METHODS OF CONTROLLING DIMENSIONS IN PROJECTION MICRO STEREOLITHOGRAPHY

Abstract

Parallel surfaces on two substrates are established with specific distances of separation, typically within a 10-micron tolerance. In general, one surface is a surface of a transparent membrane or hard window. On one embodiment, the gap defined by the distance of the transparent membrane or hard window and the other surface used to precisely control the dimensions of layers in projection micro stereolithography, however the methods for establishing the relative positions of two surfaces can be adapted to other applications.

Claims

1. A method for positioning a first surface of a first substrate relative to a surface of a second substrate, wherein the second substrate is transparent, which method comprises: positioning a first substrate and a system comprising a lens having an optical axis, a charge-coupled device (CCD) capable of performing or assisting in performing an auto focusing program, and a displacement sensor having an emission vector, in a manner such that the lens is situated between a surface of the first substrate and the CCD, the optical axis of the lens intersects the surface of the first substrate, the displacement sensor has an emission vector parallel to the optical axis, and the CCD is focusable through the lens along the optical axis, sequentially aligning three non-linear points on the surface of the first substrate with the emission vector of the displacement sensor, measuring the distance between the displacement sensor and the surface at each selected point, adjusting the substrate so that the distances between the displacement sensor and each of the three points are the same to establish a first level surface, placing the second substrate between the first substrate and the displacement sensor, sequentially aligning three non-linear points on the surface of the second substrate with the emission vector of the displacement sensor, measuring the distance between the displacement sensor and the surface at each selected point, adjusting the substrate so that the distances between the displacement sensor and each of the three points are the same to generate surface parallel with the first level surface.

2. The method according to claim 1 further comprising in any order, performing an auto focusing program using the CCD so that the first level surface is at the focus plane of the lens followed by reading the position of the first surface with the displacement sensor placing the second substrate between the lens and the first substrate so that a surface is intersected by the optical axis of the lens and positioning the second substrate so that the surface of the second substrate is at a selected distance from the first level surface as measured by the displacement sensor.

3. The method according to claim 1 wherein the CCD is a laser CCD.

4. The method according to claim 1 wherein the laser CCD is a laser CCD camera.

5. The method according to claim 1 wherein the lens is a projection lens.

6. The method according to claim 1 wherein the displacement sensor is a laser displacement sensor.

7. The method according to claim 1 wherein the displacement sensor is accurate to within 10 microns or less.

8. The method according to according to claim 1 wherein the first substrate is a sample stage for a stereolithography system, and the second substrate comprises a membrane or hard window.

9. The method according to claim 8 wherein the sample stage and membrane or hard window are positioned in a resin tank and the method further comprises a step wherein after the first and second layers are positioned relative to each other the resin tank is filled with a curable resin to the level of the membrane or hard window.

10. The method according to claim 9 wherein the second substrate comprises a membrane.

11. The method according to claim 9 wherein the distance between the first substrate and the second substrate is equal to the thickness of a first layer of a stereolithography process.

12. A method for preparing and controlling the layer thickness in a stereolithography system comprising filling a gap between a sample stage and a membrane or hard window of a stereolithography system with a sacrificial resin, curing the sacrificial resin and establishing a working gap with an accurately known distance between the cured resin and the membrane or hard window, printing a sample from the interface of the cured sacrificial resin and membrane or hard window, and then removing the cured sacrificial resin.

13. The method according to claim 12 wherein the sacrificial resin is delivered from a center channel of the sample stage.

14. The method according to claim 12 wherein the sacrificial resin is photo sensitive resin and is cured using UV light.

15. The method according to claim 12 wherein the sacrificial resin is a thermal curable resin and is cured by heat.

16. The method according to claim 12 wherein the sacrificial resin is removed by exposure to an etching solution.

17. The method according to claim 12 wherein the printing apparatus is arranged so that as the sample is printed, it will be submerged into an etching solution, which will remove the sacrificial base and release the sample.

18. The method according to claim 12 wherein the sacrificial resin comprises an acrylamide and a photoinitiator and the etching solution comprises water or acid (PH value≥4) buffer solutions.

