is fixed. Another common way of changing the distance between the adjacent damage spots is to adjust the pulse repetition rate according to the actual position of the sample. This method is commonly referred to as 'position synchronized output' or 'PSO'. The cleaving/breaking plane is formed by linear movement of motorized translation stage assembly (5). The number of passes (repeated translations) for a single cleaving line should preferably be up to 2, nonetheless the invention is not limited to that. The process of creating the cleaving/breaking plane is shown if Fig. 1 . In this case tight focusing and sharp 'spike' shape focused intensity distribution are combined and can be controlled by manipulating the aspherical lens parameters, the optical properties of the material or the properties of the incident beam.

In another embodiment, the same optical unit might be used together with means of separating the described laser beam into multiple components that would, for example, differ in their polarization or would be separated by delaying the components in time, or separated spatially. The laser beam separation into multiple components can be achieved by means of birefringent devices, beam splitters, polarizers, prisms or other optical elements. Also, previously described means to change the incident beam convergence can be applied to adjust the parameters of the beam in each optical path separately. As a result, multiple laser beams are formed and focused to form multiple "spike-like' convergence zones that yield either multiple narrow damaged structures within the material or yield a more evenly distributed fluence. Therefore, either the whole processing speed is increased, or a higher precision is achieved. Figures 10 to 12 depict three different variations of the beam splitting assembly. These three variations differ mainly in placement of the beam shaping components. Complete sets of beam shaping components (2) can be arranged in each optical path (as depicted in Fig. 10); in another embodiment, two sets of divergence control units (i.e. telescopes) (6, 7) can be arranged in each optical path separately, but the ring shaper (i.e. a group of axicon lenses) (8, 9) is arranged in the common optical path, before the focusing unit (3) (as depicted in Fig. 1 1 ); in another embodiment, the two sets of ring shaper (i.e. a group of axicon lenses) (8, 9) can be arranged in each optical path separately, but divergence control unit (i.e. telescopes) (6, 7) is arranged in the common optical path, before the beam splitter (15) (as depicted in Fig. 12).

After initial beam splitting with the beam splitter (15), each arm can be further splitted, by forming total of 3, 4, 5, 6, 7, 8, etc. optical paths, each creating a different beam divergence and thus positioning the intersection area (13, 14) at a different distance from the focusing lens (3).

In another embodiment, during the step of irradiating a workpiece (4) with a focused pulsed laser beam through a beam focusing unit (3), the beam shaping unit is arranged to include at least one diffractive optical element or birefringent element augmenting or replacing the beam shaping optics, which shapes the incoming beam in such a way that after the beam passes through the beam focusing element (3) the 'spike'- shaped intensity distribution is achieved.

Yet in another embodiment, during the step of irradiating a workpiece with a focused pulsed laser beam through a beam focusing unit, the beam shaping element is arranged to include at least one adaptive optics member that shapes the incoming beam in such a way that after the beam passes through the beam focusing element the 'spike' shaped intensity distribution is achieved. This allows using a larger variety of incoming beams (or, more particularly, differently modulated beams) or allows compensation for fluctuating processing parameters. The beam shaping member can be based on Deformable Mirrors, Piezoelectric Deformable Mirrors or similar. Yet in another embodiment, during the step of irradiating a workpiece (4) with a focused pulsed laser beam through a beam focusing unit, in accordance with the previous embodiment the adaptive optics member can be substituted with at least one phase and/or amplitude modulator member such as a liquid crystal based Spatial Light Modulator or a micro-mirror matrix.

Yet in another embodiment, during the step of irradiating a workpiece with a focused pulsed laser beam through a beam focusing unit, in accordance with the previous embodiment, the adaptive optics member can be substituted with at least one passive diffractive beam modulating element, such as a flat-top beam shaping diffractive optical element, aberration corrective optical element, or another element that would be capable of yielding appropriate parameters. The passive diffractive element is selected by a person skilled in the art in such way that a beam, modulated with such an element, can be focused with the beam focusing element achieving a 'spike'-shaped intensity distribution. It should be noted that the said element can also be arranged in the optical path after the beam focusing element (3) during irradiation.

