Abstract:
A method is disclosed for making deep microstructures in photoresist. The method utilizes a pool of photoresist on top of a transparent substrate and the laser is located below the substrate. Structures are created in the photoresist by transmitting the laser light through the substrate up into the photoresist. Since the photoresist does not have to be spin coated onto the substrate, very thick layers of photoresist can be used while the thickness uniformity is determined by the substrate surface. Alternately, a contoured substrate can be used while producing uniform structures.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of photolithography and, more particularly, to methods for using photolithographic techniques for making optical structures such as lenses that are deeper than 100 micron. 
     BACKGROUND OF THE INVENTION 
     Photolithography is a technique that is well known for making semiconductors, microlenses, diffraction gratings, diffractive lenses, microelectronic structures etc. The outstanding characteristic of photolithography is that it is capable of producing complex structures with accuracy in the nanometer range. 
     Typically, in photolithography, a coating of photoresist is applied to a silicon or fused silica substrate such as a wafer. A laser is then used to write the desired structure into the photoresist from above the photoresist. By focusing the laser to a very small spot and moving the laser in a precisely controlled manner, extremely precise structures can be created in the photoresist. After the pattern has been written onto the photoresist, a chemical wash is used to remove the photoresist preferentially between the areas that have been exposed and those that have not been exposed, thereby leaving the structure as written. 
     The limitation with photolithography is in the depth of the structures that can be created. This limitation comes from a combination of the laser power, the transparency of the photoresist to the laser light, and the thickness of the photoresist layer. While laser power can be increased to create a deeper structure, and alternate photoresists or other wavelength lasers could be used to increase the effective transparency of the photoresist; the thickness of the photoresist can only be increased a limited amount. 
     Photoresist is a liquid material as it is applied to the wafer. In order to obtain a uniform thickness of the photoresist, a spin coating technique is typically used to spread the liquid photoresist over the substrate surface. For the final structure to be flat and of uniform depth, the photoresist must be applied very uniformly over the wafer surface. The spin coating technique is very good at creating a uniform coating in the range of 1-500 micron depending on the spin speed and the type of photoresist used. For many photoresists, the thickness range that is possible with spin coating is very narrow, for example the 3612 photoresist from Shipley (Marlborough, Mass.) is only capable of thicknesses of 1-1.6 micron and the SPR220 photoresist from Shipley is capable of 7-18 micron. At the other extreme is the SU8 photoresist from Microchem Corp (Newton, Mass.) that is capable of thicknesses of 2-500 micron on a spin coater. Above the maximum thicknesses listed, the photoresist flows off the edge of the wafer during spin coating. 
     In U.S. Pat. No. 4,340,654, Campi describes a process for repairing photomasks which have clear defects. The process uses a laser applied to a photomask on a substrate so that in areas where there is a hole in the photomask, the laser penetrates the photomask to an opaque powdered material which has been applied on the other side of the photomask. The laser then melts the powdered material only in the area of the defect, thereby fusing the melted material to the photomask and repairing the defect. The process, as described by Campi, is specifically directed at opaque photomasks using a thermally based process to fuse an opaque material that absorbs the radiant energy. In a further embodiment, Campi does present a variation on photomask repair and instead describes a similar technique for producing photomasks directly. In this case, Campi is focused on fusing opaque materials to make photomasks without requiring the use of the usual prior art techniques of first coating the substrate surface with a photoresist material. Consequently, the method presented by Campi is not applicable to making optical structures which by necessity are made of transparent photoresist materials and which are three dimensional with significant depth. 
     Van Dine also describes the use of a laser which is directed through a transparent substrate in U.S. Pat. No. 4,705,698. In this case however, the laser is used to scribe the substrate through ablation to create a separation between semiconductor layers. This approach is very different from using the laser to create an optical surface in photoresist. 
     Tankovich, in U.S. Pat. No. 5,614,339, also discloses the use of a laser through a transparent substrate to remove opaque printing through ablation on the opposite side for the purpose of enabling toner materials to be recycled. Again the laser is used in an approach that is only suitable for use with opaque materials. 
