Abstract:
An electronic vernier is presented which detects and quantifies misalignment between layers of material deposited upon a semiconducting wafer. Verniers may be constructed which evaluate alignment between two conducting layers, between two conducting layers and an insulating layer and between a semiconducting layer and a capacitive layer. Circuitry is described which shows how output from a vernier may be detected and quantified in order to evaluate the amount of misalignment.

Description:
BACKGROUND 
     Processing integrated circuits requires the deposit of many layers of material upon a semiconductor wafer. Precise alignment of each layer with every other layer is necessary to assure correct functioning of a finished product. Traditionally this alignment has been done by an operator examining a circuit under a microscope or by other means using optics. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiments of the present invention an electronic vernier for the evaluation of alignment of layers in semiconductor processing is provided. The embodiments provided include verniers for aligning a first conducting layer to a second conducting layer; a conducting layer to a non-conducting layer; and a semiconducting layer to a capacitive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1F show an embodiment of a vernier for aligning two conductive layers. 
     FIGS. 2A and 2B show a second embodiment of a vernier for aligning two conductive layers in accordance with a preferred embodiment of the present invention. 
     FIGS. 3A and 3B show an embodiment of a vernier for aligning a non-conductive layer between two conductive layers. 
     FIGS. 4A and 4B show an embodiment of a vernier for aligning a semiconducting layer to a capacitive layer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows a design for an electronic digital vernier for integrated circuit (IC) process evaluation. Conducting strips 101-108 are part of a first conducting layer on an IC. Conducting strips 111-118 are part of a second conducting layer on the IC. The second layer is adjacent to the first layer. As can be seen from FIG. 1A conducting strip 101 is in contact with conducting strip 111, conducting strip 102 is in contact with conducting strip 112, conducting strip 103 is in contact with conducting strip 113, conducting strip 104 is in contact with conducting strip 114, and conducting strip 105 is in contact with conducting strip 115. There is no contact between conduction strips 106 and 116, 107 and 117, and 108 and 118. 
     Alignment may be evaluated as follows. Conducting strips 101-108 are held at a voltage Vdd (logic 1) by a voltage source 120. Conducting strips 111-118 are individually connected to a node 151 of a detection circuit 150, shown in FIG. 1C. Detection circuit 150 consists of a voltage meter 153 and a resistance 152 coupling node 151 to a reference voltage (logic 0). When node 151 is connected to conducting strips 111-115, voltage meter 153 detects a logic 1. When node 151 is connected to conducting strips 116-118, voltage meter 153 detects a logic 0. Thus detection circuit 150 detects a voltage transition between conducting strip 115 and conducting strip 116. 
     In FIG. 1B, the second conducting layer has been moved to the right relative to the first conducting layer. Therefore, conducting strips 111-118 have moved to the right with respect to conducting strips 101-108. Now, when conducting strips 111-118 are individually connected to node 151, detection circuit 150 detects a voltage transition between conducting strip 113 and conducting strip 114. Determination of where a voltage transition occurs, therefore, indicates the relative positioning of the first and second detecting layers. Placing verniers such as that shown in FIG. 1A vertically (in the Y direction) and horizontally (in the X direction) on an IC, it is possible to determine alignment of layers in both the X direction and the Y direction. 
     FIG. 1D is a block diagram showing a vernier 181 with 32 vernier elements, labelled VE 31  -VE 0  (only vernier elements VE 31  -VE 21  and VE 0  are shown), and additional circuitry which could be incorporated on an integrated circuit. Vernier outputs from vernier elements VE 31  -VE 0  are coupled to a debouncing circuit 82. In FIG. 1D example values 185 are given for vernier outputs. That is, the outputs of vernier elements VE 31  -VE 27  are labelled &#34;1&#34; (&#34;1&#34; represents a logic 1), the output for vernier element VE 26  is labelled &#34;0&#34; (&#34;0&#34; represents a logic 0), and the outputs of vernier elements VE 25  -VE 0  are labelled &#34;0/1&#34; (&#34;0/1&#34; means the output can be either a logic 0 or a logic 1). 
     Debouncing circuit 182 comprises debouncing elements DE 31  -DE 0  (only debouncing elements DE 31  -DE 21  and DE 0  are shown). Generally, debouncing circuit 182 scans output from vernier elements starting with output from vernier element DE 31 . As long as vernier elements VE 31  -VE 0  output a logic 1, corresponding debouncing elements DE 31  -DE 0  output a logic 0. However once debouncing elements detects a logic 0 output from a vernier element, the remaining debouncing elements output a logic 1. Example values 186, corresponding to example values 185, are given for debouncing circuit 182. As can be seen these values are logic 0 for vernier elements DC 31  -DC 27  and logic 1 for the rest of the vernier elements. 
     From the above description it can be seen that debouncer circuit 182 assures that in outputs from its debouncing elements DE 31  -DE 0  there is at most a single transition from logic 0 to logic 1. The location of the transition is at the highest order output of vernier elements VE 31  14 VE 0  that contains a logic 0. 
     A detector circuit 183 receives output from debouncing circuit 182. Detector circuit 183 comprises detecting elements DT 31  -DT 0  (only detecting elements DT 31  -DT 21  and DT 0  are shown). At the detecting element corresponding to the location where outputs from debouncing circuit 182 makes its transition from logic 0 to logic 1, detecting circuit 183 produces a logic 1. For all other detecting elements, detecting circuit 183 produces a logic 0, as shown. Example values 187, corresponding to example values 186 and 185, show a logic 1 at the output of detecting element DT 26 . The logic 1 output of detecting element DT 26  corresponds to the transition from logic 0 to logic 1 which occurs at the output of debouncing element DE 26 . 
     A binary encoder 184 receives output from detecting circuit 183 and produces a binary coded number 188 which indicates the location of transition from logic 0 to logic 1 in the output of debouncing circuit 182. Binary coded number 188 corresponds to example values 187, i.e., binary coded number 188 is 1101 base  2 which is equivalent to 26 base  10, i.e., the location where the outputs of debouncing circuit 182 makes a transition from logic 0 to logic 1. 
     FIG. 1E shows an embodiment of debouncing elements within debouncing circuit 182, and detecting elements within detecting circuit 183. Debouncing elements 161-165 illustrate how debouncing elements DE 31  -DE 0  may be constructed. For example, debouncing element 161 has an input 161 i  from a vernier element and an input 161 c  from a prior debouncing element. A transistor pair 168 operates as an inverter, and transistor pair 169 operates as a switch. When input 161 i  is at logic 0, transistor pair 169 is switched &#34;off&#34; and a transistor 166 is switched &#34;on&#34; so that a logic 1 (represented by a &#34;+&#34; in FIG. 1E) is propagated through to debouncing output 161 o  and to an input 162 c  of debouncing element 162. When input 161 i  is at logic 1, transistor 166 is switched &#34;off&#34; and transistor pair 169 is switched &#34;on&#34;. Transistor pair 169 thus propagates the value on input 161 c  through to debouncing output 161 o  and input 162 c . Debouncing elements 162-164 operate in a manner similar to debouncing element 161. Detecting elements 171-174 illustrate how detecting elements DE 31  -DE 0  may be constructed. A transistor 176 within detecting element 171 acts as a switch. When an input 171 c  from a prior detecting element is at logic 0, the value on output 161 o  is propagated through to an output 171 o . When input 171 c  is at logic 1, a depletion transistor 177 pulls the output 171 o  to logic 0 (logic 0 is represented by the &#34;ground&#34; in FIG. 1E). An input 172 c  to detecting element 172 is coupled to transistor 176 as shown. Detecting elements 172-174 operate in a manner similar to detecting element 171. 