19. The method according to claim 18, wherein the sacrificial resin further comprises polyethylene glycol or water.

20. The method according to claim 18, wherein the acrylamide comprises N,N-dimethylacrylamide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic drawing of a projection micro stereolithography system with laser displacement sensor.

[0020] FIG. 2 shows the steps used in leveling a sample stage using a laser displacement sensor in the PμSL system.

[0021] FIG. 3 shows operations for precisely defining the thickness of the first layer in a PμSL system.

[0022] FIG. 4 shows steps in creating a sacrificial base to control the dimension of a 3D printed sample.

DETAILED DESCRIPTION OF THE INVENTION

[0023] In one embodiment of the invention, the method is aided by a displacement sensor, e.g., as part of the lens/CCD/displacement system discussed above. Typically, the displacement sensor is a laser displacement sensor, but any other type of displacement sensor accurate to within 10-microns or less can be used. Such displacement sensors are commercially available, for example, laser displacement sensors from Keyence detect a displacement of 10 microns or less.

[0024] The displacement sensor serves two purposes. One is to align one surface parallel to another. For example, the methods herein can be used as part of PμSL printing process to establish a resin free surface, membrane or hard window as parallel to the surface of a sample stage. As shown in FIG. 2 for a PμSL printing system, three non-linear points, here forming the right-angle triangle shown, are selected on the sample stage surface and sequentially aligned with the displacement sensor emission vector by moving the x-y stages. Having a minimum distance between the points of 1 cm should guarantee good accuracy. The sample stage should be adjusted to make sure the distance readings between the displacement sensor and each point are the same. As the emission vector of the displacement sensor is parallel to the optical axis of the lens, proper controls of the system will provide a stage surface perpendicular to the emission vector of the displacement sensor, and the optical axis of the lens.

[0025] A membrane or hard window is then placed between the displacement sensor and the sample stage and the process above is repeated using three selected points on the surface of the membrane or the hard window to level the membrane. Again, proper control of the system readily provides a surface perpendicular to the optical axis ensuring that the two surfaces are parallel.

[0026] The second purpose of the laser displacement sensor is to precisely define the distance between the substrates or surfaces. In the case of 3D printing, e.g., in the PμSL system of FIG. 3, the distance between the surface of the sample stage and the membrane or the hard window will determine the thickness of the first layer as in the PμSL system as shown.

[0027] To define the distance between the substrates or surfaces, the system runs the auto focus program using the CCD, see FIG. 3, to place the sample stage at the focus plane. The laser displacement sensor then reads the position of the surface of the sample stage. The second substrate, e.g., the membrane or hard window, is then placed in position and the laser displacement sensor will help to move the membrane or the hard window to the desired distance. In the case of 3D printing the distance between the sample stage and the membrane or hard window will equal the thickness of the first layer delivered to the sample stage. Control of the distance between the two layers can be no more accurate than the error of the displacement sensor and an accurate laser displacement sensor, e.g., accurate to within 10 microns or less, is preferred.

[0028] In the process as described above, two surfaces perpendicular to the optical axis are formed, which arrangement is ideal for 3D printing. The substrate bearing the surface of interest can be moved to sequentially bring the three points in line with a stationary displacement sensor. This can be done, for example, for the surface of a sample stage in a printing system, such as those shown in the present drawings, by moving the surface in the x-y directions.

[0029] In some embodiments, the displacement sensor may be moved to various locations aligned with the selected points to take the measurements. Under these circumstances, the surfaces will be parallel if the sensor locations for measurements on the first surface lie in a first common plane and the locations for measurements on the second surface lie either in the first common plane or in a second common plane parallel to the first.

[0030] One may also envision using the present lens/CCD/displacement system to establish parallel surfaces that are not perpendicular to the optical axis.

[0031] In particular embodiments, the invention provides a method for positioning a first surface of a first substrate, e.g., a sample stage for a stereolithography (3D printing) device, relative to a surface of a second substrate, e.g., a transparent substrate such as a membrane or hard window, wherein the second substrate is transparent, which method comprises: [0032] a) positioning a first substrate and a system comprising i) a lens having an optical axis, ii) a charge-coupled device (CCD) capable of performing or assisting in performing an auto focusing program, and a displacement sensor having an emission vector, in a manner such that the lens is situated between a surface of the first substrate and the CCD, the optical axis of the lens intersects the surface of the first substrate, the displacement sensor has an emission vector parallel to the optical axis, and the CCD is focusable through the lens along the optical axis, [0033] b) sequentially aligning three non-linear points on the surface of the first substrate with the emission vector of the displacement sensor, measuring the distance between the displacement sensor and the surface at each selected point, adjusting the substrate so that the distances between the displacement sensor and each of the three points are the same, [0034] c) placing the second substrate between the first substrate and the displacement sensor, sequentially aligning three non-linear points on a surface of the second substrate with the emission vector of the displacement sensor, measuring the distance between the displacement sensor and the surface at each selected point, adjusting the substrate so that the distances between the displacement sensor and each of the three points are the same to generate parallel surfaces.

[0035] The following method of the invention for establishing the distance between two surfaces may be incorporated into the above method in any order where appropriate: [0036] aa) performing an auto focusing program using the CCD so that the surface of the first substrate is at the focus plane of the lens followed by reading the position of the first surface with the displacement sensor, [0037] ab) placing the second substrate between the lens and the first substrate so that a surface is intersected by the optical axis of the lens and positioning the second substrate so that the surface of the second substrate is at a selected distance from the surface of the first substrate as measured by the displacement sensor.

[0038] When the method is part of a printing process, the gap between the substrates typically define the thickness of, e.g., the first printing layer. For example, if part of a printing system of FIG. 1, the next steps in the above process could be to fill the resin tank to the appropriate level and begin printing.

[0039] In one embodiment, the first substrate is a sample stage for a 3D printing device and the second substrate is transparent, such as a transparent membrane or hard window.

[0040] In some embodiments the CCD is a laser CCD, e.g., a laser CCD camera.

[0041] In some embodiments the displacement sensor is a laser displacement sensor, typically with 10 micron accuracy. Generally, the lens is a projection lens.

[0042] In an embodiment the distance between the first and second layer is controlled within a tolerance of 20-microns or less, e.g., the distance between the first and second layer is controlled within a tolerance of 10-microns or less.

[0043] In many embodiments, the distance between the first substrate and the second substrate is equal to the thickness or a first layer of a 3D printing process.

[0044] In many embodiments the first substrate is a sample stage for 3D printing and the second substrate comprises a membrane or hard window, e.g., a membrane. Often the sample stage and membrane or hard window are positioned in a resin tank, and in such embodiments the method typically further comprises a step wherein after the first and second layers are positioned relative to each other the resin tank is filled with a curable resin to the level of the membrane or hard window.

[0045] An alternate embodiment of the invention uses a sacrificial resin to prepare a surface that is not only precisely parallel to a second substrate or surface, but also tangential to it at the same time. This method is easily illustrated as a method useful in 3D printing, see FIG. 4.

[0046] As shown in FIG. 4, before printing, sacrificial resin is delivered, e.g., from a center channel of the sample stage, to fully fill the gap between the membrane/hard window and the sample stage. The resin in the gap is polymerized using UV light in the case of photo reactive sacrificial resins or the heat in case of thermal curable sacrificial resins, to provide a sample stage with a base of sacrificial polymer ready for 3D printing. Due to the way it is prepared, the base will be perfectly parallel to the membrane/hard window, and at an accurately known distance. The adhesion between the sacrificial base and membrane/hard window should be controlled for good separation of the base from the membrane/hard window, for example, controlling the crosslinking density. The sample will be printed from the interface of the sacrificial base and the membrane/hard window. The printing apparatus can be arranged so that as the whole sample is printed, it will be submerged into an etching solution, which will remove the sacrificial base and release the sample.

[0047] The type of etching solution depends on the sacrificial resin. In one example, a resin comprising a mixture of N,N-Dimethylacrylamide(CAS: 2680-03-7) and 1%-3% by weight of Irgacure 819 (CAS 162881-26-7) photo-initiator was used as the sacrificial resin. Upon cure, the photo-crosslinked polymer can be dissolved in water or acid (PH value 4) buffer solutions with 100-300 um/hour etching rate. Polyethylene glycol (MW<600) or water can also be added to the photo sensitive resin to increase the porosity of the polymer, which in turn increases the etch rate.