WO2014158346A1

WO2014158346A1 - Laser ablation platform for solar cells

Info

Publication number: WO2014158346A1
Application number: PCT/US2014/014306
Authority: WO
Inventors: Rohit Dey; Jeffrey L. Franklin; Giridhar BASAPPA; Marulasiddeswara KARISIDDAPPA; Penchala N. Kankanala; Christopher T. Lane; James M. Gee
Original assignee: Applied Materials, Inc.
Priority date: 2013-03-13
Filing date: 2014-01-31
Publication date: 2014-10-02
Also published as: CN105073333A; EP2969373A4; EP2969373A1; TW201434569A; TWI630970B; CN105073333B

Abstract

Embodiments of the present invention relate to apparatus and methods for laser forming of holes in a substrate. In one embodiment, a laser scanning apparatus includes a movable transport assembly, and an optical device disposed adjacent the movable transport assembly, wherein the optical device comprises a polygonal mirror having a plurality of reflecting facets and an axis of rotation, an actuator configured to rotate the polygonal mirror relative to the axis of rotation, and a laser source positioned to direct electromagnetic radiation to at least one of the reflecting facets of the polygonal mirror, wherein the movable transport assembly is configured to position a substrate to receive the electromagnetic radiation reflected from the reflecting facets of the polygonal mirror.

Description

LASER ABLATION PLATFORM FOR SOLAR CELLS BACKGROUND OF THE INVENTION

Field of the Invention

[0001] Embodiments of the present invention generally relate to an apparatus and method of forming holes in one or more layers of a solar cell. More specifically, embodiments provided herein are directed to a platform for laser drilling of holes in a solar cell in a solar cell production line.

Description of the Related Art

[0002] Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or multicrystalline substrates, sometimes referred to as wafers. Because the amortized cost of forming silicon-based solar cells to generate electricity is higher than the cost of generating electricity using traditional methods, there has been an effort to reduce the cost required to form solar cells.

[0003] One solar cell design in widespread use today has a p-n junction formed near the front surface, or the light-receiving surface, which generates electron/hole pairs as light energy is absorbed in the solar cell. This conventional design has a first set of electrical contacts on the front side of the solar cell, and a second set of electrical contacts on the back side of the solar cell. In order to form the second set of electrical contacts on the back side of the solar cell, holes must be formed in a passivation layer that covers the back side of a solar cell substrate to allow a conductive layer to contact the underlying solar cell substrate.

[0004] It is common to need in excess of 100,000 contact points (i.e., holes formed in the back side passivation layer) on a single solar cell substrate. Conventional approaches to forming holes in the back side passivation layer of the solar cell include the use of multi-faceted mirrors to steer a laser beam across the solar cell substrate. These systems may produce 100,000 holes in about one second. However, these conventional systems produce by-products, such as large amounts of particulate matter, during ablation. The particulate matter is a potential contaminant that may cause defects in the solar cell substrate if not efficiently removed. Conventional removal of the particulate matter includes cleaning the substrates after ablation. However, the cleaning process is time-consuming, which negatively impacts throughput. Vacuum devices have also been utilized. However, the particulate matter may include a charge that causes the particulates to adhere to the substrate, which makes vacuum devices ineffective.

[0005] Accordingly, improved methods and apparatus for forming holes in a passivation layer of a solar cell substrate are needed.

SUMMARY

[0006] Embodiments of the present invention relate to an apparatus and methods of forming (i.e., drilling) holes in a substrate by delivering electromagnetic energy to a surface of the substrate.

[0007] In one embodiment, a laser scanning apparatus includes a movable transport assembly, and an optical device disposed adjacent the movable transport assembly, wherein the optical device comprises a polygonal mirror having a plurality of reflecting facets and an axis of rotation, an actuator configured to rotate the polygonal mirror relative to the axis of rotation, and a laser source positioned to direct electromagnetic radiation to at least one of the reflecting facets of the polygonal mirror, wherein the movable transport assembly is configured to position a substrate to receive the electromagnetic radiation reflected from the reflecting facets of the polygonal mirror.

[0008] In another embodiment, a laser scanning platform includes a laser scanning device comprising a polygonal mirror and configured to deliver pulses of electromagnetic radiation that are reflected by the polygonal mirror along a path that is parallel to a first direction, wherein the first direction is parallel to a surface of a substrate, a substrate transport assembly configured to transport the substrate in a second direction while the pulses of electromagnetic radiation are directed toward the substrate, wherein the second direction is at an angle to the first direction, one or more positioning sensors configured to detect a leading edge of the substrate as it is moved in the second direction towards the laser scanning device, and a controller configured to control the operation of the laser scanning device and the substrate transport assembly based on signals received from the one or more positioning sensors. In some embodiments, the second direction is substantially orthogonal to the first direction.

[0009] In another embodiment, a method for delivering electromagnetic radiation to a surface of a solar cell substrate is provided. The method includes transferring a substrate through a scanning chamber, forming holes in or on the substrate using pulses of electromagnetic radiation from an optical device comprising a polygonal mirror as the substrate moves relative to the optical device, removing particulate matter ejected from the substrate as the holes are formed, and neutralizing charges between the substrate and any particulate matter remaining on the substrate after the removing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, 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 typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0011] Figure 1 illustrates a cross-sectional view of a solar cell that may be formed using apparatus and methods described herein.

[0012] Figure 2A is a schematic side view of a laser processing platform according to embodiments described herein.

[0013] Figure 2B is an enlarged side view of the optical device of Figure 2A according to embodiments described herein.

[0014] Figure 2C is a schematic plan view of a substrate having a plurality of holes formed by the apparatus and methods disclosed herein.

[0001] Figure 3 is a schematic depiction of a laser scanning apparatus propagating a beam in accordance with embodiments described herein. [0002] Figure 4 is a schematic illustration of the Gaussian intensity profile of a beam without beam any beam shaping in accordance with embodiments described herein.

[0003] Figure 5 is a schematic illustration of the intensity profile of the beam with beam shaping in accordance with embodiments described herein.

[0004] Figure 6 is a schematic perspective view of one embodiment of the scanning chamber of Figure 2A according to one or more embodiments described herein.

[0005] Figure 7 is an isometric view of a laser processing tool having the laser processing platform of Figure 2A disposed therein, according to embodiments described herein.

