Abstract:
A small feature at a target location on a working surface of a workpiece is laser machined. A laser beam propagating along a beam path is directed for incidence at the target location on the working surface to machine the small feature. A focusing lens sized to converge the laser beam on the working surface is set in the beam path at a short working distance from the working surface to laser machine the small feature and thereby eject target material from the workpiece back toward the focusing lens. A sacrificial protective member positioned between the focusing lens and the working surface transmits without appreciable distortion or adsorption the laser beam focused by the focusing lens and incident on the working surface. The sacrificial protective member intercepts the ejected target material to prevent a sufficient amount of it from reaching and thereby appreciably contaminating the focusing lens.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure describes a laser micromachining system that includes a lens and a sacrificial protective member to prevent appreciable contamination of the lens. 
       BACKGROUND INFORMATION 
       [0002]    A conventional laser micromachining system includes a lens to focus a laser beam at a target location on a working surface of a workpiece. The focused laser beam removes material from the workpiece and produces ejected target material that is spewed in a direction towards the lens. A conventional system positions the lens at a working distance sufficiently far from the workpiece (for example, 50 millimeters (mm)) so that no portion of the ejected target material contacts and contaminates the lens. 
         [0003]    In the field of laser micromachining, however, small machined features on the workpiece are desired. Small machined features require a lens with a high numerical aperture (NA)—for example, a NA of 1—to create a small diffraction-limited spot incident on the working surface of the workpiece. Because the lenses of conventional laser micromachining systems are positioned at a far working distance to prevent the ejected target materials from reaching the lens, conventional systems use focusing lenses with large diameters to achieve high NA. For example, a lens with a diameter of 100 mm, positioned at a working distance of 50 mm, is traditionally used to achieve a NA of 1. Lenses with large diameters lead to high costs. 
         [0004]    Therefore, a need exists for a laser micromachining system that includes a high NA lens that is smaller, cheaper, and positioned at a closer working distance—without the lens becoming contaminated by ejected target material—than a lens of a conventional laser micromachining system. 
       SUMMARY OF THE DISCLOSURE 
       [0005]    The preferred embodiments disclosed perform laser machining of a small feature at a target location on a working surface of a workpiece. A laser beam propagating along a beam path is directed for incidence at the target location on the working surface of the workpiece to machine the small feature. A focusing lens sized to converge the laser beam on the working surface is set in the beam path and at a short working distance from the working surface to laser machine the small feature and thereby eject target material from the workpiece back toward the focusing lens. A sacrificial protective member positioned between the focusing lens and the working surface of the workpiece transmits without appreciable distortion or adsorption the laser beam focused by the focusing lens and incident on the working surface. The sacrificial protective member intercepts the ejected target material to prevent a sufficient amount of it from reaching and thereby appreciably contaminating the focusing lens. 
         [0006]    This approach allows a focusing lens to be set at a short working distance from a working surface of a workpiece without becoming appreciably contaminated by ejected target material. Because the focusing lens can be set at a short working distance, the focusing lens may have a small diameter, and be characterized by a high NA and high performance (i.e., small spot size). 
         [0007]    Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a simplified diagram of a laser micromachining system of a preferred embodiment. 
           [0009]      FIGS. 2   a  and  2   b  depict a flexible sheet of the laser micromachining system according to a first embodiment. 
           [0010]      FIGS. 3   a  and  3   b  depict a rigid sheet of the laser micromachining system according to a second embodiment. 
           [0011]      FIG. 4  depicts a conformal layer of the laser micromachining system according to a third embodiment. 
           [0012]      FIGS. 5   a  and  5   b  show the comparative relationship between, respectively, the laser micromachining system of the preferred embodiments and a conventional laser micromachining system. 
           [0013]      FIG. 6  depicts multiple laser beams and multiple associated lenses used in the laser micromachining system of the preferred embodiments. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    Preferred embodiments of a laser micromachining system are described below. System components with like reference numerals perform the same functions in each of the embodiments described. The preferred embodiments of the laser micromachining system include one or more lenses that are positioned at a sufficiently short working distance from a working surface of a workpiece without the lens or lenses becoming appreciably contaminated by ejected target material of the workpiece. 
         [0015]      FIG. 1  depicts a laser micromachining system  100  that includes a laser beam source  102 . Laser beam source  102  generates and emits a laser beam  104  that propagates along a beam path, represented by a beam axis  104 ′, for incidence at a target location  106  on a working surface  108  of a workpiece  110 . Laser beam source  102  may be any type of laser energy generating device known to skilled persons. Laser micromachining system  100  may also include mirrors (not shown) to change the beam path of laser beam  104  (i.e., laser beam source  102  may be at a position other than directly above target location  106 ). Laser micromachining system  100  includes a lens  112  positioned in the beam path of laser beam  104  to focus laser beam  104  at target location  106 . Lens  112  converges laser beam  104  on working surface  108  to laser machine small features that include, for example, feature dimensions ranging between about 0.25 micrometers (μm) and about 50 μm. 
         [0016]    Lens  112  may be any type of converging lens capable of focusing laser beam  104  at target location  106 . A diameter of lens  112  may be any size, but, preferably, lens  112  is a small lens having a diameter of less than 100 mm. The diameter of lens  112  is determined by a working distance x 1  between lens  112  and working surface  108  and a desired NA. For example, if a NA of 1 is desired and working distance x 1  between lens  112  and working surface  108  is approximately 25 mm, the diameter of lens  112  can be approximately 50 mm. If working distance x 1  between lens  112  and working surface  108  is approximately 5 mm, the diameter of lens  112  can be approximately 10 mm to achieve a NA of 1. For a given NA and a given performance (i.e., spot size at target location  106 ) the diameter of lens  112  varies directly in relation to a change in working distance x 1 . The mass of lens  112  scales as the third power of the diameter and hence the third power of working distance x 1 . Lens  112  may be one of multiple lenses in a compound lens system. Lens  112  may be a lens designed to operate in conjunction with a protective layer. For example, lens  112  may be a lens of a type used in compact disk (CD) and digital versatile disk (DVD) technology that is designed to operate through a protective layer provided on the CD or DVD. 
         [0017]    Laser micromachining system  100  includes a sacrificial protective member  114  positioned between lens  112  and working surface  108  of workpiece  110 . Sacrificial protective member  114  is spaced apart from working surface  108  in the embodiment shown. Sacrificial protective member  114  transmits laser beam  104  focused by lens  112  for incidence on working surface  108  at target location  106  without appreciably distorting and adsorbing laser beam  104 . Sacrificial protective member  114  may have an optical impact on laser beam  114 , but when designing lens  112  and other optical components of laser micromachining system  100 , the optical impact of sacrificial protective member  114  may be compensated for (i.e., lens  112  may be fully corrected when used with sacrificial protective member  114 ). 
         [0018]    In operation, as laser beam  104  is incident on working surface  108  at target location  106 , laser beam  104  removes target material from target location  106  and produces ejected target material that spews in a direction away from working surface  108  and generally along the beam path. The ejected target material spewed in a direction along the beam path means that at least some of the ejected target material spews generally in a direction toward lens  112  such that unimpeded ejected target material would contact and contaminate lens  112 . Sacrificial protective member  114  intercepts the ejected target material to prevent the ejected target material from reaching and appreciably contaminating lens  112 . Sacrificial protective member  114  is sacrificial in that it is used once per workpiece because after laser beam  104  produces the ejected target material, a surface  116  of sacrificial protective member  114  includes embedded ejected target material that may make sacrificial protective member  114  optically unsuitable for use with subsequent workpieces (i.e., sacrificial protective member  114  becomes unusable to transmit laser beam  104  focused by lens  112  at target location  106 ). Sacrificial protective member  114  will now be described in more detail according to the following embodiments. 
       First Embodiment 
       [0019]    According to a first embodiment depicted in  FIGS. 2   a  and  2   b , sacrificial protective member  114  is a flexible sheet  214 . Flexible sheet  214  can be any type of transparent material capable of transmitting laser beam  104  without appreciable distortion or adsorption. For example, depending on the wavelength and fluence of laser beam  104 , polymers such as polycarbonate, polymethylmethacrylate (PMMA), polystyrene (PS), polyvinylidene chloride (PVDC), optical grade polyurethane (PU), cyclic olefin polymer/copolymer (COP/COC), polyethylene terephthalate (PET) and polyetheramide (PEI) would all be good candidates for flexible sheet  214 . For example, all of these materials are transparent in the visible and near infrared but only certain grades of PMMA are transparent to 350 nanometers (nm), making PMMA the preferred choice for a 355 nm laser. All are relatively inexpensive and are available in thin sheets. PMMA, PS, and olefins have high internal transmittance, making them preferred candidates for high fluence beams where adsorption of laser energy could lead to destruction of flexible sheet  214  before it fulfills its purpose. 
         [0020]    With reference to  FIG. 2   a , flexible sheet  214  is suspended above working surface  108  (i.e., flexible sheet  214  does not contact working surface  108 ) by a frame  202 . Frame  202  also holds flexible sheet  214  taut. Frame  202  may be held in place by, or connected to, a chuck  204  that also holds workpiece  110 . Flexible sheet  214  may have a surface area that is larger than a surface area of working surface  108 . Because flexible sheet  214  is suspended above working surface  108 , flexible sheet  214  may accommodate a large amount of ejected target material. The ejected target material may be spread out over a large area on surface  116  by having a relatively large gap distance D between flexible sheet  214  and working surface  108 . Or, gap distance D between flexible sheet  214  and working surface  108  can be made relatively small so that the ejected target material is embedded in a localized location  206  on surface  116  corresponding to target location  106  to prevent the embedded ejected target material from interfering with removal of other target material at other target locations. 
         [0021]    Alternatively, flexible sheet  214  may contact working surface  108  of workpiece  110 . With reference to  FIG. 2   b , flexible sheet  214  is laid on and clings to working surface  108  of workpiece  110 . Because flexible sheet  214  clings to working surface  108 , the ejection of some target material may be physically impeded and remain on working surface  108  near target location  106 . Therefore, having flexible sheet  214  cling to working surface  108  may be best suited for situations in which a relatively small amount of material is removed. In either situation—suspended above or contacting—flexible sheet  214  is easily removable after workpiece  110  has been processed. 
       Second Embodiment 
       [0022]    According to a second embodiment depicted in  FIGS. 3   a  and  3   b , sacrificial protective member  114  is a rigid sheet  314 . Rigid sheet  314  can be any type of transparent material capable of transmitting laser beam  104  without appreciable distortion or adsorption. For example, depending on the wavelength and fluence of laser beam  104 , glass or fused silica or polymers such as polycarbonate, polymethylmethacrylate (PMMA), polystyrene (PS), polyvinylidene chloride (PVDC), optical grade polyurethane (PU), cyclic olefin polymer/copolymer (COP/COC), polyethylene terephthalate (PET) and polyetheramide (PEI) would all be good candidates for rigid sheet  314 . For example, all of these materials are transparent in the visible and near infrared but only fused silica and certain grades of PMMA are transparent to 350 nanometers (nm), making fused silica or PMMA the preferred choice for a 355 nm laser. All are relatively inexpensive and are available in thick sheet form or can be injection molded to the desired shape and thickness. Fused silica, glass, PMMA, PS, and olefins have high internal transmittance, making them preferred candidates for high fluence beams where adsorption of laser energy could lead to destruction of rigid sheet  314  before it fulfills its purpose. 
         [0023]    With reference to  FIG. 3   a , rigid sheet  314  is suspended above working surface  108 . Rigid sheet  314  may be suspended above working surface  108  by a sheet support  302 . Sheet support  302  may be connected to chuck  204  or may be a unified part of chuck  204 . Also, rigid sheet  314  may be suspended above working surface  108 , supported on a lip outside working surface  108 , and held down to chuck  204  by vacuum pressure or by a mechanical fixture. Typically, rigid sheet  314  has a surface area larger than that of working surface  108 . Because rigid sheet  314  is suspended above working surface  108 , rigid sheet  314  may accommodate a large amount of ejected target material. The ejected target material may be spread out over a large area on surface  116  by having a relatively large gap distance D between rigid sheet  314  and working surface  108 . Or, gap distance D between rigid sheet  314  and working surface  108  can be made relatively small so that the ejected target material is embedded in a localized location  306  on surface  116  corresponding to target location  106  to prevent the embedded ejected target material from interfering with removal of other target material at other target locations. 
         [0024]    Alternatively, rigid sheet  314  may contact working surface  108  of workpiece  110 . With reference to  FIG. 3   b , rigid sheet  314  is laid on working surface  108  of workpiece  110 . Rigid sheet  314  is held down against chuck  204  by vacuum pressure or by a mechanical fixture. Because rigid sheet  314  contacts working surface  108 , the ejection of some target material may be physically impeded and remain on working surface  108  near target location  106 . Therefore, having rigid sheet  314  contact working surface  108  may be best suited for situations in which a relatively small amount of material is removed. In either situation—suspended above or contacting—rigid sheet  314  is easily removeable after workpiece  110  has been processed. 
       Third Embodiment 
       [0025]    According to a third embodiment depicted in  FIG. 4 , sacrificial protective member  114  is a conformal coating  414  on working surface  108  of workpiece  110 . Conformal coating  414  may be deposited on working surface  108  by an evaporation coating process (i.e., parylene coating) or a spin coating process. Conformal coating  414  may be a polymer material similar to the materials used for flexible sheet  214  and rigid sheet  314  dissolved in a carrier or applied in a two-part process where polymerization takes place on workpiece  110 . After removal of target material, conformal coating  414  may be removed or left on working surface  108 . Conformal coating  414  may be desired when small amounts of material are removed because (i) conformal coating  414  may not sequester all ejected target material, (ii) conformal coating  414  may physically impede ejection of the target material leaving some target material in target location  106 , and (iii) the optical properties of conformal coating  414  may degrade during the removal process on a single feature, interfering with latter removal stages. 
         [0026]    The embodiments described above present numerous advantages compared to conventional laser micromachining systems.  FIGS. 5   a  and  5   b  (not to scale) show a comparison of, respectively, laser micromachining system  100  of the preferred embodiments and a conventional laser micromachining system  500 . For example, because sacrificial protective member  114  of laser micromachining system  100  intercepts ejected target material spewing towards lens  112 , lens  112  can be positioned at a working distance x 1  that is shorter than a working distance x 2  of conventional laser micromachining system  500  (i.e., if working distances x 1  and x 2  were equal, ejected target material  518  would contaminate a lens  512  of conventional laser micromachining system  500 ). In other words, working distance x 1  can be sufficiently short such that, if spewed ejected target material were unimpeded, it would reach lens  112 . 
         [0027]    Also, because lens  112  can be positioned at a close working distance, lens  112  can be smaller than lens  512  of conventional laser micromachining system  500  while still achieving a high NA and high performance. As a smaller lens, lens  112  can be less expensive than lens  512 . Lens  112  can also be lighter in weight than lens  512  so that dynamics of a lens focusing mechanism of laser micromachining system  100  can be improved. Also, because lens  112  is smaller than lens  512 , multiple lenses  112  and laser beams  104  may be provided operating in parallel on workpiece  110 , as depicted in  FIG. 6 . 
         [0028]    It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.