Abstract:
A method and apparatus for masking a workpiece with a layer of ink from an inkjet head is disclosed. The masking prevents exposure of select regions of a photosensitive workpiece. The apparatus includes a workpiece pre-aligner for movably supporting and aligning the workpiece. The inkjet head is arranged to be in operable communication with the photosensitive layer of the workpiece when positioned on the pre-aligner and is adapted for providing a select mask of opaque ink on a photosensitive layer. Where the photosensitive layer is a negative tone photoresist, upon exposure the portion of the photosensitive layer that is not exposed because of the presence of the mask is removed upon developing. In this manner, select regions of the workpiece can be kept clear of photoresist or otherwise patterned with indicia such as alphanumeric symbols or barcodes.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the lithographic exposure and patterning of workpieces, and in particular relates to an ink-based method and apparatus for masking a workpiece to prevent exposure of select regions on the workpiece. 
   2. Description of the Prior Art 
   Lithography techniques are used in the manufacturing of microdevices, such as, integrated circuits (ICs), flat panel displays, microelectromechanical systems (MEMS), the formation of bump IC interconnects for “flip chip” interconnection technology, and the like. The lithographic process involves the use of photosensitive workpieces (“wafers”) and the selective exposure of such workpieces with radiation (e.g., UV light). Workpiece photosensitivity is typically achieved by coating or otherwise applying a layer of photosensitive material called photoresist to the workpiece surface. Photoresist can either be “positive-tone,” in which the exposed resist is removed upon developing, or “negative-tone,” in which the unexposed resist is removed upon developing. Generally, the lithography process includes the steps of coating the workpiece with photoresist, exposing the photoresist with the image of a mask to form a latent pattern in the photoresist, developing the photoresist to form a three dimensional image, etching the photoresist to form a corresponding three dimensional pattern in the workpiece, and then removing the excess photoresist. These steps are repeated a requisite number of times to form the particular device structure in the workpiece. 
   In certain lithography applications involving negative photoresist, it is preferred that select regions of the workpiece outside of the individual exposure fields remain completely unexposed so that the resist in these regions is removed upon developing. One example application is bump interconnect lithography, which involves the formation of conductive (e.g., gold or aluminum) bumps on a workpiece (wafer) that are used to contact circuit lines on a circuit board. Bump lithography includes an electrochemical plating step to form the conductive bumps that requires contacting most of the edge of the wafer with an electrode. For this purpose, substantially all of the edge of the wafer must be free of all photoresist to ensure uniform electrical contact. 
   When one-to-one contact or near-contact photolithography is used the entire wafer is exposed at once and the exclusion of the edge is achieved by incorporating the desired exclusion into the mask pattern. However, in step and repeat photolithography it is not possible to define the exclusion area into the mask pattern since the stepping pattern depends on the pattern size and the step size. 
   SUMMARY OF THE INVENTION 
   The present invention relates to the lithographic exposure and patterning of work pieces using either reduction or  1 × step and repeat lithography, and in particular relates to an ink-based method and apparatus for selectively masking a workpiece to prevent exposure of select regions on the workpiece. 
   A first aspect of the invention is an apparatus for masking a workpiece coated with a photosensitive layer, to prevent exposure of select regions of the photosensitive layer. The apparatus includes a workpiece pre-aligner or other system for movably supporting and aligning the workpiece, and an ink delivery system, such as an inkjet head, arranged to be in communication with the photosensitive layer of the workpiece for providing a pattern of opaque ink on a photosensitive layer. The apparatus may also include a programmable control unit connected to the ink delivery system and the pre-aligner, for coordinating and controlling the masking process. 
   A second aspect of the invention is a method of selectively masking a photosensitive workpiece to prevent exposure of select regions. The method includes determining the one or more select regions of the photosensitive workpiece surface that need to remain unexposed, and then masking the one or more select regions with a layer of ink. The ink is opaque to a wavelength of radiation that activates the photosensitive workpiece. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic plan view of a lithography tool showing the workpiece stage, workpiece pre-aligner and main controller and housing, and ink-jet printer system arranged on the pre-aligner according to the present invention; 
       FIG. 2  is a side view of the workpiece pre-aligner with a workpiece supported on the fork of the aligner and the inkjet head arranged in proximity to the workpiece near the edge; and 
       FIG. 3  is a close-up plan view of a portion of the pre-aligner and inkjet head system of  FIG. 2  illustrating an annular mask of ink in the process being formed around the edge of a workpiece. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates to the lithographic exposure and patterning of workpieces, and in particular relates to an ink-jet-based method and apparatus for masking a workpiece to prevent exposure of select regions on the workpiece. 
   With reference now to  FIG. 1 , there is shown a lithography tool  10  that includes a workpiece stage  16  and a workpiece pre-aligner  26  in operable communication with the workpiece stage. Workpiece stage  16  and pre-aligner  26  are enclosed in a housing  34  that provides for control of the environment within tool  10 . Other elements of the lithography tool, such as the projection lens, mask stage, illuminator, etc., that reside above the wafer stage are not shown in  FIG. 1  for clarity of the present invention. 
   Workpiece stage  16  is designed to movably support a workpiece (wafer)  40 . Workpiece  40  includes a reference feature  42 , such as a notch or a flat that facilitates alignment of the workpiece to another reference. Workpiece  40  also includes an upper surface  44 . Workpiece  40  is moved beneath the projection lens by workpiece stage  16  so that multiple exposure fields  50  can be formed (exposed) on the workpiece. 
   Pre-aligner  26  is for receiving a workpiece from a workpiece handler or other workpiece source (not shown) and precisely aligning and centering the workpiece to a known reference position on the pre-aligner. This allows for the workpiece to be transferred to workpiece stage  16  in a known alignment state. Consequently, the workpiece can quickly be finely aligned on workpiece stage  16  and exposed. 
   Lithography tool  10  also includes a main controller  54  that controls the operation of the lithography tool, and is operatively connected to workpiece stage  16  and pre-aligner  26  to control the pre-alignment and delivery of workpieces to workpiece stage  16 . Main controller  54  also optionally controls the overall operation of pre-aligner  26  in carrying out the ink-based masking method described below in conjunction with the lithography process. 
   With reference now also to FIG.  2 . In an example embodiment, pre-aligner  26  includes a base  60  with an upper surface  62 . A motor unit  70  is attached to upper surface  62  towards one end of base  60 . An arm  80  with first and second ends  82  and  84 , respectively. Is connected to motor unit  70  located at first end  82 , allowing the motor to rotate the arm about an axis A 1  passing through the first end  82 . Second end  84  of arm  80  includes a fork  86  capable of supporting workpiece  40 . Motor unit  70  is also adapted to raise and lower arm  80  at first end  82  so that fork  86  can be raised and lowered when moving, adjusting the position of or otherwise engaging workpiece  40 . 
   Pre-aligner  26  also includes a workpiece rotation member  90  arranged beneath fork  86  on surface  62 . Member  90  is capable of measuring the position of the workpiece on fork  86  (e.g., the location of reference mark  42 ) and engaging the workpiece to rotate it about axis A 2  to properly orient the workpiece. Such rotation is generally necessary for pre-aligning (e.g., centering and orienting) workpiece  40  prior to it being transferred to workpiece stage  16  via the movement of arm  80 . Generally, pre-aligner  26  serves in the present invention as a convenient device for providing the necessary support and rotation of a workpiece to perform ink masking. In an example embodiment of the present invention, pre-aligner  26  may be simply a workpiece rotation device without pre-alignment capability that is remote from lithography tool  10 . However, it is advantageous to use pre-aligner  26  to perform ink masking of the workpiece, since it is already part of lithography tool  10  and is designed for moving and positioning a workpiece onto workpiece stage  16  in anticipation of lithographically processing the workpiece. 
   With continuing reference to  FIGS. 1 and 2 , an ink delivery device  110  is arranged near fork  86  so that when workpiece  40  is supported thereon, the ink delivery device is in dose proximity (e.g., from one to several millimeters) to workpiece upper surface  44 . Ink delivery device  110  is preferably movable over upper surface  44  (as indicated by arrows  116 ) so that it can print features anywhere on the workpiece upper surface. In an example embodiment of the present invention, ink delivery device  110  need only move along a radial line as workpiece  40  is rotated underneath the head. In another example embodiment of the present invention. Ink delivery device  110  includes an ink-jet head. 