[0006] Figure 8 is a side view of one embodiment of the optical alignment device of Figure 7 according to embodiments described herein.

[0007] Figure 9 is an isometric view of the optical alignment device of Figure 8 according to embodiments described herein.

[0008] Figures 10A and 10B are plan views of various embodiments of a substrate alignment device that may be used in the laser processing platform of Figure 2 according to embodiments described herein..

[0009] Figure 1 1 is a schematic side view of a control system that may be used with the laser processing platform of Figure 2A according to embodiments described herein.

[0010] Figures 12A and 12B are schematic plan views of a substrate having a plurality of holes formed by the apparatus and methods disclosed herein.

[0011] 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 disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. DETAILED DESCRIPTION

[0012] Embodiments of the present invention relate to an apparatus and methods of forming (i.e., drilling) holes in a substrate by delivering electromagnetic energy to a surface of the substrate. The apparatus includes a platform having a conveyor system for moving one or more solar cell substrates relative to an electromagnetic source, such as a moving laser beam. The platform also includes a particulate removal system that comprises at least one air knife, optionally at least one ion generator, and at least one vacuum device that is coupled along the length of the conveyor system for removing particulates generated by the moving laser beam and the solar cell substrate. The holes as described herein may include through-holes, blind holes, or elongated channels or lines formed at least partially in a surface of a substrate. The apparatus may be used as a stand-alone tool or incorporated into a larger substrate processing system, such as a cluster tool or an in-line substrate processing system.

[0013] Embodiments of the present invention provide a laser scanning apparatus for laser drilling of holes in one or more layers during a solar cell fabrication process. In one embodiment, the apparatus is used to laser drill holes in a back side passivation layer of a solar cell during back electrical contact formation. As used herein, the term "laser drilling" generally means removal of at least a portion of material using a laser. Thus, "laser drilling" may include ablation of at least a portion of a material layer disposed on a substrate, e.g., a hole through a material layer disposed on a substrate. Further, "laser drilling" may include removal of at least a portion of substrate material, e.g., forming a non-through hole (blind hole) in a substrate or a hole through a substrate.

[0014] Figure 1 illustrates a cross-sectional view of a solar cell 100 that may be formed using apparatus and methods described herein. The solar cell 100 includes a solar cell substrate 1 10 that has a passivation/ARC (anti-reflective coating) layer stack 120 on a front surface 105 of the solar cell substrate 1 10 and a rear passivation layer stack 140 on a rear surface 106 of the solar cell substrate.

[0015] In one embodiment, the solar cell substrate 1 10 is a silicon substrate that has a p-type dopant disposed therein to form part of the solar cell 100. In this configuration, the solar cell substrate 1 10 may have a p-type doped base region 101 and an n-doped emitter region 102 formed thereon. The solar cell substrate 1 10 also includes a p-n junction region 103 that is disposed between the base region 101 and the emitter region 102. Thus, the solar cell substrate 1 10 includes the region in which electron-hole pairs are generated when the solar cell 100 is illuminated by incident photons "I" from the sun 150.

[0016] The solar cell substrate 1 10 may include single crystal silicon, multicrystalline silicon, or polycrystalline silicon. Alternatively, the solar cell substrate 1 10 may include germanium (Ge), gallium arsenide (GaAs), cadmium telluride (CdTe), cadmium sulfide (CdS), copper indium gallium selenide (CIGS), copper indium selenide (CulnSe2), gallium indium phosphide (GalnP2), or organic materials. In another embodiment, the solar cell substrate may be a heteroj unction cell, such as a GalnP/GaAs/Ge or a ZnSe/GaAs/Ge substrate.

[0017] In the example shown in Figure 1 , the solar cell 100 includes a passivation/ARC layer stack 120 and a rear passivation layer stack 140 that each contains at least two or more layers of deposited material. The passivation/ARC layer stack 120 includes a first layer 121 that is in contact with the front surface 105 of the solar cell substrate 1 10 and a second layer 122 that is disposed on the first layer 121 . The first layer 121 and the second layer 122 may each include a silicon nitride (SiN) layer, which has a desirable quantity of trapped charge formed therein to effectively help bulk passivate the front surface 105 of the solar cell substrate.

[0018] In this configuration, the rear passivation layer stack 140 includes a first backside layer 141 that is in contact with the rear surface 106 of the solar cell substrate 1 10 and a second backside layer 142 that is dispose on the first backside layer 141 . The first backside layer 141 may include an aluminum oxide (AI2O3) layer that is between about 200 A and about 1300 A thick and has a desirable quantity of trapped charge formed therein to effectively passivate the rear surface 106 of the solar cell substrate 1 10. The second backside layer 142 may include a silicon nitride (SiN) layer that is between about 600 A and about 2500 A thick. Both the first backside layer 141 and the second backside layer 142 have a desirable quantity of trapped charge formed therein to effectively help passivate the rear surface 106 of the substrate 1 10. The passivation/ARC layer stack 120 and the rear passivation layer stack 140 minimize front surface reflection Ri and maximize rear surface reflection R₂ in the solar cell 100, as shown in Figure 1 , which improves efficiency of the solar cell 100.

[0019] The solar cell 100 further includes front side electrical contacts 107 extending through the passivation/ARC layer stack 120 and contacting the front surface 105 of the solar cell substrate 1 10. The solar cell 100 also includes a conductive layer 145 that forms rear side electrical contacts 146 that electrically contact the rear surface 106 of the solar cell substrate 1 10 through holes 147 formed in the rear passivation layer stack 140. The conductive layer 145 and the front side electrical contacts 107 may include a metal, such as aluminum (Al), silver (Ag), tin (Sn), cobalt (Co), nickel (Ni), zinc (Zn), lead (Pb), tungsten (W), titanium (Ti), tantalum (Ta), nickel vanadium (NiV), or other similar materials, and combinations thereof.

[0020] In forming the rear side electrical contacts 146, a number of through holes 147 must be formed in the rear passivation layer stack 140 without damaging the rear surface 106 of the solar cell substrate 1 10. In order to minimize the resistance losses in the solar cell 100 a high density of holes {e.g., between 0.5 and 5 holes per square millimeter) is required. For example, a 156 mm x 156 mm solar cell may require up to 120,000 holes, which requires a significant amount of time using conventional laser drilling systems and processes. Embodiments of the present invention provide an apparatus and method of more rapidly forming the holes 147 in the rear passivation layer stack 140 without damaging the rear surface 106 of the solar cell substrate 1 10.

