Holographic Method for Generating Three Dimensional Conformal Photo Lithographic Masks

1. Field of the Invention

The present invention relates to a photo-lithographic mask and, more particularly, to a three-dimensional conformal photo-lithographic mask formed using a hologram.

2. Description of the Related Art

It is sometimes necessary to place electrodes on a surface of a three-dimensional part. A method of designing three-dimensional electrical circuits is disclosed in U.S. Pat. No. 5,109,479. A mathematical representation of a three-dimensional object is created with a computer aided design package. A map of the flattened object is created and is then used in a computer aided circuit layout package as a printed circuit board on which an electrical circuit is placed. The electrical circuit is translated into a three-dimensional form corresponding to the surface of the object. The electrical circuit is then transferred to a three-dimensional data structure that is used to drive a numerically controlled machine tool that cuts a three-dimensional mask. The three-dimensional mask is used to print the circuit on the surface of the three-dimensional object.

Typically, a machine tool that can be used to cut the three-dimensional mask consists of a computer controlled 5-axis cutting machine. One drawback of a cutting procedure using a 5-axis cutting machine is that the 5-axis cutting machine is very costly to purchase and to operate. Since each feature must be cut individually, even using automation, the process is very time consuming. In addition, accuracy of the mask pattern is limited by the tolerances and accuracy of the cutting machine.

A holographic method of recording and reproducing etching masks is disclosed in U.S. Pat. No. 3,806,221. Substantially two-dimensional objects are holographically recorded on a hologram carrier consisting of a photographic plate carrying a photographic emulsion. To reduce the amount of distortion in the hologram image, the hologram is uniformly shrunk during development of the photographic plate. The holographic image can be reproduced as an etching mask upon a photosensitive layer on a semiconductor substrate for producing electronic components and integrated circuits. Although such a holographic mask may be useful for masking a two-dimensional surface, it is not particularly useful for masking complex three-dimensional surfaces that require a three-dimensional conformal photo mask.

A three-dimensional conformal photo mask can be formed by creating a mold having a core with a contour and forming a desired pattern of raised ribs or tracks, such as disclosed in U.S. Pat. No. 5,153,084. The ribs or tracks form a desired pattern of electrodes. The mold is used for forming a mask from a suitable transparent mask material. The mask is separated from the mold and the grooves formed by the tracks are filled with an opaque material. However, such a procedure for forming a mask is time consuming and accuracy of the mask is limited by the accuracy at which the ribs or tracks are formed.

Therefore there is a need to overcome the aforesaid problems associated with these prior art approaches to forming a photo-lithographic mask.

The present invention relates to a three-dimensional photo mask and to a method and system for forming the three-dimensional photo mask. According to an embodiment of the present invention the mask has a three-dimensional side and a planar side. A holograph is provided on the planar side of the mask. The holograph has transparent portions and portions that are opaque. The opaque portions form the holographic transformation of a pattern which is to be imaged onto the three-dimensional side of the mask or onto the part to be masked itself. According to embodiments of the present invention, a positive photosensitive resist material is placed on the three-dimensional side of the mask or on the three-dimensional part to be masked. Light is irradiated through the holograph to produce a three-dimensional hologram including an image of the desired pattern that is used to expose the photosensitive resist material The photosensitive resist material is then developed to remove the areas that were exposed.

According to another embodiment of the present invention, a computer generated holograph, containing information about a three-dimensional lithographic pattern, is provided on a planar side of a two-dimensional mask blank. The mask is brought into appropriate proximity with a three-dimensional part. Light is irradiated through the holograph to produce a three-dimensional hologram including an image of the desired pattern that is used to expose the photoresist provided on the three-dimensional part.

In other words, according to embodiments of the present invention, each mask can be accurately produced first by computer generation of a holograph, containing information about a three-dimensional lithographic pattern, at the planar side of a transparent conforming mask blank. With photoresist coated onto either the contoured side of the mask or on the part itself, by proper illumination of the holograph, a three-dimensional hologram including an image of the desired pattern is produced and is exposed into the photoresist at the contoured surface of the three-dimensional mask or on the part itself.

So that one skilled in the art to which the subject invention appertains will better understand how to practice the present invention, preferred embodiments of the invention will be described in detail below with reference to the drawings wherein:

FIGS. 1(a) and 1(b) are schematic sectional views illustrating a mask and a three-dimensional printed circuit board;

FIGS. 2(a) and 2(b) are schematic sectional views illustrating a conformal photo mask according to embodiments of the present invention;

FIGS. 3(a) and 3(b) are schematic sectional views illustrating a conformal photo mask according to an embodiment of the present invention;

FIG. 4 is a flow chart of a method for producing a three-dimensional photo mask according to an embodiment of the present invention;

FIG. 5 is a block diagram of a system for producing a three-dimensional photo mask according to an embodiment of the present invention; and

FIG. 6 is a schematic sectional view illustrating a photo mask according to an embodiment of the present invention.

FIG. 1(b) depicts a three-dimensional printed circuit board 4 on which electrodes or an electronic circuit are to be provided on three-dimensional surface 1. Printed circuit board 4 is formed from substrate 10, having a metal-cladding layer 2 formed thereon. Substrate 10 and metal layer 2 form a metal-clad substrate. Metals such as copper, silver, nickel, tin, lead, gold, titanium, platinum and aluminum are typically used for the metal cladding. A positive photosensitive resist material layer 3 is provided on metal-cladding layer 2.

A three-dimensional photo-mask is used for photolithographically imaging selected areas on the surface of the three-dimensional printed circuit board. The printed circuit board is then etched to produce the desired circuit pattern. Generally, as depicted in FIG. 1(a), a three-dimensional mask 5 is made of a transparent and compliant substrate material 6. Portions 7-9 of the mask are coated or treated to form opaque areas having a desired pattern. The desired pattern corresponds to a pattern of electrodes to be formed on printed circuit board 4, for example.

Three-dimensional mask 5 is pressed firmly against positive photosensitive resist layer 3 provided on the surface of three-dimensional printed circuit board 4. The compliant mask material complies and forms to any irregularities on the surface of the three-dimensional printed circuit board. Light is irradiated through the mask and onto photosensitive resist layer 3, photolithographically exposing the photosensitive resist layer 3 with the desired pattern. The exposed portions of the photosensitive resist material (i.e., areas corresponding to the transparent areas of the mask) are then removed with a solvent. Three-dimensional printed circuit board 4 is then etched to remove the portions of the metal cladding 2 exposed through the resist, to form the printed circuit pattern.

A three-dimensional conformal photo mask according to a preferred embodiment of the present invention is depicted in FIG. 2(a). The three-dimensional conformal photo mask 18 consists of a main substrate 20 having a substantially planar side 21 and a three-dimensional mating impression side 22. Main substrate 20 is formed from a transparent material so that light can be projected therethrough. In addition, main substrate 20 is formed from a compliant material that conforms easily to any imperfections or irregularities on the surface of a three-dimensional part to be masked. An example of a material that can be used for forming main substrate 20 is General Electric Company's room temperature vulcanized (RTV) silicones such as mixtures of organopolysiloxanes containing vinyl groups.

For reference purposes, a holograph is defined as the recorded information of a three-dimensional image that is formed, for example, on a photographic plate or material. A hologram is defined as the three-dimensional image thus produced by proper illumination of the holograph.

