Laser-based wafer dicing method and apparatus

Abstract

A method and apparatus (100) for dicing a wafer (W) having a predetermined wafer thickness (Tw). A laser beam (B) is focused within a focal region (Fr). The focal region has a predetermined length (Lr), along which a full-width-half maximum (FWHM) of a spot size (Bs) of the laser beam (B) remains below a predetermined maximum diameter (Dm). The laser beam (B) is focused by means of a set of focusing optics (10) comprising at least one spherical surface (R1f,R1b,R2f,R2b) configured to introduce a set of spherical aberrations (SA1,SA2) into the laser beam (B). The spherical aberrations (SA1,SA2) constructively interfere at the focal region (Fr) such that the length (Lr) of the focal region (Fr) is larger than the wafer thickness (Tw). The focused laser beam (B) is used to dice the wafer (W) while keeping the wafer thickness (Tw) fully inside the length (Lr) of the focal region (Fr).

Description

[0001] Title: LASER-BASED WAFER DICING METHOD AND APPARATUS

[0002] TECHNICAL FIELD AND BACKGROUND

[0003] The present disclosure relates to methods and systems for wafer dicing.

[0004] Wafer dicing is used in the semiconductor industry for the division of a (semiconductor) wafer into individual chips or die. This process is vital for the production of integrated circuits and microelectronic devices. The process of wafer dicing is typically carried out using specialized machines that utilize diamond-tipped blades or lasers to cleave the wafer with high precision. Before cutting, the wafer may be securely held in place using tape or a specialized mounting system to ensure stability during the cutting process and prevent any unintended damage. The cutting or dicing process itself typically involves the maneuvering of the cutting tool or laser beam relative to the wafer, which separates the individual die from the wafer. Once the dicing process is complete, the diced parts can be removed from the wafer for further inspection and testing to ensure quality and functionality. These individual chips are then typically packaged and integrated into various electronic devices, serving an essential role in powering various technological applications, including smartphones, computers, industrial machinery, and automotive systems.

[0005] The non-contact nature of laser-based dicing minimizes mechanical stress, making it an ideal choice for delicate microelectronic components. The intense energy of a focused laser beam causes rapid heating and vaporization of the material, effectively creating a cut along the intended path. This process needs to be carefully controlled using advanced software that manages the laser's movement and adjusts parameters such as positioning of the focus, power, and pulse duration to achieve the desired cutting depth. Waste management measures are typically implemented to handle the debris generated, often using specialized ventilation systems to prevent interference with the dicing process. Specialized optical systems, comprising lenses, mirrors, and adaptive optics, ensure precise and consistent laser focusing, with the adaptive optics adjusting the lenses dynamically to maintain the cutting surface in focus during the cutting process, resulting in clean cuts with minimal heat-affected zones and high- quality outcomes.

[0006] As the demand for smaller electronic components continues to increase, the precision and efficiency of wafer dicing have become more significant in the production of advanced microchips. Furthermore, a significant challenge associated with wafer dicing remains the generation of material waste or dust during the cutting process. There remains a need for improvement of optical system and methods for dicing a wafer allowing high precision and efficiency, while reducing material waste.

[0007] SUMMARY

[0008] The scope of this patent is defined by the independent claims.

[0009] Preferred embodiments are defined in the dependent claims. In a method or apparatus according to the present invention, a laser beam is focused within a focal region. The focal region has a predetermined length, along which a spot size of the laser beam remains below a predetermined maximum diameter. The laser beam is focused by means of a set of focusing optics. The set of focusing optics comprises at least one spherical surface introducing a set of spherical aberrations into the laser beam. The properties of the laser beam and / or focusing optics are specifically tuned to cause the spherical aberrations to constructively interfere at the focal region such that the length of the focal region is larger than the wafer thickness. By using the focused laser beam to dice the wafer while keeping the wafer thickness fully inside the length of the focal region, the wafer may be diced with improved precision and efficiency, while reducing material waste. For example, compared to a Gaussian beam, the presently enabled focal region may achieve a very high aspect ratio of the length of the focal region compared to the spot size along the focal region. Accordingly, the focused beam according to the present invention may cut the wafer specifically at the intended position while minimally affecting surrounding parts.