[0021] Figure 2A is a schematic side view of a laser processing platform 200 according to embodiments described herein. The laser processing platform 200 includes an enclosure 202 having a substrate positioning system 205 therein. The substrate positioning system 205 may be a conveyor for supporting and transporting substrates 210 through the laser processing platform 200. The laser processing platform 200 may be used to drill holes in one or more layers disposed on a substrate 210 in accordance with embodiments of the present invention. For example, the laser processing platform 200 may be used to form the holes 147 in the rear passivation layer stack 140 of the solar cell 100 of Figure 1 , which is indicated as 210 in Figures 2A-2C. Each substrate 210 is a solar cell substrate, such as the solar cell substrate 1 10 with the rear passivation layer stack 140 disposed thereon, as shown and described in Figure 1 . In one example, the rear surface 106 (Figure 1 ) of the substrates 210, shown in Figure 2A, are facing upward for processing in the laser processing platform 200.

[0022] In one embodiment, the substrate positioning system 205 is a linear conveyor system that includes a continuous transport belt 215 of a material configured to support and transport a line of the substrates 210 through the laser processing platform 200 in a flow path "A". The enclosure 202 may be positioned between a loading station 217A for providing substrates 210 and an unloading station 217B for receiving processed substrates 210. The loading station 217A and the unloading station 217B may be coupled to the enclosure 202 and include robotic equipment and/or transfer mechanisms that provide substrates 210 to the transport belt 215. The substrate positioning system 205 includes support rollers 220 that support and/or drive the transport belt 215. The rollers 220 may be driven by a mechanical drive 225, such as a motor/chain drive, and may be configured to transport the transport belt 215 at a linear speed of between about 100 millimeters per second (mm/s) and about 300 mm/s during operation. The mechanical drive 225 may be an electric motor {e.g., AC or DC servo motor). The transport belt 215 may be made of a polymeric material, stainless steel, or aluminum. In one configuration, the transport belt 215 includes two parallel belts that are spaced apart in the X-direction, wherein each of the two parallel belts have a width in the X- direction that is smaller than the X-direction dimension of the substrates 210. In this configuration, each substrate 210 in the laser processing platform 200 is disposed on a portion of both parallel belts.

[0023] The substrate positioning system 205 is a transfer device configured to sequentially transport a line of the substrates 210 (i.e., in the flow path "A") toward and through a laser scanning apparatus 230. The laser scanning apparatus 230 includes an optical device 235A coupled to a support member 240 that supports the optical device 235A above the transport belt 215 and substrates 210. The laser scanning apparatus 230 also includes a scanning chamber 235B that is fixed in position relative to the transport belt 215 adjacent the optical device 235A allowing the substrates 210 to pass therethrough on the transport belt 215.

[0024] Figure 2B is a side view of the optical device 235A of Figure 2A, wherein the optical device 235A is rotated 90 degrees from its standard position shown in Figure 2A for ease of discussion purposes. Figure 2C is a schematic plan view of a substrate 210 having a plurality of holes 147 formed by the optical device 235A of Figures 2A and 2B. The optical device 235A includes a housing 241 that provides light or electromagnetic radiation that is directed toward the surface of the substrates 210 as the substrates 210 pass through the scanning chamber 235B on the transport belt 215. In one embodiment, the optical device 235A, in conjunction with movement of the transport belt 215, is configured to form a pattern P (shown in Figure 2C) which may comprise a grid pattern of rows R and columns C of holes 147 formed in a substrate 210. The optical device 235A may form the pattern P on the substrate 210 in less than about 500 milliseconds (ms) using an optics system that provides a pulsed beam that traverses the substrate 210 at a high speed as the substrate 210 is moved on the transport belt 215. For example, the optical device 235A also includes a laser source 242 that emits light or electromagnetic radiation through an optics system that provides about 95,000 holes in the substrate 210 having a diameter greater than about 80 microns (μιτι) in less than about 500 ms. In one embodiment, the speed of the transport belt 215 may be about 140 mm/s to about 180 mm/s, such as about 160 mm/s, during operation, in order to form multiple holes 147 in a substantially linear row R in the X-direction (Figure 2A) on the substrates 210 as the substrates pass below the optical device 235A on the transport belt 215 (Figure 2A) in the Y-direction. Multiple rows of holes 147 are formed in the X-direction in order to form columns C of holes 147 at a desired pitch. In one embodiment, the holes 147 may have a pitch of about 500 μιτι in the row R direction and the column C direction in the pattern formed on the substrate 210. This provides enhanced throughput, as well as larger, cleaner holes 147 formed at a greater dimensional accuracy, that far surpass conventional laser drilling apparatus. In another embodiment, an array of lines (or overlapped holes 147) may be formed in the rows R (one example is shown in Figure 12B). Line patterns may include holes 147 having a diameter of about 40 μιτι (which may equal the line width), the holes 147 may overlap about 20% and include a pitch of about 0.7 mm to about 1 .3 mm.

[0025] The laser source 242 emits light or electromagnetic radiation 255 through a process of optical amplification based on stimulated emission of photons. In some embodiments, the emitted electromagnetic radiation 255 has a high degree of spatial and temporal coherence. In one aspect, the laser source 242 emits a continuous or pulsed wave of light or electromagnetic radiation 255 that is directed to the optics system, which includes a beam expander 244, a beam shaper 246, an optional beam expander/focuser 248, and to a movable polygonal mirror 250. In one embodiment, the laser source 242 produces a pulse at a pulse width of about 1 femtoseconds (fs) to about 1 .5 microseconds ( s) having a total energy of from about 10 microJoules per pulse (μϋ/pulse) to about 6 milliJoules per pulse (mJ/pulse). In some configurations, the pulse width and frequency of the pulses of electromagnetic radiation 255 may be controlled by providing the laser source 242 with an external trigger signal that is provided at a desired frequency from the controller 290. The repetition rate of the laser pulse may be between about 15kHz and about 5 MHz. The laser source 242 may be a an electromagnetic radiation source such as a Nd:YAG, Nd:YVO₄, crystalline disk, fiber-diode and other similar radiation emitting sources that can provide and emit a continuous or pulsed beam of radiation at a wavelength between about 255 nm and about 1064 nm. In another embodiment, the laser source 242 includes multiple laser diodes, each of which produces uniform and spatially coherent light at the same wavelength. The total average power of the laser source 242 can be up to about 50W.

