Abstract:
A three-dimensional printing approach based on stereolithography with variable printing resolutions to solve the trade-off between throughput and resolution. In this technology, the variable fabrication resolutions are achieved by switching light wavelength. The apparatus includes an optical filter based on high-contrast gratings. In one embodiment, the minimum printing resolution of the accordingly constructed stereolithography apparatus is reduced to 37 μm.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Patent Application No. 62/327,699; entitled “3D PRINTING WITH VARIABLE VOXEL SIZE BASED ON OPTICAL FILTER;” filed on Apr. 26, 2016; the content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a stereolithography apparatus and method of controlling light in a stereolithography apparatus. 
         [0003]    The following references may be referred to in this disclosure and are incorporated herein by reference.
       X. Yan and P. Gu, Comput. Aided Des. 28, 307 (1996).   N. Guo and M. C. Leu, Front. Mech. Eng. 8, 215 (2013).   K. V. Wong and A. Hernandez, ISRN Mech. Eng. 2012, 208760.   C. W. Hull, U.S. Pat. No. 4,575,330 (11 Mar. 1986).   Y. Cao, D. Li, and J. Wu, Rapid Prototyping J. 19, 100 (2013).   D. Miller, C. Deckard, and J. Williams, Rapid Prototyping J. 3, 4 (1997).   J. H. Sim, E. D. Lee, and H. J. Kweon, Int. J. Precis. Eng. Manuf 8, 50 (2007) available at http://www.dbpia.co.kr/Journal/ArticleDetail/NODE00884629.   Y. Nakayama, S. Okazaki, N. Saitou, and H. Wakabayashi, J. Vac. Sci. Technol. B 8, 1836 (1990).   C. J. Chang-Hasnain and W. Yang, Adv. Opt. Photonics 4, 379 (2012).   M. C. Y. Huang, Y. Zhou, and C. J. Chang-Hasnain, Nat. Photonics 1, 119 (2007).   V. Karagodsky and F. G. Sedgwick, Opt. Express 18, 16973 (2010).   V. Karagodsky and C. J. Chang-Hasnain, Opt. Express 20, 10888 (2012).   C. J. Chang-Hasnain, Semicond. Sci. Technol. 26, 014043 (2011).   F. Lu, F. G. Sedgwick, V. Karagodsky, C. Chase, and C. J. Chang-Hasnain, Opt. Express 18, 12606 (2010).   C. F. R. Mateus, M. C. Y. Huang, Y. Deng, A. R. Neureuther, and C. J. Chang-Hasnain, IEEE Photonics Technol. Lett. 16, 518 (2004).   K. Ikeda, K. Takeuchi, K. Takayose, I.-S. Chung, J. Mork, and H. Kawaguchi, Appl. Opt. 52, 1049 (2013); L. Carletti, R. Malureanu, J. Mork, and I.-S. Chung, Opt. Express 19, 23567 (2011).   D. Fatal, J. Li, Z. Peng, M. Fiorentino, and R. G. Beausoleil, Nat. Photonics 4, 466 (2010).   Y. Yao, H. Liu, and W. Wu, Appl. Phys. A 115, 713 (2014).   Y. Yao, H. Liu, and W. Wu, J. Vac. Sci. Technol. B 32, 06FG04 (2014).   H. Liu, Y. Yao, Y. Wang, and W. Wu, J. Vac. Sci. Technol. B 32, 06FE04 (2014).   M. Shokooh-Saremi, Opt. Express 16, 18249 (2008).   W. Yang and C. J. Chang-Hasnain, Proc. SPIE 8633, 863303 (2013).       
 
         [0026]    Additive manufacturing (AM), commonly known as three-dimensional (3D) printing, has been a fast developing area for more than three decades. It is a process that uses information from a computer-aided design file to build a 3D physical object. It has significant advantages over traditional manufacturing methods in (1) rapid creation of 3D prototypes, and (2) cost-effective production of patterns and molds with complex surfaces. Many materials have already been used in additive manufacturing such as polymer, metal, and ceramic. There are several technologies that have been developed for additive manufacturing, including stereolithography, binder printing (3DP), fused deposition modeling, selective laser sintering (SLS), etc. Among these technologies, stereolithography is the first commercially available prototyping machine and one of the most widely used AM processes. 
         [0027]    The production efficiency of stereolithography is determined by many factors, such as light spot diameter, scanning speed, hatch space, and curing depth. Among them, light spot diameter is the most direct way to determine the production efficiency. 
         [0028]    The diameter of the light spot is determined by the entire optical system and could be difficult to control precisely. Therefore, most stereolithography systems set the spot diameter as a constant. The specific value is a tradeoff between the size of the part that is being built and the desired resolution, which is typically about 0.1%-0.5% of the overall dimension. For this reason, a variable beam spot that can improve production efficiency while keeping high resolution is a promising direction for stereolithography. With a variable beam spot, a large spot can fill an open area quickly and a small spot can build details that require high resolution. Many studies have been carried out on methods to change spot size. Miller et al. developed a SLS workstation that has two laser spot sizes by pulling an aperture into and out of light path. Sim et al. used lenses with different focal length to produce different laser spot sizes. Cao et al. reported a stereolithography process that uses a dynamic focusing mirror to change spot size. Several specimens demonstrated more than 25% building time saving. 
         [0029]    Most of the studies involved the dynamic motion of optical components such as lens or physical aperture. Hence, cost of the systems would be increased due to the requirement of precise control of the motion during the fabrication process. 
       SUMMARY 
       [0030]    This disclosure includes a new method of changing light beam spot size by using two laser wavelengths and an optical filter based on high-contrast grating structure. For one of the wavelengths, the filter is almost transparent, which gives a larger spot size, while for the other wavelength, the filter works as an aperture and only part of the area is transparent, which gives a much smaller beam spot size. The advantages of this method over the previous approaches are: (1) the filter could significantly reduce the spot size of the laser beam that has been focused by lens; (2) as long as the optical system is well adjusted initially, no precise adjustment is needed during the fabrication process; (3) only an optical switch that can switch between the two wavelengths is needed during the continuous manufacturing process; (4) wavelength switching is faster than other spot size adjustment methods; (5) the shape of beam cross-section can be modified into any geometry in a fashion similar to the shaped-beam electron-beam lithography. 
         [0031]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic of a stereolithography apparatus. 
           [0033]      FIG. 2  is a side view of the stereolithography apparatus of  FIG. 1 . 
           [0034]      FIGS. 3A and 3B  depict two transmission modes of the optical filter. 
           [0035]      FIG. 4  is a schematic diagram of an exemplary optical filter  70 . 
           [0036]      FIG. 5  is a flow chart of a fabrication process. 
           [0037]      FIG. 6  is a SEM image of TiO 2  gratings and quartz substrate. 
           [0038]      FIG. 7  is a chart of experimental and simulated transmission spectrum of TiO 2  gratings. 
           [0039]      FIG. 8  includes charts of a comparison of the line widths of patterns printed by unfiltered beam and filtered beam: (a) 405 nm laser and (b) 445 nm laser. 
           [0040]      FIG. 9  depicts a printed pattern with both big feature (one vertical line, printed by 445 nm laser) and small feature (three horizontal lines, printed by 405 nm laser). 
           [0041]      FIG. 10  illustrates a digital model of a microfluidic device. 
           [0042]      FIG. 11  provides a table of calculated distance of travel of the beam spots for building the microfluidic device in  FIG. 10 . 
           [0043]      FIG. 12  is a side view of a CNC accumulation apparatus. 
           [0044]      FIGS. 13A and 13B  depict two transmission modes of an optical filter with a focusing lens. 
           [0045]      FIGS. 14A and 14B  also depict two transmission modes of an optical filter with a focusing lens. 
           [0046]      FIGS. 15A-15D  depict multiple transmission modes of an a plurality of optical filters. 
           [0047]      FIGS. 16A and 16B  depict two transmission modes of the optical filter with an optical fiber. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0049]    Additionally, the functionality described herein as being performed by one component may be performed by multiple components in a distributed manner. Likewise, functionality performed by multiple components may be consolidated and performed by a single component. Similarly, a component described as performing particular functionality may also perform additional functionality not described herein. For example, a device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. Furthermore, some embodiments described herein may include one or more electronic processors configured to perform the described functionality by executing instructions stored in non-transitory, computer-readable medium. Similarly, embodiments described herein may be implemented as non-transitory, computer-readable medium storing instructions executable by one or more electronic processors to perform the described functionality. Described functionality can be performed in a client-server environment, a cloud computing environment, a local-processing environment, or a combination thereof. 
