JP2015502258A - Method and apparatus for optimally laser marking an object - Google Patents

Abstract

The present invention relates to laser marking on an object. In particular, the present invention exposes the underlying surface of the object by laser ablating the coating applied to the object, thereby contrasting the appearance between the exposed surface of the object and the adjacent residual coating. It is related with the laser marking to the target object which forms a mark by this. The laser parameters are selected so that they have a uniform and commercially desirable appearance, maintain system throughput in an acceptable range, and do not damage the underlying surface. In particular, the laser pulse envelope is adjusted to give the desired appearance while maintaining system throughput in an acceptable range.

Description

  The present invention relates to laser marking on an object. In particular, the present invention exposes the underlying surface of the object by laser ablating the coating applied to the object, thereby contrasting the appearance between the exposed surface of the object and the adjacent residual coating. It is related with the laser marking to the target object which forms a mark by this. In addition, the first coating or the uppermost coating layer is laser ablated to expose the base of the second coating layer, and a contrast is provided between the exposed second coating and the adjacent first coating. This mark may be formed by forming a mark. The laser parameters are selected so that they have a uniform and commercially desirable appearance, maintain system throughput in an acceptable range, and do not damage the underlying surface. In particular, the laser pulse envelope is adjusted to give the desired appearance while maintaining system throughput in an acceptable range.

Background of the Invention

  Products that are sold generally require some kind of marking on the product for commercial, regulatory, decorative, or functional purposes. A mark refers to a continuous region or range on the surface of an object that is visually contrasted with an adjacent surface. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably have a selected shape, uniform color, and optical density for a mark. Durability refers to the property of remaining unchanged despite wear on the marked surface. Ease of application refers to the time and resources to generate a mark, including the cost of the material and being programmable. Programmable means that the marking device can be programmed with a new pattern to be marked by changing the software rather than changing the hardware such as the screen or mask.

  Of particular concern is the creation of marks on coated or painted objects. Objects made of metal or various types of plastics are often painted or coated with various industrial coatings to protect or change the appearance of the surface of the object. Laser ablating the coating, particularly the pattern, to remove the coating and expose the underlying surface of the object is a desirable method for marking the object. Covering the object with two or more coatings and laser ablating the first coating to expose the second coating substrate is another desirable method for marking. Marking the product by using a laser to remove the coating and expose the substrate of the object is disclosed in US Patent Application Publication No. 2008/0152859 by inventor Shogo Nagai, published June 26, 2008. It is stated in. This method uses a coating brightness that is brighter than the surface of the object. Japanese Patent Application No. 03-150842 by inventor Noboru Iwasaki, published on October 29, 1992, describes using a laser to remove one or more coating layers and exposing the underlying layer of the coating layer.

  Common to these documents is that in order to remove the coating without removing the material underlying the removed coating layer, the laser ablation threshold of the material to be removed is set to the laser ablation threshold of the underlying material. It must be lower than that. The laser ablation threshold is the minimum energy required for material removal. This removal may be ablation in which sufficient energy is injected into the material by the laser and the material dissociates into a plasma, or a thermal one that the material essentially melts and evaporates. Or a combination of the two. What is related to the ablation threshold is the damage threshold. The damage threshold is the minimum laser energy required to cause an undesirable change in material appearance. Material damage thresholds are generally lower than ablation thresholds and sometimes much lower than ablation thresholds. Damage is defined as an undesirable change in the appearance of the material comprising the underlying coating after laser removal of the object or top layer.

  In order to cleanly and completely remove the top layer of material without damaging the surface of the object, the laser fluence is greater than the ablation threshold of the overlying material and the surface of the underlying material or object. Must be less than the ablation threshold or damage threshold. In many cases this difference is small and the laser fluence needs to be accurately controlled. In addition, the actual values of the ablation threshold and damage threshold for these materials may depend on the location and slight changes in the application. Also, the actual values of ablation threshold and damage threshold for these materials can vary as a function of time. As the object is processed, the heat generated by the laser removal process may be retained by the object or coating and affect the ablation threshold or damage threshold. Thus, a single laser fluence can be selected that allows a commercially desirable marking on a particular object at a particular time using a particular laser, but there is a slight change in the object or coating or laser. If so, the process will not produce the desired results. The difference between the minimum fluence that causes ablation of the desired material and the minimum fluence that does not damage the underlying material is called the process window. When manufacturing, to accommodate changes in materials and processing systems without the need for system adjustments that adversely affect system throughput, the process window for a particular object can be increased to a practically maximum. It is highly desirable to mark

  What is desired but not disclosed in the prior art is a method that allows material to be removed reliably and repeatably without leaving undesired material on the surface of the object and without the need for frequent adjustment of the device. is there. And what is needed is to use a laser to remove the coating layer without leaving an undesirable material in the underlying material while maintaining system throughput in an acceptable range, and onto the coated object. A method for reliably and repeatably generating marks having a desired appearance.

Embodiments of the present invention use a laser marking system on a coated sample to produce marks having desired properties. The laser marking system includes a controllable beam attenuator that propagates a laser beam at a first attenuation level and changes the attenuation to a second attenuation level after a predetermined time interval, thereby providing desired characteristics. Is generated. Laser fluence can be controlled by adding a controllable beam attenuator. Fluence is defined as the cumulative laser energy irradiated per unit area and is measured in units of joules / cm ² . Aspects of the invention provide a first laser fluence associated with generating a mark having a desired characteristic in a first portion of the mark. An aspect of the present invention then provides a second laser fluence associated with generating a mark having a desired characteristic in a second portion of the mark, thereby generating a mark having the desired characteristic.

  To form a mark on a coated object by ablating the top coating layer to expose another coating or surface of the object, the ablation threshold for the material to be ablated is Must be below the ablation threshold for. In many cases this is possible by appropriate selection of materials. For example, the top coating or paint that is darker or less reflective than the underlying layer absorbs more laser energy than the underlying layer and is typically ablated at a lower fluence threshold.

  Aspects of the present invention take into account the damage threshold at the time of marking in order to generate a mark that exhibits the desired appearance. In order to generate marks efficiently, the irradiation density is adjusted to maximize the material removal rate without damaging the underlying material. Irradiation density is a measure of the fraction of energy irradiated on the surface of the object, and the tool path indicates the time that the laser beam is directed to each point on the mark, so combine the irradiation density with the tool path. This determines the fluence. The laser beam illumination density and tool path are greater than the ablation threshold of the material to be removed and smaller than the damage threshold of the underlying material, while maximizing the speed of movement of the laser beam relative to the object to maximize throughput. Is calculated as follows. The difficulty is that these thresholds can be different for different areas of the mark at different times during the marking process. Laser parameters that provide a commercially desirable appearance and an acceptable material removal rate, and thus an acceptable throughput in certain areas of the mark, can damage the underlying material in other areas of the mark. FIG. 2 shows the result of laser marking an object with a single illumination density and moving speed, the result being non-uniform and commercially unacceptable. While it may be possible to select a set of laser parameters that produce a mark with a commercially desirable appearance, the resulting material removal rate is the fastest of the acceptable removal rates for all parts of the mark Do not exceed that, which will unacceptably reduce throughput. Aspects of the present invention increase the laser fluence at the start of each set of laser pulses to remove material that would typically remain if a single laser fluence was used.

  The damage threshold for a material at a particular location is not only dependent on the irradiation density of the laser currently directed at that location, but also on the recent history of exposure to laser radiation. Therefore, the appearance of the material after laser processing cannot be appropriately predicted simply by measuring the laser fluence. This is because previous irradiation at or near that location tends to heat the material. Based on the calculated residual heating, aspects of the present invention compensate for the damage threshold reduced by previous laser irradiation by changing the laser fluence and increasing the fluence of the initial laser pulses in the group of laser pulses.

