Patent Publication Number: US-2010108820-A1

Title: Highly-integrated low-mass plastic film

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. patent application Ser. No. 10/865,429, filed Jun. 10, 2004, Klosner et al., HIGHLY-INTEGRATED LOW-MASS SOLAR SAIL. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     (Not Applicable) 
     REFERENCE TO A MICROFICHE APPENDIX 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to photolithography, photoablation, and crystallization techniques as part of a method for large-scale fabrication of highly-reflective reduced-mass-per-area plastic film with integrated microelectronics, MEMS, sensors, etc. 
     2. Description of Related Art 
     The solar sail is a propulsive device that uses a thin reflecting foil to deflect sunlight, transferring photon momentum to the sail and thereby accelerating it and the attached payload. Although the pressure per unit area of solar photons is rather small, it can be utilized to accelerate spacecraft to very high velocities—spacecraft with very low mass that are driven by very large sails can take advantage of the steady propulsion offered by the sun to attain speeds that are significantly higher than those achieved with chemical rockets. For example, at a distance of 1 Astronomical Unit the solar pressure is approximately 9 μN/m 2 , a force that will accelerate a one-gram, one-meter-square object at a rate of 9 mm/s 2 . Thus, if the material of a penny (2.5 grams) were spread over an area of one meter and reflected 100% of the incident solar light, it would accelerate to a velocity of 300 m/sec in just one day, assuming that it is steered directly away from the sun. This freely available energy makes solar sails an attractive propulsion system, but the relatively small momentum carried by photons presents a technical challenge to reduce mass. 
     To achieve useful acceleration of a solar sail by momentum transfer from solar photons, a sail made of very low-mass-per-area materials is required. Since the acceleration of the spacecraft will vary inversely with its mass, large surface area (on the order of hundreds of square meters) would be required effectively to gather sufficient photon momentum to move a spacecraft carrying even a small payload, such as a micro-satellite. A solar sail of this size, in addition to the electronics, communications equipment, sensors, and power sources required to operate the spacecraft and payload, stands to make up a significant part of the overall mass. Further, solar sail material must be resistant to ultraviolet (UV) radiation and easy to carry and deploy into space. 
     Thus, mass per unit area is a key factor in selecting materials for solar sails. Solar sails have been designed and constructed using, for example, thin films of aluminum around 100 nm thick, deposited on a 5-μm-thick substrate, such as polyester or polyimide. The coated substrate has a density of approximately 7 g/m 2 , with 0.5 g/m 2  arising from the aluminum and the remaining 6.5 g/m 2  from the substrate. With an aluminum reflectivity of close to 0.9, these sails accelerate at a rate of approximately 1 mm/s 2  at 1 AU, and could reach Mars within a year. A prototype of a solar sail deployed at NASA&#39;s Jet Propulsion Laboratory uses 3-μm-thick polyester coated with a thin-film aluminum layer achieving a total mass density slightly over 6 g/m 2 . 
     For interplanetary or interstellar travel, sails will need to be made significantly lighter than this. For example, an all-metal sail with no backing substrate would significantly reduce the mass of the sail. Various means for achieving this have been explored, such as substrates that vaporize upon exposure to UV radiation or the use of UV-degradable adhesion layers which bind the aluminum layer to the underlying substrate. In both cases, exposure to the sun after deployment results in a free standing aluminum foil. While an intermediate step might be simply to reduce the thickness of the underlying substrate, we must note that although polyester is readily available in 0.5 μm thickness, it is not an ideal sail material because it is easily degraded by the sun&#39;s ultraviolet radiation, potentially leading to a loss of the structural integrity of the sail; and while polyimide can withstand ultraviolet radiation, it is not available in layers much thinner than 8 μm. 
     A recently developed material comprised of carbon nanotubes is rigid, strong, and lightweight, only 5 g/m 2 , making it a desirable material for solar sails. However, given that it is rigid, it cannot be packaged in a very small volume like polyester and polyimide, and therefore, solar sails fabricated from the material would likely need to be assembled in space, adding significantly to the complexity of building and deploying the sail. Thus, no currently available material fulfills this role cost-effectively. 
     Several projects planned by NASA have called for large quantities of highly-reflective, lightweight, deployable, and durable materials. The fabrication method and resulting product presented here provides a cost-effective material sufficiently light, flexible, and UV-tolerant to satisfy this need. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to provide a novel solar sail material that addresses the shortcomings of existing materials by producing a low-mass substrate that will survive UV radiation, accommodate the integration of microelectronics, and that can be stored and deployed easily. 
     A feature of the invention is a weight reducing ablation pattern of ridges and wells formed by removal of material from the original substrate. Due to the laser ablation method employed in this process, this pattern may be any shape or profile suitable to the user&#39;s purposes, but polygons, specifically hexagonal ridges, are preferred in many uses. 
     Another feature of the invention is the ability to deposit and crystallize semiconductor material such as silicon on the substrate, allowing integration of microelectronic components without additional mounting hardware. 
     An advantage of the invention is its substantial weight savings. 
     Another advantage of the invention is that its laser ablation process allows the manufacturer flexibility in the ablation pattern in order to suit user-specific needs. 
     Another advantage of the invention is that the pattern ablated into the material&#39;s surface may be selected in order to prevent the propagation of rips in the material. 
     Another advantage of the invention is that it allows further mass reduction by miniaturizing and integrating electronic components that otherwise might need to be included in the payload, possibly requiring additional wiring and mounting components. 
     Another advantage of the invention is that it enables increased functionality through the integration of electronic components such as photovoltaic cells and sensors on the surface of the sail. 
     The invention has been described in its preferred utility as a solar sail, but other objects, features, and advantages of the invention will be apparent from the following written description, claims, abstract, and the annexed drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective back view of the preferred embodiment of a solar sail incorporating the invention with a hexagonal grid pattern. 
         FIG. 2  is a perspective front view of the preferred embodiment of a solar sail incorporating the invention with affixed electronic components. 
         FIG. 3  is a cross-sectional composite view of the preferred embodiment of a solar sail illustrating a hexagonal grid of cells from which material has been removed to leave flat-bottomed wells and a network of reinforcing structural ridges. 
         FIG. 4  is a stylized cross-sectional view equivalent to  FIG. 3 . 
         FIG. 5  is a schematic of the preferred embodiment of the invention in a prior art high-throughput laser patterning system. 
         FIG. 6  is a schematic of the preferred embodiment of the invention in a prior art large-area, high-throughput patterning system including a roll-to-roll material supply system and a movable stage with fixed optics. 
         FIG. 7  is a dual branch imaging apparatus that effectively recycles laser power to crystallize amorphous silicon into single-crystal configurations. 
         FIG. 8  is an illustration of a silicon crystallization technique by which a quantity of amorphous silicon is changed into a single-crystal configuration suitable for microelectronic fabrication. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a perspective illustration of the solar sail material in its preferred embodiment. The material consists of a substrate layer such as polyimide, coated with a reflective material on its first surface  1 , and textured by photoablation on the second surface to leave a tessellated pattern on the second surface. This pattern may be selected by the producer of the material to suit the specific application of the material. The pattern shown in  FIG. 1  is a hexagonal grid consisting of flat-bottomed wells denoted by  2  and ridges denoted by  3 . A hexagonal grid is suggested here for its relative structural strength and for its tendency not to propagate rips in the material. The reflective coating on the first surface  1  reflects inbound photons  4  emitted by the sun, collecting and imparting their momentum to the sail and thus to the payload. 
       FIG. 2  is a perspective illustration of the microelectronic integration capability of the invention. Since the first surface  1  remains unablated in this example, an opportunity exists to fabricate microelectronic devices such as photovoltaic cells  7 , control circuits  6 , and accelerometers  8  on this surface. This integration of components tends to save weight by integrating necessary components that may otherwise require independent packaging, mounting, and/or power supplies. Additional features may be integrated by this technique, as well. Photovoltaic cells and sensors such as accelerometers add additional power and information gathering capability while adding less mass than if these features were included in the system as discrete modules. 
       FIG. 3  is a cross-sectional view of the material illustrating the effect of ablation on cross-sectional area and thus mass. The original thickness of the substrate material is shown by arrow  9 . Ablated regions  10 , with the depth of ablation shown by  11  and the width of ablation shown by  12 , leave a well floor thickness of  13  and ridges of height  11  and width  14  attached to a layer of reflective material  15 . Below this cross-section is a projection of the cross-section onto an overhead view of a sample hexagonal pattern. This projection shows dimensions of the ablated region translated onto the pattern to illustrate how pattern ablation results in a polygonal well and ridge network. 
     The layer of reflective material  15 , preferably aluminum, is on the surface opposite the ablated surface. This material provides the reflective layer necessary to gather photon momentum. The aluminum layer also will serve as an etch stop if ablation should remove all the plastic at the floor of the well. 
     The mass reduction realized by the ablation (illustrated in  FIG. 3 ), the primary advantage of such a material, is shown by the following example. Removing a volume V R  of substrate material in the rough shape of a hexagonal prism of side-length l R  (denoted by  16 ) and depth h R  (denoted by  11 ) from a larger and thicker solid hexagonal prism of volume V S  leaves a well-shaped configuration with volume V W . The larger solid prism with side-length l S  (denoted by  17 ) and height h S  (equivalent to the thickness of the substrate film and denoted by  9 ) has a volume described by 
         V   S =(1.5 *l   S √{square root over (3)})* h   S . 
     The region removed by ablation has a volume of 
         V   R =(1.5 *l   S √{square root over (3)})* h   R . 
     Thus, removing this region leaves a well-shaped substrate cell with volume 
         V   W   =V   S   −V   R . 
     To illustrate the benefit of the invention, a hexagonal ablation pattern with a side length (l R ) of 10 μm to a depth (h R ) of 7 μm in a 10.5 μm (l S ) grid of 8-μm-thick (h S ) polyimide substrate material would remove a volume of about 181 μm 3  from an original solid hexagonal prism of about 218 μm 3 , resulting in a well with a volume of approximately 36 μm 3 —a volume (and hence mass) savings of approximately 83% over the unablated substrate material. This 83% savings changes the mass-per-unit-area density of the substrate from about 6.5 g/m 2  to about 1.1 g/m 2 . 

 
       FIG. 4  is a stylized version of  FIG. 3  illustrating a cross-section of a repeating pattern. The diagram includes the thickness of the well floors  18 , the ridges  19  separating the wells (made by the ablated areas  20 ), and the reflective coating  21 . 
       FIG. 5  is a schematic diagram illustrating a prior art system by which the material described above can be fabricated inexpensively in large-area batches. This process utilizes a technique in which large quantities of film material  57  are supplied to a lithographic stage  53  providing relative motion between the material and the laser optics  56 . The laser optics  56  carry a beam emitted by the laser  51  and shaped by an illumination system  52  and a patterning system. The patterning system illustrated here is a mask  54  that modifies the beam to include the desired pattern, here a hexagonal area  55 . The laser beam ablates masses of substrate in a pattern  55  that may be selected by the user of the process. The hexagonal grid described above exemplifies one variety of such a pattern. 
       FIG. 6  is a schematic diagram that illustrates a prior art method of supplying a flexible large-area substrate material  67  to the stage for ablation as described above. In the method shown here, a beam from a laser  61  is guided through a flexible mask  62  on rollers  63  containing a pattern  65  and through optics  66  to the surface of the material  67 . This system utilizes a movable stage  64  so the material  67  may be scanned across its width with the pattern  65  and a feed system to advance the material  67  to a take-up roll  68  where pattern-ablated material may be stored. The necessary relative motion between the material  67  and the optics  66  may also be implemented by movable optics scanning over a fixed stage. 
     As shown in  FIG. 1  above, the ridges resulting from the ablation of materials such as polyimide do not have sharp edges. Thus, the material may be rolled onto itself without concern of new layers cutting or being cut by earlier material. 
     The low temperature at which this process occurs does not risk melting the substrate material, which further facilitates large-scale production; large sheets may be produced without the risk of ruining an entire sheet by melting one portion. 
       FIG. 7  illustrates the crystallization of silicon effected by laser radiation. This invention allows the crystallization of silicon on the substrate material. If a surface of the film  76  is coated with amorphous silicon, the same optical workstation utilized for the ablation phase of the method can crystallize the silicon into a suitable configuration for microelectronic components. The preferred embodiment of the invention includes a laser  71  and two optical branches, imaging  72  and non-imaging  73 . The imaging branch  72  focuses the beam, directs it through a mask  74  and projection optics  75  to perform high-resolution exposures on the substrate  76 . The non-imaging branch  73  performs flood irradiation that overlaps the region exposed by the imaging branch  72 . The substrate  76  and stage  77  move relative to the optics described above to allow repeated exposures of the pattern onto the substrate  76 . 
       FIG. 8  shows the crystallization of silicon facilitated by the technique illustrated in  FIG. 7 . The radiation produced by the workstation can be shaped into a chevron pattern  81  and scanned over the amorphous silicon  82  on the surface of the substrate film to create a single-crystal silicon region  83 . This enables specified quantities of single-crystal silicon to be precisely and inexpensively placed on the substrate in such a way that electronic components can be located on the surface of the substrate itself. 
     The invention has been shown preferably in the form of a solar sail with its mass reduced by a polygonal ablation pattern and with microelectronic components integrated into the surface of the substrate. It will be clear that the modifications described above and other modifications, whether described as alternatives or not, will be apparent, without departing from the spirit and scope of the invention, as described in the following claims: