Abstract:
A method of controlling thermal loading of an electronic component material during ablation thereof provides a first laser light beam  42  at a certain power density and fluence and uses the first laser light beam to remove a portion of a first side  47  of the material  36 . A second laser light beam  44  is provided at a certain power density and fluence and the second laser light beam is used to remove a portion of a side  49  of the material opposing the first side thereof substantially simultaneously as the portion of the first side is being removed.

Description:
FIELD OF THE INVENTION 
     The invention relates to a laser system and, more particularly, to a method of using a laser system to control fluence and power density at a target to minimize thermal loading of the target. 
     BACKGROUND OF THE INVENTION 
     Machining of electronic component material using a laser is a function of the laser parameters and the machining characteristics of the specific material. The machining characteristics are substantially affected by the thermal properties of the material or of the material sets forming conductive layers and dielectric layers. Excess thermal loading of materials can cause quality and reliability problems with the final product. 
     Conventionally, the modulation of fluence and power density (peak power density) of a laser system to affect thermal loading is achieved by varying the power output of the laser, attenuating the power, varying the repetition rate of the laser, or changing the position of the workpiece in relation to the focal plane of the respective laser system. These methods have distinct disadvantages in controlling the fluence and or power density at the target or workpiece. For example, changing the repetition rate of the laser will cause instability and lead to a certain predetermined set of conditions. The entire range of desired fluence and power density settings cannot be scanned. In addition, the repetition rate cannot be used to create changes in less than one second intervals for a given laser source due to the resonator instability and first pulse phenomena. Thus, changing the repetition rate of the laser tends to introduce a large amount of variability in the fluence and power density. 
     Accordingly, there is a need to provide a method of minimizing thermal loading to surrounding areas while machining an electronic component material using a laser. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to fulfill the need referred to above. In accordance with the principles of the present invention, this objective is achieved by providing a method of controlling thermal loading of an electronic component material during ablation thereof. The method provides a first laser light beam at a certain power density and fluence and uses the first laser light beam to remove a portion of a first side of the material. A second laser light beam is provided at a certain power density and fluence and the second laser light beam is used to remove a portion of a side of the material opposing the first side thereof substantially simultaneously as the portion of the first side is being removed. 
     In accordance with another aspect of the invention, a method of controlling thermal loading of an electronic component material during ablation thereof includes directing a laser light beam to remove material so as to create a heat affected zone in the material. The operator then waits a certain time period to permit the material to cool and to permit the heat affected zone to expand in the material. Thereafter, the laser light beam is directed to further remove material in at least a portion of the heat-affected zone. 
     Other objects, features and characteristics of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawings, all of which form a part of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 is a schematic illustration of an embodiment of a laser system for use in a method of the present invention. 
     FIG. 2 is a schematic illustration of a method of machining material using laser systems of the type of FIG.  1 . 
     FIGS. 3A and 3B show heat affected zones as a result of ablating a target material from both sides thereof to create a through hole. 
     FIG. 4 show heat affected zones over periods of time as a result of ablating a target material from both sides thereof to create a through hole. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a preferred embodiment of a laser system  10  is shown for use in the method of the present invention. The laser system  10  includes a IR laser  12  having a resonator cavity  14  which comprises a lasant  16  positioned between a highly reflective mirror  18  and a partially transmissive mirror (or output coupler)  20  along a beam path  22 . The lasant  16  in the resonator cavity  14  is preferably a solid-state laser rod comprising Nd:YAG, Nd:YAP, Nd:YVO 4 . However, those skilled in the art will recognize that other solid-state lasants or even gas, semiconductor or tunable organic dye lasants could be used in the lasers  12 . For example, suitable gas lasants and their fundamental wavelengths could include Nitrogen (337.1 nm), HeCd (325.0-441.6 nm), Argon (457.9-514.5 nm), Krypton (350.7-799.3 nm), HeNe (632 nm), CO (4.0×10 3  -5.5×10 3  nm), CO 2  (10.6×10 3  nm), and H 2 O (118.3×10 3  nm). As another example, suitable solid-state lasants could include Ruby (694.3 nm), Nd:Glass (1.06×10 3  nm), Nd:YAG (1.06×10 3  nm), Nd:YAP (1.06×10 3  nm), and Nd:YVO 4 , while suitable semiconductor lasants could include GaAs (904 nm for a single diode or 850±50 nm for an array of 48 diodes). 
     Laser rod  16  may be pumped by a variety of pumping sources (not shown) well known to persons skilled in the art, such as thermal, electrical or optical. However, a suitable diode pump or arc lamp is preferred for the illustrated Neodymium based laser system. 
     The resonator cavity  14  is illustrated with an associated Q-switch  24  which preferably operates by electro-optical or accusto-optical means. Other well known laser components (e.g., polarizers, limiting apertures, attenuators and the like) and their uses, positioning, and operation are well known to those skilled in the art and could be utilized inside the resonator cavities as desired. 
     The IR output from the resonator  14  is passed through an upcollimator structure  28 , after which it is deflected downwardly along a beam path  30  by a beam-directing reflector  32  into a focusing lens  34  and then to an electronic component material. In the illustrated embodiment, the electronic component material is silicone or a multi-layered target  36 . 
     Instead of employing an IR laser, any suitable laser can be employed such as a green laser to produce green light (e.g., between about 500 nm to about 580 nm), a UV laser (e.g., UV light at harmonics of 1064 nm fundamental wavelength) or a multiple wavelength laser (e.g., EM radiation at a fundamental wavelength of 1064 nm). 
