Patent Publication Number: US-2020298344-A1

Title: Laser processing apparatus and laser processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Applications No. 2019-55711 filed on Mar. 22, 2019, and No. 2019-212890 filed on Nov. 26, 2019, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a laser processing apparatus and a laser processing method suitable for a case where punching (or hole making) is carried out at multiple positions of a substrate including layers whose energy required for processing is different from each other by using laser. For example, the substrate is a substrate in which a metal layer such as a copper layer is provided on a surface thereof and a resin layer is laminated beneath the copper layer. 
     BACKGROUND OF THE INVENTION 
       FIG. 8  illustrates an example of a substrate including layers whose energy required for processing is different from each other, and is a sectional view of a substrate in which a copper layer  81  is provided on a substrate surface thereof and a resin layer  82  is laminated beneath the copper layer  81 . A reference numeral  84  indicates a copper layer laminated beneath the resin layer  82 . Energy of the copper layer required for the processing is greater than that of the resin layer. 
     As a method of processing the copper layer  81  and the resin layer  82  at multiple positions of this substrate  1  by using laser to make blind holes  83 , there are conventionally methods as follows.  FIG. 7  is a timing chart for explaining processing progress thereof. 
     Namely, a UV laser whose absorption ratio with respect to copper is well is used as the laser, and processing for the copper layer  81  is mainly carried out at one hole position as a first stage. In this processing, energy density of a laser pulse is heightened by making a diameter of the laser beam smaller. For example, a mechanical laser deflection mechanism (not illustrated in the drawings) such as a galvano scanner successively irradiates laser to multiple sites along a predetermined volute (or spiral) track. Hereinafter, processing of successively irradiation to the multiple sites along the predetermined track of one hole position is referred to as “trepanning processing”.  FIG. 8B  illustrates a state after the trepanning processing at the first stage. 
     Note that in the trepanning processing at the first stage, a side of the resin layer  82  to the copper layer  81  is normally processed somewhat. 
     The laser deflection mechanism described above or a similar mechanical table drive mechanism operates for the copper layer  81  of each of the other hole positions to carry out the similar trepanning processing. After this is terminated, by mechanically varying an interval with a laser irradiation mechanism (not illustrated in the drawings) on which a substrate, the laser deflection mechanism, and the like are mounted to change a focal position, a diameter of laser beam is made larger. This causes energy density of the laser pulse to become lower. 
     The laser deflection mechanism described above or the table drive mechanism then operates to irradiate the laser pulse to the trepanning processed hole position once or multiple times as a second stage. Hereinafter, processing of irradiation to one hole position once or multiple times is referred to as “punching processing”. 
     In this punching processing at the second stage, processing for the resin layer  82  is mainly carried out.  FIG. 8C  illustrates a state after the punching processing at the second stage. 
     Each of the blind holes  83  has a depth up to this side of the copper layer  84 . The energy density of the laser pulse in the punching processing at the second stage is adjusted so as not to damage the copper layer  84  as much as possible. A driving operation of the laser deflection mechanism described above or the similar mechanical table drive mechanism is executed to carryout the similar punching processing for each of the other hole positions. The punching operation is then terminated. 
     In the conventional technology as described above, in the trepanning processing for the copper layer  81  at the first stage, the mechanical laser deflection mechanism irradiates the laser pulse to the multiple sites along the predetermined spiral track at the one hole position. Thus, it takes much time. In addition, in the processing for the resin layer  82  at the second stage, the punching processing is carried out at the same processing position as that at the first stage. Thus, the driving operation of the mechanical laser deflection mechanism or the table drive mechanism is required again. Therefore, there is a defect that a processing time thereof becomes longer. 
     Paragraphs 0037 to 0038 of International Publication NO. WO2016/185614 (hereinafter, referred to as “Patent Document 1”) discloses a conventional technology as described above, wherein in a case where blind holes are made in a substrate in which a copper layer is provided on a substrate surface thereof and a resin layer is laminated beneath the copper layer, trepanning processing for the copper layer at a first stage and punching processing for the resin layer at a second stage are carried out. 
