Abstract:
Methods for minimizing the errors associated with substrate etching are presented. The methods use intentional defocusing of the pattern image on the photoresist to minimize errors in the etching process particularly grayscale etching and/or multiple exposure contributions from neighboring patterns.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of and claims priority under 35 U.S.C. § 120 to U.S. Provisional patent application Ser. No. 10/300,865 filed on Nov. 21, 2002, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to Grayscale photolithography. More particularly it relates to using various focusing techniques to improve grayscale etch uniformity.  
         [0004]     2. Description of Related Art  
         [0005]     Grayscale photolithography has revolutionized the way curved surfaces are etched into substrates. In grayscale photolithography, grayscale masks are used to etch smooth surfaces into substrates. The smooth surfaces etched enable the creation of various shapes and curves, symmetric and asymmetric, lending themselves well for the use in fabrication of micro-optical systems. The quality of these surfaces are dependent on the qualities of etching which in turn depend, at least partly, on the qualities of the grayscale mask. Hence, errors in the grayscale mask can result in errors in the etched surfaces.  
         [0006]     The use of grayscale etching in the formation of microlenses is typified in co-pending application “Deep Grayscale Etching of Silicon” (PCT/US01/42629) to Whitley et al. herein incorporated by reference. A light (usually UV) from an illumination device (e.g. a stepper) illuminates a grayscale mask, forming a pattern image on a photoresist layer, which selectively exposes the photoresist layer such that the pattern image, or its negative, is later developed in the photoresist layer. After development, the pattern image in the photoresist is used to etch a pattern in the substrate. Significant errors in the resultant patterned substrate surface are often due to writing errors in the grayscale mask.  
         [0007]     There are several methods of producing a grayscale mask but most use an electron beam or laser beam to write the mask from a chrome base. Etched substrates are susceptible to three types of errors: the first error has to do with the roughness of the surface of the photoresist layer; the second error has to do with positioning errors of the mask writing tool (i.e. stitching error); and the third error is due to non-uniformity in wafer etching. The writing of the mask is susceptible to the first two errors typically plaguing etched substrates.  
         [0008]     The first source of error arises from general roughness in the surface of the photosensitive material. This error can be caused by the slight variations in the dose of the writing tool, usually an electron beam (e-beam) or laser. In the case of the half tone process, the chosen pixel shape scheme can cause this error. The period of irregularity caused by this general roughness error is typically on the order of 10 microns.  
         [0009]     The second source of error is a stitching error; it is geometric and is induced by slight variations in the positioning and size of the writing tool. Stitching error is due to slight inaccuracies of the stage and field of the writing tool. The stage of the writing tool refers to the horizontal sweep, wherein slight variation in the positioning of the horizontal line results in stitching error. The field refers to the width of the writing line, wherein variation in the width of the writing line also results in stitching error. The stitching error has a low frequency period and manifests itself in slight vertical lines on the etched surface.  
         [0010]     The third error forms inconsistencies between multiple lenses in an array of lenses on a wafer. This is caused by subtle variations across the mask, which are caused by slight “wafer” level variational processes such as development or chrome etch processes. High quality lens typically require &lt;1% non-uniformity in the focal length across the large array of mass produced lenses.  
         [0011]     The three errors are cumulative and add to degrade lens quality and reproducibility.  FIG. 1  illustrates the first two errors on an actual lens etched by grayscale etching of a substrate using the standard procedure of focusing the pattern image at the photoresist layer. Stitch errors  10  are evident and shown as straight lines in the lens surface. Surface roughness errors  20  are also evident and are shown as high frequency rings in the lens surface. The lens shown in  FIG. 1  has a height of 17.5 microns, a peak to valley roughness (difference) of ˜216 nm or 1.2% of total height, and a root mean squared (RMS) roughness of ˜44 nm or 0.2% of the total height.  
         [0012]     Standard practice focuses the pattern image from the mask on the photoresist layer.  FIG. 2  illustrates a standard arrangement for developing and patterning a photoresist layer  150 . A stepper  110  emits focusing light, defined by conical extent  120 . The pattern image is focused at a plane  140  on or in the photoresist layer  150 , which is deposited upon the substrate  160 . The photoresist  150  is patterned through exposure by the illumination light  120  passing through the grayscale mask  130  and is then developed. The mask (in the present example, the grayscale mask) can contain errors addressed by the present invention. The mask  130  contains a pattern that is imparted to the light passing through the mask  130 , creating a pattern image  105 . The pattern image  105  is reduced to form a reduced image  107  that is focused on the photoresist  150 . Typically, the image is focused on the top of the photoresist layer. The reduced image  107  exposes and patterns the photoresist  150  which can then be used to obtain the desired etch pattern in substrate  160  upon etching.  
         [0013]     A method for minimizing the three mentioned errors is important for developing high quality, reproducible lenses.  
       SUMMARY OF THE INVENTION  
       [0014]     It is therefore an object of the present invention to provide a method and apparatus to reduce errors associated with substrate etching. It is further an object of the present invention to provide a method and apparatus to reduce errors associated with microlens fabrication using grayscale etching.  
         [0015]     These and other objects of the present invention can be realized by the methods/devices of intentionally defocusing the pattern image produced on the photoresist layer. A first method involves intentionally setting the stepper to focus the pattern image at a point other than the optimal focus setting (at the photoresist surface). A second and third method modifies the stepper so that the stepper is out of focus for its entire design range by physically moving the stepper and/or by using optical devices. A fourth method places a thin clear cover plate on top of the photoresist-covered wafer providing defocusing of the incident illumination, and a fifth method exposes identical patterns on a mask to obtain an average image of the pattern on the photoresist reducing errors. The methods discussed can be used in combination.  
         [0016]     The first method uses an intentional defocusing setting of the stepper. If more defocus is needed the second method moves the stepper a distance greater than the stepper&#39;s defocus range. To allow a return to standard focusing, an optical device can be added to modify the focusing characteristics of the illuminating light emitted from the moved stepper, resulting in a change in the focusing characteristics of the pattern image.  
         [0017]     If a method is needed to defocus beyond the built in range of defocus and still allow focusing when needed, as mentioned above, this can be accomplished by adding optical devices and elements. The third method places an optical device between the stepper and the mask so as to alter the focusing characteristics of the pattern image. An optical device can also allow normal focusing in addition to the intentional defocusing.  
         [0018]     The fourth method places an optical device between the mask and photoresist layer. A cover plate can be used with a standard stepper, or in addition to another optical device placed between the stepper and the mask.  
         [0019]     A fifth method multiple exposes identical patterns on a mask, resulting in an averaged image of the identical pattern in a photoresist, deposited on a substrate, reducing errors, associated with variability across the mask, in the developed photoresist.  
         [0020]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The present invention will become more fully understood from the following detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention, and wherein:  
         [0022]      FIG. 1  is an illustration of a computer display showing typical errors in an etched micro-lens;  
         [0023]      FIG. 2  is an illustration showing a standard focus of a pattern image at a photoresist layer;  
         [0024]      FIG. 3  is an illustration showing the intentional defocusing of a pattern image at a photoresist layer;  
         [0025]      FIG. 4  is an illustration showing the intentional defocusing of a pattern image at a photoresist layer by using an optical device;  
         [0026]      FIG. 5  is an illustration showing the intentional defocusing of a pattern image at a photoresist layer by using a cover plate;  
         [0027]      FIG. 6  is an illustration showing the intentional defocusing of a pattern image at a photoresist layer by using a cover plate and an optical device;  
         [0028]      FIG. 7  is an illustration of a computer display showing the effect on typical errors using an embodiment of the present invention to intentionally defocus the illumination; and  
         [0029]      FIG. 8  illustrates the use of multiple exposures of patterns on a mask to form an image in a photoresist layer, where the image is the result of the combined exposures using multiple patterns, and the resultant image developed in the photoresist has decreased fabrication errors and forms one element of an array of images form by the same method, the resultant array having decreased fabrication errors, of the type discussed above. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     As noted above there are essentially three types of errors associated with quality micro-lens production. Defocusing the pattern image formed on the surface of the photoresist layer can minimize errors associated with etching of micro-lens is addressed in the embodiment of the present invention. Like figures on the appended drawings refer to like elements in the appended figures.  
