Abstract:
Effective passivation structures and guard rings can be formed in borderless gate arrays by forming the gates in an array of discrete blocks separated by thin scribe lines in which the substrate is not covered by gates. Diffusions for guard rings can be formed in the substrate for guard ring purposes, and passivation structures can be sealingly attached to the substrate. Various circuit metalizations such as discrete layers or different circuits can be produced with a single mask by covering all but a selected portion of the mask during exposure.

Description:
FIELD OF THE INVENTION 
     This invention relates to the manufacture of integrated circuit (IC) chips, and more particularly to a method of forming a plurality of passivated IC chips of various sizes, with guard rings and input-output (I/O) pads, from a borderless gate array wafer. 
     BACKGROUND OF THE INVENTION 
     IC chips are the heart of practically all modern electronic devices. They are typically manufactured by forming one or more arrays of unconnected gates or transistors on a silicon wafer, and then metalizing the array through masks to form interconnections between gates, and between gates and connection pads, that gives a chip its individuality and functionality. 
     Wafers are typically available in two types: standard-size arrays and borderless arrays. In the standard-size type, a set of individual arrays of a standard size are formed on each wafer, together with surrounding I/O pads and appropriate passivation structures for chemical isolation against environmental contaminants, as well as guard rings for electrical isolation against stray electromagnetic interference. After the interconnections have been formed, the wafer is cut between the arrays to provide individual finished chips. 
     In the borderless array type of wafer, a single array is formed to cover the entire surface of the wafer. Individual ICs are produced, after the formation of interconnections, by cutting through unused portions of the array. This method does not, however, lend itself to passivation. 
     Masks for the production of wafers and the formation of interconnections are extremely expensive, so that the manufacture of custom wafers is not economically practical for the production of chips in quantities less than hundreds of thousands. Yet there are many instances in which only a few thousand chips of any particular design are required. In order to economically produce such quantities, a wafer must be able to carry a large number of IC arrays of varying sizes for different purposes and/or different customers. This allows many different IC chips to be produced simultaneously with a single mask. 
     Problems arise in carrying out the latter method with either of the traditional types of wafers. In a standard-size array wafer, the array size must be large enough to accommodate the largest IC to be produced on the wafer. Consequently, substantial portions of the array are wasted for smaller ICs. Borderless arrays can be cut as desired to fit various-sized ICs on a wafer without substantial waste; however, borderless arrays, which are uniform throughout the wafer surface, do not lend themselves to passivation. Passivation structures can only be formed where the wafer substrate is accessible, i.e. where no transistor array has been formed on the wafer. 
     It is therefore desirable to provide a fabrication method which allows many ICs of varying sizes to be formed on a uniform generic wafer, yet allows passivation structures and guard rings to be formed around each individual IC regardless of its size or shape. 
     SUMMARY OF THE INVENTION 
     The invention overcomes the deficiencies of the prior art by forming on the surface of the wafer a borderless array composed of micro arrays or blocks about 200×200 μm in size, separated by about 10 μm wide strips in which the substrate is exposed. ICs are formed by metalizing sets of blocks which together have the requisite size and shape for the desired IC. The strips consume about 10% of the wafer surface, but the exposure of the substrate in the strips makes it possible to form passivation structures and (by forming areas of p +  and/or n +  diffusion in the strip) guard rings around any selected set of blocks. The 10 μm gap between blocks is not sufficient to interfere with the transmission of signals between gates in adjacent blocks. 
     In an additional aspect of the invention, unused blocks or portions of blocks within the layout of a particular IC may be metalized to form input/output connection pads. The versatility of the wafer can be improved by providing alternate rows or columns with various types of application-specific gate elements, such as transistors designed for use in analog or digital circuits; mixtures of transistors and resistors; or combinations of these. 
     In still another aspect of the invention, a variety of different layers and/or circuits can be metalized with a single mask by arranging all necessary patterns on the mask, and then covering all except the desired pattern during exposure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a prior art wafer; 
     FIG. 2 is a plan view of the inventive wafer; 
     FIG. 3 is an enlarged partial cross section of a wafer according to the invention; 
     FIG. 4 a  is a plan view of one form of block; 
     FIG. 4 b  is a plan view showing exemplary metallizations of the block of FIG. 4 a;    
     FIG. 5 is a plan view of another form of block; 
     FIG. 6 is a perspective view illustrating the use of separate masks for each layer; 
     FIGS. 7 a  and  7   b  are plan views of multi-layer masks according to the invention; and 
     FIG. 8 is a plan view illustrating the use of the mask of FIG.  7 a. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a typical conventional wafer  10 . Individual gate arrays or dies  12  surrounded by connection pads  14  are formed on the surface of wafer  10  in a row-and-column pattern. The dies  12  with their connection pads  14  are separated from one another by wide vertical and horizontal scribe lines  16   v  and  16   h . The marks  18  allow precise registration of the metalizing masks with the dies  12  during the subsequent metalizing operation in the manufacture of ICs. p +  and n +  diffusions for guard rings are formed in the scribe lines  16   v  and  16   h  around the dies  12  concurrently with the gate arrays  12 , and metallic passivation structures are formed along the peripheries of the dies  12  concurrently with the formation of the metallic connection pads  14 . Following metalization, the wafer  10  is cut along the scribe lines  16   v ,  16   h  to produce individual IC chips. 