     Further, Paragraphs 0004 to 0005 of Japanese Patent Application Laid-Open Publication No. 2003-48088 (hereinafter, referred to as “Patent Document 2”) discloses a technique wherein in a case where blind holes are made in a substrate in which a conductor layer is provided on a substrate surface thereof and an insulating layer is laminated beneath the conductor layer, only trepanning processing is carried out by using a mechanical laser deflection mechanism such as a galvano scanner. 
     In this technique of Patent Document 2, there is a defect that a processing time becomes longer because of the trepanning processing by the mechanical laser deflection operation. Moreover, Paragraph 0007 describes that irradiation energy is lowered in a case where the insulating layer is to be processed. However, Patent Document 2 does not disclose how to specifically lower the irradiation energy. 
     Further, Paragraphs 0044 to 0049 of Japanese Patent Application Laid-Open Publication No. 2003-136270 (hereinafter, referred to as “Patent Document 3”) discloses a technique wherein as a system to carry out trepanning processing at a high speed, an acousto-optic device (hereinafter, abbreviated as “AOD”) capable of a high-speed deflection operation is arranged at a former stage of a mechanical laser deflection mechanism such as a galvano scanner, and a deflection operation for the AOD is controlled to successively irradiate a laser pulse to multiple sites along a predetermined track. 
     With respect to this technology described in Patent Document 3, Patent Document 3 does not make reference to how energy for processing each layer is adjusted when a substrate including layers whose energy required for processing is different from each other is processed. Therefore, processing quality cannot be ensured under the circumstances. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to facilitate controls when punching against multiple positions of a substrate including layers whose energy required for processing is different from each other is carried out by trepanning processing using laser, whereby a processing time thereof is shortened and processing quality thereof is ensured. 
     In order to solve the problems described above, a representative laser processing apparatus according to the present invention disclosed in the present application includes: a laser oscillator configured to oscillate a laser pulse; a first laser deflection unit configured to deflect the laser pulse emitted from the laser oscillator in a two-dimensional direction; a second laser deflection unit configured to deflect the laser pulse emitted from the first laser deflection unit in a two-dimensional direction on a same plane as that of the two-dimensional direction of the first laser deflection unit, an operation speed of the second laser deflection unit being slower than that of the first laser deflection unit; a laser oscillation control unit configured to control an operation of the laser oscillator; a first laser deflection control unit configured to control an operation of the first laser deflection unit; and a second laser deflection control unit configured to control an operation of the second laser deflection unit. In this case, the laser processing apparatus is configured to irradiate the laser pulse emitted from the second laser deflection unit onto a substrate to process a plurality of processing positions in the substrate. Further, the first laser deflection control unit controls the first laser deflection unit to successively irradiate the laser pulse to multiple sites along a predetermined track in each of the processing positions in turn, and controls the first laser deflection unit to change energy of the laser pulse emitted from the first laser deflection unit in a middle of repeating the irradiation on the predetermined track. Moreover, the processing is completed by successive irradiation to the multiple sites in each of the processing positions. 
     Further, a representative laser processing method disclosed in the present application includes: deflecting, by a first laser deflection unit, a laser pulse emitted from a laser oscillator in a two-dimensional direction; deflecting, by a second laser deflection unit, the laser pulse emitted from the first laser deflection unit in a two-dimensional direction on a same plane as that of the two-dimensional direction of the first laser deflection unit, an operation speed of the second laser deflection unit being slower than that of the first laser deflection unit; irradiating the laser pulse emitted from the second laser deflection unit onto a substrate to process a plurality of processing positions in the substrate; successively irradiating, by the first laser deflection unit, the laser pulse to multiple sites along a predetermined track in each of the processing positions in turn; changing energy of the laser pulse emitted from the first laser deflection unit in a middle of repeating the irradiation on the predetermined track; and completing the processing by successive irradiation to the multiple sites in each of the processing positions. 
     Note that representative features of the invention disclosed in the present application are as described above, but features that is not explained herein are applied to an embodiment described below, and are recited in the following claims. 