         [0031]     In accordance with the present invention, a first method of minimizing errors, associated with substrate etching, intentionally defocuses the pattern image illuminating the photoresist layer(s) by adjusting the focusing control on the stepper  110  a shown in  FIG. 3 . Many steppers have a focusing range used to accurately focus the pattern image  107  on the photoresist layer  150  in accordance with standard practice where the position of the stepper is referred to as the focused position of the stepper. Some steppers will allow up to approximately 50 microns of defocus and others only a few microns of defocus. For some applications and masks 50 microns may be enough. Thus, the first method intentionally defocuses the pattern image away from the photoresist layer, an amount greater than would normally occur through erroneous focusing. As discussed above the standard practice is to focus the pattern image on or in the photoresist layer. Erroneous, unintentional, defocusing can result in focusing of the pattern image away from the photoresist layer but within a few microns of the optimum focusing point, which is predetermined to lie within or on the photoresist layer. The intentional defocusing of the pattern image according to the first method defocuses the pattern image tens of microns or more away from the optimum focus point on or in the photoresist layer. The intentional defocusing smoothes out the illumination intensity, decreasing errors associated with stitching, surface roughness, and non-uniform etching of the substrate.  
         [0032]      FIG. 3  illustrates a device/mask  100  using the first method according to the present invention. A stepper  110  emits focusing light, defined by conical extent  120 . A pattern image  107  is focused at a plane  140  above the photoresist layer  150 . Focus at plane  140  results in a defocus of the pattern image on the photoresist layer  150 , deposited upon the substrate  160 . The pattern image  107 , focused on plane  140 , is shown above the photoresist layer  150  but can also be below the photoresist layer. The photoresist  150  is patterned and developed by the illumination light  120  passing through the grayscale mask  130  and it is the mask (in the present example the grayscale mask) that can contain the errors seeking to be addressed by the present invention.  
         [0033]     The mask  130  contains a pattern that is imparted to the light passing through the mask  130 , creating a pattern image  105 . The pattern image  105  is reduced to form a reduced image  107  that is focused on the plane  140  that does not lie on or in the photoresist layer  150  forming a defocused pattern image on the photoresist layer  150 . The defocused pattern image  107  exposes the photoresist  150  forming a deformed pattern in the photoresist, which can be used to obtain the desired etch pattern in substrate  160  upon etching. Intentionally defocusing the pattern image on the photoresist smoothes the errors associated with substrate etching.  
         [0034]     The term “grayscale mask” as used herein may be any suitable mask, for example, a binary grayscale mask as disclosed in U.S. Pat. No. 5,310,623 to Gal, a HEBs glass grayscale mask or any other mask suitable for continuous variation in exposure of the photoresist facilitating etching of variable contours in a plane perpendicular to the surface of the substrate. The discussion herein should not be interpreted to limit the mask to a grayscale mask nor should the errors mentioned above be interpreted to be the only errors on the mask or in the etching process that can be addressed by the present invention.  
         [0035]     There are situations when the intentional defocus must be beyond the focusing range built into a particular stepper. To obtain additional range for defocusing, the stepper itself can be physically moved or equivalently optically adjusted. Optical adjustment allows the user to revert back to standard focusing procedures when desired. To optically adjust the stepper&#39;s focusing range or point, lenses can be used before the light illuminates a mask.  FIG. 4  illustrates a device  200  using an embodiment of the present invention having an optical device  180 , provided between the stepper  110  and mask  150 , to varying the focusing characteristics of the light emitted from the stepper  110 . Light, defined by the conical extents  170 , is emitted from the stepper  110  and is incident upon an optical device  180 . The optical device  180  changes the focusing characteristics of the incident light. The transmitted light, defined by the conical extents  120 , passes through a mask  130 . The mask  130  contains a pattern that is imparted to the light passing through the mask  130 , creating a pattern image  105 . The pattern image  105  is reduced to form a reduced image  107  that is focused on the plane  140  spaced away from the photoresist layer  150 , forming a defocused pattern image on the photoresist layer  150 . The defocused pattern image, patterns and develops the photoresist  150  so as to obtain the desired etch pattern in substrate  160  upon etching. Intentionally defocusing the pattern image on the photoresist, by using optical elements, smoothes the errors associated with substrate etching.  