[0026] The pulses of electromagnetic radiation 255 emitted from the laser source 242 are received at the beam expander 244 having a first diameter, such as about 1 .5 mm to about 2.5 mm. The beam expander 244 increases the diameter of the electromagnetic radiation 255 to a second diameter, such as between about 4 mm to about 6 mm. The pulses of electromagnetic radiation 255 are then delivered to the beam shaper 246 for tuning the shape of the beam as further described below with respect to Figures 3-5. From the beam shaper 246, the pulses of electromagnetic radiation 255 are delivered to the beam expander/focuser 248, which is used to adjust the diameter of the pulses of electromagnetic radiation 255 into a desired third diameter, such as between about 2 mm and about 3 mm. The beam expander/focuser 248 then delivers the pulses of electromagnetic radiation 255 to the movable polygonal mirror 250, which reflects the pulses of electromagnetic radiation 255 through a focusing lens 252 and onto the substrate 210. In some embodiments, the beam expander/focuser 248 is not utilized and the pulses of electromagnetic radiation 255 are delivered to the movable polygonal mirror 250 directly.

[0027] The movable polygonal mirror 250 reflects the pulses of electromagnetic radiation through the focusing lens 252, which is part of the optics system of the optical device 235A, and onto a surface of the substrate 210, which is continually moving in the Y-direction on the transport belt 215 in the scanning chamber 235B (Figure 2A). Thus, the transport belt 215 does not need to stop/start during the hole formation process on the substrate 210, which increases throughput. The movable polygonal mirror 250 is a mirror having multiple reflecting facets, such as between about 10 and 18, arranged such that each reflecting facet 253 is generally angled relative to another reflecting facet 253 in a direction relative to an axis of rotation 251 of the polygonal mirror 250 (into the page in the X direction in Figure 2B). The angle of each of the reflecting facets 253 of the movable polygonal mirror 250 allows the electromagnetic radiation 255 to be scanned in one direction (X-direction in Figure 2A) across the surface of the substrate 210 as the movable polygonal mirror 250 is rotated about the axis of rotation 251 by an actuator 254. The actuator 254 is used to control the speed of rotation of the movable polygonal mirror 250 to a desired speed, such as a speed of about 100 rpm to about 10,000 rpm, up to and including about 16,000 rpm. The speed of rotation may be changed for creation of a pattern P (one example is shown in Figure 2C) on the substrate 210 and the rotational speed may be fixed during the drilling process to produce the pattern P (shown in Figure 2C) on the substrate 210.

[0028] For example, the rotational speed of the movable polygonal mirror 250 may be set at a first speed for creation of a first pattern P on one or more first substrates, and the first speed may be maintained during the ablation of each of the one or more first substrates. If a different pattern P is desired on one or more second substrates, the rotational speed of the movable polygonal mirror 250 may be set at a second speed that is different than the first speed, and the second speed may be maintained during the ablation of each of the one or more second substrates.

[0029] In one embodiment, the rotation of a single facet of the movable polygonal mirror 250, as it is reflecting the delivered pulses of electromagnetic radiation 255 from the laser source 242, creates a full row R of holes 147 (i.e., a row in the X- direction) in one or more layers formed on the substrate 210. The electromagnetic radiation 255 may be scanned across the surface of the substrate 210 by use of the movable polygonal mirror 250, while the substrate 210 is transferred in an orthogonally oriented Y-direction resulting in rows R of holes 147 (i.e., in the X- direction) spanning the length of the substrate 210 (i.e., in the Y-direction). In another example, the Y-direction is positioned at an angle to the X-direction. In yet another example, the Y-direction is positioned at an angle of about 90 degrees plus or minus a few degrees relative to the X-direction. In one embodiment, the optics system of the optical device 235A is configured to deliver a beam diameter of about 2 mm and about 3 mm for formation of the holes 147. The rotational speed of the movable polygonal mirror 250 may also be set to provide a dense row R of holes 147 such that a linear channel or groove is formed in each row R.

[0030] For example, through the use of the movable polygonal mirror 250 of the laser processing platform 200 and the above-described control scheme, drilling rates of between about 60 meters per second (m/s) and about 200 m/s may be achieved. In comparison, conventional galvanometer systems (e.g., angular rotating mirrors) are typically limited to less than 20 m/s. In addition, the use of the beam shaper 246 of the laser processing platform 200 allows holes 147 to be efficiently drilled in the passivation layer stack 140 at such rates without damage to the underlying solar cell substrate 210 as further described with respect to Figures 3-5.

[0031] Figure 3 is a schematic depiction of the optical device 235A of Figures 2A and 2B propagating a beam 300, which may be similar to electromagnetic radiation 255, along a distance Z from the optical device 235A. Figure 7 is a schematic illustration of the Gaussian intensity profile of the beam 300 at the point 310 in Figure 3 (i.e., without any beam shaping). The point 310 on the beam 300 represents a typical positioning of the substrate 210 with respect to the optical device 235A, in order to achieve complete evaporation of the passivation layer stack 140 (Figure 1 ) across a desired spot 350. As can be seen, the peak intensity 410 at the center of the spot 350 is significantly higher than the peripheral intensity 420 at the periphery of the spot 350 because the periphery of the spot 350 must be set at the ablation threshold of the material of the passivation layer stack 140. Thus, although the peripheral intensity 420 is just high enough to achieve ablation of the passivation layer stack 140 along the periphery of the spot 350, the significantly high peak intensity 410 causes damage to the underlying solar cell substrate 210 at the center of the spot 350 without any beam shaping.

[0032] In order to achieve complete ablation of the spot 350 in the passivation layer stack 140 without damaging the solar cell substrate 210, the beam shaper 246 is used. The beam shaper 246 may be a refractive beam shaper that converts a Gaussian laser beam into a collimated flat top beam. Figure 5 is a schematic illustration of the intensity profile of the beam 300 at the point 310 in Figure 3 with beam shaping. As can be seen, the beam shaping or "flat topping" operation results in a beam intensity profile having a uniform energy density just at the ablation threshold of the material in the passivation layer stack 140 across the entire area of the spot 350. Thus, use of the beam shaper 246 in the optical device 235A allows for efficient drilling of holes 147 in the passivation layer stack 140 without damaging the underlying solar cell substrate 210.

