Abstract:
Microcircuits may include polysilicon features that are vulnerable to defects due to undesirable phenomena during manufacturing processes such as, inter alia, over-etching. The same phenomena that may cause defects can be exploited to automatically isolate the affected circuit and thus limit the harm caused by defects or incipient defects.

Description:
FIELD OF THE INVENTION 
   The field of invention relates to the semiconductor chip arts; and, more specifically, to the embodiment of design techniques that may result in increased yield and/or other benefits during manufacture. 
   BACKGROUND OF THE INVENTION 
   Integrated circuit (IC) manufacturers produce dice containing circuits on typically circular substrates referred to as semiconductor wafers. Each individual die may be of rectangular or square shape and a wafer may contain hundreds of them. The unsingulated dice on a wafer, (i.e. each unsingulated die), must ordinarily be tested to determine good from bad before the dice are singulated in order to manage cost and yield. 
   The use of photolithography with etching is well-known and commonplace in semiconductor wafer fabrication. A single photo-mask may be used with a stepper to create multiple (more or less) identical reticles—clusters of circuits that often contain more than one substantially identical unsingulated die. Thus a set of masks (often one per layer) for a reticle may include a plurality of circuit images, including images of application circuit components and elements. Each die will typically contain one or more application circuits composed of many circuit elements (such as gates, channels, lines etc.) 
   Because a typical circuit cluster, sometimes termed a “reticle”, includes more than one unsingulated die it is often advantageous to expose a partial (i.e. incomplete) reticle, such as at the wafer edge. This applies even though it is known that only some (at most) of the dice etched will ultimately be usable in products. A significant problem arises wherein systemic defects within a reticle but outside a particular unsingulated die impede the testing of that otherwise good unsingulated die. 
   Thus, there is a need for improved defect isolation in regards to semiconductor die fabrication. Benefits may include increased average yield for a fabrication process and/or improved reliability of the finished product (such as by eliminating marginal dice that might otherwise have passed testing). 
   Although embodiments of the invention were developed to address and remedy a particular class of wafer defects, the benefits of the invention may be expected to find a wider usage and utility and may extend far beyond solving the problem that originally motivated the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
       FIG. 1A  shows a plan view of a semiconductor wafer according to an embodiment of the invention. 
       FIGS. 1B and 1C  show close-up views of part of the semiconductor wafer of  FIG. 1A . 
       FIGS. 2A and 2B  show a symbolic representation of complete and incomplete dice within a single reticle of a semiconductor wafer. 
       FIG. 3  shows a further close-up view of another part of the semiconductor wafer of  FIG. 1A . 
       FIG. 4A  shows a plan view of a semiconductor wafer manifesting a manufacturing issue or problem. 
       FIG. 4B  shows a graph of after-etching notional polysilicon line width for the semiconductor wafer of  FIG. 4A . 
       FIG. 5A  represents a prior art CMOS microcircuit embodied near the edge of a wafer such as the wafer of  FIG. 4A . 
       FIG. 5B  represents the microcircuit structure of  FIG. 5A , further showing from where components have been removed due to the effect described in conjunction with  FIG. 4B . 
       FIG. 5C  represents the microcircuit structure of  FIG. 5B  without the removed components, which therefore causes a short circuit. 
       FIG. 6  shows a plan view of a part of a semiconductor wafer according to an embodiment of the invention. 
       FIG. 7A  represents a CMOS microcircuit embodied on a wafer according to an embodiment of the invention. 
       FIG. 7B  represents the microcircuit structure of  FIG. 7A , further showing from where components have been removed due to the effect described in conjunction with  FIG. 4B  according to an embodiment of the invention. 
       FIG. 7C  represents the microcircuit structure of  FIG. 7B  without the removed components, but which does not causes the short circuit of  FIG. 5C  according to an embodiment of the invention. 
       FIG. 7D  represents a microcircuit structure according to another embodiment of the invention. 
       FIG. 8  is a flowchart of a method for manufacturing a semiconductor wafer according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  shows a plan view of a semiconductor wafer  100  having a circular edge  144 . Semiconductor wafer  100  may typically contain a number of complete reticles  120 ,  121  as well as a number of partial reticles  110 ,  111  and  112 . 
