Abstract:
A photolithographic system uses a mask that carries a plurality of photolithographic images. In one embodiment, the mask carries images for all the layers necessary to manufacture a variety of different device cells, which can include devices of different types. A single mask may carry the images required to make a complete system consisting of multiple devices. Some devices may comprise multiple layers. The system includes an adjustable aperture system which defines the area of the mask which will be illuminated. The mask is employed in a mask aligner which includes a source of electromagnetic radiation, apparatus to carry and position a substrate, apparatus to position the mask, and apparatus to position and adjust the aperture. The process requires the successive steps of supporting a photoresist-carrying substrate, positioning the mask to register a selected photolithographic image with the substrate, positioning and adjusting the aperture to expose the desired image, and illuminating the radiation source to imprint the chosen image on the substrate. The alignment process may be repeated multiple times with the same mask and adjustable aperture so as to imprint other images of the corresponding layer of other devices elsewhere on the substrate. The substrate is processed to produce the layers of the devices so imprinted. The photolithographic process may then be repeated to imprint successive layers of the various devices, each in registry with the corresponding underlying layers.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for the photolithographic manufacture of devices, and more particularly to an apparatus and method for using a photolithographic mask in the manufacture of devices. 
     BACKGROUND 
     Photolithography is a well known technique in the manufacture of many kinds of devices, and finds particular use in the semiconductor industry. Photolithography is very effectively employed in the mass production of semiconductor devices. However, it suffers from a drawback in the manufacture of devices which are required in very small numbers, and in the prototyping of devices. Photolithography is a method which uses masks. The cost of making many expensive masks may be prohibitively high if a product is to be manufactured in quantities of only a few units, or if a prototype has to be modified multiple times during development. Some of the methods which have been developed to try to avoid these difficulties include device modeling and direct write technology. 
     Device modeling generally uses a computer to calculate, or “model,” the behavior of a circuit or other device of interest. While modeling can provide important insights into the expected behavior of devices, and is generally faster and less expensive to perform than the actual fabrication of the modeled device, one obtains only a calculated result, and not an actual device of interest. Furthermore, modeling generally requires that simplifying assumptions be made to provide a tractable mathematical representation of the device under investigation. These simplifying assumptions generally cause the calculated answers to be approximations to the actual behavior of real devices. 
     Direct write technology generally employs a method of “writing” the patterns representing the various layers or levels of a device without a mask, by the direct illumination of a substrate with a very finely focused beam, such as an electron beam. 
     The patterns to be created or “written” are generally maintained in digital form in the memory of a digital computer, and are used to scan the electron beam over the surface of the substrate, and to turn the beam on and off as appropriate to generate the desired pattern. While the method can be used to produce devices, it typically suffers from the difficulties that the equipment required to carry out the process is generally more expensive than photolithography equipment, and that the process is generally slower than production using photolithography. 
     SUMMARY OF THE INVENTION 
     A photolithographic system is presented which uses a mask that carries a plurality of photolithographic images. In one aspect, the invention includes a photolithographic mask having a plurality of images which are suited to the manufacture of multiple device cells of more than one type. In one embodiment the cells may include such types as digital circuitry, analog circuitry, micromechanical devices, microelectromechanical devices, electrooptic devices, optoelectronic devices, and electronic sensor devices. In another embodiment, the mask includes a set of images for one cell that requires more than one layer for its manufacture. 
     In another aspect, the invention comprises a photolithographic system for employing photolithographic masks which have a plurality of images. The system includes a source of electromagnetic radiation, a substrate support, a mask positioner, and an adjustable aperture which can be moved and which is positioned between the source of electromagnetic radiation and the substrate support. An embodiment of this system includes an aperture which has a plurality of individually movable screens. 