     In a typical wafer such as that depicted in FIG. 1, the dies  12  may be, for example, about 2 mm by 3 mm in size. In general, the granularity of this type of wafer is in the millimeter range. 
     FIG. 2 shows a wafer  20  according to the invention. The dies on the wafer  20  are formed as small blocks  22  without connection pads (i.e. blocks in which the transistors or gates occupy essentially the entire width and height of the block), and are about 200×200 μm in size. The blocks  22  are separated by scribe lines  26   v  and  26   h  about 10 μm wide, in which the wafer substrate is exposed for the formation of guard ring diffusions  36 ,  38 ,  40  (FIG. 3) and alignment marks  34 . The granularity of the inventive wafer  20  can thus be in the 100-200 μm range. Any desired number of blocks can be combined together during metalization, in sets such as  23  and  25 , as described below to form IC chips of any desired size. This makes it possible to fabricate a variety of chips of different sizes on a single wafer. 
     FIG. 3 illustrates the ability of the inventive structure to provide guard rings and passivation structures around any desired set of the blocks  22 . In FIG. 3,  30  denotes the p− substrate of the wafer  20 .  22   a ,  22   b  and  22   c  are blocks of transistors. Block  22   a is a circuitry block which, together with other adjacent blocks, forms part of an integrated circuit. Block  22   b  is a block used to support an input/output connection pad  32 , and block  22   c  is an unused block. The wafer  20  may eventually be cut through the block  22   c , or on the scribe line  26 . Alignment marks  34  for that purpose are made on the substrate  30 . 
     Each of the scribe lines  26  contains guard ring connections  36 ,  38 . The connections  36  are preferably p+ diffusions in the p− substrate  30 , while the connection  38  is a p− diffusion in an n+ diffusion  40  in the p− substrate  30 . The diffusions  36 ,  38  are connected to guard ring areas  42 ,  44 , respectively, on the perimeter of the outermost circuitry block  22   a  by metalization layers  46 ,  48  separated by an insulation layer  50 . 
     A selected transistor  52  of the circuitry of block  22   a  may be connected to the input/output pad  32  by a metalization  54  deposited over an insulation layer  56 . The transistors in block  22   b  are unconnected and inactive. A metalization like  54  may also be used to interconnect transistors on adjacent blocks to form a multi-block circuit. 
     A passivation structure  58  connected to the substrate  20  can be formed during metalization around the periphery of the set of blocks  22  which, after cutting of the wafer, will constitute the finished IC chips. 
     The block approach of this invention lends itself well to the manufacture of various chip configurations. For example, the blocks  22  may, for example, contain rows or columns of alternating fixed-length strips of n− transistors and p− transistors  59  for digital use (FIG. 4 a ). The transistors  57 ,  59  can be interconnected with each other and with input-output pads  61  by metalization interconnections  63  (FIG. 4 b ). Alternatively, the blocks  22  may contain analog cells or strips  65  that have special function transistors  67  at each end (FIG.  5 ), resistive or other components  69 , or mixtures of these. 
     In view of the high cost of metalization masks, it is highly desirable in chip manufacture to reduce their number. Typically, a separate mask  60   a ,  60   b ,  60   c  (FIG. 6) is provided for each metalization layer to form successive layer patterns A 1 , A 2  and A 3 . In accordance with the invention, a single mask can frequently be shared by several layers in which patterns are repetitive. This can be done by placing all of the patterns A 1 , A 2  and A 3  (or, for example A 1  through A 3  and B 1  through B 6  for a multi-project wafer) onto a single mask  62   a  or  62   b  (FIGS. 7 a  and  7   b ), appropriately aligning the mask for each layer, and then blocking off all but the desired pattern with an opaque cover  64  (FIG. 8) so that only the desired pattern is exposed. The exposure in this approach must, of course, be carefully controlled. 
     It will be seen that the present invention provides a versatile, cost-saving and area-efficient method of fabricating different kinds and sizes of IC chips on a single borderless gate-array wafer with improved passivation while providing guard rings and alignment marks on the wafer substrate, that are effectively usable for all shapes and sizes of ICs. It should be understood that the method described herein and shown in the drawings represents only a presently preferred embodiment of the invention. Various modifications and additions may be made to that embodiment without departing from the scope and spirit of the invention.