The present invention relates to a method for laser-based machining of a planar, crystalline substrate in order to separate the substrate into a plurality of parts, and to a corresponding device and also to the use of such a method or of such a device. The aim thereby is in particular to separate planar substrates in the form of semiconductor wafers into a plurality of parts (isolation of the wafers). The process thereby takes place in general with pulsed lasers at a wavelength at which the materials of the substrate are essentially transparent.
Devices and methods for separating such materials by means of lasers are already known from the state of the art.
According to DE 10 2011 000 768 A1, lasers can be used, which are absorbed greatly by the material, by dint of their wavelength or their power, or make the material highly absorbent after the first interaction (heating with for example production of charge carriers; induced absorption), and then can ablate the material. This method has disadvantages in the case of many materials: e.g. impurities due to particle formation during ablation; cut edges can have microcracks because of the thermal input; cut edges can have melted edges; the cut gap is not uniform over the thickness of the material (has different widths at different depths; e.g. wedge-shaped cut notch). Since material must be vaporised or liquefied, a high average laser power must be provided.
Furthermore, there are methods in which a laser is used, at the wavelength of which the material is extensively transparent so that a focal point can be produced in the interior of the material. The intensity of the laser must be high enough that, at this internal focal point in the material of the irradiated substrate, internal damage takes place. The last-mentioned methods have the disadvantage that the induced crack formation takes place at points at a specific depth, or at the surface, hence the entire thickness of the material is separated only via an additional, mechanically and/or thermally induced crack propagation. Since cracks tend to propagate non-uniformly, the separation surface is generally of high roughness and often must be re-machined. In addition, the same process must be applied multiple times at different depths. This in turn prolongs the process speed by the corresponding multiple.
Starting from the state of the art, it is therefore the object of the present invention to make available a method (and also a corresponding device) with which planar, crystalline substrates, in particular made of semiconductor materials, can be machined, in particular completely separated, without particle formation, without melted edges, with minimal crack formation at the edge, without noticeable cut gaps (i.e. material losses), with as straight as possible cut edges and at high process speed. This object is achieved by a method according to claim 1 and also by a device according to claim 12. Advantageous embodiment variants and/or developments of the method or of the device can be deduced respectively from the dependent claims. Essential uses according to the invention are described in claim 18.
Subsequently, the present invention is firstly described in general, then in detail with reference to various embodiments. The features shown in the individual embodiments, in combination with each other, need not thereby all be produced within the scope of the invention. In particular, individual features can also be omitted or combined in a different way with other shown features of the same embodiment or also of other embodiments.
Also individual features of one embodiment can in fact demonstrate per se advantageous developments of the state of the art.
Subsequently, firstly the basis of the present invention (mechanism of separation of the substrate into individual parts according to the invention) is described.
The method for separation according to the invention produces, per laser pulse, a laser beam focal surface (in contrast to a focal point) by means of a laser lens system suitable for this purpose (subsequently also termed optical arrangement). The focal surface defines the zone of interaction between laser and material of the substrate. If the focal surface falls in the material to be separated, the laser parameters can be chosen such that an interaction with the material takes place, which produces according to the invention crack zones along the focal surface (i.e. distributed over this surface). Important laser parameters here are the wavelength of the laser and the pulse duration of the laser.
For the interaction of the laser light with the material according to the invention, the following should preferably be present:
Subsequently, the production of the geometry of a desired separation surface (relative movement between laser beam and substrate along a line on the substrate surface) is described.
The interaction according to the invention with the material produces, per laser pulse, an individual crack zone region in the material (viewed in the direction perpendicular to the substrate surface and in the feed direction of the laser beam) over a focal surface. For complete separation of the material, a sequence of such crack zone regions is positioned, per laser pulse, so tightly adjacently along the desired separation line that a lateral connection of the crack zone regions forming a desired crack surface/contour in the material is produced. For this purpose, the laser is pulsed at a specific sequence frequency.
For production of the desired separation surface in the material, either the pulsed laser light can be moved over the stationary material with an optical arrangement which is moveable parallel to the substrate plane (and possibly also perpendicular thereto), or the material itself is moved with a moveable receiving means past the stationary optical arrangement such that the desired separation line is formed.
Finally, also the last step of separation of the substrate into the plurality of parts (separation or isolation in the narrower sense) is effected.
The separation of the material along the produced crack surface/contour is thereby effected either by intrinsic stress of the material or by introduced forces, e.g. mechanically (tension) or thermally (non-uniform heating/cooling). Since no material is ablated according to the invention, at first generally there is no continuous gap in the material, but only a highly disrupted breakage area (microcracks) which is interlocked, per se, and possibly still connected by bridges. As a result of the subsequently introduced forces, the remaining bridges are separated via lateral crack growth (effected parallel to the substrate plane) and the interlocking is overcome so that the material can be separated along the separation surface.
Subsequently, the essential features of a method according to the invention and also of a device according to the invention are described with reference to the patent claims.
Claim 1 describes the essential features of a method according to the invention, claim 12 describes the essential constructional elements of a device configured for implementing the method according to the invention.
The laser beam focal surface described in claims 1 and 12 and produced by means of the optical arrangement is termed, previously and subsequently, alternatively simplified also as focal surface of the laser beam. According to the invention, the substrate is separated or isolated into the plurality of parts by the crack formation according to the invention (induced absorption over the expanded focal surface which extends into the substrate perpendicular to the substrate plane) viewed in the substrate plane. The crack formation according to the invention is hence effected into the substrate or into the interior of the substrate perpendicular to the substrate plane. As described already, in general a large number of individual laser beam focal surface portions must be introduced into the substrate along a line on the substrate surface in order that the individual parts of the substrate can be separated from each other. For this purpose, either the substrate can be moved parallel to the substrate plane relative to the laser beam or to the optical arrangement, or conversely the optical arrangement can be moved parallel to the substrate plane relative to the substrate which is disposed in a stationary manner.
Advantageously, in addition the features of at least one of the dependent method- or device claims are produced according to the invention. Also the features of several dependent claims can be produced thereby in any combination.
There is understood by the term surface (i.e. laser beam focal surface), in the mathematical sense, a structure extended endlessly in two dimensions. The expansion of the laser beam focal surface in both mentioned dimensions (to an approximation) can be defined by the full width at half maximum. According to the invention, the expansion of the laser beam focal surface in the direction perpendicular to the mentioned two dimensions is very much smaller than in the mentioned two dimensions, preferably by at least 10 times, 50 times, 100 times or even 500 times smaller. The term “along the surface portion” thereby means the same as “viewed over the entire surface of this portion”. There is understood by induced absorption, a process which leads to the formation of microstructure defects in the crystal structure of the substrate. The microstructure defects then define parts of a weakened surface, along which or via which separation of the substrate into the plurality of parts is effected. It is thereby assumed (without restricting the generality) that by means of the local absorption of energy in the laser beam focal surface region, amorphousness is caused, by means of which the substrate material expands. The expansions lead to pressure stresses, as a result of which local cracks in or over the expanded surface portion of induced absorption occur.
Advantageously achievable features can be deduced from the dependent claim 2.
Further advantageously achievable features are described in claim 3. The expansion of the laser beam focal surface in the respective spatial direction can thereby be defined by that stretch over which the intensity in the laser beam is at least half as great as the maximum intensity achieved in the laser beam.
If a method according to claim 4 is undertaken, then the expanded surface portion of the induced absorption in the interior of the substrate extends from a surface of the substrate as far as a defined depth in the substrate (or even beyond that). The expanded portion of the induced absorption can thereby comprise the entire substrate depth from one surface to the other. It is also possible to produce longitudinally expanded portions of the induced absorption merely in the interior of the substrate (without the surfaces of the substrate being included).