Yet in another embodiment the disclosed laser processing system might include a parametric optical subsystem that would be used for wavelength adjustment of the laser source and gas or air injection subsystem that would be used for removal of the debris that might occur during the fabrication process.

In order to disclose the present invention better, the following examples are provided. Nonetheless, the disclosed examples and the mentioned parameters are provided to help understand the invention better and in no way limit its extent. These parameters can be changed in a wide interval, reproducing similar or different results, yet the main concept of the dicing process remains the same.

Example 1

In this section an example of results that were obtained using the disclosed method is provided. Fig. 9a depicts workpiece surface, which has cutting lines formed on it and fig 9b depicts the profile of the workpiece, which was broken along the cutting line. Laser induced damaged areas are also depicted in fig 9b. The workpiece was a LiNb03 wafer with approximate thickness of 190 nm. Laser radiation was chosen to be at 1030 nm wavelength, 6 ps pulse duration and produced by the laser system with a pulse repetition rate of 100 kHz. A 0.68 NA aspherical lens was selected as a beam focusing element. The average beam energy that was measured as an output of the whole optical system (behind the objective lens) was set to be 10 μϋ with the peak irradiance of the order of 105 W/cm2. A full cut of the wafer is formed by adjusting the disclosed optical system parameters. The distance between laser damage structures is 3 μιη. The translation stage speed was set to 300 mm/s.

Example 2

The focused beam properties and fluence distribution strongly depend on the shape of the aspherical lens, or more particularly, the surface sags of the aspheric lens surfaces. The surface sag of an aspherical lens surface is determined by the coefficients of surface sag equation and can be chosen freely after taking into account the desired properties of the focused beam, the optical system configuration and manufacturing capabilities. In the case of incident beam divergence - 1 mRad (as measured behind the beam divergence control unit), targeting focusing depth inside lithium niobate interval from 0 μιη to 190 pm, an example of a suitable aspherical lens is characterized by coefficients for first lens surface: R = 2.75 (radius of curvature); k = -0.5426984 (conic constant, as measured at the vertex); nonzero coefficients A4 = -3.1954606- 10~4; Αε = - 4.3977849- 10 5; As = 1.8422560- 10 5; Aio = -1 .5664464- 10 6 and for second lens surface: R= -3.21 ; k - -12.41801 ; A4 = 9.0053074- 10"3; Ae = -1 .3597516- 10"3; As = 1 .1366379- 10- 4; Aio = -4.2789249- 10"6; refractive index n= 1 .597, design wavelength 830 nm.

Claims

1 . A laser processing method, which is suitable for wafer cutting or scribing or dicing or cleaving comprises steps of
- providing pulsed laser radiation,
- shaping a laser beam, wherein the resulting beam comprises a portion extending in the propagation direction, in which the perpendicular cross-section of the beam has energy distribution in a ring shape,
- focusing the laser beam,
- irradiating a workpiece with the focused pulsed laser beam,
- translating the workpiece with respect to the focused laser beam,
c h a r a ct e r i z e d in that the beam shaping step comprises formation of a ring shaped portion of the beam, which has divergent or convergent properties, such that after focusing, the beam forms a 'spike-shaped' fluence distribution at the intersection of the opposite parts of the ring shaped beam, further the step of irradiating a workpiece comprises situating the 'spike-shaped' fluence distribution fully or at least partially between the top and bottom surfaces of the workpiece and inducing multiple intra-volume damaged zones upon irradiation with laser pulses.
2. The method according to claim 1 , c h a ra ct e r i z e d in that it the workpiece is a wafer shaped substrate, which is either bare or has microelectronic or micromechanical or micro-optical or semiconductor devices formed on it and is made from a hard, brittle, and solid material such as one of sapphire, silicon carbide, lithium niobate, gallium nitride, aluminum nitride, or more particularly, is made from a material, which has a bandgap energy higher than 0.9eV.
3. The method according to one of claims 1 and 2, c h a ra ct e r i z e d in that the beam shaping step involves manipulation of the laser beam divergence before the beam propagates through another beam shaping unit, in which the beam cross- section is converted into a ring-shape, wherein the divergence is adjusted preferably by a telescope.
4. The method according to claim 3, characterized in that the 'spike-shaped' fluence distribution can be lengthened or shortened by changing the divergence of the laser beam and thus adjusting the distance of the focal plane of the ring shaped beam from the focusing unit, wherein preferably the focal plane is adjusted to be in the range from closely above the top surface of the workpiece to closely below the bottom surface of the workpiece, at the same time adjusting the length of the intersection area of the adversative parts of the ring-shaped beam.
5. The method according to one of the claims 1 to 4, characterized in that the ring-shaped portion of the beam is formed by a group of axicon lenses, preferably at least one negative and one positive axicon lens.
6. The method according to one of claims 1 to 5, characterized in that the focusing step is carried out by using an aspheric lens or an objective with the numerical aperture in the range from 0.5 to 0.9.
7. The method according to claims 1 and 5, characterized in that the focusing step involves use of an automatic focusing system, wherein the constant distance between the aspheric lens and the workpiece is maintained by actuating said lens by means of a piezoelectric nanopositioner, motorized linear translation stage, or other device for actuating purpose.
8. A laser processing system, arranged for wafer cutting or scribing or dicing or cleaving, comprising a pulsed laser source, a beam shaping unit, a focusing unit and an actuated positioning system for wafer positioning, characterized in that the beam shaping unit is arranged at least to control the divergence of the laser beam and to convert the Gaussian beam into a beam having ring-shaped energy distribution cross-section, wherein the focusing unit is arranged to focus the convergent or divergent ring-shaped beam in order to form an intersection area, in which the opposite parts of the ring shaped beam intersect on the axis of symmetry and the focal plane is situated above or below said intersection area.
9. The system according to claim 8, characterized in that the beam shaping unit comprises a multiple lens system, preferably a telescope, arranged to change at least the divergence of the incident laser beam.
10. The system according to claim 8, characterized in that the beam shaping unit comprises a group of axicon lenses, preferably a negative and a positive axicon lens, arranged to convert the incident Gaussian beam into a beam having ring- shaped energy distribution cross-section.
11.The system according to one of claims 8 to 10, characterized in that it further comprises one or more pair of beam splitting and combining optics, arranged to split the incident beam into two or more optical paths and combine these paths into one, wherein at least partial beam shaping is carried out in each optical path separately.
12. The system according to claim 11, characterized in that both - divergence control and conversion to a ring-shaped beam energy distribution is carried out in each optical path separately.
13. The system according to claim 11, characterized in that divergence control is carried out before separation or in each optical path separately and conversion to a ring-shaped beam energy distribution is carried out in each optical path separately - or after combining said multiple optical paths.
14. The system according to one of claims 8 to 11, characterized in that the beam shaping unit further comprises at least one of a passive diffractive element, a phase or amplitude modulating element, an aberration corrective elements, a flat top beam shaping diffractive element, birefringent element and an adaptive optics element.
15. The system according to one of claims 8 to 14, characterized in that the laser source is arranged to emit laser radiation in the wavelength range of 500 to 2000 nm or the wavelength of a standard laser is converted into this range by means of parametric optics.
16. The system according to one of claims 8 to 15, characterized in that the laser is configured to emit ultrashort pulses in the range of 100 fs to 15000 fs.
17. The system according to one of claims 8 to 16, characterized in that the pulse repetition rate of the laser source is in the range of 10 kHz to 2 MHz.
18. The system according to one of claims 8 to 17, characterized in that the energy of the incident laser beam pulses to the wafer surface is in the range of 1 to 100 μϋ.
19. The system according to one of claims 8 to 18, characterized in that the system is arranged to induce optical damage to the wafer, wherein the distance between the adjacent damaged structures, which form the cutting line, is in the range of 0.1 m to 10 μιη and is adjusted by means of changing pulse repetition rate of the laser source or changing velocity of the wafer translation stage, or by means of externally triggering the laser beam source to emit the pulses with a constant density.
PCT/IB2016/054183 2016-07-13 2016-07-13 Method and system for cleaving a substrate with a focused converging ring-shaped laser beam WO2018011618A1 (en)

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