     Still another aspect of the invention is a photolithographic process for the manufacture of a device. The process includes the steps of providing a source of electromagnetic radiation, supporting a substrate of material suitable for photolithographic processing, and positioning a photolithographic mask having a plurality of images in registry with the substrate. An aperture is positioned between the source of electromagnetic radiation and the substrate so as to permit a first image to be projected onto the substrate and the substrate is exposed to electromagnetic radiation through the aperture to imprint the first image upon the substrate. The photolithographic mask is then repositioned relative to the substrate so that a second of the images is in registry with the substrate. The aperture is then repositioned to permit the second image to be projected onto the substrate, and the substrate is reexposed to electromagnetic radiation through the aperture to imprint the second image upon the substrate. This process is repeated as many times as necessary to imprint the images required to manufacture the desired device, one layer at a time. In an embodiment of this invention, a processing step is interposed between the step of imprinting the first image and the step of imprinting the second image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The advantages of the invention described above, as well as further advantages of the invention, may be better understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts a highly schematic diagram of an embodiment of the photolithographic system of the invention; 
     FIGS. 2 a  though  2   j  depict a plan view of an embodiment of the mask shown in FIG.  1  and constructed in accordance with the invention in which FIG. 2 a  depicts an overview and FIGS. 2 b  through  2   j  depict individual images; 
     FIG. 3 depicts another embodiment of a mask constructed in accordance with the invention; 
     FIG. 4 depicts a perspective view of a highly schematic diagram of an embodiment of a photolithography system suitable for the practice of the invention; 
     FIG. 5 depicts a plan view of the step of selection of an image from a set of photolithographic images on a mask according to an embodiment of the invention; 
     FIG. 6 depicts in side view diagram the steps of selecting and imprinting of a selected image on a substrate from a set of photolithographic images on a mask according to an embodiment of the invention; 
     FIGS. 7 a, b  depict another embodiment of a mask and an aperture constructed in accordance with the invention; 
     FIGS. 8 a, b, c  depict a plan view of the steps of selecting various mask images of the mask of FIG. 7 a  for manufacturing an integrated circuit according to the invention; and 
     FIGS. 9 a, b, c  depict in cross section a partially manufactured integrated circuit manufactured according to the steps shown in FIGS. 8 a, b, c.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, the term “cell” is meant to denote a basic unit of construction of a device, which includes but is not limited to single elements such as resistors, capacitors, diodes, and transistors, groups of such elements together which comprise memory elements, logic gates, bridges, and the like, and micromechanical or microelectromechanical elements such as fulcrums, levers, and gears, and the like. Similarly, the term “device” is meant to denote a structure composed of one or more cells, which includes but is not limited to components ranging from single elements such as serpentine resistors, to objects such as accelerometers, microelectromechanical motors, photodiodes, and optical sensors, to complex structures such as random access memories, central processing units, charge coupled devices, and the like. A single photolithographic image will generally represent a single level or layer which is required for the manufacture of a device by photolithographic processing. A photolithographic mask having a plurality of photolithographic images thereon may be considered to represent a library of images. Photolithographic images will generally be depicted as rectangular objects, which is a convenient shape for use in presenting embodiments of the invention, but it should be recognized that other shapes, such as triangles, hexagons, circles, or even irregular shapes, may be used in embodiments according to the invention. 
     FIG. 1 depicts a schematic general overview of the photolithographic system of the invention which includes a source of electromagnetic radiation  10 . The source  10  is selected to provide radiation of a suitable wavelength, and may be any source of suitable intensity including but not limited to a solid state laser, a dye laser, an excimer laser, or a lamp. The source  10  in one embodiment is a more complex source which includes frequency doubling of a selected source and the like. A substrate  20 , which may be a material such as a semiconductor, an insulator, or a metal, and which is provided with a suitable photoresist, not shown, is supported by a substrate support  22  in a position where the substrate  20  is illuminated by the source  10 . A photolithographic mask  30  which carries a plurality of photolithographic images, generally  50 , is disposed between the source  10  and the substrate  20 , so that radiation passing from the source  10  to the substrate  20  through the mask  30  imprints a selected image  50 ′ on photoresist on the substrate  10  as imprinted image  56 ′. 
     An aperture  42  defined by the periphery of an opening in an aperture screen  40  is used to select an image  50 ′ on the mask  30  which will be imprinted on the photoresist of the substrate  20 . The aperture  42  in one embodiment is positioned between the mask  30  and the substrate  20  so as to transmit radiation which passes through the selected image  50 ′ of mask  30  and to block radiation which passes through the undesired images  50  of mask  30 . Alternatively, in one embodiment the aperture screen  40 ′ is positioned between the mask  30  and the source  10 . In this embodiment radiation passes through the aperture  42 ′ before passing through the selected image  50 ′ of mask  30 . In one embodiment, the aperture  42 ,  42 ′ is depicted as being defined by an opening in a single screen. Alternatively, the aperture  42 ,  42 ′ in one embodiment is defined by the edges of a plurality of screens, which are all movable relative to each other, and which may be positioned to define the aperture  42 ,  42 ′ as described below. In one embodiment, the distance between the mask  30  and the aperture  42  is adjustable. This distance is chosen to control diffraction effects which generally occur as a result of the interaction of electromagnetic radiation with an edge, such as the edges of aperture screen  40  which define aperture  42 . In one embodiment, the diffraction effects are controlled by leaving a distance between adjacent imprinted images, generally  56 ′, only one of which is shown. 
     In one embodiment source  10  is turned on for an interval suited to the imprinting of an image  56 ′ upon substrate  20  and then turned off while the relative positions of substrate  20 , mask  30 , and aperture  42  are changed to allow the imprinting of another image. Alternatively, a movable shutter (not shown) may be used to temporarily intercept the radiation from source  10 , so that an image  56 ′ may be imprinted by adjusting the relative positions of the substrate  20 , the mask  30 , and the aperture  42  with the shutter closed, and then opening the shutter for a suitable length of time to imprint an image  56 ′. Another alternative uses a movable radiation diverter such as a mirror or a lens (not shown) to temporarily intercept the radiation. 
     FIG. 2 a  depicts a mask  30  comprising a set of photolithographic images generally  50  for the manufacture of a circuit requiring multiple layers and several types of devices. Each image  50  represents a discrete layer which must be fabricated in manufacturing the circuit. FIGS. 2 b  through  2   j  depict the individual images in detail and in sequence from the bottom to the top of the circuit. Each figure represents one layer in which there is a cell of elements, generally  32 , and fiduciary marks, generally  34 . The fiduciary marks perform functions which include but are not limited to aligning the cell elements, orienting the cell to prevent or to identify inadvertent inversion of mask  30 , and permitting the focusing of the image  50 ′. FIGS. 2 b ,  2   d ,  2   f , and  2   h  depict images which are used to fabricate discrete layers of metallization. FIGS. 2 c ,  2   e , and  2   g  depict the individual images which are used to fabricate “via” layers, or layers in which connections between layers of metallization are produced, creating a three dimensional serpentine conductive structure. FIG. 2 i  depicts an image which represents a pattern for the deposition of solder. FIG. 2 j  depicts an image which represents a pattern for the deposition of a multiplicity of circuit probe elements. 
     The mask  30  of FIG. 2 a  may be employed in the system depicted schematically in FIG. 1 so as to construct a circuit tester which is useful for testing a specific semiconductor circuit. 
     FIG. 3 depicts an embodiment of a mask  30  comprising a set of photolithographic images  50   a ,  50   b ,  50   c ,  50   d  for the manufacture of an integrated circuit having multiple layers and several devices. Images  50   a , which have a rectangular area, are the individual layers in one device. One example of an image  50   a  is rotated by 90 degrees with respect to the orientation of the other images  50   a , for example to use space on the mask efficiently. Mask  30  must thus be capable of being rotated by 90 degrees in order to imprint all of the images  50   a . Images  50   b , having a rectangular area which is somewhat larger and differently proportioned than the images for the first device, are individual layers for a second device. Images  50   c , of rectangular shape which is still larger, are individual layers for a third device, and images  50   d , which are the largest, are images corresponding to the individual layers of a fourth device. The mask of FIG. 3 is used in the apparatus depicted in FIG.  1 . 
     FIG. 4 depicts a schematic diagram of another embodiment of a photolithography system suitable for the practice of the invention. A substrate  20  which is prepared for receiving an imprinted image  56 ′ is provided. Mask  30 , which carries a plurality of photolithographic images, generally  50 , of which only image  54  is shown, is placed in registry with substrate  20  by the use of a microcontroller and motion drives  80 . Microcontroller and motion drives  80  determine the positioning of mask  30  by allowing mask  30  to move rectilinearly in two mutually perpendicular directions shown as x-motion  82  and y-motion  84 , and additionally in a rotational manner, depicted by curved arrow  86 , along a rotation axis z which is perpendicular to the plane of the mask  30 . Mask  30  can optionally further be positioned rectilinearly in the z-direction by a motion controller and a motion drive (not shown) so as to permit the distance between the substrate  20  and mask  30  to be controlled. A vision system  60 , which is of a conventional kind for use in the photolithographic arts, uses fiduciary marks (not shown) on mask  30  and an auxiliary source of illumination (not shown) to determine the position of mask  30  relative to the position of substrate  20 . Vision system  60  is in communication with microcontroller and motion drives  80  and sends a signal to microcontroller and motion drives  80  to adjust the position of mask  30  along any of its degrees of freedom until the mask  30  is correctly positioned. Radiation blocking screens  442 ,  444 ,  446 , and  448  are each independently rectilinearly movable in one of two mutually perpendicular directions shown as x-motions  72  and  76  and y-motions  74  and  78 . In one embodiment, the relative positions of the radiation blocking screens  442 ,  444 ,  446 , and  448  along the z direction perpendicular to the plane of the mask  30  are adjustable by use of motion controllers (not shown). Motion controller and linear drives  70  controls the motion of screens  442 ,  444 ,  446 , and  448  to define aperture  52  by the opening delineated by the overlap of the edges  500  of screens  442 ,  444 ,  446  and  448 . Vision system  60  is in communication with microcontroller and linear drives  70  and can send a signal to microcontroller and linear drives  70  to adjust the positions of screens  442 ,  444 ,  446 , and  448  along any of their degrees of freedom until the aperture  52  defined by the edges of the screens is correctly adjusted. Radiation  58  from a source (not shown) is then caused to illuminate mask  30  over the selected photolithographic image  54  so as to produce imprinted image  56 ′ upon the photoresist on substrate  20 , with radiation blocking screens  442 ,  444 ,  446  and  448  preventing the exposure of any other image carried on mask  30 . 