     FIG. 1B  shows a close-up view of part of the semiconductor wafer  100  of  FIG. 1A  bounded by edge  144 . As shown, semiconductor wafer  100  may contain one more complete reticles  120 , and at least one partial reticle  112 . Partial reticle  112  is shown as containing a number of complete die  210 ,  222  a partial die  221  and a marginally complete die  125 . Also shown are two power rails (V DD  and GND) in the form of conductive traces and a set of probe pads  290 . On-wafer test circuits  170 ,  171 ,  175  are also shown. In the exemplary reticle shown, each test circuit is associated with, and energizes precisely one unsingulated die. For example, test circuit  177  energizes and drives unsingulated die  211  during testing by impressing signals on conductive traces  179 . Test circuits, such as  175 , will typically be discarded as scrap material during die singulation (not shown). 
   Still referring to  FIG. 1B , as shown, V DD  and GND connect probe pads  290  to on-wafer test circuits  170 ,  171 ,  175 . Also (not shown) will be other similar conductive traces that also couple probe pads  290  to on-wafer test circuits  170 ,  171 ,  175 . 
     FIG. 1C  shows a further close-up view of part of the semiconductor wafer of  FIGS. 1A and 1B . As shown, on-wafer test circuit  177  is coupled to unsingulated die  211  (shown only in part in  FIG. 1C ) by conductive traces  179 . Conductive traces  179  may include a ground rail  199 , a fixed voltage power supply rail  198  and various other traces  191  including signal traces  197 , such as may be driven by control circuitry  196 . In embodiments of the invention, the traces  179  may be tailored to be purposefully narrowed (for purposes described below) at any of several points, off-die or on-die. Traces  197  may carry either inputs or outputs, such as clock, data, enable, etc.  FIG. 1C  includes examples of on-die narrowed traces (or narrowed regions)  194  and off-die narrowings  195 . 
   Thus, pulling together the features of  FIGS. 1A ,  1 B,  1 C, under the control of an automated test system (not shown) that may impress programmed signals via probe pads  290 , on-wafer test circuits may be used to generate on-wafer test sequences for a purpose of distinguishing good dice from bad prior to singulation. These on-wafer test sequences may generate various strobe signals and also constant or quasi-constant voltages at voltage nodes that are connected to conductive traces  179  including grounds, power rails and enable inputs. 
     FIGS. 2A and 2B  show a symbolic representation of complete dice  210  and incomplete dice  221  within a single reticle of a semiconductor wafer.  FIG. 2A  shows a full reticle  120 .  FIG. 2B  shows a partial reticle  112 . Scribe lines  220  are shown and serve to guide dividing a reticle into singulated dice. In  FIG. 2B , the pecked line  244  indicates the curved edge of the wafer which also forms part of the boundary of the partial reticle  112  and of the incomplete dice  221 . Dice  121 , though complete, are proximate to the edge of the wafer and so being can raise issues as discussed infra. 
     FIG. 3  shows a further close-up view of another part of the semiconductor wafer of  FIG. 1A . Good unsingulated dice  210  are shown. Scribe lines  350  may be used in the singulation process. Also shown are two test circuits  320 . These on-wafer but off-die test circuits  320  may be used to test the unsingulated dice  210 , but since the scribe lines  350  run through the test circuits  320 , the test circuits necessarily do not survive the singulation process and are discarded. Nor are these on-wafer test circuits  320  needed after die singulation. 
     FIG. 4A  shows a plan view of a sample semiconductor wafer  400  manifesting a manufacturing issue or problem. An annular outer area  447  of the wafer adjoining the edge of the wafer  444  may be vulnerable to polysilicon over-etching due to a number of reasons, some of which may be photo-resist non-uniformity, photolithography inaccuracy, and so on. A large central portion  446  of the wafer bounded by a circle  445  is unaffected by the problem of polysilicon over-etching. 
   It is commonplace in the art to use polysilicon lines to create microcircuit conductive traces and other circuit elements, such as field-effect transistor (FET) gates, especially in Metal-Oxide semiconductor (MOS) technologies. The process typically involves etching a deposited polysilicon layer using photolithography with a mask and photo-sensitive etch resist. For whatever reasons, polysilicon lines near the edge of a wafer may be vulnerable to over-etching. Over-etching a polysilicon line will typically result in reduced width, and/or part or all of the line may disappear altogether. 
     FIG. 4B  shows a graph of after-etching polysilicon line width (y-axis) across a diameter (x-axis) of the semiconductor wafer  400  of  FIG. 4A . As shown in  FIG. 4B , over the entire central portion of the wafer the resultant polysilicon line width holds a substantially consistent value meeting specifications. But towards the edge of the wafer  444 , polysilicon line width is progressively reduced. 
     FIG. 5A  represents a prior art Complementary MOS (CMOS) microcircuit  500  embodied on a wafer substrate, such as on the wafer of  FIG. 4A . Such microcircuits may be used for many purposes including forming parts of tag circuits for RFID (Radio Frequency Identification Device) tags. As shown, V DD  and GND are the conventional power rails formed as metallization. The microcircuit depicted in  FIG. 5  may be a CMOS inverter. Signal IN port  560  and OUT port  570  may be formed as metallization. Complementary channel regions  540  operate gates formed as polysilicon lines  510  to provide the two FETs that formed the heart of the inverter circuit. 
     FIG. 5B  represents a microcircuit structure  550  similar to that  500  of  FIG. 5A  but formed near the edge of the wafer, further showing by pecked lines where components have been removed due to the effect described in conjunction with  FIG. 4B . A defect illustrated by  FIG. 5B  is the result of polysilicon over-etching. Contrasting  FIG. 5B  with  FIG. 5A  it may be seen that the polysilicon line  580  is of reduced width due the over-etching and the FET gates  581  are missing altogether. Not only will this defect prevent the microcircuit from operating correctly but it will also conduct excessive current “SHORT” between the two power rails V DD  and GND. This is, more or less, a short circuit condition that may cause other circuits that rely on the same V DD  and GND rails to malfunction also. 
   Referring to both  FIG. 1B  and  FIG. 5B , suppose that defective (short-circuited) microcircuit  550  is part of test circuit  175  or unsingulated die  222 . In these circumstances, there is a significant risk that, since test circuit  175  energizes die  222  then the combination of test circuit  175  and die  222  will short the power rails V DD  and GND together. If such a short-circuit happens then none of the good dice  170  on the partial reticle  112  may be testable prior to singulation and the entire partial reticle  112  may therefore be lost-even though it contains good dice  170 . The need for defect isolation to improve manufacturing yield is self-apparent. 
     FIG. 5C  represents the microcircuit structure of  FIG. 5B  without the removed components, which therefore causes a short circuit of microcircuit  550 A between the power rails V DD  and GND as shown. 
     FIG. 6  shows a plan view of a part of a semiconductor wafer according to an embodiment of the invention. In an embodiment, a circular boundary  645  may divide the wafer  600  into an overetched annular portion  647  and a non-over-etched central portion  646 . 
   An isolated defective microcircuit structure  621  may be embodied on a wafer  600  having an edge  644  according to an embodiment of the invention. Over-etched polysilicon lines may be formed. However, the specialized conductive trace(s)  611  are also over-etched to the point of having more or less vanished; this results in an open-circuit at  611  which may function to isolate the defective circuit  621  from the V DD  rail. Conductive trace(s)  612  (which may optionally be of the same specialized form) are not overetched, and so the good microcircuit structure  622  may be successfully tested since it is isolated from the short associated with defective microcircuit structure  621 . 
     FIG. 7A  represents a CMOS microcircuit  700  embodied on a wafer according to an embodiment of the invention. Contrasting  FIG. 7A  with  FIG. 5A , the most obvious difference is the presence, in  FIG. 7A , of additional polysilicon conductive traces  770  and  771  each with narrowed regions. The conductive traces with narrowed regions  770  and  771  are each connected to a power supply rail and may each function, in a working microcircuit, to energize the circuit by conducting current to or from a power supply voltage rail or ground. In a non-working circuit they may function as disconnect links to isolate the microcircuit from V DD  and/or ground. Other embodiments of the invention may include a different number of conductors tailored to become disconnect links under certain conditions (for example just one). 
   The specialized conductive traces in microcircuits according to embodiments of the invention may serve a similar function to fuses in macro circuits. A purpose of specialized conductive traces is to isolate malfunctioning circuits. The conductive traces  770 ,  771  of the circuits of  FIG. 7A  are tailored, by design, to be at least as vulnerable to over-etching as the microcircuit that they are intended to isolate. For example if, in the circuit of  FIG. 7A , the polysilicon line  510  embodying FET gates were to be over-etched then either or both of the specialized conductive traces  770 ,  771  would in all likelihood become open-circuit also. Thus, taken as a whole, if the circuit were malformed it would more likely be open-circuit than short-circuit. Although the microcircuit is non-functional either way (short-circuit or open-circuit), failure open-circuit would tend to isolate the defect and so improve average yield as described above. In contrast, failure short-circuit would tend to interfere with the operation of neighboring microcircuits merely sharing the same power rails and thus fail to isolate the defect. 
   Disconnect links may loosely be termed “fuses” by analogy with similarly purposed components in circuits which rely on fusion to create circuit disconnects. Although conductor fusion is not a particular feature of the present invention the absence of conductor fusion is not a requirement. 
   In order to embody the invention various approaches may be used to ensure that the specialized conductive traces do indeed disconnect whenever the protected application circuit is over etched. For example, the specialized conductive trace may be formed as a polysilicon line of width or thickness that has been tailored to be purposefully more narrowed in either width or thickness (or both) as compared with conductors in other parts of the circuit thus increasing the relative vulnerability to over-etching of the specialized conductive trace. The line width, embodied such as through photolithography may thus be tailored to have a sufficiently narrowed width that results in this desired greater vulnerability to over-etching. 
   Alternatively, since over-etching tends to occur near the edges of a wafer, and referring again to  FIG. 4B , it will readily be apparent that a gradient of over-etching may occur across a reticle. This can render it advantageous to provide two specialized conductive traces (as in  FIG. 7A ), rather than merely one. If both a first and second specialized conductive trace is provided and one of them is on each side of the polysilicon circuit element being protected, then there is an improved chance that at least one conductive traces will be more vulnerable to over-etching than the circuit element it protects. The circuit layout, embodied such as through photolithography may thus be tailored to have a layout that results in this desired greater vulnerability to over-etching for the disconnect link(s). 
   There is also an issue as to whether the narrowed conductive traces (used as disconnect links) should be placed on the die or off the unsingulated die (but still on the wafer, typically in a test circuit that is discarded in a later manufacturing stage). The circuit of  FIG. 7A  could apply to either on-die or off-die placement. 
   For on-die placement, a die may typically comprise an application circuit that includes power rails (conductors), an active circuit and one or more disconnect links. Again typically the disconnect link(s) would be connected between the active circuit and the power rail(s), thus so as to be able to isolate the active circuit from one or more power rails should the manufacture be defective (typically by over-etching). Advantages of on-die placement may include a closer proximity between the disconnect link and the active circuit being protected. Disadvantages may include increased area in the completed die and the possible need to revise a microcircuit that is already proven reliable. It is well known in the art that there is no such thing as a circuit revision that is free of risk. 
     FIG. 7B  represents the microcircuit structure of  FIG. 7A , further showing from where components have been removed due to the effect described in conjunction with  FIG. 4B  according to an embodiment of the invention. In microcircuit structure  750 , disconnect links  780 ,  781  are substantially absent due to over-etching and gates  783 ,  784  are entirely absent. 
     FIG. 7C  represents the microcircuit structure of  FIG. 7B  without the removed components, but which does not causes the short circuit of  FIG. 5C  according to an embodiment of the invention. Although microcircuit  750 A is defective and thereby non-functional is does not interfere with the testing of other dice (especially those on the same reticle). 
     FIG. 7D  represents a microcircuit structure  750 D according to another embodiment of the invention. In this embodiment specialized conductive traces  790  are narrowed by etching to produce a significant resistance rather than an open circuit. As shown the specialized traces  790  have narrowed regions that may act as an equivalent current limiting resistor. Advantageously, the current may be sufficiently limited that testing of other microcircuits on the same reticle is not impacted by the performance of microcircuit  750 D. 
   It will be apparent to those of ordinary skill in the art that various alternative but substantially equivalent topologies and placements are possible without changing the theory of operation of embodiments of the invention. For example, a disconnect link may be placed in series with a power source feeding the unsingulated die. Typically that power source may be a test circuit and the disconnect link may be created in series with the test circuit, or indeed may be embodied as a part of the test circuit itself. 
     FIG. 8  is a flowchart of a method  800  for manufacturing a semiconductor wafer according to an embodiment of the invention. In box  810  the method starts. In box  820  a polysilicon layer is formed at (on) a surface of the wafer by methods well-known in the art. In box  830 , the polysilicon layer is etched to embody circuit elements such as may be rendered in a photomask. In embodiments of the invention these may typically include circuit elements of one or more application circuits and one or more conductive traces purposefully tailored to be narrowed, thinned and/or placed in judicious locations for high vulnerability to etching. 
   In optional box  840 , each die is tested and in optional box  850  the wafer is singulated into dice. In box  890  the method ends. 
   In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.