Further advantageously achievable features can be deduced according to claim 5 (see also subsequently
In the case of claim 5 and also in the case of all the other claims, the mentioned range limits include respectively the indicated upper and lower boundary value.
According to the invention, the induced absorption is produced advantageously according to claim 6. This takes place by means of adjustment of the already described laser parameters, explained subsequently also within the scope of examples and mentioned also in the dependent claims 7 to 9, of the parameters of the optical arrangement and also of the geometric parameters of the arrangement of the individual elements of the device according to the invention. Basically, any feature combination of parameters, as are mentioned in claims 7 to 9, is thereby possible. In claim 8, τ<<F/α thereby means that τ is less than 1%, preferably less than 1% of F/α (F is thereby the planar expansion of the laser beam focal surface). For example, the pulse duration τ can be at 10 ps (or even below that), between 10 and 100 ps or even over 100 ps. For separation of Si substrates, preferably an Er:YAG laser with a wavelength between 1.5 and 1.8 μm is used. In general, preferably a laser is used for semiconductor substrates, with a wavelength which is chosen such that the photon energy is less than the band gap of the semiconductor. The possibly still necessary additional method steps for the final separation or isolation of the substrate into the plurality of parts thereof are described in the dependent claims 10 and 11. As already mentioned, either the substrate is moved relative to the optical arrangement (including laser) or the optical arrangement (including laser) is moved relative to the substrate. There is thereby understood by the spacing A, not the spacing between the two directly oppositely situated (viewed in beam direction z, i.e. into the substrate) limits of two immediately adjacent (i.e. produced by temporally successive laser pulses) surface portions of induced absorption, but rather the spacing between one and the same limit (for example the limit situated at the front, viewed in the movement direction of the laser along the desired separation line) of two immediately adjacent surface portions of induced absorption. The aim is therefore to string together, as far as possible without overlapping of the individual induced absorptions produced in the interior of the substrate (otherwise too great differences in the microstructure defects in the crystal structure along the desired separation line would occur because of approximately doubled absorbed intensities in the overlapping region) and with as small as possible gaps therebetween, adjacent surface portions of induced absorption along the desired separation line. The crack formation mentioned in claim 11 should thereby be understood (in contrast to the induced crack formation essential according to the invention) as transverse crack, i.e. as a lateral crack formation in a direction situated in the plane of the substrate (corresponding to the course of the line along which the substrate is intended to be separated).
Advantageous developments of a device according to the invention according to claim 12, which describe in particular various possible embodiments of the optical arrangement for producing and positioning the laser beam focal surface, can be deduced from the dependent claims 13 to 17; see in this respect also the subsequent embodiments or
Instead of the diaphragm used according to claims 13, 14 and 16, an aperture can also be used or the diaphragm can be formed as an aperture. It is also possible, instead of the diaphragm, to use a diffractive element (with defined edge intensities) or to form the diaphragm as a diffractive element (with defined edge intensities.) (In addition to normal diaphragms, also diffractive beam formers can be combined to produce a line focus with the axicon).
According to claim 17, deflection of the two beam bundles means that the two beam bundles, viewed per se, are deflectable respectively parallel and, viewed in their entirety, are directed towards each other.
Essential uses according to the invention can be deduced from claim 18. The present invention, in comparison with methods or devices known from the state of the art, has a series of essential advantages.
Firstly, the cut formation is effected, according to the invention, without particle formation, without melted edges, with minimal crack formation at the edge, without a noticeable cut gap (hence without loss of material of the substrate) and with straight cut edges.
According to the invention, a very high average laser power is unnecessary, nevertheless comparably high separation rates can be achieved. It is thereby essential that the invention produces, per laser pulse (or per burst pulse), a laser beam focal surface (and not merely a focal point which is not expanded or only very locally). For this purpose, the laser lens systems also illustrated in detail subsequently are used. The focal surface thus defines the zone of interaction between laser and substrate. If the focal surface falls at least in portions (viewed in the depth direction) into the substrate material to be separated, the laser parameters can be chosen such that an interaction with the material takes place, which produces, according to the invention, crack zones along the entire focal surface depth and over the entire focal surface width. Selectable laser parameters are for example the wavelength of the laser, the pulse duration of the laser and the pulse energy of the laser.
Further advantages which the method according to the invention possesses relative to for example mechanical cracking and breaking are that, in addition to the lack of (or at least minimal) particle formation, in contrast to a mechanical crack line, a high aspect ratio (ratio of the expansion of the laser beam focal surface in the depth direction to the expansion in the second direction) can be achieved. Whilst during mechanical cracking and breaking, the breakline is produced into the material via extensively uncontrollable crack growth, separation can be effected according to the invention at a very precisely adjustable angle relative to the substrate normal. According to the invention, also diagonal cuts are readily possible.
Also a high machining speed is possible even with fairly thick substrates.
Ablation on the surface, burr formations on the surface and particle formations are avoided (the latter in particular if the position of the focal surface is adjusted relative to the substrate such that the method according to the invention, starting from the surface of the substrate into the interior of the substrate, ensures the expanded induced absorption and crack formations according to the invention). In this case, the first (desired) damage takes place directly at the surface and continues in a defined manner along the crack formation zone by means of induced absorption into the substrate depth.
According to the invention, different materials, in particular sapphire wafers, semiconductor wafers . . . can be machined.
Also already coated materials (e.g. coated with TCO) or also printed substrates which are not transparent on one side can be machined and separated according to the invention.
According to the invention, cutting is possible in practice without a cut gap: merely material damage is produced which is generally in the range between 1 and 10 μm of expansion. In particular, no cut loss is thereby generated with respect to material or surface. This is advantageous in particular during cutting of semiconductor wafers since cut gap losses would reduce the actively usable surface of the wafer. Due to the method according to the invention of focal surface cutting, an increased surface yield is hence produced.
The method according to the invention can be used in particular also in inline operation of production processes. In a particularly advantageous manner, this takes place during production processes which take place in a roll-to-roll process.
According to the invention, individual pulse lasers can also be used, such as lasers which generate burst pulses. Basically, also the use of lasers in continuous line operation is possible.
The following specific application fields arise, by way of example:
Subsequently, the present invention is now described with reference to some of the embodiments based on what was described above. There are thereby shown:
The beam formation effected for this purpose via elements 8, 7 and 13 which are essential to the invention (cf.
By means of the beam formation according to the invention, which is described subsequently in more detail, the previously circular halo region (
As
As
Subsequently, concrete optical arrangements 6 which can be used for the production of the focal surface 2f are described. All of the arrangements are thereby based on what has previously been described so that respectively identical references are used for components or features which are identical or correspond in their function. Subsequently, respectively only the differences are therefore described.
Since the separation surface leading ultimately to separation is or should, according to the invention, be of high quality (with respect to breaking resistance, geometric precision, roughness and avoidance of re-machining requirements), the individual (more precisely: produced by individual laser pulses) focal surfaces to be positioned along the separation line 5 (cf.
In the case shown in
According to the invention, it is thereby particularly advantageous to produce the focal surface positioning such that at least one of the surfaces 1a, 1b is covered by the focal surface, the portion of induced absorption 2c begins hence at at least one surface. In this way, almost ideal cuts can be achieved by avoiding ablation, burr- and particle formation on the surface.
The optical arrangements shown in
On the beam output side of the lens 12, a one-dimensional slit diaphragm 8 is positioned at the spacing z1 with z1>z1′ (here: z1=1.3×z1′). The slit diaphragm 8 is orientated with its preferential direction (i.e. slit direction) in the first direction, i.e. the y-direction. The slit diaphragm (subsequently also termed alternatively slit diaphragm) 8 is thereby positioned such that the optical axis 6z, viewed in the second direction x, extends centrally between the two slit edges. The slit width is chosen such that it corresponds with the inner diameter 2i of the annular beam bundle 2r on the output side of the lens 12: as
On the beam output side of the diaphragm 8 and at a spacing from the latter, a further plano-convex collimation lens 11, which serves here as focusing lens, is positioned centred about the optical axis 6z: said focusing lens focuses all of the beam components, not occluded by the diaphragm 8, of the previously annular beam bundle 2r into the first y and into the second x direction towards the planar substrate 1 which is disposed on the beam output side of this lens 11, perpendicular to the optical axis 6z, i.e. in the x-y plane. The lens 11 (the planar side of which is orientated towards the substrate 1) hence focuses the beam components, not occluded by the diaphragm 8, of the previously annular beam bundle 2r at a defined spacing from the lens 11 onto a two-dimensional laser beam focal surface 2f with a defined expansion in the z-direction (due to the effect of the axicon 9) and also with a defined expansion in the y-direction (due to the effect of the diaphragm 8); see in this respect the beam formation illustrated in
The optical properties of the optical arrangement 6 which comprises the rotationally-symmetrical elements 9, 12 and 11, positioned on the optical axis 6z, and also the diaphragm 8 (in particular the geometric forming of elements 9, 12, 8 and 11 and the positioning thereof relative to each other along the main beam axis 6z) can thereby be chosen such that the expansion 1 of the laser beam focal surface 2f in z-direction is twice as large as the thickness d of the substrate in z-direction. If the substrate is then positioned centred relative to the focal surface 2f (cf.
Instead of the plano-convex lenses 11, 12 shown in
In the beam path 2a, the plano-convex, focusing cylindrical lens 7 is positioned on the beam output side of the axicon 9 instead of the lens 12 in
At a defined spacing z2′ behind the cylindrical lens 7, the focusing plano-convex collimation lens 11 is positioned in the beam path, centred about the optical axis 6z, as in the embodiment of
Also due to the combination of the rotationally-symmetrical axicon 9 with the cylindrical lens 7 and also the subsequent focusing by the rotationally-symmetrical plano-convex collimation lens 11, the beam formation from
A further example of a device according to the invention for producing an expanded focal surface 2f is shown in
In the beam path 2a of the laser 3 (not shown), firstly a non-rotationally-symmetrical optical element 13 provided with a preferential direction (here: x-direction) is positioned. This is configured as a planar element on the beam output side which deflects on the beam input side and is centred on the optical axis 6z. The planar side therefore points towards the substrate 1. The deflecting side situated opposite the planar side (i.e. pointing towards the laser 3) is configured as a pointed-roof-shaped double wedge, the central backbone of which extends along the x-direction and though the optical axis 6z. The element 13 is subsequently termed also double wedge for simplification.
As
Viewed in beam direction at a spacing from the wedge 13 (behind the intersection point of the two beam components s1 and s2), a cylindrical lens 7 is positioned, as in the example of
Viewed in beam direction, the plano-convex collimation lens 11 is positioned behind the cylindrical lens 7 i.e. at a defined spacing z3 on the beam output side of the double wedge 13 (as in the examples from
As
The repetition rate of the laser pulses is coordinated to the feed speed of the laser such that the average spacing A of immediately adjacent expanded surface portions 2c of induced absorption, i.e. produced by temporally directly successive laser pulses, is slightly (e.g. by the factor larger than the width b of the laser beam focal surface 2f in feed direction or y-direction. Hence, without intensity overlapping, introduction of a large number of defect structures 2c which are placed immediately in a row along the channel axis 1k or the desired separation line 5 is effected and hence efficient separation of the substrate 1 along such channels 1k. The substrate residues which still remain between two adjacent defect structures 2c and are detectable here as gaps can readily effect crack formations due to the effect of mechanical forces and/or thermal stresses in order to separate finally from each other the substrate fragments produced on both sides of the separation line 5.
Number | Date | Country | Kind |
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10 2014 213 775.6 | Jul 2014 | DE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2015/065476 | 7/7/2015 | WO | 00 |