As shown in FIG. 2(a), reference numeral 23 represents a three-dimensional hologram generated by proper illumination of a holograph that is recorded on planar side 21 of main substrate 20. (Although the hologram is depicted above the mask, the hologram would actually be formed within main substrate 20 so that dark portions 25-27 of the hologram would prevent exposure of the corresponding portions of photo resist 24.) Accordingly, main substrate 20 should also be a material on which holographic information can be written or recorded. In the alternative, a layer of a suitable material (not shown) on which the holographic information can be written or recorded can be provided on planar side 21 of main substrate 20. Alternatively, the holograph can be carried on a mating planar plate (not shown) that is brought into proximity with planar side 21 of main substrate 20. According to this embodiment of the present invention, the holograph includes information defining features representing a "positive" of the pattern that is to be imaged onto photosensitive resist material 24. That is, portions 25, 26 and 27 of hologram 23 are dark and the rest of the hologram is illuminated. A positive photosensitive resist material 24 is provided on three-dimensional mating impression side 22 of main substrate 20. After exposure of the photosensitive resist material to the information contained in the hologram, the resist material is developed and portions removed, resulting in the three-dimensional conformal photo mask depicted in FIG. 3(a). The resulting conformal photo mask 50 as depicted in FIG. 3(a), consists of transparent main substrate 36 having opaque features 37-39.

According to another embodiment of the present invention, the photosensitive resist material can be provided only on the three-dimensional part itself. The hologram generated by proper illumination of the holograph can then be used to expose a photosensitive resist material provided on the three-dimensional part itself. Therefore, according to this embodiment of the present invention, it is not necessary to provide the photosensitive resist material on the three-dimensional surface of the mask.

According to another preferred embodiment of the present invention as depicted in FIG. 2(b), the holograph includes information defining features representing a "negative" of the pattern that is to be imaged onto the photosensitive resist material. That is, as shown in FIG. 2(b), the hologram consists of dark sections 28-31 and illuminated sections 32-34. In this embodiment, a negative photosensitive resist material, instead of a positive photosensitive resist material, is provided on the three-dimensional mating impression side 22 of main substrate 20. After exposure of the negative photosensitive resist material to the hologram, the unexposed portions of the resist material (i.e., the portions corresponding to dark portions 28-31 of the hologram) are removed resulting in three-dimensional conformal photo mask 50 depicted in FIG. 3(a).

Mask 50 can then be used for photolithographically exposing three-dimensional printed circuit board 40 in any conventional manner. Printed circuit board 40 (shown in FIG. 3(b) after exposure and etching) is identical to printed circuit board 4 depicted in FIG. 1(b) prior to exposure and etching. As shown in FIGS. 3(a) and 3(b), opaque areas 37-39 of mask 50 correspond to electrodes 41-43 on three-dimensional circuit board 40.

The following is a description of the steps for preparing the three-dimensional photo mask according to an embodiment of the present invention, as shown in FIG. 4. In step S2, a three-dimensional part such as a three-dimensional printed circuit board is designed. In step S4, feature areas on the surface of the three-dimensional printed circuit board on which electrodes are to be provided are numerically defined. In step S6, a conforming mask having a mating impression of the three-dimensional printed circuit board is formed. In step S8, using information generated in step S4 that numerically defines the electrode areas, Fresnell or Fourier holographic transform information of the electrode areas is generated. In step S10, the holographic transform information generated in step S8 is written or recorded onto a planar side of the three-dimensional conformal photo mask to form a holograph. In step S12, the three-dimensional side of the three-dimensional mask is coated with a positive photosensitive resist material. In step S14, a coherent light source is irradiated through the holograph and a mask to form a hologram which defines features to be imaged onto the photosensitive resist material provided on the three-dimensional side of the mask and exposes the features onto the three-dimensional side of the mask. In step S16, the photosensitive resist material is developed to remove the exposed portions of the photosensitive resist material. As shown in FIGS. 3(a) and 3(b), mask 50 forms a mating impression of three-dimensional printed circuit board 40 and includes opaque areas 37-39 that correspond to electrode areas 41-43 on three-dimensional printed circuit board 40.

A system such as that shown in FIG. 5 is provided for producing a three-dimensional conformal photo mask according to an embodiment of the present invention. The system includes a computer aided design/computer aided manufacturing (CAD/CAM) system 61 for designing and manufacturing a three-dimensional part such as a three-dimensional printed circuit board. An example of a CAD-CAM system is an Apollo workstation using the GMS CAD-CAM system by Unisys Corporation. Of course, any appropriate CAD-CAM system can be used.