[0010] BRIEF DESCRIPTION OF DRAWINGS

[0011] These and other features, aspects, and advantages of the apparatus, systems and methods of the present disclosure will become better understood from the following description, appended claims, and accompanying drawing wherein:

[0012] FIG 1 illustrates a laser-based dicing apparatus and method for dicing a wafer according to the present invention;

[0013] FIG 2A illustrates a side-view intensity profile around a focal region of a laser beam according to the present invention coinciding with the wafer;

[0014] FIG 2B illustrates, for comparison, a side-view intensity profile of a Gaussian laser beam;

[0015] FIGs 3A and 3B illustrate cross-section intensity profiles of a focal region according to the present invention;

[0016] FIG 4A illustrates possible further parts of a laser dicing apparatus;

[0017] FIG 4B illustrates a set of focusing optics comprising respective lens arrays.

[0018] DESCRIPTION OF EMBODIMENTS

[0019] Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

[0020] The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and / or crosssection illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

[0021] FIG 1 illustrates a laser-based dicing apparatus 100 and method for dicing a wafer W according to the present invention. As illustrated, the wafer W has a wafer thickness Tw. A laser beam B is focused within a focal region Fr. The focal region has a predetermined length Lr along which a spot size Bs of the laser beam B remains below a predetermined maximum diameter Dm. The laser beam B is focused by means of a set of focusing optics 10. The set of focusing optics 10 comprises at least one spherical surface Rlf,Rlb,R2f,R2b configured to introduce a set of spherical aberrations SA1,SA2 into the laser beam B. The spherical aberrations SA1,SA2 constructively interfere at the focal region Fr such that the length Lr of the focal region Fr is larger than the wafer thickness Tw. The focused laser beam B is be used to dice the wafer W while keeping the wafer thickness Tw fully inside the length Lr of the focal region Fr. Accordingly, the wafer W may be diced with a maximum cut diameter or cross-section equal to or less than the maximum diameter Dm of the spot size Bs within the focal region Fr.

[0022] In some embodiments, e.g. as shown, the set of focusing optics 10 comprises at least a first optical element 11 receiving the laser beam B having an initial beam diameter DB, and a second optical element 12 arranged at an inter-element distance DL from the first optical element 11. In a preferred embodiment, an optical power of the first optical element 11 is such that the second optical element 12 receives the laser beam B having an intermediate beam diameter DI which is larger than the initial beam diameter DB. For example, in the embodiment shown, the first optical element 11 is a negative lens. Of course also other or further optical elements could be used. For example, the first optical element 11 may be a positive lens casting a focus between the first optical element 11 and second optical element 12. In general, one or more of the lenses may be substituted for corresponding mirrors, e.g. having at least one spherical mirror surface. Preferably, at least the second optical element 12 has at least one spherical surface R2f,R2b, most preferably at least a front spherical surface R2f, facing the intermediate diverging laser beam. This arrangement may enhance the spherical aberrations, e.g. determined by different rays of light hitting the spherical surface at different angles.

[0023] In other or further embodiments, e.g. as shown, the set of focusing optics 10 comprises at least a first optical element 11 receiving the laser beam B having an initial beam diameter DB, the first optical element 11 having at least one spherical surface Rif, RIB introducing a first set of spherical aberrations SAI into the laser beam B; and a second optical element 12 arranged at an inter-element distance DL from the first optical element 11, the second optical element 12 receiving the laser beam B having an intermediate beam diameter DI and having at least one spherical surface R2f,R2b introducing a second set of spherical aberrations SA2 into the laser beam B. Preferably, a respective radius of curvature of the spherical surfaces Rlf,Rlb,R2f,R2b and inter-element distance DL between the optical elements 11,12 are configured to have a combination SA1+SA2 of the sets of spherical aberrations SA1,SA2 constructively interfere at the focal region Fr such that the length Lr of the focal region Fr is larger than the wafer thickness Tw

[0024] Preferably, the wafer W is diced while maintaining a constant axial distance DZ between the second optical element 12 and the wafer W. More preferably, the wafer W is diced without adjusting the focusing of the laser beam B. For example, the inter-element distance DL between the first optical element 11 and second optical element 12 can remain constant while dicing the wafer W. As will be appreciated, having the ability to dice, i.e. cut completely through, the wafer without having to change the focusing of the beam, may simplify construction and / or control of the optical system. For example, the first optical element 11 and / or second optical element 12 can be kept in fixed relative positions. In one embodiment, the first optical element 11 and second optical element 12 are fixated within an optical device, e.g. laser-based dicing apparatus 100. In another or further embodiment, one or both of the first optical element 11 and second optical element 12 may be adjustable, e.g. to set a specific focusing of the beam before dicing the wafer W, while keeping the focusing constant while dicing the wafer.