     According to the present invention, it is possible to facilitate controls when punching against multiple positions of a substrate including layers whose energy required for processing is different from each other is carried out by trepanning processing using laser, whereby it is possible to shorten a processing time thereof and to ensure processing quality thereof. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  is a block diagram of a laser processing apparatus according to one embodiment of the present invention; 
         FIG. 2  is a view for explaining roles of a galvano deflection unit and an AOD deflection unit in the laser processing apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a view illustrating an example of RF signals in the laser processing apparatus illustrated in  FIG. 1 ; 
         FIG. 4  is a view illustrating the content of a control table in the laser processing apparatus illustrated in  FIG. 1 ; 
         FIG. 5  is a view illustrating the content of the control table in the laser processing apparatus illustrated in  FIG. 1 ; 
         FIG. 6  is a timing chart for explaining processing progress by the laser processing apparatus according to one embodiment of the present invention; 
         FIG. 7  is a timing chart for explaining processing progress in a conventional laser processing method; and 
         FIGS. 8A, 8B and 8C  are sectional views of a substrate to be processed by the laser processing apparatus according to one embodiment of the present invention. 
     
    
    
     DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is a block diagram of a laser processing apparatus according to an embodiment of the present invention. Respective elements and connection lines are illustrated so long as it is considered that they are necessary for mainly explaining the present embodiment. However, it does not mean that all of the necessary elements and connection lines as the laser processing apparatus is illustrated. 
     As illustrated in  FIGS. 8A to 8C , in the laser processing apparatus that will be explained herein, blind holes  83  are made at multiple sites of a substrate  1  in which a copper layer  81  as a metal layer is provided on a surface thereof and a resin layer  82  is laminated immediately beneath the copper layer  81 . 
     In  FIG. 1 , a reference numeral  2  indicates a table on which the substrate  1  to be processed is placed, and a reference numeral  3  indicates a table driving unit configured to drive the table  2 . Specifically, the table driving unit is configured to move the table  2  in a two-dimensional direction in which two directions mutually make a right angle when viewed from the above of the substrate  1 . A reference numeral  4  indicates a laser oscillator configured to oscillate a laser pulse L 1  with a wavelength of a UV laser, a reference numeral  5  indicates a beam splitter configured to reflect the laser pulse L 1  emitted from the laser oscillator  4 , and a reference numeral  6  indicates an AOD deflection unit (that is, a first laser deflection unit) configured to deflect, by using an AOD, the laser pulse L 1  reflected by the beam splitter  5  in a two-dimensional direction in which two directions mutually make a right angle when viewed from the above of the substrate  1 . 
     A reference numeral  7  indicates a damper configured to absorb a laser pulse L 3  that is not deflected in a processing direction by the AOD deflection unit  6  but penetrates through the AOD deflection unit  6 . A reference numeral  8  indicates a galvano deflection unit (that is, a second laser deflection unit) configured to deflect, by using a galvano scanner, a laser pulse L 2  deflected in the processing direction by the AOD deflection unit  6  in a two-dimensional direction in which two directions mutually make a right angle when viewed from the above of the substrate  1 . A reference numeral  9  indicates a condensing lens configured to irradiate a laser pulse L 4  from the galvano deflection unit  8  to a punching position of a printed substrate. 
     The deflection to the two-dimensional direction of each of the AOD deflection unit  6  and the galvano deflection unit  8  is carried out in the same plane of the substrate  1 . 
     Note that in the present embodiment, although it is found by the following explanation, the two-dimensional direction of each of a table driving unit  3 , the AOD deflection unit  6 , and the galvano deflection unit  8  is set to become the same direction. Although it is not necessarily required for the present application, this is preferable for the control thereof. 
     In a case where deflection areas of the AOD deflection unit  6  and the galvano deflection unit  8  are compared with each other, the deflection area of the latter is overwhelmingly wider. In a case where operation speeds of the both are compared with each other, the operation speed of the former is overwhelmingly faster. 
     The laser optical system as described above is disclosed in Patent Document 3 mentioned above, for example. 
     A reference numeral  16  indicates a laser irradiating unit on which the AOD deflection unit  6 , the damper  7 , the galvano deflection unit  8 , and the condensing lens  9  are implemented. By moving the table  2  in a right-left direction with respect to the paper of FIG. (hereinafter, referred to as an “X direction”) and a vertical direction (hereinafter, referred to as a “Y direction”), relative displacement between the substrate  1  and the laser irradiating unit  16  is carried out. Further, by deflecting the laser pulse in each of the AOD deflection unit  6  and the galvano deflection unit  8  in the X direction and the Y direction, it is possible to irradiate the laser pulse onto a necessary punching position of the substrate  1 . 
     Both systems including a system for movement (or deflection) in the X direction and a system for movement (or deflection) in the Y direction are provided for each of the table driving unit  3 , the AOD deflection unit  6 , and the galvano deflection unit  8 . 
     A deflection area by the AOD deflection unit  6  and the galvano deflection unit  8  can be changed by moving the table  2 . 
     Since the AOD deflection unit  6  is not required for mechanical motion, the operation speed thereof is fast, but the deflection area thereof is small. Therefore, the galvano deflection unit  8  is used for positioning the laser pulse to a specific coordinate of the substrate  1 , while the AOD deflection unit  6  is used for positioning the laser pulse to a peripheral region in the X direction and the Y direction around the specific coordinate at a high speed. 
       FIG. 2  illustrates a relationship between the specific coordinate and the peripheral region. In  FIG. 2 , a reference numeral  20  indicates the specific coordinate, and a reference numeral  21  indicates the peripheral region around the specific coordinate  20 . 
     In case of trepanning processing, the laser pulse is positioned to the specific coordinate  20  by the galvano deflection unit  8 , and is positioned in a peripheral region  21  including a trepanning processing area by the AOD deflection unit  6 . Note that the positioning of the laser pulse to the specific coordinate  20  may be carried out not only by the galvano deflection unit  8 , but also by cooperation of the AOD deflection unit  6  and the table driving unit  3 . 
     Returning to  FIG. 1 , a reference numeral  10  indicates an entire control unit configured to control an operation of the whole apparatus. The entire control unit  10  is mainly configured by a processing apparatus based on program control, for example. Respective components and connection lines therein include logical ones. Further, a part of the respective components may be provided separately from the processing apparatus. Further, the entire control unit  10  may have control functions other than those described herein, and may be connected to blocks (not illustrated in the drawings). 
     A laser oscillation control unit  11 , a table drive control unit  12 , a control table  13 , an AOD control unit (that is, a first laser deflection control unit)  14 , and a galvano control unit (that is, a second laser deflection control unit)  15  are provided inside the entire control unit  10 . The laser oscillation control unit  11  is configured to output a laser oscillation command signal S for commanding oscillation or attenuation of the laser pulse L 1  by the laser oscillator  4 . The table drive control unit  12  is configured to output a table driving signal T for controlling the table driving unit  3 . The control table  13  is configured to register control information for controlling the AOD deflection unit  6 . The AOD control unit  14  is configured to output an AOD driving signal D for controlling the AOD deflection unit  6  in accordance with the content of the control table  13 . The galvano control unit  15  is configured to output a galvano control signal G for controlling the galvano deflection unit  8 . 
     The AOD control unit  14  and the galvano control unit  15  respectively control two AOD deflection units  6  including an X system and a Y system, and two galvano deflection units  8  including an X system and a Y system. Although only one control table  13  is illustrated in  FIG. 1 , two control tables  13  including an X system and a Y system are provided. 
     The AOD driving signals D outputted from the AOD control unit  14  consist of RF signals. A deflection angle of the AOD deflection unit  6  is changed by frequency of this RF signal, and emission energy is changed by an amplitude level of this RF signal. 
       FIG. 3  illustrates an example of the AOD driving signals D. Here, frequencies of AOD driving signals Da, Db are respectively fa and fb, and amplitudes thereof are respectively Aa and Ab. The frequency fb is higher than the frequency fa, and the amplitude Ab is larger than the amplitude Aa. 
     When the AOD driving signal Db is applied thereto, the deflection angle and the emission energy of the AOD deflection unit  6  respectively become larger than those when the AOD driving signal Da is applied thereto. 
     Data for determining amplitude to be supplied at that time are registered in the control table  13  for each frequency of the 
     RF signals to be supplied to the AOD deflection unit  6 . 
     On the basis of the present invention, data for determining two kinds of amplitudes are registered in the control table  13  for each frequency of the RF signals to be supplied to the AOD deflection unit  6 . 
     Namely, as illustrated in  FIG. 4 , data for determining amplitudes C-Ax 1 , C-Ax 2 , C-Ax 3 , . . . when the copper layer  81  is processed and data for determining amplitudes P-Ax 1 , P-Ax 2 , P-Ax 3 , . . . when the resin layer  82  is processed are registered in a control table  13 X for an X system for each frequency fx 1 , fx 2 , fx 3 , . . . of the RF signals to be supplied to the AOD deflection unit  6 . 
     Further, as illustrated in  FIG. 5 , data for determining amplitudes C-Ay 1 , C-Ay 2 , C-Ay 3 , . . . when the copper layer  81  is processed and data for determining amplitudes P-Ay 1 , P-Ay 2 , P-Ay 3 , . . . when the resin layer  82  is processed are registered in a control table  13 Y for a Y system for each frequency fy 1 , fy 2 , fy 3 , . . . of the RF signals to be supplied to the AOD deflection unit  6 . 
     Note that the content of the control tables  13 X and  13 Y herein are those for explaining a logical relationship between the mutual data. For example, in case of the control table  13 X, data indicating positions x 1 , x 2 , x 3 , . . . in the X direction are not necessarily registered for each frequency fy 1 , fy 2 , fy 3 , . . . of the RF signals. 
     The amplitude of each of the RF signals when the copper layer  81  is processed is set so that the emission energy from the AOD deflection unit  6  is adapted for the processing of the copper layer  81 . The amplitude of each of the RF signals when the resin layer  82  is processed is set to be smaller than the amplitude of each of the RF signals when the copper layer  81  is processed so that the emission energy from the AOD deflection unit  6  is adapted for the processing of the resin layer  82 . 
     Here, for example, as disclosed in Japanese Patent Application Laid-Open Publication No. 2008-36667, the emission energy from the AOD deflection unit  6  varies depending upon the frequency of the RF signal. Therefore, it is necessary to correct the amplitude of the RF signal in order to obtain predetermined emission energy regardless of high frequency or low frequency of the RF signal. 
     The amplitudes of the RF signals for respectively processing the copper layer  81  and the resin layer  82 , which are registered in the control tables  13 X and  13 Y described above, are further adjusted on the basis of the amplitudes after correction, and are obtained from experiment data as they are or by incorporating a calculation formula to them. 
     In a case where blind holes are made at the multiple sites of the substrate  1  illustrated in  FIG. 8 , the laser processing apparatus described above completes processing at each processing position by only trepanning processing, and operates as follows.  FIG. 6  is a timing chart for explaining processing progress thereof. 
     Under control of the entire control unit  10 , the galvano deflection unit  8 , cooperation of the galvano deflection unit  8  and the AOD deflection unit  6 , or cooperation of the galvano deflection unit  8 , the AOD deflection unit  6 , and the table driving unit  3  positions a laser irradiation position at one specific coordinate  20  of the substrate  1  illustrated in  FIG. 2 , and the AOD deflection unit  6  is controlled as follows to successively irradiate the laser pulse to the multiple sites in the peripheral region  21  illustrated in  FIG. 2  so as to draw a predetermined spiral track. 
     As a first step, the AOD control unit  14  first uses data for determining amplitude for a copper layer from the control table  13  to control the AOD deflection unit  6  to carry out successive irradiation along the predetermined spiral track once or multiple times. In this case, in a case where the irradiation on the same track is repeated multiple times, laser irradiation to the same position is periodically carried out multiple times. 
     The processing of the copper layer  81  is mainly carried out herein, and a state after this trepanning processing becomes that as illustrated in  FIG. 8B . 
     Then, as a next step, the AOD control unit  14  uses data for determining amplitude for a resin layer from the control table  13  to switch into the control of the AOD deflection unit to newly carry out successive irradiation along the predetermined spiral track once or multiple times in the similar manner to that described above. In this case, as well as the above step, in a case where the irradiation on the same track is repeated multiple times, laser irradiation to the same position is periodically carried out multiple times. 
     Therefore, the emission energy from the AOD deflection unit  6  herein is set to become lower than that during the processing of the copper layer  81 , and the processing of the resin layer  82  is then carried out mainly. The substrate  1  thereafter becomes a state as illustrated in  FIG. 8C , and the processing for the one punching position is completed. 
     Then, the galvano deflection unit  8 , cooperation between the galvano deflection unit  8  and the AOD deflection unit  6 , or cooperation among the galvano deflection unit  8 , the AOD deflection unit  6 , and the table driving unit  3  positions the laser irradiation position at another specific coordinate  20  provided at the other position than that of the above specific coordinate  20 , and trepanning processing is carried out in the similar manner to the above. 
     According to the embodiment described above, in a case where punching is carried out against the multiple positions of the substrate  1  illustrated in  FIG. 8  in which the copper layer  81  is provided on the substrate surface and the resin layer  82  is laminated immediately beneath the copper layer  81 , it is possible to carry out the trepanning processing for the resin layer  82  after the trepanning processing for the copper layer  81 . Therefore, a next and further driving operation by a mechanical laser deflection mechanism or a table drive mechanism for punching processing for the resin layer  82  after the trepanning processing for the copper layer  81 , which has been carried out conventionally, is never required. 
     Moreover, the trepanning processing for the copper layer  81  and the resin layer  82  is carried out by using the AOD deflection unit  6  that does not include any mechanical operation and whose operation speed is fast. Therefore, it is possible to shorten the processing time significantly. 
     Further, in a case where the resin layer  82  is processed, processing energy (that is, the emission energy) is made smaller than that when the copper layer  81  is processed. This does not cause the copper layer  84  provided beneath the resin layer  82  to be damaged. Therefore, it is possible to ensure processing quality thereof. 
     Moreover, irradiation energy is lowered when the resin layer  82  is processed, but there is no need to change the emission energy from the laser oscillator  4 . Namely, merely by adjusting information on amplitude of the RF signal, which is registered in the control tables  13 X and  13 Y for controlling the deflection operation for the AOD deflection unit  6 , it is possible to change the irradiation energy. 
     It is thought that the emission energy from the laser oscillator  4  itself is controlled in order to change the irradiation energy. However, such a method requires controls of both the laser oscillator  4  and the AOD deflection unit  6 . According to the embodiment described above, only the AOD deflection unit  6  may be controlled. Therefore, the number of adjustment targets and the number of control targets becomes smaller, and this makes it possible to facilitate controls thereof, whereby it is possible to simplify an apparatus design of the laser processing apparatus. 
     Further, it is possible to keep an oscillation state of the laser oscillator  4  constant. Therefore, it is possible to heighten stability of the laser, and this makes it possible to improve the processing quality thereof. 
     As described above, the present invention has been described specifically on the basis of the embodiment. However, the present invention is not limited to this embodiment. It goes without saying that the present invention can be changed variously without departing from the substance thereof, and may include various modification examples. 
     For example, in the embodiment described above, the case where the trepanning processing is carried out along the predetermined spiral track has been described. However, a shape of the predetermined track is not necessarily the spiral shape, and there is a method of forming the predetermined track in a concentric circular shape or a rectangular shape. 
     Further, in a case where the trepanning processing is repeatedly carried out along the predetermined track multiple times, a track this time is not necessarily the same as the previous track. Further, the track when the resin layer  82  is processed may be changed from the track when the copper layer  81  is processed. In short, a route of the track may be determined in advance. 
     Further, in a case where the trepanning processing is repeatedly carried out along the predetermined track multiple times in accordance with constituent material of the substrate  1 , the number of repeated times of the trepanning processing along the predetermined track is not necessarily an integer depending upon a kind of constituent material, the magnitude of energy of the laser pulse, or a size of a beam spot diameter. For example, the final trepanning processing may be terminated in the middle of the track. 
     Moreover, the case where the blind holes  83  are made in the substrate  1  with three layers in which the copper layer  84  is laminated beneath the resin layer  82  as illustrated in  FIG. 8  has been described. However, the present invention may be applied to a case where through holes are made in a substrate with two layers in which the copper layer  84  is not provided. Further, the case where the copper layer as a layer whose energy required for processing is higher and the resin layer as a layer whose energy required for processing is lower are used has been described. However, each of the copper layer and the resin layer may be made of the other material. Moreover, a substrate in which three or more layers are laminated may be used.