[0033] Referring again to Figure 2A, the laser processing platform 200 also includes a substrate sensing system 260 including one or more substrate position sensors. The substrate sensing system 260 uses an optical sensor 262 to detect a leading edge 265 of the substrate 210 and sends corresponding signals to a controller 290. The controller 290, in turn, sends signals to the optical device 235A to time the operation of the laser source 242 and the rotation of the movable polygonal mirror 250 to begin the laser scanning operation as the leading edge 265 of the substrate 210 is beneath the focusing lens 252. The controller further controls the rotational speed of the movable polygonal mirror 250 to scan a row R of holes in one or more layers disposed on the substrate 210 {e.g., holes 147 in rear passivation layer stack 140 in Figure 1 ) as each facet of the movable polygonal mirror 250 is rotated across the pulses of electromagnetic radiation 255. The controller further controls the speed of the substrate positioning system 205 and the rotation of the movable polygonal mirror 250, such that as a first row R of holes {e.g., aligned in the X-direction) is finished, the next row R of holes begins at a desired spacing {e.g., in the direction A) from the first row due to the linear movement of the substrate 210 by the substrate positioning system 205. Accordingly, as the entire substrate 210 is moved beneath the optical device 235A, rows R of holes are formed in one or more layers of the substrate 210 across the entire width and length of the substrate 210. The controller 290 further controls the timing of the optical device 235A, such that as a trailing edge 270 of the substrate 210 passes beneath the focusing lens 252, the scanning operation will cease after a desired period of time has elapsed until the leading edge of the next substrate 210 is positioned beneath the focusing lens 252. The controller 290 may be any controller having a suitable processor, software and memory for operation of the laser processing platform 200. The substrate sensing system 260 may also include a substrate alignment device 280 configured to align the substrates 210 prior to entry into the scanning chamber 235B. Embodiments of the substrate alignment device 280 are detailed in Figures 10A and 10B.

[0034] The controller 290 generally includes a central processing unit (CPU) (not shown), memory (not shown), and support circuits (not shown). The CPU may be one of any form of computer processor used in industrial settings for controlling system hardware and processes. The memory 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 instruction the CPU. The support circuits are also connected to the CPU for supporting the processor in a conventional manner. The support circuits may include cache, power supplies, clock circuits, input/output circuitry subsystems, and the like. A program (instructions) readable by the controller includes code to perform tasks relating to monitoring, executing, and controlling the movement, support, and positioning of the substrates 210 along with various process recipe tasks to be performed in the laser processing platform 200. [0035] A control system described Figure 1 1 may also be implemented with the laser processing platform 200 to control on/off cycling of the optical device 235A. The control system may include software and hardware (electronics, circuits, circuit boards, and the like) that may be incorporated into the controller 290 of Figure 2A. Additionally, in one embodiment of the control system, the process of forming holes on a surface of a substrate may include at least one scan (to form a full row R of holes 147 (shown in Figure 2C)) that may be interrupted at desirable times to further control hole formation in desirable location on the surface of the substrate. This capability allows for briefly and selectively stopping/starting the train of pulses that would normally be continuous for a single polygon facet. This would allow for creating pattern features or interruptions which could enable advanced solar cell structures.

[0036] Figure 6 is a schematic perspective view of one embodiment of the scanning chamber 235B of Figure 2A. The scanning chamber 235B includes a sidewall 600 having a portion cutaway to show an internal volume 605. The sidewall 600 may comprise a Class I laser enclosure. The sidewall 600 includes openings at each end thereof that are sized to receive the transport belt 215 and a substrate 210. The internal volume 605 includes a plurality of regions 610A-610C that the substrate 210 is exposed to as it is transferred in the flow path D on the transport belt 215.

[0037] A first region 61 OA of the scanning chamber 235B comprises a scanning volume where the substrate 210 receives the electromagnetic radiation delivered from the optical device 235A (shown in Figure 2A). The first region 61 OA also includes one or both of an air knife 615 and an exhaust housing 620 that is coupled to a conduit 622 that couples to a vacuum source (not shown). As the substrate 210 is being processed in the first region 61 OA, particulate matter emitted during the laser drilling process is displaced by the air knife 615 and removed through the exhaust housing 620. While ablation by-products and other particulate matter may be removed in the first region 61 OA, a portion of these contaminants may be charged and thus have an electrostatic attraction to the substrate 210.

[0038] A second region 610B of the scanning chamber 235B comprises an optional ionization volume. The second region 610B includes an optional ionization device 625, such as an ionizing bar or an ion air knife. The ionization device 625 neutralizes and/or removes particles that may remain on the substrate 210 subsequent to the laser drilling process that were not removed in the first region 61 OA. A third region 610C of the scanning chamber 235B comprises cleaning volume that eliminates residual particulate matter from the substrate 210. The third region 610C includes an exhaust housing 630 and an air knife 635. Particulate matter that has been neutralized in the second region 61 OB is displaced by the air knife 635 and removed through the exhaust housing 630.

[0039] In one embodiment, the air knife 615 comprises a first air knife of the scanning chamber 235B and the air knife 635 comprises a second air knife of the scanning chamber 235B. In one aspect, the air knife 615 is provided to flow a gas (clean air) in a flow path 638A in a first direction (i.e., in the flow path D) and the air knife 635 is provided to flow a gas in a flow path 638B in a second direction that is a counterflow to the first flow direction. Each of the first region 61 OA, the second region 610B and the third region 610C may be separated by walls 640 to prevent cross-contamination between regions and assist in more controlled air handling.

[0040] The flow of the gas from each air knife 615, 635 may be provided at a flow rate and/or pressure that will not dislodge the substrate 210 from the transport belt 215. As the substrates are generally light in terms of mass or weight [e.g., about 6 grams to 10 grams), the pressure should be low enough to not blow the substrate 210 out of position. Additionally, the height and/or the angle of the flow paths 638A, 638B of each air knife 615, 635, respectively, is provided to minimize direct flow onto the substrate 210 to prevent movement of the substrate 210. In one embodiment, the flow paths 638A, 638B are about 1 inch above the surface of the substrate 210. Further, each of the flow paths 638A, 638B may be substantially parallel to a plane of the surface of the substrate 210. In one aspect, the pressure of each air knife 615, 635 is set to about 10 pounds per square inch (psi) and a flow rate of about 80 standard cubic feet per minute (SCFM). In one example, the flow rate of each air knife 615, 635 is about 6 SCFM at about 10 psi to about 20 psi. For the same reasons, the vacuum provided by the exhaust housings 620, 630 are provided at rates that will not dislodge the substrate from the transport belt 215. In one embodiment, the vacuum provided at each exhaust housing 620, 630 is substantially the same as the flow rates for each air knife 615, 635.

[0041] In one embodiment, the air knife 635 may comprise an ionization device configured to flow a concentration of positive and negative ions in the flow path 638B. Clean air may be provided to the air knife 635 at a low pressure (about 10 psi) and a flow rate similar to the flow rate of the air knife 615, 635, to prevent dislodging the substrate 210. The clean air flows a high concentration of positive and negative ions which neutralizes any charges between the substrate 210 and particulate matter on the substrate 210. The neutralized particles may be removed by the exhaust housing 630 in the third region 610C of the scanning chamber 235B.

[0042] The ionization device 625 in the second region 610B may be an ion bar or an ion air knife. When an ion bar is used, the ionization device 625 may be positioned about 1 or 2 inches above the surface of the substrate 210. The ion bar produces a high concentration of positive and negative ions, and any charges between the substrate 210 and particulate matter on the substrate 210 are neutralized. When an ion air knife is used, the ionization device 625 may be spaced farther away from the surface of the substrate 210. Clean air may be provided to the ionization device 625 at a low pressure (about 10 psi) and a flow rate similar to the flow rate of the air knife 615, 635, to prevent dislodging the substrate 210. The clean air flows a high concentration of positive and negative ions which neutralizes any charges between the substrate 210 and particulate matter on the substrate 210. While not shown, the second region 610B may also include a vacuum housing to remove the neutralized particulate matter. When the second region 610B is configured with the ion air knife as an ionization device 325, and a vacuum housing is provided therein, the third region 610C may not be necessary.

[0043] Figure 7 is an isometric view of a laser processing tool 700 having the optical device 235A and the scanning chamber 235B disposed therein. The laser processing tool 700 comprises a main frame 701 having a first side 702A that may be coupled to a loading station 217A (shown in Figure 2A) and a second side 702B that may be coupled to an unloading station 217B (shown in Figure 2A). The main frame 701 includes panels 705 that may function as doors or removable sheets, and a portion of the panels 705 are not shown to illustrate components within the laser processing tool 700. The panels 705 may include a view window 710 to provide visual access to the interior of the laser processing tool 700. The view window 710 may comprise laser safety glass or include filters that enable viewing of the electromagnetic radiation during a laser drilling process within the laser processing tool 700 without the need for safety glasses. Power sources and control equipment, such as a laser power supply 715 (shown in dashed lines) may be housed within the main frame 701 . Additionally, an optical alignment device 720 (Figures 7 and 8) may be coupled to the main frame 701 within the laser processing tool 700. The optical alignment device 720 may be provided to adjust the position of the optical device 235A relative to the motion direction of the substrates on the substrate positioning system 205, and thus adjusting the beam path emitted relative to a substrate.

[0044] During use of the laser processing platform 200, the substrates 210 may be displaced slightly during the ablation process in the creation of one row R of holes 147 (due to movement of the substrate 210 on the transport belt 215). For example, the substrates 210 may be displaced about 0.5 millimeters in the time it takes to produce one row R of holes 147. In one example, the result is rows R of holes 147 that are somewhat diagonally oriented across the width of the substrate 210. In order to mitigate this displacement of the substrates 210 and the diagonally oriented rows R of holes 147, the optical device 235A is rotated slightly about the Z direction using an apparatus and method described in Figures 8 and 9.

[0045] Figure 8 is a side view of one embodiment of the optical alignment device 720. The optical alignment device 720 comprises a base plate 800 that may be coupled to one or more support members 805 of the main frame 701 . The base plate 800 is movably coupled to a first support plate 810 having a second support plate 815 extending therefrom in a plane substantially orthogonal to the plane of the first support plate 810. The second support plate 815 generally supports the optical device 235A. The first support plate 810 is coupled to the base plate 800 by a plurality of adjustment devices 820, which may include fasteners, linear guides and combinations thereof. The adjustment devices 820 provide at least height adjustment (in the Z direction) of the optical device 235A and may provide a theta adjustment in the X-Z plane and/or the Y-Z plane. The height adjustment is used to adjust the focal length of the focusing lens 252 (Figures 2A and 2B) of the optical device 235A. The adjustment devices 820 may also be used to level the second support plate 815 relative to a plane of the transport belt 215. An adjustable aperture device 840 is provided between the second support plate 815 and the scanning chamber 235B. The adjustable aperture device 840 may be a telescoping housing having an aperture formed therein that is sized to receive a beam path provided by the optical device 235A. The telescoping housing may be moved upward or downward based on any height adjustment of the optical alignment device 720.

[0046] In some embodiments, the optical alignment device 720 also includes an adjustable mount plate 825 disposed between the second support plate 815 and a lower surface of the optical device 235A. The adjustable mount plate 825 is secured to the lower surface of the optical device 235A and is fastened to the second support plate 815 by fasteners 830. The adjustable mount plate 825 may be adjusted for different angular orientations as well as leveling the optical device 235A to tune a scan plane 835 of a beam path emitted by the optical device 235A during processing. As described in more detail in Figure 9, the adjustable mount plate 825 may be rotated about the scan plane axis 835 (e.g., Z-direction) to adjust the orientation of the scan plane (e.g., plane aligned to the row R direction on the substrate in Figure 2C) of the output of the optical device 235A. Adjustment of the adjustable mount plate 825 may be utilized to alter the beam path(s) within the scan plane of the optical device 235A to align a row R of holes 147 on a substrate 210 (Figure 2C) and to mitigate diagonally oriented rows of holes that occur due to the movement of the substrates on the transport belt 215 during creation of the rows of holes, as discussed above.