[0025] In the following a detailed description is provided of a preferred embodiment, as shown. It will be understood that this description is provided to enable advantageous properties of a focused laser beam as described herein, in particular the properties as will be later described with reference to FIG 2A. Of course it will be understood that also variations of the preferred embodiment may be envisaged without departing from the scope of the present disclosure.

[0026] In the embodiment shown, the focusing optics 10 comprise a first optical element 11 with a first frontside spherical surface Rif facing an incoming laser beam, which has a wavelength of 588 nm. The first frontside spherical surface Rif has a (negative) radius of curvature of 42 mm. The incoming laser beam is collimated and casts a spot size DB (here 3 mm) onto the first frontside spherical surface Rif. Furthermore, the first optical element 11 has a first backside spherical surface Rib, opposite the first frontside spherical surface Rif (facing away from the incoming laser beam). The backside spherical surface Rib has the same (negative) radius of curvature of 42 mm. The spherical surfaces Rib and Rif are concave. In other words, the first optical element 11 is a negative lens. The minimum or center thickness T1 of the negative lens is 3.0 mm. The first optical element 11 has a relative refractive index of the first lens nl = 1.460 @ 588 nm. Of course also variations can be envisaged. Alternative to the embodiment shown, the incoming laser beam may be a converging or diverging beam, have a different wavelength; the first optical element 11 may have only one spherical surface (Rif or Rib) and one flat surface, or one or more aspherical surfaces, or different radius spherical or aspherical surfaces. Also the radius of curvature and / or refractive index of the lens may be different. For example, other parameters of the laser beam and / or optical arrangement may be adjusted to compensate accordingly.

[0027] In the embodiment shown, the focusing optics 10 comprise a second optical element 12 with a second frontside spherical surface R2f facing an intermediate (diverging) laser beam received from the first optical element 11. The second optical element 12 is placed at an inter-element distance DL of 100 mm from the first optical element 11 (center-to-center). The second frontside spherical surface R2f has a (positive) radius of curvature of 22 mm. Furthermore, the second optical element 12 has a second backside spherical surface R2b, opposite the second frontside spherical surface R2f (facing away from the intermediate laser beam). The backside spherical surface R2b has the same (positive) radius of curvature of 22 mm. The spherical surfaces R2b and R2f are convex. In other words, the second optical element 12 is a positive lens. The maximum or center thickness T2 of the positive lens is 8.2 mm. The first optical element 12 has a relative refractive index of the first lens n2 = 1.460 @ 588 nm. Alternative to the embodiment shown, the second optical element 12 may have only one spherical surface (preferably R2f) and one flat surface (e.g. R2b), or one or more aspherical surfaces, or different radius spherical or aspherical surfaces. Also the radius of curvature and / or refractive index of the lens may be different. Also other or further optical elements may be added or substituted for the current shown elements, e.g. having the same or similar optical characteristics. For example, other parameters of the laser beam and / or optical arrangement may be adjusted to compensate accordingly.

[0028] FIG 2 A illustrates an intensity profile around a focal region Fr of the laser beam produced in accordance with the optical arrangement described above with reference to FIG 1. FIG 2B illustrates, for comparison, an intensity profile of a Gaussian laser beam Bg.

[0029] As described herein, a set of focusing optics is configured to focus a laser beam B at a focal region Fr. In general, the focal region Fr will be understood as a region where the beam width is relatively narrow, typically around its focus (waist) where the width of the beam is narrowest. For the embodiments described herein, the spot size Bs is defined as the full-width- half-maximum (FWHM) of the beam. The spot size Bs of the beam may vary along the optical axis of the beam. In the embodiments, e.g. as shown in FIG 1 and 2 A, the length Lr of the focal region Fr is defined as the length of a section of the focusing beam, along the optical axis of the beam, where the spot size, e.g. FWHM of the beam, is less than a predetermined maximum diameter Dm.

[0030] Preferably, the length Lr of the focal region Fr is larger than the maximum diameter Dm of the spot size Bs, e.g. FWHM of the beam, within the focal region Fr by at least a factor one hundred, preferably at least a factor two hundred, up to a factor five hundred, or more. In some embodiments, the maximum diameter Dm of the spot size Bs within the focal region Fr is less than five micrometers, preferably less than three micrometer, more preferably less than two micrometer. For example, as illustrated by the optical beam simulation of FIG 2A (corresponding to the set of focusing optics 10 illustrated in FIG 1), the maximum diameter Dm of the FWHM spot size Bs within the focal region Fr is around 0.002 millimeter (2 micrometers), extending over a length Lr of around 1 mm (note the different X and Z scales in this figure). As will be appreciated, such extreme aspect ratios of Lr / Dm can provide significant improvement compared to a conventional Gaussian focal region Fg, which may comparably have one or both of a smaller depth of field (smaller length of the focal region Fr) or a larger spot size (larger maximum diameter Dm). In some embodiments, the length Lr of the focal region Fr is larger than the wafer thickness Tw by a factor between 1.05 and 2, preferably a factor between 1.1 and 1.5, most preferably a factor between 1.2 and 1.3. The inventors find that a minimum factor of 1.05, 1.1, or 1.2 may provide sufficient tolerance in the positioning of the wafer. On the other hand, using a factor larger than, e.g., 1.3, 1.5, or 2 may be difficult to achieve without causing a larger maximum diameter Dm of the spot size Bs within the focal region Fr. For example, the wafer W has a thickness between 0.1 and 1 millimeters, preferably between 0.5 and 0.9 millimeters, most preferably between 0.7 and 0.8 millimeters.

[0031] It will be understood that the methods and systems described herein are generally aimed at the purpose of laser dicing a wafer, and in particular aimed at the purpose of laser dicing a semi-conductor wafer as used in the semi-conductor industry. Typically, the wafer may be a semiconductor wafer having multiple integrated circuits formed thereon, e.g. by lithographic processes. By dicing the semiconductor wafer with the focused laser beam having an elongated focal region, as described herein, the semiconductor wafer may be cut into physically isolated pieces to form a set of individual semiconductor dies from the semiconductor wafer. Preferably, the laser beam is focused with an intensity to fully cut through the wafer, i.e. generate a physical cut or separation where the laser has passed, so that no further cutting, breaking, or other processing steps are needed to physically isolate the semiconductor dies from each other or the rest of the wafer. Each individual semiconductor die may have, e.g. exclusively consist of, a single integrated circuit of the multiple integrated circuits formed thereon. In a subsequent step, the semiconductor die may be packaged to form part of a semiconductor chip.

[0032] Standard silicon wafers used in the semiconductor industry may be available in thicknesses typically ranging from around 0.1 mm (100 micrometers) up to 0.8 centimeter. Thinner wafers are often preferred for certain applications to reduce the overall size of electronic components and enable the fabrication of more compact and efficient devices. Conversely, thicker wafers may be used in specific cases where mechanical stability and structural integrity are of utmost importance. For example, silicon wafers with diameter of 300 mm may generally have a thickness of about 775 micrometer, which is the approximate thickness “Tw” relative to the length Lr of the focal region Fr, as illustrated throughout the figures.

[0033] As will be appreciated, the present methods and systems may provide particular benefit (compared to a standard Gaussian beam) for wafers having a relatively large thickness, e.g. more than 0.5 mm. Furthermore, the present teachings may be advantageously used for dicing a stack of layers, e.g. chiplet layers. For example, a stack of at least two, three, five, ten, or more layers may be diced using a further extended focus by adjusting the parameters of the system. For example, the length Lr of the focal region Fr may be extended by a factor ten to cut through a stack of layers having a thickness between 0.1 and 1 centimeters, preferably between 0.5 and 0.9 centimeters, most preferably between 0.7 and 0.8 centimeters. To reach the extended length Lr of the focal region Fr, it may be permissible to increase the maximum diameter Dm of the spot size Bs, e.g. keeping the maximum diameter Dm of the spot size Bs within the focal region Fr less than fifty micrometers, preferably less than thirty micrometer, more preferably less than twenty micrometer.

[0034] FIGs 3A and 3B illustrate cross-section intensity profiles, corresponding to the positions indicated by arrows IIIA and IIIB in FIG 2A. In the embodiments as described herein, it may typically occur that around and outside the focal region Fr, an interference pattern SAr is formed comprising a series of concentric rings of light, resulting from the spherical aberrations. In some embodiments, the surrounding ring pattern may carry a significant portion of the total energy, but spread out over such a relatively large surface that this need not materially affect the wafer. For example, a part of the laser beam inside the focal region Fr may contain between 10% and 50% of a total energy of the laser beam B, preferably more than 50%, e.g. up to 90%, or more. For example, the interference pattern SAr of formed outside and around the focal region Fr (along the length Lr) may contain the remainder of the total energy of the laser beam B. The more energy can be contained within the focal region Fr and the less energy outside the focal region Fr, the more efficient the energy may be used and / or the less affected the surrounding parts of the wafer.

[0035] FIG 4A illustrates possible further parts of a laser-based dicing apparatus 100. In some embodiments, the wafer W is held by a wafer stage 50. Preferably, the wafer W is adhered to the wafer stage 50 by an adhesive layer 50a, e.g. tape. Also other ways of adhering or holding the wafer W can be envisaged, e.g. a vacuum chuck. In some embodiments, dicing of the wafer W comprises moving the wafer stage 50, e.g. in an X,Y plane perpendicular to the optical axis of the focal region. For example, the wafer stage 50 is a movable stage. The set of focusing optics 10 may be part of an optical device held above the wafer surface, e.g. by a connecting construction 40. Alternatively, or in addition, to moving the wafer stage 50 also the optical device and / or set of focusing optics 10 may be moved in the X,Y plane. For example, the construction 40 may comprise a moveable stage. Preferably, the wafer W is held at a constant distance DZ from the set of focusing optics 10 and / or optical device, e.g. keeping the wafer thickness Tw within the length of the focal region while dicing the wafer. Also the focusing preferably stay unchanged while dicing. Of course the distance DZ and / or focusing may be changed, e.g. set according to preferred parameters, before dicing commences, or after dicing has finished. The dicing of the wafer W may include cutting one or more lanes into the wafer resulting in separate I diced wafer parts Wd. The wafer parts Wd, e.g. chiplets may be picked up from the adhesive layer 50a and / or the adhesive layer 50a may be dissolved.

[0036] In some embodiments, e.g. as shown, the laser-based dicing apparatus 100 comprises or couples to a laser source 20. The laser source 20 is configured to output a laser beam received by the set of focusing optics 10. In some embodiments, the set of focusing optics 10 may include a folding mirror 13, e.g. between the first optical element 11 and second optical element 12. This may help compactify the system. The apparatus or system may also comprise further components such as a suction arrangement 30, configured to suck away debris formed during the dicing process. For example, the suction arrangement 30 may include a suction tube adjacent the focal region. In one embodiment, the focused laser beam B is configured, e.g. has sufficient intensity, to cause a physical cut through the wafer W to physically separate parts of the wafer on either side of the cut. Accordingly, the suction arrangement 30 may be configured to suck debris of the wafer W resulting from disintegrated material removed from the wafer W by the physical cut. Also other or further components may be included as part of the system, e.g. a source of gas for blowing away the debris.

[0037] In some embodiments, the wafer W may be cut using multiple simultaneous laser beams as described herein. This may allow relatively quickly dicing the wafer W. For example, FIG 4B illustrates an embodiment wherein the set of focusing optics 10 comprise respective lens arrays Ila, 12a. Each lenslet in the respective lens arrays may have similar properties as the first optical element 11 and / or second optical element 12.

[0038] In interpreting the appended claims, it should be understood that the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; any reference signs in the claims do not limit their scope; several "means" may be represented by the same or different item(s) or implemented structure or function; any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise.

Claims

CLAIMS1. A method for dicing a wafer (W) having a predetermined wafer thickness (Tw), the method comprising providing a laser beam (B) focused within a focal region (Fr), wherein the focal region has a predetermined length (Lr) along which a full- width-half maximum (FWHM) of a spot size (Bs) of the laser beam (B) remains below a predetermined maximum diameter (Dm); wherein the laser beam (B) is focused by means of a set of focusing optics (10) comprising at least one spherical surface (Rlf,Rlb,R2f,R2b) configured to introduce a set of spherical aberrations (SA1,SA2) into the laser beam (B), wherein the spherical aberrations (SA1,SA2) constructively interfere at the focal region (Fr) such that the length (Lr) of the focal region (Fr) is larger than the wafer thickness (Tw); and using the focused laser beam (B) to dice the wafer (W) while keeping the wafer thickness (Tw) fully inside the length (Lr) of the focal region (Fr).