  Aspects of the present invention provide laser pulse parameters such as pulse duration and pulse replenishment speed or spot size or laser beam position, or to increase laser marking system throughput while avoiding damage to the underlying material, or Control various laser parameters including tool path parameters such as laser beam velocity. The laser is selected to achieve the desired material removal rate, and the power, repetition rate, pulse time shape and pulse duration are selected. Then, the tool path, that is, the position where the laser irradiates the object to form the mark and the number of times are calculated so as to obtain a desired material removal speed while avoiding damage to the underlying material. An example of a tool path calculation is the spacing between subsequent pulses on the object that is controlled by changing the speed of relative motion between the laser pulse and the object. Another example of a tool path calculation is a spot size that controls the illumination density by moving the focal spot in the Z axis to a point above or below the surface of the object. A further example of calculating the tool path calculates the spacing between adjacent columns of pulse positions. A tool path is selected that covers the area to be marked in the line moved by the raster technique.

  Aspects of the present invention control the laser output. In order for the tool path to be easily selected by the present invention, it is necessary to attenuate the laser pulse fluence very accurately under the control of the laser marking system. Aspects of the present invention control the laser illumination density accurately enough to obtain a tool path that produces marks having commercially desirable uniformity, color, texture, and shape. An optical switch is used to switch the laser beam on and off at high speed without turning the laser on and off. Aspects of the present invention use an acousto-optic modulator (AOM) to accurately and rapidly modulate a beam so that the beam is directed to an object or propagates to a beam dump without adverse effects. Orient.

  Embodiments of the present invention are realized by modifying an existing laser micromachining system, the ESI model ML5900 laser micromachining system manufactured by Electro Scientific Industries, Portland, Oregon, 97229, USA. This system is described in detail in “ESI Service Guide ML5900” No. 178472A, published in October 2009, by Electro Scientific Industries, Portland, Oregon, 97229, which is hereby incorporated by reference in its entirety. Included in the description. Improvements include adding electro-optic devices to control fluence changes and to allow more precise control of laser fluence in real time.

  Objects that are coated with visible marks having a desired commercial quality to accomplish the above-described matters in accordance with these and other aspects specifically and broadly described herein for purposes of the present invention. Disclosed herein is a method of generating above and an apparatus adapted to perform the method. Included is a laser processing system having a laser operatively connected to a controller having stored predetermined laser pulse parameters, a laser optical system, and a motion stage. The stored laser pulse parameters are associated with a desired fluence that is selected depending on the area of the mark being processed to produce a mark having commercially desirable characteristics.

FIG. 1 shows the tool path. FIG. 2 shows a prior art mark. FIG. 3 is a photomicrograph of a prior art laser mark. FIG. 4a shows a prior art laser pulse. FIG. 4b shows the laser pulse. FIG. 4c shows the laser pulse. FIG. 4d shows the laser pulse. FIG. 5 shows a laser mark. FIG. 6 is a photomicrograph of the laser mark. FIG. 7 shows an improved laser marking system. FIG. 8 shows a pulse circuit. FIG. 9 shows a pulse circuit.

Detailed Description of the Preferred Embodiment

  Embodiments of the present invention use a laser marking system on a coated sample to generate marks having desired properties. The laser marking system has a controllable laser fluence or dose. Embodiments of the present invention determine a first laser fluence associated with generating a mark having a desired characteristic in a first portion of the mark. An aspect of the invention then determines a second laser fluence associated with generating a mark having a desired characteristic in the second portion of the mark. These fluences are stored in the storage device of the laser marking system. The object is then marked using the stored first laser fluence for the first part of the mark and using the stored second user fluence for the second part of the mark. The laser marking system is instructed to mark the object with the desired characteristics. Embodiments of the present invention provide laser pulse parameters such as pulse duration and pulse replenishment rate and spot size and laser beam position to increase the throughput of the laser marking system while avoiding damage to the underlying material, Alternatively, the laser fluence is controlled by controlling various laser parameters including tool path parameters such as laser beam velocity. Typically, the laser is selected to achieve the desired material removal rate, and the power, repetition rate, pulse time shape, and pulse duration are selected. Then, a tool path is calculated so that a desired material removal rate can be obtained while avoiding damage to the underlying material. In particular, embodiments of the present invention increase the laser pulse fluence for the first few laser pulses in a set of laser pulses in order to remove material that would otherwise remain on the surface of the object.

  Embodiments of the present invention control the output of the laser. In order for the tool path to be easily selected by the present invention, it is necessary to attenuate the laser pulse very accurately under the control of the laser marking system. Aspects of the present invention control the laser illumination density accurately enough to obtain a tool path that produces marks having commercially desirable uniformity, color, texture, and shape. Aspects of the invention use an acousto-optic modulator (AOM) to modulate the beam accurately and quickly without turning the laser on and off. Embodiments of the present invention use AOM to direct the laser beam from the normal path by diffraction into a beam dump that is emitted without adversely affecting the laser beam energy to the surface of the object. Modulating the fluence of the laser beam by reattaching. AOM is used because it can modulate the laser beam very quickly. High speed modulation is advantageous for embodiments of the present invention because the laser marking system allows the laser beam to be turned on and off quickly and accurately without disturbing the laser itself. .

FIG. 1 shows the tool path used to remove the coating from the object. The object 30 is covered with a coating 31, and the coating 31 in the shaping region 32 is removed. The laser begins to remove material from the starting point 34 along the tool path 33 and removes material as it travels to point 36 as represented by the solid line. At point 36, the laser is turned off and the laser beam is repositioned relative to the object as represented by the dotted line so that it begins to remove material at point 38 when the laser is turned on. The laser continues to cut and reposition until reaching the end point 39. FIG. 2 shows the result of removing the material as shown in FIG. In the object 40 having the coating 42, the coating is removed in the region of the mark 44 where the material base 46 is exposed. Note that a portion of the coating being coated remains at 48. This is because the laser irradiation density used is optimal to remove a lot of material in many parts of the marked area, but the material is removed in the first stage of the removal process. It is because it is thought that it is not so effective. The irradiation density is a ratio per unit area of the laser energy irradiated on the surface of the object, and is measured in units of watts / cm ² . One prior art solution to this problem is adjusting the fluence and the speed of the laser relative to the object, which may reduce throughput, which is undesirable. The need for adjustment itself is also undesirable because it adversely affects system throughput. FIG. 3 is a photomicrograph showing this effect. In FIG. 3, the object 50 is covered with a silver coating covered with a black paint layer 51. A laser is used to remove the black paint in region 52. Note that some black paint residue 54 remains in areas 52 where the black paint should have been removed by the laser.

  FIG. 4a is a graph of pulse energy against time, showing a group of laser pulses in the prior art. Referring to FIG. 1, the laser pulse begins at 62 where the laser beam is located at 34, and ends at 64 when the laser beam reaches the end of this stroke 36. The laser beam is repositioned at 38 and the laser pulse starts again at 66. With this pulse group profile, material removal is incomplete. There is also a pulse fluence that removes material 51 without leaving debris 54, but the laser fluence used to form the mark of FIG. 4a is the specific object at that particular point in time using that particular laser. It is not in the process window of the object. Embodiments of the present invention solve this problem by adjusting the pulse group profile to provide more energy at the beginning of a stroke while maintaining a specific energy for the rest of the stroke. If an adjustment pulse profile is formed by adjusting the pulse group envelope in this way, a high energy pulse is used at the beginning of the pulse group before changing to a preselected low energy pulse and ending the pulse group. This enables commercially desirable markings while maintaining system throughput. This tailored pulse profile creates a wider process window by allowing the use of low laser fluence without leaving debris. A wider process window reduces the need for system adjustment and increases system throughput. An exemplary adjustment pulse profile is shown in FIG. 4b. In FIG. 4b, two groups of adjustment pulses 70 are shown. The pulse group 70 starts at time 74 with three pulses 72 of power P2. At time 76, the power is reduced to P2, and more five pulses 78 are applied to the workpiece until time 80. Each group of pulses is adjusted in this manner, and a predetermined number of pulses having a predetermined energy are irradiated at the beginning of each step of the laser beam.

  FIG. 5 shows an object 100 coated with a coating 102 where the shape boundary 104 has been laser machined with a group of conditioning pulses in an embodiment of the present invention to remove the coating 106 within the boundary 194. Note that no apparent debris remains and no damage has occurred anywhere on the mark. FIG. 6 is a photomicrograph of a similar coating object 110 coated with a black paint 112, which is laser processed according to an embodiment of the present invention to cleanly and quickly remove the black paint 112 in a desired region, thereby producing a silver paint. The base 114 is exposed. Note that there is no debris or damage on the ground surface in appearance.

  FIG. 7 shows a diagram of an improved ESI model ML5900 laser micromachining system 120 adapted to mark an object as an embodiment of the present invention. This example application includes a laser 122 and an AOM fluence attenuator 124. A laser pulse is emitted by the laser 122 and directed to the AOM fluence attenuator 124 by a series of mirrors and other optical elements (not shown) and then the optical head by another series of mirrors and optical elements (not shown). 128. The AOM fluence attenuator includes control electronics that control the fluence of the laser pulses that are propagated under the direction of the controller 140. The optical head includes X, Y, and Z motion control elements 130 and a galvanometer block 132. These elements are combined to position a laser beam (not shown) with respect to the object to be marked 138 to generate a two-dimensional display of the mark on the surface of the object 138. The object 138 is attached to a rotary stage element 134 that aligns the object 138 from the load / unload position to below the optical head 138. In the optical head 138, the object 138 is marked and then the object is sent to the optical inspection station 136 which inspects the object 138 before alignment to return to the load / unload station for unloading. The alignment of the object 138 is performed. All of these operations are performed under the control of the controller 140. Controller 140 adjusts the operation of laser 122, AOM fluence attenuator 124, motion control element 130, galvanometer block 132, and rotary stage 134 to irradiate the appropriate laser fluence to the appropriate location on object 136. A mark having a particularly desirable appearance is generated.

The improved laser 122 is a diode-pumped Nd: YVO ₄ solid state laser operating at a triple frequency of 355 nm wavelength, model Vanguard manufactured by Spectra-Physics, Santa Clara, Calif., USA. The laser 122 is configured to generate up to 2.5 W, but typically operates at an 80 MHz mode-locked pulse repetition rate that produces about 1 W of power. In embodiments of the present invention, lasers having a power of 0.5 watts to 100 watts, more preferably 0.5 watts to 12 watts, can be advantageously used. A laser repetition rate of 10 KHz to 500 MHz, more preferably 1 MHz to 100 MHz can be used. Laser 122 cooperates with controller 100 to generate a laser pulse having a duration of about 1 picosecond to 1,000 nanoseconds, more preferably 100 picoseconds to 100 nanoseconds. The temporal and spatial distribution of pulses is typically Gaussian. Combining the motion control element 130 and the galvanometer block 132 enables beam positioning relative to the object. In an embodiment of the present invention, a laser spot is used whose size measured on the object is in the range of 5 microns to 500 microns, more preferably in the range of 10 microns to 100 microns. The beam velocity used in this system, i.e. the relative movement between the laser beam and the object, is in the range of 10 mm / s to 1 m / s, more preferably in the range of 50 mm / s to 500 mm / s. The pitch, ie the spacing between adjacent lines of laser pulses, can be in the range of 1 micron to 250 microns, more preferably in the range of 10 microns to 50 microns.

  FIG. 8 illustrates electronic circuitry used to cause the laser processing system to operate in accordance with an embodiment of the present invention. FIG. 8 shows an input signal 150 from the controller 140 indicating the start of the laser beam stroke. This signal is typically a logic signal that indicates TRUE or FALSE at the voltage level. If the input signal 150 indicates TRUE, that is, at the start of the stroke, this signal is sent to the AOM controller 162 to trigger the input “T”. This indicates that the AOM is in an active state and must be energized to propagate the laser pulse. The AOM controller 162 also has an analog voltage input “A” that causes the AOM controller 162 to send a signal 164 to the AOM (not shown) to provide laser energy proportional to the voltage produced at “A”. Propagate. A trigger signal 150 is also sent to the pulse circuit 154, which generates a pulse having a programmable duration. This pulse is sent to amplifier 156, which amplifies the pulse generated by pulse circuit 154 to a programmable voltage level. This amplified pulse is adjusted by the signal conditioning filter 158 to remove unwanted components, and is then output by the adder circuit 160 by the controller 140 and sent to the “A” input of the AOM controller 162. Combined with voltage 152 is the final output 164 to an AOM (not shown).