[0047] Figure 9 is an isometric view of the optical alignment device 720 of Figure 8. Although the optical device 235A is not shown in this view, the scan plane 835 of the optical device 235A is shown. The adjustable mount plate 825 includes a plurality of slots 900 that receive the fasteners 830 shown in Figure 8. Each of the slots 900 allow the adjustable mount plate 825 to rotate relative to the Z axis in order to adjust the scan plane of the optical device 235A. For example, a first alignment position 905 of the adjustable mount plate 825 may include a direction wherein the scan plane is substantially parallel to the leading edge 265 of the substrate 210 (shown in Figure 2A). However, testing of the laser processing platform 200 has shown the substrates 210 move about 0.5 millimeters in the time it takes to produce one row R of holes 147 (shown in Figure 2C). The resultant row R of holes 147 may be substantially non-parallel with the leading edge 265 of the substrate 210. To mitigate the diagonal orientation of the holes 147, the adjustable mount plate 825 is adjusted to a second alignment position 910, which corresponds to an angle 915. The angle 915 may be adjusted based on the speed of the substrate(s) 210 on the transport belt 215 as well as the scan speed of the electromagnetic radiation 255, which is at least partially based on pulse width of the electromagnetic radiation 255 and movement of the movable polygonal mirror 250 (shown in Figure 2B). In one embodiment, the angle 915 is about 0.5 degrees, when the speed of the transport belt 215 is about 140 mm/s to about 180 mm/s, with a pulse width of about 1 fs to about 1 .5 MS, and the scan speed of the movable polygonal mirror 250 is about 1 ,000 RPM, which results in rows R of holes 147 that are substantially linear and/or parallel with the leading edge 265 of the substrate 210.

[0048] Figures 10A and 10B are plan views of various embodiments of a substrate alignment device 280. The substrate alignment device 280 is configured to align any substrate(s) 210 that may be misaligned prior to entry into the scanning chamber 235B. Specifically, the substrate alignment device 280 is utilized to align any substrate(s) 210 that may be misaligned prior to sensing by the substrate sensing system 260 (shown in Figure 2A). The substrate alignment device 280 aligns the substrates 210 on the fly as the substrates 210 move on the transport belt 215.

[0049] In Figures 10A and 10B, the substrate alignment device 280 comprises a first alignment member 1005A on a first side of the transport belt 215, and a second alignment member 1005B on a second side of the transport belt 215. In one embodiment, the first alignment member 1005A comprises a plurality of rollers 1010 having an outer surface that is substantially parallel to edges of the transport belt 215 as shown in Figure 10A. On the opposing side of the transport belt 215, the second alignment member 1005B comprises a brush structure 1015, which includes a plurality of bristles 1020. The brush structure 1015 may be horizontally oriented such that the ends of the bristles 1020 face an edge of the substrate 210, or the brush structure 1015 may be vertically oriented such that ends of the bristles 1020 face upward (out of the paper) or downward (into the paper).

[0050] In operation, if a substrate 210 is misaligned as the substrate 210 is transferred on the surface of the transport belt 215, an edge of the substrate contacts one or both of a roller 1010 and one or more bristles 1020 of the brush structure 1015. Shear stress may be experienced by the one or more bristles 1020 causing a strain in the one or more bristles 1020. A restoring force in the one or more bristles 1020, as well as contact with more bristles 1020 (as the substrate 210 moves along the transport belt 215 transfer direction) may cause the substrate 210 to move away from the brush structure 1015 and against the surfaces of the rollers 1010, which results in a substrate 210 that is properly aligned.

[0051] In Figure 10B, the first alignment member 1005A and the second alignment member 1005B of the substrate alignment device 280 may operate the same. However, the first alignment member 1005A and the second alignment member 1005B may include a brush structure as described in Figure 10A, a plurality of rollers as described in Figure 10A, or combinations thereof. Additionally, the first alignment member 1005A and the second alignment member 1005B may include a first portion 1025 that is substantially linear and parallel with the edge of the transport belt 215, and a second portion 1030 that is angled relative to the first portion 1025. While both of the first alignment member 1005A and the second alignment member 1005B include the second portion 1030 in Figure 10B, only one of the first alignment member 1005A and the second alignment member 1005B may require the second portion 1030. In one configuration, the first alignment member 1005A and/or the second alignment member 1005B may be actuated relative to the parallel edge of the transport belt 215 to actively center the substrate(s) 210 relative to the parallel edge of the transport belt 215. In this case, the first alignment member 1005A and/or the second alignment member 1005B may be actuated by use of a pneumatic or an electrical motor that is adapted to position either or both of the first alignment members 1005A, 1005B relative to the parallel edge of the transport belt 215.

[0052] Figure 1 1 is a schematic side view of a control system 1 100 that may be used with the laser processing platform 200 of Figure 2A. In this embodiment, a sensor device 1 105, such as the substrate sensing system 260 (shown in Figure 2A), is positioned adjacent the transport belt 215. The sensor device 1 105 may be a proximity sensor that is used to detect the presence of a substrate 210 moving to a position under the optical device 235A. The sensor device 1 105 is coupled to a controller 1 1 10, which may form part of the controller 290. The sensor device 1 105 may include a common input/output controller (CIOC), and a trigger circuit 1 1 15, which is in communication with the laser source 242.

[0053] Using the control system 1 100, a method of controlling the laser by summing a start of scan (SOS) signal 1 120 from the optical device 235A and using a digital output signal 1 125 from the controller 1 1 10 when a presence signal 1 130 from the sensor device 1 105 is received, may be provided. The digital output signal 1 125 is based on input from the sensor device 1 105 and a time delay T. The time delay T may be equivalent to a travel distance of the substrate 210 when moving between the sensor device 1 105 and a starting point of the beam. The starting point of the beam refers to the position on a substrate where the laser processing will begin. The time delay T may be a variable that is dependent on the speed of the transport belt 215 and the distance of the sensor device 1 105 to the process area. In one embodiment, the time delay T may be about 0.4 seconds to about 0.8 seconds between detection of the substrate 210 and the actual triggering of the laser.

[0054] For example, when a leading edge 265 of a substrate 210 is sensed by the sensor device 1 105, the controller 1 1 10 generates the digital output signal 1 125 based on presence of the leading edge 265 and a time delay T. The optical device 235A generates the SOS signal 1 120, and a frequency of the SOS signal 1 120. The frequency of the SOS signal 1 120 is based on a rotation speed of the movable polygonal mirror 250 divided by number of facets on the movable polygonal mirror 250. In one embodiment, the optical device 235A includes a pilot laser 1 135 placed at suitable position to produce a transistor-transistor logic (TTL) signal pulse for each change of polygon facet of the movable polygonal mirror 250 relative to the incoming second beam 1 145B. A trigger signal 1 140, provided to the laser source 242, is then generated based on the presence signal 1 130 and the SOS signal 1 120. This arrangement produces a burst of laser pulses that are used to form a row R on the substrate for each provided trigger signal while preventing laser pulses impinging on the transport belt 215 when a substrate 210 is not present.