     The multi-layered target  36  can be, for example, a circuit board with a top conductor layer  27 , an upper dielectric layer  29 , an embedded conductor layer  31 , a lower dielectric layer  33 , and a bottom conductor layer  35 , respectively. The conductor layers may comprise conductive metals such as copper, aluminum, titanium, nickel, tungsten, platinum, gold, molybdenum, palladium, silver, or combinations thereof. The dielectric layers may comprise an organic composition such as PTFE, polyimides, epoxies, or combinations thereof. The dielectric materials may be reinforced with glass fibers, aramid fibers, KEVLAR, ceramics, or combinations thereof. In the preferred embodiment, the conductor layers are preferably copper and dielectric layers are either RCC or FR4, both of which contain epoxy, an organic dielectric material. FR4 also contains glass reinforcement. It should be apparent that these construction used in the PCB manufacturing are only examples of dielectric conductor layers used to form circuits. The example is only one of a multitude of constructions. 
     The laser system  10  is preferably used to create vias, e.g., blind or through holes in the electronic component material or target  36 , such as a circuit board. To control thermal loading of the target  36 , machining of the target with a laser system  10  can occur from all or various degrees of freedom, e.g., top, bottom, and sides of the target  36 . For example, with reference to the left-hand portion of FIG. 2, a laser system  10  produces a laser light beam  38  that is split by beam splitter  40  into an upper light beam  42  and a lower light beam  44 . The upper light beam passes through an optical diffuser  46  and ablates the target  36  at an upper portion  47  thereof at a certain fluence and power density. The lower light beam  44  passes through another optical diffuser  48  and ablates a lower portion  49  of the target  36  at certain fluence and power density. The fluence and power density of the beams  42  and  44  can be the same or can be different from each other. As shown in FIG. 2, the light beams  42  and  44  ablate different areas of the target  36 . It can be appreciated that the light beams  42  and  44  can be disposed along a common axis to cut a through hole in the target  36 . 
     Instead of providing the single laser system  10  to ablate the target  36 , with reference to the right-hand portion of FIG. 2, a first laser system  10 ′ can be used to ablate from the upper surface of the target  36  and a second laser system  10 ″ can be used to ablate from the lower portion of the target  36 . More particularly, the first laser system  10 ′ generates a light beam  50  that passes through an optical diffuser  52  to ablate from the top portion  47  of the target  36 . The second laser system  10 ″ generates a second laser light beam  54  that passes through another optical diffuser  56  to ablate from the lower portion  49  of the target  36 . As shown, the light beams  50  and  54  are aligned axially to create a through hole in the target  36 . The thickness d of the target of FIG. 2 is preferably greater than 200 Angstroms. The fluence and power densities of the beams  50  and  54  can be the same or can be different from each other. It can be appreciated that the beams  54  and  50  need not be aligned axially when different areas of the target are to be ablated. 
     Hence, by machining or cutting from both sides of the target  36 , thermal loading of the target  36  is minimized while increasing throughput rate. Furthermore, with reference to FIGS. 3A and 3B, a heat assist ablation zone aids in ablation and minimizes thermal loading when both sides of the target  36  are ablated to create a through hole. More particularly, FIG. 3A shows an initial ablation from both the top and bottom of the target  36 . During ablation, material is removed and heat affected zones  58  are created at both the top and the bottom of the target  36 . In addition, near the central portion of the target  36 , as the heat affected zones merge, a heat assist ablation zone  60  is defined which allows for faster and more efficient laser ablation of the target  36  since ablation is then performed on material that is already heated. 
     With reference to FIG. 4, in a temporal serial processing technique, an initial ablation, simultaneously from both sides of a target  36 , at time t 0  results in material being removed and creation of initial heat affected zones  58  at time t 1 . After a cooling period, (e.g., t 1  to t 2 ) the heat affected zones  58 ′ in the target  36  material have diffused, covering a larger area. Another ablation is performed during time t 2  to t 3  at the top and bottom of the target  36  that results in the heat affected zone  62  along with heat assisted zone  64 . Since the further ablation is performed in the heat affected zones and the heat assisted zone is created, this technique results in faster and more efficient material removal while minimizing thermal loading of the target  36 . 
     In using the technique described above, it can be appreciated that instead of having the laser light beams axially aligned during ablation to create a through hole, the light beams ablating the target can be located at different areas as in the left hand portion of FIG.  1 . Thus, in a spatial serial processing technique, a first area of the target  36  is initially ablated. Thereafter, a second area is initially ablated while the first area is cooling. Once the initial ablation of the second area is complete, the first area can be further ablated. This process is continued until all areas are fully ablated. As a result, thermal loading of the target  36  is minimized. In either the temporal or spatial serial processing techniques, a low degree of thermal loading is achieved by machining in distinct steps to allow the same amount of fluence to be transferred to the target  36 . 
     The methods described above minimize thermal loading of the target  36  during machining of through holes or vias therein by working from the top of the target and from the bottom of the target generally simultaneously. Also, different areas of a target can be worked on at different times or time intervals to avoid threshold levels of thermal loading in the surrounding material of the target. 
     With the disclosed methods, the resulting structures (e.g., vias) are better in quality and allow for improved manufacturing of the devices. The improvement is based on the adjustment of the energy density during the formation of structures i.e., vias, to accomplish: 
     1. Lower thermal loading 
     2. Higher throughput rates (adjustment of Fluence/Density during machining process) 
     3. Improve the cut quality and shape of vias in electronic material, dicing of silicon and related areas 
     4. Improve the Depth of Field on laser systems. 
     The laser and methods disclosed herein are applicable to any laser process used to cut, weld, anneal or define shapes on electronic material. The specific electronic material of interest is silicone (used in integrated electronic circuits) and advanced electronic packages/PCBs composed of multiple layers of conductive and non-conductive layers. The method is applicable to any lasers incorporated into laser systems that use a certain power density and fluence at the workpiece. 
     The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the spirit of the following claims.