Method for determining operating conditions of a method for high repetition rate femtosecond laser ablation of a given material and method for laser welding between parts of the determined material

Abstract

The present invention proposes a method for determining the operating conditions of a method for high repetition rate femtosecond laser ablation in a given material, which method comprises a first step of determining a set of parameters of a burst of laser pulses adapted to generate ablation craters in the material, the set of parameters being equal to the intra-burst repetition frequency f between several hundred MHz and 100 GHz and the number of heating and ablation pulses Nc, where Nc is determined by the formula Nc=(L 2 the number of pulses N in a burst of laser pulses, defined by (f) / D, where L represents the test depth of the material to be ablated, D represents the thermal diffusivity, and Nc is 10 or greater, and the characteristic total fluence FT of the burst of pulses. char and the characteristic fluence per pulse F below the material ablation threshold fluence Fs1 by a single laser pulse. char =FT char / Nc.

Description

[Technical Field]

[0001] The present invention relates generally to the field of laser ablation and welding methods.

[0002] The invention more particularly relates to the field of methods for laser ablation of material surfaces by bursts of laser pulses or successive bursts of laser pulses at high repetition rates of the order of GHz. [Background technology]

[0003] Surface ablation of materials with microsecond, nanosecond, picosecond, or femtosecond laser pulses is a technique widely used today to controllably modify (drill, cut, structure) the surface of a variety of materials, such as semiconductors, metals, dielectric materials, or biological tissue.

[0004] Femtosecond pulses typically produce the best results, but the ablation efficiency of these pulses is lower than that of longer pulses on the order of nanoseconds. Recently, lasers with higher average powers have become available, allowing for the use of higher repetition rates in the MHz range to shorten production times. However, the use of pulses delivered by lasers at such high repetition rates can result in undesirable heat accumulation, which reduces the quality of the material ablation. Therefore, it is essential to develop alternative methods aimed at improving the quality of surface ablation of a given material.

[0005] Scientific literature has recently shown that in some specific cases, high repetition rates of femtosecond laser pulses (exceeding 10 MHz and approaching 1 GHz) can significantly increase ablation efficiency. In this case, the number of operating parameters is extremely large, including the wavelength of the laser, the repetition rate of the pulses delivered by the laser, the pulse duration, the pulse energy, focusing, the average pulse power, and the material being ablated. This large number of operating parameters makes it difficult to determine the operating parameters best suited to a given material. Additionally, the exact nature of the mechanism of laser ablation of materials by bursts of femtosecond pulses at GHz repetition rates remains a topic of debate in the scientific literature.

[0006] According to the prior art, ablation at the surface of a material occurs when the fluence of the laser pulse is equal to the ablation threshold, which is determined for each material.

[0007] The present invention also relates to a method for high repetition rate laser welding without loss of material. Summary of the Invention [Means for solving the problem]

[0008] Thus, according to the invention, there is provided a method for determining the operating conditions of a method for high repetition rate femtosecond laser ablation in a given material, the method comprising a first step of determining a set of parameters of a burst of laser pulses adapted to generate ablation craters in the material, the set of parameters being an intra-burst repetition frequency f comprised between several hundred MHz and 100 GHz, a number N of pulses in the burst of laser pulses equal to or greater than 10, a characteristic total fluence FT of the burst of pulses char , and a characteristic fluence per pulse that is less than the material ablation threshold fluence Fs1 by a single laser pulse.

[0009] The number of pulses N in the burst of laser pulses is equal to or greater than the number of heating and ablation pulses Nc, where Nc is determined by the formula Nc = (L 2 ·f) / D, where L represents the test depth of the material to be ablated, D represents the thermal diffusivity, Nc is 10 or greater, and the characteristic fluence per pulse is F char =FT char It is convenient to make it equal to / Nc.

[0010] The present invention also relates to a method for laser ablation of a determined material, comprising the application of bursts of femtosecond laser pulses with an intra-burst repetition frequency f comprised between several hundred MHz and 100 GHz, wherein the bursts of femtosecond laser pulses comprise a number of pulses N equal to or greater than the number of heating and ablation pulses Nc, where Nc is determined according to the formula Nc = (L 2 a characteristic total fluence FT defined by (f) / D, where L represents the ablation depth in the material, D represents the thermal diffusivity of the material being ablated, Nc is 10 or greater, and a burst of femtosecond laser pulses is adapted to generate an ablation crater in the material. char and each pulse of the burst has a characteristic total fluence FT char and the number of heating and ablation pulses Nc, and have a fluence equal to or greater than the characteristic fluence per pulse (FT char =FT char / Nc), the characteristic fluence per pulse is less than the ablation threshold fluence Fs1 of the material by a single laser pulse adapted to ablate the material, the single laser pulse being adapted to ablate the material with the same spatial, spectral, and temporal characteristics as the pulses of the burst.

[0011] A preliminary step in the laser ablation method is the FT <FT char If , there is no ablation crater and FT = FT char If an ablation crater exists, the characteristic total fluence FT is defined by charTo experimentally determine , one can deliver bursts of femtosecond optical pulses to the surface of the material at the repetition rate of the pulses delivered by the laser, which is typically the threshold fluence for femtosecond pulse burst ablation methods with a repetition rate of f and a number of pulses Nc.

[0012] According to this ablation method, the ablation depth obtained is FT = FT char And when N=Nc, it is close to the value of the test depth L.

[0013] According to a particularly interesting aspect of the invention, The method calculates the characteristic total fluence FT of a burst over a number Nc of pulses. char This involves experimental determination of the value of FT char is the minimum total fluence to obtain a crater on the surface of the material, The method comprises calculating an operating total fluence FT in a burst of laser pulses comprising Nc laser pulses. opt a second step of determining -Operating total fluence FT opt is the characteristic total fluence FT char Twice the characteristic total fluence FT char Included in six times The method includes a third step of determining the number N of pulses in the burst to ablate material to a depth greater than the test depth L, where N is greater than Nc and the fluence per pulse of the burst is a characteristic total fluence FT char is equal to the number of heating and ablation pulses Nc divided by the number N of pulses in the burst is equal to N=Nc+Na, where Na is a positive or negative integer and Nc is the number of heating and ablation pulses; the number of pulses N in the burst is less than the number Nc of heating and ablation pulses to ablate material to a depth shallower than the test depth; the integer number N of pulses is between 10 and 10,000, preferably greater than 50, for example between 200 and 500, or between 300 and 500, - the ablation depth depends on the number N of pulses in the burst, -Another characteristic is the total fluence (FT) char2 is calculated as a function of the number N of pulses in the burst, which is different from Nc, where N=Nc+Na, where Na is a positive or negative integer, and FT char2 =FT char +Na·FT char / Nc, -The total fluence of the operation is another characteristic, the total fluence FT char2 Twice as much as another characteristic: total fluence FT char2 Included in six times the material to be ablated is a semiconductor material, a metallic material, a dielectric material, a polymeric material, an organic material, or a composite material; - the pulses have a dimension orthogonal to the incident beam relative to the material being ablated of 200 μm or less; the pulses of the burst have a duration d comprised between 1 femtosecond and less than 1 picosecond, the burst of laser pulses has a wavelength comprised between 200 nm and 3 μm, For a test depth L comprised between -2.2 μm and 4.4 μm, the material is silicon, the number of pulses N is comprised between 50 and 200, and the characteristic fluence per constant pulse F char is about 0.018J / cm 2 and the repetition frequency f within the burst is in the range of 0.88 GHz to 3.52 GHz. For a test depth L comprised between -2.5 μm and 5.2 μm, the material is copper, the number of pulses N is comprised between 100 and 400, and the characteristic fluence per constant pulse F char is about 0.03J / cm 2 and the intra-burst repetition frequency f is 1.76 GHz.

[0014] With the number Na fixed, another characteristic, the total fluence FT, for the number of pulses N = Nc + Na, char2But, FT char2 =FT char +Na·F char Thus, for a given material, another characteristic that defines the ablation threshold in a burst of repetition rate f and number of pulses N=Nc+Na is the total fluence FT char2 Therefore, the ablation depth obtained is large, and its estimated value is L2 = √(D·N / f). According to this method, when the test depth L is fixed at L2, the FT char2 The total fluence F is experimentally obtained as close to FT. char2 By making it larger than FT opt Regarding the other characteristic, the total fluence FT char2 Twice as much as another characteristic: total fluence FT char2 Ablation depths beyond L2 are obtained following the same rules as described for FT included in 6 times of .

[0015] According to a variation of this method, the total number of pulses N may be reduced from N by negative Na, so that N = N + Na. Thus, the effective ablation depth L2 is shallower than the test depth L, and the procedure for optimizing the total fluence Ft remains the same as described above.

[0016] Other particular and advantageous aspects of the laser ablation method are as follows.

[0017] The number of pulses N in the burst of laser pulses is equal to or greater than the number of heating and ablation pulses Nc, where Nc is determined by the formula Nc = (L 2 The characteristic fluence per pulse is defined by (f) / D, where L represents the test depth of the material to be ablated, D represents the thermal diffusivity, Nc is 10 or greater, and F char =FT char Equal to / Nc.

[0018] The number N of pulses in a burst is included in the range 100 to 10,000.

[0019] Each pulse of the burst has a fluence F that is an order of magnitude less than the ablation threshold fluence Fs1 of the material by a single laser pulse.

[0020] Each pulse of the burst has a fluence F that is one to two orders of magnitude smaller than the threshold fluence Fs1 for ablation by a single laser pulse.

[0021] The dimension of the laser pulse perpendicular to the incident beam relative to the material being ablated is less than 200 μm.

[0022] The burst has a total energy contained in 1 μJ to 20 mJ.

[0023] It is particularly advantageous if the laser ablation method comprises applying multiple bursts at a repetition rate comprised between 10 kHz and 40 MHz.

[0024] The present disclosure also relates to a method of laser welding between parts of a determined material, comprising the application of bursts of femtosecond laser pulses with an intra-burst repetition frequency f comprised between several hundred MHz and 100 GHz, wherein the burst of femtosecond laser pulses comprises a number N of pulses, the number N of pulses in the burst of laser pulses being less than the number N of heating and ablation pulses, N being defined by the formula N = (L 2 a characteristic total fluence FT defined by (f) / D, where L represents the depth of the material (heating or welding depth), D represents the thermal diffusivity of the material, and a burst of femtosecond laser pulses is adapted to generate an ablation crater in the material. char and each pulse of the burst has a fluence F less than the ablation threshold fluence Fs1 of the material by a single laser pulse.

[0025] Of course, different features, variants, and embodiments of the present invention can be associated with each other in various combinations, provided that they are not mutually incompatible or exclusive.

[0026] In addition, various other features of the present invention will become apparent from the accompanying description made while referring to the drawings showing non-limiting embodiments of the present invention.

Brief Description of the Drawings

[0027] [Figure 1] It is a graph showing an example of the time profile of a burst of femtosecond laser pulses at a repetition frequency on the order of GHz. [Figure 2] It is a graph showing the change in ablation efficiency (or the volume ablated by the total fluence) on the surface of a silicon material as a function of the total fluence FT per burst of pulses at a GHz frequency. [Figure 3] It is a graph showing the change in ablation efficiency (or the volume ablated by the total fluence) on the surface of a copper material as a function of the total fluence FT per burst of pulses at a GHz frequency. [Figure 4] It is a diagram showing an example of steps for determining the characteristic total fluence of ablation of a silicon material by the total fluence FT of a burst where FT < FTchar and at an integer number N of pulses such that N = Nc, which does not cause ablation. [Figure 5] It is a diagram showing an example of another step for determining the characteristic total fluence of ablation of a silicon material by the total fluence FT of a burst where FT ≈ FTchar and at an integer number N of pulses such that N = Nc, which causes ablation. [Figure 6]FIG. 6 shows an example of another step for determining the characteristic total fluence of ablation of silicon material as a function of the integer number N of pulses, N=Nc, and the total fluence FT of the burst, where FT>FTchar, resulting in deeper ablation than in FIG. 5. [Figure 7A-7C] FIG. 1 is a schematic illustration of the ablation mechanisms involved during high repetition rate femtosecond laser ablation at the surface of a material as a function of the total fluence of a burst of N femtosecond pulses. [Figure 8] 1 is a schematic illustration of the steps of the ablation mechanism involved in laser ablation of the surface of a material. DETAILED DESCRIPTION OF THE INVENTION

[0028] It should be noted that in these figures, structural and / or functional elements that are common to different variants may have the same reference numerals.

[0029] The invention particularly relates to the field of bursts of pulses with durations on the order of femtoseconds (sub-picoseconds) and intra-burst repetition frequencies on the order of GHz.

[0030] The present disclosure proposes a method for determining the operating conditions of a laser ablation method for a given material, comprising the step of determining a set of parameters relating to the repetition rate f of a burst of laser pulses on the order of GHz, consisting of an integer number N of pulses in the burst, and the fluence F of each pulse in the burst. The total fluence FT of the burst seen by the material is equal to the product N·F. The repetition rate C of the burst is another parameter of the laser source used.

[0031] The integer number N of pulses of the burst of laser pulses is at least 10 or more, in particular the number N of pulses is comprised between 10 and 800. In some applications, the number N of femtosecond pulses of the burst is greater than 20 or 50, comprised between 100 and 800, comprised between 200 and 600, or comprised between 300 and 400.

[0032] The duration T of a burst of N pulses at a repetition rate f is defined by T=N / f.

[0033] The pulses of the burst have a repetition frequency f comprised between 100 MHz and 100 GHz, preferably between 1 GHz and 100 GHz, or between 1 GHz and 10 GHz.

[0034] Total energy of the burst E b can be comprised between 1 μJ and 20 mJ depending on the laser used.

[0035] The bursts of pulses have a repetition rate C comprised between 10 kHz and 40 MHz, whether the material is moving or not, and whether different bursts overlap or not. However, the repetition rate is limited by the burst duration T to ensure that the burst duration T is strictly less than the reciprocal of the repetition rate C.

[0036] According to the present disclosure, each pulse of the burst has the same spatial, spectral, and temporal characteristics as the pulses of the burst, but has a fluence F that is less than the material ablation threshold fluence Fs1 by a single laser pulse transmitted as needed and that is not part of a high-rate burst.

[0037] The burst of laser pulses has a wavelength comprised between 200 nm and 3 μm.

[0038] In both figures, a coordinate system XYZ is defined, with the Z axis oriented perpendicular to the surface of the material being ablated and along which the laser beam propagates.

[0039] FIG. 1 is a graph showing an example of a burst containing N pulses, where the integer number N of pulses in the burst is 100, and the pulses in the burst have a repetition frequency f equal to 0.88 GHz. Each pulse in the burst has a duration d that is less than or equal to 550 fs. The burst has an energy E that can vary from 1 μJ to 100 μJ. b In this example, the energy E b is equal to 33 μJ. Each burst of N pulses has a total duration T equal to 114 ns. The bursts are delivered by a laser source at a repetition rate C that can vary from 1 kHz to 200 kHz.

[0040] In another example, the integer number N of pulses in a burst of pulses is 50, and the pulses in said burst have a repetition frequency f equal to 0.88 GHz. Each pulse in the burst has a duration d that is less than or equal to 550 fs. The burst has an energy E that can vary from 1 μJ to 100 μJ. b , with an adjustable repetition frequency C comprised between 1 kHz and 200 kHz, and a total duration T equal to 57 ns.

[0041] In another example, the integer number N of pulses in a burst of pulses is 200, and the pulses in said burst have a repetition frequency f equal to 0.88 GHz. Each pulse in the burst has a duration d that is less than or equal to 550 fs. The burst has an energy E that can vary from 1 μJ to 100 μJ. b , with an adjustable repetition frequency C comprised between 1 kHz and 200 kHz, and a total duration T equal to 228 ns.

[0042] The processing device can be used to control the direction of the bursts onto the sample, either by spatially superimposing the bursts when stationary, or by fully or partially spatially shifting the pulses when the sample or laser beam is moving.

[0043] cm 2Consider a material to be ablated with a given thermal diffusivity D expressed in / s.

[0044] Without being bound by theory, according to the present method, the integer number N of femtosecond pulses in a burst with a repetition rate on the order of GHz is determined by the depth L th is equal to the sum of the integer number Nc of pulses that allows heating and ablation over a range of L th corresponds to the length of the thermal depth associated with the material at the repetition rate f.

[0045] In a first step, the number of heating and ablation pulses Nc is determined, corresponding to the number of femtosecond pulses in a burst of repetition rate f required to heat the material to be ablated over a given test depth L as a starting point for optimization, in order to modify the value of the threshold fluence Fs1 until another threshold fluence Fs2 below the threshold fluence Fs1 is reached. The total fluence FT of the burst is defined such that the fluence per pulse F is less than the threshold fluence Fs1. The number Nc is calculated using the formula Nc=(L 2 In other words, according to the present disclosure, in a burst of femtosecond pulses, the fluence F per pulse is less than a threshold fluence Fs1 and greater than or equal to another threshold fluence Fs2.

[0046] The number Nc corresponds to the number of pulses of the burst that allows heating of the material to be ablated along the axis Z to a depth L. The number Nc represents a lower limit for the number of pulses N for ablation at a repetition rate f of the pulses of the burst near the depth L. In other words, Nc represents the number of pulses at which an ablation crater of a depth close to L appears on the surface of the material.

[0047] The test depth L is the characteristic total fluence value FT of a burst of pulses with repetition rate f char This characteristic total fluence FT charcorresponds to the minimum total burst fluence that allows observing ablation craters on the surface of the material. The characteristic total fluence of a burst of pulses, FT char divided by the number of pulses N is generally different from and less than the fluence Fs1 that characterizes the ablation threshold of a single pulse, i.e., FT char / N <Fs1である。

[0048] In practice, for a given material of known thermal diffusivity D, at a given repetition rate f, and for a target ablation depth L of the material, the number of pulses Nc is calculated as a function of D, f, and L. Once the number of pulses Nc is estimated, the characteristic total fluence FT char can be experimentally determined as the minimum fluence of a burst of Nc pulses that allows the formation of a crater of depth L.

[0049] Then in the second step, the total operating fluence of the burst, FT opt is determined for N=Nc, which corresponds to the optimal ablation efficiency that can optimize the ablation depth or ablation volume while maintaining the ablation efficiency near its maximum. char is estimated to be equal to the threshold fluence for GHz burst ablation. The ablation threshold fluence FT th It is generally accepted that the ablation efficiency can be estimated by knowing the ablation depth Ln(FT / FT th ) and the ablation volume varies with Ln 2 (FT / FT th ) changes with the ratio Ln(F / FT th ) / FT and Ln 2 (FT / FT th ) / FT to estimate the ablation efficiency as a function of the fluence per pulse, F, and FT th Therefore, the total fluence F T of a burst of pulses can be adjusted to have an optimum value close to F T and then saturated. opt is 2 ft char <FTopt <6 FT char is selected in the interval N=Nc.

[0050] Burst Operation Total Fluence FT opt An example of the value of is the characteristic total fluence value FT char The characteristic total fluence FT char can be estimated by multiplying by 2, i.e., FT opt =2FT char is.

[0051] The heating / ablation phase is performed at a characteristic fluence per pulse F close to the ablation threshold fluence Fs2. char This threshold fluence Fs2 is not known and is difficult to measure. In accordance with the present invention, the characteristic fluence per pulse F char is F char =FT char This value is approximated as / Nc, which corresponds to the threshold for obtaining a crater at a pulse rate f in a given material for each pair (FT char , Nc) and remains approximately constant.

[0052] A third optional step of the method consists in determining the number of pulses N for an ablation depth greater than L. FT char When the burst was optimized for a total fluence F equal to ≈F, material was heated at the surface of the crater, and the ablation threshold of a single pulse went from a known value Fs1 for the material to a lower value Fs2 that was unknown and difficult to measure.

[0053] To avoid the phenomenon of saturation caused by increasing the fluence alone and to obtain a larger ablation depth, it is advisable to increase the number of pulses N. For the same fluence per pulse F char =FT char The choice is made to increase the number of pulses N by a value Na while maintaining / Nc, i.e., N=Nc+Na. The characteristic fluence per pulse F char is the sum of all pairs (FTchar ,Nc) remains the same.

[0054] As a result, FT char +Na·F, which is equal to FT char This gives another value for the total fluence FT, which is equal to (1 + Na / Nc). For example, if Na = Nc, then FT = 2 FT char and N=2·Nc. More specifically, when N=Nc=50, the characteristic total fluence FT char =0.96J / cm 2 is obtained for silicon material. By adding Na supplementary pulses, we obtain the number of pulses N equal to 2 Nc or ​​100, and we obtain another characteristic total fluence FT char2 The value of is 2 FT char or 1.9 J / cm 2 is equal to.

[0055] The total fluence FT was re-optimized, and FT char2 The total fluence of the burst is greater than FT opt2 The above procedure is repeated to obtain another value of , resulting in the operating total fluence of the burst, FT opt2 is in the interval: 2 FT char2 <FT opt2 <6 FT char2 This becomes:

[0056] Figure 2 shows the variation of the ablation efficiency (or the volume ablated by the total fluence) at the surface of a silicon material as a function of the total fluence per burst of GHz-frequency pulses, F. In this example, the number of pulses, N, is equal to 50 (round dots) and 100 (square dots), and the repetition frequency, f, is 0.88 GHz.

[0057] In the graph, the efficiency of ablation of the surface of the material is determined by the characteristic total fluence FT at which ablation craters can be observed on the surface of the material. charIt can be seen that the ablation threshold for a value of total fluence FT corresponds to: opt The total fluence of the burst is increased until it reaches the maximum ablation efficiency value corresponding to FT. opt is 2 FT char From 6 ft char The ablation efficiency ranges from 100 to 1200, which corresponds to the optimum ablation efficiency. Once the maximum ablation efficiency is reached, it decreases as the total fluence FT continues to increase. In this case, the ablation depth or volume continues to increase, but the quality decreases due to the increasing amount of energy not required for ablation.

[0058] Here, when N=Nc=50 (round point), it is 0.96J / cm 2 The characteristic total fluence FT is equal to char In contrast, approximately 18 μm 3 A minimum ablation efficiency equal to 1000 J / cm is obtained. A maximum ablation efficiency of approximately 2 J / cm is obtained. 2 ~6J / cm 2 The total fluence of the bursts included in opt In contrast, approximately 25 μm 3 / μJ.

[0059] Here, if N=2Nc=100 (square points), then 2FT char or 1.9 J / cm 2 The characteristic total fluence FT is equal to char2 In contrast, approximately 27 μm 3 A minimum ablation efficiency equal to 1000 J / cm is obtained. A maximum ablation efficiency of approximately 4 J / cm is obtained. 2 ~12J / cm 2 The total fluence of the bursts included in opt In contrast, approximately 34 μm 3 / μJ.

[0060] Fluence per pulse Fchar It has been confirmed that the ablation efficiency is approximately constant. char2 ~6 FT char2 It has also been confirmed to be optimal.

[0061] Figure 3 shows the variation of the ablation efficiency (or the volume ablated by the total fluence) at the surface of a copper material as a function of the total fluence per burst of GHz-frequency pulses, F. In this example, the number of pulses, N, is equal to 100 (round dots) and 200 (square dots), and the repetition frequency, f, is 1.76 GHz.

[0062] In the graph, when N=100 (round dot), it is about 3J / cm 2 The characteristic total fluence FT is equal to char 0.1 mm 3 A minimum ablation efficiency equal to 100 J / min / W is obtained. The maximum ablation efficiency is approximately 6 J / cm 2 ~18J / cm 2 The total fluence of the bursts included in opt 0.5mm 3 / min / W.

[0063] In the graph, when N=200 (square points), it is about 6J / cm 2 The characteristic total fluence FT is equal to char 0.5mm 3 A minimum ablation efficiency of approximately 12 J / cm is obtained, which is equal to 100 J / min / W. 2 ~36J / cm 2 The total fluence of the bursts included in opt 0.7 mm 3 / min / W.

[0064] 4, 5 and 6 show an example of the variation of ablation depth in silicon material as a function of the integer number of pulses N and the total fluence of the burst FT.

[0065] In each example shown in Figures 4, 5 and 6, the repetition frequency C is 100 kHz, the repetition frequency f is 0.88 GHz, the period between two consecutive bursts is 10 μs, the total duration T of the bursts is 57 ns, and the duration d of a pulse in a burst is 550 fs.

[0066] Figure 4 shows that the number of pulses, N, is equal to Nc and the total fluence, FT, of the burst is the characteristic total fluence, FT char Figure 1 shows the ablation profile on the surface of a silicon material when the laser power is less than 100 W. Under these conditions, images obtained by optical profilometry show the formation of bumps on the surface of the material, which is heated but not ablated. In this example, the number of pulses N is 100 and the total fluence FT is 1.7 J / cm. 2 is equal to.

[0067] Figure 5 shows that the number of pulses, N, is equal to Nc and the total fluence, FT, of the burst is the characteristic total fluence, FT char Figure 1 shows the ablation profile at the surface of a silicon material when the pulse width is close to 1.8 J / cm. Under these conditions, the image obtained by optical profilometry shows the formation of a crater at the surface of the material, with a minimum depth. In this example, the number of pulses N is 100 and the total fluence FT is 1.8 J / cm. 2 The ablation depth produced by a burst of 100 pulses is found to be about 2.6 μm.

[0068] Figure 6 shows that the number of pulses, N, is equal to Nc and the total fluence, FT, of the burst is the characteristic total fluence, FT charFigure 1 shows the ablation profile on the surface of a silicon material when the total fluence FT is greater than 1. Under these conditions, the images obtained by optical profilometry show an increase in the ablation depth of the previously formed crater. The crater depth increases with increasing total fluence FT, with optimal efficiency. In this example, the number of pulses N is 100 and the total fluence FT is 5.8 J / cm. 2 It can be seen that the ablation depth produced by a burst of 100 pulses is about 4.5 μm.

[0069] Ablation depth: N>Nc and FT>FT char It can be seen that, for a burst containing 50 pulses, ρ increases as the number of pulses N increases. This technical effect is in no way derived from teachings in the art where femtosecond pulse ablation is highly controversial. The ablation depth decreases from 3.5 μm for a burst containing 50 pulses to 7.5 μm for a burst containing 200 pulses, and the total fluence FT of the burst is 5.8 J / cm in both cases. 2 is.

[0070] One possible interpretation is that the method is based on the coupling of the phenomenon of heat accumulation at the material surface with extremely efficient ablation, which is different from the mechanism involved in ablation by single pulses of μs or ns duration.

[0071] The method can be used to determine the operating conditions for performing laser ablation of the surface of a silicon material with bursts of femtosecond pulses at repetition rates on the order of GHz. Table 1 lists the operating conditions for performing laser ablation of the surface of a silicon material with bursts of femtosecond pulses at repetition rates on the order of GHz in the form of examples indexed from A to G. The thermal diffusivity D of silicon at room temperature is 0.86 cm. 2 Equals / s.

[0072] [Table 1]

[0073] Table 1 lists the operating conditions for laser ablation of silicon materials with bursts of femtosecond pulses at repetition rates on the order of GHz. m FT=FT char and corresponds to the experimental value of the test length L measured at N=Nc.

[0074] The threshold fluence for ablation by a single pulse in silicon, Fs1, is 0.46 J / cm 2 In Examples A-G, the fluence per pulse in the burst is significantly less than the threshold fluence Fs1 for single pulse ablation in silicon material. In other words, the fluence F per femtosecond pulse in the burst is approximately 25 times less than Fs1. The fluence F per femtosecond pulse in the burst is more than an order of magnitude less than the threshold Fs1 ablation fluence for a single pulse.

[0075] [Table 2]

[0076] Table 2 lists the operating conditions for performing laser ablation on copper material with bursts of femtosecond pulses at repetition rates on the order of GHz.

[0077] The threshold fluence for ablation with a single pulse in copper, Fs1, is 1.7 J / cm 2 The thermal diffusivity D of copper at room temperature is 1.15 cm 2 / s. In other words, the fluence per femtosecond pulse of the burst, F, is equal to approximately Fs / 50. Again, the burst fluence per femtosecond pulse, F, is more than an order of magnitude less than the ablation threshold fluence of a single pulse, Fs. The fluence per pulse, F, is one to two orders of magnitude less than the ablation threshold fluence of a single pulse, Fs.

[0078] In all of the examples A-J, the orthogonal dimension of the focused incident beam of pulses relative to the material being ablated is 24 μm. In practice, the orthogonal dimension of the focused incident beam of pulses relative to the material being ablated is less than 200 μm.

[0079] The repetition frequency C of the bursts may be comprised between 1 kHz and 200 kHz. The burst of pulses has, for example, a duration comprised between 1 nanosecond and several hundred nanoseconds. The pulses of the burst have, for example, a duration comprised between 1 femtosecond and less than 1 picosecond. The integer number N of pulses may be comprised between 10 and 400 or more. The value of the fluence F of each pulse of the burst may, for example, be greater than 0.001 J / cm. 2 ~1J / cm 2 The material to be ablated may be selected from a semiconductor material, a metallic material, a dielectric material, a polymeric material, an organic material, or a composite material.

[0080] 7A-7C show the different steps of a method for laser ablation of a material surface by bursts of femtosecond pulses with repetition rates in the order of GHz as a function of the total fluence of the burst of Nc femtosecond pulses.

[0081] Figure 7A shows the surface of a material subjected to a heating and ablation pulse of several Nc. Heating of the surface of the material begins and the total fluence FT increases to the characteristic total fluence FT char In this illustrative example, N=Nc and FT< <FT char is.

[0082] Figure 7B shows the surface of a material subjected to a heating and ablation pulse of several Nc with a total fluence FT higher than that shown in the figure above. The surface of the material is heated over a length close to the test length L, and the total fluence FT is increased to the characteristic total fluence FT char In this illustrative example, N=Nc and F <FT char is.

[0083] Figure 7C shows the surface of a material subjected to a heating and ablation pulse of several Nc. Once the material ablation threshold is reached, ablation of the material becomes possible and the total fluence FT is increased to the characteristic total fluence FT char In this illustrative example, N=Nc and FT=FT char The obtained ablation depth is close to the test length L.

[0084] Figure 8 shows the step of applying a few Na of supplementary laser pulses to the surface of the material with a burst of femtosecond pulses at a repetition rate on the order of GHz. The threshold is then calculated as the total fluence FT = FT char2 The ablation depth is greater than the test depth L.

[0085] The diagram in Figure 8 shows the surface of a material subjected to a number N of pulses equal to Nc+Na. When N equals Nc+Na, the ablation depth increases as a function of Na, and the total fluence FT is another characteristic: char2 , i.e., FT char +Na·FT char / Nc or ​​(Nc + Na) (F char In this illustrative example, N=Nc+Na and FT=FT char2 is.

[0086] The optimal ablation can be achieved by combining the intra-burst repetition rate f in the GHz range, the number of pulses per burst N, and the total pulse energy. In Examples A and B of Table 1, also shown in Figure 2, a burst with a number of pulses N equal to 100 (Example B, square points) achieves a higher ablation efficiency than a burst with a number of pulses N equal to 50 (Example A, round points) at the same intra-burst repetition rate f equal to 0.88 GHz. This example shows that, once the ablation threshold is reached, even with the same intra-burst repetition rate, an increase in the total energy of the burst, and therefore the total fluence of the burst, contributes to a more efficient ablation process. This phenomenon is limited by the saturation of the ablation efficiency at too high an energy, so the maximum fluence depends on the material and can be, for example, 50 J / cm. 2 The order will be:

[0087] A method for determining the operating conditions of a laser ablation method using bursts of femtosecond pulses with a repetition rate of the order of GHz on the surface of a material, where the number of pulses N and the energy E of the burst are b can be controlled.

[0088] The method is defined for a material with a known thermal diffusivity D.

[0089] Table 3 shows examples of values ​​of the thermal diffusivity D for different materials.

[0090] [Table 3]

[0091] The method involves calculating the energy per burst E in order to obtain a fluence F of each pulse of the burst that is less than the ablation threshold fluence Fs1 of the material under consideration by a single pulse. b It is necessary to define

[0092] This method advantageously makes it possible to adjust the ablation depth obtained as a function of the number N of pulses in the burst of a fixed fluence F per pulse.

[0093] This method advantageously allows tuning the ablation depth obtained as a function of the total fluence of the burst for a fixed number N of pulses.

[0094] For each material and for a given ablation depth, it is possible to determine the number of pulses N that corresponds to the ablation threshold as a function of the total fluence of the burst.

[0095] Contrary to expectations in the technical field of material ablation with femtosecond pulses with intra-burst repetition rates f in the GHz range, high ablation efficiencies and good processing qualities can be obtained under precise operating conditions.

[0096] For example, for silicon materials, at a test depth L comprised between 2.2 μm and 4.4 μm, the number of pulses N is comprised between 50 and 200, and the fluence per pulse F characteristic of the GHz ablation threshold is char is approximately 0.018 J / cm 2 The repetition frequency within the burst is between 0.88 GHz and 3.52 GHz.

[0097] For example, for copper materials, at a test depth L comprised between 2.5 μm and 5.2 μm, the number of pulses N is comprised between 100 and 400, and the fluence per pulse F characteristic of the GHz ablation threshold is char is approximately 0.03J / cm 2 and the intra-burst repetition rate is equal to 1.76 GHz.

[0098] The disclosed laser ablation method is particularly applicable to materials such as glass or transparent ceramics. These materials are transparent, and surface absorption occurs over a depth corresponding to the Rayleigh length. In addition, these materials are brittle. This means that cracks in the surface or volume can easily propagate and lead to severe damage. Finally, it is preferable to limit the number of pulses in a burst to the minimum necessary. In such situations, the Nc formula does not apply; only the condition for a minimum number N of pulses in a burst greater than 10 applies. In addition, in these materials, thermal diffusion must be suppressed. One way to suppress thermal diffusion is to use a laser source with a wavelength beyond the transparency range of a given glass substrate, for example, in the UV or deep UV (DUV) range.

[0099] The disclosed laser ablation method is also advantageously applicable to the ablation of glass with low surface roughness. In this case, light is trapped at the irregular surface and diffused below the surface. Surface roughness leads to optical degradation. Surface roughness can be generated using a laser with spatially controlled pulses to achieve these roughnesses. Surface roughness can also be achieved by mechanical means, such as sandblasting. The average roughness requirement is on the order of 400 nm to 10 μm surface roughness ("rms" (root mean square) roughness > 0.25).

[0100] By knowing the conditions for ablation, the present disclosure also allows determining the operating conditions for performing welding between parts of the same material without material loss. Thus, the present disclosure also applies to a laser welding method based on the application of bursts of femtosecond laser pulses with an intra-burst repetition frequency f comprised between several hundred MHz and 100 GHz, the burst of femtosecond laser pulses comprising a number N of pulses, the number N of pulses in the burst of laser pulses being less than the number N of heating and ablation pulses, N being defined by the formula N = (L 2a characteristic total fluence FT, defined by (f) / D, where L represents the test depth and D represents the thermal diffusivity of the material, and a burst of femtosecond laser pulses is adapted to generate an ablation crater in the material. char and each pulse of the burst has a fluence less than the ablation threshold fluence Fs1 of the material by a single laser pulse. In some applications, the number of pulses N in the burst of laser pulses is 10 or more.

Claims

1. A method for determining the operating conditions of a method for high repetition rate femtosecond laser ablation in a given material of known thermal diffusivity D, comprising a first step of determining a set of parameters of a burst of laser pulses adapted to generate an ablation crater in said material, said set of parameters being an intra-burst repetition frequency f comprised between several hundreds of MHz and 100 GHz, a number of pulses N of said burst of laser pulses equal to or greater than 10, a characteristic total fluence FT of said burst of pulses ｃｈａｒ and a characteristic fluence per pulse, the characteristic fluence per pulse being less than an ablation threshold fluence Fs1 of the material by a single laser pulse; This first step is the characteristic total fluence FT of the burst at the number N of heating and ablation pulses ｃｈａｒ FT ｃｈａｒ is the minimum total fluence to obtain a crater on the surface of the material, At a given repetition frequency f and for a target depth of ablation of material L, the number of heating and ablation pulses Nc is calculated and defined as a function of D, f, and L by the formula Nc=(L2·f) / D, where D represents the thermal diffusion coefficient of the material being ablated, and when Nc is 10 or more and the number of heating and ablation pulses Nc is estimated, a characteristic total fluence FT ｃｈａｒ is experimentally determined as the minimum fluence of a burst of Nc pulses that enables the formation of a crater of depth L, where the number N of pulses in the burst of laser pulses is greater than or equal to the number Nc of heating and ablation pulses, and the characteristic fluence per pulse is equal to F char =FT char / Nc.

2. Operating total fluence FT in a burst of laser pulses containing Nc laser pulses ｏｐｔ The method of claim 1 , further comprising a second step of determining:

3. The operating total fluence FT ｏｐｔ However, the characteristic total fluence FT ｃｈａｒ Twice the characteristic total fluence FT ｃｈａｒ The method of claim 2 , wherein the number of times ...

4. a third step of determining a number N of pulses in the burst to ablate the material to a depth greater than the test depth L, where N is greater than Nc and the fluence per pulse in the burst is greater than the characteristic total fluence FT ｃｈａｒ The method according to any one of claims 1 to 3, wherein the number of heating and ablation pulses is equal to Nc divided by the number of heating and ablation pulses.

5. Another characteristic: Fluence FT ｃｈａｒ２ is calculated as a function of the number N of pulses in the burst, which is different from Nc, where N=Nc+Na, Na being a positive or negative integer; FT ｃｈａｒ２ =FT ｃｈａｒ +Na.FT ｃｈａｒ The method according to any one of claims 1 to 4, wherein Nc is 0.1 - 0.

25.

6. A method for laser ablation of a determined material, comprising application of bursts of femtosecond laser pulses with an intra-burst repetition frequency f comprised between several hundreds of MHz and 100 GHz, said bursts of femtosecond laser pulses comprising a number N of pulses equal to or greater than 10, said bursts of femtosecond laser pulses having a characteristic total fluence FT adapted to generate an ablation crater in said material. ｃｈａｒ and each pulse of the burst has a fluence F less than or equal to a threshold fluence Fs1 for ablation of the material by a single laser pulse; Characteristic total fluence FT of a burst for Nc heating and ablation pulses ｃｈａｒ The value of this characteristic total fluence FT ｃｈａｒ is the minimum total fluence that allows the formation of a crater on the surface of a given material of known thermal diffusivity D, and at a given repetition frequency f, for a target depth of ablation of the material L, the number of heating and ablation pulses Nc is calculated as a function of D, f, and L. Once the number of heating and ablation pulses Nc is estimated, the characteristic total fluence FT ｃｈａｒ is experimentally determined as the minimum fluence of a burst of Nc pulses that enables the formation of a crater of depth L, where the number N of pulses in the burst of laser pulses is greater than or equal to the number Nc of heating and ablation pulses, Nc being defined by the formula Nc = (L2 · f) / D, where L represents the ablation depth target of the material, D represents the thermal diffusion coefficient of the material being ablated, Nc is greater than or equal to 10, and the characteristic fluence per pulse is equal to Fchar = FTchar / Nc.

7. 7. The laser ablation method of claim 6, wherein said number N of pulses in said burst is comprised between 100 and 10,000.

8. 8. The laser ablation method according to claim 6, wherein each pulse of the burst has a fluence F that is at least one order of magnitude smaller than the ablation threshold fluence Fs1 of the material by a single laser pulse.

9. 9. The laser ablation method of claim 8, wherein each pulse of the burst has a fluence F that is one to two orders of magnitude less than the threshold fluence Fs1 for ablation by a single laser pulse.

10. A method according to any one of claims 6 to 9, wherein the orthogonal dimension of the incident beam of laser pulses on the material to be ablated is 200µm or less.

11. A laser ablation method according to any one of claims 6 to 10, wherein the burst has a total energy comprised between 1 μJ and 20 mJ.

12. A laser ablation method according to any one of claims 6 to 11, comprising applying multiple bursts at a repetition rate comprised between 10 kHz and 40 MHz.

13. A method of laser welding between parts of a determined material, comprising the application of bursts of femtosecond laser pulses with an intra-burst repetition frequency f comprised between several hundreds of MHz and 100 GHz, said bursts of femtosecond laser pulses comprising a number N of pulses, said number N of pulses of said bursts of laser pulses being less than a number Nc of pulses allowing the formation of an ablation crater of depth L, Nc being defined by the formula Nc = (L ２ a characteristic total fluence FT defined by L / D, where L represents the depth of the material and D represents the thermal diffusivity of the material, the burst of femtosecond laser pulses being adapted to generate an ablation crater in the material; ｃｈａｒ each pulse of the burst having a fluence F less than a threshold fluence Fs1 for ablation of the material by a single laser pulse; The characteristic total fluence FT of the burst for the Nc pulses that enables the formation of an ablation crater of depth L ｃｈａｒ The value of is experimentally determined, This characteristic Total Fluence FT ｃｈａｒ is the minimum total fluence that allows the formation of a crater on the surface of a given material of known thermal diffusivity D, and at a given repetition frequency f, for a target ablation depth L of the material, the number of pulses Nc is calculated as a function of D, f, and L. Once the number of pulses Nc is estimated, the characteristic total fluence FT ｃｈａｒ is experimentally determined as the minimum fluence of a burst of Nc pulses that allows the formation of a crater of depth L.

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