2. The method according to the preceding claim, wherein the wafer (W) is a semiconductor wafer having multiple integrated circuits formed thereon, wherein, by the dicing of the wafer (W) using the focused laser beam (B), the semiconductor wafer is cut into physically isolated pieces to form a set of individual semiconductor dies from the semiconductor wafer, wherein each individual semiconductor die has a single integrated circuit of the multiple integrated circuits formed thereon for packaging in a semiconductor chip.

3. The method according to the preceding claim, wherein the laser beam (B) is focused with an intensity to fully cut through the wafer, thereby generating a physical cut or separation where the laser has passed, so that no further cutting, breaking, or other processing steps are needed tophysically isolate the semiconductor dies from each other or the rest of the wafer.

4. The method according to any of the preceding claims, wherein the focused laser beam (B) is configured to cause a physical cut through the wafer (W), wherein a suction arrangement (30) with a suction nozzle is arranged adjacent the focal region (Fr) configured to suck debris of the wafer (W) resulting from disintegrated material removed from the wafer (W) by the physical cut.

5. The method according to any of the preceding claims, wherein the set of focusing optics (10) comprises at least a first optical element (11) receiving the laser beam (B) having an initial beam diameter (DB), and a second optical element (12) arranged at an inter-element distance (DL) from the first optical element (11), wherein an optical power of the first optical element (11) is such that the second optical element (12) receives the laser beam (B) having an intermediate beam diameter (DI) which is larger than the initial beam diameter (DB).

6. The method according to the preceding claim, wherein at least the second optical element (12) has at least one spherical surface (R2f,R2b).

7. The method according to the preceding claim, wherein the first optical element (11) has at least one spherical surface(Rif, RIB) introducing a first set of spherical aberrations (SAI) into the laser beam (B), and the second optical element (12) has at least one spherical surface (R2f,R2b) introducing a second set of spherical aberrations (SA2) into the laser beam (B),wherein a respective radius of curvature of the spherical surfaces (Rlf,Rlb,R2f,R2b) and inter-element distance (DL) between the optical elements (11,12) are configured to have a combination (SA1+SA2) of the sets of spherical aberrations (SA1,SA2) constructively interfere at the focal region (Fr) such that the length (Lr) of the focal region (Fr) is larger than the wafer thickness (Tw).

8. The method according to any of the preceding claims, wherein the wafer (W) is diced while maintaining a constant axial distance (DZ) between the second optical element (12) and the wafer (W), and the wafer (W) is diced without adjusting the focusing of the laser beam (B).

9. The method according to any of the preceding claims, wherein the length (Lr) of the focal region (Fr) is larger than the maximum diameter (Dm) of the full-width-half maximum (FWHM) of the spot size (Bs) within the focal region (Fr) by at least a factor one hundred, wherein the maximum diameter (Dm) of the spot size (Bs) within the focal region (Fr) is less than five micrometers, wherein the wafer (W) is diced with a cut diameter equal to or less than the maximum diameter (Dm) of the spot size (Bs) within the focal region (Fr).

10. The method according to any of the preceding claims, wherein the length (Lr) of the focal region (Fr) is larger than the wafer thickness (Tw) by a factor between 1.05 and 1.5.

11. The method according to any of the preceding claims, wherein the wafer (W) has a thickness between 0.5 and 1 millimeters.

12. The method according to any of the preceding claims, wherein, around and outside the focal region (Fr), an interference pattern (SAr) is formedcomprising a series of concentric rings of light, resulting from the spherical aberrations.

13. A laser-based dicing apparatus (100) having a set of focusing optics (10) configured to perform the method according to any of the preceding claims.

14. The apparatus (100) according to the preceding claim, comprising a wafer stage (50) configured to keep the wafer thickness (Tw) fully inside the length (Lr) of the focal region (Fr) while dicing the wafer (W).

15. The apparatus (100) according to any of the two preceding claims, comprising a suction arrangement (30) with a suction nozzle adjacent the focal region (Fr) configured to suck debris of the wafer (W) resulting from the dicing.

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