         [0036]     The optical device  180  can be any optical device, which provides the desired amount of defocusing with respect to the positions of the stepper and photoresist. For example a simple convex lens or frenel lens would suffice. However, more complicated telescopic type configurations and more than one optical device may also be used. Hence, discussion herein should not be interpreted to limit the type of optical device(s) used in a particular embodiment.  
         [0037]     Moving the stepper beyond its defocusing range is an embodiment of the present invention, as discussed above. However, in order to revert back to standard practices when desired some method of focusing the pattern image back to the photoresist is needed. This can be accomplished, as discussed above, by an optical device placed between the stepper and a mask. An optical device can also be used between the mask and the photoresist when the stepper is not moved but defocusing is desired beyond the stepper&#39;s defocusing range. Another embodiment of the present invention provides a defocusing capability beyond a stepper&#39;s focusing range by placing an optical device between the mask and the photoresist layer. For example a thin (e.g., 250 μm-1000 μm) clear (e.g., SiO 2 , quartz) cover plate can be placed above the photoresist layer to alter the pattern image focusing characteristics. An optical device or element placed between the mask and the photoresist layer can change the focusing characteristics of the illuminated light resulting in a pattern image on a plane not coincidental with the photoresist layer. For example an optically transparent thin plate can be placed above the photoresist layer to vary the refractive (focusing) characteristics of the illuminating light, resulting in a pattern image that can be defocused on the photoresist layer.  
         [0038]      FIG. 5  illustrates a device/method  300  using an embodiment of the present invention that uses an optical device placed between the mask and photoresist layer to vary the focusing characteristics of the pattern image. An illumination device  110  emits focusing light, defined by conical extent  120 . A reduced pattern image  107  is focused at a plane  140  away from the photoresist layer  150 . The plane  140  corresponds to the location of a thin (e.g., 250 μm-1000 μm, and the like) clear (e.g., SiO 2 , quartz, and the like) cover plate  210  above the photoresist layer  150  and substrate  160 . The cover plate  210  sits on a stand  220 , which separates the cover plate  210  from the photoresist layer  150 . The photoresist  150  is patterned and developed by the illumination light  120  passing through the grayscale mask  130  and it is the mask (in the present example the grayscale mask) that can contain the errors seeking to be addressed by the present invention. The mask  130  contains a pattern that is imparted to the light passing through the mask  130  forming a pattern image  105 . The pattern image  105  is reduced and focused at a focal plane  140  on or in the cover plate  210 . The pattern image on the photoresist is defocused an intentional amount due to the cover plate  210 .  
         [0039]     The defocused image exposes the photoresist  150  forming a defocused pattern in the photoresist which can be used to obtain the desired etch pattern in substrate  160  upon etching. The stand  220  spaces the plate  210  a desired distance from the photoresist  150  and can be formed of any material, such as Si, SiO 2 , and the like, suited to the particular needs at the time of operation, and can be attached to the cover plate  210 , or attached to the substrate  160 , or be independent such as a ring or peg that then sits on the substrate. Hence, the discussion herein should not be interpreted to limit the material or attachment of the stand. The cover plate is a particular example of an optical device. The cover plate can be a birefringent crystal, a multi-lens optical device situated to modify the focus of the illuminating light, or have varying planar optical properties such as a microlens array. Therefore, the discussion herein should not be interpreted to limit the characteristics of element  210  to a single optical element, material, or optical device.  
         [0040]     In some situations it may be desirable to both defocus the general illumination beam and to provide varying focusing properties across the photoresist layer. An embodiment of the present invention combines an optical device placed between the stepper (illumination device) and the mask to generally defocus the pattern image incident on the photoresist layer, and a second optical device between the mask and the photoresist substrate that can have varying optical focusing characteristics parallel to the photoresist substrate.  
