Abstract:
A method of increasing overlay accuracy in an exposure step in a process of manufacturing a semiconductor device for determining a measurement focus plane enabling increase in accuracy for measuring displacement includes steps of: picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon while shifting a focus plane by a predetermined first distance in a predetermined first range with respect to a reference focus plane; calculating a variation in measured values of displacement of the first and second overlay inspection marks for each focus plane; and determining a measurement focus plane by a variations in the measured values.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods of increasing overlay accuracy and overlay displacement measuring devices, and more particularly to a method of increasing overlay accuracy and overlay displacement measuring device for determining a focus plane providing an optimum measuring accuracy. 
     2. Description of the Background Art 
     Recently, semiconductor devices such as ICs (Integrated Circuits) and LSIs (Large Scale Integrations) have been increasingly reduced in size. Especially, exposure devices for transferring a circuit pattern on a mask or reticle onto a circuit pattern formed on a semiconductor wafer are required to achieve increasingly high accuracy. As the degree of integration of the devices has been increased, accuracy from 0.20 μm to 0.10 μm is required, and most recently accuracy of 0.10 μm or smaller is required. 
     Referring to FIGS. 10 to  12 B, a method of measuring overlay displacement in a conventional process of manufacturing a semiconductor device will be described. 
     Referring to FIG. 10, in the conventional method of measuring overlay displacement, positions of overlay inspection marks are measured at measuring points on selected ones of a plurality of chips  102  on a wafer  101 . At each of the measuring points, first and second overlay inspection marks  201  and  202  are formed during patterning of an interconnection or the like as shown in FIGS. 11A and 11B. First overlay inspection mark  201  is formed on a substrate, and second overlay inspection mark  202  is formed thereon. A set  204  of first and second overlay inspection marks  201  and  202  is referred to as a Box-in-Box mark  204 . 
     Generally, image recognition is used for measuring a magnitude of displacement of first and second overlay inspection marks  201  and  202  (a magnitude of displacement for Box-in-Box mark  204 ). A broadband light such as a xenon lamp is used as a light source. By detecting an intensity of the light reflected from positions near edges of first and second overlay inspection marks  201  and  202  by a camera, edge positions of first and second overlay inspection marks  201  and  202  are recognized. Distances a and b shown in FIGS. 11A are obtained, and the magnitude of displacement of first and second overlay inspection marks  201  and  202  are calculated with the following expression (1). 
     
       
         Magnitude of displacement=(a−b)/2  (1) 
       
     
     Around 1991, a TIS (Tool Induced Shift) value of the magnitude of displacement would be used to increase overlay accuracy. The TIS value of displacement represents a difference between measured values of displacement for erecting and inverted images of a wafer, respectively shown in FIGS. 12A and 12B. The TIS value can be calculated with the following expression (2). 
     
       
         TIS value=(a1−b1)/2−(a2−b2)/2  (2) 
       
     
     a1, b1: distances a and b when erecting 
     a2, b2: distances a and b when inverted 
     However, when the TIS value is determined with the inspection mark which has been formed by a prescribed process, the resulting TIS value would be large. Such large TIS value has been resulted from a displaced measurement focus plane. 
     Then, a method of determining an optimum measurement focus plane using the TIS value had been developed by around 1993. Until now, this method has been used for determining the optimum measurement focus plane. 
     Now, the method of determining the measurement focus plane will be described. 
     A wafer  101  is loaded on a stage of an overlay displacement measuring device (not shown) for measuring a magnitude of overlay displacement. A rotational correction of wafer  101  is performed using an arrangement of a plurality of chips  102  formed on wafer  101  as a reference, and a reference point on wafer  101  is selected. 
     After an output level of a light source and a focus plane of a camera are set, an image of wafer  101  is incorporated. 
     Then, a magnitude of displacement of first and second overlay inspection marks  201  and  202  are measured in accordance with the above described method of measuring the magnitude of displacement for each of chips  102 , so that the TIS value of the magnitude of displacement is obtained. 