     In FIG. 1A, each pair of conducting strips--e.g. 101 and 111, 102 and 112, 103 and 113, etc.--form a vernier element. In each vernier element, proceeding from left to right across FIG. 1A, the conducting strip on the second layer (conducting strips 111-118) is shifted to the right an incremental distance 142--see FIG. 1F--relative to the conducting strip on the first layer (conducting strips 101-108). Incremental distance 142 is uniform throughout all the vernier elements. 
     FIG. 1F shows how incremental distance 142 can be calculated using two vernier elements. A first distance 131 is the distance from a leading edge 111a of conducting strip 111 to a leading edge 101a of conducting strip 101. A second distance 132 is the distance from a leading edge 112a of conducting strip 112 to a leading edge 102a of conducting strip 102. Incremental distance 142 is the difference in length between first distance 131 and second distance 132. 
     Incremental distance 142 may be used in conjunction with binary coded number 188 to determine a quantity of misalignment. For example, if incremental distance 142 has a value D, and binary coded number 188 has a value V 1  but would have had a value V 0  if the first and second layers had been properly aligned, then a quantity of misalignment M could be calculated using the following formula: 
     
         M=D x/V.sub.0 -V.sub.1 / 
    
     FIG. 2A shows an alternate arrangement of conducting strips. Conducting strips 201-212 are part of a first conducting layer and conducting strips 221-232 are part of a second conducting layer. In FIG. 2A, conducting strips 201-204 and 209-212 make contact with conducting strips 221-224 and 229-232 respectively. There is no contact between conducting strips 205-208 and conducting strips 225-228. Thus in FIG. 2A there are two transitions, a first transition between conducting strips 224 and 225, and a second transition between conducting strips 228 and 229. 
     In FIG. 2B, the second conducting layer has been moved to the right relative to the first conducting layer. Thus in FIG. 2B the first transition occurs between conducting strips 222 and 223, and the second transition occurs between conducting strips 226 and 227. 
     Utilizing a vernier with two transitions, as in FIG. 2A, allows for greater process independence. For instance, if conducting strips 101-108 and 111-118 in FIG. 1A are formed by etching, under etch or over etch might result in a change in the location of the transition thereby introducing uncertainty into the determination of alignment. In FIG. 2A, under etch or over etch may result in a change in location of both transition points, but the relative center of the transition points will remain in the same location. The relative center of the transition points may then be used to determine the alignment of the layers. 
     The verniers discussed above were designed to work between two conducting layers. Verniers may also be constructed which can be used for non-conducting layers. For instance, FIG. 3A shows a vernier element which may be used to construct a vernier for alignment of a first conducting layer having a conducting strip 301, a non-conducting layer 302 having a window 302a, and a second conducting layer having a conducting strip 303. In FIG. 3A conducting strip 301 makes contact with conducting strip 303 through window 302a. In FIG. 3B, the vernier element of FIG. 3A is shown with non-conducting layer 302 shifted to the left relative to the first and second conducting layers. In FIG. 3B conducting strip 301 does not make electrical contact with conducting strip 303 because window 302a has shifted relative to conducting strips 301 and 303. Using the vernier element shown in FIGS. 3A and 3B, a vernier similar to the vernier of FIGS. 2A and 2B may be constructed. 
     FIG. 4A shows a vernier element which may be used to construct a vernier for alignment of two semiconducting layers: an island (diffusion) layer having a semiconducting strip 401 and a poly-silicon layer having a gate 403. Through contacts 404, poly-silicon layer 403 is biased to a reference voltage (logic 0). The portion of semiconducting strip 401 immediately under gate 403 is biased to its non-conducting state. However a conducting channel exists which allows conduction between a location 401a and 401b on semiconducting strip 401. In FIG. 4B, the vernier element of FIG. 4A is shown with the island layer shifted to the right relative to the poly-silicon layer. Thus, in FIG. 4B, conducting channel 402 disappears and there is no conduction between location 401a and 401b. Using the vernier element shown in FIGS. 4A and 4B, a vernier similar to the vernier of FIGS. 2A and 2B may be constructed.