[0055] In one embodiment, the laser source 242 emits a first beam 1 145A at a first frequency and the first beam 1 145A is transformed into a second beam 1 145B having a second frequency. The first frequency of the first beam 1 145A may be in the red spectrum, such as wavelength greater than about 1000 nm, such as a wavelength of about 1064 nm. The first beam 1 145A is directed by a mirror 1 149 through an optional beam conditioner 1 150. The beam conditioner 1 150 may be a frequency doubler that emits the second beam in the second frequency. The second beam 1 145B is then directed by mirrors 1 151 and 1 152 to the movable polygonal mirror 250 and then to a mirror 1 153, the focusing lens 252 and substrate 210. The second frequency may be in the green spectrum, such as electromagnetic energy having a wavelength in a range between about 490 nm to about 570 nm.

[0056] Figures 12A and 12B are schematic plan views of a substrate 210 having a plurality of holes 147 formed thereon in a pattern P by the method described in Figure 1 1 . In Figure 12A, each of the holes 147 are discrete and spaced in a defined pitch. A line ablation pattern is shown in Figure 12B whereby holes 147 are at least partially overlapping to form a plurality of lines 1200. The line ablation pattern may be produced by increasing the number of laser burst pulses via a laser repetition rate and/or by adjusting the speed of the transport belt 215 and/or the rotation speed of the movable polygonal mirror 250. In one embodiment, the holes 147 include a diameter of about 40 μιτι (which may equal the width of a line 1200). The holes 147 may overlap about 20% and the lines 1200 may include a pitch of about 0.7 mm to about 1 .3 mm. In another embodiment (not shown), the lines 1200 may not be continuous as shown, and one or more of the lines may be a line and space pattern. For example, groups of holes 147 may be formed to be partially overlapping, and the groups of holes 147 may be separated by spaces therebetween where no ablation occurs.

[0057] Embodiments of the laser processing platform 200 described herein enable formation of multiple holes 147 on a substrate 210 that may be utilized in the production of solar cell substrates, such as in the formation of contact holes in a rear passivation layer stack of a solar cell. The holes 147 formed by the optical device 235A may be about 90 microns, or greater, and are formed at speeds that far surpass any conventional apparatus, enabling throughput of greater than 3,300 substrates per hour. The optical device 235A also forms holes 147 in the substrates with minimal damage to underlying or adjacent layers, enabling a yield of greater than 99% at the throughput mentioned above. The scanning chamber 235B removes particulate from the substrates during and after processing, which assists in the yield mentioned above. The laser processing platform 200 is also safe for personnel, utilizing safety glass for any view windows, as well as a class I enclosure utilized in the scanning chamber 235B. Further, the laser processing tool 700 comprises a small footprint, such as about 4.5 meters X 1 .0 meter, which enables retrofit into existing solar cell production lines.

[0058] While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1 . A laser scanning apparatus, comprising:

a movable transport assembly; and

an optical device disposed adjacent to the movable transport assembly, wherein the optical device comprises:

a polygonal mirror having a plurality of reflecting facets and an axis of rotation;

an actuator configured to rotate the polygonal mirror relative to the axis of rotation; and

a laser source positioned to direct electromagnetic radiation to at least one of the reflecting facets of the polygonal mirror, wherein the movable transport assembly is configured to position a substrate to receive the electromagnetic radiation reflected from the reflecting facets of the polygonal mirror.

2. The apparatus of claim 1 , further comprising:

one or more positioning sensors; and

a system controller configured to receive signals from the one or more positioning sensors.

3. The apparatus of claim 2, wherein the one or more positioning sensors are configured to detect a leading edge of the substrate as the movable transport assembly linearly transports the substrate in a direction that is at an angle to the direction of the electromagnetic radiation reflected from the reflecting facets of the polygonal mirror.

4. The apparatus of claim 2, wherein the system controller is configured to control the operation of the laser source and movement of the movable transport assembly based on signals received from the one or more positioning sensors.

5. The apparatus of claim 1 , further comprising a beam shaper positioned between the laser source and the polygonal mirror.

6. The apparatus of claim 1 , further comprising a frequency doubler positioned between the laser source and the polygonal mirror.

7. The apparatus of claim 1 , further comprising:

a housing defining an interior volume disposed about a portion of the movable transport assembly, wherein the housing includes at least two regions and each of the regions include an opening formed in opposing sidewalls of the housing to receive a substrate disposed on the movable transport assembly.

8. The chamber of claim 7, wherein the at least two regions comprise a scanning volume and an ionization volume.

9. A laser scanning platform, comprising:

a laser scanning device comprising a polygonal mirror, and configured to deliver pulses of electromagnetic radiation reflected by the polygonal mirror along a path that is parallel to a first direction, wherein the first direction is parallel to a surface of a substrate;

a substrate transport assembly configured to transport the substrate in a second direction while the pulses of electromagnetic radiation are directed towards the substrate, wherein the second direction is at an angle to the first direction;

one or more positioning sensors configured to detect a leading edge of the substrate as it is moved in the second direction toward the laser scanning device; and

a controller configured to control the operation of the laser scanning device and the substrate transport assembly based on signals received from the one or more positioning sensors.

10. The platform of claim 9, wherein the laser scanning device further comprises: a laser source; and

a beam shaper positioned between the laser source and the polygonal mirror.

1 1 . The platform of claim 10, wherein the laser scanning device further comprises: a frequency doubler positioned between the laser source and the polygonal mirror.

12. The platform of claim 9, wherein the laser scanning device further comprises an actuator configured to rotate the polygonal mirror at a desired speed.

13. The platform of claim 9, wherein the substrate transport assembly comprises a transport belt.

14. The platform of claim 13, further comprising:

a housing defining an interior volume having at least two regions, each of the regions having an opening formed in opposing sidewalls of the housing to receive a portion of the transport belt.

15. The platform of claim 14, wherein the at least two regions comprise:

an air knife configured to flow a gas in a first flow direction;

a vacuum housing positioned downstream from the first flow direction; and an ionization device configured to neutralize charges between particles and a substrate disposed on the transport belt.