Patent Publication Number: US-2018034240-A1

Title: Failure detection of laser diodes

Description:
BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to the field of maskless lithography. More specifically, embodiments provided herein relate to a system and method for performing maskless digital lithography manufacturing processes. 
     Description of the Related Art 
     Photolithography is widely used in the manufacturing of semiconductor devices and display devices, such as liquid crystal displays (LCDs). Large area substrates are often utilized in the manufacture of LCDs. LCDs, or flat panels, are commonly used for active matrix displays, such as computers, touch panel devices, personal digital assistants (PDAs), cell phones, television monitors, and the like. Generally, flat panels may include a layer of liquid crystal material forming pixels sandwiched between two plates. When power from the power supply is applied across the liquid crystal material, an amount of light passing through the liquid crystal material may be controlled at pixel locations enabling images to be generated. 
     Microlithography techniques are generally employed to create electrical features incorporated as part of the liquid crystal material layer forming the pixels. According to this technique, a light-sensitive photoresist is typically applied to at least one surface of the substrate. Then, a pattern generator exposes selected areas of the light-sensitive photoresist as part of a pattern with light to cause chemical changes to the photoresist in the selective areas to prepare these selective areas for subsequent material removal and/or material addition processes to create the electrical features. 
     In order to continue to provide display devices and other devices to consumers at the prices demanded by consumers, new apparatuses, approaches, and systems are needed to precisely and cost-effectively create patterns on substrates, such as large area substrates. 
     A control circuit is able to control a single laser diode. The control circuit is important to maintain the overall diode output for optimized illumination of a laser diode. However, tools that apply laser diodes with higher powered currents require a reduction in the overall switching current per power supply limit, as a laser diode is a current device and not a voltage device. 
     As the foregoing illustrates, there is a need for an improved laser diode control circuit for the failure detection of laser diodes. More specifically, what is needed in the art is an optical-coupled solid state relay which is employed on a laser diode and acts as a digital control for turning on and off the relays. 
     SUMMARY 
     The present disclosure generally relates to an apparatus and method of performing photolithography processes. More particularly, embodiments described herein generally relate to an apparatus and method for the digital control of optical-coupled solid state relays employed on a laser diode. Digital control of the optical-coupled solid state relays may allow for the turning on and/or turning off of the relays and allow for the failure detection of each laser diode. Furthermore, the embodiments described herein allow for an increase in current provided to the laser diodes such that overall laser diode output for optimized illumination may be maintained while life time and tool reliability are also increased. 
     In one embodiment, a processing apparatus is disclosed. The processing apparatus comprises a laser source, and a control circuit comprising at least one laser diode and a relay coupled with each of the at least one laser diode, wherein the relay provides digital control of the at least one laser diode. 
     In another embodiment, a processing apparatus is disclosed. The processing apparatus includes a control circuit. The control circuit includes two or more diodes connected in a series connection and an optical-coupled solid state relay connected with each diode. Each optical-coupled solid state relay provides digital control of a respective laser diode. 
     In yet another embodiment, a method for detecting the failure of a laser diode is disclosed. The method includes digitally scanning a relay, wherein the relay is connected with the laser diode, driving control current to the laser diode, and measuring the optic output intensity at the relay of each laser diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may to other equally effective embodiments. 
         FIG. 1  is a perspective view of a system that may benefit from embodiments disclosed herein. 
         FIG. 2  is a cross-sectional side view of the system of  FIG. 1  according to one embodiment. 
         FIG. 3  is a perspective schematic view of a plurality of image projection systems according to one embodiment. 
         FIG. 4  is a perspective schematic view of an image projection system of the plurality of image projection devices of  FIG. 3  according to one embodiment. 
         FIG. 5  schematically illustrates a beam being reflected by the two mirrors of the DMD of  FIG. 5  according to one embodiment. 
         FIG. 6  is a perspective view of an image projection apparatus according to one embodiment. 
         FIG. 7  is a perspective schematic view of a system employing serialized multiple laser diodes. 
         FIG. 8  is a perspective schematic view of a system with optical-coupled solid state relays employed on each laser diode according to one embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Embodiments described herein generally relate to the failure detection of laser diodes. Optical-coupled solid state relays are employed on each laser diode. The turning on and/or turning off of each relay may be controlled digitally. The laser control circuits for detecting the failure of laser diodes are described below and in the attached appendix. 
       FIG. 1  is a perspective view of a system  100  that may benefit from embodiments disclosed herein. The system  100  includes a base frame  110 , a slab  120 , two or more stages  130 , and a processing apparatus  160 . The base frame  110  may rest on the floor of a fabrication facility and may support the slab  120 . Passive air isolators  112  may be positioned between the base frame  110  and the slab  120 . The slab  120  may be a monolithic piece of granite, and the two or more stages  130  may be disposed on the slab  120 . A substrate  140  may be supported by each of the two or more stages  130 . A plurality of holes (not shown) may be formed in the stage  130  for allowing a plurality of lift pins (not shown) to extend therethrough. The lift pins may rise to an extended position to receive the substrate  140 , such as from a transfer robot (not shown). The transfer robot may position the substrate  140  on the lift pins, and the lift pins may thereafter gently lower the substrate  140  onto the stage  130 . 
     The substrate  140  may, for example, be made of quartz and be used as part of a flat panel display. In other embodiments, the substrate  140  may be made of other materials. In some embodiments, the substrate  140  may have a photoresist layer formed thereon. A photoresist is sensitive to radiation and may be a positive photoresist or a negative photoresist, meaning that portions of the photoresist exposed to radiation will be respectively soluble or insoluble to a photoresist developer applied to the photoresist after the pattern is written into the photoresist. The chemical composition of the photoresist determines whether the photoresist will be a positive photoresist or negative photoresist. For example, the photoresist may include at least one of diazonaphthoquinone, a phenol formaldehyde resin, poly(methyl methacrylate), poly(methyl glutarimide), and SU-8. In this manner, the pattern may be created on a surface of the substrate  140  to form the electronic circuitry. 
     The system  100  may further include a pair of supports  122  and a pair of tracks  124 . The pair of supports  122  may be disposed on the slab  120 , and the slab  120  and the pair of supports  122  may be a single piece of material. The pair of tracks  124  may be supported by the pair of the supports  122 , and the two or more stages  130  may move along the tracks  124  in the X-direction. In one embodiment, the pair of tracks  124  is a pair of parallel magnetic channels. As shown, each track  124  of the pair of tracks  124  is linear. In other embodiments, the track  124  may have a non-linear shape. An encoder  126  may be coupled to each stage  130  in order to provide location information to a controller (not shown). 
     The processing apparatus  160  may include a support  162  and a processing unit  164 . The support  162  may be disposed on the slab  120  and may include an opening  166  for the two or more stages  130  to pass under the processing unit  164 . The processing unit  164  may be supported by the support  162 . In one embodiment, the processing unit  164  is a pattern generator configured to expose a photoresist in a photolithography process. In some embodiments, the pattern generator may be configured to perform a maskless lithography process. The processing unit  164  may include a plurality of image projection systems (shown in  FIG. 3 ) disposed in a case  165 . The processing apparatus  160  may be utilized to perform maskless direct patterning. During operation, one of the two or more stages  130  moves in the X-direction from a loading position, as shown in  FIG. 1 , to a processing position. The processing position may refer to one or more positions of the stage  130  as the stage  130  passes under the processing unit  164 . During operation, the two or more stages  130  may be lifted by a plurality of air bearings  202  (shown in  FIG. 2 ) and may move along the pair of tracks  124  from the loading position to the processing position. A plurality of vertical guide air bearings (not shown) may be coupled to each stage  130  and positioned adjacent an inner wall  128  of each support  122  in order to stabilize the movement of the stage  130 . Each of the two or more stages  130  may also move in the Y-direction by moving along a track  150  for processing and/or indexing the substrate  140 . 
       FIG. 2  is a cross-sectional side view of the system  100  of  FIG. 1  according to one embodiment. As shown, each stage  130  includes a plurality of air bearings  202  for lifting the stage  130 . Each stage  130  may also include a motor coil (not shown) for moving the stage  130  along the tracks  124 . The two or more stages  130  and the processing apparatus  160  may be enclosed by an enclosure (not shown) in order to provide temperature and pressure control. 
     The system  100  also includes a controller (not shown). The controller is generally designed to facilitate the control and automation of the processing techniques described herein. The controller may be coupled to or in communication with one or more of the processing apparatus  160 , the stages  130 , and the encoder  126 . The processing apparatus  160  and the stages  130  may provide information to the controller regarding the substrate processing and the substrate aligning. For example, the processing apparatus  160  may provide information to the controller to alert the controller that substrate processing has been completed. The encoder  126  may provide location information to the controller, and the location information is then used to control the stages  130  and the processing apparatus  160 . 
     The controller may include a central processing unit (CPU) (not shown), memory (not shown), and support circuits (or I/O) (not shown). The CPU may be one of any form of computer processors that are used in industrial settings for controlling various processes and hardware (e.g., pattern generators, motors, and other hardware) and monitor the processes (e.g., processing time and substrate position). The memory (not shown) is connected to the CPU, and may be one or more of a readily available memory, such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. Software instructions and data can be coded and stored within the memory for instructing the CPU. The support circuits (not shown) are also connected to the CPU for supporting the processor in a conventional manner. The support circuits may include conventional cache, power supplies, clock circuits, input/output circuitry, subsystems, and the like. A program (or computer instructions) readable by the controller determines which tasks are performable on a substrate. The program may be software readable by the controller and may include code to monitor and control, for example, the processing time and substrate position. 
       FIG. 3  is a perspective schematic view of a plurality of image projection systems  301  according to one embodiment. As shown in  FIG. 3 , each image projection system  301  produces a plurality of write beams  302  onto a surface  304  of the substrate  140 . As the substrate  140  moves in the X-direction and Y-direction, the entire surface  304  may be patterned by the write beams  302 . The number of the image projection systems  301  may vary based on the size of the substrate  140  and/or the speed of stage  130 . In one embodiment, there are  22  image projection systems  301  in the processing apparatus  160 . 
       FIG. 4  is a perspective schematic view of one image projection system  301  of the plurality of image projection systems  301  of  FIG. 3  according to one embodiment. The image projection system  301  may include a light source  402 , an aperture  404 , a lens  406 , a mirror  408 , a DMD  410 , a light dump  412 , a camera  414 , and a projection lens  416 . The light source  402  may be a light emitting diode (LED) or a laser, and the light source  402  may be capable of producing a light having predetermined wavelength. In one embodiment, the predetermined wavelength is in the blue or near ultraviolet (UV) range, such as less than about 450 nm. The mirror  408  may be a spherical mirror. The projection lens  416  may be a 10× objective lens. The DMD  410  may include a plurality of mirrors, and the number of mirrors may correspond to the resolution of the projected image. In one embodiment, the DMD  410  includes 1920×1080 mirrors, which represent the number of pixels of a high definition television or other flat panel displays. 
     During operation, a beam  403  having a predetermined wavelength, such as a wavelength in the blue range, is produced by the light source  402 . The beam  403  is reflected to the DMD  410  by the mirror  408 . The DMD  410  includes a plurality of mirrors that may be controlled individually, and each mirror of the plurality of mirrors of the DMD  410  may be at “on” position or “off” position, based on the mask data provided to the DMD  410  by the controller (not shown). When the beam  403  reaches the mirrors of the DMD  410 , the mirrors that are at “on” position reflect the beam  403 , i.e., forming the plurality of write beams  302 , to the projection lens  416 . The projection lens  416  then projects the write beams  302  to the surface  304  of the substrate  140 . The mirrors that are at “off” position reflect the beam  403  to the light dump  412  instead of the surface  304  of the substrate  140 . 
     In one embodiment, the DMD  410  may have two mirrors. Each mirror may be disposed on a tilting mechanism, which may be disposed on a memory cell. The memory cell may be a CMOS SRAM. During operation, each mirror is controlled by loading the mask data into the memory cell. The mask data electrostatically controls the tilting of the mirror in a binary fashion. When the mirror is in a reset mode or without power applied, it may be set to a flat position, not corresponding to any binary number. Zero in binary may correspond to an “off” position, which means the mirror is tilted at −10 degrees, −12 degrees, or any other feasibly negative tilting degree. One in binary may correspond to an “on” position, which means the mirror is tilted at +10 degrees, +12 degrees, or any other feasibly positive tilting degree. 
       FIG. 5  schematically illustrates the beam  403  being reflected by two mirrors  502 ,  504  of the DMD  410 . As shown, the mirror  502 , which is at “off” position, reflects the beam  403  generated from the light source  402  to the light dump  412 . The mirror  504 , which is at “on” position, forms the write beam  302  by reflecting the beam  403  to the projection lens  416 . 
     Each system  100  may contain any number of image projection systems  301 , and the number of image projection systems  301  may vary by system. In one embodiment there are 84 image projection systems  301 . Each image projection system  301  may comprise 40 diodes, or any number of diodes. A problem arises when trying to maintain a large number of diodes as higher power is required to handle such large numbers of diodes. One solution may be to order the diodes in series; however, a need exists for the detection of a non-functioning diode when organized in a series as described below. 
       FIG. 6  is a perspective view of an image projection apparatus  390  according to one embodiment. The image projection apparatus  390  is used to focus light to a certain spot on a vertical plane of a substrate  140  and to ultimately project an image onto that substrate  140 . The image projection apparatus  390  includes two subsystems. The image projection apparatus  390  includes an illumination system and a projection system. The illumination system includes at least a light pipe  391  and a white light illumination device  392 . The projection system includes at least a DMD  410 , a frustrated prism assembly  288 , a beamsplitter  395 , one or more projection optics  396   a,    396   b,  a distortion compensator  397 , a focus motor  398  and a projection lens  416  (discussed supra). The projection lens  416  includes a focus group  416   a  and a window  416   b.    
     Light is introduced to the image projection apparatus  390  from the light source  402 . The light source  402  may be an actinic light source. For example, the light source  402  may be a bundle of fibers, each fiber containing one laser. In one embodiment, the light source  402  may be a bundle of about 100 fibers. The bundle of fibers may be illuminated by laser diodes. The light source  402  is coupled to the light pipe (or kaleido)  391 . In one embodiment, the light source  402  is coupled to the light pipe  391  through a combiner, which combines each of the fibers of the bundle. 
     Once light from the light source  402  enters into the light pipe  391 , the light bounces around inside the light pipe  391  such that the light is homogenized and uniform when it exits the light pipe  391 . The light may bounce in the light pipe  391  up to six or seven times. In other words, the light goes through six to seven total internal reflections within the light pipe  391 , which results in the output of uniform light. 
     The image projection apparatus  390  may optionally include various reflective surfaces (not labeled). The various reflective surfaces capture some of the light traveling through the image projection apparatus  390 . In one embodiment, the various reflective surfaces may capture some light and then help direct the light to a light level sensor  393  so that the laser level may be monitored. 
     The white light illumination device  392  projects broad-band visible light, which has been homogenized by the light pipe  391 , into the projection system of image projection apparatus  390 . Specifically, the white light illumination device  392  directs the light to the frustrated prism assembly. The actinic and broad-band light sources may be turned on and off independently of one another. 
     The frustrated prism assembly  288  functions to filter the light that will be projected onto the surface of the substrate  140 . The light beam is separated into light that will be projected onto the substrate  140  and light that will not. Use of the frustrated prism assembly  288  results in minimum energy loss because the total internal reflected light goes out. The frustrated prism assembly  288  is coupled to a beamsplitter  395 . 
     A DMD  410  is included as part of the frustrated cube assembly. The DMD  410  is the imaging device of the image projection apparatus  390 . Use of the DMD  410  and frustrated prism assembly  288  help to minimize the footprint of each image projection apparatus  390  by keeping the direction of the flow of illumination roughly normal to the substrate  140  all the way from the light source  402  that generates the exposure illumination to the substrate focal plane. 
     The beamsplitter  395  is used to further extract light for alignment. More specifically, the beamsplitter  395  is used to split the light into two or more separate beams. The beamsplitter  395  is coupled to the one or more projection optics  396 . Two projection optics  396   a,    396   b  are shown in  FIG. 6 . 
     In one embodiment, a focus sensor and camera  414  is attached to the beamsplitter  395 . The focus sensor and camera  414  may be configured to monitor various aspects of the imaging quality of the image projection apparatus  390 , including, but not limited to, through lens focus and alignment, as well as mirror tilt angle variation. Additionally, the focus sensor and camera  414  may show the image, which is going to be projected onto the substrate  140 . In further embodiments, the focus sensor and camera  414  may be used to capture images on the substrate  140  and make a comparison between those images. In other words, the focus sensor and camera  414  may be used to perform inspection functions. 
     Together the projection optics  396 , the distortion compensator  397 , the focus motor  398 , and the projection lens  416  prepare for and ultimately project the image from the DMD  410  onto the substrate  140 . Projection optics  396   a  is coupled to the distortion compensator  397 . The distortion compensator  397  is coupled to projection optics  396   b,  which is coupled to the focus motor  398 . The focus motor  398  is coupled to the projection lens  416 . The projection lens  416  includes a focus group  416   a  and a window  416   b.  The focus group  416   a  is coupled to the window  416   b.  The window  416   b  may be replaceable. 
     The light pipe  391  and white light illumination device  392  are coupled to a first mounting plate  341 . Additionally, in embodiments including additional various reflective surfaces (not labeled) and a light level sensor  393 , the various reflective surfaces and the light level sensor  393  may also be coupled to the first mounting plate  341 . 
     The frustrated prism assembly  288 , beamsplitter  395 , one or more projection optics  396   a,    396   b  and distortion compensator  397  are coupled to a second mounting plate  399 . The first mounting plate  341  and the second mounting plate  399  are planar, which allows for precise alignment of the aforementioned components of the image projection apparatus  390 . In other words, light travels through the image projection apparatus  390  along a single optical axis. This precise alignment along a single optical axis results in an apparatus that is compact. For example, the image projection apparatus  390  may have a thickness of between about 80 mm and about 100 mm. 
       FIG. 7  illustrates a perspective schematic view of a system  700  operating four laser diodes  702 ,  704 ,  706 ,  708 , however any number of laser diodes may be utilized, such as five laser diodes, eight laser diodes, or more depending on the electrical design and/or electronics design. The laser diodes  702 ,  704 ,  706 ,  708  may be organized in a serialization. The serialization may reduce the overall switching current from a power supply. The series of laser diodes  702 ,  704 ,  706 ,  708  may be connected with a DC power supply  710  at a first end  720 , and connected with a simplified driving circuit  712  at a second end  722 . The serialization of laser diodes  702 ,  704 ,  706 ,  708  may be between the first end  720  and the second end  722 . 
     By placing, for example, four laser diodes in series, a reduced current is achieved. A reduction in current may be required when switching between, for example, one laser diode and four laser diodes. If a system consists of multiple diodes in series, a reduction in the overall current may be significant. However, in doing so, the ability to control each individual laser diode may be lost. There is a need to operate laser diodes with higher currents, therefore requiring the laser diodes to be operated in series. Furthermore, the embodiment of  FIG. 7  may not operate efficiently when higher electrical currents are required, such as, for example, electrical currents above 0.5 amperes, such as a current of 2.0 amperes. 
     In the embodiment of  FIG. 7 , each laser diode  702 ,  704 ,  706 ,  708  may not be individually controlled. As such, a problem arises when any one of the laser diodes  702 ,  704 ,  706 ,  708  of  FIG. 7  fails. With continued reference to  FIG. 7 , when one laser diode  702 ,  704 ,  706 ,  708  fails, all laser diodes  702 ,  704 ,  706 ,  708  of the series also fail. For example, if the failure of a laser diode occurs due to an open circuit, the entire series of laser diodes also fail. In the case of a failed laser diode a determination must be made as to which laser diode has failed. However, in the embodiment of  FIG. 7  all laser diodes must function in order to determine which specific laser diode has failed. 
     In order to correct for, or account for the problem as illustrated in  FIG. 7 , a switch may be utilized in connection with each laser diode. The switch may be an optical-coupled solid state relay  814 ,  820 ,  826 ,  832  as shown in  FIG. 8 . The optical-coupled solid state relay may comprise an LED and/or a switch. The switch may receive digital information to control the turning on and/or turning off of the switch, thus creating the ability to switch and control all laser diodes at the same time. Furthermore, the ability to control each laser diode digitally may be had. 
     In order to detect laser diode failure and functionality an optical-coupled solid state relay may be utilized in connection with each laser diode within the system.  FIG. 8  illustrates a perspective schematic view of a system  800  with optical-coupled solid state relays  814 ,  820 ,  826 ,  832  employed on each laser diode  802 ,  804 ,  806 ,  808 . Each optical-coupled solid state relay  814 ,  820 ,  826 ,  832  may allow for the control of the respective optical-coupled solid state relay  814 ,  820 ,  826 ,  832  by a digital control (not shown). The optical-coupled solid state relay  814 ,  820 ,  826 ,  832  may each contain an LED  810 ,  816 ,  822 ,  828  and a switch  812 ,  818 ,  824 ,  830 . The digital control may turn on or turn off the optical-coupled solid state relays  814 ,  820 ,  826 ,  832  by closing or opening the switch  812 ,  818 ,  824 ,  830  to complete the circuit. 
     A detection of a failure of the system  800  may be made at any time. In some embodiments, the failure or fault detection may occur in the image projection apparatus  390 , for example, within or by the light level sensor  393 , discussed supra. A detection of a failure of the system  800  may be completed by a check of the laser diodes  802 ,  804 ,  806 ,  808 . A system failure may be detected by turning on an LED  810 ,  816 ,  822 ,  828  of the system  800  via a switch  812 ,  818 ,  824 ,  830 . The turning on of an individual LED  810 ,  816 ,  822 ,  828  within the laser diode  802 ,  804 ,  806 ,  808  (by closing the switch  812 ,  818 ,  824 ,  830 ) stops the laser diode  802 ,  804 ,  806 ,  808  from functioning. Upon the shutting down of a laser diode  802 ,  804 ,  806 ,  808  a measurement of optic output intensity from the laser diodes  802 ,  804 ,  806 ,  808  may be taken. If a change in the optic intensity occurs then the diode is functional. However, if no change in the optic intensity measurement is found, than the individual laser diode is non-functioning. A controller (not shown) may be used in the system  800  for the detection of a failure within the laser diodes  802 ,  804 ,  806 ,  808 . Furthermore, if a reduction in optic output intensity is detected, but the reduction is not as much as expected, then it can easily be determined that the laser diode is nearing the end of the functional life. The controller may perform a digital scan of the optical-coupled solid state relays  814 ,  820 ,  826 ,  832  as shown in  FIG. 8 . A digital scan of the optical-coupled solid state relays  814 ,  820 ,  826 ,  832  may provide information regarding the status of the optical-coupled solid state relays  814 ,  820 ,  826 ,  832 , such as, by way of example only, information regarding the optic output intensity of each of the laser diodes  802 ,  804 ,  806 ,  808 . The controller may also drive control current in the detection of the laser diodes  802 ,  804 ,  806 ,  808 . 
     Furthermore, if the failure of a laser diode occurs due to a short in the circuitry, than only the shorted laser diode may fail while the other laser diodes in the series may still function. In the case of a short, however, the current to the laser diodes and/or circuitry may be affected. In addition to laser diode failure or fault detection, however, shorting of the failed laser diode allows other diodes in the series to function normally. 
     Additionally, a laser diode may fail due to a failure in the laser cavity. In this failure mode no light may be output to an LED of the relay. However, the laser diode may be electrically normal in terms of I-V behaviors. As such, the other laser diodes in the series as well as the driving circuitry may function normally, however light output may be reduced. 
     The embodiments described herein relate to an apparatus and method for performing photolithography processes. More particularly, embodiments described herein generally relate to an apparatus and method for the digital control of optical-coupled solid state relays employed on a laser diode. Digital control of the optical-coupled solid state relays may allow for the turning on and/or turning off of the optical-coupled solid state relays and allow for the failure detection of each laser diode. Furthermore, the embodiments described herein allow for an increase in current provided to the laser diodes such that overall laser diode output for optimized illumination may be maintained while life time and tool reliability are also increased. 
     It will be appreciated to those skilled in the art that the preceding examples are exemplary and not limiting. It is intended that all permutations, enhancements, equivalents, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present disclosure. It is therefore intended that the following appended claims include all such modifications, permutations, and equivalents as fall within the true spirit and scope of these teachings.