     FIG. 5 depicts a plan view of the step of selection of an image from a set of photolithographic images according to one embodiment of the invention. Mask  30  comprises a set of photolithographic images generally  50  for the manufacture of a circuit requiring multiple layers and several types of devices. At a point in the processing of the circuit when image  50 ′ is to be imprinted, mask  30  is aligned with the substrate (not shown) so that image  50 ′ is in registry with the device of which it comprises one layer. Radiation blocking screens  442 ,  444 ,  446  and  448  are then individually positioned by being moved rectilinearly along axes of motion perpendicular to their edges generally  500 , to define rectangular aperture  52 . Aperture  52  defines the portion of mask  30  which will be illuminated by the source of electromagnetic radiation (not shown), and can thus select imaged  50 ′ for imprinting while blocking the illumination of the remainder of mask  30 . Line XX depicts the relative position of the side view (or vertical cross section) which is presented in FIG.  6 . 
     FIG. 6 depicts in cross section through line XX in FIG. 5 of the apparatus for selecting and imprinting a selected image on a substrate from a set of photolithographic images on a mask according to an embodiment of the invention. Substrate  20  is depicted with an imprinted image  56  thereon. A second image  56 ′ (shown in phantom) is positioned for selection and imprinting adjacent to image  56 . Mask  30  is disposed above substrate  20  and is positioned with image  50 ′ in registry with substrate  20  such that the location of the imprinted image  56 ′ will be adjacent to image  56 . Additional images  50  are carried by mask  20 . These images  50  are covered by the radiation blocking screens  442  and  446  which are disposed above mask  30  to define rectangular aperture  52 . FIGS. 7 a, b  depict another embodiment of a mask  30  and an aperture  42 , respectively, constructed in accordance with the invention. Mask  30  is depicted in FIG. 7 a  with a plurality of images  50 . In the embodiment shown, one image  50   a  represents a layer of a silicon semiconductor device wherein a portion of the device area is prepared for exposure to a deliberately added impurity known in the art as a dopant, the addition of which changes the electrical properties of the silicon substrate. Another image  50   b  represents a layer of a silicon semiconductor device wherein selected regions of the surface of the silicon semiconductor are covered by a deliberately added thin oxide layer, which generally passivates the silicon semiconductor and may for example permit the creation of a field effect transistor or a tunnel junction device. A third image  50   c  represents a layer of a silicon semiconductor device wherein selected areas of the device are provided with a polysilicon layer, which may for example be used as a conductor, or as a gate electrode in a field effect transistor. Other images  50  (shown schematically) are typically provided to permit the fabrication of other silicon semiconductor device elements, as will be appreciated by those skilled in the art. In this embodiment, all of the images correspond to a single device, and they are all of the same overall shape and size. Accordingly, aperture screen  40  which is depicted in FIG. 7 b  is provided, which includes aperture  42  which is defined by the periphery of the rectangular opening constructed to be substantially commensurate with the dimensions and shape of images  50  of mask  30 . Mask  30  and aperture screen  40  are employed as depicted in FIGS. 8 a, b, c,  to produce the structures depicted in FIGS. 9 a, b, c , as described below. 
     FIGS. 8 a, b, c  depict in plan view the relative positions of the mask  30  and aperture  42  during the steps of selecting various mask images of the mask  30  of FIG. 7 a  for manufacturing an integrated circuit according to the invention. FIGS. 9 a, b, c  depict in cross section a partially manufactured integrated circuit manufactured according to the steps shown in FIGS. 8 a, b, c . In FIG. 8 a , mask  30  has been positioned such that the image  50   a  is in registry with the desired location on the substrate. In this embodiment, image  50   a  has an area  90  which is transparent with regard to the electromagnetic radiation emitted by source  10  (not shown) and another area  92  which is opaque to the electromagnetic radiation emitted by source  10 . 
     For purposes of explanation, we shall assume that the photoresist used in this embodiment is softened by exposure to the radiation of source  10 , that is, the area not exposed to radiation remains covered with photoresist, and the area exposed is washed clear of photoresist in processing steps subsequent to imprinting. Alternatively, one can obtain the same result by employing a photoresist which hardens upon exposure to the radiation of source  10 , and reversing the character of areas  90  and  92 , that is making area  90  opaque rather than transparent, and making area  92  transparent rather than opaque. Aperture screen  40  is positioned above mask  30  with aperture  42  (here shown somewhat larger in dimension than image  50  for clarity) aligned with image  50   a . Image  50   a ′ is then imprinted upon substrate  20 . Line YY depicts the position of a vertical cross sectional view of the structure of the layer of the device being manufactured corresponding to image  50   a ′, which is depicted in FIG. 9 a . FIG. 9 a  depicts a substrate  20 , which for example is a silicon semiconductor wafer, which has a region  100  having one conductivity type, which for example may be n-type. Area  102  corresponds to the area  90  of image  50   a ′, which is an imprint of image  50   a  of FIG. 8 a , which was then exposed to radiation, and whose photoresist cover was removed. Substrate  20  has a surface oxide  104  which is partially removed, in particular in area  102  where no photoresist remains. Upon further processing of substrate  20  with a dopant, such as boron or aluminum in the case of a silicon semiconductor wafer, area  102  becomes a p-type region, known in the art as a p-well. As is well known in the art, oxide  104  is then replaced with new oxide  104 , and coated with photoresist (not shown). 
     After the image  50   a ′ has been imprinted on substrate  20 , and after substrate  20  has been processed, substrate  20  is returned for the imprinting of another layer, the image of which is depicted in FIG. 8 b . Mask  30  is then positioned above substrate  20  (not shown) and image  50   b  is aligned with the device of which image  50   a  constituted a layer. Aperture screen  40  is aligned with mask  30  so that aperture  42  permits image  50   b  to be illuminated and all other images  50  to be blocked from being illuminated. Image  50   b ′ is then imprinted upon substrate  20 . Again, line YY depicts the position of a vertical cross sectional view of the structure of the layer of the device being manufactured corresponding to image  50   b ′, which is depicted in FIG. 9 b . FIG. 9 b  depicts a substrate  20 , which has been imprinted with image  50   b ′, corresponding to image  50   b  of FIG. 8 b , in registry with image  50   a ′, and then processed, for example by an oxidation process such as steam oxidation, to produce layers  106  which are for example thin oxide. 
     After the image  50   b ′ has been imprinted on substrate  20 , and after substrate  20  has been processed, substrate  20  is again returned for the imprinting of another layer, the image of which is depicted in FIG. 8 c . Mask  30  is positioned above substrate  20  (not shown) as discussed previously and image  50   c  is aligned with the device. Aperture screen  40  is again aligned with mask  30  so that aperture  42  permits image  50   c  to be illuminated. Image  50   c ′ is then imprinted upon substrate  20 , the cross sectional view of which is depicted in FIG. 9 c . FIG. 9 c  depicts a substrate  20 , which has been further imprinted with image  50   c ′, corresponding to image  50   c  of FIG. 8 c , in registry with image  50   a ′, and then processed, for example by deposition of polysilicon  108  to produce a conductor or a gate. 
     The embodiment illustrated in FIGS. 7,  8  and  9  depicts images which are of one shape and size. Another embodiment uses the mask of FIG. 3, which has images of a number of different shapes and sizes. In this embodiment, there are used a plurality of aperture screens  40 , each having an aperture  42 , as in FIG. 7 b , one aperture screen  40  and aperture  42  adapted for use with one size and shape of image on mask  30 . In one embodiment, the plurality of aperture screens  40  can be conveniently housed in a cassette (not shown) and used serially as necessary. As will be apparent to those knowledgeable in the art, in other embodiments of the invention, substrates of other semiconductors, such as for example silicon-germanium, III-V compounds such as gallium arsenide and indium phosphide and their alloys and the like, substrates of insulators, and substrates of metals may also be processed in similar fashion. 
     Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.