The CAD system is used to design a three-dimensional part. The CAD system is then used to generate information numerically defining areas on a surface of the three-dimensional part that are to include electrodes, for example. After the design is complete, the three-dimensional part can be manufactured from any suitable material.

A three-dimensional mask conforming to the three-dimensional part is then made. The conforming mask can be easily designed and manufactured using the information generated by the CAD/CAM system for designing and manufacturing the three-dimensional part.

A transform generator 62 is used to generate Fresnell or Fourier holographic transform information including information-defining features to be imaged onto the three-dimensional mating impression side of the mask that correspond to the numerically defined areas on the surface of the three-dimensional part that are to have the electrodes. Transform generator 62 can be, for example, a personal computer having software suitable for generating the appropriate Fresnell or Fourier holographic transform information. An example of a software package suitable for this task is Mathmatica.RTM. by Wolfram Research or the C-Programming Language, commercially available through Borland International Inc. or Microsoft.

The generated information is written onto the planar side of the mask opposite the three-dimensional mating impression side, using holograph writer 63. The holographic information can be written onto the mask using any suitable method and apparatus. For example, the holographic information can be written or recorded using any conventional photolithographic mask writing device.

After the holographic information is written onto the mask to form the holograph, a photosensitive resist material is applied to the three-dimensional side of the mask by photo-resist applicator 64. The photosensitive resist material can be applied in any suitable manner. For example, the photosensitive resist material can be applied to the three-dimensional side of the mask either as a dry film photo resist or as a liquid photo resist.

In exposure station 65, a hologram is exposed onto the photosensitive resist material applied on the three-dimensional side of the mask, using the information in the holograph and a coherent light source such as a LASER. There is a possibility that internal reflections from the three-dimensional surface of the mask will inadvertently expose undesired portions of the photosensitive resist material. Accordingly, it may be preferable to immerse the photosensitive resist coated mask in an index matched liquid filled with a light absorbing material or in a container having optically black walls. The index matched liquid will prevent surface reflections and the light absorbing material or container having optically black walls will absorb the light preventing further reflections from the container walls. The exposed photo resist is then developed in developing station 66 to remove the undesired portions of the photosensitive resist material. For example, when a positive photosensitive resist material is used, areas of the photo resist exposed to the light undergo a physical change rendering these areas soluble to a developing solution. The exposed areas are then removed using an appropriate solvent. On the other hand, when a negative photosensitive resist material is used, areas of the photo resist exposed to the light undergo a change rendering them insoluble to the developing solution. The unexposed areas are then removed using an appropriate solvent.

It should be noted that some photosensitive resist materials that can be used for forming the opaque portions of the mask may tend to break down after repeated masking exposures. Accordingly, it may be desirable to replace the resist material with a more durable material.

Although the above descriptions relate to the use of a holograph for forming a pattern on the three-dimensional side of a mask, the present invention is also useful for forming a pattern directly on the three-dimensional part itself using a planar photo mask blank. For example, as shown in FIG. 6, a computer generated holograph 71 can be provided on a planar, transparent photo mask blank 72, the holograph containing information about a desired three-dimensional lithographic pattern. The mask can be brought into the appropriate proximity with a three-dimensional part 75 (which is similar to printed circuit board 4 as shown in FIG. 1(b)). By proper illumination of holograph 71 using an appropriate light source 76, a three-dimensional hologram 73 including an image of the desired pattern can be produced to expose the photoresist 74 directly on part 75 itself. The part can then be treated to remove the undesired portions of the photosensitive resist and the exposed metal cladding can be etched to form the desired pattern of electrodes.

It will be appreciated that the foregoing description and drawings are only intended to be illustrative of the present invention. Variations, changes, substitutions and modifications of the present invention may occur to those skilled in the art without departing from the spirit and scope of the present invention. Accordingly, it is intended that the present invention be limited only by the scope of the appended claims.