         [0041]      FIG. 6  illustrates a device/method  400  using an embodiment of the present invention having an optical device  180  to vary the focusing characteristics of the light emitted from the stepper  110  and an optical device to vary the optical focusing properties across a direction parallel to the photoresist layer  150 . Light, defined by the conical extents  170 , is emitted from the illumination device  110  and is incident upon an optical device  180 . The optical device  180  changes the focusing characteristics of the incident light. The transmitted light, defined by the conical extents  120 , passes through a mask  130 . A reduced pattern image  107  is focused at a focal plane  140  spaced away from the photoresist layer  150 . The plane  140  corresponds to the location of a thin (e.g., 250 μm-1000 μm, etc . . . ) clear (e.g., SiO 2 , quartz, etc . . . ) cover plate  210  above the photoresist layer  150  and substrate  160 . The cover plate  210  sits on a stand  220 , which separates the cover plate  210  from the photoresist layer  150 . As previously mentioned, the stand can be part of the cover plate, part of the substrate  160 , or a separate element such as a peg or ring.  
         [0042]     The photoresist  150  is patterned and developed by the illumination light  120  passing through the grayscale mask  130  and it is the mask that can contain the errors seeking to be addressed by the present invention. The mask  130  contains a pattern that is imparted to the light passing through the mask  130  forming a pattern image  105 . The pattern image  105  is reduced and focused on the focal plane  140  or a reduced image  107 , in this case on or in the cover plate  210 . The pattern image on the photoresist is defocused an intentional amount due to the cover plate  210  and the optical device  180 .  
         [0043]     As mentioned previously, intentional defocusing can be accomplished by optical elements before the mask  180  and/or after the mask  210  (e.g. coverplate). The defocused pattern image exposes the photoresist  150  forming a defocused pattern in the photoresist which can be used to obtain the desired etch pattern in substrate  160  upon etching. The cover plate is a particular example of an optical device. The cover plate could also be an optical element having anisotropic properties.  
         [0044]      FIG. 7  illustrates the results on micro-lens etching using a device/method in accordance with the present invention shown in  FIG. 4 . Comparing  FIG. 7  with  FIG. 1  one can see that the associated errors are appreciably less in  FIG. 7 . In  FIG. 7  the resultant lens has a height of 17.5 microns, a peak to valley roughness (difference) of &lt;60 nm or 0.3% of the total height, a fourfold increase in accuracy over a the standard procedure produced lens of  FIG. 1 . The lens in  FIG. 7  has a root mean squared (RMS) roughness of ˜12 nm or 0.07% of the total height; a three to fourfold increase in accuracy over the lens produced by the standard process.  
         [0045]      FIG. 8  illustrates another embodiment of the present invention. When mass arrays of identical micro-lenses are etched there are errors associated with pattern variations across the mask, as described above. The present invention reduces pattern variational errors by using multiple exposures of each identical mask lens pattern to fully develop the image pattern of a single lens in a photoresist on a substrate. Hence a multiple amount of the lens patterns are used for each lens image exposed in the photoresist. For example  FIG. 8  shows four lens patterns  800  formed in the mask  810 . Multiple illuminations  805  of each lens patterns  800  contributes to the exposure and development of a lens image  820  in the photoresist  830 , where the photoresist  830  is deposited on a substrate  840 . In the example shown in  FIG. 8  only three lens patterns  880 ,  881 , and  882  are used to develop lens image  820 . For example the exposure of lens pattern  880  could be ⅓ rd  that needed to fully expose the photoresist in the pattern of the lens image  820 . Successive exposures using different lens patterns,  881  and  882 , results in a cumulative exposure large enough to fully develop the lens image  820  into the photoresist. However any number of predetermined exposures and lens patterns can be used to accumulate exposures forming a lens image and the discussion herein should not be interpreted to limit the number of exposures or the number of lens patterns used. The multiple exposures of different lens patterns  800  smoothes the overall (fully exposed) resultant lens image  820  so as to reduce the third type of error mentioned above having to do with variations across the mask.  
         [0046]     Many variations of the processes and apparatus described herein can be made by one of ordinary skill in the art and any such variations should be deemed obvious with respect to the present invention and considered to lie within the scope of the present invention.