     The TIS value of displacement is repeatedly determined by changing the focus plane and, for each focus plane, a 3S (3 Sigma) of the magnitude of displacement is obtained. Thereafter, the focus plane providing the minimum 3S of the TIS value is selected as the focus plane providing the highest measurement accuracy for the magnitude of displacement. 
     However, our examination of the above described method of determining the focus plane by 3S of the TIS value has suggested that there is not any relation between 3S of the TIS value and 3S of the measured value of displacement, as shown in FIG.  13 . Therefore, even if the focus plane providing the smallest 3S of the TIS value is determined as the measurement focus plane, the magnitude of displacement is not always measured in the optimum manner. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the aforementioned problem. An object of the present invention is to provide an overlay displacement measuring device capable of increasing accuracy for measuring a magnitude of displacement. 
     Another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement. 
     Still another object of the present invention is to provide an overlay displacement measuring device for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values. 
     Still another object of the present invention is to provide a method of increasing overlay accuracy capable of increasing accuracy for measuring a magnitude of displacement. 
     Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement. 
     Still another object of the present invention is to provide a method of increasing overlay accuracy for determining a measurement focus plane enabling increase in accuracy for measuring a magnitude of displacement in which an inspection mark having a large magnitude of displacement does not affect a variation in measured values. 
     A method of increasing overlay accuracy according to one aspect of the present invention is performed in an exposure step in a process of manufacturing a semiconductor device. The above mentioned method of increasing overlay accuracy includes: a step of picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon by shifting a focus plane by a predetermined first distance in a predetermined first range with respect to a reference focus plane; a step of calculating a variation in measured values of displacement of the first and second overlay inspection marks for each focus plane; and a step of determining a measurement focus plane based on the variations in the magnitudes of displacement. 
     The measurement focus plane is determined by the variations in the measured values of displacement of the first and second overlay inspection marks. Unlike the conventional method using the TIS values, the focus plane is determined by using data which is directly related to the measurement of displacement. Therefore, the focus plane enabling accurate measurement of displacement of the first and second overlay inspection marks can be determined. 
     Preferably, the step of calculating the variation in the measured values of displacement includes: a step of calculating an average value of the measured values of displacement of the first and second overlay inspection marks for each pair of the first and second overlay inspection marks; a step of obtaining a value by subtracting the average value from the measured value for each measured value of each focus plane; and a step of calculating a variation in the values obtained by subtracting the average values from the measured values for each focus plane. 
     The measurement focus plane can be determined based on variations in values obtained by subtracting the average value of the measured values of displacement from the measured values of displacement. Thus, the overlay inspection mark having a large magnitude of displacement and that having a small magnitude of displacement can be equally used. As a result, accuracy for measuring the magnitude of displacement can be further increased while preventing the inspection mark having the large magnitude of displacement from affecting the variation in the measured values. 
     An overlay displacement measuring device according to another aspect of the present invention includes: a camera picking up an image of a semiconductor wafer including a plurality of chips each having first and second overlay inspection marks thereon; a displacement measuring portion connected to the camera for measuring a magnitude of displacement of the first and second inspection marks of each chip for each focus plane while controlling a focus of the camera; a measured value variation calculating portion connected to the displacement measuring portion for calculating a variation in measured values of displacement in each focus plane; and a measurement focus plane identifying portion connected to the measured value variation calculating portion for identifying a measurement focus plane by the variations in the measured values. 
     The measurement focus plane is determined by the variations in the measured values of displacement of the first and second overlay inspection marks. Unlike the conventional method using the TIS values, a focus plane is determined by using data which is directly related to the measurement of displacement. Therefore, the focus plane enabling accurate measurement of displacement of the first and second overlay inspection marks can be identified. 
     Preferably, the measured value variation calculating portion includes: an average value calculating portion connected to the displacement measuring portion for calculating an average value of the measured values of displacement for each pair of the first and second overlay inspection marks; a subtraction value calculating portion connected to the displacement calculating portion and the average value calculating portion for obtaining a value by subtracting the average value from the measured value for each measured value of each focus plane; and a subtraction value variation calculating portion for calculating a variation in the values obtained by subtracting average values from the measured values for each focus plane. 
     The measurement focus plane can be determined by the variations in the values obtained by subtracting the average value of the measured values of displacement from the measured value of displacement. Thus, the overlay inspection mark having a large magnitude of displacement and that having a small magnitude of displacement can be equally used. Therefore, accuracy for measuring the magnitude of displacement is further increased while preventing the inspection mark having the large magnitude of displacement from affecting the variation in the measured values. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a structure of an overlay displacement measuring device according to an embodiment of the present invention. 
     FIG. 2 is a flow chart shown in conjunction with a method of increasing accuracy for measuring overlay displacement according to the embodiment of the present invention. 
     FIG. 3 is a graph showing 3S of a measured value of displacement when a focus plane is shifted by 300 nm. 
     FIG. 4 is a graph not having a minimal value for 3S of the measured value of displacement. 
     FIG. 5 is a diagram showing a relation between a measurement focus and the measured value of displacement for every chip. 
     FIG. 6 is a graph showing 3S of a D 1  value. 
     FIG. 7 is a diagram shown in conjunction with a method of determining a measurement focus plane in accordance with a first procedure. 
     FIG. 8 is a flow chart showing the first procedure. 
     FIG. 9 is a flow chart showing a second procedure. 
     FIG. 10 is a top view showing a semiconductor wafer. 
     FIGS. 11A and 11B are diagrams showing overlay marks. 
     FIGS. 12A and 12B are diagrams respectively showing erecting and inverted images of a semiconductor wafer. 
     FIG. 13 is a graph used for comparison of 3S of a TIS value and that of the measured value of displacement. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an overlay displacement measuring device according to one embodiment of the present invention includes: a light source  5 ; a lens  6  converging light (broadband light)  4  from light source  5 ; a fiber optic transmission device  7  passing the light converged by lens  6 ; a lens  8  converting the light transmitted through fiber optic transmission device  7  to collimated light; a half mirror  9  receiving light  4  from lens  8 ; a lens  10  receiving a portion of light  4   x  from half mirror  9 ; a mirror  11  reflecting the light transmitted through lens  10 ; a camera  12  receiving reflected light  4   k  which is the light downwardly reflected by half mirror  9  and again reflected by a wafer  101  in FIG. 1; a CPU (Central Processing Unit)  14  connected to camera  12  for increasing accuracy for measuring overlay displacement which will later be described; a memory  13  storing a program for a process to be executed by CPU  14  and other data; and a bus interconnecting CPU  14 , memory  13  and camera  12 . 
     As in the conventional case, wafer  101  is provided with Box-in-Box marks  204  (FIGS. 11A and 11B) on a plurality of chips  102  (FIG.  10 ), which will be used as measuring points. 
     In the overlay displacement measuring device according to the present embodiment, a magnitude of displacement for Box-in-Box mark  204  is measured as in the conventional case. 
     Referring to FIG. 2, a method of increasing accuracy for measuring overlay displacement will be described. As in the conventional case, wafer  101  is loaded on a stage (not shown) of the overlay displacement measuring device. Rotational correction of the wafer is performed and a focus plane of camera  12  is set. CPU  14  produces a measurement file including these data in memory  13  (S 1 ). An image of wafer  101  is picked up for each focus plane while shifting the focus plane by  300  nm in a range of ±1500 nm with respect to the focus plane which has been set in S 1 . A magnitude of displacement of every Box-in-Box mark  204  formed on chip  102  as the measuring point is measured for every focus plane. 3S of the measured values of displacement at the measuring point is calculated for each focus plane (S 2 ). The resulting 3S of the measured values of displacement is shown in a graph in FIG.  3 . 
     CPU  14  determines if a minimal value is included in the graph in FIG. 3 which has been obtained by the process in S 2  (S 3 ). If there is not the minimal value in the graph (NO in S 3 ), the process returns to S 1 , and CPU  14  newly sets a focus plane and repeats the processes in S 1  and S 2  until the minimal value is obtained. More specifically, referring to FIG. 4, when there is not the minimal value in the measurement range (a range between a and b in the drawing), CPU  14  resets the focus plane for determining a new measurement range. 
     If there is the minimal value in the graph (YES in S 3 ), the focus plane related to the minimal value is determined as a reference focus plane, the focus plane is shifted by 100 nm in a range of ±500 nm with respect to the reference focus plane, and the image of wafer  101  is picked up for each focus plane. A magnitude of displacement for every Box-in-Box mark  204  formed on chip  102 , which is to be a measuring point, is measured for each focus plane (S 4 ). An average value of the measured values of displacement is calculated for each focus plane. For Box-in-Box mark  204  of each focus plane, a value (hereinafter referred to as a D 1  value) is obtained by subtracting the average value of the measured values of displacement from the measured value of displacement of Box-in-Box mark  204 . 3S of the D 1  values is calculated for each focus plane (S 5 ). 
     Referring to FIG. 5, when the magnitude of displacement of first and second overlay inspection marks  201  and  202  is originally large, a variation in the measured values tends to be also large. As a result, Box-in-Box mark  204  originally having a large displacement may disadvantageously affect 3S of the measured values of displacement. To avoid such problem, the D 1  value is used which is obtained by eliminating a direct current component from the measured value of displacement. 
     A graph showing 3S of the D 1  value is shown in FIG.  6 . When 3S of the D 1  values is within a range of ±10 nm (YES in S 6 ), the reference focus plane obtained by S 2  and S 3  is determined as an ultimate measurement focus plane (S 7 ). 3S of the measured values of displacement can be expressed in the following expression (3). 
     
       
         3S of measured values=(3S of errors) 2 +(3S of true magnitudes of displacement) 2   (3) 
       
     
     3S of true magnitudes of displacement is generally about 30 nm. Therefore, to permit about 10% of measurement error so as to achieve 3S of the measured values of about 33 nm, 3S of the errors may be about ±10 nm. Thus, the above described numerical value of ±10 nm is used. 
     When the range of 3S of the D 1  values is wider than ±10 nm (NO in S 6 ), the number of minimal values of 3S of the D 1  values in the graph is determined (S 8 ). When there is only one minimal value (YES in S 8 ), the ultimate measurement focus plane is determined in accordance with a first procedure which will later be described (S 9 ). When there are two or more minimal values (NO in S 8 ), the ultimate measurement focus plane is determined in a second procedure which will later be described. 
     Referring to FIGS. 7 and 8, the first procedure will be described. A slice line c dividing a range between a minimal value point a and a maximum value b in the graph for 3S in the ratio of 3 to 7 is determined (S 21 ). Intersection points d and e of slice line c and the graph for 3S are determined (S 22 ). The focus plane related to a focus value for a middle point f of intersection points d and e is determined as the ultimate measurement focus plane (S 23 ). 
     Referring to FIG. 9, the second procedure will be described. The graph for 3S is approximated to the most similar quadratic function (S 20 ). Based on the graph which has been approximated to the quadratic function, the ultimate measurement focus plane is determined in a method similar to the first procedure which has been described with reference to FIGS. 7 and 8 (S 21  to S 23 ). 
     Thus, by determining the ultimate measurement focus plane using 3S of the measured values of displacement, the focus plane can be determined by using data which is directly related to the measurement of the magnitude of displacement. As a result, a variation in the measured values is minimized and accuracy for measuring the magnitude of displacement can be increased by measuring the magnitude of displacement of Box-in-Box mark  204  in the determined ultimate measurement focus plane. 
     In addition, the overlay inspection mark having a large magnitude of displacement and that having a small magnitude of displacement can be equally used as the measurement focus plane is determined by the variation in the D 1  values. Therefore, the inspection mark having the large magnitude of displacement does not affect the variation in the measured values, so that further increase in accuracy for measuring the magnitude of displacement can be achieved. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.