   With continuing reference to  FIG. 2 , connected to ink delivery device  110  is a control unit  126  that controls the deposition of ink by the ink delivery device. Control unit  126  is also connected to main controller  54  that communicates with pre-aligner  26  so that information about the position and rotation of the workpiece, etc. (i.e., generally, the workpiece state) can be provided to control unit  126 . 
   Ink delivery device  110  and associated control unit  126  constitute an ink delivery system. An example of a commercially available ink delivery system suitable for use in the present invention is the Domino A400 inkjet system available from Domino UK Ltd., Cambridge, England. 
   Control unit  126  can be programmed with printing instructions for selectively masking upper surface  44  of workpiece  40 . In the present invention, the ink used in ink delivery device  110  is opaque to the wavelength of radiation used to expose workpiece  40 . Further, the ink layer is deposited with a thickness sufficient to block exposure of the photoresist. For negative tone photoresist, this means that the masked regions of the photoresist remain unexposed and so are removed from the workpiece upon developing. 
   Preferably, the ink layer deposited with ink delivery device  110  is fast-drying (1-2 seconds) and is suitable for (e.g., adheres to) the particular photoresist material formed upon upper surface  44  of workpiece  40 . The patterns capable of being formed by ink delivery device  110  include alphanumeric characters, bar codes, various small- and large-scale line patterns, including an annulus of a few millimeters in width formed along the very edge of the workpiece. Though some masking patterns may be more useful than others. In practice any feature capable of being formed with a conventional inkjet printer system can be formed on workpiece upper surface  44  through the appropriate programming of control unit  126 . 
   In an example embodiment of the present invention, the photoresist used on workpiece  40  to provide the required photosensitivity is a negative-tone dry film photoresist, which typically include a thin-film protective coating. An example of such a photoresist is RISTON® manufactured by DUPONT, which includes a thin MYLAR® coating that is transparent to the exposure wavelength of radiation. In the case of a MYLAR®-coated resist, the ink used should be one that readily adheres to and dries quickly on MYLAR®. MYLAR® is a trademark of a polyethylene terephthalate polyester film. 
   In operation, workpiece  40  to be processed using lithography tool  10  is first arranged on pre-aligner  26 . Because pre-aligner  26  has the capability to rotate workpiece  40  via rotation member  90 , the rotation of the workpiece can be coordinated with the operation of ink delivery device  110  via control unit  126  and optionally main controller  54  to form a desired mask. For example, to prevent photoresist from remaining on the edge of workpiece  40 , the workpiece edge can be rotated beneath ink delivery device  110  while the device deposits a narrow (e.g., several millimeters) annular ink band  180  (shown being formed in FIG.  3 ). In another example, workpiece  40  is held stationary while ink delivery device  110  scans across a region of the workpiece surface  44  to write a barcode and/or alphanumeric code  190  (FIG.  3 ), or other indicia. 
   There are a number of key advantages of using an ink delivery device for performing masking of a workpiece in order to prevent exposure of the workpiece at select photosensitive regions. A first advantage is that the exclusion region can be defined with very high accuracy, and can even be used to define alphanumeric characters A second advantage is that the particular mask pattern can be tailored to suit the array of exposure fields printed on the workpiece using the lithography tool. A third advantage is that the size of the masked regions can be changed by programming the ink delivery device controller rather than by having to make mechanical adjustments of the system. A fourth advantage is that the ink masking does not involve physical contact beyond that of the remotely applied stream (“jet”) of ink. Thus, non-flat workpiece surfaces can be accommodated. A fifth advantage is that an ink delivery system for masking the workpiece can readily be made by modifying an existing apparatus within the lithography tool, namely the pre-aligner. Because the masking can be done quickly (less than 5 seconds for most applications) and be performed in-situ at the pre-aligner while another workpiece is being exposed on the workpiece stage, there is minimal impact on the overall lithography process and thus minimal or no impact on throughput. A sixth advantage is that ink delivery devices in the form of inkjet printers are commercially available and not particularly expensive, so that the cost of the masking apparatus and process is minimal. 
   The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims.