METHOD AND SYSTEM FOR LASER WELDING OF A SEMICONDUCTOR MATERIAL

Abstract

The invention relates to a method for welding a first workpiece (11) to a second workpiece (12) by means of a laser. It is an object of the invention to provide a reliable, repeatable and reproducible approach for laser welding of two workpieces one of which consists of a semiconductor material. The method proposed by the invention comprises the following steps: Irradiating the first workpiece (11) with a beam of pulsed laser radiation, wherein the first workpiece (11) consists of a semiconductor material which is transparent at the wavelength of the laser radiation, so that the beam enters the first workpiece (11) through an entrance surface and leaves it through an exit surface, the geometric focus of the beam being positioned in the plane of the exit surface; determining a delocalization of the focus caused by nonlinear interaction of the laser radiation with the semiconductor material; placing the second workpiece (12) against the first workpiece (11); and, again, irradiating the first workpiece (11) with the laser beam of pulsed laser radiation, the focus of the laser radiation being positioned along the beam direction taking into account the determined delocalization so that the intensity maximum is located in the plane of the exit surface forming the interface of the two workpieces (11, 12), whereby the first workpiece (11) is welded to the second workpiece (12). Moreover, the invention relates to a system for welding a first workpiece (11) to a second workpiece (12).

Claims

1-13. (canceled)

14. Method for welding a first workpiece to a second workpiece by means of a laser, comprising the steps of: irradiating the first workpiece with a beam of pulsed laser radiation, wherein the first workpiece consists of a semiconductor material which is transparent at the wavelength of the laser radiation, so that the beam enters the first workpiece through an entrance surface and leaves it through an exit surface, the geometric focus of the beam being positioned in the plane of the exit surface; determining a delocalization of the focus caused by nonlinear interaction of the laser radiation with the semiconductor material; placing the second workpiece against the first workpiece; and again irradiating the first workpiece with the laser beam of pulsed laser radiation, the focus of the laser radiation being positioned along the beam direction taking into account the determined delocalization so that the intensity maximum is located in the plane of the exit surface forming the interface of the two workpieces, whereby the first workpiece is welded to the second workpiece.

15. Method of claim 14, wherein the pulse duration of the laser radiation is in the range of 1 fs to 100 ns.

16. Method of claim 14, wherein the parameters of the laser radiation, specifically the spectrum, the pulse duration, the beam size, as well as the pulse energy, are identical during the determination of the delocalization and during welding the first workpiece to the second workpiece.

17. Method of claim 14, wherein the material of the second workpiece is opaque at the wavelength of the laser radiation.

18. Method of claim 17, wherein the material of the second workpiece is a metal, or a semiconductor which is opaque at the wavelength of the laser radiation, or a semiconductor which is transparent at the wavelength of the laser radiation.

19. Method of claim 14, wherein the assembly formed by the two workpieces is moved in a plane perpendicular to the laser beam to create a welding pattern.

20. Method of claim 14, wherein the focus of the laser radiation is moved along the interface between the two workpieces to create a welding pattern.

21. Method of claim 19, wherein the intensity of the laser radiation is varied across the welding pattern.

22. System for welding a first workpiece to a second workpiece, comprising: a laser device configured to generate a beam of pulsed laser radiation; a beam controlling device configured to direct the beam through the first workpiece so that the beam enters the first workpiece through an entrance surface and leaves it through an exit surface forming the interface between the two workpieces, and to focus the beam at a focus position along the beam path; a measuring device configured to determine a delocalization of the focus caused by nonlinear interaction of the laser radiation within the material of the first workpiece; a positioning device configured to position the assembly of the two workpieces and the focus of the laser beam relative to each other, along the direction of the laser beam and in the plane perpendicular thereto.

23. System of claim 22, wherein the measuring device comprises a microscope arranged on the side of the exit surface of the first workpiece, the observation direction of the microscope being opposite to the propagation direction of the laser beam.

24. System of claim 22, wherein the beam controlling device is configured to control the parameters of the laser radiation, specifically the spectrum, the pulse duration, the temporal pulse shape, the beam size, the polarization and/or the pulse energy.

25. System of claim 22, further comprising an observation device configured to inspect the entrance surface of the first workpiece.

26. System of claim 22, further comprising a conditioning device configured to control the ambient conditions of the assembly of the two workpieces.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

[0038] FIG. 1 schematically shows a system for carrying out the method of the invention;

[0039] FIG. 2 schematically shows the measuring device of the system for determining the shift of the position of the maximum intensity along the optical axis;

[0040] FIG. 3 shows a diagram illustrating the focus delocalization as a function of laser pulse energy and duration;

[0041] FIG. 4 shows the method of the invention as a flow chart.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] With reference to FIG. 1, a system 100 for welding a first workpiece 11 to a second workpiece 12 is shown as a block diagram. The first workpiece 11 consists of a semiconductor material, e.g. silicon. The material of the second workpiece is a metal, e.g. copper. The two workpieces 11, 12 may or may not be in optical contact at their common interface. The optional optical contact can be established by introducing a suitable fluid within the gap between the two workpieces 11, 12, such as, e.g., methanol, ethanol, etc. The workpieces 11, 12 can be maintained in contact by means of any common mechanical fixturing method.

[0043] The laser welding system 100 includes in particular a laser device 21, e.g. a mode locked fiber laser in combination with one or more optical amplification stages, which emits pulsed laser radiation 201 with given properties in terms of spatial and temporal distribution, polarization, energy, spectrum, repetition rate, etc. A beam controlling device 20 is provided for controlling the characteristics of the laser pulses 201 in terms of wavelength, pulse duration, temporal pulse shape, repetition rate, spatial distribution of the beam, beam size, pulse energy, and polarization before being directed through the first workpiece 11. The beam controlling device 20 comprises several sub-modules 22-26 for controlling the pulse characteristics and a focusing optics 40 to focus the laser beam, which enters through an entrance surface of the workpiece 11 and leaves it through an exit surface forming the interface between the two workpieces 11, 12, with the geometric focus being set at a location along the beam path. The focusing optics 40 can include, e.g., an objective lens, a convex single lens, a parabolic mirror, an aspherical lens, an F-Theta lens, a cylindrical lens, etc.

[0044] The relative position of the assembly 10 of the two workpieces 11, 12 with respect to the incoming focused beam is adjusted by a positioning device 50 (e.g., a translation stage, a rotation stage, a piezoelectric stage, or a combination of these) enabling the displacement of the assembly 10 in the direction of the laser beam (in order to adapt the position of the geometric focus) as well as in the plane perpendicular thereto. For adapting the focus position further components may be used, such as an F-Theta lens, galvanometer mirrors, a beam steering phased array etc. (not depicted). The three-dimensional control of the relative position of the beam with respect to the assembly 10 makes it possible to carry out the welding procedure along a predefined welding pattern. The welding pattern is determined by a scanning strategy to obtain, e.g., a point-by-point pattern, a line-by-line pattern, a concentric circle pattern, a spiral pattern, single-scan, multi-scan with or without increase in laser intensity between the scans, etc. Ideally, the scanning speed during welding is between 1 nm/s and 10 m/s. The welding pattern can be chosen as a function of the amount of energy deposited per point of the welding pattern at the interface between the two workpieces 11, 12 which mainly depends on the laser repetition rate, pulse energy, and beam size.

[0045] The system 100 further comprises an observation device 30 for inspecting at least one of the surfaces of the first workpiece 11. The observation device 30 can be, e.g., an optical bright-field or dark-field microscope either working in transmission or in reflection, with or without phase contrast. The observation device 30 enables monitoring of the interaction of the laser radiation at the respective surface of the first workpiece 11 for precise calibration of the positioning of the geometric focus of the laser beam relative to the surfaces of the workpiece 11. The system 100 further includes a conditioning device 60 for controlling the ambient conditions of the welding process in terms of chemical composition, pressure, and temperature. The conditioning device 60 includes, for example, a chamber with a valve for gas injection, a vacuum chamber connected to vacuum pumps, an oven, etc.

[0046] As mentioned above, the beam controlling device 20 includes optional sub-modules 22-26 for tailoring the laser pulse properties. The first optional sub-module 22 is provided for controlling the wavelength, and, more generally, the spectral characteristics of the pulses. Module 22 can be, for example, an optical parametric amplifier (OPA), an optical parametric oscillator (OPO), a frequency doubling crystal, etc. The wavelength of the pulses 202 at the output of module 22 has to be adjusted so that workpiece 11 is transparent at this wavelength, whereas workpiece 12 is opaque at this wavelength. As an example, an adjusted wavelength of 1.55 μm is appropriate if the material of workpiece sample 11 is crystalline silicon, and the material of workpiece 12 is a metal (e.g., copper, gold, silver, aluminum, etc.). The second optional module 23 is provided for controlling the temporal shape of the laser pulses, in accordance with a predefined welding pattern. For an irradiation composed of single-pulses, or a pulse train, the module 23 can decrease the repetition rate of the pulses 202, and can be, for example, an optical beam shutter, an optical chopper, a Pockels cell, an acousto-optic modulator (AOM), etc. Secondly, the module 23 gives access to the control of the pulse duration, and can be, for example, a stretcher or a compressor composed of a combination of various dispersive elements such as diffraction gratings or prisms, or a pulse stretcher based on group velocity dispersion in a given material. The third optional module 24 is provided for controlling the spatial distribution of the beam. It can increase or decrease the beam size, e.g., by means of a Galilean or a Keplerian telescope, and, more generally, any afocal system. Secondly, module 24 can change the beam profile (i.e., the spatial beam distribution) from the beam at the input 203 to the output 204. E.g., module 24 can be able to generate Gaussian, tophat (i.e., uniform in intensity), Bessel or vortex beams. Module 24 can include an axicon, phase plates, a spatial light modulator, etc. The fourth optional module 25 enables the control of the polarization of the laser beam, i.e., the direction of the electric field of the laser. Module 25 can include, for example, wave plates (half-wave plate, quarter-wave plate), polarizers (e.g., linear polarizer, radial polarizer, electro-optic modulator), etc. The fifth and last optional module 26 is provided for the control of the pulse energy by decreasing the beam power from the entrance 205 to the exit 206 of this module. The attenuation in pulse energy can be carried out with, for example, an optical attenuator, neutral density filters, the combination of a half-wave plate and a polarizer (the control of the energy is achieved by rotating the half-wave plate in this case), etc. If all the aforementioned optional modules 22-26 of the beam controlling device 20 are used, the spectral, temporal, spatial, polarization and energy characteristics of the beam 206 can be optimally tailored. The sequence of the sub-modules 22-26 may be modified as required.

[0047] The laser pulse energy can be estimated by applying an energy calibration procedure. This procedure consists of measuring the average power of the laser radiation, e.g. using a power meter, between the focusing optics 40 and the workpiece 11. The corresponding pulse energy can be easily determined by calculating the ratio between the measured average power value (in Watts) and the repetition rate of the laser (i.e., the number of laser pulses per second, in Hertz). Furthermore, the energy after propagation through the entrance surface of the workpiece 11 can be estimated by multiplying the pulse energy with the Fresnel transmission coefficient calculated from the refractive indices of the environment and the workpiece 11, the laser polarization and the angle of incidence of the laser beam.

[0048] FIG. 2 shows a depiction of the measuring device 70 of the system 100. The measuring device 70 is used to measure the variation of the intensity of the laser radiation along the beam path within the first workpiece 11 to determine a delocalization of the focus caused by nonlinear interaction of the laser radiation with the semiconductor material of workpiece 11. This measuring procedure is carried out before the actual welding. The measuring procedure includes the focusing of the laser beam with identical beam parameters as used in welding (in terms of spectrum, temporal profile, pulse duration, spatial distribution of the beam, beam size, polarization, and pulse energy). The laser beam is initially focused on the exit surface of the first workpiece 11 that has to be laser welded to the second workpiece 12 (FIG. 1) afterwards. The measuring device 70, e.g. a microscope collinear to the optical axis Z and directed oppositely to the laser beam direction, enables imaging of the exit surface of the workpiece 11. Due to the fact that the imaging system 70 is also centered on the beam focused by the focusing system 40, the measuring device 70 is able to image the spatial distribution of the beam intensity at the exit surface of workpiece 11. In the depicted embodiment, the measuring device 70 is composed of a collecting optics 71 whose numerical aperture (NA) is ideally higher than the one of the focusing optics 40 for ensuring to collect all angular components of the incoming light. The collecting optics 71 can be, for example, an infinity-corrected objective lens. In this way, the beam is collimated after being collected by the focusing optics 40. An optional tube optics 72 is provided to refocus the collimated light onto a camera 73 that can be equipped, for example, with a bandpass filter for ensuring that only the light at the wavelength of the laser radiation is detected. This filter can be particularly relevant in the case of third-harmonic generation in the material, as well as for blocking most of the ambient light. When a finite-corrected objective lens is employed in the collecting optics 71, the tube optics 72 can be bypassed and the beam is directly refocused on the camera 73. Ideally, for optimized dynamic range, the camera 73 has a linear response at the wavelength of the incoming laser. Moreover, the image on the camera 73 should not show any saturated pixels during measurement. The system 100 further includes a linear translation stage 80 enabling relative displacement of the focusing optics 40 only along the optical axis Z, upstream as well as downstream along the laser beam path. In a possible alternative embodiment (not depicted), the translation stage 80 enables the workpiece 11 and the measuring device 70 to move together along the optical axis Z, with a fixed position of the focusing optics 40. The arrangement shown in FIG. 2 enables to image the spatial distribution of the incoming laser light in the (X,Y) plane for various relative positions of the focus along the optical axis Z with respect to the exit surface of the workpiece 11. By repeated alternating movements of the focusing optics 40 and image acquisitions using the measuring device 70 the intensity variation of the laser beam propagating through the first workpiece 11 can be determined in the (X,Z) and (Y,Z) planes.

[0049] A first beam propagation imaging procedure can be carried out at a pulse energy for which the beam propagation is linear, i.e. without nonlinear interaction of the laser radiation with the semiconductor material of the first workpiece 11 (typically at a pulse energy between 1 fJ and 20 pJ, depending on the sensitivity of the camera 73). The energy can be attenuated to such a low level by means of, for example, neutral density filters of module 26 (see FIG. 1). Then, the same measurement is performed at the pulse energy at which the welding procedure is to be carried out. The intensity corresponding to this pulse energy has to be sufficient for inducing a material modification at the interface between the two workpieces 11, 12 during the laser welding procedure. Due to the fact that this increased intensity will not be sufficient for inducing a modification of the material of the first workpiece 11 (being transparent at the wavelength of the laser radiation), neither at the exit surface nor in the bulk material, the measurement of the intensity variation under nonlinear propagation of the laser beam at the higher pulse energy is reliable and repeatable. The linear and the nonlinear propagation images acquired by measuring device 70 can be analyzed to determine the position of maximum pixel amplitude along the optical axis Z (which is related to the maximum fluence and intensity of the laser beam). The difference between the positions of maximum intensity along the optical axis Z in the linear and nonlinear propagation cases (designated herein below as ΔZ) can be derived from a comparison of the two images. At pulse energies required for welding, the position of the maximum intensity is strongly shifted upstream along the beam path, i.e., in the pre-focal region of the bulk semiconductor material. This is physically related to soliton formation provoked by the competition of nonlinear phenomena such as Kerr-induced self-focusing, plasma formation (due to, e.g., multi-photon absorption, avalanche ionization, tunnel ionization, etc.), and plasma defocusing.

[0050] FIG. 3 illustrates an experimental quantification of the evolution of the nonlinear propagation-induced shift AZ along the optical axis as a function of the pulse energy (measured in air before the entrance surface of the first workpiece 11). It can be seen from the diagram of FIG. 3 that, for 0.86-ps pulses at low energy (i.e., <6 nJ), ΔZ is close to zero, showing the linear propagation of the laser radiation at these energies. In the energy range 6-100 nJ, ΔZ monotonically increases with increasing pulse energy, indicating that nonlinear propagation phenomena start to become prominent. A similar behavior is observed for 9.83-ps duration pulses, but shifted toward higher pulse energies. These results demonstrate that, for maximizing the intensity at the exit surface of the semiconductor workpiece 11, i.e. at the interface between the two workpieces 11, 12, simply positioning the geometric beam focus at the interface is not appropriate. The shift AZ along the optical axis has to be taken into account for optimizing the energy deposition at the interface between the semiconductor material of the first workpiece 11 and the opaque material (metal) of the second workpiece 12. To this end, the focus of the laser radiation is (re-)positioned according to the invention during welding so that the intensity maximum is located precisely at the interface such that the two workpieces 11, 12 are optimally bonded.

[0051] FIG. 4 represents a flow chart of an exemplary embodiment of the method of the invention. Firstly, in step 301, the shift AZ of the geometric focus induced by the nonlinear propagation is determined by irradiating the first workpiece 11 only as described above with reference to FIGS. 2 and 3. In the subsequent step 302 the second workpiece 12 is placed against the first workpiece 11 to bring the two workpieces 11, 12 in mechanical and/or optical contact. Then, in step 303, a calibration of the relative positioning of the laser beam focus and the surfaces of the first workpiece 11 is carried out. To this end, the entrance surface of the first workpiece 11 is observed (using observation device 30, see FIG. 1). In this way, the precise position (of the focusing optics 40 or the workpiece 11) along the optical axis Z at which the geometric focus coincides with the entrance surface of the first workpiece 11 is determined (designated as Z.sub.entrance). This determination can be achieved by a destructive Z-scan procedure schematically illustrated at 400. The Z-scan procedure consists of creating a matrix of irradiated sites on the entrance surface of workpiece 11. The columns of the matrix correspond to different pulse energies, and the lines of the matrix correspond to different Z positions of the focusing optics 40 (or vice-versa). The Z.sub.entrance position is the one of the matrix line for which a single damage (black spots in the diagram 400) is observed for a specific pulse energy (corresponding to the damage threshold of the material of the first workpiece 11). The Z-calibration thus results from observation of the damage sites produced at the entrance surface of the first workpiece 11. A (residual) tilt of the workpiece 11 with respect to the focusing optics 40 may exist in practice. This tilt is determined in step 304. While this tilt is usually small (on the order of 1-μm difference along Z for a displacement of 100 μm in the X or Y direction), this can lead to a significant error when a mm- or cm-size semiconductor sample has to be welded to an opaque material sample. This is even more pronounced for a high refractive index of the semiconductor material, due to refraction. For example, a 1 mm displacement along Z corresponds to an actual displacement in crystalline silicon of about 3.5 mm at a wavelength of 1300 nm (where the refractive index of silicon is 3.5). In order to irradiate the interface between the two workpieces 11, 12, the thickness and topography of the first workpiece in which the laser radiation propagates has to be taken into account. For example, assuming flat and parallel surfaces of the first workpiece 11 which is tilted with respect to the optical axis, the Z.sub.entrance position has to be determined as described above at three different positions. These positions can be chosen, e.g., at the starting point of the laser welding pattern, at the maximum X position, and at the maximum Y position. The knowledge of the three Z.sub.entrance positions makes it possible to evaluate a tilt correction ΔZ.sub.tilt at each position in the (X,Y) plane to account for during the laser welding procedure afterwards. Then, the geometric focus is positioned in the Z direction at the interface (Z.sub.interface position) between the two workpieces 11, 12 in step 305. This is straightforward after determination of Z.sub.entrance, knowing the refractive index of the semiconductor material of the first workpiece 11 at the welding wavelength as well as its thickness at different positions along the X and the Y axes. In order to maximize the laser intensity at the Z.sub.interface position, the position of the geometric focus is adapted in step 306 according to the pre-determined nonlinear propagation-induced shift ΔZ. In the last step 307 the actual welding takes places, wherein the assembly of the two workpieces 11, 12 is moved in the (X,Y) plane, perpendicular to the optical axis Z, according to a predefined welding pattern, while being irradiated with the pulsed laser radiation.