         [0050]    Stereolithography Setup 
         [0051]    The stereolithography process uses photocurable resin, which consists of monomers that can be polymerized into large molecules. In the apparatus  10  of  FIG. 1 , a laser source  15  creates a laser beam  20  that is deflected by two gyro-mirrors  25  and  30 . Next, the laser beam  20  is projected on the surface  35  of a resin  40  in a liquid tank  45  and traces a cross-section of the object that is being printed. A Z stage  50  stays below the liquid resin surface at a depth that is smaller than the light curing depth. After a layer has been cured, the Z stage  50  moves down a distance of the thickness of a layer. Other stereolithography systems may operate differently from the apparatus  10  discussed herein. 
         [0052]      FIG. 2  shows the stereolithography setup used according to the schematic of  FIG. 1 . A light beam  20  was created by a collimated laser source  15 , including collimated lens  55  ( FIG. 1 ). Lasers with 405 and 445 nm wavelength were used in the experiment. Other wavelengths are envisioned for the system of  FIG. 2 . The beam  20  was then focused by the focusing lens  60 . Next, the beam was deflected by the Y mirror and X mirror  25  and  30 , respectively, to scan on the resin surface  35 . The testing target  65  was a silicon wafer coated with photocurable resin. A filter  70  was installed next to the focusing lens. 
         [0053]      FIG. 12  shows a computer numerical control (CNC) accumulation tool. The following reference nos. relate to the figure:  200 —optical fiber no. 1;  205 —optical fiber no. 2;  210 —X Axis;  215 —Y Axis;  220 —Z Axis;  225 —C Axis,  230 —A Axis;  235 —LED no. 1;  240 —LED no. 2;  255 —motion controller;  260 —force sensor;  270 —resin tank. 
         [0054]    Optical Filter Design and Fabrication 
         [0055]    The optical filter  70  in this disclosure has different transmission rates for 405 and 445 nm light, as shown in  FIGS. 3A and 3B .  FIGS. 13 and 14  show multiple ways of placing the filter  70 .  FIGS. 13A and 13B  depict a first arrangement with two transmission modes of an optical filter  70  with a focusing lens  60  to a focal plane  72 .  FIGS. 14A and 14B  depict a second arrangement with two transmission modes of an optical filter  70 A with a focusing lens  60 A to a focal plane  72 A. As shown in  FIGS. 13A and 13B , one arrangement includes placing the filter  70  in close proximity of the lens  60 . Diffraction is ignored in this case since the lens  60  and the filter  70  are close. The second arrangement includes placing the lens  60 A away from the filter  70 A. The 445 nm light still passes the filter  70 A and gives a larger spot. The 405 nm light starts to expand after the filter  60 A due to diffraction. The lens  70 A collects the light and projects the image of the aperture on the focal plane  72 A, providing a small laser spot. 
         [0056]    In addition to using one filter  70  and two laser wavelengths to achieve two laser spot sizes, multiple filters can be stacked to achieve more than two laser spot sizes. For example, in the  FIGS. 15A-15D , three filters  70 B,  70 C, and  70 D are stacked and four wavelengths are used to give four laser spot sizes. Wavelength λ 1  can pass all three filters and give the largest beam spot. The first filter  70 B works as an aperture only for λ 2  and is transparent to other wavelengths. Wavelength λ 2  gives the second largest laser spot size. The second filter  70 C works as an aperture only for λ 3  and is transparent to other wavelengths. λ 3  gives the third largest laser spot size. The third filter  70 D works as an aperture only for λ 4  and is transparent to other wavelengths. λ 4  gives the smallest laser spot size. 
         [0057]      FIGS. 16A and 16B  depict two transmission modes of an optical filter  70  on a single CNC accumulation tool having optical fibers  200  and  205  ( FIG. 12 ). For the 445 nm light, most light can pass the filter, which gives a large beam size. For the 405 nm light, incident light on most area of the filter was reflected, and only a beam with a smaller diameter passes. Only part of the filter  70  allows transmission of the 405 nm light. Therefore, the filter works as an aperture. Similarly, the filter  70  can include multiple filters as shown in  FIGS. 15A-15D . 
         [0058]    The reflection of the 405 nm light is achieved, in one implementation, by high-contrast gratings that have been used in many applications owing to their high reflectance and broad reflection band properties. In addition, the thickness of the high-contrast grating is smaller than that of other reflectors such as dielectric reflector. 
         [0059]    The schematic of an optical filter is shown in  FIG. 4 . The filter  70  includes a bottom quartz layer  75 , a TiO 2  grating layer  80 , and a top planarization layer  85 . Gratings  90  at center area  95  are etched off to provide an aperture for the 405 nm light. A period of the gratings is P. Edge length and thickness of the TiO 2  grating is L and H, respectively. 
         [0060]    The working principle of high-contrast gratings  90  can be described as following: When light is incident on the gratings  90 , lateral guided modes are generated, resulting in resonance and reradiation. When the transmitted waves interfere destructively, transmission disappears, and strong reflection occurs. In the area  95  where gratings were etched off, there is no high refractive index contrast. Therefore, no strong reflection would occur. As a result, light can get through this area  95 . However, in this area  95 , some quartz gratings might be created during the process of etching off TiO 2  gratings. Hence, a planarization layer  85  that has similar refractive index as quartz was applied to eliminate undesired resonance. The reflection spectrum can be tuned by adjusting the geometry of the gratings, including P, L, and H in  FIG. 4 . In this implementation, TiO 2  was selected because among all materials with low loss at this wavelength range, TiO 2  has the highest refractive index. Since the light sources are polarized, the filter was designed for this specific polarization (TM), and 1D grating design was used. 
         [0061]    The fabrication process  100  is summarized in  FIG. 5 . The gratings  90  were fabricated by using nanoimprint lithography. First ( 105 ), a Si mother mold was fabricated by interference lithography. Then, a glass mold ( 110 ) was duplicated from the Si mother mold by transferring pattern to a layer of UV-curable resist on a glass substrate. The filter substrate was prepared by deposition of a 400 nm thick TiO 2  thin film using direct current magnetron sputtering on top of a quartz substrate ( 115 ). The grating pattern was transferred ( 120 ) from the glass mold to the TiO 2  layer via nanoimprint, lift-off, and RIE etching process. In the lift-off process, 10 nm thick chrome mask was made by electron beam deposition and worked as etching mask in the following RIE etching. An RIE etching recipe that was developed by Liu et al. with a gas combination of SF 6 , C 4 F 8  and O 2  was used.  FIG. 6  shows a SEM image of finished TiO 2  gratings. 
         [0062]    After TiO 2  gratings were fabricated, an additional step  125  of photolithography and RIE etching was carried out to etch away TiO 2  gratings in a circular area with a diameter of 96 μm. This area can virtually be any shape depending on the requirement of application. Finally, a planarization layer was applied for aforementioned reason. Two-hundred nanometer thick UV curable resist was spin coated and cured to be the planarization layer. 
         [0063]    Transmission of the Filter 
         [0064]    Optical transmission of the grating area was measured and compared with simulation in  FIG. 7 . Numerical simulation was performed via finite-difference time-domain (FDTD) method using Lumerical FDTD solutions software. In the spectrum, the measured center wavelength of reflection matches the simulation result very well at 405 nm. Transmission at 405 nm is 10% while at 445 nm is about 80%, which means 405 nm light will be mostly reflected by the gratings while 445 nm light will pass. 
         [0065]    Printing Demonstration 
         [0066]    The design purpose of the filter is to greatly reduce the spot size of 405 nm laser while keeping the spot size of 445 nm laser similar to the unfiltered one. The values 405 nm and 445 nm are exemplary. Moreover, the filter transmission sizes and transmission rates are exemplary. Four groups of lines were printed to verify this effect. The target surface was a quarter of 4-in. Si wafer coated with 0.4 ml liquid photocurable resin. After patterns were printed, the sample was rinsed by isopropanol to remove uncured liquid resin. Finally, linewidths were characterized by an optical micro-scope.  FIG. 8  compares the linewidths of lines printed by the unfiltered beam and the filtered beam for both the 405 nm laser and the 445 nm laser. The beam spot size of the 405 nm laser was much more reduced than the 445 nm laser. For example, at laser power of 120 mW, spot size of 405 nm light was reduced from 372 to 50 μm while spot size of 445 nm light was only reduced from 332 to 258 μm due to slight reflection and material loss. There is no corresponding data for some power values since the resin cannot be fully cured in such low power. Additionally, the unfiltered beam of power below 30 mW could not print solid pattern, which confirms that small pattern below 100 μm can only be printed with the filter. 
         [0067]    In order to demonstrate changing pixel size by switching wavelength, a test pattern was printed, which is shown in  FIG. 9 .  FIG. 9  depicts a printed pattern with both big feature (one vertical line, printed by 445 nm laser) and small feature (three horizontal lines, printed by 405 nm laser). Area between the features is the surface of silicon wafer. The microscope was focusing at the top of the features. Black areas are shadows that are on the surface of the wafer and are enlarged due to out of focus. With optical filter installed in the printing system, variable spot size was achieved by only switching wavelength. The optical filter was installed in the stereolithography system during the printing process. The thinner lines (37 lm wide) were first printed by the 405 nm laser. Then, the wavelength of laser was changed to 445 nm to print the thicker lines (272 μm wide). 
         [0068]    Efficiency Test Based on Digital Model 
         [0069]    The purpose of having a variable beam spot is to improve the manufacturing efficiency when the object that is to be built has both small features and large features. To demonstrate the improvement, a digital model of a microfluidic device is created as shown in  FIG. 10 . The device has an overall dimension of 40 mm (length)×20 mm (width)×2 mm (height). A channel inside the block has a cross-section of 200×200 μm. The beam sizes are taken from the data of 120 mW laser power in  FIG. 8 . The small spot is 50 μm in diameter from the 405 nm laser, and the large spot is 258 μm in diameter from the 445 nm laser. Variable pixel sizes from 37 to 417 μm have been demonstrated in some embodiments. A maximum spot size can be as large as 700 μm. The distances of travel of the beam spots to build this device are shown in Table 1 ( FIG. 11 ). The distances are calculated using small beam spot only and using small and large beam spots together. The speed of the spots is a constant during real manufacturing process. So, the distance of travel is proportional to manufacturing time. If both of the spots are used, the small spot can build features of the channel, and the large spot can fill other volume in the block. If only one spot can be used, in order to keep the resolution of the channel the same, the small spot will be used to build the whole object. As a result, using the two different beam spots saves 87.6% of manufacturing time, which is a significant improvement. 
         [0070]    Discussion 
         [0071]    The measured transmission spectrum did not match the simulation result perfectly, but provided effectiveness. Several factors might affect the filter&#39;s performance. First, TiO 2  has some surface roughness that can be observed from the SEM image. Also, the sidewall profile was not strictly vertical. Additionally, the deposited TiO 2  has larger loss than the ideal value used in the simulation, especially in short wavelength range, which was not included in the simulation model. 
         [0072]    A high-contrast grating is chosen in this implementation for its small thickness compared with a conventional multilayer coated optical filter. Two wavelengths are used in this implementation for concept demonstration. The filter can be placed at close proximity of the surface of the resin and multiple layers of high-contrast gratings can be integrated into a single filter which can give more beam shapes for multiple wavelengths. In comparison, the conventional filter, if being used in a similar way, will suffer more reduction of resolution in beam shapes due to diffraction of light that travels through thicker material. 
         [0073]    Generally, 405 nm laser is able to cure more resin than 445 nm laser at same power level due to higher photon energy. However, when the filter was being used, the 405 nm laser printed thinner lines than the 445 nm laser, which confirms the effectiveness of the filter. 
         [0074]    In summary, an optical filter based on high-contrast gratings is designed and fabricated by nanoimprint lithography. The function of cropping 405 nm light beam and transmitting 445 nm light beam is achieved. By using the filter, the minimum printing resolution of the current setup is reduced to 37 μm. Variable beam spot size is realized by exchanging wavelength. This implementation shows a promising way to optimize the manufacturing efficiency of the stereolithography process. 
         [0075]    Accordingly, the invention provides new and useful stereolithography apparatus and method of changing light beam size. Various features and advantages of the invention are set forth in the following claims.