  4b and 4c show a group of adjusted laser pulses output from the laser controlled by the AOM control circuit as shown in FIG. In FIG. 4b, the adjustment pulse group 70 has a pulse 72 having energy P1 at the beginning of the group and a pulse 78 having energy P2 as the rest of the group. FIG. 4c shows the set of adjustment pulses that occur when the analog input 152 to the circuit in FIG. 8 drops to a level that produces a laser pulse of energy P3. Note that in this embodiment, the energy level of the laser pulse at the beginning of the stroke is constant energy due to the programmed setting of amplifier 156.

  FIG. 9 illustrates additional electronic circuitry used to cause the laser processing system to operate in accordance with embodiments of the present invention. In this circuit, the trigger input 170 from the controller 140 is transmitted to the trigger input “T” of the AOM controller 182 and to a pulse circuit 172 that generates a pulse having a programmable duration from the start of the trigger signal. The This pulse is sent to an analog switch 174 that gates the analog signal 172 from the controller 140 amplified by the amplifier 176 with a programmable gain. The analog switch 174 transmits an analog signal from the 174 when a pulse from the pulse circuit 172 exists. The signal is then sent to a signal conditioning filter 178 that removes unwanted components from the signal, combining the original analog voltage control signal 172 from the controller 140 with the amplified signal from the signal conditioning circuit 178, Sent to a combiner 180 that generates an analog control signal that is sent to the AOM controller 182 to obtain an output 184 to an AOM (not shown). This circuit generates an adjustment pulse as shown in FIG. 4d. FIG. 4d shows an adjustment group of pulses 90 that makes the first high energy pulse 92 of each group 90 a predetermined multiple P4 of the energy P3 of the predetermined pulse 94.

  It will be appreciated by those skilled in the art that many changes can be made to the details of the above-described embodiments without departing from the principles underlying the invention. Accordingly, the scope of the invention should be defined only by the claims.

Claims (11)

A method of generating a laser mark having a desired property on a sample using a laser marking system having a pulsed laser beam, comprising:
Providing the laser marking system with a controllable beam attenuator;
Instructs the controllable beam attenuator to propagate the laser beam at a first attenuation level, and after a predetermined time interval, changes the attenuation to a second attenuation level, thereby providing a desired characteristic. Producing a laser mark having,
Method.   The sample is coated with an application coating of a first layer and a second layer, and the desired property of the mark removes the first layer without damaging the second layer. The method of claim 1 comprising: The method of claim 1, wherein the laser fluence is in the range of 1.0 × 10 ⁻⁶ Joules / cm ^{2 to} 1.0 Joules / cm ² .   The method of claim 4, wherein the controllable beam attenuator is an acousto-optic modulator. An improved laser marking apparatus for marking a sample with a laser beam and having desired characteristics, the improvement comprising:
Providing the laser marking device with a controllable beam attenuator capable of attenuating the laser beam at a first attenuation level and changing the attenuation to a second attenuation level after a predetermined time interval. Improved laser marking device.   The sample is coated with an application coating of a first layer and a second layer, and the desired property of the mark removes the first layer without damaging the second layer. 6. The apparatus of claim 5, comprising: The apparatus of claim 5, wherein the pulse fluence is in the range of 1.0 × 10 ⁻⁶ Joules / cm ^{2 to} 1.0 Joules / cm ² .   The apparatus of claim 5, wherein the controllable beam attenuator is an acousto-optic modulator. A mark having a desired characteristic generated on a sample by a laser marking system having a pulsed laser beam and a controllable beam attenuator,
A first portion of the mark marked at a first attenuation level by the laser marking system;
A second portion of the mark marked at a second attenuation level by the laser marking system;
With a mark.   The sample is coated with an application coating of a first layer and a second layer, and the desired property of the mark removes the first layer without damaging the second layer. The mark of claim 9, comprising:   The method of claim 4, wherein the controllable beam attenuator is an acousto-optic modulator.

Images