     In a variation on Tankovich, Chrisey, in U.S. Pat. No. 6,177,151, discloses a similar ablation approach with a laser transmitted through a transparent substrate to an opaque layer which ablates. However, in Chrisey, the ablated material is captured onto a receiving substrate, thereby creating a method for printing with the ablated material. 
     The prior art fails to teach a method suitable for making three-dimensional optical structures using photoresist. In addition, the thermal and ablation aspects disclosed in the prior art all involve significant transfer of energy from the laser in that the material either melts or is ablated. In creating an optical structure, the transfer of energy from the laser must be minimal to protect the optical properties of the photoresist material being processed. In addition, the prior art does not disclose methods suitable for building structures of significant depth. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for producing optical structures of a photoresist material which have a depth that is greater than 100 micron. 
     It is a further object of the present invention to disclose a method for making optical structures in photoresist at substantially greater thicknesses than can be provided by spin coated coatings. 
     It is a further object of the present invention to reduce the cost of making photoresist structures by eliminating the multiple spin coating steps needed to produce thick photoresist coatings for thick photoresist structures. 
     Briefly stated, these and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by a method utilizing a pool of photoresist on top of a transparent substrate such that the laser can be located under the substrate. Optical structures can be created in the photoresist by moving the laser in a pattern that matches the desired optical structure and the laser beam is transmitted through the substrate and into the pool of photoresist. 
     By using a pool of photoresist on top of the substrate, the application of the photoresist is greatly simplified compared to spin coating in that it can be merely poured onto the substrate. If very thick layers of photoresist are desired, cylindrical walls can be attached to the edges of the substrate to create a container which can act to contain the liquid photoresist during processing. In this case, the thickness of the photoresist is limited only by the height of the walls of the container. In either case, the processing cost of the present invention in applying the photoresist is substantially less than spin coating since the process is much simpler. 
     By transmitting the laser through the substrate, the flatness of the base of the optical structure produced is determined by the flatness of the substrate rather than the flatness of the surface of the coated photoresist as in the prior art. Due to the extreme requirements of the semiconductor industry, substrates in the form of wafers of various materials can be easily obtained which are very flat. Alternately, the invention as described could be used with substrates that are intentionally contoured to create a photoresist structure on top of a contoured substrate such as in a micro device which has optical and mechanical or electrical features. 
     Thus, the present invention defines a technique to replace spin coating which will enable photoresists of all types to be effectively used at an increased thickness so that deeper optical structures can be made with uniform quality. The invention involves using wafers that are transparent to the laser that is used for writing onto the photoresist. The laser can then be located under the wafer and write up through the wafer onto the photoresist. By writing through the wafer, the photoresist can then be poured onto the wafer in any thickness and coating uniformity is no longer an issue since structure depth is determined by the laser writing process and flatness is determined by the wafer. The invention also enables optical structures to be created in photoresist on contoured substrates. The term “optical structures” as used herein is intended to include, for example, lenses, gratings, and arbitrary diffractive surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the prior art spin coating process. 
     FIG. 2 is a side elevational representation of the prior art method for producing photoresist structures using a laser above a spin coated substrate. 
     FIG. 3 is a schematic representation of the photoresist being poured onto the substrate to form a pool in the practice of the method of the present invention. 
     FIG. 4 is a schematic representation of the photoresist being poured into a container that has been attached to the edges of the substrate for very thick applications of photoresist in the practice of the method of the present invention. 
     FIG. 5 is a cross-sectional schematic showing the photoresist contained in a pool on top of a transparent substrate and the laser located underneath the substrate such that the optical structure is created in the photoresist at the interface between the substrate and the photoresist in the practice of the method of the present invention. 
     FIG. 6 is a cross-sectional schematic showing an alternate embodiment to that depicted in FIG. 5 wherein the substrate is contoured and a optical structure is being created in the photoresist over the contoured substrate using the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning first to FIG. 1 there is depicted a schematic of a prior art spin coating apparatus  10 . A liquid photoresist  11  is delivered onto a substrate  12 . It will be appreciated that spinning the substrate  12  does help to spread the liquid  11  more evenly across the substrate  12  in a thin layer  14 . However, there is a limit to the thickness of the thin layer  14  that can be maintained on the substrate  12  before the centrifugal force from spinning makes the liquid spill off the edge  16  of the spinning substrate  12 . In addition, the thickness of the coating  14  produced by spinning tends to be greater near the edge of the coating  17  particularly for thicker coatings  14 . 
     Looking next at FIG. 2 there is shown the prior art process  21  wherein a laser  18  is located above a substrate  20 . The photoresist coating  22  on the substrate  20  is shown schematically with the thickness nonuniformities that are typical for a spin coated photoresist coating  22 . The photoresist coating  22  on substrate  20  is exposed in a predetermined pattern by moving the laser  18  over the photoresist coating  22  to create a structure  23  such as an optical article, from a series of areas that have been exposed and areas that have not been exposed. The photoresist coating  22  is then developed and if the photoresist is a positive photoresist material, the exposed areas are removed, thereby leaving the unexposed areas of the photoresist coating in the form of a structure  23  in photoresist material. For the case of a negative photoresist material, the unexposed areas of the resist are removed after developing, thereby leaving the exposed areas in the form of a structure  23 . In this prior art case, since the laser  18  is located above the substrate  20  and the photoresist coating  22 , the optical structures being produced in the photoresist coating  22  tends to form from the upper surface down into the photoresist coating  22  as limited by the size of the exposing area at the focal point of the laser so that any thickness nonuniformities present in the photoresist coating  22  influence the accuracy of the optical structure  23  that is produced. 
     Turning to FIG. 3 a schematic illustration of an apparatus  29  that may be used in the practice of the method of the present invention is presented showing that a liquid photoresist  30  is poured such as, for example, from a container  32  onto a transparent substrate  34  so that the liquid photoresist forms a pool  36  on the top surface  38  of the transparent substrate  34 . In this manner, the pool  36  results in a relatively thick layer of photoresist. 
     FIG. 4 shows a schematic illustration of an alternative apparatus  41  that may be used in the practice of the method of the present invention in which to obtain a very thick layer of photoresist, a vertical wall member(s)  40  (see FIG. 4) may be attached to the edges or periphery of the transparent substrate  34  (shown in a typical but not required circular format). Again, a liquid photoresist  30  is poured such as, for example, from a container  32  onto a transparent substrate  34  so that the liquid photoresist forms a pool  36  on the top surface  38  of the transparent substrate  34 . The vertical wall  40  is sealed to the transparent substrate  34  such that the liquid photoresist cannot flow through the interface therebetween. With the vertical wall  40  in place, a chamber  42  is created that is bounded by the vertical wall  40  so that the liquid photoresist can be held in a deep pool over the transparent substrate  34 . The pool of photoresist can be processed in a liquid state or cured prior to exposure to form a thick layer of photoresist  37  as required by the type of photoresist. 
     FIG. 5 shows a schematic cross-sectional representation of the alternative apparatus  41  in which a thick layer of photoresist  37  has been placed on a transparent substrate  34 . A laser  50  located on the opposite side  46  the transparent substrate  34  is shown producing a three-dimensional optical structure  48  in the photoresist  37  (see FIG.  5 ). The three-dimensional optical structures  48  are produced in the photoresist  37  by transmitting the beam  51  from the laser  50  in a controlled pattern through the transparent substrate  34  and into the thick layer of photoresist  37 . In this method, the three-dimensional optical structures  48  are formed above the transparent substrate  34  up into the thick photoresist layer  37  so that the thickness uniformity of the photoresist  37  does not influence the accuracy of the three-dimensional optical structures  48  created. After exposing the photoresist  37  to create the predetermined three-dimensional pattern or features  48  using the laser positioned on the opposite side  46  of the transparent substrate  34 , the exposed predetermined pattern is developed leaving an undeveloped portion. The undeveloped portion of the photoresist  37  is then removed if the photoresist is a negative photoresist or alternately, the developed portion is removed if the photoresist is a positive photoresist. 
     The present invention has been described herein using flat transparent substrates. However, the method of the present invention can also be employed with a non-planar substrate  60  as shown in FIG.  6 . By way of example, substrate  60  may include three-dimensional contours or features  62 . In this case, the pool of photoresist  64  is formed over the non-planar substrate  60  so that it covers the three-dimensional contours or features  62 . By way of example, non-planar substrate  60  may be a transparent micro electromechanical system. A laser  50  emitting beam  51  is again used to create three-dimensional optical structures  68  in the photoresist. 
     It will be appreciated by those skilled in the art that the combination of moving the laser beneath a transparent substrate  34  along with the approach of starting with a pool of photoresist, makes it possible to make very deep optical structures from photoresist. In addition, the present invention substantially expands the effective thickness range over which photoresists, in general, can be used, and further, expands the effective thickness range for specific photoresists so that the number of choices of photoresists for a given optical structure is increased. In fact, the thickness limit for photoresist structures is extended by the present invention to the limit of light transmission for the photoresist at the wavelength at which the laser operates. 
     While the method described in the invention is applicable to nearly any laser or photoresist, for the laser to transmit light through the substrate and then to interact with the photoresist such that the liquid photoresist will solidify requires some balancing of the materials involved. For a typical UV laser such as an Excimer laser which operates at a wavelength of 248 nanometers, transparent substrates which can be used in the practice of the method of the present invention include, for example: sapphire, glass, quartz, lithium niobate, etc. Prefereably, the laser used in the process of the present invention is in the ultraviolet range or the visible range and operated at a wavelength of not more than about 500 nanometers. Most photoresists absorb strongly at UV wavelengths such as 248 nanometers so these combinations of lasers substrates and photoresists will perform as described in the practice of the method of the present invention. 
     The thickness of the transparent substrate can be an issue as well, particularly when a photomask (not shown) is used. In the practice of the method of the present invention, the photomask would have to be located on the same side of the transparent substrate as the laser as described above with reference to FIG.  5 . Since the photomask would actually be separated from the photoresist, diffraction at the edges of the photomask will limit the sharpness of the image obtained on the photoresist and, therefore, the accuracy of the optical structure produced will be limited. To minimize these diffraction issues, the thickness of the substrate should be kept to a minimum when using a photomask. 
     The benefit of greater depth of the optical structures created that can be achieved through the practice of the method of the present invention versus the spin coating prior art method can be illustrated by comparing both methods using a particular photoresist. By way of example, the photoresist SU8 as produced by Microchem Corp of Newton, Mass., is recommended to be spin coated up to a thickness 500 microns. In contrast, the optical transparency limit is 2 millimeters for SU8 at the photoresist&#39;s most sensitive wavelength (365 nanometers). This data supports a conclusion that the possible depth achieved in the optical structures formed in the photoresist is increased by a factor of four, using the method of the present invention wherein the photoresist is applied as a pool onto a sapphire, glass or quartz substrate as opposed to spin coating. 
     From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth together with other advantages which are apparent and which are inherent to the process. 
     It will be understood that certain features and subcombinations are of utility and may be employed with reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth and shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     Parts List 
       10  Spin Coating Apparatus 
       11  Liquid Photoresist 
       12  Substrate 
       13  Tube 
       14  Layer 
       16  Edge 
       17  Edge Of Coated Layer Of Photoresist 
       18  Laser 
       20  Substrate 
       21  Prior Art Process 
       22  Photoresist Coating 
       23  Structure 
       29  Apparatus 
       30  Liquid Photoresist 
       32  Container 
       34  Transparent Substrate 
       36  Pool Of Photoresist 
       37  Photoresist 
       38  Top Surface 
       40  Vertical Wall Member 
       41  Alternative Apparatus 
       42  Chamber 
       46  Opposite Side Of Transparent Substrate 
       48  Optical Structure 
       50  Laser 
       51  Emitting Beam 
       60  Non-Planar Substrate 
       62  Three-Dimensional Contour Of Features 
       64  Pool Of Photoresist 
       68  Three-Dimensional Optical Structures